Method using immobilized yeast to produce ethanol and alcoholic beverages

Abstract: According to the invention, yeast cells are immobilized on a substantially noncompressible carrier having anion exchange properties. The immobilized yeast can then be used to ferment a sugar-containing substrate. The immobilized yeast carrier can be regenerated.

Description: This invention relates to production of ethanol and more specifically to production of alcoholic beverages using immobilized yeast which is bound to a regenerable carrier material. Yeast immobilization can be carried out in the fermentation reactor itself thus minimizing the risk of contamination during the immobilization. Fermentation is an art that began before recorded history. It was, however, only in 1866 that Pasteur published his works on wine fermentation, analyzed the reasons for spoilage, and prescribed an appropriate treatment for wine making. Traditionally the fermentation takes place with the wild yeasts that occur on all fruits and berries. Fermentation with pure culture yeasts is used more often today in order to consistently produce good quality wines. All these general aspects of wine making and characterizations of the yeasts are presented in "Biotechnology" edited by H-J Rehm and G Reed, Vol. 5: Food and Feed Production with Micro-organisms, Verlag Chemie, and in Prescott & Dunn's "Industrial Microbiology, 4th edition, chapter 9, 1982, edited by G Reed. Traditional batch fermentation is time consuming. Typically, the time required for the alcohol fermentation in wine making requires normally at least 20 - 50 days. Although a continuous process with suspended yeast might be thought to speed this process, it is difficult to operate and maintain free from microbial contamination. Moreover, to speed up the fermentation rate, the yeast cell concentration in the fermenting should be increased while on the other hand the ethanol inhibition should be minimized. In a batch fermentation, for example with apple juice, the yeast cell concentration is 1-2 x 10⁸ cells/ml of must. A higher yeast cell concentration can, for example, be arranged by immobilizing the yeast on a suitable carrier packed in a column. When the fermenting must is passed through the column packed with immobilized yeast, the number of yeast cells in contact with the volume of must in the reactor is greatly increased. This greater contact area results in a faster fermentation. The column reactor also reduces the ethanol inhibition since the ethanol containing product is continuously removed from the yeast bed. This reduction is at its greatest when an ideal plug flow situation is achieved. It is known that yeast cells can be entrapped in calcium alginate and the resulting immobilized yeast can be used for fast fermentations. Numerous literature references describe this technique as it is applied on laboratory scale. Some efforts, however, have also been made to commercialize this technique. One of the best known is probably that of Kyowa Hakko in Japan, where alginate immobilized yeast is used for feed ethanol production. For commercial scale operations, a major difficulty with alginate entrapment is the manner in which the particles are formed. It must be carried out at the production site where a yeast slurry and a solution of sodium alginate are mixed together. When this mixture is then fed into a calcium-salt solution, the alginate precipitates and at the same time occludes the yeast cells within the precipitated particles. The particles usually are in the form of droplets/beads. A process plant that utilizes alginate entrapped yeast must have specially designed equipment just to produce these beads. Furthermore, there is a potential risk for contaminating the yeast with wild microorganisms. This is especially critical when production of an alcoholic beverage flavor is important (e.g. beer, wine, cider and similar kinds of products). For technical/fuel ethanol production the criteria of contamination is not as important as in the consumables although it does affect the productivity. A second major difficulty for alginate beads used on a commercial scale is in the physical strength of the beads. The beads are soft and easily compressible. Operating large fermentation columns can be a problem and fast downflow process streams are difficult to handle. On the other hand, a typical upflow mode in fermentation greatly wears the beads. Also, to run a reactor downflow under pressure with compressible material is virtually impossible. A third difficulty with entrapment is the diffusion limitations which slow down the accessibility of the substrate in contact with the yeast inside the bead. Finally, if the system becomes contaminated or otherwise disturbed so that a continuous operation must be discontinued, the whole lot of column material (alginate together with the yeast) must be discarded. No reuse is possible. European Brewery Convention Congress, Madrid, 1987, pp. 441-448, IRL Press, Oxford, GB reveals a process for secondary fermentation with yeast immobilized on DEAE cellulose granules, wherein diacetyl was reduced in the immobilized yeast reactor. US-A-4 546 081 concerns a continuous fermentation process for producing ethanol utilizing a fermentation vessel packed with thin film means prepared by mixing an aqueous yeast suspension with photo-crosslinkable resin. In Biotechnology and Bioengineering, Vol. 25, No. 9, pp. 2243-2262, the ethanol fermentation in a yeast immobilized tubular fermentor is described. Thereby, white birch