GB1580191A - Degassing liquids - Google Patents

Description

(54) DEGASSING LIQUIDS (71) We, BREWING PATENTS LIMITED, a British Company of 42 Portman Square, London WlH OBB, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the deliberate reduction of dissolved CO2 levels in fermentation liquids in which the gas is in a state of supersaturation.

The invention is particularly applicable to fermentation processes such as brewing. In particular the control of supersaturation in deep fermentors is contemplated. Although it is of more general applicability the invention is described herein with particular reference to brewing as a practical example.

In many fermentation processes the liquor becomes supersaturated with carbon dioxide (CO2) and it may be desirable to reduce the level of dissolved CO2 during and/or after fermentation. During the fermentation of wort by brewers yeast, for instance, it is common for the fermenting wort to contain dissolved CO2 in such an excess over the amount required for saturation that the liquid is in a highly unstable state. In this condition any mechanical disturbance transmitted to the fermentation is liable to cause sudden precipitation of a part of the excess gas in the form of a multitude of bubbles which may form so large a volume of foam that it cannot be contained within the fermenting vessel. If prior allowance is made for such an occurrence then the effective working capacity of a given vessel must be reduced to provide the extra space for the foam to expand into.

At the end of a fermentation it may be desirable to reduce the dissolved gas content in the fermented liquid. In a brewery it often happens that the CO2 content of the beer after fermentation is higher than that required in the product beer. This is most commonly a problem when the fermentation has been carried out in deep vessels such as the cylindroconical fermentors used in many modern breweries. It is well known that the levels of dissolved CO2 which build up in worts during fermentation in such vessels are usually higher than are encountered when similar worts are fermented in relatively shallow vessels under otherwise identical conditions.

Methods currently available for reducing the gas content of fermentation liquids are generally crude and on occasion may be actually dangerous in operation. Typically, the level of dissolved CO2 in a brewery fermentation may be reduced by bubbling a gas such as nitrogen into the wort via a gas diffuser made of a porous material such as sintered ceramic. Dissolved CO2 diffuses into the rising gas bubbles and in this way the CO2 is swept out of the liquid. However, this process is wasteful of gas because the bubbles released from the diffuser are relatively large compared to CO2 bubbles formed naturally and spontaneously within the fermenting liquid and therefore rise rapidly through the wort. Furthermore, substantial local turbulence and shock waves may be set up by the gas entering the fermentor, especially if the gas is blown in through an open pipe instead of a diffuser, and these disturbances may cause sudden and uncontrolled formation of CO2 bubbles in the bulk of the fermentation liquid. This is most likely to occur if the fermentation is vigorous and can be dangerous in a closed vessel where no provision has been made to deal safely with the consequences of such an event.

An improved method of releasing dissolved CO2 from a supersaturated solution was described in our patent Specification No. 1,557,156 (Application No.

14461/75). In the described method, CO2 release was nucleated in a controlled manner by introducing a liquid suspension of minute bubbles into the solution to be degassed. Typically, this liquid suspension would consist of nitrogen gas bubbles dispersed in water. Whilst this technique is an improvement over the prior art, it does require the introduction of one liquid into another and this may be considered a disadvantage in certain circumstances.

The present invention provides an alternative system whereby CO2 may be safely released from supersaturated fermentation liquids without the need for introduction of any liquid or gas which could dissolve into or otherwise contaminate the fermentation liquid. This is achieved by exposing the fermentation liquid to a multitude of renewable nucleation sites on a solid surface.

Now, it is known that in a brewery fermentation the initiation of gas bubble formation takes place mostly, if not entirely, on solid particles present in the fermenting wort. These particles are largely composed of precipitated protein, polyphenol and carbohydrate. Nucleation sites exist on the surface of such particles and each site is capable of initiating the growth of an indefinite number of CO2 bubbles in a supersaturated wort or beer. Each nucleation site has its own initiation threshold, by which is meant that for each site there is required a certain 'critical' degree of supersaturation of gas in the liquid before bubble growth can occur. Hence, during an essentially anaerobic fermentation of wort by yeast there is at any time a virtual dynamic equilibrium set up between the rate of excretion of CO2 by the yeast into the wort and the rate at which this gas is released from solution through growth of CO2 bubbles initiated by the nucleation sites on the solid particles. Under given fermentation conditions a certain degree of supersaturation will therefore occur depending mainly on the temperature and pressure. It will be appreciated that an increase in the number of active nucleation sites will cause a more effective release of dissolved CO2 resulting in lesser degrees of supersaturation through the course of the fermentation.

