Culture

Yoghurt is made by introducing specific bacteria strains into milk, which is subsequently fermented under controlled temperatures and environmental conditions (inside a bioreactor), especially in industrial production. The bacteria ingest natural milk sugars and release lactic acid as a waste product. The increased acidity causes milk proteins to tangle into a solid mass (curd) in a process called denaturation. The increased acidity (pH=4–5) also prevents the proliferation of potentially pathogenic bacteria. In Australia, to be named yogurt, the product must be made with the bacterial species Streptococcus salivarius subsp. thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. Often these two are co-cultured with other lactic acid bacteria for taste or health effects (See probiotics). These include L. acidophilus, L. casei and Bifidobacterium species. In most countries, a product may be called yoghurt only if live bacteria are present in the final product. In Australia, non-pasteurised yoghurt can be marketed as "live" or containing "live active culture". A small amount of live yoghurt can be used to inoculate a new batch of yoghurt, as the bacteria reproduce and multiply during fermentation. Pasteurised products, which have no living bacteria, are called fermented milk (drink). When yoghurt is pasteurised, even though its main aim is to kill harmful bacteria, it kills large amounts of essential bacteria too, such as Acidophilus, Bifidus and Lactobacillus Rhamnosus (LGG)