Traditional, Value-Added Applications of Dry Peas, Lentils & Chickpeas
Wheat Flour (white flour) is used to produce a wide range of popular bakery and snack products, including breads, muffins, waffles, pizza crusts, cookies, crackers, and ready-to-eat (RTE) cereals. The wheat flour gives these products their uniform, light-colored appearance and smooth, non-gritty texture.
When the protein found in wheat flour comes in contact with water and heat, it produces the protein gluten (glutenin) (i.e., the protein formed when wheat flour is made into batter or dough), which gives baked goods their elasticity, extensibility, and strength. The more gluten in flour, the greater the batter viscosity and the darker and crispier the fried food made with it. The gluten also makes it easier for the flour to build up a tough structure able to trap the waste gases of yeast during kneading. Less gluten, conversely, produces a lighter, less chewy texture such as that found in cakes.
Different types of flour contain varying amounts of protein. The exact amount of gluten in flour depends on how it was milled and the variations in growth of the crop. The strength of the flour depends on the quality of the gluten present. Weak flour contains less gluten than strong flours, with the type of flour used affecting the finished product.
The Health Benefits of Legumes
Increasingly, food industry professionals are looking for ways to make value-added processed foods more nutritious without compromising the taste. Consumers are looking for foods that are more healthful in order to improve their diets. The addition of food legumes such as dry peas and lentils into mainstream diets in the form of flour fits with this aim and is proving a popular approach for consumers and food developers alike.
It is easy to see why. Dry peas, lentils, and chickpeas and other pulses ground into flour are high in complex carbohydrates, fiber, protein, lysine, and other phytochemicals. Pulse hulls boast a nutrient profile similar to wheat bran and an extremely high fiber content. In addition, being low in fat and sodium and rich in Vitamin B folic acid, legumes play a key role in the prevention of many health issues.
Legumes are also high in protein. An essential food component, proteins are important sources of amino acids, are necessary for physical growth and maintenance, and provide functional properties in the foods in which they are incorporated. Peas, and most notably yellow peas, are especially rich in protein, as well as starch and nutrients such as fiber, vitamins, and minerals.
In addition to the nutritional benefits, there are a range of other reasons legume flours are growing in popularity. For people who are allergic to gluten or to wheat itself, legume flour offers an excellent alternative. Non-wheat flours can also provide new and unique flavors and are especially good at thickening liquid mixtures like sauces or soups.
When it comes to baking, however, legume flours can pose some challenges. Because they produce no gluten when mixed with liquid, they require special treatment when intended to be formed into workable dough or batter that will rise, hold its shape, and deliver a pleasing texture.
Successfully incorporating legume flours as ingredients for foods is contingent on the functional characteristics and sensory qualities that the flour brings to the end-product. Functional properties include foaming, emulsification, texture, gelation, water and oil absorption, and viscosity.
Given the unique nutritional and performance attributes of legumes, the specialty diet food market is taking notice and endeavoring to meet the demands of today’s health conscious consumers. In the process, healthful, gluten and casein free alternatives are booming as a growing list of products using legume flour makes its way to grocery shelves. See Appendix C for a collection of sample formulations.
The Role of Rheological Testing
A great deal of interest has been shown in recent years regarding fortifying wheat flour with high-protein, high-lysine legume flour. Such a combination increases the protein content and bolsters the essential amino acid balance, especially of baked products like bread.
Pea flour fortification does, however, alter dough rheological properties. The aim of current research is to explore the functionality and characteristics of how pea flour interacts with wheat flour in a range of bakery products. Also being evaluated are the rheological properties of pea flour compared with wheat flour at certain ratios, as well as making a physical assessment of the dough. This is done to better understand the performance of pea flour in dough systems.
Rheological testing has become a preferred approach for examining the structure and fundamental properties of various flour doughs. Due to their characteristics and sensitive response to the structure variation of flour doughs and proteins, rheological properties are considered an effective way for predicting the processing behavior and controlling the quality of food products. This testing simultaneously measures the viscous and elastic characters of dough.
The rheological characteristics affect both what is referred to as the “machinability” of the dough and the quality of the final product. Influenced by added ingredients, the rheological profile also depends to a great extent on the crop variety, properties, and milling process used.
Gluten forms when two of the proteins in the starch in wheat flour, glutenin and gliadin, combine with water. The presence of gluten increases the viscosity of the batter due to the efficient water binding capacity of the gluten protein. When the proteins are surrounded by water and stretched, they interact and form new bonds that are strong and very elastic. Because pea flour contains no gluten, increasing the amount of pea flour in wheat flours decreases the viscosity of the batter.
Though stability decreases significantly as pea flour is added, there does not seem to be much difference in terms of protein weakening. Protein content in pea flour with hulls increases water absorption and helps lm formation, which binds large amounts of water and contributes to the viscosity of the batter. This higher water absorption may be due to the higher soluble protein content.
Increasing the amount of precooked pea flour (with or without hulls) leads to progressively darker flours than seen in wheat flour mixed with uncooked pea flour. The reason behind the color change is considered to be the result of protein and carbohydrates in wheat flour undergoing carmelization in what is called the Maillard reaction. A Maillard reaction is a non-enzymatic interaction between lysine in protein and reducing sugar. During cooking, it typically leads to development of a brown color in the pea itself. Carmelization results when carbohydrates or sugar are exposed to high temperature. An increase in the amount of uncooked pea flour in wheat flour gives the product a slightly yellowish color.
Antinutritive factors in pea flour such as polyphenols, phytic acid, and trypsin inhibitors, as well as their color and flavor, can limit the use of pea flour as an ingredient in bakery products, meat products, and snack foods. Legumes can, however, be treated to reduce the content of these antinutritive factors as well as to improve the nutritional value of the protein and remove their bean flavor.
The ability of flours to absorb and retain water and oil may help improve binding of the structure, enhance flavor retention, improve mouthfeel, and reduce moisture and fat losses of certain products.
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.
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.