A scary phenomenon in the dairy industry, one which can bring a cafe to its knees, is lipolysis (the chemical breakdown of fat). There is a vast amount of scientific literature on this subject because it is so damaging to the performance of milk foams. You will know lipolysis has occurred in your milk if you finish the steaming process and the foam almost immediately dissipates.
There are degrees of lipolysis. It can sometimes bring about a rapid degradation of foam, or this can occur over a longer period of time. But even in milder cases, lipolysis will greatly reduce the perceived body of a milk drink, and at times, it will impart a rancid flavour.
Lipolysis is the loosening of the lipoprotein lipase (LPL) enzyme in milk. The enzyme is normally trapped inside the milk fat globule. One immediate trigger for lipolysis is the homogenization of milk before it is pasteurised. This breaking up of the milk fat globule into submicron particle sizes brings the LPL into more direct contact with the milk fats. Hilton C. Deeth, professor of dairy science, explains that by
‘splitting the natural fat globules into much smaller fat particles and coating them with milk proteins, rather than the natural [MFG membrane], the LPL is brought into intimate contact with fat with a large surface area’ (Deeth 2006).
Deeth supports the widespread notion that seasonal changes in the the cow’s diet can affect the milk’s performance:
‘The major factors associated with spontaneous lipolysis are late lactation (Chazal and Chilliard 1986), poor-quality feed (Jellema 1980) and mastitis (Downey 1980). Spontaneous lipolysis becomes most noticeable in the milk from farms with a majority of cows in late lactation at a time when the cows’ plane of nutrition is low. Therefore, districts where seasonal calving is practised may encounter this problem during periods of poor feed availability’ (Deeth 2006).