Controlled fermentation of rapeseed presscake by Rhizopus, and its effect on some components with relevance to human nutrition

Highlights

• Rapeseed presscake was fermented by Rhizopus oligosporus.

• Relevant proportions of polyphenols, glucosinolates, and some polysaccharides are degraded.

• Quality and safety of final product depends on fermentation time and is affected by structure.

• Monitoring the fermentation process is necessary and feasible.

Abstract

The use of rapeseed protein could contribute to meeting the increasing demand for plant proteins with high biological value in human nutrition. In order to make rapeseed presscake fit for human consumption, the presscake was fermented by using the tempeh mould, Rhizopus microsporus var. oligosporus. Fermentation was satisfactory at initial levels of added acetic acid of 40–60 mmoles/Kg, a_w of 0.97, pasteurization, surface inoculation and incubation at 32 °C and 90–95% relative humidity. It was crucial to stop the fermentation once the mould had grown and metabolized sufficiently but before a major rise in pH and subsequent growth of acid-sensitive sporeforming bacteria occurred. Some degradation of glucosinolates, cell wall polysaccharides and phenolic compounds was found, but there was some evidence that growth and metabolism of the mould also depended on the texture of the presscake, as these factors affect the oxygen supply to the mould. In conclusion, it is possible to ferment rapeseed presscake by using the “Tempeh starter” Rhizopus oligosporus, but in order to use the resulting product to enrich various foods with protein or replace other proteins, the degree of degradation of undesired compounds should be further standardized, especially by the control of the pH, oxygen supply, and fermentation time.

Introduction

High amounts of rapeseed presscake are produced as a consequence of the increasing demands for rapeseed oil for use in human nutrition or biofuel production. In the European Union (5 years average 2012–2017), 21.0 million tons of rape were produced on about 6.5 million hectares, and 23.8 million tons were crushed and processed into 9.6 million tons of oil and 13.5 million tons of meal (European Commission, 2017). Assuming a level in rapeseed meal of 33% of crude protein (von der Haar, Müller, Bader-Mittermaier, & Eisner, 2014), this means that as much as 4.5 million tons of crude rapeseed protein are produced in the European Union annually.

At present, rapeseed presscake (produced with or without solvent extraction) is mainly used for animal feed or outside the food/feed sector. However, rapeseed protein also has potential to be used in human nutrition because of its favourable amino acid composition, biological value, and high digestibility (Aider & Barbana, 2011; Barth & Metges, 2007; Campbell, Rempel, & Wanasundara, 2016; Fleddermann et al., 2013; Tan, Mailer, Blanchard, & Agboola, 2011). On the other hand, rapeseed meal and presscakes contain many undesired compounds that negatively affect the nutritive value and sensory properties. These include

• Phenolic compounds, which may interfere with protein absorption and digestibility and deliver a bitter and/or astringent taste to the meal (Naczk, Amarowicz, Sullivan, & Shahidi, 1998)

• Glucosinolates, which also convey a bitter taste to the foods. Compounds generated from the degradation of glucosinolates, especially thiocyanates, isothiocyanates and nitriles formed by myrosinase, react with proteins and reduce their digestibility (see von der Haar et al., 2014, for review) and have negative (e.g. goitrogenic) physiological effects (Heaney & Fenwick, 1995). Even though there is evidence for positive effects of some of these compounds (see e.g. Vig, Rampal, Thind, & Arora, 2009; Watzl, 2001), their presence in rapeseed products should be minimized, to ensure use for multiple foods.

• Oligosaccharides (mainly stachyose, raffinose), which cause flatulence (Siddiqui & Wood, 1977)

• Phytate, which interferes with mineral absorption (Nair & Duvnjak, 1991)

Various studies have been performed to remove undesired compounds from rapeseed meal. Some of these compounds, such as phenolics, mainly occur in the husks. Their levels can be lowered by a dehulling process before pressing (Raß & Schein, 2017). From presscakes, undesired compounds may be removed by various extraction procedures (see Krause, Kroll, & Rawel, 2007; von der Haar et al., 2014; Chéreau et al., 2016, for reviews). Extraction by aqueous alcoholic solutions has been optimized in order to minimize protein denaturation (Pickardt et al., 2010). A patent was granted for “Soluble canola protein isolate production”, the process involving the aqueous extraction of defatted rapeseed meal (Schweizer, Green, Segall, & Logie, 2014, 2017; Schweizer, Segall, Medina, Willardsen, & Tergesen, 2007). The protein isolate was positively evaluated by EFSA (2013), granted GRAS status in the US, and approved as a “novel food” in the EU. However, to date, products containing these protein isolates are, to our knowledge, not yet marketed to a significant extent.

The main drawbacks of the processing methods mentioned are that they only work with rapeseed meal with a low oil content (< 3%) which can be obtained by solvent extraction only (Natsch, 2006). For small, decentralized oil mills that have no extraction facilities or other necessary, but expensive, equipment available, this is a major obstacle to their application. Moreover, standards for the organic sector do not permit solvent extraction. These disadvantages may be overcome by fermentation methods.

Given the wide array of fungal metabolic activity, fungal fermentations appear to be promising for not only removing phenolic and other complex secondary plant metabolites from rapeseed meal, but also for a possible bio-fortification of substrates through the formation of proteins and vitamins, as well as compounds with positive health effects (Sutter, Thevenieau, Bourdillon, & De Goninck, 2017). Some strains of white-rot fungi were found to degrade sinapic acid (Żuchowski, Pecio, Jaszek, & Stochmal, 2013), and the degradation of glucosinolates by fungal strains has also been demonstrated (Croat, Berhow, Karki, Muthukumarappan, & Gibbons, 2016; Croat, Gibbons, Berhow, Karki, & Muthukumarappan, 2016; Shi et al., 2015; Wang et al., 2012). Rhizopus oligosporus / microsporus strains have a long history of use in Indonesia for manufacturing “tempeh”, from mostly soybeans (Astuti, Meliala, Dalais, & Wahlqvist, 2000; Hachmeister & Fung, 1993; Hesseltine & Wang, 1972; Nout & Kiers, 2005; Shurtleff & Aoyagi, 2011) and thus the strains may be regarded as food-grade. As discussed in the cited publications, there is ample evidence for the favourable effects of the fermentation on nutritional and health value of the resulting product (tempeh).

Various substrates other than soybeans may be fermented by Rhizopus (summarized by Nout & Rombouts, 1990; Hachmeister & Fung, 1993; Feng, 2006), particularly and mostly other legumes, but also some by-products from coconut and oilseed processing. Cereals have been fermented on an experimental and pilot scale (Berg, Eriksson, Olsson, Schnürer, & Svanerg, 2007; Cuevas-Rodríguez, Milán-Carrillo, Mora-Escobedo, Cárdenas-Valenzuela, & Reyes-Moreno, 2004; Feng, 2006; Hachmeister & Fung, 1993; Nowak, 1992) but apparently, the resulting products have not yet been commercialized. The fermentation of rapeseed presscake by Rhizopus was shown to reduce the levels of various undesired compounds (Bau et al., 1994; Rozan et al., 1996; Vig & Walia, 2001). Studies by Ahlert et al. (2008) showed that certain foods can be enriched with fermented rapeseed presscake. However, these studies did not provide information about the conditions for reproducible growth and metabolism of Rhizopus on rapeseed presscake.

The aim of the present study was to define conditions for the reproducible small-scale solid-state fermentation of rapeseed presscake by using Rhizopus, and to find simple methods for controlling this fermentation. The results may be useful in assessing both the prospects and limitations of this process for providing food-grade rapeseed proteins.