# Production of Starch Esters by Roasting Potato Starch with Unripe Apple Juice
## 3 Results and Discussion
The temperature of starch roasting also had a significant effect on the color change observed in the produced preparations. The darkening of native starch roasted with the apple juice concentrate (NC) intensified along with the increasing roasting temperature compared to the control samples (Figure 6). Compared to the native starch roasted at the lowest analyzed temperatures (N70 or N80), the native starch roasted with the apple juice concentrate at the same temperatures (NC70 or NC80) had the lowest value of the darkening index (ca. 3.5), whereas the higher roasting temperatures contributed to significant darkening of the analyzed preparations. This was probably due to the caramelization reaction [40] and Maillard reaction [41] proceeding under these conditions. Figure 5: Flow curves of pastes prepared from modified starch and mathematical parameters describing them. Figure 6: Darkening index of esters obtained by roasting native starch with apple juice concentrate. Figure 7 shows the susceptibility of native starch roasted at various temperatures and of the preparations produced from starch esterified with the apple juice concentrate to the action of amylolytic enzymes. Resistance of the roasted native starch (N) to the effects of amyloglucosidase was observed to increase along with the increasing roasting temperature. A similar dependency between starch susceptibility to amylolysis and roasting temperature was observed in our previous study [26]. Starch esterification caused a further significant increase in enzymatic resistance, with the higher resistance developed by the preparations along with the roasting temperature increase. The highest resistance to the action of amyloglucosidase, exceeding 40%, was determined for the starch roasted with the apple juice concentrate at the highest temperature tested in the study. The increase observed in the resistance of the produced malates to amylolytic enzymes was most likely due to the formation of resistant starch upon esterification. As demonstrated by [PERSON] et al. [42], the increase in resistant starch content noted during the heating of starches of various origins with malic acid was attributed to the partial damage of amylose and amylopectin structures in an acidic environment. Such conditions promote the substitution of hydroxyl groups with a malate group and starch molecule cross-linking. In turn, an increase in the content of cross-linked starch inhibits the action of amylolytic enzymes, thereby increasing the enzymatic resistance of the preparations. The increased enzymatic resistance of the produced preparations was also linked to the degree of substitution. Starch esters with a higher degree of substitution were characterized by a significantly higher resistance to amylolytic enzymes. We observed a similar dependency in our previous study with malates obtained upon potato starch heating with apple distillery wastewater [23]. Also, other authors demonstrated an increasing content of resistant starch along with an increasing degree of substitution of starch esterified with malic acid [9; 10]. The present study results suggest that heating potato starch with juice from unripe apples offers an effective method for the production of preparations with a high content of resistant starch, which could be used as a food component with beneficial health-promoting effects. The availability of apples in our geographical region makes the technology based on the use of natural apple juice for starch modification economically viable and, above all, suitable to produce health-safe preparations with desired properties that can also be classified as clean-label ingredients. The developed method for starch malate production  
Figure 7: Resistance to amylolysis of native starch and esters obtained by roasting native starch with apple juice concentrate.