Cooling Hot Wort Science
After your wort has been boiled, it must be cooled. The cooling process is simple, but it changes the wort in many ways. Cooling renders the wort hospitable to yeast, aids in the formation of solids that precipitate out of the wort and slows the formation of unwanted volatiles.
Hot wort clarification
Wort is full of suspended solids at the end of boil. It contains hop leaves (or, if pellets are used, ground-up hop particles) along with precipitated protein/tannin complex and coagulated protein. The hops need to be removed prior to transfer to the fermenter as they will interfere with the action of the yeast. The rest of the material that precipitated out of the wort during boiling, called hot break or hot trub, should be removed from the wort also. If a lot of trub carries over into the fermenter, it will be detrimental to the fermentation. Protein and polyphenols will coat the cell walls of yeast cells, making the uptake and release of compounds more difficult. Trub also tastes quite nasty and so will negatively influence the flavor of light-tasting beers.
Traditionally, wort was passed through a separate screened vessel to separate the solids from the liquid. In English ale breweries, a charge of hops was added to the screened vessel to act as a filter bed to trap trub and hop leaves. This also added the benefit of imparting additional hop aroma to the wort and hence the finished beer. This would be referred to in brewing books as a hopback. If you plan to use whole hop cones in the kettle, then the filtration separation method is really the only way to effectively remove them. As homebrewers, it is quite easy to replicate this vessel by using your mash tun, provided it has a screen and not a slotted separator. Simply transfer the wort to a cleaned and sterilized mash vessel immediately after boiling finishes, and cool it down from there.
Larger brewers these days tend to use hop pellets and whirlpool vessels to clarify the wort. The wort is spun in the vessel and centrifugal force sends the solid particles to the outside of the tank. The laws of physics flex their muscles and cause the solid particles to flow down the wall of the tank and across the bottom of the tank toward the middle, where they repeat the process. Once the spinning motion slows, the solid debris forms a mound in the middle of the bottom of the tank. Large brewers do this with pumps, but homebrewers can easily reproduce the effect by stirring the wort in the kettle with a spoon until it is all spinning fast at the same speed, and then allowing it to slow down.
Why Cool the Wort?
The wort must be cooled to an appropriate temperature for the yeast to do its work. Yeast is generally happiest growing at incubation temperatures of 98.6° F (37° C). However, for yeast to produce the kinds of flavors we enjoy in our beer, significantly lower temperatures are required. Yeast strains that produce lagers prefer to ferment in the range 45–57° F (7–14 C), while those that produce ales create their best beer flavors in the 59–72° F (15–22° C) range.
How the Wort is Cooled
A big brewery has a specialized device for wort cooling. Traditionally brewers would use a device called a coolship. This was just an open shallow pan that the wort sat in while it cooled down from boiling to around 140° F. It would then be transferred to a more rapid cooler. The rapid cooler consisted of a series of tubes containing flowing cold water over which the wort is gently allowed to flow. The risk of bacterial infection is obviously high using this method. However, the open arrangement does help to improve the volatilization of unpleasant aromas and the pickup of oxygen, both of which lead to better beer.
A modern brewery uses a plate heat exchanger. A plate cooler consists of a steel frame which carries a number of stainless-steel recessed plates pressed tightly together. The connections and passages are such that wort and the cooling medium pass each other in turbulent counterflow in shallow layers between the plates. The exchange can be set up so that two cooling media are used. For example, water cools wort down to 75–80° F, and then cold glycol will cool the wort the rest of the way.
Wort cooling is probably the area that presents the biggest challenge for homebrewers. Many appropriate heat sources are readily available, but suitable refrigeration devices are harder to find. Simply allowing the wort to cool over time will expose the wort to the risk of infections and make off-flavors more likely. Brewing strong wort, and then adding ice or cold water to cool the wort, can cause similar problems with wort sterility.
Usually, an immersion chiller is used. A clean, sterilized copper coil is dropped into the kettle and cold water is passed through the coil to chill the wort. The limit here is the temperature of the water used to cool the wort. If the water from the faucet is 60–80° F, then it is impossible to cool the wort any lower. A better option is to use two coils and first chill the faucet water in a bucket of ice water to chill it below 40° F. Counterflow chillers are the preferred option. In a counterflow chiller, wort is run inside a copper tube while water flows along the outside of the copper tube in the opposite direction, usually inside a length of hose pipe. Again, the water may need to be cooled prior to running it through the hose.
Wort volume shrinkage
One thing often overlooked by brewers, although probably of little significance to homebrewers, is the fact that wort at 200° F occupies four percent more space than wort at 68° F. For those of you counting every drop of beer, that’s a pint and a half for every five gallons.
Formation of solids
Once we have removed the solid and highly visible components from the hot wort with a whirlpooling method, or some kind of filter using hop leaves, we should have lovely, clear, hot wort. When we cool it down, though, it goes cloudy again. The reason? We are now forming what we know as cold break or cold trub.
Cold break is similar chemically to hot break, except that the particle size is much smaller. It consists of proteins, protein/polyphenol complexes, carbohydrates, bitter compounds and lipids. These are smaller molecules — about 0.5 micrometers in diameter — which are soluble in hot wort but are insoluble in wort once the temperature falls below 140° F (60° C). As wort is cooled, it precipitates and causes a solid deposit. The amount of potential cold break is dependent on the barley, the adjuncts, the degree of modification of the malt, the mashing regime and the degree of removal of hot break, but the amount that actually precipitates out is affected by the temperature. At ale fermentation temperatures (68° F) only 50 percent of the cold trub may precipitate out in the wort. At lager temperatures (50° F), as much as 85 percent of the potential cold break solidifies.
