**7. Citrus**

Citrus fruits constitute a separate case because their essential oils are normally obtained by mechanical compression of the peel, not by steam distillation. Citrus fruits are the largest fruit crop in the world (over 100 million tons per year) [38] and their main destiny is the food industry. After processing, the peels, seeds, and membrane residue represent approximately 60% of the original fresh fruit mass. Essential oil is present in the colored exterior part of the peel, called the flavedo. The main component of citrus essential oils is limonene (above 80%, depending on the species). Citrus essential oils have antioxidant, antidiabetic, insecticidal, antifungal, and antibacterial properties, which make them valuable ingredients for applications in the pharmaceutical, sanitary, cosmetic, agricultural, and food industries [39]. The inner walls of the citrus fruit peel (albedo) are rich (~30%) in pectin. Pectin is a mixture of acid and neutral-branched polysaccharides that contain glucuronic acid. Pectin is used as a thickener, emulsifier, gelling agent, or fat substitute in the food and beverage industry. Citrus peel also contains flavonoids for which antioxidant, anticancer, anti-inflammation, and cardiovascular protection activities have been reported [40]. Thus, processing citrus fruit residual material with a zero-waste approach may lead

to value addition through the isolation of essential oil, pectin, flavonoids, and other bioactive molecules.

The most common primary approaches to treat citrus fruit waste involve essential oil separation, followed by the use of the residue for composting and for animal food. Some of the limiting factors to using citrus peel waste in composting are its low nitrogen content, and its antimicrobial properties (associated with limonene), which have a negative impact on soil microorganisms. Animal food is another potential use of citrus peel waste, but there have been mixed positive and negative experiences. A study in which broiler finisher birds were fed with increasing amounts of orange peel concluded that the inclusion of sweet orange peels had adverse effects on the growth rate and nutrient utilization by the birds [41]. This was attributed to the decrease in feed intake because the compounded diets became unpalatable upon the inclusion of sweet orange peels, some of them with certain fermentation levels. The processing of citrus peel waste through anaerobic digestion and consecutive fermentations may lead to the isolation of several bioactive products, but it requires the previous removal of limonene, which can inhibit microbial activity [42].

The essential oil in the peel of citrus fruits may be isolated by mechanical pressing (cold press), which is the most common method, although higher yields are obtained with hydrodistillation and steam distillation. The latter have higher costs due to the energy needed, but when complete biomass utilization is the goal, these techniques have the advantage that the residual biomass has low amounts of limonene and can be subjected to liquid fermentation without further processing. An interesting alternative is microwave-assisted distillation, which due to its shorter duration has lower total energy costs. In one of its implementations, the moisture content of the vegetal material is sufficient to absorb the supplied microwave energy, and no additional water is needed. This lowers operation costs and the volume of the treatment chamber required [43].
