Synergetic co-production of beer colouring agent and solid fuel from brewers' spent grain in the circular economy perspective

sustainability Article Synergetic Co-Production of Beer Colouring Agent and Solid Fuel from Brewers’ Spent Grain in the Circular Economy Perspective Mateusz Jackowski 1,* , Łukasz Nied´zwiecki 2,* , Krzysztof Mo´scicki 2, Amit Arora 3, Muhammad Azam Saeed 4, Krystian Krochmalny 2, Jakub Pawliczek 2, Anna Trusek 1, Magdalena Lech 1, Jan Skˇrínský5, Jakub ˇ Cespiva 5, Ján Vereš 5, Tadeáš Ochodek 5and Halina Pawlak-Kruczek 2   Citation: Jackowski, M.; Nied´zwiecki, Ł.; Mo´scicki, K.; Arora, A.; Saeed, M.A.; Krochmalny, K.; Pawliczek, J.; Trusek, A.; Lech, M.; Skˇrínský, J.; et al. Synergetic Co-Production of Beer Colouring Agent and Solid Fuel from Brewers’ Spent Grain in the Circular Economy Perspective. Sustainability 2021,13, 10480. https://doi.org/10.3390/ su131810480 Academic Editors: Tiziana Amoriello and Roberto Ciccoritti Received: 30 August 2021 Accepted: 13 September 2021 Published: 21 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Micro, Nano and Bioprocess Engineering, Faculty of Chemistry, Wroclaw University of Science and Technology, 50-373 Wroclaw, Poland; [email protected] (A.T.); [email protected] (M.L.) 2Department of Energy Conversion Engineering, Faculty of Mechanical and Power Engineering, Wrocław University of Science and Technology, 50-370 Wroclaw, Poland; [email protected] (K.M.); [email protected] (K.K.); [email protected].edu.pl (J.P.); [email protected] (H.P.-K.) 3Department of Chemical Engineering, Shaheed Bhagat Singh State University, Punjab, Ferozepur 152004, India; amitar[email protected] 4Department of Chemical Engineering, University of Engineering and Technology, Lahore 39161, Pakistan; [email protected] 5VŠB—Energy Research Center, Technical University of Ostrava, 708 00 Ostrava, Czech Republic; [email protected] (J.S.); [email protected] (J. ˇ C.); [email protected] (J.V.); [email protected] (T.O.) *Correspondence: [email protected] (M.J.); [email protected] (Ł.N.) Abstract: Brewers’ Spent Grain is a by-product of the brewing process, with potential applications for energy purposes. This paper presents the results of an investigation aiming at valorization of this residue by torrefaction, making product for two purposes: a solid fuel that could be used for generation of heat for the brewery and a colouring agent that could replace colouring malt for the production of dark beers. Decreased consumption of malt for such purposes would have a positive influence on the sustainability of brewing. Torrefaction was performed at temperatures ranging between 180 ◦ C and 300 ◦ C, with a residence time between 20 and 60 min. For the most severe torrefaction conditions (300 ◦ C, 60 min), the higher heating value of torrefied BSG reached 25 MJ/kg. However, the best beer colouring properties were achieved for mild torrefaction conditions, i.e., 180 ◦ C for 60 min and 210 ◦ C for 40 min, reaching European Brewery Convention colours of 145 and 159, respectively. From the solid fuel properties perspective, the improvements offered by torrefaction in such mild conditions were modest. Overall, the obtained results suggest some trade-off between the optimum colouring properties and optimum solid fuel properties that need to be considered when such dual-purpose torrefaction of BSG for brewery purposes is implemented. Keywords: torrefaction; brewers’ spent grain; sustainable brewing; circular economy; beer 1. Introduction Beer has been known to humanity for millennia. The first evidence of brewing was found in the Neolithic period, and the first industrial-scale breweries were discovered in ancient Egypt [ 1 , 2 ]. Today, it is one of the most popular drinks in the world. Numerous types of beer exist that vary in colour, alcohol content, used hops, and yeasts. Those properties have their origin in the production process. Beer colour may be obtained in two ways. The first is to add roasted malt during mashing. Dark malt is produced in the malthouse from standard malt grains by roasting. Such malt is biologically inactive and does not contribute to enzyme activity during mashing. Another approach to making beer darker is to use dark malt extracts added during wort boiling [3–5]. Sustainability 2021,13, 10480. https://doi.org/10.3390/su131810480 https://www.mdpi.com/journal/sustainability Sustainability 2021,13, 10480 2 of 17 Brewers’ Spent Grain (BSG) is a residue left after lautering. BSG has been considered as a feedstock for many different processes [ 6 , 7 ]. There are various novel, sustainable ways of using BSG, including using it as an additive to sausages [ 8 ] or bakery products [ 9 – 11 ], extraction of anti-oxidants [ 12 , 13 ], including polyphenols [ 14 , 15 ], extraction of proteins [ 16 ], functional cardioprotective lipids [ 17 ], material for disposable trays [ 18 ], natural rubber modifier [ 19 ], fodder for edible insects [ 20 ], as well as feedstock for production of pigments [ 21 ] and biochar, for subsequent use as soil amendment [ 22 ] or sustainable material for electrodes [23]. Biomass is nowadays deemed an energy source with utmost importance from the point of view of energy transition towards sustainable energy supply [ 24 – 27 ]. Worldwide production of BSG has been estimated as approximately 38.6 million tons [ 28 ]. BSG can be considered as a type of biomass, and its potential use as fuel has been suggested by several authors to date [ 29 – 31 ]. As novel valorization routes for waste-to-energy are deemed promising by many [ 32 ], some authors have suggested the use of hydrothermal carbonization (HTC) as a suitable valorization process, allowing improvement of fuel parameters, with respect to unprocessed BSG, i.e., an increase in the higher heating value (HHV) and reduction of ash content [ 33 ]. Analysis of the composition of the effluent from HTC of BSG, performed by Jackowski et al. [ 34 ], using gas chromatography coupled with mass spectrometry (GC-MS), suggested its utilization in the anaerobic digestion process [ 34 ]. Poerschmann et al. concluded that hydrochars produced from BSG could be suitable for soil-improvement applications [ 35 ]. Pyrolysis GC-MS (Py-GC-MS) analysis of BSG and corresponding hydrochars was performed by Olszewski et al. [ 36 ], reporting fewer Ncompounds for pyrolysis of hydrochars performed at relatively low temperatures compared to pyrolysis of raw BSG [ 36 ]. The Maillard reaction, occurring during hydrothermal carbonization of BSG, was suspected to be the reason behind such results [ 36 ]. Ferreira et al. suggested using BSG as a feedstock for steam gasification and obtained good quality gas with a heating value ranging between 8.1 and 9.0 MJ/m 3N , reaching concentrations of hydrogen between 22.8% and 30.2% [ 37 ]. Dudek et al. investigated two alternative waste-to-energy and waste-to-carbon routes for BSG, one using BSG as anaerobic digestion substrate and another utilizing torrefaction of BSG to produce an additive enhancing anaerobic digestion, concluding that synergy could be obtained when using these routes in parallel [ 38 ]. The study also highlighted the potential role of Maillard reaction products, which form during torrefaction of BSG [ 38 ]. Products of Maillard reactions could prove useful for use as colouring agents [ 39 ], which is very important from the point of view of the sensory analysis of beers [ 40 ]. Currently, roasting of malt is the most commonly used method. The use of residues, such as spent grain, for this purpose, is tempting from the point of view of circular economy principles. Little information is available in the literature on the torrefaction of BSG to improve its fuel properties. The torrefaction process is used for thermal valorization of solid fuels, thus improving their properties, and in consequence, promoting sustainability in the energy generation sector [ 41 – 43 ]. Torrefaction is sometimes called slow pyrolysis, and it typically takes place at temperatures between 250 ◦ C and 300 ◦ C [ 44 – 47 ], with residence times ranging between 10 and 60 min [ 48 – 51 ]. Torrefied biomass can be an attractive fuel for the power industry for combustion and co-firing [ 52 – 55 ], mainly due to increased grindability [ 56 , 57 ], which extends the time between renovations of the mill [ 58 ], and energy density [ 59 – 62 ]. In addition, reduction of moisture content could be beneficial in many energy conversion processes [ 45 , 63 , 64 ]. Moreover, the advantages mentioned above are beneficial from the point of view of fuel logistics and storage [ 65 , 66 ]. Furthermore, torrefaction typically decreases oxygen content and increases the carbon content of the valorized fuel [ 53 , 67 – 71 ]. This enhancement could be deemed beneficial from the point of view of pyrolysis [ 72 – 75 ] or gasification [ 76 – 79 ]. Regarding reactivity of the torrefied material, Huescar Medina et al. [ 80 ] observed that torrefaction slightly increased the reactivity of torrefied spruce wood in terms of K St (deflagration index), P max (maximum explosion pressure), and flame speed, whereas Jian et al. observed reduced char reactivity Sustainability 2021,13, 10480 3 of 17 at elevated pyrolysis temperatures [ 81 ]. Such behaviour contributed to significant crosslinking and poly-condensation in severe torrefaction conditions, eliminating the active sites and thus reducing the conversion and reactivity of char during pyrolysis [81]. Torrefaction of BSG could potentially enhance its fuel properties, thus making it more suitable for energy purposes. However, it seems plausible to hypothesize that torrefaction of BSG could be used to produce a colouring agent, which could replace roasted malt. This could offer important benefits through the effect of synergy, as one installation could be used to turn residue of beer production into brewing substrate, and at the same time, supply a brewery with its own fuel. This, in turn, could prove beneficial in supplying the energy needs of a brewery from renewable energy source (biomass) and facilitating practical implementation of the circular economy rules by reusing residues for production processes. Therefore, the goal of this interdisciplinary paper is the experimental validation of such an approach by assessing improvements of the fuel properties of torrefied BSG and determination of the suitability of using torrefied BSG as a beer colouring agent with the additional determination of possible influence on taste through sensory analyses. 