Throughout the world, opinion is divided on whether this cold break should be removed. On the negative side, it is responsible for “chill haze” in the beer. Chill haze is a cloudiness in the beer that appears once it is refrigerated. On the positive side, the cold break carries with it vital yeast nutrients, including lipids that are needed for healthy yeast growth.
Generally, traditional ale brewers leave it in the wort since the trub sinks to the bottom of the vessel and the brewer’s yeast is reclaimed from the top of the vessel. Lager brewers traditionally remove the trub since their yeast sinks to the bottom of the tank and becomes mixed with the yeast, concentrating with each serial repitching. This is why some traditional European brewers sieve and clean their yeast.
Homebrewers generally use their yeast only once, using fresh yeast each time. If recovery of yeast is not an issue, then cold break can comfortably be left behind to settle in the bottom of the primary fermenter. Further cold conditioning of the beer causes the rest of the cold trub to precipitate out and sink to the bottom of the vessel. Complete removal of all of the cold break from the wort has been shown to cause slower fermentations, increased ester formation, poor yeast growth and poor yeast vitality.
Trub particles also operate as nucleation sites for carbon dioxide bubble formation, therefore eliminating dangerous super-saturation and increasing motion in the fermenter. If too much CO2 dissolves in the wort, then the wort will foam uncontrollably when the CO2 inevitably breaks out. The steady stream of bubbles being produced and rising through the liquid will increase mixing action and speed up the fermentation. These same fatty acids, along with some metal ions, are implicated in the reactions that cause beer staling. So too much is probably a bad idea. Large brewers remove cold trub by various methods: sedimentation in large shallow pans; chilling and filtering with diatomaceous earth; centrifuge; or a traditional method known as a flotation tank. This is an additional vessel added to the process between the hop heat exchanger and the fermenter where the wort is collected. Air is bubbled violently through the wort from the bottom and the cold break is carried to the surface of the vessel. Six to 12 hours later, the wort can be run from the bottom of the tank, leaving the cold break behind. In the UK, brewers traditionally collected the wort in a gauged government tank in order to assess the excise duty that was due on the beer. They would transfer the beer to the fermenter after 12 hours. The cold break would sediment and be left behind in the government tank, cleaning the beer.
Volatile removal
Dimethyl sulfide (DMS) is a volatile beer aroma producing a “corn like” aroma in beer. In some styles, such as a few American pale lagers or German pilsners, it is a pleasing part of the flavor profile. In other styles it is inappropriate. Normally it is significantly reduced during vigorous wort boiling. However, if that is not the case, then more can form while the wort is waiting to cool. So the quicker wort can be clarified and cooled into a fermenter, the better. A revived technology that is finding favor is that of wort stripping. Anheuser-Busch has, for a long time, clarified their hot wort and then run it through a cooler that blows air across a thin, falling film of wort. This allows volatile hop aromas and DMS to be significantly removed from the wort. Wort stripping is a new technology being built into modern German brewhouses, and a new design of kettle developed in Germany incorporates this into the boiling phase.
Aeration
Brewing yeast cells require molecular oxygen at the start of the fermentation. Indeed, immediately prior to fermentation is the only time during the entire brewing process when oxygen is deliberately added. Yeast cells require oxygen to produce several intermediate compounds that they require in order to grow effectively. And since it is the yeast growing in wort that creates beer flavors, then healthy yeast will ferment better.
The next question is how much oxygen is needed. The answer to this question is significantly more important to commercial brewers than it is to homebrewers. Commercial brewers need to re-use their yeast and home- brewers do not. If you use fresh yeast every time then wort aeration is far less important, unless you are using wort aeration as a technique to rescue an underpitched stuck fermentation. If you are serial-repitching your yeast at home, then it does become an issue that needs to be addressed. Different yeast have different oxygen requirements. In general, yeast need between five and 10 mg/litre of oxygen to be healthy. This allows them to grow sufficiently, and also to be in good health at the end of the fermentation, when they still have work to do maturing the flavor. If you force air into wort you can achieve an oxygen level of around eight milligrams per liter. Craft brewers tend not to measure oxygen levels as the meter is expensive. Big brewers, however, do measure it. So saturating the wort with air may not provide quite enough oxygen for all yeast. In this case, brewers use oxygen from an oxygen cylinder. Medical oxygen is best, although welder’s oxygen is just as pure. (Both are filled from the same pure oxygen source; it’s just that medical oxygen can only be put into cylinders that never have contained anything but oxygen.) The oxygen needs to be added to the wort in a way that allows it to dissolve and the best method is a small “sintered” stone. Traditionally these were made from pumice, hence the term “stone,” but now sintered stainless-steel devices are readily available. These are dangled into the cooled wort and oxygen is allowed to flow, foaming the wort and dissolving the gas. This method will help when making very strong beers, as air and oxygen dissolve less well in strong worts and yeast really need to grow well when the wort is strong.
To summarize, it is important to make an effort to cool the wort down as quickly as possible. The wort will be clearer and less prone to bacterial infection. The chance of off-flavors will be reduced and the beer will be less prone to chill haze. Hot trub should be separated as completely as possible. Cold break removal is less important for homebrewers, unless you’re brewing lagers and re-using the yeast.
