Hot Trub: Formation and Removal

By Rob Barchet

 

 

Hot trub, the protein precipitate formed during the boil, can impede fermentation and produce undesirable qualities in the finished beer. Simple, effective methods allow professional and home brewers to remove this unwanted by-product. 

 

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It is 9:15 a.m. in Ashburn, Virginia, and I am just starting to bring my wort to a boil. Suddenly, large chunks of protein rise to the top. They look bad, they smell bad, and they taste very bad. In this case, the senses do not deceive — these coagulated proteins and gums are bad. They are bad for the fermentation and for the finished beer. Trub coagulated in the first 5 min of boil can account for 60% of the total trub and is easily identified because of the large size of the pieces. 

 

These initial coagulates can be skimmed, reducing the amount of trub you need to remove at the end of your boil. More trub will be produced during the boil, though the shearing action of the boil presents the coagulates from growing quite as large as at the beginning of the boil. With a 1–2 h boil, however, enough of the proteins (95+%) will coagulate to be removed as trub using one of several methods. 

 

 

Technically, trub is defined as the insoluble precipitate that results from protein coagulation and simpler nitrogenous constituents that interact with carbohydrates and polyphenols. It is also referred to as “break.” Hot trub is that part of the break that occurs during the boil and is mostly proteinaceous; cold trub, which consists of proteins and protein–tannin complexes, is formed as the wort cools and the beer settles. Although most amino acids are assimilated by the yeast, remaining proteins should he removed because they later react with polyphenols, resulting in colloidal instability (haze). The elimination of all non—amino acid proteins is not warranted or even desirable, however, because they are essential for giving the beer full body and head retention.

 

Hot trub precipitates are formed during the boiling of the wort. In a study in a German brewery, hot trub particles varied in size from 30 to 80 µm. Effective removal of hot trub before fermentation is critical because the trub can smear the yeast’s cell walls, impeding the transport of substances in and out of the cell, which can lead to head retention problems, poor flavor stability, and harsh bitterness in the palate of the beer. The actual nutritive value of hot trub is greater than that of spent grains, because it contains more digestible crude protein per unit weight. The overall amount of trub (wet weight) varies from 200 to 400 g/hL (235 to 470 g/bbl), depending on various factors.

 

 

Crop location, seasonal factors, and genetics influence the amount and type of protein in the barley and hence the amount of hot trub. In addition, high-temperature kilning in the malthouse (for example, that used for darker malts) results in fewer high molecular weight proteins in the wort and therefore less trub. It has also been shown that quicker, less-intensive mashing routines yield worts containing more trub. Conversely, triple and double decoction mashing produce worts with considerably less trub, because the mash boiling process and the extended protein rests of such mashes maximize the breakdown of proteins and the removal of trub before the kettle boil. Higher gravity, all-malt worts, by virtue of having more malt, will have correspondingly greater trub.

 

Mash pH does not greatly affect the amount of trub unless it falls below 5.0; below pH 5.0, the amount of trub formed decreases sharply. Proteins coagulate most readily at their isoelectric point (the point at which the numbers of positive and negative charges are equal), which is pH ~4.9 in the case of wort proteins. In research conducted using a whirlpool, Van Gheluwe and Dadic showed that the best sedimentation occurred with wort of pH 5.0–5.2. Furthermore, the longer the wort boils, the more trub will be formed, albeit at an ever decreasing rate. In boils >2 h, coagulation will be somewhat offset by shearing action on the proteins. It is generally accepted that boiling times of 1–2 h are sufficient for most beer styles.

 

Many brewers, both professional and amateur, use kettle coagulants such as Irish moss (red marine algae Chondrus crispus and Gigartina stellata) and its derivatives to increase trub coagulation. The Irish moss, a negatively charged colloidal material, attracts positively charged proteins in the wort to form larger hot trub particles. In its raw form, 4–8 g/hL (4.7-9.4 g/bbl) Irish moss should be added to the boiling wort ~15 min before knockout. Because treatment with Irish moss can cause the wort to foam and boil over, watch the boil and be prepared to reduce the heat going to the kettle. Irish moss is more effective if first diluted in water and let to stand for at least a couple of hours (preferably overnight), thus promoting swelling and gelling action. Incidentally, for those Reinheitsgebot purists, the addition of kettle coagulants is not allowed. 

 

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Hop backs. The original method for removing trub is a hop back. Historically, when beer was made using whole hops, wort would be discharged into a vessel with a false bottom, not very different from a lauter tun. This system is still prominent in breweries that use whole hops. The hops create a filter bed that removes both hop and proteinaceous trub. After wort flows through the hop back, it is pumped through a heat exchanger and into the fermentor. The hop back must not allow any leaves or flowers to pass through, because such materials can clog a plate heat exchanger. To avoid this, brewers often set a bed of fresh hops on the false bottom before letting the wort flow in. This practice also brings greater hop aroma into the finished beer. Despite the splashing, the wort’s uptake of oxygen during this step is less than one might expect because the steam generated during runoff forms a protective barrier between the wort and outside air.

 

Design of the hop back must take into account the amount of whole hops that the brewery uses. It should have enough open surface area on the false bottom to allow an entire gyle to filter in <1 h, preferably closer to 0.5 h. Textbooks recommend a filter bed of at least 6 in. (preferably 12–24 in.), though I have seen home brewers effectively use much less. It is important to avoid suction of the wort because it increases the risk that hops will pass through to the heat exchangers. Wort flow through the hop back can be controlled by applying back pressure (that is, by placing a valve on the positive side of the wort pump).

