Comparative Study of Bioavailability and Forms of zinc in Parts of Watermelons from Nairobi and Kirinyaga Counties, Kenya

Abstract

Chemical speciation of elements in foods sheds light on their bioavailability in organisms. Watermelon is a widely consumed fruit with substantial levels of essential elements. However, there is very little if any on studies dealing with the bioavailability of these elements in this fruit and therefore, the need for this study. This research aimed to identify forms of Zn element in watermelons, their bioavailability, and the effect of drying on bioavailability from the different parts of Crimson sweet (CS), Sugar baby (SB), and Charleston grey (CG) watermelons. The fruit samples were bought from Mwea and Ngara markets in Kenya, separated into seeds, peels, white-flesh and red-flesh parts. Total Zn levels were determined in the various parts of the fruit before sequential extraction with hexane to dichloromethane to ethanol to water, and the final residue digested with acid. Zn levels in the fractions were determined by AAS while plausible functional moieties chelating with Zn were identified by GC-MS. Data was subjected to ANOVA and t-test at 95% CL. There were significant variations (p<0.05) in Zn levels in (i) different parts, and varieties, (ii) various solvents used and, (iii) fresh and dry parts. Water and ethanol extracts (considered potentially bioavailable forms in the human body) had significantly higher levels of zinc compared to other solvent fractions. Total Zn levels in watermelon parts ranged from 4.74–9.71 mg/100 g, which were markedly higher than levels previously reported in many common fruits such as banana (0.11 mg/100 g), mango (0.13 mg/100 g), and tomato (0.15 mg/100 g), and vegetables such as cabbage (0.36 mg/100 g) and lettuce (0.27
mg/100 g). Fresh and dry SB watermelons bought from Ngara market had significantly higher levels of zinc than other varieties in either Ngara or Mwea markets. Drying of watermelon parts significantly (p<0.05) increased extraction in dichloromethane and water, and decreased in hexane and ethanol. The presence of organic acids and related complexing agents may have enhanced extraction of potentially bioavailable Zn fractions. These findings suggest that consumption of different edible watermelon parts may contribute substantially to dietary Zn intake.