Food Processing and Preservation
Food Processing and Preservation tests the principles used to slow or stop spoilage so that food stays safe and acceptable for longer. Questions ask you to match a method to a product, explain why it works, or identify a storage fault.
The principles
- Remove or destroy microorganisms: heat (pasteurisation at 72 °C for 15 seconds, sterilisation and canning at about 121 °C), filtration, irradiation.
- Slow growth: chilling at 0–5 °C, freezing at −18 °C or colder, and the two-stage cooling rule for cooked food.
- Reduce available water: drying, salting, sugaring, smoking. Lower water activity deprives microorganisms of moisture.
- Increase acidity: pickling and fermentation below about pH 4.6 inhibits many spoilage organisms and Clostridium botulinum.
- Exclude oxygen: vacuum packing, modified atmosphere packaging, waxing, oil sealing.
- Chemical preservatives such as nitrite, sulphur dioxide and benzoate are controlled by food law and must be declared.
Worked example
A 20 % weight-for-weight brine is requested. Percentage w/w is solute divided by total mass, so dissolve 200 g of salt in 800 g of water to give 1000 g of brine. Adding 200 g of salt to 1000 g of water gives only 16.7 % and would under-preserve the product. The reasoning shows why the denominator is total mass, not the water alone; the lower water activity then inhibits many spoilage bacteria.
Traps that cost marks
- Believing a freezer kills bacteria - it only stops them multiplying.
- Refreezing thawed meat, which multiplies drip loss and microbial load.
- Scraping mould off a soft food and serving the rest; the mycelium extends beyond the visible growth.
- Storing dried goods near the steamer or over the fryer, where humidity ruins crispness.
- Home-canning low-acid foods without pressure equipment, risking botulism.