Key words

cereal-based biorefineries; bioethanol; polyhydroxybutyrate; succinic acid; food waste

10.1 Introduction

Conventional cereal processes are mature industrial technologies that have been developed for the production of food or non-food products. Cereal processes are based on different fractionation schemes exploiting mainly dry and wet milling operations to break down the grain into various fractions with many end-uses (e.g., food, feed, chemicals, textiles, cosmetics, fermentation). Dry milling generally produces incomplete fractionation of grain components through physical processing involving mainly grinding and sieving unit operations. For instance, in traditional wheat milling processes, the main aim is to maximize the separation of bran from endosperm. Wet milling processes can be divided according to the type of solvent (i.e., aqueous and non-aqueous) used for selective separation of cereal grain components. The corn wet milling process constitutes an industrially mature technology that has been developed mainly in the United States for the production of a spectrum of products.

Corn refining started in 1848 by Thomas Kingsford who introduced the art of starch manufacture in a small firm of only 70 employees that by 1880 grew to become the largest company of its kind worldwide (Peckham, 2001). The success of this company led to the creation of new companies that benefited from the increasing demand for starch, as laundry aid and food ingredient, and corn syrups that were used in the candy, baking, brewing and vinegar industries (Peckham, 2001). During the twentieth century, corn refining evolved into a mature industrial process (Johnson, 2006). The wet milling process of corn fractionates the original grain into its components, namely starch, fibre, protein and oil. Starch is subsequently converted into a spectrum of products including bioethanol, dextrose, glucose syrups of 20–70 dextrose equivalent and high fructose corn syrups. NatureWorks LLC operates the only large-scale industrial facilities for the production of lactic acid from corn (180,000 t per year) and Ingeo polylactide resins (140,000 t per year) in Blair, Nebraska, USA (Vink et al., 2010).

Cereal-based biorefineries were among the first biorefining schemes – together with lignocellulosic, whole-crop, thermochemical-based and green biorefineries – proposed at the beginning of the current biorefinery era as alternative sustainable technologies to petroleum refineries (Kamm et al., 2006). The utilization of cereals as renewable resources for biorefinery development is dependent on the combination of physical, chemical, thermal and biological processing for the fractionation, extraction or (bio)conversion of the original grain and associated residues for the production of a spectrum of products including biofuels, chemical, biodegradable plastics, biomaterials, functional proteins, oils, antioxidants, polysaccharides, food and feed.

This chapter presents research carried out in the Satake Centre for Grain Process Engineering (SCGPE) at the University of Manchester (UK) on the development of cereal-based biorefineries for the production of fuel ethanol, polyhydroxybutyrate as a biodegradable polymer and succinic acid as a platform chemical for the future sustainable chemical industry. Future trends will also be presented based on the utilization of cereal-based food waste generated from cereal processors, bakeries, confectionary industries, households and restaurants that do not compete with food production.

10.2 Wheat-based biorefineries

Understanding the structure of wheat grain is important in order to comprehend the development of modern wheat milling operations for flour production and to design biorefinery strategies. The main parts of the wheat grain are the bran, the endosperm and the germ. Bran and germ are the terms used in traditional wheat milling operations. Bran is a generic term that includes all outer layers of the wheat grain that do not contain starch and gluten, the main components of endosperm. Germ is also a generic term to describe the embryo, the main parts of which are the embryonic axis and the scutellum. The bran is constituted by several layers including the outer and inner pericarp, the seed coat, the nucellar tissue and the aleurone layer. The aleurone layer surrounds the entire endosperm and part of the embryo. The bran fraction, including the aleurone layer, is approximately 17% of the whole grain weight on a dry basis. Detailed description of the structure of wheat grain is given by Evers and Bechtel (1988). Pomeranz (1987, 1988) and MacMasters et al. (1971) presented in detail the chemical composition of a typical wheat grain. Table 10.1 presents the composition of each major wheat grain fraction.

The viability of a whole-crop biorefinery based on wheat is dependent on the efficient fractionation of the grain and the production of added-value products with diversifying market outlets and improved production economics. Research at the SCGPE has focused on the production of, or the extraction of, added-value products from wheat bran, germ and part of the gluten, while the majority of the endosperm fraction has been used for the production of a generic fermentation feedstock. Koutinas et al. (2007a) presented a biorefining strategy that integrates the utilization of wheat as the sole raw material for production of both speciality and commodity products. The main target is the fractionation of wheat in order to maximize the products that could be produced either via extraction if they are present in one of the wheat layers or via microbial bioconversion employed for the production of bioethanol, biodegradable polymers or platform chemicals. In this way, minimization of waste production can be achieved.

