This is not something that we necessarily ask ourselves as we tuck into a lovely green salad, but maybe we should! Plants cannot move so they need to be resilient against stresses. To protect themselves they switch on a complex network of genes which in turn are translated into changes in metabolism, new enzymes and chemical compounds. When a salad leaf is harvested it is suddenly exposed to multiple stresses: it is transported cold and in the dark then subjected to mechanical damage as it is washed and processed. This switches on new metabolic pathways changing nutritional quality.
In our recently completed FP7 EU funded project – Quafety – we have taken multiple approaches to understand how these stresses affect the quality of fresh fruit and green salads. Quafety partners engaged in developing methods to improve quality and work with industrial partners to assess how best to exploit the technology developed. Salads are very perishable so reliable methods of predicting shelf-life and assessing internal quality are of interest to the industry. My interest though is also in stress responses and senescence: what is happening to metabolism in the cells? How is this affected by different storage parameters?
At Cardiff University in the School of Biosciences we have worked together with Markes International to explore the use of analyses of volatile organic compounds (VOCs) for detecting changes in quality in fresh cut rocket salad leaves and melon cubes. VOCs were collected on metal tubes packed with sorbent materials; these tubes are easily transportable and storable meaning that we could collect samples remotely from other labs and industrial processors. The volatiles were then desorbed from the sorbent in the tubes by heat and then the VOCs were separated by gas chromatography and detected by time-of flight mass spectrometry to find changes in composition as well as identifying the major chemical groups.
Using this approach we were able to show clear differences when the plant material was stored at different temperatures and also changes throughout its shelf life. As time of storage increased, volatile composition changed and this reflected internal changes in nutritionally relevant compounds including vitamin C. Volatile profiles were also strongly affected by growth conditions as well as stress imposed shortly after harvest, and these changes were reflected in the genes that were switched on and off suggesting that pre-harvest stresses are also important in determining aroma.
We hope that the methods we are developing will be useful to deliver better quality ready-to-eat salads to maximise the intake of the nutritional content which we know has a range of health benefits. Studying the genes involved we may also be able to understand better how aroma develops, and how it relates to metabolic changes that affect quality.