Beer Stability: A perspective on beer flavour stability

Beer flavour stability

In a perfect world a beer would taste the same at package release as it does at the end of its shelf life. In reality, beer begins to undergo flavour modifications as soon as it leaves the brewery with more brewers now competing in expansive domestic and global markets. Shipment of beer to distant markets takes time and thus requires best-buy/pull dates of six months to a year. When shipped and stored under refrigeration, beer is likely to stay fresh up to anticipated shelf life. That said, the shipment and storage of beer under refrigeration adds cost and is not always feasible.

Most beers have expected shelf lives of 17-26 weeks. Imports have longer shelf lives and draught shorter at 9-13 weeks. A typical domestic lager stored at room temperature (75°F) is expected to stay fresh for up to 17 weeks. Yet, for each day stored 10 degrees above 75°F, a domestic lager will lose up to two days of shelf life (Figure 1). Prediction of a beer’s actual shelf life can be achieved by force aging under accelerated conditions (augmented temperature). Evaluation of beer flavour changes at several time points provides useful information for shelf life prediction and insight into potential process modifications a brewer may need to make to increase product flavour stability. All that is required to gather this type of information is a discerning palate, or a collective group of discerning palates in the form of a sensory panel. For those brewers especially concerned about flavour consistency and stability, the next step is to look at the installation of instrumentation for quality control and quality assurance monitoring.

Impact on flavour

Despite numerous focused studies and application of highly sophisticated analytical instrumentation, forming a comprehensive picture of beer flavour instability remains a challenge. In general, beer aging is characterised by decreased bitter taste, increased sweet/caramel taste, ribes (black currant), toffee and sherry-like aromas. However, each brew is truly unique and no single compound or measurement exists to adequately evaluate the many facets of beer aging for all styles. Pale and dry-hopped beers are generally of higher susceptibility than dark or heavily kettle-hopped beers.

Hundreds of compounds from various chemical groups have been associated with beer flavour modification during aging¹. These components may take part in one or more chemical reactions including, but not limited to: Maillard reactions, formation of linear aldehydes and esters, ester degradation, acetal formation, etherification, degradation of hop bittering acids and polyphenol formation/interaction. Occurrence of each reaction largely depends on the beer type, raw materials used and exposure to the major beer enemies: oxygen, light, temperature and time^2,3. Although many of the aging mechanisms are associated with oxidation, non-oxidative mechanisms may also occur⁴.

Non-oxidative flavour modification reactions include esterifications, etherifications, Maillard reactions and glycoside and ester hydrolysis³. These types of reactions can be impactful for beers that are bottle conditioned and for dry-hopped beers that are high in glycosidically-bound aroma precursors. During bottle conditioning, hungry yeast will attack bound sugars of glycosides to release volatile floral aromatics such as linalool and geraniol. Several studies show that refermentation or bottle conditioning can markedly improve the overall profile of the beer. Other reactions, such as esterifications between carboxylic acids and alcohols may positively change the aroma profiles from pungent to fruity. Maillard reactions tend to lead to flavour formation of caramel or cooked notes, but are of potentially low impact given their high flavour thresholds.

Oxygen in beer

Limiting dissolved oxygen levels in packaged beer to below 50ug/L should prevent most undesirable effects on flavour stability. However, this is not necessarily achievable in all packaging facilities, and thus quality control criteria for packaged oxygen can be set at 0.2 mg/L or less, with modern filling equipment capable of achieving 0.1 mg/L total package oxygen^1,5. Reactive oxygen species (ROS) are thought to be largely responsible for aged beer flavour formation^6,7. Such reactive oxygen species can be of the radical form (nitrogen and oxygen) or non-radical form in that they have the potential to convert to oxidising radicals. The measurement of total package oxygen in finished product is a good quality assurance practice for the modern brewer. However, in practice molecular oxygen levels do not directly equate to flavour deterioration. Oxygen related reactions are thus thought to proceed via free radical formation through transition metal catalysis.

