FNH 200 Chapter 8: Preservation-Dehydration
1 Feb 2019
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Ch 8: Dehydration as a Preservation Method
November 25, 2017
3:03 PM
8.1 Intro
• Based on principle that microbial growth, chemical/enzymatic reactions occur only when free
water is present
• Aw of dehydrated foods: 0.2-0.6
• Chemical reactions(Maillard browning) occur at 0.3
• MO not killed, resume growth with rehydrated
Dehydration: physical removal of as much water as possible
Concentration : some water removed, not inherently shelf stable, need other preservation
methods
8.2 Changes in Food during Dehydration
Cell/tissue Shrinkage
• Water moves from interior to surface, takes water soluble substances too, evaporates
• Loss of water soluble substances decreases attraction of water into food
Case hardening
• Rapid drying cause compounds( sugars_ to form hard, impermeable case around food
• Result in decrease in dehydration rate
• Occur in high sugar products
• Dehydration minimize case hardening
Chemical Changes
• In warm air drying agent
• Maillard reactions occur fast when water content is 15-20%
o Reactants more concentrated after water removed, closer proximity, increase reaction
probability
o Drying systems designed to remove water thru 15-20% range as fast as possible, minimize
maillard reaction
• poor rehydration occur b/c loss of ability of hydrophilic food to absorb water
• Heat denaturation of proteins, starches, gums can decrease water holding capacity
• Dehydrated foods sweeter, sugars concentrationed on surface
8.3 Factors Affecting Dehydration
Food Composition
• More difficult if water bound to solutes in food
• Porosity: enhanced->faster drying rate
Surface Area
• Maximize SA to V ratio
Temp
Document Summary
8. 1 intro: based on principle that microbial growth, chemical/enzymatic reactions occur only when free water is present, aw of dehydrated foods: 0. 2-0. 6, chemical reactions(maillard browning) occur at 0. 3, mo not killed, resume growth with rehydrated. Dehydration: physical removal of as much water as possible. Concentration : some water removed, not inherently shelf stable, need other preservation methods. Cell/tissue shrinkage: water moves from interior to surface, takes water soluble substances too, evaporates. Loss of water soluble substances decreases attraction of water into food. Case hardening: rapid drying cause compounds( sugars_ to form hard, impermeable case around food, result in decrease in dehydration rate, occur in high sugar products, dehydration minimize case hardening. Food composition: more difficult if water bound to solutes in food, porosity: enhanced->faster drying rate. Surface area: maximize sa to v ratio. Temp: higher temp-> faster, upper limit temp established. Humidity of drying air: drier->more moisture can absorb.
