Water is a macronutrient making up a large percentage of meat (up to 75%), fruits and vegetables (up to 95%). In food it is a medium for chemical reactions and transportation of other food components. It also adds structure by imparting turbidity to cells. In the food industry, water content is generally referred to as moisture. A number of functional properties of water are important in food processing such as the ability to dissolve and disperse other food components, the ability to be bound by other food components, the ability to transfer heat, the ability to sublimate, and its ability to transport materials.
Solvent
Water dissolves more substances than any other liquid. Therefore it is referred to as the universal solvent. This property is important for dissolving and dispersing dry ingredients in food formulations. Water is an important constituent of cleaning chemicals and the cleaning process. During cleaning, water dissolve and disperse soils so that they can be easily removed from food contact surfaces. This ability to dissolve substances increase with increasing temperature. Therefore washing equipment and your hands with warm water will increase the effectiveness of cleaning.
Water Activity (aw)
Water can be bound to other substances such as proteins, carbohydrates and lipids. Therefore in foods, water will be present in both the free form and the bound form.
The amount of free water available is referred to as water activity or aw. Water activity range from 0 to 1 with 1 having the highest aw. High water activity increase the rate of food spoilage, since bacteria will have more water to use and multiply.
Water activity is a better indicator of a product’s shelf life than the total moisture which combines both free and bound water. The rate at which certain chemical reactions proceed such as lipid oxidation and Maillard browning reaction are affected by aw. The activity of enzymes and microorganisms (mold, yeast, bacteria) are also influenced by aw. For example, a high aw of 0.9 or more will initiate bacteria growth.
Salt is a very good preservative due to its ability to effectively bind water and reduce its water activity quickly. The graph below represents real data collected in my food science class comparing the effect of salt and sugar concentrations on water activity.
Heat Transfer
Water is an important medium for heat transfer in cooling, cooking, and commercial sterilization of food. Water freezes at 0 oC (32 oF). When it freezes, water becomes less dense. This explains why ice floats on water. During freezing, water expands by about 9% due to changes in its molecular bond configuration. Therefore containers must be designed to accommodate this expansion. When ice is heated, its temperature is increased. The energy required to raise the temperature of 1 gram of water by 1 oC is called the specific heat capacity. The specific heat of water is 4.184 Joules/gram oC. Once ice reaches a temperature of 0 oC it will begin to melt but it will not increase in temperature until all the melting is complete. This is called the latent heat of fusion of water. As you apply heat to water, its temperature will increase until it gets to 100 oC where it will boil and convert to steam. However during the transition from liquid to steam, it will not increase in temperature. Therefore the energy absorbed during the transition from liquid to steam is called the latent heat of vaporization.
The specific heat of water and its latent heat is relatively high compared to most liquids. This allows it to take more heat to boil. In other words it has the capacity to pull a large amount of heat from its surroundings. This This makes it a good cooling agent. Once water is boiling or in steam form, it contains a lot of stored up energy which can then be given back to its environment. This makes water a good heating agent.
Sublimation
A very interesting property of water is that it is capable of sublimation. That means that it can move from solid to gas without melting. From the phase transition graph below, you will see that the ability of water to move from solid to a liquid depends on the pressure. Melting from ice to water occurs at 0 oC at normal atmospheric pressure of 1 atmosphere. However, if you drop the pressure below 0.006 atmospheres, you can get ice to move directly to a gas. This is what is done in freeze drying. First, water is frozen and then a strong vacuum is applied to cause the ice to sublimate. Sublimation can occur at any combination of low temperature and pressure along the sublimation line shown in the graph. Freeze drying offers some functional benefits such as better texture and flavor retention compared to drying using heat.
Expansion During Freezing
Water expands during freezing. This occurs because the molecules reorient themselves in an open hexagonal arrangement having more internal space than exists in a liquid state.
Impurities like salt reduce the degree of expansion possible during freezing.
Boiling Point
Water boils at 100oC at sea level. However, at higher altitudes where atmospheric pressure is less, boiling occurs at lower temperatures. This means that cooking your foods at higher altitudes will take longer due to the lower temperature that water is able to reach.
Boiling temperature is elevated in the presence of dissolved solutes such as sugar and salt.
Conductivity
You may have often heard the warning to keep electrical appliances away from water in order to avoid getting shocked. After hearing this so many times, I bet you assume that water is a good conductor of electricity. It turns out however that water is a poor conductor of electricity in the pure state. It is the charged particles in water such as dissolved ions from salts that makes it an excellent electrical conductor.
The graph below shows that distilled water (0.0 % salt) conducts electricity poorly but conductivity drastically increases with even a small incremental increases in salt.
Transportation
Water is used as a transport medium to move food and waste material throughout the plant. This is a common way to transport and clean produce at the same time. Materials that are denser than water (>1g/cm3) such as dirt, metal, stones, and rotten food will sink to the bottom while materials that are less dense than water (<1g/cm3) such as many whole and sound fruits and vegetables, will float.
