2.4. Enzyme activity and hydrolysis
Two commercial enzymes and four enzymes, produced in-house from A. flavus, P. funiculosum, P. verruculosum, and T. longibrachiatum, were evaluated. The protein concentration was measured by the Lowry method using bovine serum albumin as standard [20]. Enzyme activities were assayed with specific substrate solution, 50 mM sodium citrate buffer pH 4.8 (at 37 °C for 30 min). Endoglucanase (CMCase) and exoglucanase (Avicelase) activities were determined with 1% carboxymethylcellulose and microcrystalline cellulose (Avicel) as substrate, respectively. Xylanase activity was determined by the same procedure described for endo- and exo-cellulase, but using beechwood xylan (Sigma) as substrate. Polygalacturonase acitivity was measured with a 0.5% polygalacturonic acid (Sigma) in 50 mM sodium citrate buffer (pH 4.8) at 37 °C for 5 min. Reducing sugars were quantified with DNS (dinitrosalicylic acid) and the absorbance was measured at 540 nm by modified Miller [21] using Thermo Scientific Multiskan EX (Thermo Fisher Scientific, Finland). One unit of activity was defined as the amount of enzyme required to release one μmol of glucose, xylose or galacturonic acid per min. Specific activities were expressed as enzyme units per milligram of protein.
Commercial and extracellular enzymes were added to okara at concentration of 40–140 ug protein/g okara. Enzymatic hydrolysis was performed on 1% (w/v) dry matter with 50 mM sodium acetate buffer (pH 4.8) at 180 rpm for 72 h at 37 °C. Optimization of enzyme loading volume and time course change okara during enzymatic hydrolysis were measured using colorimetric method (DNS) for determination of sugars. Based on a small scale study, the enzymatic hydrolysis process was scaled up to a 5 L bioreactor (1% dry matter).