Determination of Phytochemicals and Antioxidant Activity in Butterhead Lettuce Related to Leaf Age and Position

Abstract

The effect of leaf position on phytochemical distribution in butterhead lettuce was assessed by rings (from ring 1: outer leaves, to ring 6: inner leaves) or by zones (external, middle, internal). Maximum ascorbic acid was found in middle rings while chlorophyll and carotenoids gradually decreased from external to internal rings. Outermost leaves accumulate more phenolics and present more antioxidant activity as determined by 2,2-diphenyl-1-picrylhydrazyl (DPPH) and trolox-equivalent antioxidant capacity methods. However, the presence of phenolics in this zone also contributes to the enzymatic browning. Quantitative but not qualitative differences were found in the phenolic profiles between inner and outer lettuce zones. The major phenolic compounds identified were the phenolic acids chicoric, chlorogenic and isochlorogenic. For all phenolic compounds, greater content was always found in the external zone except for caffeic acid. A high correlation between DPPH and total phenolics, chlorophyll and carotenes indicated that these compounds were major contributors of antioxidant activity in lettuce. Practical Application During lettuce development, each leaf had a different level of exposure to environmental conditions (light, humidity, nutrients absorption and temperature) and also inner leaves are younger than outer ones. These factors may affect the distribution of phytochemicals and antioxidant capacity in the lettuce head. Knowledge of the bioactive content and antioxidant capacity profile in lettuce plants could be of interest to consumers and the food industry for selecting the more suitable leaves to make salads or other ready-to-eat mixed vegetable dishes with high nutritional value. Additionally this study reveals, from a nutritional point of view, the losses value of regular greengrocers' practices that include the removal of the external lettuce leaves as storage advances and signs of senescence are evident.