Food Chemistry 145 (2014) 378–387

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Flavour generation during commercial barley and malt roasting operations: A time course study Hafiza Yahya b, Robert S.T. Linforth a, David J. Cook a,⇑ a b

Brewing Science Section, Division of Food Sciences, The University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire LE12 5RD, UK Faculty of Science and Technology, Islamic Science University of Malaysia, Bandar Baru Nilai, 71800 Negeri Sembilan, Malaysia

a r t i c l e

i n f o

Article history: Received 2 April 2013 Received in revised form 22 July 2013 Accepted 12 August 2013 Available online 20 August 2013 Keywords: Malt Roasting Thermal flavour generation Speciality malt

a b s t r a c t The roasting of barley and malt products generates colour and flavour, controlled principally by the time course of product temperature and moisture content. Samples were taken throughout the industrial manufacture of three classes of roasted product (roasted barley, crystal malt and black malt) and analysed for moisture content, colour and flavour volatiles. Despite having distinct flavour characteristics, the three products contained many compounds in common. The product concentrations through manufacture of 15 flavour compounds are used to consider the mechanisms (Maillard reaction, caramelisation, pyrolysis) by which they were formed. The use of water sprays resulted in transient increases in formation of certain compounds (e.g., 2-cyclopentene-1,4-dione) and a decrease in others (e.g., pyrrole). The study highlights rapid changes in colour and particularly flavour which occur at the end of roasting and onwards to the cooling floor. This highlights the need for commercial maltsters to ensure consistency of procedures from batch to batch. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The flavour of malt products derives from the chemical composition of barley as a raw material and the way it is processed during commercial malting. This chemical composition varies according to malting process factors, such as the length of germination, or the extent of modification of the grain. However, during its manufacture, it is the thermal processing steps (kilning or roasting) which have the greatest impact upon product colour and flavour. Roasted malts are a category of ‘speciality’ malts which are generated by roasting in a drum roaster (of similar design to those used in coffee bean roasting) to high finishing temperatures, e.g., 130– 230 °C (Blenkinsop, 1991; Gretenhart, 1997). The feedstock for the process can be either raw barley, green malt (which has been germinated but not kilned) or pale malt (which has been dried to low moisture on a conventional kiln at temperatures less than 100 °C). Other roasted cereal products (e.g., wheat, rye) may be manufactured similarly, but are of less commercial significance and are not the focus of this paper. The high colour and flavour developed in roasted cereal products, together with a low enzyme survival, mean that roasted malts are used as low percentage adjuncts to conventionally kilned pale malts in brewing (Gruber, 2001). Thus, colour and flavour are product-defining characteris-

⇑ Corresponding author. Tel.: +44 (0) 115 9516245; fax: +44 (0) 115 9516685. E-mail address: [email protected] (D.J. Cook). 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.08.046

tics and the specific combination of raw material and thermal processes which give rise to the desired characteristics are of interest. Brewing science literature contains several accounts of the flavour and colour characteristics of the family of roasted malt products and of their applications in brewing (Blenkinsop, 1991; Gretenhart, 1997; Gruber, 2001). However, only a few studies (O’Shaughnessy, Chandra, Fryer, Robbins, & Wedzicha, 2003; Vandecan, Daems, Schouppe, Saison, & Delvaux, 2011) have directly addressed the question of how the colour and flavour of roasted malt products relate to the raw material feedstock properties and roasting conditions. Fewer still have operated on a commercial scale, as for the trials described here, which were conducted on-site at a commercial maltings using 2-tonne roasting drums. Time-point samples were taken through manufacture and immediately snap-frozen in liquid nitrogen to halt colour and flavour formation and preserve their transient states. In this way it was possible for the first time to report changes in product flavour composition which occur late in the manufacturing process, as the product is tipped and cooled to ambient. Impacts, at a commercial scale, of the use of water sprays during roasting are also reported for the first time. The experimental design was selected to include feedstocks of raw barley, green malt and pale kilned malt, generating the corresponding roasted products: roasted barley, caramel malt and black malt. Barley and kilned malt are both low-moisture feedstocks; however, the germination of grains during the malting process means that the pale malt starts with a higher concentration of reducing sugars and amino acids (each significant

