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An Introduction to Sensory Analysis
by Scott Bickham, column editor
This first installment of "Focus on Flavor" introduces the complex world of beer flavor and how brewing methods create and influence it -- for good or for ill. In each issue, BJCP exam director Scott Bickham will show you how to identify and classify flavors, determine their desired levels for given beer styles, and pinpoint their origins in the raw materials and production methods behind your beer. We hope this column will help you to become better tasters, better judges, and better brewers. Prosit!!
Think about how many senses are required to enjoy a beer fully. From the moment the bottle is opened and the beer is poured into a glass, our ears are greeted by the hiss of escaping carbon dioxide. Our eyes are attracted by the sparkling clarity of a Pilsener, the hazy sheen of a Weizen, or the black depth of a stout. We patiently wait for the creamy head to slowly collapse, leaving wisps of Belgian lace on the sides of the glass. As we bring the beer to our lips, our nose detects the aroma of citrusy hops in an American pale ale, bittersweet chocolate in a porter, or perhaps fruity, spicy esters in a Trappist ale. Finally, we imbibe, savoring the malt, hop, and ester flavors before swallowing to let the hop alpha-acids wake the taste buds on the back of the tongue.
We take another drink and swish the beer through our mouth to evaluate the body and mouthfeel. Ah, this seems like a well-made beer -- but wait! What is that lingering aftertaste? Does it taste like cooked cabbage, or is it perhaps reminiscent of newly mown grass? Is that a hint of paper or leather in the background? What are these flavors and how did they get into your beer?
This curiosity about flavor marks the first step in progressing from a pleasure drinker to a beer troubleshooter (or, in scientific lingo, from an organoleptic taster to a sensory analyst). The former uses a nonscientific method of beer evaluation based primarily on hedonic quantities: Is this beer good or bad? A sensory analyst, on the other hand, works like a scientific instrument and measures, rather than records, the components of beer flavor (1). A trained taster should be able to identify flavors, quantify them with respect to their thresholds, and objectively determine their desirability in a particular beer style.
It should be noted, however, that while this sensory training will improve tasting and judging skills, it can't guarantee that anyone will become a "supertaster." Adult humans have a finite number of taste buds that decrease from approximately 10,000 to only a few thousand as we age. The advantage lies with younger tasters, although experience can compensate for much of the difference. Statistical differences will also have an impact: A sizable percentage of the population cannot detect compounds such as diacetyl and methyl mercaptan in any concentration. Furthermore, the perception skills of females are generally regarded to be superior to those of males. Inherent differences in our tasting abilities exist, but by keeping them in mind as we learn about beer flavors we can at least recognize our limitations.
Sensory analysts make good beer judges, but also contribute another important service -- providing the brewers technical feedback to help them correct flaws in either the ingredients or brewing process. Brewers can take advantage of this information to better control the outcome of their beers.
This column will focus on the origins and characteristics of flavors in beer.*The goal is to train readers to become sensory analysts through familiarization with the Beer Flavor Wheel, the industry's standard sensory analysis tool (see figure at right). We will start by exploring the four basic flavors and move through off-flavors and fermentation by-products, finishing up with a look at mouthfeel and fullness. Each column will cover a new section of the wheel, in detail, until the entire wheel has been circled.
Improving one's sensory skills is a demanding task. This column will offer readers a well-rounded introduction to sensory analysis, providing a technical foundation for some of the most important beer flavors and identifying where they may be found in commercial examples or reproduced by doctoring beers.
Beer Flavor Defined
Taste: Taste, or gustation, is the combined impression we receive when free nerve endings and taste buds in the mouth detect various stimuli. The free nerve endings possess no receptors, but are responsible for the perception of sensations such as pain, temperature, pungency, and astringency. The taste buds are clusters of approximately 100 taste cells that occur as protuberances, called papillae, on the tongue. The mechanism of flavor perception is not well understood, but it is believed that the arrival of a chemical stimulant on the surface of a receptor temporarily modifies the cell wall and produces an electrochemical impulse. This impulse is then transmitted through a nerve cell to the brain, where it is decoded into sensory information in the cerebral cortex.
Smell: The sense of smell, or olfaction, is even more complex than that of taste. Humans can distinguish between thousands of odors, which are perceived by neurons in the olfactory epithelium (skin cells) of the upper respiratory passages. Whether or not a volatile compound produces a stimulus depends on the size, shape, and degree of ionization of the molecule. Sniffing is required to draw a significant number of air molecules into this upper nasal cavity. Once detected, the stimulus passes directly from bundles of nerve cells called glomerulli into the olfactory lobe of the brain. Some molecules also reach this olfactory ephithelium by way of the back of the mouth during swallowing; this is known as gustatory aroma perception.
Other factors: Taste and aroma may be perceived separately, but as stated in the scientific definition of flavor above they are often integrated to produce the total flavor impression. That total flavor impression may also include other influences beyond taste and aroma. Tactile sensations such as viscosity and the prickle from dissolved carbon dioxide, for example, are determined by the physical composition of the beer and also play an important role in zymological evaluations. Temperature is extremely important for all aspects of flavor perception. Warmer temperatures tend to emphasize the aromatic components of the beer, while lower temperatures suppress them. It is therefore imperative to serve beer at the proper temperature for the style.
This column will not cover psychological factors or the senses of vision and hearing, which may indirectly affect the results of a beer evaluation. A trained taster should be able to minimize the influence of these external factors while performing a sensory analysis.
