**1. Introduction**

All natural cellulose fibers have their origin in the plant cell wall. Different plants' cell walls are radically different, but the cellulose content remains constant in the range of 35% to 50% of plant dry weight [1]. With rare exceptions, cellulose fibers are encased in a matrix composed of hemicelluloses and lignin, which constitute approximately 20–35% and 5–30% of plant dry weight [1–3]. Plant cell wall composition and architecture are fundamental characteristics of cellulose-containing biomass. While some other characteristics, such as size and moisture content, could be easily adjusted, carbohydrate and lignin content, pore density, and cellulose crystallinity are much harder to change and make the utilization of cellulose-containing materials more complex than that of pure cellulose.

The main requirements for biotechnology feedstock are low cost, availability, and high sugar yield [4,5]. The cost and availability depend on the logistics. In turn, the sugar yield in enzymatic hydrolysis or the convertibility of cellulose-containing materials are determined by fundamental

characteristics. It is necessary to identify cellulose-containing materials that match the requirements for biotechnology processes. It is reasonable to obtain simple sugars from low-cost non-food feedstock firstly due to the lack of competition with edible biomass and secondly to resolve some ecological problems—i.e., environmental pollution, greenhouse gas emission, or the alienation of productive land for landfills.

The first group of widely available potential feedstock is agricultural waste which consists mainly of herbaceous biomass (straw, husks, cobs, peels, leaves) with a low lignin content. The worldwide production of crop plant residues is about 3.4 billion tons yearly [6], including 100 million dry tons of corn stover [7], 280 million dry tons of bagasse [8], and 354 million dry tons of wheat straw [9]. As sugarcane bagasse in tropical countries, sugar beet pulp is a major by-product of the sugar industry in countries with a continental climate. The annual production of sugar beet pulp is 21–22 million wet tons, and its main use is as animal feed [10]. More than 100 million tons of agricultural plant residues are generated every year in the Russian Federation [11]. In general, the dry weight residue/grain ratio for cereal varies from 1 to 2 [12]. This agricultural biomass requires reutilization, otherwise it is left on field, dumped, and incinerated.

The annual harvest of wood biomass for saw logs and paper and pulp is 960 million tons. The estimated forestry processing wastes in the EU are about 88.2 million tons per year [13]; in the Russian Federation, they are about 35.5 million m<sup>3</sup> /yr [14]. Woodworking industry residues are widespread and often have no need to be additionally milled, but felling residues have to. Operating felling residues are important for sustainable forest use and preventing forest soil contamination. Aspen (*Populus tremula*) has a great potential in future bioeconomy as an energy crop due to its fast growth or as a felling residue because of its poor wood.

Food production generates a huge amount of residues and by-products that are usually used as animal feed due to residual protein, starch, and sugar [15,16] but could find application as a biotechnology feedstock. A total of 90 to 150 million tons of cultivated wheat is converted into bran every year [17]. Starch-containing bran was studied for bioethanol production [18] and protein extraction [19]. The main coproducts of grain-derived fuel and food-grade ethanol production are distillers' dry grain and solubles, which are limited in monogastric livestock. The growing production is expected to drive its supple up and prices down, making it a promising cellulose-containing resource [20].

It is common opinion that raw wet and dry cellulose-containing materials need to be pretreated prior enzymatic saccharification due to their rigidity. The most studied types of pretreatment are dry or wet mechanical milling, steam explosion and its varieties, dilute acid or alkaline pretreatment, and pretreatment with organic solvents. Irrespective of the pretreatment process, the cellulose-containing material's fundamental characteristics change. While relatively novel pretreatment processes require additional optimization and upscaling, pulp and paper mills are already fine-tuned for the production of delignified cellulose-rich materials located close to forest resources and could be quickly integrated into forest biorefinery. The estimated global production of mechanical and semi-chemical pulp is 36.3, with chemical pulp at 142.4, Kraft pulp (bleached and unbleached) at 140, and sulfite pulp at 2.4 million t/yr in 2017 [21]. The only technology missing in pulp mill production chain is enzymatic hydrolysis. The production of cellulosic sugars and further biofuels and biochemicals on a reorganized mill is economically feasible because of the compatible supply, facilities, and workforce [22]

In some cases, it is reasonable to process unconventional local cellulose-containing materials, such as municipal solid waste or algal biomass [23].

The aim of this research is to evaluate the enzymatic convertibility of cellulose-containing materials of different origin—including agricultural and industrial byproducts and waste in enzymatic hydrolysis with *P. verruculosum* cellulolytic complexes—to simple sugars which could be used as raw materials in different sectors of bioindustry.
