Crystallisation and glass transition behaviour of Chilean raisins in relation to their sugar compositions.

The effects of sugar component ratio, water fraction, and storage conditions on crystallisation and glass transition temperature (Tg) of three Chilean dried raisins were examined by using differential scanning calorimetry (DSC), polarised light microscopy and X-ray diffraction (XRD) techniques. Thompson, Flame and Golden raisins differed in fructose:glucose and glucose:water ratios, impacting on their measured Tgs (P < 0.05) and propensity of sugaring. The ratios of fructose:glucose (1:3, 1:2 and 1:1) and glucose:water (1.7, 1.9 and 2.1) also influenced the measured Tgs (P < 0.05) and sugaring of fructose-glucose-water model solutions. Measurement of Tgs in the raisins as a function of water activity (aw 0.11-0.74) showed that water acts as a strong plasticiser decreasing the Tgs (-16.4 to -61.6 °C). XRD results revealed that sugaring in Thompson raisins was delayed at low temperatures (5 & 15 °C) compared to that stored at 25 °C. The refrigeration may be a simple approach to delay the sugaring in raisins.