The generic wheat-based biorefinery concept is presented in Fig. 10.1. The main focus of this biorefining concept is the fermentative production of a major commodity product from the major component of the wheat grain, namely starch, and part of the gluten depending on the requirements for nitrogen sources by the microorganism used in each case. Thus, the process presented in Fig. 10.1 has been modified accordingly for the production of fuel ethanol (Arifeen et al., 2007a,b), polyhydroxybutyrate (Xu et al., 2010) or succinic acid (Du et al., 2008). The other wheat components can be used for the extraction or production of commodity or speciality products with diversified market outlets.

The first unit operation employed in the process presented in Fig. 10.1 is pearling, in which bran layers are sequentially removed through friction and abrasion stages depending upon duration (Dexter and Wood, 1996; Gills and McGee, 1999; Koutinas et al., 2006). Pearling eventually produces two fractions, one rich in bran and the other rich in endosperm. Bran-rich pearlings can be used for the extraction of various components including functional foods, monosaccharides, ferulic acid, arabinoxylan, and germ-rich fractions (Koutinas et al., 2006). Pearled wheat grains that are enriched in starch and gluten are macerated with a hammer mill into flour which is subsequently used for the production of case-specific fermentation feedstocks and for gluten extraction. Gluten is a valuable co-product as it can be used in conventional applications (e.g., food industry) or as a raw material for the production of novel products, such as biodegradable plastics, edible films, adhesives, biomedical materials, composites, binders and raw material for the separation of amino acids as precursors for chemical production (Bietz and Lookhart, 1996; Kim, 2008; Koutinas et al., 2008; Zhang and Mittal, 2010; Lammens et al., 2012). The quantity of gluten that can be extracted from each alternative processing scheme is dependent on the requirement for nitrogen sources by the microorganism that is employed in the fermentation stage. For instance, succinic acid production is notorious for the high concentrations of yeast extract required for microbial growth and thus product formation. This means that in the wheat-based biorefinery concept, a significant quantity of gluten should be used as nitrogen supplement in the fermentation in order to eliminate the need for yeast extract.

A small fraction of the pearled wheat flour is used as the sole fermentation medium in submerged (preferably operated in continuous mode) or solid state fungal fermentations using a fungal strain of Aspergillus awamori for the production of crude enzyme consortia, predominantly amylolytic enzymes but with significant others such as proteases and phytase. In this way, the macromolecules (e.g., starch, gluten, phytic acid) contained in wheat can be hydrolysed into directly assimilable nutrients (e.g., glucose, amino acids, peptides). The crude filtrate from fungal fermentation is mixed with pearled wheat flour from which gluten has been entirely or partially extracted. Koutinas et al. (2007b) reported that sufficient starch hydrolysis to glucose can be achieved by following a temperature ramp process at temperatures lower than 70°C. In this process operated in batch mode, simultaneous gelatinization, liquefaction and saccharification can be achieved when enzyme consortia produced by A. awamori are employed.

The biorefinery concept presented in Fig. 10.1 exploits fungal autolysis as a natural process to generate nutrient supplements as substitute for commercial nutrients such as yeast extract. The fungal biomass that is produced during enzyme production can be autolysed under oxygen limiting conditions at 55°C and natural pH (Koutinas et al., 2004, 2005). Re-generating nutrients via fungal autolysis could replenish the nutrients consumed for fungal growth and also replace the nutrients lost during bran separation and gluten extraction. The glucose-rich solution produced via wheat flour hydrolysis could be mixed with fungal autolysate to produce fermentation media to suit the needs of many microorganisms. In the following sections, alternative processing schemes utilizing wheat grains or associated residues as raw material for biorefinery development will be presented.

10.3 Fuel ethanol production from wheat

The current energy supply relies mainly on fossil fuels, with oil, coal and gas contributing over 80% of the world energy consumption. The sustainable development of the society requires continuous supply of affordable renewable energy. One of the commercialized alternative biofuels is fuel ethanol production from plant-based biomass, e.g. corn, wheat or sugar cane. Although the debate of food versus fuel never settles, the worldwide bioethanol production has continued to increase over recent years (Fig. 10.2). In Europe, bioethanol is predominately produced from wheat. The wheat used in such first generation biorefineries is not generally of food grade. In fact, most first generation biorefineries use feed grade wheat, which is originally produced or imported for animal feed. In conventional wheat-based fuel ethanol production processes, starch is converted into bioethanol, while the protein and fibre fractions end up in the ‘dried distillers grains with solubles’ (DDGS). DDGS is sold as a valuable animal feed due to its high protein content. A conventional wheat-based biorefinery production process is shown schematically in Fig. 10.3.

