Abstract:
African Walnut (Tetracarpidium conophorum- Mull. Arg) is a perennial climbing shrub
which grows mainly in the Western region of Africa. It is found mainly in Nigeria,
Gambia, Sierra Leone, Gabon, Equatorial Guinea and Cameroon as well. The nuts are
encased in pods which may contain between 2 to 5 nuts. The seed is enclosed in a hard
shell-like case. The nuts are commonly processed by boiling or roasting and consumed
as a snack or used as soup thickener. In ethnobotanical medicine, the nut extract is
extensively used in decoctions for treatment and/or management of common and chronic
ailments such as malaria, dysentery, high blood pressure, diabetes and cancer. The nuts
are generally exposed to high temperatures (25 – 37 °C) and relative humidity (RH) which
increases susceptibility to fungal contamination and nutrient degradation, hence, raising
concerns over product quality and safety. Experiment simulating the common retail
postharvest storage and processing practices was conducted to: (i) determine the effects
on the fatty acid profile; (ii) assess the impact on the fungal population contaminating the
nut shells at different maturity stages, and potential mycotoxigenic implications; (iii)
evaluate the cytotoxicity of four extract of the nut on lung cancer (A549) cells; and finally
(iv) assay the total phenolic content and profile potential individual phenolic components
of the nut.
Results indicated the presence of essential and non-essential fatty acids namely;
palmitate, oleate, stearate, linoleate, arachidate and α-linolineate with α-linolineate being
the most abundant (1.1 – 8.2 mg/g freeze-dry weight). Boiling and roasting generally
improved the concentration of the fatty acids best when nuts are cold stored at 5 °C for
maximum of 10 days.
Potential mycotoxigenic species - Aspergillus section Nigri, Aspergillus
flavus/Parasiticus, Fusarium spp. and Penicillium spp. - were frequently isolated from
cultured shell pieces of stored nuts. When compared with unprocessed nuts, roasting
completely prevented fungal contamination in shell pieces from nuts in the non-stored
(NSN) group at early maturity stage, while boiling significantly reduced the level of
contamination to about 58 % (P < 0.05). Simulating open market conditions caused 100% fungal contamination in all boiled samples and roasted samples at early maturity.
Mycotoxin analysis using Yeast Extract agar (YES) and High Performance Liquid
Chromatography (HPLC) - Fluorescence detector (FLD) showed that Aflatoxins - G1
(AFG1), B1 (AFB1), G2 (AFG2), and B2 (AFB2) were produced by 20 isolates with both
AFG1 and AFB1 being predominant at concentration ranges 4 – 32,200 and 4 – 22,700
ng/g plug weight, respectively. No Ochratoxin A (OTA) was detected.
Phenolic component analysis indicated unprocessed (20.79 ± 1.0 mg gallic acid
equivalent per gram freeze-dry weight – GAE/g FDW) samples showed the highest value
for total phenolics while both boiling (9.90 ± 1.8 mg GAE/g FDW), and roasting (9.32 ±
2.7 mg GAE/g FDW) reduced the amount by more than 50 % when compared with
unprocessed. Potential individual phenolic compounds were unambiguously separated
using high performance liquid chromatography – diode array detector (HPLC-DAD).
There were no differences between chromatograms of defatted and non-defatted
unprocessed, roasted and boiled samples. Cytotoxicity evaluation showed no decrease in
cell densities in plates treated with extracts from unprocessed nuts at all concentrations.
Diethyl ether-ethyl acetate (10 µg/mL) and n-butanol (1000 and 500 µg/mL) extracts of
roasted nuts as well as dichloromethane and water (1, 10 µg/mL) of boiled nuts caused a
non-significant decrease of < 10 % in cell densities when compared with the phosphate
buffered saline-media control. However, all extracts showed no cytotoxic effect on the
A549 cells
African walnut is basically produced at subsistence level in Nigeria, but considering the
presence of desirable fatty acid profile and phenolic compounds, need for increased
industrial scale production is herein recommended. Although fungal attack and potential
mycotoxin risk on the nut may be high, retail processing by roasting has prospects to
greatly accentuate the risk. Cold storage of the nut may help to improve the shelf life
although it may not be cost effective for local farmers in Nigeria and Africa, however, it
provides opportunity for export business. Although the nut extracts showed no cytotoxic
effect on A549 lung cancer cell lines, there is need to investigate further to confirm it
non-cytotoxicity activity on other cancer lines and normal cell lines.