Something for free

Recently, I was doing a bit of research on the proteins responsible for beer foam. I plan on writing a post about that sometime soon.[1]

I came across the Journal of the Institute of Brewing. This is published by the Institute of Brewing and Distilling.

This journal is a treasure trove of articles dealing with everything from PCR analysis of bacterial infections, to GC-MS analysis of yeast metabolites, to SDS-PAGE analysis of beer proteins.

And best of all, the Journal can be accessed free of charge. At least temporarily. I have no idea how long access had been free nor do I know how long it will last, but I've been taking advantage of it. There's some good stuff there.

They provide access back to 1990, but they also have thrown in a volume from 1896. I like the articles that reference Louis Pasteur's latest discoveries.

[1] yeah right. I've heard that before.

Irish Moss: A brief description

Clear beer is often a goal for many homebrewers, the standard being set by the commercial brewers. With modern brewing techniques and common industrial practice we have come to expect our beer to be crystal clear. When homebrewing, clear beer is not common. In reality, most homebrews are cloudy.

The two main culprits of cloudy beer are yeast and proteins.

As the yeast near the end of their job (i.e. when fermentation is ending due to lack of sugars) the single cells of yeast will clump together in groups of thousands and settle out. This is known as flocculation[1]. Different yeasts flocculate differently. Some yeasts settle out nicely and others do not. One strategy to get more yeast to settle out is to cool the beer (such as lagering) before bottling or kegging.

The other culprit of cloudy beer are proteins along with polyphenols and lipids (fats). While proteins are not necessarily small molecules, they are small enough to remain in suspension.

Beer can be clairified using Irish moss, a fining agent. Fining agents all work by making the smaller molecules aggregate into larger particles so they settle out of solution. This can be mathematically described by Stokes Law:

Where v is the rate of sedimentation, r₁ is the density of the particle and r₂ is the density of the wort, r is the radius of the particle, g is 9.8 m/sec² (a.k.a. acceleration due to gravity), and h is the viscosity of the medium. In other words, as the density and size of the particle increases it will settle out faster. In addition, a thinner wort will allow settling to occur faster.

Irish moss is Atlantic red seaweed[2] that contains k-carrageenan:
The k-carrageenan is a polymer of β-D-galactose-4-sulphate-3,6-anhydro-a-D-galactose. It is similar to starch or cellulose (i.e. comprised of thousands of carbohydrates). The negatively charged sulfate groups are thought to interact with the proteins in suspension. As the wort cools, more and more proteins interact with the k-carrageenan and the k-carrageenan adopts a more compact structure. The result is the molecular equivalent of marbles in syrup. After the churning of an active fermentation ends (4-5 days) the carrageenan-protein chunks settle out with the yeast.

Homebrewed beer is still often cloudy, but Irish moss does make a noticable difference.



[1] He he he he, I said flocculation.
[2] Yes, seaweed.

An interesting observation

I bottled my garage-lagered pilsner tonight. I think this is going to be a very fine beer.

As I was transporting bottles, I noticed a very interesting phenomenon. One I had never noticed before.

If you take a bottle of beer and tap it sharply with your finger, it makes a clear ringing sound. A sound you would expect to hear.

However, if you do the same thing to a bottle of beer that has just been bottled, the sound is a dull thud. There is no ring to it. It sounds as if the bottle is filled with sand.

I have some beer that was bottled 3 days ago and it has partially regained the ring.

So, what causes this? Well, I'm not sure.

The just bottled beer is not carbonated yet and it contains a fair amount of suspended solids. That may be the cause, but I will know more tomorrow. I am going to check again tomorrow to see if allowing some of the solid to settle out makes a difference.

UPDATE: I checked the bottles today and the ring is back. Since it is too early to carbonated, I suspect the sound has to do with suspended solids. Apparently, the colloidal suspension the results after bottling has a dampening effect on the vibration of the bottle. After settling out for 24 hours, the dampening effect is gone. Interesting.

