PHYSICOCHEMICAL PROPERTIES OF TEPARY BEAN STARCH.

Abstract

The composition of tepary beans Phaseolus acutifolius var. latifolius was determined; starch (41%) and protein (26.3%) were the predominant components. The physical, physicochemical, rheological, structural and nutritional properties of the isolated starch and its suitability in food products were evaluated. Starch granules were spherical to oval with a typical legume starch birefringence pattern. They appeared smooth and showed a normal distribution of diameters (range 15.5 - 59.9 μ; mean 33.5 μ). Tepary starch had an amylose content of 30.7% and a gelatinization temperature range of 70.5 - 84.0°C. Other physicochemical values were as follows: phosphorus, 0.01%; iodine binding capacity, 6.14%; alkali number, 8.97; inherent viscosity, 2.44 ml/g; water binding capacity, 82.1%; swelling power, 13; solubility, 17.9%; and sag, 6.26%. Fractionation provided an amylose fraction with a β-amylolysis of 76.3%, limiting viscosity of 83.5 ml/g and a DP(η) of 618. Amylopectin (iodine binding capacity, 0.88%) had a DP(η) of 1094, limiting viscosity of 91.1 ml/g and CL, OCL, and ICL values of 33, 22 and 10 glucose units, respectively. Gelatinization of tepary starch was a single-step process. Pasting viscosity was stable when starch suspensions were cooked at 95°C for 15 min; viscosity progressively increased when pastes cooled to 50°C. At 4 and 6%, tepary starch pastes resembled a type C viscosity pattern, whereas at 7% and above, pasting behavior exhibited a type B pattern. Pastes were stable at pH 5 to 9 but showed faster breakdown and no retrogradation at pH 3. Addition of sucrose and salt changed rheological behavior; 2% oil had no effect on pasting. Sensory evaluation of tepary starch vanilla pudding established its consumer acceptability. In vitro determination indicated that raw tepary starch was poorly digested (8%). Digestibility improved (from 8 to 82%) upon cooking. Raw corn starch was more digestible (25.6%) than tepary starch. Freeze-drying reduced the hydrolysis of all materials evaluated (i.e. 8.64 to 3.21%, 8.03 to 2.63% and 25.6 to 17.8% for tepary flour, tepary starch and corn starch, respectively). Autoclaving decreased the digestibility of tepary flour (8.64 to 5.72%), and tepary starch (8.03 to 7.38%), but it slightly improved that of corn starch (25.6 to 27.7%).