Chapter 1 Food Spoilage

Question 1

What are the signs of microbial spoilage? Provide at least 4 signs.

Answer 1

• Visible growth
• Foam Production
• Slime
• Exudate
• Off-flavor

Question 2

What are the 5 factors that affect microbial spoilage?

Answer 2

• Significance of microorganism (i.e. multiplication rate)
• Microbial load
• Predominant microorganisms
• Food nutrients
• Microbial growth in succession

Question 3

What are the 4 major causes of food spoilage?

Answer 3

Physical, Chemical, Microorganisms, and Others.

Question 4

What types of nutrients do microorganisms prefer?

Answer 4

Metabolizable carbohydrates > NPN and small peptides > Large proteins > Lipids

Question 5

Why does the predominant microbial type and nature of food spoilage change?

Answer 5

Due to the production of metabolites from the predominant m/o. The metabolites changes the environment and may favor certain species while inhibit other species of m/o.

Question 6

What is the sequence of events in microbial spoilage?

Answer 6

1. Introduction of m/o
2. Favorable food environment
3. Storage time
4. Multiplication of m/o

Question 7

Define the protein-sparing effect.

Answer 7

The protein-sparing effect is when the body utilizes sources other than protein to generate energy.

Question 8

How does LAB affect the microflora of fresh meat?

Answer 8

When carbs are added, LAB can arrest the growth of many normal microflora that metabolizes NPN and proteinaceous compounds, which prevents putrefaction as proteins are not decomposed into volatile compounds.

Question 9

What are some (4) examples of carbohydrate degradation?

Answer 9

• Alcoholic fermentation
• Lactic acid fermentation
• Butyric acid fermentation
• Propionic acid fermentation

Question 10

State the common groups (8) of microorganisms involved in food spoilage.

Answer 10

• Yeasts
• Molds
• Enterobacteriaceae
• Pseudomonas sp.
• Lactic Acid Bacteria
• Bacillus sp. (spore former)
• Clostridium sp. (spore former)
• Alicyclobacillus (spore former)

Question 11

What are the differences between proteolysis and putrefaction?

Answer 11

Proteolysis is the breakdown of proteins into polypeptides and into peptides; Putrefaction is the breakdown of amino acids into volatile products.

Question 12

How does lipid degradation contribute to rancidity and off-flavor.

Answer 12

Lipid degradation by lipases produces glycerol and fatty acids. The latter can then degrade into aldehyde and ketone, which is the contributor of rancidity and off-flavor.

Question 13

Name the three (3) spore former species discussed in this chapter

Answer 13

• Bacillus sp
• Clostridium
• Alicyclobacillus

Question 14

Give some examples of m/o (4) that can degrade nitrogen compounds.

Answer 14

Pseudomonas, Shewanella, Bacillus, Clostridium.

Question 15

What are some changes in foods caused by spoilage m/o?

Answer 15

• Color change from m/o growth
• Color change from chemical changes
• Softened texture
• Changes in odor and flavor.

Question 16

Give some examples of volatile products produced from the breakdown of amino acids.

Answer 16

H2S (Hydrogen Sulfide) 
Methyl Mercaptans
Indole
Cadaverine
Putrescine
Histamine

Question 17

Define shelf-life

Answer 17

The period of time in which food remains safe and retains desired quality.

Question 18

Isolation of microorganisms that causes food spoilage is based on Koch’s postulates. Describe the steps involved in the isolation process.

Answer 18

1. Isolate m/o from food
2. Grow m/o in pure culture.
3. Inoculate m/o in another sample of food
4. m/o must cause spoilage in the new food.
5. Isolate and identify the m/o

Question 19

There are five(5) major microbial factors that affect the shelf life of food, state and describe them.

Answer 19

pH - Death is rapid in low pH as bacteria uses more energy to maintain intracellular pH.
Water Activity - Low water activity causes dehydration
Microbial interactions - e.g. mutualism or competition may have a positive impact on shelf-life.
Gas tensions - MAP/ vacuum packaging inhibits the growth of aerobes but is less inhibitory to anaerobes.
Storage conditions

Question 20

What is the purpose of shelf-life test?

Answer 20

To assess if a product is safe and stable during normal production and storage conditions.

Question 21

What is the purpose of challenge test?

Answer 21

To assess if a food product is likely to be safe if contaminated with undesirable microorganism.

Question 22

How does ASLT differ from shelf-life test?

Answer 22

ASLT uses an “abuse” condition to speed up the rate of deterioration to predict the shelf-life of a product.

Question 23

What is predictive microbiology?

Answer 23

Predictive microbiology is the utilization of statistical techniques and computer data analysis to predict the growth and activation of m/o.

Question 24

What is predictive microbiology?

Answer 24

Predictive microbiology is the utilization of statistical techniques and computer data analysis to predict the growth and activation of m/o.

Question 25

How to reduce the fishy odor caused by Trimethylamine N-oxide (TMAO)?

Answer 25

Evaporate it by heating it with alcohol.

Question 26

Some apples rot to become soft and mushy, while some become dry and firm. What is a possible factor that gives rise to the soft and mushy texture of rotten apples?

Answer 26

Microorganisms that produce the enzyme pectinase breaks down the pectin cell-wall of the apple, giving rise to a softened texture.

Question 27

Why is the microbial number important in food spoilage?

Answer 27

The microbial number must reach the “spoilage detection level” to result in observable changes. For example, formation of lactic acid may be detected at higher microbial load, while formation of H2S can be detected at lower microbial load.

Question 28

A food is usually spoiled by one or two species of dominant microorganisms. What are the factors that can make a microorganism more dominant than others?

Answer 28

• Suitable growth environment

- Shorter generation time