Assessing Color Quality of Beer - American Chemical Society

Chapter 15

Assessing Color Quality of Beer Downloaded by NANYANG TECHNOLOGICAL UNIV on October 19, 2015 | http://pubs.acs.org Publication Date: June 13, 2008 | doi: 10.1021/bk-2008-0983.ch015

1

Thomas H. Shellhammer and Charles W. Bamforth

2

1

2

Department of Food Science and Technology, Oregon State University, 100 Wiegand Hall, Corvallis, OR 97331 Department of Food Science and Technology, University of California at Davis, 127 Cruess Hall, Davis, CA 95616

The visual quality of beer depends on color, clarity and foam characteristics and how these support or negate consumer expectations regarding the particular style and brand. Beer color originates principally with Maillard reaction products formed during malting within the. grain used to prepare the wort for fermentation. Kilning malted barley creates the greatest input to beer color while boiling wort can add substantial color in lighter colored worts, along with the oxidation of grain and hop polyphenols. In some instances, beer color is modified post-fermentation using caramel color or roasted malt extracts. The conventional method for measuring beer color throughout much of the world examines the absorbance at 430 nm. While this works well for light colored lagers it lacks the ability to measure nuances of darker, redder beer or cloudy beer color. Tristimulus measurement improves on A measurement in this case. 430

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© 2008 American Chemical Society

In Color Quality of Fresh and Processed Foods; Culver, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Downloaded by NANYANG TECHNOLOGICAL UNIV on October 19, 2015 | http://pubs.acs.org Publication Date: June 13, 2008 | doi: 10.1021/bk-2008-0983.ch015

The Appearance of Beer The aesthetics of beer as judged by the consumer and the quality as interpreted by the brewing scientist lies in large part with its appearance. One's first impression of whether the beer is true to type is dependent on the product's color. The presence or absence of haze can be viewed either as a defect or characteristic feature depending upon the style. For instance, world-wide pale lager beer is expected be brilliantly clear and the presence of a haze represents a defective product to both the consumer and scientist alike. Similarly, beers with the purposeful inclusion of yeast, such as German and North American hefeweizens, are meant to be cloudy, and one measure of quality is the extent and duration of the biological haze present in this style. While the absolute color may be key to a consumer's perception of beer quality, brewers recognize that reproducible and recognizable color may be of greater importance. Consumers' expectation of flavor can hinge on the color of the product they are consuming. Experiments in which the color of a pale lager beer is modified illustrate this impact. A collection of British beer tasters, some fully trained as sensory panelists, were given two beers to evaluate: a pale colored lager beer and the same beer to which caramel coloring had been added to increase the color value by 9 °EBC (European Brewing Convention) color units thereby giving it a color of a typical British pale ale. When asked to rate each beer on a scale of 1 (most lager-like) to 10 (most ale like) nine out of ten panelists rated the colored beer higher (more ale like) than the uncolored lager beer. In fact, four of the panelists offered aromatic descriptors of the colored lager which one would normally associate with an ale, i.e., "lacks dimethyl sulfide", "more bitter", "full", "heavy", and "malty" (7). In a separate experiment a leading brand of American commercial light colored lager beer was colored using flavorless food dyes at four different levels up to a color that matched the color of a leading British commercial ale. When American beer drinking consumers were asked to rank the five samples in increasing order of quality (low to high), the lager beer with the greater amount of color was ranked as being higher in quality and the degree of quality improvement was correlated with the extent of color increase (2). Clearly, the color of the beer has a significant impact on the perceived, or expected, flavor and quality regardless of the beer type.

Standard Methods for Measuring Beer Color The color of beer ranges from the palest yellow for lightly flavored lager beer through brown, with reddish and amber hues, for many ales and porters to deep black for stouts. Stylistic differentiation is due in large part to the grist


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composition which impact both flavor and color. Grist color is due principally to Maillard reaction products formed during kilning and in some instances to caramelization of sugars in specialty malts. Transmission spectra for commercial beer display significant differences from 400 - 550 nm, violet through green, and slightly lower differences from 550 - 700 nm (Figure 1).

