On Food and Cooking

The Curdling Specialist Traditional rennet is
made from the fourth stomach or abomasum
of a milk-fed calf less than 30 days old, before
chymosin is replaced by other protein-
digesting enzymes. The key to rennet’s
importance in cheesemaking is chymosin’s
specific activity. Where other enzymes attack
most proteins at many points and break them
into many pieces, chymosin effectively
attacks only one milk protein, and at just one
point. Its target is the negatively charged
kappa-casein (p. 19) that repels individual
casein particles from each other. By clipping
these pieces off, chymosin allows the casein
particles to bond to each other and form a
continuous solid gel, the curd.
Since plain acidity alone causes milk to
curdle, why do cheesemakers need rennet at
all? There are two reasons. First, acid
disperses the casein micelle proteins and their
calcium glue before it allows the proteins to
come together, so some casein and most of the