Water Activity and Food Preservation

Water is a universal solvent and most important constituent of any food. When it comes to food safety and preservation water is the most significant factor. How? Let’s find out.

In food, water is present in two forms i.e. free water and bound water. Bound water is not available for biological functions. It is usually bound to other food constituents like carbohydrates and proteins. Whereas the Free water is available for biochemical reactions and microbial growth, bound water is not.

When we talk about food safety and preservation, a common term comes across i.e. moisture content. Moisture content includes both free water and bound water and thus indicates total amount of water in the food.  However, the shelf stability of a product can be achieved by controlling the water activity of the product. 

What is water activity?

In simple words water activity is the measurement of the water that is not bound to other food constituents and is available for microbial growth, chemical and biochemical reactions. It ranges from 0 to 1 (pure water). In scientific terms, it is the ratio of vapour pressure of water in food product to the vapour pressure of pure water at same temperature (Sandulachi, 2012).

In 19th century, scientists began to correlate the water content of the food with its tendency to spoil. Over the period of time, they realized that free water is more important for the stability of food than total amount of water (Rahman & Labuza, 2007). So, determining water activity of a medium helped in controlling the spoilage due to microbial growth. 

Water activity can determine the safety and stability of a food product with regards to microbial growth, and chemical, biochemical and physical properties. When it comes to food safety microbial spoilage is one of the main concerns especially in the case of perishable or high moisture foods. 

Water has a significant contribution to the growth of microorganisms. Microorganisms require water for transporting nutrients, cellular wastes and maintaining cell shape. So, when microorganisms get sufficient amount of water they flourish in that environment leading to microbial spoilage and may cause food borne illnesses.

Now the question comes: how can water activity help us in avoiding the microbial growth? Yeast, moulds, bacteria require certain amount of water for their growth and water activity can indicate that minimum amount of free water available for microbial growth. This is called as limiting water activity level below which microorganism will not grow. So, every microorganism needs optimum moisture for growth. Hence, knowing the water activity helps in predicting the potential source of spoilage. Thus, it is considered as an important parameter in HACCP plan in food industries. 

Based upon water content foods can be classified into three categories- Low moisture food (water activity <0.60), Intermediate moisture foods (water activity 0.60- 0.85) and High Moisture foods (water activity > 0.85) (Erkmen & Bozoglu, 2016). Higher the water activity, the chances of spoilage are higher. Bacteria require higher values of water activity for growth than fungi. Below 0.60, there is no microbial growth. The table below indicates the water activity levels of different foods and microorganisms- 

Table- Water activity levels of different foods and inactivation of microorganisms

(Source – Food Preservation by Reducing Water Activity (2016), Food Microbiology: Principles into Practice, 44–58)

Also, controlling water activity can help in maintaining chemical stability of the product. Water acts as a solvent and reactant in many biochemical reactions. So, water activity affects the non-enzymatic browning (Millard reaction), lipid oxidation, protein denaturation, starch gelatinization and retrogradation. With the reduction in water activity, the rate of these reactions also decreases (Safefood 3600, 2014).

Water activity also affects the physical properties of a food product. The foods with high water activity can be soft, juicy whereas foods with lower water activity are hard and dry in texture. Increasing or decreasing the water activity of these products can affect their textural properties. 

In a multi-component product, moisture migration may occur, as each component will have its own water activity. If there is a difference in water activity then water will migrate from component of high-water activity to the one with low water activity causing physical changes in the final product. We can take the example of bhel where the murmura (puffed rice) becomes soggy after some time due to other moist ingredients like onion.  Two components with same moisture content can have different water activities. If the water activity of two components is same then there will be no moisture migration. In case of powdered products, increased water activity can result in caking and reduction in rehydration properties of the dried products. 

Similarly, moisture migration may take place between the food and atmosphere. This is due to the relation between water activity and relative humidity. A water activity of 0.80 is in equilibrium with the 80%relative humidity of air. When you keep biscuits open outside they become soggy. Why does that happen? Because moisture migrates from atmosphere to biscuits. Say biscuit has a water activity of 0.75 and relative humidity is 85% then water will migrate from atmosphere to biscuits making them soggy. Reverse happens when drying foods.

Water activity is important in determining the shelf life of a product. Changes in water activity over the period of time affect the shelf life of a product. So, controlling water activity prevents the growth of vegetative cells, germination of spores, toxin formation, physicochemical changes and thus results in a shelf stable, high quality product.  

Controlling water activity-

The shelf stability and quality of a product can be maintained by controlling water activity below critical limits. It can be done by either removing the water or making it unavailable for microbes and chemical reactions. Now you may think how to do so? Well we have been doing it for ages even before the concept of water activity was discovered. 

Most common and easy method of controlling water activity is dehydration or drying. It removes water and thus reduces water activity resulting in increased shelf life. Dried fruits, beef jerky are great examples. Also, dried spices like red chillies have a lower water activity and can be stored for a longer time.  Drying can be done by various methods like sun drying, mechanical drying, freeze-drying etc.

Another most common and household method for controlling water activity is adding solutes. We preserve vegetables in brine solution or fruits in sugar syrups.  The method of preserving foods by adding salts and sugars has been used since ancient times. But why does it preserves the food? The answer is it reduces the water activity of product. Added solutes (salt, sugar) bind with the water molecules making it unavailable for microbes. A 0.005 reduction in water activity from 0.955 to 0.95 in the environment reduces the intracellular water content by about 50% and reduces cell volume by about 45%. This indicates the sensitivity of microorganisms to a slight water activity change. Jam and jelly are great examples of this technique (Erkmen & Bozoglu, 2016).

A combination of the two techniques works even better. Salted fish can be a good example. Also, our very own amla candy is prepared by adding sugar and then drying. So it has good shelf life. 

We have been preserving foods using these techniques for decades. Now that we know that the main reason is controlled water activity. So, having sound knowledge of the concepts of water activity helps us in deciding the method of preservation, packaging material and shelf stability of a product and eventually designing a safe product with high quality.