Natural Deep Eutectic Solvent-Based Extraction of SunGold Kiwifruit Polyphenols: Optimisation, Bioactivity, and Functional Application in Yoghurt Systems

Abstract

Polyphenols are widely recognised as plant-derived bioactive compounds with potential antioxidant and antidiabetic properties. However, their application in functional foods depends not only on their natural abundance in plant materials, but also on how effectively they can be extracted, retained, and delivered within a suitable food matrix. SunGold kiwifruit is a nutrient-rich fruit containing vitamin C, dietary fibre, organic acids, and phenolic compounds that may contribute to metabolic health benefits. At the same time, yoghurt provides a practical food matrix for bioactive ingredient delivery due to its nutritional value, high protein content, ready-to-eat format, and wide consumer acceptance. Therefore, combining SunGold kiwifruit polyphenols with yoghurt may offer a promising approach for developing functional dairy products with potential antioxidant and antidiabetic properties.

Natural deep eutectic solvents (NADES) have recently gained attention as green extraction solvents for plant bioactive because their polarity, viscosity, and hydrogen-bonding capacity can be tailored through solvent composition and water addition. This thesis investigated the use of choline chloride:glycerol-based NADES for extracting polyphenol-rich compounds from SunGold kiwifruit and evaluating their functional application in yoghurt systems. The research focused on extraction optimisation, antioxidant activity, antidiabetic activity, phenolic profiling, and the physicochemical properties of fortified yoghurts.

Chapter 2 reviewed the potential of NADES for extracting polyphenols with antidiabetic relevance. The review discussed the classification, dietary sources, and biological activities of polyphenols, particularly their role in carbohydrate-hydrolysing enzyme inhibition, glucose regulation, and antioxidant mechanisms. It also examined the concept of NADES, including the influence of hydrogen bond acceptors, hydrogen bond donors, water content, polarity, viscosity, and extraction conditions on polyphenol recovery. Compared with conventional organic solvents, NADES were identified as promising alternatives due to their tunability and potential compatibility with food applications. However, several challenges remain, including high viscosity, toxicity considerations, regulatory limitations, bioaccessibility, and stability after incorporation into food systems. This review highlighted the need to evaluate NADES not only as extraction solvents, but also as part of functional food development.

Chapter 3 optimised ultrasound-assisted NADES extraction of polyphenol-rich compounds from SunGold kiwifruit using response surface methodology. A choline chloride:glycerol NADES system was evaluated by varying NADES molar ratio, water content, solid-to-liquid ratio, extraction time, and extraction temperature. The responses measured were total phenolic content, DPPH radical scavenging activity, ferric reducing antioxidant power, and α-glucosidase inhibitory activity. The fitted models were statistically significant and showed good predictive ability, confirming that extraction variables strongly influenced the recovery and bioactivity of SunGold kiwifruit polyphenols. The optimum extraction conditions were a NADES ratio of 1:1, approximately 50% water content, a solid-to-liquid ratio of 1:20 g/mL, an extraction time of approximately 22 min, and an extraction temperature of 30°C. Under these conditions, the experimental values were 4.892 ± 0.038 mg GAE/g for total phenolic content, 9.797 ± 0.156 mg TE/g for DPPH activity, 9.599 ± 0.129 mg TE/g for FRAP activity, and 2.651 ± 0.024 mg QE/g for α-glucosidase inhibitory activity. LC-MS/MS analysis indicated the presence of several phenolic compounds, including catechin, epicatechin, chlorogenic acid, caffeic acid, and related flavonoids.

Chapter 4 evaluated the incorporation of NADES- and water-extracted SunGold kiwifruit polyphenols into yoghurt and assessed their effects on antioxidant activity and physicochemical properties. The extracts were added at different concentrations and at two fortification stages, before fermentation and after fermentation. The water extract showed higher total phenolic content and DPPH radical scavenging activity than the NADES extract, whereas the NADES extract showed higher total flavonoid content, FRAP, and CUPRAC activity. This indicated that the extraction solvent influenced the type of bioactive compounds recovered and that antioxidant activity depended on the mechanism of the assay used. Fortification generally improved the antioxidant properties of yoghurt in a concentration-dependent manner. However, extract type and fortification stage also affected yoghurt quality. NADES-fortified yoghurts generally showed higher pH, higher °Brix, higher viscosity, and lower syneresis than water-fortified yoghurts, suggesting better structural stability. In contrast, water-fortified yoghurts showed greater changes in acidity, colour, and syneresis, particularly at higher extract concentrations.

Chapter 5 investigated the in vitro antidiabetic activity of NADES-extracted SunGold kiwifruit phenolics and their application in fortified yoghurt. The NADES extract showed stronger α-glucosidase and α-amylase inhibitory activities than the water extract, with IC50 values of 15.331 ± 0.393 mg/mL and 116.530 ± 2.110 mg/mL, respectively. It also showed greater glucose diffusion retardation, increasing from 35.37% to 62.84% over 180 min, compared with 9.47% to 37.89% for the water extract. When incorporated into yoghurt, enzyme inhibitory activities generally increased with extract concentration, with NADES-fortified yoghurts showing stronger activity than water-fortified yoghurts. The highest α-glucosidase and α-amylase inhibitory activities were observed in NADES-fortified yoghurt prepared by post-fermentation addition at the highest concentration. LC-MS/MS analysis confirmed the presence of several phenolic compounds in the extracts and fortified yoghurts, including catechin, epicatechin, epigallocatechin, chlorogenic acid, caffeic acid, and gallic acid. These compounds may have contributed to the observed antidiabetic activity, although the results suggest that the activity was more likely due to the combined effects of multiple phenolics rather than a single compound.

Overall, this thesis demonstrated that choline chloride:glycerol-based NADES can be used as a green and functional extraction system for recovering bioactive phenolic compounds from SunGold kiwifruit. Although water extraction was more effective for total phenolic content and DPPH radical scavenging activity, NADES extraction favoured flavonoid recovery, reducing power, carbohydrate enzyme inhibition, and yoghurt structural stability. Compared with yoghurts fortified with the water extract, yoghurts fortified with the NADES extract showed higher responses in several antioxidant and in vitro carbohydrate-digesting enzyme inhibition assays, alongside differences in physicochemical properties. Further studies should evaluate sensory acceptance, storage stability, gastrointestinal bioaccessibility, safety, and in vivo effects to determine whether NADES-based yoghurt products can provide meaningful benefits for glycaemic health.