Metals as protagonistic catalysts

Metals play a defining role in beer style. When concentrations reach beyond what is needed for pH adjustment and yeast health (e.g., zinc), metals become protagonists of beer flavour and flavour stability. Excess iron in beer can lead to metallic off-taste and high manganese has also been attributed to sherry-like, off-flavour formation during beer aging iron can also behave as a catalyst that facilitate oxidation mechanisms through radical generation via the Fenton and Haber-Weiss reactions. Copper and manganese are also capable of catalysing reactions to produce ROS and is suspected to act synergistically with iron to catalyse oxidative staling reactions⁸. Because even trace amounts of transition metals may cause detriment to beer flavour stability, the brewer should make every effort to keep levels under 50 ppb at all stages of the brewing process. That said, raw materials (water, malt and hops) can have higher than acceptable levels of transition metals. Typically, transition metal concentrations will decrease during fermentation, as yeast absorb and intracellularly distribute transition metals. Over several repitches, this can affect yeast health and subsequently affect the flavour stability of the finished product. Brewers also need to pay attention to interaction with filtration media such as Diatomaceous earth, which has the potential to contribute to beer soluble iron. In a well-equipped quality assurance lab metal content can be monitored by inductively coupled plasma – atomic emission spectroscopy (ICP-AES).

Aldehydes as staling indicators

Aldehydes are monitored as staling indicators in beer because of their tendency to increase during aging and their relatively low flavour and aroma thresholds (sub-ppb). During beer aging, amino acids such as leucine and phenylalanine can undergo Strecker degradation to form aldehydes of high aroma impact: Isovaleraldehyde (threshold 46 ppb as malty, cherry, apple, almond) and phenylacetaldehyde [threshold <100 ppb as floral, roses⁹]. Strecker degradation begins during the brewing process and will progress during beer storage. Aldehydes pertinent to beer aging can result from oxidation reactions, Maillard reactions and from the degradation of proteins. The formation of trans-2-nonenal (cardboard/papery flavour) in beer is thought to align well with oxidative aging. Trans-2-nonenal levels of less than 1 ppb can significantly impact the flavour of a pale lager beer. Hence, brewers with access to sophisticated instrumentation can monitor levels of trans-2-nonenal and its precursors throughout the brewing process. Despite its relatively high flavour threshold, the Maillard product furaldehyde forms due to heat stress and is thus used as a staling indicator relevant to temperature exposure. Other equally flavour-inactive aldehydes are monitored and used as aged beer flavour indicators because their concentrations increase along with increases in known oxidative flavours⁹. Although a majority of aldehydes found in beer are thought to derive from malt precursors, hop acid side chain degradation (organic acids) and yeast metabolism (short chain fatty acids) can also lead to aldehyde formation. Aldehydes can be quantified via use of Gas Chromatography with Flame Ionization Detection (GC-FID) or mass spectrometer detection (GC-MS). This can be done in conjunction with a derivatisation agent and solid phase micro extraction fibres (SPME) or stir bar sorptive extraction (also known as twister) (SBME). GC-Olfactometry (GC-O) can also be incredibly useful for monitoring off flavour formation.

Yeast and flavour formation

Healthy, happy yeast will cleave sugars to produce ethanol, carbon dioxide, and heat as well as various secondary metabolites: esters, carboxylic/fatty acids, phenolics, sulfur components, etc.¹. Yeast ability to produce specific secondary metabolites, both desirable and undesirable, varies by strain, and thus strain can be highly impactful on overall beer flavour. In general Ale yeasts (saccharomyces cerevisae) produce more esters than lager yeasts (saccharomyces pastorianus). Both ale and lager yeasts produce Ethyl acetate (solvent, nail polish). Ethyl acetate is ethanol ester, and is most predominant in beer because ethanol is the predominant alcohol produced by beer yeast. Ethyl acetate has a relatively high aroma threshold in beer which is why even higher concentrations do not impact the overall aroma profile of the beer. Other esters such as isoamyl acetate (banana) are detectable by nose in even the lightest of lager beers because they have a relatively low flavour threshold and can reach significant levels during fermentation. As beer ages, levels of fruity and floral esters tend to decrease. However, some esters may actually form during aging due to ethanolic esterification of carboxylic acids during beer storage. Acetate esters of higher alcohols and ethyl esters of long chain fatty acids (butyric, caprylic and proprionic) can result in pungent off flavours described as ‘cheesy’, ‘goaty’ and ‘milky’. Higher alcohols or fusel alcohols can be important flavour contributors, imparting solvent and floral notes to beer. During aging, ethanol can also oxidise into acetaldehyde (green apple note). Increases in acetaldehyde can be monitored as a staling indicator in beer. Yeasts also produce the notoriousvicinal diketones (VDKs) such as diacetyl, which has a flavour note of butter. Typically VDKs and precursors are monitored at the end of fermentation to ensure that levels are low enough for aging and package release. Consequently, any VDK precursors remaining in beer can lead to off-flavour formation post-package. VDK analysis can be done using spectrophotometry; however, it is relatively less sensitive than GC-ECD. Not all, but some ale yeast used for Saisons, Heffeweisens and Belgian style ales possess the enzyme necessary to convert ferulic acid to the phenolic 4-vinylguiaicol (clove-like), which carries through to the finished beer. However, this phenolic note tends to decrease during beer aging.

