Lecture_10___milk_drying

Lecture_10___milk_drying - Drying in the Dairy Industry...

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1 Drying in the Dairy Industry Carmen I. Moraru Cornell University Dehydration of foods Dehydration - Represents the nearly complete removal of (free) water from foods – typically to a final moisture content of less than 5% - Used as a preservation technique, and also to ease the transportation and handling (lower volume) Dairy products that are typically dried: - Milk, skim milk, whey, cream, ice cream mix, protein concentrates, infant foods
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2 How is drying achieved? Triple point of water (0.0098°C and 0.64 kPa = 4.8 mm Hg) Sublimation Vaporization Gas Liquid Solid Temperature Pressure Types of drying - Air and contact drying under atmospheric pressure - heat is transferred through the drying material either from heated air or from heated surfaces. Water vapors are removed by air. - Vacuum drying - water evaporation occurs easier at lower pressures. Heat transfer occurs generally by conduction, sometimes by radiation. - Freeze drying - the water vapors are sublimated from a frozen material. In case of foods, the advantage of this method is that food quality is better maintained.
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3 Vapor properties relevant to drying Absolute pressure Latent heat of vaporization Saturation temperature (kPa) (kJ kg -1 )( ° C ) 1 2485 7 2 2460 18 5 2424 33 10 2393 46 20 2358 60 50 2305 81 100 2258 99.6 101.35 (1 atm) 2257 100 110 2244 102 120 2238 105 200 2202 120 500 2109 152 Note: due to the high energy consumption and the high operation cost of drying, water is partially removed in a prior concentration step. Factors that control drying - Transfer of heat to provide the necessary latent heat of vaporization: - Mass transfer of water vapors through the material and then their removal from the dried food: ) P P ( RT M ' h m VA 0 V V v = Where: h = heat transfer coefficient h’ = mass transfer coefficient λ = latent heat of vaporization Moist product surface P V0 , T 0 v m q Air stream P VA , T A V 0 A m ) T T ( h q λ = =
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4 Technical challenge in drying of foods - Prevent the product from undergoing undesirable changes, such as: - Insolubilization of proteins - Maillard reactions 10 20 30 0 % Water Relative reaction rate Rate of protein insolubilization in concentrated skim milk at high temperature Rate of Maillard reaction Note: drying also causes inactivation of enzymes and killing of microorganisms (though cannot be used for effective killing of microorganisms).
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This note was uploaded on 05/02/2009 for the course FDSC 4250 taught by Professor Moraru during the Spring '09 term at Cornell.

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Lecture_10___milk_drying - Drying in the Dairy Industry...

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