## 4 Discussion

Table 2: Summary of the principal actors involved in the valorization of coffee by-products and waste products using green extraction techniques, extracted from the co-occurrence analysis of words appearing in titles, sources and abstracts of publications. According to the bibliometric analysis, the most widely used coffee product is the SCG, which is one of the most abundant residues and has the greatest potential for resource recovery [43], estimated to constitute about 40-45% of the total mass of coffee after brewing [44]. Consequently, a significant proportion of research efforts are directed towards this particular type of waste, given its extensive industrial applications, particularly in the production of cosmetics or food ingredients [42; 45]. In addition, the bibliometric analysis highlighted research into other products such as green beans, husk, and pulp, which have been shown to possess a high abundance of bioactive compounds [46; 47; 48]. These residues are, as a general rule, utilized by various industries, which have invested significantly in the generation of novel high-value products such as bio-sugars or bio-oils [49]. Due to the characteristics of these residues and their significant potential, the food, pharmaceutical, and cosmetic industries are the primary sectors prioritizing their waste management through reuse and recycling [50]. Another crucial element is the extraction method. The results of the bibliometric analysis show that ultrasound-assisted extraction and microwave-assisted extraction are particularly noteworthy, which may be due to their reduced solvent usage, rapid extraction, and energy efficiency [48; 51]. For example, the employment of ultrasound-assisted extraction techniques has been demonstrated to result in a substantial enhancement of the total phenol content and antioxidant activity, with a recorded increase of almost 60% in comparison with conventional extraction methods [52]. Microwave-assisted extraction has been identified as a highly efficient and ecologically sustainable extraction method [53]. This finding may be a motivation for researchers to publish more high-quality studies using this method. Pressurized liquid extraction, along with high-temperature and high-pressure solvent extraction, also stands out [51; 54]. The bibliometric analysis has enabled the observation that sustainable technologies such as cold plasma-assisted extraction (CPAE) have not been described in relation to coffee products. This could be a significant technique for extracting polyphenols [53], from coffee waste. In terms of the type of solvent employed, the most commonly utilized are water [55], and natural deep eutectic solvents. This is due to their biodegradability and low toxicity [47; 56]. These findings are consistent with those obtained through co-occurrence analysis. Furthermore, deep eutectic solvents and absolute or hydrated ethanol, a traditional solvent with favorable extraction efficiency [47; 51; 57], are also employed. The employment of this eco-friendly methodologies has been demonstrated to reduce operating costs by decreasing solvent and energy consumption, thereby optimizing process efficiency and enhancing profitability [58]. An illustrative example is a study on the extraction of oil with supercritical fluids from used coffee grounds, which demonstrates a preliminary economic assessment with a net process yield of approximately 56 million euros per year [59]. Subsequently, the extraction processes yield a range of by-products. The high occurrence of polyphenols and caffeine is indicative of their importance as bioactive compounds of interest. According to the results, the compounds that were found to be most prevalent were caffeic acid and chlorogenic acid and their derivatives. This finding can be attributed to the strong antioxidant properties exhibited by these compounds [56; 57; 60]. This highlights its importance and value in its recovery from coffee waste for both the scientific community and industry.