C3 Microorganisms of Fermented Foods

Question 1

State the general functions of LAB, yeasts and molds in the fermentation of foods.

Answer 1

• LAB: Produce lactic acid and other antimicrobial substances, reduces sugar content and prolong shelf-life
• Yeasts: Produce aroma compounds and alcohols
• Molds: Producing proteolytic and lipolytic enzymes, contributing flavor and texture.

Question 2

What are the three types of starter cultures?

Answer 2

• Single-strain starter culture
• Multiple-strain starter culture
• Mixed, undefined bacterial starter culture

Question 3

What are lactic acid bacteria?

Answer 3

• Gram-positive, non-sporeforming, catalase negative, facultative anaerobic rods or cocci bacteria.
• Homofermentative lab produce mainly lactic acid while heterofermentative LAB produce equal amounts of lactic acid, acetic acid and CO2.

Question 4

What are the precautions that can be taken to prevent phage attacks in milk fermentation?

Answer 4

• Use phage-resistant starter cultures
• High level of cleaning and sanitation
• Minimal access of persons
• High personal hygiene
• Closed vats equipped with filters.

Question 5

Describe acetic acid fermentation

Answer 5

A two stage process where the first stage includes an alcoholic fermentation followed by the oxidation of ethanol via acetaldehyde to produce acetic acid.

Question 6

How can one reduce the amount of spoilage microorganisms in fermented vegetables?

Answer 6

• Addition of salt or brining
• Incubation at anaerobic conditions

Question 7

How are yeasts applied in food fermentation?

Answer 7

Yeasts initiate fermentation process for optimal aroma and alcohol production. Its applications include:
- Brewing (Alcohol production)
- Baking (leavening)
- Fermentation of dairy products (Initiate ripening of cheese)
- Fermentation of meat products

Question 8

State the concerns of using yeasts for fermentation.

Answer 8

• Habitat and function of yeast species
• Probiotic property
• Pathogenic hazard.

Question 9

What are the two types of yeasts used in brewing?

Answer 9

• Top-fermenting/Ale yeast: S. cerevisiae
• Bottom-fermenting/Lager yeast: S. pastorianus, S. carlsbergensis

Question 10

Describe the usage of yeasts in fermented meat products

Answer 10

• Osmotolerant yeast D. hansenii is used to help in color stabilization and for its proteolytic activity against proteins and peptides.
• Other yeasts may also be used for the ripening of cured ham and bacon.

Question 11

State the characteristics of mold.

Answer 11

• Effective producers of enzymes
• High proteolytic and lipolytic activities
• Involved in the production of aroma compounds.
• Obligate aerobes

Question 12

Why should products produced using mold be carefully monitored?

Answer 12

• To ensure that penicillin (antibiotics) and mycotoxins are not formed

Question 13

What is immobilization?

Answer 13

Associating the enzyme or cell with an insoluble matrix to retain it in a proper reactor geometry for its economic reuse under stabilized conditions.

Question 14

What are the advantages of immobilization (enzymes and cells)

Answer 14

Enzymes:
- Convenient, economic and stable

Cells:
- Prolonged activity and stability
- Higher cell densities which contribute to higher productivity.
- Increased tolerance to high substrate concentration
- Reduced risk of microbial contamination
- Ability for low-temperature fermentation/maturation
- Reduction of fermentation/maturation times.

Question 15

Differentiate between immobilization and encapsulation.

Answer 15

Immobilization: Trapping a material within or throughout a matrix
Encapsulation: Forming a continuous coating around an inner matrix containing the encapsulated material.

Question 16

What are the criteria for the selection of materials as matrix for immobilization and encapsulation?

Answer 16

• Low price
• Large surface area
• Easy to handle
• Does not affect cell activity and viability
• Contains sufficient number of functional groups to bind the cells
• Allows recycling and safe disposal
• Large void volume
• Acceptance of matrix by consumers
• No negative effects on the final food product

Question 17

What are the methods of immobilization?

Answer 17

SEAM

Self Aggregation:
- Slow mixing brings cells together. Collisions of cells cause them to bond and produce large, visible flocs

Entrapment:
- Trapping cells/enzymes in a porous matrix such as sodium alginate, k-carrageenan, polyacrylamide gel, etc.

Attachment/adsorption:
- Physical Adsorption: Adsorbing cells/enzymes on a solid carrier surfaces by electrostatic forces
- Chemical adsorption: Adsorption of cells/enzymes via covalent binding between the cell membrane and the carrier.

Mechanical Containment:
- A technique used to immobilize cells in a specific location by physically confining them within a space using a barrier such as microporous membrane and microcapsule.

Question 18

What are the common materials used for immobilization?

Answer 18

• Starch-gluten-milk matrix for yeast and LAB
• Orange peel for Baker’s yeast

Question 19

State the applications of immobilization.

Answer 19

• Alcoholic fermentation
• Lactic acid fermentation
• Probiotic fermentation