Yogurt is produced by the controlled fermentation of milk by two species of bacteria Lactobacillus bulgaricus and Streptococcus thermophilus.
Yogurt The sugar in milk (called lactose) is fermented to acid (lactic acid) and it is this that causes the characteristic curd to form. Streptococcus thermophilus brings the pH of the milk down to 5.5 Lactobacillus bulgaricus converts lactose to lactic acid
Yogurt The two bacteria have a mutually stimulating effect on one another. Proteolytic enzymes from L. bulgaricus break down milk proteins into peptides.
Yogurt These peptides stimulate the growth of S. thermophilus which in turn produces formic acid and carbon dioxide These are growth stimulants for the L. bulgaricus.
Yogurt At the end of the incubation pH may fall to as low as 4.2 – 4.4 The lactic acid coagulates the remaining milk proteins, causing the yogurt to thicken.
Yogurt Acetaldehyde, a metabolic by product of both species, gives raw yogurt its characteristic flavor.
Yogurt Incubation takes 12 hours at 45 o C to reach the set point of natural yogurt Sugar, color, and fruit pulp is often added to increase popularity of product.