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Nutr 330 study guide #2

Nutr 330 study guide #2 - Page 1 of 3 uNutrition 330L...

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Page 1 of 3 uNutrition 330L, Principles of Food Science Carole Conn Spring 2008 Study Guide for Exam #2 STARCH 1. Distinguish between amylose and amylopectin in terms of: linear vs. branched structure, type(s) of glucosidic bonds, iodine-binding ability . Amylose *Linear form ( no branching); coils of 6 to 7 molecules *Glucose units linked by 1, 4 alpha glucosidic bonds *Molecular weight = 1000 to 150,000 (Corn and rice cereal starches have shorter, lighter amylase molecules than are found in potato and other starches from roots and tubers) -Cereals have lower molecular weight starch -Potatoes and tubers have higher *Slightly soluble * Iodine test to detect amylase in a starch solution -every six carbons binds one iodine in a coil -long helix turns solution blue (or short coils turn it red) *usually 17-30% of starch is amylase -cereals: cornstarch, wheat starch about 25-28% -potato starch: around 20% -tapioca (cassava): 17% -special peas and corn: 75% Although amylase is described as a linear molecule, individual molecules appear to form a loose, rather flexible coil when they are dispersed in a solution. This coiled arrangement allows iodine to be trapped within the helix when it is added to amylase, resulting is a blue color. Each six-D- glucose units in the helix can bind one iodine atom. The iodine test is used to determine the presence of amylase in starch dispersions. Starches from various sources differ in relative content of amylose. Although it is possible to breed plants that produce starch containing essentially no amylose, the more common sources of starch range in amylose content from about 17 percent to around 30 percent. In general, the root and tuber starches contain somewhat less amylose than do the cereal starches. Cornstarch typically ranges between 24 and 28 percent amylose; the range for wheat starch is similar but a bit narrower, 25 to 26 percent. The amylose content of potato starch is somewhat lower, usually between 20 to 23 percent. Tapioca , which is only about 17 percent amylose, provides the least amylose of the starches commonly used. At the other end of the spectrum, some special species of peas and corn have been bred with amylose content as high as 75 percent. Amylopectin *branched form (dendritic)
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Page 2 of 3 *glucose units linked by alpha 1,4 glucosidic with alpha 1,6 glucosidic bonds at branch points. * molecular weight in the millions -like amylose, only one free reducing group (aldehyde group) per molecule. * does not form coil in solution , therefore iodine test solution becomes purple/red, not blue. The failure to show the blue color that iodine produces with amylose is due to the lack of a helical configuration within amylopectin molecules. * more prevalent in foods than amylose (waxy maize is about 100% amylopectin) Genetic variations containing starches composed of virtually only amylopectin waxy maize have been developed and are of commercial significance. The relative proportions of amylopectin and amylose in starches are of considerable importance because of the different behaviors of these two starch fractions in cooked starch products.
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