Analytical Approaches to the Study of Flavor Instability
Chapter
A diversity of instrumental methods have been applied to the pursuit of flavor stability in beer. Individually, most of these techniques offer some information, but no single method totally informs the brewer of the full story when it comes to predicting shelf life. Some of the procedures—notably, the measurement of oxygen in packaged beer—certainly have substantial importance. However, most of them tell only part of the story (at best). They are probably best considered as investigative tools: procedures to use when you are troubleshooting or trying to get a handle on where undesirable changes are occurring in the process stream.
The Measurement of Oxygen I can do no better than refer to the late and greatly missed Chaz Benedict for the plainest description of oxygen measurement, published posthumously (Benedict, 2016). Above all else, it is essential that measurement is made “in real time.” Oxygen will frequently react rapidly with components of wort and beer (or indeed, the likes of yeast), and so the only reliable means of assessing whether oxygen has entered the process is by measuring it as soon as possible. 31
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Chapter 5 Analytical Approaches to the Study of Flavor Instability
A range of oxygen analyzers and sensors are available (Figs. 5-Â1 through 5-Â5). The myriad challenges involved in using them reliably are described extensively by Benedict (2016), making his chapter essential reading.
Fig. 5-1. An oxygen analyzer attached in-line in a brewery. (Photo courtesy of Hach)
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Chapter 5 Analytical Approaches to the Study of Flavor Instability
Fig. 5-2. An in-line oxygen probe. (Photo courtesy of Hach)
Fig. 5-3. An at-line oxygen measurement being made using a portable dissolved oxygen analyzer. (Photo courtesy of Hach)
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Chapter 5 Analytical Approaches to the Study of Flavor Instability
Fig. 5-4. A portable dissolved oxygen analyzer. (Photo courtesy of Hach)
Redox Potential One way to consider the impact of oxygen (i.e., to assess the extent to which it has encroached without being in a position to measure it directly) is to assess redox potential. Sidebar 3-Â1 describes the concept of oxidation and reduction. The measurement of the overall balance of oxidation and reduction can be quantified as redox potential, as described by van Strien (1987).
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Chapter 5 Analytical Approaches to the Study of Flavor Instability
Fig. 5-5. Oxygen measurement for a bottle of beer. (Photo courtesy of Hach)
Redox probes are commercially available (but see my comments in Chapter 6).1 If you insist on gaining some idea of how much reducing power has been siphoned away from your process stream, then some of the following simple tests may give clues. 1 I well recall true gentleman Preston Besford, part of the quality team at the Bass
Preston Brook Brewery in the late 1980s, asking me whether he could purchase some redox probes. I replied, “But Preston, you have two already. One is here”— and I pointed to one eye—“and there is another one just like it on the other side.” My meaning was that simple observation can tell you so much about where oxygen is creeping into a process stream. Is a mash tun being filled by splashing in from the top of the vessel? Oxygen is being picked up, more so than with a gentle bottom fill, and so on.
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Chapter 5
The Indicator Time Test
Analytical Approaches to the Study of Flavor Instability
The indicator time test dates back to 1939 and measures the so- called reducing power in wort or beer. The sample is mixed with a blue solution of something called 2,6-dichlorophenolindophenol (DCPIP). The more reducing power in the sample, the more extensively the blue color is removed.
The TBA Test In the food industry over the years, there has been much use of thiobarbituric acid (TBA), which can react with carbonyl compounds to generate a pink color. The pinker the color that develops, the more carbonyl substances that are present. And so the test can be used with force-aged samples (perhaps by heating): The more color that develops in the heated sample, the more potential carbonyls that are present in the sample and the less the predicted stability. This is method Wort-21 of the American Society of Brewing Chemists (ASBC) Methods of Analysis. There have been many criticisms of the method because of the interference of things such as browning reaction products, proteins and sugar degradation products, and amino acids.
Measurement of Thiols One class of materials that are known to be oxidized relatively easily are the thiols—that is, those substances that contain an –SH grouping. Oxygen removes the hydrogen from two adjacent thiol compounds, causing them to link (see Fig. 5-6). The more oxygen has reacted, the less free –SH can be detected. The regent used to measure the thiols is called 5,5´-dithiobis(2-nitrobenzoic acid) (DTNB). When an –SH reacts with DTNB, a yellow color develops. The more yellow the color, the more thiol and the less oxidation. (See Muller [1997] for more about this method.)
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Chapter 5 Analytical Approaches to the Study of Flavor Instability
Fig. 5-6. Some of the reactions that may occur in mashing. The abbreviation POD stands for peroxidase. The bold solid line indicates that polyphenols are said to inhibit lipoxygenase (LOX). The bold dotted line indicates that polyphenol can scavenge reactive oxygen species, such as hydroxyl. (Drawing by Emily Kultgen; Courtesy C. W. Bamforth)
Nonenal Potential The measurement of nonenal potential involves a forcing test in which wort is adjusted to the typical beer pH (4.0) before heating for a couple of hours at 100°C (212°F) in an atmosphere of argon to exclude oxygen. Then the level of nonenal is measured. As such, the method measures the amount of nonenal that might be lurking in wort—either in the form of precursor that can be converted into nonenal or in the form of nonenal attached to a molecule or molecules that can hold on to it.
Total Polyphenols By measuring the total polyphenol level, it is possible to get a rough indication of how much has been lost to oxidation. Clearly, this could not be used with beers treated with polyvinyl polypyrrolidone (PVPP). (See ASBC Method Beer-35.) 37
Chapter 5
Electron Spin Resonance (ESR) Spectroscopy
Analytical Approaches to the Study of Flavor Instability
Electron spin resonance (ESR) spectroscopy (sometimes referred to as EPR) has attracted a lot of attention since the 1990s, and of all the techniques mentioned in this chapter (other than the measurement of oxygen in beer), it is perhaps the most relevant. It, too, warrants an entry in the ASBC Methods (Beer-46). ESR is basically a way to measure the reactive materials that I talk about in Chapter 3, reactive oxygen species (ROS), or the presence of the materials that lead to the production of ROS. And so a good friend of mine in the industry tells me that ESR is a great way to detect iron. Alternatively, some brewers claim that this method detects the anti oxidant potential of a beer. In the ESR method, a signal is produced for which the intensity is proportional to the amount of these reactive materials (also known as radicals). In the procedure, the signal is measured as the beer is force-aged—for example, at 60°C (140°F) (see Chapter 3). This produces the damaging materials (the ROS/radicals). The more antioxidant materials in a beer (materials that grab hold of these radicals and thereby neutralize their damaging impact), the more slowly the signal starts to increase—and the longer the so-called lag time. For much more detail on the use of ESR in analyzing flavor stability issues, the reader can dip into an online ASBC workshop (see “Further Reading”).
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