By definition, water activity in foods; is the ratio of the vapor pressure of the water in the food to the vapor pressure of pure water at the same temperature and its symbol is aw.
Theoretically, the water activity value is between 0 and 1; however, water activity in foods varies between 0.1 and 0.99. Water activity has no units because it expresses a ratio.
aw = P/P0
P: The vapor pressure of the water in the food,
P0: Vapor pressure of pure water
The water activity of a food is determined instrumentally with a device called “water activity meter”. In this analysis, some food is placed in the sample cup of the device and water activity is determined as a result of the food reaching the equilibrium vapor pressure at varying times.
Every food contains more or less water. Unexpectedly, dried fruits and cereals, which we define as dry food, contain relatively large amounts of water at varying rates such as 10-15%.
Similarly, powdered milk contains 1-4% water. However, fruits such as cucumbers and tomatoes, which have a solid structure, contain 95-96% water. (For detailed information see Water in Foods; Forms, Characteristics and Importance)
However, the water activity of two foods with the same water content can be different from each other due to the different structures of the foods. For example, while the water activity of cereals containing 10-13% water is between 0.65-0.75 aw, the water activity of dried fruits containing 15-20% water is between 0.60-0.65 aw.
Therefore, it becomes more important to know the degree of activity of the water it has, rather than the water content of the food. The table below shows the water activity values of some foods;
|Food||water activity (aw)|
|Milk, fresh meat, fresh vegetables and fruits, yogurt, cheese, butter, bread, foods containing up to 8% salt or up to 40% granulated sugar||0,99-0,95|
|Maturated cheeses, ham, foods containing up to 55% granulated sugar or up to 12% salt, cakes, tomato paste, mayonnaise||0,95-0,91|
|Salami, sausage, cured beef, bacon, margarine, foods containing up to 65% granulated sugar or up to 15% salt||0,91-0,87|
|Molasses, flour, legumes, rice, concentrated fruit juices, foods containing 15-17% water||0,87-0,80|
|Jam, marmalade, some dried fruits, foods containing up to about 26% salt||0,80-0,75|
|Nuts, chocolate, marshmallows, jelly, cereals containing 10-13% water||0,75-0,65|
|Honey, most dried fruits, butterscotch||0,65-0,60|
|Pasta, noodles, spices||0,60-0,50|
|Biscuits, rusks, toast||0,30|
|Powdered milk, crackers, corn chips||0,20|
The Effects of Water Activity
Water activity in foods is important in two ways. First, microorganisms need a certain aw value to grow in food.
Secondly, for some chemical reactions, whether enzymatic or non-enzymatic, the water activity value has quite an effect on the acceleration or deceleration of the reaction for some chemical reactions.
In other words, water activity in foods is effective and important in two different ways, microbiological and chemical.
1. The Effects of Water Activity on Microbial Activities
Like every living thing, microorganisms need water to grow and reproduce. The development and reproduction of microorganisms in food are very risky both in terms of food spoilage and human health.
This is different in fermented foods; on the contrary, the growth of “good bacteria” that carry out fermentation in fermented foods, as in the example of yogurt, are encouraged.
The more suitable water in food for the development of microorganisms, the higher the risk. Here, what is meant by “suitable water” is that the microorganism can use that water. In order for the microorganism to use water, water must be of pure water quality.
Processes such as drying, freezing, salting and adding sugar (such as jam), which have been used by mankind for a long time in the preservation of foods, are processes applied to remove or reduce the water that is “suitable” for microorganisms from food.
The “suitable” water in drying evaporates and separates from the food. In salting and adding sugar, the existing water acts as a solvent and becomes a solution rather than pure water. Ultimately, the growth of microorganisms is suppressed or completely stopped.
What is remarkable here is that the amount of water in the food does not change in salting or adding sugar, but since the substance dissolves in it, a significant amount of it is no longer usable for microorganisms.
In fact, a very common situation in jams can be given as a good example to understand the importance of water activity. Jam contains a high amount of sugar and therefore its water activity is so low that microorganisms cannot grow.
However, when a drop of water drips on the jam or water condenses on it, mold growth occurs immediately on that part. This is very important in terms of understanding how important water activity is.
There is no certain amount of “suitable” water for growing all microorganisms; The “suitable” water needs of microorganisms may differ from each other. In this context, the minimum aw values required for the growth of some microorganisms were determined experimentally as follows;
|Most of the harmful bacteria||0,91|
|Most of the harmful yeasts||0,88|
|Most of the harmful molds||0,80|
|Some bacteria species|
|Clostridium botulinum tip E||0,97|
|Clostridium botulinum tip A ve B||0,94|
|Some mold species|
|Some yeast species|
The data in the table above are the data obtained as a result of laboratory experiments. However, the behavior of microorganisms in their natural environment differs from its behavior in the experimental environment.
