Ch 20

 During dehydration;  1. Water- transfer from the food into the dehydrating environment  2. Heat- transferred to the food ( promoting water removal from the food)

 DH foods have lower water activity (a w >  Not enough free water for :  Microbial Growth ( needs a w > 0.93)  Staphylococcus aureus (a w > 0.85) PATHOGEN!  Mold ( need a w >0.6)  Enzymatic reactions  Chemical reactions ( eg) Mailllard browning) at a w >0.3  BUT…  Microorganisms are not killed  MO will resume growth after food is rehydrated This is an exception of food pathogens

ProductaWaW Skim milk powder Dehydrated soups 0.03 noodles0.5 Fruit: berries, apples, apricots, raisins ( grapes)0.6 Fruit juice concentrates, condensed milk0.8

 Both involve physical removal of water from food  Dehydration : removal of as much water from the food as possible  Imparts a long storage life  Concentration: some of the water is removed from the food  Concentrates the food constituents  Foods are not fully shelf stable…. Other forms of processing are needed (can/freeze)  Fruit juices, canned soup, jams, evaporated mik, condensed milk, maple syrup

 Cell/ tissue shrinkage  Case hardening  Chemical changes

 shrinkageWater migrates- interior of the food surface  Evaporates by the drying medium  Water carries with it water soluble substances dissolved in it  Shrinkage

 Case hardening  Rapid drying- compounds (such as sugar) form a hard, impermeable “case” around the food piece  Can slow down the dehydration  Common in high sugar products  Tropical and temperate fruit products

 Chemical changes  Browning and flavor changes due to reactions  Maillard browning = from increased solution concentration  Denaturation of proteins, aggregation of polysaccharides  Loss of water soluble binding capacity  Loss of water soluble components or  Concentration on the surface of the food ( case hardening)  Loss of volatiles ( especially flavor compounds) NOTE: Not all dehydrated foods show these changes it depends on the method of drying

 Look at differences in  Appearance  Texture  Flavor ( aroma and taste)  4.5 # of grapes = 1 lb raisins  Which is sweeter?  Which is shelf stable at room temperature?  Why?

 Surface area  Smaller food piece, more rapid the rate of moisture loss  Temperature  Increase in temperature will increase the dehydration rate  Air velocity  Maximize velocity of heated air moving around the food particles  Humidity of drying air  The drier the air, the moisture it can absorb  % RH (relative humidity) of the drying air determines the final moisture content of food

 Atmospheric pressure and vacuum  Water boils at 100 o C ( at a pressure of 1 atm= 760Hg)  At lower pressure the boiling temperature will decrease  Eg. Under vacuum, water will boil at 32 o C  Important for heat sensitive food products  ONLY USED FOR FREEZE DRYING

 Sun dry  Spray drying  Air ( Tray) Drying  Drum Drying  Freeze drying

 Dry, warm climates  Slow drying method ( several days)  Fruits, vegetables, fish  Inexpensive  Invasion by insects, birds, rodents, microorganisms

Hot dry air 347 o F Liquid food Small droplets; rapid dehydration Exit moist air Dry food collection Air cyclone seperator Food must be liquid Fastest method to dehydrate Height of chamber is based on time needed to dehydrate given food Used for milk, eggs, protein powders, Flavorings, and coffee

Trays or racks Heated air ( o F) at set velocity and with a low %Relative humidity Poor/ moderate rehydration properties ( case hardening) Food shrinkage, dense product, shape not held Quick, inexpensive Small scale hrs needed Ex) Pasta, vegetables, fruit, herbs and spices

248 o F Food is more likely to be flakey Ex) purees, pastas and mashed foods Mashed potatoes, tomato pastes Size of drum and speed of rotation is determined by drying time needed

 Food must be in frozen state  Vacuum chamber  Heat from radiant heaters- energy  Water will sublime:  Water ( solid)  water (vapor)  No transition through a liquid state (no evaporation)

Voids from ice crystals when water sublimes ( act as channels) Excellent rehydration No translocation of water soluble constituents to the surface Shape of food is retained (no shrinkage) food closely resembles starting material very expensive– used for high value foods Instant soups, high quality coffee, vegetables, military rations, space food

 Protect against moisture absorption  Dehydrated foods- hygroscopic  Prevent transmission of water vapor  Physical protection  Prevents crushing  Freeze dried foods- porous structures with spaces; easily crushed  Protection from oxygen and light  photo- oxidation