Coffee has become one of the most popular beverages [1], with a world coffee consumption of 166,346 thousand 60 Kg bags (~10 million tons) in the coffee year 2020/2021 (20 October to 21 September) [1]. World coffee exports amounted to 10.07 million bags in the same coffee year, from which 32.7% were supplied by Brazil, 19.3% by Vietnam, and 9.9% by Colombia, as the top three exporting countries. This industry has a current world trade of USD 30.9 B, while 59.8% was imported to the European Union (mainly Germany, which exported 28.3% of the total from the EU, Italy, and France), 28% to the United States of America, and 5.6% to Japan [2].

With such consumption rates worldwide, rising awareness about the environmental impact of residues from the coffee industry is evidenced by several scientific studies dedicated to the valorization routes of coffee by-products within the circular economy paradigm [3]. The fruit of coffee plants, referred to as coffee cherries, comprises the pericarp and the seeds, the latter harvested for human consumption. The pericarp includes the skin (exocarp), mucilage (mesocarp), parchment (endocarp), and the two seeds of the coffee cherry consisting of a peripheral spermoderm or “silverskin”, an endosperm (responsible for taste and aroma), and the embryo. Upon harvesting and processing, two main by-products are generated, coffee husk (CH) or coffee pulp (CP), which depends on the post-harvest method to transform the cherry into a green bean [4]. CH is obtained through the dry or semi-dry method (unwashed) and comprises all the pericarp layers, while CP is generated by the wet method (washed) and contains the mucilage and the skin of the cherry.

The green bean is then stored and roasted in place or transported to consumer countries for roasting, where the coffee silverskin (CS) is released from the bean as a by-product. The roasting process alters the bean’s chemical composition and determines the organoleptic properties and color of the roasted bean [5]. The preparation of coffee beverages consists of brewing ground roasted beans with hot water, which leaves a major by-product known as spent coffee grounds (SCG). Although one can already perceive the large proportion of residues generated in the coffee industry that might end up in landfills, the numbers are quite cruel. From a rough average of the published data, it is possible to estimate that one ton of coffee produced by the wet method generates ~0.5 tons of CP (wet weight), from which the pulp accounts for 40–50% of the fresh weight of coffee berries. In comparison, the dry method produces ~0.2 tons of CH (dried weight) from one ton of harvested coffee fruit. The roasting process generates one ton of silverskin for 120 tons of roasted beans (0.008 per 1 ton), and the brewing leaves between 0.5–0.7 tons of SCG per one ton of green bean (~0.35 tons per one ton of cherries) [6].

Over the last two decades, by-products from coffee consumption have grown as an independent field, receiving considerable worldwide attention from researchers ^[7][8][7,8]. Some researchers have described the main functional properties of coffee by-products. For example, Esquivel and Jiménez [8] explain the properties of each part of the cherry. Murthy and Madhava [9] show the process and standard value addition of coffee by-products. However, both studies are descriptive and subjective. Other studies focus on a specific part of the cherry bean. For instance, Narita and Inouye [10] provide an overview of CS chemical composition, biological activity, and its reuse; Campos-Vena et al. [11] review the SCG theory and propose future research, legislative frameworks, and policy recommendations [12]. Other studies were reported in the literature with more specific applications, such as in the food industry (tea, jam, juice, jelly, and others) [13]. Finally, recent studies show difficulties in transforming coffee by-products ^[14][15][14,15]. However, none of these papers use quantitative techniques to identify the topic’s importance, evolution, and intellectual structure.

These studies failed to provide a quantitative perspective of coffee by-products. In this vein, citation analysis is suggested to mitigate the limitations. For example, Kohn and Gordon [16] pointed out that citation analysis incorporates the importance and influence of papers throughout a ranking without the limitation to a set of specific journals or published years. Unlike other similarly growing fields, papers generated in this field have not yet been analyzed systematically to reveal the different coffee by-products. This gap is surprising because coffee is the most popular beverage in the world [17].

Although researchers have studied the by-products from coffee during the last two decades, only a few studies have tried to organize the academic literature to explain the development in the research of coffee by-products ^[13][14][15][13,14,15]. The insights of these works have tended to be fragmented and subjective compared with the results of objective, data-based scientometric approaches.