Fermentation - Principles and types

Question 1

What are the workhorses in fermentation?

Answer 1

1. LAB
2. Yeast
3. Moulds

Question 2

Give all five types of fermentation and examples.

Answer 2

1. Alcoholic –> Wine, beer, bread
2. Lactic –> Yoghurt, vegetables
3. Propionic –> Cheese
4. Alkaline –> Kinema
5. Acetic –> Vinegar

Question 3

What is the difference in principle between natural fermentation and defined starter fermentation?

Answer 3

Where natural fermentation relies on the selective environmental conditions and ‘back-slopping’, defined starter fermentation relies on heat treatment before inoculation with a chosen starter.

Question 4

What are the advantages and disadvantages of using natural fermentation?

Answer 4

+ Low cost
+ Simple
+ Small-scale

• Unpredictable quality failures
• Sometimes slow

Question 5

What are the advantages and disadvantages of defined starter fermentation?

Answer 5

+ Predictable
+ Constant quality
+ Efficacious

• High initial investment
• Only feasible at large scale

Question 6

Describe the steps in alcohol fermentation and their yield.

Answer 6

Step 1: Embden-Meyerhof pathway or “common glycolysis”
Yield:
- 2 Pyruvate which is further used in step 2
- 2 NADH which is recycled in step 2
- 2 ATP

Step 2: Pyruvate reduction or ethanol production
(net) yield:
- 2 CO2
- 2 NAD+
- 2 Ethanol

Ehrlich pathway:
production of higher alcohols by degradation of amino acids into either fusel acid or alcohol.

Question 7

What is the difference between homo fermentative and hetero fermentative bacteria?

Answer 7

Homo fermentative bacteria use the Embden- Meyerhof pathway. Homofermentative bacteria can either use homolactic (lactate) or mixed acid fermentation (acetate, formate, ethanol) . The latter is more energy efficient, some MO can switch under conditions of limited substrate and slow growth. The key enzyme is Aldolase

Heterofermentative bacteria use the Phosphoketolase pathway. Hereby, three products are formed: equimolar quantities lactate, CO2, and acetate (presence of O2) or ethanol (lack of O2).

The main difference are the formation of either 1 or two acids and the key enzyme needed for the reaction.

Question 8

What type of starter cultures are usually used to produce fermented milk products?

Answer 8

Thermophilic LAB:
e.g.:
- Streptococcus thermophilus
- Lactobacillus delbrueckii

Question 9

What are the eight antimicriobial properties of LAB?

Answer 9

1. Low pH
2. Organic acids
3. Bacteriocins (e.g. Nisin creating pores inducing leakage in cytoplasm of MO including spores)
4. Hydrogen peroxide (absence of catalase)
5. Ethanol
6. Diacetyl
7. Nutrient depletion
8. Low redox potential

Question 10

How are propionic acid bacteria used?

Answer 10

• They usually cooperate with LAB.
  Step 1: Lactose –> LA by LAB
  Step 2: 3 LA –> 2 propionic acid, 1 acetic acid, 1 CO2 (bubbles in cheese ‘‘eyes’’).

Question 11

How are acetic acid bacteria used?

Answer 11

• Cooperate with yeast
  Step 1: conversion of sugars in ethanol (yeast)
  Step 2: ethanol is oxidized to acetic acid (Acetobacter and gluconobacter)
• Similar to spoilage of vinegar
• Also a two-stage process:
  1. Ethanol –> acetaldehyde + H20
  2. acetaldehyde + O2 –> Acetic acid