Heat-moisture modification of pigeonpea cajanus cajan (L.) Millsp. starch in the presence of different calcium compounds and physico-chemical properties of modified starch

Abstract

The effects of the presence of calcium ions (Ca2+) during heat-moisture treatmant (HMT) at a 30% moisture content and 110°C for 1 h on the structures and properties of pigeonpea [Cajanus cajan (L.) Millsp.] starch were examined. Calcium hydroxide (Ca(OH)2), calcium chloride (CaCl2) and calcium lactate (C6H10CaO6) were used at Ca2+-starch ratios (g/g) of 0:1, 0.0001:1, 0.0004:1 and 0.0007:1. The effects of HMT temperature (100-120°C) and time (1-2 h) were also studied using Ca(OH)2 at Ca2+-starch ratios of 0:1 and 0.0007:1. The starches were analysed with microscopy (light,polarized light and scanning electron) and X-ray diffraction, as well as for swelling power, % solubility, amylose leaching, pasting properties, freeze-thaw stability, rheological properties (steady shear and dynamic) at 25°C and thermal properties. Granules of native starch, HMT starch in the absence of calcium compound (HMT-starch) and HMT starch in the presence of calcium compound (HMT-Ca starch) were of round and oval shape. The HMT-starch and HMT-Ca starches showed loss of birefringence at the granule center. The presence of calcium compounds led to a greater loss of birefringence. The native starch exibited a C-type crystalline pattern with 38.02% relative crystallinity (RC). Upon HMT, a crystalline pattern changed to A- type and % RC was at least 18% and 11% lower for HMT-starch and HMT-Ca starches, respectively. An increase in Ca2+-starch ratio increased % RC. Using Ca(OH)2 or C6H10CaO6 led to a decrease in swelling power, % solubility and amylose leaching. For pasting properties. the peak, trough, breakdown and setback, which were prominent in the native starch, were absent in the HMT-starch and HMT-Ca starches. For all the calcium compounds used, an increasing Ca2+-starch ratio decreased the viscosities at the end of the heating period at 95°C (ηh) and the cooling period at 50°C (ηc). Using Ca(OH)2 or C6H10CaO6 had a stronger effect of the Ca2+-starch ratio on ηh and ηc than using CaCl2. The Ca(OH)2 gave the best freeze-thaw stability starch. All starch suspensions (5% w/w) showed pseudo-plastic with a yield stress following the Herschel-Bulkley model. As regerds the use of Ca(OH)2 and C6H10CaO6, an increase in Ca2+-starch ratio decreased the consistency coefficient (K) along with increased the flow behavior index (n), but did not affect yield stress (τ0). At their linear viscoelastic region (0.1–10 Hz and 1% strain), all starch gels also exhibited an elastic-like solid. The HMT-Ca starches had lower storage modulus (G′) than the native starch and the HMT-starch. The native starch gave the lowest gelatinization temperatures, while the HMT-Ca starches gave the highest gelatinization temperatures. An increase in the Ca2+-starch ratio increased gelatinization temperatures but decreased the gelatinization enthalpy. After storage at 5°C for 7 and 14 days, the regelatinization peak had a lower temperature than the gelatinization peak. After 14 days storage, the % retrogradation of native starch was 61.98, and that of HMT and HMT-Ca starches was at least 14.18% and 49.64% lower, respectively. Using Ca(OH)2 and C6H10CaO6 retarded starch retrogradation better than using CaCl2. Increases in HMT temperature and time caused greater loss of birefringence at the granule center and the gelatinization temperatures of HMT-starch and HMT-Ca starches. They also decreased ηh, ηc, swelling power, % solubility, amylose leaching, % retrogradation and % RC of these starches. Using Ca(OH)2 starch at the Ca2+-starch ratio of 0.0007:1, HMT at 120°C for 2 h gave the best freeze-thaw stability starch.