FST 151 FOOD FREEZING FOOD SCIENCE AND TECHNOLOGY 151 Special topics: Freeze Concentration

1. FST 151 FOOD FREEZING FOOD SCIENCE AND TECHNOLOGY 151 Special topics: Freeze Concentration Lecture Notes Prof. Vinod K. Jindal (Formerly Professor, Asian Institute of Technology) Visiting Professor Chemical Engineering Department Mahidol University Salaya, Nakornpathom Thailand Freeze Concentration - Vinod Jindal

2. Concentration of liquid foods • Concentration of liquid foods is a vital operation in many food processes. Concentration is different from dehydration. Generally, foods that are concentrated remain in the liquid state, whereas drying produces solid or semisolid foods with significantly lower water content. Freeze Concentration - Vinod Jindal

3. Liquid Concentration Technologies • Several technologies are available for liquid concentration in the food industry, with the most common being evaporation and membrane concentration. Freeze concentration is another technology that has been developed over the past few decades, although significant applications of freeze concentration of foods are limited. Freeze Concentration - Vinod Jindal

4. Freeze Concentration • Water is partially frozen to produce an ice crystal slurry in concentrated product. Separation of ice crystals is then accomplished using some washing technique. Current applications of freeze concentration are limited to fruit juices, coffee, and tea extracts, and beer and wine. Freeze concentration produces a superior product. Freeze Concentration - Vinod Jindal

5. Freeze Concentration Freeze concentration of liquid foods involves the fractional crystallization of water to ice and subsequent removal of the ice. Removing of ice using mechanical separation technique or washing columns. The degree of concentration achieved is higher than in membrane process but lower than concentration by boiling Crystallization requires 151kJ/kg, where the evaporation requires 2055kJ/kg water.  Disadvantages: High refrigeration cost, high capital cost, high operating cost, low production rate. Advantages: process at low temperature, High retention of volatile aroma Freeze Concentration - Vinod Jindal

6. In freeze concentration it is desirable for ice crystals to grow as large as is economically possible, to reduce the amount of concentrated liquor entrained with the crystals. This is achieved in a paddle crystallizer by slowly stirring a thick slurry of ice crystals and allowing the large crystals to grow at the expense of smaller ones. Freeze Concentration - Vinod Jindal

7. Definition of Freeze Concentration • A liquid food is cooled with sufficient agitation. Ice crystals nucleate and grow, and a slurry of relatively pure ice crystals is removed. Separation of these pure ice crystals leaves a concentrated product. Freeze Concentration - Vinod Jindal

8. Advantages & Disadvantages of Freeze Concentration • High product quality due to low-temperature operation • Absence of a vapor-liquid interface maintaining original flavors. • Higher cost of than the other two methods (evaporation and membrane separation). Freeze Concentration - Vinod Jindal

9. Employed on Wide Range of Products • Fruit juices, milk products, vinegar, coffee and tea extracts, beer and wine, and other flavor products. • Concentration of alcoholic beverages is one application where freeze concentration is superior to other techniques. Freeze Concentration - Vinod Jindal

10. Equipment for freeze concentration A direct freezing system or direct equipment to freeze the liquid food A mixing vessel to allow the ice crystal to grow A separator to remove the crystals from the concentrated solution Freeze Concentration - Vinod Jindal

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13. Freezing-point Depression • Products containing low-molecular weight compounds, like sugars and salts, experience a reduction in freezing point as product is concentrated. Freeze Concentration - Vinod Jindal

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17. Ice-concentrate separation Ice-concentrate separation is achieved by centrifugation, filtration, filter pressing or wash columns. Wash columns operate by feeding the ice-concentrate slurry into the bottom of a vertical enclosed cylinder. The ice crystals are melted by a heater at the top of the column and some of the melt water drains thought the bed of ice crystals to remove entrained concentrate. Max. obtainable concentration using this method is 45oB TSS. Freeze Concentration - Vinod Jindal

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20. Crystallization - Introduction • Crystallization refers to the formation of solid crystals from a homogeneous solution. • It is a solid-liquid separation technique • Used to produce • Sodium chloride • Sucrose from a beet solution • Desalination of sea water • Fruit juices by freeze concentration • Crystallisation requires much less energy than evaporation • e.g. water, enthalpy of crystallisation is 334 kJ/kg and enthalpy of vaporization is 2260 kJ/kg Freeze Concentration - Vinod Jindal

21. Saturation • An unsaturated or undersaturated solution can dissolve more solute. • A saturated solution is one which contains as much solute as the solvent can hold. • A supersaturated solution contains more dissolved solute than a saturated solution, i.e. more dissolved solute then can ordinarily be accommodated at that temperature. • Two forms of supersaturation: • Metastable – just beyond saturation • Labile – very supersaturated • Crystallization is normally operated in the metastable region. Freeze Concentration - Vinod Jindal

22. Solubility curve – sucrose Ref: http://www.nzifst.org.nz/unitoperations/conteqseparation10.htm Freeze Concentration - Vinod Jindal

23. Solubility curve - Saturation diagram Supersaturated Or Labile Concentration Kg solute/100kg solvent Metastable Stable Temperature Stable zone – crystallisation not possible Metastable zone MSZ – crystallisation possible but not spontaneous Labile – crystallisation possible and spontaneous We need a supersaturated solution for crystallisation Freeze Concentration - Vinod Jindal

24. Crystallization Technique • In freeze concentration, crystallization is achieved by • Cooling a solution • If supersaturation is a function of temperature Freeze Concentration - Vinod Jindal

25. Nucleation • Crystallization starts with Nucleation • There are two types of nucleation – Primary and Secondary • Primary relates to the birth of the crystal, where a few tens of molecules come together to start some form of ordered structure • Secondary nucleation can only happen if there are some crystals present already. It can occur at a lower level of supersaturation than primary nucleation. • Often, industrial crystallizers jump straight to secondary nucleation by ‘seeding’ the crystallizer with crystals prepared earlier. Freeze Concentration - Vinod Jindal

26. Supersaturation and Crystal Growth • For low supersaturation primary nucleation is not widespread. Secondary nucleation on existing crystals is more likely. Result is small numbers of large crystals. • For high supersaturation primary nucleation is widespread. This results in many crystals of small size. • Slow cooling with low supersaturation creates large crystals. • Fast cooling from high supersaturation creates small crystals. • Agitation reduces crystal size by creating more dispersed nucleation. Freeze Concentration - Vinod Jindal

27. Seeding • The type or quality of seed used can influence the crystallization process. • Good seed results in a good crystallization, i.e. a particle size distribution that does not include fines. • Bad seed can increase the amount of fines produced. • Good and bad can be defined by the seed crystal size. Freeze Concentration - Vinod Jindal