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Effects of Roasting on Fermented Avocado Seeds and Apricot Kernels: Chemical Components and Bioactivities

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Thesis is embargoed until 8 July 2028(15.68 MB)

Date

2024

Authors

Zhao, Zhiyu

Supervisor

Hamid, Nazimah
Le, Thao
Kam, Rothman

Item type

Thesis

Degree name

Doctor of Philosophy

Journal Title

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Publisher

Auckland University of Technology

Abstract

This research examines the impact of fermentation and roasting processes on the alterations in chemical composition and bioactivities of non-edible seeds, with a particular focus on avocado and apricot kernels. Through a series of comprehensive studies, the research uncovers the complex relationship between Lactobacillus plantarum fermentation and the antioxidant, anticancer properties, chemical composition as well as bioactive peptides of the seeds. The investigation in Chapter 3 begins by fermenting avocado seeds through natural and inoculated processes with L. plantarum and kefir cultures, as well as natural fermentation. This was followed by roasting of the L. plantarum sample that exhibited the highest antioxidant activities. Microbial analysis, phenolic content evaluation, and antioxidant activity assessments were conducted, followed by roasting to produce a shelf-stable powder. Avocado seeds were subjected to natural fermentation, and inoculated fermentation with either Lactobacillus plantarum or kefir. The fermented samples were evaluated in terms of total plate count values of lactic acid bacteria (LAB), acetic acid bacteria (AAB) and yeasts. The number of microorganisms increased significantly (p<0.001) over the seven days of fermentation for all samples. Fermentation with L. plantarum resulted in significantly (P<0.05) higher total phenolic content (TPC) and antioxidant activities compared to kefir and naturally fermented samples. The L. plantarum fermented avocado seeds were further subjected to roasting to yield a shelf stable dried powder. Roasted samples fermented by L. plantarum had significantly (P<0.001) higher total polyphenolic content and antioxidant capacity (CUPRAC and FRAP assays) compared to kefir and naturally fermented samples. The avocado seed powder also demonstrated significant inhibition effects on the liver cancer cell line Hep G2, followed by the breast cancer cell lines MDA-MB-231 and MCF-7. A Central Composite Design (CCD) was utilised to establish the experimental framework for investigating the effects of roasting time and temperature. The roasting conditions for optimal antioxidant and anticancer activities, which were determined using the Response Surface Methodology (RSM), were identified to be 127°C for 24.7 minutes. The findings demonstrated that fermentation of avocado seeds in combination with roasting yielded a powder with good antioxidant and anticancer activities, which can potentially be incorporated in food for added health benefits. The effects of roasting on the metabolites and anticancer activities of fermented avocado seeds were investigated in Chapter 4. All three anti-cancer activities of fermented avocado seeds were higher in avocado seed roasted at 99°C for 9 min. The best inhibition effect was found against liver cancer cell line Hep G2 followed by the breast cancer cell lines MDA-MB-231 and MCF-7. Untargeted metabolite profiling using gas chromatography-mass spectrometry resulted in the identification of 208 metabolites. In total, 41 metabolites with Variable Importance in Projection (VIP) values greater than 1 using Partial Least Squares regression (PLS-R) were found to be related to anticancer activities. These key metabolites were also positively correlated with higher levels of all amino acids and most sugars in roasted avocado seeds. The roasting conditions for optimal antioxidant and anticancer activities were determined using RSM, resulting in an optimal condition of 121°C for 9 minutes. Apricot kernels are known for being a rich source of oil, protein, and bioactive compounds. In Chapter 5, apricot kernels were fermented using L. plantarum to enhance their bioactivities Additionally, the study explored the correlation between polyphenols, amino acids, antioxidant activities, and total phenolic content (TPC) by using a Random Forest model combined with SHAP (SHapley Additive exPlanations) values to identify and interpret the key contributors to antioxidant activities. Results demonstrated that apricot kernels fermented by L. plantarum exhibited higher antioxidant activities using FRAP and CUPRAC assays, as well as a higher total phenolic content (TPC) compared to the naturally fermented samples. The analysis showed that certain polyphenols, such as hydroxybenzoic acid, DL-3-phenyllactic acid, and benzoic acid, significantly increased over the fermentation period, reaching peak levels on Day 5. Similarly, most amino acids demonstrated an increase in concentration as fermentation progressed, peaking on the ninth day. The study further employed Random Forest Regression as a form of Explainable Artificial Intelligence (XAI) to explore the relationships between phenolic compounds, amino acids, and antioxidant activities. Amino acids like L-cystine and L-anserine were found to positively impact FRAP values, while L-histidine and 1-methyl-L-histidine contributed to CUPRAC antioxidant activity. Notably, hydroxybenzoic acid emerged as a key contributor to both FRAP and TPC, highlighting its significance in the overall antioxidant capacity. These findings suggest that when subjected to optimised fermentation conditions, apricot kernels exhibit promising potential as functional food ingredients, owing to the beneficial antioxidant properties identified in this study. Chapter 6 analysed the impact of fermentation on the amino acid and bioactive peptide profiles of apricot kernels. Most amino acids increased in concentration as fermentation progressed. The optimal extraction of peptides from fermented apricot kernels was achieved using 60% acetonitrile, resulting in the identification of 1087 peptides. Among the samples, apricot kernels fermented by L. plantarum on Days 5 and 9 stood out, with 536 and 512 peptides respectively identified at a 1% false discovery rate (FDR) level. The fermentation of apricot kernels, especially with L. plantarum, led to the production of a significant quantity of bioactive peptides with diverse activities such as antihypertensive, anticancer, antimicrobial, and antibacterial properties. Moreover, the study analysed the effects of the top 20 explanatory amino acids on the abundance of the five most prominent peptides—GLHLPS (A), GAGAGP (B), GFGGGGIGGGGFGGGY (C), GGAGFG (D), and GRRGGGYGGGGYGGGGYGGGG (E)—using a Random Forest (RF) model. By utilizing SHAP (SHapley Additive exPlanations) values, the research provided insights into the relative contributions of each amino acid, with L-histidine and L-citrulline identified as key drivers positively associated with peptide abundance. The RF model and SHAP values enabled a comprehensive understanding of the relationship between amino acids and peptide production, providing insights into the relative contributions of each amino acid, and highlighting L-histidine and L-citrulline as key drivers positively associated with peptide abundance. In conclusion, the results of the study highlight the potential to repurpose plant seeds as functional food products with the implementation of fermentation and roasting techniques. The use of lactic acid fermentation not only enhances the nutritional value of the seeds but also offers a sustainable solution for reducing food waste. This innovative approach opens opportunities for developing a wide range of nutrient-rich foods and ingredients from various plant byproducts, contributing to both food sustainability and the enhancement of healthy food options for consumers.

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http://hdl.handle.net/10292/19485

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