FOOD PROCESSING AND PRESERVATION: DRYING
Sun drying and mechanical drying: free heat against controlled heat
Sun drying
Food spread thin in sun and wind, free heat, open to weather
versus
Mechanical drying
Warm air, usually 50 °C to 70 °C, blown through food in a cabinet or tunnel dryer
Figure 1: Sun drying uses free sun and wind, while mechanical drying uses fuel and fans to control the air. The two boxes are alternatives, not steps in order.
Introduction
Sun drying and mechanical drying both remove water from food with warm air, but one uses the sun and the wind for free, and the other uses fuel, fans, and a dryer to control the air. Sun drying is the oldest method: food spread thin in sun and wind. It costs little, but it is slow, it depends on weather, and the food is open to dust, insects, and rain. Mechanical drying blows warm air, usually 50 °C to 70 °C, over or through the food in a cabinet or tunnel dryer. It is faster, cleaner, and repeatable, but it costs fuel and equipment.
Quick comparison
Table 1: Sun drying compared with mechanical drying
| Aspect | Sun drying | Mechanical drying |
|---|---|---|
| Heat source | Sun and wind, free | Fuel, electricity, or steam heating air |
| Air temperature | Set by the weather, changes through the day | Controlled, usually 50 °C to 70 °C |
| Speed | Slow, often days | Fast, hours |
| Control | Almost none: depends on weather | Air temperature, speed, and humidity are set by the operator |
| Cleanliness | Open to dust, insects, and rain | Enclosed, clean, protected from insects and rain |
| Evenness | Uneven: pieces dry at different rates | Even drying, especially in tunnel dryers |
| Cost | Almost no cash cost | Fuel, fans, and equipment cost money |
| Typical foods | Grain, fruit, fish, and vegetables in sunny climates | Fruit, vegetables, herbs, and spices in factories |
| Quality risk | Slow drying loses more vitamin C; rain can rewet the food | Too-hot air after the surface dries causes case hardening |
Scientific explanation
Definition
Sun drying is spreading food thin in sun and wind so the sun warms it and the wind carries the vapour away. Mechanical drying is blowing warm air over or through food in a cabinet, tray, or tunnel dryer, with the air temperature and speed controlled by the operator.
Working principle
Both methods work the same way: heat warms the food and turns its water into vapour, and moving air carries that vapour away. The rate depends on temperature, air speed, humidity of the air, and the size of the food pieces. Thin slices dry fast because water has a short path out; thick pieces dry slowly. Solar dryers improve sun drying with a covered tray and a chimney that pulls warm air through, keeping the food cleaner and drying faster. In a cabinet dryer, food sits on stacked trays in a heated cabinet with a fan. In a tunnel dryer, trays or belts move through a long heated tunnel, with hot air entering at one end, so food dries continuously as it travels.
Apparatus
Sun drying needs a clean raised surface, sun, and wind, and nothing else. A solar dryer adds a covered collector that warms the air and a chimney that pulls it through the food. Mechanical drying needs a dryer: a cabinet or tray dryer for small batches of herbs, fruit, and spices, or a tunnel dryer for continuous factory drying. A fan moves the air, a heater sets its temperature, and vents let the damp air out.
Measurement procedure
Both methods are judged by the same numbers. Water activity is measured with an instrument: a sample sits in a sealed chamber, and the instrument measures the humidity of the air above it once it has settled, which equals the aw of the food. A fully dried food aims below aw 0.60, where no microbe grows. Grain is dried to a safe moisture level, usually below about 13 percent for cereals, before storage. In a factory, aw is a release test: the batch does not leave until its aw is at or below the target.
Factors affecting readings
In sun drying, the weather decides everything: cloud, rain, and still air all slow drying, and rain can rewet food that was nearly dry. Fruit dried slowly in sun loses more vitamin C than fruit dried fast in hot air. In mechanical drying, the air temperature is a trade-off: hotter dries faster but damages colour, flavour, and vitamins, and risks case hardening, the hard skin that traps moisture inside. Drying runs in two periods: a constant-rate period while the surface stays wet, set by the air, then a falling-rate period set by the inside of the food, split by the critical moisture content. Pushing hot air after that point causes case hardening.
Formulas and calculations
The time to dry through the constant-rate period is t = ρλx(Mi – Mc) / (hc(Ta – Ts)), where ρ is the bulk density, λ the latent heat of vaporisation, x the bed thickness, Mi the starting moisture, Mc the critical moisture content, hc the surface heat transfer coefficient, Ta the air temperature, and Ts the wet-bulb temperature of the air. The formula shows why hotter, drier, faster air dries faster in the constant-rate period: Ta – Ts is larger and hc is larger. It also shows why thin beds dry faster: the time grows with the bed thickness x.
Worked example: water activity from Raoult’s law. Water activity is aw = n_water / (n_water + n_solute). Dissolve 100 g of salt in 100 g of water: salt is 100 / 58.5 = 1.71 mol, and each salt unit splits into two particles, so the solute counts as 3.42 mol. Water is 100 / 18 = 5.56 mol. Then aw = 5.56 / (5.56 + 3.42) = 0.62. The same weight of sugar only reaches aw 0.95, which shows why salt preserves with far less weight than a syrup needs.
Industrial applications
- Sun drying: grain and pulses dried after harvest in sunny climates; sun dried fruit, fish, and vegetables where fuel is scarce; solar dryers for mango slices and herbs.
- Mechanical drying: cabinet and tunnel drying of sliced fruit, vegetables, and herbs in factories; tray drying of spices and small batches; grain dryers that dry harvested grain promptly to below about 13 percent moisture.
Advantages and limitations
Sun drying, advantages: almost no cash cost; needs no fuel or equipment; suits grain and fruit in sunny climates. Limitations: slow and weather dependent; open to dust, insects, and rain; uneven drying; more vitamin C loss from slow drying.
Mechanical drying, advantages: fast, clean, and repeatable; works in any weather; even drying in tunnels; the operator controls temperature and air speed. Limitations: fuel and equipment cost money; too-hot air causes case hardening; heat loses some vitamins and darkens colour.
FAQ
What is a solar dryer, and how is it different from open sun drying? A solar dryer is a covered dryer that uses the sun to warm air and a chimney to pull it through the food. It keeps the food cleaner and dries faster than open sun drying, while still using free sun heat.
Why is sun drying slow? Because the air temperature and speed are set by the weather, not by the operator, and the food dries only as fast as the sun and wind allow.
What is case hardening, and which method risks it more? Case hardening is a hard dry skin on the surface that traps moisture inside. Mechanical drying risks it more, when the air is too hot after the surface dries in the falling-rate period.
Why is grain dried promptly after harvest? Grain cut wet and left in a heap heats up and moulds, while grain dried promptly after threshing to below about 13 percent moisture stays sound for months or years in a dry store.
Can sun dried food be as safe as mechanically dried food? Yes, if it reaches the same water activity: below aw 0.60 no microbe grows. The difference is control and cleanliness, not the final aw target.
References and further reading
FTN subjects: Principles of Food Preservation, Unit 4 (Drying and Water Activity); Principles of Food Processing, Unit 3 (Mass Transfer, Drying, Evaporation). Books used: Fellows, P. J., Food Processing Technology: Principles and Practice; Rahman, M. S., Handbook of Food Preservation; Chakraverty, A., Mujumdar, A. S., Raghavan, G. S. V., and Ramaswamy, H. S., Handbook of Postharvest Technology: Cereals, Fruits, Vegetables, Tea and Spices.