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precursors of thermally generated flavours) than are present in barley. The moderate temperatures (e.g. finished at 85 °C) reached through the kilning cycle mean that the pale malt feedstock already contains some thermally pre-formed volatile compounds and their precursors, as distinct from raw barley. By comparison, green malt has been germinated but not dried and is thus a high moisture feedstock with high enzyme activities, since these have been developed through germination and not partially inactivated through kilning. The crystal malt roasting process thus starts with a low temperature ‘stewing’ step where continued activity of starch-degrading enzymes is specifically favoured. Using the present experimental design it was possible to make comparisons with regards to flavour generation between the different feedstocks and the range of processing conditions employed; in particular the time courses of temperature and humidity experienced by product in the roasting drum, which are known to be key factors determining colour and flavour formation (Channell, Yahya, & Cook, 2010; Cook, Channell, Abd Ghani, & Taylor, 2005). By developing further knowledge of the time course of flavour formation through roasting operations, and how that relates to feedstock composition and process conditions, it should be possible to: (i) better understand the sources of variability in the processes and their products and (ii) envisage how process modifications might either reduce such variability or perhaps lead to entirely new categories of roasted malt products, based on novel or modified feedstocks and roasting conditions. 2. Materials and methods 2.1. Roasting feedstocks A UK winter barley variety was used in the commercial roasting experiments. The same variety was used for the manufacture of each roasted product. 2.2. Chemicals All chemicals were analytical grade (>97% purity), including the following standards used for GC–MS identification and calibration: methylpyrazine, pyrrole, 5-methylfurfural, 2-furanmethanol, 5methyl-2-furanmethanol, acetic acid, maltol, 2,3-pentanedione, benzeneacetaldehyde, 2,5-dihydro-4-hydroxy-3(2H)-furanone (Sigma–Aldrich, Poole, Dorset, UK) and 2-furaldehyde (Acros Organics, Loughborough, UK). 2.3. Industrial scale roasting experiments Samples were taken at frequent time points during replicate runs of the commercial roasting of three types of roasted malt products: roasted barley, black malt and crystal malt. Roasted products were prepared in a gas-fired, 2-tonne batch-size Barth drum roaster. The product temperature was monitored during roasting by a thermocouple situated in the product at the front of the drum and adjacent to the sampling port. This temperature was used to control the (automated) progression of the roasting processes. Samples were obtained by scooping about 100 g of grain from the sampling port at the front of the drum. After collection, samples were immediately snap frozen in liquid nitrogen, sealed in foil-lined pouches and subsequently stored at 80 °C until required for analysis. The sampling time points were varied according to each manufacturing process, in order to take most samples at periods likely to coincide with rapid changes in flavour compound concentrations due to thermal flavour generation. Sampling points were as follows: roasted barley: 0, 30, 65, 70, 80, 85, 90, 95, 100, 110, 120, 130 min; black malt: 0, 30, 65, 70, 80, 85, 90, 95, 100,

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105, 110, 120, 130 min; crystal malt: 0, 30, 55, 65, 70, 75, 80, 85, 90, 100 min. Finally, samples were taken after each product had been tipped from the drum and cooled to approximately room temperature by blowing air through the product. These ‘product’ samples were taken at (approximate time point from start of process) 140 min (crystal malt) and 160 min (roasted barley and black malt). During the production of roasted barley and black malt, the kernels were sprayed twice with water (20 L) at 85 and 105 min after the start of roasting for roasted barley, and at 65 and 85 min for black malt. This is a standard manufacturing protocol used both to minimise the risk of fire and to achieve target colour and flavour for the products. 2.4. Determination of sample moisture content Sample moisture content was analysed following standard methods according to European Brewery Convention (EBC) Analytica-EBC Barley Method 3.2 or Malt Method 4.2. 2.5. Analysis of flavour volatiles by solvent extraction/gas chromatography-mass spectrometry (GC–MS) Roasted barley or malt samples (2 g) were ground into a fine powder using a Knifetec 1095 sample mill (FOSS Tecator). Methanol (4 ml) containing an internal standard (5-nonanone, 5 lg ml 1) was added to the samples. The samples were then mixed on a roller bed for 30 min before being centrifuged at 6708g for 10 min. The supernatant was transferred into GC–MS vials ready for analysis. 2.5.1. GC–MS operating conditions Volatile compounds extracted from the samples were separated using a Trace GC Ultra gas chromatograph (Thermo Scientific, Waltham, MA) fitted with a ZB-Wax column (30 m  0.25 mm i.d.; film thickness 1.0 lm; Phenomenex, Macclesfield, UK). The injector was operated in splitless mode (240 °C; 1 min) with helium gas as the carrier (head pressure 20 psi). The oven temperature was programmed as follows: 40 °C and hold for 1 min followed by a temperature ramp at 4 °C/min to 220 °C and hold for 2 min. The EI-MS was operated in full scan mode over a range of m/z 40–250 (scan time 0.45 s; inter-scan delay 0.05 s). Compound concentrations were calculated after correction against the internal standard and using external standards for 2methylpyrazine, 2-furaldehyde, pyrrole, 5-methyl-2-furfural and 2-furanmethanol, injected at concentrations of 0.5, 1, 2, 5, 7 and 10 lg ml 1. The extraction and analysis of flavour volatiles was performed in triplicate for each sample. 2.6. Measurement of sample tristimulus colour co-ordinates (L⁄a⁄b⁄) Changes in the surface (husk) colour of grain samples taken throughout the roasting processes were monitored as an index of colour formation. It should be noted that the derived colour indices are likely to differ from those which would result if the grain sample was first ground and then the colour of either a flour or extract were determined. The aim here was not to provide an industry standard measure of colour; rather to attain a rapid index of colour change that could be cross-referenced with flavour formation. Grain samples were assessed at each time point by International Commission on Illumination (CIE) L⁄a⁄b⁄ colour determination using the light reflectance port of the Hunter Lab, Colour Quest XE (Hunter Associates Laboratory, Reston, VA) at wavelengths from 400 to 700 nm. The equipment was calibrated using a white tile. The L⁄ or lightness parameter corresponds to the overall lightness or darkness (0 – black or opaque; 100 – white or clear). a⁄ and b⁄ values represent the red (+a⁄) to green ( a⁄) and

