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Producing Animal Feed as a Coproduct of Biorefining
Chapter 13
Producing Animal Feed as a Coproduct of Biorefining Dawn Scholey and Emily Burton Nottingham Trent University, School of Animal, Rural and Environmental Sciences, Nottingham, United Kingdom
13.1 INTRODUCTION The process of biorefining has been defined as the sustainable processing of biomass into a spectrum of bio-based products (food, feed, chemicals, and minerals) and bioenergy (biofuels, power, and/or heat) [1]. In 2008, a lack of public awareness led to a controversy over the use of grain as a biorefinery feedstock to produce bioethanol [2], but contrary to popular belief, the process is actually an excellent example of biorefinery production of both fuel and nutrients. Although this issue is still apparent, it has been mitigated by the increasing need to address feed security in Europe, particularly in the case of protein supply for animal feed. The formulation of animal feeds with biorefinery coproducts allows the balance of feed and fiber from fuel production to be converted into the human food supply via animal production. This chapter will address the potential for feed protein production from ethanol biorefineries, with emphasis on the nutritional composition of the product and suitability for monogastric nutrition. Animal feed protein requires several considerations in addition to the raw protein content of the material, including the impact of nonprotein components which may have potentially antinutritional qualities which then need to be taken into account during feed formulation. In biorefining, these would include high-fiber-content material as fiber can have deleterious effects on monogastric animal production due to increased gut fill, increased gut transit, and reduced nutrient digestibility. However, these nonprotein components can in some cases have a beneficial effect on animal health, particularly when they contain an additional energy source, such as in oil seed meal production. Some components may even have a beneficial biological effect, such as yeast used in the biorefinery process, which will be discussed in Advanced Bioprocessing for Alternative Fuels, Biobased Chemicals, and Bioproducts. DOI: https://doi.org/10.1016/B978-0-12-817941-3.00013-9 © 2019 Elsevier Inc. All rights reserved.
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more detail later in this chapter. Second, the amino acid profile of any protein is vital when the material is incorporated into animal feed, with lysine and methionine being of major importance as the first limiting amino acids. Another important factor is the biological availability of the protein source, as associations with plant carbohydrates will influence the digestibility of the protein in animals [3]. Mitigation of antinutritional factors will also need to be considered [4]. Finally, and probably most importantly to the animal feed industry, the production must be financially viable and able to be reliably produced in sufficient quantities while also being able to compete with other protein sources in terms of cost, variability, and nutritional value. Without production of at least 50,000 t a year, a product would not be considered a commodity protein to be used in feed formulation, although a high-value additive with additional nutritional benefits could still have applications.
13.2 APPLICATION OF PROTEINS IN FEED FOR LIVESTOCK The feed industry can be separated into two distinct sectors: one supplying ruminant species (mainly cattle and sheep) and the other addressing nonruminant species, such as fish, pigs, and poultry. Ruminants are able to utilize energy from fiber and amino acids from nonprotein nitrogen, and therefore, the typically fiber-rich outputs of biorefineries [e.g., distiller’s dried grains with solubles (DDGS)] are usually incorporated into diets in this sector. Higher values protein coproducts, which tend to be produced in smaller quantities, tend to be directed toward the monogastric animal feed market. Animal needs vary depending on life stage and nutritional requirements of the animals, with the extreme growth rates in commercial fish, pig, and poultry production making this sector particularly sensitive to variations in feed and protein quality. This makes many coproducts unsuitable for incorporation into monogastric diets. Other considerations include protein density and the presence of factors, such as lectins, trypsin inhibitors, and β-conglycinin, which have antinutritional properties [5]. However, in addition to the animal requirements, considerations from the feed industry and the end consumer also need to be understood. The feed industry requires several factors to ensure widespread adoption of a raw material, including reliable supply and quality, critical production volume, and suitable quality-control systems. As feed density is so important, particularly in neonatal nutrition, it is vital that protein sources do not limit feed intake and therefore diet nutrient intake. In monogastric nutrition, protein must have little fiber contamination as this can affect feed intake and ultimately acceptance of the product by the formulator. Consumer choice is a relatively new area for consideration in animal and feed production, gaining prominence only in recent years [6], with their opinions now having a large impact on the choice of products used in animal feed. Consumers are increasingly aware of the importance of food safety and also of the role of animal feed in the food chain due to a number of food-
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safety scares [7]. Over the last 30 years, there have been several food scares in the EU, including most recently the Campylobacter contamination of poultry meat [8] and the dioxin residue issue [9]. There is now much greater consumer demand for choice and information regarding food safety [including genetic modification (GM)], animal welfare, and environmental sustainability. Issues around sustainability include overfishing, hence the use of fish meal as a protein source [10], and the reduction in food miles for raw materials. The welfare of production animals is also of greater import to consumers, with more public concern regarding the feeding of GM grains in animal diets.
13.2.1 Protein Value There are several factors, which influence protein value, including geographical location, which varies from the least expensive (United States) to the costliest (Asia and the EU) for soya and DDGS. Second, the composition of the protein is important to the value of the protein, and there is a further financial benefit from additional nutrients and health benefits, as is the case with yeast. Finally, there is a strong drive to source a more environmentally sustainable product than soya, with the potential for strong local supplies, which adds a further financial premium to locally produced protein. This would suggest a strong market for yeast protein from biorefineries, particularly in those markets where soya and DDGS are most expensive.
13.3 THE VALUE OF A BIOREFINERY A new biorefinery is a significant financial investment which requires a solid business case for both the production of the primary product and the associated coproduct(s). In construction of new biorefinery plants, there are economies of scale which may benefit coproduct production, including combined power use and already established supply chains for both supply of raw materials and transport of finished products. In larger plants, the increased scale and high volume can result in continuous supply, improved product consistency, and the addition of novel technology to add further value to coproduct streams. This will be of additional financial benefit both for new and existing biorefineries planning future construction projects. Biorefining processes are already used extensively in livestock feed production with the biorefining process having a significant positive impact on the protein quality in many cases. For example, the initial process for the production of soya oil from beans was modified to improve the coproduct, soya bean meal (SBM), and to enable its widespread use in animal feed. In this case, processing was used to reduce antinutritional components by inactivation of the trypsin inhibitors, improving the digestibility of the SBM. Originally, processing was a combination of physical processing, heat, and
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solvent extraction [11]. However, recent developments have led to the selective use of in-feed enzymes to eliminate detrimental plant components, such as phytate [12]. There are several other examples of biorefining to produce a coproduct, including meal production from rapeseed oil and the production of monosodium glutamate from rice, which results in a rice protein concentrate. Later, this chapter will specifically look at a more recent process, the production of yeast as an additional value coproduct from bioethanol production [13]. It is vital that all aspects are considered when the production of a coproduct is added to a biorefining process. For example, it was proposed that a high-protein coproduct of green beet tops could be used to add value to sugar beet production [14]. It was suggested that this high-protein meal could be produced; however, the system proved to require 56 t of leaf material to provide a single tonne of feed protein, and therefore, the system was only cost effective if the leaves did not require transport to the factory. It may be in the future that these beet leaves can be pressed to make a semimeal product [15], but this example shows that without a sustainable supply chain, both into and out of the refinery, the coproduct may not be economically viable.
13.4 COPRODUCT STREAMS FROM DRY GRIND ETHANOL PRODUCTION Renewable fuel in the form of bioethanol can be produced via the fermentation and distillation of a starch feedstock, such as cereal, grain, or tuber. The value of the resulting ethanol is very dependent on the price of mineral oil, and when the price of the latter falls, this places greater emphasis on the value of the coproduct of the process in order for the refinery to remain profitable as a whole. The bioethanol process results in a number of coproducts that are typically used in animal feed, the major one of which is DDGS. Ethanol production via a feedstock is typically from a dry grind process [16] which ferments starch to ethanol resulting in the production of carbon dioxide and DDGS in approximately equal quantities [17].
