Reduced-fat milks make especially firm
yogurt because manufacturers mask their lack
of fat by adding extra milk proteins, which
add density to the acid-coagulated protein
network. (Manufacturers may also add
gelatin, starch, and other stabilizers to help
prevent separation of whey and curd from
physical shocks during transportation and
handling.)
Heating the Milk Traditionally the milk for
yogurt was given a prolonged boiling to
concentrate the proteins and give a firmer
texture. Today, manufacturers can boost
protein content by adding dry milk powder,
but they still cook the milk, for 30 minutes at
185ºF/85ºC or at 195ºF/90ºC for 10 minutes.
These treatments improve the consistency of
the yogurt by denaturing the whey protein
lactoglobulin, whose otherwise unreactive
molecules then participate by clustering on
the surfaces of the casein particles (p. 20).