FST 151

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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
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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/conte
qseparation10.htm
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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
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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.