13.4.1 Distiller’s Dried Grains With Solubles In maize alcohol production, currently 50% of the revenue of the process is derived from the coproducts [18]. However, the promotion of renewable fuels as a means of reducing carbon emissions has massively increased bioethanol production and consequently coproduct volume, which negatively impacts its value [19]. The grain residue left after fermentation includes all the grain proteins, aleuronic layers, fibrous carbohydrates and lipids. Together, with the yeast produced in the process, they are combined and dried to produce DDGS. DDGS tends to be fed almost solely to ruminants due to the high fiber to
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protein ratio (27:33) which is ideal for ruminant feed [20] but limits the use of the product for monogastric species [17,21]. One of the main issues with DDGS is that in order to store and transport it, the product is dried, using approximately 40% of the energy costs from the whole refining process. Increasing DDGS inclusion in animal feed has also been negatively correlated with pellet durability [22], which is highly correlated with feed efficiency [23], and increasing energy usage in the condenser due to the viscosity of the mash [24]. The high energy expenditure limits the value of DDGS as a feed ingredient, especially in the ruminant market, where lowcost materials are vital due to the high volume of feed required for every kg of meat (10 kg/kg for cattle, compared with 2 kg/kg for poultry [25]). There have been transport difficulties observed with DDGS, including caking and poor flow due to water adsorption [26]. Particle separation has been observed with DDGS [27], and there have been large variations in the physical characteristics reported [27,28]. Any variation in bulk density of products will influence transport costs, particularly if transporters cannot be filled consistently [29].
13.4.2 Differentiation of Distiller’s Dried Grains With Solubles There has been considerable study into the production of a differentiated DDGS with higher protein, using a combination of sieving and air flow [30,31]. This Elusieve process comprises a rotary sifter with three decks for stack sieving of DDGS. In use, it has been shown to produce four fractions, including a small particle size, modified DDGS, with an increased protein content by between 2% and 2.6% while also increasing the neutral detergent fiber value of the larger particle size fiber fraction by as much as 37% [32]. Economically, based on a bioethanol plant using 2030 t/day of corn and a capital investment of 1.4-million dollars, the modified process produces DDGS at a cost of $0.84/t [33]. The highest potential revenue stream from this method is suggested to be three coproduct streams of a high-protein (5% higher) DDGS, a traditional DDGS product, and a fiber-rich product [31]. Inclusion of this product in feed improved some nutritional characteristics, including higher metabolizable energy [34], increased bodyweights in broilers [35], and increased energy digestibility in fish [36]. Alternatively, this process can be amended to separate corn kernels preprocessing to improve ethanol output by around 4.5% based on a 50-million gallons a year plant, which would recoup the capital output in an estimated 3 years, depending on corn and ethanol prices [37].
13.5 THE POTENTIAL FOR YEAST PROTEIN Yeast multiplies during the bioethanol process and contributes 20% of the protein in the dry matter (DM) of DDGS [38]. If this material can be isolated
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FIGURE 13.1 A comparison of the nutritional content of yeast protein concentrate and DDGS, both from maize bioethanol production. DDGS, Distiller’s dried grains with solubles.
in an economically viable process, this would be a substantial potential protein stream. Bioethanol relies on economies of scale, and a typical dry grind plant will utilize up to 1-million tonnes of grain a year, which a significant potential production of yeast coproduct. It is estimated that for every gram of fermented starch, 0.071 g of yeast is produced [39]. Therefore, a 400million-liter bioethanol plant with an intake of 1.1-million tonnes of wheat will theoretically produce 48,000 t of yeast per annum. The potential for yeast to be marketed for around d600 a tonne makes yeast separation a favorable financial proposition. Yeast is rich in both vitamins and minerals and is the most valuable component of DDGS. It also contains valuable proteins, nucleotides, and is high in both glutamic acid and inositol [40]. A comparison of the composition of DDGS and yeast protein concentrate (YPC), both from maize bioethanol production, is shown in Fig. 13.1. The nucleotides and β-glucan together with the mannan-oligosaccharides (MOS) in the yeast cell wall can have positive effects on performance in both poultry and pigs [41,42]. MOS have prebiotic properties and therefore have been investigated as potential mitigators of the ban on antibiotic growth promotors in the EU. Studies have shown positive effects on gut microbiota, immunity and disease resistance, as well as improved gut morphology, and mucus production [43,44].
13.5.1 Yeast Separation Often yeast is removed from potable alcohol distilleries to reduce the overall yeast concentration, usually by mechanical means via one of four methods: pressured leaf filters, vacuum drum filtration, membrane filtration, and centrifugation. The first two listed are inefficient and use diatomaceous earth.
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Membrane filtration involves cross flow technology and can involve vibrating filters. Centrifugal techniques include a decanter centrifugation and disc nozzle separation which will be discussed individually.
13.5.1.1 Separation Using Decanter Centrifugation A decanter centrifuge can be used to separate fiber from stillage and removes water. This type of separation is rapid and can therefore allow high throughput while giving some control over the “cut” of the output by altering feed rates and the size of the output value. The equipment construction is robust and flexible, allowing different consistency slurries to be separated consecutively with no issues. These types of separator are also simple to clean and maintain. 13.5.1.2 Separation Using a Sedicanter A sedicanter is a combination of separator and decanter, which clarifies the suspension while producing a solid output. It can also process a large amount of solid material and has an adjustable impeller to enable variability in feed input. It is reported by the manufacturers to use 20% 50% less installed power than conventional decanters, and less energy is required for drying due to the higher final DM of the output. There are reports of a 28% DM output with increased alcohol yield [45]. The clarified suspension may be suitable for use in liquid feedanaerobic digestion (AD). Recent studies have shown that a yeast concentrate separated using a sedicanter had a similar amino acid content to soya. 13.5.1.3 Separation Using a Vibration Sheer Enhanced Process Unit The vibration sheer enhanced process dewatering unit consists of a stack of Teflon membranes with a strong vibration to create highshear forces which facilitate the transport of liquids through the membranes and reduce fouling of the system. The output of this system is similar to a decanter centrifuge [46], but the liquid stream contains no solid particles, whereas a decanter will not separate all fibrous material completely. However, using these systems result in high operating and energy costs, with particularly high water consumption. 13.5.1.4 Separation Using Filtration and Hydrocyclones For batch fermentations, centrifuges are often used for yeast recovery but filter aid filtration followed by release of the yeast using hydrocyclones may be a more cost-effective process. Studies used a 1.14 m2 filtration area with perlite as a filtration aid and two 30 mm hydrocyclones. The use of the hydrocyclones alone only produced an efficiency of 1%, but when they were used to remove perlite from the filtration cake, the efficient of separation was increased to 95%, suggesting that for batch production, this process may be feasible [47].
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13.5.1.5 Separation Using a Disc Stack A disc separator is a continuous bowl centrifugal separator comprising a stack of discs which separate solids by size and eject a liquid stream via centripetal force and the solid stream to an ejecting nozzle. An automatic disc separator requires a relatively low inlet concentration due to limited space, and frequent ejections of the solid material increase wear and maintenance, and increasing the size of the machine to mitigate these issues is a costly solution. A nozzle disc separator is an improved process with continuous yeast discharge although this system is still prone to blockage and the resulting solid stream may be wetter.
