The enzyme extract was obtained from 20 g of potato (Solanumtuberosum L., Monalisa variety) with 100 ml of buffer solution pH7 (0.1 M phosphate-citrate buffer). After 2 min of grinding in ablender, the mixture was filtered (by cotton) and centrifuged(15 min, 4 C, 3200g). The crude enzyme extract was used as theenzyme source, with the soluble protein content estimated in mgof albumin (Lowry, Rosenbrough, Farr, & Randall, 1951). The peroxidaseenzyme activity was determined using 0.2 ml of enzyme extract,1 ml of 30 mM H2O2, 2 mL of a 5 mM guaiacol solution, withthe final volume of the tube being completed to 4 ml with bufferpH 7, and the reaction absorbance detected at 470 nm after10 min of reaction at 30 C. The polyphenol oxidase activity wasdetermined using 1 ml of enzyme extract, 2 mL of a solution of10 mM catechol, 1 mL of buffer pH 7 with the absorbance reactiondetected at 425 nm after 10 min of reaction at 30 C. The inhibitoryeffect of phenolic compounds extracted from rice bran and fermentedrice bran (96 h) in the activity of these enzymes was evaluatedusing different concentrations of the inhibitor. The final pHof the reaction was adjusted at 7 by the addition of a solution of0.1 M NaOH.The inhibition mechanism of phenolic compounds on theperoxidase enzyme was also evaluated by the km and Vmax parameters.Different concentrations of the substrate (guaiacol) wereused in the enzymatic reaction with the addition of phenolic extractsolutions. The results were plotted according to Lineweaver
& Burk (1934) graphic method.
2.6. Statistical analysis
One-way Analysis of Variance (ANOVA) test was used to determine
significant differences between variables. Differences with a
probability value of <0.05 were considered significant and all data
were reported as mean ± sd.
3. Results and discussion
3.1. Biomass and phenolic content
After fermentation time of 48 h, there was not detected a significant
increase in phenolic content, whereas the fungal biomass
demonstrated an important increased until 96 h of fermentation
(Fig. 1). The glucosamine, a constituent of chitin, an insoluble linear
polymer composed of a-1,4 acetylglucosamine bonds, was determined
to estimate the multiplication in fungal SSF (Schmidt & Furlong,
2012). At 96 h, 8.8 mgglucosamine/g were obtained from
fermented biomass, showing that the R. oryzae fungus can grow
using rice bran as a carbon source.
The phenolic compounds content at the beginning of fermentation
was of about 2.4 mg/g and at the end of 120 h was of 5.1 mg/g,
resulting in an increase of over 110% (Fig. 1). Rice phenolics include
derivatives of benzoic and hydroxycinnamic acids, mainly ferulic
acid and diferulates. These are commonly present in a chain form,
and are normally components of complex structures such as
hydrolyzable tannins and lignins, and linked to the cell wall structural
components such as cellulose, lignin and proteins by ester
linkages (Zhang et al., 2010). The more soluble phenolics are compartmentalised
within the cell vacuoles, and they are in free or
conjugated form, while the insoluble phenolics are connected to
structures in the cell walls, esterified with arabinose or galactose
residues of hemicellulose or pectic components (Mira, Massaretto,
Pascual, & Marquez, 2009; Mira et al., 2008).
There are two ways in which phenolic compounds can be
formed; from the decomposition of the linkages between lignin,
cellulose and hemicellulose or by producing a part of rice bran
oil (Pourali et al., 2010). In the case of rice bran fermentation, the
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