Ohmic heating, also called Joule heating, is one of the few food processing methods that uses electricity. James Prescott Joule discovered that electricity gives off heat, and this heat is produced from resistance (ohms) in 1841. To put it simply, electricity is converted into heat energy. This discovery has led to believe that this process can be used to process and preserve food.

This has turned into fruition when Ohmic heating was used to pasteurize milk in the late 19th century. The process involved flowing milk between two plates of different electric voltages. At the turn of the 20th century, progress in Ohmic heating was considerably limited though.

This was due to various reasons, mainly technological constraints (lack of suitable electrode materials), lack of the understanding of the process, and financial considerations. In 1930s, the process was called “Electro-Pure”. In the 1950s, a process of sterilizing milk using Ohmic heating was developed. However, the process was discontinued because of the high operational costs and the excessive use of electricity.

Interest in Ohmic was revisited in the 1980s because there was a need for a process that could efficiently sterilize liquid–large particle mixtures. Today, Ohmic heating has become an attractive option for food manufacturing. This is in large part of the unending research and study about the technology.

Let’s discuss Ohmic heating further.

What is Ohmic heating?

Ohmic heating is a food process wherein electric current is converted into heat energy to heat the food. It is also known as Joule heating, electrical resistance heating or resistance heating, and electro-heating. A Ohmic heating device commonly consists of a power source, an electrode, and a way to contain the sample, a vessel or tube, for example.

Ohmic is performed by passing alternating current (AC) or direct current (DC) to the food material placed between electrodes. As the electricity hits the food, salts, water and other components resist this energy and generates heat.

This food process is based on the principle of electrical resistance. Electrical resistance is the measure of how resistant a material is to the flow of electric current. In the case of Ohmic heating, the food material is the electrical resistor. The resistance depends on various factors, including the composition of the food, and the amount of moisture present.

Because of this resistance, the electric energy is dissipated and converted into heat. This heat generation is rapid that allows quicker and uniform heating of food than in thermal-based food processes. Ohmic heating is particularly effective for liquid–particle mixtures as they allow uniform heat transfer.

Ohmic heating vs conventional heating methods

Conventional thermal processing methods such as boiling, sterilization, and canning have been known to cause product quality degradation because of slow convection and conduction heat transfer.

Slow heat transfers in conventional heat treatments are fully known in the food industry. In canning, food is heated by conduction, wherein heat is transferred from one material to another. In the case of canning, heat is transferred from the can to the food. This heat transfer is slow, especially if the can is considerably larger or the food is densely packed inside.

This is where Ohmic heating is a much better option. With Ohmic, the entire food mass can be heated volumetrically. It is possible to heat large portion of food by up to 1 in.3. And since Ohmic heating can heat food rapidly, it can transfer heat to the inner portion without difficulty.

This is unlike conventional methods that sometimes overprocess the surrounding liquid portion of the food being heated as it transfers heat to the inner portion. This uneven heating diminishes quality.

In nutrient retention for example, it has been studied that food that has undergone conventional heat treatments contain lower nutrients, particularly heat-sensitive ones.

One of the findings in this study is the reduction vitamin C in food processed under Ohmic and conventional heating. In conventional heating, the product’s vitamin C reduced by 13.58%. Ohmic heating, on the other hand, was not that different from unheated sample. Another study found heating under Ohmic helps retain significantly higher antioxidant activities and essential minerals essential minerals calcium, potassium, and phosphorus.

This just suggests that Ohmic heating is better at preserving food quality, especially nutrients, than conventional methods.

Uses and applications

The first use of Ohmic heating was for sterilizing milk in the 19th century. Since then, its usage than expanded to various sectors in the food industry. It is now also used for food items such as fruit juices, purees, meat, canned goods, and vegetables.

Let’s briefly discuss some of the processes Ohmic heating is used for.

Thermal processing

Pasteurization and sterilization have long been used for processing liquid like milk and fruit juices. Pasteurization involves temperatures that destroys and reduce the levels of spoilage microorganisms, whereas sterilization destroys all microorganisms, including beneficial ones (probiotics, for example).

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Integrating Ohmic heating with these thermal processes helps rapid and uniform heating to maintain product quality. In the United States, Ohmic heating is used to produce liquid egg product.

Fermentation

Fermentation is an anaerobic process that uses microorganisms to convert carbohydrates such as sugar into alcohol. Examples of fermented products includes kimchi, yogurt, and wine.

The application of Ohmic heating in fermentation has been less common than when used with thermal processes. One beneficial use of it in fermentation is the reduction of acidity in cocoa and coffee beans. While acidity may be desirable to some varieties of coffee, reducing its levels in many varieties often results in a smoother and rounded flavor. Arabica coffee is one example of coffee that naturally contain high acidity.

