Antioxidant and antimicrobial properties of by-products (peels and seeds) from 'Hass' avocado (Persea americana Mill.) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, New Zealand

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Background: ‘Hass’ avocado (Persea americana Mill.) is the most common variety of avocado grown for the global market. Avocado is rich in fibre; vitamins C, B, and E, unsaturated fatty acids (oleic, linoleic, linolenic acids); potassium; and other nutrients. Avocado oil production discards a large amount of by-product, which could be a potential source of bioactive molecules. Avocado peel and seed are rich in vitamins, fatty acids, proteins, and other bioactive substances. The main phytochemicals found in avocado seed and peel are procyanidins, flavanols, catechins, hydroxybenzoic acids, and hydroxycinnamic acids. Extracts of avocado by-product have antioxidant, antimicrobial, antimutagenic, cancer-preventing, and anti-cariogenic properties. Objective: There is a lack of New Zealand (NZ) research on the chemical composition and biological activity of ‘Hass’ avocado seed and peel. The utilization of avocado by-products (peel and seed) could provide a potential value-added resource for the NZ avocado industry. This thesis aimed to investigate the bioactive components, antioxidant activity, and antibacterial activities of NZ ‘Hass’ avocado seed and peel extracts. Methods: In this thesis, the total polyphenol content (TPC) was measured by the Folin−Ciocalteu assay, the tannin content (TTC) was measured with a microplate assay kit, and the procyanidin content (PC) was measured by the HCl-n-butanol assay. High-performance liquid chromatography–mass spectrometry was used for characterizing and quantifying the procyanidin B2 content of NZ ‘Hass’ avocado by-products. The in-vitro antioxidant activity of ‘Hass’ avocado by-products (peel and seed) was evaluated by 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2’-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) assays. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay was developed for characterizing the in vitro antibacterial activity of NZ ‘Hass’ avocado by-products. In addition, thin-layer chromatography was combined with direct microbiological detection to show the areas of antibacterial activity from the separated compounds of hexane extracts of NZ ‘Hass’ avocado by-products. Mass spectrometry analysis was used to characterize the target components from hexane extracts of avocado peel and seed. The main antibacterial compounds (Fragment Ion Search score > 80 or match score > 80) have been tentatively identified by Compound Discoverer 3.3. Results: In this study of NZ ‘Hass’ avocado by-products, the ripe seed extract showed the highest TPC (64.55 ± 2.26 mg GAE/g) than the extracts of ripe peel, mature unripe seed, and mature unripe peel. After flash chromatography separation, the low-polarity fraction of the ripe peel had the highest DPPH and ABTS scavenging capacities (90 ± 0.08%; 93 ± 0.05%) compared with other extracts. In addition, the phytochemical content, in vitro antioxidant activity, and antibacterial activity were compared among NZ, Chinese, and Chilean ‘Hass’ avocado by-products. The extract from ripe seeds of NZ avocado had a higher TPC (52.82 ± 0.04 mg GAE/g) than the corresponding extracts from Chinese and Chilean avocados. The TTC and PC values showed non-significant differences (p > 0.05) among extracts from Chinese, Chilean, and NZ ‘Hass’ avocado by-products. Chilean ripe avocado seed extracts had the greatest inhibition of Staphylococcus aureus (98.21% ± 0.98) of all the extracts. Chilean ripe avocado seed extracts had the highest DPPH and ABTS scavenging capacities (90.94 ± 0.94 % and 92.83 ± 0.62 %, respectively). Moreover, (15Z)-9,12,13-trihydroxy-15-octadecenoic acid, (±)9-HpODE, and NP-011548 are three antibacterial fatty acids tentatively identified from hexane extracts of avocado peel, and (2E,4E,6E)-12-hydroxy-2,4,6-heptadecatrienoic acid, pentadecanoic acid, ascorbyl oleate, and ascorbyl stearate are the antibacterial fatty acids found in hexane extracts of avocado seed. Conclusion: NZ ‘Hass’ avocado seed and peel contain several valuable phytochemical components (polyphenols, tannins, procyanidins, and fatty acids). Moreover, they showed good antioxidant and antibacterial activities. The peel and seed from NZ ‘Hass’ avocado could be considered as a natural antibacterial and antioxidant in developing food additives in the future.
Chapter 3 was published as Shi, D., Xu, W., Balan, P., Wong, M., Chen, W., & Popovich, D. G. (2021). In Vitro Antioxidant Properties of New Zealand Hass Avocado Byproduct (Peel and Seed) Fractions. ACS Food Science & Technology, 1(4), 579-587.
Avocado, Plant extracts, New Zealand, Composition, Plant bioactive compounds, Antioxidants, Antibacterial agents, avocado peel, avocado seed, phytochemical components, antioxidant ability, antibacterial activity