I’ve seen the importance of diacetyl rests mentioned dozens of times. the description always states to increase fermentation temperatures to the 65–68 °F (18–20 °C) range for 24–48 hours or so. However, there is never a discussion of how quickly to raise the temperature. If I have a lager at 50 °F(10 °C), do I raise the temperature only 2–3 degrees per day until I hit the range (adding ~7 days) to avoid shocking the yeast or do I raise it much faster to no ill-effect?
I have really never understood the idea of shocking yeast. Warming or chilling yeast within the temperature range that brewers ferment beer can and does occur quickly in commercial breweries without negatively affecting the yeast or the resultant beer. Fermenting too warm can of course lead to excessive production of esters, higher alcohols, sulfur, phenols, and other aromatic compounds, so controlling fermentation temperature is an important facet of brewing process control. The short answer to your question about the warming rate is not to worry about it. The easiest way to warm a batch of lager is to move your carboy from your lager fridge to a cool closet or corner in your basement and just let things happen.
So what happens during the diacetyl rest and why is it important to warm the beer in the first place? There is a precursor to diacetyl, alpha acetolactate or simply “DP” (diacetyl precursor) that is excreted from yeast during fermentation. Outside of the cell, DP is oxidized and converted to diacetyl by oxidizing compounds, such as metal ions or sometimes oxygen, in beer. Once DP is converted to diacetyl, yeast cells that are metabolically active and in need of a hydrogen acceptor convert diacetyl into the flavorless compound 2,3 butanediol.
Diacetyl is referred to as a vicinal diketone (VDK) because the molecule contains two ketone groups, R-C(=O)-R’ in biochemical shorthand, located next to each other (vicinal). Diacetyl contains four carbon molecules, hence the root name butane. 2,3 pentanedione is another VDK in beer and contains five carbon atoms. While diacetyl smells similar to butter, 2,3 pentanedione smells similar to honey and is less pronounced than its big brother “D.” This is why 2,3 pentanedione doesn’t have a common name and a rest named after it! But a similar reaction follows, where 2,3 pentanedione is reduced to 2,3 pentanediol. When these reduction reactions occur, nicotinamide adenine dinucleotide (NAD+) is regenerated. Suffice it to say, this is important for yeast metabolism and relates to the general topic of biochemical pressure that helps to understand how we as brewers can influence yeast biochemistry. It is worth noting that acetaldehyde reduction to ethanol also regenerates NAD+ and excess levels of acetaldehyde that build in beer during fermentation drop off with aging during the diacetyl rest.
Crawling out of the rabbit hole of biochemistry and back to the practical world of brewing . . . As a commercial brewer, albeit on the smaller end of the scale, I scratch my head about the diacetyl rest because most fermentation vessels are equipped with cooling jackets, not heating jackets. Most commercial brewers simply have no way of using the sort of diacetyl rest employed by so many homebrewers. This essentially means more time for lager beers. There are some breweries that have invested in heat exchangers used to warm beer after primary fermentation so that aging is shortened. Since time is money for commercial breweries, one would think that this method would be more common, but brewers are a cautious lot and process changes are not undertaken lightly.
One reason that lager brewers may not be able to reduce aging time by simply investing in methods to warm beer is tank pressure rating. Lager beers can be fully car-bonated during lagering using tanks rated for 15 psi/1 bar when the temperature is gradually cooled to about 43 °F (6 °C). Although yeast metabolism is slow in this temperature range, lager yeast do remain active. If the beer temperature were warmed to 68 °F (20 °C) the tank pressure would need to increase to 31 psi/2 bar. This is a significant difference and requires beefier tanks stamped for use at pressures in excess of 15 psi/1 bar. In the US, this means that the vessels are ASME (American Society of Mechanical Engineers) stamped. Yet another reason why a relatively simple homebrewing method is a very real challenge in a commercial setting.
Homebrewers often are envious of the equipment used by commercial brewers. But sometimes the opposite is true, and the ease at which homebrewers can accelerate diacetyl and acetaldehyde reduction in lager fermentations is one example. Go for it, I say!