This study proved that using Ohmic heating can effectively reduce this acidity to a more desirable level. The time and temperature variations used in the study resulted in acid levels between 0.18% and 0.73%.

Thawing

Food thawing may be a relatively simple step in food preparation. But doing it in a much better way can help food processors save resources and retain quality. In most cases, there would be moisture loss or drip loss. This is evident when the food is drier. Drip loss is commonly associated with meat, seafood, fruits, and vegetables.

Drip loss occurs because the ice crystals that ruptured that cell walls turns into water and escapes from the food. There is more drip loss if air thawing is done because the process is very slow. With Ohmic heating, thawing is rapid because the heat is generated from the food itself, instead from the surface inward.

Another benefit of Ohmic heating for thawing is that there is not a need for water. Hence, no wastewater is generated. This also saves resource and operating cost.

Blanching

Similar to thawing, blanching may also result in moisture loss. However, the mechanism of how it occurs is different. With blanching, the elevated temperature ruptures the cell wall of the food that allows the water to escape and nutrients to leech into the blanching water.

With Ohmic heating, it is possible to reduce or prevent moisture loss and leeching. Its rapid heat generation allows uniform heating and prevents overcooking of the surface. A study found that blanching with Ohmic heating also removes the need of cutting (a process that is commonly done prior to blanching) large pieces of food, especially vegetables. The reason here is uniform and rapid heating that traditional blanching cannot achieve. This particularly important for delicate vegetables.

Extraction

Uneven temperatures can also lead to inefficient extraction of certain components from food materials. Examples of extraction are juicing from fruits, soymilk from soybeans, and beet dye from beets.

Traditional extraction methods apply heat from the outside to the inside of the sample. Since this manner of heat transfer is relatively slow, there is an extended processing time. Not only this affects the yield, it also affects the quality of extraction. This is especially true if what involved in the extraction are heat-sensitive compounds like vitamins.

With Ohmic heating, the quality of extract is better preserved. This statement is demonstrated in this study of Ohmic heating-assisted extraction of anthocyanins from black rice bran. Anthocyanins are natural pigments responsible for the color of eggplants, blueberries, purple grapes, and cherries. Anthocyanins are heat sensitive. But with Ohmic heating used in the study, the method had better colorant yield, as well as bioactive compound activity.

References:

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

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

P. Fellows (2000). Food Processing Technology (2nd edition). CRC Press

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

The post Ohmic Heating (Joule Heating) Of Food: How Does It Work? appeared first on The Food Untold.

Sodium chloride or simply salt is probably the most important food additive ever; the functions of salt apply to various food processes. But unlike how we know it today, its uses were very limited when it was first used in food. Our ancestors from 5000 years ago started using salt in food for 2 main reasons: to improve flavor and preserve for later use. The latter was especially important because they did not have access to technology that we normally enjoy now. While today, we simply store food in the refrigerator, our ancestors had to resort to other forms of preservation techniques such as salting, curing, and brining to store food for an extended period of time.

We have come a long way since then. Today, hundred years of research have led us to a better understanding of how salt works with food. We no longer add salt merely as a flavor enhancer and antimicrobial agent. Here are the 5 main functions of salt in food.

SALT ENHANCES FLAVOR

This maybe is a no-brainer, but salt is the universal flavor enhancer. It elevates the flavor of virtually every food that we consume— from meat to seafood. Specifically, vegetables that are not normally enjoyed by children taste better by just sprinkling salt. What makes salt special is that aside from adding and increasing the saltiness in food, it also enhances the other desirable flavors, particularly sweetness and savory or umami flavor.

How?

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In naturally sweet foods like baked products such as cookies and cakes, salt in low concentrations elevates the sweetness and adds depth to other flavors . Salt does this by suppressing the bitter taste and other undesirable tastes, all thanks to the sodium ions. This is why most recipes of sweet goodies always call for a pinch of salt.

At higher concentrations, salt may start to suppress the sweet taste. But such concentrations is ideal for savory foods. Special mention to naturally salty food that are rich in umami such as anchovies.

While salt can intensify the umami or savory taste, umami-containing compounds do not work the same way; they do not enhance the fruity, bland, or sweet taste in food.

SALT PRESERVES FOOD

Like I already mentioned, we have been using salt as a preservative for several thousand years now. So there is no wonder why salt has countless applications. Beef jerky, dried fish, bacon, dates, pickles, etc.

How does it work?

Water is one of the requirements for bacteria and other microorganisms to thrive. In order to preserve the food and extend its shelf life, water must be drawn out to a point where microorganisms can no longer use it for biological functions such as spore germination, toxin production, and more importantly, growth of microbial cells.

Water activity (a_w) is the simplest way to describe this. Water activity refers to the amount of free water (unbound), hence available for use, more specifically to support microbial growth.

