Moskva, Moscow, Russian Federation
Russian Federation
Current technological aspects of lactose crystallization are considered. A promising lactose crystallization method involving simulation seed crystals is reported. Advanced engineering solutions for continuous crystallization using spraying in vacuo and scraped-surface heat exchangers are presented.
lactose, heterogeneous and homogeneous crystallization, simulation seed crystals, continuous crystallization of lactose, scraped-surface plate-type cooling crystallizer, vacuum spray crystallizer.
INTRODUCTION
Lactose crystallization is among the necessary technological operations in the production of sweetened canned condensed milk. This operation consists of creating appropriate conditions for extensive formation of crystallization centers and for subsequent controllable crystal growth at certain processing parameters and under post-production storage conditions.
The lactose crystallization kinetics can be described in terms of a C = f(t) function, where С is the lactose concentration in the solution (%) and t is time (s). The corresponding crystallization curve can conventionally be divided into the following three segments: induction period, in which C = const; rapid increase in the concentration with time; slow variation of the concentration at the late stages of the process.
The first period is characterized by the formation of crystal nuclei; the second and third periods, by their growth. A nucleus (crystallization center) is the minimum amount of a new phase that is capable of independently existing [1, 10]. Once stable nuclei have formed in the new phase, they begin to grow. The main processes determining the crystal growth rate are the diffusion of the constituent particles to the surface of the growing crystal and their incorporation in the crystal lattice. In turn, the latter process includes the adsorption of particles by the surface, their migration on the surface, and their incorporation in the lattice as such.
The factors on which the crystal growth rate depends are the solution temperature, stirring intensity, the presence of impurities, degree of supersaturation, viscosity, etc. The effect of a given factor depends on crystallization conditions. For example, the variation of the crystal growth rate with the degree of supersaturation depends on whether the solution is stirred or not. On the whole, the value of supersaturation is so significant that its variation alters the growth mechanism [2-4, 9-11]. A large number of crystal growth theories have been devised to account for the complicated dependence of the growth rate on various factors. However, there is still no unified theory completely describing the multiformity of the crystallization process.
Contributions to classical crystallization theory were made by J. Gibbs, M. Volmer, W. Kossel, I.N. Stranskoi, and R. Kaishev. The theory is based on the thermodynamic conception that an isolated system is absolutely stable when its entropy is invariable [5].
The present-day technologies of production of sweetened condensed dairy products necessarily include the introduction of seed crystals for preventing the consistency flaws arising from uncontrolled lactose crystallization. The seeds used in these technologies are microcrystalline lactose with a crystal size of 2-3 μm, supersaturated solutions or suspensions of lactose containing crystal nuclei, and water-soluble crystalline macromolecular organic compounds mixed with lactose [6, 7].
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2. Polyanskii, K.K. and Shestov, A.G., Kristallizatsiya laktozy: fiziko-khimisheckie osnovy (Lactose Crystallization: Physicochemical Foundations), Voronezh: Voronezh. Gos. Univ., 1995.
3. Gnezdilova, A.I., Razvitie nauchnykh osnov kristallizatsii laktozy i sakharozy v mnogokomponentnykh vodnykh rastvorakh (Development of the scientific foundations of lactose and sucrose crystallization from multicomponent aqueous solutions), Doctoral (Eng.) Dissertation, Vologda, 2000.
4. Polyanskii, K.K., Kristallizatsiya laktozy v proizvodstve molochnykh produktov (Lactose crystallization in the production of dairy products), Doctoral (Eng.) Dissertation, Moscow, 1981.
5. Stranskii, I.N. and Kaishev, R., K teorii rosta kristallov i obrazovaniya kristallicheskikh zarodyshei (On the theory of crystal growth and nucleation), Uspekhi khimii (Russian chemical reviews), 1939, vol. 21, no. 4, pp. 408-465.
6. Khramtsov, A.G., Molochnyi sakhar (Lactose), Moscow: Agropromizdat, 1987.
7. Chekulaeva, L.V., Polyanskii, K.K., and Golubeva, L.V., Tekhnologiya produktov konservirovaniya moloka i molochnogo syr’ya (Production of Canned Milk and Dairy Products), Voronezh: Voronezh. Gos. Univ., 1996.
8. Golubeva, L.V., Chekulaeva, L.V., and Polyanskii, K.K., Khranimosposobnost’ molochnykh konservov (Storability of Canned Dairy Products), Moscow: DeLi Print, 2001.
9. Shabalin, V.N. and Shatokhina, S.N., Morfologiya biologicheskikh zhidkostei cheloveka (Morphology of Human Biological Fluids), Moscow: Khrizostom, 2001
10. Rosenberger, F., Fundamentals of Crystal Growth, Berlin: Springer, 1979.
11. Hartel, R.W., Crystallization in Foods, New York: Aspen, 2001.
