Development of a rational cooling regime in milk sugar production

ABSTRACT

Introduction: Lactose crystallization is a key stage in milk sugar production, determining both yield and product quality. It takes place during cooling of the crystallizate and is associated with substantial lactose losses (70–75% of total process losses). In industrial practice, cooling regimes are mainly selected empirically and are not based on quantitative relationships describing crystallization kinetics and changes in supersaturation, which leads to increased losses and non-uniform crystal size.

Purpose: To develop a theoretically justified cooling regime for the crystallizate based on a mathematical model of the cooling rate that accounts for lactose crystallization kinetics and the temperature dependence of lactose solubility, and to experimentally verify the proposed regime under pilot-scale conditions.

Materials and Methods: The object of the study was lactose crystallization in concentrated ultrafiltrate of cheese whey with a total solids content of 55–60%. The crystallizate, crystals, and intercrystalline solution (mother liquor) obtained after centrifugation were analysed. Total solids in the syrup and mother liquor were determined using an RL-3 refractometer; lactose content was measured polarimetrically according to GOST R 54667. The mean crystal size and particle size distribution were assessed microscopically according to GOST 33567 using an OLYMPUS CX31 microscope and ToupView software; all experiments were performed in triplicate. The theoretical part included the analytical derivation of a cooling rate equation based on lactose crystallization kinetics and mathematical modelling.

Results: It was shown that the technological parameters of the crystallizate during cooling must be aligned with crystal growth conditions and prevent the formation of new nuclei, which is achieved when the cooling rate matches the crystallization rate of the supersaturated solution. An equation for the cooling rate was obtained that incorporates lactose crystallization rate, mass fractions of total solids and crystals, and saturation and supersaturation coefficients, and on this basis a stepwise cooling regime (in terms of both rate and temperature) was developed. Its application increased the mean crystal size to 275.5 µm (39% above the control), the uniformity coefficient to 0.79 (11.3% above the control), reduced lactose losses in the mother liquor by 7.5%, and increased crystal yield to 41.3% (8% above the control). The regime was tested in the experimental plant of JSC “Training and Experimental Dairy Plant of Vologda State Dairy Farming Academy”.

Conclusions: The results provide a basis for a scientifically grounded choice of time–temperature parameters of lactose crystallization when scaling the process up to industrial conditions.

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