Recent advances in rapeseed meal as alternative feedstock for industrial biotechnology

Highlights

• Pretreatment and fermentation using rapeseed meal have greatly developed recently.

• Product and fermentation type determines the role of rapeseed-derived compounds.

• Future efforts should focus on scale, integrated biorefining and techno-economics.

Abstract

Rapeseed meal (RSM) have high potential as an important alternative feedstock in industrial biotechnology. Recently, the pace of development for RSM processing and fermentation have increased significantly. A wide variety of pretreatment and hydrolysis methods have been developed for media production; including fungal pretreatment, enzymatic hydrolysis, acidic hydrolysis, autohydrolysis as well as microwave. Meanwhile, RSM has been applied to a wide variety of fungal, bacterial and microalgal fermentation schemes including, solid-state, semi-solid-state and submerged fermentation. As a result, a myriad of products has been derived from RSM, including enzymes, antimicrobials, bioactive compounds, platform chemicals, biosurfactants and biopolymers. The role of RSM within these fermentation schemes has also evolved, from providing a simple matrix for solid state fermentation, to being the main source for nitrogen, micronutrients and carbon. RSM is also promising due to potential economic and environmental advantages that could be gained from its use in integrated biorefining. This may include fermentation of other rapeseed-derived by-product streams such as glycerol or rapeseed straw, the fractionation of RSM to produce additional high-value products including protein isolates and phenolic extracts, as well as process integration with existing rapeseed oil refining or biodiesel production. Quantification of economic and environmental benefits of using RSM over purified substrates will need to be conducted in the future, via technoeconomic and life cycle analysis. Other knowledge gaps such as the feeding of RSM-derived media, improving process performance, scale-up related challenges also will need to be addressed. This will be crucial for the realization of RSM’s potential in industrial biotechnology and may provide insights for development and commercialization of alternative feedstocks in general.

Introduction

Rapeseed (Brassica napus) is a major source of vegetable oil. Significant increase in rapeseed cultivation occurred during the beginning of this century, with an average annual increase of 4.9%. While a lot of rapeseed oil is used in the food industry, this increase was largely driven by the demand of rapeseed oil for biodiesel production, especially in the EU [1], [2]. This led to a peak production of 75 million metric tons of rapeseed in 2017/18 [3]. In recent years, the EU’s demand of rapeseed oil in biodiesel has been reduced by competition from soybean oil methyl ester and palm oil methyl ester [2]. Nevertheless, with current annual consumption of approximately 70–71 million metric tons, rapeseed remains an important oil crop globally [3].

The principal by-product of the industry is rapeseed meal (RSM), which is the post-pressing residue, produced at 39 million metric tons per annum [3]. Currently, RSM is mainly use as animal protein feed [4]. However, direct usage of RSM is limited by other components of the meal. These antinutritional components, including glucosinolates, phenolics, phytates and lignocellulosic fiber, could negatively impact protein solubility and digestibility or lead to formation of toxic compounds [4]. This has limited both the types of animals that can be fed RSM, as well as the proportion of RSM in the total feed. For example, while the complex digestive system of ruminants can tolerate RSM [5], its can only make up to 50% of pig feed and is not recommended for poultry [6]. Such limitations have kept the price of RSM low compared to the more preferable soybean meal, with RSM typically being 100 USD per ton lower in price [3].

Alternatively, RSM could potentially be valorized as a feedstock for industrial biotechnology. RSM contains an extremely high amount of protein (~35% w/w) as well as significant amounts of lignocellulosic material (~12% w/w) [7]. Using RSM-derived media could lead to improved commercial viability of bioproducts, especially those which still rely on purified substrates [8]. The high protein content, coupled with the aforementioned high availability and competitive price versus soybean meal have garnered RSM a significant amount of interest as an alternative source of nitrogen. In many such cases the RSM also acted as a source of micronutrients. The lower portion of lignocellulose typically mean that RSM could not act as a sole source of carbon, especially for the production of bio-products which are secondary metabolite and requires nitrogen-limited conditions.

RSM proteins also have value in itself if isolated for human consumption [4]. Post-protein extraction RSM could have higher proportion of lignocellulose and may have improved potential as a carbon source. RSM also contains other potentially valuable fractions which could be valorized such as phenolics (approximately 5000–7000 μg of sinapic acid equivalents per g) which have potential value due to its antioxidative properties [7], [9] and phytates which could be a valuable source of phosphates for agriculture or biotechnology [10]. Therefore, future potential exists for a whole-crop biorefining scheme based on RSM, involving extraction of valuable compounds followed by fermentation of the residues.

To the best of our knowledge, the last review on the application of RSM in biotechnology was conducted by Lomascolo et al. on both RSM and sunflower meal [7]. At the time, RSM was still largely applied in enzyme production. The pace of progress in RSM valorization had quickened in recent years, with a noticeable increase in the number of publications from 2017 to the present.

In this review the progress made since 2012 is illustrated, as the studies of RSM have expanded to a much greater range of processes, producer organisms and product types, including antimicrobial compounds, biosurfactants, and biopolymers. Fermentation studies which solely focused on improving the feeding values of the RSM are not included. This is because while extremely prevalent within literature, these studies are geared towards the traditional use of RSM as animal feed. In this review various aspects of these recent developments are discussed, including the impact of various pretreatment, hydrolysis and fermentation types, and the role which RSM plays. Areas for future work, such as important knowledge gaps, fields of research with high potential as well as avenues of integrated biorefining are also discussed. Overall, this review highlights the large degree of potential benefit that RSM valorization can offer to a wide array of industrial biotechnological processes in the future.