Starch Retrogradation: Understanding the Science Behind Stale Food

Did you ever wonder why bread that’s left over becomes hard and dry, or why rice gets grainy when it’s been in the fridge for a while? Well, it’s because of something interesting called starch retrogradation. This is a natural process that happens in foods with a lot of starch (like bread and rice), and it’s what causes the texture to change and the food to not taste as fresh as before. It’s like the reason why food becomes “stale.”

In this blog post, we will discuss starch retrogradation, exploring its science, effects on food, and how to minimize its impact on our culinary delights.

UNDERSTANDING RETROGRADATION

Starch is the most common carbohydrate in plants. It is made up of two kinds of molecules: amylose and amylopectin. Amylose is arranged in a straight chain, while amylopectin has a more complex, branched structure. I’ve written another article that talks about how these two differ. You can find it here. When we cook or work with starchy foods, the starch molecules soak up water and expand, which is why dishes can become thicker and turn into a gel-like texture.

On the flip side, retrogradation stands as the process wherein starch molecules within cooked foods undergo a reorganization, adopting a more structured and crystalline arrangement. This intricate occurrence takes place when gelatinized starch gradually cools and sheds moisture, compelling the starch chains to bond and reform crystals. Consequently, the once tender and vibrant texture of the food undergoes a shift, losing its initial allure.

High amylose starches are predisposed to undergo retrogradation. This phenomenon becomes evident in baked goods that lose their initial fresh taste and texture, signifying the transition from a gel-like starch state. Similarly, residual long-grain rice experiences this process due to its elevated amylose content, causing it to become rigid and less palatable.

Several factors impact the speed of retrogradation. These include the ratio of amylose to amylopectin molecules, which form the starch; the way these molecules are structured due to the plant source of the starch; temperature; how concentrated the starch is; and the existence and amount of other components, especially surfactants and salts.

THE SCIENCE BEHIND THE PROCESS

Starch retrogradation initiates promptly after the baking phase concludes and the product commences its cooling journey. This phenomenon is particularly pronounced in products containing a high concentration of amylose starch. Amylose, a linear starch molecule, undergoes retrogradation more swiftly than its counterpart, amylopectin. Notably, by the time the baked product reaches room temperature, the process of amylose retrogradation is often nearing completion.

However, the story doesn’t end there. The retrogradation of amylopectin, a branched starch molecule, requires a more extended period compared to amylose retrogradation. This temporal discrepancy between the two starch components imparts a significant impact on the overall quality of baked goods, contributing significantly to the phenomenon known as staling.

Staling, the undesirable transformation of baked goods from their fresh and soft state to a more rigid and less palatable one, is predominantly driven by the retrogradation of amylopectin. Over time, during the staling phase, the once pliable and amorphous amylopectin molecules revert to their original crystalline state, forming rigid granular structures. This process results in the expulsion of moisture from the product’s crumb, causing a loss of moisture content.

As a consequence of the expelled moisture, the texture of the baked product undergoes a noticeable change. The product gradually becomes firmer and less elastic, a stark departure from its desirable characteristics. This loss of moisture and alteration in texture are key attributes of staling, rendering the product less appealing to consumers.

COMMON FOODS THAT UNDERGOES STARCH DEGRADATION

Starch retrogradation is something that happens to many common foods we eat. Let’s take a look at some examples: bread, pasta, rice, potatoes, and crackers.

Think about bread. When it’s fresh out of the oven, it’s soft and chewy. But as it sits for a while, it becomes dry and crumbly. Even the outside part, the crust, turns tough and less yummy.

---

You might also like: How To Make Stale Bread Soft?

---

Pasta is another example. When you cook pasta and it’s hot, it’s nice and soft with a little bit of chewiness. But as it cools down, it starts getting harder and not as tasty. That’s why leftover pasta isn’t as good – it loses its good texture.

