CHEMICAL COMPOSITION, STARCH STRUCTURE AND ANTIOXIDANT ACTIVITIES OF SOME LIMA BEANS (Phaseolus lunatus L. WALP) VARIETIES

Abstract

The key nutritional potential of Phaseolus lunatus is unquestionable, due to the high content of bioactive compounds, macro- and micronutrients. Four lima beans (P. lunatus) varieties (NSWP100, NSWP98, NSWP47 & NSWP96) were subjected to standard analytical techniques in order to evaluate the physicochemical, some nutrients composition, starch structure, phytochemical constituents and in vitro antioxidant activity of their constituents (whole meal, flours, starches and seed coats) for comparative study in order to determine their potential practical applications. The starch from NSWP96 was subjected to debranching, hydrothermal treatments (annealing, heat-moisture and temperature cycling) and enzyme hydrolysis. Chemical composition of the seed constituents (g/100 g) contained crude protein 14.53 – 30.20; moisture content 3.17 – 11.20; crude fibre 2.80 – 33.59; crude fat 0.59 – 2.91; total ash 0.13 – 4.06; carbohydrate 47.11 – 97.66 and resistant starch 60.10-82.50. The most abundant mineral element in the studied varieties was potassium 7.53- 626.31 mg/100 g followed by sodium 7.53 – 119.22 mg/100 g, while lead, cobalt and nickel were not detected in the seed constituents. The amino acids profile demonstrates nutritionally acceptable results for the four varieties due to the high levels of essential amino acids such as lysine, leucine, arginine and phenylalanine; in comparison with FAO/WHO standard; with total essential amino acid (TEAA) 49.47 - 53.29 g/100 g protein. Glutamic acid and aspartic acid were the most abundant amino acids in all the constituents while marginally limiting in S-containing amino acids cysteine and methionine (Cyst + Meth). The molecular weight of proteins was in the range of 54 - 18.5 kDa under reducing conditions. The scanning electron micrograph showed a spherical rough surface, oval smooth granular starch surface without pin holes in flours and native starches respectively; C-type of polymorph (A- and B-types) granular packing; while the granule structure was affected by the treatments. Native starches showed strong peak at diffraction angles 2θ =17.10o and 23.18o while treated starches had strong peak at 2θ = ~ 17.02o. The common features of polysaccharides with modes generating absorption bands at 1045–1200 cm-1 (flours), 1000-1300 cm-1 (native starches) and 1056-1340 cm-1 (treated starches) was shown by Fourier-transform infrared spectrum. The constituents exhibited minimum solubility at pH 4 and maximum solubility at pH 10 for all the varieties. Water and oil absorption capacity ranged from 1.29 to 3.14 g/g and 1.57 to 3.97 g/g respectively. The least gelation concentration was in the range of 6 - 16%. The proteins showed higher emulsion and foaming properties with concentrate and isolate from NSWP96 (PPC and PPI) which exhibited significant foaming properties. Maximal swelling capacity and solubility in all varieties was observed at 90 oC. Pasting characteristics showed final viscosity value of 4010 - 4025 RVU and 4019 – 4179 RVU; set back 1923 -2112 RVU and 2003 -2173 RVU for flours and starches respectively. The phytochemical estimation of the constituents showed alkaloids (0.13 -55.25%) and saponin (2.05-80.54 mg/g) as the most concentrated phytochemicals observed in flours and seeds coat from all the varieties. The concentration of total phenol was higher in the seeds coat than other constituents 35.50 - 41. 74 mg GAE/g sample. Quercitrin and caffeic acid constitutes the most abundant phenolic compound found in free fraction whereas ellagic and caffeic acid were the most abundant phenolic in bound fraction from P. lunatus flours. The radical scavenging ability of the constituents against ABTS and DPPH free radicals was in a concentration-dependent manner, TEAC values range of 0.17-0.36 and 0.13- 0.28 observed in seeds coat and whole meal respectively; an IC50 value of 0.37 mg/ml recorded in seeds coat which is an indication of higher activity. The results varied among the varieties, thus, the significant occurrence of the nutritive composition , bioactive compounds and antioxidant capacity along with the interesting functional properties of the P. lunatus constituents’ could be considered useful as functional ingredients for food formulation.