Water activity and its relation to spoilage of foods:

Water activity and its relation to spoilage of foods:

         All the food products contain varying amount of water in their tissues. Various qualities and processing attributes like deterioration, dehydration, spoilage etc., in raw as well as processed foods, are influenced by the water content. Water exists in three forms:

  • As solid (ice) below  0 º C
  • In liquid form as water between 0º C and 100º C and
  • As gas above 100º C at the atmospheric pressure.

 We consume food in its raw as well as in the cooked form. The entire food product contains certain amount of water. e.g., vegetables and fruits like spinach, cucumbers,

watermelons etc. The water contained in raw foods is referred to as the moisture content of those foods. The moisture content of certain foods depends on the temperature and partial vapor pressure of water in the surrounding. These are called hygroscopic foods. On the other hand, the foods in which the moisture content does not get affected by these factors are called

Non-hygroscopic. The hygroscopic foods can absorb water in a number of ways. This is referred to as sorption and can occur by formation of a hydrate, binding to surface, diffusion in the food, capillary condensation, formation of a solution etc.

Types of Water in Foods

The water content or the moisture content of a food influences its appearance, texture and flavour. It varies a great deal in different food items. The green leafy vegetables contain more than 90 per cent of water while it may be negligible in oils and fats like ghee, butter, oil etc.

Water Content of Some Common Foods

               Products

                 Water (%)

Green leaves

95

Tomato

95

Water melon

93

Orange

87

Potato

78

Banana

75

Butter

16

Rice

12

Legumes (dry)

9-11

          Water in foods can be either in free or bound form, depending on its interaction with the surrounding molecules. The bound water refers to water that is physically or chemically bound to other food components Many compounds like starch, proteins and some salts have water bound to them in the form of hydrates.

          The free water is the bulk water free from any other constituents. It is held in narrow channels between certain food components due to capillary forces and is held trapped within the spaces in food. It is surrounded by physical barrier e.g., biological cell that prevents it from escaping. The free water is actually responsible for the microbial growth and deterioration of food. This form of water is also called the available water. Another form of water in food is called imbibed water. This water is found in hydrophilic gums like gelatin which is a type of protein with ability to absorb a large amount of water. Gelatin forms a jelly like mass on absorbing or imbibing water. Imbibed water is more or less like the hydrate formation and Involves hydrogen bonding. A yet another form of water in food is adsorbed water. Some solid foods have the ability or tendency to absorb water on surface. The powdered forms of the solids adsorb more water because of a larger surface area.

A portion of the total water content present in a product is strongly bound to specific sites on the chemicals that comprise the product. These sites may include the hydroxyl groups of polysaccharides, the carbonyl and amino groups of proteins, and other polar sites. Hydrogen bonds, ion-dipole bonds, and other strong chemical bonds tightly bound water. Some water is bound less tightly, but is still not available (as a solvent for water-soluble food components). Many preservation processes attempt to eliminate spoilage by lowering the availability of water to microorganisms. Reducing the amount of free--or unbound--water also minimizes other undesirable chemical changes that occur during storage. The processes used to reduce the amount of free water in consumer products include techniques like
concentration, dehydration and freeze drying. Freezing is another common approach to controlling spoilage. Water in frozen foods is in the form of ice crystals and therefore unavailable to microorganisms and for reactions with food components.

Moisture Content

           The moisture content of a food item is defined as the amount of water lost per gram of the food product at about 100o C. Though important, water content or per cent moisture is not a reliable predictor of microbial responses and chemical reactions in food products. The water

content of a safe product varies from product to product and from formulation to formulation. One safe, stable product might have a water content of 15 per cent while another with a water content of just 8 per cent is susceptible to microbial growth. This is so because the microbial stability or physicochemical properties of food are often determined by amount of free water

Present rather than the total amount of water. It is the free water, not the bound one that supports the growth of bacteria, yeasts and molds (fungi). This unbound or available water is expressed in terms of water activity.

WATER ACTIVITY

            The water present in foods interacts with major hydrophilic nutrients such as carbohydrates, proteins, water-soluble vitamins, and minerals etc. by means of polar hydrogen-bonding interactions. The alkyl chains of fats and proteins, on the other hand, are hydrophobic in nature, and therefore do not tend to interact with water. In other words the hydrophilic molecules cause the water molecules to bind while hydrophobic do not. We may categorize the water within food into bound water, affected water, and free water in the order of their decreasing strength of interaction. Depending upon the strength of interaction the water may escape with varying degree of tendency. Since the structure and properties of the bound water get altered due to the interactions we can say that the properties of water in food are different from those of pure water. Though we have used the terms like ‘bound’ and ‘free’ but we should be clear of the fact that these refer to the relative strength of the interaction between different food constituents and water. Different molecules of bound water may be free to different extents and similarly the free water molecules are bound to some extent.

