8 min readConcept sheetOften a 5-mark note on one dryer; part of a 10-mark 'types of dryers' answer

Tray, tunnel and fluidised-bed drying

Food pieces are dried by hot air in tray (cabinet), tunnel or fluidised-bed dryers. Tunnels suit large volumes; fluidised beds dry small particles fast and evenly.

Tray dryerBatch cabinet with stacked trays; small units

Tunnel dryerSemi-continuous: trolleys of trays move through a long tunnel

Fluidised-bed dryerUpward hot air makes small particles float and dry evenly

Three hot-air dryers for solid foods

Dryer	How it works	Foods	Advantages	Disadvantages
Tray (cabinet) dryer	Insulated cabinet with stacked mesh trays; a fan blows air over a heater and across the trays; batch	Fruit and vegetable slices, leaves; small units and labs	Cheap, simple, flexible	Batch work, labour to load; uneven unless trays are rotated
Tunnel dryer	Trolleys of trays enter one end of a long tunnel and leave the other while hot air flows along; air moves with the food (co-current) or against it (counter-current)	Vegetables and fruits on a large scale	Semi-continuous, high output, good control	High cost and space; uneven if loading varies
Fluidised-bed dryer	Hot air blown up through a perforated plate lifts small particles so the bed behaves like a boiling liquid	Peas, diced vegetables, grains, granules, sugar, salt, instant powders	Fast, even drying; good control; compact; batch or continuous	Only small, even, non-sticky particles; breakage and dust

Co-current vs counter-current tunnel drying

Co-current (parallel) flowHot, dry air meets the wettest food at the entry. Drying is fast at first with little heat damage, because evaporation keeps the food cool. But the food leaves meeting cool, humid air, so the final moisture stays higher.

Counter-current flowFood and air move in opposite directions, so the driest food meets the hottest, driest air at the exit. A lower final moisture is reached, but scorching of the dry food is more likely.

Two-stage tunnels use co-current flow first and counter-current flow to finish.

Worked example: water to remove from tomato slices

Problem

1 kg of tomato slices with 94% moisture is dried in a tray dryer to 6% moisture. Find the weight of the dried product and the water removed.

Working

Solids in fresh tomato = 1000 g × (100 − 94) ÷ 100 = 60 g.
Solids do not change on drying; in the dried product they make up 100 − 6 = 94% of the weight.
Dried weight = 60 ÷ 0.94 = 63.83 g (rounded to 2 decimals).
Water removed = 1000 − 63.83 = 936.17 g.
Check: water left = 63.83 − 60 = 3.83 g, and 3.83 ÷ 63.83 = 0.06, i.e. 6%.
Drying ratio (fresh ÷ dried) = 1000 ÷ 63.83 = 15.7 : 1 (to one decimal).

Answer

About 64 g of dried tomato; about 936 g of water is evaporated; drying ratio about 15.7 : 1.

Drying vegetables in a tray (cabinet) dryer

1
Select, wash, peel and cut into thin, even slices
2
Blanch (most vegetables), cool and drain
3
Spread in a single thin layer on mesh trays
do not overload
4★
Dry with moving hot air at the temperature in your manual
too hot scorches and case-hardens
5
Turn the pieces and rotate the trays
for even drying
6★
Check dryness
vegetables brittle; fruits leathery
7
Cool, condition in a closed container so moisture evens out, and pack airtight

★ one-way, controls the pace

In food and nutrition

Hot-air dryers give cleaner, faster drying than the open sun, so more vitamin C and carotene survive. Small tray dryers let self-help groups and hospital kitchens make dried vegetable mixes and curry-leaf or drumstick-leaf powders hygienically.

Exam angle

2 marks: “What is a fluidised-bed dryer?” A dryer in which hot air is blown up through a bed of small particles fast enough to lift them, so each particle is surrounded by air and dries quickly and evenly.

5 marks: “Explain tray and tunnel drying with their advantages and disadvantages.” Tray: batch cabinet, stacked trays, fan and heater; tunnel: trolleys through a tunnel with co-current or counter-current air; uses; merits and demerits; a sketch.

Remember it

Tray = batch cabinet, tunnel = trolley train, fluid bed = floating peas.