# DBT JRF 2022 — Question 113

Exam: DBT JRF 2022 · Section: B · Topic: Industrial Microbiology › Fermented Foods - Yogurt · Difficulty: Medium

## Question

The principle microorganism for yogurt is:

## Options

- **A.** Streptococcus thermophilus
- **B.** Lactobacillus acidophilus
- **C.** Streptococcus lactis
- **D.** Leuconostoc citrovorum

## Answer

Option A

## Explanation

Principal microorganism in yogurt production: Yogurt is produced by bacterial fermentation of milk. The two principal organisms used together: 1. Streptococcus thermophilus: gram-positive, homofermentative; grows at 42–45°C; produces lactic acid and acetaldehyde. 2. Lactobacillus delbrueckii subsp. bulgaricus: gram-positive, homofermentative; protease activity releases peptides that stimulate S. thermophilus. Synergistic relationship: • L. bulgaricus produces free amino acids (especially valine and histidine) from casein → stimulates growth of S. thermophilus. • S. thermophilus produces formate and CO₂ → stimulates L. bulgaricus. • Together they produce more lactic acid faster than either alone. Legal standard in many countries: yogurt MUST contain both organisms. Among the given options, Streptococcus thermophilus is identified as the principal organism in yogurt. Answer: Streptococcus thermophilus is the principal microorganism in yogurt production (synergistic with L. bulgaricus).

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