METHOD AND APPARATUS FOR CONTROLLING GROWTH OF AN ORGANISM IN A NUTRIENT MEDIUM

Abstract: A method and apparatus for controlling growth of an organism in a nutrient medium comprising adding to the nutrient medium one or more gaseous materials which are consumed by the organism, or which do not affect the organism; monitoring the one or more gaseous materials, or monitoring one or more gaseous materials produced by the organism; and controlling the addition of the one or more gaseous materials in response to the one or more of the monitored gaseous materials to keep the growth within predetermined limits; wherein the one or more added and/or produced gaseous materials are monitored in the exhaust gas of the nutrient medium.

Description: METHOD 7AND APP7ABATUS FOR CONTROLLING GROWTH OF AN ORGANISM IN A NUTRIENT MEDIUM DESCRIPTION 1. BACKGROUND OF THE INVENTION The present invention relates to a method and apparatus for controlling the growth of an organism in a nutrient medium comprising adding to the nutrient medium one or more gaseous materials which are consumed by the organism, or which do not affect the organism; monitoring the one or more gaseous materials, or monitoring one or more gaseous materials produced by the organism; and controlling the addition of the one or more gaseous materials in response to the one or more gaseous materials to keep the growth within predetermined limits. In particular the invention relates to such a method and apparatus for controlling the cultivation or propagation of an organism, in particular yeast for brewing beer. The Technical Field Organisms such as yeast grow and reproduce very rapidly in aerated nutrient medium e.g. aerated wort. The yeast cells use energy obtained from suitable nutrients e.g. carbohydrates in the wort. Aerobic organisms deplete oxygen. Once the population reaches a high density, the cells at the air-liquid interface uses all of the oxygen immediately. As a result, cells below the surface cannot grow. Aerobic metabolism of yeast cells yields very fast growth of cells which can be controlled by providing just the right amount of oxygen that the yeast cells demand. When the available oxygen has been consumed, the yeast cells adapt to the oxygen deficiency by anaerobic utilisation of the nutrients of the wort. For example the yeast cells ferment carbohydrates of the wort into various metabolites such as ethanol and carbon dioxide. Generally, when growing organisms in a nutrient medium, both under aerobic and anaerobic conditions, it is known to control various parameters that influence the growth. In propagation of yeast for brewing beer, for example, it is known to control the flow of sterile air added to the nutrient according to a scheme based on passed experience of the brew master to provide the desired properties of the final product produced by the propagated yeast. This method is lengthy and expensive since empirical studies of the resulting product have to be based on a batch-to- batch production basis the variability of which is difficult to control. For use in propagation of yeast for brewing beer, e.g. lager, ale, wheat, stout, and other beers, it is known to control the parameters: content of carbon dioxide, content of oxygen, concentration of extract, temperature, and pH by measurement in the fluid of the wort to keep the growth within predetermined limits. This method has a number of drawbacks. In propagation of yeast under aerobic conditions, the organism can be in a state where it cannot utilize e.g. the oxygen in the nutrient medium, typically because of an inhomogeneous distribution of oxygen throughout the nutrient medium. Also, the oxygen may not be available to the organism, e.g. oxygen may be bound to other molecules. Consequently, control of the growth of the organism based on measurements in the fluid of the wort, the lack of ability to use the oxygen result in a negative response of the organism to addition of oxygen based on measurements indicating excess and/or deficit of oxygen in the wort. Thus, if some parts of the wort are low in oxygen due to inhomogeneities in the oxygen distribution therein, the yeast cells may adapt themselves to the oxygen deficiency. Then, if oxygen is added, an excess of oxygen may adversely affect or "stress" the oxygen-deficiency adapted cells leading to a slower fermentation and delayed settling, e.g. of yeast for lager beer, in the fermentation process. Other stress factors may be caused by temperature, pressure, carbon dioxide, and pH of the nutrition medium. Further, in case of an oxygen sensor being placed in the wort, the sensor may be sensitive to other substances and e.g. proteins may clog the surfaces of the sensor and affect the accuracy and precision of the monitored oxygen. Consequently, other substances affect the control of the growth process. Thus, for control of growth of organisms in a nutrient medium, both in batch processes and continuous processes, there is a need for an improved control of the growth of the organisms to provide organisms which are optimised for their intended application. Prior Art Disclosures Anders Andersen, "Yeast Propagation Plants - Development in Relationship to History, Process and Construction", MBAA Technical Quarterly, Vol. 31, No. 2, pp. 54-57, 1994. DE-A1-197 40 319 discloses a method and apparatus for propagating yeast cells in a brewing process. Oxygen is measured in a circulated cell suspension of a propagating tank as an indication of the oxygen excess therein. EP-A2-0 647 708 discloses a method and apparatus for propagation of yeast in beer wort in a tank-like assimilator. The parameters of content of carbon dioxide, content of oxygen, and concentration of extract, temperature, and pH of the wort fluids measured within the interior of the assimilator. The contents of extract and oxygen is kept within limits specified for a particular propagation programme US-A-4 764 471 discloses a method and apparatus for continuously propagating micro organisms in a medium wherein a rigorous control over the concentration of gaseous material in the medium is attainable by virtue of increased solubility of the gaseous material in the medium flowing under pressure. Use of a highly enriched gas stream and a small total volume of gas required relative to the flow of nutrient medium. Monitoring of oxygen is not disclosed. SU-Al-1 648 980 discloses a method of automatic control of a process of culturing fodder yeast in continuous yeast fermenters, the method comprising measurement and control of a large number of parameters including temperature, pH and level of culture medium, and feed rate of inoculant yeast. The method further include additional measurements of pH of nutrient medium fed to the fermenter, pH of inoculant yeast, feed rates of air for aeration, inoculant yeast, nutrient medium, and acid for adjusting pH in the fermenter, and concentration of oxygen in gases exiting the fermenter; and additional control of feed rate of air, level and pH of culture medium, and inoculant yeast with correction of feed rate and pH of nutrient medium and inoculant yeast, and allowance for oxygen concentration in off-gasses, for increasing the yield of biomass of the microorganisms irrespectively of their specific properties. Nothing is indicated nor suggested about controlling addition of gaseous materials in response to the monitored gaseous materials to keep the growth of the organisms within predetermined limits, e.g. to keep the growth of organisms with desired properties, such properties depending e.g. on the amount of oxygen available. It is known that too much oxygen may adversely affect or "stress" oxygen-deficiency adapted cells leading to a slower fermentation and delayed settling in a fermentation process. 