Milk owes its foaming power to its
proteins, which collect in a thin layer around
the pockets of air, isolate them, and prevent
the water’s strong cohesive forces from
popping the bubbles. Egg foams are also
stabilized by proteins (p. 101), while the foam
formed by whipping cream is stabilized by fat
(below, p. 31). Milk foams are more fragile
and short-lived than egg foams because milk’s
proteins are sparse — just 3% of the milk’s
weight, where egg white is 10% protein —
and two-thirds of the milk proteins are
resistant to being unfolded and coagulated
into a solid network, while most of the egg
proteins readily do so. However, heat around
160ºF/70ºC does unfold the whey proteins
(barely 1% of milk’s weight). And if they
unfold at the air-water boundary of a bubble
wall, then the force imbalance does cause the
proteins to bond to each other and briefly
stabilize the foam.
barry
(Barry)
#1