13.5.2 Yeast Protein Concentrate Production A process which combines the use of a decanter with a disc nozzle separator has been developed to produce a high-protein YPC. This method maintains three product streams: the YPC, a pentose sugar syrup, and a fiber product very similar to DDGS (with protein reduction of around 5% compared to traditional DDGS). This process can be included as an additional module to existing first-generation bioethanol plants, with little alteration to the engineering of the plant. The additional product streams will increase profitability of ethanol production, although any potential loss in the value of the reduced protein DDGS will have to be considered. In this process, a horizontal bowl decanter removes the majority of the fiber to obtain a liquid fraction with around 5% DM and little fibrous contamination. This liquid stream was then dewatered using a continuously operating nozzle centrifuge designed for liquid separation (GEA Westfalia, Germany). This process produced a liquid output with less than 0.2% DM and a solid discharge with approximately 40% DM. The solid material was dried to a powder, referred to throughout this chapter as YPC.
13.5.3 Drying Yeast Drying is an issue for any biorefinery coproduct as the material needs to be uniformly dried to ease transport and handling while increasing shelf life. A powder can also be easily and thoroughly mixed with other feed materials prior to pelleting of diets. However, drying can affect nutritional content of the finished product, although product deterioration is usually due to the application of excess heat rather than the moisture removal [48]. Overheating can lead to Maillard reactions, which binds the amino acid lysine in a form which cannot be digested by animals. This is a particular problem for poultry where lysine is the first limiting essential amino acid. A comparison of bioethanol yeast products dried by ring, spray, and freeze drying showed that protein quality was deleteriously affected by ring drying [46]. Spray drying has been shown to be a suitable method for
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preserving yeast protein from bioethanol, with a similar amino acid profile to soya [49]. However, it is a high energy demand process [50], and ring drying is a more suitable process for large throughput processes, though this needs to be carefully managed to avoid burning. Washing the yeast prior to drying may remove some soluble sugars and improve the flow of the material while increasing protein, energy, and amino acid content [51].
13.5.4 Yeast and Yeast Protein Concentrate in Animal Feeds The yeast component of the DDGS has been estimated to make up 5.3% of the DDGS [52] contributing 20% toward DDGS proteins in maize bioethanol [38]. This yeast is a valuable source of protein if it can be economically separated from DDGS, leaving a high fiber fraction suitable for ruminant feeding. Yeast contains valuable protein as well as additional vitamins and nucleotides [39], but there have been several reports of reduced intake in poultry fed diets containing yeast [53,54]. Feeding the yeast in either a pelleted or crumb diet has been shown to increase intake [53], which suggests that the issue is one of feed form. Spray drying of yeast creates a very small particle size, and this has been shown to reduce intake in chickens [55]. In bioethanol yeast, there may be additional issues, as high ethanol exposure may create a thicker, toughened cell wall, which is more resistant to enzyme proteolysis [56]. Certainly there is evidence that disrupting the yeast cell wall can improve nutrient utilization compared with whole yeast [57,58]. Rumsey et al. [59] suggest that access to the contents of the yeast cell is paramount for nutrient use, but the cell wall itself also contains biologically important fractions which have their own role in nutrition. Yeast cell walls make up 20% of the dry weight [51], containing sugars including MOS, which have been shown to have a prebiotic potential via three modes of action: improved pathogen adsorption [60], improved gut health [61], and immune modulation by binding to specific immune receptors [62]. MOS inclusion in diets has also been shown to improve bodyweight gain and feed conversion ratio in poultry [40] and pigs [41] while shifting intestinal microflora toward beneficial organisms [63]. Cell wall components have been shown to improve intestinal morphology and the proportion of mucosa producing goblet cells in chicks [64,65]. Finally, there is a suggestion that bioethanol yeast may be more exhausted of cellular components, and protein content has been found to be lower for ethanol yeast than traditional brewer’s yeast [66]. Yeast has been separated from bioethanol and distillery stillage and fed to several species with success. A bioethanol YPC was included in carp diets with 15% 20% the optimal inclusion [67] for performance, and similar results were found in sunshine bass, although inclusion levels over 20% were found to impair palatability and production performance [68,69]. In poultry, distillery sludge showed a decrease in weight gain with rate of inclusion
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[70], and a recent study showed that yeast can reduce serum cholesterol in broilers while increasing performance [71]. Knott and Shurson [72] also fed a yeast cream separated from a corn biorefinery, with a resulting increase in gain: feed in late grower pigs. However, gut health improvements were seen only with a residual soluble fraction, which questions whether some of the beneficial yeast components may be soluble and therefore removed during any dewatering process. Bioethanol YPC has a digestible amino acid content comparable with soya for broiler chicks [73] although this is heavily influenced by the drying process used [48]. The shear forces of the disc stack disrupt the yeast cell walls; and therefore, the cell contents are more easily digestible. Up to 17.5% inclusion of YPC in pelleted diets improved broiler performance [72]. However, mash diets containing spray dried YPC gave a reduction in bird bodyweight gain, possibly due to the small particle size of the spray-dried material, which increases feeding time and energy required for feed manipulation. Digesta viscosity can also be increased by fine particles [55], and measurement of the digesta of the birds showed this to be the case [74]. The increased digesta viscosity reduces nutrient availability and accelerates feed passage rate with detrimental effects on bird growth and welfare, but recent studies show that this issue may be mitigated by enzyme addition [75]. There have been historical suggestions that the nonprotein nitrogen portion of yeast increases uric acid production with detrimental effects on bird welfare. However, there is research which suggests that uric acid may be a powerful antioxidant therefore potentially beneficial [76]. Recent investigations feeding bioethanol yeast to broilers have provided little evidence that serum uric acid is actually increased with increasing yeast inclusion [77]. YPC inclusion up to 62.5% (replacing soya) does not appear to have a detrimental effect on pellet quality, with a small reduction in the number of fines observed [73]. However, this study was carried out in a small-scale facility, and this may not be indicative of a higher throughput commercial process. High-quality pellets improve bird performance [78] due to reduced feeding time, and diets with many fines can lead to poor flock uniformity. Another unconsidered benefit from feeding YPC is the potential of additional value from the phosphorus available in the product. Phosphorus is an expensive and limited resource but is an absolute requirement for pigs and poultry to maintain skeletal integrity so it is supplemented routinely. Inclusion of YPC increased bone mineralization [73] and was shown to be able to replace up to 5 g/kg phosphorus [73], with potential cost savings to diet manufacturers.
13.6 CONCLUSION Currently ethanol biorefineries produce mainly DDGS as a valuable feed ingredient for cattle. However, protein is an expanding, high value market with potential to grow 40% by 2050. Development of a highly nutritional
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value coproduct will mitigate controversy over the diversion of cereal crops from animal feed to first-generation bioethanol. A high-protein coproduct also opens up the fast expanding market for pig, poultry, and fish feed and will contribute toward reducing the reliance on imported soya in the EU. There is a drive with the fluctuation in ethanol margins globally to increase efficiency and improve profit via addition of multiple coproduct streams. As energy costs rise, the cost of drying any coproducts needs to be considered, and alternative methods of water removal will become more attractive. Biorefinery production of ethanol could be rebranded to emphasize the traditional, sustainable process of producing feed, protein, and ethanol.
13.7 FUTURE OUTLOOK There is a potential for production of yeast as a bioethanol coproduct to also create new product streams. The fiber fraction produced remains suitable for ruminant feeding as the protein content is still more than sufficient for those species. There is also potential for this fibrous material in manufacturing or as a more sustainable alternative to peat as a growing media for plants. Initial studies utilizing fibrous waste from AD have shown potential for utilization of remaining nutrients by crops. There are a number of potential uses for the liquid fraction including as a feed for an anaerobic digester, or as a potential replacement for molasses in equine diets. The yeast separation process has potential for improvement, particularly in the opportunities for modern enzyme applications. Enzymes have been shown to reduce the supernatant of birds fed YPC, with the suggestion that a cocktail of carbohydrase enzymes will be efficacious in treating viscosity issues. Currently, enzymes are often used in bioethanol production to increase yield, but these could be optimized to positively affect both the bioethanol process, coproduct production, and the composition of the final product. Green biorefineries can use a wide range of biomass including grass, sugar beet leaves, or clover with several projects in the EU concentrating on zero waste production with zero emissions. These processes rely on a mechanical refiner to break down the leafy material, and the process can produce a protein with potential for monogastric feed, fiber for manufacturing, and a soluble’s output containing amino acids, sugars, minerals, and organic acids with the potential for nutritional or feedstock use. These types of refineries along with more traditional cellulosic ethanol production have the future potential for addition of a portfolio of coproducts to improve the sustainability of the process.
