Cloud-Point Extraction: History
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Subjects: Chemistry, Analytical
Contributor:
Theodoros Chatzimitakos
,
Vassilis Athanasiadis
,
Martha Mantiniotou
,
Dimitrios Kalompatsios
,
Eleni Bozinou
,
Ioannis Giovanoudis
,
Stavros I. Lalas

The production of food biomass waste has been increasing rapidly. This necessitates urgent measures to be taken so as to utilize them. Since most food biomass waste contains useful bioactive substances, cloud-point extraction (CPE) has emerged as a promising solution to valorize waste. CPE is an extraction method employed for the extraction and preconcentration of various chemical compounds, including polyphenols and flavonoids.

  • food biomass waste
  • cloud-point extraction
  • bioactive compounds

1. Introduction

Food biomass waste is an abundant, biodegradable organic resource produced worldwide. Food biomass waste constitutes about one-third of all food production and contributes to the greenhouse effect by producing greenhouse gases [1]. Therefore, food-biomass-waste valorization would not only minimize pollution, but it would also have the potential to generate biobased compounds, foods, and sustainable energy [2]. Food byproducts, for instance, have been valorized into biocomposites and bioplastics, as has the potential use of industrial biomass waste as an upcoming environmentally friendly energy source [3][4][5][6][7]. Zhang et al. [8] investigated energy and heat generation by food biomass waste using a mobile food-waste-to-energy system in conjunction with an anaerobic digester and a biogas engine. The bioconversion of waste food into insect-based protein growth for food or feed is one possible strategy to build a circular economy [9]. Furthermore, it is now usual to convert food waste into liquid biofuels [10][11][12][13][14]. As a result, the utilization of food-biomass-waste products is gaining popularity in the scientific, industrial, and government sectors [15]. Due to their technological, scientific, and economic benefits, conventional extraction techniques, such as Soxhlet extraction and hydrodistillation, have been used so far. Tavares et al. [16] employed hydrodistillation extraction to extract bioactive compounds from Cymbopogon citratus industrial waste. However, industries are shifting towards more environmentally friendly and efficient methods, able to extract bioactive compounds, such as flavonoids, polyphenols, carotenoids, phytosterols, and dithiones, that are found in foods. To this end, ultrasound-assisted extraction, microwave-assisted extraction, pressurized liquid extraction, solid–liquid extraction, liquid–liquid extraction, and enzyme-assisted extraction are more and more being employed [17][18]. Enzyme-assisted extraction capitalizes on the selectivity of the enzymes to release bound bioactive compounds under mild conditions so as to preserve their biological activity [19]. Enzyme-assisted extraction has been employed to separate bioactive compounds from bay leaves [20], polyphenols from brocade orange peel [21], and banana peel [22]. Regarding microwave-assisted extraction, this method relies on the quick absorption of microwave radiation that is then transformed into heat and elevates the temperature via ionic conduction and dipole rotation. Microwave energy absorption, and consequently heat generation, can be evaluated using dissipation, implying that the presence of polar solvents is essential [23]. The benefits of this method include good repeatability and minimal sample treatment, as well as carrying out the extraction procedure by reducing the solvent volume, exposure time, and energy usage [24][25][26]. Several research groups have been interested in the extraction of pectin from food waste and byproducts using microwave-assisted extraction [27]. This method has been also employed to extract polyphenols from eucalyptus bark [28], peach byproducts [29], and cellulose from almond shell waste [24]. Solid–liquid extraction is a method that is based on transferring the desired compounds from the solid phase to the liquid-extractant phase, usually by employing organic solvents [30]. One major disadvantage of these methods is the potential degradation of the extracted compounds due to their light, temperature, and oxidation sensitivity [17]. This happens because high temperatures are incompatible with thermally sensitive substances, such as food bioactive vitamins, carotenoids, polyphenols, tocopherols, antioxidant molecules, and so on [31]. Another highly efficient extraction technique is pressurized liquid extraction, which occurs in a firmly sealed stainless-steel cell so that it can be exposed to both high-temperature and high-pressure conditions [32]. While the extraction procedure is on, the liquid phase is kept below its critical point. Pressure and temperature need to be determined to raise the rate of the mass transfer. This can be accomplished by lowering the surface tension, minimizing the viscosity of the solvent, and also by elevating the solubility of the components [18]. Pressurized liquid extraction has been utilized to isolate polyphenols from granadilla waste [33]. On the other hand, liquid–liquid extraction can also be used so as to partition the compounds from the aqueous phase into the immiscible, organic phase [30].
Cloud-point extraction (CPE), also known as the liquid-concentration technique, micelle extraction, or micelle-mediated extraction [34], is a highly efficient extraction method that exhibits specificity, simplicity, minimal solvent requirements, and applicability at low temperatures. Thus, CPE is a promising, environmentally friendly method that holds great promise in the extraction of food bioactive compounds efficiently [35]. CPE is one of the many methods known for recovering compounds from their matrices. These compounds can be either organic or inorganic, and are found in various foods. In the CPE method, the separation of compounds from the bulk solution occurs by the introduction of a surfactant, which leads to the formation of clouds (vide infra) when the solution is heated to or above a critical temperature, known as the cloud point. Surfactants used in CPE can either be ionic or nonionic. The separation of the desired compound from the bulk solution can also be enhanced by the addition of a salt via the salting-out effect [36][37][38]. Surfactants often accumulate at the interface between the hydrophilic (or aqueous) and the lipophilic phase, where the polar side is directed towards the aqueous part and the hydrophobic side is directed towards the lipophilic layer. Centrifugation is then applied to separate the solution into its two distinct phases. The structure of the micelles can range from roughly spherical to oval, depending on the type of surfactant and solution. The low requirements of CPE in reagents make it one of the most innovative technologies for extracting functional components. Moreover, CPE is typically accomplished at mild or low temperatures and without the use of hazardous or toxic reagents.

