Production of yogurts and fermented milks

Fermented milks are dairy products obtained by essentially lactic fermentation of milk, resulting in acidification and gelation. Unlike cheeses, coagulation is due solely to the action of lactic bacteria and does not involve the use of coagulants. Historically, the aim was to improve the preservation of milk, a rapidly perishable raw material. Since the last century, these products have become increasingly popular with consumers, thanks to their better-known nutritional properties, their pleasant organoleptic characteristics (freshness, acidity and creaminess) and their diversity. As a result, they account for a significant proportion of dairy product consumption, representing around 21 kg per capita per year in France in 2018.

Their historical and geographical origins are not exactly known. However, it seems that the first fermented milks appeared in the Middle East around 10,000 years ago. Today, they are marketed in a great many countries, from industrial or artisanal production units.

The most widely consumed fermented milk in Western countries is yoghurt. However, a wide variety of fermented milks are produced around the world: acidic fermented milks (yoghurt, laban, leben, dahi), concentrated (Greek yoghurt, labneh, srikhand), drinkable (ayran, dough, laban arbil, lassi, shenina, tan), possibly thickened by the use of specific cultures (viili), acidic and slightly alcoholic products (kefir, koumiss) and low-acid fermented milks (buttermilk, ribot milk). Some fermented milks contain micro-organisms known as probiotics, which have a positive effect on the consumer's health when ingested in sufficient quantities. These fermented milks are marketed all over the world, but in Europe, the health claim is not authorized. In addition to these fresh products, there are by-products such as pasteurized, frozen, powdered and carbonated fermented milks.

The importance of these products, both for consumers who choose them for their nutritional value, sensory characteristics, ease of use and diversity, and in industrial and economic terms, justifies our ongoing interest in them. Product quality and consistency of production are also areas in which efforts are continuing.

Based on the importance of these products for human consumption, and on the favorable technical and economic context in which they operate, this article aims to provide an overview of the manufacture of fermented milks, and yoghurts in particular. It begins by defining these products and outlining their regulations, presenting the micro-organisms involved in their production and their specific characteristics, and explaining the biochemical mechanisms involved in the transformation of milk by lactic fermentation, and the physico-chemical mechanisms that justify...