2. Materials and Methods Since the beer production process could significantly influence the properties of the corresponding BSG [ 82 ], the beer production process was performed in a controlled manner in a pilot-scale brewery and characterization of the main product of brewing (beer) is also an intrinsic part of this investigation. The residual BSG for further tests was produced in a pilot-scale brewing installation at the Wrocław University of Science and Technology. This pilot scale unit can be used for brewing approximately 1 hL of beer, with full control over each unit operation of the process (Figure 1). Sustainability 2021, 13, x FOR PEER REVIEW 3 of 18 Medina et al. [80] observed that torrefaction slightly increased the reactivity of torrefied spruce wood in terms of K St (deflagration index), P max (maximum explosion pressure), and flame speed, whereas Jian et al. observed reduced char reactivity at elevated pyrolysis temperatures [81]. Such behaviour contributed to significant cross-linking and poly-condensation in severe torrefaction conditions, eliminating the active sites and thus reducing the conversion and reactivity of char during pyrolysis [81]. Torrefaction of BSG could potentially enhance its fuel properties, thus making it more suitable for energy purposes. However, it seems plausible to hypothesize that torrefaction of BSG could be used to produce a colouring agent, which could replace roasted malt. This could offer important benefits through the effect of synergy, as one installation could be used to turn residue of beer production into brewing substrate, and at the same time, supply a brewery with its own fuel. This, in turn, could prove beneficial in supplying the energy needs of a brewery from renewable energy source (biomass) and facilitating practical implementation of the circular economy rules by reusing residues for production processes. Therefore, the goal of this interdisciplinary paper is the experimental validation of such an approach by assessing improvements of the fuel properties of torrefied BSG and determination of the suitability of using torrefied BSG as a beer colouring agent with the additional determination of possible influence on taste through sensory analyses. 2. Materials and Methods Since the beer production process could significantly influence the properties of the corresponding BSG [82], the beer production process was performed in a controlled manner in a pilot-scale brewery and characterization of the main product of brewing (beer) is also an intrinsic part of this investigation. The residual BSG for further tests was produced in a pilot-scale brewing installation at the Wrocław University of Science and Technology. This pilot scale unit can be used for brewing approximately 1 hL of beer, with full control over each unit operation of the process (Figure 1). Figure 1. Pilot-scale installation diagram, with special emphasis on unit operations with heat/cold demand. Heat supply marked with arrows, cold supply marked with a arrow (1—mashing; 2—lautering; 3—boiling; 4—wort separation in a whirlpool; 5—wort cooling; 6—fermentation; M—malt; BSG—Brewers’ Spent Grain; H—hops; Y—yeasts). Figure 1. Pilot-scale installation diagram, with special emphasis on unit operations with heat/cold demand. Heat supply marked with Sustainability 2021, 13, x FOR PEER REVIEW 3 of 18 Medina et al. [80] observed that torrefaction slightly increased the reactivity of torrefied spruce wood in terms of K St (deflagration index), P max (maximum explosion pressure), and flame speed, whereas Jian et al. observed reduced char reactivity at elevated pyrolysis temperatures [81]. Such behaviour contributed to significant cross-linking and poly-condensation in severe torrefaction conditions, eliminating the active sites and thus reducing the conversion and reactivity of char during pyrolysis [81]. Torrefaction of BSG could potentially enhance its fuel properties, thus making it more suitable for energy purposes. However, it seems plausible to hypothesize that torrefaction of BSG could be used to produce a colouring agent, which could replace roasted malt. This could offer important benefits through the effect of synergy, as one installation could be used to turn residue of beer production into brewing substrate, and at the same time, supply a brewery with its own fuel. This, in turn, could prove beneficial in supplying the energy needs of a brewery from renewable energy source (biomass) and facilitating practical implementation of the circular economy rules by reusing residues for production processes. Therefore, the goal of this interdisciplinary paper is the experimental validation of such an approach by assessing improvements of the fuel properties of torrefied BSG and determination of the suitability of using torrefied BSG as a beer colouring agent with the additional determination of possible influence on taste through sensory analyses. 2. Materials and Methods Since the beer production process could significantly influence the properties of the corresponding BSG [82], the beer production process was performed in a controlled manner in a pilot-scale brewery and characterization of the main product of brewing (beer) is also an intrinsic part of this investigation. The residual BSG for further tests was produced in a pilot-scale brewing installation at the Wrocław University of Science and Technology. This pilot scale unit can be used for brewing approximately 1 hL of beer, with full control over each unit operation of the process (Figure 1). Figure 1. Pilot-scale installation diagram, with special emphasis on unit operations with heat/cold demand. Heat supply marked with arrows, cold supply marked with a arrow (1—mashing; 2—lautering; 3—boiling; 4—wort separation in a whirlpool; 5—wort cooling; 6—fermentation; M—malt; BSG—Brewers’ Spent Grain; H—hops; Y—yeasts). arrows, cold supply marked with a Sustainability 2021, 13, x FOR PEER REVIEW 3 of 18 Medina et al. [80] observed that torrefaction slightly increased the reactivity of torrefied spruce wood in terms of K St (deflagration index), P max (maximum explosion pressure), and flame speed, whereas Jian et al. observed reduced char reactivity at elevated pyrolysis temperatures [81]. Such behaviour contributed to significant cross-linking and poly-condensation in severe torrefaction conditions, eliminating the active sites and thus