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Check back in the next few days for my most labor intensive post ever. It's a journey of sorts.

Analysis of beer

As a scientist and as a brewer, I can't help but try to combine the two. I've done some informal analysis of my beer (aside from tasting and metabolizing it), but since I'm not a professional brewer or a brewing scientist, I can't justify investing a lot of time and money into analyzing my beer.

I would love to get a copy of "Methods of Analysis of the American Society of Brewing Chemists." There are all sorts of interesting things in there. This resource covers thing such as:

1. Barley
2. Malt
3. Adjunct Materials
4. Cereals
5. Sugars and Syrups
6. Brewers' Grains
7. Hops
8. Wort
9. Beer
10. Flavored Alcohol Beverages
11. Microbiology
12. Yeast
13. Microbiological Control
14. Filter Aids
15. Packages and Packaging Materials
16. Bottles
17. Bottle Closures
18. Cans
19. Fills
20. Sensory Analysis
21. Statistical Analysis

Appendixes:

• Tables for Extract Determination in Malt and Cereals
• Tables Related to Determinations on Wort, Beer, and Brewing Sugars and Syrups

Granted, that is waaaaaaaay more than I as a homebrewer would ever need, and it costs about $650. This is designed for the professional brewer who needs to make a living brewing beer. There is no way in the world I can get something like this, but it sure would be cool.

I am going to grow my own hops this year. I did last summer, but I built a shed on top of them and I doubt they will survive. I'd love to be able to measure the amount of alpha acids present in my own hops. I'm sure I can figure out how to do this, but I've always been a fan of not reinventing wheels.

I also want to measure the amount of unfermented sugars left in my beer. I could easily do this using the "phenol/sulfuric acid test," but again, why reinvent the wheel.

So, if you have a copy of "Methods of Analysis of the American Society of Brewing Chemists" and want to give it to me for zero dollars and zero cents, please feel free. I'll pay shipping.

Basement lagering

Lager beers should be fermented and aged at low temperatures. Depending on the yeast, the beer should be fermented at about 45-55°F. This requires a spare refrigerator and temperature controller. That is something I do not currently have.[1] As a poor but necessary substitute I take advantage of the cold Minnesota winters to lager.[2] Placing the carboy in a closet on a cement basement floor gets the fermentation down to 56-60°F. That's not perfect, but it works, sort of. I have made a number of lagers this way and have always been content with the product.

The fermentation is not as "clean"[3], but it is still fine beer. I drink it. It is not as crisp as a real lager, but I drink it. It tends to be hazier, but I drink it.

Another important step in making a lager correctly is the "lagering." Lagering is done after fermentation is complete. Usually the beer is transfered to a secondary and cooled to 34-40°F. This is stored at this temp for up to 2 weeks. During this time, the beer "cleans up." Chemically the goal is to allow the levels of diacetyl, acetaldehyde and sulfur compounds to decrease.

One trick homebrewers can do is allow the fermented beer to warm to room temperature for a couple days before lagering. This is called a diacetyl rest and allows the yeast to convert acetolactic acid into valine instead of converting it into diacetyl (a.k.a. butadione). This helps clean up the beer.



Yesterday I brewed a dark lager. Something along the lines of a Beck's Dark. It is fermenting at 60°F right now in my basement. We are at the start of another cold streak, so that temp should drop further as the current cold arctic blast removes kinetic energy from my house.


[1] this summer will be spent watching the classifieds for a free refrigerator :) I hope to have lagering capabilities by the end of summer.
[2] just like the good old days when lagering was invented.
[3] what a "clean" fermentation is could be debated.

Sulfur

I just recently learned that oak barrels (used in wine and beer making) can be "sanitized" by burning sulfur in them to produce gaseous sulfur dioxide. This doesn't actually sanitize the barrel, but it inhibits the growth of mold and bacteria.

This is especially useful if the barrel is being stored for any length of time. Keeping an atmosphere of SO₂ inside the barrel keeps the peskys away.

