Formula Foods

Why do other systems require temperature control while the standard AquaLab Series 3 does not?

Almost all other instruments measuring water activity use a capacitance relative-humidity sensor. For capacitance a_w measurements to be accurate, the sensor and sample must be at exactly the same temperature. A one degree difference between sample and sensor temperatures leads to a 5% (0.05 a_w) error in calculating water activity. Capacitance systems require temperature control to keep the sample and sensor at the same temperature to avoid these serious errors.

AquaLab uses a chilled-mirror dewpoint sensor and does not require precise sample temperature control. The AquaLab measures the sample temperature and determines the vapor pressure in the headspace using a chilled-mirror dewpoint sensor. A built in microcontroller computes the water activity from these measurements. Differences between sample and sensor temperature, therefore, have very little effect on AquaLab's accuracy.

When do I need the AquaLab temperature control model (Series 3TE) instead of the standard AquaLab Series 3?

There are three reasons for sample temperature control with the AquaLab: 1) to minimize extreme ambient temperature fluctuations; 2) to compare water activities of different samples independent of any effects temperature may have on their water activity; and 3) to comply with special government or corporate regulations on temperature control for specific products.

Most AquaLab customers purchase our standard model (Series 3). The temperature controlled version (Series 3TE) is equipped with a thermoelectric module that monitors and maintains a constant temperature in the sample chamber. Water activity measurements are therefore be made at a fixed constant temperature. Because of the added features, the Series 3TE is slightly more expensive than the standard Series 3.

What does "extreme ambient temperature fluctuations" mean?

If the lab (and AquaLab) temperatures fluctuate by as much as ±5°C daily, water activity readings will vary less than ±0.01 a_w. Often, this much uncertainty in sample water activity is acceptable, so there is no need to purchase the temperature controlled model. Such variations in ambient temperatures are uncommon. But, if your lab temperature varies to this degree, and you require better than 0.01 a_w precision, you may want a Series 3TE.

How does temperature changes affect water activity?

Temperature effects vary depending on the material being tested. Water activity of unsaturated salt solutions is not temperature dependent. In saturated salt solutions, temperature affects solubility, so a_w can change with temperature, depending on how solubility changes (see Greenspan, 1977). Labuza (1984) provided a thermodynamic model for estimating the temperature effect on water activity in foods, but the parameters in the model must be empirically determined. In the examples given by Labuza, water activity increases as temperature of the sample increases, but the water activity of most saturated salts decreases as temperature increases. One can therefore not predict even the direction of the change of water activity with temperature since it depends on how temperature affects the factors that control the water activity in the food.

Solubility of solutes can sometimes be a controlling factor, but control is usually from the state of the matrix or food polymer to which the water is bound. Since temperature can have a profound effect on the state of that matrix, (glassy vs. rubbery state) one should not be surprised that temperature affects the water activity of the food. Labuza showed that the effect of temperature on water activity is negligible in high moisture foods, but in intermediate and low moisture foods a 10°C change in temperature can result in a few percent change in a_w.

The temperature dependence of water activity in foods.

To illustrate the temperature dependence of water activity in low, intermediate and high moisture foods the following experiment was conducted. We procured samples of low (dry powder soup mix, sweetened toasted oat cereal, and chocolate peanut butter cups), intermediate (bakery chocolate chip cookies, granulated dog food, and frosted coconut cake snack) and high moisture (beef jerky, chocolate syrup, and sausage) foods.

An AquaLab CX-2T (an older temperature-controlled version) determined the water activity at temperatures of 10, 20, 30 and 40°C. Samples were sealed in Decagon's plastic sample cups with the same set of samples was used for all temperatures.

Samples were run in triplicate for the low and intermediate moisture foods, while single measurements of the high moisture foods were measured. All measurements were completed within one working day to assure that samples hadn't lost water during the experiment. The measurement sequence was from cold to hot.

Some measurements were repeated at 10°C to confirm that sample water activity had not changed during the experiment. The table below shows the results of the experiment.

Temperature Dependence of Water Activity

The foods clearly show a wide range of behavior. Some foods increase water activity with temperature, others decrease, while others hardly change. The soup mix changes substantially between 10 and 30°C. The beef jerky, chocolate syrup, and sausage showed negligible change in a_w with temperature. The average standard error of the mean for the three samples was 0.003, thus most of the changes seen are statistically significant. The errors for the peanut butter cups, however, were about 3 times the average. For most of the samples the change in water activity with temperature is typically less than 0.002 per degree. The data from the table above may help you determine whether you need a Series 3 or a Series 3TE.

Why is a 25°C measurement even important?

The primary reason is compliance with government regulatory agencies requiring 25°C sample readings.

How can I compare AquaLab's room temperature a_w readings with readings at 25°C?

The change of a_w with temperature is usually so small that no correction is needed. This is especially true at high water activities. If you do need to make these corrections, contact Decagon for assistance or consult Labuza (1984).