Lecture_11___crystallization - Crystallization in the Dairy...

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1 Crystallization in the Dairy Industry Carmen I. Moraru Cornell University Crystallization in food/dairy processing - Crystal: a material in solid state, in which the building atoms or molecules are closely packed in a repeating pattern (lattice), and the free energy of the system is at a minimum. - Crystallization: a separation process in which a solute is transferred from a solution into a pure, solid crystal phase. - Crystallization occurs frequently in food processing; it can involve water, sugars, triglycerides and other minor components. - Has a significant impact on food properties such as consistency, mouth feel and physical stability. - In dairy foods, crystallization of lactose has the most technological importance.
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2 How does crystallization occur? - Soluble components can be removed from solution by: - lowering the temperature - concentrating the solution until the solution becomes supersaturated and the excess solute crystallizes out in a pure form. - Crystallization ends when an equilibrium is established between the crystals and the surrounding solution (mother liquor). Thermodynamics of crystallization - In order to induce crystallization, heat must be removed from the system: Heat removed = sensible heat to cool the solution + heat of crystallization - When using evaporation to achieve supersaturation, the heat of vaporization must also be taken into account. - For most food materials, the heat of crystallization is positive, i.e. heat is given out during crystallization.
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3 Stages of crystallization - Crystallization occurs as a result of two processes: nucleation and crystal growth. - Crystallization requires the presence of nuclei on which the crystals will subsequently grow. - Nucleation can be homogeneous and heterogeneous. - If no nuclei are present, crystal growth cannot occur and the precipitated material (solute) will form an amorphous glass. Factors that affect nucleation - Three main factors: - temperature - degree of supersaturation - interfacial tension between the solute and the solvent - If supersaturation is: - high , further nucleation takes place and the growth of existing crystals is limited small crystals - low , nucleus formation is not encouraged, but the available nuclei will continue to grow and large crystals will result - In practice, slow cooling, which maintains a low level of
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Lecture_11___crystallization - Crystallization in the Dairy...

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