Last updated on April 26, 2023
Some Properties of Water
Water, one of the four major components in foods, is extremely important in terms of its vital necessity in nutrition and its physical, chemical and microbiological effects on foods.
To understand why water is so important for food, it is necessary to know its main characteristics.
Water is a compound formed by the covalent bonding of two hydrogen atoms with an oxygen atom. The shape of the water molecule and the fact that oxygen is more electronegative than hydrogen in O – H bonds give water a polar character with negative and positive ends. Some characteristics of water are provided below;
A hydrogen bond is formed between water molecules, the oxygen of a water molecule and the hydrogen of a neighboring water molecule.
The hydrogen bonding of all the water molecules together in the three-dimensional system ensures that the heat capacity, surface tension, melting point values, evaporation, melting enthalpy, adhesion and cohesion strength of the water are high.
Water’s high adhesion and cohesion strength give water the ability to adhere to a foreign substance, wet it quickly and move easily against gravity, as in the capillaries of plants.
The solid-state of water (ice) has less density than the liquid state is also related to the number of hydrogen bonds formed between the molecules and the length of the hydrogen bonds.
The presence of positive and negative ends in the water molecule ensures that ionic compounds dissolve well in water. However, non-ionic but polar compounds such as sugar and primary alcohols can also dissolve in water.
The effect here is due mainly to the establishment of hydrogen bonds between the polar ends (between the oxygen of the water and the carbonyl group of the solute compound).
Forms of Water in Foods
All foods contain more or less water. For example, 70% of meat and 87% of milk is water. The water content of some foods is given on the table;
However, not all of the water in these foods acts like water and does not have the properties of water. In other words, water exists in different forms in the food structure. These forms are as follows;
1. Constitutional Water: It is a water part of components other than water. It has no solvent characteristics, does not freeze at -40oC and has zero water activity. It constitutes a tiny percentage of the water in the food.
2. Vicinal Water; It is water that binds to compounds with strong hydrophilic poles. It has no solvent characteristics, does not freeze at -40oC and has zero water activity. Its high amount can envelop the hydrophilic poles in a single layer.
3. Multilayer Water; It is the form of water that connects to the neighboring water. Due to its proximity to components other than water, it can partially show the characteristics of pure water. Most of them do not freeze at -40oC and can show solvent properties.
4. Free Water; Away from components other than water in food; it is the form of water only in water-water interaction. The characteristic of this water is similar to dilute saltwater. Free water can freeze at temperatures below 0 and show solvent properties.
5. Entrapped Water; It is the water that remains between large macromolecules. It can be easily removed by drying or quickly frozen. Trapped water is water in pectin and starch gels, animal and plant tissues.
Based on this classification, we can say that not all water in food has water properties. This information means that not all of the food’s water is used for physical and chemical reactions and activities of microorganisms.
Therefore, it is extremely important to know how much of that water has water properties and the amount of water contained in the food in terms of preservation and processing of the food. Therefore, the storage period of two foods containing the same amount of water may differ from each other.
The parameter known as “water activity” is used to evaluate the activity of the water in the food. Water activity is defined as the ratio of the pressure of water in the food to the vapor pressure of pure water at the same temperature.
The symbol of water activity is “aw”. Water activity ranges from 0 to 1 and is expressed as unitless. For example, the water activity of pure water is 1, honey is about 0,60 and milk powder is about 0,2. The importance of water activity can be more clear if it is explained as follows;
Most harmful bacteria must have an aw of at least 0,91 to survive. If the water activity of a food is less than 0.91 or if the water activity is reduced below this value by processing (such as drying or concentrating), that food is now in an environment where most harmful bacteria cannot grow.
Therefore, these bacteria will no longer cause any harm to food. From a chemical point of view, the Maillard reaction reaches its maximum rate between 0.65-0.70 aw values.
The water activity is significant, especially in foods containing high amino acids and reducing sugars. Similarly, the amount of vitamin C in the food is closely related to the water activity value of the food. As water activity increases, the rate of vitamin C degradation also increases (For more detailed information see Water Activity in Foods; Definition and Effects on Microbial Activities).
However, it should not be forgotten that; While the water activity parameter is used effectively at temperatures above the freezing point, it loses its validity to a large extent at temperatures below the freezing point.
Another important concept for the relationship between foods and water is “equilibrium humidity”. As it is known, foods take water from the environment or give water to the environment.
While biscuits in the environment absorb water from the environment and moisten; Tomatoes lose water and dry up in the same environment. After a certain period, the food’s moisture content and the environment’s moisture content reach a balance and stabilize.
The point at which it reaches equilibrium is called “moisture content equilibrium” and is expressed in g moisture/g dry matter unit. Naturally, the amount of equilibrium humidity varies according to the temperature and relative humidity of the environment.
“Absorption isotherms” are formed by charting the equilibrium moisture content of food due to moisture exchange with the environment. Bringing a dehydrated food to equilibrium by absorbing moisture from the environment is called “adsorption isotherm”.
Charting the phase of a moist food losing water and reaching balance with the ambient humidity is expressed as “desorption isotherm”. Data for sorption isotherms are obtained experimentally.
Equilibrium moisture content and sorption isotherms are extremely important in food storage and preservation.