Before wine or beer is added to the barrel, it is critical that the SO₂ gas be fully removed or H₂S can form. That would smell and probably taste awful.

I'm planning on experimenting with oak chips in some future batches of beer. Conditioning in oak barrels would be awesome, but not practical (at this point).

The picture is from the Ithaca Beer Brewers Blog.

Tannins and icky beer

Tannins are astringent, bitter tasting compounds. Chemically, tannins refer to a broad class of polyphenols. They are often differentiated from other polyphenols in that, tannins precipitate proteins. Ann Hagerman at Miami of Ohio University has a great pdf online that explains tannins much better than I could.

They are not desired in beer. However, they are found in the hulls of the grains whence comes the goodness that eventually becomes beer. During the mashing process, the starches get enzymatically broken down to maltose. However, if the mashing or sparging water is too hot, detrimental amount of tannins can be leached from the grains. It is well known that for most things, the solubility in water increases as the temperature of the water increases. So, this makes sense.

However, the pH of the mash can also affect the solubility of tannins. If the pH is too high (the "normal" pH of a mash is around 5.2) the solubility of tannins increases. This has to do with the presence of the phenolic functional group.

Shown is catechin, a common (and fairly simple) tannin found in a brew kettle near you [edit: The structure shown is missing one OH group. I'll fix it later].

As any sophomore organic chemistry student will tell you, the OH groups bonded to the aromatic rings (the group that puts the "phenol" in "polyphenol") are willing to lose their protons to a base. The pKa of phenol is about 10. I don't know what it is for catechin, but I'm sure it is in that vicinity.

As a result, if the pH of the mash get too high, more and more of the tannins will lose the proton. The result is charged, ionic compound which has a higher water solubility than the neutral species. As a result, the result of this result results in more tannins dissolved in the water.

So, what affects the pH of the water in the first place? You'll just have to wait and find out.

Beer is bad? NEVER!!!! OK, maybe

A lot of chemistry blogs (many of them are very good) include updates from the literature. These are papers that are important in certain fields or just interest the blog author.

Recently, one of the (eh-um) "journals" I read had an interesting article about a dangerous chemical in beer. OK, the "journal" is Brew Your Own magazine, but it was still a well written and interesting article.

I'm a big fan of BYO magazine and BYO.com. It is a great publication for those of us interested in learning everything we can about our favorite hobby.

The article was part of the "Help Me, Mr. Wizard" feature. The articles in BYO magazine are great, but the "Ask Mr. Wizard" feature is my favorite. Mr. Wizard, Ashton Lewis, knows what he is talking about, but (and this is most important) he also researches what he says.

In the latest issue (which I have read completely and will proceed to read again), Mr. Wizard was asked about the dangers of tyramine in beer and the differences between draft beer and bottled beer.

For the most part, Wizard Lewis doesn't shy away from technical terms, but there is nary a chemical structure to be seen. That's OK. That's where this blog comes in.

Tyramine is formed when the amino acid tyrosine is decarboxylated by some bacterial enzymes, namely lactic acid bacteria.



Tyramine is not a problem for most people since monoamine oxidase enzymes catalyze the metabolism of tyramine. However, people taking monoamine oxidase inhibitors (MAOI) have difficulty dealing with tyramine.

The result is high blood pressure and possibly a stroke.

The Mr. Wizard response goes on to explain that tap beer has a higher chance of containing dangerous amounts of tyramine. Commercial bottled beer has a nearly zero risk thanks to pasteurization. Kegged beer and homebrew beer are not pasteurized and run the risk of harboring the offending bacteria.

Tyramine is also found in other foods.

The bottom line is avoid tap beers and homebrew if you are on MAO inhibitors.

Hop aroma research

I decided to make a quick post while grading General Chemistry and Organic Chemistry exams. It is times like this that I really regret not giving multiple choice exams.

The Gen Chem exam is on acid-bases, Ka and solubility product.
Organic is on electrophilic aromatic substitution and intro to reactions with carbonyl carbons. Good stuff.

