"But it was the immortal Louis Pasteur who in 1876 electrified the world with the publication of his 'Studies on Beer.' He proved conclusively that yeast is the cause of fermentation. ...After these discoveries, came the word 'enzyme,' derived from the Greek word 'en,' meaning 'in,' and 'zyme,' meaning 'yeast'; or together 'in yeast.' These enzymes are tireless transformers. They will break down the materials employed in brewing, modify them and digest them into that solution we call 'beer.'" --Eloise Davison, Beer in the American Home
Let's talk about some biology for a minute. We'll start simple. Drinking beer makes us feel good (most of the time) because it is a type of drug known as a depressant. [highly amusing link] The active ingredient, specifically, is ethanol.
Fermentation
The ethanol is present because the beer has undergone fermentation. The fermenting agent is the ingredient called yeast. Yeast is a single-celled organism that ferments the beer as a source of energy. The primary source of energy for all living matter on Earth is adenosine triphosphate, or ATP for short. [link] Many organisms generate ATP from sugars, such as the simple sugar glucose, through a process known as glycolysis. Specifically, during glycolysis the glucose is converted to a substance called pyruvic acid, or pyruvate. Also, a couple of molecules of nicotinamide adenine dinucleotide, or NAD+, accept some electrons to create NADH. Some ATP (a net gain of 2 molecules) is gained in this process, but many organisms (like humans) can get much more in the presence of oxygen through a process called cellular respiration. Others, such as yeast, do not do this, and must do something else with their pyruvate. [link] This process, formally called anaerobic glycolysis, is also the chemical reaction referred to by the term fermentation.
Fermentation sometimes occurs in humans. When you are exercising hard, your body is undergoing a whole lot of cellular respiration, using oxygen faster than you can supply it, and if it still needs more ATP, it'll get it by undergoing some fermentation as well. In humans, the byproduct of fermentation is lactic acid, which can be converted back into glucose. It is still commonly believed that a buildup of lactic acid causes muscle fatigue, but this may not actually be true. In addition to creating the lactic acid, or lactate, fermentation also converts a couple of molecules of NADH back into NAD+, allowing the cycle to begin again. [link]
Yeast ferments differently than humans do--rather than converting it to lactate, yeast will turn its pyruvate into acetaldehyde, with the creation of a couple of molecules of carbon dioxide as a side effect--this is what causes carbonation, and what bubbles through the fermentation lock during my homebrewing process--and turn the acetaldehyde into ethanol, which it simply excretes. It might seem like a waste to go to all this trouble and simply discard the results, but it's necessary for the yeast to do all this if it wants to reclaim its NAD+ and keep fermenting.
This is the how and the why of fermentation, as it appears in humans and as it appears in brewing.
Inebriation and its effects
Alcohol affects everyone differently depending on a number of factors--weight, gender, and the amount of food currently in the stomach feature among them--but the ultimate universal measure of inebriation is blood alcohol content (BAC for short), or how much of one's bloodstream consists of alcohol that has managed to make its way there. Inebriation is generally agreed to start at a BAC of .03%, or one part in 3,333; a BAC of .5%, or one part in 200, is fatal. One's BAC determines how intoxicated by alcohol, or inebriated, one becomes. The stages of inebriation are as follows [link]:
1. Euphoria. BAC from .03% to .12%. Inhibitions lowered; confidence increases; general improvement of mood.
2. Lethargy. BAC from .09% to .23%. Increased fatigue; slurred speech; loss of coordination.
3. Confusion. BAC from .17% to .28%. Dizziness; nausea; more extreme emotions; forgetfulness.
4. Stupor. BAC from .25% to .39%. Consciousness comes and goes.
5. Coma. BAC from .35% to .5%. Persistent unconsciousness; decrease in heartbeat and breathing.
6. Death. BAC above .5%.
Due to the intervening nature of stages 4 and 5, it is relatively rare for stage 6 to occur, but it does happen, mostly in cases where fatal amounts of alcohol are consumed quickly and all at once, before stupor and/or coma can set in. Due to the relatively low alcohol content of beer as opposed to distilled liquors such as whiskey and rum, it is especially rare for fatal alcohol poisoning to result from the consumption of beer alone. Note that these stages overlap, so one can, for example, be both euphoric and lethargic at once.
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