Starch Gelatinization

Starch is the most abundant carbohydrate in the legume seed (22 percent to 45 percent). The total carbohydrate level varies based on the variety. Among dry peas, the Miami and Nitouche cultivars have the highest starch content (44.7 percent and 43.5 percent, respectively), Majoret the lowest (approximately 40 percent).

Starch is composed of two basic molecular components: amylose and amylopectin (i.e., the insoluble or gel component of starch). Though identical in their basic constituent (glucose), they differ in their structural organization, or linkages, which impacts their functionality in food applications. In addition, each is hydrolyzed, digested, and absorbed differently.

Both the amylose and amylopectin are located in the starch granules, with the size, shape, and characteristics of the granules varying based on the plant source. The growth and development of the granule originates at the hilum (i.e., center of the granule). Under magnification and polarized light, native starch granules typically appear to have a cross-like structure, the size and shape of which varies among starch sources.

Use of Starch in Food Products

Starch is the main carbohydrate found in plants and is a major source of nutrition for humans and animals. As a result of their high amylopectin content, some legume starches demonstrate a restricted swelling and an increased overall stability during processing. This and other beneficial physicochemical properties make them highly suitable for use in a variety of food products.

One of the important functional properties of starch is pasting, which is the formation of a high-viscosity solution after heating in water. This characteristic is exploited in many foods as well as in non-food applications such as

adhesives. Another important functional characteristic is starch’s ability to form gels, which is also used in a range of food and non-food applications.

As environmental issues have led to a growing interest in renewable raw materials, researchers have sought new sources of starch and development of new methods for its modification. This pursuit has led many to leguminous plants, which are an increasingly sought-after source of starch, thanks to a range of unique benefits.

Legume starch is mainly available as a by-product of protein extraction and is therefore considered a relatively cheap source of starch compared to corn, wheat, and potato starches.

Pea starch offers numerous special features (e.g., formation of high-viscosity pastes, stronger gels, etc.) that can benefit food technology, especially as an alternative to chemically modified starch. The central reason is pea starch’s high amylose content.

Increasingly, legume starch is being employed to modify the texture of food products such as frozen foods, extruded snacks, pasta, noodles, cookies, crackers, sauces, and soups. Because of its importance in food processing and consumer acceptance, research into starch characteristics (e.g., pasting profiles, thermal behaviors, thickening and gelling properties, swelling factors, etc.) continues to grow in the U.S. food industry.

New patents are being developed for products focused on modifying legume starch (mostly pea starch). Among them, a novel starch-based texturizing agent has been produced from high-amylose starch. It was created by dissolving the starch in water under acidic conditions, while agitating it at an elevated temperature and pressure, followed by retro-gradation at low temperature and drying.

The aim of texturing agents is to create fat-like attributes such as structure, viscosity, smoothness, and opacity so as to reduce and or replace the actual fat content in foods including pourable salad dressings, yogurt, cottage cheese, sour cream, cream cheese, peanut butter, frosting, cheesecake, mousse, and sauces.

The texturing agent can also be effectively used in full-fat foods as a stabilizer, and as an opacifying agent such as for low-fat and fat-free foods and beverages like coffee creamer, cottage cheese dressing, nutritional drinks, and ice cream.

Additionally, legume starches can be used in the preparation of food with a reduced lipid (i.e., an organic compound of the fat group) content. In this case, the lipid portion in the food is replaced by non-gelling, pre-gelatinized starch.

Native and modified legume starches can be used in a range of applications, including preparation of the following:

  • Gels (e.g., puddings) that can be prepared with about 50 percent less starch in comparison to corn starch
  • Extruded products and instant starches that can be produced without the significant loss in viscosity that occurs with other starches
  • Roll-dried starches and fruit and vegetable flakes that have a pulpy texture after rehydration and considerable stability at cooking temperatures
  • Pulpy products created via freeze-thaw processing that keep their pulpy texture even after prolonged cooking
  • Roll-dried instant starches with cold swelling gelling properties that can be used in instant desserts for a flake-like texture

Extensive research has already been conducted on potato, corn, cereal, and cassava starches as they are used extensively in food and non-food industries. The differences in rheological properties of starches may result from the varied amounts of non-starch components (e.g., proteins and lipids) in each. To continue to find new uses, it remains necessary to extend the use of rheometers to investigate the rheology of gelatinized pea starch.

