Kidney Bean Substitution Ameliorates the Nutritional Quality of Extruded Purple Sweet Potatoes: Evaluation of Chemical Composition, Glycemic Index, and Antioxidant Capacity

Abstract

The extrusion process may influence the nutritional profiles of carbohydrate-rich food ingredients, including the glycemic index (GI) and antioxidant capacity. This study aimed to evaluate the nutritional profile of extruded purple sweet potato (EPSP) substituted with kidney bean flour (KBF) (0, 30, and 40%). These foods were further characterized by their proximate composition, resistant starch, polyphenols, GI, and antioxidant capacities. The 40% KBF substitution enhanced the protein and fiber contents of the EPSP by up to 8% and 6%, respectively. Moreover, it also revealed that EPSP with 40% KBF substitution had a low-GI category (53.1), while the 0 and 30% substitution levels had a high-GI category, i.e., 77.4 and 74.7, respectively. However, the extrusion processing reduced the anthocyanin content and antioxidant capacity of purple sweet potato flour containing 40% KBF by 48% and 19%, respectively. There was a significant relationship between the GI values of proteins, fats, and fibers (p < 0.05). The insignificant effect of resistant starch and phenol contents on GI value was recorded due to the low concentrations of those components. KBF substitution could ameliorate the profile of protein, fiber, and GI, but not for antioxidant capacity. The other innovative processes for preserving antioxidant capacity might improve the product quality.

Keywords: extrusion; fiber; glycemic index; indigenous crop; legumes.

Figure 1

The visual appearance of extruded purple sweet potato with various proportions of kidney bean flour according to the formulation given in Table 1.

Figure 2

Glycemic response (means ± SE) of subjects after consuming glucose as reference food (Ref 1 and Ref 2) and F0, F1, and F2 as test foods. Ref 1, First replication of the reference food; Ref 2, Second replication of the reference food; F, Formula according to the formulation given in Table 1; F0, Formula with 0% substitution of kidney bean flour; F1, Formula with 30% substitution of kidney bean flour; F2, Formula with 40% substitution of kidney bean flour.

Figure 3

Content of protein and total dietary fiber (%db) (means ± SD) (a) and GI (means ± SEM) (b) are affected by processing type, i.e., drying for PSP flour, wet heating for steamed PSP, and extruded PSP with different levels of kidney bean substitution (0% for F0, 30% for F1, and 40% for F2). ^a–d, Different superscript letters in each column are significantly different at p < 0.05, analyzed using one-way ANOVA and the Duncan post hoc test; ‡ Data was adopted from previous unpublished research that used the same methodology and the same PSP variant (ayamurazaki); PSP, Purple Sweet Potato.

Figure 4

Profile of anthocyanin content, IC₅₀, and AEAC of PSP flour and extruded PSP flour (selected formula: F2). * Significantly different at p < 0.05, analyzed using a t-test; F2, Formula with 40% substitution of kidney bean flour; IC₅₀, the concentration of an antioxidant-containing substance required to scavenge 50% of the initial DPPH radicals; PSP, purple sweet potato; AEAC, Ascorbic Acid Equivalent Antioxidant Capacity.