Water and Water ActivityThe Component of Food

Definition

Water : a colourless, transparent, odourless, liquid which forms the seas, lakes, rivers, and rain and is the basis of

the fluids of living organisms

Water is a compound of oxygen and hydrogen (chemical formula: H2O) with highly distinctive physical and

chemical properties: it is able to dissolve many other

substances; its solid form (ice) is less dense than the

liquid form; its boiling point, viscosity, and surface

tension are unusually high for its molecular weight, and it

is partially dissociated into hydrogen and hydroxyl ions

Water Structure

O

H H

1 molecule of water is

made up of 2 hydrogen atoms

bonded with 1 oxygen atom

OWATER STRUCTURE

The bond that forms water

is a covalent bond

OH H

1 molecule of water

A LOT OF MOLECULES

OF WATER

The state of the water

Gaseous state

Liquid state

Solid state (ice)

Form of Water in Foods

1. Constitutional water

2. Hydration water

3. Bulk/solvent/free water

Form of Water in Foods

Form of Water in Foods

1. Constitutional water

Most strongly bound

Is not available as solvent or to support enzyme activity

Un-freezable water (at -40oC)

Form of Water in Foods

2. Hydration water

Mostly not freezable at -40oC

Influences on the physical properties of solutes

Increases the rate of most reactions

Form of Water in Foods

3. Bulk/solvent water

Fills capillaries and pores

Retains all the properties of water

Increases molecular mobility

Decrease viscosity

Freezable

Physically constrained within the macromolecular matrix

Water content of Food & Drinks

Water Activity

Water Activity (Aw)

If you place a sample of food in a sealed container the moisture in the air space will

come to an equilibrium value

Aw = defined as the equilibrium relative humidity (ERH) of the air space divided by

100

Aw = ERH/100

Range of water activity values

Aw is expressed as a decimal fraction of the equilibrium relative humidity (ERH) of

a particular material.

0 (bone dry)1.00 (pure water)

E.g., a sample containing a water activity reading of 0.91 is equivalent to an (ERH)

reading of 91%.

The concept of water activity (Aw)

Physical properties of biological materials is are modified by the presence, amount and activity of

water

Aw is a measure of the availability of water for chemical reactions and microbial growth

NOT a measure of the total moisture content which includes bound water.

Aw is important because it affects:

1. Stability

2. Degradation by chemical reactions,

moulds, yeasts, bacteria

3. Texture

4. Shelf life

Aw of common foods

>0.95 Fresh fruit, meat, milk

0.95-9 Cheese

0.9-0.85 Margarine,

0.85-0.8 Salted meats

0.8-0.75 Jam

0.75-0.65 Nuts

0.65-0.60 Honey

0.5 Pasta

0.3 Cookies

0.2 Dried veg., crackers

Gram ve 0.97Gram +ve 0.90

Yeasts 0.88

Moulds 0.80

Halophiles 0.75

Xerophiles 0.65

Critical Aw

Control of water activity

Substances which bind water include:

Glycols (PEGs, propylene glycol)

Alcohols

Sugars (sorbitol, high fructose corn syrup)

Calcium and sodium salts

Glycerin

Gums (xanthan)

Proteins

Using Aw in food preservation

N.B. low Aw does not prevent the presence of microorganisms in a sample, it only reduces

growth.

The more tightly water is bound, the lower its chemical potential hence Aw.

Therefore modification of Aw can result in preservation of food without the addition of

preservatives.

Physical preservation and Aw

Utilized in a wide variety of food

preservation procedures : Evaporation

Condensation

Drying/dehydration

Salting

Brining

Syruping

Freezing

Remove water

Binding water

Is water content the same

as water activity (Aw)?