Philosopher Aristotle coined the idea of the spontaneous generation theory in 4th century BCE, 22 centuries before time of Louis Pasteur. This theory stated that living matter could arise from non-living matter spontaneously. One of the most famous examples of this theory is that maggots could appear on decaying piece of meat.

This idea went on to persist for a very long time. This is due largely to the fact that it easily explained how mold grow on bread or that flies appear on spoiled food.

But this idea did not align to many scientists, many of them tried to disprove this idea, including Louis Pasteur.

Early challenges to spontaneous generation

For a millennium, Aristotle’s theory of spontaneous generation was widely believed around the world. Was this because of the lack of technology that science enjoys today? For example, microscopes were far from being invented to allow researchers to observe and study microorganisms. Hence, experiments to test theories were not really much of a thing back then.

By the 1600, scientists and scholars have started questioning the factualness of the theory. One of these individuals who challenged the theory was Italian physician Francesco Redi. He showed that maggots do not spontaneously arise from decaying meat by doing the so-called “Redi experiment” in 1668.

In this experiment, Redi set up 3 jars of various conditions. The first jar was open and let flies to enter the jar. The second jar was tightly to prevent flies from entering. And the last jar was covered with a mesh. After letting the jars sit for a short period, maggots appeared in the open jar and mesh-covered jar, but not the tightly sealed one.

Redi concluded that flies laid eggs that would hatch into maggots. This result suggested that living matters like maggots come from other living matters, and do not arise spontaneously. Although the Redi experiment demonstrated that living matters could only arise from pre-existing living matters, this was not sound enough to disprove the spontaneous generation theory.

Hence, the debate continued.

Antonie Van Leeuwenhoek contribution

Antonie Van Leeuwenhoek was a Dutch scientist known as the Father of Microbiology. He developed microscopes during the 17th century that were considered advance during that time. Leeuwenhoek made about 500 microscopes in this career. One of these could magnify objects up to 300 times. This capability was unrivaled back then. In comparison, the microscope English physicist Robert Hooke developed could only magnify up to 50 times. This magnification could only reveal basic details on minute organisms.

Leeuwenhoek’s, on the other hand had greater magnification. This allowed him to study various microorganisms in greater detail. Leeuwenhoek described bacteria, yeasts, and other microorganisms. Their shapes, movement, and behavior were documented for the first time. But his discovery of the existence of microorganisms was not solid evidence to dispel the spontaneous generation theory.

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When his work was made public, scientist still wondered the same question. Do microorganisms come from pre-existing living things? Or they just generate spontaneously from non-living things? Yes, the scope of microbiology back then was very limited. However, Leeuwenhoek’s contribution to understanding microorganisms paved the way for another scientist to disprove the theory of spontaneous generation at once, Louis Pasteur.

Louis Pasteur’s works prior to debunking the spontaneous generation theory

Louis Pasteur was a French chemist and microbiologist. He lived during the 19th century. At this point, the debate on the spontaneous generation theory was at its peak. Prior to disproving the theory, he already worked on fermentation, and pasteurization.

In 1850s, Pasteur studied extensively the process of fermentation. Fermentation is a preservation method wherein sugar in food is converted into alcohol or acid. Prior to Pasteur’s research on the process, it was widely accepted that fermentation was solely a chemical process. The belief was that fermentation would occur because components in food decompose in the absence of air. Hence, microorganisms were not believed to be responsible in fermentation.

But Pasteur’s work changed this when he studied spoilage in wine and beer. In the mid-19th century, the brewing industry in France was suffering from economic losses due to spoilage of wine. The losses were massive that it hit wine exports badly. To resolve the problem, Napoleon III and the French government asked for help from Pasteur. He then presented clear evidence that undesirable or spoilage microorganisms were responsible for the off-flavor and souring in wine.

What Pasteur did was preheat the wine at between 122°F (50°C) and 140°F (140). This prevented souring and extended the shelf life of wine.

Based on his research on microorganisms, spoilage microorganisms found in wine are heat sensitive. Hence, he hypothesized that treating the wine with elevated heat high enough to destroy these microbes would effectively extend the shelf life of wine. The temperature range he used was well thought of because not only it killed unwanted microbes, but it was also not high enough to preserve the flavor of the wine. This heat treatment is now called pasteurization.

Pasteur’s Swan-Neck Flask experiment debunked the spontaneous generation

Louis Pasteur became aware of the spontaneous generation when he came to know fellow Frenchmen Felix Archimède Pouchet, a strong follower of the spontaneous generation theory. Pasteur had been very skeptical about the theory, and the French Academy of Sciences opened a competition called Alhumbert Prize to ultimately put an end to this debate. Pasteur took up the challenge and performed an experiment that would ultimately debunk the theory— the Swan-Neck flask experiment.

In this experiment, Pasteur gathered a number of long, curved S-shaped flasks that looked like swan’s neck, hence the name of the experiment. He filled each flask with an infusion or nutrient rich broth. After that, he pasteurized the flasks to destroy the harmful microorganisms that were present in the broth.

After letting the pasteurized broth in the flask to sit for some time he observed what happened. And just as he predicted, the broth did not change in appearance or appear to have been contaminated. The unique S-shape of the flask prevented contaminated to happen here. The curve neck allowed air to flow through, but not dust and any other elements that may contaminate the broth.

But if the curved long neck of the flasks were removed, or the flask were tilted that the broth got into contact with the curve neck, airborne microorganisms would have been introduced to the broth and contaminate it.

The Swan-Neck flask experiment by Pasteur ultimately debunked the spontaneous generation theory. Because of this, he was awarded the Alhumbert prize, which also carried a value of 2,500 francs. This was considered a huge sum already in 1862.

The post How Louis Pasteur Debunked the Spontaneous Generation Theory appeared first on The Food Untold.

Salmonella is a type of bacteria that can cause foodborne illnesses in humans. There are various strains of Salmonella. Among these, Salmonella enterica is one of the most common species that can infect people. It is responsible for the many cases of food poisoning each year, with symptoms ranging from mild gastrointestinal discomfort to severe illness. According to the Centers for Disease Control and Prevention (CDC), Salmonella cause about 1 million cases of foodborne illness every year. The bacteria can contaminate a wide range of foods, especially raw or undercooked foods such as meat, egg, and poultry.

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With the wide range of foods that salmonella can contaminate, you might ask if cooking can kill the bacteria. In this blog, we will explore the role of cooking in killing Salmonella and preventing foodborne infections.

WHAT IS SALMONELLA?

Salmonella, a Gram-negative bacterium, presents rod-shaped cells with lengths ranging from 2 to 5 micrometers when viewed under a microscope. These cells are non-sporulating, meaning they do not form spores, and they are motile, equipped with flagella that enable them to move.

Salmonella exhibits adaptability in its growth conditions. It is a facultative anaerobe, capable of thriving in both oxygen-rich and oxygen-poor environments. Its optimal growth temperature falls within the range 98°F (37°C) to 107.6°F (42°C), mirroring human body temperature. This characteristic is significant as it enables Salmonella to colonize the human gastrointestinal tract effectively. The optimal pH for Salmonella is between 6.5 to 7.5. The bacteria can also withstand low pH levels of around 4.5. This enables them to survive the acidic conditions of the stomach.

Moreover, Salmonella exhibits relative heat resistance, which raises concerns for food safety.

From a pathogenic standpoint, certain Salmonella strains are known to be harmful to humans and animals, causing diseases such as salmonellosis, typhoid fever, and gastroenteritis. These pathogenic strains possess virulence factors that facilitate host cell invasion and disease progression, including fimbriae, flagella, and type III secretion systems.

Transmission of Salmonella primarily occurs through the ingestion of contaminated food and water. Many food items, particularly those of animal origin, such as poultry, eggs, and meat, can serve as reservoirs for Salmonella. Fecal-oral transmission may transpire when food is mishandled, or when contaminated hands or surfaces come into contact with food.

Symptoms of Salmonella infection

Salmonellosis, an infection caused by the Salmonella bacterium, can be contracted by individuals when they consume food that has been contaminated, including inadequately cooked eggs, meats, poultry, or unpasteurized milk.

The symptoms of salmonellosis usually develop within 12-14 hours of eating the contaminated food, but can range from a few hours to several days.

Symptoms include nausea, vomiting, abdominal pain, headache, chills, diarrhea, prostration, muscular weakness, faintness, fever, restlessness, and drowsiness. These Symptoms typically last for 2-3 days.

The mortality rate for Salmonella food poisoning is 4.1%, but varies depending on age. The mortality rate is highest in infants (5.8%), followed by people over 50 years old (15%).