Most food products have a water activity of 0.95. Generally, to prevent microbial activity, the water activity must be lowered down to at least 0.75. Although this largely depends on the food and the target microorganism. Some need more water, while others require less. A water activity of 0.85 inhibits the growth of pathogenic bacteria. While certain yeasts and molds can still thrive in an environment with a water activity of 0.75. Although they can no longer produce toxins to spoil the food.

By applying salt, water is drawn out through a process called osmosis. Osmosis occurs when there is a diffusion or movement of solvent (water) molecules across a semipermeable membrane from a region of lower concentration of solute (salt) to another of higher concentration. This equalizes the concentration on both sides of the membrane. As a result, the water activity lowers in the food, and the growth of microorganisms is inhibited.

SALT ENHANCES TEXTURE

In cheese, salt functions mainly as a preservative and flavor enhancer. Additionally, salt also aids in the development of the physical characteristics of cheeses— stretching, meltability, shredding, and flow. At proper salt concentrations, modification of the protein structure occurs to make a good rind of cheese. Some cheddar types of cheese with reduced amount of salt find trouble maintaining their shape as a result of softening. At low salt concentrations, non-starter lactic culture may activate, which contributes as well to cheese softening.

Salt also has a similar favorable effect on the texture of meats, generally. In processed meats such as sausages, the addition of salt makes the myofibrillar proteins (which are naturally insoluble in water alone) soluble. By combining salt with blending and tumbling, these salt-soluble proteins move toward the surface of the meat. This process is particularly useful in restructured meats, such as bologna and frankfurters. The salt-soluble proteins that have been extracted to the surface work as a glue—they keep the chopped or ground pieces of meat together as they form a gel network.

In baking, salt produces a binding effect by altering the protein structure, strengthening the gluten network. The gluten is the main protein responsible for the texture of the final product. The interaction with salt makes the gluten more capable of holding water and carbon dioxide. In return, the dough expands without ripping and tearing. Salt also reduces the stickiness of the dough, allowing more control during kneading.

In many baked products such as cookies, and cakes, popular leavening agents that are used to produce airy texture contain sodium. These include baking soda (sodium bicarbonate) and baking powder (sodium bicarbonate combined with: sodium aluminum sulfate, potassium hydrogen tartrate, calcium acid phosphate or sodium acid pyrophosphate).

SALT IMPROVES COLOR

When it comes to the development of the appealing bright red color in processed meats, particularly cured ones such as hot dogs, salt is very underrated. What would first come to mind is nitrate or nitrite. When you look closer, it is actually a product of the teamwork of nitrate/nitrite, sugar and salt.

How?

When salt is added, the chlorine ions dissociate from NaCl. When this happens, the rate of conversion of nitrite to nitric oxide speeds up. In return, the development of the cured color in the meat accelerates.

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In breads, the presence of salt encourages enhanced color (golden crust) formation by increasing the rate of caramelization. Caramelization is a reaction in which the oxidation of carbohydrates (or sugars) at elevated temperature results in the development of brown color in food. This study proved that other salts, including potassium chloride (KCl), calcium dichloride (CaCl₂) and magnesium chloride (MgCl₂) can also significantly improve the color of baked products.

Another way salt enhances the color of bread is by controlling fermentation. The residual sugar in the dough is responsible for the browning of the crust. However, during fermentation, the yeasts consume sugar, turning it into carbon dioxide that is responsible for rising of bread. By adding salt at an appropriate amount (too little makes dough to rise faster, but too much may inhibit yeast activity), the yeast activity is controlled. Hence, there is less destruction of sugar, and the crust color development improves.

SALT PROVIDES ESSENTIAL NUTRIENTS

Salt is our main source of sodium, an essential nutrient. It helps control the bodily fluids; controls blood pressure and volume and maintains balance of minerals (electrolytes) and water. Like most of the nutrients we need, only a small amount of sodium is necessary for proper function. According to the Food and Drug Administration (FDA), the recommended daily intake for Americans is less than 2,300 mg per day or 1 teaspoon of table salt. Studies have shown that Americans consume sodium mosftly from prepackaged, processed foods, and restaurant. The same thing can be said for other parts of the world.

The most common form of table salt, particularly in the U.S., is iodized salt. Iodized salt is a product of food fortification which contains a significant amount of the element iodine. Its main purpose is to prevent iodine-deficiency. Globally, 2 billion people around the world is iodine-deficient.

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A low intake of iodine increases the risk of developing health illnesses such as goiter and problems with mental development, especially in young children. In fetuses and infants, iodine is essential for proper development of the skeletal and central nervous system. For most adults, the recommended daily intake of iodine is 150 μg, while 90 μg for children aged 1 to 8.

Do you know all these functions of salt in food? Which one did you find interesting? Feel free to leave a comment down below.

The post 5 Main Functions of Salt (Sodium Chloride) In Food appeared first on The Food Untold.