Figure 1. Transmission spectra for four different colored commercial beer styles.

Historically, beer color was determined by visual comparisons against a set of color standards developed by Joseph Lovibond in the late 19 century as well as against solutions of potassium chromate in the early 20 century. Color blindness in the human comparator, roughly 10% in men and 1% in women (3), inconsistent illumination spectra, and aging of the color comparison standards were key flaws in this method. These issues prompted the need for precise and reproducible measurements and led color measurement to evolve into one of spectrophotometric absorbance and/or transmission measurements. The standard method of the American Society of Brewing Chemists (ASBC) and the European Brewery Convention (EBC) relies on absorption at 430 nm (4, 5). In the case of beer, selection of the proper wavelength of absorption is complicated by the fact that beer does not possess a wavelength of maximum absorbance (Figure 2). The single wavelength selected for color measurement is often one that is associated with the product's complimentary color. Since beer color typically varies from yellow to brown, and within some cases slight reddish hues (Table I), absorbance in the blue - indigo region (430 nm) is reasonable. th

th



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Figure 2. Absorbance spectra for four different colored commercial beer styles. Standard methods of beer analysis utilize absorbance at 430 nm.

The ASBC standard was developed in the 1950's using a half-inch cell. Absorbance readings were adjusted such that they were close in value to the standard at the time, °Lovibond (Equation 1). ASBC color = A

4 3 0

(Vi inch cell) * 10

(1)

Since most modern cuvettes have a path length of 10 mm, color measurements using a 10 mm cuvette are multiplied by 1.27 to accommodate for the shorter path length. The ASBC method is often referred to as being the (U.S.) Standard Reference Method (SRM), and color values may be expressed as °SRM to denote the method. Color expressed as °SRM agrees closely with color in °Lovibond and the two can be used interchangeably.

Table I. Beer color across a range of beer styles Style American/European light lager British Pale Ale American Porter Irish Stout

Color Yellow, straw, golden Amber Dark brown Black

Color units SRM EEC 4--8 •2-•4. 10-• 15 2 0 --30 35-•70

20--30 40--60 7 0 - 140


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The caveat to spectrophotometric color measurement is that the sample must be visually clear. Thus, the ASBC color analysis is also performed at 700 nm. Should the A700 < 0.039 x A430, the beer is considered "free of turbidity" and the color of beer can be determined from its absorbance at 430 nm. If the sample is not "free of turbidity" it requires further clarification by centrifligation or filtration followed by a repeat absorbance measurement at 430 nm. The EBC also uses absorbance at 430 nm but with a slight modification (Equation 2). EBC color = A o (10 mm cell) x 25 43

(2)

Thus for the same color EBC units will be approximately twice (1.97 times) as large as SRM color units.

Origins of Color in Beer Four key ingredients serve as the basis for all beer: water, malted barley, hops and yeast. Of these, malted barley represents the single greatest impact on beer color because it is the ingredient that is used in the largest quantity and can be produced in a wide range of colors and flavors. Hops have a negligible direct contribution to beer color, but they can potentially impact the color of very pale beer via oxidation of their polyphenols. Yeast does not contribute to color unless it remains present in the final product, as in a hefeweizen, where it contributes to turbidity that is critical in the visual appearance of this particular style. Yeast can result in color loss via adsorption of colored materials to their cell wall. The color of malted grain develops as a result of the biochemical and thermal processes during malting. Briefly, malting is controlled germination of grain with the three key steps being steeping, germination, and kilning. In the production of standard malt, with wort colors ranging from 2 - 1 0 °SRM, the precursors to color are reducing sugars and free amino nitrogen (FAN) that will participate in the Maillard reaction during kilning. The final amounts of these reactants are determined by the moisture content of the grain as it leaves the steeping operation and the extent to which the maltster allows the grain to germinate. Higher steep-out moisture, in the range of 45 - 50% w/w, leads to more rapid malting with greater amounts of reducing sugars and FAN at the end of germination. Similarly, lower steep-out moisture (40 - 43%) and cooler malting conditions lead to lower levels of these Maillard reaction precursors. The biochemical sequence of events during germination of barley is initiated by water entering the kernel via the micropyle, the release of gibberellic acids via the migration of moisture across the scutellum and activation and/or release of enzymes in the aleurone layer at the periphery of the endosperm (Figure 3). Sequentially, beta glucanases, proteases, and amylases are produced within the