2. Application of CPE for Bioactive Compound Extraction in Food Byproducts

2.1. Olive-Based Waste

Olive tree harvesting is expanding globally, resulting in a large amount of waste. Olive leaves (20–25% by weight) are produced initially during tree pruning, and then in mill leaves and thin branches (olive mill leaves) [39]. Since the global demand for olive oil has increased, the olive processing industry has generated a large amount of agroindustrial wastewater. Olive mill wastewater (OMW) residues consist of both solid waste (olive pulp) and liquid waste (vegetables and additional water generated during decantation). OMW has a dark-brown (often black) color and a strong, pungent perfume reminiscent of olives [40]. Additionally, table olive processing involves many procedures that generate several waste streams: lyes, washing wastewaters, brines, and machine washwaters [41]. Large volumes of olive mill effluent are generated during the continuous three-phase olive-oil-extraction method, causing major environmental issues [42]. The valorization of such wastes is of great importance, both for the environment and food technology, as natural polyphenolic antioxidants from agricultural wastes are gaining great interest [43].
Kiai et al. [41] examined the applicability of CPE using three different nonionic surfactants, Genapol X-080, Triton X-100, and Tween 80, for the extraction of polyphenols from table OMW. The optimal conditions for the CPE of polyphenols were investigated in relation to various experimental parameters, including the surfactant content, solution pH, equilibrium temperature, and incubation period. The optimum conditions were defined as 10% surfactant (w/v), with a pH of 2, at 70 °C, and an equilibration time of 30 min. It was found that Triton X-100 and Tween 80 had a higher recovery for the polyphenols than Genapol X-080. This is reasonable, considering that Triton X-100 and Tween 80 have a similar chemical structure, as they are both nonionic polyethylene oxide emulsifiers. Genapol X-080 is a nonionic emulsifier that consists of acrylates. The recovery rate of hydroxytyrosol was nearly identical for all three surfactants. Furthermore, Tween 80 demonstrated the capability of extracting caffeic acid with a recovery ratio of 75.5%, in addition to the other two surfactants that showed lower recoveries. Katsoyannos et al. [43] investigated the yield of polyphenols isolated from OMW using Triton X-114. Triton X-114 is also nonionic and an excellent surfactant choice, as it requires low temperatures, and it has good detergent and wetting characteristics. The results of their study revealed that the best conditions are a three-step CPE with 2% Triton X-114 at 55–60 °C for 20 min. When a three-step CPE is used with 2% v/v surfactant in each step, a total surfactant volume of 6% of the sample volume is required to separate more than 90% of the polyphenols from the aqueous phase. Multiple extraction procedures should be employed to achieve >90% recoveries. Moreover, it is shown that CPE can be effectively applied to aqueous polyphenolic solutions and, when fatty substances are absent, polyphenols can be quantitatively extracted. Comparing these two studies, it is obvious that Triton X-114 is a more favorable surfactant than Triton X-100. This stems from the fact that it can achieve higher recoveries in lower temperatures and with a lower surfactant concentration. Nevertheless, these conditions are only applicable to the extraction of polyphenols. Next, Gortzi et al. [44] evaluated Genapol X-080 as a CPE surfactant for the separation of polyphenols and tocopherols from OMW and measured the recovery values. The results showed that the recovery of polyphenols was proportionate to the percentage of Genapol X-080 and that the recovery of tocopherols was quantitative even when the concentration of Genapol X-080 was only 5% w/v, and the equilibrium temperature was 55 °C for 20 min. When multistep CPE was applied, higher yields were achieved, which is in line with Kiai et al. [41]. In another study, Katsoyannos et al. [45] evaluated the possibility of applying CPE on OMW utilizing low-biohazard surfactants. In this category, surfactants such as Tween 20, Tween 80, Span 20, and PEG 400 were used. All these surfactants are also nonionic, and they were used in the separation of natural antioxidants, polyphenols, and carotenoids present in OMW and red-fleshed orange juice. Among the selected surfactants, Tween 80 was found to be the most appropriate for both OMW and red-fleshed orange juice, at a concentration of 5% w/v and 7% w/v, respectively. The equilibrium temperature was 55 °C and the equilibrium time was 30 min. All other surfactants provided low recoveries. Optimization of experimental CPE parameters would improve extraction efficiency and could provide the basis for more the