Home Lab Tests

Every homebrewer knows that unwanted microorganisms are an enemy to our beer. Most of us clean and sanitize our equipment scrupulously. And, for most, the result is that our beer does not suffer from obvious contamination, as evidenced by sour flavors, gushing bottles or “ring around the collar” (a line of growth around the neck of a beer bottle). However, all beer — commercial or homebrewed — is contaminated to some extent, even if it tastes, smells and looks fine. Sometimes it is worth taking extra steps to examine just how clean your beer really is. In this article, I’ll describe a few easy tests that can give you an idea of the level of contamination in your homebrewed wort and beer. They may also be able to tell you where the contaminating microorganisms came from.

These tests can be especially helpful to homebrewers who plan on repitching their yeast to another batch of beer. A level of contamination that was undetectable by taste in an initial batch of beer may blossom in the next, leading to off flavors and aromas. Knowing the condition of your wort can help you decide whether to repitch or to start with a fresh culture of yeast.

Likewise, these tests can also help you evaluate if you are cleaning all of your equipment adequately. These days, many of us use counter-flow wort chillers. These chillers cool wort quicker and with less water than immersion chillers. However, many homebrewers worry because they cannot see inside the chiller. (With most other pieces of homebrewing equipment, you can visually inspect every surface the wort will touch.) The deep recesses of the chiller are a perfect place for bacteria to take hold and potentially be passed on to all our subsequent beers.

Finally, given how easy the tests are, you may simply want to run these tests to check on your cleaning and sanitation practices, even if you can’t detect any problems in your beer.

Wort Stability Test

The first test is extremely simple and requires only a small sanitized container, a sample of your wort and a relatively warm place to store the sample. On brewing day, take a sample of your wort after cooling but before pitching. You can do this by simply intercepting a small amount of wort as it exits your counter-flow chiller or kettle on its way to your fermenter. Alternately, you can use a sanitized wine thief or other sampling device to grab some unpitched wort from your fermenter. Any kind of small glass jar will work as a sampling vessel. Used spice jars, baby food jars or other household containers will work, as will Erlenmeyer flasks, laboratory media bottles, sterile test tubes, sterile culture tubes or sterile specimen cups. Make sure the container is spotlessly cleaned and sanitized.

Once you’ve taken your wort sample, lightly cap the container or cover it with sanitized aluminum foil. Leave the cap loose enough so that any gas produced in the test can escape. Place the sample in a warm place for a few days.

Optimally, you would like the temperature to be 80–90 °F (27–32 °C). Setting the jar on top of a refrigerator or near any appliance that gives off some heat may be sufficient. In the summer, a garage or shed may stay in that range. If you want to get a little more fancy, it’s not too hard to attach a thermostat to a hot pot (or even a Crock Pot slow cooker) and construct a make-shift, temperature-controlled hot water bath. Do not set the sample in direct sunlight or near any bright light source, however.

Once the sample is in its incubator, examine it periodically. Look for signs of haze, bubble formation or full on fermentation. The length of time your wort remains clear is an indication of how clean it is. If your unpitched wort sample lasts longer than 72 hours (3 days) at 80–90 °F (27–32 °C) without showing any signs of contamination, you are producing very clean wort. Yeast harvested from this beer can be repitched with confidence. This would also be good evidence that, if you are using a counter-flow chiller, the cleaning and sanitation practices you use on it are sufficient.

(This assumes the wort has not been contaminated subsequent to you taking the sample. For example, if you took the sample prior to it reaching your fermenter and the fermenter wasn’t properly sanitized, the result of the test is irrelevant.)

If the sample lasts 48 hours (2 days) without showing signs of contamination, your beer will probably not taste or smell contaminated, but repitching your yeast from this batch is not advisable. You will likely want to re-examine your cleaning and sanitizing routine to look for ways that contaminating microorganisms may be entering your wort.

If the sample only lasts 24 hours (or less), it’s time to start taking your cleaning and sanitation much more seriously. Once the test is over, you may want to taste the sample to determine what sort of contamination is present.

Pitched Wort Stability Test

One possible source of beer contamination is your yeast, especially if you are repitching yeast from a previous batch or pitching yeast from starter that has been stepped up a few times. (Every time you transfer a yeast sample, there is a chance of contamination.) A Pitched Wort Stability Test can tell you if your yeast sample is clean. A Pitched Wort Stability Test is conducted in much the same way as the previously explained Wort Stability Test. In fact, both tests are usually run in parallel (i.e. at the same time and under the same conditions). The only difference between the two tests is that a yeast inhibitor is added to the pitched wort sample.

To run this test, simply take a sample of pitched wort from your fermenter after you take the sample of unpitched wort for the first test and after you’ve pitched your yeast. (A sanitized wine thief will work for taking a sample, so will a sterile pipette.) As with the unpitched wort sample, place it in a sanitized jar with a lightly sealed lid or foil wrap. Label the unpitched and pitched samples so you can tell them apart.

