An update on functional, nutraceutical and industrial applications of watermelon by-products: A comprehensive review
Graphical abstract
Introduction
The demand for fruits and vegetables has been considerably increased due to an increase in the world's population, changing dietary habits, and consumer perception of the health benefits offered with such important food commodities. In 2018, the production of different fruits and vegetables around the world exceed one billion tons (FAO, 2019), depending on geographical regions, growing traditions, and consumption that eventually lead to the generation of a large number of byproducts and waste (Fierascu, Sieniawska, Ortan, Fierascu, & Xiao, 2020). Fresh fruit and vegetables are not always eaten by people directly but are first processed to separate the desired valued product. After household sewage, the food processing industry including fruits and vegetable processing is the second largest waste generator in the world (Sagar, Pareek, Sharma, Yahia, & Lobo, 2018). Fruit and vegetable processing results in massive amounts of waste materials produced along the processing chain, such as peels, seeds, stones, and unused flesh. These wastes pose potential disposal and pollution problems along with loss of valuable biomass and nutrients (Rolim, Seabra, & de Macedo, 2019). One of the major concerns in the agri-food industries is the handling of by-products arising from processing (Peng, Ma, Wang, & Xie, 2019). Indeed, food wastes are of rising concern, since they constitute social, economic and environmental issues worldwide. Food waste management, therefore, appears continuously to be a crucial issue for food security (Silva, Albuquerque, Alves, Oliveira, & Costa, 2020).
Watermelon (Citrullus lanatus) is a fruit vegetable, belongs to the genus Citrullus and the family Cucurbitaceae (Renner, Sousa, & Chomicki, 2017). Watermelon biomass can be categorized into three main components, which are flesh, seed, and rind. Flesh contributes to approximately 40% of the total weight, the rind and seeds represent approximately 60% of total watermelon fruit fruits which is massive agro-waste as compared to other similar fruits peels such as melon (40%), pumpkin (45%), bitter apple (30%), musk melon (35%), and papaya (47%), etc., (Chakrabarty et al., 2020; Mohamed,; Hasanin and Hashem, 2020a, Hasanin et al., 2020b; Zamuz et al., 2021). Half of the watermelon fruit is edible while the other half goes to waste. Considering the production of watermelon in 2017–18, approximately 42 million tonnes of watermelon by-products (rind and seeds) was generated by numerous fruit juice processing industries, cottage fruit juice producers, and restaurants during production, preparation and consumption of watermelon. These by-products are discarded indiscriminately into the environment and thereby constituting environmental challenges despite being edible (Esparza, Jiménez-Moreno, Bimbela, Ancín-Azpilicueta, & Gandía, 2020). However, various factors such as the continuous growth of the world population, depletion of renewable sources, reduction in land for cultivation and over-accumulation of this waste urge the need for the valorization of these byproducts to be used as a substitute ingredient and to produce functional products in food, pharmaceutical and nutraceutical industries. Large quantities of these waste byproducts containing a rich composition of vitamins, minerals, fibers, oils, bioactive compounds with certain therapeutics properties (Rico, Gullón, Alonso, & Yáñez, 2020). The watermelon byproducts with high nutritional value have gained a lot of interest because these are highly valued and have great potential to be added to the human diet. Watermelon rind (WMR) and watermelon seeds (WMSs) can represent a low-cost raw material (Hasanin and Hashem, 2020a, Hasanin et al., 2020b), rich in potentially valuable components for other industries. The main challenges faced by the food industries are the lack of awareness and capacity of handlers and integrated management for this solid waste to implement strategies to transform complex, heterogeneous and disposable biomass into value-able and marketable high value-added products, providing environmentally adequate and sustainable end. Therefore, It is very important for sustainability to properly treat, handle and efficient conversion of WMR and WMSs to design and develop innovative food products with added value, being a solution to maximize agricultural profit and to minimize food losses, wastes and environmental impact (Kausar, Hassan, & ud Din, 2020; Kumari, 2020).
In this context, this review intends to present a critical overview of the composition of nutritional and bioactive compounds of watermelon by-products. It also emphasizes the nutraceutical perspective of their functionally active compounds against various ailments to fully support the development of evidenced-based functional food products and potential industrial exploitation as a valuable ingredient of the future. Also, this review is anticipated to stimulate research on other peel and seeds-based waste byproducts.
Section snippets
Nutritional composition
WMR is a thick white flesh between the outer green covering and inner red flesh, accounts for approximately one-third of the total fruit mass and is usually discarded, despite being edible; however, most times it is avoided due to its unappealing flavor. The fresh WMR has a higher moisture content of 95.63% and the proximate composition showed that the WMR powder has moisture content (9–17%), crude protein (6–21%), fat (0.66–15%), crude fiber (12–23%), ash (12–20%) and carbohydrates (42–65%) (
Nutritional composition
The nutritional composition of (WMSs) is resumed in Table 1. WMSs are rich in crude protein (17–50%), crude fiber (4%–46%) and total fat content (24–58%) (Falade, Otemuyiwa, Adekunle, Adewusi, & Oluwasefunmi, 2020; Rico et al., 2020). Tabiri, Agbenorhevi, Wireko-Manu, and Ompouma (2016) evaluated the chemical composition of seeds of three different varieties of watermelon (Crimson sweet, Charleston gray and black diamond). Charleston gray seeds have the high protein content (17.75 ± 0.97%)
Nutraceutical worth
The chemical composition discussed earlier demonstrated that the watermelon by-products are a rich source of vitamins, minerals, fatty acids, phytosterols, protein, phenolic compounds, dietary fiber and citrulline. All these compounds have certain therapeutic properties in the prevention and treatment of disease. The following are the nutraceutical properties of watermelon by-products. Table 2 and Table 3 explain the potential industrial application and market available products of WMR and WMSs.
Conclusion, future perspective and recommendations
Watermelon byproducts (WMR and WMSs) are a potential source of functional compounds and these can be used to replace, enrich and create novel/value-added food products. The chemical composition of WMR and WMSs showed that they are a good source of important nutrients with beneficial properties having in view of health and wellbeing promotion and can be used as dietary supplements in relevant food applications. The nutraceutical potential revealed it to treat cancer, ulcer, kidney damage and
Acknowledgment
The authors are thankful to the University of Agriculture, Faisalabad, Pakistan for their support.
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