cost-effective isolation of antioxidants from natural sources. Athanasiadis et al. [40] extracted the polyphenols from OMW by using CPE with lecithin, a common emulsifier, as a surfactant, and the extracted polyphenols were exploited to enrich olive oil. Lecithin is a zwitter anionic surfactant and it can be produced in animal and plant tissues. It is nontoxic and edible. The CPE extraction procedure took place for 20 min at 40 °C and with a pH of 3.5, and the concentration of the emulsifier was 3% w/w. The lecithin emulsifier resulted in a recovery of up to 42.2%. The concentration of total polyphenols in olive oil samples increased with the addition of micellar dispersions. This increase led to a considerable reduction in free radicals in this sample when compared to other samples. The colors of the samples were consistently maintained, there was no visible sediment, and the flavor of the olive oil was delicious and delightful. This study showcases the applicability of the extracts obtained by CPE and their advantages. Because further study was necessary to maximize the extraction of polyphenols from OMW, Karadag et al. [46] analyzed the effect of the extraction temperature, pH, sodium chloride, lecithin concentration, and equilibration time on the recovery of polyphenols using single-factorial experiments and the optimization of these parameters by response-surface analysis. They concluded that the high recovery of total polyphenols from OMW was achieved by utilizing lecithin in the CPE method under the method conditions optimized by the response-surface methodology. The optimum parameters were established to be 80 °C, with a pH of 5.5, a 20% w/v sodium chloride concentration, and a 12.5% w/v for lecithin. As the equilibration time did not affect the recovery of polyphenols, it was excluded from the optimization study. The higher oxidative stability of this OMW-enriched lecithin was confirmed in a salad dressing sample. According to these results, lecithin is not as good as Tween 80 for the extraction of polyphenols, as it had lower recoveries even when higher concentrations were employed. El-Abbassi et al. [42] used the CPE method to extract polyphenols from an ultrafiltered sample of OMW. The ultrafiltration minimized the organic load and suspended solids, and a more clarified OMW was produced. This clarified extract had easy phase visualization during the cloud-point extraction, while it could keep the total polyphenol content as high as possible. Response-surface methodology was used to optimize the yield of polyphenols from OMW using Triton X-100 as a surfactant. The proper conditions were found to be a one-step CPE using 10% of Triton X-100 at 90 °C for 30 min. CPE using Triton X-100 was successfully employed for OMW, resulting in a quantitative phenol extraction, with a yield of 66.5%. Up until now, the most efficient way to extract bioactive compounds from OMW appears to be a multistep CPE, and the most appropriate surfactant appears to be Triton X-114 for the polyphenols and Genapol X-080 for tocopherols. A notable advantage of CPE is its minimal surfactant consumption on an industrial scale compared to other conventional extraction techniques. Additionally, the unique potential to directly incorporate the extract into food is of high importance. The superiority of CPE is further demonstrated in the following study conducted by De Marco et al. [47]. This study involved the implementation of liquid–liquid extraction, wherein equal volumes of OMW were mixed with solvents, such as hexane or ethyl acetate. In spite of the promising results, this approach required a significant solvent consumption and mainly lacked the direct applicability in food.
Stamatopoulos et al. [48] proposed a rapid, clean, energy-efficient, and nontoxic method for the extraction of oleuropein and related polyphenols from olive leaf extract using cloud-point extraction (CPE) assisted by the salting-out effect. In these methods, Tween 80 was selected as a surfactant. Oleuropein and related polyphenols were heated to improve their thermal stability. The principle of the method relied on increasing the sulfate concentration, capitalizing on the salting-out effect, followed by the separation of the enriched phase from the bulk solution through centrifugation. This method not only facilitated the phase separation but also contributed to the reduction in the solubility of polyphenols. The method also helped to lower the cloud-point temperature of Tween 80. Optimal conditions were selected as a pH of 2.6, an ambient temperature of 25 °C, 4% Tween 80 (w/v), 35% sodium sulfate (w/v), and a settling time of 5 min. The total recoveries of oleuropein and the rest of the polyphenols were between 93 and 100%. In comparison, the polyphenols that remained in the surfactant-rich phase displayed higher thermal stability.

