Physicochemical and Functional Characteristics of Phosphate Acetate Starch at Different Concentrations of Acetic Anhydride

Starch is a natural biopolymer with wide potential applications; however, its native properties remain limited, particularly in terms of stability and functionality. Arenga starch (Arenga pinnata) is a locally abundant starch source in Indonesia, but its utilization requires modification to enhance its quality. The objective of this study was to evaluate the physicochemical and functional characteristics of dual-modified arenga starch through phosphorylation–acetylation. Phosphorylation was carried out using a 99:1 ratio of sodium trimetaphosphate (STMP) and sodium tripolyphosphate (STPP) at 5%, followed by acetylation with acetic anhydride at concentrations of 5, 7, 9, and 11% (w/v) based on starch weight. The analyzed variables included moisture content, ash content, water-holding capacity (WHC), oil-holding capacity (OHC), swelling power, solubility, and emulsion separation volume. The results showed that moisture content varied nonlinearly: low (5%) and high (11%) concentrations reduced it, while the intermediate concentration (9%) increased it to near-native levels. Ash content decreased sharply at 5% (0.22%) compared to 0.67% in native starch but increased again at 7–11%. WHC significantly increased from 0.69 g/g (native) to 1.17 g/g (11%), while OHC peaked at 9% (3.21 g/g) before slightly declining at 11%. Swelling power rose to 22.07 g/g at 9% and then decreased, whereas solubility consistently decreased with higher reagent concentrations. In addition, emulsion stability improved with increasing acetic anhydride concentration, with phase separation at 11%, only 26.9% after 72 hours, indicating much higher stability than native starch (57.5%). The XRD diffraction pattern shows similar peak characteristics with relatively homogeneous crystallinity values in the range of 26–27%. The FTIR spectrum showed that an increase in treatment concentration (5–11%) led to changes in intensity and shift in absorption bands, particularly in the region of about 1701–1725 cm⁻¹, indicating the formation of new functional groups (such as carbonyls/esters) compared to native samples.