Water Activity In Food Stuff


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Reproduced with permission from Food Science Australia

http://www.foodscience.csiro.au/water_fs-text.htm

            


Water in food which is not bound to food molecules can support the growth of bacteria, yeasts and molds (fungi). 
The term water activity (aw) refers to this unbound and available water.

The water activity of a food is not the same thing as its moisture content. Although moist foods are likely to have greater water activity than are dry foods, this is not always so; in fact a variety of foods may have exactly the same moisture content and yet have quite different water activities.

The Typical Water Activity of Some Foodstuffs
Type of Product Water Activity (AW)
Fresh meat and fish 0.99
Bread 0.95
Aged cheddar 0.85
Jams and jellies 0.8
Plum pudding 0.8
Dried fruit 0.6
Biscuits 0.3
Milk powder 0.2
Instant coffee 0.2

Measuring Water Activity (AW)
The water activity scale extends from 0 (bone dry) to 1.0 (pure water) but most foods have a water activity level in the range of 0.2 for very dry foods to 0.99 for moist fresh foods. Water activity is in practice usually measured as equilibrium relative humidity (ERH).

The water activity (aw) represents the ratio of the water vapor pressure of the food to the water vapor pressure of pure water under the same conditions and it is expressed as a fraction. If we multiply this ratio by 100, we obtain the equilibrium relative humidity (ERH) that the foodstuff would produce if enclosed with air in a sealed container at constant temperature. Thus a food with a water activity (aw) of 0.7 would produce an ERH of 70%.

Predicting Food Spoilage
Water activity (aw) has its most useful application in predicting the growth of bacteria, yeasts and molds. For a food to have a useful shelf life without relying on refrigerated storage, it is necessary to control either its acidity level (pH) or the level of water activity (aw) or a suitable combination of the two. This can effectively increase the product's stability and make it possible to predict its shelf life under known ambient storage conditions. Food can be made safe to store by lowering the water activity to a point that will not allow dangerous pathogens such as Clostridium botulinum and Staphylococcus aureus to grow in it. The diagram below illustrates the water activity (aw) levels which can support the growth of particular groups of bacteria, yeasts and molds. For example we can see that food with a water activity below 0.6 will not support the growth of osmophilic yeasts. We also know that Clostridium botulinum, the most dangerous food poisoning bacterium, is unable to grow at an aw of .93 and below.

The risk of food poisoning must be considered in low acid foods (pH > 4.5) with a water activity greater than 0.86 aw. Staphylococcus aureus, a common food poisoning organism, can grow down to this relatively low water activity level. Foods which may support the growth of this bacterium include cheese and fermented sausages stored above correct refrigeration temperatures.

Semi-Moist Foods
For foods with a relatively high water activity correct refrigeration is always necessary. These include most fresh foods and many processed foods such as soft cheeses and cured meats. However many foods can be successfully stored at room temperature by proper control of their water activity (aw). These foods can be described as semi-moist and include fruit cakes, puddings and sweet sauces such as chocolate and caramel.

When these foods spoil, it is usually the result of surface mold growth. Most molds cease to grow at a water activity level below about 0.8, but since some molds will grow slowly at this aw, it is usually recommended that products of this type do not have an aw greater than 0.75.

While this will not ensure complete freedom from microbial spoilage, those few yeasts and molds which do grow at lower water activities need only to be considered when special shelf life conditions must be met For example a commercial shelf life over twelve months might be required for confectionery; in these circumstances an aw below 0.6 would be required.

 Water activity chart

How Can We Control Water Activity?

In a nutshell to lower water activity, we can remove water, add solutes that will cause water to be bound, or lower the temperature of the product.

Removing water means that the remaining water is more tightly bound than before and therefore the water activity is lower.

Lowering the temperature results in reducing the rate of movement of the water and this acts to lower the water activity. It must be kept in mind that when products warm up, there water activity will rise too and then leave that product prone to whatever negative reactions occur within the new water activity range.

Adding ingredients can result in lowering water activity by lowering the energy state of absorbed water. This is known as the forces of adhesion and cohesion (van der Waal-London forces). Solutes, such as sugar or salt, lower the total free energy of the water and therefore "bind" it. The energy that would be needed to remove the bound water form the food is called water potential.

Water potential is an equilibrium measure. Equilibrium would imply that the water potential (energy needed to remove water) is the same throughout a food system. This often is not the case with foods. Sometimes however, the water is moving through the system so slowly that equilibrium cannot be achieved in the foods normal lifetime. In this case it can for practical purposes be said that the water is bound. For example, it is hypothesised that when foods enter the glassy state the movement of water is so slow that it is effectively bound.

Water potential is a quantitative measure of the binding energy of water in food.

Types of Instruments to Measure Water Activity

There are essentially two types of sensors used for measuring water activity. One measures changes in resistance (capacitance), the other measures chilled mirror/dew point.

 

Chilled Mirror / Dew Point Method

This is the primary method approved by the AOAC International. It's major advantage is speed. Some can take readings in as little as 5 minutes. This compared to 30-90 minutes for the capacitive sensors. Some manufacturers produce instruments that come precalibrated. Others require calibration using a variety of saturated salt solutions. According to some, the chilled/mirror sensors can determine water activity over a wider range than can the capacitive sensors.

Disadvantages of the chilled/mirror type of instrumentation is that the mirror can get coated with dust or otherwise contaminated and must then be cleaned. Be sure to note the design of the instrumentation with particular attention to how easy the mirror is to get to for cleaning. Another disadvantage of this style is that foods that contain propylene glycol cannot be analysed because the propylene glycol will condense on the mirror. (Prop. glycol is often a component of confectionary products) Extremely dry substances (i.e.: those with water activity of less than 0.03) equilibrate so slowly that measurements can take longer than the usual 5 minutes.

Capacitive sensor theory
Some aw instruments use capacitance sensors to measure water activity. Such instruments use a sensor made from a hygroscopic polymer and associated circuitry that gives a signal relative to the ERH. The sensor measures the ERH of the air immediately around it. This ERH is equal to sample water activity only as long as the temperatures of the sample and the sensor are the same. Since these instruments relate an electrical signal to relative humidity, the sensor must be calibrated with known salt standards. In addition, the ERH is equal to the sample water activity only as long as the sample and sensor temperatures are the same. Some capacitive sensors need between 30 and 90 minutes to come to temperature and vapor equilibrium. Accurate measurements with this type of system require good temperature control.

     

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