reducing the conversion and reactivity of char during pyrolysis [81]. Torrefaction of BSG could potentially enhance its fuel properties, thus making it more suitable for energy purposes. However, it seems plausible to hypothesize that torrefaction of BSG could be used to produce a colouring agent, which could replace roasted malt. This could offer important benefits through the effect of synergy, as one installation could be used to turn residue of beer production into brewing substrate, and at the same time, supply a brewery with its own fuel. This, in turn, could prove beneficial in supplying the energy needs of a brewery from renewable energy source (biomass) and facilitating practical implementation of the circular economy rules by reusing residues for production processes. Therefore, the goal of this interdisciplinary paper is the experimental validation of such an approach by assessing improvements of the fuel properties of torrefied BSG and determination of the suitability of using torrefied BSG as a beer colouring agent with the additional determination of possible influence on taste through sensory analyses. 2. Materials and Methods Since the beer production process could significantly influence the properties of the corresponding BSG [82], the beer production process was performed in a controlled manner in a pilot-scale brewery and characterization of the main product of brewing (beer) is also an intrinsic part of this investigation. The residual BSG for further tests was produced in a pilot-scale brewing installation at the Wrocław University of Science and Technology. This pilot scale unit can be used for brewing approximately 1 hL of beer, with full control over each unit operation of the process (Figure 1). Figure 1. Pilot-scale installation diagram, with special emphasis on unit operations with heat/cold demand. Heat supply marked with arrows, cold supply marked with a arrow (1—mashing; 2—lautering; 3—boiling; 4—wort separation in a whirlpool; 5—wort cooling; 6—fermentation; M—malt; BSG—Brewers’ Spent Grain; H—hops; Y—yeasts). arrow (1—mashing; 2—lautering; 3—boiling; 4—wort separation in a whirlpool; 5—wort cooling; 6—fermentation; M—malt; BSG—Brewers’ Spent Grain; H—hops; Y—yeasts). Overall, this installation works in the same manner as industrial breweries, including controlling the process and technical solutions. Obtained brewers’ spent grains were a by-product of standard pilsner beer production. For brewing, 20 kg of pilsner malt and 100 L of water were used. The mashing regime was as follows: 65 ◦ C for 30 min, 72 ◦ C for 20 min, 78 ◦ C for 10 min. The mixture was transferred into a lauter tun in the next Sustainability 2021,13, 10480 4 of 17 step, filtrated and sparged with 10 L of hot water. Through the filtration, 98 L of wort and approximately 10 kg of brewers’ spent grains dry mass were achieved. Wort extract was 11.5 Brix. In the next step, the wort was hopped, and yeasts were added in order to produce standard pilsner beer. All samples of BSG and reference materials (raw BSG, barley, pilsner malt and dark malt) were mashed according to the congress mash technique (European Brewery Convention 4.5.1). Raw materials (50 g) were ground and mixed with 200 mL of distilled water in temperature 45 ◦ C. The temperature was held for 30 min and mash was stirred at 90 rpm. The temperature was then raised up to 70 ◦ C and held for 10 min. After that step, 100 mL of distilled water at a temperature of 70 ◦ C was added and temperature was held for another 15 min. Finally, wort was cooled down to 25 ◦ C and filled to 450 mL with distilled water. Before analysis, samples were centrifugated at 3000 rpm for 3 min in order to remove turbidity. Wort extract was determined using a refractometer scaled in Brix (Bx) degrees. Wort colour was obtained using spectrophotometer Hitachi U-1900 and a 1-cm-length quartz cuvette. Absorbance was measured in a wavelength of 430 nm. The results were calculated to the units of European Brewery Convention (EBC) scale using following equation: EBC =A430·f·25 (1) where A430—absorbance in wavelength 430 nm f—dilution factor Aroma profiles were determined in an organoleptic manner by two researchers. Only aromas felt distinctly by both were taken into consideration. Compounds responsible for aroma are mostly created in Maillard reactions. Among them there are compounds whose aroma is described as acrid, toffee, coffee, roasted, nutty, bread, caramel [3]. Torrefaction was performed in stainless steel Petri dishes, each with a diameter of 120 mm and a height of 20 mm. Each was filled with dried BSG. The BSG was dropped freely into each dish from a height of approximately 10 cm until the container was overfilled and a cone was formed. Subsequently, a straight ruler was used to scantle the excess BSG. The mass of empty and full dish was checked for each sample using Radwag AS 310/C/2 scale with 0.1 mg resolution. The bulk density of the spent grain was determined to be 246 kg/m 3 . The dishes were subsequently closed with stainless steel lids to prevent air from accessing the material during torrefaction (apart from the air left in the interstitial space). Torrefaction was performed in a laboratory oven with a controlled temperature and connected ventilation. In each case, the oven was heated up to the selected temperature. The closed dishes were placed in the hot oven, which were subsequently closed. After a prescribed time of the experiment, the closed dish was taken out of the oven and placed in a desiccator for cooling. The mass was subsequently checked for cooled torrefied samples, followed by the characterization of torrefied