The first step in bioethanol production is milling. The grain is milled either by wet milling or dry milling. In wet milling, wheat is soaked in a weak sulphurous acid solution at 48–52°C for about 2 days prior to milling. In comparison to dry milling, wet milling leads to relatively higher starch utilization efficiency. Then, the milled grains are cooked and starch hydrolysis enzymes, e.g. α-amylase, are added to break down the starch polymer to soluble dextrins (a process known as liquefaction). Cooking is also a way to sterilize the medium. Then, in the saccharification stage, glucoamylase is used to further hydrolyse dextrins into glucose. Saccharification could also be integated with fermentation, enabling ‘simultaneous saccharification and fermentation’ (SSF). In most bioethanol plants, the yeast strain Saccharomyces cerevisiae is most often utilized. The bacterial strain Zymomonas mobilis is also reported to be used in industrial-scale fermentations. After around 2–4 days of fermentation, the ethanol concentration could reach around 8–10 wt%. The fermentation step is followed by distillation, which separates the stillage for DDGS production. Distillation produces an ethanol stream that contains up to 95.6% ethanol. Fuel grade ethanol is nowadays most often produced via molecular sieve dehydration.

Research carried out at the SCGPE focused on restructuring the conventional wheat-based industrial process for fuel ethanol production through the development of a novel biorefinery concept (Fig. 10.1). Arifeen et al. (2007a) presented the development of a continuous process converting wheat into a nutrient-complete feedstock suitable for fuel ethanol production. As presented in Fig. 10.1, starch and fungal autolysates are mainly employed as fermentation media for ethanol production, while other major wheat components such as bran and gluten are extracted and processed for different end-uses. Starch liquefaction and saccharification have been integrated in a continuous process through the utilization of complex enzyme consortia produced by on-site fungal fermentations. Arifeen et al. (2007a,b) also showed that it is possible to integrate starch hydrolysis with fungal autolysis in a single reaction because the operating conditions employed for both processes are similar. In the case of fuel ethanol production, gluten separation is feasible due to the low nitrogen requirements of ethanol fermentation. Furthermore, the separation of bran and gluten from the original wheat grain results in the production of pure yeast cells after the end of fermentation that could be used as a much higher market value co-product compared to DDGS.

Arifeen et al. (2007a) estimated that a process producing 120 m³/h nutrient-complete fermentation feedstock for fuel ethanol production containing 250 g/L glucose and 0.85 g/L free amino nitrogen (FAN) would result in a production cost of $0.126 per kg glucose. This cost was estimated assuming a wheat cost of $0.16/kg. From 1997 to 2006, the average cost of starch hydrolysate produced by corn wet millers in the US was approximately $0.14 per kg glucose. Since 2006, the cost of cereal grains has become significantly more volatile, increasing the concern over the utilization of food crops for the production of biofuels.

Arifeen et al. (2007b) presented the optimization of the novel biorefining concept for fuel ethanol production from wheat using the equation-based software General Algebraic Modelling System (GAMS). The process flow sheet was designed in continuous mode and contained the following unit operations:

At production capacities in the range of 10–33.5 million gal per year (37.85–126.8 million L per year), the proposed wheat-based biorefinery could lead to a fuel ethanol production cost of $0.96–0.50 per gal ethanol ($0.25–0.13 per L ethanol). It should be stressed that the production cost of fuel ethanol is strongly dependent on the commercial value of the generated co-products (e.g., gluten, yeast and bran-rich pearlings). Bran-rich pearlings have been evaluated for the extraction of arabinoxylans (Du et al., 2009; Misailidis et al., 2009) and gluten has been enzymatically converted into a fermentation medium for the production of recombinant proteins (Satakarni et al., 2009). Wheat bran has also been employed for the extraction of ferulic acid, which can be converted into vanillin (Di Gioia et al., 2007).

Du et al. (2009) and Misailidis et al. (2009) developed a process for the extraction of arabinoxylans from wheat bran as a potential co-product in the wheat-based fuel ethanol plant (Fig. 10.4). Arabinoxylan (AX) is a component of wheat bran that could be used as a food ingredient for viscosity enhancement and gel formation. More promisingly, the extraction of AX uses only ethanol as the main extractant. This immediately suggests scope for economical AX recovery within a fuel ethanol production plant. Experiments showed that high arabinoxylans content was presented in the outer layer of the wheat bran (Du et al., 2009). A wheat bran sample which was produced by pearling to a level of 4% (w/w) contained around 27% AX (w/w). That was 50% more than the bran recovered by conventional roller milling of whole wheat. The economic assessment indicated that the cost of AX product (80% purity) was around £3.7–4.5 per kg in an integrated fuel ethanol and AX plant (Misailidis et al., 2009). If the selling price of AX could reach £6/kg, the overall return on investment (ROI) would increase from 17 to 26%. Alternatively, if the ROI was kept constant at 17%, the ethanol could be sold 14% cheaper. These results suggested that the development of AX co-production in a fuel ethanol biorefinery would increase the economic competitiveness and commercialization feasibility.

