Water activity is defined as the ratio between
Answer: A
Water activity compares the vapour pressure of the food itself with the vapour pressure of distilled water under identical conditions. It therefore measures how available the water is to microorganisms, which is not the same thing as how much water the food contains.
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Water activity is related to equilibrium relative humidity by
Answer: B
Equilibrium relative humidity is expressed as a percentage and water activity as a decimal, so dividing by 100 converts between them. A food in equilibrium with air at 85 per cent relative humidity therefore has a water activity of 0.85.
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Most foods have a water activity above
Answer: C
Above 0.95 there is enough available moisture to support the growth of bacteria, yeasts and moulds alike, which is why most fresh foods spoil readily. Lowering water activity is one of the oldest ways of extending shelf life.
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A finished product with a water activity of 0.85 or less is
Answer: D
Controlling water activity to 0.85 or below takes a product outside those regulations, because at that level the organisms of concern in canned foods cannot grow. It is a regulatory threshold rather than a guarantee of sterility.
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Water activity in foods is usually controlled by the use of
Answer: A
Correct answer: salt or sugar
Water activity is the proportion of the water in a food that is free rather than bound, and it is lowered by dissolving solutes in that water. Salt and sugar are the classical agents - curing, brining, jam-making and syrup packing all work this way - along with drying, which removes the water altogether. Acids act on pH, which is a separate hurdle, and antibiotics and ultraviolet light are not water activity controls at all.
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The minimum water activity for growth of Clostridium botulinum is approximately
Answer: B
FDA gives the minimum as approximately 0.93, noting that depending on product characteristics it can be as high as 0.96. That is a relatively demanding requirement, which is why reducing water activity is an effective control against this organism.
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Depending on product characteristics, the minimum water activity for Clostridium botulinum can rise as high as
Answer: C
Correct answer: 0.96
Clostridium botulinum is among the more demanding pathogens in its water requirement, and the minimum activity at which it will grow depends on the product and on the solute used to depress the activity, rising to around 0.96 in some. Lowering water activity below what the organism needs is therefore one of the standard hurdles against it, alongside a pH of 4.6 or below, refrigeration, and nitrite in cured meats.
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Pure water has a water activity of
Answer: D
Correct answer: 1.0
Water activity is defined as the ratio of the vapour pressure over the food to that over pure water at the same temperature, so pure water scores exactly 1.0 by definition and every food falls below it. Fresh meat and vegetables sit around 0.98 to 0.99, bread near 0.95, jam around 0.80, and dried foods below 0.60, where nothing grows at all.
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The temperature range known as the Danger Zone is
Answer: A
Correct answer: 40 to 140 degrees Fahrenheit
The Danger Zone runs from 40 to 140 degrees Fahrenheit, that is roughly 4 to 60 degrees Celsius - cold enough at the bottom to slow growth to a crawl, hot enough at the top to prevent it. Between those limits foodborne pathogens multiply, most rapidly around body temperature. The whole of temperature control in a kitchen consists of keeping food out of that range, or limiting the total time spent in it.
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Within the Danger Zone, bacteria can double in number in as little as
Answer: B
Correct answer: 20 minutes
Under favourable conditions in the Danger Zone many foodborne bacteria have a generation time of about twenty minutes, so a population doubles three times an hour. Starting from a hundred cells, four hours produces well over a million. This exponential behaviour is why the two-hour rule is expressed as an absolute limit rather than a guideline, and why the last hour of an excursion matters far more than the first.
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