2. DISCLOSURE OF THE INVENTION Object of the Invention It is an object of the present invention to seek to provide an improved method and apparatus for controlling growth of an organism in a nutrient medium. In particular, it is the object to seek to provide an improved method and apparatus for controlling growth of an organism which organism is suited for production of specific products. 295 More particular, it is the object of the present invention to seek to provide such a method and apparatus for improving the control of propagation of yeast, in particular yeast for brewing beer. It is still another object of the present invention to seek to provide means for optimising the operation and control of such a method and apparatus. Further objects will appear from the description elsewhere. Solution According to the Invention "Method of controlling growth of an organism in a nutrient medium" In an aspect of the invention these objects are fulfilled by providing a method of controlling growth of an organism in a nutrient medium as claimed in claim 1 wherein the one or more added and/or produced gaseous materials are monitored in the exhaust gas of the nutrient medium. It has surprisingly turned out that when the one or more added and/or produced gaseous materials are monitored in the exhaust gas of the nutrient medium, as opposed to monitoring said one or more added and/or produced gaseous materials within the nutrient medium, the control of the addition of the one or more gaseous materials in response to the one or more of the monitored gases to keep the growth of the organism within predetermined limits is improved. Monitoring in the exhaust gas of the nutrient medium According to the present invention, the one or more gaseous materials are monitored in the exhaust gas of the nutrient medium. The exhaust gas of the nutrient medium can be monitored at any suitable location outside of the nutrient medium. In a preferred embodiment, a suitable location is one wherein an integral value of the one or more gaseous materials can be measured. This can be obtained by mixing the contributions from various locations in the nutrient medium, and then monitor the one or more gaseous materials in a common exhaust gas of the mixed gases in one or more outlets. This embodiment is particularly useful in a continuous growth process with a flowing medium e.g. in a plug flow reactor. In another embodiment, suitable locations are locations wherein individual values of the one or more gaseous materials can be measured of the exhaust gas of the nutrient medium by avoiding one or more gaseous materials from the various locations . In a preferred embodiment, the nutrient medium is contained in an essentially closed container with at least one outlet for the exhaust gas wherein the one or more added and/or produced gaseous materials are monitored in said at least one outlet for the exhaust gas outside the container whereby it is obtained that an integral value of the one or more monitored gaseous materials is obtained. Further, it is obtained that foam optionally present in the headspace of the container above or in the interface of the nutrient medium does not influence the sensor of the monitored gaseous material. Also, cleaning and sterilization of sensors to be inserted into the nutrient medium is avoided. In another preferred embodiment, the one or more added and/or produced gaseous materials are monitored in the exhaust gas in one or more locations of the headspace of the container above the nutrient medium whereby it is obtained that individual values for each location are obtained and that the distribution of the added gaseous material can be monitored. This is particularly useful in a continuous growth process with a flowing medium. In still a further preferred embodiment, the one or more added and/or produced gaseous materials are monitored in both in said at least one outlet of the exhaust gas outside the container and in said one or more locations of the headspace of the container above the nutrient medium whereby the combinatorial effect of simultaneously obtaining both integral and individual values of the one or more monitored gaseous materials is obtained. .Response to the moni tored one or more gaseous materials Generally, the various parameters used to control the growth of an organism in a nutrient medium may be adapted in response to the monitored parameters in order to keep the growth within predetermined limits. In a preferred embodiment, the control of addition of the one or more gaseous materials is adapted in response to a decrease and/or an increase in one or more of the monitored gaseous materials, or a derivative thereof, whereby it is obtained that the added gaseous material is adapted to the one or more gaseous materials which have been made available to the organism, or which the organism has produced. This is different to the previous methods where the gaseous material is added in response to the monitored one or more gaseous materials available to the organism in a nutrient medium. Thus, assuming that the organism has been presented to a sufficient amount of a particular gaseous material, the gaseous material, which is not used by the organism, appears in the exhaust gas. Consequently, controlling the addition of the one or more gaseous materials in response to desired levels of the one or more gaseous materials in the exhaust gas, it is ensured that the amounts of gaseous materials are always sufficient to cover the demand of the organism. It should be noted that the levels of the one or more gaseous materials in the exhaust gas can be selected as desired to expose the organism to certain stress factors, if so desired. Various levels of stress to the organism can be obtained. If the addition of a certain gaseous material provides a small value of the gaseous material in the exhaust gas, the stress level is low. It can even be negative if the presence of the gaseous material is essential to the organism. However, if a high level of the gaseous material is present in the exhaust gas, more gaseous material is made available to the organism than it requires which may consequently stress the organism. In a preferred embodiment, the control of the addition of one or more gaseous materials is further adapted in response to one or more constant levels of one or more other monitored gaseous materials whereby especially addition of one or more gaseous materials in the start up phase can be controlled. An example of this being the combined addition of air and pure oxygen, the latter of which is typically the most expensive of the two, wherein the level of added pure oxygen may be adapted in response to a monitored constant level of e.g. nitrogen thereby ensuring an optimum use of the more expensive pure oxygen and the cheaper air. Gaseous materials The one or more gaseous materials which the organism consumes, or which do not affect the organism, includes gaseous materials which the organism absorbs and which participate in the metabolic processes, either directly or indirectly. Especially, the added gaseous materials may be fully or partially involved in these processes. Also, all or a part of the added gaseous materials may be consumed. Thus, these gaseous materials include primary metabolites that are involved directly in the growth of the organisms through the growth phase, and gaseous secondary metabolites that are produced in the stationary phase of the growth. In a preferred embodiment, the one or more gaseous materials which are consumed by the organism, or which do not affect the organism, is selected from the group consisting of oxygen, carbon dioxide, and nitrogen, and mixtures