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the yellow (+b) to blue ( b⁄) colour axes, respectively. Triplicate readings were taken for each time point sample. Image Pro Plus software (Version 4.5, Media Cybernetics, Inc, Rockville, MD) was used to analyse the data. 2.7. Statistical analysis of data In order to make statistically valid comparisons between concentrations of flavour volatiles in the different roasted products, a 2-way ANOVA was performed (Minitab Version 16.2.2; Minitab Ltd, Coventry, UK) to analyse the variance attributed to product type (black malt, crystal malt, etc.) versus that attributed to run to run variance between the replicate manufacturing processes. Post-hoc analysis was conducted using Fisher’s LSD, where significant differences were identified between sample means (p < 0.05). 3. Results and discussion 3.1. Changes in the surface colour of grain through roasted product manufacture The variations through manufacture of grain surface colour coordinates (L⁄a⁄b⁄) are shown in Fig. 1 for each roasted product. The lightness-darkness (L⁄) colour co-ordinate was found to give the most useful index of the progressive darkening of the grain through the roasting processes and is plotted alongside product temperature in Fig. 1 to depict the links between applied temperature and colour formation. The L⁄ profile was similar for roasted barley and black malt products, reflecting the similar temperature profiles employed in each case. The surface of these products became significantly darker as the product temperature increased above 150 °C and L⁄ values fell from approximately 75–40 over the cycle. For crystal malt product, changes in the L⁄ value were more subtle, starting from a lower value (since the feedstock was green (non-kilned) malt) and falling by less than 8 L⁄ units across the process, which employed lower temperatures. For both black malt and roasted barley products, a⁄ and b⁄ colour co-ordinates initially increased (increasing redness and yellowness, respectively), but from approximately one hour onwards decreased, such that the surface redness (a⁄ co-ordinate) of the product was not dissimilar to that of the feedstock in each case, whilst the yellow co-ordinate (positive b⁄ values) declined sharply. Grain surface colour during crystal malt manufacture showed only minimal changes in a⁄ and b⁄ co-ordinates, the main change being a small increase in product redness during the final temperature ramp to 150 °C. Product redness (a⁄) is most likely a measure of shorter chain melanoidins or caramelised products, which are converted to darker, more polymerised products on prolonged roasting at high temperatures (note the reduction in a⁄ beyond 60 min for black malt and roasted barley products; Fig. 1). 3.2. Product temperature and moisture content through manufacture Time-course data for the temperature of each roasted product through manufacture are shown in Fig. 1, whilst corresponding measurements of sample moisture content are plotted in Fig. 2. Moisture contents throughout are quoted on a wet weight basis. The main distinctions between roasted product types can be summarised as follows: raw barley for roasting entered the process at around 9% moisture content and was subjected to a rapid temperature ramp, causing product temperature to rise to 230 °C over a period of 2 h. The moisture content fell, such that it was around 5% after 40 min and had reached a steady and very low ( 200 °C did not cause changes in the mean product moisture content, which is apparent in Fig. 2. This is most likely because just 20 kg of water were added in each case to around 2000 kg of grain, at product temperatures well above the boiling point of water. The impact of the water sprays was a rapid evaporative cooling of the product (see product temperature traces for black malt and roasted barley in Fig. 1) with only transient increases in product moisture – which were proportionately too small to be identified in the product time-moisture curves (Fig. 2) where samples were taken for moisture analysis once every 15 min. Crystal malt manufacture represents a substantially different process, starting from high moisture content in the green malt (38% in this instance). The product is deliberately ‘stewed’ at 65 °C for 50 min, whereby the roasting drum is sealed so that a moist environment is retained. These conditions favour a certain degree of residual enzyme activity within the malt. Of particular relevance to flavour formation are the continued activity of diastatic enzymes (degrading starch to dextrins and simple sugars) and proteolytic enzymes. Such action increases the concentrations of precursors to the Maillard reaction, one of the main flavour formation pathways in thermally processed foodstuffs (Gerrard, 2006; Muir, 2007; Van Boekel, 2006). Following this period, the drum door is opened and flue gases passed around the malt to dry it down. The product was heated to a final temperature of 150 °C after an overall process time of 100 min. A key difference as compared with the roasted barley and black malt processes is that the product retained relatively high moisture content throughout the process (Fig. 2) and finished with a product moisture content of 5% (w/w). Moisture content plays a key part in the extent of Maillard reactions, both because water is a direct participant in flavour formation pathways but also because it acts as a solvent, influencing the mobility of the primary reactants (Bell, 2008). Thus, at high product moistures the Maillard reaction occurs in water as a solvent, whilst at low product moistures and high temperatures Maillard chemistry occurs in the solid phase – termed pyrolysis (Wnorowski & Yaylayan, 2000). The influence of the phase in which Maillard reactions occur is apparent in the difference in flavour between boiled and roasted food products. Wnorowski and Yaylayan (2000) reported that, whilst pathways to many Maillard reaction products were similar in the two phases, solid-phase reactions were in general faster, yielded more products and favoured thermal degradation of reactants. Aqueous-phase Maillard reactions favoured pathways requiring either solvent-assisted isomerisation reactions or those involving transition states of a polar nature that are stabilised by the presence of water. 3.3. Flavour composition of roasted products Samples of the finished roasted products were extracted into methanol and analysed for flavour volatiles using GC–MS. The coefficient of variation for the analytical method ranged between 2% and 18% and was highest for compounds analysed at concentrations close to their detection limits, e.g., pyrrole. Although there were clear differences between the GC–MS chromatograms for the different roasted products, there were considerable similarities