 

An in-line sight glass is a nice feature and is useful for checking clarity. Large breweries have more complex hop backs, with features such as sparge, recirculation, underletting, and spent-hops discharge. Others use hop separators, where wort enters, passes through a sieve, and discharges. Hop separators maximize hop efficiency by incorporating a screw conveyer that compresses and sparges the hops before expelling them.

 

Coolships. Another time-honored method of hot trub removal is the coolship. Coolships are still quite prevalent in small, old-time Bavarian and Belgian breweries. Hot wort from the kettle is pumped into the coolship, which is usually a long, wide, shallow open copper or stainless vessel. Here the wort remains from 1 to 3 h (depending on conditions), cools to ~140–170 °F (60–77 °C), and trub settles out. The wort is then cooled further by using a plate heat exchanger or baudelot cooler. In some eases the wort stands in the coolship for 12 h, cooling it enough to precipitate some cold trub.

 

The coolship is very effective at removing hot trub; however, coolships have disadvantages. First, the nature of the coolship’s geometry makes some product loss inevitable. Second, because the wort sits at extremely vulnerable temperatures for quite some time, the risk of infection is much higher than with other methods (to prevent infection, some coolships are housed in rooms with sterile air ventilation). Finally, wort will pick up color as it contacts air, and hop alpha acids are substantially isomerized as the wort stands, darkening the color and increasing (up to 20%) the hop bitterness of the finished beer.

 

Whirlpool tanks. The whirlpool tank is the most widely used method for hot trub removal, particularly in breweries that use hop pellets, powder, and extracts. Whirlpool tanks come in various designs, and opinions vary as to the best geometry. Some call for a conical bottom, some a slightly welled bottom, and still others prescribe a flat or even an inverted cone bottom. The original whirlpools were constructed to have a height-to-diameter ratio of 1:1. Although this ratio remains the more popular of geometries, later whirlpool designs have ranged from very shallow (0.6:2), to accommodate the trub associated with hop powders, to tall (3:2).

 

The wort usually enters the tank tangentially at speeds between 13 and 50 ft/s; faster wort flows result in better trub separation. Placing a ping pong ball on the outside surface of the whirling wort will give you an idea of the speed your wort is moving. You can calculate the speed by first determining the inner circumference of your whirlpool tank (circumference = π X diameter). By counting the number of revolutions in a fixed period of time (20 seconds, for example), you can calculate the distance covered per second (rate = [circumference X number of revolutions]/number of second measured).

 

In general, the effectiveness of whirlpools decreases as the original gravity of the wort increases because the relative difference in density between the trub particles and the wort decreases. Injecting carbon dioxide into the wort as it goes to the whirlpool has been shown to increase the amount of trub deposited.

 

To minimize oxygen uptake in a whirlpool, the inlet should be placed one-quarter to one-third of the way from the tank bottom. The rotation about the vertical axis should continue for 20–40 min after the wort is in the tank. As the whirling action slows, the trub, which is heavier than beer, forms a fairly hard conical cake at the center of the bottom of the tank. If the time the wort is left standing is too short, separation will be incomplete; too long a stand, and the risk of infection rises and the breakdown of S-methyl-methionine continues, raising dimethyl sulfide levels.

 

The whirlpool tank is fitted with one or more take-off points (some with interchangeable standpipes that can be sized for particular beer styles), so that the clarified wort can be discharged to the heat exchanger, leaving the trub behind. By having more than one discharge point, the wort can be removed with minimal turbulence, leaving the trub cake intact and resulting in a clearer wort. The trub is then flushed down a center drain, fitted with a valve that is usually twice the size of the wort outlet.

 

Some breweries have a combination kettle–whirlpool, with essentially the same properties of the dedicated whirlpool. Unfortunately, the presence of kettle baffles or an agitation propeller will completely disrupt the whirling action, making a combination kettle–whirlpool impossible. If the kettle has no baffles and the kettle agitator is equipped with a high-power, high-speed motor with an inverter and a clutched gear drive, the wort can be whirled using the agitator. When the desired whirling action is reached, the agitator can be turned off, activating a clutch and allowing the propeller to turn freely with the wort. Alternatively, the wort is discharged from the bottom of the kettle and pumped back into the kettle tangentially. Pumping this way for 15–20 min will create a whirlpool. In either case, when the whirling subsides and the trub cake has formed, the clear wort can be pumped from a port above the trub cone.

 

Advantages of the combination system include reduced oxygen uptake, because no wort transfer is necessary. It also saves time and eliminates the need for a separate whirlpool tank. On the other hand, because of necessary engineering compromises, it is unlikely to be quite as effective as a dedicated whirlpool. For it to work at all, it must be carefully engineered. 

 

 

Most home brewers will find that creating a whirlpool in their kettles is the most effective means of hot break removal. Whirlpools can be created in homebrew kettles by vigorously stirring the wort and allowing it to stand for about 20 min, until the trub settles. Wort is then siphoned from the side of the kettle, away from the trub cone. When whole hops are used, home brewers can tie a metal scrubby (standard pot scrubbing type) around the inlet of the siphon to ensure that no hops and less trub will pass through. When purchasing your scrubby, make sure that it is copper or stainless and contains no detergents. If the siphon gets plugged, it is usually an indication that the whirling action was insufficient. After replacing the scrubby, try stirring faster and allowing the wort to settle longer before siphoning.

 

A home brewer friend of mine has an interesting system for removing hot trub in his 1-bbl system. He boils his wort with a lauter screen (standard perforated sheet) in the kettle. At the end of the boil, he vigorously stirs his wort, making a whirlpool. After the wort settles, runoff is drawn beneath the screens from a side port. This system essentially uses both a whirlpool and a hop back and has been successfully used for both whole hops and pellets. 

 

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