Microbiology plating in a small brewery lab
Plating is how a brewery sees bacteria and wild yeast before they reach a customer. You do not need a full laboratory. You need the right media, a sampling plan and the discipline to read the plates every time.
What plating tells you that tasting cannot
By the time you can taste a lactic or phenolic infection, the batch is lost and the organism is probably living in a valve, a gasket or the filler. Plating catches it at low numbers, often in places that have not yet reached a finished beer.
The aim is not sterile beer. It is a record that shows your process is clean and an early warning when it stops being clean. Our guide to a QC lab on a budget covers the wider kit list. This one is about the plates.
Choosing your media
Different media answer different questions. A small brewery can cover most needs with three or four.
- WLN (Wallerstein Laboratory Nutrient). Grows brewing yeast and most other organisms. Useful for checking a yeast slurry and for general counts. Colonies of different yeasts often look different on it.
- WLD. The same base with cycloheximide added, which suppresses brewing yeast so bacteria can show. Cycloheximide is toxic, so buy ready-made media or handle the powder with gloves and a mask.
- HLP or UBA. Media designed for beer spoilage bacteria such as Lactobacillus and Pediococcus, the organisms behind most sour and buttery infections.
- Lysine medium. Brewing Saccharomyces cannot use lysine as its nitrogen source but many wild yeasts can, so growth points at wild yeast in your culture.
Plates or tubes poured in-house are cheaper but take sterile technique. Pre-poured media cost more and remove one source of error.
Where and when to sample
Build a sampling plan along the path the beer takes.
- Cooled wort at the heat exchanger outlet, before pitching.
- The yeast slurry you are about to pitch.
- Each fermenter near the end of fermentation.
- The bright tank before packaging.
- The filler or keg head by swab. Finished packages from the start and end of a run.
Swab after cleaning, not before, because you are testing whether the CIP worked. For bright beer, filter 100 mL or more through a 0.45 micron membrane and lay the membrane on the plate. Low numbers of spoilers are invisible in a 1 mL sample.
Frequency matters more than volume. A brewery making a few batches a week can plate wort, slurry and bright beer for every batch and swab the filler weekly without the lab becoming a full-time job. Start there, keep it up for three months, then decide where you can sample less often and where the results tell you to sample more.
Sterile technique without a clean room
Work in a still corner with no fan, or build a simple still-air box from a clear plastic storage tub. Wipe surfaces with 70% alcohol. Flame the sample valve with a spray of ethanol before sampling, then run off beer to flush it.
A domestic pressure cooker works as an autoclave for media and for killing used plates before disposal. Fifteen minutes at full pressure, around 121 °C, is the usual rule. Never throw live plates in the bin. You are deliberately growing the organisms you most want out of your building.
Incubation and reading plates
Most beer spoilage bacteria grow best with little or no oxygen. Use an anaerobic jar with a gas pack for HLP, UBA and WLD. Incubate at around 25 to 30 °C. A cheap incubator is fine. In an Indian summer you will want one with cooling, not only heating.
Read plates at set times. Yeast colonies on WLN usually show in two to three days. Lactic bacteria can take five to seven days, so do not call a plate clean on day two. Count colonies, note their shape and colour, then write the result down against the sample point and date.
Turning results into action
A single colony on a bright beer plate is a prompt to look, not a reason to panic. Two weeks of colonies from the same filler head is a cleaning problem. Growth on a yeast slurry plate means that crop stops being repitched.
Set simple action limits in writing before you need them: what count triggers a re-clean, a re-test or a hold on the batch. Then keep the records. The value of plating shows in the trend over months. A lab that plates but never changes anything is only buying agar.
Common questions
What media should a small brewery use for plating?
WLN for yeast and general counts, WLD or a dedicated medium such as HLP or UBA for beer spoilage bacteria and lysine medium to check for wild yeast. Pre-poured plates are a good start.
How long should I incubate brewery plates?
Yeast colonies usually appear in two to three days. Lactic acid bacteria can take five to seven days, so wait the full time before calling a plate clean.
Do I need an autoclave?
A domestic pressure cooker at full pressure for about 15 minutes does the same job for small volumes of media and for killing used plates before disposal.
Why sample after cleaning rather than before?
Because the question is whether your cleaning worked. Swabs taken after CIP show what is left on the surface that the next beer will touch.
