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

Freeze Concentration - Vinod Jindal

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FST 151FOOD FREEZINGFOOD SCIENCE AND TECHNOLOGY 151

Special topics: Freeze ConcentrationLecture Notes

Prof. Vinod K. Jindal(Formerly Professor, Asian Institute of Technology)Visiting ProfessorChemical Engineering DepartmentMahidol UniversitySalaya, NakornpathomThailand


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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.


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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.


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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.


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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


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• 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.


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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.


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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).


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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.


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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


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Freezing-point Depression

• Products containing low-molecular weight compounds, like sugars and salts, experience a reduction in freezing point as product is concentrated.


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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.


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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


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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.


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Solubility curve – sucrose

Ref: http://www.nzifst.org.nz/unitoperations/conteqseparation10.htm


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Solubility curve - Saturation diagram

Temperature

Con

cent

ratio

nK

g so

lute

/100

kg so

lven

tSupersaturatedOr Labile

Metastable

Stable

Stable zone – crystallisation not possibleMetastable zone MSZ – crystallisation possible but not spontaneousLabile – crystallisation possible and spontaneousWe need a supersaturated solution for crystallisation


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Crystallization Technique

• In freeze concentration, crystallization is achieved by– Cooling a solution

• If supersaturation is a function of temperature


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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.


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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.


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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.

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

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