# Production of Starch Esters by Roasting Potato Starch with Unripe Apple Juice
## 3 Results and Discussion  
Malic acid is widely used in the food industry as a preserving and flavor-enhancing agent. It has a mild taste and easily dissolves in organic solvents. The Food and Drug Administration (FDA) has given it the \"Generally Recognized as Safe\" (GRAS) status, whereas the Food Chemicals Codex (FCC) has classified it as a food-grade organic acid [22]. In the present study, malic acid turned out to be the major acid of the unripe apple juice concentrate used to produce starch malates. Its content was determined to be 38.87 g/100 g apple concentrate dry weight (Table 1). The concentrate also contained small amounts of other organic acids, i.e., 2.75 g of succinic acid, 0.28 g of lactic acid, and traces of oxalic and citric acid per 100 g of dry weight. The malic acid found in the concentrate was a natural esterifying agent, which enabled the production of esters differing in the degree of substitution depending on reaction conditions. The number of malic acid residues esterified with starch during native starch roasting with the unripe apple juice concentrate is presented in Figure 1. As shown by the data in the figure, the degree of starch substitution increased along with the roasting temperature increase. However, lower roasting temperatures resulted in starch preparations with lower degrees of substitution (from 0.06 g/100 g to 0.12 g/100 g), whereas roasting at temperatures above 100 \({}^{\circ}\)C (120 \({}^{\circ}\)C and 140 \({}^{\circ}\)C) enabled producing preparations with a few-fold higher degree of esterification (0.55 g/100 g and 1.24 g/100 g, respectively). We observed a similar dependency in our previous study, where potato starch roasted with apple distillery wastewater at temperatures of 130 \({}^{\circ}\)C and 150 \({}^{\circ}\)C showed ca. 10-fold higher degree of substitution compared to the starch roasted at 110 \({}^{\circ}\)C [23]. The effect of roasting temperature on the degree of substitution of starch vitrates was also reported by other authors, who demonstrated that when starch was treated with citric acid at lower temperatures, the main phenomenon observed was mild hydrolysis [24], whereas its treatment at temperatures exceeding 100 \({}^{\circ}\)C resulted in starch esterification, with citric acid observed as the main reaction [25]. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Acid**} & \multirow{2}{*}{**Molecular Formula**} & **Content of Acid in Unripe Apple Juice** \\  & & **Concentate** \\  & & **[g/100 g]** \\ \hline malic & C4H6O5 & 38.87 \\ lactic & C6H807 & 0.28 \\ succinic & C3H6O3 & 2.75 \\ oxalic & C2H2O2 & \textless{}0.1 \\ citric & C3H4O4 & \textless{}0.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Content of organic acids in unripe apple juice concentrate determined with the liquid chromatography–mass spectrometry technique. Figure 1: Degree of substitution of esters obtained by roasting native starch with apple juice concentrate. Figure 2 presents the results of determinations of solubility in water (Figure 2B) and swelling power (Figure 2A) of the modified starch preparations. As can be seen from the data in the figures, the solubility of roasted native starch (N) increased along with increasing roasting temperature, reaching a value 20% higher in the case of starch roasted at the highest temperature compared to the native starch roasted at the lowest temperature tested. A similar dependency of starch solubility increase on the roasting temperature was also observed in our previous study [26], which demonstrated that the increased solubility of the analyzed preparations was due to the progressing starch thermolysis. Native starch esterification with the apple juice concentrate (NC) caused its solubility to increase compared to the control, except for the sample obtained upon starch roasting with the concentrate at the highest temperature tested (NC140), the solubility of which did not differ significantly from that of the control sample.