Analysis of the moisture evaporation process during vacuum freeze-drying of koumiss and shubat

Abstract

The equation for the calculating of a moisture evaporation rate in the vacuum freeze-drying, wherein as a driving force instead of the generally accepted in the drying theory of ∆t temperature difference, ∆p pressure difference, ∆c concentration difference, a difference of water activity in the product and the relative air humidity ($a_{\text{w}} - \varphi$) is suggested. By using the proposed equation, the processes of vacuum freeze-drying of koumiss and shubat were analyzed, and it was found two drying periods: constant and falling. On the first drying period, a moisture evaporation rate of koumiss is j = 2.75 × 10⁻³ kg/(m² h) and of shubat is j = 2.37 × 10⁻³ kg/(m² h). On the second period, values decrease for koumiss from j = 2.65 × 10⁻³ kg/(m² h) to j = 1.60 × 10⁻³ kg/(m² h), and for shubat from j = 2.25 × 10⁻³ kg/(m² h) to j = 1.62 × 10⁻³ kg/(m² h). Specific humidity for koumiss is u_eq = 0.61 kg/kg and for shubat is u_eq = 0.58 kg/kg. The comparative analyze of the experimental data of the moisture evaporation rate versus the theoretical calculation shows that the approximation reliability is R² = 0.99. Consequently, the proposed equation is useful for the analyzing a moisture evaporation rate during a vacuum freeze-drying of dairy products, including cultured milk foods.