Anyways, the real purpose of this post....

I'm reading an article titled Hop Aroma in American Beer. J. Agric. Food Chem. 1980, 28, 774-777 (pdf of first page).

OK, well, I'm not actually reading it, but I will. I couldn't get past the first line in the experimental section:

Eight liters of each beer was vacuum distilled (0.02 torr) at 20°C into a trap cooled in liquid nitrogen.

Eight liters of beer?!?!?!?! nay, of EACH beer.

This is why I can't do research of this type. I would have a very hard time dumping 8 liters of beer into anything other than my mouth (not in one sitting of course).

Maybe bad beer, but the analysis of bad beer would be pointless.

OK... back to the correcting.

Brewing chemistry: Part 2- Kilning

Once the barley has been malted, and the barley has just started to sprout, the next step is to kiln dry the grain. In some cases, the grain is roasted.

The purpose of the kilning (if that is a word) is to dry the grain, but also, in some cases, to roast it.

During the heating, some enzymes become denatured. However, most of the amylase enzymes survive. These will be utilized during mashing to liberate fermentable sugars from starch. Fortunately, during the drying phase, most lipase and lipoxygenase enzymes are destroyed. These enzymes are implicated in the formation of off flavors in beer as it ages.

A second goal of kilning, in some cases, is to roast the grain. Pale malt is typically not roasted, whereas, roasted barley is (umm, that's why they call it 'roasted' barley). During the roasting process a glorious reaction called the Maillard reaction occurs.

The Maillard reaction is a general reaction between an amino acid and a reducing sugar. Considering there are a lot of types of sugars and amino acids available, the Maillard reaction can form a variety of products. The Maillard reaction is a very important reaction in food chemistry. The products contribute to the color and flavor of browned bread, chocolate, seared meat, caramel and deep-fried death. Now, I'm not a food chemist. I'm just a chemist who likes food. My understanding is the Maillard reaction is central to what food chemist study. The following is my understanding of the Maillard reaction.

In the first step, an amino acid reacts with a reducing sugar (glucose is shown) to make an amino glucose. This part of the reaction is known as the Amadori reaction.



The Amadori complex can react with dicarbonyl compounds in a reaction known as the Strecker degradation. This results in a number of aldehydes such as, isobutyraldehyde and furfural and others. Some examples include:



Another important product of this breakdown are the melanoidins. This class of poorly characterized heterocycles contribute a dark color and a toasty aroma.

In reality, the Maillard reaction is more important during the boiling of the wort (a future post), but it does play a role in the roasting of grain.

UPDATE:
Try #2 for the images. I don't know why, but my images stopped appearing. I reloaded them above as .gif files and below as a .bmp file. Can you see it?

Gibberellic acid follow-up

Someone who read my post on gibberellic acid and its relationship to brewing was nice enough to send me a copy of the Corey papers I mentioned (JACS 1978, p.8031 and 8034). Thanks.

I was struck by one of the statements made. In explaining why gibberellic acid was so hard to make they mentioned that "a singularly diabolical placement and density of functionality serves to thwart all but the most sophisticated of approaches." I love the use of the word "diabolical." It fits perfectly.

Without going into the details of the synthesis, rest assured the paper details sophisticated approaches.

Brewing chemistry: Part 1- Gibberellic acid

Gibberellic acid is a plant hormone that induces the formation of a number of key enzymes, and in a sense, it gets things started in the brewing process. So, I thought I would start with gibberellic acid.

Beer is made from the sugars in malted barley (along with a few other key ingredients). In brief, the malting process involves soaking the barley in water to induce germination. During germination cell walls are broken down and starch is released within the grain [there is a bunch of plant anatomy that I could get into, but I'm not really interested in that]. Enzymes are formed that will eventually be used by the brewer to break down the starch into maltose. At a certain point germination is stopped and the grain is kiln dried and/or roasted. This is now malted barley, and it is ready to be mashed. I will deal with mashing in another post.

