BACKGROUND OF THE INVENTION The buildup of carbon dioxide (CO2) formed during brewing and other alcoholic fermentations is associated with a number of disadvantages. The CO2 generated during conventional fermentations first saturates the fermenting medium and then forms gas bubbles, which rise to the headspace of the fermenter, causing the formation of foam.

Foam production is undesirable for a several reasons.

Foam formed during lager or ale fermentations may occupy as much as one third of the fermenter volume. If insufficient headspace is allowed, overfoaming of the fermenter can occur, resulting in product loss, environmental problems, increased BOD load, sanitation problems, loss of collectable CO2, and increased manpower requirements.

The large headspace required to avoid overfoaming severely limits fermenter capacity utilization. Maximizing fermenter capacity is an important goal within the brewing industry, because increased fermenter capacity reduces the cost of production. Brewers have attempted to increase production capacity by increasing the gravity of the wort or increasing fermentation temperatures. Unfortunately, both of these modifications cause increased foam formation;

the increased headspace required to avoid overfoaming partially offsets the potential capacity increase.

Several methods to reduce foaming have been investigated or employed by brewers. One method that has been used is adding anti-foaming agents to the beer during fermentation to reduce foaming. Such agents, which are often based on poly-dimethyl-siloxane, are not always acceptable to brewers, because the agents may be considered additives. Another foam reduction method employs a mechanical foam breaker such as a cyclone to reduce foaming. Ultrasonic foam breakers placed in the top of fermenters have also been used to reduce foaming.

Although the aforementioned methods reduce foam, some foam remains. In general, these methods destroy foam after it has formed, which results in the loss of certain desirable hydrophobic substances that tend to concentrate in the foam, including hop bitter acids and high molecular weight proteins responsible for a good head in the final product. After foam collapse, these substances may precipitate and not go back into solution. Preventing the formation of. foam will prevent the loss of these components.

Foam is formed after CO2 bubbles that are generated in the fermenting medium rise to the top and become surrounded by a thin liquid film containing surface-active materials.

Carbon dioxide bubbles are formed at nucleation centers in the fermenting medium at points where the dissolved CO2 concentration exceeds the local equilibrium partial pressure.

Another problem associated with the buildup of CO2 during fermentation is that high concentrations of CO2 alter or inhibit yeast metabolism. For example, high levels of CO2 found in fermenting wort cause a reduction in yeast production of esters, important components in the flavor and aroma of beer. Absent a means for regulating

the CO2, the concentration of CO2 in fermenting medium varies greatly according to the fermenter size and geometry. Therefore, beer flavor profiles are also affected by the type of fermenter used.

In conventional brewing methods, agitation of the fermentation medium is provided by the evolution of CO2 bubbles. Agitation is important in the brewing process, because the level of agitation is directly related to the fermentation rate. The level of agitation depends on formation of CO2 gas bubbles; therefore, it follows that systems in which CO2 bubbling is not regulated do not permit control of agitation. Because the rate of bubble formation is affected by fermenter geometry and size, agitation and fermentation rates in conventional brewing systems will vary from fermenter to fermenter. This makes it difficult to achieve consistency between fermentations conducted in fermenters of different sizes or geometries.

Carbon dioxide produced during fermentation may be collected and used in other commercially important applications, such as carbonating beverages. In conventional-methods, CO2 is used to sweep out the headspace to eliminate unwanted oxygen prior to collecting CO2. This results in a loss of about 10% of collectable CO2.

What is needed in the art is a method of removing C02 from fermentation media during fermentation so as to increase capacity, provide greater flexibility by the ability to control agitation independent of bubble formation, produce more consistent flavor profiles, and recover more high quality CO2. As shown in the examples, fermentations in which nonaerated wort was pitched with yeast oxygenated by the method of the invention

BRIEF SUMMARY OF THE INVENTION The present invention provides a method for removing CO2 from a fermenting liquid medium, comprising the steps of: transferring at least a portion of the fermenting medium through a membrane contactor, the contactor comprising at least one polymeric membrane, the membrane having a liquid side and a gas side, wherein at least a portion of the medium is in proximity to the liquid side of the membrane. under conditions that allow CO2 in the medium to transfer from the liquid side of the membrane to the gas side of the membrane.