The present invention accordingly provides a method of reducing the degree of supersaturation of carbon dioxide in a fermentation liquid which comprises bringing the supersaturated liquid into contact with at least one solid surface having a multitude of renewable nucleation sites and being a surface of, a substrate coated with a sprayed coating of an halogenated ethylene polymer, foamed polystyrene, acetone etched solid polystyrene, metal sheet sprayed with red oxide primer paint or aluminium paint, porour polyvinyl chloride sheet or Keselguhr, graphite powder or polyvinylchloride cenospheres applied to a supporting matrix.

The invention particularly provides a method of controllably reducing the degree of supersaturation of carbon dioxide in a fermentation liquid held in a container which comprises bringing a solid body having a said surface into contact with the supersaturated liquid in the container and reducing the degree of supersaturation to a desired level by altering the position of the body within the container.

It is known that rough surfaces such as etched glass can act as sources of nucleation sites and thus contribute to the release of gases from supersaturated solution. However, such surfaces do not always produce renewable sites and also their overall activity is low. It is an important feature of this invention that we have found that certain surfaces, especially rough surfaces, coated with sprayed polytetrafluoroethylene (p.t.f.e.) possess a multitude of renewable nucleation sites.

It is not, in practice, possible to determine the precise number of nucleation sites on a surface, especially where that surface is as active as those used in the invention, but we have found that a convenient practical measure is the volume of gas given off per unit area under comparable conditions. The surfaces used in the invention, especially p.t.f.e. coated surfaces have activities, measured on a volume basis, many times and often many orders of magnitude greater than merely roughened surfaces.

We have found that not all p.t.f.e. surfaces are good nucleajing surfaces.

Moulded or extruded p.t.f.e. generally has a very poor nucleating surface.

Substrates coated, by spraying, with p.t.f.e. are much more active nucleating surfaces. Aerosol sprayed surfaces are particularly active.

As yet we have not tried other materials similar to p.t.f.e. but believe that equivalent surfaces of other highly halogenated ethylene polymers would also work. A body particularly suitable for use in the method of the invention comprises a substrate and on the surface thereof a sprayed coating of an halogenated ethylene polymer preferably polytetrafluoroethylene. Preferably the surface of the substrate is roughened e.g. by abrasion with emery paper or a file and conveniently at least part of the surface comprises a gauze or net e.g. expanded metal. The nature of the substrate is not believed to be critical and is conveniently of metal although other materials can be used if desired. To avoid the necessity of forcibly immersing the body it preferably has an overall density greater than that of the liquid to be treated.

The nature of nucleating surfaces is complex and we have found that activity in degassing liquids is at least in part a function of the size of bubbles produced on the surface. As mentioned above not all p.t.f.e. surfaces are highly active nucleating surfaces, this being in part a function of bubble size. Moulded and extruded surfaces tend to have relatively few nucleating sites. Sprayed, especially aerosol sprayed surfaces, are active producing small bubbles which are released from the surface. One disadvantage of such surfaces is that they do not adhere to metal substrates very readily. Adhesion to the substrate can be improved by heating to 'bake' the coating onto the substrate. However, we have found that baking to high enough temperatures to substantially improve adhesion (up to about 450"C) causes a reduction in gas release activity because the bubbles grow to larger sizes before being released. Much to our surprise we have found that baking at much higher temperatures (up to about 500"C i.e. close to the melting point of the aluminium alloy substrate actually used,) restores the property of producing small bubbles. The activity of the surface is somewhat reduced at relatively low degrees of supersaturation but is comparable with non-baked surfaces at relatively high degrees of supersaturation. One convenient way we have found to make the surface is to high-temperature bake a sprayed coating of p.t.f.e. onto the substrate and then re-spray with aerosol p.t.f.e. to give a further coating. The reasons for these changes in the activity of p.t.f.e. are not clear.

Other surfaces which are highly active and can be used in the invention are: foamed polystyrene, solid polystyrene surfaces briefly etched with acetone, metal, e.g. aluminium, sheet sprayed with red oxide primer paint, metal e.g. aluminium, sheet sprayed with aluminium paint, and porous polyvinyl chloride sheet; also, certain particulate materials when applied to a supporting matrix by, for instance, applying the particles in the form of powders to a painted metal surface whilst the paint is still tacky. Such materials include certain grades of Keselguhr (diatomaceous earth), graphite and polyvinyl chloride cenospheres. We cannot adequately explain why these surfaces (including p.t.f.e.) work and have found no basis for predicting whether or not other surfaces will or will not work before trying them.

