Effects of processing treatments on the nutritive composition and consumer acceptance of some Nigerian edible leafy vegetables

Abstract

Leafy vegetables are highly perishable food items and require special processing treatments to prevent post harvest losses. Leafy vegetables to be preserved by canning, freezing or dehydration are normally blanched in order to obtain good quality products. In Nigeria, leafy vegetables are preserved by sun-drying and used like freshly harvested vegetables in soups. The effects of these treatments on the nutritive composition and sensory qualities of some edible Nigeria leafy vegetables are reported. Samples (1kg each) of amaranths (Amaranthus hybridus), fluted pumpkin (Talfaria accidentalis), gnectum vegetable (Gnectum africana), vine  spinach (Basella alba), bush okro (Corchorus olitorus), slippery vine (Asystacia gangetica) and cocoyam leaves (Colocasia esculenta) were washed, cut into thin slices and sundried for 5 hrs daily for 2 days. Samples (1kg) of tomatoes  (Lycopersicon esculentum) were similarly dried for 4 days. The fresh vegetables (1kg each) were blanched in hot water at 100^oC for 5-6 min and rapidly cooled to 2⁰C in a freezer. Samples (250g each) of fresh (raw), sun-dried or blanched vegetables were cooked separately for 5min in soups. Sun-drying at 30 ± 1⁰C and RH 80 – 85% for 10hr resulted in a mean moisture loss of 35.6% with insignificant (P>0.05) increases of protein, lipid, crude fiber and total ash. Gnectum vegetable, vine spinach, bush okoro, cocoyam leaves and slippery vine had protein contents ranging from 3.0-5.0. K, Fe,Na, Mg and Ca were the predominant mineral elements analysed while contents of Zn and P were low. Blanching and cooking caused significant (P ≤ 0.05) reductions in the K, Na, Ca, Mg, Zn, Fe, and P contents of the vegetables. The implications of these results on the nutritive value of these  vegetables are discussed. Among the treatments, cooking accounted for 64.3 – 67.5% loss of vitamin C while blanching and sundrying accounted for 44.8-47.1% and 36.8 – 39.6% respectively. Panelists rated the soups significantly (P≤ 0.05) higher for colour, taste, texture, flavour/aroma and overall acceptability  characteristics. Fresh fluted pumpkin soup was rated significantly (P≤ 0.05) higher for colour, taste, texture and flavour/aroma characteristics, while fresh gnectum vegetable soup was highly rated for taste, flavour/aroma and overall acceptability characteristics. Compared with other dried vegetable soups, dried gnectum  vegetable soup had the highest score for colour and taste while dried fluted pumpkin soup had the highest score for flavour/aroma and overall acceptability  characteristics. Generally, panelists preferred the fresh to dried vegetable soups.

Key words: Blanching, Sun-drying, Cooked, Vitamin C, Sensory attributes.