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.