Rice also goes through changes. Right after you cook rice, it’s fluffy and moist. But as it gets cold, it becomes dry and the grains might stick together, making it not so great to eat.

Potatoes, like the ones you might have as fries or mashed, also change. After they’re cooked and then cool, they can turn from creamy and soft to kind of gritty and dry. That’s not as yummy.

And let’s not forget crackers. When they’re fresh, they’re crunchy and easy to break. But if you leave them out, they get softer and chewier over time.

These foods show us how starch retrogradation works. It’s like they’re going through a texture change after they’re cooked and then cool down. So, if your sandwich bread isn’t as soft or your pasta isn’t as good the next day, you can blame starch retrogradation for that!

FIGHTING STALING IN THE FOOD INDUSTRY

Emulsifiers, enzymes, and hydrocolloids emerge as key players in this pursuit, each wielding distinct functions that contribute to the modification of the retrogradation process, ultimately enhancing product quality and extending shelf life.

By dispersing fat molecules within a starch matrix, emulsifiers hinder the reassociation of starch molecules into a crystalline structure. Consider mayonnaise, a classic example of an emulsion. When emulsifiers are introduced, the resulting product showcases reduced starch retrogradation, leading to a smoother, longer-lasting consistency that defies the clumping and firming often associated with retrograded starches.

Another example of emulsier is glycerol monostearate (GMS). GMS is produced by adding glycerol to fat or oil which results in a mixture of monoglyceride and diglyceride. Incorporating GMS at 0.25–0.5% allows amylose to form a helical complex that retards the retrogradation of the starch.

Enzymes catalyze specific reactions, transforming complex molecules with precision. In the context of starch retrogradation, enzymes like amylases can break down starch molecules into smaller fragments, impeding their propensity to form rigid crystalline networks upon cooling. This enzymatic intervention not only enhances the texture but also extends the freshness of products. For instance, the addition of amylase enzymes mitigates the retrogradation-induced staling, resulting in loaves that remain softer and more enjoyable over an extended period.

Glycosyltranferase is another enzyme that adds more branching points to create modified starch. This results in enhanced functional characteristics such as increased solubility, decreased viscosity, and minimized retrogradation.

Hydrocolloids, a diverse group of substances with exceptional water-absorbing capabilities, contribute significantly to the fight against starch retrogradation. They do this by preventing the formation of tight crystalline structures during retrogradation. Imagine a fruit pie filling; the incorporation of hydrocolloids maintains the desired consistency and texture, resisting the undesirable textural changes stemming from starch retrogradation.

PREVENTING STALING OF FOOD AT HOME

An effective approach involves appropriate storage methods. For instance, when it comes to baked goods such as bread, placing them in an airtight container or plastic bag within a cool, dry location can effectively delay moisture loss and limit exposure to air. Although some might suggest refrigerating baked items, this might not be the optimal choice, as it could accelerate retrogradation. In fact, staling of bread happens most rapidly at 32°F (0°C) to 39°F (4°C).

For extended preservation, freezing is very effective at slowing down starch retrogradation and staling. However, it’s important to recognize that certain changes in texture may occur during the thawing process. Thus, it is recommended to take these potential alterations into account when planning the use of frozen starchy items.

When reheating starchy leftovers, it’s wise to choose gentle methods that safeguard the original texture. Employ techniques that minimize exposure to high temperatures, such as microwaving with a small amount of water or utilizing mild oven reheating. By adopting these methods, the risk of overcooking and excessive moisture loss is mitigated, ensuring that the starchy foods maintain their desired texture and overall quality.

References:

A. Chakraverty (2014). Postharvest Technology and Food Process Engineering. CRC Press.

W.Zhou, Y. H. Hui (2014). Bakery Products Science and Technology(2nd Edition). John Wiley & Sons, Ltd.

M. Kuddus (2018). Enzymes in Food Technology. Springer.

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

P. Cheung, B. Mehta (2015). Handbook of Food Chemistry. Springer