In general, microorganisms require higher aw values for growth in food media than in vitro. For example, Staphylococcus aureus can grow with 0.86 aw in the experimental environment, while the same bacteria cannot grow in shrimp with 0.89 aw.
A similar situation is also seen in the gene transfer between microorganisms, exhibiting different behaviors in experimental environments and different behaviors in natural environments such as food and intestines.
You know, different behaviors and characters can be seen in people at home, at work or in other social environments; Likewise, microorganisms can exhibit different behaviors in different environments.
The fact that many other factors together with aw are effective in the growth of microorganisms leads to this result.
On the other hand, the minimum aw value required for some toxin-producing microorganisms to produce toxins is higher than the water activity value required for their growth. Examples of these microorganisms are Staphylocooccus aureus, Penicillium patulum, Aspergillus flavus and Aspergillus clavatus.
In addition, the farther the temperature and acidity of the environment are from being suitable for the microorganism, the higher the aw value required for growth.
For example, while the minimum water activity required by Clostridium botulinum type A is 0.94 at 37oC temperature and 7.0 pH, which are the optimum growth conditions, it decreases to 0.99 when the pH is reduced to 5.3 at the same temperature.
As a result, when evaluating the effect of water activity on microorganisms, it can be said that;
- In general, food spoilage caused by bacteria is not observed when the aw is below 0.90. Bacteria can survive for a long time in foods with a water activity of less than 0.90, but they are unlikely to grow and reproduce, causing food spoilage.
- In foods with water activity between 0.90-0.80, spoilage is usually caused by yeast and molds. Xerophilic, halophilic and osmophilic microorganisms up to an aw value of 0.60 may pose a risk of spoilage.
- In general, foods with a water activity below 0.60 aw are foods where microorganisms cannot grow and therefore microbial spoilage is unlikely.
However, it is worth repeating that microorganisms cannot grow in foods with low water activity, but they can survive. The most well-known example of this is seen in the process of freezing food. By freezing food, its water activity can be reduced to between 0.1 and 0.25.
Microorganisms cannot grow at these aw values. Therefore, frozen food can be stored for a long time . However, many of the microorganisms in the food survive even if they cannot grow.
Therefore, even if the food is free from the risk of spoilage while it is frozen, it immediately faces the risk of microbial spoilage as soon as at room temperature. We can observe this very well when storing meat at home.
2. The Effects of Water Activity on Chemical Activities
Although the effect of water activity in terms of chemical reactions is not clearly known, some studies show that water activity has an effect on the rate of chemical reactions. Water activity affects the following reactions;
a) Oxidation of lipids
The concept of oxidation of lipids generally refers to the saturation of unsaturated fatty acids by reacting with oxygen.
The rate of oxidation reactions of lipids fluctuates as the water activity increases. Generally, the reaction rate increases when going from 0.1 to 0.3 aw values.
The reaction rate decreases from 0.3 to 0.5 aw; Going from 0.5 to 0.75 aw, the reaction rate increases again, and after 0.75, the reaction rate decreases again.
b) Maillard reaction
The Maillard reaction is one of the non-enzymatic browning reactions. The Maillard reaction takes place between the reducing ends of carbohydrates and the amino groups of proteins and amino acids.
The main factor affecting the Maillard reaction is temperature. However, aw also has an effect on the rate of the Maillard reaction. According to a study, the reaction rate reaches its maximum between 0.60-0.70 aw.
After 0.7 aw, the reaction rate decreases. This is explained by the increase in dilution with the increase in water activity.
c) Enzymatic reactions
It is thought that the function of water in enzymatic reactions is to ensure the movement of substrates and products. In this context, the rate of enzymatic reactions increases with the increase of water activity.
However, more research is needed on the effect of aw on enzymatic reactions.
d) Oxidation of ascorbic acid
The degradation rate of ascorbic acid, also known as vitamin C, increases in direct proportion to the increase in water activity. In a study, the half-life of vitamin C was 76 days at a constant temperature of 30oC and 0.1 aw, while this period was found to be 6 days at 0.65 water activity.
 In the freezing storage of butter, occurring the bitterness of the butter after a while is not of microbial origin. The bitterness of the butter stored by freezing is due to the fact that the lipase enzyme found in the natural structure of milk continues its activity even at -40oC. Lipase breaks down fat molecules and bitter tasted fatty acids are released.
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Us, F., 2014. Water and ice. In: Food Chemistry, Ed: Saldamlı İ., Hacettepe University Publisher, Ankara.
Yıldırım, İ., 2009. Food Microbiology Lecture Notes. Akdeniz University, Antalya.
Uysal Seçkin, G. and Taşeri, L., 2015. Semi-dried vegetables and fruits. Pamukkale University Journal of Engineering Sciences, 21(9), 414-420.