thereof. Moni tored quanti ty Generally, the monitored gaseous materials are measured in any suitable quantity including concentration and flow, and any suitable derivative thereof, either based on moles, weight or volume. In a preferred embodiment, the response of the one or more of the monitored gaseous materials is measured in concentration of gaseous component with respect to the total gaseous material. Signals of the moni tored one or more gaseous materials Generally, the signals of the one or more monitored gaseous materials comprises a constant level, a decreased level, and an increased level, or a combination thereof. For example, a decrease in the level of monitored oxygen may be monitored, simultaneously with an increase of monitored carbon dioxide and a constant level of monitored nitrogen. For various phases of the growth process, the skilled person can contemplate other combinations. Other parameters such as temperature and pressure Generally, the growth of an organism in a nutrient medium is influenced by a large number of parameters. In a preferred embodiment, the addition of the one or more gaseous materials is adapted in response to the temperature of the nutrient medium whereby it is obtained that the stress level of the organism is controlled depending on the rate of growth of the organism. Typically, the rate of growth of an organism is higher at a higher temperature. In another preferred embodiment, the addition of the one or more gaseous materials is adapted in response to the pressure of the nutrient medium whereby the influence of e.g. the hydraulic pressure in the nutrient medium on the control of the addition of the one or more added and /or produce gaseous materials can be accounted for. For example the one or more gaseous materials may be added to the nutrient medium at different positions in the nutrient medium, e.g. at different heights of a propagation tank, depending on the hydraulic pressure at the position. Batch and continuous processes Generally, the method according to the present invention can be carried out in any suitable process, including batch process, continuous processes, or semi-continuous processes . In a preferred embodiment, the method is carried out in a batch process. In another preferred embodiment, the method is carried out in a continuous process. Organisms The organism to be grown according to the present invention can be any organism that can grow in a nutrient medium. This includes organisms that can grow both under aerobic and anaerobic conditions. In addition to metabolic processes of producing C0 2 , the metabolic processes may include both oxidative respiration processes producing reduced inorganic gaseous products such as N 2 0, N ; , H 2 S, and CH 4 and fermentation processes producing reduced organic products such as CH 4 . The growth may be measured both by the number of cells and by the size of single cells. In preferred embodiments the organisms include bacteria and fungi. Examples of organisms are Saccharomyces, Streptococcus, Lactobacillus, and Penicillium. Specific strains of Saccharomyces include S. carlbergensis for bottom fermentation of e.g. lager beers and S. serevisiae for both bottoms and top fermentation of other beers.. In a particularly preferred embodiment the organism is yeast for brewing beer. Predetermined limi ts Generally, for a given organism and its desired application, it is known to try to keep the growth of the organism within predetermined limits from a desired growth, e.g. the growth curve falls within desired limits. In the present context, the term "predetermined limits" is not limited to number of cells per time unit in the growth curve but may include conditions of the growth which provide organisms with desired properties. Such organisms may exhibit a desired viability and vitality. The growth conditions may also include the aim to produce a large number of living organisms suited for particular applications such as the production of specific products including e.g. specific flavours and smelling substances. Or the growth conditions may include aiming at producing a culture of organisms that maintain anaerobic fermentation characteristics after the cells have been subjected to aerobic growth. This applies to facultative organisms in particular yeast for brewing beer . Predetermined limits can include growth conditions that produce organisms that exhibit no stress symptoms, stress symptoms, or a combination thereof depending on the desired application of the organism. Predetermined limits are determined based on empirical studies, including microscopy of cells for number, shape and size. "Apparatus for growing an organism in a nutrient medi um " The present invention also relates to an apparatus for growing an organism in a nutrient medium; said apparatus comprising an essentially closed container, at least one gas additions means, at least one exhaust gas means, at least one process monitoring means for monitoring one or more parameters of the growth process comprising at least one gas monitoring means of gaseous materials, and at least one process control means, as claimed in claim 14, wherein the at least one gas monitoring means is adapted to monitor the one or more gaseous materials in the exhaust gas of the nutrient medium. Gas monitoring means In a preferred embodiment, the at least one gas monitoring means is adapted to monitor the one or more gaseous materials in the exhaust gas of the exhaust gas means outside the container. In another preferred embodiment, the at least one gas monitoring means is adapted to monitor the one or more gaseous materials in the headspace of the container above the nutrient medium. In still another preferred embodiment, the at least one gas monitoring means is adapted to monitor the one or more gaseous materials both in the exhaust gas of the exhaust gas means outside the container and in the headspace of the container above the nutrient medium. In a preferred embodiment, the gas addition means include means for providing small or large bubbles, or mixtures thereof, whereby the gas addition can be further adapted to the addition and distribution of gaseous material. Gas addition means In another preferred embodiment, the gas addition means is adapted to add one or more gaseous materials in response to a decrease and/or an increase in the one or more of the monitored gaseous materials, or a derivative thereof, in the exhaust gas of the nutrient medium. In still another preferred embodiment, the gas addition means is further adapted to add one or more gaseous materials in response to one or more constant levels of the one or more monitored gaseous materials. In a preferred embodiment, the gas addition means include means for providing small and large bubbles, or a mixture thereof, whereby the gas addition can be further adapted to the addition and distribution of gaseous material. Exhaust gas means The exhaust gasses of the nutrient medium are guided away from by any suitable means. It is preferred that the exhaust gas means are adapted to collect the exhaust gas in the headspace above the nutrient medium and to discharge the exhaust gas through a common outlet whereby the exhaust gas generated in various locations of the nutrient medium is mixed and whereby a monitored gaseous material is monitored for an integral value of the total exhaust gas. In another preferred embodiment, the exhaust gas means are adapted to let out the exhaust gas through individual outlets from different locations above the nutrient medium whereby the exhaust gas from various locations are kept as individual partially mixed exhaust gases. In still another preferred embodiment, the exhaust gas means are adapted