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Water occur :• - as an intracellular or extracellular
component in vegetable and animal products, - a dispersing medium or solvent in a variety of products,
• - the dispersed phase in some emulsified products such as butter and margarine,
• -as a minor constituent in other foods.
an understanding of water properties and behavior is necessary.
• The presence of water influences the chemical and microbiological deterioration of foods.
• removal (drying) or freezing of water is essential to some methods of food preservation.
• Water consists of an oxygen atom bound to two hydrogen atoms by two single covalent bonds.– Oxygen has unpaired &
paired electrons which gives it a slightly negative charge while Hydrogen has no unpaired electrons and shares all others with Oxygen
– Leaves molecule with positively and negative charged ends
A water molecule is a polar molecule with opposite ends of the molecule
with opposite charges.
5
slightly positive charge
slightly negative charge
hydrogen bond between (+) and (-) areas of different water molecules
Water molecules form Hydrogen bonds
The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond.
– Each water molecule – can form hydrogen
bonds with up to four neighbors.
• Extraordinary Properties that
are a result of hydrogen bonds.– Cohesive behavior– Resists changes in temperature– High heat of vaporization– Expands when it freezes– Versatile solvent
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Uses of Water in Food Prep• Universal solvent• Heat transfer• Freezing• Cleansing agent• Promotion of chemical changes
– Ionization of salt– Baking powder
• Water and pH• Hydrolysis reactions
• TYPES OF WATER
3 Types of water :
1.Langmuir or monolayer water (Aw 0-0.2)
2.Capillary water (0.3 - 0.8)
3. loosely bound water (aw 0.8 to 1.0)
• Bound water :
1. Bound water is the water that remains unfrozen at some prescribed temperature below O0C, usually -2O0C.
2. Bound water is the amount of water in a system that is unavailable as a solvent.
Water Activity
• Water activity– Ratio of the vapor pressure of water in a food
at a specified temperature to the vapor pressure of pure water at same temperature
• Foods more perishable if higher water activity– Microorganisms need water!– To reduce water activity• Dry• Freeze• Add sugar or salt
Water Activity Definition• Water Activity is a measure of the energy
status of the water in a system.
• Old definition: Water activity is the amount of “free” or “available” water in a product as opposed to “bound” water.
• “Free”: same physical properties as pure water
• “Bound”: different physical properties
Water Activity Definition
• Equilibrium Conditions
aw Component A = aw Component B = aw air
• At equilibrium the energy status (chemical potentials) are the same and thus no net exchange of water.
µcracker = µcheese = µair
Water Activity Definition
aw = ————————————————
aw = p/po = %ERH/100
Range: 0 (bone dry) — 1.0 (pure water)
Vapor pressure of water above sample @ °C
Vapor pressure of pure water @ same °C
Water Activity Definition Experiment - Place together in a sealed container Component A &
Component B
Which way does water move?
25% Moisture 52% Moisture
ConclusionWater activity should be used as a stability map to predict potential changes, reactions, and hazards for new products.
• instant coffee, where the critical area is at about 50% RH. RH >50% the product cakes and loses itsnflowability.
WATER ACTIVITY AND PACKAGING
• Water Activity is critical for food safety.• Water activity along with pH, temperature, and
other hurdles are important tools for making food safe.
• Water activity relates to safety and quality of foods, pharmaceuticals & cosmetics:– Microbial Growth– Chemical / Biochemical Stability– Physical Properties– Shelf Life
Physical Changes
The most common physical changes occurring in foods as they spoil are evaporation, drip loss, and separation.
• Dehydrate: To remove at least 95% of the water from foods by the use of high temperatures.
• Freeze-dry: To remove water from food when it is in a frozen state, usually under a vacuum.
WATER BINDING OF MEAT
first hour after slaughter, bivalentmetal ions of muscle -> intothe muscle proteins at pH 6, causing a contraction and a dehydration
Further changes in hydrationduring aging for up to seven days canbe explained by an increaseavailable carboxyl and basic groups (proteolysis)
• Water is an effective solvent as it can form hydrogen bonds.– Water clings to polar
molecules causing them to be soluble in water.• Hydrophilic -
attracted to water– Water tends to
exclude nonpolar molecules.• Hydrophobic -
repelled by water
Solid water (ice) is less dense than liquid
same mass but a larger volume
• Ice is less dense than water: the molecules are spread out to their maximum distance
Density = mass/volume