🧀 Buttermilk

Buttermilk is the liquid left over after cream or soured milk has been churned into butter. Almost all the fat goes into the butter, so little stays behind: according to FAO, traditional sour buttermilk in Africa and Asia contains 1–3% fat. What remains are proteins, milk sugar, minerals and part of the milk fat globule membrane (MFGM).

In India, the buttermilk separated when dahi is churned is called chhaas; it is about 92% water, and diluted with cold water it becomes the drink lassi. In Ethiopia, Somalia and Niger, raw milk is churned without heating, and the buttermilk then sours within 3–24 hours and keeps for up to two days. In Chad and Sudan, the sour buttermilk roab is left to stand for about a day.

In industrialised countries, cream for butter is mostly not soured, so the sour buttermilk on supermarket shelves is a separate product: pasteurised milk fermented with lactic acid bacteria. In the US it may be labelled “cultured buttermilk”, and its acidity has to be at least 0.5% expressed as lactic acid. According to USDA, 100 g of low-fat buttermilk has 40 kcal, 3.3 g protein, 1.1 g fat and 116 mg calcium. Buttermilk from butter-making is also a raw material: the fat globule membrane is extracted from it.

Buttermilk or kefir, buttermilk or plain milk? Traditional buttermilk is simply what is left after butter is churned, so it is thin and low in fat, 1–3%. The buttermilk on supermarket shelves is a different product: pasteurised milk fermented with lactic acid bacteria, which the US calls cultured buttermilk. Kefir is fermented on purpose with bacteria and yeasts. Low-fat store-bought buttermilk has 40 kcal and 1.1 g of fat per 100 g, whereas ordinary milk is not soured at all.

Look for the name "buttermilk", or "cultured buttermilk" on US-style packs, in the chilled dairy aisle of a supermarket, usually near kefir and yogurts. The ingredients should be simply milk and lactic cultures. Pay attention to the fat figure on the label: the low-fat kind is about 1 g per 100 g. It is sold in cartons or bottles, and ethnic grocers may stock the Indian chhaas as well.

Why it matters to you. In baking, buttermilk adds tang; if you have none, the University of Maine Cooperative Extension suggests a substitute: a tablespoon of lemon juice or white vinegar topped up with milk to one cup, then wait 5–10 minutes until the milk curdles slightly. Don't trust promises of “healing” buttermilk: the fat globule membrane is being actively studied, but a 2023 review admits its mechanisms still need research.