1. Introduction
Our lab has conducted significant research focused on maximizing the value of kiwifruit production, with a particular emphasis on the underutilized agricultural byproduct of thinned immature kiwifruits (TUK/TIK). This work addresses the critical need to reduce agricultural waste while identifying novel sources of bioactive compounds for functional food and nutraceutical applications. The research spans characterization of bioactive compounds, development of efficient and green extraction methodologies, evaluation of health-promoting properties, and exploration of postharvest preservation strategies for mature fruit.
2. Characterization of Bioactive Compounds in Thinned Immature Kiwifruits
A core theme of our research is the comprehensive characterization of phenolic compounds and polysaccharides in thinned immature kiwifruits, demonstrating their exceptional potential as sources of valuable bioactives. We established that TIK possess remarkably higher levels of total phenolics (116.39 mg GAE/g DW) and total flavonoids (33.88 mg RE/g DW) compared to mature fruits, approximately 7.4 and 4.8 times greater, respectively (Deng et al., 2024a). Similarly, the total major phenolic components in TIK were found to be 5.4 times higher than in mature fruits, with neochlorogenic acid, epicatechin, procyanidin B1, and procyanidin B2 identified as predominant compounds (Deng et al., 2024a). Comparative analysis across nine commercial cultivars revealed significant variation, with 'Hongao' and 'Cuiyu' exhibiting the highest levels of key phenolics like catechin, epicatechin, procyanidin B2, and neochlorogenic acid (Deng et al., 2024b). Furthermore, we characterized the pectic polysaccharides (YKP) extracted from TUK, identifying them as primarily composed of homogalacturonan (HG) and rhamnogalacturonan I (RG I) domains (Wu et al., 2024).
3. Development of Innovative and Efficient Extraction Methodologies
Our lab has pioneered green and efficient extraction techniques specifically tailored for recovering bioactives from thinned kiwifruits. We successfully optimized ultrasound-assisted deep eutectic solvent extraction (UADE), achieving a total phenolic yield of 105.37 mg GAE/g DW from thinned young kiwifruits – significantly higher than conventional organic solvent extraction (14.51 mg GAE/g DW) or ultrasound-assisted ethanol extraction (43.85 mg GAE/g DW) (Wu et al., 2023). Similarly, deep eutectic solvent-assisted extraction (DAE) and microwave-assisted deep eutectic solvent extraction (MDE) were developed for polysaccharides, yielding significantly higher quantities of pectic polysaccharides (YKP-D and YKP-DM) compared to traditional hot water extraction (YKP-H) (Wu et al., 2024). We also optimized ultrasound-assisted extraction (UAE) conditions for antioxidants from mature 'Jinfeng' kiwifruit, identifying the optimal parameters (68% ethanol, 20 mL/g ratio, 30 min, 42°C, 420 W) which increased ABTS antioxidant capacity by 18.5% over conventional methods (Mai et al., 2022). These methods consistently demonstrated superior efficiency and environmental friendliness.
4. Evaluation of Health-Promoting Properties and Applications
The bioactive extracts derived from thinned kiwifruits exhibit potent biological activities with significant health implications. Crucially, polyphenol-enriched extract from TIK (YK) demonstrated superior hepatoprotective effects against alcoholic liver disease (ALD) in mice compared to extracts from mature fruit (Deng et al., 2024a). YK administration increased body weight, reduced serum markers of liver damage (ALP, AST, ALT), lowered hepatic lipids (TG, TC), diminished lipid accumulation and tissue damage, restored antioxidant enzyme levels (SOD, CAT), and reduced pro-inflammatory cytokines (IL-6, IL-1β, TNF-α), indicating amelioration of ALD through reduced oxidative stress and inflammation (Deng et al., 2024a). Furthermore, extracts from TUK showed strong in vitro antioxidant capacity, significant inhibitory effects on digestive enzymes (α-glucosidase and α-amylase), and anti-inflammatory activities, with procyanidin B2 identified as a key contributor (Deng et al., 2024b; Wu et al., 2023). The pectic polysaccharides (YKP-D and YKP-DM) extracted via deep eutectic solvents also exhibited stronger antioxidant, anti-glycosylation, and immunomodulatory effects than those obtained by hot water extraction (Wu et al., 2024).
5. Postharvest Preservation Strategies for Mature Kiwifruit
Complementing the byproduct valorization work, our lab has also contributed to understanding and improving the postharvest quality of mature kiwifruit. We conducted a comprehensive review of innovative postharvest strategies developed over the past five years, analyzing the characteristics, advantages, and mechanisms of physical (e.g., low-temperature), chemical (e.g., essential oils), and biotechnological (e.g., endophytic yeast) methods for controlling disease, delaying chilling injury, and inhibiting oversoftening and off-flavor development (Xia et al., 2024). This review highlighted the limitations of single techniques and proposed that synergistic applications or novel approaches like pulsed light and cold plasma represent promising future directions for maintaining kiwifruit quality during storage (Xia et al., 2024).
