Drying and dehydration of foods
How food drying works: constant and falling rate periods, critical moisture, dryer types from sun to spray and freeze drying, and quality changes.
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Drying is the oldest way to preserve food and is still one of the most widely used. Taking water out lowers the water activity until microbes cannot grow and most enzymes and chemical reactions slow down. It also cuts weight and volume, so dried foods are cheaper to store and transport. Milk powder, instant coffee, raisins, dried chillies, pasta, dehydrated onion and soup mixes are all made this way.
Drying involves heat transfer into the food and mass transfer of water vapour out of it at the same time. Understanding both explains the shape of the drying curve, the choice of dryer, and most of the quality problems.
The physics of drying
When warm, unsaturated air passes over a wet food, heat flows from the air to the surface and water evaporates. The vapour then moves into the air stream. The driving forces are the temperature difference between air and surface, and the vapour pressure difference between the surface and the air.
The air's ability to dry depends on its temperature, its relative humidity and its velocity. Hot, dry, fast-moving air removes water fastest. The psychrometric chart describes this: as air picks up moisture during adiabatic drying, its dry-bulb temperature falls and its humidity rises along a line of constant wet-bulb temperature.
Dry-basis moisture is used in drying calculations because the dry solids stay constant while water is removed.
Water removed from carrots
Carrots at 88% moisture (wet basis) are dried to 6%. Per 100 kg of fresh carrots, the dry solids are 12 kg. At 6% moisture the product weighs 12 ⁄ 0.94 = 12.8 kg. Water removed = 100 − 12.8 = 87.2 kg, so the drying ratio is about 7.8 : 1.
Periods of drying
A drying curve plots moisture content or drying rate against time or moisture. It usually has three parts.
Settling-down period
The food warms up or cools to the wet-bulb temperature of the air. This phase is short.
Constant-rate period
The surface behaves like a free water surface and stays at the wet-bulb temperature. The rate is set by external conditions: air temperature, humidity and velocity. All the heat that arrives goes into evaporating water, so the food stays relatively cool even in hot air. Many fruits and vegetables have a short constant-rate period or none at all.
Falling-rate period
Below the critical moisture content, the surface is no longer fully wet. Water must move from the interior by liquid diffusion, capillary flow or vapour diffusion, and internal resistance controls the rate. The product temperature rises toward the air temperature, and this is where heat damage happens. Drying stops at the equilibrium moisture content, which depends on the air's relative humidity and is read from the food's sorption isotherm.
Most of the drying time for foods is spent in the falling-rate period. Cutting smaller pieces shortens the diffusion path. Because diffusion time scales with the square of thickness, halving the thickness cuts falling-rate time to roughly a quarter.
Types of dryers
| Dryer | How it works | Typical products |
|---|---|---|
| Sun and solar dryers | Solar radiation and natural air flow; solar cabinets add a collector and cover | Grapes, apricots, chillies, fish, grain |
| Cabinet (tray) dryer | Hot air blown across trays in a batch | Vegetables, fruit, small factories |
| Tunnel dryer | Trolleys of trays move through a hot-air tunnel, co-current or counter-current | Fruit and vegetable pieces |
| Conveyor (belt) dryer | Continuous belt through zones of hot air | Vegetables, cereals, snacks |
| Fluidised-bed dryer | Upward air suspends small particles | Peas, diced vegetables, granules |
| Drum (roller) dryer | Thin film on a steam-heated rotating drum, scraped off | Baby cereals, potato flakes |
| Spray dryer | Liquid atomised into hot air; droplets dry in seconds | Milk powder, instant coffee, egg powder |
| Freeze dryer | Frozen food under vacuum; ice sublimes | Instant coffee, strawberries, astronaut and military food |
| Osmotic dehydration | Pieces soaked in sugar or salt solution before air drying | Candied fruit, intermediate-moisture foods |
Spray drying is the main way to make powders from liquids. The large surface area of tiny droplets means drying takes only a few seconds. Evaporative cooling keeps the particle near the wet-bulb temperature, so heat-sensitive milk proteins survive even with inlet air at 150–220 °C.
Freeze drying (lyophilisation) gives the best quality. There is no liquid water, so there is little shrinkage, and the porous product rehydrates quickly and keeps most of its aroma. It is the most expensive method, however, and is used only for high-value products.
Quality changes during drying
- Shrinkage and changes in texture, especially when surface tissue collapses.
- Case hardening: if the surface dries too fast, a hard, impermeable skin forms and traps moisture inside. It is common with fruits high in sugar.
- Browning: Maillard reactions peak at intermediate water activity, which every food passes through during drying. Enzymatic browning is controlled by blanching or sulphiting.
- Loss of nutrients and flavour: vitamin C and carotenoids degrade with heat and oxygen, and volatile aromas escape.
- Poor rehydration: cell damage means the product does not fully regain its original shape.
Pre-treatments reduce these problems. Blanching inactivates enzymes. Sulphuring or sulphiting stops browning in apricots and golden raisins. Dipping grapes in an alkaline oil emulsion breaks down the waxy skin and speeds drying.
Key points
- Drying lowers water activity to stop microbial growth and slow chemical reactions.
- Moisture is expressed on a dry basis (water ⁄ dry solids) or wet basis (water ⁄ total mass).
- Drying periods: settling down, constant rate (surface at wet-bulb temperature, external control) and falling rate (internal diffusion control) down to equilibrium moisture.
- The critical moisture content marks the change from constant to falling rate.
- Spray drying turns liquids into powders in seconds; freeze drying gives the best quality at the highest cost.
- Quality problems include case hardening, shrinkage, browning, nutrient loss and poor rehydration.
What the exams ask
- Define critical and equilibrium moisture content, and sketch the drying rate curve.
- Convert between wet-basis and dry-basis moisture content, or calculate the water removed.
- Explain why a food stays at the wet-bulb temperature during the constant-rate period.
- Compare spray, drum, tunnel and freeze dryers and match each to a product.
- Explain case hardening and how to prevent it.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why does freeze-dried food taste better than air-dried food?
Freeze drying removes water as vapour straight from ice, at low temperature and without liquid water moving through the tissue. The structure stays rigid and porous, little heat damage occurs, and aroma compounds that would evaporate or react in hot air are largely kept.
Is sun drying safe?
It can be, with good practice. Open sun drying exposes food to dust, insects, birds and rain, and slow drying in humid weather can allow moulds and aflatoxin. Solar dryers with covered trays dry faster and more hygienically.
How dry does food need to be to keep?
It needs to reach a water activity at which microbes cannot grow, generally below about 0.6 for long shelf life. Each food has its own target moisture: about 13–14% for cereal grains, around 4% for milk powder and 15–20% for many dried fruits, which are protected by their sugar.
Sources
- Fellows, P. J. Food Processing Technology: Principles and Practice, 4th edition. Woodhead Publishing, 2017. Chapters on dehydration and freeze drying.
- Singh, R. P. and Heldman, D. R. Introduction to Food Engineering, 5th edition. Academic Press, 2014. Chapter on dehydration.
- Mujumdar, A. S. (ed.) Handbook of Industrial Drying, 4th edition. CRC Press, 2014.
- Geankoplis, C. J. Transport Processes and Separation Process Principles, 4th edition. Prentice Hall, 2003. Drying of process materials.
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