While milk kefir grains and water kefir grains share a common purpose in fermenting beverages, they differ significantly in composition and use. Milk kefir grains are used for fermenting dairy milk and are a complex mixture of bacteria and yeast embedded in a matrix of proteins, lipids, and polysaccharides. This combination ferments the lactose in milk to produce a tangy, thick, and nutritious drink packed with probiotics.

On the other hand, water kefir grains are used for fermenting sugar water, juice, or coconut water. These grains are also symbiotic cultures of bacteria and yeast, but they differ in the specific species and the matrix material, which is generally a polysaccharide dextran. Water kefir is a lighter, often carbonated beverage with a wide variety of beneficial bacteria and yeasts.

The key difference lies in their substrates and the end products they produce; milk kefir grains are for dairy fermentations, resulting in a creamy, yogurt-like drink, while water kefir grains ferment sugar solutions, creating a light, carbonated beverage. Neither can effectively ferment the other’s preferred medium without losing vitality or altering the expected probiotic benefits.