Freeze Drying Tech and Freeze Drying Tech and UsesUses

Corey BehrensCorey Behrens

April 22, 2007April 22, 2007

Freeze Drying ApplicationsFreeze Drying Applications

Food preservation methodFood preservation method Lowers water activityLowers water activity Reduces potential for microbial growthReduces potential for microbial growth Prevent Browning/DegradationPrevent Browning/Degradation

Maillard and CarmelizationMaillard and Carmelization Heat sensitive productsHeat sensitive products

Long Shelf LifeLong Shelf Life

Freeze Dried ProductsFreeze Dried Products

Fruits/VegetablesFruits/Vegetables PeasPeas TomatoesTomatoes CherriesCherries BerriesBerries

Other food productsOther food products Ice CreamIce Cream SpaghettiSpaghetti CoffeeCoffee

PharmaceuticalsPharmaceuticals PetsPets

DogsDogs SquirrelsSquirrels

Effective Method Effective Method CharacteristicsCharacteristics

Factors for efficiency of drying Factors for efficiency of drying processesprocesses Heat and Mass transfer considerationsHeat and Mass transfer considerations Maximum PMaximum Pvapvap gradient gradient Maximum Maximum ΔΔT between air and interior of T between air and interior of

productproduct High convective coefficients at surfaceHigh convective coefficients at surface

Lyophilization : How it worksLyophilization : How it works

Reducing product tempReducing product temp Majority of product moisture in solid stateMajority of product moisture in solid state

Decrease ambient pressureDecrease ambient pressure Sublimation (HSublimation (H22O evaporation from solid to O evaporation from solid to

gas)gas) Carried out over vacuumCarried out over vacuum maintain Pmaintain Pvapvap gradient between the ice front in gradient between the ice front in

the material and the surrounding environment the material and the surrounding environment Apply heat to aid sublimationApply heat to aid sublimation

State Diagram for HState Diagram for H22OO

Heat TransferHeat Transfer

Two possibilities:Two possibilities: Through frozen product layerThrough frozen product layer

H.T. Rapid, not limitingH.T. Rapid, not limiting Through dry product layerThrough dry product layer

SlowSlow Low thermal conductivity of highly porous Low thermal conductivity of highly porous

structurestructure

Mass TransferMass Transfer

Occurs in dry product layerOccurs in dry product layer Diffusion of water vapor is rate-Diffusion of water vapor is rate-

limitinglimiting Low molecular diffusion in vacuumLow molecular diffusion in vacuum

Drying RateDrying Rate

Drying time equation for moisture diffusion Drying time equation for moisture diffusion limited cases:limited cases:

t = [(RTt = [(RTAALL22)/(8DMV)/(8DMVWWPPii-P-Paa)]*(1+4D/k)]*(1+4D/kmmL)L)

L = thicknessL = thickness TTAA = absolute Temp = absolute Temp M = molecular weightM = molecular weight VVWW = specific volume = specific volume PPii = P = Pvapvap of ice of ice PPaa = P = Pvapvap of air at condenser surface of air at condenser surface kkmm = mass transfer coefficient = mass transfer coefficient D = diffusivity [=] LD = diffusivity [=] L22/t/t R = universal gas constantR = universal gas constant

The ProcessThe Process

Primary ConsiderationsPrimary Considerations

AdvantagesAdvantages Higher quality productHigher quality product Does not form ice crystals that disrupt the food Does not form ice crystals that disrupt the food

matrix of the product.matrix of the product. I.e. freezing fruitI.e. freezing fruit

When crystals grow, cell wall breakageWhen crystals grow, cell wall breakage Result? Higher quality productResult? Higher quality product

DisadvantagesDisadvantages High cost of product/processHigh cost of product/process Energy intensive in comparison to other drying Energy intensive in comparison to other drying

methodsmethods

ReferencesReferences

http://home.howstuffworks.com/freeze-dryihttp://home.howstuffworks.com/freeze-drying1.htmng1.htm

Singh, R. Paul and Dennis Heldman. Singh, R. Paul and Dennis Heldman. Introduction to Food EngineeringIntroduction to Food Engineering. . Academic Press, Boston. 2001. pp567-8.Academic Press, Boston. 2001. pp567-8.

http://www.foodsci.wisc.edu/courses/fs532http://www.foodsci.wisc.edu/courses/fs532/12freezedrying.php/12freezedrying.php

httphttp://www.cheng.cam.ac.uk/research/groups/://www.cheng.cam.ac.uk/research/groups/biosci/lyophilisation/images/fdstatic.jpgbiosci/lyophilisation/images/fdstatic.jpg