Measurement of water activities of foods at different temperatures using biospeckle laser

Water is one of the main components of food and its presence is related to the rate of deterioration of foods. Thus, control of the water activity ( a w ) is a fundamental feature of food technology. The methods currently used for determining a w of foods mostly use the relative vapor pressure of the product to quantify the a w at constant temperature. This last factor limits evaluation of the a w because the instrument used is not capable of determining the a w at temperatures lower than 25 °C and/or at negative temperatures. In this study, the potential of the biospeckle laser for a w measurement was evaluated. Contrast analysis using the biospeckle laser technique provided statistically significant differences in the concentrations and a w for lithium chloride solutions and model solutions of pectin and sucrose at temperatures of 0, 5, 10, 15, 20, and 25 °C, as well as at freezing temperatures. After calibration of the biospeckle laser with the lithium chloride solutions, a w of corn starch, oatmeal, and wheat flour was determined at different temperatures, with errors lower than 20%. This achievement provides a platform for developing protocols to measure the a w of foods at different temperatures using the biospeckle laser.