Gelatinization increases starch availability for amylase hydrolysis. As a result, gelatinization of starch is frequently employed in cooking to make starch digestible or to thicken/bind water in roux, sauce, or soup. Starch gelatinizes at approximately 140°C (284°F), but many foods contain temperatures above or below this range during preparation or storage.
Starchy vegetables such as potatoes and corn become soft when cooked. This is because the starch inside these vegetables readily gelatinizes at around 140°C (284°F). When cooled down, the gelatinized starch remains solid at room temperature.
When you cook rice, it becomes fluffy because the starch inside the rice granules releases rapidly and easily into a fluid state under heat treatment. This makes the starch more available for digestion by enzymes in saliva that break down starch into sugars. The more resistant the starch, the longer it takes for it to release its energy content. Thus, less intense cooking methods, such as boiling, can be used with resistant starches because they don't need to be completely broken down yet. For example, studies have shown that boiled rice causes fewer intestinal problems for people who cannot eat raw rice because of a gluten allergy.
Resistant starch also has beneficial effects for our bodies beyond simply providing food for our microbes.
Starch gelatinization is the process of breaking down starch molecules' intermolecular connections in the presence of water and heat, allowing the hydrogen bonding sites (hydroxyl hydrogen and oxygen) to engage additional water. The starch granules are permanently dissolved in water as a result of this. Starch that has been gelatinized can be cooled and re-melted without losing its consistency.
Gelatinization starts at about 63 degrees Celsius and is complete by about 115 degrees Celsius. Above 120 degrees Celsius, starch is completely degraded into sugar.
When starch is heated to its gelatinization temperature, it begins to lose its crystalline structure and become more fluid. As it continues to heat up, the viscosity increases even more, until at its peak temperature of 140 degrees Celsius or higher, it becomes very viscous like gelatin.
As soon as the starch is cooled below its gelatinization temperature, it will return to a liquid state. If it's kept cold, then it will eventually solidify.
In cooking, gelatinization of starch is useful for thickening sauces and stews, since the increased viscosity of the cooked mixture allows it to be strained through a fine sieve. The starch also absorbs many flavors from the food it contacts.
Gelatinization is also responsible for the stiffening of cooked rice.
The starch granule absorbs water, expands, and loses crystallinity during gelatinization; however, during dextrinization, which is preferred by extrusion at lower moisture levels, the starch granule is physically broken apart. Both procedures make the starch more easily digestible. For example, when cooked, dextrinized corn starch will yield approximately two-thirds as much glucose as raw corn starch.
Dextrinization was originally developed by food scientists to create stable, smooth hot cereal products such as corn flakes. However, today there are many ways that chefs use dextrinized corn starch instead of gelatin to add texture and stability to sauces and soups.
Gelatin is made from collagen, which is found in bones, skin, teeth, and other connective tissue. It can be extracted from animal sources such as cows, pigs, and sheep or from plants such as soybeans. Gelatin is used as a thickener, stabilizer, and binding agent in foods such as desserts, puddings, and beverages. It also adds body to emulsions and solutions.
Dextrin is made by reducing the amount of available glucose on the starch granule surface. This makes more surface area for other molecules to interact with. During cooking, these other molecules are primarily maltodextrins, which are polymers of glucose linked together.
Under specific heat and moisture conditions, starch gelatinization is an endothermic process defined by the loss of starch crystallinity or the disruption of molecular orderliness inside the starch granule. One of the most important technical indicators of starch quality is gelatinization temperature. When cooled water-starch mixtures are warmed again, the gelatinization temperature can be as high as 140° C (300° F). Above this temperature, the starch molecules become completely soluble in water.
The gelatinization temperature of a starch sample can be determined by using a differential scanning calorimeter (DSC). The test involves heating a sample of starch at a constant rate, measuring its heat flow with time, and identifying the peak value as the gelatinization temperature. The higher the gelatinization temperature, the more resistant the starch molecule is to digestion by amylase enzymes in the digestive system. Starchy foods that retain their shape after being cooked include potatoes, carrots, corn, peas, and cassava. Foods that turn into a liquid when cooked include all wheat products and most rice products.
In general, starches that gel at lower temperatures are easier to digest than those that require higher temperatures to do so. Thus, the body can more easily break down starch into sugar for energy production. In fact, the human body uses only the sol portion of starch, which exists mainly at low temperatures.
Gelatinisation is the process by which starch granules form a sort of deferment in cold water. When the granules come into contact with water, they start to thicken and swell, forming a gel. The temperature at which this happens is called the gelatinization temperature.
In cooking, the gelatinization point of starch is important because it determines when cooked grain can be kept hot without going back to liquid stage. Before that point is reached, the starch will continue to absorb more moisture from the surrounding material (in this case, water), causing the mixture to become slushy and thus unsuitable for further processing or consumption. After the gelatinization point is passed, the starch will no longer absorb any additional moisture, and will therefore be considered fully cooked.
At temperatures below the gelatinization point, the starch molecules remain isolated, meaning that they cannot absorb any more moisture and will eventually get dry. This is why when you cook corn on the cob, the outer layer will begin to turn yellow before the inside is done. The starch inside has not yet gelatinized, so it can still absorb water, causing the corn to become mushy.
At temperatures above the gelatinization point, the starch starts to lose its crystallinity and becomes completely amorphous, which means that it has no defined structure and cannot absorb any more moisture.