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[22] M.Y. Shim, G.M. Pesti, R.I. Bakalli, Evaluation of corn distillers dried grains with solubles as an alternative ingredient for broilers, Poult. Sci. 90 (2011) 369 376. [23] B. Carre, N. Muley, J. Gomez, F.X. Ouryt, E. Laffitte, D. Guillou, et al., Soft wheat instead of hard wheat in pelleted diets results in high starch digestibility in broiler chickens, Br. Poult. Sci. 46 (2005) 66 74. [24] R.E. Loar, J.S. Moritz, J.R. Donaldson, A. Corzo, Effects of feeding distillers dried grains with solubles to broilers from 0 to 28 days’ post hatch on broiler performance, feed manufacturing efficiency and selected intestinal characteristics, Poult. Sci. 89 (2010) 2242 2250. [25] Soil Association, Feeding the animals that feed us. ,http://treatyrockbeef.com/wp-content/uploads/2011/01/Feeding-the-Animals-that-Feed-Us.pdf., 2010 (accessed 29.06.17). [26] P. Saragoni, J.M. Aguilera, P. Bouchon, Changes in particles of coffee powder and extensions to cake, Food Chem. 104 (2007) 122 126. [27] K.E. Ileleji, K.S. Prakash, R.L. Stroshine, C.L. Clementson, An investigation of particle segregation in corn processed dried distiller’s grains with solubles (DDGS) induced by three handling scenarios, Bulk Solids Powder Sci. Technol. 2 (2007) 84 94. [28] K.A. Rosentrater, Some physical properties of distillers dried grains with solubles (DDGS), Appl. Eng. Agric. 22 (4) (2006) 589 595. [29] C.L. Clementson, K.E. Ileleji, Variability of bulk density of distillers dried grains with solubles (DDGS) during gravity-driven discharge, Bioresour. Technol. 101 (2010) 5459 5468. [30] R. Srinivasan, R.A. Moreau, C. Parsons, J.D. Lane, V. Singh, Separation of fiber from distillers dried grains (DDG) using sieving and elutriation, Biomass Bioenergy 32 (2008). 468-172. [31] R. Srinivasan, F. To, E. Columbus, Pilot scale fiber separation from distillers dried grains with solubles (DDGS) using sieving and air classification, Bioresour. Technol. 100 (2009) 3548 3555. [32] R. Srinivasan, R.A. Moreau, K.D. Rausch, R.L. Belyea, M.E. Tumbleson, V. Singh, Separation of fiber from distillers dried grains with solubles (DDGS) using sieving and elutriation, Cereal Chem. 82 (2005) 528 533. [33] R. Srinivasan, Fractionation of DDGS using sieving and air classification, in: K. Li, K. Rosentrater (Eds.), Distiller’s Grains: Production, Properties and Utilization, 2016. [34] E.J. Kim, C.M. Parsons, R. Srinivasan, V. Singh, Nutritional composition, nitrogencorrected true metabolizable energy, and amino acid digestibilities of new corn distillers dried grains with solubles produced by new fractionation processes, Poult. Sci. 89 (1) (2010) 44 51. Available from: https://doi.org/10.3382/ps.2009-00196. [35] R.E. Loar, R. Srinivasan, M.T. Kidd, W.A. Dozier, A. Corzo, Effects of elutriation and sieving processing (Elusieve) of distillers dried grains with solubles on the performance and carcass characteristics of male broilers, J. Appl. Poult. Res. 18 (2009) 494 500. [36] K.M. Randall, M.D. Drew, Fractionation of wheat distillers dried grains and solubles using sieving increases digestible nutrient content in rainbow trout, Anim. Feed Sci. Technol. 159 (2010) 138 142. [37] T.S. Pandya, S. Aditya, Economics of implementing Elusieve process for fiber separation from corn flour in dry grind ethanol plant, J. Bioprocess Eng. Biorefinery 2 (2013) 305 308. [38] J. Han, K. Liu, Changes in composition and amino acid profile during dry grind ethanol processing from corn and estimation of yeast contribution towards DDGS proteins, J. Agric. Food Chem. 58 (2010) 3430 3437.
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[39] I. Spencer-Martins, N. Van Uden, Yields of yeast growth on starch, Eur. J. Appl. Microbiol. 4 (1977) 29 35. [40] V.K. Silva, J. Della Torre da Silva, K.A.A. Torres, D. de Faria Filho, F. Hirota Hada, V. M. Barbosa de Moraes, Humoral immune response of broilers fed diets containing yeast extract and prebiotics in the prestarter phase and raised at different temperatures, J. Appl. Poult. Res. 18 (2009) 530 540. [41] D.M. Hooge, M.D. Sims, A.E. Sefton, A. Connolly, P.S. Spring, Effect of dietary mannanoligosaccharide, with and without bacitracin or virginiamycin, on live performance of broiler chickens at relatively high stocking density on new litter, J. Appl. Poult. Res. 12 (2003) 461 467. [42] M.E. Davis, C.V. Maxwell, G.F. Erf, D.C. Brown, T.J. Wistuba, Dietary supplementation with phosphorylated mannans improves growth response and modulates immune function of weanling pigs, J. Anim. Sci. 82 (2004) 1882 1891. [43] E. Santin, A. Maiorka, M. Macari, Performance and intestinal mucosa development of broiler chickens fed diets containing Saccharomyces cerevisiae cell wall, J. Appl. Poult. Res. 10 (2001) 236 244. [44] R. Morales-Lopez, E. Auclair, F. Garcia, E. Esteve-Garcia, J. Brufau, Use of yeast cell walls: beta-1, 3/1, 6-glucans; and mannoproteins in broiler chicken diets, Poult. Sci. 88 (2009) 601 607. [45] A. Gertsman, Management of surplus yeast in modern breweries, in: Alexander, World Brewing Congress, Aug 13 17, 2016, 2016. [46] M. Galbe, L. Pilcher, C. Roslander, Separation of solid residues after SSF and SHF in fuel ethanol production from spruce, in: 24th Symposium on Biotechnology for Fuels and Chemicals, Gatlinburg, TN, April 28 May, 2002, 2002. [47] V.M. da Matta, R.A. Medronho, A new method for yeast recovery in batch ethanol fermentations: filter aid filtration followed by separation of yeast from filter aid using hydrocyclones, Bioseperation 9 (2000) 43 53. [48] A. Morris, A. Barnett, O.J. Burrows, Effect of processing on nutrient content of foods, Cajanus 37 (3) (2004) 160 164. [49] D. Scholey, P. Williams, E. Burton, Effect of drying process on amino acid digestibility of yeast protein separated from potable alcohol distilleries, in: Burton, et al. (Eds.), Sustainable Poultry Production in Europe, 2016, pp. 285 286. [50] G. Luna-Solano, M.A. Salgado-Cervantes, G.C. Rodriguez-Jimenes, M.A. Garcia-Alvarado, Optimisation of brewer’s yeast spray drying process, J. Food Eng. 68 (2005) 9 18. [51] M. Sharif, M.A. Shahzad, S. Rehman, S. Khan, R. Ali, M.L. Khan, et al., Nutritional evaluation of distillery sludge and its effect as a substitute of canola meal on performance of broiler chickens, Asian-Australas. J. Anim. Sci. 25 (2012) 401 409. [52] W.M. Ingledew, Yeast could you base a business on this bug? Under the Microscope Focal Points for the New Millennium. Biotechnology in the Feed Industry. Proceedings of the 15th Alltech Annual Symposium, Nottingham University Press, Nottingham, UK, 1999, pp. 27 50. [53] N.J. Daghir, J.L. Sell, Amino acid limitations of yeast single-cell protein for growing chickens, Poult. Sci. 61 (2) (1982) 337 344. [54] G. Succi, S. Pialorsi, L. Di Fiore, G. Cardini, The use of methanol grown yeast LI-70 in feeds for broilers, Poult. Sci. 59 (7) (1980) 1471 1479. [55] S. Yasar, Performance, gut size and ileal digesta viscosity of broiler chickens fed with whole wheat added diet and the diets with different wheat particle sizes, Int. J. Poult. Sci. 2 (2003) 75 82.
