Hot Wort Clarification
At the end of the boil the wort should be absolutely clear (“bright”) but contain, suspended in it, the remains of hops and flocs of trub or hot break. Hot wort should be clarified as soon as possible before cooling, not removing the hot break (trub) and hop solids will cause problems downstream. The trubaceous matter (hot break) which includes coagulated proteins, polyphenol-protein complexes. There will also be significant quantities of hop debris and grain materials. The total wet mass of solids is typically in the range of 1 to 1.5 kg/hL (2.5–4 lb/bbl) of wort but varies depending on the strength of the wort and the hopping rate.
Whole Hop Separation
When whole hops are used, the cast wort must first be cleared of spent hops by passing it through a hop back or hop separator. The methods of separating whole hops vary considerably. In smaller breweries hops are removed by straining the wort through a cloth bag or a sieve while in larger breweries a number of whole hop separation systems are employed— such as a hop back or hop separator. They are usually placed in-line after the kettle before the hot break separation vessels such as coolships, settling tanks, whirlpools, and centrifuges.
The hop back is a vessel with a slotted false bottom or a mesh screen for separating whole hops and is often employed by British brewers. Wort from the kettle is run into the hop back and strained by the slotted base. The wort is recycled from under the plates through the filter bed of hops until the wort runs clear. As the spent hop material accumulates, it progressively improves the straining action so that hot trub is retained. Briggs reports a bed depth of 30 to 60 cm (77–152in) for optimum performance (Briggs et al., 2004).
Hot Trub Separation
If pellets or extract are used, the wort may be clarified by sedimentation, a whirlpool, a centrifuge, or a combined kettle/whirlpool, or by filtration. It is recommended that clarification take place at the highest possible wort temperature in order to obtain optimum trub removal (Vernon, 1985).
In older breweries, it is still possible to find, at the top of the brewhouse tower, a redundant shallow rectangular vessel of no more than a depth of 15 to 35 cm (38–89in) called a coolship (Figure 10.7). The wort remains in the coolship from 1 to 3 hours (depending on conditions) and in some cases up to 12 hours, cooling to approximately 60 and 77 degrees C (140 and 171°F). In Germany, brewers occasionally inject air from the bottom of the tank to aid the sedimentation process.
Successors to coolships are settling tanks. These may have flat or conical bases and the sides of the tanks, which are generally cylindrical, may or may not be cooled by water in wall jackets. The tanks are enclosed and have vapor-escape stacks. Usually, the tanks are filled from the bottom to minimize oxygen pick-up and wort oxidation. The wort from the kettle is transferred to the sedimentation tank and allowed to stand for 20 to 40 minutes.
The whirlpool, as shown in Figure 10.8, is the most widely used method for hot trub removal particularly in breweries that use hop pellets, powder, or extracts liquid products. Whirlpools cannot be used with whole hops unless a hop back or strainer is used initially. The whirlpool represents the most efficient method for trub removal for small brewers. The whirlpool is a circular vessel with very few moving parts, so once optimized, there is very little to be maintained or serviced.
Stand Time. After the boil wort may remain in the whirlpool for up to an hour before being cooled. If the stand time is too short, separation will be incomplete. Trub carryover into fermentation is responsible for “vegetable” or “stewed” character in the final beer and excessive trub can coat the yeast and can lead to inadequate fermentation performance. Chapman reports that stand time should not exceed rotation time; otherwise, the trub cone may spread (Chapman et al., 1997).
The latest development in small brewhouses is the use of a combination brew kettle and whirlpool. The vessel, as its name implies, is used for wort boiling and as a whirlpool. The vessel can have an external wort boiler or calandria and no internal fountain or spreader. Some systems can be direct-fired or steam-jacketed. A pump is used to create the whirlpool action by drawing off the bottom of the kettle and injecting it into a tangentially inlet port entry, drives the rotation of the liquid.
Sometimes the wort is clarified by centrifugation, which spins the wort with high velocity, forcing the hop and trub debris to the side of the vessel. Figure 10.9 illustrates a typical centrifuge. If the wort is heavily hopped, it may be necessary to pre-treat it through a strainer to remove some of the abrasive hop solids. Qualitatively, a centrifuge does not guarantee better hot break removal than a whirlpool, but it allows for a more consistent turbidity level.
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