Whey

Whey is the thin, pale yellow liquid that separates out when milk or a milk-based ferment like yogurt or kefir curdles, either from fermentation or from straining. In milk kefir, whey separation typically happens when a batch ferments a bit longer than intended, causing the milk solids to clump into curds and release the liquid whey around them.

Rather than discarding it, many home fermenters save whey for other uses, such as adding a small amount to vegetable ferments to introduce extra beneficial bacteria, using it in baking, or as a base for other cultured drinks. Straining finished kefir or yogurt through a fine mesh strainer or cheesecloth is the most common way to separate and collect whey cleanly.

A frequent point of confusion is treating heavily separated, curdled kefir as a failed batch, when it is simply kefir that fermented longer than usual and can still be strained, used, or blended back together. Whey does not keep indefinitely, so most guidance suggests refrigerating it and using it within a reasonably short window, similar to other fresh dairy products.