wood chips are used in the fermentor in order to immobilize Saccharomyces cerevisiae yeast cells. Therefore, it is an object of the present invention to develop a method for continuous column fermentation that does not require on-site formation of the column material. Another object is to develop a method that eliminates sources of contamination. Yet another object is to develop a method of this type that can withstand pressure. Still another object of this invention is to develop a method where the carrier material can be regenerated and reused. Further objects include development of processes that produce alcohol at reasonably rapid rates such as at least a minimum of about 1 bed volume of about 5 percent alcohol per day. These and other objects are achieved by the present invention which is directed to a method for the primary production of an ethanolic product. According to this method, an aqueous substrate containing a dissolved yeast-fermentable carbohydrate is contacted with yeast immobilized upon a carrier having anionic exchange properties. The contact can be preferably accomplished by passing the aqueous substrate through a packed bed reactor of the yeast immobilized upon the carrier. However, it may also be accomplished by use of the immobilized yeast in a fludized bed reactor. The yeast-carrier combination is comprised of yeast bound to the surfaces of the porous carrier. The carrier is substantially non-compressible. It is composed of a continuous porous matrix, or alternatively, of dimpled or reticulated, porous particles. The matrix or particles, in turn, are composed of individual microparticles or microfibers. This carrier structure provides a maximal surface area for carrying yeast cells. The resulting high number of yeast cells per unit volume of carrier makes rapid fermentation possible. The particulate or matrix character of the carrier is produced by loosely binding, felting, weaving, gluing or agglomerating (hereinafter binding) the microparticles or microfibers together. The binding is accomplished by establishing chemical, adherent or mechanical links at some of the contact points between the individual microparticles or microfibers. Chemical binding is accomplished by causing a chemical cross-linking reaction at these points. Adherent binding is accomplished by agglomerating or gluing the microfibers or microparticles together through the use of an additional ingredient such as a thermoplastic resin. Mechanical binding is accomplished by entangling or knotting the fibers at the contact points or by joining the particles by meshing their surfaces together. In the latter form, the matrix will comprise a continuous structure throughout the reactor, much like cotton fluff or filter paper packed into a tube. Also then, in their final form, the particles will be discrete and individual. The microfibers or microparticles are composed of any anion exchange substance that can be formed into the desired, rough-surfaced microfibers or microparticles. These substances include native or regenerated cellulose or rayon that is derivatized to provide anion exchange character; synthetic anion exchange resins such as phenolformaldehyde resins and agarose or dextrin based anion exchange resins. Excluded are white birch wood chips. The preferred carrier is a porous, particulate anion exchange resin derived from cellulose or rayon that has been chemically modified to provide anion exchange character. Especially preferred embodiments include microfibers or microparticles of diethylaminoethylene substituted cellulose, adherently bound by agglomeration with polystyrene. It is believed that the electric forces established between the positively charged resin and the negatively charged yeast cells are primarily responsible for the binding of yeast cells to the surfaces of the resin. This binding minimizes substantial leaching of the yeast yet permits intimate contact between the yeast and the aqueous medium. The aqueous substrate used as a starting material is composed at least of water and a fermentable carbohydrate such as hydrolyzed starch, sucrose, glucose, fructose, maltose or maltotriose (lactose and xylose). The concentration of sugar will be sufficient to permit continuous production of alcohol but will not be so high that the fermentive activity of the yeast is completely inhibited. According to the method of the invention, it is important to control the process parameter involving carbon dioxide produced by the fermentation. The carbon dioxide can be maintained in the fluid product stream or removed. In a preferred method, a series of columns is employed, the interconnections of which are adapted for removal of gas from the column outlet stream. If the column interconnection taps are all employed, a product without carbonation will be produced. By maintaining the columns under pressure, a carbonated product can be produced. The column pressure, however, cannot be so high as to substantially diminish the yeast fermentation. Facile routineering can establish this limit, which, in general, will be at least about 14 bar. In addition to pressure, other process parameters can be varied to affect the output of ethanol and taste of the product. These parameters include column temperature, aqueous substrate feed rate, column residence time, direction of substrate flow in the column (with or against gravity), periodic reversal of flow direction, yeast strain, yeast concentration and yeast nutrients in the aqueous substrate. Suitable ranges for these parameters