1 INTRODUCTION
Canola meal, the by-product of the
canola oil extraction process, is gaining tremendous interest in the
food industry due to its high protein content (35%-45%) and low cost.
The use of canola meals as an alternative protein source can provide
more protein options to the plant-based food industry. Canola proteins
are dominated by two major proteins, a salt-soluble globulin protein
(cruciferin, 12S , MW ~300 kDa) and a smaller
water-soluble albumin (napin, 1.7-2S , MW ~14–17
kDa). The amino acid composition of canola meal is well balanced with a
relatively high protein efficiency ratio (PER) of 2.64 and therefore can
be used for human nutrition (Aider & Barbana, 2011). In recent
research, canola proteins have presented high solubility, foaming
capacity and stability, and comparable or moderate emulsifying
properties compared to other plant proteins such as soybean and pea
protein (Chang et al. , 2015; Cheung et al. , 2014; Khattab
& Arntfield, 2009; Tan et al. , 2011a; Wu &Muir, 2008). These
characteristics make canola protein a potential ingredient for the food
industry.
Canola oil can be extracted by hexane extraction (HE) or cold-pressing
(CP). The conventional way, HE, is a chemical process involving the use
of non-polar solvents, e.g., hexane. During HE, the seeds are ground up
and washed with hexane under controlled conditions to release oil
molecules stored within. A desolventizing step is followed to remove the
hexane by distilling at approximately 100-110oC, after
which the solvent is recycled and reused (Cheng et al., 2019). The HE
process efficiently produces a high yield of oil, however, the use of
chemicals and the application of heat lead to considerate damage to the
proteins within the meal, resulting in reduced functionality (e.g.,
emulsifying properties) (Östbring et al., 2019). The CP technique, on
the other hand, solely utilizes mechanical forces (e.g., crushing)
without the application of organic solvents or high heat for their
subsequent removal. The seeds are pressed at low temperatures
(<40oC), leaving nutrients in the remaining
meal in a less disturbed form compared to HE, however, the oil yield was
lower in HE with a high level of residual oil (6-20%) (Hickling, 2007;
Östbring et al., 2019).
Fermentation is the process accomplished by the metabolism of
microorganisms that catalyze nutrients, synthesize secondary
metabolites, and complete other physiological activities under anaerobic
or aerobic conditions. Solid-state fermentation (SSF) is a popular
process to modify the functionality and nutritional composition of
protein ingredients. As opposed to submerged fermentation (SmF), there’s
no free water in SSF, making it suitable for microorganisms that don’t
require high moisture or high water activity to grow, for example,
fungi. Recently, SSF has been applied to canola meals to improve their
quality. Pal Vig and Walia (2001) used Rhizopus oligosporus as
the fermenting culture to produce a high-protein product from HE canola
meal. The results showed a significant decline in contents of
glucosinolates (GLS) (~43%), thiooxazolidones
(~31 %), phytic acid (~42%) and fibre
(~26%) along with a ~65% increase in
crude protein after 10 days. Croat et al. (2016) studied the use
of SSF on HE and CP canola meals to modify their nutritional
composition. The strains Trichoderma reesei , Aspergillus
pullulans , and A. pullulans improved protein content by 22.9,
16.9 and 15.4%, while reducing the total GLS content from 60.6 to 1.0,
3.2 and 10.7 μmol/g, respectively. Significantly higher content of dry
matter yield was reported for the HE meal compared to the CP meal, which
is mainly due to the high oil residues left by CP. Much work has been
done on enhancing the nutritional quality of canola meals, however, the
use of SSF to alter the functionality of proteins in HE or CP canola
meals has been limited.
Several methods, including alkaline extraction-isoelectric precipitation
(AE-IP), salt extraction-dialysis (SE), protein micellation method
(PMM), and low pH extraction combined with membrane separation and
ultrafiltration (UF), have been widely used for the production of canola
protein isolates (Tan et al. , 2011a, 2011b; Wanasundara, 2011).
The structural composition and functionality of the canola proteins may
vary significantly depending on the extraction method used (Can Karacaet al. , 2011; Hoglund et al. , 1992; Wu & Muir, 2008). In
addition, the extraction of canola proteins is especially difficult due
to the differences in protein fractions (widely differing isoelectric
points of pH 4-11 and molecular weights of 13-320 kDa) and the presence
of antinutrients (GLS, phytic acid, and polyphenols), pigments, and
fibre in the canola meal (Wu & Muir, 2008). It is expected that
pre-treatment, such as fermentation, and specific purification processes
may improve protein solubility in the meals and consequently protein
extractability.
In our previous study, both CP and HE canola meals had a
~45% degree of protein hydrolysis upon 72 hours of
fermentation with Aspergillus niger NRRL 334 andAspergillus oryzae NRRL 5590. In this study, those fermented
canola meals were used in the wet fractionation process to produce
protein products. Two wet extraction processes, AE-IP and SE, were
employed. The resulting protein products were characterized for select
functional properties and compared with protein products extracted from
meals that were not pre-treated by fermentation. We hypothesize that
pre-treatment with SSF has the potential to improve the protein
extractability from canola meal because of partial protein hydrolysis
and the loosening of protein-carbohydrate interactions while also
modifying protein functionality. Moreover, the CP meals are expected to
yield protein products with higher functional properties than those
processed by HE due to the lack of heat and chemical damage to the
proteins. The findings of this work will provide a relatively
comprehensive view of the effect of SSF of canola meals on the
functionality of extracted proteins with direct comparisons between the
test fungal strains, meal type, and protein fractionation techniques.