Ultrasound-assisted extraction of anthraquinones from roots of Morinda citrifolia

Abstract

This study investigated the use of ultrasound-assisted extraction (UAE) to improve the extraction efficiency of the classical solvent extraction techniques such as maceration and soxhlet extraction to extract anti-oxidant activity compounds, anthraquinones, from the root of Morinda citrifolia. The effects of different extraction conditions were determined, i.e., temperature of (25, 45, 60 °C), ultrasonic power, solvent types, and compositions of ethanol in ethanol–water mixtures. The results show that the yield increases with increasing extraction times and extraction temperatures. The percent recovery of anthraquinones using ultrasound was found to be highly dependent on the type of solvents (acetone > acetonitrile > methanol > ethanol). Furthermore, the use of ethanol–water solution as extraction solvent increased the yield of anthraquinones due to the relative polarity, the swelling effect of plant tissue matrix by water, and increased sound absorption. To achieve the same recovery as that achieved by UAE, soxhlet extraction and maceration required much longer time.

Introduction

Morinda citrifolia (Noni), a plant which has been used in folk remedies for over 2000 years, has recently gained increased interest from the scientists and medical professionals. The true health benefits of these remedies are being investigated widely. All parts of the plants, which include the fruits, the leaves, the bark, and the roots have been shown to contain active compounds that have high medicinal values. Wang et al. [1] has recently published a review of Noni research, which summarizes the therapeutic effects of various compounds in this plant. Noni fruits and leaves are edible and have been used as a food and in herbal remedies to maintain overall health and to treat various diseases. The roots of Noni plants were used by Polynesians to produce yellow or red dye, but more importantly, they contain medicinally active components, anthraquinones, which show several therapeutic effects. These include anti-bacterial, anti-viral, and anti-cancer activities as well as analgesic effects, which make the roots potentially useful in several medical applications [2], [3]. However, making use of the roots is unlike making use of the fruits or the leaves. When the roots are harvested, the entire plants are generally destroyed, thus no longer offer medicinal value thereafter. It is therefore important that the most efficient extraction methods be utilized.

Recent studies have shown that ultrasound-assisted extraction (UAE) enhances the extraction efficiency by increasing the yield and by shortening the time of extraction of secondary metabolites from various plant tissues, such as excised leaves of tea, mint, sage, chamomile, ginseng, arnica, and gentian [4], [5], [6], [7], [8]. These studies demonstrated that ultrasound is capable of accelerating the extraction of organic compounds contained within the plant tissues by disrupting the cell walls and enhancing mass transfer of cell contents. The review of ultrasonic isolation of bioactive compounds from plant materials can be found in a recent paper by Vinatoru et al. [9].

The enhancement of extraction efficiency of organic compounds by ultrasound is attributed to the phenomenon called acoustic cavitation. As ultrasound passes through a liquid, the expansion cycles exert negative pressure on the liquid, pulling the molecules away from one another. If the ultrasound intensity is sufficient, the expansion cycle can create cavities or micro-bubbles in the liquid. This occurs when the negative pressure exceeds the local tensile strength of the liquid, which varies with the type and purity of the liquid. Once formed, these bubbles will absorb the energy from the sound waves and grow during the expansion cycles and recompress during the compression cycle. The increase in pressure and temperature caused by the compression leads to the collapse of the bubbles, which causes shock wave that passes through the solvent, enhancing the mass transfer within the system [10], [11], [12].

Even though the effects of ultrasound have been studied in over hundreds of herbal species, to the author’s knowledge, its effect on the UAE of this important plant, M. citrifolia, has not been investigated. The aim of this study was therefore to determine the extraction efficiency of anthraquinones from the roots of M. citrifolia by means of conventional ethanol extraction per se as compared with UAE. We specifically investigate the effects of times of ultrasonic treatment, temperature, ultrasonic power, and type and compositions of solvents on the release of anthraquinones.