Tomatoes (Lycopersicon esculentum) are the world’s second-most important vegetable crop and a key component of the Mediterranean diet, with a global production of over 186 million tons in 2020, according to the Food and Agriculture Organization (FAO, 2020, https://www.fao.org/faostat/en/#data/QCL, accessed date 25 April 2022). Tomato consumption is linked to a series of health benefits such as lower risk of inflammatory processes and chronic diseases, reduced risk of developing carcinogenesis, and the inhibition of low-density lipoprotein oxidation, which helps to lower blood cholesterol levels in humans [1,2].

According to recent studies, more than one-quarter of the global production of tomatoes is processed as tomato-based products such as tomato sauce, ketchup, and canned or crushed tomatoes, which are in high consumer demand. The industrial processing of tomatoes generates large quantities of by-products, namely tomato pomace composed of tomato peels, seeds, and small amounts of pulp, which are primarily used as livestock feed, compost or disposed [3]. The amount of waste ranges between 5.4 and 9.0 million tons yearly; however, precise statistics on this subject matter are difficult to collect [4]. The by-products can represent 5–30% of the main product and are a source of environmental discomfort caused by their high moisture content, which favors microorganism growth and further impacts global warming through methane emissions [5]. On the other hand, several scientific studies highlight the rich bioactive composition of tomato processing by-products and the possible reutilization of extracted phytochemicals such as carotenoids, phenolic compounds, dietary fiber, and polyunsaturated fatty acids [6,7,8]. These nutraceutical chemicals can be used as natural antioxidants, preservatives, colorants, and functional food ingredients along with pharmaceutical and cosmetic applications [9,10,11]; therefore, the search for new uses in industries is a critical trigger in moving beyond the image of “waste as a problem” to “waste as a resource” [12,13,14].

The major bioactive constituents of tomato processing by-products are carotenoids, predominantly lycopene, followed by β-carotene and lutein (Figure 1). The human organism cannot produce these bioactive compounds; therefore, their uptake in the body relies on dietary sources such as carotenoid-rich foods and supplements. Given that carotenoids are fat-soluble pigments, specific fatty acids or vegetable lipids play an essential role in their bioavailability, and are involved in regulating the oxidative mechanism at the same time [15,16,17].

Lycopene is a bright red pigment that accumulates in tomato peels at concentrations approximately 5-foldhigher than in tomato seeds [18] and tomato pulp [19] and is the primary carotenoid found in tomatoes. Due to its superior antioxidant activity, lycopene has been found to significantly benefit human health, reducing the risk of several non-communicable diseases and other degenerative disorders mediated by free radical reactions [20]. Therefore, lycopene has been proposed and is already used in a wide range of industrial applications as a food supplement or as a functional ingredient in new food products [4]. This large number of possible applications of lycopene as a high-added-value product, combined with its abundance in tomato peels and with the growing consumer demand for natural food additives, underpins the great interest of researchers and manufacturers in the recovery of lycopene from tomato processing by-products discarded mostly from the peeling operation [4,21].

In relation to the principles of the bio-based economy, the conversion and valorization of recovered compounds into value-added products in a sustainable manner is of significant importance and constitutes the main scope of the circular economy action plan [22]. The design of new processes or the adaptation of existing technologies oriented toward the use of environmentally-friendly laboratory practices is the primary goal of the present study regarding carotenoid recovery from by-products and the upscale of these processes to the industrial level as future perspectives. In this regard, an alternative solution to the classic organic solvent extraction is offered by the green chemistry approach, and previous results indicate the use of green solvents such as ethyl lactate or ethyl acetate (Figure 2) to optimize lycopene extraction from waste materials [23,24].

Sustainable solvents employed for the extraction of carotenoids include the so-called “green solvents” and ionic liquids. Green solvents are generally produced from resources as biomass (wood, starch, vegetable oils, or fruits) or derived from non-toxic and/or biodegradable petrochemical-based solvents [25]. When used for extraction purposes, they should be able to penetrate into cell tissues, leading to high yields, short extraction times, cost-effectiveness, and sustainability, also preserving the biological activity of the extracted compounds.

Ethyl acetate belongs to the group of green solvents, which allows the extraction of lipophilic compounds in a sustainable way, having yields comparable to those of the extraction with hexane. Furthermore, it is considered as a low-toxicity solvent due to its rapid hydrolysis to ethanol and acetic acid during metabolism. It can replace benzene or methyl ethyl ketone and ethyl acetate/ethanol mixtures, and also act as a green alternative to dichloromethane in chromatographic separations [25].

Ethyl lactate is produced from the fermentation of carbohydrate feedstock, and it is entirely biodegradable in CO₂ and water. Its miscibility with both hydrophilic and hydrophobic compounds makes it appropriate to extract a diverse range of metabolites additionally, it has the advantage of extracting both trans-and cis-lycopene isomers [26].

In 2005, the US Food and Drug Administration approved the use of ethyl lactate in food and pharmaceutical products, as a generally recognized as safe (GRAS) solvent, and earlier studies concerning sustainable chemistry and pharmacy concluded that ethyl lactate is a safe, efficient and potentially inexpensive solvent capable of extracting both polar and non-polar compounds from fruit and vegetable by-products [27].

Carotenoid extraction from agro-industry-derived wastes is a well-documented subject, and a growing body of scientific work can be found on this topic [5,6,25]. One of the most widely used techniques for carotenoid recovery from wastes and by-products is the classic organic solvent extraction, singly or in combination with other procedures, [7,28]; however, the vast majority of the used solvents in the process is originated from the petrochemical industry (which are highly flammable, corrosive, carcinogenic, and harmful substances). Furthermore, the major inconvenience of the classic solvent extraction is the elimination of the residual solvents to obtain a safe extract that can be integrated into food products [24].

According to a recent review about carotenoid extraction methods [23], ultrasound-assisted extraction (UAE) was found to be an efficient technique with regard to extraction time and temperature, as well as to the reduction in solvent consumption and the degradation and isomerization of lycopene [5,24].

Our study aimed to evaluate the total and individual carotenoid content, by high performance liquid chromatography (HPLC), found in tomato waste extracts, which were obtained by using green solvents such as ethyl acetate and ethyl lactate. The tomato by-products were freeze-dried and the extraction protocol was assisted by ultrasound. The recovered carotenoids were further used as functional ingredients in three different fatty acid profiled oils (flaxseed, grape seed, and hempseed oil) to examine how the incorporated bioactive compounds affect the oils’ rheological behavior.