The definition of water activity in foods is the ratio of the vapor pressure of the water present in a food to the vapor pressure of pure water at the same temperature, and its symbol is aw.
Theoretically, the value of water activity ranges between 0 and 1; however, in foods it varies between 0.1 and 0.99. Since it expresses a ratio, water activity has no unit.
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 using a device called a “water activity meter.” In the analysis, a certain amount of food is placed in the sample chamber of the device, and water activity is determined after the food reaches equilibrium vapor pressure over a period of time.
Every food contains water in varying amounts. Dried fruits and cereals, which we define as dry foods, contain relatively high amounts of water, unexpectedly ranging between 10–15%. Milk powders obtained by drying contain between 1–4% water. Fruits such as cucumber and tomato, which exhibit an obviously solid structure, contain 95–96% water. (For detailed information see Water in Foods; Forms, Characteristics and Importance)
The water activity of two different foods with the same moisture content may differ due to differences in their structures. For example, cereals containing 10–13% moisture have a water activity between 0.65–0.75, whereas dried fruits containing 15–20% moisture have a water activity between 0.60–0.65. Therefore, in addition to the moisture content of a food, knowing the activity of the water it contains is highly important.
The table below presents the water activity values of some foods;
| Food | water activity (aw) |
|---|---|
| Pure water | 1,00 |
| 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 |
| Egg powder | 0,40 |
| Biscuits, rusks, toast | 0,30 |
| Powdered milk, crackers, corn chips | 0,20 |
Importance of Water Activity
Water activity in foods is important from two perspectives. The first is its significant effect on microbial growth. Microorganisms require a certain water activity value in order to grow in foods.
The second is that, whether enzymatic or non-enzymatic, some chemical reactions are influenced by water activity in terms of acceleration or deceleration.
In other words, water activity in foods is important from both microbiological and chemical perspectives.
1. Importance of water activity from a microbiological perspective
Like all living organisms, microorganisms require water to grow and reproduce. The growth and reproduction of microorganisms in foods pose risks both in terms of food spoilage and human health.
The situation is different in fermented foods; as in the case of yogurt, the growth and reproduction of the “beneficial bacteria” responsible for fermentation are intentionally encouraged.
The more favorable the water in a food is for microbial growth, the greater the risk. By favorability of water, what is meant is the ability of the microorganism to utilize that water. For a microorganism to use water, the water must have the characteristics of pure water.
Processes such as drying, freezing, salting, and sugaring (such as jam production) are methods that humans have long used to preserve foods by removing or reducing the water that is “available” to microorganisms.
In drying, water evaporates and is removed from the food. In salting and sugaring, the existing water acts as a solvent and ceases to be pure water, becoming a solution. Ultimately, microbial growth is inhibited or completely stopped.
It is noteworthy that in salting or sugaring, the total amount of water in the food does not change. However, because solutes (salt or sugar) are dissolved in it, a significant portion of that water is no longer available to microorganisms.
A common situation observed in jam provides a good example of the importance of water activity. Jam contains a high amount of sugar; therefore, its water activity is generally too low to allow microbial growth. However, when a drop of water falls onto the jam or when condensation forms on its surface, mold growth immediately occurs in that area. This clearly demonstrates the importance of water activity.
The minimum amount of available water required for the growth and reproduction of microorganisms may vary depending on species and even strain.
The table presents the minimum water activity values required for the growth of certain microorganisms;
| Microorganism | Minimum aw |
|---|---|
| Most of the harmful bacteria | 0,91 |
| Most of the harmful yeasts | 0,88 |
| Most of the harmful molds | 0,80 |
| Halophilic bacteria | 0,75 |
| Xerophilic molds | 0,62 |
| Osmophilic yeasts | 0,61 |
| Some bacteria species | |
| Clostridium botulinum tip E | 0,97 |
| Clostridium botulinum tip A ve B | 0,94 |
| Clostridium perfingens | 0,95 |
| Pseudomonas spp. | 0,96 |
| Pseudomonas fluorescens | 0,97 |
| Pseudomonas fragi | 0,91 |
| Acinetobacter spp. | 0,96 |
| Escherichia coli | 0,95 |
| Bacillus subtilis | 0,95 |
| Bacillus cereus | 0,95 |
| Bacillus stearothermophilus | 0,93 |
| Salmonella spp. | 0,92-0,95 |
| Lactobacillus viridescens | 0,94 |
| Listeria monocytogenes | 0,92 |
| Staphylococcus aureus | 0,86 |
| Enterobacter aerogenes | 0,95 |
| Pediococcus cerevisiae | 0,94 |
| Vibrio parahaemolyticus | 0,94 |
| Some mold species | |
| Rhizopus stolonifer | 0,93 |
| Rhizopus nigricans | 0,93 |
| Botrytis cineria | 0,93 |
| Aspergillus citri | 0,84 |
| Aspergillus flavus | 0,78 |
| Aspergillus niger | 0,78 |
| Aspergillus versicolor | 0,78 |
| Aspergillus ochraceous | 0,77 |
| Aspergillus glaucus | 0,70 |
| Penicillium expansum | 0,83 |
| Penicillium islandicum | 0,83 |
| Penicillium patulum | 0,81 |
| Penicillium citrinum | 0,80 |
| Penicillium chrysogenum | 0,79 |
| Some yeast species | |
| Candida utilis | 0,94 |
| Saccharomyces cerevisiae | 0,90 |
| Saccharomyces baiht | 0,80 |
| Debaryomyces hansenii | 0,83 |
| Xeromyces bisporus | 0,61 |
| Zygosaccharomyces rouxii | 0,62 |
The data in the table were obtained from laboratory experiments. However, the behavior and requirements of microorganisms in natural environments such as foods differ from those observed under laboratory conditions.
