How did fermented milk help preserve fresh milk before refrigeration?

Before the advent of modern refrigeration, fermented milk served as a vital method for preserving fresh milk. This preservation was achieved through the natural action of lactic acid-producing bacteria, such as Streptococcus and Lactobacillus.1

When these beneficial bacteria were introduced to warm raw milk—often by adding a small portion from a previous batch—they began the process of fermentation. During this process, the bacteria partially converted lactose (milk sugar) into lactic acid. The resulting increase in acidity effectively suppressed the growth of spoilage and pathogenic organisms that would otherwise cause the milk to spoil quickly.12

By creating an environment hostile to harmful bacteria, this natural fermentation process allowed milk to be preserved for several days or even weeks without the need for refrigeration. This practice enabled communities to store and consume dairy products safely, eventually leading to the development of various cultured foods like yogurt, kefir, and cultured buttermilk.13