material from the point of view of its use as a fuel and as a colouring agent. The performance of the torrefaction process is typically determined directly using mass yield (Y m ) and energy yield (Y e ) [ 83 – 86 ]. The indirect method was proposed by Weber et al. for simple assessment of performance of thermal biomass valorisation processes, such as production of biochar [ 87 ]. The use of an indirect method for performance assessment of the future industrial-scale installations is tempting, as in many cases, direct online measurement of mass and moisture content of the product could be problematic. A method proposed by Weber et al. [87] is described by following equation: Ym Weber =1−VMfeedstock 1−VMproduct (2) where Ym—mass yield (next part of the subscript indicating a method), -. Sustainability 2021,13, 10480 5 of 17 VM—respective volatile matter content of feedstock and product, %dry Moreover, the mass yield was also assessed indirectly, using ash tracer method [88]: Ym ash tracer =Afeedstock Aproduct (3) where Ym—mass yield (next part of the subscript indicating a method), -. A—respective ash content of feedstock and product, dry basis, %db Additionally, the mass yield was determined using the direct method: Ym direct =mdry product mdry f eedstock (4) where Ym—mass yield (next part of the subscript indicating a method), -. m—respective dry mass of feedstock and product, kgdry A well established formula was used for the calculation of the energy yield [89,90]: Ye=Ym· HHVproduct HHVfeedstock (5) where Ye—energy yield, -. HHV—respective higher heating value of feedstock and product, MJ/kg. Ash content was determined according to a procedure described in EN 18122 [ 91 ]. The combustion was performed at 550 ◦ C, using porcelain dishes. Volatile matter content was determined according to EN 15148 [ 92 ]. The furnace was heated up to 900 ◦ C. The samples were subsequently inserted into the furnace and kept for 7 min. The ultimate analysis was performed according to EN ISO 16948 using a Perkin Elmer 2400 analyzer [ 93 ]. Channiwala and Parikh [ 94 ]’s Equation (6) was used to estimate the HHV of both BSG and corresponding hydrochars: HHV =0.3491·C+1.1783·H+0.1005·S−0.1034·O−0.0151·N−0.0211·A , MJ kg (6) where C, H, O, N, S and A represent carbon, hydrogen, oxygen, nitrogen, sulphur and ash contents of material, respectively, expressed in mass percentages on dry basis. An absolute error limit of 1.45% was confirmed for Equation (6), when concentrations were within specified limits of C (0.00%–92.25%), H (0.43%–25.15%), O (0.00%–50.00%), N (0.00%–5.60%), S (0.00%–94.08%) and Ash (0.00%–71.4%) [94]. 3. Results and Discussion 3.1. Characterization of the Torrefaction Process Torrefaction of the BSG, performed within this study’s scope, leads to the loss of dry mass of the material, which is typical for the process [ 95 – 99 ]. Mass yield depended on both the temperature of the process as well as residence time (Figure 2). Looking at the mass yield result (see Figure 2), it could be stated that very little effect could be observed at 180 ◦ C. A similar observation was reported by Hejna et al. [ 34 ], who investigated a thermo-mechanical treatment of BSG with a maximum temperature of 180 ◦ C. The study concluded that the loss of mass could primarily be attributed to loss of moisture and only to a limited extent to thermo-chemical influence, mostly Maillard reaction [100]. Sustainability 2021,13, 10480 6 of 17 Sustainability 2021, 13, x FOR PEER REVIEW 6 of 18 at 180 °C. A similar observation was reported by Hejna et al. [34], who investigated a thermo-mechanical treatment of BSG with a maximum temperature of 180 °C. The study concluded that the loss of mass could primarily be attributed to loss of moisture and only to a limited extent to thermo-chemical influence, mostly Maillard reaction [100]. Direct comparison of the results presented in this study with the literature is not possible due to the lack of results available for the torrefaction of BSG. Work published on wet torrefaction reported substantially lower mass yields compared to the results obtained within the scope of this paper. Jackowski et al. [34] reported a mass yield of 0.809 for wet torrefaction of BSG, obtained from the brewing of beer using a mixture of pilsner, Munich and dark malt, carbonized at 200 °C for 150 min [34]. In another work, Jackowski et al. [82] performed wet torrefaction of BSG from two types of beer, one brewed using barley-based pilsener malt, with the addition of some juniper, and the other wheat-based, using wheat malt, with the addition of pilsener and caramel malt [82]. HTC of barleybased beer BSG at 180 °C for 10 min resulted in a mass yield slightly higher than 0.8, whereas the increase of the process temperature up to 200 °C resulted in the mass yield reaching 0.6 [82]. HTC of wheat-based beer BSG, performed at 200 °C for 60 min, resulted in a mass yield slightly lower than 0.6 [82]. This is not surprising, as the yield of wet torrefaction is much higher than dry torrefaction for similar process conditions [101–104]. Such behaviour could be attributed to different reaction pathways for wet torrefaction and probably to some extent to the excellent heat conductivity of water, enhancing the delivery of heat to the whole batch of feedstock when the HTC process is compared with dry torrefaction. Figure 2. Mass yield for the torrefaction of BSG (A—direct method, B—Weber method, C—ash tracer method). Looking at the assessment of the mass yield using indirect methods, both the ash tracer (see Figure 2) and the Weber method (see Figure 2) seem to give satisfactory results, with