On food and cooking the science and lore of the kitchen ( PDFDrive ) 46

micelles to flock together and the milk to curdle One way is souring Milk’s normal pH is about 6.5, or just slightly acidic If it gets acid enough to approach pH 5.5, the cappingcasein’s negative charge is neutralized, the micelles no longer repel each other, and they therefore gather in loose clusters At the same acidity, the calcium glue that holds the micelles together dissolves, the micelles begin to fall apart, and their individual proteins scatter Beginning around pH 4.7, the scattered casein proteins lose their negative charge, bond to each other again and form a continuous, fine network: and the milk solidifies, or curdles This is what happens when milk gets old and sour, or when it’s intentionally cultured with acid-producing bacteria to make yogurt or sour cream Another way to cause the caseins to curdle is the basis of cheese making Chymosin, a digestive enzyme from the stomach of a milkfed calf, is exquisitely designed to give the casein micelles a haircut (p 57) It clips off just the part of the capping-casein that extends into the surrounding liquid and shields the micelles from each other Shorn of their hairy layer, the micelles all clump together — without the milk being noticeably sour The Whey Proteins Subtract the four caseins from the milk proteins, and the remainder, numbering in the dozens, are the whey proteins Where the caseins are mainly nutritive, supplying amino acids and calcium for the calf, the whey proteins include defensive proteins, molecules that bind to and transport other nutrients, and enzymes The most abundant one by far is lactoglobulin, whose biological function remains a mystery It’s a highly structured protein that is readily denatured by cooking It unfolds at 172ºF/78ºC, when its sulfur atoms are exposed to the surrounding liquid and react with hydrogen ions to form hydrogen sulfide gas, ...casein micelles a haircut (p 57) It clips off just the part of the capping-casein that extends into the surrounding liquid and shields the micelles from each other Shorn of their hairy layer, the micelles all clump together —... layer, the micelles all clump together — without the milk being noticeably sour The Whey Proteins Subtract the four caseins from the milk proteins, and the remainder, numbering in the dozens, are the whey proteins Where the. .. supplying amino acids and calcium for the calf, the whey proteins include defensive proteins, molecules that bind to and transport other nutrients, and enzymes The most abundant one by far is lactoglobulin,