include a temperature of -2 to 40°C, feed rate 0.01-10 reactor bed volume (BV)/hour, a residence time of 0.1-100 hours and yeast concentration of 10⁹ to 10¹² yeast cells per litre of carrier. Yeasts such as Saccharomyces or Candida can be used. Generally, these parameters will be adjusted to produce an ethanol concentration of from 0.05 to 15 percent. In a preferred method of the invention, a slurry of the particulate carrier is first placed in a column and the carrier allowed to settle into a packed column. The carrier is sterilized by a method known per se, such as washing with hot caustic. After neutralizing with sterile, dilute acidic solution and rinsing with sterile water, the column is then flowed with a yeast broth so that the yeast becomes attached to and immobilized on the carrier particles. After this pretreatment, the column of yeast is used as described above. In a further preferred method of the invention, an ethanolic product that is consumable is produced. This product can be wine, sake, an alcoholic fruit, berry or vegetable drink, carbonated versions thereof, beer, or low alcohol versions of the foregoing products. The aqueous substrate used to produce the consumable product is derived from a source such as a fruit, berry or vegetable juice or extract, a wort, hydrolyzed plant material or an aqueous syrup containing a fermentable sugar derived from a natural or synthetic source. The juice will be a liquid pressed from a fruit, berry or vegetable. The extract will be a liquid produced by combining the fruit, berry or vegetable with water and processing by mashing, cooking, pressing, mixing and the like. The hydrolyzed plant material will be material that is derived from cellulose, hemicellulose and/or starch through a technique such as acid, enzyme or auto-hydrolysis. Another method of the invention is directed to production of low alcohol beverages such as those containing for example less than 0.2 percent by volume alcohol. According to this method, the steps practiced and the yeast-carrier material are the same as those of the primary production method. The sugar content and feed rate of aqueous substrate, however, are modified to provide the "low alcohol" production. The feed rate is increased to an extent that decreases the alcohol concentration to the value desired. Also by lowering the temperature the fermentation can be slowed down. The sugar concentration is also appropriately adjusted so that the beverage is not overly sweet yet enough sugar is present for fermentation to alcohol. By continually adjusting the process parameters above one can choose a wanted alcohol level. The invention is further directed to the combination of the carrier and yeast in any form. Preferred forms include that made in situ , as described above, and a dried combination. The dried combination in aseptic condition can be packaged, transported to a fermentation plant and reconstituted for use by immersion in aqueous nutrient. Alternatively, the dried form can be packaged for home use. The Figure is a flow scheme for preparing the immobilized yeast columns. The Figure shows a hydration vessel 1, an immobilized yeast reactor 2, a sterilization liquid vessel 3, a neutralization acid vessel 4, a yeast slurry vessel 5, a substrate vessel 6, a carbon dioxide separation port 7, a receiving vessel 8 and a pressure releasing valve 9. The present invention is based upon a yeast-carrier system, described above, that is non-compressible and sterilizable. A specially preferred example of a carrier for use in producing the yeast-carrier system is particulate DEAE cellulose (weak anion exchanger) agglomerated with polystyrene. This carrier is commercially available and is well-known for immobilizing enzymes typically isomerase (see U.S. Patent No. 4,355,117 for details regarding this carrier). In a preferred process for preparing the yeast-carrier system as depicted in the figure, the dry carrier is hydrated in a hydration vessel 1 and pumped into a column reactor 2. The preferred DEAE cellulose carrier is stable in acid and alkaline media and can withstand a temperature up to 100°C. It can thus be easily sterilized in the column, for example, with hot caustic alkali. Other resins can appropriately be sterilized in a similar way. Yeast immobilization onto the carrier is accomplished after sterilization of the carrier with hot dilute caustic alkali 3. The carrier bed 2 is then rinsed with water and neutralized by pumping a suitable dilute acid 4 through the carrier bed; finally the carrier bed is rinsed with sterile water. The immobilization is carrier out by pumping a cultured, active yeast slurry 5 into the reactor 2. The carrier then adsorbs the yeast. The yeast cells attach to the carrier surfaces as a result of the optimal surface shapes of the carrier. Preferably, the yeast cells are grown on the carrier to provide a density of between about 10⁹ - 10¹² cells per litre of carrier, with a density of about 10⁹ cells being particularly preferred. The density must be sufficient to provide enough enzymatic activity to substantially convert the sugars in the aqueous substrate to ethanol. The substrate from vessel 6 is pumped through the reactor either through the bottom or top inlet. By adjusting the flow rate one can control the level of fermentation. Slow flow rate (about 1BV/day) means long contact time between the carrier and the substrate and thus high alcohol levels. Faster flow again lowers the alcohol level. The reactor can be run under atmospheric pressure feeding from