The Microbiology of Making Yogurt re-examined
Introduction
Yogurt is the product when milk ferments in the absence of air (‘anaerobically’); typically, bacterial cultures of Lactobacillus Acidophilus are used to promote the process. In the traditional scheme (Diagram 1) following pre-treatment, fermentation commences with the addition of starter culture to fresh milk in a bowl and this is followed by some time in a warm draught-free environment (a few hours or overnight) during which time the culture spreads throughout the milk with yogurt curd as the main product.
Diagram 1 – Traditional yogurt making process
The Commercial production of yogurt
The Traditional approach to the production of yogurt is also the starting point for the commercial production, by which means most yogurt is made today. In this scheme however the principles of production are designed to provide the largest marketable product yield in the minimum time. Some assumptions about the process that were acceptable in pre-modern times but less so today given our advanced understanding include –
- 1. Dispersal of the starter culture is impossible without stirring.
- 2. A large liquid surface area exposed to air during fermentation cannot be avoided
- 3. The geometry of the fermentation chamber is largely irrelevant
- 4. The natural growth pattern of bacterial culture is not relevant
- 5. Extracting and storing starter culture is essential for re-starting fermentation
- 6. Turbulent motion of the ferment is not relevant
- 7. The presence of ‘live’ culture in the product is not essential
- 8. Contamination is best detected and controlled by instrumentation
- 9. Shutdown and sterilisation between serial fermentations is essential
- 10. Optimal conditions (eg fermentation temperature) are essential