It is an object of the present invention to provide a method of removing CO2 from fermenting medium.

Another object of the invention is to prevent the formation of foam during fermentation.

It is a further object of the invention to remove CO2 ~ formed during fermentation to permit greater flexibility in developing beers with different flavor profiles.

It is an advantage that the invention provides a method that will allow increased capacity utilization.

It is a further advantage that the method of the invention essentially eliminates foam formation, thereby preventing the loss of desired beer components.

An additional advantage of the method is that it allows the possibility of controlling CO2 levels and.. agitation independently.

Another advantage of the invention is that a greater percentage of collectable CO2 produced during fermentation may be collected as high quality CO2.

The method of the invention is advantageous in that it provides a method of controlling the amount of COs present during fermentation, rendering the beer flavor profiles independent of fermenter size or geometry.

Other objects, features and advantages of the present invention will become apparent upon review of the

specification.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS Fig. 1 shows a circulating system for removing CO2 from fermenting liquid medium employing a polymeric membrane contactor.

DETAILED DESCRIPTION OF THE INVENTION The buildup of CO2 during wort fermentation reduces capacity utilization; causes foaming, which reduces the concentration of desired components in beer; and affects beer flavor profiles in a way that imposes constraints on the size and geometry of fermenters that can be used for brewing certain types of beer.

The present invention includes a method of removing CO2 from a fermenting liquid medium so as to prevent foam- formation, increase capacity utilization, reduce the loss of desired components during fermentation, decouple agitation rate from the rate of CO2 formation, permit control of CO2 levels, allow recovery of higher quality C02, allow greater recovery of recoverable CO2, and allow greater flexibility in brewing by controlling CO2 levels regardless of fermenter size or geometry.

By a fermenting liquid medium, it is meant a liquid medium comprising a fermentable carbon source and a.. microorganism that ferments the carbon source, producing C02 as a byproduct. Preferably, the microorganism is yeast.

In the method of the invention, CO2 is removed from a fermenting liquid medium by passing a portion of the medium through a membrane contactor comprising at least one polymeric membrane comprising a liquid side and a gas side, such that the transferred medium is in proximity with the liquid side of the membrane and CO2 is transferred from the medium to gas side of the membrane.

Preferably, the partial pressure of CO2 is maintained at a lower level on the gas side of the membrane than the CO2 partial pressure in the head space of the fermenter.

This may be done by application of a vacuum.

Alternatively, CO2 may be removed by flushing with a suitable gas, such as nitrogen.

As described in the examples and as shown in Fig. 1, the CO2 may be removed from fermenting liquid medium according to,the method of the invention by recirculating a portion of the fermenting medium through a CO2 removal system. Removal of CO2 from fermenting liquid medium in a fermenter 10 is accomplished by transferring a portion of the medium from the fermenter 10 to a membrane contactor 20, the contactor 20 comprising a microporous hydrophobic membrane or a nonporous hydrophobic or hydrophilic membrane 30, the membrane having a liquid side 40 and a gas side 50.

Carbon dioxide in the medium passes from the liquid side 40 of the membrane 40 to the gas side 50 of the membrane 30.

Carbon dioxide removal from the gas side 50 of the membrane 30 is facilitated by maintaining the partial pressure of CO2 on the ga-s side lower than the CO2 partial pressure of the head space in the fermenter. This may be accomplished by the application of a vacuum from a vacuum source or by flushing with a gas supplied by a gas source. Optionally, CO2 stripped from the membrane may be collected in a C'02 collector. Following CO2 removal in the membrane contactor 20, the medium is returned to the fermenter 10.

Preferably, recirculation of the medium is conducted in a continuous process. In an alternative embodiment, the membrane contactor may be housed within the fermentation tank.

