Homebrew Science Experiments

A lone figure stands above the steaming kettle. A sample of the malty liquid is carefully drawn off, cooled, and then poured into a graduated cylinder. He drops a glass hydrometer slowly into the murky mixture. He nods his head and smirks as he records a precise measurement. “At last,” he says, curling his fingers into a fist, “I’ve done it.”

If this sounds like a description of you, you must be either a mad scientist or a homebrewer. Then again, perhaps you are both. Every batch of homebrew you make is an experiment, whether you are trying a new recipe or attempting to reproduce a beer. You might be trying out myriad homemade gadgets every time you brew, as well.

Anticipating the outcome of these experiments makes brewing fun. One of the challenges of trying new ingredients and gadgets all the time is determining what exactly made your beer better or worse than the last. Was it the different hops, the extra crystal malt, the new yeast strain, or the shorter cooling time? With so many aspects of the beer constantly changing, it is difficult to tell. If you are interested in seeing the effect of a change in your brewing method or recipe, you should tackle the problem scientifically. Consider turning your beer into an experiment. However, keep in mind some guidelines:

• Only change one parameter at a time.

• Be consistent in your methods for sanitation, mash temperature, and cooling time. All of these things can change the flavor of the beer.

• Take scrupulous notes and label everything. By labeling and taking notes, you will be able to keep track of different treatments.

• Try to draw a conclusion and write it down. You can easily refer
back to it and use your results to make better beers.

Each of the following experiments requires only one batch of beer. By splitting up a single five- or 10-gallon batch for different trials, you can avoid the variation that occurs between batches. Of course you will only be able to do this when experimenting with the beers after the boil. To see differences in hopping rates, hop additions, boil time, or mash temperature, you will have to brew more than one batch. The number of possible brewing trials seems limitless. The following experiments just scratch the surface of possibilities.

Pitch Rate and Forced Wort

Next to sanitizing, pitching a proper amount of viable yeast may be the most important component of making great homebrew. A low yeast count at the start of fermentation will give bacteria and wild yeast an opportunity to grow and spoil the wort. A low pitch rate also results in more yeast growth and therefore more yeast by-products or flavor compounds. Pitch rate also determines how long you must wait for a finished beer because more yeast results in a faster fermentation.

This experiment requires that you set aside a portion of unfermented wort. This is called a forced wort test. It is an easy way to check your sanitation methods and demonstrates the value of adding culture yeast.


• One homebrewed wort
• Two carboys
• One 22-ounce bottle
• Airlock
• Two seven-gram packets of dried ale yeast or two liquid yeast cultures of the same type


1. Prepare a beer and split it between two carboys. Sanitize a 22-ounce bottle and fill halfway with wort. Stopper the bottle with an airlock. This small portion of the wort will remain unpitched and will demonstrate the need for culture yeast in a successful fermentation. Microbial growth in the unpitched wort indicates contamination.

2. The two carboys should each contain about 2.5 gallons of wort. Open a packet of dried ale yeast. Using a sanitized measuring spoon, measure out one-half teaspoon of yeast and add it to the wort in one carboy. To the wort in the other carboy, add the full contents of a seven-gram yeast packet.

Although dried yeast is easier to measure, you can attempt this experiment with liquid yeast. Mix the liquid yeast thoroughly. Add a teaspoon of it to one wort and a full packet to the other.

3. Ferment the two worts in the same location along with the bottled wort.

4. When they finish fermenting, rack them off the yeast as you normally would. Note which beer finished fermenting first. Take a preliminary sample of the two beers to determine any taste difference. Remember to taste (or at least smell) the unfermented wort from the bottle along with the other samples. Note any differences. Discard the remaining unpitched wort.

5. Bottle or keg the beer as you normally would. Taste again and make notes. Predicting the exact flavor difference between your beers is difficult because results will vary. In general expect more off-flavors from both yeast and contaminants in the under-pitched beer.

Wort Aeration

In general, brewers try to keep their beer as far away from oxygen as possible. The one exception to this rule is wort aeration. Yeast need oxygen to conduct a swift and complete fermentation. At the start of fermentation, yeast use oxygen to make cell-wall components. If they fail to make enough, their ethanol tolerance may be lowered and they may fail to complete fermentation. Furthermore, the yeast may produce more esters (flavor compounds that contribute to the fruity, solvent-like, and floral aroma and flavor of beer) than normal and cause unwanted off-flavors.

For this experiment split a batch prior to fermentation, aerating one half and not the other. To add oxygen to the aerated half, you can splash the wort around or go a little more high-tech: a fish tank aerator.

One problem with the fish tank aerator is that there is a chance of contamination from the air. You can decrease the chances of contamination by adding an easy-to-make in-line cotton air filter. Take a three-inch length of brass pipe with 1/8-inch pipe threads on both ends. Stuff the inside with cotton. Screw a 1/8-inch brass hose barb to each end of the pipe. Sterilized it in a pressure cooker for 15 minutes. The plastic fish aerator tubing between the aerating pump and the stone slips easily onto the barbs. All the air from the aeration pump will pass through the cotton on its way to the wort aeration stone, which should be submerged in the beer. The air may not be completely sterile, but the cotton will remove many of the larger particles that float in the air, thereby reducing the chance of contamination.


