Food Processing, Preservation and Storage
Here the exam checks that you can name unit operations, choose a preservation method with reasons, calculate a thermal process, and explain storage losses of staples such as cassava, maize, plantain and tomato. Answers score when they link each step to the principle it exploits - heat, cold, reduced aw, reduced pH, oxygen exclusion or radiation.
Unit operations
Cleaning, sorting, grading, peeling, size reduction (milling, grinding), mixing, blanching, heating, evaporation, drying, cooling, freezing, packaging. Blanching (about 95–100 °C, 1–5 min) inactivates enzymes such as polyphenol oxidase and peroxidase before drying or freezing.
Preservation principles
- Thermal: pasteurisation (LTLT 63 °C/30 min; HTST 72 °C/15 s) kills vegetative pathogens but not spores; sterilisation/canning at 121 °C; UHT 135–150 °C for 1–2 s plus aseptic packaging.
- Drying: sun, solar, cabinet, drum, spray, freeze-drying. Shelf-stable below about aw 0.60.
- Low temperature: chilling 0–5 °C slows growth; freezing at −18 °C stops growth and enzyme activity (blanch first).
- Fermentation: lowers pH (lactic acid bacteria in garri, koki, fufu) and produces antagonistic compounds.
- Salting, sugaring, smoking: reduce aw; smoke also deposits antimicrobial phenols.
- Irradiation: Codex permits up to 10 kGy; cold process, but needs labelling.
- Packaging and storage: moisture barriers, oxygen barriers, protection from light and pests, first-in-first-out stock control.
Worked example
A suspension contains 10⁶ spores of Clostridium botulinum and the D-value at 121 °C is 0.2 min. To achieve a safe 12D process, required time = 12 × 0.2 = 2.4 min at 121 °C (F₀ = 2.4). To reduce the same load of a more heat-resistant organism with D = 1.5 min would need 12 × 1.5 = 18 min. Reasoning: log reduction = time ÷ D, so time = required logs × D. Show that you know D is organism- and temperature-specific.
Traps that cost marks
- Confusing pasteurisation with sterilisation, or claiming pasteurisation makes a product shelf-stable at room temperature.
- Forgetting blanching before freezing - residual enzymes cause off-flavours and colour loss.
- Assuming sun drying is harmless: slow drying at high humidity favours mould and aflatoxin.
- Mixing "preservation" with "processing"; both must be justified by principle, not just named.
- Confusing percentage losses in a chain as additive rather than multiplicative.