The rate of release of gas from a supersaturated liquid e.g. CO2 from wort during brewing, depends primarily on two factors: the degree of supersaturation and the nature and condition of the surface. Considering first the degree of supersaturation.

At higher levels of supersaturation, both the number of actively nucleating sites is greater and the rate of growth of individual bubbles is increased. Thus, if a suitable area of nucleating surface is placed in e.g. a fermenting wort, it will have the effect of initially reducing the degree of supersaturation of CO2 and then maintaining the dynamic equilibrium value at any subsequent time at a lower level than would otherwise occur. The maintenance of this lower degree of supersaturation will be to some extent self-regulating because changes in the rate of CO2 formation by the yeast will be reflected in changes in the rate of gas release from the nucleating surface. If decarbonation is carried out on a beer after fermentation has ceased then the level of dissolved CO2 will fall until it approaches the equilibrium value determined primarily by the temperature and pressure at which the beer is held.

The nature of the surface possessing nucleating sites is a more complex matter and is critical to the invention. The desirable characteristics of a surface which would make it suitable for use in the present invention are that it should contain a large number of nucleating sites per unit area, that the critical degree of supersaturation for the nucleation sites should, at least on average, be as low as possible and that preferably the nucleation sites should not be susceptible to deactivation by contact with liquid which is not supersaturated with gas. The phenomenon of deactivation is shown by a dry surface which nucleates effectively when introduced into, for example a carbonated beverage, but when it is first wetted with a liquid which is not supersaturated with a gas it ceases to nucleate effectively. The original potential for nucleation will usually be found to return, however, if the surface is dried and then reintroduced into the supersaturated liquid. The susceptibility of a particular surface to temporary inactivation in this manner is a function of the specific nature of that surface and of the actual conditions employed in testing it. No general rule governing the liability to reversible inactivation is known that can be applied to predict the behaviour of a previously untested surface. As has been intimated above the tendency to reversible deactivation can be overcome by ensuring that the surface is dry and by avoiding, as far as possible, contact with liquid that is not supersaturated. The surfaces used in the invention are preferably either not subject to deactivation or can readily be reactivated by drying. Sprayed p.t.f.e. surfaces are deactivated to some extent by wetting with non-supersaturated liquid but can readily be reactivated by drying. Further, sprayed p.t.f.e., particularly is sprayed onto already rough surfaces. has a very large density of nucleation sites of relatively low critical degree of supersaturation especially for supersaturated solutions of CO2 in aqueous alcohol (wort). In practice, the nucleation surface is typically not introduced into the wort until the wort is supersaturated with respect to CO2 and to ensure maximum activity the surface is dried before it is introduced into the wort.

Contact between a supersaturated solution of CO2 in wort and a nucleating surface will initially result in the growth of a multitude of bubbles on those nucleation sites whose critical nucleation threshold is exceeded by the degree of supersaturation. As the bubbles grow they will tend to detach themselves from the surface, consequently re-exposing the nucleation sites so that further gas release can occur. As briefly mentioned above the size to which bubbles grow before being released is a significant factor. If the bubbles are small they do not significantly obstruct the surface. However, if the bubbles grow to be relatively large they obstruct the surface, thus reducing the effective number of nucleation sites. The surfaces used in this invention can be operated to produce satisfactorily small bubbles. For each nucleation site, bubble formation will continue until such time as the degree of supersaturation of the CO2 in the fermentation liquid falls below the critical threshold value for that particular site at which time bubble formation on that site will cease.

The procedure for reduction of CO2 level in a fermentation liquid is basically simple and effective. It is usually sufficient to lower the nucleating surface, made up in a convenient and compact form, into the supersaturated liquid from above whereupon decarbonation takes place. The overall rate of gas release is controllable by raising or lowering the nucleating surface within the body of the liquid. For any particular set of conditions within the liquid there will be an optimum depth at which the rate of gas released at the surface of the liquid will be at a maximum. The optimum will occur on lowering the nucleating surface because of the balance between two factors. As the depth from the liquid surface is increased so the rising gas bubbles have a greater opportunity to grow before reaching the surface thus releasing more of the dissolved gas; however, at the greater depth the number of nucleation sites that are actually seeding the formation of bubbles will diminish because of the increased hydrostatic pressure which reduces the driving force for gas release by lowering the degree of supersaturation.