2.2. Wine-Based Waste

Winemaking generates a huge amount of waste, mostly known as wine sludge. Wine sludge is defined as any residue left in wine containers after fermentation, storage, and processing, plus whatever remains after the filtering and/or centrifugation of the wine product [49]. Despite their polluting nature, grapes, wine, grape seeds, and skin extracts have been shown to have beneficial effects on human health, such as protection against cardiovascular disease, anti-inflammatory activity, and anticarcinogenic effects, due to their high content of polyphenols [50].
Chatzilazarou et al. [50] were the first to investigate the CPE method in the wastes from the wine industry. They intended on isolating polyphenols from wine sludge waste. A double-step CPE method was performed at the optimal conditions with 5% PEG 8000 or 2% Genapol X-080 in each CPE stage. The optimal conditions were used for each surfactant. In the case of PEG 8000, these were a pH of 2.5 and heating at 55 °C for 30 min. When it came to Genapol X-080, the optimal conditions were a pH value of 3.5, a temperature of 55 °C, and 30 min of equilibration. The extraction of the polyphenols yielded 75.8% in the double-step CPE with total consumption of 4% of Genapol X-080, and in the case of PEG 8000, the yield was 98.5% with a general consumption of 10%. The use of PEG 8000 led to a higher polyphenol yield. The acidic conditions applied during the CPE method may have been a beneficial factor in this. However, in the two CPE steps, the total consumption of Genapol X-080 was much lower than that of PEG 8000. For this purpose, it would be worthwhile to repeat these experiments under the same conditions, but with three CPE steps applied. In this way, it could be assessed whether having three CPE steps and using Genapol X-080 would lead to a higher recovery of polyphenols with a lower consumption of the surfactant. Alibade et al. [49] employed CPE for the extraction of polyphenols from wine sludge. They used lecithin as a surfactant and they investigated the optimum conditions for the method. Lecithin was chosen as a surfactant in this investigation since it is an edible, harmless substance that is also quite inexpensive. Furthermore, there is no need to separate recovered polyphenols from the surfactant environment before incorporating them into foods. To obtain that, they implemented multiple extraction steps, varying from one to three. Then, they tested the temperature between 35 and 55 °C and the pH value from 2.5 up to 5.0. They also used four different concentrations of the surfactant, more specifically 2, 5, 10, and 15%, to examine which one provided better conditions for the extraction. The method was then optimized as 5% lecithin concentration, at 40 °C, and with a pH of 3 for a 30 min equilibrium time. Three steps of CPE were employed. When the method was finished, a high recovery of polyphenols was achieved, up to 76%. Similarly, to the application of lecithin as a surfactant in the olive mill waste, as in the case of wine sludge, the use of an emulsifier is not as effective as the conventional surfactants.
The simplicity and swiftness of the CPE technique compared to other techniques is worth noting. A research study conducted by Brianceau et al. [51] evaluated a novel methodology for the utilization of fermented grape pomace through the application of a pulsed electric field. Following the implementation of the pulsed-electric-field treatment, the extraction of polyphenols from grape pomace was conducted in a cylindrical extraction cell using a mixture consisting of 50% ethanol v/v at a temperature of 50 °C, with the ratio of liquid to solid being consistently maintained at a value of 5. The combination of densification and the pulsed-electric-field treatment demonstrated significant efficacy in augmenting the extraction of total anthocyanins (~0.57 g/100 g of raw material) from fermented red-grape pomace, thereby establishing its industrial relevance. The application of the pulsed-electric-field treatment on fermented-grape pomace enables the targeted extraction of total anthocyanins across a spectrum of temperatures.

