Review articleGreen extraction processes for the efficient recovery of bioactive polyphenols from wine industry solid wastes – Recent progress
Introduction
The processing of grapes into wine and main relevant co-products (distilled beverages) is a process characterised by the generation of waste streams at the major steps involved, including agricultural practices such as pruning and exfoliation (branches and leaves), vinification (skins, seeds, stems, lees) and distillation (solid and liquid residues) (Figure 1). Out of these residues, the so-called pomace, comprised of skins and seeds, as well as the stems, represent almost 25% of the amount of grapes used in the winemaking process [1], and thus it is in fact an abundant resource. The aforementioned WISW have attracted considerable interest owed to their unique variety and richness in polyphenolic substances [2], which are broad class of secondary plant metabolites. Polyphenols are regarded as exceptionally beneficial nutrients because of salient bioactivities, such as antioxidant, anti-inflammatory, antimicrobial, cardioprotective and chemoprotective properties [3].
The polyphenolic composition of grape skins, seeds and stems is characterised by a multitude of structures, belonging to several polyphenol subclasses (Figure 2). The major constituents, in terms of abundance, are not the same for skins originating from white and red grape vinification. The total polyphenol content of red skins is usually much higher than for corresponding white skins. Stems and seeds also display diversified polyphenolic profiles. By virtue of their abundance and significance, WISW polyphenols have been a subject of intensive research over the past decade, and there is a rather extended number of references pertaining to the implementation of various techniques of WISW polyphenol extraction, involving both low- and high-technology methodologies, with the aim of achieving increased polyphenol recovery [4].
However, many of these techniques are based on the use of conventional volatile solvents of petrochemical origin, which render them incompatible with the development of an integrated green extraction process. Considering that green extraction should be characterised by reduced energy demands, the use of reusable and non-toxic solvents, and no compromise regarding the safety and quality of the final product (extract), then the development of extraction processes involving novel, alternative solvents and environmentally benign physico-chemical technologies of process intensification, would appear as a promising path towards sustainable industrial production. On such a conceptual basis, this review summarises recent, eco-friendly procedures for polyphenol recovery from WISW (Figure 3).
Section snippets
Conventional green solvent extraction
The green solvent mostly used in this type of extractions is bioethanol, which is a bio-solvent produced through alcoholic fermentation of sugar or starch-containing food wastes [5]. Ethanol is always combined with an appropriate amount of water to ensure high yields in polyphenol recovery. However, the final solvent composition is defined by the nature of the solute (polyphenols) and the tissue used. A 80% (v/v) aqueous ethanol solution was found to be very effective for pomace (skins and
Pressurized-liquid extraction (PLE) and supercritical fluid extraction (SFE)
The term “pressurised liquid” has become almost completely synonymous with “subcritical water” (otherwise stated as pressurised hot water), as far as the extraction of bioactive compounds from plant material is concerned. Subcritical water is commonly referred to water at temperatures between 100 and 374 °C and pressures higher than its vapour pressure, so that water is maintained in its liquid state. Under such conditions viscosity and dielectric constant are reduced, turning water into a
Microwave-assisted extraction (MAE)
This type of extraction is gaining increasing acceptance owed to its benign character, which is the rapid penetration of microwaves into the solid, the direct generation of heat inside the solid and a consequent rise in temperature. As heating can be relatively uniform, heat and mass transfer may be coordinated towards the same direction and this is believed to cause acceleration of extraction that usually entails shorter extraction time. For the extraction of RGP polyphenols, MAE in
Ultrasound-assisted extraction (UAE)
The power of ultrasound may be used either as a pre-treatment step or as an effective means of enhancing the performance of the solid–liquid extraction itself. UAE is regarded as an environment-friendly extraction technology, offering several advantages over other conventional and non-conventional ones, such as lower cost, versatility and ease of deployment [20].
UAE has been receiving credits for its effectiveness to recover RGP polyphenols mainly for its effect on the extraction duration, as
Deep eutectic solvent (DES) extraction
Eutectic (low-melting point) mixtures are liquid materials that can be formed by natural substances, including organic acids (e.g. citric acid), polyols (e.g. glucose) and salts (e.g. choline chloride, sodium citrate). They are characterised by reduced toxicity, low vapour pressure, tunability, ease of preparation and low cost, and this is the main reason why these liquids have been currently attracting attention as green extraction media [26].
A combination of a glycerol-based DES (organic acid
Conclusions – future perspectives
In the light of recent findings, it becomes obvious that there is an ongoing research towards the development of green procedures, aiming at the exploitation of WISW for the commercialisation of formulations enriched in polyphenolic phytochemicals. Central role in the relevant processes plays the use of low-energy and environmentally benign physico-chemical techniques and reusable and non-toxic solvents, but in several cases the integrated development of eco-compatible technologies is still in
Declaration of interest
The author declares no conflict of interest.
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2022, Science of the Total EnvironmentCitation Excerpt :A study to characterize these large differences is currently under development. Probably, the most important unit operation in the polyphenol recovery process from agri-food waste is the solid-liquid extraction, particularly when it is not optimized, since it is often time and energy consuming: i) inducing the use of huge amount of solvent (e.g. water or organic compounds, harmful for the environment and users) and; ii) generating large quantity of waste (Makris, 2018). The review of the state of the art of polyphenols extraction technologies indicates that most of the present proposals are centered in the integration of mixtures of aqueous, benign organic solvents and CO2(g), with advanced physical methods (Pagano et al., 2021).