sour cream today, in usual commercial packaged versions, only vaguely resembles cream that happens to have naturally gone sour. Before universal pasteurization, cream and milk spontaneously soured by the action of certain lactic acid bacteria that were ubiquitous in most of northern Europe and the British Isles. Loosely labeled “mesophilic,” or adapted to temperatures between about 70° and 90°F (21° to 32°C), these organisms ferment part of the original lactose (milk sugar) to lactic acid, thickening the cream to a gel and lending it a noticeable but discreet sourness.