Other potent flavour compounds that derive from yeast belong to the sulphur family. Yeasts metabolise sulphur containing amino acids such as cysteine and methionine to produce some rather pungent molecules ranging from rotten eggs, onion, burnt match to cooked cabbage and sweet corn. Some sulphur components such as dimethyl sulfide (DMS, sweet corn note) originate from malt constituents. DMS can be removed by vigorous wort boiling in the kettle. However, if there is any S-methylmethionine precursor left in the wort, it may convert to DMS in the packaged beer.

Because yeast is the engine of beer making, yeast health is vital. The presence of spoilage bacteria can significantly alter pH and flavour attributes of beer. If a consumer detects flavours such as musty, mouldy, medicinal, sour, bready, horsey or farm-like in their beer, spoilage organisms have surely been at work.

Hop bittering acid degradation

Most commercial beers contain hops or hop products. Hops help the brewer in many ways: they provide flavour, aroma and bitterness, aid in wort clarification, act as antioxidants and antimicrobials, provide foam capacity, texture and can improve the overall flavour stability of beer. The main bittering components of beer, the isomerised-alpha acids, are also susceptible to aging.

Oxidation, heat stress and light are all enemies of the hop acids. Over time, iso-alpha acids in beer begin to degrade which results in decreased beer bitterness. Because the trans-stereoisomers of the three main iso-alpha acid analogues are less energy favourable and degrade more quickly during storage, the ratio of trans/cis-iso-alpha acids can be used as a staling indicator. Hop bittering acid content of beers can be monitored using High pressure liquid chromatography (HPLC) in conjunction with solid phase extraction (SPE). Thermal degradation or heat stress of the hop acids (both in the hops and in the beer) can yield pungent organic acids such as butyric (vomit), valeric (cheesy) and isovaleric acids (cheesy/sweaty). In the presence of ethanol, these organic acids may undergo esterification into somewhat less offensive, fruity esters. Hop acids in leaf (whole cone), pellet and extract form are all susceptible to the same degradation reactions and thus, if not stored properly, may develop similar cheesy aromas. Light is an enemy that can be battled, if a brewer so chooses. Exposure to light energy instigates a reaction between iso-alpha acids degradation products and sulphur in the beer matrix to produce lightstruck or skunky character. Brown glass, cans and kegs all block the wavelengths that cause lightstruck character. Conversely, beer packaged in blue, green or flint-glass is susceptible. The culprit, 3-methyl-2 butene-1 thiol (MBT), begins to accumulate within minutes in beer exposed to sunlight. The reaction occurs less readily under UV lights from a lit display case, but it will eventually occur. Modified hop extracts that are not susceptible to photo-degradation are commercially available from several vendors. However, use of modified extracts for beer bittering may result in decreased antioxidant potential and thus sacrificed flavour stability. Hop derived off flavours are best monitored by GC-FID or GC-MS by sampling the headspace. Detection of lightstruck character is easily done by sensory panel due to thiol low flavour thresholds. Analytical detection of thiols such as MBT requires highly sophisticated instrumentation with sulphur detection capabilities.

Endogenous antioxidants in hops and malt

Polyphenol capacity to improve food oxidative stability has been well documented. Therefore, it seems realistic for brewers to look toward polyphenols as potential antioxidants with a capacity to improve beer flavour stability¹⁰. Hops and malt contain polyphenols of the flavonoid class (proanthocyanidins, flavonols, and flavan-3-ol monomers, such as (+)-catechin and (-)-epicatechin). Although the same polyphenols are known to influence oxidative mechanisms responsible for aged food flavours, the brewing industry is still wavering on their practical effectiveness. To be certain, too little is understood regarding their impact, or their exclusion (as in the use of hop extracts) on aged beer flavour formation.

