Water Sorption Isotherms

• Foods, when exposed to different relative humidities, adsorb or desorb water/moisture depending on their water activity.

• When their aw is less than the surrounding RH, the product will adsorb water and when reverse is the case, it will lose the moisture (desorb).

• Thus, adsorption is the binding of water at the physical interface between a solid and its environment. On the other hand, desorption is the removal of water from the interface.

• The water sorption by food depends on the chemical composition, physico-chemical state of ingredients and the physical structure of the food.

Sorption phenomenon

Water activity and moisture content• It is possible to describe the relationship between aw and

the moisture content of the food (ie. Moisture/water sorption isotherm).

• The water sorption isotherm is a plot between water content in a product on solids basis on the ordinate and the water activity/ RH values on the abscissa at a particular temperature.

• Water sorption isotherms are of two types: a. Adsorption isotherm – This isotherm is made by equilibrating a completely dry material into various environments of increasing RH, and measuring the weight gain due to water adsorption.

b. Desorption isotherm – This isotherm is made by placing an initially wet material/sample into various RH atmospheres, and measuring the loss in weight.

• The water sorption isotherms manifest the steady state amount of water held by the food as a function of aw or RH.

• The water sorption isotherms for most foods are non- linear and are sigmoidal in shape.

• The isotherm can be divided into three regions representing the ranges in which the three principal types of water binding predominates.

Region I. Aw 0 to 0.25 – water is bound by ionic groups such as NH3 associated with proteins. The water is of structural, hydrophobic hydration and monolayer bound. This water is not available for microorganisms but is available for some chemical reactions.

Region II. Aw 0.25 to 0.70 – The water is covalently bound to amide groups in proteins and -OH groups in proteins and carbohydrates. The water is still bound tightly and hence is not available for most of the microorganisms but is available for chemical activity. The uptake of water by foods in this region is linear.

The changes in physical state such as loss of crispiness, stickiness of powders and hard candies and recrystallization of amorphous sugars, causing irreversible caking takes place.

Region III. Aw 0.70 to 1.0 – Water is present in multilayers on proteins and carbohydrate polymers and in capillaries and void clefts. The water in this region is loosely bound and is available for both chemical and microbiological activities.

In the 0.65-0.75 aw range, mold growth would dominate. Above 0.85 aw , both pathogenic and spoilage bacteria grow

• The course of water sorption by a dry material is first by the formation of a monolayer, followed by multilayer adsorption, the uptake into pores and capillary spaces, dissolution of solutes, and finally, mechanical entrapment of water at the higher levels of aw .

• These phases may overlap and will differ in magnitude among foods depending upon the chemical composition and structure.

• The monolayer region can be determined from the BET (Brunaeur-Emmett-Teller) equation/plot.

• where, m = equilibrium moisture (g/100 g solids)mo = amount of monolayer water (g/100 g solids)C = thermodynamic constant

Figure. Three main regions of the water sorption isotherm

(Adopted from Barbosa-Cánovas et al 2007)

Product BET monolayer value (kg water/ kg solids)

Skim milk 0.024Casein 0.04-0.089Lactose 0.022Chhana powder 0.019Khoa 0.026Raw paneer 0.042Fried paneer 0.031Whey proteins 0.091

Temperature dependence of water vapor sorption• The water sorption properties are influenced by food

composition, temperature, pressure and RH.

• In describing the water sorption isotherm, the temperature has to be held constant because temperature affects the mobility of water molecules and the dynamic equilibrium between the vapor and adsorbed phases.

• The aw decreases with decreasing temperature. Any increase in temperature result in a fairly significant increase in the aw at a constant water content, allowing higher rates of microbiological activity.

• This need not be true always because at higher aw and temperature, some new solutes may dissolve causing a cross over.

• The effect of temperature on increasing the aw at constant moisture content is greatest at lower to intermediate water activities.

• The temperature dependence of vapor pressure of water and water activity follows the Clausius- Clapeyron equation.

• From the equation, the isotherm value at any temperature can be predicted if the corresponding excess heat of sorption is known at constant moisture content.

Figure. Temperature dependence of water sorption (Adopted from Barbosa-Cánovas et

al 2007)

Figure. Shift in water sorption with temperature, illustrating the cross-over

where, aw1 and aw2 are the water activities at temperature T1 and T2 respectively

‘Qs ’ is the isosteric heat of sorption (J/mol)

‘R’ is the universal gas constant

• Thus, to predict the aw of a food at any given temperature, the water sorption isotherm must be determined for at least two temperatures.

Applications of water sorption isotherms

• The isotherms provide information on what water contents certain desirable or undesirable levels of aw are achieved.

• In addition, the significant changes in water content with respect to changes in aw can be inferred from the isotherms at any given temperature.

• The bound water, capillary water/multilayer water which decide the texture of products can be estimated.

• Thermodynamic properties of the product such as enthalpy of water binding, Gibbs free energy and heat of sorption, all of which will be useful in computing the energy requirement for drying the product.

• Sorption behaviour of powders is important in understanding the storage stability and in selecting the most suitable packaging material.

• The isotherm data will be useful in controlling the growth of microorganisms in food products so as to minimize quality deterioration in foods.

• The water sorption data is critical for the evaluation of water uptake, porosity, estimation of specific surface area and the crystalline state of various components of food.

• The shelf-life and stability of dehydrated foods can be predicted from the water sorption isotherms.

• The water sorption isotherms enable us to predict the enzymatic reactions and deteriorative chemical changes possible in the food product.