• One homebrewed wort
• Two glass carboys
• A fish tank aerator setup with homemade in-line air filter
• Two seven-gram packets or two liquid cultures of the same yeast


1. Prepare a five-gallon batch of beer. It is a good idea to make a higher-gravity wort (1.060 to 1.070) so that the yeast will be stressed and reveal how oxygen affects their performance.

2. Split the beer evenly between two carboys. You want to minimize aeration in one of the worts. When transferring the less-aerated wort, use a plastic tube that touches the bottom of the carboy to minimize splashing and oxygen pick up. Add the same amount of yeast to both beers. Use a neutral strain, such as Wyeast 1056, to keep matters uncomplicated. Split a seven-gram packet of dried yeast between the two.

3. Using the previously described setup, aerate one of the worts for an hour.

4. Observe the fermentation. Which beer starts fermenting first and which finishes first?

5. When the beers are finished, keep them separate. Note any flavor differences between the aerated and unaerated beers. The unaerated beer should have more flavors from the yeast, such as esters, fusel alcohols (higher alcohols that contribute spicy, fruity, and wine-like flavors), and ethyl acetate (nail-polish remover).

Yeast strains

Choice of yeast strain is one the most important factors that determines beer flavor. Most beers are made from the same ingredients. Many breweries even buy their ingredients from the same suppliers. However, because of the unique yeast strains, their beers will never be likened to one another.

Different yeast strains produce different flavor compounds. Some yeast produce buttery diacetyl, others make fruity esters. Still others are known for clove-like phenolics. Or perhaps the yeast make few of these compounds and allow the malt and hops to predominate.

With so many new yeast strains available to the homebrewer, choosing a strain can be a challenge. Some brewers perform this simple yeast trial on a regular basis with whatever beer they make. For example a brewer who makes 10 gallons might add a different yeast to each five-gallon fermenter. Sometimes the two versions taste like completely different beers. You can easily perform this experiment by splitting a five-gallon batch.


• One homebrewed wort
• Two carboys
• Equal amounts of two yeast strains


1. Split the wort equally between the two carboys. Add the different yeast strains to each carboy.

2. Unless you are comparing an ale and a lager yeast, try to ferment the beers in the same place (i.e. same temperature). Observe which yeast starts first and which finishes first. Does the yeast form a large head during fermentation? Does it settle out quickly or remain in solution?

3. After fermentation rack the beer, age, and bottle. Note the flavor differences in each beer. Also observe which beer is drier or sweeter. Record your results and use the knowledge you have gained to choose the yeast strain for your next batch.

Dry Hopping  

Dry hopping is the traditional British practice of adding fresh hops to the secondary or conditioning tank. Certain flavor compounds are extracted by this method that cannot be achieved by late hopping in the kettle. These compounds are typically very aromatic and volatile. When the hops are added to the kettle, these compounds either dissipate into the air with steam or later blow off with carbon dioxide during fermentation.

Dry hops add complexity to beer. Different hop varieties will add different characters to the smell of the beer. For this experiment choose four hop varieties that interest you. You can use whole or pellet hops. Try Hallertauer, Tettnanger, Saaz, and Cascade for an interesting comparison of aromatics. You might also try comparing domestic and imported versions of the same hop variety, pellet and whole versions of the same hop, or different amounts of the same hop. If you use a high-alpha variety, you might find that it has harsher flavors than the traditional low-alpha aroma hops.


• Five gallons of freshly fermented homebrewed beer made with a neutral ale yeast
• Five one-gallon glass apple-juice jugs with airlocks
• Four different hop varieties


1. Make five gallons of a pale beer with a mild malt character, such as pale ale, so that the flavors from the hops are evident. Add the bittering hops at the start of the boil to avoid extracting too much late-hop character.

2. As the fermentation comes to a close, rack the beer into five one-gallon jugs. Fill one of the jugs to the top and set it aside as the control test. Make sure to allow space in each of the other jugs for the addition of hops.

3. Add one-fourth ounce of a different hop variety to each of the four glass jugs.

4. Age for a week.

5. Rack each beer into a sanitized empty jug. Dissolve one-half tablespoons of priming sugar in water for each beer. Then bottle and label according to hop variety.

6. After the beers have carbonated, gather some friends and have a blind tasting. Tape a piece of paper over the bottle labels and number them one to five. Try to come up with a distinct aromatic profile for each beer. Rate them for preference on a scale of one to five. When you brew again, use your favorite hop.


Many homebrewers tend to go the extra mile to make a perfect beer when they buy ingredients. They buy special imported malts, hops sealed under nitrogen in protective bags, and unusual yeast strains isolated from foreign brews. As many professional brewers have learned the hard way, all this extra effort is of little value if the beer is not processed correctly after fermentation. This includes pinpointing the carbonation. Surely, few brewing misfortunes are worse than serving  finely brewed beer at a party only to find its high carbonation brings a chorus of belches rather than accolades. Or even worse, the foam shoots to the ceiling and your beers are mistaken for party favors.

