The Nature of Light
Light is a type of radiant energy — a class that includes everything from radio waves to X-rays. These energies follow a wave model of behavior; they oscillate uniformly about a central line that indicates the direction of travel for the wave.
The character of energy depends upon the length of the oscillating wave, or wavelength, which is the distance between two successive peaks in the oscillation pattern. Most wavelengths of radiant energy — including microwaves and ultraviolet and infrared light — are invisible to the human eye. Only a small band of the radiant energy spectrum contains the visible wavelengths. This band has wavelengths ranging from about 380 to 760 nm.
White light contains energy from all the visible wavelengths in approximately equal proportions. Color occurs when one wavelength (or group of wavelengths) in this range is absorbed by a solution or object. The absorption creates an imbalance in the spectrum of white light, and we perceive the nonabsorbed wavelengths as their characteristic colors.
Beer absorbs light very well in the violet to blue spectrum (400–480 nm) and moderately well in the adjoining green spectrum (480–540 nm). Because these blue and green wavelengths are absorbed, we perceive the remaining wavelengths, which are in the yellow to red range. Thus, beers are often yellow, and they become more red as they darken. Very dark beers show significant absorbance across the entire spectrum of visible light; they absorb all wavelengths of light, so we perceive no color at all and the beers appear black.
The wavelength and quantity of light absorbed by a beer depends upon the chemistry of the solution. Absorbance occurs at the molecular level, where various structures have differing abilities to absorb specific wavelengths of energy. Thus, the wavelengths absorbed depend upon the specific composition of the solution, and the extent to which a given wavelength is absorbed depends upon the identity, quantity, and concentration of specific compounds present.
It is apparent, then, that chemistry is the source of a beer’s color. Although the exact structures responsible for beer color have yet to be well understood, the origins of color have been clearly characterized. Let’s examine the essential chemistry of beer color.
The Chemistry of Beer Color
Three areas of chemistry have potential bearing on beer color: first, Maillard or browning reactions, which produce both color pigments and flavor compounds; second, caramelization products; and third, oxidation products.
Maillard reactions: Maillard reactions are by far the most important source of color in beer. They account for the formation of color during malting as well as during beer production. The formation of browning products begins with reactions between sugars (glucose, fructose, maltose, and others) and amino acids (the building blocks of proteins and enzymes). The colored end products are nitrogen-containing polymers called melanoidins. provides an idea of the complexity of these reactions.
Louis-Camille Maillard first described melanoidins in 1912, and their formation occurs in virtually every heated food product. Despite their common occurrence, scientists still know little about their chemical formation or structure. The latest food science research holds that the color-causing melanoidins have no aroma or flavor. Many sources concur, however, that browning reactions are a primary source of aroma and flavor for beer and food through intermediary compounds and minor end products. Thus, the conditions that lead to color also lead to flavor, although the exact products and proportions will vary based upon the conditions of the reaction environment.
Most malt flavors can be attributed to these products. A wide variety of flavors may result from these products, including burnt, sweet, toasted, musty, aromatic, buttery, fruity, bready, chocolate, caramel, rye bread, maple syrup, rock candy, and violets. When combined with certain amino acids, maltose is reportedly able to produce some more unusual aromas such as those typical of beef broth, baked ham, stale potato, and horseradish.
Separately, melanoidins may have a more direct flavor influence in finished beer. If melanoidins are oxidized during wort production, they can contribute to beer staling through the oxidation of higher alcohols into aldehydes during beer storage.
The raw materials of melanoidin production — sugars and amino acids — occur in ample concentrations in beer worts. The production of color compounds consumes only a small fraction of these ingredients during brewing; one study measured the amounts lost as 8.6% of the total amino acids and 3.8% of the sugars. Within the population of amino acids, certain specific members (such as threonine) seem to be preferentially consumed, and some sources suggest that the basic amino acids (e.g., lysine, arginine) react most readily. Among the sugars, maltose appears to play the major role in the formation of color compounds, followed by fructose and glucose.
The exact products formed by Maillard reactions are the result of a number of variables including time, temperature, water content, and pH. Maillard reactions do not require the presence of oxygen.
Time and temperature are the most significant variables in many settings, and different combinations of time and temperature produce different sets of end products. If we remember that Maillard reactions are at work in the production of malt as well as beer, the role of time and temperature helps explain the variations in color and flavor found in different types of malt — even the variations between different crystal malts that have supposedly been treated in similar ways. Also, browning products form rapidly at temperatures of 212 °F (100 °C) and higher, so the length of time that wort is boiled will have a direct influence on the production of color.
Browning reactions require the presence of water, and water concentration affects the speed and mix of browning-reaction end products. Maillard-reaction products form most quickly when water is present in relatively small quantities, such as in dried or evaporated food products. Malt extract syrup falls within the ideal range for the formation of these color and flavor products. Because beer worts primarily consist of water, Maillard or browning reactions occur, but at a slower rate than would be possible under other circumstances.
Alkaline conditions (high pH) hasten browning reactions, although these reactions can take place under both alkaline and acidic conditions. Some researchers have shown that a change in wort pH from 5.57 to 6.44 can produce dramatic changes in wort color during the boil — from 5.9 °L to 15.6 °L.
Other possible contributors to beer color: While Maillard reaction products are the major source of color for beer, other sources are significant. Oxidation of polyphenols is probably the second most significant source of color formation in beer. Polyphenols are sometimes referred to as tannins and may be derived from malt husks and hops. These multi-ringed structures can react with oxygen to contribute red-brown colors in beer. If you boil hops alone in water, rather than in wort, for an hour or so you can usually see this effect.
Contemporary beer research into phenol oxidation focuses on the staling and haze properties of the reaction products rather than on their contribution to color. Nonetheless, many sources note the darkening of color that attends oxygenation of worts and beers at any stage of production. Thus, reduction of polyphenol levels and reductions in wort oxidation can help to reduce color formation from this source. These issues will be of particular importance to brewers who wish to produce very light-colored finished beers.
Caramelization is a chemical process that affects sugars subjected to temperatures of about 400 °F (200 °C) or greater. Unlike melanoidin formation, this reaction, as it occurs in beer, does not involve nitrogen-containing compounds (commercial production of the now-outlawed caramel coloring did use ammonia, however).
Caramelization occurs in the boil, to a limited extent in most cases, and may be accentuated by the shape of the brew kettle and the method of heating. Gas-fired kettles, for example, will produce more carmelization than those heated with steam. Longer boils and higher gravity wort will also increase the amount of caramelization produced.
Increased pH has been shown to accelerate the caramelization process as well.
Naturally occurring pigments of barley and hops such as flavins, anthocyanins, and carotins have been examined as a potential source of color in beer, but they appear to play little if any role in the color of the finished product.
A Matter of Chemistry
Humans perceive color when a substance absorbs certain wavelengths of light and reflects others. Because light absorprion is the result of a substance’s chemistry, a particular beer’s color is the direct result of that beer’s chemistry.
The three primary contributors to beer color are Maillard (browning) reactions, caramelization products, and oxidation products. Armed with an understanding of how these color contributors affect beer worts, we can consider techniques for controlling and predicting beer color. The third installment of this series in the next issue will explore this final area of beer color mastery.
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