Antioxidant and antimicrobial activity of Carica papaya extracts in minimally processed fresh produce

Abstract

Food (especially fruit derived) waste has a detrimental influence on the environment and food security of a country. As the world’s population continues to rise, the challenge becomes feeding more people nutrient dense meals and preventing wastage of what we already produce. Fresh produce is important for a healthy diet, however, challenges arise around retaining their freshness and quality during processing and distribution. A lot of produce gets wasted along the supply chain and so technologies that help in retaining the quality are needed. The conversion of by-products obtained during processing to high end food additives is one way of reducing fresh produce waste. Papaya (Carica papaya L.) is one such produce whose by-products are underutilised. Based on the nutritional quality of the edible part, there was potential in utilising the papaya peel in the study. There is not much that has been reported on papaya with regards to utilization of its by-products as preservatives within the food industry. Therefore, the antioxidant and antimicrobial properties of C. papaya peel crude extracts were studied in this work. The bioactive compounds in the peel crude extracts were also identified. The ability of the crude extracts to inhibit more common foodborne pathogens was investigated. Furthermore, the crude extracts were also tested as anti-browning agents using apples and potatoes as model systems. Using the agar well diffusion technique, the antibacterial activity of the peel crude extracts against bacteria (Listeria monocytogenes and Escherichia coli) was examined. The crude extracts were active against both L. monocytogenes and E. coli. Because of the difference in cell wall structure between the two bacteria (the former is a gram positive bacterium and the latter is a gram negative bacterium), antimicrobial activity was more pronounced in L. monocytogenes than in E. coli. The crude extracts were also stored at 4°C and 10°C for 5 days and thereafter their activity tested against the two bacterial species. The results showed that the crude extracts generally retained their antimicrobial properties even after exposure to the low temperatures for the 5 day period. The antimicrobial activity of the crude extracts was found to be due to presence of bioactive compounds. The bioactive component profiles of the peel crude extracts were investigated using a high-resolution ultra-performance liquid chromatography system with diode array detection, quadrupole time-of-flight and mass spectrometer (UPLC-DAD-QTOF-MS). Metabolites such as citric acid, ascorbic acid, gluconic acid, malic acid, ρ-coumaric acid, ferulic acid, caffeic acid glucoside, vanillic acid, sinapic acid, rutin, quercetin-3-O- rhamnosyl rutinoside, Isorhamnetin-3-O- dirhamnosyl glucoside and benzyl glucosinolate were identified. Phenolic compounds were found to be the most predominant in the crude extracts. Overall, the bioactive compounds identified within the crude extracts were organic acids, phenolic acids, flavonols and a glucosinolate. The Folin-Ciocalteu (F-C) technique, the aluminium chloride test and the ferric reducing antioxidant power (FRAP) assay were used to measure the total phenolic (TPC), total flavonoid content (TFC), and metal chelating activity of the crude extracts, respectively. The TPC, TFC and FRAP in the crude extracts were 6865 ± 153 mg GAE/g dw, 3638 ± 252 mg QE/g dw and 7968 ± 38 mM TE/g dw, respectively. The crude extracts therefore showed antioxidant activity. The presence of the enzymes chymopapain and papain in the crude extracts was determined using SDS-PAGE (Sodium dodecyl-sulphate polyacrylamide gel electrophoresis). This was done to test for toxicity of the crude extracts. Additionally, the BCA (Bicinchoninic acid) assay was used to determine the overall protein concentration in the crude extracts. There were low concentrations of proteins observed in the crude extracts. Ultimately, the absence of enzymes chymopapain and papain in the crude extracts gave evidence that the extracts were not toxic. To determine the crude extracts’ ability to inhibit enzymatic browning, potato and apple pieces were immersed in treatments containing the extracts and these were compared with other standard and control treatments. Browning was measured as change in colour using a CR-400 Chromameter where Lab values were obtained and used to calculate the overall colour change (ΔE) and the browning index (BI). Browning was also measured through visual assessment where browning scores (BS) for the apples and potatoes were recorded. The crude extracts inhibited enzymatic browning but were less effective than the standard treatment where the apples and potatoes had been immersed in a solution containing sodium metabisulphite, citric and ascorbic acids. The ability of these crude extracts to inhibit browning is supported by the presence of antioxidants and their ability to slow down the enzymatic browning process. The outcomes and findings of this research open further avenues and opportunities for the increased utilisation of C. papaya peels which is are derived from the consumption of papaya.