The Plasticisation Effect of Glycerol and Water on the Gelatinisation of Wheat Starch
| Main Authors: | , , |
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| Format: | Article |
| Language: | English Slovak |
| ISSN: | ISSN 0038-9056 |
| Online Access: | http://www.viks.sk/chk/star_3_4_03_131_137.doc |
| Abstract: | Summaries. In the extrusion manufacture of starch-based thermoplastics, such as biodegradable packaging materials, glycerol is an effective additive as a plasticiser, that is, to diminish the brittle nature of the product and provide the desired extent of flexibility. However, the addition of glycerol may also affect the gelatinisation behaviour of the starch-water mixture, and hence the required processing conditions for producing a homogeneously gelatinised starch-based material. The effect of glycerol on the gelatinisation of wheat starch was studied using differential scanning calorimetry (DSC). Mixtures of starch, water and glycerol were investigated with a water content ranging from 12 -40% and a glycerol concentration up to 75%. Dependent on composition, the enthalpy of gelatinisation ranged from 1.7-12.6 J/g (on a dry starch basis), while the onset and peak temperatures varied from 54 to 86 °C and 60 to 90 °C, respectively. As expected, water acted as a plasticiser in that the onset temperature for gelatinisation (To) decreased with increasing moisture content. Glycerol, however, increased To. It is shown that the T0 of starch-glycerol-water mixtures may be predicted on the basis of the effective moisture content of the starch fraction of these mixtures resulting from the relative speed of moisture absorption by glycerol and starch, respectively. Moisture sorp-tion kinetics of wheat starch and glycerol in 100% relative humidity were determined and used to predict the preferential water absorption by glycerol in starch-glycerol-water mixtures and hence the resulting To of the system. Conclusion. It was observed, through differential scanning calorimetry, that glycerol behaves as an "anti-plasticiser" as it hinders the gelatinisation process rather than assisting it. A linear increase of To with increasing glycerol content was observed. It is proposed that in starch-glycerol mixtures, the hydrophilic nature of glycerol interferes with the moisture uptake of starch, and hence diminishes the effective moisture contents of the starch available to aid gelatinisation. This hypothesis was tested through sorption studies comparing the rate of moisture absorption of starch, glycerol and mixtures thereof. It was evident that glycerol absorbed water at a much faster rate than starch. The Peleg model was fitted to the sorption data to predict the effective amount of water available to the starch in starch-wa- ter-glycerol mixtures. It was shown that the gelatinisation temperature of the starch-glycerol-water mixtures was equivalent to that of starch-water mixtures with the calculated effective moisture content. Hence it was shown that the gelatinisation temperature of starch-glycerol-water mixtures can be predicted based on sorption behaviour of the ingredients. The method presented here may be applicable to a wider range of starch mixtures. This method may also be used to control the gelatinisation process, by compensating for the hygroscopic nature of the various ingredients by altering the total moisture added. |
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| ISSN: | ISSN 0038-9056 |
