How to measure Water Activity

Objective

To measure the water activity of food.

Background

Apart from moisture content, it is also very important to check for water activity. Water activity tells you how active the water is in a food system. Is the water free to move around or is it bound? Water can be bound by other food components like carbohydrates, salt, proteins and lipids. The freer the water is, the more it is available for use by microorganisms, and for chemical and metabolic reactions such as enzymatic activity. Often, moisture content alone may not be enough to predict shelf life. Moisture content may be high but the product could be very shelf stable. For example, jams and jellies may have a very high moisture content (up to 90%), but the water activity may be relatively low (0.7). This is caused by the ability of sugar to bind water.

Water activity ranges from 0.0 – 1.0, where 1 is the water activity of pure water. It can be calculated as the ratio of the vapor pressure of air surrounding the food to the vapor pressure of pure water. So what do we mean by vapor pressure? Well, you know that foods contain water. This water is constantly evaporating and condensing. You don’t need to boil foods for water to evaporate. If you put the food in a closed system e.g. a sealed jar, the water that is evaporating from the food will exert a pressure on the container, known as the vapor pressure. Higher water activity tells us that there is more vapor in the closed system and therefore more free (unbound) water in the food. To calculate water activity, the vapor pressure exerted by the water in the sample is divided by the pressure that is exerted by pure water in a closed system.

Since vapor pressure will vary depending on the temperature, you should always report the temperature when you are reporting water activity. The diagram below shows the effect of water activity on oxidation, browning reactions, enzymatic and microbial activity.

Water activity is determined in the lab using a water activity meter. This machine measures the vapor pressure of the water surrounding the food and divides it by the vapor pressure of pure water to give a value between 0.0 – 1.0.

Materials and Equipment

1. Food samples – navy bean flour, ketchup, biscuit, orange juice and cooking oil
2. Water activity cups
3. Water activity meter

Method

1. Transfer food sample to water activity cup, filling it to about half full. Note: Biscuit should be ground uniformly before measurement
2. Place in water activity meter, close and measure
3. Record the water activity and the temperature
4. Repeat measurement, calculate and report the average

Lab Questions

1. Differentiate between moisture and water activity (2 points)
2. What advantage is there to collect water activity readings in addition to moisture? (3 points)
3. Organize your results in an excel table and calculate the mean and standard deviations of water activity readings (5 points)
4. Explain reason(s) for major differences in water activity observed (10 points)

Reference: Labuza, T.P., S.R. Tannenbaum and M. Karel. 1970. Water content and stability of low moisture and intermediate moisture foods. Food Technology 24:543-550