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Figure 3. Cross-section of a barley kernel (Reproduced from (6). Copyright 2006 American Society of Brewing Chemists)

aleurone and slowly migrate through the endosperm toward the center of the kernel. In order to grow, the embryo must ultimately utilize amino acids produced via the action of exo- and endo-proteases on storage protein plus sugars produced from the amylolytic action on the starch granules. Enzymatic attack on these internal structures is only possibly once the endosperm cell walls, which are comprised of (1—•3),(l-^4)-P-D-glucan (-75%), arabinoxylan (-20%), and protein (-5%), have been sufficiently degraded to allow entry of this large molecular weight machinery. However, it is the maltster's objective to achieve near complete destruction of the grain's endosperm cell walls, and hence minimize the amount of beta glucans, without undue loss of starch via the amylases. The final step in malting is the kilning operation, one that has two distinct phases - drying followed by curing. In the former phase, surface water and free moisture is removed under relatively cool drying conditions in order to preserve enzyme activity. In this first phase the air-on temperature is between 50-60°C. Once the surface moisture has been removed, the moisture content of the barley is approximately 12%, and the second phase of kilning begins by increasing the temperature to 80-110°C and reducing the air flow (7). The objective of this second phase is the formation of color and flavor via the Maillard reaction. The timing of the second phase, the final curing temperature, and the length of time at this curing temperature dictate the type and extent of color formation. Color from this type of malt is due to high molecular weight melanoidins which are yellow, orange and red initially and turn to brown as the Maillard reaction is allowed to proceed (#).



198 In the production of specialty malts, such as Caramel malt, the kilning operation is replaced with a combination of stewing and roasting in a roasting drum. Green malt, having just completed germination (45% moisture), is placed in a drum roaster and heated to a temperature which is optimal for the newly created and released amylases, 65-75°C. During the stewing phase all of the starch is converted into fermentable reducing sugars such as maltose, maltotriose, and to minor degree glucose, and nonfermentable dextrins, a process not unlike mashing which occurs in the brewery. Once this conversion is complete, the temperature is increased substantially to 80-145°C and the large concentration of reducing sugars results in the creation of substantial amounts of Maillard reaction products. The high temperatures results in the formation of nitrogen heterocyclic products that have strong toffee and nutty flavors, and in extreme cases the formation of pyrroles and pyrazines bring burnt and bitter flavors (9). The high concentration of maltose and high temperatures also result in caramelization reactions occurring. Caramel malts tend to be amber with red hues and are often used in the production of "red" beers. Another class of specialty malts is produced by roasting green or kilned lager malt in a drum roaster. These roasted malts are subjected to very high

Table II. Appearance and flavor of malted barley Color (SRM)

Appearance

Beer type

Flavor

Wheat

1

Pale straw

Weizen

Malty

Pale Lager

2

Pale yellow

Light lagers

Cereal, DMS

Pale Ale

3

Yellow, golden

Ales

Biscuit, toasted

Vienna

4

Amber

Dark lagers

Nutty Toffee

Malt type Standard Malts

Color/Caramel Malts Munich

10-20

Amber, brown

Amber beer

Intense malty

Cara Pils

5-15

Pale

Lagers

Sweet, biscuit

Caramel/ Crystal

20-120

Amber, brown, red

Ales & lagers

Toffee, nutty, burnt

350

Brown, black

Porters & stouts

Coffee

Black

400-600

Black

Porters & stouts

Neutral

Roasted barley

300-800

Black

Irish stout

Bitter, burnt

Roasted Malts Chocolate



199 temperatures, as high as 230°C which is within 20°C of the combustion temperature (248°C) for malt (9). The flavors and colors from these malts are caused by pyrroles and pyrazines as well as thermal degradation of carbohydrates and phenolic acids. The former produce coffee and burnt aromas while the later can result in smoky and clove-like aromas. Roasted malts offer colors which range from brown to black. Unique to this class of malts are black malts which act essentially as coloring agents offering little in flavor. High molecular weight extracts of this type can be used in small amounts to trim color of pale beer without impacting flavor just prior to packaging. Conversely, low molecular weight extracts can have the complimentary effect of adding flavor with minimal color.

