C8 Fermented milk products

Question 1

Describe the effects conferred to yoghurt by microorganisms.

Answer 1

• Soft gel texture: Lactic acid lowers pH to isoelectric point of casein (Casein-casein interaction) and cause coagulation, forming a continuous gel. Exopolysaccharides also increase retention of water and lower whey separation, increasing viscosity, creaminess and smoothness
• Unique aroma and flavor compounds: production of acetaldehyde, diacetyl, free amino acids and unsaturated fatty acids.
• Antimicrobial properties from acid and bacteriocins

Question 2

Describe the symbiotic relationship between yoghurt cultures.

Answer 2

S. thermophilus:
- Produces formic and folic acid, which helps purine biosynthesis in L. bulgaricus
- Produce CO2 and consume O2, benefits the less aerotolerant L. bulgaricus
- Exopolysaccharides ensure close proximities, facilitating exchange of metabolites
- Glutathione relieve acid stress in L. bulgaricus

L. bulgaricus
- Increase oligopeptides that support S. thermophilus

Question 3

Describe the factors affecting the fermentation of yoghurt.

Answer 3

• Composition of starter culture (St & Lb ratio = 1:1)
• Incubation temperature (40-43C)
• Fat content of milk (More fat = More viscous and higher incubation time)
• Amount of inoculum used (Higher inoculum = increased rate of fermentation)

Question 4

How does amount of inoculum affect yoghurt fermentation?

Answer 4

• Reduce lag phase of bacteria in the starter culture
• Increase rate of acid production
• Faster fermentation
• Prevents pathogen or spoilage microbes from growing/surviving

Question 5

What are the two important indicators of successful yoghurt fermentation?

Answer 5

pH: 4-4.6
Amount of live cultures >= 10^6 CFU/ml