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.