COMMENTS ON “A THEORETICAL MODEL TO PREDICT THE FORMATION OF PORES IN FOODS DURING DRYING BY M. S. RAHMAN [INTERNATIONAL JOURNAL OF FOOD PROPERTIES (2003) 61-72]”

The above article presented a theoretical based model for the prediction of pores during drying. The developed model was based on the conservation of mass and volume principle including shrinkage expansion coefficient (f). Using this model, the author presented values of f during drying of selected food materials. The model assumed that food materials can be considered as multiphase systems (i.e., gas-liquid-solid).[1] The material density was calculated using the known equation presented by Choi and Okos:[2]

(1)

where r_m is material density, r_T is true density, X_i and r_i are the mass fraction and density of food components, respectively. The above equation assumed there is no pore exists and no interactions occur between the components. The mass fraction of each component at any given water content during drying was estimated as

(2)

where M is the mass ratio at any moisture content relative to the initial mass before drying. The values of M was estimated from the solids balance as:

(3)

where X_w ^o is the initial mass fraction of water before drying and X_w is mass fraction of water at any time during drying. Later M, r_m (i.e., Eq (1) and (3) and r_w were used in the development of the shrinkage-expansion coefficient. Subsequently, Eq. (1) was used for the calculation of material density during drying. Equation (1) proposed by Choi and Okos[2] includes all food components with water contest. During drying, the water content decreases and, hence, the value of M will be always lower than unity. The mass fraction of other food components will increase during drying, and it can be estimated using Eq. (2). Equation (2), however, cannot be used to calculate the mass fraction of water in food during drying. The author did not elaborate how Eq. (1-3) should be used for the calculation of material density. In order to make it clear and consistent, Eq. (1) should be rewritten as

(4)

The mass fraction of any component except water during drying can be estimated using Eq. (2) and (3). The values then can be put into Eq. (4) with the mass fraction of water and mass density of individual components to estimate the material density during drying.