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 capping-
casein’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 milk-
fed calf, is exquisitely designed to give the
barry
(Barry)
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