Disclaimer The author holds no claim on any trademarks used in this presentation. They are used solely as educational examples and do not imply any sort of evaluation of the product.
Natural History of Flavor Flavor has a long pedigree. Every living cell from day one to today needs information about the surroundings outside of itself. Is it hot or cold? Does it contain food or poison? When yeast cells detect maltose, they install maltose pumps in their membranes.
Cells Each is bounded by a membrane. The membrane is incomprehensibly thin. It is made it of molecules that have a head that sticks to water (inside and outside the cell) and a tail that sticks to other tails. sticks to water [hydrophilic] sticks to other end [hydrophobic] 10 nm
Channels, Pumps, Receptors The membrane is a barrier. Embedded in the membrane are structures that transport material and information across. Channel: A gate that allows a substance through the membrane. Most channels can open and close. Pump: Uses energy to drive a substance across the membrane, even if the concentration is lower outside. Receptor: Attaches to molecules outside causing something to happen inside. Receptors transmit information (Yo, there s sugar out here.)
Membrane Potential Pumping of charged particles (ions) gives the inside of the membrane a negative electrical charge that comes to 0.07 volts. Excitable cells can open channels and admit positively charged particles (ions) to make the cell less negative. This depolarization is the primary response to a stimulus.
Depolarization by Positive Ions Closed channel + + + + + + + + + + + Open channel
Flavor in Humans Flavor has three components. Taste Aroma Mouth feel Taste and aroma are specific chemical senses. Mouth feel comprises many types of senses, including temperature, pressure, and chemical irritation. Face Fragment Arlene Love
Taste Materials dissolved in water. Sensed mostly by taste buds on the tongue. Bitter Salty Sour Sweet Umami Taste bud
Ions: Salty Taste Sodium chloride consists of sodium ions: Na + and chloride ions Cl. Sodium ions can enter salty taste cells through selective channels. The positive ions depolarizes the cell resulting in a signal. salt crystal model grey = sodium ions green = chloride ions
Ions: Sour Taste Acids give H 3 O +, the hydronium ion. Like sodium ion, hydronium ion enters and depolarizes the taste cell giving rise to a sour taste. Lactic acid: sour
Receptors Other tastes and all aromas result from binding of molecules to receptors at the surface of the cell, causing channels to open, leading to depolarization. isohumulone: bitter
Aroma Gas molecules. Sensed by nerve cells at the top of the nose. Hundreds of types of aroma receptors. In combinations give rise to a virtually unlimited variety of flavors. Human Olfactory system Image: Brain human sagittal section.svg Image:Head lateral mouth anatomy.jpg by Patrick J. Lynch, medical illustrator
Aroma Receptors Aroma receptors are not very selective, most will respond to a range of compounds. Also, an aroma compound will usually bind to more than one type of receptor. Any particular aroma seems to result from a composite response of several receptors.
Aroma Diversity A certain aroma compound may bind strongly to some receptors, and less strongly to some others. So there will be a spectrum of responses from a number of receptors. Another compound may bind to the same receptors but with a different spectrum of responses, giving a different perceived aroma. A B C D E A B C D E
1. Aroma molecule binds to receptor. 2. Receptor changes shape. 3. Change activates GTP in protein complex. 4. Protein complex releases beta subunit. 5. Beta subunit bonds adenylyl cyclase. 6. Adenylyl cyclase makes camp. 7. camp binds gated calcium channel. 8. Calcium channel opens, admitting Ca 2+ ions. 9. Cell begins to depolarize locally. 10. Depolarization opens voltage-gated sodium channels, depolarization spreads. 11. Cell becomes fully depolarized, signaling next cell.
Flavor Intensity Some substances are much more strongly flavored than others. This may be because some molecules fit certain receptors very well, so nearly every one of them activates the flavor cascade. Another possibility is that the brain is looking for certain signals or combinations of signals. Box Car Brewing West Chester PA
Flavor Threshold The lowest concentration of a compound that can be tasted is the flavor threshold. Ethanol has a threshold of 1.4%. Isohumulone (hop bitter compound): 4 ppm. MBT (skunky): 4 parts per trillion. MBT is 3.5 billion times more strongly flavored than ethanol. MBT: skunky flavor in lightstruck beer.
