Which bacteria are present in yoghurt?

Question

BlackTom AI · Accepted Answer

The correct answer is a. Lactobacillus bulgaricus because yogurt is produced by lactic acid bacteria that ferment milk lactose into lactic acid, causing milk proteins to coagulate and form the characteristic texture and tang.

1. Relevant knowledge points and concepts
- Yogurt is a fermented milk product created by specific bacterial cultures, primarily Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus (commonly called Lactobacillus bulgaricus).
- These are lactic acid bacteria (LAB) that metabolize lactose to lactic acid; the acidification denatures milk proteins (casein), producing the gel-like structure and sour flavor.
- Saccharomyces cerevisiae and Torulopsis are yeasts, not the typical bacterial starter cultures for yogurt.

2. Step-by-step reasoning
- Traditional and industrial yogurt manufacture inoculates pasteurized milk with a starter culture containing S. thermophilus and L. bulgaricus.
- L. bulgaricus works synergistically with S. thermophilus: S. thermophilus grows quickly, producing formate and other nutrients; L. bulgaricus thrives on these and produces aromatic compounds and more acid, completing fermentation.
- Because the question asks specifically “Which bacteria are present in yoghurt?”, Lactobacillus bulgaricus directly fits as a bacterial species used in yogurt production.

3. Intermediate biological classification steps
- Identify candidate organisms from options:
  a) Lactobacillus bulgaricus — lactic acid bacterium, known yogurt starter.
  b) Saccharomyces cerevisiae — baker’s/ brewer’s yeast (fungus), not used as yogurt starter.
  c) Torulopsis — an old yeast genus name (yeast-like fungi), not standard yogurt bacteria.
  d) There are no bacteria in yoghurt — false; fermentation requires bacteria.

4. Why the correct answer is correct
- Lactobacillus bulgaricus is one of the two principal bacterial species required by standard yogurt cultures; it is widely documented and used in yogurt production worldwide.

5. Why other options are incorrect
- Saccharomyces cerevisiae (b) and Torulopsis (c) are yeasts, not the lactic acid bacteria that make yogurt. Option d is incorrect because yogurt is defined by bacterial fermentation.

Conclusion: This analysis confirms the correct choice is a. Lactobacillus bulgaricus, matching the provided answer.

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