Swazi leafy vegetables are cheap, seasonal, locally grown and easily available, easy to
propagate and store, highly nutritious food substances that are often used to supplement
diets and whose nutrient content is affected by processing treatments. The effects of
cooking on the nutrient composition of some Swazi vegetables were investigated.
Common Swazi vegetables and some exotic cultivars were collected from the Manzini
region, then cooked strictly according to Swazi traditional practice, digested and
analysed for calcium, copper, iron, magnesium, manganese, potassium and zinc content
using flame atomic absorption spectrophotometer and inductively coupled plasma
spectrometer. The vegetables investigated were:
Hibiscus escolentus
[upright and exotic
cultivars],
Corchorius olitorus
,
Amaranthus spinosus
,
Amaranthus caudatus
,
Bidens pilosa
,
Solanum nigrum
and
Momordica involucrata
.
Brassica oleracea
and
Spinacia oleracea
were also analysed and compared with other vegetables. Sampled vegetables
were divided into two categories. All vegetables in category one were cooked without
any additives. In category two
Hibiscus escolentus and
Corchorius olitorus were cooked
with bicarbonate of soda and common salt;
Amaranthus spinosus,
Amaranthus caudatus,
Bidens pilosa,
Solanum nigrum,
Brassica oleracea and
Spinacia oleracea were prepared
with sunflower cooking oil and salt. Only salt was added to
Momordica involucrata.
Results of the study show that cooking significantly (p ≤ 0.05) reduces zinc, calcium,
iron, magnesium, manganese, potassium and copper content of all the vegetables
analysed. The percent reductions were: 4.2 – 100% for zinc, 2.0 – 75.6% for calcium,
2.4 – 79.2% of iron, 1.2 – 73.2% for magnesium, 6.6 – 100% for manganese, 1.9 - 67.5%
for potassium and 46.4 – 100% for copper. It was also found that addition of salt and
bicarbonate of soda enhances loss of minerals. The cooked Swazi vegetables had
significantly higher minerals content when compared to exotic varieties such as
Brassica
oleracea which lost most of the minerals when cooked. The loss of minerals in the
vegetables may be due to leaching effect during the cooking process. It appears that this
leaching effect is exacerbated by addition of sodium bicarbonate, sodium chloride and
sunflower cooking oil. Cooking the common vegetables without additives or with small
quantities of additives is, therefore, recommended.
