2009 Proteins Part 1 - On the ultimate composition of...

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On the ultimate composition of simple alimentary substances; with some preliminary remarks on the analysis of organised bodies in general Philosophical Transactions of the Royal Society 355 (1827) “The subject of digestion, however, had for a long time occupied my particular attention: and by degrees I had come to the conclusion, that the principal alimentary matters employed by man, and the more perfect animals, might be reduced to three great classes, namely, the saccharine , the oily , and the albuminous “… William Prout (1785-1850)
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Proteins in Foods General functionalities of proteins in foods: Source of nutritionally essential amino acids Provide structure & texture to foods
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Protein content of common foods Food Amount yielding 25 g protein* Meats 4-5 oz Cheddar cheese 4 oz Milk 3 cups Black beans 1-2 cups Tofu (soybean curd) 1-2 cups White bread 9 slices Egg 4 eggs Wheaties 8 cups Spaghetti 3 cups Lettuce 33 cups Apples 75 apples Celery 90 stalks Coffee 750 oz * about 50% of RDA for adults
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Structure of Proteins Proteins are polymers of amino acids : A generic amino acid: Amino acids are distinguished by their side chain, or “ R ” group : Polar vs non-polar Ionizable vs non-ionizable (“R group”) 20 common amino acids 9 are essential (required in the diet) R group :
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Amino acids with polar (hydrophilic) R groups : Amino acids with non-polar (hydrophobic) R groups : serine cysteine alanine leucine phenylalanine
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Amino acids with ionizable R groups : Those with carboxyl ( COOH ) or amine ( NH 2 ) groups Mostly Neg charged above pH 4 Mostly Pos charged below pH 9 Important ! : pH of most foods is between 5 and 7 lysine ( Note: all of these are also polar ) Q: In what ionization state would these R-groups exist in a food of pH 6.8?
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Free amino acids are relatively rare in foods (many have very low flavor thresholds) Amino acids are linked together in
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2009 Proteins Part 1 - On the ultimate composition of...

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