to both collect the exhaust gas in the headspace and to discharge the exhaust gas through a common outlet and to let out the discharge gas through individual outlets from different locations above the nutrient medium. Gas moni toring means The one or more gaseous materials can be monitored by any suitable monitor known in the art, either in-line by continuous monitors, or off-line by sampling of suitable samples and subsequent measurement with suitable analytical equipment, e.g. gas chromatograph. Monitors of gaseous materials comprise monitors for gases such as oxygen, carbon dioxide, and nitrogen but are not limited hereto. Other gas monitors include monitors for e.g. methane. The gas monitors can be any monitor based on any suitable measuring principle that relates a response to the concentration or flow of the gaseous material in question, or any other suitable quantity. Suitable gaseous monitors for e.g. oxygen monitors are ceramic oxygen monitors. Addi tion and distribution of gaseous material According to the invention, the one or more added gaseous materials may be added to the nutrient medium in any suitable way. This includes addition of said gases at different heights of the nutrient medium as well as at different position within a certain height. In a preferred embodiment, the apparatus according to the invention further comprises means for distribution of the one or more gaseous materials in the nutrient medium whereby the gaseous material can be more homogeneously distributed in the nutrient medium. Gas distribution means comprises gas nozzles, gas distribution conduits, and agitation means, but are not limited thereto. Process control means for temperature and pressure In another preferred embodiment, the process control means comprises temperature controlling means for controlling the temperature of the nutrient medium and the at least one process monitoring means comprises temperature monitoring means. In another preferred embodiment, the process control means comprises pressure controlling means and at least one process monitoring means for controlling the pressure within the container. Process control means The process control means of the apparatus according to the present invention include any suitable computing means which is able to control the growth of an organism by receiving measuring signals from the at least one process monitoring means, including signals from gaseous monitors, flow monitors, temperature and pressure monitors, and by transmitting control signals to various flow valves and pressure valves to control flows of various process streams and pressures, respectively, of the gas addition means and exhaust gas means . In a preferred embodiment, the process control means comprises a computer whereby the sequence of process monitoring and controlling steps can be easily executed. "Further aspect of the present invention " The object of the present invention is further solved by providing a computer program for controlling the growth of an organism in a nutrient medium in a method or apparatus according to the present invention. In a preferred embodiment, the computer program controls a process control means for controlling one or more parameters of the growth process in an apparatus according to the invention. In a particular preferred embodiment, the computer program is adapted to control the addition of gaseous material in response to the monitored one or more gaseous materials . 3. BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention is further disclosed with detailed description of preferred embodiments by way of examples only, reference being made to the drawings in which Fig. 1 shows a sketch of an apparatus for growing an organism in a nutrient medium according to the prior art, here illustrating the principle of propagation of yeast for brewing beer, Fig. 2 shows a side view sketch of an embodiment of an apparatus for controlling growth of an organism in a nutrient medium according to the invention, Fig. 3 shows an embodiment as shown in Fig. 2 incorporated in a system for propagation of yeast for brewing beer, Figs. 4A and 4B show schematic diagrams of an embodiment of a process monitoring means and process control means for monitoring one or more parameters of the growth process and for controlling these parameters in response to the monitored values thereof, and Fig. 5 shows a plot of experimental results for a growing process in a preferred embodiment. 4. DETAILED DESCRIPTION "Propagation of brewing yeast" The production (propagation) of brewing yeast is typically based on batch processes in closed containers under strict sanitary conditions. The purpose is to produce of a certain amount of yeast with good fermentation characteristics which to some degree is counteracting yeast growth because yeast growth is obtained primarily under aerobic conditions and fermentation is an anaerobic process. Under these conditions, the optimal condition for brewing yeast propagation is to supply only as much oxygen as the yeast is able to take up without* damaging the fermentation characteristics (oxygen stress) of the yeast. According to a preferred embodiment, to ensure the correct supply of oxygen, oxygen is measured in the exhaust gas leaving the nutrient medium. If oxygen is registered in the exhaust gas leaving the nutrient medium under optimal air-transfer conditions, it is because the yeast has not been able to utilise the oxygen. The typical course of oxygen consumption during propagation of brewing yeast is as follows: 1. Start phase Very low oxygen consumption because small amount of yeast and high concentrations of monosaccharides (mainly glucose) are present. Monosaccharides suppress aerobic metabolism to a much higher degree than disaccharides (mainly maltose) . Nonetheless the yeast prefers monosaccharides and these are consumed before the disaccharides . 2. After monosaccharides are consumed After a period the monosaccharides are consumed and this gives way to aerobic metabolism and the oxygen uptake - and therefore also the yeast mass - grows. 3. After growth stops The logarithmic growth period stops after some time - most likely due to the fact that the substrate runs out of growth substances, either free amino nitrogen or others, but it may also be because when placed in a concentrated sugar solution only a certain number of cell divisions are possible - and the oxygen consumption therefore declines. The oxygen consumption thus varies considerably in the period of propagation and in all phases it is important not to overdose the oxygen supply in order not to give stress to the yeast which may cause suboptimal fermentation characteristics. The control system can involve not only an oxygen monitor but also a carbondioxide monitor in the exhaust gasses. A certain pressure is maintained in the tank during the propagation in order to avoid infections coming from outside and to avoid foaming and controlling the pressure of the gaseous materials. The carbondioxide is toxic to the yeast and it is therefore important to minimize the content in the tank. When the oxygen uptake and yeast growth stops, the carbondioxide production declines and the foam problems therefore become less pronounced. In the end of the propagation period it is therefore beneficial to minimize the pressure in order to get as much carbondioxide as possible out of the solution. With signals from the oxygen and carbondioxide monitors indicating an increased oxygen concentration and a decline in carbondioxide concentration in the exhaust gasses, the overpressure is minimized, the propagation is finished and the yeast ready to be used for fermentation. "Prior art apparatus" Fig. 1 shows a sketch of an apparatus