Overview
This lecture covers the production process, microbiology, chemistry, and quality factors of yogurt, including various styles and the roles of key bacteria.
Yogurt Definition & Characteristics
- Yogurt is an acidified dairy product made by fermenting milk with specific bacterial cultures.
- Differentiated from other acidified milks (e.g., sour cream, buttermilk) by its unique texture and required microorganisms.
- U.S. regulations require Streptococcus thermophilus and Lactobacillus bulgaricus for standard yogurt.
Yogurt Production Process
- Start with fresh milk; adjust fat and protein content through standardization.
- Pasteurize milk using extended heating (30-60 minutes at ~180°F) to enhance texture by denaturing whey proteins.
- Optionally homogenize milk to prevent fat separation.
- Cool milk to fermentation temperature (40–44°C).
- Add bacterial cultures (mainly S. thermophilus and L. bulgaricus).
- Ferment for ~6 hours until desired pH (≤4.4) and acidity (~1% lactic acid) are reached.
- Chill yogurt using a two-stage process: rapid cooling to 20°C, then to 5°C for storage.
Yogurt Styles & Variations
- Set style: Inoculated milk is fermented and cooled in final containers, undisturbed, yielding a thick consistency.
- Stirred (Swiss) style: Fermentation occurs in vats; yogurt is stirred, ingredients can be added post-fermentation.
- Drinkable, frozen, strained (Greek), and low-fat/light yogurts are variations based on production modifications.
Microbiology & Proto-Cooperation
- S. thermophilus and L. bulgaricus engage in proto-cooperation, enhancing each other's growth and fermentation rates.
- S. thermophilus lowers pH and creates anaerobic conditions, favoring L. bulgaricus.
- L. bulgaricus supplies amino acids via protease activity, benefiting S. thermophilus and reducing bitterness.
- Joint activity ensures proper texture, flavor, and acidity.
Chemistry & Texture Formation
- Gelation occurs as bacteria lower pH, causing casein micelles to reach their isoelectric point and aggregate.
- Extended heat treatment denatures whey proteins, which interact with casein to improve gel structure.
- Some bacteria produce exopolysaccharides, enhancing yogurt's texture.
Flavor Compounds & Quality
- Lactic acid provides tartness; acetaldehyde and diacetyl add distinct flavors.
- Overproduction of diacetyl or acetaldehyde leads to off-flavors.
- Faults may include graininess (rapid acidification), syneresis (whey separation), or weak texture (inactive cultures, incorrect fermentation).
Key Terms & Definitions
- Standard of Identity — Legal criteria defining what constitutes yogurt.
- Standardization — Adjusting milk's fat/protein content before processing.
- Pasteurization — Heating to destroy microbes, with extended time in yogurt making.
- Homogenization — Breaking fat globules to prevent separation.
- Proto-cooperation — Mutual metabolic support between S. thermophilus and L. bulgaricus.
- Set Style Yogurt — Fermented in the final container.
- Stirred Style Yogurt — Fermented in vats, then stirred and packaged.
- Isoelectric Point — pH where proteins have no net charge and aggregate.
- Sinuresis (Weighing Off) — Separation of whey from the yogurt gel.
Action Items / Next Steps
- Review U.S. Code of Federal Regulations 21 CFR 131.200, .203, .206 for yogurt standards.
- Prepare for yogurt-making lab by understanding production steps and temperature control.
- Read about cheese fermentation for the next lecture.