Food Processing Equipment and Unit Operations
Unit operations
A food process is a sequence of unit operations: receiving, cleaning, sorting, size reduction, mixing, separation, heat exchange, evaporation, drying, extraction, forming and packaging. The exam tests mass and energy balances and the choice of equipment.
Equipment and rules
- Conveyors (belt, screw, bucket elevator) move solids; pumps move liquids.
- Crushers, pulpers, homogenisers and colloid mills reduce particle size or stabilise emulsions.
- Filtration, centrifugation, sieving and decantation separate phases.
- Plate heat exchangers are compact and suitable for liquids; shell-and-tube exchangers handle higher pressures.
- Evaporators remove water by boiling; dryers remove water by evaporation into air. Spray dryers make powders from liquids; fluidised-bed dryers dry particulate solids.
- Retorts and autoclaves provide pressure cooking for canned foods; rotary retorts improve heat transfer.
- Mass balance: input mass = output mass + losses.
Worked example
1000 kg of tomatoes at 5% total solids are concentrated to a paste at 10% solids. Solids input = 1000 × 0.05 = 50 kg. If all solids are retained and losses are ignored, paste mass = 50 ÷ 0.10 = 500 kg. Water removed = 1000 − 500 = 500 kg. The reasoning is a solids balance: the solids remain constant while water is removed. In a real plant, screening and evaporation losses would be subtracted and the yield would be lower.
Traps
Do not confuse a unit operation with a whole process. Efficiency is not the same as capacity. Fouling lowers heat-transfer efficiency and requires cleaning. A homogeniser does not sterilise; a pasteuriser does not homogenise. Always state units and whether the balance is on total mass or solids.