the bottom inlet the carbon dioxide being separated from the system freely through a separation port 7. The product is collected to a receiving vessel 8 from where it is further processed through filtration etc. before final bottling. When the system is operated under pressure, the substrate 6 is fed from the top inlet in order to keep the carrier bed in packed form and thus reach a plug flow. The fermentation rate is controlled in such a manner (by flow rate) that the applied pressure is high enough to maintain the formed carbon dioxide dissolved. The pressure is released through valve 9 on the receiving vessel unless the final product is intended to contain carbon dioxide. In order to keep the pressure and dissolved carbon dioxide in reasonable limits, it is advisable to have two or more similar reactors 2 in series and release the excess carbon dioxide between each reactor. The carrier can be regenerated by feeding a hot caustic alkali through the carrier bed until the material colour is uniformly bright. Then the carrier is rinsed with water until a pH of about 10 is achieved and neutralized by pumping a suitable dilute acid through the carrier bed. Finally, the carrier is rinsed with sterile water. The carrier is important in terms of providing an adequate environment for yeast growth and contact with the aqueous substrate. The non-compressible anion exchange carrier, such as agglomerated DEAE-cellulose, has several beneficial qualities compared with soft, gel-like carriers, such as alginate beads. These qualities include fewer mass transfer problems, easier immobilization, faster start-up, easier scale-up, much improved regenerability and a long life-time of the carrier. In laboratory scale experiments a gel-like carrier, i.e. alginate beads and a preferred carrier of the invention, i.e. DEAE-cellulose granules, have been compared. The carrier made from DEAE-cellulose was more facile to use for ethanol production and with many substrates had a more desirable production capacity. These factors suggest a better contact between yeast and aqueous substrate during treatment. It is believed that the mode of interaction of the immobilized yeast and the carrier according to the invention may explain the increased yield. Electron micrographs show that in the alginate beads, the yeast grows in colonies, some of which grow through the alginate layer to the bead surface. The yeast colonies presumably act as the "active sites" in the alginate beads. The microphotographs show that the DEAE-cellulose granules are porous, reticulated matrices of microfibers. This structure permits the aqueous substrate to reach yeast cells growing inside the granules. Thus, the yeast grows rather loosely and separately in the internal and external pockets of the microfibers and these individual cells act as "active fermentation sites" in the column. Accordingly, more yeast cells per unit carrier surface area are available for fermentation when the carrier of the invention is used. The mechanism of the yeast cell immobilization onto the surface of the carrier, such as granular DEAE-cellulose, offers many benefits. Firstly, the yeast cells are substantially all on the surface of the carrier. Thus, the cells act as if they were freely suspended in a solution. Secondly, there are no substantial diffusion limitations and the fermentable fluid substrate can freely come into contact with the yeast. Also the nutrients that the yeast needs for life are available without hindrance from the need to permeate into the interior of the carrier particles. Thirdly, the carrier properties according to the invention also permit easy start-up and regeneration because the yeast cell immobilization can take place in situ within the column. This procedure lowers the risk of contamination and improves the conditions of the system as a whole. In addition, the column can be easily regenerated which is an important economic advantage. Regeneration is sometimes expedient because of chemical impurities or contamination from the aqueous, microbial contamination or from mutations of the immobilization yeast itself. To regenerate a column reactor made from a carrier such as DEAE cellulose, for example, the spent column can be washed and sterilized with a hot caustic solution or with another sterilizing medium such as an organic agent in water and the like. After washing and neuralization, the carrier is ready for repitching and reimmobilization. In pilot scale set-ups, such a column has been utilized for at least 13 weeks without need for regeneration. In laboratory scale, a similar column has been utilized for about 30 weeks. In general, the entire reactor system, including the carrier, can be sterilized with the above-mentioned treatment. When an aseptically cultured yeast is then pumped or eluted through the carrier bed, the risk of contaminating the yeast/carrier with wild strains or bacteria will be minimal. When the yeast is attached to the column (a suitable charge is about 10⁹ to 10¹² yeast cells/litre carrier), the column can be further conditioned by slowly pumping a nutrient solution such as an aqueous medium of ammonium phosphate and sugar through the reactor for approximately one day. This process causes the yeast to flourish to a maximum density. As is generally true of all yeast-carrier systems used according to the invention, one of the preferred carrier embodiments, granulated DEAE cellulose, is non-compressible (non-swelling). This property allows much variation as to how the fermentation can be