This will generally be the case excepting in possible occasional instances where circumstances cause the dissolved CO2 to increase considerably at progressively greater depth.

To secure rapid and complete fermentation it is often desirable to ensure that effective circulation of fermenting liquid takes place in the containing vessel in order to discourage yeast particles from sedimenting to the bottom of the vessel before the intended degree of fermentation has been completed. According to one aspect of the present invention the introduction of a suitable nucleating surface will help achieve the desired circulation, at the same time as reducing the CO2 supersaturation, by creating a rising cloud of bubbles which induce a corresponding motion in the bulk of the liquid. This encourages a faster fermentation. It may, thus, be beneficial to enhance release of excess dissolved CO2 by introducing nucleation sites on a solid surface at an early stage during fermentation.

In a further aspect of this invention the nucleating surface forms an integral and immobile part of the fermenting vessel. This surface may, for example, be in the form of a coating bonded to the inside surface of the vessel. In yet another alternative form of the invention the nucleating surface is constructed in the form of a unit which is built into the interior of the fermenting vessel and which would not normally be removed between or during fermentations. Naturally, the provision of a nucleating surface which is essentially an integral part of the fermenting vessel necessitates the use of an effective nucleating material which retains a substantial proportion of its ability to induce bubble formation after immersion for several hours in a liquid such as a wort which is not saturated with fermentation gasses. In this version of the invention it will be understood that decarbonation will be promoted throughout the time that the fermentation liquid remains supersaturated with dissolved gas.

At the end of fermentation it is sometimes the case in a brewery fermentation that the CO2 level is undesirably high. This is most often the case when the fermentation is carried out in deep fermentors where the absolute pressure at the base of the fermentor may be as high as 3 atmospheres. At this pressure, the CO2 derived from the fermentation is not so readily flushed away during the fermentation. If the beer is then transferred and, for instance, filtered into a bright beer tank it is not unusual for the CO2 content to remain too high for the product to be racked into keg or cask at the correct level of carbonation. In another aspect of the present invention, treatment of the beer at any stage after primary fermentation to reduce CO2 levels is possible by the method of the invention.

It is typically the case during ale fermentations that a substantial proportion of the yeast rises to the surface of the fermenting wort where it forms a continuous and semi-stable layer, commonly referred to as a yeast head, which is primarily an admixture of CO2 bubbles and yeast particles but also contains other substances. In many brewing systems, particularly those which employ shallow, flat-bottomed fermenting vessels, the yeast head is collected by one or other of a number of processes known collectively within the trade as 'skimming'. Collection of the yeast head may be performed more than once during the course of a fermentation and it is normally an important part of the brewing process in these systems because a proportion of the skimmed yeast is used for pitching subsequent brews of wort.

However, it can happen that, for reasons which may or may not be apparent to the brewer, the yeast head is not formed satisfactorily. In such circumstances it is of value to the brewer to be able to encourage the proper formation of the yeast head.

It is well known that when yeast particles rise to the surface of a fermenting wort it is because of an association which is formed between rising gas bubbles and yeast cells or agglomerates. Hence, an increase in the rate of bubble formation will encourage the carriage of yeast to the surface of the wort. In another aspect of this invention the enhancement of yeast head formation is achieved by the introduction of a nucleating surface in suitable form into the fermenting wort whereupon a consequently increased rate of bubble formation will aid the transportation of yeast cells to the yeast head.

The method of reducing the gas content of fermentation liquids, described in this invention, is if correctly applied, safe, simple and inexpensive to operate. It is expected that the invention will be of especial benefit to reduce or control the dissolved CO2 content of fermenting worts and beers.

The invention will be described further with reference to the accompanying drawing which shows a plan view of a preferred embodiment of a decarbonation unit for use in the invention. The decarbonation unit comprises sheets 1 of a suitable material e.g. aluminium or other metal, preferably roughened e.g. by abrading with silicon carbide paper, and spray coated with p.t.f.e. These flat sheets alternate with folded sheets of gauze or net e.g. expanded or perforated metal 2 also spray coated with p.t.f.e. The component sheets of the unit are held together by suitable means e.g. binding wire, clamps, a holding frame, bolts, etc. or are welded or glued together to provide a unitary construction. The particular means used to hold the unit together is not believed to be critical to the operation of the unit provided it does not obstruct the active surface to any great extent.

The unit can be used to control the degree of supersaturation of fermentation liquids as described in general above. The use of units of this general construction is specifically illustrated in Examples I and 3 below.

The invention is illustrated by the following Examples.

