Water

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 Definitionaw = p/po

1. Equilibrium2. Constant T & P

Water Activity Definition Experiment - Place together in a sealed container Component A &

Component B

Which way does water move?

25% Moisture 52% Moisture

Water activity

Water Activity and Reaction Rate

aw and food processing

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 as solvent

• 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

Fig. 12-7, p. 392