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[56] G.M. Caballero-Cordoba, V.C. Sgarbieri, Nutritional and toxicological evaluation of yeast (Saccharomyces cerevisiae) biomass and a yeast protein concentrate, J. Sci. Food Agric. 80 (2000) 341 351. [57] P. Coutteau, P. Lavens, P. Sorgeloos, Baker’s yeast as a potential substitute for live algae in aquaculture diets: artemia as a case study, J. World Aquacult. Soc. 21 (1990) 1 9. [58] J. Salinas-Chavira, C. Arzola, V. Gonza´lez-Vizcarra, O.M. Manrı´quez-Nu´n˜ez, M.F. Montan˜oGo´mez, J.D. Navarrete-Reyes, et al., Influence of feeding enzymatically hydrolyzed yeast cell wall on growth performance and digestive function of feedlot cattle during periods of elevated ambient temperature, Asian-Australas. J. Anim. Sci. 28 (9) (2015) 1288 1295. [59] G.L. Rumsey, J.E. Kinsella, K.J. Shetty, S.G. Hughes, Effect of high dietary concentrations of brewer’s dried yeast on growth performance and liver uricase in rainbow trout (Oncorhynchus mykiss), Anim. Feed Sci. Technol. 33 (1991) 177 183. [60] P. Spring, C. Wenk, K.A. Dawson, K.E. Newman, The effects of dietary mannanoligosaccharides on cecal parameters and the concentrations of enteric bacteria in the ceca of Salmonella-challenged broiler chicks, Poult. Sci. 79 (2000) 205 211. [61] M. Brummer, C. Jansen van Rensburg, C.A. Moran, Saccharomyces cerevisiae cell wall products: the effect on gut morphology and performance of broiler chickens, South Afr. J. Anim. Sci. 40 (2010) 14 21. [62] G. Kogan, A. Kocher, Role of yeast cell wall polysaccharides in pig nutrition and health protection, Livest. Sci. 109 (2007) 161 165. [63] M.S. Geier, V.A. Tork, G.E. Allison, K. Ophel-Keller, R.J. Hughes, Indigestible carbohydrates alter the intestinal microbiota but do not influence the performance of broiler chickens, J. Appl. Microbiol. 106 (2009) 1540 1548. [64] A.W. Zhang, B.D. Lee, S.K. Lee, K.W. Lee, G.H. An, K.B. Song, et al., Effects of yeast (Saccharomyces cerevisiae) cell components on growth performance, meat quality and ileal mucosa development of broiler chicks, Poult. Sci. 84 (2005) 1015 1021. [65] H. Lea, P. Spring, J. Taylor-Pickard, E. Burton, A natural carbohydrate fraction Actigen from Saccharomyces cerevisiae cell wall: effects on goblet cells, gut morphology and performance of broiler chickens, J. Appl. Anim. Nutr. 1 (2013) 1 7. [66] E.A. Yamada, V.C. Sgarbieri, Yeast (Saccharomyces cerevisiae) protein concentrate: preparation, chemical composition and nutritional and functional properties, J. Agric. Food Chem. 53 (2005) 3931 3936. [67] S.S. Omar, D.L. Merrifield, H. Kuhlwein, P.E.V. Williams, S.J. Davies, Biofuel derived yeast protein concentrate (YPC) as a novel feed ingredient in carp diets, Aquaculture 330333 (2012) 54 62. [68] B. Gause, J. Trushenski, Replacement of fish meal with ethanol yeast in the diets of sunshine bass, N. Am. J. Aquacult. 73 (2011) 97 103. [69] B. Gause, J. Trushenski, Production performance and stress tolerance of sunshine bass raised on reduced fish meal feeds containing ethanol yeast, N. Am. J. Aquacult. 73 (2011) 168 175. [70] K.S. Rameshwari, S. Karthikeyan, Distillery yeast sludge as an alternative feed resource in poultry, Int. J. Poult. Sci. 4 (2005) 787 789. [71] M.E. Ahmed, T.E. Abbas, M.A. Abdlhag, D.A. Mukhtar, Effect of dietary yeast (Saccharomyces cerevisiae) supplementation on performance, carcass characteristics and some metabolic responses of broilers, Anim. Vet. Sci. 3 (2015) 5 10. [72] J.S. Knott, G.C. Shurson, Nutritional value of yeast by-products from ethanol production for young pigs, Proceedings of the 65th Minnesota Nutrition Conference, St. Paul, MN, USA, Sep. 21-22, 2004, 142 159.
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[73] E.J. Burton, D.V. Scholey, P.E.V. Williams, Use of cereal crops for food and fuel—characterization of a novel bioethanol co-product for use in meat poultry diets, Food Energy Secur. 2 (2013) 197 206. [74] D.V. Scholey, N. Morgan, P. Williams, E.J. Burton, Effects of incorporating a novel bioethanol co-product in poultry diets on bird performance, Br. Poult. Abstr. 7 (2011) 57. [75] D.V. Scholey, P. Williams, E.J. Burton, Effect of carbohydrase enzymes on digesta viscosity of broiler chicks fed graded levels of a yeast protein concentrate, Br. Poult. Abstr. 9 (2013). [76] A. Cohen, K. Klasing, R. Ricklefs, Measuring circulating antioxidants in wild birds, Comp. Biochem. Physiol. 147 (2007) 110 121. [77] D.V. Scholey, P. Williams, E.J. Burton, Welfare implications for broiler chicks fed diets containing graded levels of potable alcohol yeast protein, in: Recent Advances in Animal Welfare Science III, June 21, 2012, York, 2012. [78] A.M. Amerah, C. Gilbert, P.H. Simmins, V. Ravindran, Influence of feed processing on the efficacy of exogenous enzymes in broiler diets, Worlds Poult. Sci. J. 67 (2011) 29 46.
