By Harold McGee
“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 wines. To accentuate this flavor note, manufacturers sometimes add citrate to the milk or cream before fermentation, and they ferment in the cool conditions that favor diacetyl production.