Supercritical CO₂ extraction of V. vinifera leaves: Influence of cosolvents and particle size on removal kinetics and selectivity to target compounds

Highlights

• Measurement and modeling of SFE curves from V. vinifera leaves.

• Relevant extractives are long chain aliphatic alcohols, α-tocopherol, β-sitosterol, and triterpenes.

• Influence of CO₂ cosolvents and particle size upon SFE kinetics and selectivity.

• Results comprise 40 experimental SFE curves, modeled using broken plus intact cells model.

• Remarkable selectivities were obtained for triterpenes (lupeol and β-amyrin).

Abstract

Leaves from Vitis vinifera L. contain potential bioactive compounds, namely: long chain aliphatic alcohols (e.g., 1-hexacosanol, 1-octacosanol and 1-triacontanol), α-tocopherol, β-sitosterol, and the triterpenes β-amyrin and lupeol. Supercritical fluid extraction (SFE) runs were measured at lab scale using: crushed and ground biomass; pure CO₂ at 300 bar and 40−80 °C; and CO₂ modified with 5 and 10 wt.% of ethanol or ethyl acetate. Total SFE yields ranged from 1.86 to 7.52 wt.%. The broken plus intact cells model (BICM), provided a good fitting of the SFE curves with 4.06 % error for total yield and 1.98–5.49 % for the individual yields of the said compounds. The BICM results revealed that the SFE is limited by intraparticle diffusion. Remarkable experimental and calculated selectivities were obtained for triterpenes (lupeol and β-amyrin), starting with a score of 1.7 and increasing along time to 10−100.

Introduction

Among the high-volume agricultural crops farmed in Europe, grape vine (Vitis vinifera L.) is one of the most important species. Since the fruit (grape) is extensively explored for wine production, by-products (roots, stalks, bark and leaves) are inherently produced in huge amounts. For example, in Europe, 6.2 million hL of wine were produced in 2015 [1] leaving behind 14.5 million tons of waste [2].

Vine leaves are one of the least investigated by-products and are generally considered organic wastes. However, previous works addressing the composition of the extracts of V. vinifera leaves identified several compounds with possible economic interest. For instance, Fernandes et al. [3] showed that ethanolic extracts of grape leaves are rich in phenolic compounds with biological activity, mainly hydroxycinnamic acid derivatives and quercetin glycosides. Pensec et al. [4] focused on triterpenoids extracted with chloroform, namely, β-amyrin, lupeol, taraxerol, α-tocopherol and β-sitosterol. For example, α-tocopherol is a liposoluble antioxidant belonging to the Vitamin E class and is important for protecting biological membranes (such as cell walls) [5]. β-sitosterol is an anticholesteremic drug and also an antioxidant [6]. As for the triterpenes, lupeol is a known bioactive compound used to treat cancer and malaria [7], β-amyrin (a precursor of oleanolic acid) exhibits antinociceptive and anti-inflammatory properties [8], and taraxerol evidences anti-inflammatory properties [9].

Supercritical fluid extraction (SFE) is a mature high-pressure technology that in most cases relies on carbon dioxide (CO₂) as the working solvent [10,11]. For the particular case of SFE of V. vinifera biomass, the solvent can be synergistically obtained from fermentative processes (e.g, wine production [12]), which adds value to the universal advantages of this technology, namely the preservation of the natural label on ensuing products due to the innocuousness of the CO₂, mild critical conditions ($T_{\text{c}}=31.1$ ºC and $P_{\text{c}}=73.8$ bar), the null solvent surface tension and inertness, and the dismissal or reduction of distillation/drying units to isolate the extract from the solvent and both from the exhausted solid matrix. The last decades have been marked by prolific research activity on natural extracts produced by SFE from plants such as aloe vera [13,14], cork [15,16], eucalypt [17,18], hemp [19,20], water hyacinth [21,22], stevia [23,24], and many others [[25], [26], [27], [28]]. The most important operating conditions/parameters under investigation have been pressure, temperature, flow rate, cosolvent type and concentration, particle size, and extraction time [28]. The usual responses are total extraction yield, specific/individual yields (of molecules or families of molecules), concentration of target compounds in the extracts, and much more rarely selectivities [16,28]. An important variable influencing selectivity is frequently cosolvent type and/or concentration, which leads to a trade-off between higher total yields and individual yields of target compounds [16,29,30].

The SFE of grape seeds has been a hot investigation topic devoted to bulk vegetal oil [[31], [32], [33], [34], [35]], phenolics [36], flavonols [37] or phytosterols [38]. In this work, SFE technology was used for the first time to investigate natural extracts of leaves from Vitis vinifera L. The novelty of the study also encompasses the mapping of selectivity profiles for different target compounds and the identification of conditions for kinetic enhancement of the process. The influence of green cosolvents addition (ethanol and ethyl acetate) at two concentration levels, biomass particle size and temperature is accomplished and analysed using the broken plus intact cells model.