Gelatinization degree
Gelatinization degree refers to the percentage of gelatinized starch to the total amount of starch in starch. The higher the gelatinization degree of starch, the easier it is to be hydrolyzed by enzymes, which is beneficial to digestion and absorption.
2. Gelatinization characteristic index (derived from GBT 24852-2010 Rapid viscometer method for determination of gelatinization characteristics of rice and rice flour)
1) Gelatinization temperature: After the sample is heated, The temperature at which the viscosity of the sample begins to increase.
2) Under the specified conditions of peak viscosity, heating causes the sample to begin to gelatinize to the maximum viscosity value reached before cooling.
3) Peak time: Under specified conditions, the time from when the sample starts to be heated to reaches the peak viscosity.
4) Minimum viscosity: the minimum viscosity value between the coolers after the sample reaches the peak viscosity under specified conditions.
5) Final viscosity: the viscosity value of the sample at the end of the test under specified conditions.
6) Attenuation value: the difference between the peak viscosity and the lowest viscosity.
7) Pick-up value: the difference between the final viscosity and the lowest viscosity.
2. Factors affecting starch gelatinization
1. Moisture
1) Moisture content has a greater impact on the gelatinization of rice starch. The higher the moisture content, the greater the gelatinization of rice starch. The lower, the harder it is for rice starch to gelatinize. When the moisture content is less than 50%, it is difficult to completely gelatinize rice starch under normal pressure.
2) When the moisture content is 50%, it is difficult for rice flour to gelatinize, and there are a large number of ungelatinized starch granules in the microscope field of view; when the moisture content is 80%, some ungelatinized starch can still be seen granules, but significantly reduced compared with the sample with 50% moisture content; when the moisture content reached 100%, there were basically no ungelatinized starch granules; when the moisture content was 150% and 200%, the rice flour was observed by smearing. Completely gelatinized.
3) Using high-pressure conditions such as puffing, the moisture can be reduced to 23%.
2. Temperature
Temperature is the factor that has the greatest influence on the degree of gelatinization. When the temperature is low, the thermal motion of molecules is not strong. Starch melts rapidly when the temperature is high. The thermal motion of molecules is violent and the degree of gelatinization increases.
The gelatinization temperature of most cereal flours when there is sufficient moisture is between 6O and 8O℃ (see the table below). The higher the temperature, the faster and deeper the starch gelatinization rate.
Ding Wenping (2002) reported that the final gelatinization temperature of rice amylose is 58-79°C. The gelatinization and retrogradation of rice amylose are closely related to the lipid content. Morrison et al. determined that the complex rate of amylose and lipid in rice with high amylose content (19.5%-28.3%) reaches 19.4%~30.2%, and its crystallization melting temperature is between 80~120℃. .
3. Amylose content
As the gelatinization time gets longer, the degree of gelatinization deepens. Amylose content will affect starch gelatinization properties. The main reasons are:
(1) The ratio of amylose to amylopectin in corn starch affects the starch crystal structure; at the same time, the increase in amylose content will cause the amylopectin side chain to grow, causing the starch crystal structure to change from A to A. Type B changes to type B, and the gelatinization characteristics of starches with different crystal structures are also different.
(2) Amylose is mainly distributed on the particle surface, intertwined with amylopectin and penetrates into the crystalline and amorphous regions; amylose has a "binding" effect on amylopectin, and along with the amylose As the starch content increases, the "binding" effect will also increase, so that the amylopectin cannot be fully stretched, thereby inhibiting the expansion and gelatinization of starch, and the viscosity will also decrease.