Processing Impacts on Wort and Beer Color Several steps exist throughout beer production that have a measurable impact on the color of the final product. First and foremost is the mashing step. At this point the grist composition will determine the color and flavor of the final product. Utilizing the unique color and flavor impacts of various malts (Table II), the brewer blends these to achieve a specific target. Color intensity is determined in part by wort concentration; the values listed in Table II are obtained from worts produced via a Congress mash at roughly 8°P (% wt/wt). Wort boiling potentially has the second largest impact on wort, and hence beer, color. Boiling typically lasts 1 - 2 hours at temperatures of 93 - 110°C, with lower temperatures occurring at altitude and higher temperatures as a result of pressure boiling. The extent of melanoidin production that occurs during wort boiling is a function of temperature, time, wort pH, and concentrations of FAN and sugars. While the majority of color input to the wort/beer system comes during the kilning phase of malting, color increase due to boiling is measurable albeit minor in many cases. Nevertheless, in pale worts (2 SRM) with high FAN, it is possible to significantly increase the wort's color with a long boil. Remaining operations downstream, namely fermentation and filtration, result in minor color depletion due to adsorption on yeast cell walls and filtration media, such as cellulose fibers in pad filtration. Oxidation of barley and hop polyphenols during beer storage will result in increased reddish, brown color. Color changes as a result of polyphenol oxidation are most apparent in pale lager beers following extended storage postpackaging. Such oxidation will be promoted by high levels of dissolved oxygen in the packaged beer as well as the presence of soluble iron imparted by brewhouse equipment, brewing water, or diatomaceous earth filtration media. Fining with polyphenol adsorbents such as polyvinylpolypyrrolidone (PVPP), prior to packaging helps mitigate oxidative browning by reducing levels of


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potential browning polyphenolic precursors. In darker beers, oxidative browning is masked by the colors from the colored & roasted malts. Coloring agents such as malt extracts and caramel coloring can be added post-fermentation as a means of adjusting the beer's final color. These products are intensely colored with colors ranging from 250 - 3,500 °SRM for malt extracts and 5,000 - 30,000 °SRM for caramels (10). At the levels used for beer coloring applications they present a negligible impact on flavor. Caramel as a coloring agent is discussed in depth in Chapter 20 of this monograph.

Comparison of Conventional Color Measurement Techniques The brewing industry retains A readings as the conventional standard for online and offline color measurements. Single wavelength measurement is rapid, straightforward, and easily transferable to automated online color monitoring. While the use of a single wavelength color measurement is satisfactory in representing color for some beers, such as pale yellow lager beer, it becomes less satisfactory for darker and reddish beers. The retina of the human eye is sensitive to wavelengths ranging from 400 to 700 nm (Figures 1 & 2); and the perception of color relies on the assessment of electromagnetic absorption across the entire visible spectrum. Cone cells at the center of the retina respond individually to red, green, or blue light and together they are interpreted as color (77). Color measurement using tristimulus data representing lightness, red-green quality, and yellow-blue quality (CIE L*a*b*, respectively) better characterize how the human eye perceives color. The ASBC approved tristimulus color measurement of beer using the CIE L*a*b* color space as a standard method in 2002(72). Transmission data can be collected using a precision spectrophotometer at 5 nm intervals over the visible range and converted to chromacity values (X, Y, Z) using the spectral power distributions and color matching functions at each wavelength for the standard illuminant C. The chromacity values in turn are used to create L*, a* and b* values. Data collection software on many spectrophotometers has this function built in. The 9 edition of the ASBC Methods of Analysis includes a tristimulus color calculator written in an Excel spreadsheet format whereby the user pastes the transmission spectra into a preprogrammed spreadsheet to obtain ASBC color and tristimulus color (L*a*b* and L*C*h). Beer color determination using tristimulus colorimetry is typically more accurate, allows for better comparisons of samples, and gives information about measured color shifts which are not available from single-wavelength methods. An example illustrating this was presented by Stephen Smedley at the Brewing Research Foundation International (75) in which pairs of beers having equal A readings had different L*a*b* values. Six sets of beer pairs ranging in color from 3.5 to 17 °SRM were found statistically to be visually different by a panel of 430

