Starch grains degradation

Porosity characteristics also influence the degradation rate of blends containing intact starch grains. Amylase removal of starch from these films was not highly correlated with starch content, since films whose starch content was above possible "percolation thresholds" (6) were degraded at very different rates when starch content was not very different (Table I). 

For starch to be effectively degraded it must first be heated above 65 °C. The starch grains then burst and they form a viscous gel. This must be thinned so that the viscosity is low enough for the dispersed starch to be handled. The a-amylase from barley is fairly stable to heat, and it is active at 65 °C. However some a-amylases from the organisms which grow at temperatures up to 80° or 90 °C (e.g. Bacillus stearothermophilus) are stable for short periods at temperatures above 100°C, and these heat-stable enzymes are very useful for the controlled thinning of starch gels. 

Hours derived from wheat without sprouted grains have some [i- but very little a-amylase activity (cf. 15.2.2.1). Thus, only a small amount of starch is degraded to fermentable maltose by handling dough. An insight into the extent of starch degradation is provided by the maltose value (cf. 15.4.1.1.1). Addition of a-amylase in the form of malt flour or as a microbial preparation increases the flour capacity to hydrolyze the starch. 

When starch is heated in water to 60-80°C the starch grains swell and yield two components. One component (amylose) is water-soluble, is entirely degraded to maltose by the ]S-amylase and its molecules consist of unbranched spiral chains of glucose units (300-1000 units) linked by 1 4 a-linkages. The second component (amylopectin) is not soluble, yields both maltose and dextrin on treatment with )8-amylase and has a branched chain structure due to the occurrence not only of 1 4 but also of 1-6 linkages between the glucose units (Fig. 5.11). 

The variation of sucrose-P-synthase in the leaf adjacent to the ear parallels the variation of assimilate demand by the ear. Carbon fixation capacities decreased continuously but the degradation of Rubisco is less rapid than other soluble proteins. A close agreement exists between the activity of grain ADP glucose pyrophosphorylase and rate of starch accumulation in the grain. 

Two requirements must be met to obtain biologically-mediated mineralization of a polymer it must be exposed to a biotic environment conducive to degradation and its chemical structure must be amenable to breakdown by the available set of microbial enzymes in that environment. Thus, even the inherently biodegradable starch in grains (barley and wheat) found in Egyptian pyramids was preserved over millennia because of the dry and dark environment they were stored in. 

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