I want to zero in on what causes the starch-hydrolyzing enzymes to form. As the barley is germinating, any free carbohydrates are consumed during respiration (i.e converted to pyruvate and then CO2). Once the carbohydrates are depleted, the starving barley grain turns to its starch reserves. Since starch doesn't just fall apart into glucose, enzymes are needed. When the free glucose gets low, a signal is sent to start forming enzymes (amylase) to break up starch. The "signal" that triggers the formation of these enzymes is the plant hormone, gibberellic acid (GA3).

The structure of GA₃ is:



This is a very interesting molecule. It was first synthesized by a fellow by the name of Elias James Corey in 1978. I'd love to write a bit about his synthesis, but thanks to very limited (and embarrassingly so) library access at my institution, I can't easily get the papers. Thanks to Peter J. Stang, I can at least see the first page of the 1978 communications [JACS, 1978, v.100, p.8031 and p. 8034]

GA₃ gets the germinating plant to form mRNA that codes for the formation of things like amylase and other starch hydrolyzing enzymes. How any of the gibberellins work is not well understood. What is known is that the cells in the aleurone layer of the barley seed contain a membrane-bound receptor for GA₃. When GA₃ binds, a Myb transcription regulator is produced. This Myb protein induces transcription of the amylase gene. The amylase is sent on a tour of duty to destroy starch, but a well trained maltster will stop the malting before this takes place. The amylase is going to be used during the mashing process by the brewer.

To be continued...

SPME...the endgame... finally!!

OK, I realize I've been talking about this for too long. On the 3 month anniversary of the 3rd day after I started this blog (depending on timezone), I am publishing some SPME GC/MS data. This is from an old post, but I am now publishing the computer generated results. First, here is the GC trace:



Second, here are the results...... WAIT JUST A FREAKIN' MINUTE. Before you blindly read and accept the results, I must tell you, these were picked out of an Agilent library based on comparison to MS fragmentation patterns. If Rosko and Derek have taught us anything, it's that computers cannot be trusted. They are very useful, but not infallible ("I'm sorry Dave, I'm afraid I can't do that"). Regardless, let's blindly accept the data from the computer, shall we?

Here it is:


I've only picked a few of the big peaks. However, there are really no surprises. A lot of esters and higher alcohols. The SPME conditions have NOT been optimized. I have not experimented with any other fibers. I'm sure I could mess around with the conditions and find other interesting compounds.

I'd like to look for hop compounds and polyphenols (tannins). I know I'm not the first one to do this, so I'll keep looking for information related to this.

Until then, realize as you drink your beer: what a wonderful chemical concoction it is.

SPME of my beer

Finally, I am getting around to posting about the SPME (solid-phase microextraction) I did of one of my beers. SPME is pretty sweet. It involves dipping a fiber coated with what is usually on the inside of a GC column. Clever Canadian. The fiber is then placed in the GC/MS inlet port at about 300°C for 3 minutes. Any analytes are desorbed and sent to the MS detector via the 30m GC column (I can't remember what type of column I used).

In this case I used a fiber coated with 75μm of a carboxen/polydimethylsiloxane. The beer I chose was my second attempt at cloning Fat Tire Belgian Ale. A 4 mL sample was acidified to pH=2 and the fiber was placed in the sample while stirring for 15 minutes. The GC/MS trace is shown below:



I'd love to tell you what all of the peaks are, but you are going to have to wait until tomorrow. I have the list, but I'm tired and am going to bed.....

Hop chemistry

Sure, we all love a "40" of "O.E." every now and then. But, that bad boy is missing something that makes beer unique. Hops. Most malt liquor is unhopped or only slightly hopped.

Hops are a critical ingredient in beer. These flowers from the female plant botanically classified as Humulus lupulus provide beer with the resins and essential oils that give beer its bitterness, aroma and to some extent flavor. There are a ton of resins and terpenes found in hops. Each variety of the 50+ types of hops, offers a different chemical profile. The possibilities are endless.