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5 0

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Fig. 1. Variation in product temperature and L a b lightness-darkness of the grain surface through the commercial manufacture of roasted barley, black malt and crystal malt products. L⁄a⁄b⁄ Data are the mean values of three analytical measurements of each of two replicate manufacturing processes.

in terms of the compounds present. Crystal malt contained the greatest overall range of volatile compounds and roasted barley the least. To facilitate a comparison of the influence of process conditions on product flavour, concentrations of 15 flavour volatiles (which were found to be present in all 3 products; Table 1) were quantified in the finished products (Table 1). Data for methyl pyrazine, furaldehyde, pyrrole, 5-methyl-2-furfural and 2-furanmethanol were quantified against external standard series using authentic compounds. The remaining concentration data (Table 1) are semi-quantitative in that peak area data have been corrected against an internal standard, and compounds assumed to have similar response factors to the external standards.

Two-way analysis of variance, conducted for flavour volatile concentrations at selected time points, or in the finished products, confirmed that there were significant differences (p < 0.05) in volatile concentrations between replicate runs of the same production process for each product. This was not entirely unexpected and reflects inherent variations in process that exist under standard manufacturing conditions, as were being operated by production during the trials. Notwithstanding the run-to-run variation, there were also significant differences (p < 0.05) between the flavour volatile concentrations of different product types. The major compositional differences reported for finished products in Table 1 are those between ‘dry’ roasted products (roasted barley and black

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2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one (DDMP) (p < 0.05; Table 1). The reasons for these analytical differences relate to the influence of product moisture and temperature on thermal flavour generation pathways and will be discussed in greater detail in the following section. 3.4. Time course of flavour volatile formation during malt roasting operations

Fig. 2. Variation in product moisture content (%, wet weight basis) through the commercial manufacture of roasted barley, black malt and crystal malt products. Data are the mean values of three analytical measurements of each of two replicate manufacturing processes.

malt) and the crystal malt, which was manufactured at higher product moisture and with a lower finishing temperature. Low moisture, high temperature roasting resulted in significantly elevated levels of methylpyrazine, furfural, pyrrole, 5-methyl-2-furfural, benzeneacetaldehyde and maltol (p < 0.05; Table 1). Conversely, the crystal malt product contained greater amounts of 2-cyclopentene-1,4-dione, isomaltol, 2-furanmethanol, 5methyl-2-furanmethanol, 2-hydroxy-2-cyclopenten-1-one and

Fig. 3A–C plot time course data for six thermally-generated, volatile compounds through the manufacture of each of the three roasted products in turn (Fig. 3A: roasted barley; Fig. 3B: black malt; Fig. 3C: crystal malt). The y-axis ranges are the same in each Figure to facilitate a direct comparison of flavour generation during the different processes, and six compounds have been selected which exemplified varying temporal behaviours. The final time points in Fig. 3A–C represent the flavour composition of the final products (see also Table 1). At first glance the time course traces appear somewhat complex. This is because the transient concentrations of each volatile compound in the roasting product are a balance between their instantaneous rates of formation and depletion, where depletion can either be due to onward chemical reactions, thermal degradation or volatilisation and loss to the flue gases. Furthermore the relative formation rates of individual compounds change as the main reaction phase changes from liquid to solid (pyrolysis) as the product becomes dehydrated (Wnorowski & Yaylayan, 2000). This accounts for the rapid increase in concentration of compounds such as methylpyrazine and maltol towards the end of the high temperature roasting processes. Further features of the compound time-course curves can be explained by process factors such as

Table 1 Identification details for 15 flavour compounds which were present in all three roasted products (Roasted Barley, Black Malt & Crystal Malt) and their mean analytical concentrations in the final products. Data are the mean values of three analytical measurements on each of two replicate manufacturing processes. Superscript letters (lower case) indicate post-hoc groupings by Fisher’s LSD (P < 0.05). Compound

A

EI-MS quantitation ion (m/z)

LRI ZBWaxA

2,3-Pentanedione

100

1063

Methylpyrazine Acetic acid 2-Furaldehyde (furfural) Pyrrole

94 60 96 67

1279 1466 1478 1531

5-Methyl-2-furfural

110

1587

2-Cyclopentene-1,4-dione Isomaltol Benzeneacetaldehyde 2-Furanmethanol

96 126 120 97

1602 1630 1653 1663

5-Methyl-2-furanmethanol 2-Hydroxy-2-cyclopenten-1-one Maltol 2,5-Dimethyl-4-hydroxy-3(2H)-furanone (DMHF or Furaneol) 2,3-Dihydro-3,5-dihydroxy-6-methyl-4Hpyran-4-one (DDMP)