In general, microorganisms require higher aw values to grow in food environments than under laboratory conditions. For example, while Staphylococcus aureus can grow at a water activity of 0.86 in laboratory conditions, the same bacterium cannot grow in shrimp with a water activity of 0.89.
Microorganisms also exhibit differences in gene transfer behavior in natural environments compared to laboratory conditions. Species prone to gene transfer in laboratory environments have often been observed to avoid gene transfer within the food matrix.
Just as humans may display different behavioral patterns at home, at work, or in different social environments, microorganisms can also exhibit different responses and characteristics depending on environmental conditions.
It is highly likely that the simultaneous influence of many factors in addition to water activity on microbial growth leads to this outcome.
In general, the less suitable the environmental temperature and acidity are for a microorganism, the higher the water activity required for growth. For example, the minimum water activity required by Clostridium botulinum type A under its optimal growth conditions of 37°C and pH 7.0 is 0.94. However, at the same temperature, when the pH is reduced to 5.3, the minimum required water activity increases to 0.99.
On the other hand, for some toxin-producing microorganisms, the minimum water activity required for toxin production is higher than that required for growth. Examples include Staphylococcus aureus, Penicillium patulum, Aspergillus flavus, and Aspergillus clavatus.
When evaluated on a species basis, the effect of water activity on microorganisms can be summarized as follows;
In general, bacterial food spoilage is rarely observed below a water activity of 0.90. In foods with water activity below 0.90, bacteria may remain viable for long periods; however, it is unlikely that they will grow and reproduce to levels capable of causing spoilage.
In foods with water activity between 0.90–0.80, spoilage is generally caused by yeasts and molds. Down to 0.60 aw, xerophilic, halophilic, and osmophilic microorganisms may pose a spoilage risk.
In general, foods with water activity below 0.60 are those in which microorganisms cannot grow and microbial spoilage is therefore unlikely.
However, it should be reiterated that in foods with low water activity, microorganisms cannot grow and reproduce, but they may survive.
The most well-known example of this is food freezing. By freezing a food, its water activity can be reduced to values between 0.1–0.25. At these values, microorganisms cannot grow or reproduce. For this reason, frozen foods can be preserved for long periods [1].
However, even though microorganisms present in frozen food cannot grow and reproduce, they may still survive. Although there is no spoilage risk while the food remains frozen, microbial growth and reproduction resume immediately once thawing begins. When ice melts, water becomes available for microbial use. From that point onward, water activity ceases to be the primary factor suppressing microbial growth.
2. Importance of water activity from a chemical perspective
Although the effect of water activity on chemical reactions is not fully understood, some studies indicate that it influences reaction rates. As currently known, water activity affects the following reactions;
a) Lipid oxidation
Lipid oxidation generally refers to the reaction of unsaturated fatty acids with oxygen, leading to their saturation.
The rate of lipid oxidation reactions fluctuates as water activity increases. Generally, as aw increases from 0.1 to 0.3, the reaction rate increases. From 0.3 to 0.5 aw, the reaction rate decreases; from 0.5 to 0.75 aw, the reaction rate increases again; and beyond 0.75, it decreases once more.
b) Maillard reaction
The Maillard reaction is a non-enzymatic browning reaction. It occurs between the reducing ends of carbohydrates and the amino groups of proteins and amino acids.
Temperature is the main factor affecting the Maillard reaction. However, water activity also influences its rate. According to one study, the reaction rate reaches a maximum between 0.60–0.70 aw. After 0.7, the reaction rate decreases. This is explained by increased dilution as water activity increases.
c) Enzymatic reactions
In enzymatic reactions, water is thought to function by facilitating the movement of substrates and products. In this context, as water activity increases, the rate of enzymatic reactions also increases.
However, further research is needed regarding the effect of water activity on enzymatic reactions.
d) Oxidation of ascorbic acid
Ascorbic acid, commonly known as vitamin C, shows an increase in degradation rate proportional to the increase in water activity. In one study conducted at a constant temperature of 30°C, the half-life of vitamin C was 76 days at 0.1 aw, whereas it was found to be 6 days at 0.65 aw.
[1] The rancidity that develops in butter after a period of frozen storage is not of microbial origin. The rancidity of frozen butter results from the continued activity of the lipase enzyme naturally present in milk, which remains active even at –40°C. Lipase breaks down fat molecules, releasing free fatty acids with bitter flavors.
References;
Kisla, D., 2013. Preservation of food with low water activity. In: Food Microbiology, Ed: Erkmen O. Efil Publisher, Ankara.
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.
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