Up to 5% of people who recover from Salmonella food poisoning become carriers of the bacteria. This means that they can carry the bacteria in their intestines without showing any symptoms, but they can still spread the bacteria to others.

Now, back to the question.

CAN SALMONELLA BE KILLED BY COOKING?

Yes, Salmonella can be effectively killed by cooking when the correct temperatures are reached and maintained. According to a study, exposure to a temperature of 140°F (60°C) to 149°F (65°) for several minutes is sufficient to destroy salmonella bacteria.

The process of killing Salmonella through cooking relies on the application of heat. When food reaches these recommended temperatures and is held at them for a sufficient duration, the heat effectively breaks down the Salmonella bacteria’s cell walls and denatures the proteins within the cells. This process destroys the bacteria’s ability to cause infections, making the food safe to eat.

When preparing food, especially those susceptible to Salmonella contamination, cook it to the recommended temperature. For example, when cooking poultry, such as chicken and turkey, ensure it reaches a minimum internal temperature of 165°F (73.9°C). For ground meats like beef, pork, or lamb, aim for a temperature of 160°F (71.1°C). When cooking eggs, make sure both the egg white and yolk have solidified. Seafood should be heated to 145°F (62.8°C) or until it easily flakes apart with a fork. When reheating leftovers, be certain they reach a temperature of 165°F (73.9°C) to eliminate any potentially harmful bacteria that may have multiplied during storage.

References:

G. Cooper (2018). Food Microbiology. Library Press.

J. Jay, M. Loessner, D. Golden (2005). Modern Food Microbiology (7th edition). Springer.

Y. Motarjemi (2014). Encyclopedia of Food Safety. Academic Press.

The post Can Salmonella Be Killed By Cooking? appeared first on The Food Untold.

Unlike walnuts and pecans, peanuts are grown beneath the soil. And for this reason, they are prone to infections, especially aflatoxins. Aflatoxins are produced by the soil-dwelling fungi (mold) Aspergillus flavus and A. parasiticus. In peanuts and other nuts and grains, it is the most important of the mycotoxins from a human health perspective.

Aflatoxins are quickly and widely absorbed from the gut and are metabolized to toxic epoxides in the liver. These epoxides bind to and harm vital cell components like DNA, RNA, and protein enzymes. The primary clinical outcome of aflatoxin consumption is liver damage in all animal species. According to epidemiological studies, aflatoxins and the hepatitis B virus are cocarcinogens, and the risk of liver cancer is higher in areas where both aflatoxins contamination and hepatitis B are prevalent.

Internationally, aflatoxins in peanuts are required to not exceed 15 μg/kg. This considerably very low MRL (maximum residue limit) ensures that peanuts with high concentrations of aflatoxins cannot be traded. In the United States, the Food and Drug Administration (FDA) has set the allowable limit of aflatoxin levels in peanut products at 20 ppb.

Fortunately, there are several ways to remove aflatoxins in peanuts.

In commercial operations, aflatoxins are regularly removed from food or food ingredients. Solvent extraction methods have been used in most cases. Some foods or food ingredients have been treated with appropriate chemicals to inactivate aflatoxins. Some chemicals, such as ammonia, hydrogen peroxide, and sodium hypochlorite, have been found to be effective at inactivating aflatoxins.

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Let’s discuss further.

WHAT IS AFLATOXIN?

Mycotoxins are toxic secondary metabolites produced by certain fungi. When consumed in large amount or regularly may cause certain cancer. Of all mycotoxins, aflatoxins have unquestionably been the most studies. Consumers have been aware of their existence since 1960, when over 100, 000 turkey poults in England perished after consuming imported peanut from Africa and South America.

Aflatoxin is classified into two types, B1 and G1. Each of them has several subtypes. Aflatoxin B1, produced by A. flavus and A. parasiticus, is thought to be the most potent. In fact, it has been classified as carcinogenic (Group 1) to humans by the International Agency for Research on Cancer (IARC). For more discussion on these types of aflatoxin, refer to this file.

Aflatoxin production has been shown to occur in many food products, which include nuts, dried fruits, and cereals. In general, toxin production is directly related to a mold strain’s growth rate. In microbiological media suitable for mold growth, Asp. flavus strains can produce optimum concentrations of aflatoxin at 91°F (33°C), pH 5.0, and Aw (water activity) of 0.99. Some toxin can be detected within 24 hours under optimal growth conditions.

In peanuts, aflatoxin formation occur primarily during the curing of peanuts after they have been removed from the soil. Hence, it is crucial to perform proper handling and storage to control the growth of aflatoxins. Moisture and temperature during storage should especially be given more attention. They are the two most important factors that affect aflatoxin formation. The presence of aflatoxins in peanuts is obvious if there is discoloration, visible mold growth, and the nuts are shriveled.

Aspergillus spp.-contaminated peanuts. Source

With that being said, here are 5 effective methods that you can do you remove aflatoxins from peanuts.

5 EFFECTIVE METHODS OF REMOVING AFLATOXINS IN PEANUTS

Aflatoxin contamination can occur in the field prior to harvest, during harvesting, or during storage and processing. While certain treatments have been found to reduce aflatoxin formation in peanuts, the most effective way is to avoid it in the first place. This is not always possible. But there are technologies and methods that can prevent contamination that would otherwise occur.

A variety of industrial food-processing methods can also be used to manage the aflatoxins in peanuts. Some of these can also be performed at home. As simple as sorting by color, screening, and lot separation (either with or without blanching) can reduce the toxin levels. In subsistence farming situations, the manual sorting of inferior nuts is also an option. Aflatoxins are thought to be heat stable, but roasting peanuts can reduce B1 aflatoxins by 50–80%.

Sorting and cleaning

Sorting and cleaning is the simple yet effective way for removing aflatoxins from peanuts. This entails removing any discolored, moldy, or damaged and shriveled peanuts from the batch because they are more likely to contain aflatoxin. To remove any dirt or debris, peanuts can be washed in clean water. Although this method is simple and easy to use at home, it may not remove all of the aflatoxin found in peanuts. Hence, this should be combined with another method.

In some practices, water is treated so that the process is more effective. For example, peanuts may be soaked in electrolyzed acidic water. Electrolyzed acidic water has a low pH of around 2.5, and is used as an effective disinfectant in food contact surfaces. One study has found that soaking grains in electrolyzed acidic water for 15 minutes can reduce 80% to 90% of B1 aflatoxin in peanuts.

Roasting

One of the most widely used techniques for removing aflatoxin from peanuts is roasting. High heat causes the aflatoxin molecules in peanuts to disintegrate, rendering them safe to consume. It’s crucial to remember that not all roasting techniques are equally efficient. As already mentioned, aflatoxins are heat stable. Studies say the optimum temperature for the production of aflatoxins is between 81°F (27°C) and 86°F (30°C). But they can withstand heat at normal cooking temperatures.

It has been demonstrated that dry roasting, which involves roasting peanuts without any additional oil, is more efficient at lowering aflatoxin levels than oil roasting. Additionally, to ensure that all aflatoxin molecules are destroyed, peanuts must be thoroughly and evenly roasted. Dry roasting is typically done on a very high heat.

To remove aflatoxins from peanuts properly, ensure to reach a temperature of 320°F (160°C) or higher. This study has shown that at a temperature of 320°F or higher, aflatoxins can be completely degraded. Another study showed that adding sodium chloride and citric acid can substantially improve the effectiveness of the process. If you wish to replicate the process, the authors recommend using up to 5% of citric acid. More than that may adversely affect the taste of peanuts.

Chemical treatment

Commercially, chemical treatment to remove aflatoxins in peanuts is widely practiced. What is involved in this method are common disinfectants or sanitizers in the food industry. One of them is hydrogen peroxide, a strong oxidizer. The peanuts are soaked in a hydrogen peroxide and water solution for a set amount of time before being thoroughly rinsed. One study said that treating peanuts with 0.075% of hydrogen peroxide for 1 minutes can reduce aflatoxin levels by up to 90%. Another good thing is that hydrogen peroxide residue can be removed by drying. Furthermore, it is environmentally friendly since it breakdowns into oxygen and water once exposed to light.

Although this method has been shown to significantly reduce aflatoxin levels in peanuts, it may also have a subtle effect on the taste and quality of the peanuts.

Other chemicals used to remove aflatoxins in peanuts include ammonia, sodium hypochlorite, and benzoyl peroxide.