Producing Animal Feed as a Coproduct of Biorefining Dawn Scholey and Emily Burton Nottingham Trent University, School of Animal, Rural and Environmental Sciences, Nottingham, United Kingdom
13.1 INTRODUCTION The process of biorefining has been defined as the sustainable processing of biomass into a spectrum of bio-based products (food, feed, chemicals, and minerals) and bioenergy (biofuels, power, and/or heat) [1]. In 2008, a lack of public awareness led to a controversy over the use of grain as a biorefinery feedstock to produce bioethanol [2], but contrary to popular belief, the process is actually an excellent example of biorefinery production of both fuel and nutrients. Although this issue is still apparent, it has been mitigated by the increasing need to address feed security in Europe, particularly in the case of protein supply for animal feed. The formulation of animal feeds with biorefinery coproducts allows the balance of feed and fiber from fuel production to be converted into the human food supply via animal production. This chapter will address the potential for feed protein production from ethanol biorefineries, with emphasis on the nutritional composition of the product and suitability for monogastric nutrition. Animal feed protein requires several considerations in addition to the raw protein content of the material, including the impact of nonprotein components which may have potentially antinutritional qualities which then need to be taken into account during feed formulation. In biorefining, these would include high-fiber-content material as fiber can have deleterious effects on monogastric animal production due to increased gut fill, increased gut transit, and reduced nutrient digestibility. However, these nonprotein components can in some cases have a beneficial effect on animal health, particularly when they contain an additional energy source, such as in oil seed meal production. Some components may even have a beneficial biological effect, such as yeast used in the biorefinery process, which will be discussed in Advanced Bioprocessing for Alternative Fuels, Biobased Chemicals, and Bioproducts. DOI: https://doi.org/10.1016/B978-0-12-817941-3.00013-9 © 2019 Elsevier Inc. All rights reserved.
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more detail later in this chapter. Second, the amino acid profile of any protein is vital when the material is incorporated into animal feed, with lysine and methionine being of major importance as the first limiting amino acids. Another important factor is the biological availability of the protein source, as associations with plant carbohydrates will influence the digestibility of the protein in animals [3]. Mitigation of antinutritional factors will also need to be considered [4]. Finally, and probably most importantly to the animal feed industry, the production must be financially viable and able to be reliably produced in sufficient quantities while also being able to compete with other protein sources in terms of cost, variability, and nutritional value. Without production of at least 50,000 t a year, a product would not be considered a commodity protein to be used in feed formulation, although a high-value additive with additional nutritional benefits could still have applications.
13.2 APPLICATION OF PROTEINS IN FEED FOR LIVESTOCK The feed industry can be separated into two distinct sectors: one supplying ruminant species (mainly cattle and sheep) and the other addressing nonruminant species, such as fish, pigs, and poultry. Ruminants are able to utilize energy from fiber and amino acids from nonprotein nitrogen, and therefore, the typically fiber-rich outputs of biorefineries [e.g., distiller’s dried grains with solubles (DDGS)] are usually incorporated into diets in this sector. Higher values protein coproducts, which tend to be produced in smaller quantities, tend to be directed toward the monogastric animal feed market. Animal needs vary depending on life stage and nutritional requirements of the animals, with the extreme growth rates in commercial fish, pig, and poultry production making this sector particularly sensitive to variations in feed and protein quality. This makes many coproducts unsuitable for incorporation into monogastric diets. Other considerations include protein density and the presence of factors, such as lectins, trypsin inhibitors, and β-conglycinin, which have antinutritional properties [5]. However, in addition to the animal requirements, considerations from the feed industry and the end consumer also need to be understood. The feed industry requires several factors to ensure widespread adoption of a raw material, including reliable supply and quality, critical production volume, and suitable quality-control systems. As feed density is so important, particularly in neonatal nutrition, it is vital that protein sources do not limit feed intake and therefore diet nutrient intake. In monogastric nutrition, protein must have little fiber contamination as this can affect feed intake and ultimately acceptance of the product by the formulator. Consumer choice is a relatively new area for consideration in animal and feed production, gaining prominence only in recent years [6], with their opinions now having a large impact on the choice of products used in animal feed. Consumers are increasingly aware of the importance of food safety and also of the role of animal feed in the food chain due to a number of food-
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safety scares [7]. Over the last 30 years, there have been several food scares in the EU, including most recently the Campylobacter contamination of poultry meat [8] and the dioxin residue issue [9]. There is now much greater consumer demand for choice and information regarding food safety [including genetic modification (GM)], animal welfare, and environmental sustainability. Issues around sustainability include overfishing, hence the use of fish meal as a protein source [10], and the reduction in food miles for raw materials. The welfare of production animals is also of greater import to consumers, with more public concern regarding the feeding of GM grains in animal diets.
13.2.1 Protein Value There are several factors, which influence protein value, including geographical location, which varies from the least expensive (United States) to the costliest (Asia and the EU) for soya and DDGS. Second, the composition of the protein is important to the value of the protein, and there is a further financial benefit from additional nutrients and health benefits, as is the case with yeast. Finally, there is a strong drive to source a more environmentally sustainable product than soya, with the potential for strong local supplies, which adds a further financial premium to locally produced protein. This would suggest a strong market for yeast protein from biorefineries, particularly in those markets where soya and DDGS are most expensive.
13.3 THE VALUE OF A BIOREFINERY A new biorefinery is a significant financial investment which requires a solid business case for both the production of the primary product and the associated coproduct(s). In construction of new biorefinery plants, there are economies of scale which may benefit coproduct production, including combined power use and already established supply chains for both supply of raw materials and transport of finished products. In larger plants, the increased scale and high volume can result in continuous supply, improved product consistency, and the addition of novel technology to add further value to coproduct streams. This will be of additional financial benefit both for new and existing biorefineries planning future construction projects. Biorefining processes are already used extensively in livestock feed production with the biorefining process having a significant positive impact on the protein quality in many cases. For example, the initial process for the production of soya oil from beans was modified to improve the coproduct, soya bean meal (SBM), and to enable its widespread use in animal feed. In this case, processing was used to reduce antinutritional components by inactivation of the trypsin inhibitors, improving the digestibility of the SBM. Originally, processing was a combination of physical processing, heat, and
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solvent extraction [11]. However, recent developments have led to the selective use of in-feed enzymes to eliminate detrimental plant components, such as phytate [12]. There are several other examples of biorefining to produce a coproduct, including meal production from rapeseed oil and the production of monosodium glutamate from rice, which results in a rice protein concentrate. Later, this chapter will specifically look at a more recent process, the production of yeast as an additional value coproduct from bioethanol production [13]. It is vital that all aspects are considered when the production of a coproduct is added to a biorefining process. For example, it was proposed that a high-protein coproduct of green beet tops could be used to add value to sugar beet production [14]. It was suggested that this high-protein meal could be produced; however, the system proved to require 56 t of leaf material to provide a single tonne of feed protein, and therefore, the system was only cost effective if the leaves did not require transport to the factory. It may be in the future that these beet leaves can be pressed to make a semimeal product [15], but this example shows that without a sustainable supply chain, both into and out of the refinery, the coproduct may not be economically viable.
13.4 COPRODUCT STREAMS FROM DRY GRIND ETHANOL PRODUCTION Renewable fuel in the form of bioethanol can be produced via the fermentation and distillation of a starch feedstock, such as cereal, grain, or tuber. The value of the resulting ethanol is very dependent on the price of mineral oil, and when the price of the latter falls, this places greater emphasis on the value of the coproduct of the process in order for the refinery to remain profitable as a whole. The bioethanol process results in a number of coproducts that are typically used in animal feed, the major one of which is DDGS. Ethanol production via a feedstock is typically from a dry grind process [16] which ferments starch to ethanol resulting in the production of carbon dioxide and DDGS in approximately equal quantities [17].
