Isolation, characterization, functional properties and biological evaluation of tepary bean (Phaseolus acutifolius) proteins.

Abstract

Tepary bean (Phaseolus acutifolius) proteins were fractionated sequentially according to solubility in sodium phosphate buffer (SPB), sodium chloride (salt), ethanol, 2-mercaptoethanol (2-ME), and sodium dodecyl sulfate (SDS) solutions. Isolated protein fractions were analyzed for protein, amino acids (AA), trypsin inhibitor (TIA) and hemagglutinating activities (HA), phytic acid (PA), and characterized by SDS-PAGE. Tepary flour (TF) and SPB and salt protein fractions were tested for protein solubility, apparent viscosity, heat coagulability, emulsion capacity, foaming and foam capacity, and water and oil absorption capacity in comparison to soy protein isolate (SPI). Raw, cooked, and autoclaved TF or SPB protein fraction were evaluated for their nutritive value using mice. SPB and salt soluble protein fractions represented 83.2 and 13.7% of extractable protein, respectively. AA of SPB, salt, and 2-ME protein fractions were not significantly different (P < 0.05). Cysteine was concentrated in ethanol protein fraction. SDS-PAGE patterns of SPB and salt protein fractions contained 37 and 27 polypeptides, respectively. Major bands were at 29, 45, and 45 KDa. TIA was extremely high in ethanol protein fraction (161 TIU/mg sample). HA and PA were high in SPB protein fraction and TF (29,000 and 20,000 HU/g sample; 4.61 and 6.83 mg/g sample, respectively). Solubilities of SPB and salt protein fractions compared to SPI were higher at pH 1-4 but similar at pH 8-12. Although SPI had significantly higher viscosity than tepary proteins, TF and SPB protein fraction exhibited significantly higher heat coagulability and foaming properties. Salt protein fraction did not coagulate upon heating to 100°C. It also formed a weaker and less stable foam and had significantly lower emulsion capacity than TF, TA, and SPI. Tepary proteins absorbed significantly higher amounts of oil than SPI. Both raw and 20-min-autoclaved TF or SPB protein fraction, when included in the diets were deadly to mice while soaked then 20-min-cooked tepary beans or 40-min-autoclaved SPB protein fraction eliminated toxicity and led to a perceptible weight gain (0.30 and 0.23 g/day), PER (0.97 and 0.82), and digestibility (83.54 and 54.37%), respectively. Autoclaving TF or PE for 60 min had a significant depressing effect on feed intake, weight gain, and PER. Processing eliminated HA and reduced TIA by 91 to 99%.