## 1 Introduction  
Coffee is one of the most popular beverages around the world [1]. More than 70 countries are active in the field of coffee cultivation and export. According to statistics from the International Coffee Organization (ICO), global production for the years 2021/2022 was estimated at 168.5 million bags (60 kg per bag). The global consumption of coffee for 2021/2022 was 175.6 million bags (60 kg per bag), while Europe accounts for 31.5% of the annual global consumption [2]. Both the cultivation and coffee processing generate a large amount of waste. Coffee Silverskin (CS) and Spent Coffee Ground (SCG) are the main by-products from the coffee industry [3]. SCG consists of roasted and ground coffee beans that have been extracted during the brewing stage of the coffee beverage [4]. Considering the need to protect the environment and find renewable sources of energy and food, modern coffee industries are driven to develop by-product utilization strategies that allow not only the recovery of high nutritional value components but also their recycling through the creation of new products that find applications in various biotechnological sectors, such as the pharmaceutical, food, and cosmetic industries [5]. Spent Coffee Ground (SCG) consists of dark coffee powder with a high moisture content, as it has just been extracted with water or steam. It is observed that the SCG derived from small-scale extractions is richer in bioactive components than that obtained during the industrial production of instant coffee, since in the latter case, extraction conditions are chosen to maximize its performance. From 2000 to 2012, there has been an increase in the cultivation of green coffee on a global scale (17%), due to a 24% increase in production efficiency. Therefore, the increase in green coffee production also led to an increase in SCG production. SCG production amounts to six million tons per year worldwide, while one ton of fresh green coffee corresponds to 650 kg of SCG. This by-product contains many organic substances (e.g., lipids, amino acids, polyphenols, minerals, and polysaccharides) and thus becomes a suitable by-product for utilization [1; 6; 7; 8]. The content of dietary fiber in SCG amounts to 62% \(w/w\), where 20% of this is soluble fiber and the rest, 80%, is insoluble fiber [1]. Cellulose, mannans, galactomannans, type II arabinadactans, and hemicellulose (which consists of mannose, galactose, and arabinose) are the major polysaccharides of the corresponding residue. Bioactive compounds, such as antioxidants and proteins, can be simultaneously isolated alongside dietary fibers as accompanying compounds. For example, coffee by-product fibers have antioxidant properties (2.4 mmol of \(\mathrm{Trolox}/100\) g dry weight) [7]. Thus, dietary fiber from SCG can be categorized as antioxidant dietary fiber, which is likely to be used as a food additive [6; 7; 8; 9]. The antioxidant activity of the by-product is mainly due to its high phenolic content. According to research, where extractions (solvent extraction procedure using ethanol/water% \(v/v\) 0:100 and 60:40) were carried out on coffee residue to recover the natural antioxidants, the phenolic content was quantified, and the results showed that SCG contains 16-35 mg expressed as gallic acid equivalents per gram of dry sample [10; 11]. The main phenolic components are chlorogenic acids (mainly 3-Caffeoylquinic acid--3-CQA) and free cafic acid. These ingredients are very important for human health thanks to their antioxidant action. Caffeic acid corresponds to the phenolic part of all chlorogenic acids and belongs to the C6-C3 class of phenols. In chlorogenic acids, caffeic acid is esterified with quinic acid (Caffeoylquinic acids, CQAs). The percentage of phenolic components in the extract of the by-product depends on the variety of coffee and the type of extraction used. The concentration of chlorogenic acids amounts to 6.22-13.24 mg/g, which is four times higher than the corresponding concentration in the coffee beverage [6; 12].