for growing of an organism in a nutrient medium according to prior art, here illustrating the principles of propagation of yeast for brewing beer. The apparatus comprises an essentially closed container; here a tank 100 comprising an upper part 102 and a lower part 101 with a conically shaped bottom. The container contains the organism, the nutrient medium, added gaseous materials, and materials produced by the organism as a result of its metabolic activities, here the organism is yeast for brewing beer either top-fermenting yeast or bottom-fermenting yeast. The nutrient medium is wort which is obtained by methods known in the art, usually obtained by separating the liquid phase from the resulting suspension in the marking process. The wort is placed in the lower part 101 of the tank forming a more or less well defined interface region 103 with gaseous material in the upper part of the tank. The interface region may include a region with a foamed phase of the wort. The gaseous material provided by the organism is primarily carbon dioxide that is collected in the headspace of the upper part 102 of the tank. The apparatus comprises at least one gas addition means for adding one or more gaseous materials into the container. Here two gas addition means are shown. A first gas addition means includes a gas inlet 110 allowing pressurised sterile air supplied through an air supply connection 111 to be conducted via a control valve 112 to the lower part of the tank. Typically, the tank further comprises a stirring means 150 for distributing and mixing the added gaseous material with the various components of the wort, said stirring means being operable from the outside of the tank. A second gas addition means includes a gas inlet 120 of sterile air supplied through an air supply connection 121 at the top of the tank used in the process of sterilisation and emptying of the tank. Here sterile air is supplied through another air control valve 122, optionally connected to an air supply source common with the first air supply connection 112. The second inlet of sterile air is supplied in the upper part of the tank to the top of the wort either for addition of gaseous material for the growth of the organism and/or for controlling the pressure in the headspace of the tank. The apparatus further comprises at least one exhaust gas means for discharging exhaust gas of the nutrient medium from the container. Here the tank is equipped with an exhaust outlet 130 for discharging gases in the top of the tank, typically said exhaust outlet includes a back pressure control valve 131 for controlling the pressure in the tank and for discharging the exhaust gas in the outlet conduit 132. The apparatus further comprises at least one process monitoring means for monitoring one or more parameters of the growth process comprising at least one gas monitoring means for monitoring the gaseous materials of the one or more added gaseous materials, and/or at least one gas monitoring means for monitoring other gaseous materials produced by the organism. Here the gaseous monitoring means consists of a sampling tube 140 for taking samples in the wort within the tank. Samples are subsequently analysed typically off-line by suitable analytical equipment 141, here shown logically coupled to the sampling tube. On-line analysis can also be done. Sampling of the wort can be for monitoring various parameters of the nutrient medium such as pH, and temperature, and various concentrations of e.g. oxygen and carbon dioxide as well as yeast cells within the tank. Alternatively, the gaseous monitoring means can include monitoring probes placed in the wort (not shown) . The apparatus further comprises a process control means for controlling the one or more parameters of the growth process in response to outputs of the at least one process monitoring means. Here a computer 142 is shown logically connected to the sampling tube of the process monitoring means . The apparatus also includes an inlet/outlet valve 160 for adding nutrient medium to the tank and for removing the produced product therefrom. The apparatus can include other means and devices (not shown) e.g. cleaning means such as spray balls for cleaning the tank between batches; heating means and cooling means for regulating the temperature of the wort, tank, and conduits. According to this prior art control of the growth of an organism added to the nutrient medium is obtained by aeration of the wort by adjusting the air control valve 112 until a suitable flow of sterile air is produced in the tank. The parameters of carbon dioxide, oxygen, concentration of extract, temperature and pH are measured in the wort by means of the sampling tube 140, the analytical equipment 141 and the computer 142. Exhaust gases of nitrogen, carbon dioxide and excess of oxygen are discharged at the exhaust valve 131 into the outlet conduit 132. The exact criterion for a suitable flow of sterile air is based on the individual wort. Further, it is based on the empirical experience of the brew master to adjust the aeration to keep the growth within predetermined limits. In case of brewing beer, the experience of the brew master in propagation of the yeast is decisive to obtain the desired properties of the final beer product e.g. with respect to the amount of alcohol and other substances such as aroma substances and taste providing substances . "Preferred apparatus embodiments according to the invention" Fig. 2 shows a side view sketch of an embodiment of an apparatus for controlling growth of an organism in a nutrient medium according to the present invention. The apparatus comprises an essentially closed container for containing the organism, the nutrient medium, added gaseous materials, and the materials produced by the organism as a result of its metabolic activities. The container is a tank similar to that shown in Fig. 1 having a lower part 101 and an upper part 102. In a preferred embodiment, a heating/cooling jacket 203 surrounds the container. A temperature sensor 204 is located in the lower part of the tank, preferably near to the inlet 224. The apparatus comprises at least one gas addition means for adding one or more gaseous materials into the container. Here one gas addition means is shown including a gas inlet 220 allowing pressurised sterile air supplied through an air supply connection 221 to be conducted via a control valve 222 and flow meter 223 to the inlet 224 in the lower part 101 of the tank. Typically, the tank further comprises a stirring means 150 for distributing and mixing the added gaseous material with the various components of the wort, said stirring means being operable from the outside of the tank. The apparatus further comprises at least one exhaust gas means for discharging exhaust gas of the nutrient medium from the container. Here the tank is equipped with an exhaust outlet conduit 230 for discharging gases in the upper part 102 of the tank, typically said exhaust outlet includes a back pressure control valve 231 for controlling the pressure in the tank. The outlet conduit 230 is shown to let out gasses in the upper most part of the upper part 102 of the tank thereby providing an integral of the discharged gasses across the tank. However, the point of outlet in the tank can be selected from one or more other locations in the headspace of the container above the nutrient medium. Also, gas traps in optional foam of the nutrient medium can be a point of outlet. The apparatus further comprises at least one process monitoring means 232,223 for monitoring one or more parameters of the growth process comprising at least one gas monitoring means 232 for monitoring the gaseous materials of the one or more added