CBS is the fibrous external tegument and constitutes about 10–20% of the total cocoa bean weight [10,22,23]. CBS is recovered during the cocoa bean roasting process at the chocolate factory, i.e., for every kg of chocolate produced, almost 100 g of CBS is obtained [24,25,26]. It is estimated that each year, between 700 and 900 thousand tons of CBS are produced worldwide, approximately 300 of which are generated in Europe. The accumulation of this by-product represents an important disposal problem in terms of economic and environmental issues, which is further accentuated by new legal regulations [26].

Although CBS is generally considered and treated as a residue, it has been reported that its nutritional composition does not differ too much from that of cocoa beans. CBS nutritional composition has been deeply investigated in different research works with the aim of extracting carbohydrates, dietary fibres, phenolic compounds (mainly epicatechin and catechin) and methylxanthines, such as theobromine. Table 1 summarizes the composition of CBS according to literature data, which varies depending on the origin and the processing of cocoa fruit.

It has been reported that the moisture content of CBS, which is determined by the roasting process, normally ranges from 4 to 8 g/100 g, values considered acceptable for CBS storage. However, in some cases, moisture levels can reach up to 13 g/100 g [29]. In this sense, CBS has been described as a considerably hygroscopic material; therefore, storage at moisture levels over 15% could lead to mould growth [27]. According to the literature, the ash content is in the range of 6 to 9 g/100 g of CBS, which is a parameter also influenced by the roasting process and is relatively similar among varieties. CBS contains a higher ash content than other fruit residues, such as apple pomace (0.5 g/100 g) and orange peel (2.6 g/100 g) [52,53]. Osundahunsi et al., 2007 [54] reported that sodium and potassium are the main components of CBS ash.

The fat content is quite low in CBS compared to cocoa beans, which accounts for almost 50% (dry weight). It has been reported that the content of lipids in CBS can be reduced by 40% after the roasting process [29]. The chemical and physical characteristics of CBS lipids are similar to cocoa butter fat, except for the acidity levels, which are higher in CBS (9% oleic acid) than in cocoa butter (1.7% oleic acid) due to the hydrolysis of triacylglycerols [49]. The saponifiable fraction of CBS is mainly formed by oleic, capric, palmitic and stearic fatty acids, whereas the unsaponifiable fraction is composed of phytosterols, specifically stigmasterol [20]. From a health perspective, it has been demonstrated that the consumption of cocoa increases high-density cholesterol and reduces low-density cholesterol because of its fatty acid content [55].

The carbohydrate content in CBS varies between 13.2 and 70.3 g/100 g (Table 1), mainly constituting glucose (17%), galactose (3%), mannose (3%), xylose (1.2%) and arabinose (1.7%) [10,30]. Carbohydrates are one of the most important macronutrients in cocoa beans since these compounds are responsible for the flavour during the fermentation and roasting processes. Additionally, it has been demonstrated that roasted CBS contains more carbohydrates than unroasted CBS, which is associated with the transfer of sugars toward the outer shell during roasting [29]. Furthermore, it has been reported that some CBS carbohydrates, such as feruloylated oligosaccharides, could improve the intestinal microbiota due to their prebiotic characteristics [53]. With respect to the protein content, during fermentation, an increase of up to 30% can be achieved in CBS. However, roasting processes result in a reduction in the protein content of CBS. According to the literature, proteins constitute between 18.2 and 27.4% of CBS. Furthermore, it is known that just approximately 1% of the total protein in CBS remains in a free condition, while 90% is firmly bound to oxidized polyphenols found in the shell [49,56].

CBS has been reported to be an important source of dietary fibre (13.8–65.6 g/100 g) with a small fat content (2–7 g/100 g). The amount of dietary fibre in CBS also depends on the roasting process. Certainly, it has been proved that, in roasted cocoa shells, Maillard reaction compounds and tannin or protein complexes are responsible for an increment in fibre content [57]. Insoluble dietary fibre accounts for 80% of the total dietary fibre of CBS extracts and almost half of the total dry weight. According to the literature, the main components of this CBS insoluble fibre are uronic acid and glucose, followed by galactose, xylose, mannose and arabinose to a lesser extent. This composition indicates that the cell wall mainly consists of cellulose with lower amounts of hemicellulose and pectin [17]. It has been reported that cocoa fibre has physicochemical and antioxidant properties, which make it an interesting alternative for the preparation of low-calorie and high-fibre cocoa products [57].

Along with dietary fibre, CBS polyphenols are the most studied and interesting compounds since they are responsible for biofunctional properties attributed to this by-product, such as its antidiabetic, anticarcinogenic and anti-inflammatory effects [58]. Flavonoids, one of the most remarkable groups of phenolic compounds, have been widely investigated due to their extensive bioactive properties, including cardio-protective, anti-oxidation and anti-inflammatory activities [59]. The reviewed literature describes that the total phenolic content (TPC), total flavonoid content (TFC) and total tannin content (TTC) in CBS range between 22 and 100 mg GAE/g CBS, 1.6 and 44 mg CE/g CBS and 2.3 and 25.3 CE/g CBS, respectively. The number of phenolic compounds in CBS extracts varies depending on several factors, such as origin, variety, extraction procedure (technologies, conditions, solvents…) and the presence of other bioactive compounds [40,60,61]. Flavanols have been identified as the major class of flavonoids in CBS, mainly including catechin (0.8–5.7 mg/g CBS), epicatechin (0.6–30 mg/g CBS) and procyanidin B2 (0.2–1.4 mg/g CBS). These polyphenols are not essential for short-term well-being, but it has been suggested that a long-term intake of them could contribute to health benefits, such as those related to cognitive function [62]. Other flavonoids also detected in CBS include kaempferol, quercetin and anthocyanins [26,40,58].

Theobromine and caffeine are the main methylxanthines, also known as alkaloids, found in CBS in quantities that fluctuate from 0.6 to 2.7 mg/g of CBS and 0.1 to 0.1–1.1 mg/g of CBS, respectively. Both alkaloids are related to physiological effects on the cardiovascular, gastrointestinal, respiratory, renal and central nervous systems in addition to their anticarcinogen and diuretic properties [20,61]. However, it is well known that excessive consumption of methylxanthines, particularly caffeine, is related to tachycardia, kidney dysfunction and other disorders. In addition to theobromine and caffeine, theophylline is another methylxanthine detected in CBS extracts, but at lower concentrations (0.1–0.3 mg/g CBS) or even at trace levels [51,60]. Methylxanthines, along with polyphenols, contribute to the characteristic bitter taste of cocoa and its derivatives, such as chocolate [63].

CBS can also contain some toxic compounds and/or antinutrients, such as biogenic amines, trypsin inhibitors and phytic acid. However, they are at concentrations so low (for example, the amount of phytic acid is below 0.6 g/100 g CBS) that this does not affect the use of CBS as a potential source of nutrition [53,64].

Recently, CBS valorization has been investigated to find novel applications for this by-product, including its use in food formulation, the obtention of biofuels, the extraction of bioactive compounds and employment as an adsorbent. Figure 2 shows an overview of different potential applications of CBS.