## 3 Results and Discussion  
Potato starch extrusion with apple distillery wastewater followed by roasting led to obtaining starch esters substituted with malic acid residues and with trace amounts of other acids (Figure 1). The effectiveness of the esterification process was mainly affected by the roasting temperature of the starch extrudates. The preparations roasted at the highest temperature tested (140 \({}^{\circ}\)C) had an almost five-fold higher degree of substitution than those heated at 80 \({}^{\circ}\)C. The effect of temperature on the intensity of chemical reactions is commonly known. In the present study, the impact of the extrusion temperature was negligible, probably because of the short period of preparation exposure to its action. The low effectiveness of the starch extrusion process has been reported by other authors [20,21]. The relatively good outcome of the esterification reaction achieved in the present study can be due to a high concentration of reacting substances, their very well mixing, and the roasting process applied [22]. The extrusion of starch increases its solubility in water and decreases its swelling power, with the intensity of changes being dependent on the type and moisture content of the raw material, and process parameters [23]. The roasting of native starch also increases its solubility, which is associated with carbohydrate thermolysis [24]. The solubility of the extruded and roasted starch in water increased along with an increasing extrusion temperature and decreased along with an increasing roasting temperature (Figure 2a). Each time, an increase in the extrusion temperature by 20 \({}^{\circ}\)C caused an increase in the solubility by several to a few dozen percent on average (from ca. 16 to ca. 39 g/100 g). In the case of roasting, the differences resulted from exceeding the roasting temperature of 100 \({}^{\circ}\)C. At roasting temperatures of 80 and 100 \({}^{\circ}\)C, the solubility was at ca. 35 g/100 g on average, but, when the temperature of 100 \({}^{\circ}\)C had been exceeded, it decreased to ca. 25 g/100 g. These differences could be caused by the starch re-polymerization with the low-molecular-weight products of its depolymerization observed at higher temperatures. The re-polymerization process could, additionally, be intensified by low-molecular-weight dextrins formed during the extrusion process and present from the very beginning of the roasting process. This hypothesis can be corroborated by the results of analyses of extruded and roasted starch resistance to amylolysis (which will be discussed further in the Discussion portion of this section), which was exceptionally high (Figure 3). The starch esters produced in all of the experimental variants were characterized by lower solubility than the control samples. At roasting temperatures up to 100 \({}^{\circ}\)C, the value of this trait decreased by several dozen percent on average (from ca. 35 to 21 g/100 g), whereas, at higher roasting temperatures, it reached only a few percent (6-9 g/100 g on average). An analogous tendency was observed during starch esterification with citric acid [25] or malic acid [26]. It should also be remembered that the process of esterification was performed in an acidic medium, which catalyzes starch hydrolysis processes, thereby increasing the content of the water-soluble fraction. The low solubility of starch esters can only be explained by the crosslinking (attachment) of the soluble starch fraction with starch chains by means of malic acid. The mechanism of crosslinking of starch roasted with citric acid was described in our previous work [25]. Figure 1: Degree of substitution of esters produced by starch extrusion with apple distillery wastewater and roasting (EW). The extrusion of starch decreases its swelling power, with the decrease being greater along with an increasing extrusion temperature [27].