Moisture penetration and crystallization in sugar glasses

In most studies, moisture sorption into amorphous foods is expressed as a change in the bulk moisture content, which does not provide detail on how a moisture front moves into the food. This study was targeted towards characterization of the moisture behavior in amorphous saccharide materials of different molecular weight (MW), glass transition temperature (Tg) and moisture content. Various sugar glass systems (initial moisture content 4.0 to 6.5%) made with mixtures of sucrose and other sugars (fructose, glucose, maltose and lactose) (60:40 solid basis), stored at 21±3°C and 71±3% RH, were analyzed for moisture uptake over a period of several weeks by conventional gravimetric method and unidirectional moisture penetration using Raman micro-spectroscopy. Bulk moisture uptake was greatly influenced by tendency of crystallization, especially for crystallization-prone samples like lactose and maltose. Penetration profiles (moisture content vs. penetration depth) were obtained from the Raman spectra based on standard curves (peak ratios versus moisture content) for each sample. Mixtures with lower molecular weight sugars (fructose and glucose) exhibited faster penetration compared to those with higher molecular weight sugars (lactose and maltose). However, the absolute penetration depth values for respective days varied among the sugar mixtures having similar molecular weight sugars (lactose and maltose). In the case of sugar glasses consisting of different sugars, penetration rate decreased with an increase in the glass transition temperature. Initial moisture penetration rate was found to be marginally higher for the sugar mixtures having higher initial moisture content. Crystallization appeared to be faster in the sugar mixtures with higher molecular weight sugar (lactose) compared to lower molecular weight sugar (fructose and glucose).