Agricultural and food by-products can be valuable natural sources of bioactive and functional compounds to be employed as ingredients in functional foods, supplements, cosmetics, and nutraceutical products. On the other hand, synthetic ingredients are usually refused by consumers who more often look for products or functional ingredients originating from natural sources. Thus, the possibility to valorize waste streams and produce upcycled natural ingredients offers interesting perspectives. This is particularly true for chemicals such as terpenes, flavonoids, and other polyphenols, and omega-3 and other lipids that need to be extracted from botanicals or other organic sources. Thus, green and efficient technologies for the extraction of agroindustrial byproducts are desirable for the production of nutrients or supplements with high nutritional value or healthcare products, since their recovery may be both economically and environmentally advantageous.
Conventional solvent extraction methods such as maceration, percolation, and Soxhlet have historically been implemented to extract bioactive compounds from plant materials and are still implemented today. But these methods have several drawbacks, such as long batch duration, usage of large amounts of organic solvents which can leave toxic residues in the extracts, high temperature processes that lead to degradation of thermo-sensitive molecules and more. These drawbacks make conventional processes cost-inefficient and environmentally unsustainable.
To overcome these drawbacks, green and efficient extraction is implemented nowadays for the recovery of bioactive upcycled ingredients. [1-5] Microwave-assisted extraction, subcritical water extraction, and supercritical fluid extraction (SFE) represent the most promising green technologies [2-4]. More specifically, SFE is considered an excellent alternative to conventional methods for the extraction of natural products since it yields extracts free of organic solvents, it protects thermolabile compounds, requires minimal post-extraction processing, and it economizes significant amounts of energy and solvent through continuous recycling. [5]
A few prominent examples of major agroindustries and respective by-products or waste streams that have been valorized through green and efficient extractions technologies for recovery of specific bioactive compound are presented below. The focus is on SFE, since it seems to be the dominant technology in the field due to its efficiency and sustainability.
Olive Oil Industry
Worldwide, about ten million metric tons of olives are produced each year. A million metric tons are used for table olives and nine million (93% of the total crop) are pressed for olive oil. The European Union (EU) produces roughly 67% of the world’s olive oil. Around four million hectares, mainly in the EU Mediterranean countries, are dedicated to the cultivation of olive trees, combining traditional and intensive groves. Olive leaves gather the interest of the scientific community and the industries worldwide as their health promoting benefits are constantly being shown by an ever-increasing volume of scientific data. [6]
Olive leaves are considered byproducts of olive farming, representing almost 10% of the total weight of materials arriving to the olive mill. Research into olive leaves has revealed that their health properties are attributed to a group of secondary metabolites, namely biophenols that display a wealth of structural variety and diversity of important activities. [7] They contain phenolic compounds including flavones (luteolin-7- glucoside, apigenin-7-glucoside, diosmetin-7-glucoside, luteolin, and diosmetin), flavonols (rutin), flavan-3-ols (catechin), substituted phenols (tyrosol, hydroxytyrosol, vanillin, vanillic acid, and caffeic acid), and secoiridoids (oleuropein). [8]
The concentrations of these high added-value compounds drive the potent radical scavenging power that olive leaf extracts exhibit. Among the bioactive compounds of olive leaves, oleuropein presents very interesting pharmacological activities. Studies have shown that oleuropein exhibits anti-ischemic, antioxidative, hypolipidemic, antiviral, antimicrobial, antiatherogenic, cardioprotective, antihypertensive, and anti-inflammatory properties. [9–12] Olive leaves have been extracted with supercritical carbon dioxide (CO2) to obtain tocopherols [13], and with supercritical CO2 with added ethanol for waxes, hydrocarbons, squalene, carotene, triglycerides, tocopherol, sitosterol, and alcohols [14].
