# Production of Starch Esters by Roasting Potato Starch with Unripe Apple Juice
## 3 Results and Discussion
When investigating the properties of starch citrates, Falade & Ayetigbo [27] also obtained esters exhibiting higher solubility in water compared to native starch. The solubility of esters produced upon starch roasting with the apple juice concentrate at a temperature of 100 \({}^{\circ}\)C ranged from 26% to 30%. Presumably, gentle heating of starch with apple juice concentrate leads to mild hydrolysis and amylose leaching from starch granules, which in turn contributes to the increased solubility of the modified preparations [24]. A further increase in roasting temperature caused a significant decrease in the solubility of the starch esters to a value below 19% in the case of those obtained upon starch roasting with the apple juice concentrate at the highest temperature tested. Most likely, the changes observed in the solubility of the produced malates were related to the degree of substitution of the obtained esters (Figure 1) because, as proved by other authors [28], starch esters with a low degree of substitution exhibit higher solubility in water compared to starches with a high degree of substitution. The roasting of native starch (N) caused an insignificant increase in its swelling power along with a roasting temperature increase (from ca. 20 g/g to 25 g/g; Figure 2A). The samples obtained after starch roasting with the apple juice concentrate showed a lower swelling power compared to the control samples, but still, the difference between the samples increased with the increasing roasting temperature. Starch esters produced at roasting temperatures of 70-100 \({}^{\circ}\)C showed a similar swelling power, reaching 17.3 g/g on average, whereas roasting temperature increase over 100 \({}^{\circ}\)C caused a decrease in the swelling power of the produced malates to ca. 6.5 g/g (NC140). A reduced swelling power of starch esters, compared to the native starch, was confirmed in ample research [26; 29]. High-temperature roasting of starch with apple juice concentrates probably leads to the cross-linking of starch chains, whereas the links formed impair water penetration and contribute to a decrease in the swelling power of the modified preparations [30]. In addition, [PERSON] and [PERSON] [31], who analyzed the properties of starch citrates, concluded that a decrease in their swelling power was due to the enhanced interaction between amylose and amylopectin molecules and strengthened intramolecular bonds. In turn, [PERSON] et al. [32] ascribed the lower water capacity of starch citrate to the presence of short-chain molecules formed during acidic hydrolysis. Furthermore, [PERSON] et al. [5] found that a change in the swelling power of starch esters was influenced by the degree of substitution, with the tendencies and extent of these changes depending on reaction conditions. Figures 3 and 4 present parameters describing the thermal characteristics of pasting of starch malates and roasted native starch. As can be concluded from the data in Figure 3A, the initial pasting temperature decreased along with an increasing roasting temperature in both types of preparations. Starch esterification with the apple juice concentrate at roasting temperatures from 70 to 100 \({}^{\circ}\)C (NC70, NC80, or NC100) caused an increase, whereas modification at temperatures above 100 \({}^{\circ}\)C (NC120 or NC140) caused a decrease in the initial pasting temperature compared to the control samples. An analogous tendency was observed for the end pasting temperature of the obtained preparations (Figure 3B). Similar changes in the pasting temperatures of starch and malates, as affected by roasting temperature, were observed in our previous work. These parameters of the pasting characteristics of the analyzed samples were most likely affected by the degree of substitution of starch esters, i.e., the higher the substitution degree, the higher the roasting temperature. Also, other authors reported a declining pasting temperature of esterified starch along with an increasing roasting temperature [33].