gaseous materials, and/or at least one gas monitoring means for monitoring other gaseous materials produced by the organism. Here the gaseous monitoring means include an oxygen meter 232 for monitoring the oxygen in the outlet conduit 132 and a flow meter 223 for measuring the flow of sterile air. Other monitoring means include means for monitoring pressure (e.g. 308 as shown in Fig. 3) and temperature 204. The apparatus further comprises a process control means for controlling the one or more parameters of the growth process in response to outputs of the at least one process monitoring means. Here a computer 242 is shown logically connected to the oxygen meter 232 and the flow meter 223. Further the computer is connected to valves 222 and 231. The apparatus also includes an inlet/outlet valve 160 for adding nutrient medium to the tank and for removing unused nutrient medium including the produced product there from similar to that of the prior art shown in Fig. 1. The computer is programmed in any suitable programming language to perform monitoring and control functions of the various parameters, including pressures, temperatures, and flows of the respective sensors and by transmitting signals to the respective control valves, heaters/coolers, etc. Generally, the computer is programmed to control the flow of gaseous material added, here sterile air added to the tank in response to the flow of exhaust gasses, here oxygen leaving the tank. In a preferred embodiment, the computer is programmed to control the addition of sterile air until the flow of oxygen in the outlet is close to zero. This condition is typically kept through out the growth process by increasing/decreasing the flow of the added sterile air. After saturation, the flow of sterile air is typically low at the beginning and adapted so that the organisms in the nutrient medium absorb the added one or more gaseous materials, here oxygen of the sterile air. Then the flow of sterile air increases to further adapt the amount of oxygen so that the organisms absorb that added. After the growth rate of the organisms has reached its maximum, the flow of sterile air then decreases and finally stops as the demand for oxygen by the organisms levels off. In another preferred embodiment, the computer is programmed to control temperatures over time and in response to the values of the monitored parameters whereby it is obtained that the temperature conditions for the organism can be controlled. In particular it is obtained that the combined response of both the addition of gaseous material and the increase/decrease of temperature can be controlled thereby ensuring a particular efficient control of the growth of the organism. In this embodiment, for controlling the temperature, the heating/cooling jacket 203 is controlled in response to the temperature transducer 204. In still another preferred embodiment, the computer is programmed to monitor and control the pressure in tank. Fig. 3 shows the embodiment in Fig. 2 incorporated in a system for propagation of yeast for brewing beer. Compared to the system of Fig. 2, this embodiment further includes means which are particular useful in further improving the control of the growth process in the apparatus according to the invention. Such means include distribution means. Here, the distribution means comprises a rotor means, such as a propeller 350, a rotor connection means, here a tube 325 including a rotary shaft, e.g. a rod or cylindrical shaft (not shown) , and a motor, here a computer controlled externally driven motor 351. The distributor means is connected via the distribution connection means to the externally driven motor for agitation of the wort in the tank. The distributor means is preferably placed centrally in the tank thereby allowing the site of agitation in the nutrient medium to be centrally placed near the inlet 224 of the added gaseous material, here sterile air, in the lower part of the tank 301. However, the site of agitation, or optionally more sites of agitation, can be chosen in other parts of the tank, e.g. in order to provide suitable agitations of added gaseous materials through more inlets thereby obtaining a more homogeneous distribution of the components. A more efficient control of the agitation, the mixing, and the distribution of the added gaseous material, wort, and organisms in the wort is obtained which provides a further improvement of the control of the growth process. For the purpose of cleaning the interior of the tank, the valves, and conduits for contaminating organisms, the apparatus further comprises a cleaning means for so- called cleaning-in-place operation that is particularly useful in combination with the process monitoring means, in particular the gas monitoring means. The cleaning-in- place means comprise means for supply 322 (CIP) of cleaning fluid, here a hot caustic solution. Further, it comprises means for closing the conduit 220 for adding gaseous material, here a pneumatic leak proof T-valve 305 coupled to the CIP supply through a pneumatic angle valve 306 and coupled to the outlet conduit 230 through another pneumatic angle valve 304. The top pressure conduit is further connected to a safety valve 310 in case of overpressure. In operation during cleaning of the conduit 224 for adding gaseous material to the tank, here the lower part of the tank, the T-valve 305 is arranged to close for passage of sterile air from the conduit 220 and open for passage of the cleaning fluid supply CIP through the pneumatic valve 306 and down into the tank through the inlet 224. The pneumatic valve 304 is normally closed during this phase. If it is open, the cleaning fluid can pass directly to the top pressure conduit 230 in order to clean this line. Also, during cleaning, the distribution connecting means 325 comprises a passage for cleaning fluid from the CIP supply to a spray ball 324. The spray ball may be mounted in fixed or variable position to provide a spray of cleaning fluid in a desired spray pattern and level of the tank. During cleaning the pneumatic valves 333 and 332 are pulsating to allow the cleaning fluid to pass to the exhaust outlet 132 through the backpressure control valve 331a. Furthermore, cleaning fluid passed through anti vacuum valve 331b to the top pressure conduit 230. In operation of the apparatus during growth of the organism and addition of sterile air, the exhaust gas collected in the upper part of the tank 302 is discharged through the back pressure control valve 331a and top pressure conduit 230 while the pneumatic angle valves 333, 332, and 304 and the safety valve 310 are closed. Leak-proof valve 305 and angle valve 306 are open. The backpressure control valve 331a can for example be a pressure exhaust valve of the type X4.27.39.100 supplied by Alfa Laval Scandi Brew, Søborg, Denmark. The addition of sterile air is continuous in the amount needed by the growth process. For visual inspection, the tank also comprises a tank light 309 and a tank sight glass 315 whereby the growth of the organism can be visually inspected by manually viewing the foam and evaluating the growing process for its characteristics, e.g. the characteristic appearance of the foam for unstressed conditions of the yeast. A high-level probe 307 and pressure monitor here a pressure gauge 308 are mounted on the top of the tank. Fig. 4A shows a schematic diagram of an embodiment of a process monitoring means and process control means, here generally referred to by reference numeral 424, for monitoring one or more parameters of the growth process and for controlling these parameters in response to the monitored values thereof. The process monitoring means comprises an oxygen meter 401 (or meter