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Proximate composition and antioxidant potential of selected tubers peel

This study is aimed to investigate the nutritional composition, phytochemicals and
antioxidant activity of red and purple variety of Ipomoea batatas (sweet potato) peels and
Manihot esculenta (cassava) periderm and cortex layers. The proximate composition
(carbohydrate, protein, fat, moisture, ash and minerals) was determined using standard
methods while phytochemical contents (phenolic, flavonoid, anthocyanin and carotenoid)
and antioxidant activity [2,2-diphenylpicrylhydrazyl (DPPH), ferric reducing antioxidant
power (FRAP), and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS)
assays] were determined spectrophotometrically. Proximate analysis showed that both I.
batatas and M. esculenta contained high carbohydrate content ranged from 75.07% to
87.04% but zero protein and low-fat contents (0.83-2.2%). For the mineral contents, I.
batatas contained higher potassium, sodium, iron and magnesium than M. esculenta.
However, calcium was found higher in M. esculenta as compared to I. batatas. For the
phytochemical study, total anthocyanin content showed the lowest value (0.43-3.08 µg c-3
-gE/g) for both tubers as compared to total phenolic (37.28-71.79 µg GAE/g), flavonoid
(4.98-16.50 µg RE/g) and carotenoid (14.54-46.66 µg BC/g) contents. For the antioxidant
study, both purple and red variety of I. batatas showed higher percentages of inhibition
using DPPH assay with 87.93% and 87.62%, respectively compared to M. esculenta.
Meanwhile, FRAP assay displayed lower values than ABTS assay for both tubers. In
conclusion, both tubers, I. batatas and M. esculenta have close values of nutritional
compositions. However, I. batatas showed higher mineral contents (potassium, iron,
magnesium and sodium) and antioxidant activity than M. esculenta. For the
phytochemical content, I. batatas contained higher total flavonoid and total anthocyanin
contents while M. esculenta contained higher total phenolic and carotenoid contents.
Hence, this study suggested that tuber’s by-products have great potential for use in
functional food as they are cheap and readily available as well as reducing waste disposal.