What are the metabolic pathways involved with making yogurt?

Yogurt production primarily relies on the lactic acid fermentation pathway, driven by specific bacteria. Here's a breakdown:

1. Milk Composition:

* Milk contains lactose (milk sugar), proteins (casein and whey), fats, and other nutrients.

2. Bacteria Introduction:

* Yogurt cultures (typically *Lactobacillus bulgaricus* and *Streptococcus thermophilus*) are added to the milk.

3. Lactic Acid Fermentation:

* The bacteria metabolize lactose using glycolysis, the first stage of cellular respiration.

* Glycolysis breaks down lactose into pyruvate, producing ATP (energy) and NADH (electron carrier).

* Pyruvate is then converted to lactic acid, the key byproduct of this fermentation.

4. Acidification and Curdling:

* The accumulating lactic acid lowers the pH of the milk, making it more acidic.

* This acidification causes the milk proteins (casein) to denature and coagulate, forming the characteristic yogurt texture.

5. Other Reactions:

* Some bacteria also produce other compounds during fermentation, contributing to yogurt's flavor and aroma:

* Acetic acid: adds a slight vinegar-like taste

* Diacetyl: contributes to the buttery flavor

* Acetaldehyde: contributes to the yogurty aroma

6. Yogurt Development:

* The fermentation process continues until the desired acidity and texture are achieved. This typically takes several hours at a controlled temperature (around 100-110°F or 40-43°C).

7. Cooling and Storage:

* Once the fermentation is complete, the yogurt is cooled to stop the bacteria's activity.

* Yogurt is then stored refrigerated to prevent further fermentation and spoilage.

Note: Yogurt production can involve other metabolic pathways, depending on the specific bacteria used and the desired flavor profile. For example, some yogurts may use *Bifidobacterium* species, which utilize a different pathway called lactose utilization to produce lactic acid.