for other gases) logically coupled to an oxygen sensor 232 (or sensors for other gases) and to a process controlling means here a computer 402. In a preferred embodiment, the process monitoring means comprises a carbon dioxide meter for monitoring carbon dioxide coupled to a carbon dioxide sensor (not shown) . The carbon dioxide sensor is positioned at a similar location as the oxygen sensor. Similarly the process monitoring means comprises a flow meter 403 logically coupled to a flow meter 223 and to the process controlling means, also here a computer 402. Also, the process monitoring means comprises a temperature controller including another process control means, here another computer 404, and logically coupled to a temperature transducer 223 and to a heating/cooling jacket 203. Further, the two process-controlling means, i.e. the computers 402 and 404, are logically coupled. It is preferred that the process monitoring means comprises further monitors e.g. a level monitor connected to a level detector 307 e.g. a level switch LS, and a pressure monitor connected to a pressure sensor e.g. a pressure transducer PI 308. FIG. 4B shows a particularly preferred embodiment wherein the back pressure control valve 331a is controlled. 5 . EXAMPLE An embodiment of the invention is further illustrated by an example of controlling the growth of yeast cells (lager yeast) according to an embodiment of the invention. Procedure A tank equipped as described in connection with figs. 2 was filled with about 20 hi nutrient medium, in form of aerated standard wort with 14 degree plato. Any equivalent wort or suitable nutrient can be used. The wort was previously sterilised and cooled to a fermentation temperature of initially 15-16 °C. A sufficient amount of sterile C0 2 was let through the nutrient medium under gentle agitation in order to drive off non-dissolved 0 2 from the medium and oxygen in the headspace. (See Fig. 5 curve B at zero time origin) . After this the oxygen concentration in the headspace was 5 mg 0; per liter. 20 liter second generation Tuborg yeast cells (a variety of S. carlsbergenesis) were inoculated into the wort through a sample cock (not shown) . Sterile air was added by adjusting the air supply valve 222 to provide a desired sterile air flow rate (curve A of Fig. 5) corresponding to 20 1 sterile air per minute as measured by the flow indicator 223. The reading on the oxygen meter 232 was recorded showing a slight decline above zero (Curve B on Fig. 5) At the beginning of the second day analysis showed an increased cell count. Further to maintain the oxygen concentration in the outlet 132 within a range of above zero and below 1 mg 0 per liter, the amount of sterile air was increased (end of first day to start of second day) . The demand for oxygen increased for the increasing number of cells curve C. Following a propagation phase of usually 3-7 days depending on the yeast cell, the high kraussend stage was reached, here 2,5 days and the propagation process was stopped by terminating the addition of sterile air. The number of cells (curve C of Fig. 5) was measured by sampling and measurement using a standard cell counting apparatus . For comparison, growth experiments carried out according to the method of the present invention have shown a much better control over prior art methods could be obtained. Further, the quality of the cells was investigated by microscopy and indicated healthy unstressed yeast cells. Also, a considerably smaller number of dead cells was obtained. A brew master's recipe - propagation example explanatory description An embodiment of the invention is further illustrated by example of a brew master's recipe which is summarized in the subsequent table. Step 1 - Wort sterilisation The wort is sterilised by supplying steam to the heating jacket on the tank cone. Heating continues until the wort has reached a temperature of 120 °C which is held for 30 minutes. In order to reach this temperature and avoid boiling, pressure is increased to 1.5 bar G (over pressure) . Step 2 - Wort cooling After sterilisation steam supply to the cone is stopped and cooling through the cooling jacket of the tank cylinder is started. When temperature reaches 25 °C, aeration is started in order to supply oxygen to the wort and - as a secondary effect - to agitate the tank content thereby reducing the cooling time as the temperature gradient becomes smaller. Pressure is reduced to 0.5 bar G. Cooling continues until propagation temperature is reached (e.g. 20 °C) . Step 3 - Inoculation The tank is inoculated with yeast from the laboratory grown in a Carlsberg flask. Temperature and pressure are maintained at propagation conditions (see step 4), no aeration is performed. Step 4 - Propagation During propagation temperature is maintained at a fixed temperature (e.g. 20 °C) and pressure 0.5 bar G. Aeration is performed by interval aeration and aeration is stopped when oxygen is registered in the exhaust gas indicating that more oxygen is supplied than the yeast can take up. Step 5 - End of propagation When propagation period is approaching the end two events occur: 1. The period between aeration start and registration of oxygen in the exhaust gas becomes shorter (e.g. 30 seconds) indicating that the yeast does not take up more oxygen. 2. A reduction in carbondioxide production is registered by the carbondioxide monitor. When one of these two events occur it is a signal that yeast growth is declining and propagation period is coming to an end. At this stage a signal is given to the back pressure control valve to reduce pressure to 0.2 bar G in order to minimize the amount of carbondioxide dissolved in the tank. The carbondioxide is toxic to the yeast the content of dissolved carbondioxide must therefore be minimized. Propagation example

Claims: METHOD AND APPARATUS FOR CONTROLLING GROWTH OF AN ORGANISM IN A NUTRIENT MEDIUM CLAIMS 1. A method of controlling growth of an organism in a nutrient medium comprising adding to the nutrient medium one or more gaseous materials which are consumed by the organism, or which do not affect the organism; monitoring the one or more gaseous materials, or monitoring one or more gaseous materials produced by the organism; and controlling the addition of the one or more gaseous materials in response to the one or more of the monitored gaseous materials to keep the growth within predetermined limits; wherein the one or more added and/or produced gaseous materials are monitored in the exhaust gas of the nutrient medium. 2. A method according to claim 1 further comprising that the nutrient medium is contained in an essentially closed container with at least one outlet for the exhaust gas " wherein the one or more added and/or produced gaseous materials are monitored in said at least one outlet outside the container. 3. A method according to claim 1 further comprising that the nutrient medium is contained in an essentially closed container with at least one outlet for the exhaust gas wherein the one or more added and/or produced gaseous materials are monitored in the exhaust gas one or more locations of the headspace of the container above the nutrient medium. 4. A method according to claim 2 and 3 wherein the one or more gaseous materials are monitored both in said at least one outlet of the exhaust gas outside the container and in said one or more locations of the headspace of the container above the nutrient medium. 5. A method according to any of the preceding claims wherein the addition of the one or more gaseous materials is adapted in response a decrease and/or an increase in one or more of the monitored gaseous materials, or a derivative thereof. 