112 98 126 128

1716 1767 1933 1981

145

2164

Odour characteristicsB

Buttery, nutty, toasty, caramellic Popcorn, nutty, cocoa Vinegar Bread, almond, sweet Sweet, warm, nutty, ethereal Almond, caramel, burnt sugar Burnt sugar, fruity Floral, honey, sweet Bready, estery, sweet, caramellic Brothy, malty Caramel-like, sweet Caramel Sweet, fruity, caramellike Sweet, cookies

Compound identification

Concentration in finished product (lg g 1) Roasted barley

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LRI + STD + MS

0.13a

0.18a

0.40a

LRIE + STD + MS LRID + STD + MS LRIC + STD + MS LRIF + STD + MS

29.2a 55.2a 9.76a 0.09a

7.71b 8.13c 9.54a 0.10a

0.10c 35.7b 2.10b 0.03b

LRIC + STD + MS

6.55a

4.47a

0.78b

MS MS LRIC + STD + MS LRI + STD + MS

0.11b 1.15b 1.92a 3.75b

0.20b 0.64b 0.83b 1.07b

0.60a 5.11a 0.33c 21.0a

LRIC + STD + MS LRI + MS LRIF + STD + MS LRIE + STD + MS

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0.10b 1.32b 319a 5.80a

0.84a 6.88a 155b 3.86a

LRIE + MS

5.82b

5.33b

28.2a

Linear retention index against alkanes measured on ZB-Wax column in this experiment. These are standard descriptors listed to give an indication of the kinds of flavour characters contributed by these chemicals. Sourced from: www.flavornet.org/ and http:// www.thegoodscentscompany.com/. Compound identification: MS – EI-MS library match (reverse fit factor > 900). LRI – verified by comparison of Kovat’s Linear Retention Index with literature values. STD – Authentic standard run to verify retention time under identical chromatographic conditions. C LRI value on BP-20 as cited by Beal and Mottram (1994). D LRI value on DB-WAX as cited by Perez-Cacho, Mahattanatawee, Smoot, and Rouseff (2007). E LRI value on DB-WAX as cited by Rega, Guerard, Delarue, Maire, and Giampaoli (2009). F LRI value on DB-WAX as cited by Poinot et al. (2008). B

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A

methyl-pyrazine Isomaltol DDMP

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increasing temperature, which can induce higher temperature reaction pathways such as pyrazine formation (Fors & Eriksson, 1986; Muller & Rappert, 2010; Weenen et al., 1994). The opening

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of the drum seal during crystal malt manufacture caused a drop in most product volatile concentrations after around 60 min process time (Fig. 3C), due to volatilisation. Some further features can be directly attributed to the injection of water during processing (to attain target colour); e.g., note the peaks for DDMP, Fig. 3B, attributable to water sprays at 85 and 105 min process time) and see also Section 3.5, below. Quantitatively, DDMP was a major product of the early thermal flavour generation pathways which formed rapidly under higher moisture conditions (Fig. 3A–C). This compound is a product of the 1-deoxyosone pathway of hexose sugars (Yaylayan & Mandeville, 1994) and is found in many heated and stored foods (Kim & Baltes, 1996). Nishibori and Kawakishi (1990) identified DDMP as a major odorant contributing to the aroma of cookies baked at 150 °C. Moreover, it was only produced from mixtures of ingredients at this temperature and not by the thermal degradation of individual cookie ingredients, indicating the role of Maillard chemistry in its formation. In particular they reported that large amounts of DDMP were formed during the dry heating of ingredients high in the amino acid proline – which is quantitatively the most prevalent amino acid in barley (Tressl, Bahri, & Helak, 1983). It is notable in the roasted barley and black malt products that DDMP concentrations reduced significantly beyond 1 h of process time (Fig. 3A and B). This coincided with product temperature elevation to >150 °C. Kim and Baltes (1996) studied the thermal decomposition pathways of DDMP, reporting that DDMP itself was odourless, but that when heated at 150 °C, 56 volatile degradation products were isolated and the mixture smelled of sweet, caramel and melted butter flavours. Quantitatively significant components were 2,3-pentanedione, 2-methyl-4,5-dihydro3(2H)-furanone, 1-hydroxy-2-propanone, cyclotene, and 2,5-dimethyl-4-hydroxy-3(2H)-furanone (furaneol). Isomaltol was also amongst the compounds formed. When roasted at 220 °C, quantitatively significant degradation products were 5-hydroxymaltol, 2,4-dihydroxy-2,5-dimethyl-3(2H)-furanone and maltol. It thus appears that the major significance of DDMP to the flavour of roasted barley/malt products is due to its breakdown products, such as maltol, isomaltol and furaneol, which are formed in varying ratios dependent on the temperature and moisture of the system above 150 °C. Maltol was formed in substantial quantities in all 3 roasted products and the finished product concentrations were highest in the roasted barley and black malt samples (p < 0.05) which were finished at the highest temperatures (>230 °C). They contained approximately twice as much maltol as crystal malt. This primarily resulted from the accelerated formation of maltol during dry roasting at product temperatures above 200 °C (Fig. 3A and B). Maltol has long been known to be produced by the pyrolysis of materials such as malt, cellulose, starch and wood (Patton, 1950). Production of 2-methylpyrazine was favoured by dry roasting at process temperatures >130–140 °C (e.g., roasted barley, Fig. 3A) and rose dramatically after approximately 100 min of process time. A similar trend was noted for black malt, except that here the starting concentration of methylpyrazine was higher (due to prior malt kilning) and the final product concentration was lower than for roasted barley (probably due to the marginally shorter overall roasting time). With crystal malt (Fig. 3C), the concentration of 2-methylpyrazine declined progressively from concentrations present in green malt at the start of the process. These results are in accordance with the observations of Fors and Eriksson (1986), who reported that pyrazine formation in an extruded green malt product increased with temperature between 130 and160 °C and was favoured at lower moistures. The major route to pyrazine production in thermally treated systems is via the condensation of aminocarbonyl compounds formed through Strecker degradation (Weenen et al., 1994).

