Rocket salad aroma is affected by sampling method, species and degree of leaf damage

Abstract

Rocket salad is a valuable commercial product often sold as a bagged ready to eat salad. Two different species from the Brassicaceae family are sold as rocket salad: Diplotaxis tenuifolia and Eruca sativa. Both are aromatic, and previous studies have shown that their aroma is composed of a complex mixture of volatile organic compounds (VOCs) from several different chemical families. Of particular interest are isothiocyanates that are generated from the metabolism of glucosinolates produced by Brassicaceae species. Both types of rocket salad have a limited shelf-life and there has been interest in assessing whether analysis of VOCs could be used to help set ‘use by dates’ assess quality changes, and report on issues within the supply chain. However, different methods have been used to sample and analyse the VOC profiles. Here we compare the profiles from D. tenuifolia using two VOC sampling methods (solid phase microextraction, SPME) and sampling onto thermal desorption (TD) tubes. We also compare the VOC profiles sampled onto TD tubes from both species from leaves subjected to different levels of damage: intact, chopped and blended. We find that both the method of VOC sampling and the level of leaf damage have important effects on the VOC profile. In the comparison, fewer VOCs are detected when sampling with SPME compared to TD tubes. Overall, 41 different VOCs are detected in the leaf damage experiment but many fewer are detected in intact leaves. While the two rocket species are distinct based on the VOC profile of chopped leaves, they are not discriminated by VOC profiles of blended or intact leaves. When species are considered separately VOC profile discriminates level of damage. Very few isothiocyanates are detected in intact or chopped leaves, presumably due to the requirement for leaf damage to activate their production from glucosinolates. To conclude, level of leaf damage will strongly influence VOC profiles from rocket leaves, and medium damage seems to elicit the most discriminatory profiles