KEYWORDS
Antinutritional factors, Aspergillus niger, Aspergillusoryzae , canola meal, protein digestibility, solid-state
fermentation
1 INTRODUCTION
Canola meal, leftover from oil extraction process is rich in protein
(~36-39%) and total dietary fibre
(~33%) and is typically used as animal feed. The meal
byproduct is prepared using a conventional process involving solvent
extraction (e.g., hexane extraction (HE), 60°C) and a desolventization
step (110°C) (Kalaydzhiev et al., 2019), or a cold pressing (CP) process
involving mechanical crushing of seeds at low temperatures (50-60°C)
(Febrianto et al., 2011). The former achieves higher levels of oil
extraction, however, leaves the remaining meal protein severely heat
treated, whereas the latter achieves sub-optimal oil extraction but
leaves proteins of higher quality in the meal. In addition to having
different compositions and physicochemical properties, the two meal
types have been reported to vary in their protein digestibility
properties (Kasprzak et al., 2016).
In general, proteins of canola meal have a favorable pattern of
essential amino acids and are considered a suitable feedstock for
further processing for human consumption. However, food uses of canola
meal are limited due to naturally present compounds such as phytic acid,
sinapine, phenolic acids and polyphenols (Croat et al., 2017). These
compounds can lead to poor protein digestibility, interfere with mineral
absorption, and give undesirable colour and taste attributes (Wu &
Muir, 2008). Phenolic compounds are the major antinutritional compounds
found in canola meals with levels of ~5 mg gallic acid
equivalents (GAE)/g dry meal in common canola meals, which is almost
30-times higher than in soybean meals (Shahidi & Naczk, 1992). Thus, it
is necessary to further process canola meals to minimize the levels of
or eliminate these phytochemicals to make the meals more suitable for
human consumption.
Solid-state fermentation (SSF) is a clean label process where there is
nearly no free water in the solid substrate, making it acceptable for
fungi growth. Recently, researchers have employed SSF of canola meals
with various fungal strains to achieve significant reductions in
glucosinolates (Shi et al., 2015), thiooxazolidones, phytic acid
(El-Batal & Abdel, 2001), total phenolic compounds, and neutral
detergent fibre (NDF) (Pal Vig & Walia, 2001) levels, with an
enhancement in the protein content. Furthermore, Shi et al. (2015) found
improved amino acid in-vitro digestibility after A. niger SSF on a
composite substrate containing rapeseed cake and wheat bran. Hyphae can
penetrate into structural matrices to loosen them and to allow higher
utilization of nutrients entrapped in the matrices by fungi. Canola
meals contain ligno-cellulosic materials and other hard-to-digest
matrices that are hardened by the heating process during oil extraction
of canola.
The direct comparison of the effect of SSF on the nutritional attributes
of differently processed meals (CP and HE) has not yet been
investigated. In this study, SSF with two types of fungal strains
(Aspergillus niger NRRL 334 and Aspergillus oryzae NRRL 5590, both have
generally recognized as safe (GRAS) status) was used to ferment CP and
HE extracted meals to different times to improve their nutritional
value. It was hypothesized that SSF would induce protein hydrolysis and
produce canola meals with increased protein content, decreased levels of
antinutritional compounds, and improved protein digestibility with
variations as a function of fungus and meal type.