Food Processing Equipment and Unit Operations
Unit operations thinking
A food plant is a chain of unit operations, and the exam tests whether you can name the operation behind a process. Cleaning and sorting, size reduction (crushing, grinding, homogenisation), mixing, heat exchange, evaporation, drying, separation (filtration, centrifugation, membrane), extrusion, conveying, filling and sealing are the building blocks. Mass balance (input = output + loss) and energy balance are used to size equipment and estimate steam and power costs.
Key equipment
- Heat exchangers: plate for milk and juice (high area, compact), shell-and-tube for viscous or high-pressure duties, scraped surface for ice cream and jam.
- Evaporators: multiple-effect falling-film; steam economy rises roughly with the number of effects, but product heat damage also rises.
- Dryers: tray, tunnel, drum, spray (milk, coffee), fluidised bed, and freeze dryer for heat-sensitive or high-value products.
- Size reduction: hammer mill, roller mill, colloid mill; homogeniser at 200–250 bar for milk.
- Separation: cream separator, decanter centrifuge, ultrafiltration for whey protein.
Worked example
Milk powder by spray drying: preheated concentrate at about 45% total solids is atomised into a chamber with inlet air at 180–200 °C and outlet air at 85–90 °C; powder leaves at 3–4% moisture. Reasoning: atomisation creates a huge surface area, so drying takes seconds, and evaporative cooling keeps particle temperature well below the inlet air temperature, protecting heat-sensitive proteins. A tray dryer could not do this economically.
Traps
Using drying and evaporation interchangeably; choosing a plate exchanger for viscous jam; forgetting that CIP design (drainability, no dead ends, surface finish) is part of equipment selection; quoting steam economy without the trade-off in product quality; ignoring utilities and waste streams when costing.