Extraction of oil from grape seeds (Vitis vinifera L.) using recyclable CO2-expanded ethanol

https://doi.org/10.1016/j.cep.2020.108147Get rights and content

Highlights

  • CO2-expanded ethanol (CXE) was used for extracting oil from grape seed waste.

  • Oil yield of CXE extraction could approach that got with organic solvent method.

  • CXE extraction was an eco-friendly and cost-effective process.

  • Linoleic acid accounts for up to 75.5 % in CXE oil representing higher quality.

Abstract

A novel green solvent, CO2-expanded ethanol (CXE), was employed to extract oil from grape seeds, which is one of the main by-products generated by the grape-processing industry. The optimum extraction conditions were obtained by response surface methodology (RSM) coupled with Box-Behnken design (BBD): a pressure of 7.4 MPa, a temperature of 314 K and a CO2 mole fraction of 0.3. The highest oil yield of 13.6 % by CXE extraction was equivalent to or even higher than that obtained with conventional methods, including organic solvent and supercritical CO2 extraction. In terms of reducing solvent consumption and extraction time, the CXE-based process was superior than the conventional methods. CXE oil contained 87.5 % total unsaturated fatty acids, especially linoleic acid was up to 75.5 %, presenting a higher antioxidant capacity.

Introduction

Grapes (Vitis vinifera L.) are one of the most extensively cultivated fruits in the world, with global production reaching 79 million tons in 2018 alone [1]. In addition to 13 % of total grapes marketed as fresh fruit, there are more than 85 % destined to produce wine, juices and jams. The annual grape residues produced are as high as to 15.8 million tons, which contains about 2.66 million tons of grape seeds [2,3]. Grape seeds are considered as a source for phenolic and other compounds, containing a high content of oil (10∼15 %) in particular [4]. Since around 85 % of grape seed oil are unsaturated fatty acids, the extraction of edible oil is becoming a promising way of exploiting grape seeds. In particular, linoleic acid, one of the essential fatty acids for the human body accounts for over 70 % of total fatty acids in grape seed oil [5]. Nowadays, grape seed oil, as a premium nutritional oil, has broad prospects for development with an increasing market share.

Traditionally, the grape seed oil is extracted mainly by cold pressing and organic solvent techniques. The former method entirely relies on physical and mechanical forces without heating or chemical treatment, obtaining the cold pressing oil that retains the original flavour, but the oil yield is generally less than 9 wt.% [6]. Although the latter can thoroughly extract the grape seed oil with a higher yield of 10 wt.%, it usually requires more extraction time and a large amount of organic solvent [7]. In recent years, increased interest in green extraction for natural products has been observed as the organic solvent is quite harmful to human health and the environment. Gómez et al. [8] extracted grape seed oil using supercritical CO2 for 3 h at 35 MPa and 40℃ with an approximate yield of 6.9 wt.%. Moreover, Cao et al. [9] attempted to employ supercritical CO2 with a small amount of ethanol as entrainer at 40 MPa and 40℃ for 3 h, and found that oil yield increased significantly to 10.4 wt.%. Some research indicated that the addition of ethanol not only contribute to break the intermolecular interaction between the target solute and the sample matrix, but also increase the selectivity of the solvent for neutral lipids and free fatty acids [10]. CXE is a modified solvent containing 50∼70 % (v/v) of dominated ethanol, by varying the compressed CO2 fraction, the polarity range of the fluid can be adjusted flexibly and rapidly, facilitating the solubility of solids and liquid analytes. Furthermore, adding compressed CO2 into ethanol causes the volume expansion, reducing the surface tension and viscosity, thereby enhancing diffusivity and promoting the release of solutes into the extraction medium.

Different from adding a small proportion of ethanol as an entrainer to supercritical CO2, CXE is a mixed solvent formed by dissolving a small fraction of supercritical or subcritical carbon dioxide in the main component ethanol, providing many beneficial features including polarity-adjustable, solubility-enhanced, and diffusion-fast [11]. In recent years, CXE has been extensively used as a new environmentally friendly medium for performing separations, extractions, reactions, and other applications. CXE can achieve a continuous transition from polar ethanol to non-polar supercritical CO2 by varying the ratio of critical CO2 to ethanol at a particular pressure and temperature [12]. Thus, CXE is easily accessible to a wide range of solvent properties, enhancing the extracts solubility from the sample matrix. On the other hand, the volumetric expansion of the liquid phase caused by dissolving compressed CO2 into ethanol can decrease the viscosity of the CXE fluid and thereby, to some degree, improve the mass transfer efficiency [13]. In addition, CXE has the advantages of hypoxic and mild operating conditions, which are suitable to extract the oxidizable compounds such as unsaturated fatty acids (UFAs), as already reported for the extraction of γ-linolenic acid [14] and DHA-containing lipids [15]. So far, there were a few studies about CXE extraction of microbial lipids, but no report has been found for the CXE extraction of vegetative oil. A recent work from our research team was also successfully extracted DHA-containing lipids by CXE from microalgae, Schizochytrium sp. [16]. In this study, an attempt was made to use CXE as solvent for extracting UFAs-rich oil from grape seeds.

The present work aims to assess the feasibility of CXE as a green medium for the extraction of grape seed oil. The effects of independent variables, including pressure, temperature and CO2 mole fraction, on the oil yield were investigated. In contrast to conventional methods such as organic solvent and supercritical CO2 extraction, the process of CXE extraction was preliminarily evaluated in terms of environmental performance and extraction efficiency and operating cost. The fatty acid composition and antioxidant capacity were also analysed to evaluate the quality of extracted oils from the different methods.

Section snippets

Sample preparation and reagents

In September 2017, red grapes were harvested in Turpan (43.15 °N, 88.87 °E, and 878 m above sea level), Xinjiang, China. The grape pulps and skins were separated manually. Thereafter, the seeds were dried at 328 K for 48 h until the matrix humidity reached 1.5∼2% and then milled up to a particle diameter less than 0.5 mm. The powder was stored in dark conditions under vacuum at 253 K.

CO2 (99 % purity) was purchased from Jinhua Datong Gas Co., China. 1,1-diphenyl-2-picrylhydrazyl (DPPH) and the

Kinetic analysis of the extraction process

Three levels of variables, including pressure (3.8∼10 MPa), temperature (313∼333 K) and CO2 mole fraction (0.1∼0.5 mol/mol), were selected as the operating conditions to extract grape seed oil using CXE prior to the RSM trials. Fig. 2 showed the overall extraction curves of the oil yield extracted versus time. The CXE extraction curves were characterized by three stages [22,23]. In the first 40 min, called a constant extraction rate period, mostly the oil adsorbing to the surface of the matrix

Conclusion

The optimal conditions in CXE extraction of grape seed oil were at 7.4 MPa, 314 K, and CO2 mole fraction of 0.3, providing a highest yield of 13.6 %. Not only a greater oil yield was obtained using CXE than the conventional methods, but a lower amount of solvent and less time were required. Linoleic acid in CXE oil was up to 75.5 %, possibly contributing to the rise of the antioxidant capacity. In summary, the obtained results highlight the great potential applications of CXE extraction as a

Author statement

I have made substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; AND I have drafted the work or revised it critically for important intellectual content; AND I have approved the final version to be published; AND I agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

This work was financially supported by Public Technical Project of Zhejiang province (No. LGN19B060001) and Natural Science Foundation of Zhejiang province (No. LY19C200014).

References (33)

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