Both moisture content and water activity are the critical measurements done by food processors and manufacturers in order to ensure food quality and safety. All the food preservation techniques implies reduction in moisture content and so does water activity. Generally people think that if the moisture content of a food commodity is reduced obviously the water available for the microorganisms will also be reduced and hence the food commodity will be less prone for the growth of microorganisms. Higher moisture content does not necessarily means high water activity. Take an example of marmalade, marmalade has high moisture content but its water activity is less because of the high concentration of sugar thus it implies that water activity has much to do with amount of particles dissolved in water rather than actual moisture content.
- It is the total amount of water present in any food commodity or it is defined as the amount of water lost per gram of food product at 100 degree Celsius
- Though it is important but it is not a reliable indicator of microbial contamination.
- It is measured by measuring the initial weight of the sample and then weighing the sample after the moisture has been removed.The difference is then divided by initial weight of sample and then its percentage is calculated.
- Watermelon has 95% moisture content. Some fresh green leafy vegetable have also moisture content greater than 90%.Meat and meat products have moisture content around 70%.
- It is defined as the ratio of vapor pressure of water in a food item to the vapor pressure of pure water at same temperature.It is unit less
- Water activity ranges between 0-1 , 0of bone dry and 1 of pure water but most food products have water activity ranging between 0.2 (for dry fruits) to 0.99 (moist fresh foods)
- Good indicator of microbial growth.
Generally people gets confused between these 2 terms because they seem to be similar , but the difference stated below will solve all your confusion.
Difference Between Moisture Content And Water Activity
Relationship Between Moisture Content And Water Activity
There is relationship between moisture content and water activity can be developed by testing moisture content and measuring activity at many different moisture levels and temperature. Moisture-sorption isotherms are the relationships of moisture content and water activity at a given constant temperature.These isotherms are non-linear. At low moisture content (less than 50% ) even minor change in the moisture content leads to major change in water activity.
Classification Of Food Product On The Basis Of Moisture Content
On the basis of moisture content,the food products are classified as:
- LOW MOISTURE FOODS-They have 5-15% of moisture content.
Example-Freeze dried products
Related Read: Freeze Drying Explained In Depth
- INTERMEDIATE MOISTURE FOODS-Food products having 20-40% of moisture content.
Example-Cakes and Dates.
- HIGH MOISTURE FOODS-They have moisture content greater than 40%
Example-Fresh fruits and vegetables.
Though moisture content is important, it is water activity which is the critical factor for deciding the shelf life of food products. By measuring water activity, it is important to predict which organisms will and will not be potential source of contamination.Water activity plays a major role in determining activity of enzymes and vitamins in food and have a major impact on the color,taste and aroma.
Food processing technology principles and practice by PJ Fellows