Beer Sensory Analysis Tools
Two important tools have been developed over the past 30 years to help establish a standard vocabulary for beer sensory analysis, both by Dr. Morten Meilgaard: first, an early system based on flavor units and then, the Beer Flavor Wheel.
The Meilgaard system: This system, developed by Meilgaard in 1975, was the first attempt to link flavor characteristics to certain styles of beer (2). This classification scheme assigned a Flavor Unit (FU) rating to each flavor constituent, defined as the ratio of a given compound's concentration to its threshold value. Thus, if an American amber ale contained a caramel compound in concentrations twice that of its threshold value, for example, it would have an FU of 2. In this context, the relevant threshold is one of recognition, which corresponds to the lowest physical intensity at which a stimulus is correctly identified. This is to be compared with the stimulus threshold, the lowest physical intensity at which stimulus is perceptible and the difference threshold, the smallest change in physical intensity of a stimulus which is perceptible.
In the Meilgaard system, compounds primarily responsible for "beery" flavors are called primary constituents. These have concentrations above 2 FU (therefore they are generally present at concentrations at least twice the threshold). Removing any primary constituents from the beer would have a significant impact on the flavor. It doesn't require much sensory training to recognize the essential contributions of alcohol, hop bitterness (technically), and carbon dioxide to any beer's flavor. Although not as intense, secondary constituents (classified as having 0.5-2.0 FU) constitute the bulk of the flavor and act together to provide the characteristics that distinguish different beer styles. Removal of any of these secondary constituents would result in a lesser but still noticeable change in the flavor.
The next group is the tertiary constituents (0.1-0.5 FU) responsible for contributing subsidiary flavor notes. Removal of these, or of any of the background constituents with concentrations below 0.1 FU, produces no perceptible change in beer flavor, though these compounds nevertheless contribute to the overall character of the beer. Just as if you had removed the spices from a casserole, they may not be a major part of the recipe, but without them the end result will be flat.
Though somewhat revolutionary, this early Meilgaard system is of limited value in practical applications. For one thing, some controversy exists in the use of thresholds as sensory criteria. Thresholds are usually determined in artificial environments where the chemicals are tasted in isolation. In practice, however, each threshold is but one data point on a complex and dynamic concentration continuum. The descriptions of taste and aroma compounds may also vary with the medium in which they are analyzed. For example, the aroma of geraniol, an alcohol derived from hops, has been characterized as cheap perfume when tasted in isolation, but nonetheless provides pleasant floral notes to beer (4).
The Beer Flavor Wheel: Meilgaard later developed a flavor wheel in an attempt to standardize the language of flavor analysis. The flavor wheel was jointly adopted in the 1970s by the European Brewery Convention (EBC), the American Society of Brewing Chemists (ASBC), and the Master Brewers Association of the Americas (MBAA) (3,5). Though it was recognized that no set of flavor descriptions is static, the version of the wheel released in 1979 is still the standard today.
The flavor wheel gives each distinct flavor a descriptive name and groups it with similar flavors into 14 classes. (Under the descriptive name "phenolic," for example, are the five descriptors "tarry," "bakelite," "carbolic," "chlorophenol," and "iodoform.") These flavor names are consistent with the EBC thesaurus and include no subjective terms such as good/bad or balanced/unbalanced. The flavor wheel contains a total of 46 first-tier flavor descriptors -- common terms such as fruity, solventlike, and metallic with which most people will be familiar. A second tier further breaks out the first-tier descriptors into 76 specific flavors found in beer.* A trained beer taster should be able to identify approximately 100 of these flavors (1). Fewer than half of these are commonly found in beer; the remainder are used to describe flavors in defective or specialty beers.
Intensity chart: The language of the flavor wheel and the thresholds of the Meilgaard scheme can be combined to create an intensity bar chart that represents the desired flavors in a particular beer style. In their new book An Analysis of Brewing Techniques, George and Laurie Fix recommend using the following five-point system to rate flavor intensities (4):
This system is a practical and qualitative version of the Meilgaard system (which is based on analytical measurements not accessible to most tasters). A complex beer such as an ESB would have many prominent peaks of intensity 4 or higher, corresponding to flavors in most of the 14 classes. These would include sweet, bitter, body, hoppy, malty, nutty, and caramel. Secondary peaks would correspond to diacetyl, acidic, alcoholic, estery, fruity, and grassy. Flavors in the oxidized, phenolic, and sulphury classes should have negligible intensities. Bland or unbalanced efforts at brewing this style would have no or only a few significant peaks, respectively. As a taster becomes more experienced, this intensity bar chart should correspond to the characteristics he or she expects from the various beer styles and can be used by beginner tasters to become familiar with style guidelines.*
What to Expect from the "Focus on Flavor" Column
Instead of moving through the flavor classes in order, we will work backwards on the flavor wheel beginning with the four basic tastes. From there, we will work through common off-flavors derived from oxidation and sulfur compounds, progress through flavors primarily derived from malt and hops, and then cover fermentation by-products such as alcohols and esters. We will finish with a description of the last two flavor classes, fullness and mouthfeel, which are perhaps the most complex and least understood by most beer evaluators. This coverage, along with some hands-on sensory training, will provide a balanced, practical introduction to beer flavors that should enable readers to become expert at the sensory analysis of beer.
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