Field of the Invention

[0001] The present invention relates generally to anaerobic and aerobic microbial propagation, conditioning and/or fermentation. In particular, the present invention involves a method of reducing the concentration of undesirable microorganisms while simultaneously encouraging propagation and/or conditioning of desirable microorganisms and increasing the efficiency of desirable microorganisms during fermentation.

Background of the Invention

[0002] Microorganisms, such as yeast, fungi and bacteria, are used to produce a number of fermentation products, such as industrial grade ethanol, distilled spirits, beer, wine, pharmaceuticals and nutraceuticals (foodstuff that provides health benefits, such as fortified foods and dietary supplements). Yeast are also commonly utilized in the baking industry.

[0003] Yeast are the most commonly used microorganism in fermentation processes. Yeast are minute, often unicellular, fungi. They usually reproduce by budding or fission. One common type of yeast is Saccharomyces cerevisia, the species predominantly used in baking and fermentation. Non-Sacharomyces yeasts, also known as non-conventional yeasts, are also used to make a number of commercial products. Some examples of non-conventional yeasts include Kuyberomyces lactis, Yarrowia lipolytica, Hansenula polymorpha and Pichia pastoris.

[0004] However, other microorganisms can also be useful in making fermentation products. For example, cellulosic ethanol production, production of ethanol from cellulosic biomass, utilizes fungi and bacteria. Examples of these cellulolytic fungi include Trichoderma reesei and Trichoderma viride. One example of a bacteria used in cellulosic ethanol production is Clostridium Ijungdahlii.

[0005] Most of the yeast used in distilleries and fuel ethanol plants are purchased from manufacturers of specialty yeasts. The yeast are manufactured through a propagation process. Propagation involves growing a large quantity of yeast from a small lab culture of yeast. During propagation, the yeast are provided with the oxygen, nitrogen, sugars, proteins, lipids and ions that are necessary or desirable for optimal growth through aerobic respiration.

[0006] Once at the distillery, the yeast can undergo conditioning. The objective of both propagation and conditioning is to deliver a large volume of yeast to the fermentation tank with high viability, high budding and a low level of infection by other microorganisms. However, conditioning is unlike propagation in that it does not involve growing a large quantity from a small lab culture. During conditioning, conditions are provided to re-hydrate the yeast, bring them out of hibernation and allow for maximum anaerobic growth and reproduction.

[0007] Following propagation or conditioning, the yeast enter the fermentation process. The yeast are combined in an aqueous solution with fermentable sugars. The yeast consume the sugars, converting them into aliphatic alcohols, such as ethanol.

[0008] During these three processes the yeast can become contaminated with bacteria or other undesirable microorganisms. This can occur in one of the many vessels used in propagation, conditioning or fermentation. This includes propagation tanks, conditioning tanks, starter tanks, fermentations tanks, piping and heat exchangers between these units.

[0009] Bacterial or microbial contamination reduces the fermentation product yield in three main ways. First, the sugars that could be available for yeast to produce alcohol are consumed by the bacteria or other undesirable microorganisms and diverted from alcohol production. In addition to reducing yield, the end products of bacterial metabolism, such as lactic acid and acetic acid, inhibit yeast growth and yeast fermentation/respiration, which results in less efficient yeast production. Finally, the bacteria or other undesirable microorganisms compete with the yeast for nutrients other than sugar.

[0010] After the fermentation stream or vessel has become contaminated with bacteria or other undesirable microorganisms, those bacteria or other microorganisms can grow much more rapidly than the desired yeast. The bacteria or other microorganisms compete with the yeast for fermentable sugars and retard the desired bio-chemical reaction resulting in a lower product yield. Bacteria also produce unwanted chemical by-products, which can cause spoilage of entire fermentation batches. Removing these bacteria or other undesirable microorganisms allows the yeast to thrive, which results in higher efficiency.

[0011] As little as a one percent decrease in ethanol yield is highly significant to the fuel ethanol industry. In larger facilities, such a decrease in efficiency will reduce income from 1 million to 3 million dollars per year.

[0012] Some previous methods of reducing bacteria or other undesirable microorganisms during propagation, conditioning and fermentation take advantage of the higher temperature and pH tolerance of yeast over other microorganisms. This is done by applying heat to or lowering the pH of the yeast solution. However, these processes are not entirely effective in retarding bacterial growth. Furthermore, the desirable yeast microorganisms, while surviving, are stressed and not as vigorous or healthy. Thus, the yeasts do not perform as well.