Irradiation

Irradiation is one of the most effective methods in making foods shelf life stable, in general. But its high cost required makes it less utilized. Irradiation can remove aflatoxins by exposing the peanuts to ionizing radiation to break down the aflatoxin molecules. Most applications of irradiation require a maximum overall average dose of 10 kGy (kilogray).

For the purpose of microbial reduction, the doses range between 1 to 10 kGy. However, the outcome is positively correlated with an increase in the radiation dose. In one study, a 10 kGy dose reduced the toxin levels in peanuts to 58.6. Another study used lower doses of 4, 6, and 8 kGy, which reduced the aflatoxin levels to 7.6%, 17.3%, and 23.25% respectively. Nonetheless, irradiation is a viable option in reducing the aflatoxins in peanuts to below the maximum allowed levels.

While effective, the irradiation process may become less appealing to consumers who are reluctant to consume irradiated foods due to possible exposure radiation. It is crucial to remember that the radiation used to irradiate food is extremely low and has been approved as safe by regulatory bodies such as the Food and Drug Administration (FDA) and World Health Organization (WHO).

Furthermore, the taste, texture, or nutritional value of the peanuts are unaffected by irradiation. Peanuts are rich in proteins and fats. And these macronutrients are barely unaffected by radiation at usual doses.

Biological control

Biological control is a technique for removing aflatoxins from peanuts that involves using naturally occurring predators or competitors of the fungus that causes the toxin. Many factors influence a fungus’s ability to compete for a host. These include the soil type, pH level, water content, mineral available, nitrogen and carbon availability. The soil microbiome, primarily fungi and bacteria, influences fungi’s ability to produce secondary metabolites.

Using a strain of the A. flavus that does not produce aflatoxins is one example of bio-control. Aflatoxin levels in peanuts are decreased when this strain is introduced to the soil where peanuts are grown because it outcompetes the strain that produces the toxin. This 2020 study evaluated 18 non-aflatoxigenic strains of A. flavus. 6 of them reduced the aflatoxin levels produced by the native aflatoxigenic strains by 50%. Although promising, this strategy faces challenges that prevent it from being a clear solution to reducing aflatoxins in peanuts. For example, its biology is not well understood due to the diversity of A. flavus.

An alternative to this is using other species that can effectively affect the fungi from producing aflatoxins.

In this 2018 Korean study, the authors used Aspergillus oryzae M2040, a strain isolated from fermented soybean. 1% inoculation level of the A. Oryzae strain showed that it can effectively displace A. flavus and inhibit aflatoxin production.

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Another example is the bacterium Bacillus subtilis, whose many strains are safe for human consumption. It also produces enzymes that break down aflatoxins. This Japanese study used strains of B. subtilis that proved to be inhibitory on the growth of both A. flavus and A. parasiticus.

References:

Y. Motarjemi, G. Moy, E. Todd (2014). Encyclopedia of Food Safety. Academic Press.

Y. Motarjemi, H. Lelieveld (2014). Food Safety Management: A Practical Guide for the Food Industry. Academic Press

G. Cooper (2018). Food Microbiology. Library Press.

J. Jay, M. Loessner, D. Golden. (2005). Modern Food Microbiology (7th edition). Springer.

B. Ray (2005). Fundamental Food Microbiology (3rd edition). CRC Press.

I. Shaw (2013). Food Safety: The Science of Keeping Food Safe. John Wiley & Sons, Ltd.

The post How To Remove Aflatoxins From Peanuts? appeared first on The Food Untold.

Aspergillus species play a less significant part in the production of fermented foods (at least in foods popular in Western cultures). But they are still a component of some of the fermented foods that are consumed the most around the world. One example of important specie in food is Aspergillus oryzae (A. oryzae). A. oryzae is a filamentous fungus (a mold), also known as “koji” mold, commonly used in Chinese, Japanese, and other East Asian cuisines.

Its primary uses include fermenting soybeans to produce soy sauce and miso (fermented bean paste). It is also used to sweeten rice, barley, other grains, and potatoes to produce alcoholic beverages like hōchū, huangjiu, sake, and makgeolli (Korean rice wine). Billions of people actually consume these Asian fermented foods.

In Japan, this mold is culturally important. In the journal of the Brewing Society of Japan, Dr. Eiji Ichishima of Tohoku University referred to the koji fungus as a “national fungus” (kokkin). This is due to its significance in the production of koji for miso, soy sauce, and a variety of other traditional Japanese foods in addition to koji for sake brewing.

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Studies suggest that the Japanese domesticated Aspergillus flavus. A. flavus is a pathogenic fungus that produces aflatoxins. But what the Japanese used mutated to stop producing the aflatoxins. This eventually gave rise to the beneficial A. oryzae mold.

Let’s discuss further the Aspergillus oryzae genome as well as its various uses in food.

ASPERGILLUS ORYZAE: THE KOJI MOLD

A. oryzae or simply Koji mold, is an aerobic filamentous fungus in the Aspergillus subgenus Circumdati section Flavi. Aspergillus section Flavi also contains industrially important species such A. flavus and A. parasiticus, both of which generate aflatoxins.

While closely related to other Aspergillus species, A. oryzae does not produce aflatoxins. Rather, the ability of Aspergillus oryzae to ferment makes it an important fungus in food manufacturing. In fact, the Food and Drugs Administration (FDA) has listed it as Generally Recognized as Safe (GRAS).

A. oryzae grows best at 89°F (32°C) to 97°F (36°C), and cannot grow over 111°F (44°C). It prefers a pH of 5 to 6 for growth, and can germinate in pH between 2 and 8. Aspergillus Oryzae has been observed to grow in dry food, such as corn flour with a water content of about 16%. It can grow on media with a water activity (aw) greater than 0.8, although it rarely grows below 0.8.

A. oryzae haploid genome contains 37 million base pairs and 12,000 predicted genes organized into 8 chromosomes. A consortium of Japanese biotechnology companies revealed this information in late 2005.

A. oryzae‘s genome is one-third larger than the genomes of two related Aspergillus species, the genetics model organism A. nidulans and the potentially deadly A. fumigatus. Many of the additional genes found in A. oryzae have been linked to secondary metabolism. The sequenced strain, RIB40 or ATCC 42149, was obtained in 1950. The specific increase of genes for amino acid metabolism, secretory hydrolytic enzymes, and amino acid/sugar uptake transporters make A. oryzae an ideal microorganism for fermentation.

SOY SAUCE MANUFACTURING

Soy sauce is a dark brown liquid that is made by fermenting soybeans and wheat in a salt brine. Manufacture starts with the preparation of raw materials. Soybeans or defatted soybean flakes are cooked after being moistened. For each form of soy sauce, the cooked beans are mixed with toasted, cracked wheat in varying proportions.

Soybeans are cooked in a continuous cooker at high temperature and pressure for a short period of time. In koikuchi, usukuchi, and saishikomi shoyu soy sauce manufacture, the same amount of cooked soybeans and roasted wheat is blended. And then the mixture inoculated with an A. oryzae or Aspergillus sojae pure koji starter. Certain Aspergilli species are also utilized in the production of usukuchi and shiro shoyu to prevent the formation of a deep color in the following steps of fermentation.

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After three days of fermentation, a 17% to 19% salt solution is applied to the koji (fermented grains) to create liquid mash called moromi. During the moromi process, osmophilic lactic acid bacteria and yeasts provide a distinct taste, scent, and color in moromi. High concentrations of proteins and carbohydrates from raw materials are significantly destroyed by koji enzymes and microbial action.

To ensure homogenous conditions and accelerate microbial growth, the moromi mash is occasionally stirred and aerated. Lactic acid bacteria, such as Pediococcus soyae or Lb. delbrueckii are permitted to grow on the moromi to make it acidic enough to prevent spoilage and sour taste. Afterwards, yeasts like Saccharomyces rouxii and Torulopsis sp. grow on the moroni to produce alcohol and aid in flavor production. The moromi process takes 4 to 8 months to complete, depending on temperature, agitation, and air supply from an air compressor.

SAKE FERMENTATION

Alcoholic beverages from East Asia are very distinct, compared to those of Western origin. It is because of two reasons. First, their main ingredient is fermented starchy grains, usually rice. Another reason is that fermentation does not occur from grain enzymes into sugars. Instead, a mold is introduced to start the fermentation process in unison with yeasts. A good example of a product that is produced by this is Japanese sake (rice wine).