As described in the examples, CO2 was removed from wort using a polymeric hollow fiber membrane across which gas may be exchanged. A membrane provides a high surface

area contact with the liquid wort. A high ratio of membrane surface area to wort volume provides a high specific area for mass transfer.

As described in the examples below, the method of the invention was evaluated using a hydrophobic membrane of the type used as a blood oxygenator (Model Max-FTE, Medtronic, Minneapolis, MN). A mass transfer devise of the type used in the examples is described in detail in U. S. Patent 4, 975,247, which is incorporated by reference.

In the examples, the membrane contactor was equipped with a plurality of microporous, hydrophobic hollow fiber membranes made from polypropylene, with a pore size of about 0.03 um and a porosity of about 40%.

Examples of suitable hydrophobic membranes include, but are not limited to polysulfone, polytetra- fluoroethylene (PTFE or Teflon), polyvinylidenefluoride, polyvinylidenechloride, and polyethylene. A nonporous hydrophobic or hydrophilic membrane may also be used in the present invention. With nonporous membranes, the rate of transfer of CO2 from the liquid to the gas side will depend on the rate of diffusion through the nonporous membrane. However, it is reasonably expected that acceptable rates of CO2 removal may be obtained using nonporous membranes.

One skilled in the art will appreciate that in-., addition to the membrane contactor disclosed in the examples, there are other membrane configurations that are also suitable for use in the invention. It is expected that spiral wound, vibrating, rotary and tubular hollow fiber membranes may also be used in the practice of the invention.'The membrane contactor used in the examples is suitable for relatively small fermentation volumes. One of skill in the art would appreciate that for relatively large fermentations, one would wish to use a larger contactor.

Optionally, volatile compounds from the CO2 gas stream may be collected and used in other applications. For example, the CO2 present in the gas stream could be trapped using a cold trap placed between the membrane contactor and the vacuum source. Alternatively, CO2 could be absorbed using a suitable liquid or selectively adsorbed by solid adsorbents. Preferably, the oxygen concentration in the CO2 recovered from the fermentation is less than about 50 ppm. Still more preferably, the oxygen concentration in the CO2 recovered from the fermentation is less than about 25 ppm or even as low as 5 ppm or less.

The method of the invention allows relatively high recovery of recoverably CO2 generated during fermentation because it eliminates the loss of CO2 that occur during conventional venting. Preferably, 95% or even as much as 99% or greater recovery of recoverable CO2 is achieved by- this method. The initial concentration of oxygen is relatively low because the fermenter is filled to a greater capacity than is possible in conventional methods.

To reduce contamination of CO2 with oxygen, CO2 removal is preferably begun after yeast in the fermenter have taken up essentially all of the oxygen in the medium. In general, oxygen uptake is essentially completed from about 0 to 6 hours after pitching, depending on a variety of factors, including pitching rate.

In-order to afford greater consistency between fermentations or to achieve certain flavor profiles, one may wish to monitor and control removal of CO2 over the course of the fermentation. Carbon dioxide may be monitored by sampling the fermentation medium and measuring its CO2 content. Levels of CO2 may be controlled most suitably by controlling the partial pressure of CO2 on the gas side of the membrane or by adjusting the flow rate of the liquid medium. Partial pressure of CO2 on the gas side may be controlled by application of a vacuum, or by

flushing with a suitable gas, such as nitrogen.

By the method of the invention, the capacity utilization is increased dramatically relative to that achievable using conventional venting. By"conventional venting"is meant releasing CO2 from the headspace to the atmosphere or to a CO2 collection system. It is reasonably expected that tanks may be filled to 80%, 90%, 95%, or even as high as 99% or greater capacity without overfoaming. The method of the invention is expected to allow high capacity utilization, even when the specific gravity of the medium or the temperature of fermentation is increased.

Conventional fermentations depend on the generation of CO2 to achieve agitation. The rate of fermentation is directly related to the degree of agitation. However, high concentrations of CO2 interfere with yeast metabolism.- The method of the invention decouples CO2 generation and agitation rates.