conducted. Under normal atmospheric pressure, the reactor/continuous fermenter may be operated as follows. The substrate is fed in from the bottom of the reactor and the CO₂, which is formed during the fermentation, is freely allowed to evolve from the carrier bed. With this technique, however, an ideal plug flow situation cannot be reached (CO₂ bubbles always disturb the bed) and backmixing causes ethanol inhibition and thus lower productivity. The non-compressible carrier system of the present invention can also be operated under increased pressure in order to maintain the carbon dioxide in a dissolved state. The column can then also be run in a downflow condition and in this packed bed system, an ideal plug flow is reached. According to a specific embodiment the flow of the aqueous substrate through the reactor is periodically reversed. Several pressurized columns may also be run in series to avoid high pressure. The series arrangement is especially useful if a large concentration of alcohol in the final product is desired. This method enables the operation in a downflow mode without the harmful release of carbon dioxide which could disrupt the packed character of the column, i.e. cause channelling. Between the columns, carbon dioxide is separated from the stream. The reactor size can also be kept smaller as no extra fluidization and carbon dioxide-separation space is needed. Basically any fermentable fluid substrate can be used as feed material for production of the ethanolic product provided that any particulate impurities are filtered out before feeding into the immobilized yeast column. Examples of typical substrates would be: (i) wort (ii) fruit juice (iii) berry juice (iv) sugar syrup (v) starch syrup (vi) any hydrolysate out of plant material (vii) sugar syrups flavoured by any fruit, berry, malt or similar extracts (viii) any aqueous substrate that is used in beer, wine or liquor production A typical total sugar concentration would be 100 - 250 g/l including both original and added sugars depending on the basic raw material. The yeast nutrients (including sources of phosphorous, nitrogen etc.) need to be balanced unless they are favourable in the original raw material. The sugar concentration can, of course, be varied widely depending on how much alcohol and how much sweetness the end product is expected to have. It may also contain natural flavours, oils and the like. The sugar will be present at a concentration of from at least 1 percent by weight to an amount that will inhibit fermentation function (e.g. about 30 to 40 percent by weight), preferably about 4 to 25 percent by weight relative to the total weight of the medium. The flow rate of the aqueous substrate through the column is an important parameter because the ethanol production capacity is dependent on the flow rate. It has also been shown that the flow rate determines the number of yest cells which remain bound in the carrier. With high flow rates, the yeast cell outflow is increased; consequently ethanol production may also be dependent on the cell number bound in the immobilized system, and not only on the throughput rate of the substrate. If the flow rate is too high, the ethanol production is incomplete and/or inadequate; by controlling the flow rate, the ethanol concentration can be controlled, typically by seeking a concentration which is from 0.05 percent to 15 percent by volume relative to the total volume of the aqueous substrate. By adjusting the flow rate, the fermenter can obtain an ethanol concentration within this range. The amount of yeast cells which remain bound to the carrier material appears to be relatively stable at low flow rates, suggesting that yeast growth and leaching of the cells are balanced. With higher flow rates leaching increases but the cell number returns back to the normal level again when the flow rate is decreased. The flow rate should be high enough to enable the leaching of dead cells in order to avoid autolysis. During the operation of the immobilized yeast column, the yeast is maintained viable by fermentable sugars in the aqueous substrate. The immobilized yeast column reactor can be pressurized as discussed above. The operation pressure should be high enough to keep the carbon dioxide soluble in the column and to avoid the possible channelling of the CO₂ bubbles through the immobilized yeast column. Typical measure is 14 bar. A suitable temperature is that used in traditional batch fermentation processes. It will range from 0 to 40°C, preferably 10 to 35°C. This temperature can be maintained either by warming the columns with heating jackets or maintaining the columns in an environmentally controlled room, or by cooling. Of course, the yeast also develops a certain degree of heat that also can be used to advantage. The pressure is chosen according to the operation temperature. Following fermentation, the eluted ethanolic product is cooled down to the storage temperature and collected into a buffer tank for the conventional post-fermentation treatment, e.g. filtering, pasteurization and packaging. The ethanol production system described above is a faster and more convenient system than traditional fermentation processes and produces a product with acceptable taste equivalent to the taste resulting from conventional fermentation processes. The time needed for fermentation is reduced from weeks to a matter of hours by the present invention which, along with the continuous nature of the process, provides a vast potential for time and money savings in