13.4.1 Distiller’s Dried Grains With Solubles In maize alcohol production, currently 50% of the revenue of the process is derived from the coproducts [18]. However, the promotion of renewable fuels as a means of reducing carbon emissions has massively increased bioethanol production and consequently coproduct volume, which negatively impacts its value [19]. The grain residue left after fermentation includes all the grain proteins, aleuronic layers, fibrous carbohydrates and lipids. Together, with the yeast produced in the process, they are combined and dried to produce DDGS. DDGS tends to be fed almost solely to ruminants due to the high fiber to
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protein ratio (27:33) which is ideal for ruminant feed [20] but limits the use of the product for monogastric species [17,21]. One of the main issues with DDGS is that in order to store and transport it, the product is dried, using approximately 40% of the energy costs from the whole refining process. Increasing DDGS inclusion in animal feed has also been negatively correlated with pellet durability [22], which is highly correlated with feed efficiency [23], and increasing energy usage in the condenser due to the viscosity of the mash [24]. The high energy expenditure limits the value of DDGS as a feed ingredient, especially in the ruminant market, where lowcost materials are vital due to the high volume of feed required for every kg of meat (10 kg/kg for cattle, compared with 2 kg/kg for poultry [25]). There have been transport difficulties observed with DDGS, including caking and poor flow due to water adsorption [26]. Particle separation has been observed with DDGS [27], and there have been large variations in the physical characteristics reported [27,28]. Any variation in bulk density of products will influence transport costs, particularly if transporters cannot be filled consistently [29].
13.4.2 Differentiation of Distiller’s Dried Grains With Solubles There has been considerable study into the production of a differentiated DDGS with higher protein, using a combination of sieving and air flow [30,31]. This Elusieve process comprises a rotary sifter with three decks for stack sieving of DDGS. In use, it has been shown to produce four fractions, including a small particle size, modified DDGS, with an increased protein content by between 2% and 2.6% while also increasing the neutral detergent fiber value of the larger particle size fiber fraction by as much as 37% [32]. Economically, based on a bioethanol plant using 2030 t/day of corn and a capital investment of 1.4-million dollars, the modified process produces DDGS at a cost of $0.84/t [33]. The highest potential revenue stream from this method is suggested to be three coproduct streams of a high-protein (5% higher) DDGS, a traditional DDGS product, and a fiber-rich product [31]. Inclusion of this product in feed improved some nutritional characteristics, including higher metabolizable energy [34], increased bodyweights in broilers [35], and increased energy digestibility in fish [36]. Alternatively, this process can be amended to separate corn kernels preprocessing to improve ethanol output by around 4.5% based on a 50-million gallons a year plant, which would recoup the capital output in an estimated 3 years, depending on corn and ethanol prices [37].
13.5 THE POTENTIAL FOR YEAST PROTEIN Yeast multiplies during the bioethanol process and contributes 20% of the protein in the dry matter (DM) of DDGS [38]. If this material can be isolated
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6.0
60.0
DDGS maize
50.0
Maize YPC
5.0
Percentage
Percentage
70.0
40.0 30.0 20.0 10.0
4.0 3.0 2.0 1.0
0.0
n ei ot Pr
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FIGURE 13.1 A comparison of the nutritional content of yeast protein concentrate and DDGS, both from maize bioethanol production. DDGS, Distiller’s dried grains with solubles.
in an economically viable process, this would be a substantial potential protein stream. Bioethanol relies on economies of scale, and a typical dry grind plant will utilize up to 1-million tonnes of grain a year, which a significant potential production of yeast coproduct. It is estimated that for every gram of fermented starch, 0.071 g of yeast is produced [39]. Therefore, a 400million-liter bioethanol plant with an intake of 1.1-million tonnes of wheat will theoretically produce 48,000 t of yeast per annum. The potential for yeast to be marketed for around d600 a tonne makes yeast separation a favorable financial proposition. Yeast is rich in both vitamins and minerals and is the most valuable component of DDGS. It also contains valuable proteins, nucleotides, and is high in both glutamic acid and inositol [40]. A comparison of the composition of DDGS and yeast protein concentrate (YPC), both from maize bioethanol production, is shown in Fig. 13.1. The nucleotides and β-glucan together with the mannan-oligosaccharides (MOS) in the yeast cell wall can have positive effects on performance in both poultry and pigs [41,42]. MOS have prebiotic properties and therefore have been investigated as potential mitigators of the ban on antibiotic growth promotors in the EU. Studies have shown positive effects on gut microbiota, immunity and disease resistance, as well as improved gut morphology, and mucus production [43,44].
13.5.1 Yeast Separation Often yeast is removed from potable alcohol distilleries to reduce the overall yeast concentration, usually by mechanical means via one of four methods: pressured leaf filters, vacuum drum filtration, membrane filtration, and centrifugation. The first two listed are inefficient and use diatomaceous earth.
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Membrane filtration involves cross flow technology and can involve vibrating filters. Centrifugal techniques include a decanter centrifugation and disc nozzle separation which will be discussed individually.
13.5.1.1 Separation Using Decanter Centrifugation A decanter centrifuge can be used to separate fiber from stillage and removes water. This type of separation is rapid and can therefore allow high throughput while giving some control over the “cut” of the output by altering feed rates and the size of the output value. The equipment construction is robust and flexible, allowing different consistency slurries to be separated consecutively with no issues. These types of separator are also simple to clean and maintain. 13.5.1.2 Separation Using a Sedicanter A sedicanter is a combination of separator and decanter, which clarifies the suspension while producing a solid output. It can also process a large amount of solid material and has an adjustable impeller to enable variability in feed input. It is reported by the manufacturers to use 20% 50% less installed power than conventional decanters, and less energy is required for drying due to the higher final DM of the output. There are reports of a 28% DM output with increased alcohol yield [45]. The clarified suspension may be suitable for use in liquid feedanaerobic digestion (AD). Recent studies have shown that a yeast concentrate separated using a sedicanter had a similar amino acid content to soya. 13.5.1.3 Separation Using a Vibration Sheer Enhanced Process Unit The vibration sheer enhanced process dewatering unit consists of a stack of Teflon membranes with a strong vibration to create highshear forces which facilitate the transport of liquids through the membranes and reduce fouling of the system. The output of this system is similar to a decanter centrifuge [46], but the liquid stream contains no solid particles, whereas a decanter will not separate all fibrous material completely. However, using these systems result in high operating and energy costs, with particularly high water consumption. 13.5.1.4 Separation Using Filtration and Hydrocyclones For batch fermentations, centrifuges are often used for yeast recovery but filter aid filtration followed by release of the yeast using hydrocyclones may be a more cost-effective process. Studies used a 1.14 m2 filtration area with perlite as a filtration aid and two 30 mm hydrocyclones. The use of the hydrocyclones alone only produced an efficiency of 1%, but when they were used to remove perlite from the filtration cake, the efficient of separation was increased to 95%, suggesting that for batch production, this process may be feasible [47].
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13.5.1.5 Separation Using a Disc Stack A disc separator is a continuous bowl centrifugal separator comprising a stack of discs which separate solids by size and eject a liquid stream via centripetal force and the solid stream to an ejecting nozzle. An automatic disc separator requires a relatively low inlet concentration due to limited space, and frequent ejections of the solid material increase wear and maintenance, and increasing the size of the machine to mitigate these issues is a costly solution. A nozzle disc separator is an improved process with continuous yeast discharge although this system is still prone to blockage and the resulting solid stream may be wetter.
13.5.2 Yeast Protein Concentrate Production A process which combines the use of a decanter with a disc nozzle separator has been developed to produce a high-protein YPC. This method maintains three product streams: the YPC, a pentose sugar syrup, and a fiber product very similar to DDGS (with protein reduction of around 5% compared to traditional DDGS). This process can be included as an additional module to existing first-generation bioethanol plants, with little alteration to the engineering of the plant. The additional product streams will increase profitability of ethanol production, although any potential loss in the value of the reduced protein DDGS will have to be considered. In this process, a horizontal bowl decanter removes the majority of the fiber to obtain a liquid fraction with around 5% DM and little fibrous contamination. This liquid stream was then dewatered using a continuously operating nozzle centrifuge designed for liquid separation (GEA Westfalia, Germany). This process produced a liquid output with less than 0.2% DM and a solid discharge with approximately 40% DM. The solid material was dried to a powder, referred to throughout this chapter as YPC.
