Leafy vegetables form part of the diet of most people of tropical Africa. The
contribution of these leafy vegetables to their mineral needs has not been well
elaborated. The aim of this study was to determine the effects of different processing
methods on the mineral composition and anti-nutritional components of some tropical
leafy vegetables. Twenty-one species of different vegetables locally grown and
consumed in the northern region of Cameroon were collected and analyzed for their
iron, copper, manganese, zinc, magnesium, calcium, sodium and potassium content.
The levels of anti-nutrients like phytates, oxalates, tannins and saponins that are likely
to affect the bioavailability of these minerals were also determined. The effects of
processing methods on the mineral composition and anti-nutritional components of
these leafy vegetables were also determined. Amongst these species,
Moringa oleifera
,
Hibiscus canabinum
,
Solanum nigrum
,
Cucurbita maxima
and
Vernonia calvoana
that are used for soup, and found in relative abundance in the region were
subjected to two cooking methods (boiling plus squeeze-washing and a combination
of boiling in alkaline salt (kanwa) plus squeeze-washing). The levels of iron, zinc,
manganese, copper, magnesium, calcium, sodium and potassium varied in the
different vegetables. In all vegetables studied, magnesium was the most abundant
mineral. The iron levels ranged from 14.99±2.00mg/100g dry weight (DW) in
M.
oleifera to 167.42 ± 18.63mg/100g DW in
H. sabdariffa
. Boiling and squeeze-washing
led to significant losses of minerals with sodium being the most affected. On
the contrary, higher values of iron, calcium and sodium were found in samples that
were boiled in alkaline salt and squeeze-washed. The levels of antinutrients were also
reduced as a consequence of processing. Percentage losses during boiling with or
without alkaline salt were between 25.4 and 55.5% for phytates, 11.1 and 80% for
oxalates, 16.4 and 68.5% for saponins and 25 and 26.44% for tannins. Boiling,
therefore, with or without alkaline salt is an effective means of reducing the levels of
these antinutrients in vegetables to tolerable levels.