This alcoholic beverage is transparent, pale yellow. The substrate food is steamed rice starch, which is hydrolyzed to sugars by Aspergillus oryzae to produce the koji. In general, whole or brown rice is milled to remove 25% to 50% of the surface material (germ and bran), which is required because the fat and protein components are undesired. The rice is then rinsed and steeped for several hours to reach a moisture content of about 30%. The damp rice is then cooked for an hour and chilled to 86°F (30°C) to 95°F (35°C). Three-quarters of this rice are removed and chilled to 41°F (5°C) to 50°F (10°C) for later use. The final fourth is utilized to make koji.

Saccharomyces sake ferments the substance for 30-40 days, giving in a product with 15-20% alcohol and roughly 0.3% lactic acid. The main fermentation takes place in open tanks under cool circumstances, starting at around 50°F (10°C) and rising to around 59°F (15°C). S. cerevisiae strains used in sake production differ from those used in wine and beer production. They have greater osmotic, acid, and ethanol tolerance.

Following fermentation, moromi is separated from the solids to create clarified saké, which is settled, refiltered, pasteurized, mixed, and diluted with water before bottling.

MISO MAKING

Miso originated in China and Korea thousands of years ago. But Japan is today’s leading producer and consumer. Miso is a popular fermented soy bean product. If you have not tasted one before, miso tastes just like soy sauce, but liquid or paste-like, and with a texture similar to that of thick peanut butter. It is used in Japan to make soups and broths. It is also used as a seasoning or flavoring agent. Products similar to miso include doenjang (Korean bean paste), taoco (Indonesian bean paste), and taosi (fermented black soybeans from the Philippines).

Miso is manufactured in a manner similar to that of soy sauce, with one notable exception. Dry salt, rather than brine, is added straight to the koji-soy bean mixture during miso production. For this reason, the product has roughly double the total solids of soy sauce (50% to 60% against 24% to 28%).

The production process begins with the manufacture of koji. Rice, barley, or soybeans can be used as substrate. At 59°F (15°C), the rice or barley is soaked in water overnight before steaming in a batch or continuous cooker. After cooling, a spore culture of specific strains of Aspergillus oryzae and A. sojae is utilized as the inoculum at 0.1%. The koji is then cultured in fermentation chambers at 86°F (30°C) to 104°F (40°C) for 40 to 48 hours.

After that, miso is made by combining salt, koji, steamed soybean, and water, and adding the halo-tolerant yeast, Zygosaccharomyces rouxii or Candida versatilis. The salt is then added to help the yeast and LABs ferment and to prevent any unwanted form of fermentation.

The mixture is allowed to mature for three to twelve months.

References:

M. Shafiur Rahman (2007). Handbook of Food Preservation (2nd edition). CRC Press.

R. Hutkins (2006). Microbiology and Technology of Fermented Foods. Blackwell Publishing.

J. Jay, M. Loessner, D. Golden. (2005). Modern Food Microbiology (7th edition). Springer.

M. Gibson (2018). Food Science and the Culinary Arts. Academic Press.

B. Ray (2005). Fundamental Food Microbiology (3rd edition). CRC Press.

G. Cooper (2018). Food Microbiology. Library Press.

Y. H. Hui (2012). Handbook of Plant-Based Fermented Food and Beverage Technology (2nd edition). CRC Press.

The post Aspergillus Oryzae And Its Uses In Food appeared first on The Food Untold.

Salting is one of the oldest forms of food preservation. Before the invention of the refrigerator, the majority of households used salt to extend the shelf life of various foods. Today, salting is one of the most prevalent pretreatments for fish products. It reduces the moisture content of fresh fish and acts as a preservative, converting it into shelf-stable items.

Salting does this by preventing the growth of spoilage bacteria and molds while allowing friendly bacteria and local enzymes to populate and break down the relatively flavorless protein molecules and lipids found in fish. This, in turn, reacts to produce even more complex flavors.

Since ancient times, people have been aware of the impact of a food’s water content on its perishability. Between 15,000 and 10,000 BC, our forefathers began to dry foods to preserve them. They used the wind and sun to dry excess fish and meat, and later did the same with excess fruits. The combination of drying and salting contributes to the development of distinctive sensory characteristics of products, which impact their use as food.

Fish that has been salted for a day (with a little salt) often stays fresh for a few days, while fish that has been steeped in salt last for about a year or more. Lean cod and other fish from related families have always been salted before being air dried. In contrast, fish like herrings and their relatives are preserved from rancidity caused by the air through brining and/or smoking.

Let’s discuss further.

WATER ACTIVITY IN FRESH FISH

In food science, the term water activity is a commonly discussed term. It refers to the amount of free or unbound water, water that can be used for various processes, particularly to support microbial growth. Most foods contain a water activity of around 0.95. The water activity of most fresh fish is over 0.85. This is enough for microorganisms to thrive and spoil food. This is why they are highly perishable.

To prevent this, the water activity must be lowered down. Pathogenic bacteria are inhibited by a water activity of 0.85 or lower. While certain yeasts and molds can survive in a 0.75 water activity environment, they can no longer create toxins. In fish salting, the water activity decreases because salt draws the water out in a process called osmosis. The idea is to increase the concentration of salt, a solute. In this manner, water diffuses between cells in the environment. This results in the same concentration of salt on both sides of the cell. According to the Food and Agriculture Organization of the United Nations (FAO), a concentration of between 6 to 10 % salt will prevent the growth of most spoilage bacteria. 

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The procedure results in a saline equilibrium between the muscle and the surrounding salt solutions. Under typical temperature conditions, the maximum concentration obtainable is that of a saturated brine solution. Because of the osmotic pressure between the brine and the fish muscle, sodium chloride or salt diffuses through the fish flesh via a dialysis mechanism, and water diffuses to the outside.

This process does not go on indefinitely. The sodium and chlorine ions create a water-binding complex with protein. This in turn produces an osmotic pressure that attains an equilibrium.

MICROBIAL SPOILAGE OF FISH

Most microorganisms responsible for the spoilage of fish halophobic. Halophiles are microorganisms that can thrive in an environment of high salinity— with a high concentration of salt. And they will not grow until there is 10% salt present. Examples of halophiles include Halobacterium salinaria, H. cutirubum, Pseudomonas spp Sarcina morrhuae and S. litoralis.

These bacteria are aerobic—they require oxygen for growth, and are rarely found in pickled fish, where the brine provides only limited oxygen access. They are also thermophilic, with an optimum growth temperature of approximately 42°C and a minimum growth temperature of approximately 5°C.

These bacteria produce pink spoilage. Pink spoilage are so-called because of the color of their colonies and the resulting appearance of the cured fish. A delicate pink sheen on the surface of the fish in wet stack or during pining is the first indicator of pink spoilage. This can be readily removed without harming the fish.

Maintaining the ambient temperature below 50°F (10°C) is likely to prevent initial germination and growth of halophiles. Treatment with formaldehyde or sulphur dioxide vapors, or dipping the fish in a solution of sodium metabisulphite also work. Although food poisoning cases that were allegedly brought on by eating pink-spoiled fish were likely actually brought on by the spread of Staphylococcus aureus, a bacteria that produces exotoxins.

This bacteria start growing at somewhat greater water activity levels than those required for the growth of pink bacteria. Salt fish has the water activity of a saturated common salt solution of 0.75, regardless of how much it has been dried before and after salting.

Many microorganisms will rupture in very high salt solutions due to the difference in pressure between the exterior and inside of the microorganism.

METHODS OF MAKING SALTED FISH

Depending on the fish composition and size, salting may be dry or wet.

The rate of salt diffusion is proportional to the concentration gradient between the salting medium at the surface and the point in the fish most distant from the salting medium. Hence, the stronger the brine, the faster the salt uptake and to achieve a water activity low enough for preservation.

Fish are salted whole and uneviscerated, eviscerated and split open, or in smaller pieces ranging from fillets to mince, depending on their size. Only small species, such as anchovies and small herring, can be salted whole without gutting. Large fish usually have fish skins that prevents proper salt penetration. Hence, large fish treated in this manner would decay before the salt could have an effect.

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The most frequent approach is a mix of the dry and wet method. The fish are immersed in dry salt before being immersed in the liquid pickle produced by the salt solution in the liquid extracted from the fish. This is also known as ‘blood pickle’.

The dry method involves stacking the fish in salt by directly applying it onto the surface.

And the brine produced is allowed to run away. Anchovies are commonly dry-cured by layering it with sufficient salt to saturate the tissue of the fish.

This is then placed under weight and stored at high temperatures around 86°F (30°C) for 6 to 10 months. This is a very traditional Mediterranean dish that can be eaten as is or combined with oil or butter to make a paste. Muscle, skin, blood cells, and enzymes, along with bacteria and the warm curing temperature, all work together to stimulate the early stages of browning reactions and generate several aromatic molecules.

