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The function of the Lactobacillus bulgaricus and Streptococcus thermophilus cultures is to ferment lactose to produce lactic acid. The increase in lactic acid decreases pH and causes the milk to clot, or form the soft gel that is characteristic of yogurt.

In lactose-free milk, all the lactose was transformed in glucose and galactose using lactase. Is it possible for those bacteria to ferment glucose and galactose instead of lactose? Or is this based on other biological reactions?

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Conversion of lactose to galactose and glucose is a part of the bacterial metabolism of lactose. So digested lactose would work.

In fact, it should work for all kinds of 6-carbon sugars as lactic acid is produced from pyruvate (end product of glycolysis). See Wikipedia:lactic acid fermentation.

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