## 5 Biomass Porosity and Cellulase Efficacy
Each cellulose glucan is a polymer consisting of D-anhydroglucopyranose joined together by \(\beta\)-1,4-glycosidic bonds, with anhydrocellobiose as the repeating unit [19]. Each microfibril contains a single elementary fibril and is surrounded and embedded in a matrix of non-crystalline hemicelluloses & pectin polysaccharides with microfibril dimensions typically ranging from 2-3 nm to 20-50 nm, as affected by plant species and tissue types [19; 105; 106]. When microfibrils interact with one another to form aggregation via van der Waals forces, macrofibrils are formed and their dimensions vary from 50 to 250 nm in diameter [105; 106; 99; 105]. Thus, plant cell materials typically consist of macrofibril, microfibril and elementary fibril units. Cell wall biomass materials are porous materials. Porous materials are classified into three categories including micropores (pore diameters less than 2 nm), mesopores (pore diameters between 2-50 nm) and macropores (pore diameters larger than 50 nm) [108]. The micropores (around 1 nm in pore diameters) and the mesopores (between 2 and 5.2 nm in pore diameters) were likely to be the capillaries between adjacent microfibrils while the macropores (around 20 to 100 nm in pore diameters) were likely to be the space along the external surface between macrofibrils in plant cell wall biomass materials, as was originally proposed by [PERSON] and [PERSON] (1976) [98] and further discussed by [PERSON] and [PERSON] (2011) [107]. It is also reasonable to believe that micropores represent the majority whereas mesopores and macropores likely only represent a small proportion of the total pores in the raw biomass materials. Hence, another layer of biomass resistance is the restricted nanoporosity, which reflects the crosslinks formed by hemicellulose & pectin polysaccharides embedding each elementary fibril via non-covalent bonds and the lignin polymers surrounding the exterior of each microfibril via covalent bonds, and is related to free space across microfibrils and macrofibrils in the biomatrix [109]. Pores in biomass can be open or closed. Open pores are accessible, whereas closed pores are inaccessible, to accommodate cellulases for cellulose hydrolysis [107]. The drying process, especially drying cell wall biomass materials at higher temperature for a long duration (e.g., 150 \({}^{\circ}\)C for 1 h), can cause fibre ramification that is a consequence of the irreversible change of the cell wall structure due to the collapse of pores [109]. [PERSON] et al. (1979) reported that pore size diameters in various living plant cells ranged between 3.5 and 5.2 nm, as measured by using a solute exclusion technique [18]. Biomass material accessibility and porosity can be experimentally measured with several techniques, including a solute exclusion, water vapor sorption and a liquid N\({}_{2}\) adsorption, and these have been conducted for a number of biofuel materials [19; 109]. As reviewed in an earlier section, several previous studies documented that pretreated lignocellulosic biomass materials and purified crystalline cellulose model substrates such as Avicel, \(\alpha\)-cellulose, Sigmacell (20, 50 and 100) and Solka-Floc\({}^{\circledR}\) were associated with much larger pore diameters, ranging 5.1-11.0 nm [94; 95; 96; 97]. Thus, it is safe to predict that fresh fibre feeds such as grasses for grazing herbivores have an upper porosity diameter at about 5.2 nm according to the study by [PERSON] et al. (1979) [18]. To the best of our knowledge, these types of porosity measurements have not been well reported in other raw plant feed materials for agricultural food animals such as corn silage, ground hay, DDGS, wheat shorts and middlings, canola meal and soybean meal, etc.