Microbiology of Foods Questions and Answers

Answer: C

Correct answer: two hours

Two hours is the standard cumulative limit for food held between 40 and 140 degrees Fahrenheit. It is chosen so that even a substantial initial contamination cannot multiply to a hazardous level within the time allowed. The limit halves to one hour when the ambient temperature exceeds 90 degrees, since growth is then faster still.

Answer: D

Correct answer: 90 degrees Fahrenheit

Above 90 degrees Fahrenheit ambient - a hot day, an outdoor buffet, a delivery vehicle in summer - the food warms towards the middle of the Danger Zone quickly and grows faster once there, so the allowance falls from two hours to one. The underlying reasoning is unchanged: the limit is set so that the population cannot reach a hazardous level in the time permitted.

Answer: A

Correct answer: 40 degrees Fahrenheit

Forty degrees Fahrenheit, about 4 Celsius, is the upper limit for cold holding and the lower boundary of the Danger Zone. Below it growth of most pathogens is very slow, though not absent - Listeria monocytogenes will grow slowly at refrigeration temperatures, which is why ready-to-eat chilled foods carry use-by dates rather than being treated as indefinitely safe.

Answer: B

Correct answer: 140 degrees Fahrenheit

One hundred and forty degrees Fahrenheit, about 60 Celsius, is the lower limit for hot holding and the upper boundary of the Danger Zone. Note that this is a holding temperature, not a cooking temperature: cooking must reach considerably higher - 165 degrees for poultry - to reduce the population, whereas holding merely has to prevent regrowth in food already cooked.

Answer: C

Correct answer: shallow containers, for quick cooling

A large deep container of hot food cools from the outside inward and its centre may remain in the Danger Zone for hours, which is exactly the condition in which Clostridium perfringens spores germinate and multiply. Dividing the food between shallow containers gives a large surface area and a short conduction path, so the whole mass passes through the Danger Zone quickly. An insulated flask or a covered pot does the opposite.

Answer: D

Correct answer: slowing growth, not killing

Refrigeration is bacteriostatic, not bactericidal: it slows metabolism and lengthens the generation time so that the population grows too slowly to matter within the shelf life. The organisms are still present and still viable, and they resume growth as soon as the food warms. This is why chilled food has a limited shelf life and why refrigeration cannot repair food that has already been abused.

Answer: A

Correct answer: 4.6

A finished equilibrium pH of 4.6 is the regulatory dividing line, chosen because Clostridium botulinum will not grow or produce toxin below it. Foods at or below 4.6 are acid foods and need only a pasteurising heat process; foods above it are low-acid and, if packed hermetically and stored at ambient, require a full botulinum cook. The whole of canning regulation turns on which side of 4.6 a product falls.

Answer: B

Acidity below that point prevents germination, which is why such foods can be held at ambient temperature without the severe heat process a low-acid canned food requires. It restrains the organism rather than sterilising the product.

Answer: C

Correct answer: 0.85

An acidified food is a low-acid food to which acid or acid ingredients have been added to bring the finished equilibrium pH to 4.6 or below, and the definition also requires a water activity above 0.85 - below that figure the product is controlled by its dryness instead and falls outside the category. Pickles, acidified vegetables and many sauces are made this way, obtaining ambient stability from pH rather than from a severe heat process.

Answer: D

Exceeding the threshold removes the safeguard the whole process depends on, so the product cannot simply be released. The options are full reprocessing, treating it as low-acid, or holding it for assessment of public health significance.