6. A method according to claim 3 wherein the control of the addition of one or more gaseous materials is further adapted in response to one or more constant levels of one or more other monitored gaseous materials. 7. A method according to any of the preceding claims wherein the one or more gaseous materials which are consumed by the organism, or which do not affect the organism, is selected from the group consisting of oxygen, carbon dioxide, and nitrogen, and mixtures thereof. 8. A method according to any of the preceding claims wherein the response of the one or more of the monitored gaseous materials is measured in concentration of gaseous component with respect to the total gaseous material. 9. A method according to any of the preceding claims wherein the method is carried out in a batch process. 10. A method according to any of the preceding claims wherein the control of the addition of the one or more gaseous materials is further adapted in response to the temperature of the nutrient medium. 11. A method according to any of the preceding claims wherein the control of the addition of the one or more gaseous materials is further adapted in response to the pressure of the exhaust gas. 12. A method according to any of the claims 1-8 wherein the method is carried out in a continuous process. 13. A method according to any of the preceding claims wherein the organism is yeast for brewing beer. 14. An apparatus for growing an organism in a nutrient medium, comprising (a) an essentially closed container (100) for containing the organism, the nutrient medium, added gaseous materials, and the materials produced by the organism as a result of its metabolic activities; (b) at least one gas addition means (220, 221, 222) for adding one or more gaseous materials into the container; (c) at least one exhaust gas means (230) for discharging exhaust gas of the nutrient medium from the container; (d) at least one process monitoring means for monitoring one or more parameters of the growth process comprising at least one gas monitoring means (232) for monitoring the gaseous materials of the one or more added gaseous materials, and/or at least one gas monitoring means for monitoring other gaseous materials produced by the organism; (e) and at least one process control means (242) for controlling the one or more parameters of the growth process in response to outputs of the at least one process monitoring means to keep the growth within predetermined limits; wherein the at least one gas monitoring means (232) is adapted to monitor the one or more gaseous materials in the exhaust gas of the nutrient medium. 15. An apparatus according to claim 14 wherein the at least one gas monitoring means (232) is adapted to monitor the one or more gaseous materials in the exhaust gas of the exhaust gas means outside the container. 16. An apparatus according to claim 14 wherein the at least one gas monitoring means is adapted to monitor the one or more gaseous materials in one or more locations in the headspace of the container above the nutrient medium. 17. An apparatus according to claims 15 and 16 wherein the at least one gas monitoring means is adapted to monitor the one or more gaseous materials both in the exhaust gas of the exhaust gas means outside the container and in one or more locations in the headspace of the container above the nutrient medium. 18. An apparatus according to claims 14-17 wherein the gas addition means (220, 221, 222) is adapted to add one or more gaseous materials in response to a decrease and/or an increase in the one or more of the monitored gaseous materials, or a derivative thereof in the exhaust gas of the nutrient medium. 19. An apparatus according to claim 18 wherein the gas addition means (220, 221, 222)) is further adapted to add one or more gaseous materials in response to one or more constant levels of the one or more monitored gaseous materials . 20. An apparatus according to claims 14-19 wherein the exhaust gas means are adapted to collect the exhaust gas in the headspace above the nutrient medium and to discharge the exhaust gas through a common outlet. 21. An apparatus according to claims 14-10 wherein the exhaust gas means are adapted to let out the discharge gas through individual outlets from different locations above the nutrient medium. 22. An apparatus according to claims 14-19 wherein the exhaust gas means are adapted to both collect the exhaust gas in the headspace and to discharge the exhaust gas through a common outlet and to let out the discharge gas through individual outlets from different locations above the nutrient medium. 23. An apparatus according to any of claims 14-22 further comprising means (150, 350, 351) for distribution of the one or more gaseous materials in the nutrient medium. 24. An apparatus according to any of claims 14-22 wherein the process control means further comprises temperature controlling means (203) for controlling the temperature of the nutrient medium and the at least one process monitoring means further comprises temperature monitoring means (204) . 25. An apparatus according to any of claims 14-24 wherein the process control means further comprises pressure controlling means and at least one process monitoring means (308) for controlling the pressure within the container. 26. An apparatus according to any of claims 14-25 wherein the process control means comprises a computer. 27. A computer program for controlling the growth of an organism in a nutrient medium in a method as claimed in claims 1-13. 28. A computer program for controlling a process control means for controlling one or more parameters of the growth process in an apparatus for growing an organism in a nutrient medium as claimed in claims 14-26. 29. An apparatus for growing an organism in a nutrient medium, comprising (a) an essentially closed container (100) for containing the organism, the nutrient medium, added gaseous materials, and the materials produced by the organism as a result of its metabolic activities; (b) at least one gas addition means comprising a conduit (220) for adding air, an air supply conduct (221), and a control valve (222) for supplying air into the container; (c) at least one exhaust gas means comprising an outlet conduit (230) for discharging exhaust gas of the nutrient medium from the container, and a back pressure control valve (331a) for controlling the pressure of the exhaust gas that allows outlet of the exhaust gas; (d) at least one process monitoring means for monitoring one or more parameters of the growth process comprising at least one gas monitoring means (232) comprising an oxygen meter (401) for monitoring oxygen, said oxygen meter being connected to said back pressure control valve, and/or at least one gas monitoring means for monitoring other gaseous materials produced by the organism; (e) and at least one process control means (242) for controlling the one or more parameters of the growth process in response to outputs of the at least one process monitoring means to keep the growth within predetermined limits; said control means comprising a computer (403) ; said computer being connected to said back pressure control valve (331a) and air-supply control valve (222) ; wherein the at least one gas monitoring means (232) is adapted to monitor the one or more gaseous materials in the exhaust gas of the nutrient medium. 30. An apparatus according to claim 29 wherein said at least one gas monitoring means for monitoring other gaseous materials comprises a carbon dioxide meter for monitoring carbon dioxide.

Inventors: ANDERSEN ANDERS, ANDERSEN, ANDERS
Applicants: ALFA LAVAL COPENHAGEN AS, ANDERSEN ANDERS, ALFA LAVAL COPENHAGEN A/S, ANDERSEN, ANDERS
Application year: 2001
Main classification: C12M1
IPC codes: C12M1