Moisture adsorption characteristics of gingerbread, a traditional bakery product in Pardubice, Czech Republic

Adsorption isotherms of gingerbreads were obtained in the temperature range of 20–30 °C. The adsorption curves exhibited sigmoid shape of to the type III isotherm, typical for many foods. Generally, increasing temperature decreased adsorbed moisture (negative temperature effect) at low water activities, particularly in the curve for 20 °C. However, the negative temperature effect was reversed above water activity of 0.80 implying higher equilibrium moisture content at higher temperature. This altered temperature dependence of equilibrium moisture content in relation to aw could be attributed to the presence of sugars. Of the four sorption models, the GAB model gave the best fit at all the three temperatures. Monolayer moisture and other properties of sorbed water were also obtained showing the negative effect with increase in temperature. Isosteric heat of sorption obtained by applying Clausius–Clapeyron equation decreased exponentially with increasing moisture content. It could be implied from the results that keeping the moisture content of gingerbreads below 12–15% d.b. (equivalent to 60–64% of relative humidity) may ensure long shelf life of this products during storage at room temperature.