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Cod can also be dry-cured in 25% salt marinade for at least 15 days. The fish can be stored after for several months as Micrococcus bacteria break down molecules, generating trimethylamine (TMA) and flavor-generating free amino acids. Furthermore, oxygen works on the fat molecules, forming fatty acids that contribute to the aroma of the fish as well. A further 3 days of artificial drying in a controlled cold chamber completes the process.

The wet method involves immersing the fish in a strong brine, or ‘pickle’.

Wet salting is accomplished through brining and pickle curing. The method employed is determined on whether the product will be further processed by drying or smoking, or just preserved by salting. The wet method is ideal for fatty fish such as mackerel and sardine.

Because herrings and comparable species can contain up to 20% fat by weight, traditional dry curing causes rancidity. During the early days of making salted fish, air would be removed from the equation. And the fish would be wet-cured in a mild brine (16%-20% salt solution), allowing the fish to be stored for up to a year.

Then, a method was devised that allowed a portion of the intestine rich in digesting enzymes (the pyloric caecum) to remain inside the fish. In addition to the fish muscle and skin enzymes, the digestive enzymes of the pyloric caecum collaborate to break down proteins. The result is a softer texture and a complex flavor-aroma that was slightly cheesy, fishy, and meaty. This technique is still followed today, and the herrings are eaten as they are, without being salted or cooked.

References:

G.M. Hall (1997). Fish Processing Technology (2nd edition). Blackie Academic and Professional.

M. Gibson (2018). Food Science and the Culinary Arts. Academic Press.

M. Shafiur Rahman (2007). Handbook of Food Preservation (2nd edition). CRC Press.

The post The Science Of Making Salted Fish appeared first on The Food Untold.

When talking about cheeses, most people think about cheddar, Swiss, American, mozzarella, or Parmesan cheese. These types of cheese have one thing in common—they taste and smell good. So there is no wonder why they are the most commonly eaten. In the United States, the most popular cheese is Cheddar, according to a poll conducted by Yougov. 1 for every 5 Americans or 19% prefer the cheese that originated from Somerset, England. Came in 2nd at 13% preference is Uncle Sam’s own American cheese, and 3rd (9%) is mozzarella cheese, which was followed closely by Swiss cheese (8%).

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Now where is the Limburger cheese? Is it surprising? Limburger, Muster, and other similar cheese types are known for their undesirable smell, to many, at least. This pronounced stench that Limburger cheese gives off is because of a certain bacteria involved during the ripening process.

Well, aged cheeses involve certain microorganisms during the ripening process that provide the distinct characteristics of the final product. Take the Swiss cheese for example. The production of Swiss cheese involve cultures of Propionibacterium shermanii. During cheese ripening, the propionibacteria consume the lactic acid of cheese to produce acetic and propionic acid. Along with the acid is carbon dioxide, the gas responsible for the characteristic eyes of Swiss cheese.

In the case of Limburger cheese, the bacteria responsible for the pungent smell are Brevibacterium linens. Although the bacteria make the cheese smell like rotting, they would not make anyone feel ill. An aversion to the odor of rotting has the apparent biological benefit of keeping us safe from food illness. So it is no surprise that a food made from an animal that smells like shoes or soil takes some getting used to.

BREVIBACTERIA IN LIMBURGER CHEESE

The surface growth of B. Linens are a necessary condition for the creation of the distinctive color, flavor, and aroma of smear surface-ripened cheeses, Limburger cheese particularly. Brevibacteria are smear bacteria that are natives of salty environment (up to 15% salt concentrations). These salty conditions inhibit the growth of most other microorganisms. Brevibacteria also grow in warmer conditions, and grow optimally at temperatures between 68°F (20°C) and 86°F (30°C). However, they do not withstand acidic environments well, unlike most finishing bacteria. They grow well at neutral pH and pH between 6.5 to 8.5.

When used for producing Limburger cheese, one key point cheese makers have to know is that the bacteria are an obligate aerobic bacteria—they require oxygen to grow. For this reason, the bacteria are introduced into the cheese during the ripening process by wiping it with a salt brine. This is unlike cheeses that are ripened by lactic acid bacteria, which are nonaerobic or aerotolerant microorganisms.

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The presence of Brevibacteria at the surface lead to excessive lipolysis and proteolysis. Lipolysis is the breakdown of lipids and fats, producing fatty acids, whereas as proteolysis is the breakdown of proteins.

HOW BREVIBACTERIUM LINENS PRODUCE THE SMELL

The breakdown of fats and proteins on the surface is what makes Limburger cheese distinct from most cheeses. Lipolysis and proteolysis form several carboxylic acids, such as volatile 3‐methylbutanoic, butanoic, and hexanoic acids. These acids give off aromas which are very similar to those of sweaty feet or gym socks smell.

The reason for this is because another Brevibacteria habitat of these bacteria is the human skin. And if you are wondering, they are the same bacteria responsible for body and foot odor. Our feet, when salty, sweaty, and moist just become the perfect place for them to thrive.

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It is said that the human skin is most likely the original source of the culture. Well, yes, it is possible that a cheese maker ate a cheese after growing it in a salty, warm, and oxygenated environment. This was probably how the bacteria were transferred from skin to cheese. Besides, this was how most foods we consume today started.

Other aroma molecules responsible for the smell of Limburger cheese include methanethiol and methyl thioacetate. Methanethiol is a very volatile molecule with a characteristic odor that is characterized as or “rotten egg-like” or”cabbage-like”.

The post The Bacteria That Make Limburger Cheese Smell appeared first on The Food Untold.

Yogurt is a dairy product made by fermenting and coagulating milk. Our ancestors figured out by accident that fermentation makes bread and beer. Historians believed that yogurt was invented 5000 years ago in Mesopotamia (modern-day Iraq). However, its origin is not singular. In ancient Greece, the Greeks produced oxygala, one form of yogurt. In ancient Egypt, Egyptians learned that fermenting milk is still safe to consume and that it provides similar health benefits to fresh milk. Some historians also claimed that yogurt originated in Turkey. In fact, yogurt got its name from the obsolete Turkish word “yogmak”, which means to thicken”.

Today, yogurt manufacturing is done by cultivating one or more pasteurized dairy products like cream, milk, skim milk, or milk with the fat removed with a bacterial culture. The process is similar to making buttermilk and sour cream. However, the incubation period and the types of bacteria is different.

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Microorganisms in yogurt exist in friendly form (good bacteria), known as probiotic flora. They convert lactose, a sugar found in milk, into lactic acid. The tart, acidic flavor of yogurt is a result of the production of lactic acid. Just like cheese, yogurt depends on the growth of bacteria to produce flavor compounds, ripening enzymes, and acidity. The more bacterial strains, the stronger the acidic flavor. The acid also contributes to the thickening of the milk, giving it a consistency yogurt is known for.

Starter cultures used in making yogurt are lactic acid bacteria (LABs). The lactase enzyme, which is necessary to break down lactose into its component sugars, glucose and galactose, is absent in most bacteria. LABs, however, readily digest lactose and utilize glucose as an energy source.

Here are the common starter cultures used in yogurt.

STREPTOCOCCUS THERMOPHILUS AND LACTOBACILLUS BULGARICUS

The process of making yogurt commercially entails gradually acidifying previously heated milk. The milk must be heated prior to inoculation to kill any competing microbes, often at 85°C to 90°C for 5–20 minutes. This causes the whey proteins to lose their natural properties and permits disulfide exchange events with the cysteine-containing caseins. These processes alter the micellen surface, which improves the coagulum’s acidified textural characteristics.

 

The two most widely used bacteria strains in the United States are Streptococcus thermophilus and Lactobacillus bulgaricus. The Food and Drug Administration (FDA) requires that these two specific LABs must be present in the product to be called yogurt. These LABS co-exist in the milk in a stable associative relationship called photocooperation, wherein there is an exchange of metabolites and/or stimulatory factors.

S. thermophilus grow preferentially in milk because most strains have fewer nutritional requirements. In fact, no growth of Lb. bulgaricus is seen during the initial exponential growth of Thermophilus.

The growth of S. thermophilus (less acid tolerant) slows down in the second phase when the pH of the milk starts to drop, and it provides a variety of growth factors, including formate, pyruvate, folate, CO2, and even some long-chain fatty acids that encourage the growth of Lb. bulgaricus. The latter then releases cytoplasm and cell wall peptidases and proteases, which hydrolyze caseins into peptides and amino acids, respectively.