commercial alcohol or alcoholic beverages production. The following examples further illustrate many aspects of this invention. The examples, however, are not meant to stand as limitations or characterizations of the invention as it has been fully characterized in the foregoing text. Example 1 Preparation of Column Reactor Granular DEAE-cellulose (GDC) manufactured according to U.S. Patent 4,355,117 by Finnish Sugar Co. Ltd. of particle size 315 - 840 µm or 470 - 840 µm was used as a carrier. In all the experiments the carrier was filled, sterilized and yeast immobilized thereon according to the following procedure for handling GDC: With reference to the Figure, the hydration vessel (1) was first filled half with water. The mixer was started and dry carrier (GDC) was transferred to the vessel (1). When the hydration was completed (about 5 hours) the immobilized yeast reactor (2) was filled half with water and the carrier water slurry from the hydration vessel (1) was transferred to the reactor (2). In order to maintain the water level in the reactor, the bottom valve on the reactor was adjusted so that inlet and outlet flows from the reactor were about the same. The carrier in the reactor was then sterilized with hot dilute caustic alkali (3) by pumping it through the reactor (2). The carrier bed was then rinsed with water and neutralized by pumping a suitable dilute acid (4) through the carrier bed in the reactor (2) and finally the carrier bed was rinsed with sterile water. A yeast slurry was made up in the vessel (5). The yeast slurry was then pumped through the carrier bed in about 1-4 hours whereby the yeast was adsorbed onto the carrier. The yeast was now immobilized onto the carrier. The reactor (2) was basically ready for fermentation. Example 2 Fermentation of apple juice The yeast was cultured as it is done for batch fermentations and the carrier beds of about 500 ml each were prepared with an amount of yeast cells as described in Example 1. The carrier had a particle size of 0.315 to 0.840 mm. Immediately after the yeast had been adsorbed onto the carrier bed in the column (diameter 50 mm and height 250 - 300 mm) the substrate feed was started with a flow rate of 1 bed volume/day. The substrate was pasteurized apple juice where 1 g/l of ammonium phosphate was added as yeast nutrient. The total sugar concentration was adjusted to 220 g/l by adding sugar to the apple juice. The fermentation started slowly and after 3 weeks the ethanol level was about 4.5%. Example 3 Example 2 was repeated using a column loaded with a carrier of the particle size 0.470 - 0.840 mm. After 50 days operation the flow rate was reduced to about 0.8 bed volume/day which immediately increased the ethanol level during the following week from 5% to 7-8%. Example 4 Example 3 was repeated using a column which was wider and shorter (diameter 75 mm and height 152 mm). When the carrier bed had been sterilized and washed and neutralized (sodium-metabisulphite), the yeast immobilization was carried out with substantially higher yeast cell concentration: for 500 ml of GDC, 600 ml of yeast slurry was used which contained 150 million cells/ml. The leakage through the column was 5 million cells/ml so the total immobilized amount was 9 x 10¹⁰ cells. This is equal to 2 x 10⁹ cells/ml GDC or 5 x 10⁹ cells/g GDC. Prior to feeding apple juice substrate the column was treated for one day with yeast nutrient containing substrate solution. The fermentation started rapidly when compared to Examples 2 and 3 and on the second day the ethanol level was already at 8%. The flow rate was 1 bed volume/day and nutrients level 1 g/l juice. Some material was also lost from this column by carbon dioxide reflux. Reduction of feed rate after 3 weeks caused the alcohol level to raise again. After 1.5 months the nutrient level was raised to 2 g/l which raised the alcohol level further and the system stabilized at about a 10% ethanol level during its third month of operation. Example 5 Example 4 was repeated using an 8 litres reactor. The reactor was constructed so that in the outlet line there was also a screen plate to hold back any GDC particles. The system stabilized into a 10 % ethanol level within a week. A portion of the product from the 8 litres reactor was further fed to a second 500 ml column to increase the level of ethanol. All the experiments were carried out at room temperature (20 - 23°C). When the second reactor was operated under pressure high enough to keep the carbon dioxide dissolved a sparkling champagne type product was obtained, which if bottled under the same pressure as it is fermented makes the secondary fermentation (in the bottles) unnecessary and thus makes the 'champagne' process less complicated. Example 6 Primary beer fermentation The column preparation and yeast immobilization was carried out following the procedure described in Example 1. A 500 ml carrier bed was packed in a glass column which was wider in the upper end in order to ease the carbon dioxide separation. The wort was fed into the bottom of the fermentation column. The yeast cell concentration was adjusted to 10⁹ cells/g GDC. Traditional wort for lager beer brewing was used as a feed material. The wort was produced from 18 kg of barley malt (pilsner type) which gave a final volume of 100 l of finished wort (12.0°P). The mixture of water and malt grist was mashed in one vessel by programmed infusion method with rests at temperature 48°C for 15 minutes, 63°C for 30 minutes, 72°C for 20 minutes and 78°C at the end of mashing. The