Supercritical fluid extraction of raspberry seed oil: Experiments and modelling

Highlights

• Oil extraction kinetics was fitted by empirical and mass-transfer models.

• Kinetics of the process was highly affected by the process parameters.

• Constant extraction rate period was described by initial mass-transfer rate.

• ANN optimization was applied to achieve the highest initial mass-transfer rate.

Abstract

The aim of this study was the optimization of supercritical fluid extraction (SFE) of raspberry seed oil. Sequential extraction kinetic modelling and artificial neural networks (ANN) were used for this purpose. SFE was performed according to the broaden Box-Behnken experimental design with pressure, temperature and CO₂ flow rate as independent variables, while the influence of particle size on extraction kinetics and adjustable model parameters was additionally evaluated. Five empirical kinetic equations and mass-transfer model proposed by Sovová were utilized for extraction kinetics modelling. According to appropriate statistical parameters (R², SSE and AARD), the mass-transfer model exhibited the best fit of experimental data. The initial mass-transfer rate of extraction curve was used as a response variable in ANN optimization. SFE should be performed at elevated pressure and CO₂ flow rate, while temperature and particle size should be held at a lower level in order to achieve a maximal initial mass-transfer rate.

Introduction

Under nutrition has been recognized as emerging problem of modern age. Expansion of human population is followed by growing demand in food supplies. Thus, there is a need to develope sustainable systems capable to ensure production of food products meeting safety criteria and diminishing negative impact on environment. Processing of food industry by-products can partly resolve this problem [1]. Serbia is one of the largest producers and exporters of raspberries [2]. Processing of raspberries, generate large amount of seeds which are further utilized as a by-product or discarded as a waste [1]. The analysis of chemical composition shows that raspberry seeds are rich in dietary fibers, proteins, carbohydrates and starch. Also, raspberry seeds contain about 15 % of oil [3]. Previous studies reported that the raspberry seeds contain certain health beneficial bioactive compounds [4], as it is significant source of essential fatty acids. Yan et al. [5] found that the major ω–3 fatty acid in plants is α-linolenic acid, which humans can’t synthesize in the body and have to be obtained through the diet. Furthermore, α-linolenic acid is the only ω–3 fatty acid that may be present in botanical materials, including oil from seeds [6]. Furthermore, raspberry seeds are rich source of tocopherols especially its γ-isomer [7]. Tocopherols are recognized for their health benefits, such as preventing cardiovascular diseases, cancer, diabetes and obesity [8].

Cold-pressing and solvent extraction have been conventionally used procedures for oil recovery. However, high cost of organic solvents, environmental pollution and toxicity limited the use of solvent extraction [9], while poor yield achieved by cold-pressing represents the main disadvantage of this process. Supercritical fluid extraction (SFE) has been recognized as an excellent alternative to conventional processes being able to provide high recovery of vegetable oil and prevent chemical alteration caused by high temperature and interaction with solvent and/or oxygen. Supercritical CO₂ has been established as the most promising solvent for SFE as it has low surface tension and viscosity and high diffusivity in supercritical state [10]. In this region, CO₂ has polarity similar to n-pentane which makes it suitable for isolation of non-polar compounds [11].

ω–3 Fatty acids are important bioactives with high market value, thus there is constant demand for alternative resources which could be used for their recovery. According to Gallego et al. [12], fish processing residues were used as raw materials and SFE in temperature range from 30 to 60 °C and pressure from 20 to 35 MPa is preferred technique. High content of PUFA-rich oil in raspberry seeds indicated that this by-product could be valorized as alternative raw material. According to available literature, valorization of raspberry pomace was performed in order to develop optimized high-pressure extraction procedures for separation of into lipophilic and hydrophilic fractions [13]. Physical separation of raspberry seeds and skins could be performed prior extraction since seeds are particularly rich in non-polar oil and skin is rich in moderately polar polyphenols. Limited work on SFE of raspberry seeds was found in literature and it was focused on changes of bioactive components in raspberry seed oil during SFE [14].

Mathematical modelling of SFE process could be particularly useful since it provides insight of transport phenomena occurring in SFE. Models applied for evaluation of SFE kinetics are generally combination of mass-transfer based models, empirical models and models based on heat-transfer analogy [15]. SFE modelling is also interesting from an industrial aspect, since it improves scaling of process from laboratory to industrial scale. Therefore, the aim of this research was application of different empirical and mass transfer models for fitting the SFE of raspberry seed oil and evaluation of SFE parameters (pressure, temperature, CO₂ flow rate and particle size) influence of kinetic curves and adjustable model parameters. The final goal of this research was the ANN optimization of SFE in order to maximize the initial mass transfer rate of extraction curves.