Post on 14-Sep-2015
description
transcript
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)?