[0013] The predominant trend in the ethanol industry is to reduce the pH of the mash to less than 4.5 at the start of fermentation. Lowering the pH of the mash reduces the population of some species of bacteria. However it is much less effective in reducing problematic bacteria, such as lactic-acid producing bacteria, and is generally not effective for wild yeast and molds. It also significantly reduces ethanol yield by stressing the yeast.

[0014] Another approach involves washing the yeast with phosphoric acid. This method does not effectively kill bacteria and other microorganisms. It can also stress the yeast, thereby lowering their efficiency.

[0015] Yet another method is to use heat or harsh chemicals and sterilize process equipment between batches. However this method is only effective when equipment is not in use. It is ineffective at killing bacteria and other microorganisms within the yeast mixture during production.

[0016] In yet another method, antibiotics have previously been added to yeast propagation, conditioning or fermentation batch to neutralize bacteria. Fermentation industries typically apply antibiotics to conditioning, propagation and fermentation processes. Antibiotic dosage rates range between 0.1 to 3.0 mg/L and generally do not exceed 6mg/L.

[0017] However, problems exist with using antibiotics in conditioning, propagation and fermentation. Antibiotics are expensive and can add greatly to the costs of large-scale production. Moreover, antibiotics are not effective against all strains of bacteria, such as antibiotic-resistant strains of bacteria. Overuse of antibiotics can lead to the creation of additional variants of antibiotic-resistant strains of bacteria.

[0018] Antibiotic residues and establishment of antibiotic-resistant strains is a global issue. These concerns may lead to future regulatory action against the use of antibiotics. One area of concern is distillers grain that is used for animal feed. European countries do not allow the byproducts of an ethanol plant to be sold as animal feed if antibiotics are used in the facility. Distiller grain sales account for up to 20% of an ethanol plant earnings. Antibiotic concentration in the byproduct can range from 1-3% by weight, thus negating this important source of income. Regulatory concerns have led to a desire to decrease use of antibiotics in fermentation processes. [0019] In addition, there are other issues to consider when using antibiotics. Calculating the correct dosage of antibiotic can be a daunting task. Even after dosages have been determined, mixtures of antibiotics should be constantly or at least frequently balanced and changed in order to avoid single uses that will lead to antibiotic- resistant strains. Sometimes the effective amount of antibiotic cannot be added to the fermentation mixture. For example, utilizing over 2 mg/L of Virginiamycin will suppress fermentation but over 25 mg/L is required to inhibit grown of Weisella confusa, an emerging problematic bacteria strain.

[0020] Fermentation plants can experience infections. This occurs when undesirable microorganism levels increase to above a normal or allowable level. This can occur due to process design, poor quality feed stock or other contributing factors. When this occurs antibiotic usage is usually increased to compensate for the infection. These conditions instigate overuse of antibiotic which can stress yeast and impact efficiency or cause regulatory non-compliance.

[0021] Chlorine dioxide (ClO ₂ ) has many industrial and municipal uses. When produced and handled properly, ClO ₂ is an effective and powerful biocide, disinfectant and oxidizer.

[0022] ClO ₂ has been used as a disinfectant in the food and beverage industries, wastewater treatment, industrial water treatment, cleaning and disinfections of medical wastes, textile bleaching, odor control for the rendering industry, circuit board cleansing in the electronics industry, and uses in the oil and gas industry. It is an effective biocide at low concentrations and over a wide pH range. ClO ₂ is desirable because when it reacts with an organism in water, it reduces to chlorite ion and then to chloride, which studies to date have shown does not pose a significant adverse risk to human health.

[0023] Previously, brewers added an aqueous 2-6% by weight sodium chlorite solution, otherwise known as stabilized chlorine dioxide, to their fermentation batches in an attempt to kill bacteria and other microorganisms. When sodium chlorite reacts in an acidic environment it can form ClO ₂ . The ClO ₂ added using this method was not substantially pure, which made it difficult to ascertain the amount added or control that amount with precision. If the amount is not precisely maintained, the ClO ₂ can kill the desired yeast or inhibit the glucoamylase enzyme that is present to prepare the fermentable sugars. If these undesirable consequences occur, the addition of ClO ₂ will not result in more efficient production. This method is also not effective at a neutral or basic pH level.

