ICAR AICE-JRF/SRF Food Microbiology: Spoilage, Pathogens & Detection: practice questions

1 · Intrinsic and extrinsic factors controlling microbial growth · hard

Hurdle technology, as developed by Leistner, is best described as:

1. Applying one severe preservation step that is sufficient on its own to sterilise the food completely
2. Combining mild preservation factors that together prevent growth with less damage to quality
3. Adding a sequence of chemical preservatives at doses each high enough to stop growth individually
4. Storing a food under a fluctuating temperature regime that repeatedly shocks the microflora

2 · Spoilage of canned, bakery and beverage products · hard

Canned strawberries soften and disintegrate after a normal pasteurisation process. The organisms most likely responsible are:

1. Heat-resistant moulds such as Byssochlamys with ascospores that survive mild heat
2. Pseudomonas species entering through the seam during water cooling of the cans
3. Lactic acid bacteria that grow in the syrup and produce dextran from sucrose
4. Proteolytic Clostridium botulinum that grows because the fruit is a low-acid food

3 · Bacterial food-borne pathogens: Salmonella, Listeria, E. coli (STEC), Campylobacter, Shigella, Vibrio and Yersinia · hard

Salmonella has caused large outbreaks from chocolate, peanut butter and milk powder although it cannot grow in these foods. The best explanation is that:

1. It survives for long periods at low water activity, and fat may protect it during digestion
2. It grows slowly at water activities down to 0.60 when the storage temperature is high
3. It produces a preformed heat-stable toxin in the raw ingredients before processing
4. It forms heat-resistant spores that may germinate once the product is reconstituted with water

4 · Spore-formers and toxin producers: Clostridium botulinum, C. perfringens, Bacillus cereus and Staphylococcus aureus · hard

Enterotoxin-producing Clostridium perfringens strains were classically called type A. Under the revised toxinotyping scheme published in 2018, most such food-poisoning strains are placed in:

1. Type B
2. Type G
3. Type F
4. Type D

5 · Mycotoxigenic moulds: Aspergillus, Penicillium and Fusarium · hard

Why is sampling, rather than analysis, the largest source of error in determining aflatoxin in a lot of groundnuts?

1. Aflatoxin moves between kernels during storage, so the lot changes after sampling
2. The toxin is concentrated in a few highly contaminated kernels, so its distribution is very uneven
3. Immunoaffinity columns retain the toxin unpredictably between replicate extractions of one ground sample
4. Aflatoxin degrades rapidly in ground samples before they reach the laboratory

6 · Rapid methods: ATP bioluminescence, immunoassays and lateral flow · hard

A food contains 1 Salmonella cell in a 25 g portion homogenised in 225 mL of enrichment broth. The organism doubles every 30 min with no lag, and a lateral flow test detects 10^5 CFU/mL. About how long must the enrichment run?

1. 12.3 h
2. 8.3 h
3. 6.1 h
4. 24.6 h

7 · Predictive microbiology: primary and secondary models · medium

In the Weibull inactivation model, a shape parameter p less than 1 describes:

1. A survivor curve with a shoulder
2. A survivor curve with tailing
3. A curve showing regrowth after heating
4. A straight log-linear survivor curve

8 · Biofilms in food plants · hard

Milk powder plants often find thermophilic spore-formers in the product even though the milk was pasteurised. A common source is:

1. Growth of Listeria in the finished powder during dry storage at room temperature for months
2. Biofilms of Pseudomonas in the raw milk silos that survive spray drying
3. Biofilms of lactic acid bacteria on the outer packaging of the powder
4. Biofilms of thermophilic bacilli such as Anoxybacillus and Geobacillus in evaporators

9 · Microbiological criteria and sampling plans · hard

A lot has 10% of its units contaminated. Using a two-class plan with c = 0, how does increasing n from 5 to 60 change the probability of acceptance?

1. From 0.59 to about 0.002
2. From 0.50 to about 0.05
3. From 0.90 to about 0.54
4. From 0.59 to about 0.10

10 · Intrinsic and extrinsic factors controlling microbial growth · hard

Non-proteolytic Clostridium botulinum has a minimum growth temperature of about 3 °C. A chilled vacuum-packed fish product is stored at 2 °C. Which statement best describes the safety position?

1. The product is safe because vacuum packing raises the redox potential above the organism's limit
2. The product is unsafe because non-proteolytic strains grow fastest at refrigeration temperatures
3. Safe only while 2 °C is held throughout; abuse makes shelf-life limits or extra hurdles necessary
4. The product is safe indefinitely because spores of non-proteolytic strains cannot survive storage at 2 °C or below