Kinetic Characterization of Enzymatic Hydrolysis of Apple Pomace as Feedstock for a Sugar-Based Biorefinery

Abstract

The enzymatic hydrolysis of cellulose from biomass feedstock in the sugar-based biorefinery chain is penalized by enzyme cost and difficulty to approach the theoretical maximum cellulose conversion degree. As a consequence, the process is currently investigated to identify the best operating conditions with reference to each biomass feedstock. The present work reports an investigation regarding the enzymatic hydrolysis of apple pomace (AP). AP is an agro-food waste largely available in Europe that might be exploited as a sugar source for biorefinery purposes. A biomass pre-treatment step was required before the enzymatic hydrolysis to make available polysaccharides chains to the biocatalyst. The AP samples were pre-treated through alkaline (NaOH), acid (HCl), and enzymatic (laccase) delignification processes to investigate the effect of lignin content and polysaccharides composition on enzymatic hydrolysis. Enzymatic hydrolysis tests were carried out using a commercial cocktail (Cellic®CTec2) of cellulolytic enzymes. The effect of mixing speed and biomass concentration on the experimental overall glucose production rate was assessed. The characterization of the glucose production rate by the assessment of pseudo-homogeneous kinetic models was proposed. Data were analysed to assess kinetic parameters of pseudo-mechanistic models able to describe the glucose production rate during AP enzymatic hydrolysis. In particular, pseudo-homogeneous Michaelis and Menten, as well as Chrastil’s models were used. The effect of lignin content on the enzymatic hydrolysis rate was evaluated. Chrastil’s model provided the best description of the glucose production rate.

Bioreactor and Bioprocess Design Issues in Enzymatic Hydrolysis of Lignocellulosic Biomass

“…Other attempts to adjust the long-term prevision of sugar production dynamics and yields included the use of product inhibition [14,15] and biomass recalcitrance [16] as additional terms of the kinetic model. In addition, diffusion limited models, such as Chrastil's model [17] (see equation in Table 1), can be safely applied if the limit conversion (maximum sugar concentration) is provided by bench scale tests [18,19]. This type of model is aimed at describing the effect of substrate structure on the rate of glucose production, so, in addition to the kinetic parameter k, the n parameter describes the effect of soluble species diffusion into the porous structure of the biomass (n→1 negligible diffusion limitation, 0 < n < 1 diffusion limited regime).…”

“…As an example, the selection of proper mixing conditions is hardly considered among the operational parameters that influence the validity of bench-scale hydrolysis tests, and Gan et al [21] observed how most of the studies ignore the effects of external mass transfer on the overall sugar production rate. Pratto et al [18] and Procentese et al [19] proved that the mixing rate should be selected first since the initial glucose production rate (over 60 min) is highly affected by the mixing speed. They assessed the threshold value of mixing speed, above which a constant glucose production rate was observed.…”

“…Then, the effect of substrate and enzyme concentrations on the initial rate and long term dynamics was characterized at a fixed mixing speed that was slightly larger than the threshold value. These values were about 200 rpm for hydrothermally pretreated sugarcane straw [18] in mixed Erlenmeyer flasks, and varied between 180 and 300 rpm for apple residues [19] processed in a stirred bench scale glass vessel (250 mL), depending on the delignification pretreatment applied.…”

“…Catalysts 2021, 11, x FOR PEER REVIEW 2 of 19 and is currently the benchmark technique, in sugar-based biorefinery, to perform a selective hydrolysis of the glucosidic bonds and minimize the formation of by-products [1][2][3] (Figure 1). Access of cellulolytic enzymes to polysaccharides is hindered by the lignin fraction of the biomass.…”

Ultrasonically-Assisted Dissolution of Sugarcane Bagasse during Dilute Acid Pretreatment: Experiments and Kinetic Modeling

“…Enzymatic hydrolysis is the process of using enzymes to break down cellulose into soluble glucose, which can then be used to produce paper, cotton textiles, and biofuels such as ethanol [16]. Enzymatic hydrolysis strongly depends on the operating conditions of the process (such as temperature, solids loading, enzyme loading, and pH), enzyme consumption (specific activity, enzyme recycling strategies, and stability), and the effect of product inhibitor on enzyme catalysis [17]. Normal enzymatic hydrolysis applied directly to raw lignocellulosic biomass is not effective in extracting the cellulose as the lignin remains intact and unaffected by the process [18].…”