[0024] Producing ClO ₂ gas for treating yeast during the propagation, conditioning and/or fermentation process is desirable because there is greater assurance of ClO ₂ purity when in the gas phase. ClO ₂ is, however, unstable in the gas phase and will readily undergo decomposition into chlorine gas (Cl ₂ ), oxygen gas (O ₂ ), and heat. The high reactivity of ClO ₂ generally requires that it be produced and used at the same location.

[0025] Recently, it was discovered that chlorine dioxide effectively reduce undesirable microorganisms during propagation, conditioning and/or fermentation while encouraging propagation and/or conditioning of the desirable microorganisms and increase their efficiency in fermentation. This is discussed in co-owned U.S. Patent Application Serial Number 11/626,272, filed January 23, 2007, entitled "Apparatus and Method for Treatment of Microorganisms During Propagation, Conditioning and Fermentation," which claims priority benefits from U.S. Provisional Patent Application Serial Number 60/775,615, filed February 22, 2006, entitled "Apparatus and Method for Treatment of Yeast During Propagation, Conditioning and Fermentation." Both of these applications are hereby incorporated by reference in their entirety.

[0026] Since as little as a one percent decrease in ethanol yield is highly significant to the fuel ethanol industry, ethanol producers are constantly looking for ways to increase efficiency.

[0027] Accordingly, it would be desirable to provide a less costly and more effective method of reducing undesirable microorganisms during propagation, conditioning and/or fermentation than those currently used. It is also desirable that this method encourages propagation and/or conditioning of the desirable microorganisms and increase their efficiency in fermentation.

Summary of the Invention

[0028] In evaluation of chlorine dioxide for use in conditioning, propagation and fermentation, it was determined that not only is chlorine dioxide compatible with antibiotics, such as Erythromycin, Penicillin and Virginiamycin, but is synergistic when applying both technologies simultaneously. The process also allows for use of a decreased amount of antibiotic compared to when antibiotic is used alone.

[0029] An embodiment of the current method for reducing undesirable microorganism concentration, promoting yeast propagation, and increasing yeast efficiency in an aqueous fluid stream comprises (a) introducing a quantity of fermentable carbohydrate to an aqueous fluid stream, (b) introducing a quantity of yeast to the aqueous fluid stream, (c) generating ClO ₂ gas, (d) dissolving the ClO ₂ gas to form a ClO ₂ solution, (e) introducing an aqueous ClO ₂ solution into the aqueous fluid stream, and (f) introducing an aqueous antibiotic stream into the aqueous fluid stream. These steps can be performed sequentially or in a different order. [0030] In the foregoing method, the "undesirable" microorganisms intended to be reduced are those that compete for nutrients with the desirable microorganisms, such as yeast and Trichoderma that promote in the fermentation processes involved here. In this regard, the aqueous ClO ₂ solution and antibiotic employed in the present method does not appear to detrimentally affect the growth and viability of desirable, fermentation-promoting microorganisms, but does appear to eliminate or at least suppress the growth of undesirable microorganisms that interfere with the fermentation process. Moreover, the elimination or suppression of undesirable microorganisms appears to have a favorable effect on the growth and viability of desirable microorganisms, for the reasons set forth in the Background section.

[0031] The ClO ₂ gas can be generated by reacting chlorine gas with water and then adding sodium chlorite. Alternatively the ClO ₂ gas could be generated by reacting sodium hypochlorite with an acid and then adding sodium chlorite. The ClO ₂ gas can also be generated by reacting sodium chlorite and hydrochloric acid. The ClO ₂ gas can also be generated using electrochemical cells and sodium chlorate or sodium chlorite. Equipment-based generation could also be used to create ClO ₂ gas using sodium chlorate and hydrogen peroxide.

[0032] In one embodiment, the ClO ₂ solution has a concentration of less than about 15 mg/L and the antibiotic has a concentration of from about 0.1 to about 1.5 mg/L. In another embodiment the ClO ₂ solution has a concentration of between about 5 and about 50 mg/L and the antibiotic has a concentration of from about 0.5 to about 6 mg/L. In one embodiment the ClO ₂ solution has an efficiency as ClO ₂ in the stream of at least about 90%. As used in this application "to have an efficiency as ClO ₂ of at least about 90%" means that at least about 90% of the ClO ₂ solution or ClO ₂ gas is in the form of ClO ₂ .

