In vitro dry matter digestibility of sprouted cowpeas
The IVDMD coefficients of raw (control) and sprouted cowpeas, the measured key-spot chemical components and the inhibitor activity auxiliary variables are presented in Table 1. Quantitative effects on the main auxiliary explanatory variables (TIA, crude protein, acid and neutral detergent fibre) are plotted in Table 2. Sprouting influenced the compartmental and total digestion (p<0.001). The steps 1-2 (gastric-ileal) IVDMD dropped (p<0.05) in the two and three-day sprouts, the sprouts which had highest (p<0.05) step three IVDMD. The total (steps 1-3) IVDMD coefficient increased (p<0.05) in 2-4 day sprouts (p<0.05).
Sprouting affected the compartmental IVDMD in unpredictable fashion, with the high total digestibility observed in 2-4-day sprouts (Table 2). The pattern of total IVDMD digestibility was largely quantitatively consistent with confounding effects of the increasing fibre content and reduction in TIA (Table 2). In previous studies, soaking and sprouting similarly altered the compositional quality of cowpea sprouts
(Devi et al., 2015). The positive trend in CP in cowpea sprouts (Table 2) was quantitatively similar to the trend reported in 2-5 day sprouts
(Malomo et al., 2013), though without effect on the chemical score, essential amino acid index, biological value and requirement index
(Malomo et al., 2013).
Abdelatief (2011) reported higher chymotrypsin inhibitor (15.02 TIU/mg protein) in unprocessed cowpeas compared to those previously (
Sumathi and Pattabiraman, 1976) reported for cowpeas (7.2 TIU/mg protein) and for and soybeans (6.6 TIU/mg protein). The reasons for higher levels of these enzyme inhibitors in the experimental local cowpea seeds could be attributed to adverse environmental conditions as well as varietal differences (
Abdelatief, 2011). In different cowpea cultivars,
Devi et al., (2015) reported 29-56% decrease in TIA with soaking.
Khattab and Arntfield (2009) reported similar TIA reduction in soaked cowpeas. Though protease inhibitor activity tends to decrease as germination proceeds (
Kayembe and van Rensburg, 2013), in the present study, the lowest level TIA was obtained after day 4 sprouting. In sprouts, initial TIA (day 2) was recorded before gradual depletion (Table 2). Antinutritional factors may be endogenous, partially as byproducts during the processing of proteins
(Gilani et al., 2011) in legume seeds. Sprouting increases, the permeability of cell membrane, increasing the amount of antinutrient leaching
(Ali et al., 2022). On a residual DM basis, similar to the nutrient, such change in TIA could merely reflect depletion of readily soluble and, or metabolized organic matter, in as much as soluble inhibitors might also not be recovered in solid digesta.
Crude protein and embryonic structural carbohydrate constituents also increased in 4-day sprouts
(Nonogaki et al., 2010). Similar to the present study, sprouting cowpeas for 4 days increased CP (
Uppal and Bains, 2012), in addition to crude fibre (
Uppal and Bains, 2012;
Devi et al., 2015), calcium
(Devi et al., 2015). In previous studies, IVDMD also depended on the sprouting period
Recharla et al., (2019). Soaking (8-10 hours) and sprouting (1-3 hours) increased cowpea
in vitro protein digestibility from 6 to 17% (
Uppal and Bains, 2012). However, in the present study, given small indigestible residue sample recovery from the in vitro digestion, computation of DM digestibility and nutrient and anti-nutrient content expression on DM basis should be carefully interpreted.
In vitro dry matter digestibility of roasted cowpeas
The IVDMD coefficients of raw (control) and roasted cowpeas are presented in Table 1. Roasting influenced the partial step 3 and the total IVDMD (p<0.05), with no effect on steps 1-2 IVDMD (p>0.05). The roasting increased (p<0.05) both step 3 and the total IVDMD to peak in the 20-minute (105°C) roasts (p<0.05). The lowest (p<0.05) total IVDMD was recorded in 15-minute (95°C) roasts (p<0.05). Quantitative effects on the main auxiliary explanatory variables (TIA, crude protein, acid and neutral detergent fibre) are plotted in Table 3.
Roasting influenced the partial step 3 and the total IVDMD, without effect on steps 1-2 IVDMD. The roasting increased both step 3 and the total IVDMD to peak in the 20-minute (105°C) roasts (p<0.05), which was considered most effective processing. Heat inactivated cowpea enzyme inhibitors (
Khatoon and Prakash, 2005).
Udensi et al., (2007) reported heat lability phytic acid. Trypsin inhibitor thermal stability persisted to 80°C, after which activity decreased to detectable low levels at 100°C
(Kansal et al., 2008), which is consistent with the apparent deactivation in the present study (Table 3). Trypsin inhibitor thermal stability is attributed to the rigid, compact protein structure which is stabilized by several disulfide linkages (
de la Sierra et al., 1999).
In legumes, perhaps of equal importance to macromolecular depolymerization facilitated nutrient release is antinutrient detoxification to expose biomolecules to plant endogenous and animal enzymatic degradation. The main proteinaceous anti-nutritional factors in cowpeas are trypsin inhibitors and lectins
(Mihailovi et al., 2005). The molecular architecture
Kimura et al., (2008); Oliveira et al., (2012) of these key proteins is relevant to optimal seed processing. The trypsin inhibition assay is the standard test for the efficacy of thermal, hydrothermal and methods used to process grain legumes for food
(Miki et al., 2009). Nutritionally, TIA is classified
(Miki et al., 2009) into very low (2-4 TIU mg
-1 DM), low (4-7 TIU mg
-1 DM), medium (7-10 TIU mg
-1 DM) and high (10-13 TIU mg
-1 DM). A 2 TIU level is considered the minimal threshold enzyme-inhibiting TIA
(Miki et al., 2009).
Roasting affects DM digestibility by gelatinizing resistant starch (
Uppal and Bains, 2012), protein denaturation and breaking of cross-linkages (
Khatoon and Prakash, 2005) and through complex mallard oxidation reactions involving carbohydrates, lipids and nitrogenous compounds (
Khatoon and Prakash, 2005), processes which when excessive, may negatively affect digestibility. Overall, the major legume antinutrients are heat labile (
Khatoon and Prakash, 2005).
Roasting increased the lipid and protein content of cereal seeds such as millet (
Sade, 2009), maize
(Oboh et al., 2010) and sesame (
Makinde and Akinoso, 2014), an effect attributed to the destruction of cell structure which enables efficient release of the oil reserve
(Cuevas-Rodriguez et al., 2004). In the current study, roasting had insignificant quantitative effect on cowpea lipid content. The quantitative effect of roasting was less pronounced on fibre components, in contrast to greater reduction of the TIA (Table 3).