th

430


201 human judges. Across these six sets, the net difference in L*a*b* values defined as AE in Equation 3 ranged from 0.43 to 7.40. The darkest pair (49.4 SRM) was not judged to be visually different despite a AE of 1.39. Transmission spectra for an analogous case are presented in Figure 4.

2

2


AE = V ( A L * ) + ( A a * ) + ( A b * )

400

450

500

550

600

2

(3)

650

700

Wavelength (nm) Figure 4. Transmission spectra for two beers that have different tristimulus color but have identical absorbance at 430 nm. Color = 29.7 °SRM, 38.8 °EBC AE = 1.98

Beer color is determined primarily by the type and concentration of Maillard reaction products created during the malting of barley or other brewing grains, such as malted wheat. Secondary inputs to color arise from boiling wort and/or oxidation of polyphenols during beer aging. Both of these situations are more apparent in lightly colored wort and beer. Post-fermentation color adjustments are often made with caramel color or intensely colored malt extracts. Color measurement in the brewing industry is primarily based on a single measurement at 430 nm despite its apparent shortcomings. Although tristimulus color measurement has been approved by the ASBC as a standard method for quantifying beer color, it is still relegated to the laboratory. Nevertheless, equipment for rapid online tristimulus data collection using tristimulus filters, diode array detectors, or narrow bandwidth filters, is available to the brewer.


202 Incorporation of this technology and the use of a critical AE will allow the brewer to produce a visually consistent color over time.

References


1. 2. 3. 4.

5.

6. 7.

8.

9. 10. 11. 12.

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Bamforth, C. W.; Butcher, K. N.; Cope, R. Ferment 1989, 2, 54-58. Traina, C. D. The sources and measurement of beer color and its impact on consumer perception of quality. M.S., University of California, Davis, 2004. Hough, J.; Briggs, D.; Stevens, R.; Young, T., Malting and Brewing Science. 2 ed.; Chapman and Hall: London, 1982; Vol. 2, p 914. American Society of Brewing Chemists, Beer-10A. Spectrophotometric color method. In Methods of Analysis, 9th ed.; American Society of Brewing Chemists: St. Paul, MN, 2004. European Brewery Convention, Section 9 Beer Method 9.6 Colour of Beer. Spectrophotometric Method: Instrumental Method. In Analytica-EBC, Verlag Hans Carl Getranke Fachverlag: Nurnburg, Germany, 2004. Bamforth, C. W. Scientific Principles of Malting and Brewing. American Society of Brewing Chemists: St. Paul, MN, 2006; p 246. Bamforth, C. W.; Barclay, A. H. P. Malting Technology and the Uses of Malt In Barley: Chemistry and Technology, MacGregor, A. W.; Bhatty, R. S., Eds. American Association of Cereal Chemists: St. Paul, MN, 1993; pp 297-354. Nursten, H. The Maillard Reaction In Chemistry, Biochemistry and Implications. The Royal Society of Chemistry: Cambridge, UK, 2005; p 214. Gretenhart, K. E. MBAA Technical Quarterly 1997, 34, 102-106. Smedley, S. M. Brewers' Guardian 1995, 124, 42-45. Smedley, S. M., Jounal of the Institute of Brewing 1992, 98, 497-504. American Society of Brewing Chemists, Beer-10A. Tristimulus Analysis (Colorimetric or Spectrophotometric). In Methods of Analysis, 9th ed.; American Society of Brewing Chemists: St. Paul, MN, 2004. Smedley, S. Brewers' Guardian 1995, 124, 44-47.


Assessing Color Quality of Beer - American Chemical Society

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