Since S. thermophilus strains lack extracellular proteases, cocultures with Lb. bulgaricus strains significantly accelerate their growth as they serve as an amino acid source to support a second exponential growth phase for S. thermophilus. In the third growth phase, the growth of Lb. bulgaricus continues.

HOW STARTER CULTURES ARE ADDED

The addition of starter culture happens after pasteurization and homogenization of milk.

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Any competing bacteria are eliminated during the heat treatment, which typically lasts 5–20 minutes at 185°F (85°C) – 194°F (90°C). This heating process enables disulfide exchange events with the cysteine-containing caseins and denatures the whey proteins as well. These reactions modify the micelle surface and contribute to favorable textural characteristics in the coagulum.

After pasteurization is homogenization typically at pressure between 15 and 20 MPa using a single-stage homogenizer. This technique successfully reduces fat globule size, increases fat surface area, and covers the surface with mostly proteins; casein micelles cover approximately 25% of the surface. In addition to evenly blending all the ingredients. Homogenized milk yogurt is firmer, smoother, and more stable (has less wheying and creaming) during storage.

After homogenization, the mixture is cooled to incubation temperature between 104°F (40°C) to 113°F (45°C) and then pumped into fermentation tanks.

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The starter culture (3% w/w of S. thermophilus and Lb. bulgaricus in a 1:1 ratio) is either introduced to the fermentation tanks or is metered directly into the mixture while pumping. However, fermentation can be in retail containers (set-style) or in bulk tanks (stirred). A temperature around 42°C is maintained for 2 to 2.5 hours. During this period, the titratable acidity and/or pH of the yogurt must be checked. Once a pH of 4.6 or titratable acidity of 0.85 to 0.90% is reached, there should be a solid mass of gel that has formed. At this point, the yogurt is cooled to around 41°F (5°C). The product can be stored or further processed to produce other forms of yogurt.

References

N.A. Michael Eskin, F. Shahadi (2013). Biochemistry of Foods (3rd edition). Academic Press.

Experts from Mayo Clinic, University of California, Los Angeles, and Dole Food Company, Inc (2002). Encyclopedia of Foods. Academic Press.

V. Vaclavik. E. Christian (2014). Essentials of Food Science (4th edition). Springer.

S. Damodaran, K. Parkin (2017). Fennema’s Food Chemistry (5th edition). CRC Press.

Starter cultures are essential ingredients in yogurt making.

The post Starter Cultures In Yogurt appeared first on The Food Untold.

Osmosis is one type of diffusion wherein there is a net movement of molecules that pass through a semipermeable membrane to an area of lower concentration. Examples of semipermeable membrane include synthetic membrane for dialysis and cell membranes in cells of living things like plants and animals. The movement of molecules keeps on until the solute concentration on both sides of the membrane is equal, reaching an equilibrium.

Osmosis is a regular thing in many foods. When fruits are cooked in water, the water migrates into the tissues through osmosis. And sugar, which usually around 12 to 15%, goes through diffusion.

Dried fruits such as raisins become plump. Pectins become soluble and diffuse into the water, resulting in less dense cells and a softer product. Lignins are unaffected by the softening of cellulose. The fruit begins to lose its form.

The main importance of osmosis is its capability to preserve foods and extend their shelf life. Osmosis usually occurs in the presence of salt or sugar solutions. The addition of solutes such as salt or sugar allows the removal of high percentage of water out of bacterial cells to equal the low level of water in the surrounding medium. Sucrose is the most widely utilized osmotic agent in fruits, while sodium chloride is used in vegetables, fish, and meat. Other examples of osmotic agents in food processing include glucose, fructose, lactose, maltose, dextrose, sorbitol, whey, maltodextrin, polysaccharide, and combination of these agents.

This post discusses how osmosis preserves foods and extend their shelf life.

BY LOWERING THE WATER ACTIVITY OF THE FOOD

Just like humans and other living things, microorganisms require water to thrive and multiply. The so-called water activity (Aw) refers to the amount of unbound or free water in the food. This means it is the water available for spoilage microorganisms to consume.

Water activity can be anywhere between 0 to 1.0. Water activity is 1.0 in pure water, whereas the water activity of fresh fruits, vegetables, and meats range from 0.98 to 1.00. This is why they are highly perishable items.

To preserve foods and extend their shelf life against pathogenic bacteria, lowering the water activity via osmotic dehydration is one way to achieve this. Osmotic dehydration is the removal of water from a lower concentration of solute to higher concentration through a semipermeable membrane. This movement of water lowers the water activity and inhibits the growth of spoilage microorganisms.

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Sugar syrup with fruit slices or cubes and salt or brine with vegetables are the most common solutes used in osmotic dehydration. The diffusion process is multicomponent. That means water flows from the fruits or vegetables to the solution, and some of their components, such as minerals, vitamins, and fruit acids, also flow towards the solution. Sugar and salt tend to gravitate toward fruits and vegetables.

This dehydration procedure does not typically result in a shelf-stable product with a low moisture content. As a result, the osmotically treated product should be further processed (usually by air, freeze, or vacuum drying processes) to achieve shelf stability, or the dehydration procedure could be utilized as a pretreatment for canning, freezing, and minimum processing.

Another preservation method that uses osmosis to foods is curing by salt. By lowering the moisture content and serving as a preservative, salting transforms fresh meat into shelf-stable items. These processes, when combined with drying, aid in the formation of distinct sensory properties in the goods, which influence their use as food.

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BY DIRECTLY ATTACKING THE SPOILAGE MICROORGANISM

The internal osmotic pressure of microbial cells is higher than that of the surrounding medium (food). So the cell wall is subjected to turgor pressure, which provides support the mechanical force required for cell expansion and development. But when the microorganism is in a concentrated aqueous solute solution of reduced water activity because of osmosis, water migrates out of the cytoplasm.

Cytoplasm is the gel-like fluid inside of the cell where reactions take place. The water movement results in loss of membrane turgor. From here, the internal equilibrium (or homeostasis) is disrupted, and the organism will not reproduce.

However, the microorganism will stay in lag-phase until equilibrium is re-established. Lag phase is part of the bacterial growth curve. In the lag phase, microbial population is constant. And the cells are more into adapting to the environment. This post discusses the bacterial growth curve in more detail.

How exactly microbial cells react to osmosis and low water activity?

There are two main ways microbial cells react to adapt to osmosis and changing water activity in food. Here they are.

By accumulating low molecular weight solutes in the cytoplasm

One of the most common responses of cells to lowered water activity is the accumulation of low molecular weight solutes in their cytoplasm at concentrations slightly beyond the external medium’s osmolality. In this manner, the cells regain or prevent water loss through osmosis so the turgor in the cell membrane is maintained for proper functioning.

In 1981, Chirife et al. theoretically calculated the intracellular water activity from the solute content of several bacterial cells cultured in medium with aw values ranging from 0.85 to 0.993. Their study found that the intracellular water activity is equal or somewhat less than that of the growth medium. In terms of cell water content, the overall reaction appeared to represent a homeostatic mechanism.

Salt and sugar are solutes that help preserve food via osmosis. However, not all solutes cause harm to microbial cells.

There are these so-called compatible solutes because they do not interfere with the cell’s metabolic and reproductive functions. What they do is attract water and restore or partially restore isoosmotic conditions across the cell membranes to allow essential metabolic reactions to continue.

They specifically have these properties:

• Small and usually neutral or zwitterionic molecules
• Soluble at high concentrations and can be accumulated in the cytoplasm
• They cell membrane demonstrates permeability to them.

Examples of compatible solutes in bacterial cells include amino acids such as proline, glutamic acid, and amino butyric acid. Predominant protoplasmic solutes in fungi include polyols such as mannitol, arabitol, sorbitol, and glycerol. Many foods that we consume already contain compatible solutes like proline, choline, and betaine to allow growth at low water activity. In some instances, compatible solutes can be delivered from the environment or synthesized de novo in the cell’s cytoplasm.

Adaptation of the membrane composition

Another major reaction of microbial cells to lowered water activity and osmosis is the adaptation of the membrane composition. For many bacterial cells, the most common modification is the increase in the proportion of anionic phospholipids and/or glycolipids in the membrane. Glycolipids serve as receptors for cell-cell communication as well as a structural role in membrane integrity. This adaptation is a way of maintaining the correct bilayer phase to maintain its vital functions.

References:

P. Zeuthen, L. Bogh-Sorensen (2003). Food Preservation Techniques. CRC Press

V. Vaclavik, E. Christian (2014). Essentials of Food Science (4th edition). Springer

M. Shafiur Rahman (2007). Handbook of Food Preservation (2nd edition). CRC Press

The post How Osmosis Preserves Foods (And How Microbial Cells React) appeared first on The Food Untold.

