## 3 Results and Discussion
### Rheological Properties
#### 3.4.2 Amplitude Sweep  
Amplitude sweep testing was done to assess the visco-elastic properties of the _P. purpureum_ samples after storage. Figure 6a shows the storage modulus (G') and loss modulus (G\") in function of shear strain for the concentrate storage paste at \(t_{0}\). Within the linear visco-elastic region, i.e., the region where G' and G\" are independent of the shear strain, G' was higher than G\", indicating predominantly elastic behavior (Figure 6a). For the analysis of the storage effect, G' and the phase angle (\(\delta\) = \(\tan-1\) (G\"/G')) are displayed for 1% shear strains in Figure 6b,c, respectively. The storage modulus G' gives an indication of the gel stiffness, while \(\delta\) gives information about the visco-elastic behavior. During concentrate storage, all studied storage factors including their interaction had a significant impact on G' (\(p\) < 0.001) and on \(\delta\) (\(p\) < 0.05). The G' modulus raised during concentrate storage where the rise was fast during 20 \({}^{\circ}\)C storage and slow during 4 \({}^{\circ}\)C and 8 \({}^{\circ}\)C storage (Figure 6b). The decreasing \(\delta\) values indicate that the samples became more solid-like during storage (Figure 6c). This evolution was the fastest during 20 \({}^{\circ}\)C concentrate storage and was slowed down by cooling. During dilute culture storage, the G' value of the algae pellet was not influenced by storage time (\(p\) = 0.091) nor by the storage temperature (\(p\) = 0.14), while \(\delta\) was only affected by the storage temperature (\(p\) = 0.006). Slightly lower \(\delta\) values were observed after 20 \({}^{\circ}\)C storage than after 4 \({}^{\circ}\)C or 8 \({}^{\circ}\)C storage (Supplementary Materials Figure S2).  
Figure 5: **(a)** Viscosity of the storage concentrate at \(t_{0}\) in function of the applied shear rate and **(b)** after storage at a shear rate of 10 s\({}^{-1}\). **(c)** Viscosity of the dilute culture storage concentrate at \(t_{0}\) in function of the applied shear rate and **(d)** after storage at a shear rate of 10 s\({}^{-1}\). Viscosity values in panel **(b,d)** are averages of duplicate measurements with error bars depicting the standard deviations. Bars that are not marked with a common lowercase letter within one graph are statistically different, \(t_{0}\) data are not included in the statistical analysis.  
The observed formation of a gel network by _Porphylidium_ suspensions is in line with the findings of [PERSON] et al. [33; 38]. They found that EPS can act as a kind of binder enhancing cell aggregation [33; 38]. Furthermore, they noticed an increase in gel stiffness when the suspensions were heated (15 min, 95 \({}^{\circ}\)C) probably due to an enhanced cell aggregate formation [38]. EPS inter-chain interactions probably play a key role here as the disintegration of such interactions in EPS solutions by sonication can reduce the viscosity dramatically and can shift such a solution from a weak gel to a liquid-like system [35].  
Figure 6: (**a**) Storage modulus (G’) and loss modulus (G”) for the concentrate storage paste at \(t_{0}\). (**b**) Storage modulus at a 1% shear strain during concentrate storage. (**c**) Phase shift \(\delta\) at a 1% shear strain during concentrate storage. Values in panel (**b**,**c**) are averages of duplicate measurements with error bars depicting the standard deviations. Bars that are not marked with a common lowercase letter within one graph are statistically different, \(t_{0}\) data are not included in the statistical analysis.  
Our data suggest that a gel network is formed fast in freshly harvested and concentrated _P. purpureum_ suspensions and that this process is slowed down by cooling. This has important consequences for further processing of the biomass in a down-stream processing context. The stronger the gelling behavior, the more difficult it becomes to dissolve the biomass. Moreover, strong cell aggregation will compromise extraction such as in the case of aqueous B-PE extraction.