Re: How does lactobacillus bulgaricus work on milk to make yogurt?

Hi Paul

Thanks for your question. The basic process by which milk is transformed into yougurt is fermentation. Several species of bacteria can be used for yogurt production: you mention Lactobacillus bulgaricus, others include Lactobacillus acidophilus and Streptococcus thermophilus. Take a look at the name of that second one. It tells you that lactose is involved in the process and that the bacterium likes to live in acidic conditions.

Milk is basically a suspension of fat globules in water, plus some minerals (such as calcium), proteins and sugars (principally lactose). When the aforementioned bacteria are added to milk, they can take up the lactose and split it into simpler sugars such as glucose. Glucose is then fermented to lactic acid, so yogurt bacteria are often classified as lactic acid bacteria. This can occur by 2 biochemical pathways (named homofermentative or heterofermentative) and it occurs because oxygen is depleted in the yogurt culture. The same process occurs in our muscles during strenuous exercise and the build-up of lactic acid causes cramps. At thi s excellent website you can discover more about fermentation in yogurt and see the biochemical processes involved.

Now, lactic acid (also called lactate) is not good for bacteria, so they excrete it into their environment. This causes the pH to fall and protein molecules in the milk become denatured. This means that they unfold from their normal structures, become disordered and begin to stick to each other. This is what causes the yogurt to become partially solid and gelatinous. It's exactly the same process that causes eggs to become solid when cooked, egg whites to become solid when whipped or cream to curdle if you add lemon juice. In each case, proteins are denaturing and sticking to one another to form a semi-solid matrix.

Hope this helps with your question,

Neil Saunders