Carbonation can be a particular challenge if your beer volume changes from batch to batch while the amount of priming sugar added stays the same. The solution is simple. The commonly recommended three-fourths cup of corn sugar is for five gallons. This is about 2.5 tablespoons of sugar per gallon. To figure out how much beer you have in your carboy, you must mark the volume on the side of the carboy in one-gallon increments. This simply requires that you take an empty carboy and add one gallon of water (one full grocery store plastic water bottle). Then with a permanent pen mark the water level on the glass. Add a second gallon and mark the new volume until you have measured and marked five gallons one by one. The next time you want to bottle, refer to the marks on the carboy and calculate the amount of priming sugar by multiplying the volume in gallons by 2.5 tablespoons. If the volume of the beer is about 4.5 gallons, you will add about 11.25 tablespoons of sugar.

There is no holy commandment of brewing that requires the brewer to add priming sugar at a ratio of three-fourths cup per five gallons. This experiment explores some different carbonation levels and leaves the ultimate decision up to you.


• One homebrewed beer of your choice (ready to bottle)
• Priming sugar
• One-gallon jug
• A bottling setup


1. Prepare four different priming doses using 1 tablespoon, 1.5 tablespoons, 2.5 tablespoons, and 3 tablespoons of priming sugar. Dissolve each amount into 1 cup of water separately and boil over low heat for 15 minutes.

2. Pour the solution into a sanitized one-gallon jug. Cover and allow to cool.

3. Rack a portion of the beer from the carboy into the jug until it is filled. Swirl the jug to make sure the sugar solution is mixed.

4. Bottle the beer from the glass jug as you normally would. Make sure to label the amount of priming sugar used on each bottle. Repeat steps one to four for all four amounts of priming sugar.

5. Bottle the remaining beer as you please, because it is not part of the experiment. You may wish to consume it immediately to lessen the toils of bottling.

6. After two weeks to one month, check a few of the bottles to see if they are carbonated. If not, wait another two weeks and try again. When they are ready, line up a row of glasses and pour each of the four beers. They should all have different carbonation levels. Choose the level you prefer. When you bottle your next beer, multiply the amount of sugar you liked by the number of gallons in your carboy as previously described. Bottle and enjoy a beer carbonated to your taste.

Bottled Beers

If beer could talk (not to mention feel and think), it would probably request to be served on draft rather than bottled. After all, bottling exposes beer to flavor-damaging oxygen and numerous sources of contamination. Somehow the convenience of bottled homebrew outweighs any contrived preferences our beer might have. Still, there are ways to keep beer fresh even in the bottle. In the following experiments bottled beers are exposed to different conditions and the consequent effects on flavor are evaluated.


The first experiment requires that you bottle beer with a large headspace, so that oxygen will react with the beer. Although bottling beer with live yeast helps reduce oxygen, there is a limit to how much oxygen the yeast in the bottle can gobble up. The rest will react with the beer, causing stale flavors reminiscent of cardboard.


• Homebrewed beer ready to bottle
• Bottling setup


1. While bottling, fill three bottles three-fourths full and cap. Fill three more bottles almost to the top and cap.

2. Keep the bottles at room temperature. Taste a set of samples, a low fill and a normal fill, at two weeks, one month, and two months. Note any flavor differences between the two beers. The beer that has more air should degrade faster and have flavors typical of oxidized beer.


In the second experiment beer is bottled in green or clear bottles and exposed to sunlight. The light causes a chemical degradation of certain hop compounds in the beer. The result is a beer that smells like skunk. One sniff should be enough to convince you that brown bottles are superior.


• Homebrewed beer ready to be bottled
• Green, clear, and brown bottles


1. Bottle the beer in at least one clear bottle, one green bottle, and two brown bottles.

2. Place clear, green, and brown bottles filled with beer in the sun for a few hours. Leave another bottle in a dark place indoors.

3. Arrange four glasses in a row. Pour all of the beers and taste them. The beer from the clear and green bottles should have a distinct skunkiness that is not present in the beer from the brown bottles. It is a wonder so many beers are still packaged in these bottles.

Bottled-Beer Force Test

Commercial breweries usually keep a library of bottled beers on hand that are labeled by bottling date. One set might be kept at room temperature or in a warm incubator and the other in a refrigerator so it will not spoil. You can do the same thing with homebrew. By comparing room temperature and refrigerated samples over time, you can track the flavor progression of your beer as it ages.


• Four pairs of bottled beers


1. As soon as the bottled beers are carbonated, reserve eight bottles and label them with the date. Place four of the bottles in the fridge. Store the others at room temperature.

2. Pull out pairs of the bottles and taste the beers at two weeks, one month, two months, and three months. The flavor change over the course of time will vary. Some beers may oxidize, creating cardboard and toffee or sherry notes in the beer. Others may become buttery, sour, or phenolic due to wild yeast or bacterial growth. The autolysis of yeast in the beer will also add new flavors such as soapy, sulfury, and rubbery.

Issue: June 1997