wort was clarified in a lauter tun and rinsed twice with 78°C water. The wort was boiled in a wort kettle for about 90 minutes. Hop pellets were added at the beginning of boiling (total amount of alpha acids was about 10 g). Any precipitation formed during boiling was separated in a whirlpool. The clarified wort was cooled in a plate heat exchanger from 100°C to 10°C. The composition of the wort was as follows: Original extract 12.0 °Plato Colour 10.0 °EBC Bitterness 25 EBU pH 5.3 Apparent attenuation rate 85% The composition of the wort, however, can vary as follows: Original extract 6-18 °Plato pH 4.5-5.5 Apparent attenuation rate 65-100% Fermentation of the wort in the reactor column was started slowly. After one week's operation, the rate was capable of producing an acceptable beer at less than one bed volume/day feed rate. The column behaviour was similar to that of the wine reactors in Examples 2 and 3. Example 7 Sake fermentation Low alcohol sake (Japanese rice wine) was produced. The column was prepared and the yeast was immobilized following the procedure set forth in Example 1. To make the substrate, the rice was first liqufied by steaming and hydrolyzing with alphaamylase and amyloglucosidase. The resulting hydrolysate was filtered in order to remove all mechanical impurities and unhydrolyzed residues. The substrate was finally diluted so that the glucose concentration was 23% when it was fed into the column. The column had a carrier colume of about 500 ml. The column diameter was 70 mm and diameter to height ratio of the carrier about 1:2. Comparison was carried out with Ca-alginate entrapped yeast. The feed rates, parameters and results for both fermentations are given in the following table. Result Support GDC Reactor Ca-alginate Reactor Fermentation time (days) 15 25 Temperature (°C) 15 - 20 15 - 20 Mean value of ethanol (v/v %) 9.99 13.6 Mean of flow rate (BV/h) 0.069 0.099 Productivity (g/h/l support) 5.44 10.62 Example 8 Mead fermentation Low alcohol mead (traditional Scandinavian beverage) was produced. The fermentation rate was controlled by regulating the feed rate and the required flavour and alcohol content in the product was achieved. The column preparation and yeast immobilization was carried out following the procedure described in Example 1. The yeast cell concentration was adapted to provide 10⁹ cells/g GDC. The aqueous substrate for fermentation was formulated as follows: Crystal (white) sugar 500 g Honey 125 g Soft brown sugar 625 g Juice of 2 lemons Water 10 l The column has a carrier volume of 500 ml. The substrate was fed into the bottom. The feed rate at the beginning was 1 bed volume per 5-10 hours. After stabilization of fermentation the feed rate was regulated to 1 bed volume per 1 - 5 hours depending on the ethanol content and sweetness desired in the product. The ethanol content was between 0.2 and 2.0% w/w in the beverage. To maintain the viability of the yeast, small amounts of ammonium phosphate (10-100 ppm) were added as a nutrient. The production was carried out at room temperature with the parameters described above. The immobilized yeast reactor can also be operated under pressure at lower temperature. Then the carbon dioxide formed is dissolved and final addition of carbon dioxide can be avoided. In addition to the foregoing substrate, other raw materials can also be used as substrates. All contain fermentable sugars, e.g. malt, fruits and berries. The flavour of the final product will depend on raw material, sugar concentration and fermentation rate. Example 9 Demonstration of Yeast Immobilization on several Carriers A brewer's yeast from the "Collection of Industrial Micro-organisms at the Technical Research Center of Finland" (Collection No. A-75050) was immobilized on two resins with anion exchange functionality. The resins were: Granulated DEAE cellulose having the trademark SPEZYME GDC 220 and a synthetic anion exchange resin having the trademark DUOLITE A 568. The immobilizations were performed according to the following procedure: The yeast was incubated for 48 hours in a malt extract broth at 30°C. The resin was sterilized by washing with 1 M sodium hydroxide, buffered to pH 5.0 - 5.1 and washed with sterile water. A 10 g dry weight sample of the resin was flushed in a 20 mm i.d. glass column equipped with a glass sinter bottom plate. 100 ml of the yeast suspension was passed by gravity through the column at an approximate rate of 3 bed volumes/hour, after which the column was washed with 100 ml of sterile water. The cell concentration of the yeast suspension before and after the immobilization was determined by an agar plate count. From the difference, the number of immobilized cells was calculated. The results as immobilized cells per gram of resin were as follows: Resin Yeast Offered Immobilized SPEZYME® A-75050 2.3 x 10⁸ 2.1 x 10⁸ DUOLITE® A-75050 2.3 x 10⁸ 2.2 x 10⁸ Example 10 Regeneration of the carrier The DEAE cellulose column used in the experiments was regenerated by feeding a hot (about 60°C) caustic soda solution (2% sodium hydroxide) through the column until the material colour was uniformly bright. The column was rinsed with sterile water until the pH of the leaving solution was about 10, and neutralized with sodium pyrosulphite to a pH of about 7. The column was rinsed with sterile water and filled with wort, pitched with yeast cells (about 10¹⁰ cells/litre of carrier), the cells were grown in the presence of aerated wort for 24 hours and were then ready for fermentation of the substrate.