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2-Furanmethanol formation and retention in products was favoured by moist conditions and intermediate process temperatures, as used in crystal malt manufacture. Crystal malt product contained around 20 lg g 1 2-furanmethanol (Table 1), whereas black malt product contained 1.1 lg g 1. 2-Furaldehyde (furfural) is a relatively volatile aroma compound, which can be formed via either Maillard reactions or sugar caramelisation. This compound is readily volatilised from roasted products at higher temperatures. Whilst the low temperature (65 °C) ‘stewing’ conditions of green malt manufacture encouraged early formation of furfural (Fig. 3C), once the drum was opened and temperatures raised towards 150 °C, furfural concentrations in the product dropped to 2 lg kg 1, as compared with close to 9.5 lg kg 1 for black malt and roasted barley products. Furfural is formed in the Maillard reaction from pentose sugars via dehydration of a cyclic precursor formed from the 3-deoxyosone (1,2enolisation pathway). It can also be formed directly from the pyrolysis of glucose, starting once again with a dehydration step, the cyclic Grob fragmentation (Paine III, Pithawalla, & Naworal, 2008). Such dehydration reactions are thought to be favoured under the drier roasting conditions of black malt and roasted barley manufacture. A similar effect was noted when using on-line APCI-MS to monitor real-time generation of flavour in barley malt (Channell et al., 2010). At 180 °C, approximately 5 times as much furfural was monitored when purging with dry make-up gas, as opposed to when it was humidified by injecting water into the gas flow. The flavour volatile time-course data in Fig. 4 enable a direct comparison between the different manufacturing processes for six compounds, which showed substantive differences between the dry/high temperature processes and crystal malt manufacture as noted in Section 3.3. Isomaltol, 2-furanmethanol, 2-hydroxy-2cycloepenten-1-one, and DDMP were all formed at significantly higher amounts in crystal malt (Fig. 4). Consistent with the product sensory characteristics reported previously (Blenkinsop, 1991; Gretenhart, 1997), these flavour compounds contribute sweet, caramel, biscuit and marshmallow aromas to roasted products (Table 1). Whilst each of these compounds was present at lower levels in roasted barley and black malt products, their concentrations were in general highest early in these manufacturing processes, after which they were either volatilised or converted to other products as roasting temperatures were increased at very low moisture. The time-concentration profiles for these 4 compounds are similar, which may reflect interconnected pathways for their production. Kim and Baltes (1996) showed the breakdown of DDMP to produce, amongst other compounds, isomaltol and this may represent the main mechanistic pathway to isomaltol during malt roasting. 2-Furanmethanol production was greatly favoured in crystal malt production, relative to either black malt or roasted barley. Wnorowski and Yaylayan (2000) proposed a mechanism of formation for 2-furanmethanol from glycine and D-glucose, based on 13C-labelling experiments, in which carbon atoms 2–6 of the glucose molecule end up in 2-furanmethanol, but the first carbon atom is lost following retroaldol cleavage of the Amadori product. 2-Methylpyrazine and furfural are examples of volatiles which were present at higher concentrations in the dry/high temperature roasted products (Fig. 4), because of mechanistic reasons already discussed. The roasted/burnt/smoky flavours developed by prolonged low moisture roasting are contributed by heterocyclic nitrogen, sulphur and oxygen compounds whose generation in roasted products is exemplified by this type of behaviour. Reviewing the flavour formation data as a whole in comparison with colour formation during the malt roasting processes, it is difficult to make overall conclusions or correlations. Periods of maximum change in L⁄a⁄b⁄ colour co-ordinates (Fig. 1) did not always coincide with increases in monitored flavour volatiles (Figs. 3