predicted mass yields reasonably close to the values obtained directly from the experiments (Figure 2). This is different to wet torrefaction, as the ash tracer method was reported to be unsuitable for this purpose by Jackowski et al. [82]. However, in the case of dry torrefaction, ash is not partially washed out, as is often the case for wet torrefaction [105,106]. Energy yield and its predictions, based on Ym determined using ash tracer and Weber method, show that most of the primary chemical energy in BSG should be retained in the torrefied product after the process, for most of the process conditions (Figure 3). This is Figure 2. Mass yield for the torrefaction of BSG (A—direct method, B—Weber method, C—ash tracer method). Direct comparison of the results presented in this study with the literature is not possible due to the lack of results available for the torrefaction of BSG. Work published on wet torrefaction reported substantially lower mass yields compared to the results obtained within the scope of this paper. Jackowski et al. [ 34 ] reported a mass yield of 0.809 for wet torrefaction of BSG, obtained from the brewing of beer using a mixture of pilsner, Munich and dark malt, carbonized at 200 ◦ C for 150 min [ 34 ]. In another work, Jackowski et al. [ 82 ] performed wet torrefaction of BSG from two types of beer, one brewed using barley-based pilsener malt, with the addition of some juniper, and the other wheat-based, using wheat malt, with the addition of pilsener and caramel malt [ 82 ]. HTC of barley-based beer BSG at 180 ◦ C for 10 min resulted in a mass yield slightly higher than 0.8, whereas the increase of the process temperature up to 200 ◦ C resulted in the mass yield reaching 0.6 [ 82 ]. HTC of wheat-based beer BSG, performed at 200 ◦ C for 60 min, resulted in a mass yield slightly lower than 0.6 [ 82 ]. This is not surprising, as the yield of wet torrefaction is much higher than dry torrefaction for similar process conditions [ 101 – 104 ]. Such behaviour could be attributed to different reaction pathways for wet torrefaction and probably to some extent to the excellent heat conductivity of water, enhancing the delivery of heat to the whole batch of feedstock when the HTC process is compared with dry torrefaction. Looking at the assessment of the mass yield using indirect methods, both the ash tracer (see Figure 2) and the Weber method (see Figure 2) seem to give satisfactory results, with predicted mass yields reasonably close to the values obtained directly from the experiments (Figure 2). This is different to wet torrefaction, as the ash tracer method was reported to be unsuitable for this purpose by Jackowski et al. [ 82 ]. However, in the case of dry torrefaction, ash is not partially washed out, as is often the case for wet torrefaction [105,106]. Energy yield and its predictions, based on Y m determined using ash tracer and Weber method, show that most of the primary chemical energy in BSG should be retained in the torrefied product after the process, for most of the process conditions (Figure 3). This is beneficial from the point of view of the product. However, this might suggest that relatively high torrefaction temperatures would be needed to achieve an autothermal process. As a consequence of not being able to achieve an autothermal process, a part of the product would be needed to supply the process with all necessary heat to maintain the sustainable character of the process. More research on the heat demand for torrefaction of BSG would be needed to give more specific guidance on the process conditions allowing autothermal torrefaction, as it has been reported for other materials, such as wood [ 107 ]. Sustainability 2021,13, 10480 7 of 17 Generally speaking, for a mild severity of process conditions, it seems plausible to expect that combustion of torgas could not be the sole source of heat for the process. Sustainability 2021, 13, x FOR PEER REVIEW 7 of 18 beneficial from the point of view of the product. However, this might suggest that relatively high torrefaction temperatures would be needed to achieve an autothermal process. As a consequence of not being able to achieve an autothermal process, a part of the product would be needed to supply the process with all necessary heat to maintain the sustainable character of the process. More research on the heat demand for torrefaction of BSG would be needed to give more specific guidance on the process conditions allowing autothermal torrefaction, as it has been reported for other materials, such as wood [107]. Generally speaking, for a mild severity of process conditions, it seems plausible to expect that combustion of torgas could not be the sole source of heat for the process. Figure 3. Energy yield for the torrefaction of BSG (based on mass yield determined by: A—direct method, B—Weber method, C—ash tracer method). 3.2. Torrefied BSG as a Solid Fuel As expected, torrefaction of BSG led to an increased fixed carbon content and decreased volatile matter content (Figure 4). The effect was especially profound for higherseverity torrefaction conditions (Figure 4). This is well in line with the published sources [42,108,109]. As a consequence of partial loss of volatiles, ash content was also a subject of increase. The ultimate analysis shows that the carbon content increased, whereas oxygen content decreased (Figure 5). A decrease in hydrogen content was also observed (Figure 5), which could be attributed to dehydration. Higher heating value (HHV) also increased as an effect of BSG torrefaction (Figure 6), as could be reasonably expected, based on the published literature [42,108,110]. Figure 3. Energy yield for the torrefaction of BSG (based on mass yield determined by: A—direct method, B—Weber method, C—ash tracer method). 