Q.Explain the changes that milk undergoes when suitable starter/inoculum is added to it. How does the end product formed prove to be beneficial for human health ?
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Start your 14-day free trial to unlock the full solution →Adding a starter culture (lactic acid bacteria) to milk triggers lactic acid fermentation — lactose is converted to lactic acid, which coagulates milk proteins (casein) into curd. The resulting curd is easier to digest, provides probiotics that improve gut health, and is safe for lactose-intolerant individuals.
The process you are asking about is the transformation of milk into curd (yogurt/dahi) — a classic example of fermentation driven by microorganisms. The key is to understand that the starter culture is not just any bacteria; it is a specific set of lactic acid bacteria (LAB), such as Lactobacillus bulgaricus and Streptococcus thermophilus. These bacteria are the workhorses behind the change.
Why does this happen? Milk is a complex fluid containing water, proteins (mainly casein), fats, and the sugar lactose. The bacteria in the starter culture are added to a warm environment (around 40°C). They are anaerobic (they work without oxygen) and their primary metabolic trick is to break down lactose into lactic acid. This single chemical change sets off a cascade of physical and chemical transformations.
Let us walk through the changes step by step.
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Lactose breakdown and acid production
The starter bacteria secrete the enzyme lactase (β-galactosidase), which hydrolyses the disaccharide lactose into its monosaccharides: glucose and galactose. The bacteria then ferment these sugars, primarily glucose, via the glycolytic pathway to produce pyruvate. Under anaerobic conditions, pyruvate is reduced to lactic acid by the enzyme lactate dehydrogenase.
Lactose --(lactase)--> Glucose + Galactose --(glycolysis)--> 2 Pyruvate --(LDH)--> 2 Lactic acid
The lactic acid accumulates, causing the pH of the milk to drop from about 6.6 to around 4.5–4.0.
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Coagulation of milk proteins (curdling)
Milk proteins, especially casein, are normally kept suspended in a stable colloidal state. Casein molecules carry a negative charge at the natural pH of milk, which keeps them repelling each other. As lactic acid builds up, the pH falls toward the isoelectric point of casein (pH 4.6). At this point, the net charge on casein becomes zero. The repulsive forces vanish, and the casein molecules aggregate, forming a network that traps water and fat. This is the curd — a semi-solid gel.
NoteThe whey (the watery liquid that separates) is not a waste product; it contains water-soluble proteins (whey proteins), lactose, and minerals. In traditional curd, some whey remains trapped, giving the curd its characteristic texture.
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Changes in flavour and aroma
Besides lactic acid, the bacteria produce small amounts of other compounds: acetaldehyde (the characteristic "yogurt" aroma), diacetyl (buttery flavour), and trace amounts of ethanol and carbon dioxide. These give curd its tangy, fresh taste — very different from the bland sweetness of milk.
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Partial breakdown of proteins and fats
The starter bacteria also possess proteolytic enzymes that partially digest casein into peptides and amino acids. This makes the proteins in curd easier to digest than those in raw milk. Similarly, some lipases break down milk fats into free fatty acids, contributing to flavour.
Now, how is the end product — curd — beneficial for human health? This is where the real value lies.
The health benefits of curd are not just from the nutrients present in milk, but from the living bacteria (probiotics) and the pre-digested nature of the food. …
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