Food Engineering

Lesson 25. WATER ACTIVITY AND MASS TRANSFER

Module 3. Food dehydration

Lesson 25

WATER ACTIVITY AND MASS TRANSFER

25.1 Introduction

Drying is defined as the removal of moisture from a product, and in most practical situations the main stage during drying is the internal mass transfer. The mechanisms of water transfer in the product during the drying process can be summarized as follows:

• Water movement due to capillary forces
• Diffusion of liquid due to concentration gradients
• Surface diffusion
• Water vapor diffusion in pores filled with air
• Flow due to pressure gradients
• Flow due to water vaporization–condensation

In the pores of solids with rigid structure, capillary forces are responsible for the retention of water, whereas in solids formed by aggregates of fine powders, the osmotic pressure is responsible for water retention within the solids as well as in the surface. The type of material to be dried is an important factor to consider in all drying processes, since its physical and chemical properties play a significant role during drying due to possible changes that may occur and because of the effect that such changes may have in the removal of water from the product.

A hygroscopic material is one that contains bound water that exerts a vapor pressure lower than the vapor pressure of liquid water at the same temperature. It is expected that products made mainly of carbohydrates will behave in a hygroscopic way, since the hydroxyl groups around the sugar molecules allow formation of hydrogen bonds with water molecules. The interaction between the water molecules and the hydroxyl groups causes solvation or solubilization of sugars. In water soluble proteins, as in most of the globular proteins, the polar amino acids are uniformly distributed in the surface, while the hydrophobic groups are located towards the inside of the molecule. This arrangement allows formation of hydrogen bonds with water, which explains the solubility of this type of proteins.

Dehydration involves the simultaneous transfer of heat, mass and momentum in which heat penetrates into the product and moisture is removed by evaporation into an unsaturated gas phase. Owing to the complexity of the process, no generalized theory currently exists to explain the mechanism of internal moisture movement. Although it is now accepted that in most practical situations of air drying of foods the principal rate-determining step is internal mass transfer, there is no agreement on the mechanism of internal moisture movement. In the case of capillary-porous materials such as fruits and vegetables, interstitial spaces, capillaries and gas-filled cavities exist within the food matrix and water transport takes place via several possible mechanisms acting in various combinations. The possible mechanisms include:

• liquid diffusion caused by concentration gradients,
• liquid transport due to capillary forces,
• vapour diffusion due to shrinkage and partial vapour-pressure gradients (Stefan’s law),
• liquid or vapour transport due to the difference in total pressure caused by external pressure and temperature (Poiseuille’s law),
• evaporation and condensation effects caused by differences in temperature,
• surface diffusion in liquid layers at the solid interface due to surface concentration gradient,
• liquid transport due to gravity.

Most foods are classified as capillary porous rigid or capillary porous colloids. Therefore, it is often proposed that a combination of capillary flow and vapour diffusion mechanisms should be used to describe internal mass transfer. Water activity, rather than moisture content, influences biological reactions. In the regions of water adsorption on polar sites or when a mono-molecular layer exists, there is little enzyme activity. Enzyme activity begins only above the region of mono-molecular adsorption. When the moisture content of a substrate is reduced below 10 %, microorganisms are no longer active. It is necessary however to reduce the moisture content to below 5 % in order to preserve nutrition and flavour.

25.2 Water Activity

Water activity and moisture content of the material are very important in food unit operations. During processing and storage, many chemical and physical factors are influenced by the water activity and moisture content level. Chemical changes that are enhanced by water activity include enzymatic reactions, nonenzymatic browning, and microbial activity. In many food products, enzymes are not inactivated during the heating process. Consequently, enzymatic reactions can take place at even low moisture contents. Water activity also affects the nonenzymatic browning reactions in foods. When water is present, carboxyl and amino compounds are involved as reactants, products, or catalysts in the browning process. Bacterial growth is also affected at fairly high water activity levels. If water activity is maintained at a value below 0.90, most bacteria remain dormant. Most yeasts and molds, however, can grow and multiply at water activity levels as low as 0.80. Physical changes such as texture and aroma can depend greatly on water activity. Textural changes are most often seen in freeze drying and subsequent storing of foods, particularly meats and fish. The water activity in dried foods can also affect the retention of aroma. Different foods that are stored together will be altered if their individual relative humidities (water activities) are different. Under the action of a driving force created by a difference in water activities changes in the moisture and water activities of these foods will follow their own isotherm curve until an equilibrium water activity is achieved.

Water activity (a_w) is measured as the equilibrium relative humidity (ERH), the percent relative humidity (RH) of an atmosphere in contact with a product at the equilibrium water content. a_w is also ratio of the partial pressure of water in the headspace of a product (P) to the vapour pressure of pure water (P₀) at the same temperature

a_w = ERH = P/P₀

The relationship between a_w and the rate of deteriorative reactions is very important to design dehydration systems. Reducing a_w below 0.7 prevents microbiological spoilage. However, although microbiological spoilage does not occur at a_w = 0.7, prevention of other deteriorative reactions needed to preserve a dried food product successfully reduction of a_w to below 0.3.

25.3 Mass Transfer

During dehydration, water is vapourized only from the surface. The transfer of water vapour from the wet surface to a stream of moving air is analogous to convection heat transfer, therefore, a mass transfer coefficient is used. Moisture flux is proportional to the driving force which is the difference in vapour pressure on the surface and the vapour pressure of water in air surrounding the surface. At the same time that water is removed from the surface, water diffuses from the interior of a solid towards the surface. The later is a general form of diffusion which is analogous to conduction heat transfer. The differential equations for conduction also apply to diffusion, but mass diffusivity is used in place of thermal diffusivity.

25.4 Classification of Dehydration Systems

Example 25.5

A dry food product has been exposed to a 30% RH environment at 15^oC for 5 h without a weight change. The moisture content has been measured and is 7.5% (wet basis). The product is moved to a 50% RH environment and a weight increase of 0.1 kg/kg product occurs before equilibrium is reached.

1. Determine water activity of the product in the first and second environment.
2. Compute moisture contents of the product on dry basis in both environments.

Solution

Equilibrium RH = 30% in first environment.

Product moisture content is 7.5% (wet basis) in first environment.

Weight gained in second environment = 0.1 kg / kg product.

The water activity is equilibrium RH divided by 100. so water activities are 0.3 and 0.5 in first & second environment respectively.