Characterization and nixtamalization of pigeon pea Cajanus cajan L. Millsp. starch

Abstract

Pigeon pea, Cajanus cajan L. Millsp, starch was isolated through wet milling and fractionated by sieving. Native starch and its fractions were characterized for their composition, size, shape and crystallinity and properties (thermal properties, water absorption capacity, swelling power, solubility, amylose leaching, pasting properties, freeze-thaw stability and enzymatic digestibility). Nixtamalization, boiling and then steeping grains in 0 – 1.0 % w/v calcium hydroxide (Ca(OH)₂) solutions, was used to modify pigeon pea starch granules. In order to evaluate effects of nixtamalization on starch granule structures as well as properties, changes in structures and properties mentioned above (except enzymatic digestibility) were determined. For this purpose, oil absorption capacity, percentage retrogradation and rheological properties were also determined. In dry basis, native pigeon pea starch contained less than 2 % non-carbohydrate compounds, 96.74 % starch and 30.74 % amylose. Starch granules had round, spherical-like, and oval shape. Granule size ranged from 4.88 to 60.87 μm with the volume moment mean diameter, D[4,3], of 27.81 μm. Crystalline pattern was C-type and relative crystallinity was 35.24 %. Through differential scanning calorimeter, granules started to gelatinize (onset temperature, To) around 73°C and required enthalpy of gelatinization (HGel) of 17.35 J/g. At 70°C, water and oil absorption capacities were 229 g water/g dry starch and 51 g oil/g dry starch, respectively. Increases in swelling power, solubility and amylose leaching with temperature followed the Arrhenius-type temperature dependence. At 95°C, swelling power, solubility and amylose leaching were 35 g/g dry sample, 23 % and 72 %, respectively. Enzymatic hydrolysis was 0.36 %. Peak viscosity, final viscosity, and setback were 3,856, 5,604, and 2,791 mPa.s, respectively. At 4°C, retrogradation was 56 %, after 12 day storage. After 7 freeze-thaw cycles, syneresis was 12 %. At 75 °C, 8 %w/w paste had pseudoplastic flow with 24 mPa.s0.44 consistency index. At 25°C, strain of 2 % and frequency range of 0.1 – 100 rad/s were rubbery region for 15 % w/w paste with storage modulus (G΄) of 38,900 Pa. By sieving, three granule fractions having D[4,3] of 24.08, 27.07 and 30.05 μm were obtained. As D[4,3] increased, amylose content increased, however phosphorus content and percentage relative crystallinity decreased. Gelatinization temperatures, HGel, pasting properties with the exception pasting temperature, percentage syneresis increased. Though water absorption capacity, swelling power, solubility, amylose leaching and percentage hydrolysis decreased as D[4,3] increased. Soaking grains in water did not affect structures and properties of pigeon pea starch. Soaking grains in Ca(OH)₂ solutions caused deposition of calcium on granule surface resulting in slight increasing in pasting properties and G΄, but decrease in percentage retrogradation and syneresis. Interactions among components during boiling altered chemical compositions of starch granule resulting in modification of its physical properties. Boiling grains caused partial gelatinization of starch granules resulting in higher contents of protein, lipid, crude fiber, ash, calcium and phosphorus more than 6, 2, 2, 2, 2 and 10 times, respectively, due to less complete separation of starch and other components. However starch and amylose contents were lower by 30 % and 20 %, respectively. Gelatinized starch granules had D[4,3] higher than native starch by at least 60 %. Boiling grains decreased percentage of relative crystallinity about 30 %. Nixtamalized, or modified, flours had higher gelatinization temperatures as well as water and oil absorption capacities. Below 70°C, swelling power, solubility and amylose leaching of modified flours were higher than those of native starch, the opposite direction was observed at 70°C and higher. Modified flours had RVA final viscosity less than 20 mPa.s. Due to low amylose content and amylose leaching, modified flours had low consistency index. An increase in Ca(OH)₂ used for boiling grains increased calcium and phosphorus, as well as percentage of relative crystallinity. This resulted in increases in gelatinization temperatures, HGel and water absorption capacity at 70°C. Although it decreased swelling power, solubility, amylose leaching, RVA final viscosity, percentage retrogradation and syneresis.