Chocolate syrup, like any other food product, has a critical water activity below which microbial growth is inhibited; provided this level is not exceeded, the syrup can be stored and consumed safely. Given the safety-related need to measure water activity, a study has sought to identify the best-practice method for measuring water activity using an AquaLab water activity analyser – which is generally considered to be an excellent instrument for making fast, accurate, reliable water activity measurements in different environments including research and development departments, quality assurance laboratories and production areas.
The study sought to establish whether the test conditions made any significant difference to the measured water activity value. In particular, the conditions under investigations were ambient humidity, test temperature and measuring time. In addition, the study compared the water activity average values, test times and repeatability across several water activity instruments, and also considered the potential benefits of using the custom mode testing option in the AquaLab Series 4TEV when testing chocolate syrup.
Two AquaLab Series 4TEV instruments and a single AquaLab Series 3 were used for the study. While the Series 3 uses a chilled mirror dewpoint measurement technique, the Series 4TEV instruments – which are designed to be suitable for use with samples containing volatiles – uses both a dewpoint sensor and a capacitance sensor. All three instruments were calibrated daily using unsaturated salt solutions at 0.25aw, 0.5aw, 0.76aw and 1.00aw. Tests were conducted on three replicates taken from three independent samples. Because the study, in part, investigated the effect of ambient humidity, all of the sampling and testing was conducted in a glovebox with controlled and monitored humidity – which is not necessary when using the AquaLab water activity analysers in ‘real world’ applications. The humidities used for the study were 30%, 50% and 70% RH.
In the first part of the study, the investigation focused on measurement time. Normally the sample is placed in the AquaLab, the test procedure started, and the instrument concludes the test automatically when the reading stabilises – which typically takes three to five minutes. However, for this study the instrument was set to continue taking readings for up to one hour, then the initial and final mean water activity and standard deviation across the three samples were compared using ANOVA (analysis of variance). This part of the study concluded that water activity measurements for chocolate syrup were not significantly better after an extended measurement time. Furthermore, as well as the readings themselves being no better, the standard deviations indicated that repeatability was no better either.
The second part of the study sought to evaluate the benefits, if any, of using the AquaLab’s custom mode when analysing chocolate syrup. Custom mode allows the user to set stability specifications for ending the test; a water activity range is specified and this must be met by a specified number of tests. To determine the optimum custom mode for achieving the best combination of repeatability and speed when testing chocolate syrup, four custom mode settings were compared: three tests within +/-0.001aw, five tests within +/-0.001aw, three tests within +/-0.003aw and five tests within +/-0.003aw. All of these tests were conducted using one AquaLab Series 4TEV instrument with three replicates from three samples, with all tests conducted at 25 degrees C. Analysing the results, the researchers found that there was no significant difference in the value or standard deviation when measuring the water activity using custom mode, so they concluded that custom mode provided no benefit when measuring the water activity of chocolate syrup.
Water activity can be temperature-dependent but the sensitivity varies between food products; the third part of the study therefore considered the influence of temperature on the water activity of chocolate sauce. In this part of the study, the water activity of each replicate from each of three samples was measured at temperatures of 15, 25 and 45 degrees C. The mean water activity and measurement time were compared for the three temperatures to see whether the temperature made any significant difference to the water activity. In fact the results indicated that the water activity of chocolate syrup is temperature-independent within the range studied.
Looking at all the results from the studies undertaken in a wide variety of conditions, the chocolate syrup water activity always measured 0.838+/-0.003aw. In addition, there was no benefit in either extending the measurement time beyond 3-5 minutes or using the AquaLab’s custom mode.
In conclusion, the AquaLab is an excellent instrument for making accurate, repeatable measurements of water activity in chocolate syrup across a wide range of operating conditions. The user simply has to place sufficient chocolate syrup in the sample cup to cover the bottom, then run the default test routine. Provided the instrument has been calibrated and is clean, good results will be obtained very quickly and easily.
Chocolate syrup has a relatively high water activity and the AquaLab can be used on food samples with activities ranging from 0 to 1.0aw. Nevertheless, caution should be exercised because measurements for water activity in other food products may be more susceptible to changes in environmental conditions than was found to be the case for chocolate syrup.
Labcell is the sole UK distributor for the AquaLab water activity analysers. Contact Labcell for more information or to request a demonstration by telephoning 01420 568150, emailing mail@labcell.com or go to www.labcell.com. Labcell can also support customers who wish to conduct their own studies into the influence of environmental conditions on water activity measurements for particular food products.
