Litchi is a fragile fruit that does not keep for a long time. Stored at ambient temperature, it loses its pinkish red colour in a few days and browns very rapidly. This classic enzymatic browning is revealed by cell delocation. Cold results in only a very modest increase in conservation of the red colour and adjuvants must also be used to maintain the initial colour. The most commonly used method for preventing browning is the fumigation of fruits with SO2 a few hours after harvesting.
SO2 treatment of litchi is sometimes followed by soaking in an acid bath that restores the red colour after decolorisation by sulphur treatment and results in commercially attractive fruits. However, it must not be forgotten that the use of SO2 can be harmful or allergenic for consumers sensitive to sulphur. Today, SO2 treatment has been totally eliminated in the United States, except for the treatment of table grapes. In France, the legislation permits residues of 10 ppm in litchi pulp and 250 ppm in the shells. As the trend is towards the precaution principle, it is very likely that these levels will be lowered or even that the use of SO2 may no longer be authorised in the EU. In this context, it is particularly important to be able to propose substitutes for fumigation to the profession, especially as France is the main destination and consumer country for litchi, with nearly 80% of the European total.
The replacing of sulphur treatment has been studied by different research teams, in particular in Israel and South Africa. The treatments proposed are based on various chemical or thermal techniques. None of this research has yet led to a method that is acceptable or has been developed commercially. In the light of these observations, Cirad-flhor is exploring new lines of research on conservation techniques that would totally eliminate the use of SO2.