Olive oil deodorizer distillates (OODD) are a byproduct of the olive oil refining process, representing approximately 1.5% of the crude olive oil. It is normally disposed as a waste, though it contains high amounts of free fatty acids (FFA) and unsaponifiable ingredients. Among these, the most abundant is squalene, an aliphatic triterpene. OODD is produced in several facilities around the Mediterranean basin and quantities occurring are approximately 8,000-10,000 tons/year. The refining process for the olive oil that has a relatively high acidity consists of several steps, mainly neutralization, discoloration, and deodorization. During deodorization in industrial units that use high vacuum (0.3 mbar) and temperature (> 230°C), distillation of squalene and other unsaponifiable molecules occurs together with FFAs. Olive oil contains 0.08-1.20% squalene [15,16] while 60-70% of the unsaponifiable fraction of olive oil consists of squalene [17]. Certain studies have been conducted for the separation and purification of squalene from OODD, most often implementing countercurrent SFE.
Tomato Industry
Today, approximately 150 million metric tons of tomatoes are produced globally. During tomato processing for ketchup and sauce production, 40% of the tomatoes is obtained as waste in the form of tomato peel and seeds. [18] Disposal and handling of the tomato processing waste is an issue; currently, tomato peel and seeds are wasted or used for animal feed production. Nevertheless, the real value of the tomato peel and seed is underestimated, because tomato seed contains high nutritional quality oil and tomato peel is a very rich source of flavonoids and carotenoids, containing five times more lycopene than tomato pulp. Extraction of lycopene has received a lot of attention due to reports demonstrating lycopene’s potential to decrease risk of cardiovascular diseases and chronic diseases such as prostate, lung, and stomach cancer. [18-20] Because it is a high value bioactive compound and it degrades easily during processing, supercritical CO2 has been used as a green solvent to extract tomato oleoresin containing lycopene. [21,22]
Onion Industry
Onions (Allium cepa L.), with a world annual production approximately 66 million tons are, after tomatoes, the most important horticultural crop worldwide. Due to the high amount of bioactive compounds in onion byproducts, an idea for their reuse could be to use them as a source of high-value functional and health ingredients. The major byproducts resulting from industrial peeling of onion bulbs are dried skin, the outer two fleshy leaves, and the top and bottom bulbs, which are not edible and are removed before processing. [23]
Onion wastes, due to a rapid phytopathogenic growth and to their strong unique smell, are not suitable for fodder or landfill disposal. Therefore, the valorization of onion byproducts is indispensable and could provide an economic benefit for onion producers and processors. Onions are one of the most common and also the richest natural source of biologically active phytomolecules, including phenolic acids, anthocyanins, cepaenes, thiosulfinates, and flavonoids. [24-26]
In the last several years, multiple studies have been published on the physiological role of phytochemicals from onions. The major bioactive compounds found in onion dry peels are phenolics like quercetin. Quercetin and other food-derived phenolics are getting greater attention due to their antimicrobial and antioxidant activity, which play pivotal roles in cancer prevention, inflammatory disorders, and cardiovascular diseases. [27] Other benefits of quercetin include anti-inflammatory activity, antihistamine effect, allergy medication, and anticancer and antivirus activities. It has also been claimed that quercetin reduces blood pressure in hypertension. [26] To use onion by-products as source of bioactive compounds, green and efficient extraction methods have been developed to reduce time, cost, and environmental impact. [28]
Winemaking Industry
Grapes have gained high economic production in grape juices, seed oil, raisins, vinegar, etc. The total grape production worldwide in the year 2018–19 was 22.15 million metric tons. Grape pomace is an industrial byproduct from the winemaking process. It consists mainly of grape seeds, skin, and stems. Metabolites such as polyphenols, including resveratrol, remain in the pomace at concentrations that depend on the grape variety, the mechanical pre-treatment, and the winemaking process. Major fluctuations are observed in the resveratrol content from one vintage to another from different locations and because of different weather conditions. Generally, the concentration is higher in red than in white wines, since the must is fermented with the skins. [29] Currently, the focus is on the recovery of bioactive molecules of grape pomace using green technologies such as microwave, ultrasound, supercritical fluids, enzymatic methods, and other hybrid techniques. Most commercial applications concern nutraceuticals and extension in shelf-life of perishable foodstuffs. More specifically, there are numerous reports on the recovery of polyphenolic compounds from different varieties of grapes by SFE. [30,31]
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