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Food Microbiology

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1) List 6 things that can cause food spoilage

Ans: Rodents, Insects, Physical injury due to bruising or freezing (Texture change), Tissue enzymatic activity (flavour change), Non-enzymatic chemical change, Microorganism.

2) How do you detect food spoilage? (give 5 ways)

Ans: Appearance (colour changes, pockets of gas swelling, visible mold growth), Texture, Odour,

Flavour, Slime production

3) Contrast perishable, semi-perishable and non-perishable foods


Perishable(Stable) Semi-Perishable Non-Perishable

Long Shelf life, No storage Req. Limited Shelf life Req. Preservation(Mjority food)

Eg. Flour, Canned food, Rice Potato, Apple Milk, Meat, Egg

4) What is the main bacterium for Bacterial soft rot?

Ans: Most common spoilage organism:- Erwinia species (Enterobacteriaceae)

Specific Bacterium: - Erwinia caratovora, Pseudomonas marginalis

5) What causes gray mould rot? Why is it gray?

Ans: When vegetables are stored at the high RH & warm temperature the Botrytis cinerea grows on the decayed areas and forms the prominent GRAY MOLD ROT.

It is gray because Botrytis cinerea produces a GRAY MYCELIUM on the veg.

6) What does pectinase have to do with soft rot or sour rot?

Ans: It destroys the skin tissue so organism like Geotrichum candidum can enter to the vegetable and ferment suger which results in soft rot or sour rot.

7) Why does Pseudomonas grow on meat surfaces but Clostridium does not?

ANS: Pseudomonas is psychrophilic bacteria which can resist the refrigerator temperature.

8) List 4 spoilage indications for meats

Ans: a) Off-odour and slime(due to growth of aerobic bacteria on cut surfaces.)

b) Fungal growth (favoured at low Aw's)

c) Bone taint ... or deep-spoilage (due to anaerobes or fac-anaerobes.)

d) Discolouration due to alterations of muscle pigments

9) What are 'whiskers' on sides of beef?

Ans: Thamnidium, Rhizopus, Mucor

10) What is slime made up of? (on cured and sliced meats)

Ans : yeast and lactic acid organism

11) What causes greening? How does it occur?

Ans: Lactobacilli and Luconostoc produces the PEROXIDE which is responsible for the greening.

Catalase enzyme is inactivated a heat or nitrite which allows peroxide to build up

Then H2O2 + meat pigments = Porphyrin Green

12) What causes souring of cured meats? Give 2 microbes involved

Ans: Fermentation of milk solid (Lactose) to lactic acid causes souring

>>Lactobacilli and Streptococci<<

13) Why don't eggs spoil more often?

Ans: - Avidin combines with vitamin Biotin so bacteria can't use it

- Conalbumin sticks to Fe so bacteria can't use it.

14) What causes odours in poultry? (Name one microbe...)

Ans: Decomposition product diffuses slowly in the meat which offers the odours

P. putrifaciens


15) How do altered temps affect microbial systems?

Ans: Altered temperature lowers the metabolic rate and 10oC drop halves the enzymatic activity

16) How are proteins and lipids changed in psychrophiles?

Ans: Proteins:- Conformational



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