Conversion of Exhausted Sugar Beet Pulp into Fermentable Sugars from a Biorefinery Approach

Abstract

In this study, the production of a hydrolysate rich in fermentable sugars, which could be used as a generic microbial culture medium, was carried out by using exhausted sugar beet pulp pellets (ESBPPs) as raw material. For this purpose, the hydrolysis was performed through the direct addition of the fermented ESBPPs obtained by fungal solid-state fermentation (SSF) as an enzyme source. By directly using this fermented solid, the stages for enzyme extraction and purification were avoided. The effects of temperature, fermented to fresh solid ratio, supplementation of fermented ESBPP with commercial cellulase, and the use of high-solid fed-batch enzymatic hydrolysis were studied to obtain the maximum reducing sugar (RS) concentration and productivity. The highest RS concentration and productivity, 127.3 g·L^-1 and 24.3 g·L^-1·h^-1 respectively, were obtained at 50 °C and with an initial supplementation of 2.17 U of Celluclast^® per gram of dried solid in fed-batch mode. This process was carried out with a liquid to solid ratio of 4.3 mL·g^-1 solid, by adding 15 g of fermented solid and 13.75 g of fresh solid at the beginning of the hydrolysis, and then the same amount of fresh solid 3 times every 2.5 h. By this procedure, ESBPP can be used to produce a generic microbial feedstock, which contains a high concentration of monosaccharides.

Keywords: enzymatic hydrolysis; generic microbial feedstock; solid-state fermentation; sugar beet; sugars hydrolysate.

Figure 1

Schemes of the enzymatic hydrolysis of exhausted sugar beet pulp pellets (ESBPPs) by crude enzyme extracts obtained by solid-state fermentation (SSF) (A) and by the addition of fermented solid (B).

Figure 2

Evolution of reducing sugar (RS) concentration through hydrolysis time fresh ESBPPs in 300 mL of citrate-phosphate buffer (pH 5, 0.05 M). Effect of temperature (T) with various fermented to fresh solid ratios (FFRs): (a) 5:30, (b) 15:15, and (c) 30:0.

Figure 3

Evolution of RS concentration through hydrolysis time of fresh ESBPPs in 300 mL of citrate-phosphate buffer (pH 5, 0.05 M) at 55 °C with various fermented to fresh solid ratios. FFRs of (A) 5:30 (star), 10:30 (triangle), and 15:30 (square) and (B) 15:30 (star), 15:35 (triangle), 15:40 (square), 15:45 (circle), 15:50 (diamond), and 15:55 (cross) are shown. Lines indicate theoretical values obtained from the kinetic model and symbols represent experimental values.

Figure 4

Evolution of RS concentration over hydrolysis time of fresh ESBPPs in 300 mL of citrate-phosphate buffer (pH 5, 0.05 M) at 50 °C with an FFR of 15:55 and with addition of Celluclast^® (0, 0.72, 1.44, 2.17, and 2.89 U·g⁻¹ of dried fresh ESBPPs). Lines indicate theoretical values obtained from the kinetic model and symbols represent experimental values.

Figure 5

Evolution of RS concentration over hydrolysis time of fresh ESBPPs in 300 mL of citrate-phosphate buffer (pH 5, 0.05 M) at 50 °C with addition of Celluclast^® (2.17 U·g⁻¹ dried fresh ESBPPs). (A) Fed-batch hydrolysis with a total FFR of 15:55. (B) Fed-batch hydrolysis with a total FFR of 15:90. Lines indicate theoretical values obtained from the kinetic model and symbols represent experimental values.

Figure 6

Evolution of reducing sugars concentration (circle), glucose (triangle), and the mixture arabinose and galactose (diamond) in enzymatic hydrolysate.