In the December 2013 issue you described a low-cost method for wort aeration consisting of a barbed tee set up in-line on the way to the fermenter. I recently tried this method and ended up with a wort geyser! I thought that it may have happened because I had created too much back pressure by leaving the standoff at the end of the racking cane. I experimented again later without the standoff, using only water and had the same issue. I am pumping wort into my heat exchanger with 1⁄2-inch tubing and it exits into 3⁄8-inch tubing, through a 3⁄8-inch in-line thermometer, then to the 3⁄8-inch tee with the open end facing up, to the 3⁄8-inch racking cane. What am I doing wrong?
Rocky Hill, Connecticut
The low-cost method I described uses a tee where the wort flows horizontally into the center branch of the tee and down from the bottom leg. As liquid flows through the tee, air is sucked into the liquid flow from the branch that is exposed to the air. I described the use of a cotton wad filter and explained how microbiologists have used these simple yet effective filters for the past 150 years in microbiology labs across the globe. I like the history of science and suggest reading up on some of the laboratory methods used by Louis Pasteur as well as the design and explanation of Venturi nozzles.
When I was a graduate student in Dr. Michael Lewis’ brewing lab in the early ‘90s at University of California-Davis we used this type of device to aerate wort that was gravity flowing from a 5-gallon (19-L) hop back through a wort cooler and into the glass carboys used for fermentation. No wort geysers that I can recall ever set loose from the aeration tee because the fill tube extending into the carboy was a few inches shorter than the carboy and flow was not restricted. The main differences in your method are your pump and reduction in line size from the heat exchanger and into the tee. The pressure drop downstream of the tee is greater than pressure developed by your pump, so the excess pressure and accompanying liquid flow is pushing up and out of the tee.
There are many ways to “scrub” pressure from pumps. Control valves, orifice plates and choker lines are examples of things that can be added in line before the tee to reduce pressure. A simple needle valve is probably the easiest solution since it is adjustable and you can see how closing the valve down prevents the wort geyser. Orifice plates and choker lines need to be sized to work properly and require a pretty solid understanding of fluid dynamics to get right. Or you can use trial and error to size.
In commercial operations, pump speed is often varied with the use of a variable frequency drive (VFD). Over the last 20 years the price of VFDs has dropped low enough that many process systems use them to modulate motor speeds for a wide range of controls applications. Modulating pump speed is a very convenient way to control discharge pressure. You can purchase single phase AC VFDs for small motors for about $100. This is really pretty inexpensive considering the control provided. With a VFD-controlled pump you can vary flow rate and use a single pump for a wide range of applications. Mash-in, sparging, whirlpooling, and wort cooling usually require different flow rates, and a pump sized for the worst case duty can easily be dialed back with a VFD. I realize this information is not directly related to your aeration question, but the topic of pump pressure control relates to other homebrewing applications.
I am struggling to convert recipes tried and true on a 5-gallon (19-L) all-grain homebrew system for use on a 15-gallon (57-L) system. My hope for consistency is being dashed by bittering level differences. Is there a technique for determining the hop utilization rate of a given brewery set up to facilitate recipe scaling, and is there a method of determining actual bitterness levels compared to the calculated bitterness that most brewing software gives you?
The hard part about answering this question involves determining IBUs in a beer. For the moment, let’s ignore the elephant in the room and pretend that that is not so difficult for homebrewers! The thing that makes scaling recipes fairly easy is knowing hop utilization. Assume that a 20-liter (a bit over 5 gallons) test brew is made using a single 30 gram hop addition at the beginning of the boil. The hops used have 8% alpha acids and the resultant beer ends up with 36 IBU. The reason for this test brew is to determine hop utilization and that is pretty simple given the data above.
Utilization (%) = liters wort x IBU in beer ÷ grams hops ÷ hop alpha acid content x 10
Utilization (%) = 20 x 36 ÷ 30 ÷ 8 x 10
Utilization = 30%
I have intentionally left out units of this not to confuse the meat of this question. Previous columns and articles in BYO have covered the unit cancellation in this calculation for those interested in units. The point is that hop utilization can be calculated with the information above in hand. Multiple hop additions make the process a bit more complex, but the basic method is the same.
Now let’s jump into a new brewhouse, run the same experiment and calculations. The new brewhouse is determined to have a better hop utilization, where the first hop addition has a 35% hop utilization. Scaling up recipes from the old brewhouse is pretty darn simple. You can use the equation above to run hop calculations, or apply a few simple scaling factors. The new brewhouse produces 60 liters (almost 16 gallons) of wort, so all hop additions are simply multiplied by 3. To account for the improved yield, the additions can be adjusted by 30/35 (0.86). Combining the two multipliers (3 x 0.86) results in 2.6 and this is all that is needed to scale up those 20-liter recipes to 60-liter batches.