3.2. Torrefied BSG as a Solid Fuel As expected, torrefaction of BSG led to an increased fixed carbon content and decreased volatile matter content (Figure 4). The effect was especially profound for higher-severity torrefaction conditions (Figure 4). This is well in line with the published sources [42,108,109] . As a consequence of partial loss of volatiles, ash content was also a subject of increase. The ultimate analysis shows that the carbon content increased, whereas oxygen content decreased (Figure 5). A decrease in hydrogen content was also observed (Figure 5), which could be attributed to dehydration. Higher heating value (HHV) also increased as an effect of BSG torrefaction (Figure 6), as could be reasonably expected, based on the published literature [42,108,110]. Sustainability 2021, 13, x FOR PEER REVIEW 8 of 18 Figure 4. Proximate analysis of torrefied BSG. Figure 5. Ultimate analysis of torrefied BSG. Figure 6. Higher Heating Value of torrefied BSG. The torrefaction process caused a significant decrease of H/C and O/C molar ratios for torrefied BSG, as depicted in Van Krevelen’s diagram (Figure 7). The diagram (Figure 7) clearly shows that valorized biomass becomes more similar to coal with increased severity of torrefaction. Relatively severe torrefaction conditions (temperature between 270 °C and 300 °C and residence time between 40 and 60 min) resulted in a BSG biocoal composition similar to lignite and low-quality hard coal (Figure 7). This confirms trends observed in the literature for other torrefied materials. A similar trend, in terms of the direction of change for torrefied material, was observed by Brachi et al. [111] for torrefaction of Figure 4. Proximate analysis of torrefied BSG. Sustainability 2021,13, 10480 8 of 17 Sustainability 2021, 13, x FOR PEER REVIEW 8 of 18 Figure 4. Proximate analysis of torrefied BSG. Figure 5. Ultimate analysis of torrefied BSG. Figure 6. Higher Heating Value of torrefied BSG. The torrefaction process caused a significant decrease of H/C and O/C molar ratios for torrefied BSG, as depicted in Van Krevelen’s diagram (Figure 7). The diagram (Figure 7) clearly shows that valorized biomass becomes more similar to coal with increased severity of torrefaction. Relatively severe torrefaction conditions (temperature between 270 °C and 300 °C and residence time between 40 and 60 min) resulted in a BSG biocoal composition similar to lignite and low-quality hard coal (Figure 7). This confirms trends observed in the literature for other torrefied materials. A similar trend, in terms of the direction of change for torrefied material, was observed by Brachi et al. [111] for torrefaction of Figure 5. Ultimate analysis of torrefied BSG. Sustainability 2021, 13, x FOR PEER REVIEW 8 of 18 Figure 4. Proximate analysis of torrefied BSG. Figure 5. Ultimate analysis of torrefied BSG. Figure 6. Higher Heating Value of torrefied BSG. The torrefaction process caused a significant decrease of H/C and O/C molar ratios for torrefied BSG, as depicted in Van Krevelen’s diagram (Figure 7). The diagram (Figure 7) clearly shows that valorized biomass becomes more similar to coal with increased severity of torrefaction. Relatively severe torrefaction conditions (temperature between 270 °C and 300 °C and residence time between 40 and 60 min) resulted in a BSG biocoal composition similar to lignite and low-quality hard coal (Figure 7). This confirms trends observed in the literature for other torrefied materials. A similar trend, in terms of the direction of change for torrefied material, was observed by Brachi et al. [111] for torrefaction of Figure 6. Higher Heating Value of torrefied BSG. The torrefaction process caused a significant decrease of H/C and O/C molar ratios for torrefied BSG, as depicted in Van Krevelen’s diagram (Figure 7). The diagram ( Figure 7 ) clearly shows that valorized biomass becomes more similar to coal with increased severity of torrefaction. Relatively severe torrefaction conditions (temperature between 270 ◦ C and 300 ◦ C and residence time between 40 and 60 min) resulted in a BSG biocoal composition similar to lignite and low-quality hard coal (Figure 7). This confirms trends observed in the literature for other torrefied materials. A similar trend, in terms of the direction of change for torrefied material, was observed by Brachi et al. [ 111 ] for torrefaction of tomato peels. However, in this case, the raw biomass had a much lower O/C and H/C ratio prior to valorization, and the change was not as extensive as in the case of this study [ 111 ]. Similarly, the trend was overall maintained in torrefaction of grape pomace, reported by Botelho et al. [ 61 ], despite the significantly different location of the feedstock on Van Krevelen’s diagram. The location of raw BSG on Van Krevelen’s diagram seems to be in good agreement with the area typical for carbohydrates, as depicted by Trubetskaya et al. [ 112 ]. This seems sensible, as BSG would consist mainly of fibre, with small amounts of residual sugars. BSG could contain some proteins, which generally has lower O/C in comparison to carbohydrates [ 112 ]. However, thr relatively low N content ( Figure 5 ) suggests that the content of proteins was not significant. Similar trends, in terms of the location on Van Krevelen’s diagram with increased severity of torrefaction, were observed for various lignocellulosic species of biomass, such as wood [ 113 , 114 ], beechwood [ 115 ], miscanthus [ 115 ], Sida hermafrodita [ 97 ], Erianthus arundinaceus [ 96 ], pepper pellets [ 114 ], perilla stem pellets [114], and Kenaf [116]. Sustainability 2021,13, 10480 9 of 17 Looking at the results obtained within the course of this study (Figure 7), it can be observed that dehydration dominated, which makes it plausible to suspect that a significant amount of hydroxyl groups were removed. Such loss of hydroxyl groups could potentially contribute to the hydrophobic character of torrefied products [111,117–119]. Sustainability 2021, 13, x FOR PEER REVIEW 9 of 18 tomato peels. However, in this case, the raw biomass had a much lower O/C and H/C ratio prior to valorization, and the change was not as extensive as in the case of this study [111]. Similarly, the trend was overall maintained in torrefaction of grape pomace, reported by Botelho et al. [61], despite the significantly different location of the feedstock on Van Krevelen’s diagram. The location of raw BSG on Van Krevelen’s diagram seems to be in good agreement with the area typical for carbohydrates, as depicted by Trubetskaya et al. [112]. This seems sensible, as BSG would consist mainly of fibre, with small amounts of residual sugars. BSG could contain some proteins, which generally has lower O/C in comparison to carbohydrates [112]. However, thr relatively low N content (Figure 5) suggests that the content of proteins was not significant. Similar trends, in terms of the location on Van Krevelen’s diagram with increased severity of torrefaction, were observed for various lignocellulosic species of biomass, such as wood [113,114], beechwood [115], miscanthus [115], Sida hermafrodita [97], Erianthus arundinaceus [96], pepper pellets [114], perilla stem pellets [114], and Kenaf [116]. Looking at the results obtained within the course of this study (Figure 7), it can be observed that dehydration dominated, which makes it plausible to suspect that a significant amount of hydroxyl groups were removed. Such loss of hydroxyl groups could potentially contribute to the hydrophobic character of torrefied products [111,117–119]. Figure 7. Torrefied BSG on Van Krevelens’ diagram (locations of different solid fuels adapted from [42,61,97,111,115,120]). 3.3. Torrefied BSG as a Beer Colouring Agent The results show that BSG roasted at 180 °C for 60 min and 210 °C for 40 min have the best colouring properties, reaching an EBC colour value of 145 and 159, respectively (Figure 8). These temperatures are slightly higher than the roasting temperature of Chocolate malt and slightly lower than roasting temperature of black malt, with a roasting time of 90 min, as reported by Ekielski et al. [121]. This suggests that the initial content of sugars also profoundly influences the colouring properties of the produced colouring agent. Although the colour of the roasted BSG produced within the course of this study, is two times less intense than those obtained from dark malt, the use of the spent grains could still prove beneficial since, in such a scenario, process residues would be used. Moreover, the addition of roasted spent grains may help in the formation of filtration cake during lautering. That, in turn, could positively contribute to the wort filtration process. Figure 7. Torrefied BSG on Van Krevelens’ diagram (locations of different solid fuels adapted from [ 42 , 61 , 97 , 111 , 115 , 120 ]). 3.3. Torrefied BSG as a Beer Colouring Agent The results show that BSG roasted at 180 ◦ C for 60 min and 210 ◦ C for 40 min have the best colouring properties, reaching an EBC colour value of 145 and 159, respectively ( Figure 8 ). These temperatures are slightly higher than the roasting temperature of Chocolate malt and slightly lower than roasting temperature of black malt, with a roasting time of 90 min, as reported by Ekielski et al. [ 121 ]. This suggests that the initial content of sugars also profoundly influences the colouring properties of the produced colouring agent. Although the colour of the roasted BSG produced within the course of this study, is two times less intense than those obtained from dark malt, the use of the spent grains could still prove beneficial since, in such a scenario, process residues would be used. Moreover, the addition of roasted spent grains may help in the formation of filtration cake during lautering. That, in turn, could positively contribute to the wort filtration process. Sustainability 2021, 13, x FOR PEER REVIEW 10 of 18 Figure 8. Colouring properties and content of sugars in raw and torrefied BSG, as well as colouring malt (typically used to change colour), pilsen malt and non-malted barley. The bar graph shows colour in EBC scale, whereas points show values of extract in °Bx (Brix). Another benefit of using torrefied BSG, apart from its colouring properties, is the profile of flavours and aromas (see Table 1) that could enrich the taste experience of a particular beer. Same as in the case of colouring properties, severe torrefaction conditions resulted in unwanted charcoal flavour. However, in some cases, flavours and aromas from sweet/fruity (mild torrefaction conditions) to whiskey/chocolate/coffee (moderate torrefaction conditions) could be sensed. Obtaining a more profound and more unique flavour could be one of the advantages from a commercial perspective. Table 1. Flavour/aroma profile of torrefied brewers’ spent grains. Temperature Time Aroma [°C] [min] 180 20 cereal 40 sweet 60 dried plum 210 20 dried fruits 40 wood 60 whisky 240 20 sweet/cereal 40 roasted/wood 60 charcoal 270 20 chocolate/cocoa 40 roasted 60 charcoal 300 20 whisky/coffee 40 charcoal 60 charcoal 3.4. 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