As mentioned earlier, the microbial and chemical stability of a food product is directly related to how much water is available in a food item, for biological or chemical reactions. This is measured in terms of water activity which describes the escaping tendency of the water in a sample.

Definition of Water Activity:

          Water activity is defined as the ratio of the vapour pressure of water in a food item (p) to the vapour pressure of pure water at the same temperature.  Relative humidity of air is defined as the ratio of the vapour pressure of air to its saturation vapour pressure. Therefore, when a sample in a sealed chamber comes to equilibrium with the water vapour in the surrounding, the water activity of the sample can be equated to the relative humidity of surrounding air. Since water activity is a ratio of vapour pressures; it has no units. Its value

ranges from 0.0 (bone dry) to 1.0 (pure water) but most food products have aw value in the range of 0.2 for very dry fruits to 0.99 for moist fresh foods.

Water Activity of Some Food Products:

Products

Water Activity

Highly perishable vegetables, meat, fish, milk, bread

0.95

Cheese, fruit juice

0.91

Dry cheese, sponge cakes

0.87

Fruit juice concentrates, flour, rice, pulses

0.80

Jam, marmalade

0.75

Dried fruits, nuts

0.65

Noodles, spaghetti, etc.

0.50

Role of Water Activity in food spoilage:

        On the basis of moisture content, the food products can be broadly put into three categories as:

  • Low moisture foods e.g., dried or freeze dried foods having moisture content of 5-15 per cent.
  • Intermediate moisture foods e.g., cakes and dates with a moisture content of 20-40 per cent.
  • High moisture foods e.g. fresh fruits and vegetables of greater than 40 per cent of moisture.          

        Though moisture content is important, it is the water activity (aw) that is critical factor for the Shelf life of a food item. It may be used to predict stability with respect to physical properties like texture and caking, rates of deteriorative reactions and microbial growth.

Some Water Activity Values and their Effect on Shelf Life:

Water Activity Value

                                       Effect on Food

0.2–0.3

A monolayer of moisture is formed. It represents the optimal moisture content for the maximum shelf life of the dehydrated foods.

0.35–0.45

The water is available outside the monolayer and the water phase requiring chemical reactions begin to occur. This may cause changes in physical state, e.g., loss of crispness, stickiness, and re-crystallization of amorphous state sugars.

0.4–0.5

The soft materials like raisins etc., become hard due to drying out.

0.6

Considered a critical point where there is high potential for growth of microbes if the moisture content increases.

0.6–0.8

The rates of chemical reactions that require an aqueous phase increase and cause deterioration of foods, reaching a maximum. The rates however fall at higher moisture content.

0.85

Another critical point, above which bacterial spoilage occurs and pathogens begin to grow. 

If we want to prevent the food spoilage or the action of microbes and maintain food quality, the water activity of the food product needs to be lowered. Conventionally, the water activity in foods has been controlled by drying, addition of sugar or salt, and by freezing. The added salt or sugar dissolves in the free water and makes it bound or unavailable. In other words, it decreases the water activity and makes the food less prone to spoilage. Lowering the temperature checks the activity of the enzymes in the food and also makes conditions unfavorable for the growth of microorganisms. Drying of the product acts as a means of preserving the food by lowering the water activity.

Water Activity and Quality:
Water activity is a critical factor that determines shelf life (Fig. 8-10). Most bacteria, for example, do not grow at water activities below 0.91, including pathogens such as Clostridium botulinum. Below 0.80 most molds cannot be grown and below 0.60 no microbiological growth is possible. However, there remain a number of food spoilage microbes that can grow within the range 0.8 - 0.6. The risk of food poisoning must be considered in low acid foods (pH > 4.5) with a water activity greater than aw 0.86. Staphylococcus aureus, a common food poisoning organism, can grow down to this relatively low water activity level. Foods that may support the growth of this bacterium include cheese and fermented sausages stored above correct refrigeration temperatures.

By measuring water activity, it is possible to predict which microorganisms will and will not be potential sources of spoilage. In addition to influencing microbial spoilage, water activity can play a significant role in determining the activity of enzymes and vitamins in foods and can have a major impact their color, taste, and aroma. It can also significantly impact the potency and consistency of pharmaceuticals. With the exception of lipid oxidation, all of the rates decrease at least 100 fold as the zone 2 water is removed and effectively stop at the monolayer value. This is because whatever the reagents responsible for a reaction, they always need a solvent to move around in order to encounter each other and react. As the solvent is removed the rate decreases and, as monolayer water is not adequately liquid-like to act as a solvent, the reaction stops. Several rates may slightly decrease at high water activities due to dilution
of the reagents.

 

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Comment by Palanivel Raj on October 30, 2012 at 7:36pm

Precise..  

Comment by RAVINDRA PATIL on October 26, 2012 at 9:37am

Thanks for sharing this useful knowledge.....................

Comment by Nirav Dineshbhai Joshi on October 25, 2012 at 2:52pm

Thank you so much

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