Leaf harvesting severity affects total phenolic and tannin content of fresh and dry leaves of Moringa oleifera Lam. trees growing in Gauteng, South Africa

Abstract

The most important chemicals common in Moringa oleifera Lam. leaves are polyphenols and tannins. They are synthesised during development and the amount and composition are primarily dependent on environmental conditions and factors such as leaf harvesting. Trials were conducted in the Gauteng Province at the University of Pretoria Experimental Farm on Hillcrest Campus (250 45′ 08.6″S, 280 15′ 30.S" E), with an altitude of 1372 m above sea level and an average annual rainfall of 640 mm. Five year old M. oleifera tree orchard were used for this study and treatments were laid in a randomised complete block design, where 12 plants were used in each plot within a row, with a plant spacing of 2 × 2 m2. Partial harvesting was done at 25% leaf removal, while severe leaf harvesting was done at 75% leaf removal. The control trees were not harvested, but only leaf (mature and immature) samples were harvested for analyses. Leaf samples from all plots were harvested in September, November, February and May in years 1 and 2. Leaf samples (equal quantities of immature and mature leaves) from partially, severely harvested and control plots were harvested and separated into batches, one to be analysed as fresh material and the other as dry material. The dry material was placed in paper bags and allowed to immediately air dry under normal room conditions of 26 °C. Partially harvested M. oleifera leaves produced an increased accumulation of total phenolic content (TPC) 35–44 g/kg dry weight, from spring to summer season, in comparison to severe harvesting 26–35 g/kg dry weight. With severe leaf harvesting (75%), because of probable lower photosynthetic capacity, phenols across September to May of years 1 and 2 decreased. The onset of low temperatures in May resulted mostly higher TPC and tannin build up, which means that cold stress may improve more bio-active compounds in the leaves. Dark and fully developed mature leaves mainly contained the highest amount of TPC. Moringa oleifera leaves collected and dried at each harvesting interval, instead of freshly milled, do show the same quantity of TPC and tannins across the season illustrating that air drying can be used as a post-harvest handling technique, extending shelf life without losses of vital phenols. With harvesting severity as a concern in Moringa, this study confirms that the TPC and tannins in mature and immature leaves did not rise to toxic levels for human consumption.