[0033] Another embodiment of the current method that reduces undesirable microorganism concentration, promotes yeast propagation, and increases yeast efficiency in an aqueous fluid stream comprises (a) introducing a quantity of fermentable carbohydrate to an aqueous fluid stream, (b) introducing a quantity of yeast to the aqueous fluid stream, (c) introducing ClO ₂ having an efficiency as ClO ₂ of at least about 90% into the aqueous fluid stream and (d) introducing an antibiotic into the aqueous fluid stream. These steps can be performed sequentially or in a different order.

[0034] The ClO ₂ having an efficiency as ClO ₂ in the stream of at least about 90% can be produced by equipment or non-equipment based methods. Examples of non-equipment based methods of ClO ₂ generation include dry mix chlorine dioxide packets that include both a chlorite precursor packet and an acid activator packet. Equipment- based methods include using electrochemical cells with sodium chlorate or sodium chlorite, and a sodium chlorate/hydrogen peroxide method. [0035] In one embodiment, the ClO ₂ solution is in the form of an aqueous solution having a concentration of less than about 15 mg/L and the antibiotic has a concentration of from about 0.1 to about 1.5 mg/L. In another embodiment the ClO ₂ solution is in the form of an aqueous solution having a concentration of between about 5 and about 50 mg/L and the antibiotic has a concentration of from about 0.5 to about 6 mg/L. In another embodiment the ClO ₂ is in a gaseous form.

[0036] An embodiment of the current apparatus for reducing undesirable microorganisms, promoting producing microorganism propagation, and increasing efficiency comprises a ClO ₂ generator, a batch tank and a process vessel having a ClO ₂ solution inlet and an antibiotic inlet for containing an aqueous microorganism solution. The ClO ₂ generator comprises an inlet for introducing at least one chlorine- containing feed chemical and an outlet for exhausting a ClO ₂ gas stream from the generator. The batch tank is fluidly connected to the ClO ₂ generator outlet and receives the ClO ₂ gas stream from the ClO ₂ generator outlet. The batch tank comprises an inlet for introducing a second water stream and an outlet for exhausting an aqueous ClO ₂ solution from the batch tank. The process vessel is fluidly connected to the batch tank. The process vessel has an antibiotic inlet to introduce antibiotic. In operation, introducing the ClO ₂ and antibiotic to the process vessel promotes propagation of producing microorganisms present in the vessel. [0037] The batch tank preferably has an inlet for introducing a second water stream and an outlet for exhausting an aqueous ClO ₂ solution. In one preferred embodiment, the batch tank is capable of exhausting an aqueous ClO ₂ solution that has a concentration of less than about 5,000 mg/L. In one embodiment, the exhausted ClO ₂ solution is dosed to have a concentration between about 5 and about 50 mg/L and the antibiotic has a concentration of from about 0.5 to about 6 mg/L. In another embodiment, the exhausted ClO ₂ solution is dosed to have a concentration of less than about 15 mg/L and the antibiotic has a concentration of from about 0.1 to about 1.5 mg/L.

[0038] The process vessel can be a conditioning tank, heatable, capable of performing liquefaction or a yeast propagation vessel. The process vessel could also be a fermentation tank having an inlet for producing microorganisms, an inlet for water, an inlet for fermentation chemicals and an outlet for the fermentation product connecting to processing equipment.

[0039] Another embodiment of the current method for reducing undesirable microorganism concentration, promoting desirable microorganism propagation, and increasing desirable microorganism efficiency in an aqueous fluid stream comprises (a) introducing a quantity of cellulose to an aqueous fluid stream, (b) introducing a quantity of desirable microorganisms to the aqueous fluid stream, (c) generating ClO ₂ gas, (d) dissolving the ClO ₂ gas to form a ClO ₂ solution, (e) introducing an aqueous ClO ₂ solution into the aqueous fluid stream and (f) introducing an aqueous antibiotic stream into the aqueous fluid stream. These steps can be performed sequentially or in a different order. In one embodiment the ClO ₂ solution has an efficiency as ClO ₂ in the stream of at least about 90%.

[0040] Another embodiment of the current method that reduces undesirable microorganism concentration, promotes desirable microorganism propagation, and increases desirable microorganism efficiency in an aqueous fluid stream comprises (a) introducing a quantity of cellulose to an aqueous fluid stream, (b) introducing a quantity of desirable microorganisms to the aqueous fluid stream, (c) introducing ClO ₂ having an efficiency as ClO ₂ of at least about 90% into the aqueous fluid stream and (d) introducing an aqueous antibiotic stream into the aqueous fluid stream. These steps can be performed sequentially or in a different order.

