The waste portion in the processing of some fruits and vegetables can be as large as 70% of the harvested material, as is the case of artichokes, passion fruits and some lettuce cultivars. In leaf vegetables (lettuce, spinach, Swiss chard, etc.) the external leaves are often removed as they are too hard and green and often have some defects (bruises, cuts, etc.). In other products, the edible portion is the flower (or the flower heart, as in the case of artichokes) and in this case the leaves, stems and some parts of the flower (the external and harder bracts) are discarded. In celery only the stems, especially the whiter ones, are selected, and the leaves and greener and thinner stems constitute waste product. In onions, the residues are external membranes and sometimes scales. The peels are frequently wastes, as in the case of most fruits, potatoes, tomatoes, etc. In other products the wastes are the fruit husks (banana, citrus, pomegranate, etc.) and shells (pistachio, almonds, walnuts, etc.). The fruit stones are also wastes from fruit processing (peach, apricot, etc.). After fruit pressing for juice, wine and oil production, the press-cake constitutes a relevant waste (grapes, berries, olives, etc.). In some cases the wastes generated are the waters of industrial processing (brines, blanching waters, cooking waters, etc.), and these are generally rich in secondary metabolites.
There is a need to decrease waste production as environmental legislation becomes more and more restrictive, and the trend is to take waste generation to levels as close as possible to zero. The use of wastes as an animal feedstuff is a common practice, being the main use for plant food industry residues (Martínez-Teruel et al., 1998; Iñiguez et al., 2001). Bio-fuel (Hang, 1987) and compost production (Vlyssides et al., 2004), as well as the preparation of dietary fibres (Femenia et al., 1998; Marconi et al., 2000), are also alternatives already in use. The volume of waste production is increasing with agricultural production and processing. The use as animal feedstuff has some limitations as the wastes are generally very rich in water and are highly perishable, fungal development is very likely to occur with the risk of mycotoxin production (Tomás-Barberán et al., 2005). These mycotoxins could then pass to the animal tissues and therefore to man. The use of waste for composting also has some applications, and composts and manures from different wastes are available on the market. These waste tissues are also rich in bioactive secondary metabolites, and therefore there is an opportunity for the extraction of high-value compounds from wastes. Once they are extracted the residues could still be used for animal feeding or for composting or fuel production.
In addition, there is a demand for functional foods (foods with specific physiological effects in humans) and nutraceutical preparations both from society and the industry (Roberfroid, 2000), and the phytochemicals present in these plant food residues can be an excellent raw material for the preparation of these extracts.
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