4.1. Lycopene Characteristics, Applications and Extraction

Lycopene is a lipophilic antioxidant belonging to the carotenoid family, synthesized by many plants and essential for light absorption during photosynthesis and protection against photo-oxidative damage. It is a bright red carotenoid found in red fruits and vegetables and represents more than 85% of the total carotenoids in tomatoes [44].

For many years, lycopene was commonly used as a pigment and natural food colorant [4], highly accepted by consumers as a food additive due to its well-known antioxidant properties and consequent health benefits, such as the reduction of the risk of coronary heart disease and atherosclerosis. Likewise, epidemiological findings have linked a lower risk of the incidence of specific types of cancer with the consumption of lycopene [44,45]. Moreover, in vivo experiments have proven that lycopene exerts considerable biological effects, including immunomodulatory and anticancer activity against prostate cancer, breast cancer, cardiovascular illness, and neurological degenerative diseases [46]. Lycopene has been categorized as a class A nutrient by the World Health Organization (WHO) and the Food and Agriculture Organization of the United Nations (FAO) based on its diverse applications in the food, medicine, and cosmetics industries [47].

Since lycopene is not synthesized in the human body, industrial lycopene production from tomatoes is highly demanded by food and pharmaceutical companies to produce functional foods and nutraceuticals.

In fact, according to the report of Global Opportunity Analysis and Industry Forecast, the lycopene global production in 2020 generated USD 107.2 million, and it is predicted to make USD 187.3 million by 2030, witnessing a compound annual growth rate (CAGR) of 5.2% from 2021 to 2030 [45].

According to the data found in the literature, the highest concentration of lycopene, ranging from 72% to 90%, is found in tomato skins and water-insoluble parts of tomatoes and is five times higher than that found in tomato pulp [48]. Therefore, lycopene recovery from tomato processing by-products has great potential interest [11].

Lycopene is a non-polar compound commonly extracted from tomato peels using organic solvents. In addition to the individuation of the most appropriate solvent, the optimization of effective processing parameters, including temperature, solvent to sample ratio, mixtures of different solvents, and extraction time, is necessary to improve the extractability of target compounds [49,50].

According to most of the studies present in the literature, the traditional way to recover lycopene from tomato peels is through the use of solvent mixtures that have been proven to enhance the extraction yield [49,51]. Zuorro (2020) utilized optimized mixed-polarity solvent mixtures, namely n-hexane–ethanol–acetone and ethyl acetate–ethanol–acetone, to extract lycopene from tomato peels and demonstrated that the highest lycopene extraction yield (95%) was obtained with a mixture containing 30.6% hexane, 32.8% ethanol, and 36.6% acetone (w/w) at 40 °C. This mixture was effective in producing a tomato oleoresin with high lycopene content (12.7 wt%) and antioxidant capacity (1582 µmol TE/g of tomato peels) [51].

The optimization of the solid-liquid extraction process to maximize the recovery of lycopene from tomato pomace was also investigated by Pandya et al. (2017) by selecting the most effective combination of solvents, extraction temperature, time, and solid-liquid ratio. The authors showed that using a mixture of acetone–ethyl acetate (1:1) as solvent at 40 °C for 5 h and a feed-to-solvent ratio (w/v) of 1:30 resulted in the highest yield of lycopene (6.11 mg/g of tomato pomace) [49]. The effects of these key parameters on the recovery of carotenoids from tomato by-products were also analyzed by Strati et al. (2011). The authors reported that, regardless of the type of solvent used, lycopene extraction was considerably influenced by the number of extraction steps. Among all the investigated solvents, ethyl lactate allowed obtaining the highest lycopene yield (0.243 mg /g dw) at 70 °C after 30 min of extraction. However, even at 25 °C, a similar extraction yield (0.202 mg/g dw) was obtained using ethyl lactate, suggesting that this solvent is capable of extracting at ambient temperature more lycopene than other solvents used at higher temperatures, thus reducing energy consumption and costs [52].

Although most studies focused on tomato peels and tomato pulp, another by-product of the tomato processing industry that contains a not-insignificant amount of lycopene, was investigated in several studies as a source of this carotenoid [48]. Poojary et al. (2015) utilized factorial design methodology for the optimization of extraction processing conditions to obtain high purity all-trans-lycopene from tomato pulp waste. In this work, the authors evaluated the effect of four variables, namely extraction time (1–60 min), temperature (30–50 °C), concentration of acetone in hexane (25–75%, v/v), and solvent amount (10–30 mL), on lycopene recovery yield. Experimental results showed that the highest lycopene amount (0.038 mg/g) was acquired at 30 °C for 60 min by using 30 mL of a mixture of 25% acetone in hexane (v/v) as solvent. However, numerical results demonstrated that the optimal extraction conditions were 20 °C for 40 min using 40 mL of solvent mixture, resulting in 0.039 mg/g lycopene in tomato pulp and limiting the isomerization and degradation of all-trans-lycopene. From an industrial point of view, these optimal extraction conditions are particularly appropriate, resulting in a high level of purity and recovery of all trans-lycopene (98.3% and 94.7%, respectively) [53]. Moreover, over the past few years, researchers have been motivated to use green solvents for lycopene extraction. In this regard, Amiri-Rigi and Abbasi (2019) evaluated the application of olive oil microemulsions as a green solvent by applying different proportions of olive oil, lecithin, 1-propanol, and water to examine their abilities to extract lycopene from tomato pomace. The results showed that the highest extraction efficiency (88%) was obtained when mixing 1 gr of tomato pomace with 5 gr of microemulsion prepared with the combination of olive oil: water: lecithin: 1-propanol (10:10:53.33:26.67 wt%). More importantly, this food-grade microemulsion enriched in lycopene can be used in a variety of formulations in the food industry because of its good solubility in aqueous and non-polar media and can enhance the health-promoting qualities of both lycopene and olive oil [54].