Application of a novel approach to modelling the supercritical extraction kinetics of oil from two sets of chia seeds
Introduction
At present, there is a constant search of promising and inexpensive vegetal materials as a source of polyunsaturated fatty acids (PUFAs). Chia (Salvia hispanica L.), a plant indigenous to Guatemala and Mexico, which belongs to the Lamiaceae family, is attracting a great attention, especially the seeds. Chia seeds are being studied as a source of food ingredients, such as proteins and dietary fiber [1], [2], [3], but they mainly stand out for their high oil content (20–35 mass %). Chia seed oil contains significant amounts of PUFAs, mainly the omega-3 α-linolenic acid (ALA) (60–65% of total fatty acids) [4], which role in the prevention of cardiovascular, nervous system and inflammatory diseases has been thoroughly described [5], [6].
Thus far, different solvents (ethyl acetate, acetone, propane, petroleum ether and hexane) and extraction techniques (cold and hot pressing, Soxhlet, Ultrasound-Assisted Extraction and Pressurized Liquid Extraction) have been applied to obtain chia seed oil [7], [8], [9], [10], [11]. A viable and eco-friendly alternative to the use of organic toxic solvents is the extraction with supercritical CO2 (scCO2), which is non-toxic, non-flammable, non-mutagenic and carcinogenic and is abundant and inexpensive. Moreover, due to the possibility of working at low temperatures and in the absence of oxygen, supercritical extraction with scCO2 (SCE) prevents or minimizes the degradation of bioactive compounds and allows obtaining solvent-free products [12]. New developments regarding the application of this advanced technique to obtain oils were reported recently in the literature. For example, Wei et al. [13] employed ultrasound-assisted supercritical carbon dioxide extraction for removing oleanolic acid and ursolic acid from Hedyotis corymbose. The experimental solubility data, called by the authors fictitious, were read from the initial slope of the curve of the extraction yield versus the amount of scCO2 used, and were modelled applying several semi-empirical density-based models. Moon et al. [14] studied the scCO2 extraction, with and without co-solvent (ethanol) of the essential oil from Asiasarum heterotropoides and the results obtained were compared with conventional extraction. In another study [15], turmeric (Curcuma longa L.) was extracted with scCO2 and turmerones were concentrated using semi-preparative supercritical chromatography. Supercritical fluid extraction with a co-solvent was also applied to extract oil from rice bran with the aim to promote the valorization of this abundant feedstock [16]. Concerning SCE of chia seed oil, as far as we are aware, only a few studies have been carried out till present [17], [18], [19], [20]. In those, the concentration of ALA achieved in the extracted oil was approximately (60–65) %, hence SCE process allowed obtaining healthy and high-quality oils regardless of the operational parameters applied.
It is known that kinetic data are essential for the realization of a feasible industrial process. However, despite the aforementioned and the possibilities to realize a viable industrial process, kinetic data are not only scarce and superficial, but also, as far as we are aware, there are no attempts related to the modelling of SCE of oil from chia seeds reported in the literature till present. Accordingly, two were the objectives of this work: i) to provide new data on the scCO2 extraction of chia oil and ii) to apply a novel approach to the extraction kinetics modelling, advocated originally by Sovová and Stateva [21]. This approach reflects the interaction between kinetics and phase equilibria (solubility) and applies a reliable and versatile modelling framework to estimate the solubility of the oil in the supercritical fluid, as discussed in detail in Section “Supercritical fluid extraction of chia oil — modelling framework”.
To achieve the objectives of our work, edible chia seeds (ECS), and discarded seeds (DCS) were studied. Particularly for the latter, the present investigation can be of considerable interest as it will provide information on how to intensify further their valorization and industrial applications. So far, the studies reported in the literature have been carried out by using ECS. Chia seeds employed in the present study were subjected to a selection process in which the seeds were classified in different qualities mainly based on their weight, intactness, color, visual aspect and size. Thus, DCS consist of damaged, partially broken and/or smaller-size and lower-weight seeds that are usually discarded during post-harvest handling and finally intended to animal feeding. Their price, as a consequence of the market surplus of this product, is considerably lower than that of ECS. Even though they are an underutilized raw material, DCS possess a noteworthy amount of oil and may constitute a viable alternative source for obtaining highly polyunsaturated chia seed oils to be used in human health care formulations.
Section snippets
Sample preparation
The two different sets of chia seeds (Salvia hispanica L.) originating from Mexico were purchased from a local supplier (Primaria). The oil content for each set of seeds (28.3% and 19.9% mass for ECS and DCS, respectively) was determined by pressurized liquid extraction by using a 2:1 chloroform:methanol mixture at 60 °C and 10 min of extraction time [22]. These results are in agreement with the values provided by the supplier for each set (25.2% and 20.6%, respectively). In what follows we
Supercritical CO2 extraction
The experimental kinetic curves, obtained for the SCE of chia oil from DCS and ECS are displayed on Fig. 1a and b, respectively. For DCS, the extraction yield (mass of oil/mass of seeds) increased with pressure and temperature, and was in the range from 13.3% (at 25 MPa and 40 °C) to 18.6% (at 45 MPa and 60 °C) after 240 min extraction time (Fig. 1a).
At the experimental conditions studied in this work, a crossover effect on the overall extraction yield was not observed. Rocha Uribe et al. [19]
Conclusions
This work presents for the first time the results of modelling the experimental kinetics data of SCE of oil from two sets of chia seeds, ECS and DCS. The SCE experiments demonstrated that the highest oil yield (18.6%) obtained from DCS was achieved at the highest pressure and temperature applied (45 MPa and 60 °C). Furthermore, at these operational conditions practically all the oil was exhausted (93.5% oil recovery). As can be expected, the extraction yields achieved from ECS, as compared to
Declarations of interest
None.
Acknowledgements
J. A. P. Coelho, R. M. Filipe and R. P. Stateva acknowledge the funding received from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 778168. J. A. P. Coelho, and R. M. Filipe acknowledge the funding received from Fundação para a Ciência e a Tecnologia, Portugal, under projects UID/ECI/04028/2013 and UID/QUI/00100/2013. Chia seeds were generously supplied by PRIMARIA (www.primaria.biz).
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