Claims: A method for producing an ethanolic product, characterized in that in the primary production step an aqueous substrate containing a fermentable sugar is passed through a reactor containing yeast cells immobilized on the surfaces of a substantially noncompressible carrier being composed of a continuous porous matrix or dimpled or reticulated porous particles, the said matrix or particles having a structure formed from a loosely associated plurality of microparticles or microfibers which are chemically, adherently or mechanically bound together at least at some contact points between the individual microparticles or microfibers, said microparticles or microfibers being composed of an anion exchange resin, under the proviso that white birch wood chips are excluded. A method according to claim 1, characterized by loading a reactor with an aqueous mixture of a substantially noncompressible carrier as defined in claim 1; optionally sterilizing the loaded reactor; adsorbing a yeast broth on the carrier; feeding an aqueous substrate containing a fermentable sugar into the reactor containing immobilized yeast, and recovering the ethanolic product. A method according to claim 1 and 2, characterized by that the microparticles or microfibers composed of an anion exchange resin are selected from the group consisting of native or regenerated cellulose derivatized to provide anion exchange character. A method according to claims 1 to 3, characterized by that the microparticles or microfibers are bound together by adherent links. A method according to claims 1 to 3, characterized by that the carrier comprises particles formed from microfibers agglomerated with polystyrene and the anion exchange resin is diethylaminoethylene substituted cellulose. A method according to claims 1 to 5, characterized by regenerating the fermentation capacity of a reactor by removing the immobilized yeast material from the carrier packed within the reactor, and passing new yeast broth through the column to cause new yeast cells to become immobilized upon the carrier. A method according to claim 6, characterized by removing the immobilized yeast material by passing hot caustic alkali through the column, rinsing the column with water, neutralizing by passing a dilute acid through the column, and rinsing with water, and passing new yeast broth through the column. A method according to claims 1 to 7, characterized by passing the aqueous substrate through a plurality of reactors connected in series and having means for removing gas from the fluid leaving each reactor. A method according to claims 1 to 8, characterized by passing the aqueous substrate through the reactor at a pressure sufficient to maintain in a dissolved state a substantial portion of the carbon dioxide produced, thereby producing a carbonated ethanolic product. A method according to claims 1 to 9, characterized by that the flow of aqueous substrate through the reactor is in a direction against the force of gravity. A method according to claims 1 to 9, characterized by periodically reversing the flow of aqueous substrate through the reactor. A device for producing an ethanolic product including: a) a reactor (2) containing yeast cells immobilized on the surfaces of a substantially non-compressible carrier being composed of a continuous porous matrix or dimpled or reticulated porous particles, the said matrix or particles having a structure formed from a loosely associated plurality of microparticles or microfibers which are chemically, adherently or mechanically bound together at least at some contact points between the individual microparticles or microfibers, said microparticles or microfibers being composed of an anion exchange resin, under the proviso that white birch wood chips are excluded, this reactor (2) being combined with b) a vessel (1) for hydrating the dry carrier before this is pumped into the reactor (2), c) a vessel (3) including a sterilization liquid for sterilizing the carrier included in the reactor (2), d) a vessel (4) including a neutralization acid for neutralizing the carrier after sterilization, e) a vessel (5) including the yeast slurry to be pumped into the reactor (2) and to be fixed on the surface of the carrier, f) a vessel (6) comprising the substrate to be treated by the reactor (2), g) separation ports (7) for carbon dioxide, h) a vessel (8) for receiving the final product after the treatment in the reactor (2), and i) pressure releasing valves (9). A device as claimed in claim 12, wherein the reactor (2) comprises several pressurized columns in series. A device as claimed in claim 13, wherein means for separating carbon dioxide from the stream are placed between the columns.

Inventors: AHVENAINEN JUHA, LOMMI HEIKKI, AHVENAINEN, JUHA, LOMMI, HEIKKI
Applicants: CULTOR OY, CULTOR LTD.
Application year: 1989
Main classification: C12P7
IPC codes: C12G1, C12G3, C12P7, C12C11, C12N11