How does the pH of yoghurt compare to the pH of fresh milk?

Yoghurt has a significantly lower pH than fresh milk because *Lactobacillus* bacteria convert lactose into lactic acid. This increased acidity is what gives yoghurt its sour taste and acts as a preservative.

What is the difference between the purpose of the $85-95 \, ^{\circ}C$ stage and the $40-45 \, ^{\circ}C$ stage?

The $85-95 \, ^{\circ}C$ stage is used to kill unwanted bacteria and pathogens in the milk, whereas the $40-45 \, ^{\circ}C$ stage provides the optimal temperature for the added *Lactobacillus* to grow and ferment.

How does the metabolic byproduct of yoghurt production differ from that of bread production?

In yoghurt, bacteria produce lactic acid which thickens the milk. In bread, yeast produces carbon dioxide which makes the dough rise, and ethanol which evaporates during baking.

What would happen if *Lactobacillus* were added to milk immediately after it was heated to $90 \, ^{\circ}C$?

The bacteria would die because the extreme heat would denature their enzymes and destroy their cell structures. The milk must be cooled to roughly $45 \, ^{\circ}C$ to ensure the bacteria remain active.

Why is it incorrect to say that *Lactobacillus* respire aerobically during yoghurt formation?

Yoghurt formation relies on fermentation, which is an anaerobic process. If oxygen were present, the bacteria might not produce the lactic acid necessary for the milk to thicken and sour.

Commonly, students confuse the substrate and the product. What are they in this process?

The substrate is lactose (the sugar found in milk), and the product is lactic acid. The bacteria consume the lactose to produce the acid as a byproduct of their respiration.

Define 'Pasteurisation' in the context of dairy production.

Pasteurisation is the process of heating milk to a high temperature (around $85-95 \, ^{\circ}C$) to kill harmful microorganisms and competing bacteria before further processing. It ensures the safety and consistency of the milk.

What is *Lactobacillus*?

It is a genus of beneficial bacteria used in food production to ferment sugars into lactic acid. In yoghurt making, it is the primary organism responsible for the chemical changes in the milk.

What does 'Incubation' mean during yoghurt making?

Incubation is the period where the milk and bacteria mixture is kept at a constant warm temperature ($40-45 \, ^{\circ}C$). This allows the bacteria to grow, reproduce, and ferment the lactose over several hours.

Why does yoghurt have a much longer shelf life than fresh milk?

The lactic acid produced by bacteria lowers the pH of the yoghurt. This high acidity creates an environment that inhibits the growth of most other harmful or spoilage-causing microorganisms.

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5. Use of Biological Resources

Role of Bacteria in Yoghurt Production

Summary

Yoghurt production is a biotechnological process that utilizes the metabolic activity of specific bacteria, primarily Lactobacillus, to transform milk. Through the fermentation of lactose into lactic acid, the chemical and physical properties of milk are altered, resulting in a thickened texture, characteristic sour flavor, and enhanced shelf life due to increased acidity.

1. Definition & Core Concepts

Lactobacillus: This is the genus of Gram-positive, facultative anaerobic bacteria used specifically for their ability to convert sugars into lactic acid.
Fermentation: In the context of yoghurt, this is the anaerobic metabolic process where bacteria digest milk sugars to produce energy and chemical byproducts.
Lactose: The primary disaccharide sugar found in milk which serves as the energy source (substrate) for the bacterial culture.
Lactic Acid: The metabolic byproduct of fermentation that causes the pH of the milk to drop, leading to the denaturation of milk proteins.

2. Underlying Principles

Metabolic Conversion: The bacteria utilize enzymes to break down lactose into glucose and galactose, which are then processed into lactic acid via anaerobic pathways.
Protein Coagulation: As the concentration of lactic acid increases, the pH of the milk drops. This acidity causes milk proteins (casein) to denature and cross-link, forming a semi-solid mesh that thickens the milk into yoghurt.
Preservation via pH: Most pathogenic and spoilage microorganisms cannot survive in highly acidic environments. By lowering the pH, Lactobacillus effectively acts as a biological preservative, extending the shelf life of the dairy product.

Diagram showing the biochemical conversion of lactose to lactic acid by Lactobacillus, leading to yoghurt formation and protein thickening.

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7. Connections & Extensions