Water activity converted
- 1. Water Activity (aw) P. Karthika Assistant Professor Department of Food Science and Nutrition Periyar University, Salem -11
- 2. Introduction • 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.
- 3. • The water present in foods interacts with major hydrophilic nutrients such as carbohydrates, proteins, water- soluble vitamins, and minerals etc. (polar hydrogen-bonding interactions). • The alkyl chains of fats and proteins are hydrophobic in nature, and therefore do not tend to interact with water.
- 4. Definition 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 (po) at the same temperature.
- 5. 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.
- 6. Water Activity and Quality • 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. 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.
- 7. • measuring water activity - to predict which microorganisms will and will not be potential sources of spoilage. • water activity - determining the activity of enzymes and vitamins in foods and can have a major impact their color, taste, and aroma. • Stability and food security depends on water activity and pH in the food environment. • water activity is higher with the products are perishable.
- 8. Aw 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.