The yeast inhibitor — cyclohexamide, sometimes called actidione — you will add prevents the yeast from multiplying in the wort, but does not interfere with bacterial growth. Thus, if there is any bacterial contamination of your yeast sample, the wort sample will eventually show signs of this.

Cyclohexamide is a poison. (It inhibits protein synthesis in many types of eukaryotic cells, including brewers yeast and human cells.) As such, you need to clearly mark the cyclohexamide-containing sample with the word “poison,” biohazard tape, a “Mr. Yuk” sticker or something similar. Add 1 mL of a 0.1% cyclohexamide solution to every 100 mL of your pitched wort sample. Often, the working cyclohexamide solution is stored in a syringe and pushed through a 0.22 micron filter.

Incubate the pitched wort sample, treated with cyclohexamide, alongside the unpitched wort sample, which was not treated with anything. There are essentially four different outcomes to this paired test.

One possible outcome is that both the unpitched and pitched, treated samples last 72 hours (or longer) at 80–90 °F (27–32 °C) before showing signs of fermentation. This would indicate that both your wort and your yeast culture are very clean.

Another possible outcome is that your pitched, treated sample shows signs of contamination well before the straight unpitched sample. This indicates that your wort is fine, but your yeast is contaminated.

A third possible outcome is that both samples show signs of fermentation within 24 hours. This indicates that your wort is contaminated. Your yeast may (or may not) also be contaminated, but you can’t tell from the test results.

The final possible outcome is that the unpitched sample shows signs of contamination well before the pitched, treated sample. This indicates something has gone wrong with the test. If the unpitched sample is contaminated, the pitched sample should also be because it’s the same wort. In this case, your sample jar for the un-pitched sample was probably not sanitized adequately.

This last outcome points out one key aspect required for a valid test — clean, sanitized (or better yet, sterilized) labware. Wort is a very nutrient-rich media — and when it’s held at 80–90 °F (27–32 °C), many kinds of bacteria or yeast present will grow quickly. This includes any contaminants in your sample jar or introduced from any device that you used to sample the wort (such as a wine thief).

One way to give your test a better chance of validity is to use sterile test equipment. Most scientific supply stores sell individually packaged sterile test tubes, culture tubes or vials in a variety of sizes. Likewise, it’s not too hard to track down sterile sample cups (the kind that urine samples are typically collected in) at many medical supply stores. These can only be used once, but are fairly cheap. (If you use vials or tubes, you will want to get a simple rack to help keep them upright during the test. These are cheap.) Scientific supply stores also sell individually wrapped sterile pipettes in a variety of sizes (including 5, 10 and 25 mL). These are also fairly cheap and can be used to sample wort from your fermenter.

If you don’t like using disposable equipment, you could get some small Erlenmeyer flasks and glass pipettes. These can be cleaned the same as any of your brewing equipment and easily sanitized in your oven. Put aluminum foil caps on the Erlenmeyer flasks, wrap the pipettes in aluminum foil and heat the glassware in your oven at 350 °F (177 °C) for about an hour. Obviously, you will want to let the glass cool before you use it. Using sterile labware ensures that you aren’t contaminating your wort sample with your test equipment, yielding false positives.

Beer Stability Test

You can also use the same procedure to test the stability of your beer. By the time your beer is in bottles or kegs, it has has several opportunities to become contaminated. Beer can pick up “bugs” when it is transferred to secondary, to a bottling bucket or a keg. Likewise, your bottles or kegs may be contaminated. Knowing the level of contamination in your beer can give you some idea of how long it will remain stable.

To run a beer stability test, just take a sample of beer and place it in a sealed tube. Incubate the sample at 80–90 °F (27–32 °C) and watch for signs of contamination. Signs include turbidity (haze or “rope” forming in the beer), bubbles, visible growth, off aromas and, of course, off flavors.

Because this test is done in a sealed container (to keep oxygen away from the beer as it incubates), there is always a chance the container will burst. For this reason, it’s a good idea to watch this test carefully and remove any sample that is obviously producing gas. As before, it is very important to use well-sanitized (or better yet sterile packaged) containers for this test.

To evaluate the results of the test, keep in mind a simple rule of thumb (attributed to a Dr. Farnsworth) — beer lasts ten times as long at 38 °F (3.3 °C) than at 80 °F (26 °C). So, if it takes 2 weeks for your incubated sample of beer to go bad, it should last about 5 months if refrigerated.

These tests are extremely easy to run once you have the materials on hand. If you use sterile 25 mL disposable pipettes for sampling, sterile 15 mL disposable culture tubes for holding the sample and your Crock Pot water bath is up to temperature, setting up the wort tests takes less than a minute.

Issue: December 2004