and 4). This is partly because the product volatile concentrations are also influenced by factors such as volatilisation from the product to the flue gases, concentrations of their precursors, further chemical reactions or thermal degradation and process factors, such as whether the roasting drum door was open or closed. Across all three products, periods of increasing redness (a⁄) of the grain surface broadly coincided with the production of certain Maillard reaction products such as DDMP. However, for the crystal malt product there was significant flavour volatile formation but only limited colour formation during the low temperature stewing period. It is possible that monitoring colour using ground samples or extracts of the grain, as opposed to surface measurements, might have provided a better measure of colour with which to correlate flavour formation. However, as already discussed, the transient volatile concentrations are influenced by several chemical, physicochemical and process factors, which mean that direct correlations with colour formation of a generic nature are unlikely. 3.5. The impacts of water sprays during manufacture of roasted barley & malt products By superimposing the time profiles of generation of volatiles with the water spray points for roasted barley (Fig. 5A) it was possible to see examples of how the introduction of water changed the relative balance of volatiles in the product. Directly after the water sprays at 85 and 105 min, there were peaks in the production of some compounds (exemplified by 2-cyclopentene-1,4-dione in Fig. 5A) and declining concentrations in others (e.g., pyrrole; Fig. 5A). It is interesting to note that, whilst the bulk of the water spray is volatilised on contact with the product at >200 °C (as discussed in Section 3.2) some water is evidently consumed in chemical reactions. Presumably this occurs close to the grain surface because there was no evidence from Fig. 2 of any substantial increase in bulk grain moisture content following the water sprays. Such reactions might occur rapidly – as suggested in Fig. 5A – due to the prior build-up of chemical precursors in the absence of freely available water. Pyrrole can be formed thermally via a number of mechanisms, including the direct pyrolysis of the amino acid serine (Yaylayan & Keyhani, 2001), the reaction of ammonia with furan or with a 3-deoxyhexosone (Finot, Aeschbacher, Hurrell, & Liardon, 1990). In the mechanism proposed by Yaylayan and Keyhani (2001), acetaldehyde and a-aminoacetaldehyde react via aldol condensation to form pyrrole with the loss of 2 molecules of water. From such a mechanism it is possible to understand why pyrrole formation would be favoured under drier conditions and impeded by water sprays (Fig. 5A). 2-Cyclopentene-1,4-dione was analysed at greater quantities in crystal malt, suggesting a role for Maillard reactions at higher moisture contents in its formation. This might explain the elevated concentrations of 2-cyclopentene1,4-dione observed following water sprays. Furthermore, Tehrani, Kersiene, Adams, Venskutonis, and De Kimpe (2002) reported the generation of 2-cyclopentene-1,4-dione as a breakdown product of melanoidins from a glucose/glycine system, when heated at temperatures between 100 and 300 °C. Thus, it is further possible that a burst of colour formation following the water spray caused an increase in 2-cyclopentene-1,4-dione via subsequent pyrolysis of melanoidins. With regard to this potential source of compounds, it is worth noting that, of the 15 volatiles considered here, furfural, 5-methyl-2-furfural, methylpyrazine and 2-furanmethanol were also detected as thermal breakdown products of melanoidins. 3.6. Control of roasting end-point and product flavour In Figs. 3 and 4 it is notable that the concentrations of several flavour volatiles were increasing steeply towards the end of the respective roasting processes. This was caused both by the ele-

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Concentration (µg/g)

isomaltol 6

2-furanmethanol OH

BLACK MALT

5

O

35

ROASTED BARLEY

30

CRYSTAL

25

4

20

3

15 2

10

1

5

0 0

50

100

150

200

0 0

DDMP

Concentration (µg/g)

150

200

12

BLACK MALT

35

10

CRYSTAL

30

8

25 20

6

15

4

10 2

5

0

0 0

50

100

150

0

200

50

methylpyrazine

BLACK MALT

25

150

200

12

ROASTED BARLEY

30

100

2-furaldehyde

35

Concentration (µg/g)

100

2-hydroxy-2-cyclopenten-1-one

ROASTED BARLEY

40

50

10

CRYSTAL

8

20 6

15

4

10

2

5

0

0 0

50

100

150

200

Time (min)

0

50

100

150

200

Time (min)

Fig. 4. A comparison of flavour formation in different roasted malt products for selected volatiles which showed clear differences between the dry/high temperature roasted products and crystal malt. Data are the mean values of three analytical measurements of each of two replicate manufacturing processes.

vated product temperatures and lower moisture contents; in the black malt and roasted barley products this increase in formation likely reflects the increased rate of pyrolysis reactions in the solid phase noted by Wnorowski and Yaylayan (2000). During crystal malt manufacture, the moisture content remained above 5% until close to the end of processing; hence rapid changes in volatile content towards the end of crystal malt manufacture may still have been attributable to Maillard reactions in the liquid phase. Following the stewing period at the start of the process, which generates many Maillard reactants and intermediates, there is a ‘reservoir’ of compounds available to support flavour formation, which thus

accelerates when product temperature is subsequently raised and the drum door opened to dry the product. A further observation of this study was the significant change in volatile composition between the last sample taken from the roasting drum, and the finished product after it had been tipped and cooled to ambient temperature. This emphasises the fact that thermal flavour formation continues whilst the product is being cooled but also that flavour concentrations can decline due to vaporisation into the stream of cooling air. Thus, for crystal malt manufacture (Fig. 3C) maltol and isomaltol continued to increase in concentration from drum to finished product, whereas a decline was noted

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further sensory evaluation studies, not conducted here, would be required to confirm the general applicability of such an index.

