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The Second Life of Citrus Fruit Waste: A Valuable Source of Bioactive Compounds - PubMed

️Fri Jan 01 2021

Review

The Second Life of Citrus Fruit Waste: A Valuable Source of Bioactive Compounds

Caterina Russo et al. Molecules. 2021.

Abstract

Citrus fruits (CF) are among the most widely cultivated fruit crops throughout the world and their production is constantly increasing along with consumers' demand. Therefore, huge amounts of waste are annually generated through CF processing, causing high costs for their disposal, as well as environmental and human health damage, if inappropriately performed. According to the most recent indications of an economic, environmental and pharmaceutical nature, CF processing residues must be transformed from a waste to be disposed to a valuable resource to be reused. Based on a circular economy model, CF residues (i.e., seeds, exhausted peel, pressed pulp, secondary juice and leaves) have increasingly been re-evaluated to also obtain, but not limited to, valuable compounds to be employed in the food, packaging, cosmetic and pharmaceutical industries. However, the use of CF by-products is still limited because of their underestimated nutritional and economic value, hence more awareness and knowledge are needed to overcome traditional approaches for their disposal. This review summarizes recent evidence on the pharmacological potential of CF waste to support the switch towards a more environmentally sustainable society.

Keywords: Citrus; by-products; circular economy; flavonoids; nutraceuticals; phytochemicals; polyphenols; valorization; waste.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

**Figure 1**
Structures of the main monoterpenes and sesquiterpenes present in CF peels. Monoterpenes represent the simplest terpenes, with two isoprene units and ten carbon atoms. They are divided into hydrocarbons (on the **upper left**) and oxygenated derivatives (on the **lower left**). Among the former, d-limonene (circled) is the main constituent of essential oils. Sesquiterpenes (on the **right**) are more complex terpenes possessing three isoprene units and fifteen carbon atoms.

**Figure 2**
Chemical structure of pectin (D-galacturonic acid units linked together by α-1,4 glycosidic bonds; on the **left**) and cellulose (glucose units −300–3000 molecules- linked by a β-1,4 glycosidic bond; on the **right**), soluble and insoluble dietary fibers, respectively.

**Figure 3**
Carotenoids present in CF juices. Based on their chemical composition, they are classified as xanthophylls (oxygenated carotenoids; on the **left**) and carotenes (hydrocarbon carotenoids; on the **right**). Some examples of the two classes of carotenoids are here shown.

**Figure 4**
Chemical structures of the main classes of flavonoids present in CF, namely flavones, flavanones, flavonols, isoflavones, anthocyanidins and flavanols. The common scaffold of flavonoid class consists of a benzene ring joined to a benzo-γ-pyrone moiety.

**Figure 5**
Chemical structure of phenolic acids present in CF juices. Based on their origin, they are divided into hydroxybenzoic (gallic, vanillic and syringic acids; on the **right**) and hydroxycinnamic (caffeic, ferulic, p-coumaric, sinapic and chlorogenic acids; on the **left**) acids.

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The Second Life of Citrus Fruit Waste: A Valuable Source of Bioactive Compounds - PubMed