4.3 Yogurt

It is a popular coagulated milk fermented by LAB.

yogurt

Is yogurt considered a probiotic or prebiotic?

Probiotic

What type of fermentation occurs in yogurt production?

A homofermentative process converting lactose to lactic acid

Which microorganisms are primarily involved in yogurt fermentation?

• Streptococcus thermophilus

• Lactobacillus delbrueckii subsp. bulgaricus

What type of relationship exists between S. thermophilus and L. bulgaricus in yogurt fermentation?

symbiotic

What is the main product of yogurt fermentation?

Lactic acid, produced from lactose via pyruvate reduction

Which enzyme converts pyruvate to lactate in yogurt fermentation?

lactate dehydrogenase

What is the yield of sugar converted to lactic acid in yogurt fermentation?

95%

What are the by-products of yogurt fermentation that contribute to its flavor?

• acetaldehyde

• acetoin

• diacetyl

Both L. bulgaricus and S. thermophilus produce peptidase, but which of the two is responsible for the initial protein degradation in yogurt?

L. bulgaricus

L. bulgaricus’ proteolytic activity produces stimulatory __ for S. thermophilus.

peptides and AAs

When is L. bulgaricus most active during yogurt fermentation?

in the later stage of fermentation, further lowering the pH to around 4.5.

Among L. bulgaricus and S. thermophilus, which grows faster and produces CO2 and acid?

S. thermophilus

It is responsible for the initial pH drop to 5.0 during yogurt fermentation.

S. thermophilus

Which microorganism dominates during the early stage of yogurt fermentation?

Streptococcus thermophilus

What is the approximate pH during the early fermentation stage of yogurt production?

5.0 (by Streptococcus thermophilus)

Which microorganism dominates during the later stage of yogurt fermentation?

Lactobacillus delbrueckii subsp. bulgaricus

What is the approximate pH during the later fermentation stage of yogurt production?

4.5

What percentage of milk proteins are broken down by proteolytic activity during yogurt fermentation?

1-2%

What causes the breakdown of milk proteins in yogurt fermentation?

Microbial proteolytic activity

What does protein breakdown during fermentation provide for bacteria?

Small peptides and amino acids needed by bacteria

What are the main protein substrates in yogurt fermentation?

Casein and some whey proteins

What amino acids increase in yogurt compared to milk?

• valine

• histidine

• serine

• proline

What happens to carbohydrates during yogurt fermentation?

Lactose → Lactic acid via the homofermentative pathway

What percentage of fermentation products in yogurt are lactic acid?

95%

How much change occurs in vitamins and minerals during yogurt fermentation?

Minimal changes in content

Which vitamins may be lost due to pasteurization in yogurt production?

• B6 - pyridoxine

• B12 - cobalamin

• B9 - folic acid

What are the causes of yogurt spoilage?

• syneresis

• thermophilic Bacillus spp.

• yeasts and molds

• excessive acidity

• post-fermentation

The weeping or oozing out of water from the gel, which increases surface water activity and makes yogurt prone to microbial spoilage.

syneresis

Which microorganism can survive pasteurization and cause bitter flavors in yogurt?

Thermophilic Bacillus species

How do yeasts and molds contribute to yogurt spoilage?

They may come from fruit or flavoring additives and cause off-flavors.

They may come from fruit or flavoring additives and cause off-flavors in yogurt.

molds and yeasts

What causes excessive acidity in yogurt?

High incubation temperatures or slow cooling after reaching the target pH.

What is post-fermentation acidification?

A process where microorganisms continue producing acid slowly during prolonged incubation, increasing yogurt acidity over time.