Concentration in roasted barley (µg/g)

A 0.25

Pyrrole

4. Conclusions

2-cyclopentene1,4-dione

0.2

0.15

0.1

0.05

0 0

20

40

60

80

100

120

140

160

Time (min)

B

Fig. 5. Time-course data for: (A) pyrrole and 2-cyclopentene-1,4-dione concentrations during the manufacture of roasted barley, illustrating the impacts of water sprays injected into the roasting drum as indicated by the dotted lines (t = 85 & 105 min) and (B) the analysed ratio of maltol to methylpyrazine through the manufacturing process of three roasted products. Data are the mean values of three analytical measurements of each of two replicate manufacturing processes.

over the same period for the more volatile products, such as furfural and 2-furanmethanol. A commercial maltster cannot routinely take these nuances of analytical flavour concentrations into account; however they can attempt to ensure consistency of handling from batch to batch, such that they achieve flavour consistency and control. Much as for the process itself, this involves adhering to a consistent time course of product temperature and humidity during cooling. 3.7. Flavour balance The ratio of maltol to pyrazines has been proposed as a crude indicator of the flavour balance of roasted malt products (Walker & Westwood, 1992). The basis of this is the balance between maltol as an exemplar of sweet malty aroma, contrasted with pyrazines that are representative of smoky and roasted aromas formed during ‘dry’ roasting at high temperatures. In the present experiment the ratio of maltol to 2-methylpyrazine was by far the greatest in crystal malt (Fig. 5B), where it increased to more than 1500 in the finished product, as opposed to 41 for black malt, or 12 for roasted barley. These results are in accordance with the expected flavour balance of these products and support the general usage of such an index as a crude flavour balance indicator. However,

By tracking flavour generation over time through malt or barley roasting operations it was possible to identify how conditions of temperature and humidity create different ratios of thermally generated flavour compounds which ultimately give each product their distinct flavour characters. Conditions of intermediate moisture content and moderate temperatures (exemplified by the manufacture of crystal malts) favoured aqueous-phase Maillard reactions and most of the compounds monitored showed a peak during the early manufacturing cycle relating to this. For ‘dry roasted’ products such as black malt and roasted barley, product moisture had dropped to less than 2% by the time their temperatures were in excess of 200 °C. These conditions led to extensive pyrolysis reactions and the accelerated generation of compounds such as maltol and methylpyrazine. Interestingly most compounds continued to be formed at a certain rate under these conditions, although product concentrations of compounds such as furanmethanol and furfural did not increase to such an extent during the ‘pyrolysis phase’ because the net balance between rates of their generation and volatilisation from the product were lower. The study demonstrated that concentrations of several key flavour volatiles were changing rapidly at the end-point of roasting and onwards through the cooling operations. This highlights the need for skilled operators who understand the need for consistent process control, in order to avoid significant batch-to-batch variations in roasted product flavour. Furthermore it was apparent that the use of water sprays influenced flavour generation and thus provides one mechanism through which product flavour may be varied. Whilst the underlying chemistry may be complex, commercial maltsters have developed the art of controlling roasting conditions to generate a variety of products with distinct and meaningful flavour characteristics (‘caramellic’, ‘biscuity’, ‘smoky’, ‘chocolate’, etc.). Acknowledgements We wish to acknowledge the Malaysian Government for sponsorship of Hafiza Yahya’s PhD research. References Beal, A. D., & Mottram, D. S. (1994). Compounds contributing to the characteristic aroma of malted barley. Journal of Agricultural and Food Chemistry, 42(12), 2880–2884. Bell, L. N. (2008). Moisture effects on food’s chemical stability. In G. V. BarbosaCánovas, A. J. J. Fontana, S. J. Schmidt, & T. P. Labuza (Eds.). Water activity in foods: Fundamentals and applications (pp. 173–198). John Wiley & Sons. Blenkinsop, P. (1991). The manufacture, characteristics and uses of speciality malts. Technical Quarterly of the Master Brewers Association of the Americas, 28, 145–149. Channell, G. A., Yahya, H., & Cook, D. J. (2010). Thermal volatile generation in barley malt: On-line MS studies. Journal of the American Society of Brewing Chemists, 68(4), 175–182. Cook, D. J., Channell, G. A., Abd Ghani, M., & Taylor, A. J. (2005). Thermal flavour generation: Insights from mass spectrometric studies. In W. L. P. Bredie & M. A. Petersen (Eds.). Flavour science. Recent advances and trends (43rd ed.), pp. 569–572). The Netherlands: Elsevier B.V.. Finot, P. A., Aeschbacher, H. U., Hurrell, R. F., & Liardon, R. (1990). The Maillard reaction in food processing, human nutrition and physiology. Springer, p. 516. Fors, S. M., & Eriksson, C. E. (1986). Pyrazines in extruded malt. Journal of the Science of Food and Agriculture, 37(10), 991–1000. Gerrard, J. A. (2006). The Maillard reaction in food: Progress made, challenges ahead-conference report from the eighth international symposium on the Maillard reaction. Trends in Food Science and Technology, 17(6), 324–330. Gretenhart, K. E. (1997). Speciality malts. Technical Quarterly of the Master Brewers Association of the Americas, 34, 102–106.

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