Question 8 of 8: Sterilization Methods and Refrigeration of Pasteurized Milk

Question 8: Sterilization Methods and Refrigeration of Pasteurized Milk (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Sterilization methods — advantages and limitations. (i) Boiling (100°C, atmospheric pressure): simple, cheap, no special equipment; but it does not reliably kill bacterial endospores (which survive prolonged boiling), so it disinfects rather than truly sterilizes — unsuitable wherever spore-forming contaminants (e.g. Clostridium, Bacillus) matter. (ii) Autoclaving (steam under pressure, typically 121°C at 15 psi/103 kPa for 15+ min): the gold-standard sterilization method because the elevated pressure raises steam temperature well above boiling, reliably destroying endospores; limitations are that it requires dedicated pressure equipment, cannot be used on heat- or moisture-sensitive materials (some plastics, certain pharmaceuticals, powders that steam cannot penetrate), and requires proper loading/venting to ensure steam actually contacts every surface. (iii) Irradiation (UV or ionising, e.g. gamma): penetrates without heating the material (useful for heat-labile items, and for surface/packaged-product treatment), and ionising radiation penetrates deep into bulk materials; but UV has poor penetration (surface-only, blocked by any opacity/shadowing) and ionising radiation requires expensive specialized facilities, radiation-safety infrastructure, and can alter the chemistry/quality of sensitive materials (flavour, nutrient content, polymer properties) at sterilizing doses. (iv) Dry heat (hot-air oven, typically 160–180°C for 1–2+ h): effective for materials that would corrode, be damaged by moisture, or that steam cannot penetrate (glassware, oils, powders, some metal instruments), and destroys endospores given sufficient time/temperature; limitation is that dry heat transfers energy far less efficiently than moist heat, so it needs much higher temperatures and far longer exposure times than autoclaving to achieve the same lethality, and is unsuitable for heat-sensitive materials. (v) Ethylene oxide (gas sterilization): penetrates well and works at low temperature, making it ideal for heat- and moisture-sensitive medical devices/plastics that cannot survive autoclaving; limitations are that it is toxic, flammable/explosive, requires long exposure and aeration times to clear residual toxic gas from the sterilized items before use, and needs specialized, tightly-controlled equipment. (vi) Filtration (typically 0.22 µm membrane filters): the only method that sterilizes without heat, essential for heat-labile liquids (antibiotic solutions, some media components, vaccines); limitation is that it physically removes cells (and typically works only for liquids or gases), but does not remove viruses (much smaller than the pore size) or destroy toxins/free nucleic acids already present, and filters can clog or be damaged by highly particulate liquids.

(b) Why refrigerate pasteurized milk? Pasteurization is deliberately a disinfection, not a sterilization, process: its time–temperature combination (e.g. 72°C for 15 s, HTST) is engineered specifically to destroy vegetative pathogens (notably Mycobacterium tuberculosis, Coxiella burnetii) and reduce the overall spoilage microbial load to a safe/extended-shelf-life level, but it is deliberately mild enough to preserve milk's flavour and nutritional quality — and in doing so it does not, and is not intended to, kill heat-resistant bacterial endospores or all thermoduric organisms, some of which survive the process. Refrigeration is therefore still required after pasteurization because: (1) surviving spores and thermoduric/psychrotrophic organisms can still germinate and grow, and any post-pasteurization contamination (from packaging, handling, or the environment) introduces additional live organisms into a nutrient-rich medium; and (2) low temperature (typically ≤4°C) does not kill these survivors but sharply slows their metabolic and growth rate, extending the safe shelf life until consumption — without refrigeration, the surviving/re-introduced microbial population would resume exponential growth at room temperature and spoil (or become unsafe) within a much shorter time than the pasteurization process was designed to provide for.