Abstract
African leafy vegetables (ALVs) have existed since time immemorial. Despite the fact that
these vegetables are a rich source of phenolic compounds, vitamins and minerals, they are
still highly underutilised owing to many reasons one of them being that very little knowledge
and literature exists around their benefits. In this study, an ALV, Corchorus olitorius was
subjected to different cooking (steam-blanching, boiling and frying) and drying methods to
replicate what happens at household level to determine the effect it would have on the Total
Phenolic Content (TPC), Total Flavonoid Content (TFC), antioxidant, proximate, mineral,
dietary fibre and vitamin A and C composition. Furthermore, Fourier transform infrared
spectroscopy (FTIR) was used to identify the functional groups present. Cooking and drying
significantly (p<0.05) increased the level of phenolic compounds. The steamed sample
depicted the highest TPC mean value of 118.89 mg GAE/g whilst the dried sample had the
highest TFC with mean value of 78.42 QE/g There was significant difference at p<0.05 for
all the antioxidant assays. Across the DPPH, FRAP and ABTS assay, the steamed sample had
the highest antioxidant activity of mean values 119.58, 167.31 and 122.23 μM TE/g
respectively. The control sample had the highest ash and carbohydrate content (16.83 and
41.49 percent, respectively), whereas the sample fried with vegetable oil had the highest
moisture, crude protein, and fat content (15.32, 14.52, and 27.23 percent, respectively). It had
the highest energy value of 423.47 kcal/100 g as well. The crude fibre, fat, and calorie
content of the control sample were enhanced by cooking and drying. The crude fat content of
blanched, boiled, and dried samples did not vary significantly. Furthermore, there was no
significant variation in carbohydrate content of the cooked and dried samples. ICP-MS was
used to quantify and qualify the minerals. Cooking and drying reduced the P, Mg, Ca and K
content of the leaves. There was no significant difference at p< 0.05 in the Mn levels amongst
the blanched, steamed and boiled samples. There was no significant difference in Cu
concentration amongst the control, boiled, blanched and steamed sample at p<0.05. The
steamed sample showed the highest Fe and Zn content of 1.501 and 0.053 mg/g respectively.
The control had the highest K content of 35.34 mg/g whilst boiling had the highest P and Mg
levels with means of 3.787, 2.997 mg/g respectively. Cooking and drying significantly
reduced the vitamin C content, with values of 9.21- 22.74 mg/100 g compared to the control
(25.51 mg/100 g). There was no significant difference at p<0.05 for vitamin A content with
regards to the blanched and steamed sample. Furthermore, there was no significant difference
between frying with olive oil and frying with vegetable oil at p<0.05. The steamed sample
had the highest vitamin A content of 103.66 mg/100 g compared to other processing methods
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(35.89- 101.83 mg/100g). The insoluble and total dietary fibres were greatly affected by
cooking and drying. All the treated samples had significantly lower insoluble and total dietary
fibre content compared to the control (17.36 and 21.52% respectively). The boiled sample
showed the highest soluble fibre content (2.67%) compared to other treatment methods (0.33-
1.85%). Liquid chromatography-mass spectrometry (LC-MS) was used to give a
comprehensive profile of the metabolites in the different samples after selected cooking
(blanching, steaming, frying with olive oil) and drying methods. Crypto-chlorogenic acid
(CGA) was the most abundant phenolic acid in the cooked samples
(blanched>steamed>fried) with values of 601.9, 580.8 and 563.6 mg/kg respectively). Ltryptophan
was the dominant in the dried sample with concentration of 278.1 mg/kg. Other
compounds identified and quantified include caffeic acid, syringic acid, 3, 5-dicaffeoylquinic
acid, quercetin-3-O-robinobioside, quercetion-3-galactoside and chlorogenic acid. The results
obtained show how C. olitorius can be used in food security, as a functional food due to its
health promoting properties that can be employed in reducing chronic ailments.
Keywords: Phenolic compounds, vitamins, minerals, dietary fibre, cooking, drying.