## 2 Agri-Food Side Streams: Residues, By-Products, and Wastes  
The agri-food production required to sustain the estimated population of nine billion people by 2050 will consequently increase the amounts of unavoidable agri-food wastes [29]. The agri-food system is currently producing in an inefficient manner [30; 31] and polluting the planet while depleting natural resources [1; 32; 33]. The United Nations (UN) estimated that 7 to 10 billion tons of waste were generated globally in 2010 [34]. [PERSON] et al. [35] calculated that about one-third of the food produced globally is lost or wasted along the whole food chain. These losses equal 1.3 billion tons of wastes and by-products every year [36], and they are valued at about USD 936 billion dollars annually [37]. The European Union (EU) reported that 129 Mt of food wastes were generated in the EU in 2011 [38]. These vast amounts of waste include agricultural lignocellulosic residues, agri-food by-products, food wastes, and industrial sludge and wastewater, all of them streams of organic waste suitable for exploitation [7]. Ensuring a sustainable food future for the world requires that the industry should be making the most out of the resources used [32]. Agricultural residues are produced during crop harvesting and consist of leaves, roots, stalks, stems, straw, seed pods, and husks. Asia is responsible for 47% of all the agricultural residues produced worldwide, while the United States produces 29%, Europe 16%, Africa 6%, and Oceania 2% [39]. Agricultural residues are inedible and unavoidable and have low bulk density, low ash melting point, fibrous nature, high volatile matter content, high calorific value, and low moisture [29]. Lignocellulosic residues are also being used for the production of bioenergy (heat and electricity generated by combustion) or biofuel production, but these are procedures that require considerable amounts of fossil fuel and other resources such as land and water [40]. Industrial residues derive from the food industries that use various methods and techniques to transform raw materials into food. Industries processing fruits and vegetables, grains, and dairy, meat, poultry, marine, winery, and brewery products are the main sources of huge quantities of solid and liquid effluents, which have to be sustainably managed without being an environmental burden [16]. Process residues include husks, peels, pulp and shells, brass, various pomaces, bagasse, and wastewaters. Agri-food side-streams are generally considered to have little value and are incorporated in animal feed [41], burned, end up in a landfill, or even worse, disposed of uncontrollably [42], causing severe environmental problems and health issues [43; 44]. Agri-food residues are often called lignocellulosic biomass because they are mainly composed of cellulose, hemicelluloses, and lignin, which form composite, recalcitrant matrices that create structural complexity and deem them indigestible and hard to biodegradation. All these side streams and co-products exhibit a wide variety of physicochemical characteristics that determine their technical and economic potential for valorization [7]. The carbon/nitrogen ratio is a critical factor for macrofungi (mushroom) cultivation, as it can impact their growth, yield, and polysaccharide and protein content [26; 45; 46; 47; 48]. The properties of some of the most widely produced side streams and wastes are presented in Table 1. An important characteristic that agri-food side streams share is wide variability in their physicochemical properties. Moisture, carbon, and nitrogen content are not consistent throughout the year, and the production cycles vary between production facilities and manufacturers. That is probably because the properties of the produced side streams and residues are affected by the location, the climate produced, and the properties of the original raw material [7]. Therefore, physicochemical analyses are needed as well as the mixtures' formation before their use as a substate for mushroom cultivation [46; 47; 48].