Rice. 1.3. Structure of starch grain:

1 - structure of amylose; 2 - structure of amylopectin; 3 - starch grains of raw potatoes; 4 - starch grains of boiled potatoes; 5 - starch grains in raw dough; 6 - starch grains after baking

When heated from 55 to 80°C, starch grains absorb a large amount of water, increase in volume several times, lose their crystalline structure, and, consequently, their anisotropy. The starch suspension turns into a paste. The process of its formation is called gelatinization. Thus, gelatinization is the destruction of the native structure of the starch grain, accompanied by swelling.

The temperature at which the anisotropy of most grains is destroyed is called the temperature gelatinization. The temperature of gelatinization of different types of starch is not the same. Thus, gelatinization of potato starch occurs at 55-65°C, wheat - at 60-80, corn - at 60-71°, rice - at 70-80°C.

The process of gelatinization of starch grains proceeds in stages:

* at 55-70°C grains increase in volume several times, lose optical anisotropy, but still retain a layered structure; a cavity ("bubble") is formed in the center of the starch grain; a suspension of grains in water turns into a paste - a low-concentration amylose sol, in which swollen grains are distributed (the first stage of gelatinization);

* when heated above 70 ° C in the presence of a significant amount of water, starch grains increase in volume by dozens of times, the layered structure disappears, the viscosity of the system increases significantly (the second stage of gelatinization); at this stage, the amount of soluble amylose increases; its solution partially remains in the grain, and partially diffuses into the environment.

With prolonged heating with excess water, starch bubbles burst, and the viscosity of the paste decreases. An example of this in culinary practice is the liquefaction of jelly as a result of excessive heat.

The starch of tuberous plants (potato, Jerusalem artichoke) gives transparent pastes of a jelly-like consistency, and cereals (corn, rice, wheat, etc.) - opaque, milky-white, pasty consistency.

The consistency of the paste depends on the amount of starch: when its content is from 2 to 5%, the paste turns out to be liquid (liquid jelly, sauces, puree soups); at 6-8% - thick (thick jelly). An even thicker paste is formed inside the potato cells, in cereals, pasta dishes.

The viscosity of the paste is affected not only by the concentration of starch, but also by the presence of various nutrients (sugars, mineral elements, acids, proteins, etc.). So, sucrose increases the viscosity of the system, salt reduces it, proteins have a stabilizing effect on starch pastes.

When starch-containing products are cooled, the amount of soluble amylose in them decreases as a result of retrogradation (precipitation). In this case, aging of starch jellies (syneresis) occurs, and the products become stale. The rate of aging depends on the type of products, their humidity and storage temperature. The higher the humidity of the dish, the culinary product, the more intensively the amount of water-soluble substances in it decreases. The most rapid aging occurs in millet porridge, slower - in semolina and buckwheat. An increase in temperature slows down the process of retrogradation, so dishes from cereals and pasta, which are stored on food warmers with a temperature of 70-80 ° C, have good organoleptic characteristics for 4 hours.

hydrolysis of starch. Starch polysaccharides are able to decompose into the molecules of their constituent sugars. This process is called hydrolysis, as it comes with the addition of water. Distinguish between enzymatic and acidic hydrolysis.

Enzymes that break down starch are called amylases. There are two types of them:

α-amylase, which causes a partial breakdown of starch polysaccharide chains with the formation of low molecular weight compounds - dextrins; with prolonged hydrolysis, the formation of maltose and glucose is possible;

β-amylase, which breaks down starch into maltose.

Enzymatic hydrolysis of starch occurs in the manufacture of yeast dough and baking products from it, boiling potatoes, etc. Wheat flour usually contains β-amylase; maltose, formed under its influence, is a nutrient medium for yeast. α-amylase predominates in flour from sprouted grains, and dextrins formed under its influence give products stickiness and an unpleasant taste.

The degree of hydrolysis of starch under the action of )