Sour grapes about Sour Milk

By Alan Lugton, Cookery Manager

I was asked recently about ways to use sour milk in recipes and after some confused discussion it soon became clear that what I was being asked was how to use shop-bought milk that had passed its expiry date!

This is different from sour milk and the confusion arises due to the process of pasteurisation. If you have ever had the courage or misfortune to taste milk that has passed its expiry date, ‘sour’ is not the best word to describe it. Bitter, acrid, putrid and quite simply ‘wrong’ are more appropriate.

The process of pasteurising milk is well known, every child learns about it in school, after all it is one of the great health benefits of our time. In reducing potentially harmful pathogens and prolonging the milk’s shelf life, Louis Pasteur’s process can be credited with saving the lives of many in its time. However pasteurisation also has the side affect of killing off some of the useful bacteria that are naturally present in the milk.

Unpasteurised (raw) milk has a natural life span in which it stays fresh (sweet milk) for several days before the natural bacteria (lactobacilli) get busy making lactic acid to give it that lovely sourness . With pasteurised milk, the life is extended past the sour stage, keeping it sweet longer, and then it moves right into the spoiling stage where the milk starts to putrefy and separate, thus giving it an acrid, bitter taste. At this point the milk will most likely give you an upset tummy at the very least.

Many of the recipes that use sour milk date back to a time when raw milk was more widely used and farmstead kitchens regularly included sour milk in their baking. If you ever need sour milk in a recipe these days and you can’t get hold of raw milk, use buttermilk or you can sour fresh milk manually by adding a little vinegar or lemon juice.

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