Efficient and green strategy based on pulsed electric field coupled with deep eutectic solvents for recovering flavonoids and preparing flavonoid aglycones from noni-processing wastes

Highlights

• Pulsed electric field improves recovery of flavonoids from noni pomace.

• Pulsed electric field coupled with deep eutectic solvents can convert rutin.

• A record-rutin conversion efficiency (8.72%/min) was achieved in this study.

• Degraded rutin has been almost 100% converted to quercetin in our method.

• Our method avoids the use of enzymes, acidic organic solvents and high-temperature.

Abstract

The recovery of flavonoids and preparation of flavone aglycones from pomace waste are accompanied by the use of acidic organic solvents and high energy consumption, which lead to environmental pollution. This study aimed to develop an efficient and eco-friendly method based on pulsed electric field-assisted extraction (PEF-AE) combined with deep eutectic solvents (DESs) to recover flavonoids from noni pomace. The feasibility of using pulsed electric fields (PEF) combined with DESs as an alternative to conventional enzymatic, acidic, and pyrolytic methods, for the conversion of flavone glycosides to flavone aglycones was also evaluated. The results showed that, by using PEF-AE combined with DESs, the recovery efficiencies of the two main flavonoids (rutin and quercetin) in noni pomace were significantly superior to those of conventional organic extraction. Choline chloride/ethylene glycol (Ch/Eth) and choline chloride/oxalic acid (Ch/Oxa) were found to be optimal extractants for the recovery of rutin and quercetin. After response surface methodology optimisation, the highest extraction amounts of rutin and quercetin were 16.21 mg/g and 19.85 mg/g, respectively. During the extraction process, it was found that almost 100% of degraded rutin can be converted to quercetin in Ch/Oxa. Under identical treatment time and temperature, the rutin conversion efficiency of PEF was 25.4–206.4 folds higher than that of hot-water treatment (HWT). An excellent rutin conversion efficiency (8.72%/min) for PEF was achieved by adjusting the key reaction conditions. Overall, PEF-AE combined with DESs is an efficient, selective, and sustainable method for recovering flavonoids from noni pomace waste. This study also demonstrated that PEF coupled with DESs is a promising method for the efficient and eco-friendly preparation of flavone aglycones.

Introduction

Morinda citrifolia L. (Noni) is an evergreen plant native to the tropical and subtropical regions. In Polynesia, India, Malaysia, Indonesia and China, Noni fruit has been used as a traditional herb for more than 2000 years (Motshakeri and Ghazali, 2015). Noni juice is a significant player in the growing functional beverage market owing to its excellent nutritional and pharmacological properties (Dussossoy et al., 2011). As a result of effective marketing and official safety authentication, it is now readily available and sold as bottled pasteurized juice or mixed juice in Europe, the United States, Japan and China (Chan-Blanco et al., 2007). In the production process of noni juice, a large amount of pomace is generated after squeezing the fruit. Generally, these by-products are discarded as waste, thereby increasing the risk of environmental pollution. Analyses of noni pomace have shown that it is rich in flavonoids (Chan-Blanco et al., 2006). The reuse of noni pomace has garnered increasing interest because of its potential as a source of bioactive substances with high added value; however, it is currently underutilised.

Conventionally, hot-water extraction (HWE) is the most common method for extracting flavonoids from plant tissues. However, traditional extraction techniques are usually associated with high organic solvent consumption, long extraction times, and high cost (Do et al., 2014). Therefore, other preferable methods need to be explored to improve the extraction efficiency. Currently, as a novel food processing technique, pulsed electric field-assisted extraction (PEF-AE) is a promising alternative to conventional extraction because of its low energy consumption and high efficiency (Niu et al., 2018). The enhancement in extraction caused by a pulsed electric field (PEF) is mainly attributed to the effects of electroporation and permeabilisation (Li et al., 2020). Although PEF is regarded as a “green” technique, organic solvents are commonly used in the PEF-AE of flavonoids (Zhang et al., 2017). In addition, in the PEF extraction process, bubbles that are not removed from the solvent are electrically broken down to produce sparks. In a closed treatment chamber filled with highly concentrated flammable organic solvents, sparks can easily cause potential safety risks, resulting in damage to the treatment chamber (Fig. S1). Therefore, to fulfil the principle of “green chemistry” and “safe production”, it is crucial to find safe and eco-friendly alternative solvents, which is a challenge.

Deep eutectic solvents (DESs) have been recognised as green and sustainable solvents for the extraction of flavonoids owing to their low cost, low toxicity, and biodegradability (Huang et al., 2022; Nolan et al., 2022; Bittner et al., 2022). They also exhibit a wide range of polarities and are non-flammable (Cao et al., 2020). Owing to these properties, DESs are considered potential alternatives to organic solvents when using PEF to extract flavonoids. Recent studies have shown that some novel extraction techniques, such as microwave and ultrasound, can significantly improve the ability of DES to extract bioactive substances from plants and herbs (Table 1). Cui et al. found that under the optimum experimental conditions, extraction yields of microwave combined with DESs for genistin, genistein and apigenin from pigeon pea roots increased 1.15, 1.16, 1.10 folds to those of HWE combined with DESs. Similar trends were observed in other reports on the extraction of flavonoids by microwave coupled with DESs (Yao et al., 2015; Bener et al., 2022). Ultrasound-assisted extraction has also been proved to perform better than HWE in improving the extraction efficiency of DESs. Under the same extraction time and temperature, the extraction rates of anthocyanins and flavonoids from DESs under ultrasonication were increased by 70.1% and 170%, respectively. (Bubalo et al., 2016; Lanjekar et al., 2022). However, the effect of PEF-AE combined with DESs on the extraction of bioactive substances has not yet been studied in detail (Table 1). Thus, to test the ability of PEF-AE combined with DESs to recover flavonoid compounds from noni pomace, the extraction efficiencies of PEF-AE combined with different types of DESs for Noni flavonoids were compared and evaluated in our study. The extraction process was then statistically optimised using response surface methodology (RSM) to select the most efficient conditions. In addition, most of the current studies only focus on the effect of different extraction methods on the extraction rate of target substances, while few studies focus on the influence of the extraction methods used on the degradation and conversion of target substances (Table 1). Based on the analysis of the relationship between the extracted flavonoids, the conversion of rutin to quercetin under the synergistic effect of PEF and DESs was another significant discovery in our study. The influences of PEF treatment conditions, DESs types and DESs concentrations on rutin conversion were also explored. In summary, our research contributes to developing an eco-friendly and efficient method based on PEF coupled with DESs to recover bioactive flavonoids from fruit processing waste. This work also provides innovative insights into the conversion of flavonoid glycosides to flavonoid aglycones in a green and efficient manner, which is a great prospect for the food, pharmaceutical, and chemical industries.