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Thermal events in calcium alginate beads during encapsulation dehydration and encapsulation-vitrification protocols

Gamez-Pastrana, R., Gonzalez-Arnao, M.T., Martinez-Ocampo, Y., Engelmann, F.
Acta horticulturae 2011 no.908 pp. 47-54
calcium alginate, differential scanning calorimetry, encapsulation, freezing, glass transition, glycerol, melting point, sucrose, temperature profiles, thermal analysis, vitrification
The effect of dehydration conditions employed in encapsulation-dehydration (ED) and encapsulation-vitrification (EV) protocols on thermal events occurring during freezing was studied on empty calcium alginate beads using differential scanning calorimetry (DSC). With ED, beads were submitted to 1 day pregrowth in liquid MS medium supplemented with 0.75 M sucrose and desiccated for 1, 2, 4, 6, 8 or 24 h with silicagel. With EV, beads were subjected to a loading treatment in 0.4 M sucrose + 1 M glycerol solution for 20 min, and then exposed to vitrification solutions PVS2 or PVS3 for 0, 5, 10, 15 or 30 min at 0°C or room temperature. Thermal analyses were performed using a DSC Q2000 V23.4 apparatus (TA Instruments), with a ramp of -5°C.min-1 from 35 to -70°C and 10 °C.min-1 from -70 to +70°C. The thermal profiles obtained revealed that penetration of sucrose in beads during preculture with 0.75 M sucrose resulted in a decrease of the melting temperature, compared with that of untreated beads. Exposure of samples to PVS at 0°C produced a slower removal of freezable water from beads compared with water removal at room temperature. However, after 30 min exposure, all freezable water was extracted from the beads, irrespective of the PVS employed and of temperature. It was observed that more freezable water was removed using PVS2, compared with PVS3 under the same treatment conditions. Beads dehydrated with PVSs displayed a glass transition (Tg) temperature below -65°C, while Tg of beads dehydrated with silicagel was above -50°C.