Studies in history have found that humans first started fermenting foods as early as 6000 B.C in the Fertile Crescent (crescent-shaped region in the Middle East). However, fermentation was a poorly understood method back then. It was until the mid-19th century when French scientist Louis Pasteur showed how fermentation works. In a series of experiments he performed, he proved that fermentation of food occurs in the presence of certain microorganisms. In 1877, through years of studies on fermentation, he published his famous book, Etudes sur la Bière (Studies on fermentation). One of the key findings in his studies is that lactic acid is produced by microorganisms (bacteria) in lactic acid fermentation. Based on Pasteur’s findings, many studies have followed, which have allowed us to have a better understanding of fermentation.

Today, fermentation can be classified into 3 types. These include lactic acid fermentation, alcoholic fermentation, and acetic fermentation. Sure, fermentation is quite a topic. So in this blog post, we’ll cover lactic fermentation, whose applications include fermented vegetables such as pickles, kimchi, sauerkraut, and fermented milks such as cheese and yogurt. Before we begin, let’s define what fermentation is.

WHAT IS FERMENTATION?

Fermentation is one of the earliest and simplest forms of food preservation—no heat or artificial energy source necessary. It is a process wherein microorganisms change the sensory and functional properties of food, producing an end product that is desirable to the consumer. Basically, microorganisms do this by transforming organic substances into smaller molecules. A good example of this is alcoholic fermentation in which yeasts ferment glucose to produce carbon dioxide and alcohol.

In the case of lactic acid fermentation, lactic acid bacteria (LAB) ferment glucose to produce carbon dioxide and lactic acid. Lactic acid fermentation can be divided into two: homolactic and heterolactic fermentation.

During homolactic fermentation, one mole of glucose converts into two lactic acid moles. Up to 85% lactic acid can be produced in this reaction.

During heterolactic fermentation, on the other hand, one molecule of glucose yields one mole each of lactic acid, ethanol, and carbon dioxide. However, this reaction only produces 50% lactic acid.

What’s interesting with LABs is that they are able to produce organic acid, thereby lowering the pH or acidity level in the food. The decrease in the level of acidity creates an environment that would inhibit the growth of spoilage microorganisms and foodborne pathogens.

LACTIC ACID BACTERIA

LABs share common characteristics. They are gram-positive, catalase-negative, non-spore forming, and fermentative anaerobes. Most of their cellular energy come from the fermentation of sugars, which in return produce lactic acid. Except streptococci, LABs do not cause harm to humans. This makes them an ideal preservative agent.

How they ferment and preserve foods come from several mechanisms. The most important one is by rapidly raising the acidity level at which it inhibits the growth of undesirable microorganisms. Lactobacilli are also capable of producing hydrogen peroxide, which, too, is inhibitory to spoilage microorganisms.

Hererofermenters aside from ethanol and lactic acid also produce carbon dioxide. The following are the antifungal compounds produced by LABs during fermentation.

• Acetic acid
• Caproic acid
• Carbon dioxide
• Cyclic dipeptides
• Diacetyl
• Hydrogen peroxide
• Lactic acid
• Phenyllactic acid
• Proteinaceous compounds
• Reuterin
• 3-hydroxy fatty acids

In the food industry, LABs also serve as acidulant, and dough conditioner. In various foodstuff, LABs are added deliberately to produce many kinds of fermented foods such cereals, fish, meat, vegetables, and legumes. Interestingly, these microorganisms in foods are called probiotics, the good bacteria. They have many ways to help our body healthy, especially the digestive system.

PRODUCTS OF LACTIC ACID FERMENTATION

Common products of lactic acid fermentation are pickled vegetables such as kimchi, olives, and sauerkraut. Fermenting milk also produces products such as yogurt and cheese.

Yogurt

Yogurt is a dairy product produced by coagulating milk through lactic acid fermentation. It can be produced using whole, low-fat, or skim milk. The nonfat milk solids are raised to 12%–15% by concentrating the milk, or adding condensed milk or powdered skim milk. The milk is then pasteurized at 82°C–93°C for 30–60 minutes, and cooled at 40°C–45°C. Then, yogurt starter is added at 2% by volume and incubated for 3 to 5 hours. The length of incubation is also finished if the final product reaches a pH of 4.4-4.6 or titratable acidity of 0.85%–0.90%.

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The two organisms involved in yogurt fermentation is Streptococcus thermophilus and Lb. delbrueckii subsp. bulgaricus. The existence of the two in a 1:1 ratio results in lactic acid and acetaldehyde production at a greater rate than that produced by either organism when growing alone. Streptococci produce carbon dioxide, lactic acid, and formic acid. The presence of formic acid stimulates the growth of lactobacilli. Lactobacilli, on the other hand, liberate amino acids necessary for the growth of streptococci. It also leads to production of acetaldehyde, which contributes mostly to the typical yogurt flavor, and lactic acid to lower the pH to 4.4-4.6. In some yogurt culture, Lactobacillus acidophilus is present to reduce excessive aldehyde and for added health benefits.

Cheese

Cheese is a dairy product made from coagulating milk protein casein. According to the Food and Drug Administration, cheese can be coagulated with rennet, lactic acid, or other suitable enzyme or acid. For thousands of years, processing of milk into cheese makes milk readily available and less perishable. Today, fresh cheeses are also available. Cheeses like mozzarella, ricotta, and cream cheese do not require fermentation. However, many cheeses are produced by introducing starter culture bacteria to milk.

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During cheesemaking, the addition of starter bacteria to pasteurized milk ferments it and at the same time reduces the pH level to induce curdling. To achieve curdling, two main groups of LABs can be used: moderate -temperature lactococci (such as mesophilic) and the heat-loving lactobacilli (thermophilic). Both of these groups are heterofermenters. However, the bacteria to use will depend on the particular step necessary to produce the cheese. If the milk has to be subjected to a temperature of 70°C, lactococci will effectively ferment the milk, reduce the pH to induce the formation of curd. Mesophilic bacteria can also thrive under these conditions. However, for cheeses (like hard cheeses) that require high temperature during cooking, thermophilic bacteria is ideal since they thrive in this condition. In almost all types of cheeses, an enzyme called rennet is also used to make the curd elastic and strong.

The breakdown of lactose during the fermentation process makes the cheese more digestible.

Fermented Vegetables

Perhaps vegetables are the most common applications of lactic acid fermentation. In western countries, the most commercially important ones are cabbage, cucumbers, and olives. Vegetables such as carrots, peppers, okra, onions, cauliflower, and celery are also preserved by lactic acid fermentation. Unlike other foods, these vegetables rely on the natural flora, hence fermentation may occur spontaneously. Lactic acid bacteria may thrive if the conditions are favorable to them. When fermenting vegetables, factors such as aerobic condition, temperature, moisture or water activity, and salt concentration must be taken into account. Salt concentration is particularly important.

The vegetable is submerged in a brine solution of appropriate concentration. This creates an environment unfavorable to unwanted microorganisms (spoilage microorganisms). The salt also extracts water from the vegetable, serving as a substrate for the growth of LABs. The concentration to use varies depending on the vegetable. For cabbage (sauerkraut), the brine solution should be around 2.5% and 10% for olives. Koreans use higher brine concentration when making kimchi. The brine solution can be as high as 26%. Although the standard brine solution is 15% for traditional kimchi.

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Olives receives special treatments. Prior to brining, olives are treated with lye solution for up to 8 hours to remove the bitter-tasting oleuropein.

What species of LABs are involved in vegetable fermentation?

The species involved in lactic acid fermentation depend on the stage of fermentation. For production of sauerkraut, Leuconostoc mesenteroides grows first, which produces carbon dioxide, acetic acid, and lactic acid. This is followed by the growth of Lb. brevis and then lastly Lb. plantarum. The presence of these LABs lowers the pH to below 4.0 allowing the cabbage to be stored for longer periods under anerobic conditions.

For high-salt pickles, Pediococcus cerevisiae initially grows. As the acidity lowers, the more acid-tolerant LABs Lb. plantarum and Lb. brevis thrive. Leuconostoc mesenteroides is more active in low-salt pickles, but contributes little to high-salt pickles.

In green olives fermentation, the LABS Leuconostoc mesenteroides and Pediococcus cerevisiae initially dominate. Then followed by lactobacilli Lb. plantarum and Lb. brevis.

