Effects of processing parameters in the sonic assisted water extraction (SAWE) of 6-gingerol

Highlights

• Sonic energy and entrainer as internal intervention in ginger extraction using water as solvent.

• Effects of low frequency and high frequency were compared in the Sonic Assisted Water Extraction.

• The most significant parameter was applied power followed by entrainer and other five parameters.

• High frequency SAWE is an option for thermolabile compounds with milder operating conditions.

Abstract

The use of water in subcritical conditions for extraction has several drawbacks. These include the safety features, higher production costs and possible degradation of the bioactive compounds. To overcome these problems, sonic energy and an entrainer were used as external interventions to decrease the polarity of water at milder operating conditions. The effect of low (28 kHz) and high (800 kHz) frequencies of sonication in the extraction of the main ginger bioactive compound (6-gingerol) were compared. Six parameters were studied: mean particle size (MPS, mm), time of extraction, applied power, sample to solvent ratio (w/v), temperature of extraction, and the percentage of entrainer. The optimum conditions for high frequency SAWE prototype were MPS 0.89–1.77 mm, 45 min, 40 W applied power, 1:30 (w/v), 45 °C, and 15% of ethanol as entrainer. Two-way analysis of variance (ANOVA) gave the most significant parameter, which was power with F (1, 45.07), p < 2.50 × 10⁻⁹. Although the effect of low frequency was stronger than high frequency, at the optimum conditions of the sample to solvent ratio 1:30 (w/v) with 700 mL solvent and temperature 45 °C, the concentration and recovery of 6-gingerol from high frequency of SAWE prototype was 2.69 times higher than at low frequency of SAWE. It was found that although the effects of high frequency (800 kHz) were negligible in other studies, it could extract suitable compounds, such as 6-gingerol, at lower temperature. Therefore, the effects of sonication, which cause an enlargement in the cell wall of the ginger plant matrix, were observed using a Scanning Electron Microscope (SEM). It was found that the applied power of sonication was the most significant parameter compared to the other parameters.

Introduction

There has been increasing consumer interest towards functional foods which, beyond the basic function of supplying nutrients, are claimed to have health-promoting or disease-preventing properties. In this respect, it is of paramount importance to have processing methods which preserve not only the nutritional and sensorial quality but also the bioactivity of these food materials. Applying heat is the most common method to process food, owing to its ability to kill microorganisms and inactivate enzymes. Additionally, the use of heat processing is favoured due to the easier management of the equipment used [1]. However, heat processing particularly under severe conditions may give rise to physical and chemical changes that impair the organoleptic properties as well as reduce the content or bioavailability of some of the nutrients. Therefore, food industries are constantly looking into milder processing technologies such as high pressure processing, pulsed electric and magnetic fields, not only to obtain high-quality food with “fresh-like” characteristics, but also food with improved or even novel functionalities [2].

Sonication technology or ultrasonic assisted extraction (UAE) has been extensively explored in the last two decades as an efficient extraction method in the food and pharmaceutical industries, as indicated by the exponential increase in papers published in this area [3]. It has attracted the attention in industrial applications with it advantages of less solvent requirement in a shorter time [4] and more importantly it is a versatile technique which can be scaled-up on an industrial level [5]. Due to the lower temperature and pressure of UAE it can also be conducted safely.

Many studies done on natural products have used low frequency UAE such as 20 kHz [6], [7], 35 kHz [8], 40 kHz [9], and 45 kHz [10] due to the availability of such probes and the known effects of much stronger cavitation in these frequencies. Low frequency (20–100 kHz) is also known as high power or high intensity ultrasound which imparts high energy and has mechanical, biological and chemical effects [11], [12]. This is due to the formation, growth and collapse of larger bubbles from acoustic cavitation phenomenon [13] which causes higher temperatures and pressures [14]. Simultaneously, this is also due to the conversion of kinetic energy of motions into heating the contents of the bubbles [15]. The main parameters that affect low frequency are energy, intensity, pressure, velocity and temperature [11].

High frequency (400–800 kHz) is also known as low power or low energy ultrasound which is used for controlling the properties and to improve the qualities of food [11]. The effect of high frequency is less violent and gentler on antioxidant and phenolic compounds where the local temperatures and pressures are much lower [16]. Ultrasound at frequency 400 kHz was used for palm oil separation of emulsion and increases oil quality [17]. The uses of high frequency in ultrasonic extraction are still scarce with limited literatures although some studies had been done. Toma et al. [18] compared the effect of 20 kHz and 500 kHz using solvents and found that the effect of both frequencies on the vegetal tissues was stronger when using the low frequency (20 kHz). González-Centeno et al. [16] and Cravotto et al. [5] studied the effects of different frequencies, and concurred with the earlier findings where the effects were more significant at low frequencies (<100 kHz) compared to high frequencies (120 and 300 kHz). Even though, low frequency is more effective [19], it may cause harm and degradation to the bioactive compounds.

The potential of one variety found in Malaysia of Zingiber Officinale Roscoe known as Halia Bentong as medicinal compounds especially as an antioxidant and anticancer was studied extensively using solvent extraction [20], [21], [22], [23], [24], [25]. It was found that solvent polarity plays an important role and give different extraction yield of phenolic compounds [26]. However, the commonly used solvents were methanol, acetone, and chloroform. Gingerols and shogaols were obtained from oleoresin which produced commercially using organic solvent extraction while steam distillation was used to obtain the essential oil [27]. Water based extraction is more appropriate in the separation process of medicinal compounds and safe for human consumption. Subcritical water technology uses water as a solvent at temperatures between 100 and 374 °C below 22.1 MPa to maintain water in its liquid state to extract both polar and non-polar compounds [28]. This technology has been scaled up from 22 mL, Dionex model 350 by a factor 80–1760 mL in 2015 [29] for the extraction of curcuminoids from Curcuma long L. Meanwhile, Ko et al. (2016) has scaled up the SWE system to 8000 mL or 8 L for the extraction of flavonoids from satsuma mandarin (Citrus unshiu Markovich) peel [30]. The bioactives from ginger, gingerols were known for their pharmacological activities such as anticancer, anti-inflammatory and antioxidant. 6-Gingerol has been reported to inhibit the activities of human breast cancer cells [31].

Sonic assisted water extraction (SAWE) was done to overcome the drawbacks of using high temperature and pressure in thermolabile herbal extraction such as ginger bioactive compounds. Thus, the effect of low (28 kHz) and high (800 kHz) frequencies to the extraction of ginger (Z. officinale Roscoe) bioactive compound which was 6-gingerol was compared to evaluate the extraction efficiency from both frequencies. Six parameters studied which affect SAWE were mean particle size (MPS), extraction time, applied power, sample to solvent ratio, extraction temperature and percentage of entrainer. The addition of ethanol as an entrainer to water in improving the extraction efficiencies was also investigated. The effects of sonic energy at low and high frequencies of SAWE were described from the micrographs of Scanning Electron Microscopy (SEM). The findings of this study would contribute to the new application of high frequency SAWE in herbal extraction as well as other potential separation of thermolabile compounds.