Difference Between Drying and Dehydration

FOOD PROCESSING AND PRESERVATION: DRYING

Figure 1: Drying is the whole family of water-removal methods, and dehydration is the controlled hot-air member of that family. The two boxes are levels of the same idea, not steps in order.

Introduction

Drying and dehydration are close cousins, and the two words are often used for the same job, which is why students mix them up. Drying is the broad term for taking water out of a food, by any method: sun, hot air, spray, or freeze drying. Dehydration is the controlled form of that job: removing water with heated air in a dryer, to a low moisture that keeps the food stable on a shelf. In plain words, all dehydration is drying, but not all drying is dehydration, because sun drying and freeze drying are drying methods that are not usually called dehydration.

Quick comparison

Table 1: Drying compared with dehydration

Scientific explanation

Definition

Drying moves water out of a food in two steps: heat warms the food and turns its water into vapour, and moving air carries that vapour away. Dehydration is the same removal done under control in a dryer, where the air temperature, speed, and humidity are set by the operator, so the food dries evenly to a chosen moisture.

Working principle

The rate of drying 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, and their surface can dry into a hard skin that traps moisture inside, a fault called case hardening. Drying runs in two periods. A wet food first dries at a constant rate, while its surface stays wet and water evaporates as fast as the hot air can carry heat to it. Then the surface dries out, and the rate falls. In this falling-rate period, water must travel from inside the food to the surface before it can evaporate, and the inside of the food sets the speed, not the air. Turning the air hotter now helps little and harms much: it hardens the surface into case hardening.

Apparatus

Sun drying needs only a clean surface, sun, and wind. A solar dryer improves it with a covered tray and a chimney that pulls warm air through, keeping the food cleaner and drying faster. Dehydration uses cabinet or tray dryers, in which food sits on stacked trays in a heated cabinet with a fan, and tunnel dryers, in which trays or belts move through a long heated tunnel with hot air entering at one end, so food dries continuously as it travels. 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.

Measurement procedure

Water activity, written aw, measures the water in a food that is free for microbes to use, on a scale from 0 to 1, where pure water is 1.00. The aw limits are the most useful numbers in preservation. Most bacteria stop growing below aw 0.91. Most yeasts stop below about 0.88. Most moulds stop below about 0.80. Below about aw 0.60, no microbe grows. That last number is the goal of full drying: a food dried below aw 0.60 is stable against microbes at room temperature. In a factory, aw is a release test: the batch does not leave until its aw is at or below the target. Grain is dried to a safe moisture level, usually below about 13 percent for cereals, before storage.

Factors affecting readings

Aw is not the same as water content: a food can hold a lot of water and still have low aw, if that water is bound to sugar, salt, or other solids. Honey is about one fifth water, yet its aw is around 0.6, because the sugar binds the water so tightly that microbes cannot use it. Drying also does not switch enzymes off on its own, so fruit is often dipped in a sulphur dioxide solution or blanched before drying, to stop browning. Drying does not kill all microbes: it stops them from growing, but spores and some hardy cells survive dry, and they wake up when the food is wetted again, which is why dried food must be kept dry in storage.

Formulas and calculations

Water activity follows Raoult’s law: aw = p / p₀, where p is the vapour pressure of water in the food and p₀ is the vapour pressure of pure water at the same temperature. For an ideal solution this becomes aw = n_water / (n_water + n_solute), where n counts moles.

Worked example 1: sugar in water. Dissolve 100 g of sucrose in 100 g of water. Sucrose has a molar mass of 342 g/mol, so 100 g is 100 / 342 = 0.292 mol. Water has a molar mass of 18 g/mol, so 100 g is 100 / 18 = 5.56 mol. Then aw = 5.56 / (5.56 + 0.292) = 5.56 / 5.85 = 0.95.

Worked example 2: salt beats sugar, mole for mole. Dissolve 100 g of salt in 100 g of water. Salt has a molar mass of 58.5 g/mol, so 100 g is 100 / 58.5 = 1.71 mol, and each salt unit splits into two particles in water, sodium and chloride, so the solute counts as 2 x 1.71 = 3.42 mol of particles. Then aw = 5.56 / (5.56 + 3.42) = 5.56 / 8.98 = 0.62.

Drying time in the constant-rate period. 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.

Industrial applications

• Drying: grain dried after harvest to below about 13 percent moisture for months of storage; sun dried fruit and vegetables; spray dried milk powder and instant coffee; freeze dried coffee, herbs, and fruit for snacks.
• Dehydration: hot-air cabinet and tunnel drying of sliced fruit, vegetables, and herbs in factories; drum drying of potato flakes; tray drying of spices and small batches.

Advantages and limitations

Drying, advantages: covers every method, so the maker can pick the cheapest that fits the food; dried food is light and cheap to store and move; no fridge needed once dry. Limitations: some methods are slow and weather dependent; drying alone does not kill all microbes or switch enzymes off; dried food must be kept dry, or its aw rises again and microbes wake up.

Dehydration, advantages: controlled and repeatable; even drying in trays or tunnels; faster than sun drying and cleaner. Limitations: needs fuel and equipment; too-hot air after the critical moisture content causes case hardening; heat and air lose some vitamin C and darken colour.

FAQ

Are drying and dehydration the same thing? Nearly, but not exactly. Drying is the broad term for removing water by any method, while dehydration usually means controlled drying with heated air in a dryer.

What is water activity, and why does it matter more than water content? Water activity, aw, is the water microbes can actually use, on a scale from 0 to 1. A food can hold a lot of water and still have low aw if sugar or salt binds it, as honey shows at aw around 0.6.

Below what aw does nothing grow? Below about aw 0.60, no microbe grows. Most bacteria stop below 0.91, most yeasts below about 0.88, and most moulds below about 0.80.

What is case hardening? The surface of a food dries into a hard skin that traps moisture inside, so the inside stays wet enough for microbes while the outside looks dry. It happens when the air is too hot in the falling-rate period.

Why must dried food be packed against moisture? Because dried food takes up water from humid air, so its aw rises again and microbes wake up. The pack must block moisture, and the store must be dry.

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; Frazier, W. C., and Westhoff, D. C., Food Microbiology; Adams, M. R., and Moss, M. O., Food Microbiology.