In the examples, agitation is achieved by recirculating the fermentation medium between the fermenter and the membrane contactor. However, one could further increase agitation by including a mechanical agitator, such as a motorized impeller.

In the examples, the medium was recirculated between the contactor and fermenter at a rate of 800 ml/min. The '- residence time of the wort in the contactor was about 9 seconds. The average velocity was about 41 cm/min in the fiber bed, and the superficial velocity (flow rate/cross sectional area of the fiber bed) was about 13.7 cm/min.

One of ordinary skill in the art will appreciate that one may vary these parameters and still achieve acceptable carbon dioxide removal, provided that the KLa value is high. Preferably, the KLa value is in the range of from about 0.05 sec-1 to about 0.4 sec'1.

Using the method of the invention, foam formation

during fermentation is essentially eliminated. However, we expect that the method of the invention may be useful in applications in which foam formation is reduced but not eliminated. Therefore the present invention is not intended to be limited to conditions in which foam formation is eliminated.

Because hop acids and beer foam proteins are lost when beer foam formed during fermentation collapses, it is reasonably expected that a beer made by the process of the invention will retain higher concentrations of hop acids or beer foam proteins than a control beer made by the same process without removing CO2 formed during fermentation.

In the examples described below, the flow path of the liquid medium within the a hollow membrane fiber was in a dirction substantially perpendicular to the fiber axis (cross flow). It is expected that parallel and counter current flows may also be successfully employed. Although a method employing cross flow is likely to give better results, parallel flow is also expected to be suitable for use in the method of the invention.

The following nonlimiting examples are intended to be purely illustrative.

Materials For the CO2 removal experiments described below, fermentations were conducted in volumes of 2 and 20 liters using wort having an original gravity of about 15 degrees Plato pitched at a rate of about 0.8 g yeast dry weight/liter wort.

The membrane contacter employed in the CO2 removal studies (Model MAX-FTE, Medtronic, Minneapolis, MN) was equipped with a standard polypropolyene microporous membrane having a surface area of about 2.4 m2 and a liquid hold-up volume of about 120 ml.

Bubble-free CO2 removal during fermentation The method by which C02 was removed from fermenting liquid medium is shown schematically in Figure 1. Pitched wort was contained in a 26 L stainless steel tank equipped with a magnetic stir bar for continuous mixing and a temperature control system for maintaining the fermenting wort at a predetermined temperature. The yeast was allowed assimilate oxygen for a period of 3 hours after pitching. A membrane contactor that had been purged with ambient pressure CO2 to remove essentially all traces of atmospheric oxygen was then connected to the fermenter. A portion of the fermenting wort was pumped to the membrane contactor at a flow rate of about 800 ml/min. The residence time of the wort in the contactor was about 9 seconds. The average velocity of was about 41 cm/min in the fiber bed, and the superficial velocity (flow- rate/cross sectional area of the fiber bed) was about 13.7 cm/min. Carbon dioxide present in the wort was transferred from the liquid side to the gas side of the membrane contactor, which was connected to a regulated vacuum source.

Periodically, liquid samples from the fermenter were collected to monitor cell growth, extract consumption, and pH.

Foaming potential was visualized using the above- described membrane and vacuum set up using a glass tube (50 mm ID x 2 m long) as the fermenter in place of the stainless steel tank. Essentially no foam was generated in a system comprising the membrane contactor coupled with vacuum application, relative to a control fermentation in which CO2 removal was accomplished by conventional venting, or a control fermentation undergoing conventional venting and recirculation at the same rate as the test. The liquid recirculation rates used in the fermentations were 800 mL/min regardless of fermentation volume (2 L or 20 L of

fermenting wort). Vacuum levels of 0.5 inches (14.45 psia) to 28 inches (0.9 psia) of mercury were employed. No foam was generated during fermentations employing the contactor membrane system, even using a vacuum level as low as 0.5 inches of mercury.

The present invention is not limited to the exemplified embodiment, but is intended to encompass all such modifications and variations as come within the scope of the following claims.