10.4 Succinic acid production from wheat

Succinic acid (SA) is a 1,4-dicarboxylic acid which has been demonstrated to be one of the key platform molecules to be transformed into a variety of useful chemicals (Beauprez et al., 2010; Bozell and Petersen, 2010; Cukalovic and Stevens, 2008; Lin et al., 2012). From esters to amides through pyrrolidones, alcohols and/or biopolymers, the rich chemistry of the two carboxylic groups within the molecule (Fig. 10.5) as well as its partial solubility in water are two key relevant assets from the list of building blocks. Increasing petroleum prices favour the fermentative production of SA as a replacement for petrochemical production from maleic anhydride. The current production of maleic anhydride is based on n-butane. The process requires high energy consumption due to demanding harsh operating conditions: high temperature (250°C) and pressure (200 bar). Therefore, low temperature and pressure bioproduction of SA could be economically favourable.

Research carried out at the SCGPE has focused on the development of wheat-based bioprocessing schemes for the production of succinic acid that integrate upstream processing of whole wheat grains or wheat milling by-products, fermentative production of succinic acid and chemical transformations of succinic acid either directly from fermentation broths or after purification in the form of crystals (Luque et al., 2009; Lin et al., 2010). Figure 10.6 presents a schematic diagram of a representative wheat-based bioprocess employed for the production of succinic acid followed by its conversion into various value-added products. As mentioned earlier, wheat contains all essential nutrients required for the formulation of a nutrient-complete fermentation medium. Thus, research focused on the production of fermentation media from wheat that can enhance succinic acid production. Four wheat-based upstream processing strategies (Fig. 10.7) have been developed at the SCGPE to produce carbon-rich and nitrogen-rich streams (Dorado et al., 2009; Du et al., 2008; Koutinas et al., 2007a; Webb et al., 2004). Although Strategy I has fewer processing steps, it has been demonstrated that it was not suitable for generic feedstock production due to the low glucose and free amino nitrogen (FAN) concentrations in the fungal filtrate. On the other hand, both Strategies II and III could fulfil the nutrient requirement of typical SA fermentations. In Strategy III, two fungal strains (Aspergillus awamori and Aspergillus oryzae) have been utilized to produce amylolytic and proteolytic enzymes, respectively. These enzymes were subsequently employed to hydrolyse gluten-free flour and gluten, respectively. In terms of nutrient concentrations, the highest glucose concentration that could be obtained by solid state fermentation was 170 g L^− 1.

As mentioned in Section 10.3, Arifeen et al. (2007a) presented an optimized upstream process for the production of a nutrient-complete wheat-based medium involving the combined hydrolytic/autolytic reaction using A. awamori submerged fungal fermentation solids. The simulations suggested that this process could produce 120 m³ h^− 1 generic fermentation feedstock containing 250 g L^− 1 glucose and 0.85 g L^− 1 FAN, resulting in a production cost of $0.126/kg glucose. Therefore, based on a succinic acid yield of 0.81 g/g wheat (Du et al., 2008), the substrate cost is $0.156/kg. The substrate cost is reduced by 5.6- and 1.7-fold as compared to the processes using maleic anhydride and glucose, respectively (Table 10.2).

The search for alternative and more sustainable water sources have been the focus of many studies in recent decades due to the shortage of fresh water in many areas. A recent report from the United Nations Office has summarized that overcoming the crisis in water and sanitation is one of the greatest human development challenges of the early twenty-first century (Harlem, 2012). The development of a seawater-based biorefinery strategy could have a potentially strong impact in this area with a holistic utilization of seawater, aiming at more efficient, low cost and low carbon footprint processes. Lin et al. (2011) presented the first report regarding the novel usage of seawater instead of plain water in fermentative biochemical production. Interestingly, there were no significant differences in terms of SA production in fermentations using seawater/water mixtures containing between 65% and 100% synthetic seawater. Indeed, a complete replacement of distilled/tap water by seawater could be achieved which will facilitate media preparation procedures, as well as make the process more economically and environmentally sound.

In bench-top fermentations with 50% seawater and wheat-derived media, around 45 g/L SA was produced with a yield of 1.02 g/g (consumed glucose) and a productivity of 0.84 g L^− 1 h^− 1. These values were similar to those obtained in fermentations using semi-defined media and tap water. Most promisingly, 49 g/L SA was produced with a yield of 0.94 g/g and a productivity of 1.12 g L^− 1 h^− 1 in a fermentation using only wheat-derived medium and natural seawater (Lin et al., 2011). In summary, mineral compounds and salts in seawater, together with wheat-derived media, were found to meet well the mineral requirements for A. succinogenes fermentations, without significant inhibition of cell growth. Interestingly, compounds present in seawater had a major effect on rates of reactions of a range of downstream transformations of SA including esterifications and amidations in comparison with reactions run under similar conditions using distilled water (Lin et al., 2011).

Table 10.3 shows the SA production using various raw complex materials. It indicates that almost any type of raw biomass could be used for the bioproduction of SA. The SA concentration, yield and productivity depend on the sugar, nutrient and inhibitor concentrations in the biomass-derived media.

