Course:FNH200/Lessons/Lesson 08/Page 08.3
8.3 Factors Affecting Dehydration
Dehydration of food requires that water (mass) be transferred from the food into the dehydrating environment, and that heat (the driving force that encourages water removal) be transferred to the food to promote water removal from the food. The objectives of food dehydration operations are to dry the food as fast as possible, at the least cost, while creating the fewest changes in food quality.
The composition of the food itself can have an effect of the rate at which dehydration occurs. For example, if water is bound to solutes in the food it will have a lower vapour pressure and therefore will be more difficult to remove. The porosity of the food is also important. Efforts are made to enhance the porosity of foods to be dehydrated in order to facilitate mass transfer and speed of drying rate, thus maximizing the efficiency of dehydration. Porous (sponge-like) structures are formed by creating steam pressure within the product during the drying process. The steam will "puff" the product. Another way of creating porosity is by making a stable foam from a liquid food prior to drying.
In addition to the composition of food, the following factors can also affect heat and mass transfer within food materials undergoing dehydration, and therefore are important to consider in order to control some of the undesirable changes described previously:
- Surface area. It is desirable to maximize the surface-to-volume ratio of the food to be dehydrated to minimize the resistance to heat and mass transfer. Generally, the smaller the food piece, the more rapid the rate of moisture loss.
- Temperature.The hotter the air, the more moisture it will hold before becoming saturated. Drying systems are designed to maximize temperature differences between the product and the drying air to increase the rate of dehydration. An upper limit to drying air temperature is dictated by adverse chemical reactions that can take place in a food at high temperatures. The upper temperature limit is also dictated by the chemical and physical nature of the food.
- Air velocity. The faster the air velocity within a dehydrator, the more rapid the rate of moisture removal. Food dehydrators are designed to maximize the velocity of heated air moving around the food particles to be dried.
- Humidity of the drying air. The drier the air, the more moisture it can absorb before it becomes saturated. The relative humidity of the drying air determines the final moisture content of the food being dried. Knowledge of the equilibrium relative humidity of food is important for the proper design of dehydrators and for the design of packaging systems that will prevent moisture adsorption by the dehydrated food during storage. You may have experienced the loss of crispness of crackers in opened packages during lengthy storage in your cupboards. Crackers have a low equilibrium relative humidity and they tend to adsorb water from the air.
- Atmospheric pressure and vacuum. Water boils at 100°C when it is at a pressure of 1 atm (760 mm Hg). As the pressure lowers, the boiling temperature will decrease. If the temperature is maintained constant, a decrease in pressure will increase the rate of boiling. Some concentrators and dehydrators are operated at pressures below atmospheric pressure in order to increase the rate of boiling and moisture removal. This is especially important in the case of heat-sensitive food products.