Freeze Drying Tech and Uses Corey Behrens April 22, 2007

Freeze Drying Applications Food preservation method Food preservation method Lowers water activity Lowers water activity Reduces potential for microbial growth Reduces potential for microbial growth Prevent Browning/Degradation Prevent Browning/Degradation Maillard and Carmelization Maillard and Carmelization Heat sensitive products Heat sensitive products Long Shelf Life Long Shelf Life

Freeze Dried Products Fruits/Vegetables Fruits/Vegetables Peas Peas Tomatoes Tomatoes Cherries Cherries Berries Berries Other food products Other food products Ice Cream Ice Cream Spaghetti Spaghetti Coffee Coffee Pharmaceuticals Pharmaceuticals Pets Pets Dogs Dogs Squirrels Squirrels

Effective Method Characteristics Factors for efficiency of drying processes Factors for efficiency of drying processes Heat and Mass transfer considerations Heat and Mass transfer considerations Maximum P vap gradient Maximum P vap gradient Maximum ΔT between air and interior of product Maximum ΔT between air and interior of product High convective coefficients at surface High convective coefficients at surface

Lyophilization : How it works Reducing product temp Reducing product temp Majority of product moisture in solid state Majority of product moisture in solid state Decrease ambient pressure Decrease ambient pressure Sublimation (H 2 O evaporation from solid to gas) Sublimation (H 2 O evaporation from solid to gas) Carried out over vacuum Carried out over vacuum maintain P vap gradient between the ice front in the material and the surrounding environment maintain P vap gradient between the ice front in the material and the surrounding environment Apply heat to aid sublimation Apply heat to aid sublimation

State Diagram for H 2 O

Heat Transfer Two possibilities: Two possibilities: Through frozen product layer Through frozen product layer H.T. Rapid, not limiting H.T. Rapid, not limiting Through dry product layer Through dry product layer Slow Slow Low thermal conductivity of highly porous structure Low thermal conductivity of highly porous structure

Mass Transfer Occurs in dry product layer Occurs in dry product layer Diffusion of water vapor is rate-limiting Diffusion of water vapor is rate-limiting Low molecular diffusion in vacuum Low molecular diffusion in vacuum

Drying Rate Drying time equation for moisture diffusion limited cases: Drying time equation for moisture diffusion limited cases: t = [(RT A L 2 )/(8DMV W P i -P a )]*(1+4D/k m L) t = [(RT A L 2 )/(8DMV W P i -P a )]*(1+4D/k m L) L = thickness L = thickness T A = absolute Temp T A = absolute Temp M = molecular weight M = molecular weight V W = specific volume V W = specific volume P i = P vap of ice P i = P vap of ice P a = P vap of air at condenser surface P a = P vap of air at condenser surface k m = mass transfer coefficient k m = mass transfer coefficient D = diffusivity [=] L 2 /t D = diffusivity [=] L 2 /t R = universal gas constant R = universal gas constant

The Process

Primary Considerations Advantages Advantages Higher quality product Higher quality product Does not form ice crystals that disrupt the food matrix of the product. Does not form ice crystals that disrupt the food matrix of the product. I.e. freezing fruit I.e. freezing fruit When crystals grow, cell wall breakage When crystals grow, cell wall breakage Result? Higher quality product Result? Higher quality product Disadvantages Disadvantages High cost of product/process High cost of product/process Energy intensive in comparison to other drying methods Energy intensive in comparison to other drying methods

References drying1.htm drying1.htm drying1.htm drying1.htm Singh, R. Paul and Dennis Heldman. Introduction to Food Engineering. Academic Press, Boston pp Singh, R. Paul and Dennis Heldman. Introduction to Food Engineering. Academic Press, Boston pp eezedrying.php eezedrying.php eezedrying.php eezedrying.php osci/lyophilisation/images/fdstatic.jpg osci/lyophilisation/images/fdstatic.jpg osci/lyophilisation/images/fdstatic.jpg osci/lyophilisation/images/fdstatic.jpg