Effects of sonication with acetic acid on functional properties of maize starch

Abstract

Starch is chemically or physically modified to improve functional properties for wider applications. The objective of this work is to chemically and physically modify starch with a combination of sonication and acetic acid. Native starch was sonicated at 382 W/cm2 in distilled water, 5.16 M and 10.16 M acetic acid for 0, 3, 5 and 10 days, sonicated for 1 h each day. The experiment was a 2 (sonication level) x 3 (acetic acid concentration) x 4 (time interval) factorial. The treated and untreated starch were analysed for pasting properties, gel strength, water solubility index, water absorption index, Fourier transform infrared (FTIR), thermal properties with differential scanning calorimetry (DSC), X-ray diffraction, thermal properties, microstructure using light microscopy and scanning electron microscopy analyses. Starch sonicated in water had the highest peak, breakdown and setback viscosity as compared to starch that was sonicated in both concentrations of acetic acid and non-sonicated starches in acetic acid and water. Pasting temperature of all treated starches were lower than pasting temperatures of native and non-sonicated starch in water. Water solubility index and water absorption index at 90 ºC for starch sonicated in 5.16 M and 10.16 M increased from day 0 to day 3, and decreased from 5 days to 10 days. Scanning electron microscopy revealed that starch granules sonicated in acetic acid were partially eroded and had pits on the surface, suggesting that these pits increase surface area for increase water absorption and peak viscosity. Starch sonicated in 10.16 M had the formation of new absorption peak when analysed with FTIR that was associated with the acetyl group. Due the possible incorporation of acetyl group, firmness of the starch gels decreased with increasing number of days for both sonicated and non-sonicated starches and this incorporation did not affect the stickiness of the gel. There were no notable changes in A type molecular arrangement that was displayed by all treated starches. By comparing 0, 3, 5 and 10 days, starch sonicated in both acetic acid concentrations had relatively high relative crystallinity and non-sonicated in the same acetic acid concentration and native starch. Gelatinisation endotherm of sonicated starch both acetic acid concentrations decreased for 3 days and increased from 5 to 10 days while the enthalpy of non-sonicated starches decreased with increasing days of treatment. There were no obvious changes in shape and size of sonicated and non-sonicated starch in both 5.16 M, 10.16 M and water for 3, 5 and 10 days as determined by SEM. The maltese crosses reduced gradually from 3 to 10 days however, profound change was more observed in starch sonicated in both acetic acid concentrations (5.16 M and 10.16 M) than in non-sonicated starches with and without acetic acid. The combination of sonication with acetic acid had the potential to chemically and physically modify starch and be use as “clean label”.