MATERIALS AND SAMPLE PREPARATION
The crystallisation behaviour of amorphous concentrated starch syst ems was studied using pregelatinised waxy corn starch with an amylopectin content of more than 99 w% (C*Gel-Instant 12410), supplied by Cerestar Belgium.
The MDSC study was performed on compressed pellets of the freeze dried starch, fitting in the reusable high-pressure stainless steel (HPS) DSC pans from PerkinElmer. The starch concentration was adjusted by adding water to the sample with a micro syringe. The samples were equilibrated overnight, at room temperature, in hermetically closed HPS pans to obtain a homogeneous distribution of water in the sample. Small errors in the water content during sample preparation are unavoidable with this method, causing small variations in the initial values of the glass transition temperature of each sample. Dextrose up to malto-heptaose, with a degree of polymerisation (DP) from 1 to 7, were obtained from Sigma. Their respective molecular mass is 180, 352, 504, 666, 828, 990 and 1152 gmol-1.
(Modulated) differential scanning calorimetry
(Modulated) Differential Scanning Calorimetry ((M)DSC) measurements were performed on a DSC 2920 of TA Instruments, with modulated DSC option (MDSC™) and equipped with a Refrigerated Cooling System (RCS).
The purge gas was helium (25 mlmin"1). Temperature was calibrated with indium and cyclohexane. Enthalpy was calibrated with indium. Water at 35°C was used for the calibration of heat capacity, with the same modulation parameters as for the experiments.
The weights of the empty sample pan and reference pan were matched. The initial sample mass varied between 20 and 30 mg. No mass loss was observed for all thermal treatments applied.
The choice of modulation conditions and heating rate are of importance to obtain reliable results especially with HPS pans. A period of100s and an amplitude of 0.5°C were chosen. In non-isothermal conditions, an underlying heating rate of 1°Cmin-1 was never exceeded.
All MDSC measurements were started at -60°C. The samples were first heated to 170°C to erase the thermal history. After measuring the glass transition temperature of the amorphous materials during the second heating, the samples were instantaneously cooled to the (quasi)isothermal crystallisation temperature and kept (quasi)isothermally for a predetermined crystallisation time. Depending on the crystallisation temperature used, the end of crystallisation was reached in about 1500 to 5000 min. After the isothermal step, the samples were immediately cooled to -60°C. A subsequent heating to 200°C was performed under the same conditions. Some samples, however, were heated without modulation (conventional DSC), at a higher heating rate of 5°Cmin-1.
For studytng the crystallisation with conventional DSC, partial crystallisation experiments were carried out with the temperature program as described above (except the modulation). After partial isothermal crystallisation, the amount of crystallised material is calculated from the enthalpy of melting measured in a subsequent heating at 5°Cmin-1.
The water content of all starch samples was determined by TG, measuring the mass loss at 125°C in dry helium. The TG experiments were performed on a PerkinElmer TGA-7.
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