2.3. Pomegranate-Based Waste

Pomegranate fruit is divided into three parts: juice, seeds, and peels. Pomegranate contains a high concentration of bioactive components, such as organic acids, polyphenols, and flavonoids. Despite its high concentration of beneficial components, this fruit has 60% waste in the form of peels. Pomegranate peels have a high concentration of bioactive compounds, such as polyphenols, flavonoids, and organic acids. Because of their multifunctional, high nutritional, and bioactive properties, peels have recently been used in the food, pharmaceutical, and chemical processing industries [35][52].
Pomegranates are known to have high amounts of bioactive compounds. Pomegranate peel waste was studied by Motikar et al. [35]. At first, a series of experiments was carried out so as to choose the best method and the optimum parameters for the extraction of the bioactive compounds, such as polyphenols and flavonoids, from the peel waste. The methods examined were conventional extraction, ultrasound extraction, microwave extraction, ultrasound-assisted CPE, and microwave-assisted CPE. Next, they followed another series of tests so they could optimize the conditions of the CPE method. They tested four different surfactants, Triton X-100, Triton X-114, Tween-20, and Tween-80. The most suitable one was chosen (i.e., Triton X-100) because of its high density, which makes phase separation by centrifugation feasible. Then, they investigated the surfactant concentration that was necessary for the maximum recovery of the bioactive compounds. Afterwards, the solid–liquid mixing ratio was tested, along with the pH value. A range of temperatures and equilibration time experiments followed, where the tested temperature values were between 35 and 80 °C and the tested equilibrium times were between 20 and 60 min. Finally, the salt concentration that would lead to the maximum recovery of bioactive compounds was investigated. The tested sodium chloride concentrations were between 6 and 18% w/v. The surfactant concentration of 2% v/v was insufficient for further phase separation, while at 10% v/v, the phase was too viscous, resulting in no micelle formation. Because the extraction yield is affected by the mass-transfer phenomenon, the solid-to-liquid ratio was investigated. The larger the concentration gradient between solution and solvent, the greater the mass transfer and, consequently, the extraction yield. In terms of the pH, at low pH values, the polyphenols are protonated, resulting in hydrophobic molecules that interact more strongly with hydrophobic micellar surfactant, and therefore easily become trapped in the micelles. On the other hand, the polyphenols are deprotonated and their ionic properties rise at high pH values, resulting in a decrease in the solubility of the hydrophobic polyphenols in the micelles due to increased proton activity. The equilibrium temperature may influence the time it takes for organic molecules and surfactant micelles to form. The extraction efficiency decreased as the incubation time was beyond 30 min; the stability of the polyphenols due to degradation may occur if the sample is retained for a longer time at a higher temperature up to 60 min. The frequency of ultrasonication is an important component in tissue disruption. As a result, the effect of ultrasonication combined with cloud-point extraction on polyphenol recovery was tested. The maximum recovery was achieved when the ultrasound-assisted CPE was followed, with a ratio of 1 to 70 pomegranate peel powder to the solvent used. The extraction solvent was 70% ethanol in water. The surfactant utilized was Triton X-110 at a content of 8% v/v. The optimum temperature was 55 °C, at an acidic of pH value 4.5, and the method was performed for 30 min. Furthermore, sodium chloride at a content of 14% was added so the salting-out effect would be provoked. The total polyphenol and flavonoid recovery were expressed as antioxidant activity. The greatest antioxidant activity was calculated to be 94.48%. Next, Sun et al. [52] worked on the optimization of this method. To do so, the response-surface methodology was employed. The maximum recovery was obtained when the optimal parameters were established. So, a solid-to-liquid ratio of 1:40 g/mL was applied. The most suitable surfactant was found to be Triton X-100 at a content of 10% (v/v), and the optimum sodium chloride concentration was 14% (w/v). The pH value was 4.0. All these parameters were applied for 40 min at 65 °C. The yield of the bioactive was up to 82.37 mg GAE/g. This result enhanced the applicability of CPE on pomegranate peel waste, in order to valorize the bioactive compound existing in them.

This entry is adapted from the peer-reviewed paper 10.3390/biomass3030019

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