Prepared by Assoc Prof Darryl Small (v4 July 2012) questions/feedback to

Molecule of the week Water

Compared to most molecules in food, water is one of the simplest and smallest. In most food materials water is found in high concentrations. The significance of water is often overlooked, but it is very important for a number of reasons: 1. It is an essential nutrient. This is emphasised in

Australian dietary guidelines most recently issued by the National Health and Medical Research Council (NHMRC) in 2005[1]. The guidelines are to drink plenty of water and to choose water as a drink.

2. Water affects the textural characteristics and makes food enjoyable. Viscosity, gel formation, snap, crispiness all involve water in one way or the other. Thus the palatability and digestibility of foods depend on the amount of water present.

3. The extent to which other molecules attract water and bind them is one of the major determinants of both the structure and sensory attributes of foods.

4. Product quality is determined by the water molecules because it is important to the growth of micro-organisms as well as determining the rate of chemical reactions.

Water and micro-organisms The availability of water also plays a key role in determining whether particular groups of micro-organisms can survive / grow / multiply. The response of particular bacteria, fungi, moulds, yeasts will vary quite widely. A knowledge of these factors is important in either encouraging growth during fermentation or controlling those organisms causing spoilage or responsible for food poisoning. Water is also important in the deterioration of food quality caused by a wide variety of enzymatic and non enzymatic chemical reactions.

Water and the stability of foods During the processing of many foods and in final products, water molecules provide the environment that allows many chemical reactions to occur by bringing together reactant molecules. These may have desirable or less desirable consequences for food quality, involving both sensory and nutritional significances. In many cases, food molecules are more stable at low water levels. However some hydrophobic molecules react more readily at low water contents.

Structure of water This can be represented in various ways. Although the structure is simple, it is important to understand the shape. Diagram 4 shows all eight of the electrons in the outermost shell of the oxygen atom. There are four pairs and two of these form the single bonds with the hydrogens. The molecule of water is bent (structures 3, 4 and 5).

Polarity of water The four pairs of electrons repel each other and get as far away from each other as possible giving the tetrahedral structure . The electrons in the bonds between the hydrogen and oxygen atoms are not shared fairly. The oxygen is greedy and takes more of the electron cloud. This is shown by the - symbol where the Greek letter implies a small amount (rather than a full charge). This results in hydrogen bonding between water and many other molecules in foods. This is shown in where water is forming close links with two ethanol molecules.

H2O HOH

OHH

2

+ +

OH

H lone pairs

OH

H

a tetrahedron water, atetrahedralmolecule

Types of water in food Although the molecules are chemically identical a number of distinct categories of water may occur in foods. Free water is that which is mobile and can be readily removed during processing. In contrast, constitutional water molecules are very difficult to remove because they are part of the internal structure of other molecules including proteins. Another category is the vicinal or monolayer water: tightly bound to the surface of hydrophilic components. In foods the interactions with proteins and carbohydrates are particularly significant.

C OH

HH

H

O HH O

H

CH

H

H

+

+

+

alcohol/water

References and further reading [1] The guidelines are available at nhmrc.gov.au. [2] The food chemistry texts by Fennema and [3]

Coultate both have very useful chapters on water. [4] The NUTTAB database includes water content data

for Australian foods (see foodstandards.gov.au).