Effect of processing steps on the phenolic compounds and protein digestibility of cowpea

Cowpeas, Vigna unguiculata (L.) Walpers, belong to the family Leguminosae, originated from sub-Saharan Africa. It is known as a source of protein as well as phenolic compounds and typically have been consumed as dry seeds, which is common especially in undeveloped countries. However, the consumption is limited by presence of some antinutritional compounds such as trypsin which adversely affect the digestibility and bioavailability of nutrients. ıt has been suggested through many studies that traditional processing steps contribute not only to the reduction of the adverse effects of antinutritional compounds, but also increase the nutritional value of cowpea. In this study, in order to investigate the effect of both landrace and improved types of different varieties on the health-related constituents, the total phenolic, flavonoid, antioxidant capacity, major phenolic compounds were determined. Moreover, the effect of processing steps on total phenolic, flavonoid, antioxidant capacity, major phenolic compounds were determined for a selected cowpea variety along with simulation of in vitro gastrointestinal (GI) digestion as well as its protein content, protein digestibility and trypsin inhibitor activity were examined. The variety results showed that the free total phenolic content ranged from 0.36 to 1.13 mg GAE/g dw while the bound total phenolic content ranged from 6.05 to 9.13 mg GAE/g dw. Bound total phenolic content was higher than total phenolic content of the free forms, and this for each of the varieties. Also, there was no significant difference in free total phenolic content between landrace and improved types of each varieties (p>0.05). The total flavonoid content ranged from 0.41 to 1.48 mg CAE/g dw for the free fraction and 2.90 to 12.6 mg CAE/g dw for bound fraction in cowpea varieties. When compared free total flavonoid content between landrace and improved types of each varieties, no significant difference was observed (p>0.05). The results of the total antioxidant capacity which was measured by three different methods (ABTS, DPPH and FRAP), show different trends. When compared the types of those varieties, landraces had highest free total antioxidant capacity. On the other hand, no significant difference was observed between landrace and improved types of each varieties for antioxidant capacity (p>0.05). In briefly, Asrat had highest TPC, TFC, DPPH and FRAP values but not the highest ABTS values. Moreover, the improvement application seems to be ineffective in terms of increasing amount of phenolic compounds and antioxidative capacity. Results of processed cowpeas showed that raw cowpea had the highest protein content (39.06 g/100g dw), the protein contents were decreased with processing steps. The highest total phenolic content was found after the fermentation process (1.97 mg GAE/g dw) compared to the raw samples (0.80 mg GAE/g dw). For total flavonoid content, the highest value was found in cooked cowpea (1.10 mg CAE/g dw). The total antioxidant capacity was found to be the highest in fermented cowpea, ranging from 15.96 mg TE/g dw for DPPH, 32.57 μmol TE/g dw for ABTS to 60.61 μmol Fe II/g dw for FRAP. Furthermore, the increases in TAC were found after cooking and germination by 76.29% and 41.13% for ABTS, and after cooking by 4.93% for FRAP compared raw cowpea. The analyses of individual phenolic compounds in analyzed processed cowpea, belonging to phenolic acids and flavonoids. Luteolin was found in high quantities after soaking, dehulling, germination and fermentation process. Other phenolic compounds measured were vanillic acid, caffeic acid, sinapic acid, gallic acid, naringenin and p-coumaric acid, quercetin, catechin. The impact of processing on these phenolic compounds was different. Also, other phenolic compounds measured were vanillic acid, caffeic acid, sinapic acid, gallic acid, naringenin and p-coumaric acid, quercetin, catechin. The impact of processing on these phenolic compounds was different. According to the results of after in vitro gastrointestinal digestion, 38% of total phenolic content were present in the dialyzed fractions for raw cowpea while after dehulling (33%), soaking (25%), fermentation (23%), cooking (23%) and germination (21%) processes the recovery ratios were decreased. After the in vitro simulation of GI digestion, the dialyzed free flavonoid fraction represented 71% of the initial value for raw cowpea which had highest recovery. To determine the effect of in vitro gastrointestinal digestion on total antioxidant capacity, DPPH, ABTS and FRAP were used. Comparing the DPPH results, the lowest TAC of the dialyzed fractions was found in soaked (2.11 mg TE/g dw) and dehulled (1.74 mg TE/g dw) cowpea while raw sample had highest TAC of the dialyzed fraction (5.87 mg TE/g dw). After in vitro GI digestion, the highest recovery in the dialyzed fraction was measured after cooking process (107%). On the other hand, overall view of the results of ABTS, the amount of the dialyzed fraction was remained same after processing steps, except fermentation made an increase in amount of the dialyzed fraction (142.94 μmol/g dw) (p<0.05). Contrary to ABTS results, FRAP The amount of the dialyzed fraction decreased with processing steps, especially the lowest TAC was observed after dehulling. Protein digestibility was significantly increased by the process steps and the highest increase was observed in cooked cowpea (5%). On the other hand, there was a significant reduction measured in the trypsin inhibitor activity after soaking (8%), germination (30%), cooking (89%) compared to raw cowpeas (p<0.05). After fermentation, no trypsin inhibitor activity was detected. However, an increased activity of the trypsin inhibitor was observed in dehulled cowpea (11%) compared to the raw samples. According to SDS Page analysis, different process steps contained almost similar bands except cooked cowpea in the range of 21.5 kDa to 191 kDa. Moreover, when compared to bands belonging to trypsin inhibitor, the clearest band was seen in dehulled cowpea and followed by raw, soaked and germinated cowpea while there was no band equal to trypsin inhibitor in cooked cowpea and fermented cowpea.