10.5 Polyhydroxyalkanoate (PHA) production from wheat

Polyhydroxyalkanoates (PHAs) are biodegradable polyesters that contain 3-, 4-, 5- and 6-hydroxy-alkanoic acids as monomers. One of the main advantages of PHA production is the wide spectrum of applications, including agricultural uses (e.g., controlled release of insecticides, mulch films), food packaging, medical uses, manufacture of articles such as combs, pens and bullets, and production of flushables, scaffolds for tissue engineering applications, binders, biocomposites, adhesives, flexible packaging, thermoformed articles, synthetic paper and medical devices, among others (Du et al., 2012; Wolf et al., 2005; Philip et al., 2007). They are accumulated for energy and carbon storage in the form of intracellular granules by many microorganisms. The bacterial strain Cupriavidus necator (previously designated as Ralstonia eutropha) is the most widely studied microorganism for the production of PHAs that are accumulated by this microorganism as secondary metabolite in the presence of an abundant source of carbon and the limitation of another nutrient such as N, P, Mg, K, O or S. The homopolymer polyhydroxybutyrate (PHB) made of 3-hydroxybutyric acid (3HB) units and the copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) made of 3HB and 3-hydroxyvaleric acid (3HV) units are the most widely studied members of the PHA family that are produced by various strains of C. necator usually by consuming two different carbon sources as precursors.

PHA production can be achieved by many carbon sources including monosaccharides (e.g., glucose, fructose), disaccharides (e.g., sucrose, lactose), alcohols (e.g., methanol, ethanol), triacylglycerols derived from vegetable oils and animal fats, alkanes (e.g., hexane to dodecane) and organic acids (e.g., butyrate upwords) (Wolf et al., 2005; Castilho et al., 2009; Koller et al., 2010). Glucose consumption by C. necator strains leads to the production of PHB. The biosynthetic pathway (Steinbüchel and Schlegel, 1991) of 3HB monomers leading to PHB production in C. necator begins with the condensation of two acetyl-CoA molecules, catalysed by the enzyme β-ketothiolase, leading to the formation of acetoacetyl-CoA. The intracellular accumulation of acetyl-CoA molecules occurs only under nutrient limiting conditions that prevent their utilization in the tricarboxylic acid (TCA) cycle. Subsequently, acetoacetyl-CoA is converted into 3-hydroxybutyryl-CoA via acetoacetyl-CoA reductase. PHB is finally produced through esterification of 3-hydroxybutyryl-CoA monomers by PHA synthase. The main driving force for the production of PHB is the availability of reducing equivalents in the form of NADPH (Madison and Huisman, 1999).

Nowadays, it is widely accepted that commercial production of PHAs can only be achieved within biorefinery strategies and through cost reduction in raw material selection and processing, fermentation in industrial-scale bioreactors and downstream separation. In the case of the raw material, research has focused on the evaluation of various agricultural products and agro-industrial waste and by-product streams as renewable resources for the production of PHAs, including cereals, whey, green grass, pulp fibre sludge, silage, molasses, and meat and bone meal (Rusendi and Sheppard, 1995; Zhang et al., 2004; Nikel et al., 2006; Solaiman et al., 2006; Koutinas et al., 2007c; Koller et al., 2010). Pure starch and commercial protein supplements (e.g., yeast extract, casein hydrolysate, casamino acids) have been tested for PHB production (Bormann et al., 1998; Lapointe et al., 2002; Yu et al., 2003; Quillaguaman et al., 2005; Huang et al., 2006).

Research at the SCGPE focused on the development of a wheat-based biorefining strategy for the production of PHB as the core commodity product and bran-rich pearlings and gluten as added-value co-products (Xu et al., 2010). The proposed biorefinery concept is included in Fig. 10.1. The main challenge of PHB production from wheat was to formulate two types of fermentation feedstocks from wheat; one rich in all nutrients necessary for microbial growth and a second one containing high glucose concentration and optimum concentration of nitrogen sources. The nutrient-complete fermentation medium was produced by combining fungal autolysates with flour hydrolysates that were produced from pearled wheat flour after partial extraction of gluten. The second fermentation medium can be used as feeding medium during PHB accumulation where nutrient limitation is necessary. It was observed that supplying low amount of nitrogen sources is necessary in order to maintain microbial cell viability and achieve high PHB concentrations and yields.

