Pasteurization is designed to
Answer: A
The aim is a safe and longer-lasting product with its flavour intact, so the heating is deliberately mild and survivors remain. Pasteurized milk is therefore refrigerated and has a limited shelf life, unlike a sterilized product.
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The high-temperature short-time method of pasteurization uses about
Answer: B
Flash pasteurization at 72 degrees for 15 seconds achieves the same reduction as the older holder method of 63 degrees for 30 minutes, while treating the liquid continuously and affecting flavour less. Both are pasteurization, not sterilization.
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Tyndallization achieves its effect by
Answer: C
Heating on one day kills vegetative cells, surviving spores germinate overnight, and the next day's heating kills the new vegetative cells; three cycles usually suffice. It was devised for materials that cannot tolerate autoclave temperatures.
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Refrigeration preserves food chiefly because it is
Answer: D
Low temperature slows metabolism and division without destroying the organisms, so spoilage is delayed rather than prevented and the population resumes growing on warming. Some pathogens, notably Listeria, still multiply slowly at refrigerator temperatures.
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Ultraviolet light damages microorganisms mainly by
Answer: A
Ultraviolet radiation is absorbed by nucleic acid and links adjacent pyrimidines into dimers, which blocks replication and transcription. Absorption peaks near 260 nanometres, which is why germicidal lamps are built to emit close to that wavelength.
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The main practical limitation of ultraviolet radiation as a sterilizing agent is its
Answer: B
Ultraviolet light is stopped by glass, plastic, dust, liquid and any layer of organic soil, so it acts only on directly exposed surfaces and air. It is useful for cabinets and room air, and useless for anything wrapped or shadowed.
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Ionizing radiation such as gamma rays is described as cold sterilization because it
Answer: C
Gamma irradiation penetrates deeply through packaging and needs no heat, which makes it suited to disposable plastic syringes, sutures and catheters that an autoclave would deform. It damages nucleic acid directly and through the radicals it generates in water.
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Compared with ultraviolet light, gamma radiation has
Answer: D
Gamma photons carry far more energy and pass through packaging and dense material, whereas ultraviolet light is absorbed at the surface. That is exactly why gamma is used to sterilize sealed single-use devices and ultraviolet is not.
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Bacteria are removed from a heat-labile solution by membrane filtration using a pore size of about
Answer: A
A 0.22 micrometre membrane retains vegetative bacteria while letting the fluid through, which is how sera, vaccines and antibiotic solutions are handled. Note that it does not reliably retain viruses or mycoplasmas, so a filtered solution is not necessarily sterile.
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Filtration is chosen in preference to heat when the material is
Answer: B
Vaccines, sera, enzyme solutions and some antibiotics are destroyed by autoclave temperatures, so the organisms are removed physically instead of being killed. The trade is that anything smaller than the pore, including viruses, passes straight through.
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