Using a flexible shaft agitator to enhance the rheology of a complex fungal fermentation culture

Abstract

The rheology behavior of biological fluids particularly when the viscosity is high and rheology is complex, is an important issue to understand, particularly for studies in mass-transfer and for solving technical problems with mixing in stirred bioreactors. In this paper, the use of a Swingstir^® impeller during the fermentation of Aspergillus oryzae resulted in decreases from the parameters of a power-law model, in viscosity and in the thixotropic behavior of a cultivation broth. The results showed that both the K _L a and the alpha amylase activity were improved when using the Swingstir^® in comparison with Fullzone^® impeller (FZ) at the same level of energy consumption. Increasing the pellet porosity during mixing via the Swingstir^® resulted in increases in oxygen mass transfer and the average shear stress.

Appendices

Appendix 1: Statistic efficacy

It was noticed that all of the obtained data were extracted by doing at least three times independent experimental processes. Each of repeated experimental process and analysis the fermentation parameters were done at the same condition. Alpha amylase activity and glucose concentration were measured a minimum of five times during each sampling. Average of these values was recorded with standard deviation as the data shown in the diagrams (Fig. 5).

Fig. 5

Illustration of waste of biomass during fermentation of A. oryzae using a Maxblend and b Double Rushton turbine impellers at the same P _v

Appendix 2

It was important to note that the equation shown as Eq. (7), is the simplified equation of C _D measurement for a single bubble in a liquid (Figs. 6, 7, 8, 9). This equation is the default of R-Flow software for calculating the C _D during gas–liquid flow simulation in stirred tank using shear-thinning fluid (Tables 3, 4, 5). Equation (7) was extracted from the originate equation shown as follows [18];

$$C_{\text{D}} = { \hbox{max} }\left\{ {\frac{24}{{Re_{\text{b}} }}(1 + 0.15Re^{0.687} )/Re_{\text{b}} , \frac{8}{3} \frac{{E_{\text{O}} }}{{E_{\text{O}} + 4}}} \right\},$$

(9)

where $E_{\text{O}}$ is Eotvos number (Eq. 10), $Re_{\text{b}}$ is the bubble Re.

$$E_{\text{O}} = \frac{{gd_{\text{b}}^{2} \left( {\rho_{c} - \rho_{d} } \right)}}{\sigma }$$

(10)

Fig. 6

Definition of hyphae diameter (thickness), hyphae length and pellet diameter in this study

Fig. 7

Simulation of viscosity in the fermentation culture stirred by a FZ and b Swingstir^® at t = 48 h

Fig. 8

a High hyphal branches intensity at t = 48 h (×500) and b uniform hyphae length unit in branches of A and B at t = 72 h, (×800) when using Swingstir^® during fermentation

Fig. 9

Simulation of fluid flow velocity during the fermentation using Swingstir^® in Y–Z plane, at P _v = 150 Wm⁻³

Table 5 Mixing time correlations for Swingstir^® and FZ