Understanding the mechanism of cross-linking agents (POCl3, STMP, and EPI) through swelling behavior and pasting properties of cross-linked waxy maize starches

The effects of cross-linking waxy maize starch with phosphorous oxychloride (POCl3), sodium trimetaphosphate (STMP), or epichlorohydrin (EPI) on degree of swell and pasting properties were studied. As expected, increased concentration of cross-linking agent resulted in decreased granule swelling potential, Q(mL/g). The slower acting reagents, STMP (4-hr reaction time) and EPI (17-hr reaction time), showed a similar relation between Q value and molar concentration of agent, which was different from the faster-acting POCl3 (30-min reaction time). Brabender viscoamylograph results show decreased peak viscosity with increasing amounts of cross-link agent due to increased inhibition to swelling. Brabender viscosities (BU) continued to increase after the time interval in which an uncross-linked sample would dissolve, which may be a sign of flocculation. The magnitude of BU for all of the treatments after 41 min, plotted versus calculated molar concentration of cross-linking agent, showed a similar trend for all three reagents, indicating that type of reagent plays little effect on the overall pasting behavior of cross-linked waxy maize. However, when BU was plotted versus Q, starches treated with POCl3 again separated themselves with much higher viscosities than the collectively grouped EPI- and STMP-treated starches. The combination of the reduced swell and higher viscosity indicates that POCl3-treated granules have a more rigid external surface area, with hard crust formed on the outer layers of the granule. This information shows that the mechanism of action of the individual reagents plays a major role in the physicochemical behavior of the starches.