The main physicochemical indicator of change, apart from acidity, is a decrease in lactose content. (Of course, academically speaking, some vitamins will increase.) I. Principles of yogurt fermentation. Lactose in milk under the action of microbial lactose decomposition enzyme, first of all, lactose decomposition into 2 molecules of monosaccharides, lactic acid bacteria in the further action of lactic acid to generate lactic acid, when lactic acid reaches a certain concentration, so that the milk coagulation phenomenon occurs.C12H22O11 (lactose) + H2O ----- 2C6H12O6 (glucose + galactose), C6H12O6 --- C3H6O3 (lactic acid) ii. Fat is not involved in fermentation. It just has an effect on the state of its organization (so the state of the organization of yogurt made from fermentation of completely skimmed milk tends to be a little worse). So there is no change after fermentation. Third, part of the protein is broken down into free amino acids, as the lactic acid bacteria "nitrogen nutrient source". Theoretically, there is a reduction, but the consumption is very small. So on a macro level, the protein content of the finished product is basically unchanged.