Functional ingredients from valorization of melon (Cucumis melo L.) by-products : production, bioactivity and potential application

• Main Author: Gomez García, Ricardo
• Publication Date: 2021
• Language: eng
• Source: Repositórios Científicos de Acesso Aberto de Portugal (RCAAP)
• Download full: http://hdl.handle.net/10400.14/39809
• topic: Melon by-products; Biowaste valorisation; Circular bioeconomy; Functional ingredients; Bioactive proteins; Subprodutos melão; Valorização biorresíduos; Bioeconomia circular; Ingredientes funcionais; Proteínas bioativas

Currently, fruit processing industries are responsible for the production and accumulation of large amounts of vegetable by-products, which are in some cases disposed of in landfills as waste, causing environmental and economic problems. Melon by-products are no exception and are part of this problem due to the proportion generated, which represents about 30% in residue (fresh weight), during its industrial processing. The development of an integral volorisation strategy for melon by-products through green and sustainable processes in the context of zero waste and circular bioeconomy is an important task. This will allow its full valorisation and reincorporation into the industrial value chain as a rich source of natural and bioactive ingredients, demonstrating the applicability of high economic value and reduced environmental impact. The main objective of this Ph D work was to fully explore and with zero¬waste the value of melon by-products and the development of functional ingredients rich in bioactive molecules with high added value, including different fractions rich in pigments, polyphenols, functional proteins (antifreeze and coagulating enzymes), pectin, microcellulose and a flour rich in fiber. In addition, it also allowed the development of an integrated downstream extractive process using clean technologies and the evaluation of the biological and technological properties of the obtained functional ingredients to establish their value and potential application. Initially, fresh melon peels were fractionated by mechanical processes within the zerowaste and biorefinery approaches, generating three fractions, which were characterized. I) The pellet fraction (PF), which represents the smallest fraction (2.0% of the total fresh weight of the peels (FW)), is the richest fraction in proteins (34.90% w/w), chlorophylls (174.84 mg/100 g FW) and total carotenoids (98.59 mg/100 dry matter (DM)) with βcarotene, lutein, β-cryptoxanthin and violaxanthin quantified as the main ones. This fraction has also been valorised through disruption processes combined with solid-liquid extraction methods to separate and isolate all important constituents, including proteins, chlorophylls and xanthophylls. II) The solid fraction (SF) represents 21.5% of the FW and is the richest fraction in fiber (44.42% w/w), which revealed the composition of cellulose, hemicellulose and lignin (27.67%, 8.2% and 26.5% w/w, respectively), showing great potential as an ingredient rich in antioxidant fiber. Associated with its bioactive compound profile, this fraction was also subjected to simulated conditions of the gastrointestinal tract (GIT) and subsequently evaluated by in vitro fecal fermentation with the objective to develop a prototype of functional flour for health promotion, mainly associated with its prebiotic potential. The results of the 16rRNA gene analysis showed that SF did not impact negatively the intestinal microbiota diversity and allowed the positive production of short-chain fatty acids (acetate > propionate > butyrate). In addition, the SF fraction also showed high pectin content, so in a second approach, it was also extracted by testing different acids combined with temperature, being citric acid the one that exhibited the best efficacy, allowing 34% w/w extraction yield and 60-70% methyl esterification, and showing good emulsifying and gelling properties. After extracting pectin from the SF fraction, the lignocellulosic residues from melon were subjected to extraction of microcrystalline cellulose (MCC) by two methods using NaOH: traditional and thermoalkaline. The results showed a higher degree of crystallinity by the thermoalkaline method than by the traditional process (51¬61% and 54-55%, respectively). Finally, III) the liquid fraction (LF), the most representative fraction in fresh weight with about 70% due to its high-water content (>90%). This fraction showed high antioxidant activity by the ABTS, DPPH and ORAC methods attributed to the high polyphenol content (798.43 mg GAE/100 g DM). The LF fraction was subjected to the conditions of the GIT. The total phenolic content (TPC) was maintained in the mouth and gastric phase (453.69 and 425.89 mg/100 g DM, respectively), while in the small intestine a drastic decrease was observed (reaching 313.37 mg/100 g DM), which corresponds to 54-57% of recovery index (RI), however, the antioxidant activity of ABTS and DPPH was still significant, which confirm the capacity of such compounds to exert their bioactive properties (54-76% accessibility index (ACI)). On the other hand, this fraction had a prebiotic effect (2% w/v) on the main beneficial groups of the intestinal microbiota, the Lactobacillus sp. and Bifidobacterium sp., modulating positively its growth and metabolism. Furthermore, cucumisin (CUC) was extracted through a green chemistry approach using carrageenan as a precipitating agent, proving to be an effective strategy to isolate it with a yield of 0.17 g CUC/100 g of by-products, and with better purification factor of proteolytic activity than the traditional precipitation process (17.65 and 1.60¬ folds, respectively), maintaining its biological properties. The melon proteins extracted by this green precipitation process also exhibited antifreeze properties, possibly attributed to the great similarity in structure and molecular weights (6.2 and 3.5 kDa) with other proteins previously reported such as type III antifreeze proteins (C-terminal domain, few alanine residues and no cysteine residues) and type I (alanine-rich), respectively. Briefly, the results of this work on the integral valorization of melon peels will contribute to a sustainable development in the fruit processing industries, allowing the obtention of new functional ingredients/additives through clean strategies and preserving their bioactivity and functionality.