Example I A nucleating surface decarbonation body generally as illustrated in the accompanying Drawing was constructed as follows. Seven 3" squares of aluminium sheet were cut from a large sheet of 1/32" thick metal. The surfaces of these squares were roughened by abrading with silicon carbide paper. The sheets were then cleaned, dried and sprayed with a quick-drying p.t.f.e. dry lubricating agent (Fluorplast 82, manufactured by Pampous Fluorplast Ltd., Stoke-on-Trent, England). Several coats were applied to build up a dry stable film on each metal surface.

Six rectangular pieces of expanded aluminium mesh (cut from 24 gauge metal sheet, with holes approximately 1/16" by 3/16", inclined at an angle to the plane of sheet; obtained from Expanded Metal Ltd, London, England) were cut from a larger sheet. These pieces were then corrugated by successive bending in opposite directions to an obtuse angle at intervals of about 1/2" about axes parallel to the shorter side of each piece so that the overall dimensions of the pieces were altered to 3" by 3" by approximately 1/4" (the depth of the corrugations). The corrugated metal mesh sheets so formed were then coated with a p.t.f.e. film in a similar manner to that described above in the case of the plain sheets.

The two types of coated sheets were then stacked together in pairs to form a block comprised of alternating plain and corrugated sheets flanked at each end by plain sheets. The block so formed, measuring (approximately) 3"x3"x1.3/4", was then bound together with wire to hold the sheets in place.

The p.t.f.e. coated surfaces of this block were found to possess a high density of potential sites for the nucleation of gas bubbles. Put together in the way described, the treated metal sheets formed an efficient unit suitable for promoting gas release in a supersaturated solution such as, for instance, a normal fermenting wort containing dissolved CO2 in excess of that which would be dissolved if the liquid was in true equilibrium with the gas. Furthermore, it was found that the p.t.f.e. coated surfaces could be effectively sterilized with wet steam at 1000C provided that they were dried before use. No loss of nucleating activity occurred.

The effectiveness of the decarbonation unit was demonstrated in a fermentation, thus: 36 gallons of an ale wort, at an original gravity of 1.060, was pitched at 16"C with top-fermenting ale yeast in a flat-bottomed cylindrical stainless steel vessel of internal diameter I'll". The wort depth was 2'. After 18.5 hours the specific gravity of the fermenting wort had fallen to 1.051 and the temperature had risen to 190C. The level of dissolved CO2 was measured and found to be 1.7 volumes (at S.T.P.). An hour later, the decarbonation unit, with its component sheets aligned vertically to allow free passage of gas and liquid up the channels formed by the corrugations, was lowered by stages into the wort to a depth of approximately 1' over a period of 12 minutes. Considerable evolution of CO2 was noted and when the dissolved CO2 level was next measured (at 20.5 hours from pitching) it was found to have fallen to 1.2 volumes. By the time the gravity had fallen to 1.029 (at 42.5 hours), the CO2 content of the wort was discovered to have declined still further to 0.9 volumes. Subsequent measurements showed that the dissolved CO2 concentration never rose above this value during the remainder of the fermentation indicating that the decarbonation block was functioning effectively to the end of the fermentation.

Example 2 The activity of a number of different types of surface were compared by immersing 1/2" diameter rods 2" long into bottled beer for a period for 30 seconds.

The weight loss of fob (foam) expelled from the bottle was measured. The weight loss is an approximate measure of the nucleating activity of the surface. The results are given in the Table below.

Average Weight Material Loss (g) p.t.f.e. coated roughened aluminium 48 p.t.f.e. coated roughened stainless steel 21 roughened aluminium 4 roughened stainless steel 5 p.t.f.e. rod 3 Example 3 A full scale decarbonation unit was constructed on the same principle as that described in Example 1. This unit comprised 26 rectangular sheets of plain abraded aluminium, 1/32" thick, and an equal number of similar sized sheets of expanded folded aluminium mesh (as in Example 1) each sheet measuring 12"x4" and being spray-coated with p.t.f.e. These two types of sheets were stacked in pairs in a rectangular frame, also made of aluminium sheet, so that they were arranged in an alternating sequence of plain and folded mesh sheets and were held vertically on their shorter axis. This construction had overall measurements of 14"x 12"x4" high and weighed about 10 Ib.

The decarbonation unit so formed was suspended from stout nylon cord with adjustable floats arranged on this line so that the whole unit could be suspended at any chosen depth below the surface of a wort in a fermentor with the free end of the line tied off to a suitable fixing point above the wort surface.