I have to take a break at this point and do a little explaining. Why am I using metric in these calculations? Because it is the easiest way to do calculations involving units that are fundamentally metric, which really means just about everything in brewing. The United States, Myanmar (Burma), and Liberia are the only nations on the planet that have not officially embraced the metric system. When brewing calculations are performed with pounds, gallons, inches, and degrees Fahrenheit things are just more difficult. So metric calculations are in a sense lazy!
Another bit of explaining involves assumptions. Liquid volumes get routinely tossed about by homebrewers and it is important that we use the same basis for communication. A 20-liter (or 5-gallon-ish batch) of beer means different things to different brewers. From a brewing calculation standpoint, batch size really refers to a very specific volume and that is wort volume produced after wort boiling. All of the malt extract and hop acids extracted during the brewing process are present in this volume of wort.
When wort is transferred out of the kettle (or whirlpool) some wort (representing malt and hop goodies) is loss. When beer is racked more malt and hop goodies are lost. And when beer is packaged more hop and malt goodies are lost. However, volumetric losses downstream of the brew kettle do not affect brewhouse yield (malt efficiency) or hop utilization. So it is important to actually measure how much wort is produced in the brewhouse because wort volume is the basis of all brewing calculations involving malt and hops added in the brewhouse. Your hydrometer does not care about beer or wort volume and neither do methods that measure IBUs in beer, as both of these measure concentration.
Back to the question at hand, and we are left with that pesky thing called the IBU. If you happen to have access to a UV spectrophotometer it is not very difficult to analyze your beer and determine IBUs. But what if you cannot analyze bitterness, is there another way? Fortunately hop bitterness can be diluted and analyzed with sensory methods. If you use a consistent beer with a known IBU, like a big name lager beer, as your reference standard you can dilute your homebrews (I am assuming that most homebrews are not less bitter than big name lagers) and some pretty basic sensory methods to determine beer bitterness. Any readers who work in a lab and are cringing right now, please put a sock in it!
If you want to try this at home you need something to dilute your beer with. Carbonated water is the best thing to use because carbonation does affect the perception of bitterness and you don’t want big differences in carbonation to alter the results. And by using a bracketing method of dilution you can dilute your homebrew so some samples are more bitter than your standard and others are less bitter. Once you tighten your dilutions around the standard you can come up with a reasonable estimate of the bitterness level, and from that you can calculate hop utilization. Or better yet, calculate the differences in your two homebrew systems. Sorry if this answer seems to have a heavy dose of voodoo and crystal ball quality sprinkled on top! Think this through and I am sure you will discover that you can get your recipes scaled up without disappointment.
Is there any way to correct astringent wort/beer post mash or post fermentation? Or, is the batch lost and better luck next time? I brewed an altbier where the taste after primary fermentation was quite astringent, which I believe occurred while my recirculating wort in the heated mash tun got too hot when the pump clogged. Can this beer be saved? Will time heal this wound?
All is not lost when a batch of beer is spotted that may eventually be ascribed with the scarlet letter “A” for astringency. The key with this statement is that the beer is identified and slated for corrective actions before you end up with a palate-puckering bottle or keg of astringent alt. The good news is that there are numerous cures to astringency, and they all begin with understanding the root cause.
Astringency is a mouthfeel associated with tannins, and is commonly found in tea, young red wine, really hoppy beers, especially dry hopped brews, and grainy and unbalanced beers. Tea astringency can be rectified with milk. Wine astringency can be reduced with egg white finings. And beer astringency can be associated with haze in finished beer. The common element of these three examples is reaction with proteins. Tea tannins react with milk proteins, wine tannins react with egg proteins, and hop tannins react with malt proteins to form haze. And tannins bind with proteins in the mouth to cause that unpleasant texture associated with astringent foods and beverages.
The solution to astringency is protein-tannin interaction. The general idea is to get those tannins that will, given the chance, assault your palate to embrace another protein and gracefully exit the scene before you keg or bottle your beer. Cold temperature also helps with this process and explains why aged lagers are rounder than young lagers. Things you can use at home to combat your astrin-gent alt include PVPP (a protein analog), cold aging, isinglass, egg whites, and time.