On Food and Cooking

hours of gravity separation, bacteria would
grow spontaneously in the milk and give the
cream and the butter made from it a
characteristic aroma and tartness.
“Cream cultures” is a convenient
shorthand for products that are now
intentionally seeded with these same bacteria,
which are various species of Lactococcus and
Leuconostoc, and have three important
characteristics. They grow best at moderate
temperatures, well below the typical
temperature of yogurt fermentation; they’re
only moderate acid-producers, so the milks
and creams they ferment never get extremely
sour; and certain strains have the ability to
convert a minor milk component, citrate, into
a warmly aromatic compound called diacetyl
that miraculously complements the flavor of
butterfat. It’s fascinating that this single
bacterial product is so closely associated with
the flavor of butter that all by itself, diacetyl
makes foods taste buttery: even chardonnay