Fermentations carried out in a 1 L bioreactor and operated in fed-batch mode using a nutrient-complete wheat-based medium at the beginning of the fermentation and a concentrated (pure) glucose feeding solution during the PHB accumulation phase (Fig. 10.8(a)), showed that PHB accumulation is enhanced with increasing microbial biomass concentration achieved during the initial growth phase (Xu et al., 2010). Microbial biomass concentration was enhanced with increasing initial FAN concentration at the beginning of the fermentation up to an initial FAN concentration of less than 1000 mg/L. A maximum PHB concentration of 68.2 g/L was achieved when an initial FAN concentration of 960 mg/L was used at the beginning of the fermentation and concentrated pure glucose was used as feeding medium (Fig. 10.8(a)). Microbial cell autolysis that occurred at the end of the fermentation increased the PHB content up to 93% (w/w). Fermentations with FAN concentration higher than 1000 mg/L resulted in enhanced microbial cell growth but no PHB accumulation.

Figure 10.8(b) shows that sequential feeding with a wheat hydrolysate followed by a pure glucose solution during PHB accumulation leads to the production of PHB concentrations up to 162.8 g/L at a productivity of 0.89 g/L/h. Xu et al. (2010) showed that running fed-batch fermentations with addition of wheat hydrolysates at different flow rates results in varying PHB production. It is, therefore, evident that there is a critical concentration of organic nitrogen that leads to substantial increase of PHB production. In addition, the consumption of amino acids and peptides for bacterial growth and glucose mainly for PHB accumulation leads to increased glucose to PHB conversion yields. Furthermore, bacterial cell autolysis was also observed in fed-batch fermentation using wheat hydrolysate and pure glucose as feeding media leading to a final PHB content of 93% (w/w).

Considering that PHA concentrations of 60 to preferably 80 g/L should be achieved to facilitate commercial viability (Wolf et al., 2005), the PHB production achieved from wheat-based media surpasses this limiting threshold. In addition, the glucose to PHB conversion yields (higher than 0.37 g/g) achieved when wheat-based media are used are among the highest reported in the literature. However, the productivity achieved (0.89 g/L/h) should be improved further to enhance the commercial potential of this process.

10.6 Utilization of wheat straw

The sustainability of wheat-based biorefineries could be improved by integrating wheat straw and other residue utilization through the optimization of their application in the agricultural field to maintain soil quality and avoid erosion losses, and as renewable resources for the production of chemicals, materials, biofuels and energy. The amount of crop residue that should remain in the field could be calculated through simple carbon models (Hettenhaus, 2006). Surplus amounts of wheat residues could be used for energy production via combustion or chemical production through thermochemical conversion technologies either alone or integrated, in many cases, with enzymatic hydrolysis and microbial bioconversions. Pyrolysis and gasification are potential technologies for fuel and chemical production from wheat straw (Demirbas, 2006). Hydrogen and methanol production via gasification could be commercially viable, but production of simple alcohols, aldehydes, mixed alcohols and Fischer–Tropsch liquids are not cost-competitive yet and require further research and development activities (Werpy and Petersen, 2004).

With the pressure of continuous increases in food prices, it is increasingly desirable to produce fuel ethanol from lignocellulosic raw materials (e.g., wheat straw) rather than food-based materials. Thus, the concept of the second generation of bioethanol production process was developed (Fig. 10.9). Wheat straw, which is an abundant lignocellulosic materials in Europe, has attracted great interest as a raw material for fuel ethanol production. In comparison with the first generation wheat-based bioethanol production process, the utilization of wheat straw in ethanol fermentation is relatively complex and energy intensive. Usually, the wheat straw needs to be physically pre-treated (e.g., cutting, milling) to reduce the particle size. Then, wheat straw powder will be subjected to one or more chemical pretreatments, such as dilute acid pretreatment, to separate cellulose from lignin and hemicellulose. Next, cellulases are used for enzymatic hydrolysis of cellulose to obtain glucose for subsequent yeast fermentation. One of the main challenges in the wheat straw to biofuel process is the efficient conversion of lignocellulosic raw materials into simple sugars with minimum or even negligible production of inhibitors to the bioethanol fermentations. Various pre-treatment methods have been investigated, including dilute acid pretreatment, alkali pretreatment, steam explosion, ammonia fibre explosion, thermochemical pretreatment, organosolv process and biological pretreatment. These processes were recently reviewed by Talebnia et al. (2010). It is reported that a wheat straw-based fuel ethanol pilot plant has been constructed, but commercial-scale production has not yet been confirmed.

Although not yet economically viable, the hemicellulose fraction of wheat straw could be converted into directly assimilable sugars by many microorganisms through integrated chemical pre-treatment and enzymatic hydrolysis methods (Saha, 2003). Enzymatic hydrolysis of the hemicellulose fraction of wheat straw would result in the release of xylose, fermentative conversion of which could lead to the production of xylitol (Canilha et al., 2006), which has been listed by the US Department of Energy among the top 12 value-added platform molecules for the production of a range of chemicals, including xylaric acid, propylene glycol, ethylene glycol, mixture of hydroxyl-furans, and polyesters (Werpy and Petersen, 2004). Wheat straw hydrolysates could also be used for fermentative production of 2,3-butanediol and lactic acid (Saha, 2003; Saito et al., 2012) or chemical conversion to levulinic acid (Chang et al., 2007). Agricultural residues, such as wheat straw, could be used for the production of biofibres with potential applications as composites, textiles, food, enzymes, fuels, chemicals, pulp and paper (Reddy and Yang, 2005).

