1. Introduction
Aqueous enzymatic extraction is a promising method for extraction of oils and proteins from oilseeds, in which water is used as the extraction solvent (Liu, Gasmalla, & Li, 2016). Compared with the traditional oil extraction process, aqueous enzymatic extraction has the advantages of being environmentally friendly, nonuse of organic solvents, low energy consumption, and mild reaction conditions (Li, Qi, & Sui, 2016; Yusoff, Gordon, & Niranjan, 2015). Furthermore, peanut oil requires low degree of refinement and peanut protein can be recycled at the same time (Campbell & Glatz, 2011; Latif, Anwar, & Hussain, 2011; Balvardi, Rezaei, & Mendiola, 2015). Cell wall degrading enzymes, such as cellulase, hemicellulase, and pectinase, can degrade the main components and destroy the structure of the cell wall without affecting the peanut protein. Bisht et al. (2015) showed that cellulase, hemicellulase, and pectinase alone or their complex enzymes could effectively increase the oil yield at appropriate concentrations; e.g., the combination of cellulase and pectinase increased the oil yield by 14.22%. Szydłowska-Czerniak et al. (2010) reported a higher yield of rapeseed oil extracted by pectinase and cellulase compared with the yield obtained by traditional methods, with pectinase having better effect on extracting rapeseed oil than cellulase. The degradation of the peanut cell wall in aqueous enzymatic extraction is a critical step that facilitates the release of peanut proteins and oil bodies. Viscozyme® L, a compound cell wall degradation enzyme, promotes the release of nonhydrolyzed protein and oil bodies by degrading the main components and destroy the structure of the cell wall without affecting the peanut protein (Zúñiga, Soto, & Mora, 2003; Gaur, Sharma, & Khare, 2007), but enzyme active sites of Viscozyme® L and the mechanism of degradation of the peanut cell wall in aqueous enzymatic extraction remain unclear.
Fourier-transform infrared spectroscopy (FT-IR) has been applied to monitor the extraction of cell wall polysaccharides and to observe the changes in the cell wall during the processing and quality control of fruits and vegetables (Barros, Mafra, & Ferreira, 2002; Ferreira, Barros, & Coimbra, 2001). The optimal region of FT-IR spectrum used for carbohydrate analysis is 1200 - 850 cm-1 and 1800 - 1200 cm-1; the former region is not affected by the spectrum bands of proteins and water molecules (Coimbra, Barros, & Barros, 1998). The region 1200 - 850 cm-1 mainly reflects the stretching and vibration characteristics of C-O, C-C, and CH2 groups or the ring structure formed by these groups (Fry, 1988). To obtain relatively complete cell wall information, both regions, 1800–1200cm-1 and 1200–850cm-1, were used for FT-IR spectrum analysis in this study. Principal component analysis (PCA) is a common data statistics method used to simplify and analyze highly dimensional data sets by constructing principle components (PCs) that explain the maximum variability of the data. PCA is particularly suitable for analysis of the infrared spectrum characteristics in relation to sample diversity and complexity (He, 2015). FT-IR spectroscopy combined with PCA is very useful for determining structural and compositional changes in the cell wall (Ferreira, Barros, & Coimbra, 2001; Ana, Encina, & Penélope, 2004), for assessing the degree of amidation and methyl esterification of pectic polysaccharides in plant cell wall extracts (Gnanasambandam & Proctor, 2000; Winning, Viereck, & Salomonsen, 2009), and for evaluating cell wall polysaccharides composition of pectic and hemicellulosic components derived from plant materials (Hori & Sugiyama, 2003).
In the present study, the changes in cellulose, hemicellulose, and pectin content in the peanut cell wall hydrolyzed by Viscozyme® L were examined under different solid-liquid ratios, enzyme concentrations, enzyme hydrolysis temperatures, and enzyme hydrolysis times. The characteristic FT-IR absorption bands of cellulose, hemicellulose, and pectin in the cell wall of peanut were analyzed by PCA to explore the key sites of Viscozyme® L activity on peanut cell wall during enzymatic hydrolysis. The mechanism of cell wall enzymolysis studied in this paper will provide theoretical basis for further explorations of the mechanism of aqueous enzymatic extraction and help to enhance the extraction of peanut protein and oil bodies.