DSC data showed that there was only one inflection point for Tg. Apparently extrusion cooking resulted in a homogenous extrudate and the pellet was amorphous17,18. Figure 1 illustrates the dependence of glass transition temperature (Tg) and expansion temperature (Te) on the equilibrium moisture content of pellet, which was extruded at in-barrel water content of 55%. The equilibrium moisture acted as a plasticizer and lowered both Tg and Te. The Tg obtained was little higher than that for amylopectin9, but showed similar moisture dependence pattern. The difference may be due to the compositions and preparation of sample. Tg reached a finite value when the equilibrium moisture content was higher than 20%. The pellet without dextrin yielded a finite Tg higher than freezing point of water and was classified as water-sensitive polymer19. With the addition of 20% of dextrin, Tg was lowered and yielded a finite value lower than the freezing point of water. The addition of dextrin modified the pellet from water-sensitive polymer to water-compatible polymer. The intrinsic viscosity of pellet was reduced from 92.7 mL/g to 86.5 mL/g by adding 20% dextrin.
It appeared that the major effect of dextrin was to decrease the average molecular weight of the pellet. Te exhibited similar dependence on the equilibrium moisture content, but was higher than Tg. The Addition of dextrin also decreased Te due to the reduction in molecular weight. Te increased linearly with Tgwith a correlation coefficient (r2) of 0.95 (Fig. 2). The regressed equation is
Te = 0.53 Tg + 95.02 (6)
where Te is the initial expansion temperature (oC) and Tg is the glass transition temperature in
oC. Thus Tgwas a good reference point for studying the expansion of pellet.
The data showed that Te were 20o~100oC above Tg (Fig. 3) at the experimental conditions. This difference was similar to which (Te – Tg ≒ 10 ~ 70 oC) occurred during frying20. Since melting temperature is mostly 100oC higher than Tg, Te appears to be in rubbery state. The value of (Te-Tg) increased as Tg decreased. Both increasing equilibrium moisture content and adding dextrin resulted in an increase in (Te-Tg). More energy was required to evaporate water at high moisture content. Thus, the value of (Te-Tg) increased with moisture content. The expansion ratio (data not shown) was reduced by the softening effect of moisture. The addition of dextrin caused a formation of crust, which could hold more water vapor and resulted in higher (Te-Tg). In this case, the expansion ratio was increased. For example, the expansion ratio was increased from 5.12 to 5.83, 6.07, and 6.56 by adding 5%, 10%, and 20% dextrin, respectively, for the pellet extruded at 55% water content. Lowering in-barrel water content resulted in lower expansion ratio. The expansion ratio of pellet with 20% dextrin extruded at 45% in-barrel water content was 4.41. Both extrusion condition and formulation affected the expansion behaviour of pellet.
There existed an equilibrium moisture (10%) yielding the maximum expansion of the pellet (Fig. 4). The expansion ratio was dramatically decreased when the moisture was raised from 10 to 15%. The expansion ratio might be related with mechanical properties of materials at rubbery state. The pellets were weak and soft with low elongation and low energy to break point as the equilibrium moisture increased21. In other words, the pellets were not completely elastic but acted more like stiff gel-like materials. Thus, it was more difficult to expand, which resulted in lower expansion ratio. The expansion ratio did not drop too much
when the equilibrium moisture increased from 15 to 25%. It was difficult to have a uniform size and distribution of air bubbles at high moisture content. As the moisture content was reduced from 10 to 5%, the pellet became brittle and harder for expansion. Therefore, the pellet yielded lower expansion ratio. Intrinsic viscosity affected the expansion ratio as shown in Fig. 5 for the 10% equilibrium moisture pellet extruded at 55% water content with the presence of dextrin or not. The expansion ratio decreased linearly (r2 = 0.89) as the intrinsic viscosity increased at the experimental conditions. In other words, the lower the molecular weight, the higher the expansion. Addition of dextrin reduced the average molecular weight as well as viscosity at rubbery state. The reduction in viscosity increased the expansion20.
The pellets consisted of rice flour and water, and thus were considered as binary mixtures. Gordon-Taylor equation was used to fit the experimental glass transition temperatures for the samples without dextrin. Fig. 6 illustrates the comparison between the experimental data and calculated values. Although Gordon-Taylor equation was reported to underestimate Tg of extruded corn meal10, the fitting was very well (r2 = 0.88). The regressed value of k in equation 2 (Gordon-Taylor equation) was 0.245 which was almost exactly equal to the ratio (0.242) of △Cpstarch(0.47 J/g.k) to △CpH2O (1.94 J/g.k) from literature9. The results demonstrate that Gordon-Taylor equation can be used to predict Tg of the extruded rice pellet. This may be due to low protein and fat content in rice flour. Thus, starch dominated the properties of pellet after extrusion processing.
Once dextrin was added in the formulation, the pellets became ternary mixtures.
An equation has been developed to calculate Tgof ternary mixtures consisting of rice flour, dextrin, and water and expressed as follows:
(5)
The ternary mixture was considered to be a linear combination of two binary mixtures. The first term in equation (5) represented the binary mixture of water (w1) and rice flour (w2) and the second term was for the binary mixture of water and dextrin (w3). The distribution of water in the two binary-mixtures was assumed to be dependent upon the weight fraction of rice flour and dextrin. Thus the terms of w1[w2/(w2+w3)] and w1[w3/(w2+w3)] represented the distribution of water in two binary-mixtures, respectively. These terms were used to modify Gordon-Taylor equation for calculating Tg of binary-mixtures. The total weight fraction (the term in the square bracket) in each binary-mixture was used as a weight-factor for calculating Tg of the whole system. Equation 5 appears to be a modified Gordon-Taylor eq’n, in which k1 and k2 was similar to the ratio of △Cpstarch/△CpH2O and △Cpdextrin/△CpH2O, respectively. It is more complicated than the expanded Couchman-Karasz eq’n but much simpler than the dual applications of Gordon-Taylor eq’n9. Fig. 7 illustrates the comparison between experimental and calculated Tg’s with r2 of 0.88. Both the regressed values of k1 and k2 were equal to 0.221. The regressed k1 was 8.7% lower than 0.242 as mentioned above. Again, the starch dominated the property of pellet. When the literature data of dextrin with DE of 35 was used (△Cpdextrin = 0.53 J/g.k)22, the regressed k2 was 19% lower than the calculated value (0.273). The high correlation coefficient (r2 = 0.88) indicated that equation (5) was acceptable for predicting Tg of the pellet consisting rice flour, dextrin, and water.
ACKNOWLEDGMENT
This study is part of the project sponsored by the National Science Council of the Republic of China (project no. NSC 87-2313-B-002-067). The financial support is greatly appreciated.
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