10.7 Conclusion and future trends

Cereal grains were obviously among the initial renewable raw materials that were considered for the production of biofuels, chemicals and materials via biorefinery development. The existence of industrial cereal processors facilitated these initiatives because the successful implementation of a new industrial concept is highly dependent on the availability of raw material, the existence of skilled workforce and logistics (e.g., transportation). Although cereals fulfilled most of the above prerequisites, their main utilization as food and feed has created a major barrier for widespread industrial implementation.

Cherubini (2010)pointed out that most of the existing biofuels and biochemicals are currently produced in single production chains but not within a biorefinery concept, and usually require materials in competition with the food and feed industry. Food waste and by-product streams constitute renewable resources that could be utilized for chemical and material production. In this way, biorefinery concepts could be developed that do not compete but, on the contrary, coincide with food production. Dorado et al. (2009) and Leung et al. (2012) demonstrated the future potential of integrating succinic acid production in existing cereal-based industries (e.g., wheat milling industry, bakeries) through the utilization of by-products or waste streams. Two types of bioprocesses or biorefineries could be developed:

10.7.1 Valorization of industrial cereal-based waste and by-product streams

Cereal-based industries, including starch production, wheat milling, confectionery industries, pasta production and bakeries among others, generate significant quantities of starch- or flour-based waste and by-product streams. For instance, wheat middlings and bran are by-products from the wheat dry milling industry. Koutinas et al. (2006) reported that out of the 5,624 × 10³ t of wheat processed by UK flour millers in 1999–2000, approximately 1,148 × 10³ t of by-products were generated, the predominant fraction of which is middlings that contain significant quantities of starch (20–35%), protein (15–19%) and phosphorus (approximately 1%). The poor nutritional value of wheat milling by-products reduces its versatility as animal feed only to pigs and cattle (Koutinas et al., 2006). For this reason, in many countries, surplus quantities of wheat milling by-products that cannot be used as animal feed could be treated as renewable resources for the production of chemicals and materials.

Dorado et al. (2009) devised a strategy to convert wheat milling by-product streams into a generic fermentation feedstock (Strategy IV in Fig. 10.7). Similar to Strategy III presented also in Fig. 10.7, wheat bran has been utilized as substrate for the production of amylolytic and proteolytic enzymes using the fungal strains A. awamori and A. oryzae, respectively. Simultaneous starch and protein hydrolysis together with fungal autolysis then occurred when the A. awamori and A. oryzae solid state fermentation solids were mixed with suspensions of wheat middlings and bran at 55°C. Hydrolysis of wheat middlings and bran in a paddle reactor resulted in the highest glucose and FAN concentrations of around 100 g/L and 300 mg/L, respectively. The carbon and nitrogen concentrations of the hydrolysate mixture met well the nutrient requirement of an industrial SA fermentation (Fig. 10.10). Figure 10.10 presents the glucose and FAN concentrations produced in each one of the fermentation feedstock production strategies presented in Fig. 10.7 along with the respective requirements of a typical succinic acid fermentation process. Using wheat milling by-products as the sole raw material for microbial feedstock production resulted in the production of 50.6 g/L succinic acid at a productivity of 1.04 g/L/h and a glucose to succinic acid conversion yield of 0.73 g/g (Dorado et al., 2009). Therefore, it was demonstrated that wheat milling industry by-products could be effectively utilized in fermentative succinic acid production. In this way, succinic acid production could be integrated into existing wheat milling plants to provide an alternative market for surplus wheat milling by-products that are not used as animal feed.

Research at the Agricultural University of Athens in Greece focuses on the utilization of flour- or starch-based waste streams from confectionery industries as renewable resources for the production of microbial oil as raw material for biodiesel or oleochemical production. Microbial oil is produced by oleaginous microorganisms and has similar fatty acid content to vegetable oils. Koutinas and Papanikolaou (2011) presented a detailed review of microbial oil production from various renewable resources and discussed its prospect for biodiesel production.

10.8 Sources of further information and advice

While the use of biological raw materials for the production of value-added products through fermentation has been practised for many thousands of years, and even the production of industrial chemicals through biotechnology is almost a century old (starting in 1917 with the development, in Manchester, of the acetone-butanol fermentation by Chaim Weizmann), the biorefinery concept is rather new. It was not until the mid-1990s that the term ‘biorefinery’ came to mean what it does today. In Europe, the Danish Bioraf Foundation project was exploring links between agriculture and industry and published its report on ‘The whole crop biorefinery’ (Gyling, 1995). Meanwhile, in the US, Wyman and Goodman (1993) had recently published their seminal paper on the production of fuels, chemicals and materials from biomass. It was at this time that Colin Webb began work on the cereal-based biorefinery and established the SCGPE in Manchester (Webb, 1994).