References

M. Shafiur Rahman (2002). Handbook of Food Preservation (2nd edition). CRC Press.

Y. H. Hui, E. Ozgul Evranuz (2012). Handbook of Plant-Based Fermented Food and Beverage Technology (2nd edition). CRC Press.

M. Wallert, K. Colabroy, B. Kelly, J. Provost (2016). The Science of Cooking: Understanding the Biology and Chemistry Behind Food and Cooking. John Wiley & Sons, Inc.

V. Vaclavik and E. Christian (2014). Essentials of Food Science (4th edition). Springer.

N. A. V. Eskin (2005). Biochemistry of Foods. Academic Press.

(2002). Hand

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For the most of us, whenever we hear the words “bacteria”, “fungi” or “microorganisms”, the first things that come to mind are negative things such as diseases. Well, it is definitely true that there microorganisms that do us harm. But not all the time. Let’s talk about Escherichia coli (E. coli) for example. One particular strain of E. coli is O157:H7. Someone who ingested food that is contaminated with this strain of E. coli may experience food poisoning with severe symptoms. But for most of the time, E. coli does that cause harm or adverse health effects. In fact, this bacteria lives in our intestines and those of animals.

The negative connotations associated with microorganisms is undeserving. And on the contrary, many of them are beneficial in different fields or industries. In medicine, without microbes, we would be able to produce vaccines and antibiotic. Soil microbes help farmers recycle plant materials and decompose organic matter.

In the food industry, a lot of food products that we enjoy now would not have existed without them. One species of yeast that we have worked with for thousands of years is Saccharomyces cerevisiae. This fungus is one of the most important in the food industry. It has been used extensively in the manufacture of fermented beverages such as wine and beer, distilled beverages such as vodka and rum, and baked goods. But the strains of Saccharomyces cerevisiae involved in the manufacture of these products vary tremendously.

Let’s discuss further.

WHAT IS SACCHAROMYCES CEREVISIAE?

Like other species of yeasts, Saccharomyces cerevisiae is a eukaryotic, unicellular microorganism. The cells can exist in two forms: haploid or diploid. Most cells exist in diploid form, in which the cells are ellipsoid-shaped with a diameter of 5-6um, Cells in haploid form are spherical with a diameter of 4um.

Cells reproduce both sexually and asexually.

More often, S. cerevisiae reproduce asexually. In a process called budding, a haploid cell undergoes mitosis, forming new haploid cells or daughter cells that bud off the mother cell. The new cell grows bigger until it reaches the size of the mother cell and separates.

During sexual reproduction, two different haploid yeast cell mate, forming a diploid cell. This diploid cell then undergoes mitosis to form zygotes.

S. cerevisiae is a facultative anaerobe—it grows well aerobically and anaerobically. In nature, S. cerevisiae is commonly found in ripe fruits, particularly grapes. All strains can feed aerobically on sugars, including glucose, maltose, and trehalose, but not on disaccharide lactose and cellobiose. Anaerobically, some strains do not grow on trehalose and sucrose. Among these sugars, S. cerevisiae prefers glucose the most.

A 1977 study found out the optimum temperature for rapid growth of all strains of S. cerevisiae to be between 86 °F (30 °C) to 95 °F (35 °C).

Since it is easy to culture, S. cerevisiae is the most studied eukaryote. In fact, S. cerevisiae was the first ever eukaryote genome to be fully sequenced in 1996. The S. cerevisiae genome is made up of over 12 million base pairs and over 6000 genes, packaged in 16 chromosomes. Visit the Saccharomyces Genome Database for more on this.

Brewing

It is hard to pinpoint the exact origin of beer fermentation. But according to history, the oldest piece of evidence was a chemically confirmed barley beer found in modern day Iran.

Basically, beer is produced using germinated cereal grains (referred to as malt), flavoring like hops, water (which accounts for 93% of beer by weight), and yeasts. Yeasts are perhaps the most important ingredient in beer brewing. It is largely responsible for beer’s final characteristics—the alcohol content, appearance, aroma, and flavor—through a process called alcoholic fermentation.

When yeasts are added, they start feeding off the sugar available. The sugars in beers are mostly maltose, a dissacharide. The sugars consumed by yeasts are converted into alcohol and carbon dioxide. The final ethanol content by weight of beers vary from about 3% to 8%. Carbon dioxide is responsible for that fizz sound whenever we open a can of beer. However, CO2 produced during fermentation is allowed to escape. Oftentimes, brewers increase the carbonation by introducing pressurized CO2. Beer fermentation takes a week to several months to complete. This mainly depends on the type of beer (strength) and the yeast involved.

Once fermentation has finished, the beer is conditioned. This is where the yeast settles at the bottom of the fermentation tank, clarifying the beer. The yeast can be collected and reused for the next brewing process.

Around the world, there are over a hundred beer styles that exist. These include lagers, ales, and stouts. One main difference between these beers is how they are fermented. Ale beers are produced using S. cerevisiae yeast at temperatures of 53.6 °F to 64 °F. Whereas lagers are produced using Saccharomyces carlsbergensi yeast at a colder temperature of 46.4 °F to 53.6 °F. Both both ales and lager beers can be dark or light in appearance.

Baking

There are generally 3 main types of leavening agents in baked products. These include physical leaveners such as air or steam, chemical agents such as baking soda and baking powder, and biological agents such as yeast. Unsurprisingly, the species of yeasts more synonymous with baking is S. cerevisiae. This is why S. cerevisiae is also called baker’s yeast.

Occasionally, bakers use other species of yeast in baking. Saccharomyces exiguus is typically used as sourdough yeast.

Baker’s yeast come in several forms. In commercial baking, where the daily production volume is immense, cream yeast is used. Cream yeast looks similar to a yeast slurry. It is about 85% water and 15% S. cerevisiae yeast. Cream yeast only lasts for up to 10 days, so refrigeration and additional equipment during storage is necessary.

Another form of yeast widely used in commercial baking is compressed yeast. Compressed yeast is similar to cream yeast, but contains less liquid. It is generally 70% water and 30% yeast. Like cream yeast, compressed yeast has a very short life span. For this reason, compressed yeast is now less common, especially in developing countries.

Active dry yeast and instant yeast are common forms of yeast for baking at home. In many home recipes, both forms can be used interchangeably. The main difference between the two is that active dry yeast requires dehydration before use. Whereas instant yeast can be added and mixed directly with other ingredients. Instant yeasts also requires less time to rise.

One advantage of active dry yeast has a longer shelf life than other forms of yeast. It can last for a year at room temperature.

Winemaking

Wine is an alcoholic drink generally made from fermented grape juice. Like in brewing beer, the addition of S. cerevisiae yeast converts the sugar in the fruit into ethanol and carbon dioxide.

Some winemakers use wild yeast to ferment wine for more interesting complex flavors. Thousands of years ago, wines were fermented using wild or “natural” yeasts. They tend to be more active once the grapes have matured enough. However, one major flaw of using wild yeast is its unpredictable nature. And a lot of wild yeasts do not produce quality wine. Most of these yeasts belong in the Kloeckera and Candida genera.

In order to produce quality wines consistently, commercial wineries inoculate strains of S. cerevisiae yeast.

Throughout history, vintners or winemakers have used fruits (apple wine) other than grapes, vegetables, and grains (rice wine such as sake). But wine varieties made from these do not usually produce wine with qualities similar to those made from grapes. The main reason for this is that they contain less fermentable sugars and water to maintain proper fermentation.

Grapes are high in sugars. The initial sugar content of the grape juice dictates the alcohol level of the resulting wine. Unripe grapes contain predominantly glucose. Ripe grapes contain equal amount of glucose and fructose, both of which are fermentable sugars. Other sugars in the grapes in smaller amounts include pentoses, cellobiose, and galactose, all of which are unfermentable sugars.

After fermentation, the wine can have an alcohol or ethanol content between 11-13% on average. This depends on several factors such as the wine variety and the winemaker. For example, some winemakers intentionally stop the fermentation process before the yeast converts of all the sugars into alcohol. This results in a sweeter wine.

Other references

M. Shafiur Rahman (2007). Handbook of Food Preservation (2nd edition). CRC Press.

M. Wallert, K. Colabroy, B. Kelly, J. Provost (2016). The Science of Cooking: Understanding the Biology and Chemistry Behind Food and Cooking. John Wiley & Sons, Inc.

J. Jay, M. Loessner, and D. Golden (2005). Modern Food Microbiology (7th Edition). Springer

O. Zaragoza, A. Casadevall (2021), Encyclopedia of Mycology, Elsevier.

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