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.