What Is Gelatinization?

The amylose and amylopectin of the starch are tightly packed in granules marked by a high degree of molecular order. Insoluble in cold water, when starch granules arevheated in water, beyond a critical temperature their organized molecular structure is destroyed. As a result, the granule absorbs a large amount of water and undergoes an irreversible swelling to many times its original size. This transformation is referred to as gelatinization.

More specifically, starch gelatinization is an order-to-disorder phase transition that starts in the amorphous regions of starch granules because of the presence of weak hydrogen bonds. These intermolecular hydrogen bonds break down in the presence of water and high temperatures. The heating immobilizes the glucan (i.e., a polysaccharide that is a polymer of glucose) chain segments of the granule and allows water absorption, which leads to the reduction of the ordered structure, amylose leaching, and the destabilization of the amylopectin.

During pea starch gelatinization, the molecular disruption begins from the hilum area and then spreads quickly through the central part of the granule, causing the central part of the granule to swell. Not until higher temperatures are reached is the outer part of the granule disrupted and gelatinization completed. In the presence of sufficient starch concentration, the amylase will then form an elastic gel upon cooling.

Temperature

The differences in gelatinization temperatures among the flours may be attributed to differences in size, form, and distribution of starch granules in the flours, and to the internal arrangement of starch within the granule. Low-protein and high-amylose starches require high inputs of energy to undergo starch gelatinization. Low-amylopectin starch has a higher gelatinization temperature, and is more resistant to enzyme and acid digestion compared to other starches.

Smooth pea starches, for example, are packed with a higher energy capacity but have been shown to also have higher gelatinization temperatures. The process of gelatinization occurs at about 149 degrees to 158 degrees F (65 degrees to 70 degrees C) for smooth pea and chickpea starches. Pea starch, being higher in amylose content, requires temperatures greater than 212 degrees F (100 degrees C).

Impediments to Gelatinization and Digestibility

Non-starch components such as sugars, salts, proteins, and lipids influence starch gelatinization and, by extension, starch’s behavior in food applications. The presence of lipids, for instance, affects water absorption, which impacts gelatinization. This influences the formation of resistant starch and the starch’s susceptibility to enzymatic digestion.

In many common plant foods the starch is only partly gelatinized because of limited water content during processing. The starch granules swell only slightly, while the internal structure remains partly intact.

For example, some baked products and breakfast cereals contain incomplete gelatinized starch granules due to lower water absorption. If such products, or certain pre-cooked convenience foods, are produced under more severe conditions (elevated temperatures and pressures, and using extrusion cooking or popping), they typically achieve full gelatinization despite the low water content.

The cell walls may also inhibit starch gelatinization by limiting both the degree of swelling of the starch granules and the movement of water. By grinding the raw legumes to break down the cell walls, and then cooking them, swelling of the granules can be increased. Researchers hypothesize that such a practice could also increase the rate of starch digestion and the glycemic response.

The difference in the degree of legume gelatinization after processing has been put forth as a possible explanation for the observed differences in starch digestibility. It is suggested that the low metabolic response and in-vitro starch digestibility of legumes might be caused by the entrapment of starch in the cells.

The effect of complete gelatinization on metabolic response has been, and continues to be, studied in humans, both in diabetic and nondiabetic subjects, as well as in animals. Starch from potatoes and corn, for example, give much lower postprandial glucose and insulin responses in raw form than after cooking. Raw corn starch has, in fact, been used clinically to provide glucose with prolonged absorption in the treatment of type 1 glycogenosis (i.e., an inherited disorder of glycogen metabolism that results in excess accumulation of glycogen in various organs of the body).

A Subject of Ongoing Research

Starch is an excellent raw material for modifying food texture and consistency, and for improving nutritional values, among other benefits. Research devoted to gathering information on starch properties, such as thermal behavior, rheological properties of pasting, thickening and gelatinization, is therefore of continuing significance to food processors and consumers alike.