Starch – A Food Thickener
WHAT IS STARCH?
The word starch is derived from the Germanic root word meaning “stiff,” and commercial starch lives up to the original meaning by acting as a thickening or gelling agent in food preparation.
Starch is a polysaccharide consisting of long chains of repeating units of glucose molecules linked together either in the form of amylose, which is made up of primarily linear molecules, or amylopectin, whose molecules are highly branched.
Most starches contain about 75% amylopectin and 25% amylose.
Starches containing higher levels of amylose tend to gel, whereas starches containing higher levels of amylopectin are considered non-gelling, but are still somewhat gummy.
Genetic engineering techniques can be used to produce any ratio of amylase or amylopectin required to achieve the starch’s desired functionality.
Plants serve as the source of starch granules, which are the plant cell’s unit for storing starch. Cereals such as wheat, rice, and corn are common sources of starch. Root starches include potatoes, arrowroot, and cassava (tapioca).
Other sources of complex carbohydrates serving as dietary starch sources include dried beans, peas, and the sago palm.
Note: Knowing the source of starch is important when preparing foods because starches vary in flavor and viscosity.
Some starches derived from plants can be considered food additives and are used in a wide variety of ways.
STARCH IN FOOD PRODUCTS
Starch serves several purposes in the food industry, including thickening agents, edible film, and sweetener sources (dextrose and syrup).
- Starch’s main use in processed foods is as a thickening and/or gelling agent.
- Foods that are frequently thickened with starch include soups, sauces, pie fillings, gravies, chili, stews, cream-style corn, cream fillings, custards, fruit pie fillings, whipped toppings, and icings.
- Certain puddings, candies, gums, and salad dressings are also thickened with starch.
- Starch films are used as a protective coating for chewing gums, bind foods such as meat products and pet foods, and act as a base on the food for holding substances such as flavor oils in chocolates.
- Because starch consists of repeating units of glucose, it can be broken down into these individual units for use as a sweetener in the production of confections, wine, and some canned goods.
- The food industry refers to this glucose derived from starch as dextrose and measures the degree of conversion from starch to glucose in dextrose equivalents (DEs)
* Dextrose equivalent (DE): A measurement of dextrose concentration. A DE of 50 means the syrup contains 50% dextrose.
- A hydrolysis product of starch is considered a starch hydrolysate.
- These starch hydrolysates have been used as sweeteners for over 3,000 years, starting most likely with candy made in
China with the aid of maltose-bearing syrups derived from rice starch.
- The most common solid starch hydrolysates used now are dry maltodextrins (DE < 20) and corn syrup solids (DE = 24 to 48).
- The wet milling process is used to derive starch from corn, which is the major source (95%) of starch in the United
- The dried kernels are softened by soaking them in warm water containing sulfur dioxide.
- Once softened, the kernels are cracked, any extraneous material is removed, and the cracked kernels are ground and screened or sifted down to yield starch and protein.
- The protein is removed and the starch is filtered, washed, dried, and packaged as cornstarch.
- Over half of all the starch produced in the United States is eventually converted to syrups.
- Corn syrup made from corn starch is added to a large assortment of foods, including soft drinks, canned fruits, jams, jellies, preserves, frozen desserts, confections, frozen fruits, fountain syrups, and many others.
CHARACTERISTICS OF STARCH
- It is the increase in volume, viscosity, and translucency of starch granules when they are heated in a liquid.
- Pasta, rice, oats, scalloped potatoes, and most sauces, soups, and puddings are very different in consistency before and after cooking, and this is because of gelatinization.
- A fluid starch paste is a sol, whereas a semisolid paste is known as a gel.
- Not all starches will gel, but among those that do, the gel forms after the gelatinized sol have been cooled, usually to below 100°F (38°C).
- Gelatinization must occur before the next step, gel formation, also called gelation.
- As the gel cools, bonds continue to form between amylose molecules, and retrogradation occurs.
- This retrogradation is accelerated by freezing, so the starches used in frozen food products usually come from sources low in amyloses, such as waxy corn or sorghum.
- The best way to prevent retrogradation is to use the gelled food as soon as possible.
- Another process characteristic of starches is dextrinization, which results in an increase in sweetness.
- It is the breakdown of starch molecules into smaller, sweeter tasting dextrin molecules in the presence of dry heat.
- A side effect is that dextrinized starches lose much of their thickening power because they have been broken down into smaller units; thus, more flour is required to thicken gravy if the flour has been browned in the gravy-making process.
- A small amount of starch called resistant starch is not digested in the small intestine and therefore does not contribute calories.
- Some researchers suggest that it may be used as fiber in foods for weight loss purposes. Suggested benefits of resistant starch include improved glycemic (blood glucose level) management, colon health, weight management, and energy.
- It is the starch that has been chemically or physically modified to create unique functional characteristics.
- Some starches are sold only to foodservice operations and food companies and are not encountered at the retail level.
- These starches have been altered to yield a wide variety of modified starches, extending their usefulness in food processing.
- The modifications may affect the starch’s gelatinization, heating times, freezing stability, cold-water solubility, or viscosity.
Testing of starch
Food scientists can determine if starch granules are either amylose or amylopectin by staining them under a microscope. If they turn blue, they contain more amylose. An amber violet color indicates more amylopectin.
Why does toasted bread taste sweeter than untoasted bread?
Toasting or browning breaks down amylose and amylopectin and the resulting dextrins cause toast to taste noticeably sweeter than the original bread.
Gravies and cooked commercial breakfast cereals also taste sweeter than their unprocessed ingredients because of this process.