On food and cooking the science and lore of the kitchen ( PDFDrive ) 46
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micellestoflocktogetherandthemilkto
curdle.Onewayissouring.Milk’snormalpH
isabout6.5,orjustslightlyacidic.Ifitgets
acidenoughtoapproachpH5.5,thecappingcasein’snegativechargeisneutralized,the
micellesnolongerrepeleachother,andthey
thereforegatherinlooseclusters.Atthesame
acidity,thecalciumgluethatholdsthe
micellestogetherdissolves,themicelles
begintofallapart,andtheirindividual
proteinsscatter.BeginningaroundpH4.7,the
scatteredcaseinproteinslosetheirnegative
charge,bondtoeachotheragainandforma
continuous,finenetwork:andthemilk
solidifies,orcurdles.Thisiswhathappens
whenmilkgetsoldandsour,orwhenit’s
intentionallyculturedwithacid-producing
bacteriatomakeyogurtorsourcream.
Anotherwaytocausethecaseinstocurdle
isthebasisofcheesemaking.Chymosin,a
digestiveenzymefromthestomachofamilkfedcalf,isexquisitelydesignedtogivethe
caseinmicellesahaircut(p.57).Itclipsoff
justthepartofthecapping-caseinthatextends
intothesurroundingliquidandshieldsthe
micellesfromeachother.Shornoftheirhairy
layer,themicellesallclumptogether—
withoutthemilkbeingnoticeablysour.
TheWheyProteinsSubtractthefourcaseins
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
defensiveproteins,moleculesthatbindtoand
transport other nutrients, and enzymes. The
most abundant one by far is lactoglobulin,
whosebiologicalfunctionremainsamystery.
It’sahighlystructuredproteinthatisreadily
denatured by cooking. It unfolds at
172ºF/78ºC,whenitssulfuratomsareexposed
to the surrounding liquid and react with
hydrogen ions to form hydrogen sulfide gas,