Hops have typically 3 uses in making beer. 1) Bittering 2) Flavor 3) Aroma. How a hop is used, depends on how long it is boiled during the brewing process.

Bittering hops are boiled in the wort for at least 60 minutes. A critical isomerization occurs during this time (see below). Flavor hops are added during the last 15 minutes of a boil, and aroma hops are added during the last 1-5 minutes of the boil. The role is determined by how oxidized the hop chemicals get during the boil.

I want to just consider the bittering aspect right now. Hops contain varying amounts of what are known as alpha acids. Many hops range from 4-15% alpha acids. Humulone (as seen in the figure below) is one example of an alpha acid. There are of course many others, but they are all phenolic compounds that are only slightly soluble in water. The alpha acids get isomerized to iso-alpha acids during the boil in the slightly acidic wort. The iso-alpha acids are more soluble in water and contribute the bitterness of beer. A good 60 minute boil is necessary to extract and isomerize the alpha acids. The % alpha acids is a good measure of how bitter a hop will make the beer.

Below is my propose mechanism for the isomerization of alpha acids to iso-alpha acids.

Beer is beautiful

As pointed out by Retrospectacle, late nights in the lab + a light polarizing microscope + beer = real perrty pictures.

Bottling sucks and oxygen rocks!!!!

I hate the bottling part of homebrewing. But, until I can afford a kegging system, bottles it is. The best bottles to use are the returnable type. Returnable bottles are not as common these days, but they are still easy to find. The really "hard" part is having to buy the beer and empty the bottles. Oh my! What pains I must go through for my craft.

My beer of choice that comes in returnable bottles is Hauenstein. "Hauey" is a semi-local beer contract brewed by the fine folks at Schell's brewery. At $13 a case, the high quality bottles with beer are cheaper than buying empty bottles from a homebrew supply store. The beer isn't too bad. A typical American lager. Not exceptional, but $13 for a case of good bottles makes it worth it.

I bottled my Pilsner Urquell clone last night. As I've mentioned before, I sanitize my bottles in the dishwasher. That's a lot easier than washing them by hand. However, thanks to Chemistry, sanitizing is pretty easy.

There are a lot of sanitizing agents used in homebrewing. The cheapest and easiest to obtain is bleach (sodium hypochlorite, NaClO). However, using bleach requires a lot of rinsing. When sanitizing 50 bottles, excessive rinsing can get annoying. I prefer to use sodium percarbonate (a.k.a. One Step sanitizer). The sodium percarbonate (2Na₂CO₃•3H₂O₂ -sorry, I haven't figured out HTML subscripts yet [UPDATE: Got it. Thanks Ψ*Ψ.) releases hydrogen peroxide when dissolved in water. The H₂O₂ does its thing on the wee beasties, killing them dead. The byproducts are sodium carbonate (Na₂CO₃) water and oxygen. These don't have to be rinsed away. And that means more time for tasting.....

Butanedione

Butanedione, a.k.a. diacetyl, is a chemical occasionally found in beer. For most styles, it is undesired. For some styles, low levels are acceptable, but typically, high levels of butanedione (diacetyl) indicate something went wrong during the fermentation.



The first two reactions occur inside the yeast cell. They are catalyzed by yeast enzymes. The oxidation of the acetolactic acid to butanedione occurs after the acetolactic acid leaves the cell. Another fate of acetolactic acid is the formation of valine. Malt that contains high levels of valine favors the formation of more butandione.



The butanedione can be reabsorbed by the cell and enzymatically reduced to 2,3-butanediol. This usually occurs after 7-10 days, and diacetyl levels can noticeably decrease.

High levels of butanedione in the finished beer indicate either poor sanitation (which favor bacterial production of butanedione), too much oxygen, poor malt, or poor yeast.

Someday, I am going to find a procedure for the analysis of butandione. I will test all of my beer at various stages. Now all I need is the time....