Addition of hops and hop vegetative matter to the kettle undoubtedly can impart overall flavour, as well as improve overall beer flavour and shelf life. Brewing trials from various geographic origins indicate that the most potent punch seems to come from kettle hopping with pellets and that beneficial flavour attributes do derive from the hop vegetative matter that is considered ‘spent’ by the hop extract industry (CO₂ extraction removes most of the bittering components and oils)¹¹. To date, at least five patents have been filed in reference to the advantages of brewing with hop polyphenols. The main challenge with research on polyphenols lies in the difficulty of extracting and analysing them. Analysis of polyphenols can be done via spectrophotometry, however the methods are largely unspecific. HPLC-MS is much more specific, yet is costly, complicated and time-consuming. Rather than characterising and quantifying specific polyphenols, analysing the total antioxidant or anti-radical capacity of the beer or hop products can be useful. Antioxidant capacity or anti-radical capacity of beer and raw materials may be assessed via spectrophotometric methods (DPPH and FRAP) or via ESR. However, comparison of results from ESR to other antioxidant capacity methods reveals a sort of polyphenol/flavour stability paradox in that the different analytical methods do not always align.

In summary, flavour instability is of growing concern for many brewers. Achievement of beer quality in terms of flavour can be extremely challenging, especially considering the multifaceted mechanisms at play. Having an understanding of beer flavour origins and modifications that occur post-package will help brewers work towards their flavour stability goals, no matter the distance between brewery and thirsty consumer.

References

1. Lewis, M.J. and Young, T.W. (2002) ‘Brewing, Second Edition.’ New York, NY: Kluwer Academic and Plenum Publishers.’ pp. 398.
2. Kaneda, H., et al. (1995) Chemical evaluation of beer flavour stability.’ MBAA Technical Quarterly. 32: p. 76-80.
3. Bamforth, C.W. and Parsons, R. (1985), New procedures to improve the flavour stability of beer. Journal of the American Society of Brewing Chemists. 43: p. 197-202.
4. Bamforth, C.W. (1999) ‘The science and understanding of the flavour stability of beer: a critical assessment.’ Brauwelt Int.: p. 98-110.
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6. Irwin, A.J., Barker, R.L., and Pipasts, P. (1991) ‘The role of copper oxygen and polyphenols in beer flavour instability’ Journal of the American Society of Brewing Chemists. 493: p. 140-149.
7. Hughs, P.S. and Baxter, E.D.(2001) ‘Maintenance of Beer Quality, in Quality, Safety and Nutritional Aspects.’ Royal Society of Chemistry. p. 138.
8. Mochaba, F.O.C.-C., E.S.C.; Axcell, B.C.(1996) ‘Metal ion concentration and release by a brewing yeast: Characterization and Implications.’ Journal of the American Society of Brewing Chemists. 543: p. 155-163.
9. Saison, D., De Schutter, D.P., Uyttenhove, B., Delvaux, F., Delvaux, F.R. (2009) ‘Contribution of staling compounds to the aged flavour of lager beer by studying their flavour thresholds.’ Food Chem. 114: p. 1206-1215.
10. Lermusieau, G., Liegeois, C., and Collin, S., Reducing power of hop cultivars and beer ageing. Food Chem. 724: p. 413-418,2001.
11. McLaughlin, I.R., Lederer, C., and Shellhammer, T.H., Bitterness-modifying properties of hop polyphenols extracted from spent hop material. J Am Soc Brew Chem. 663: p. 174-183,2008.

About the author

Patricia Aron obtained a B.S. in Biochemistry from Elmira College, Elmira, NY. She obtained both her MS and PhD degrees at Oregon State University, Corvallis, OR. During her MS, Patricia studied Enology (Wine Chemistry) and focused on investigating polyphenol extraction during red wine maceration. Patricia’s PhD work focused on lager beer flavour and flavour stability as it pertains to hopping technology. Patricia began working at MillerCoors in Milwaukee, WI in 2010. She currently functions as the Senior Hop Chemist in the Brewing Materials Group for MillerCoors Brewing Research, Innovation and Quality department.