Malt Aroma Nearly all malt flavor and aroma comes from the chemical reactions that occur when malt is kilned. The same chemistry as bread toasting or coffee roasting gives a wide range of possible sensations: bready, nutty, malty, toasty, roasty coffee, chocolate, burnt. Different combinations of time, temperature and other factors create this range. Part of the brewer’s art is to layer them to create a unique and memorable experience.
Hop Flavor and Aroma Hops are small fluffy cones (strobiles, technically) of a climbing vine related to marijuana. These cones contain a resiny goo with bittering and aromatic properties. Hops come in a bewildering array of specific varieties, often with distinctly different personalities. Growing location also makes a difference—people in the wine world would call it “terroir.” Some hop aroma chemicals are also found in flowers (roses and geraniums), citrus, pine, herbs and spices, so it is not surprising that these same words are used to describe the aromas of hops.
Yeast Aroma Yeast is a complex chemical engine that converts sugar to alcohol and carbon dioxide, but also generates additional flavors. Ethanol itself has a flavor and aroma, but is not usually apparent in beer below about 7% by volume. Higher (more complex) alcohols are found in much smaller quantities, but also have more potent aromas. Think rubbing alcohol.
Esters are an important group of flavor chemicals produced by yeast. Present in minute quantities, these powerful aromatic chemicals are metabolic byproducts that vary widely by strain and fermentation temperature. It is the lack of esters—due to a cool fermentation—which differentiates lager beer from ale. Esters are largely responsible for aroma in fruit, so it’s not surprising that “fruity” is the most common term used to describe them. With a more educated palate, simple fruitiness gives way to “peachy,” “banana,” “raisiny,” “apple-like” and other specific terms. In larger quantities esters are perceived as more solvent-like, with one common one, ethyl acetate, actually being the chemical used to make nail polish remover.
When healthy, yeast can produce a number of other chemicals that can affect beer aroma and flavor: sulfury notes such as the vegetal DMS (dimethyl sulfide), the buttery tang of diacetyl, spicy phenols, plus a host of other aromatics that add up to the complex experience of beer. When yeast is stressed or contaminated by rogue microorganisms, some of these chemicals may be present in sufficient quantities to be perceived as flavor defects.
Water For ages, regions with certain types of water were prized as brewing centers. You have to dig down pretty deep to recognize what water adds to beer, but it is possible, especially in extreme cases. The famous pale ales of Burton-on-Trent, England have a crisp dryness impossible without a water high in gypsum, and indeed you can sometimes smell the plaster-of-Paris (same mineral, CaSO3) in a well-kept cask Bass, as it is the same mineral. The delicate pale lagers of Plzn, in the Czech Republic, would be quite different without their nearly mineral-free water. If you’d like to prove it to yourself, taste distilled water alongside a chalky one like Evian—there is a surprising amount of difference. Since beer is about 95 percent water, it’s no wonder water is critical. In addition to flavor of its own, water affects the brewing process chemically in critical ways.