Boiling water is hard

Hydrogen bonds are strong. It takes a lot of energy to break them. Especially when trying to boil 11 L of wort (Wort, BTW, is beer before it gets fermented). Thanks to the high heat capacity of water, getting to the boiling point takes a lot of hydrocarbon fuel.

Tonight I brewed a Brown Ale, and while waiting nearly 30 minutes for the brew kettle to get it's boil on, I had some time to consider the thermochemistry (I linked that page because it has some good math in it and it hasn't been updated in 8 years!!! That is my pet peeve... the lack of updates, not the math.).

When my wife and I bought the stove, we went with for the JGB900SEF GE stove complete with "Power Boil!!!" This baby kicks out 15,800 kJ per hour (15,000 BTU/hr). I was under the impression that this thing should be able to boil 11 L (3 gal) of water in about 3 minutes. Not so (and how silly of me).

Assuming the water starts at 19°C and boils at 104°C (> 100°C due to the boiling point elevation colligative property) the amount of heat needed to raise the temperature by 85° C is: 11,000g x 4.184 J/g °C x 85°C x .001 kJ/J = 3910 kJ.

If the stove kicks out 15,800 kJ/hr, assuming 100% efficiency, it should take about 15 minutes. Obviously, some heat is lost to the surroundings. What I didn't do tonight was measure the amount of time needed to boil the solution. If I do measure that, I will be able to determine how efficient my burner really is.

I don't know how I can survive without knowing this piece of information. Next time I will keep track of the time.

Once the reaction reached a boil. It was kept at a boil for 1 hour. During this one hour 15,800 kJ of heat was produced from the combustion of methane. Methane has a heat of combustion of 900 kJ/mol. During just the boil, I used 15,800 kJ / 900 kJ/mol = 17.5 moles of CH4. That means at 1 atm and 20°C (293 K), I used 420 L of methane gas. That's a lot.

Why taking specific gravity readings are important

Today I was a negligent scientist and brewer.

A measurement used ubiquitously in brewing is specific gravity (SG). SG is used to gauge the extent of fermentation and to estimate the alcohol content.

Specific gravity is defined as the density of a substance divided by the density of water. The density of water is 1 g/mL (technically only at 4° C), so for practical purposes, SG is the density of the substance divided by 1 g/mL. Not your most interesting mathematical operation. What it essentially does is give a value without units. This value would be (and is) more accurately described as "relative density."

SG is measured using a hydrometer, a sealed glass tube that floats in the liquid. The deeper it sinks, the less dense the liquid is.

Wort (the beer before it is fermented... so, it really isn't beer) is comprised of water and sugar (as well as many other things). This has a typical SG of 1.050. As the sugars get converted to ethanol, the SG drops. This is because the ethanol is less dense than the aqueous sugar solution. Pure ethanol (100% v/v) has a density of 0.789 g/mL. The fermentation does something else of note. Before fermentation, water is the only solvent and the sugars (and other things) are dissolved solids. When the dissolved sugars gets converted to ethanol, the ethanol (a liquid completely miscible with water) becomes a co-solvent.

The bottom line of all of this is that when the fermentation is finished and all of the fermentable sugars are gone, the SG stops dropping. When a homebrewer measures the SG 3 days in a row with no change in the reading, the fermentation is finished.

So, why was I a negligent scientist and brewer? Well, I didn't take SG readings of my Fat Tire Clone #2 (FTC2). I assumed it was time to bottle. So, I went through the process of cleaning bottles and sanitizing them in the dishwasher. I went downstairs to prepare the bottling bucket and get 1 cup of malt for the bottle priming. It was then that I noticed a positive pressure in the airlock of the secondary containing the FTC2. I watched for a few seconds and, sure enough, a bubble of fresh CO2 bubbled out: a sure sign the fermentation was NOT done. The SG is currently 1.014. I'll measure again in a few days.

So, I have a dishwasher full of sanitized bottles. I'm not sure I can get away with leaving them in there until next week.