Flavor Units The concentrations of flavor compounds are expressed in multiples of the threshold. This concentration is called the flavor units. Ethanol (TH = 1.4%) at a concentration of 2.8% would contribute 2 flavor units. At 4.2% would contribute 3 flavor units. If two substances have the same flavor units, they would be perceived as contributing roughly the same amount to the flavor. ethanol, aka John Barleycorn
Beer Primary Flavors Primary flavors are those present at 2 flavor units or more. Here are some flavors that are primary in nearly all beers. Flavor typ conc thresh flav units carbon dioxide 0.5% 0.1% 5 hop acids 16 ppm 4 ppm 4 ethanol 4.0% (w/w) 1.4% 2.9
Carbon dioxide Hop acids [isohumulone] Ethanol
Bitterness Hop bitter compounds often given in bitterness units. BU roughly equivalent to 1 ppm isohumulone. One flavor unit is about 4 BU. Bitterness units do not always correspond to perceived bitterness.
Other Beer Flavors Other flavors differentiate the beer styles. hop aroma from hops myrcene malty from heat treatment maltol
fruity from fermentation (also hops) isoamyl acetate phenolic (clove) from fermentation 4-vinyl guaiacol
Flavors and Styles From the chemistry point of view we can fit beer styles into the following categories. Bitter ale Malty ale Bitter lager Malty lager Specialty Malty lager
Ale vs Lager Ale is fermented at temperatures above 60 F. Yeast grows rapidly and produces esters. These introduce fruity notes. ethyl acetate an ester Lager beer is fermented at temperatures below 60 F. Yeast growth is slow, less esters are made, so the flavor is clean. Different species of yeast work best at the different temperature ranges.
Malty vs Bitter The balance between maltiness and bitterness is a key style issue. At high temperature malt kiln or kettle this reaction can occur: sugar amino acid Shiff base
The Shiff base reacts in two directions. One direction gives big, highly colored molecules called melanoidins that make the beer dark. The other direction gives small, highly flavored molecules that make the beer malty.
Malty Compounds maltol furaneol Malty flavors are abundant in dark malts, like Munich. They are very abundant in crystal malt, especially the higher roasted varieties.
Off-Flavors cidery too much adjunct, weak yeast acetaldehyde ribes (catty) oxygen, aging also hops 3-methyl-3-mercaptobutyl acetate
buttery inadequate conditioning diacetyl vinegar bacteria and oxygen acetic acid
cardboard trans-2-nonenal stalenes s cooked veg dimethylsulfide (DMS) inadequate boiling or slow chilling
skunky exposure to light 3-methyl but-2-ene-1-thiol (MBT) MBT results from the action of light on hops bitter compounds.
Skunk-proofing +2H isohumulone rho isohumulone won t skunk
Traditional Skunk-proofing Brown bottles keep out most light.
Most Effective Skunk-proofing Aluminum cans admit no light.
Beer Aging Beer has a limited shelf life; it goes stale. Aging (staling) reactions mostly involve transfer of electrons from stuff in the beer to species derived from oxygen. All brewers try to hold the oxygen content at packaging to very low levels. Oxygen can get through the seal in a crown cap. polymer seal
Protecting Beer from Staleness Low temperature: chemical reactions are slower. Staling can be slowed down by a factor of ten by keeping the beer at 40 F instead of 100 F. For oxygen infiltration, cans are better than glass bottles, which are better than plastic bottles. Yeast eats oxygen. Some styles of beer may keep better if bottle conditioned with yeast. 650 bbl lauter tun, Yuengling Beer Co
Protecting Beer from Staleness Dr. Barth s recommendation: Don t keep it waiting.
Beer Course The Chemistry of Beer course at West Chester University teaches the basics of chemistry and biochemistry as applied to beer brewing to about 70 students each semester. Any student can enroll, no prior science is required. There is no laboratory, but we do take field trips. I would be delighted to hear from anyone with a connection to a college or university that could be interested in a course on beer. rbarth@wcupa.edu Victory Brewing Co. Downingtown, Pa
Acknowledgments Many thanks to Marcy Barth for art and design work, to Matt Bolling and the AHA for giving me this opportunity, and to you for your attention, interest, and advocacy for beer.
Publication: November 2013 1 Introduction 2 What is Beer? 3 Chemistry Basics 4 Water 5 Introduction to Organic Chemistry 6 Sugars and Starches 7 Milling and Mashing 8 Wort Separation and Boiling 9 Fermentation 10 Tests and Measurements 11 The Chemistry of Flavor 12 The Chemistry of Beer Styles 13 Foam and Haze 14 Beer Packaging 15 Beer Flavor Stability 16 Brewing at Home