I’m an extract brewer (that also steeps) and I recently noticed a bunch of white spots on top of my brew (which was a wheat recipe) after I transferred it to the secondary (see picture below). I thought it was mold forming but read that it’s probably “yeast rafts” or flocculated yeast clumps. The beer turned out nicely but I’m curious at what causes this to happen? Are certain beer styles more susceptible to this phenomenon?
I am not completely sure about the nature of these little floaters. I think, however, that it is possibly either yeast, as you suggest, or protein clumps. Some yeast strains have a tendency to form a dense top crop. When these beers are racked some of the top cropping yeast is carried over and the appearance is at times strange. Weizen
yeast is an example of an aggressive top-cropper. In the case of Brettanomyces, a pellicle is formed on the surface of the beer and also has an odd appearance.
This could very well be the early signs of a Brettanomyces pellicle forming on the surface of your beer. If this is the case it is an indication of a problem, unless you intentionally added this yeast type. Brettanomyces is one of those strains considered to be “wild” and can be a problem to control if you bring it into your brewery and do not have rigorous sanitation procedures.
The other thing that may be responsible for the unusual appearance in the fermenter is trub carry-over from the brewhouse. Wheat contains a significant amount of gluten proteins and when wheat malt is mashed the glutens tend to be retained in the mash and discarded with the spent grain. If you steeped some wheat malt it is possible that some of these gluten proteins made it into the fermenter and resulted in the odd appearance.
The good news about things like trub and yeast that collect on the top of fermenting beer is that you can rack the beer away from the solids and there is no detrimental effect on beer quality. If this beer begins to start smelling leathery, earthy and like a barnyard after a few months of storage the source was likely Brettanomyces. If that proves to be
the case you should be careful handling the bottles since Brettanomyces is a super-attenuating strain that will slowly ferment dextrins in your beer that Saccharomyces species cannot. This can result in bottle grenades.
For those of us without temperature controls through fermentation, has anyone done any experiments with changes in wort temperatures? It would be great to hold 65 to 68 °F (18 to 20 °C), but what about a 70 °F (21 °C) start, dipping into the 60s (~18 °C) then allowing the temperature to increase back into the 70s (~20s °C)? Or, starting in the 60s (18 to 20 °C) and creeping up into the mid 70s (23 to 24 °C)? Is there any way to predict the yeast flavors when the temperature is moving?
The batch size for most homebrewers has been around 5 gallons (19 L) for a very long time, and keeping this volume of fermenting beer cool is not difficult. Recently, however, many homebrewers have become interested in controlling fermentation and aging temperature and all types of fancy rigs are being built to make little fermentation cellars for use at home. To boot, batch sizes for many homebrewers has increased and the heat of fermentation is more difficult to remove in these larger vessels.
The short answer to your question is yes; there is a long history of fermenter temperatures moving up and down during fermentation. At one time it was common for tanks to be equipped with attemperation coils, or pipes inserted into fermenters for the purpose of temperature control. A fairly common method used for attemperature coil operation is to monitor the fermenter temperature with a thermometer and to simply turn the cooling valve on for enough time to cool the fermenter and then after some time the valve is turned off. This method of tank control is called “on/off” control and is how most thermostatic controls are operated. Since manually controlling fermenters with attemperation coils requires brewers to manually monitor and control fermenters the result is a temperature profile that moves up and down around the desired temperature.
Predicting how temperature affects flavor is not an exact science, but in general terms the effects of fermentation temperature on beer flavor are well known. Beer flavor is cleanest when the fermentation temperature is as low as possible for a given yeast strain without causing problems with sluggish and/or incomplete fermentations. As temperature increases the production of esters increases and fruity aromas increase, especially for beers made with strains that are noted for the production of aromatics. The production of these compounds is greatest at the peak of fermentation and it follows that the most critical point to consider for temperature control is this peak in activity.
Commercial brewers often begin fermentations cooler, usually about 5 °F (3 °C) cooler, than the controlled temperature, which you can try. One reason for allowing the fermentation to experience this “free rise” is that tank temperature is easier to control when the fermenter is being mixed by the activity of fermentation. After fermentation is complete the temperature is lowered. If you plot temperature over time the result is a curve that goes up and down.
To summarize, temperature fluctuations are normal and the most important temperature to control is the peak temperature experienced during peak activity. Also, starting cool and allowing your fermenter to warm up is a good method to consider if you lack refrigerated cooling. To keep the peak from getting too high you can use a low tech method such as plunging your carboy into an ice bath for 20 minutes or so at a time, a method that mimics turning on the valve of an attemperation coil.