Cleansing rounds, squares, or pontoons, are covered vessels, each furnished at its upper side with an opening through which the yeast formed by the fermentation of the contained beer can escape. The rounds are completely filled with beer, and as there is a certain loss of liquid during the pro gress of the deansing, they have from time to time to be filled up, in order that the proper level may be maintained. The refilling is termed topping up, and it is performed sometimes by an arrangement of ball-cock or similar self-acting valve, and sometimes by hand. The supply of beer necessary for maintaining the level in the pontoons is drawn from rounds placed at a higher level, aod termed topping-up rounds, and filled with a well-fermented beer. The division of the beer amongst a number of small vessels greatly checks fermentation, and Indus care is taken to push it sufficiently far in the fermenting tun, there is probability of the process being incomplete, and of the beer consequently remaining too sweet. The rounds being completely filled with beer, the yeast formed rises through the openings in the heads, and is cooducted by spouts to troughs or backs in which it is collected. By thia process the beer is gradually freed from the particles of yeast and glutinous matter held in suspension, which if not removed would keep the beer turbid. It seems probable that the composition of the water used in brewing affects to some extent the process of cleansing, for when the water contains a considerable proportion of compounds of lime, a double decomposition is set up with the salts present in the malt, and consequently in the wort, with the result of the formation of a lime salt, which is precipitated, and carries down impurities with it.
Cleansing rounds were formerly made of wood, a common arrangement being that of casks placed on end in groups of four, with spouts leading from openings in the upper heads of the casks to a vertical spout carried down through the apace in the centre of the group. From these rounds the beer is pumped through an attemperator or refrigerator to a aettling tank, and thence into the vats. Cleansing squares constructed of slate are now largely used at the principal breweries. Slate appears to be the best material for the construction of cleansing squares, or for tanks for hold ing cold beer. The slates forming the divisions between the squares are best so connected that no metal is exposed inside. The squares should be arranged in groups, aod the slabs forming the divisions betweeo the squares jointed to that formiog the front of tbe group. The bolts securing the front slab may be screwed jot° nuts sunk in the division slabs, the hole containing the nut being filled in with putty. In some cases the slabs of slate squares are connected by angle-pieces, the heads and nuts of the bolts beiog tinned. The squares may be arranged in a series of double rows, baying spaces between, into which the yeast is discharged. The lips, or short spouts by which the yeast issuing from the openings in the tops of the cleansing squares is discharged into the yeast troughs, may be of copper tinned, of tinned wrought iron, of wrought iron enamelled, or of cast iron painted and varnished. Au objection has been raised to the employment of slate for
cleansing squares, that it is a too good conductor of heat, and that tbe beer is subject to con siderable atmospheric variations of temperature ; but this objection has no practical value.
The fermentation of porter aad stout at the Loudon breweries is generally completed in cleansing rounds, but in some cases it is commenced and finished in squares holding from 170 to 320 barrels ; these squares are usually fitted with attemperatore and with parachutes into which tbe yeast is skimmed. At the City of London Brewery, the fermentation of porter is commenced in tuna holding 600 barrels, and completed in cleaasiog rounds haviog a capacity of 5i barrels each, or in hogsheads arranged in a similar manner to the Burton unions.
For ale, it is usual to employ much smaller fermenting tuns than for porter, and frequeotly the fermentation is completed in these tuns. The tuns are sometimes fitted with small parachutes, by which the yeast can be removed and tbe fermentation checked. At Charriogton's, the ale, after making, is allowed to cleaose further, the casks being arranged on stillions or gutters, by which the yeast is received. The casks are filled up by hand. At Hoare's, the fermentation of the pale ales is completed in union casks, on the Burton system. At the City of London Brewery, the fermentation of ales is, in some instances, commenced in tuna of 140 barrels, and is completed in cleansing casks. At this brewery are copper fermenting tuns capable of holding 40 barrels each ; these tuns consisting of a copper vessel enclosed in an outer casing containing water. In these tuns the fermentation is commenced and completed, the temperature being regu lated by water circulating between the tuos and their casing.
The parachute consists of a kind of a copper funnel, having a stem which extends through the bottom of the tun. This stem is provided with a telescopic joiut. By a lifting arrangement, the height of the pamchute can be adjusted, so that its lip is slightly above the level of the beer in the tun ; as the yeast formed flows over into the parachute and down through the tubular stems, the cleansing proceeds in the same manner as in the cleansing rounds. Aa the cleansing proceeds, there is a cettain loss of liquid from the tun, aud the parachute has to be lowered from time to time to maintain its level. Parachutes are sometimes balanced to float on the surface of the beer, and descend automatically. Usually, the top of the parachute has only a small area in proportion to the surface exposed by the beer, and the yeast is then akimmtd into it. When fitted to ferment ing squares, parachutes are sometimes made rectangular, and placed so that they extend across the squares. By the aid of parachutes the yeast can be removed from the tuns in a much more cleanly and convenient manner than by skimming.