Enzymatic production of lactulose by fed-batch and repeated fed-batch reactor

The effects of the most significant operational variables on reactor performance of fed-batch and repeated fed-batch were evaluated in the lactulose production by enzymatic transgalactosylation. Feed flowrate in the fed stage (F) and fructose to lactose molar ratio (Fr/L) were the variables that mostly affected the values ​​of lactulose yield (Y_Lu), lactulose productivity (π_Lu) and selectivity of transgalactosylation (S_Lu/_TOS). Maximum Y_Lu of 0.21 g lactulose per g lactose was obtained at 50% w/w inlet carbohydrates concentration (IC) of, 50 °C, Fr/L 8, F 1 mL⋅min^-1, 200 IU∙g_Lactose^-1 reactor enzyme load and pH 4.5. At these conditions the selectivity was 7.4, productivity was 0.71 g_Lu∙g^-1∙h^-1and lactose conversion was 0.66. The operation by repeated fed batch increases the efficiency of use of the biocatalysts (E_B) and the accumulated productivity compared to batch and fed batch operation with the same biocatalyst. E_B obtained was 4.13 g_Lu∙mg_{biocatalyst protein}^-1, 10.6 times higher than in fed-batch.

Enhanced rhamnolipids production using a novel bioreactor system based on integrated foam-control and repeated fed-batch fermentation strategy

BACKGROUND

Rhamnolipids are the best known microbial-derived biosurfactants, which has attracted great interest as potential ''green" alternative for synthetic surfactants. However, rhamnolipids are the major contributors to severe foam problems, which greatly inhibit the economics of industrial-scale production. In this study, a novel foam-control system was established for ex situ dealing with the massive overflowing foam. Based on the designed facility, foam reduction efficiency, rhamnolipids production by batch and repeated fed-batch fermentation were comprehensively investigated.

RESULTS

An ex situ foam-control system was developed to control the massive overflowing foam and improve rhamnolipids production. It was found that the size of individual bubble in the early stage was much larger than that of late fermentation stage. The foam liquefaction efficiency decreased from 54.37% at the beginning to only 9.23% at the end of the fermentation. This difference of bubble stability directly resulted in higher foam reduction efficiency of 67.46% in the early stage, whereas the small uniform bubbles can only be reduced by 57.53% at the later fermentation stage. Moreover, reduction of secondary foam is very important for foam controlling. Two improved designs of the device in this study obtained about 20% improvement of foam reduction efficiency, respectively. The batch fermentation result showed that the average volume of the overflowing foam was reduced from 58-640 to 19-216 mL/min during the fermentation process, presenting a notable reduction efficiency ranging from 51.92 to 73.47%. Meanwhile, rhamnolipids production of batch fermentation reached 45.63 g/L, and the yield 0.76 g/g was significantly better than ever reported. Further, a repeated fed-batch fermentation based on the overall optimization was carried out. Total rhamnolipids concentration reached 48.67 g/L with the yield around of 0.67-0.83 g/g, which presented an improvement of 62% and 49% compared with conventional batch fermentation by using various kinds of defoamers, respectively.

CONCLUSIONS

The ex situ foam-control system presented a notable reduction efficiency, which helped greatly to easily solve the severe foaming problem without any defoamer addition. Moreover, rhamnolipids production and yield by repeated fed-batch fermentation obtained prominent improvement compared to conventional batch cultivation, which can further facilitate economical rhamnolipids production at large scales.

Hydrogen production from starch by co-culture of Clostridium acetobutylicum and Rhodobacter sphaeroides in one step hybrid dark- and photofermentation in repeated fed-batch reactor

Hydrogen production from starch by a co-culture hybrid dark and photofermentation under repeated fed-batch conditions at different organic loading rates (OLR) was studied. Effective cooperation between bacteria in co-culture during initial days was observed at controlled pH 7.0. However, at pH above 6.5 dark fermentation phase was redirected from H₂ formation towards production of formic acid, lactic acid and ethanol (which are not coupled with hydrogen production) with simultaneous lower starch removal efficiency. This resulted in decrease in the hydrogen production rate. The highest H₂ production in co-culture process (3.23LH₂/L_medium - after 11days) was achieved at OLR of 1.5g_starch/L/day, and it was twofold higher than for dark fermentation process (1.59LH₂/L_medium). The highest H₂ yield in the co-culture (2.62molH₂/mol_hexose) was obtained at the OLR of 0.375g_starch/L/day. Different pH requirements of bacteria were proven to be a key limitation in co-culture system.

Efficient arachidonic acid-rich oil production by Mortierella alpina through a repeated fed-batch fermentation strategy

Arachidonic acid (ARA)-rich oil production by Mortierella alpina is a long fermentation period needed process due to the low growth rate of the filamentous fungus used. This causes the low productivity of ARA-rich oil and hinders its industrial mass scale production. In the present study, different fed-batch strategies were conducted to shorten the fermentation period. The result showed that compared with the batch culture, the fermentation period was shortened from 7days to 5days with the productivity of ARA-rich oil increased from 0.9g/(L·d) to 1.3g/(L·d) by using the fed-batch fermentation strategy. Furthermore, repeated fed-batch fermentation strategy was adopted to achieve the purpose of continuous production. By using this strategy, the fermentation period was shortened from 40days to 26days in a four cycle repeated fed-batch fermentation. This strategy proved to be convenient and economical for ARA-rich oil commercial production process.