13.5.3 Drying Yeast Drying is an issue for any biorefinery coproduct as the material needs to be uniformly dried to ease transport and handling while increasing shelf life. A powder can also be easily and thoroughly mixed with other feed materials prior to pelleting of diets. However, drying can affect nutritional content of the finished product, although product deterioration is usually due to the application of excess heat rather than the moisture removal [48]. Overheating can lead to Maillard reactions, which binds the amino acid lysine in a form which cannot be digested by animals. This is a particular problem for poultry where lysine is the first limiting essential amino acid. A comparison of bioethanol yeast products dried by ring, spray, and freeze drying showed that protein quality was deleteriously affected by ring drying [46]. Spray drying has been shown to be a suitable method for
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preserving yeast protein from bioethanol, with a similar amino acid profile to soya [49]. However, it is a high energy demand process [50], and ring drying is a more suitable process for large throughput processes, though this needs to be carefully managed to avoid burning. Washing the yeast prior to drying may remove some soluble sugars and improve the flow of the material while increasing protein, energy, and amino acid content [51].
13.5.4 Yeast and Yeast Protein Concentrate in Animal Feeds The yeast component of the DDGS has been estimated to make up 5.3% of the DDGS [52] contributing 20% toward DDGS proteins in maize bioethanol [38]. This yeast is a valuable source of protein if it can be economically separated from DDGS, leaving a high fiber fraction suitable for ruminant feeding. Yeast contains valuable protein as well as additional vitamins and nucleotides [39], but there have been several reports of reduced intake in poultry fed diets containing yeast [53,54]. Feeding the yeast in either a pelleted or crumb diet has been shown to increase intake [53], which suggests that the issue is one of feed form. Spray drying of yeast creates a very small particle size, and this has been shown to reduce intake in chickens [55]. In bioethanol yeast, there may be additional issues, as high ethanol exposure may create a thicker, toughened cell wall, which is more resistant to enzyme proteolysis [56]. Certainly there is evidence that disrupting the yeast cell wall can improve nutrient utilization compared with whole yeast [57,58]. Rumsey et al. [59] suggest that access to the contents of the yeast cell is paramount for nutrient use, but the cell wall itself also contains biologically important fractions which have their own role in nutrition. Yeast cell walls make up 20% of the dry weight [51], containing sugars including MOS, which have been shown to have a prebiotic potential via three modes of action: improved pathogen adsorption [60], improved gut health [61], and immune modulation by binding to specific immune receptors [62]. MOS inclusion in diets has also been shown to improve bodyweight gain and feed conversion ratio in poultry [40] and pigs [41] while shifting intestinal microflora toward beneficial organisms [63]. Cell wall components have been shown to improve intestinal morphology and the proportion of mucosa producing goblet cells in chicks [64,65]. Finally, there is a suggestion that bioethanol yeast may be more exhausted of cellular components, and protein content has been found to be lower for ethanol yeast than traditional brewer’s yeast [66]. Yeast has been separated from bioethanol and distillery stillage and fed to several species with success. A bioethanol YPC was included in carp diets with 15% 20% the optimal inclusion [67] for performance, and similar results were found in sunshine bass, although inclusion levels over 20% were found to impair palatability and production performance [68,69]. In poultry, distillery sludge showed a decrease in weight gain with rate of inclusion
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[70], and a recent study showed that yeast can reduce serum cholesterol in broilers while increasing performance [71]. Knott and Shurson [72] also fed a yeast cream separated from a corn biorefinery, with a resulting increase in gain: feed in late grower pigs. However, gut health improvements were seen only with a residual soluble fraction, which questions whether some of the beneficial yeast components may be soluble and therefore removed during any dewatering process. Bioethanol YPC has a digestible amino acid content comparable with soya for broiler chicks [73] although this is heavily influenced by the drying process used [48]. The shear forces of the disc stack disrupt the yeast cell walls; and therefore, the cell contents are more easily digestible. Up to 17.5% inclusion of YPC in pelleted diets improved broiler performance [72]. However, mash diets containing spray dried YPC gave a reduction in bird bodyweight gain, possibly due to the small particle size of the spray-dried material, which increases feeding time and energy required for feed manipulation. Digesta viscosity can also be increased by fine particles [55], and measurement of the digesta of the birds showed this to be the case [74]. The increased digesta viscosity reduces nutrient availability and accelerates feed passage rate with detrimental effects on bird growth and welfare, but recent studies show that this issue may be mitigated by enzyme addition [75]. There have been historical suggestions that the nonprotein nitrogen portion of yeast increases uric acid production with detrimental effects on bird welfare. However, there is research which suggests that uric acid may be a powerful antioxidant therefore potentially beneficial [76]. Recent investigations feeding bioethanol yeast to broilers have provided little evidence that serum uric acid is actually increased with increasing yeast inclusion [77]. YPC inclusion up to 62.5% (replacing soya) does not appear to have a detrimental effect on pellet quality, with a small reduction in the number of fines observed [73]. However, this study was carried out in a small-scale facility, and this may not be indicative of a higher throughput commercial process. High-quality pellets improve bird performance [78] due to reduced feeding time, and diets with many fines can lead to poor flock uniformity. Another unconsidered benefit from feeding YPC is the potential of additional value from the phosphorus available in the product. Phosphorus is an expensive and limited resource but is an absolute requirement for pigs and poultry to maintain skeletal integrity so it is supplemented routinely. Inclusion of YPC increased bone mineralization [73] and was shown to be able to replace up to 5 g/kg phosphorus [73], with potential cost savings to diet manufacturers.
13.6 CONCLUSION Currently ethanol biorefineries produce mainly DDGS as a valuable feed ingredient for cattle. However, protein is an expanding, high value market with potential to grow 40% by 2050. Development of a highly nutritional
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value coproduct will mitigate controversy over the diversion of cereal crops from animal feed to first-generation bioethanol. A high-protein coproduct also opens up the fast expanding market for pig, poultry, and fish feed and will contribute toward reducing the reliance on imported soya in the EU. There is a drive with the fluctuation in ethanol margins globally to increase efficiency and improve profit via addition of multiple coproduct streams. As energy costs rise, the cost of drying any coproducts needs to be considered, and alternative methods of water removal will become more attractive. Biorefinery production of ethanol could be rebranded to emphasize the traditional, sustainable process of producing feed, protein, and ethanol.
13.7 FUTURE OUTLOOK There is a potential for production of yeast as a bioethanol coproduct to also create new product streams. The fiber fraction produced remains suitable for ruminant feeding as the protein content is still more than sufficient for those species. There is also potential for this fibrous material in manufacturing or as a more sustainable alternative to peat as a growing media for plants. Initial studies utilizing fibrous waste from AD have shown potential for utilization of remaining nutrients by crops. There are a number of potential uses for the liquid fraction including as a feed for an anaerobic digester, or as a potential replacement for molasses in equine diets. The yeast separation process has potential for improvement, particularly in the opportunities for modern enzyme applications. Enzymes have been shown to reduce the supernatant of birds fed YPC, with the suggestion that a cocktail of carbohydrase enzymes will be efficacious in treating viscosity issues. Currently, enzymes are often used in bioethanol production to increase yield, but these could be optimized to positively affect both the bioethanol process, coproduct production, and the composition of the final product. Green biorefineries can use a wide range of biomass including grass, sugar beet leaves, or clover with several projects in the EU concentrating on zero waste production with zero emissions. These processes rely on a mechanical refiner to break down the leafy material, and the process can produce a protein with potential for monogastric feed, fiber for manufacturing, and a soluble’s output containing amino acids, sugars, minerals, and organic acids with the potential for nutritional or feedstock use. These types of refineries along with more traditional cellulosic ethanol production have the future potential for addition of a portfolio of coproducts to improve the sustainability of the process.
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