The effectiveness of the unit was tested by lowering it into an actively fermenting wort (1040 barrels) in a conical bottomed cylindrical fermenting vessel.

The wort, of original specific gravity 1.037, was at a temperature of about 25"C and had been fermenting for 36 hours during which time the specific gravity had fallen to 1.020. Silicone based antifoam had been added earlier to prevent excessive foam formation; consequently, there was a yeast head of only about 3" depth on the surface of the wort.

As the decarbonation unit was lowered into the wort, vigorous release of dissolved CO2 occurred as the wort was drawn through the unit. This gas release was so rapid that it was not possible to lower the unit more than about one foot below the wort surface for several minutes. As the gas release rate declined the unit was gradually lowered into the wort until it was suspended by the floats at a depth of 15 feet and roughly on the centre axis of the vessel so that the gas release would encourage the natural circulation pattern which is known to occur in conicalbottomed fermentors. At this depth the unit was at about the half-volume level of the wort in the fermentor. During these first few minutes of immersion the yeast and foam head rose about 2 feet but soon fell back until after half an hour the fob depth was estimated at about 4".

The quantity of CO2 released by the unit was determine

Claims (5)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   The effectiveness of the unit was tested by lowering it into an actively fermenting wort (1040 barrels) in a conical bottomed cylindrical fermenting vessel.

   The wort, of original specific gravity 1.037, was at a temperature of about 25"C and had been fermenting for 36 hours during which time the specific gravity had fallen to 1.020. Silicone based antifoam had been added earlier to prevent excessive foam formation; consequently, there was a yeast head of only about 3" depth on the surface of the wort.

   As the decarbonation unit was lowered into the wort, vigorous release of dissolved CO2 occurred as the wort was drawn through the unit. This gas release was so rapid that it was not possible to lower the unit more than about one foot below the wort surface for several minutes. As the gas release rate declined the unit was gradually lowered into the wort until it was suspended by the floats at a depth of 15 feet and roughly on the centre axis of the vessel so that the gas release would encourage the natural circulation pattern which is known to occur in conicalbottomed fermentors. At this depth the unit was at about the half-volume level of the wort in the fermentor. During these first few minutes of immersion the yeast and foam head rose about 2 feet but soon fell back until after half an hour the fob depth was estimated at about 4".

   The quantity of CO2 released by the unit was determined from wort samples taken via a small cock situated on the side of the vessel a few feet above the conical section. Just prior to immersion of the unit the dissolved CO2 level in the wort was measured as 2.0 volumes; one hour later the CO2 concentration had fallen to 1.65 volumes. The CO2 level fell slowly after this until the end of the fermentation (S.G.

   1008), about 64 hours after pitching, when the measured value was 1.4 volumes, this being about 0.3 volumes lower than that normally found in 'control' Fermentations.

   During the whole period of immersion it was clear from visual observation that the unit was stimulating gas release since a turbulent region was apparent at the wort surface above the unit and the unit was noted to be wandering about on the end of the cord, this presumably reflecting small changes in the general circulation pattern caused by the gas release and the effect that this had on the position of the floats which were free to move to a limited extent.

   WHAT WE CLAIM IS: 1. A method of reducing the degree of supersaturation of carbon dioxide in a fermentation liquid which comprises bringing the supersaturated liquid into contact with at least one solid surface having a multitude of renewable nucleation sites and being a surface of a substrate coated with a sprayed coating of an halogenated ethylene polymer, foamed polystyrene, acetone etched solid polystyrene, metal sheet sprayed with red oxide primer paint or aluminium paint, porous polyvinyl chloride sheet or Kieselguhr, graphite powder or polyvinyl chloride cenospheres applied to a supporting matrix.
2. 2. A method as claimed in claim I for controllably reducing the degree of supersaturation of the carbon dioxide in the fermentation liquid wherein the liquid is held in a container which comprises bringing a solid body, having a said surface thereon, into contact with the supersaturated liquid in the container and reducing the degree of supersaturation to a desired level by altering the position of the body within the container.
3. 3. A method as claimed in either claim 1 or claim 2 wherein the fermentation liquid is wort, beer or ale.
4. 4. A method as claimed in any one of claims 1 to 3 wherein the halogenated ethylene polymer is polytetrafluoroethylene.
5. 5. A method as claimed in any one of claims I to 4 and substantially as hereinbefore described in any one of the Examples or with reference to or as illustrated in the accompanying drawing.