# Production of Starch Esters by Roasting Potato Starch with Unripe Apple Juice
## 3 Results and Discussion
Figure 2: Swelling power (**A**) and solubility in water (**B**) of native starch and esters obtained by roasting native starch with apple juice concentrate. Figure 4: Heat of phase transition of pasting of native starch and esters obtained by roasting native starch with apple juice concentrate. Figure 3: Initial (**A**) and end temperature of pasting (**B**) of native starch and esters obtained by roasting native starch with apple juice concentrate. The heat of phase transition of the produced esters ranged from 12.44 J/g to 16.11 J/g (Figure 4). Its lowest value among all analyzed samples was determined for the starch roasted with the apple juice concentrate at the highest temperature tested. The heat of phase transition is a measure of starch crystallinity, and its decrease may result from the loss of the ordered starch structure [34]. The observed decrease in the heat of phase transition was probably due to the higher degree of substitution of this preparation, whereas the attachment of functional groups modified the arrangement of amylose and amylopectin chains, leading to the enlargement of the amorphous region of starch granules [35]. The rheological properties of pastes produced from starch roasted at various temperatures and starch esters produced with the use of the apple juice concentrate are presented in Figure 5. All analyzed samples, except for the starch roasted with the apple juice concentrate at the highest temperature tested, showed properties of shear-thinned non-Newtonian fluids, which is typical of starch pastes [36; 37]. Among the pastes prepared from roasted native starch, the highest viscosity was determined in the entire course of the flow curves for the paste prepared from starch roasted at 80 \({}^{\circ}\)C (N80), whereas the lowest value of this parameter was determined for the paste produced from starch roasted at 140 \({}^{\circ}\)C (N140). Native starch roasting with the apple juice concentrate (NC) caused a significant decrease in the viscosity of pastes compared to the control samples, with the samples roasted at the highest temperatures tested (NC120 and NC140) exhibiting a drastic paste viscosity decrease. Changes in the viscosity of the investigated samples may also be observed by analyzing values of rheological parameters determined based on [PERSON]'s and [PERSON]'s rheological models. The decreased viscosity of the preparations obtained upon starch roasting with the apple juice concentrate was also indicated by the consistency coefficient (K), which decreased along with the increasing roasting temperature of the malates, reaching very low values (0.03 Pa-s\({}^{2}\) and 0.01 Pa-s\({}^{2}\)) in the case of the preparations produced at temperatures higher than 100 \({}^{\circ}\)C (NC120 and NC140, respectively). The esters produced at the highest roasting temperatures tested were also not resistant to shear forces, as indicated by very low values of their plastic viscosity (0.02-0.01 Pa-s) determined based on [PERSON]'s model. Likewise, starch roasting with the apple juice concentrate at a temperature above 120 \({}^{\circ}\)C caused a drastic decrease in paste viscosity, making determination of the above-discussed rheological parameters impossible in some cases [23]. The difference observed in the viscosity of pastes was most likely due to thermolysis progressing during starch roasting and the presence of organic acids in the apple juice concentrate [38], which might have accelerated the degradation of starch molecules at high temperatures. The decreased viscosity of the pastes prepared from starch esters could also be due to the formation of new functional groups during starch reaction with malic acid, which enhanced the interaction between two starch molecules by cross-linking. The bonds formed between starch chains make it difficult for these chains to unfold during gelatinization and increase the steric hindrance of water molecules, which in turn reduces the viscosity of starch malates [39].