Composition and conventional processing technologies for cereal grains is presented in various books including Pomeranz (1987, 1988), White and Johnson (2003), MacGregor and Rattan (1993), and Webster (1989). Cereal-based biorefineries as well as lignocellulosic-based biorefineries including utilization of cereal straws have been presented in a number of books, for example those by Campbell et al. (1997) and Kamm et al. (2006). Johnson and May (2003) present the corn-based biorefinery development established by exploiting the wet milling process of corn. Other excellent texts to consult on the biorefinery concept include those by Demirbas (2010) and Stuart and El-Hawagi (2012). The latter addresses design aspects related to the integrated biorefinery concept. Several journals are now dedicated to the area. Amongst these are Wiley’s Biofuels, Bioproducts and Biorefining (Biofpr) and Springer’s Biomass Conversion and Biorefinery.

For general information and latest developments, the reader should consult the National Non-Food Crops Centre (http://www.nnfcc.co.uk/biorefinery), the International Forum on Industrial Biprocesses (http://www.ifibiop.org/) and the National Biorefineries Database (http://en.openei.org/datasets/node/50). For biofuels specific biorefinery information, an excellent information source can be found at the Biofuels Platform (http://www.plateforme-biocarburants.ch/en/home/).

Recent research projects focus on the utilization of cereal-based food waste for the production of biofuels, chemicals and biomaterials. BREAD4PLA (2012) is a European project (http://www.bread4pla-life.eu/) aiming to demonstrate the potential production of polylactic acid (PLA) for bio-based plastic formulation using bakery waste as raw material for fermentative production of lactic acid. The PLA produced will be used in the production of new biodegradable packaging for bakery products. This means that the wastes generated from a cereal-based food industry will be utilized as raw material for the production of a product that will be used by the same industry as a substitute for petroleum-derived plastics.

Pilot-plant facilities have been created in order to evaluate the conversion of renewable raw materials including cereal-based food waste for the production of a spectrum of products. At the Leibniz Institute for Agricultural Engineering Potsdam-Bornim (ATB) in Germany, waste bread is used as substrate for the production of lactic acid. A production capacity of 10 t of lactic acid in 200 days per year was achieved (Venus and Richter, 2007). The Biorenewables Development Centre (BDC) in York (UK) was constructed in 2012. It was built upon the research and development expertise of the Centre for Novel Agricultural Products (CNAP) and the Green Chemistry Centre of Excellence (GCCE) at the University of York, with support from the Science City of York. It contains pilot-scale processing equipment with the aim to develop a broad range of products and processes based on the use of biorenewable resources (BDC, 2012).

The School of Energy and Environment at the City University of Hong Kong has recently started collaboration with the coffee retail giant ‘Starbucks Hong Kong’ (Lin et al., 2013; Zhang et al., 2013). The partnership, which was facilitated by the NGO The Climate Group, focuses on the valorization of spent coffee grounds and unconsumed bakery wastes via bioprocessing. The collaboration is based on a support scheme and part of the ‘Care For Our Planet’ campaign: for every set of Care For Our Planet Cookies Charity Set sold, Starbucks will donate HK$8 to the School of Energy and Environment of City University of Hong Kong to support research on valorization of food waste for sustainable production of chemicals and materials. The aim of the research is to valorize the disposed coffee grounds and unconsumed baked goods to bio-plastics and detergent ingredients, facilitating the development of biomass use in Hong Kong and reducing the release of greenhouse gases and other air pollutants into the atmosphere.

10.9 Acknowledgements

This research was supported by the Engineering and Physical Sciences Research Council (EP/C530993/1) and gr/s24909/01 in association with the Crystal Faraday Partnership in the United Kingdom. We are also grateful for the provision of the Overseas Research Students Awards Scheme to Dr. Lin by Universities UK. The authors gratefully acknowledge the generous contribution of the Satake Corporation of Japan in providing financial support for much of the research carried out in the Satake Centre for Grain Process Engineering (SCGPE, University of Manchester, UK). Dr Carol Lin additionally acknowledges the Industrial Technology Funding from the Innovation and Technology Commission (ITS/323/11) in Hong Kong, the donation from the Coffee Concept (Hong Kong) Ltd. for the ‘Care For Our Planet’ campaign, as well as a grant from the City University of Hong Kong (Project No. 7200248). Dr Apostolis Koutinas acknowledges the financial support from ‘NUTRI-FUEL’ (09SYN-32-621) research project implemented within the National Strategic Reference Framework (NSRF) 2007–2013 and co-financed by national (Greek Ministry – General Secretariat of Research and Technology) and community funds (EU – European Social Fund).