## 3 Results and Discussion
### Impact of the bioreactor Settings on the Yield and Mechanical Properties of Bacterial Nanocellulose  
Selection of the most suitable process parameters for BNC production can significantly influence both the yield and properties of BNC. Tensile strength is one of the parameters which characterize the quality of BNC, besides crystallinity and high water holding capacity (WHC) in a lot of applications. In this paper, we propose a two-stage approach to simultaneously study the impact of a wide range of factors on the yield and mechanical properties of bacterial cellulose. In the first step, we focused on the fast screening of variables affecting the production yield. Afterwards, for the chosen significant process parameters, the mechanical strength was correlated with the yield depending on the value of individual variables. Screening of process parameters which influence the biosynthesis yield was designed using PBD. We assessed two responses connected with the yield, namely dry weight of cellulose and glucose conversion ratio. The first allows direct evaluation of the yield, whereas the glucose conversion ratio shows economical aspects of the process. In our study, we used modified standard bioreactors for static culture. The SAF bioreactor allows introduction of fresh air inside the bioreactor without movement of the growing cellulose membrane. The Plackett-Burman design matrix contained nine variables at two levels. Analysis of the data shows that seven or eight of them significantly influence the dry weight of cellulose or glucose conversion ratio, respectively. Bioreactor area was the factor which most affected the dry weight of cellulose, but in case of glucose conversion ratio and the yield, calculated as the dry weight per surface volume, its impact was lower. In static culture, BNC is formed across the surface of the media at the air-liquid interface. Therefore, the greater the surface area of the bioreactor, the higher cellulose mass should be obtained [25]. [PERSON], [PERSON], and [PERSON] also studied the effect of bioreactor area on the dry weight of cellulose, yield, and glucose conversion ratio [13]. The authors reported that this parameter strongly affects the dry weight of cellulose as a result of oxygen transfer for the growth of cells. On the other hand, the impact of bioreactor area on the production yield and glucose conversion ratio was insignificant. This is in accordance with our results, where the bioreactor area had the positive effect on the glucose conversion ratio, but its contribution was only 0.7%. The obtained results allow the conclusion that at the production scale, the bioreactor area does not affect cost in terms of the amount of cellulose achievable per unit volume of medium. Surface to volume (s/v) ratio is the only variable which was insignificant for both studied responses. This parameter is inverse to the high of medium in bioreactor. The higher the depth of the medium, the lower s/v ratio obtained. If the volume of the medium is not the limiting factor during culture, BNC could grow up to the stagnation point [25]. In this condition, the thickness and dry weight of pellicle are not strongly affected by the s/v ratio. For optimization of the production yield, the s/v ratio should be maximized by taking into account other process parameters. First of all, the thickness of membranes, and therefore the medium depth, depends on the used _Komagataeibacter_ strain. Previous studies reported an optimal surface to volume ratio for _K. xylinus_ E25 at 0.71 cm\({}^{-1}\)[26], while for other strains the s/v ratio of 2.2 cm\({}^{-1}\) and 0.39 cm\({}^{-1}\)[25] gave the highest yield. Also, for higher volumes (lower s/v ratio) of medium, there are more bacteria cells added with inoculum than in case of lower medium volume, although the density of cells is the same. Due to the aerobic nature of _Komagataeibacter_ strains, most of the bacteria grow at the air-liquid interface, not in the entire volume of medium.