# Production of Microfibrilated Cellulose from Fast-Growing
## 3 Results and Discussion
### MFC Characterization
#### 3.2.3 Nanofibrillation Yield and Water Retention Value (WRV)  
MFCs obtained by mechanical treatments consist of a mixture of fibers, fiber fragments, fines and fibrils [PERSON] et al. (2018) that create an inhomogeneous material [PERSON] et al. (2018). The nanofiber content is assessed by centrifugation of a dilute suspension of cellulose micro/nanofibers, where nanofibers remain suspended, whereas fibers and microfibrillated material precipitate. The nanofibrillation yields in this work ranged from 11.3% to 13.3% (Table 3), indicating that the samples contain a significant amount of fibers and microfibers, in agreement with SEM observations. These values are lower than those observed by [PERSON] et al. [PERSON] et al. (2018) and [PERSON] et al. [PERSON] et al. (2018). However, these authors used different raw materials (elm) or more intensive mechanical treatment conditions to produce CNFs. Other authors, such as [PERSON] et al. [PERSON] et al. (2018), showed similar nanofibrillation yields when employing olive tree pruning to produce MFC by exclusively mechanical means, although they used more severe conditions in the fibrillation process than those used in this work (10 Passes vs. 6 Passes through the microfluidizer). The carboxylic acid content is known to enhance the nanofibrillation yield [PERSON] et al. (2018). When cellulose micro/nanofibers are produced using only mechanical processes, as in this case, the carboxylic content does not increase compared to those observed in the starting pulp \begin{table}
\begin{tabular}{c c} \hline \hline  & **Degree of Polymerization** \\ \hline O-Pulp & 2742 \(\pm\) 52 \\ P-Pulp & 2076 \(\pm\) 51 \\ E-Pulp & 2129 \(\pm\) 43 \\ O-MFC & 1010 \(\pm\) 69 \\ P-MFC & 1100 \(\pm\) 60 \\ E-MFC & 1290 \(\pm\) 26 \\ \hline \hline \end{tabular} O-Pulp: olive pulp; P-Pulp: polar pulp; E-Pulp: eucalypt pulp; O-MFC: olive microfibrillated cellulose; P-MFC: poplar microfibrillated cellulose; E-MFC: eucalypt microtub microfibrillated cellulose.  
\end{table} 
Table 2: Degree of polymerization of the different pulps and their corresponding MFCs. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Nanofibrillation Yield** & **Water Retention Value** \\  & **(\%)** & **(\%)** \\ \hline O-Pulp & - & 294 \(\pm\) 11 \\ P-Pulp & - & 230 \(\pm\) 12 \\ E-Pulp & - & 222 \(\pm\) 8 \\ O-MFC & 12.9 \(\pm\) 0.3 & 520 \(\pm\) 43 \\ P-MFC & 13.3 \(\pm\) 3.3 & 499 \(\pm\) 39 \\ E-MFC & 11.3 \(\pm\) 0.6 & 481 \(\pm\) 10 \\ \hline \hline \end{tabular} O-Pulp: olive pulp; P-Pulp: poplar pulp; E-Pulp: eucalypt pulp; O-MFC: olive microfibrillated cellulose; P-MFC: poplar microfibrillated cellulose; E-MFC: eucalyptTPUT microfibrillated cellulose.  
\end{table} 
Table 3: Nanofibrillation yield (%) and water retention value (%) of MFCs. (lower than 90 umol g\({}^{-1}\)), thus explaining the low nanofibrillation yields. On the other hand, the lower nanofibrillation yield observed for E-MFC could be attributed, amongst other factors, to the lower hemicellulose content, since the lignin content was very similar for all samples. Moreover, the nanofibrillation yield depends not only on the raw material (composition, crystallinity, carboxylic acid content, etc.) but also on the type of process used to obtain it. Thus, a more severe nanofibrillation process or other types of pretreatment such as chemical oxidation mediated by TEMPO could be used to increase nanofibrillation yields [20; 43]. On the other hand, pulp swelling is connected to the water retention value and it depends on the fibrillation degree and the surface chemistry of fibers [8]. Swelling also depends on the fiber structure, on the degree of crystallinity and on the presence of amorphous and porous regions. The mechanical treatments used during the cellulose microfiber production resulted in an increase in the WVR value for all samples (77%, 117% and 116% for O-MFC, P-MFC and E-MFC, respectively). This fact has been attributed mainly to an increase in the specific surface area in the samples [8].