6. Novelty and Significance of Our Work
The novelty of our lab's research lies in the systematic and comprehensive focus on thinned immature kiwifruits (TUK/TIK) as a high-value, underutilized agricultural resource, rather than solely concentrating on mature fruit. We were among the first to rigorously quantify the vastly superior concentration of phenolic compounds (up to 7.4x higher total phenolics) and key bioactives in these byproducts compared to mature fruit (Deng et al., 2024a; Deng et al., 2024b). Furthermore, we pioneered the application of green extraction technologies, specifically deep eutectic solvents combined with ultrasound or microwave assistance, for efficiently recovering polyphenols and polysaccharides from TUK/TIK, achieving significantly higher yields and bioactivity compared to conventional methods (Wu et al., 2023; Wu et al., 2024). The demonstration of potent in vivo hepatoprotective effects against ALD using TIK extract provides critical translational evidence for the functional food potential of these byproducts, moving beyond in vitro studies (Deng et al., 2024a).
The significance of this work is multifaceted. First, it offers a sustainable solution to agricultural waste by transforming TUK/TIK into valuable functional ingredients. Second, it provides scientific validation for developing novel nutraceuticals and functional foods targeting liver health, metabolic disorders (via enzyme inhibition), and inflammation. Third, it establishes efficient and eco-friendly extraction protocols applicable to the food industry; Finally, it contributes to enhancing the overall economic viability and sustainability of the kiwifruit industry by creating value from a previously discarded resource. This research directly addresses global challenges in food waste reduction and the demand for natural, health-promoting compounds.
Acknowledgement
We would like to thank the hard work of all my team members and collaborators involved in this work.
Our Publication List
Deng, W., Yang, Q. N., Wu, D. T., Li, J., Liu, H. Y., Hu, Y. C., Zou, L., Gan, R. Y., Yan, H. L., & Huang, J. W. (2024a). Comparison of Protective Effects of Polyphenol-Enriched Extracts from Thinned Immature Kiwifruits and Mature Kiwifruits against Alcoholic Liver Disease in Mice. Foods, 13(19), 3072. http://dx.doi.org/10.3390/foods13193072
Deng, W., Yang, Q. N., Liu, H. Y., Xia, Y., Yan, H. L., Huang, J. W., Hu, Y. C., Zou, L., Gan, R. Y., & Wu, D. T. (2024b). Comparative analysis of phenolic compounds in different thinned unripe kiwifruits and their biological functions. Food Chemistry-X, 24, 101815. http://dx.doi.org/10.1016/j.fochx.2024.101815
Mai, Y. H., Zhuang, Q. G., Li, Q. H., Du, K., Wu, D. T., Li, H. B., Xia, Y., Zhu, F., & Gan, R. Y. (2022). Ultrasound-Assisted Extraction, Identification, and Quantification of Antioxidants from 'Jinfeng' Kiwifruit. Foods, 11(6), 827. http://dx.doi.org/10.3390/foods11060827
Wu, D. T., Deng, W., Li, J., Geng, J. L., Hu, Y. C., Zou, L., Liu, Y., Liu, H. Y., & Gan, R. Y. (2023). Ultrasound-Assisted Deep Eutectic Solvent Extraction of Phenolic Compounds from Thinned Young Kiwifruits and Their Beneficial Effects. Antioxidants, 12(7), 1475. http://dx.doi.org/10.3390/antiox12071475
Wu, D. T., Geng, J. L., Li, J., Deng, W., Zhang, Y., Hu, Y. C., Zou, L., Xia, Y., Zhuang, Q. G., Liu, H. Y., & Gan, R. Y. (2024). Efficient extraction of pectic polysaccharides from thinned unripe kiwifruits by deep eutectic solvent-based methods: Chemical structures and bioactivities. Food Chemistry-X, 21, 101083. http://dx.doi.org/10.1016/j.fochx.2023.101083
Xia, Y., Wu, D. T., Ali, M., Liu, Y., Zhuang, Q. G., Wadood, S. A., Liao, Q. H., Liu, H. Y., & Gan, R. Y. (2024). Innovative postharvest strategies for maintaining the quality of kiwifruit during storage: An updated review. Food Frontiers, 5(5), 1933–1950. http://dx.doi.org/10.1002/fft2.442
