Conclusions

The crystallisation window of plasticised starch is bordered by its glass transition and melting temperature. Both transitions depend on the water content. The plasticizing properties of water on starch were investigated using MDSC. Phase separation occurs from a certain concentration. However, it was remarked that during cooling, phase separation does not occur immediately and Tg still decreases, even beyond the concentration where phase separation should occur thermodynamically. The minimum Tg measured in this study with MDSC is therefore -25°C at 66w% ds.

To overcome the problem that Tg of dry starch cannot be measured without degradation, Tg is estimated using two different approaches. The first way is via extrapolation of Tg data of a series of maltooligosaccharides with increasing molecular mass. The second way is via extrap olation of Tg of starch samples with decreasing water content. A Tg value for dry starch of240°C and 250°C, respectively, is obtained.

The glass transition temperatures at different water contents are comparable for different types of starch studied (corn, waxy corn and potato).

It is shown that Modulated Differential Scanning Calorimetry (MDSC) enables to follow in situ the slow isothermal crystallisation process of concentrated amorphous starch systems. The accurate, reproducible, and continuous measurement of the heat capacity change during (quasi) isothermal crystallisation can be related to the crystallisation process, as confirmed by other techniques, like X-ray, DMA and Raman spectroscopy.

The MDSC method with high-pressure stainless steel pans enables a systematic study of the slow crystallisation of amorphous starch in the presence of small amounts of water. The major benefits of this procedure are the combination of (i) easy preparation of amorphous samples with a homogeneous water distribution before crystallisation; (ii) excellent control of temperature and water content, even for extended crystallisation times (several days) in combination with high crystallisation temperatures (up to 100°C); (iii) easy measurement of the evolu tion of the glass transition region using the same experimental set-up, enabling to establish relations between crystallisation and other thermal transitions.

The crystallisation rate was quantified as a function of isothermal crystallisation temperature, giving rise to a bell-shaped curve.

For waxy corn starch it was found that the temperature of maximum crystallisation rate decreases with the starch concentration, from 75°C for 76w% starch to 22°C for 60w% starch. The maximum crystallisation rate depends on the starch concentration. The highest crystallisation rate is obtained for a concentration of 70w% (t1/2 = 330min).

To be able to estimate the temperature of maximun crystallisation rate and the influence of the crystallisation temperature on the crystallisation rate for a chosen concentration, a universal crystallisation rate curve was proposed by plotting normalised rate data as a function of (Tc-Tg)/(Te-Tg). The temperature of maximum crystallisation rate for this concentrated starch systems lies at approximately (Tg+Te)/2.

Crystallisation rates of different types of starch (waxy corn, corn and aewx) were compared. Retrogradation rates were found to increase with the amylose content and the size of the chain length of the amylopectin fraction. Therefore, aewx crystallises faster than corn starch, which in turn crystallises faster than waxy corn starch. Nevertheless, the maximum crystallisation rate was obtained at the same temperature (75°C at 76w% starch).

The melting endotherms of recrystallised starches are at least bimodal. The onset of melting starts only about 10°C above the crystallisation temperature. The end temperature of melting is almost independent of the crystallisation temperature.

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