Tangerine (Citrus reticulata)

Tangerine or Citrus reticulata is a small oblate, thin-skinned, bright orange, easy-to-peel type of mandarin indigenous to China and Southeast Asia. Tangerine cultivation slowly spread westwards along trade routes as far as the Mediterraneanwhere, in the 1800s, acquired the name tangerine due to the export of this sweet mandarin from the port of Tangiers in Morocco. Nowadays, tangerine production is distributed in more than 140 countries. However, the vast majority of crops are grown in the northern hemisphere in subtropical regions, namely southern Europe, southern United States, and Southeast Asia, with the major producers being China (the largest, with 27 million metric tons (MMT)), European Union, Turkey, United States of Americaand Japan. Currently, tangerine production plays a crucial role in the agri-food industry, with global production being expected to increase by 2.0 MMT by 2021/22, reaching a groundbreaking record of 37.2 MMT. Although synonymously used, tangerines and mandarins are not necessarily the same, with the latter encompassing tangerines, clementines, and satsumas. The phytotomy of tangerines consists of an exocarp which entails the epidermis and flavedo (containing oil sacks that produce aromatic oils), the albedo (a sponge-like layer that is a source of flavanones), and the vascular bundles. Segments containing the juice sacs and seeds, with septa running along these segments converging onto the central axis of the tangerine, comprise the endocarp. Tangerine chemical composition is dependent on maturity level, storage, horticultural, and climate conditions, all of which impact the chemical profile of the fruit. Nonetheless, tangerines possess low amounts of proteins and lipids, with carbohydrates (namely, sucrose, glucose, and fructose) being the most prevalent macronutrient. The albedo is rich in dietary fibers composed of cellulose, lignin, and pectin, with the latter being recognized as a valued gelling agent and cholesterol (low-density lipoprotein; LDL) lowering agent. Concerning organic acids, citric is by far the most predominant in tangerines, although trace amounts of malic acid may also be present. Tangerines also comprise a vast array of secondary metabolites, namely, essential oils, flavonoids, phenol acids, alkaloids, limonoids, and carotenoids, among others. Each of these groups of compounds presents a notable biological activity of considerable therapeutic importance to human health and well-being, with flavonoids (namely hesperidin, the most abundant flavonoid in tangerine peel) possessing potent antioxidant, anticarcinogenic and anti-inflammatory properties, as well as neuroprotective effects. Moreover, tangerine bioactive potential has been documented concerning antidiabetic, antiproliferative, antihypertensive, antiatherogenic, antiviral and antimicrobial properties, as well as skin hydrolipidic balance improvement, gut microbiota modulation and cardiomyopathy prevention. Owing to increasing awareness of healthy eating habits by the average consumer, citrus fruits are regaining more and more a prime role in the daily diets, either being consumed fresh, in the form of juices, or other processed products. Although 70% of all produced tangerines are allocated to be consumed fresh, roughly 30% of all tangerines are subjected to some form of processing. This processing will undoubtedly lead to the formation of waste residues of either solid (peels and seeds), semi-solid (pulp), or liquid nature (wash waters and other effluents). The solid and semi-solid waste residues are mainly composed of the remains of the endocarp and exocarp (seeds, fibers, membranes, and vesicles). Conventionally, waste management methods for citrus waste englobe composting, incineration, direct discharge into wells, lakes, or other bodies of water. Although incineration may be used for energy production and composting is a valuable end-product, these disposal methods do not harness the full potential of these added-value byproducts. Consequently, in recent years there has been significant development of new processing methods for the total exploitation of the various tangerine waste products. These waste residues are rich in enzymes, carbohydrates, lipids, dietary fibers, organic acids, amino acids, minerals, essential oils, vitamins, and flavonoids. All of those compounds are of utmost interest to the pharmaceutical, cosmetic, and food industries. Nonetheless, in order to be available for the mentioned purposes, those compounds need to be extracted. In this sense, distinct technologies may be applied to obtain the highest yield and purity. The most conventionally used methodology is solvent extraction, which includes the well-recognized “Soxhlet”, together with emerging technologies, namely, supercritical and pressurized fluid extractions (SFE and PFE, respectively). There are also methodologies associating physical stimuli with posterior, or concomitant, use of solvents. Hence, these techniques are often classified as pre-treatments and not as extraction methods per se. This consideration may be applied to microwave and ultrasound-assisted extractions (MAE and UAE, respectively). Other technologies comprise enzyme-assisted extraction (EAE), high hydrostatic pressure (HHP), and pulsed electric field (PEF), among others. In this sense, besides being utilized to extract compounds from unprocessed tangerine, these methodologies/technologies may also be applied to convert waste residues originated in tangerine processing into added-value products. Consequently, harnessing the various citrus components will undoubtedly contribute to a reduction in the carbon footprint posed by tangerine orchard waste which, in turn, aligns with the thirteenth sustainable development goal aimed at reducing the carbon footprint of food industries. Tangerine has been exploited towards the formulation of novel food products, namely, marmalades, low-calorie antioxidant and probiotic wort-based beverages, spray-dried tangerine powder-based juices, and extruded snacks. Moreover, waste residues have demonstrated a notable potential for the development of novel products, namely, food preservatives and stabilizers, edible films, supplements, titanium dioxide nanocrystals, carbon quantum dots, bioadsorbents of heavy metals, bio-fertilizers, biogas, bioethanol, and biohydrogen.