National University of Singapore
Food Science & Technology
Fundamentals of Drying
The removal of moisture from food via evaporation to obtain a solid end product state with very low moisture content is referred to as drying. It is a complex phenomenon involving simultaneous heat and mass transfers where latent heat of vapourisation is provided to the food and the vapour produced is removed. Drying prolongs shelf-life of food while removing the need for refrigeration as the low moisture content and high osmotic pressure inhibit the growth of most microbes and spoilage reactions. Drying of foods also reduces food volume and mass which in turn enhances storage stability and minimises packaging requirement.
Commercially, there exists numerous drying methods, one of which is forced convection air drying where hot air is allowed to pass over the food to achieve drying. The temperature difference causes the transfer of heat from the air to the cooler surfaces on the food, thereby resulting in vapourisation. Subsequently, the higher vapour pressure in food results in the movement of vapour into dry air until an equilibrium is achieved. The equilibrium moisture content of food could be defined as the moisture content of food in equilibrium with the external surrounding environment of a given vapour pressure and temperature after a theoretical infinite period. The equilibrium moisture content is achieved when there is no net moisture loss or gain between the food and external surrounding environment.
The rate of moisture removal per unit area and per unit time is defined as the drying rate (rate of migration of water from food interior to the surface or the vapourisation plane) and is governed by both internal and external factors.
Internal factors
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size and shape of food
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moisture content
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food matrix
affect the rate of migration of water from the food interior to the surface or vapourisation plane.
External factors
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temperature of hot dry air into the dryer
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velocity of hot dry air
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relative humidity
Affect the rate of evaporation and heat transfer
Typically, drying rate changes at different stages of the drying process. Figures 1 and 2 demonstrate the changes of the drying rate with respect to free mositure content (g water/g dry solids) and time, respectively.
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A-B: The initial rate period is a short transition at the beginning of the drying process where temperature of food samples increase.
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B-C: The constant rate period follows, where water is continuously removed from saturated surface of the food and drying occurs at wet bulb temperature. The rate of evaporation determined by air condition is the major rate limiting factor.
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C-D: Falling rate period follows and it is during this period where the rate of migration of water becomes the major limiting factor.
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D-E: The moisture content at which drying rate suddenly decreases is termed the critical moisture content. After drying for a theoretically infinite time, the food product is said to have reached its equilibrium moisture content at the given external air conditions, after which no more exchange of water is observed between the food and the surrounding air.
Fig 1. Curve of drying rate versus free moisture content on a dry basis. A, B, C, D & E are points indicating different drying periods. The black line is the theorectical drying curve whilst the blue line is a practical experimental drying curve.
Fig 2. Curve of drying rate versus time. A, B, C, D & E are points indciating different drying periods.