Detailed Description of Preferred Embodiment(s)

[0041] In the present method, the concentrations of bacteria and other undesirable microorganisms are reduced while simultaneously propagation and/or conditioning of desirable microorganisms is encouraged, and the efficiency of those desirable microorganisms in fermentation and an apparatus for carrying out this method increased. [0042] Recently, chlorine dioxide was determined to be effective at reducing the concentration of bacteria and other undesirable microorganisms while simultaneously encouraging propagation and/or conditioning of desirable microorganisms and increasing the efficiency of those desirable microorganisms in fermentation. In evaluation of chlorine dioxide for use in conditioning, propagation and fermentation, it was determined that not only is chlorine dioxide compatible with antibiotics, such as Erythromycin, Penicillin and Virginiamycin, but is synergistic when applying both technologies simultaneously. Erythromycin, Penicillin and Virginiamycin are used as an example throughout this application. However, it is contemplated that other antibiotics could be used. For example beta-lactums, penicillins, natural penicillins, antistaphylococcal penicillins, aminopenicillins, antipseudomonl penicillins, beta-lactum/beta-lactamase inhibitors, cephalosporins (first, second, third and fourth generation), carbapenems, monobactums, glycopeptides, fluoroquinolones, aminoglycosides, tetracylines, glycyclines, macrolides, ketolides, oxazolidinones, nitroimidazoles, nitrofurans, streptogramins, cyclic lipopeptides and lincosamides could be used.

[0043] Plant scale evaluations have determined that adding a small amount of antibiotic, for example about .1 to about 1.5 mg/L or alternatively about .5 to about 6 mg/L, in addition to and simultaneously with chlorine dioxide results in a synergistic effect. The addition of chlorine dioxide and antibiotic simultaneously results in improved microbiology efficacy, enhanced ethanol production and reduced glycerol formation. This effect has been document in both low and high infection plants.

[0044] The production of fuel ethanol by yeast fermentation is used as an example. However, this is merely one illustration and should not be understood as a limitation. Other fermentation products could include distilled spirits, beer, wine, pharmaceuticals, pharmaceutical intermediates, baking products, nutraceuticals (foodstuff that provides health benefits, such as fortified foods and dietary supplements), nutraceutical intermediates and enzymes. The current method could also be utilized to treat yeast used in the baking industry. Other fermenting microorganisms could also be substituted such as the fungi and bacteria typically used in cellulosic ethanol production, Trichoderma reesei, Trichoderma viride, and Clostridium Ijungdahlii.

[0045] The fermentation process begins with the preparation of a fermentable carbohydrate. In ethanol production, corn is one possible fermentable carbohydrate. Other carbohydrates including cereal grains and cellulose-starch bearing materials, such as wheat or milo, could also be substituted. Cellulosic biomass such as straw and cornstalks could also be used. Cellulosic ethanol production has recently received attention because it uses readily available nonfood biomass to form a valuable fuel.

[0046] In corn-based ethanol production the corn is ground into a fine powder called meal. The meal is then mixed with water and enzymes, such as alpha-amylase, and passed through a cooker in order to liquefy the starch. A product known as corn mash results.

[0047] A secondary enzyme, such as glucoamylase, will also be added to the mash to convert the liquefied starch into a fermentable sugar. The glucoamylase cleaves single molecules of glucose from the short chain starches, or dextrins. The glucose molecules can then be converted into ethanol during fermentation.

[0048] Yeast, small microorganisms capable of fermentation, will also be added to the corn mash. Yeast are fungi that reproduce by budding or fission. One common type of yeast is Saccharomyces cerevisia, the species predominantly used in baking and fermentation. Non-Sacharomyces yeasts, also known as non-conventional yeasts, are naturally occurring yeasts that exhibit properties that differ from conventional yeasts. Non-conventional yeasts are utilized to make a number of commercial products such as amino acids, chemicals, enzymes, food ingredients, proteins, organic acids, nutraceuticals, pharmaceuticals, cosmetics, polyols, sweeteners and vitamins. Some examples of non-conventional yeasts include Kuyberomyces lactis, Yarrowia lipolytica, Hansenula polymorpha and Pichia pastoris. The current methods and apparatus are applicable to intermediates and products of both Sacharomyces and non-conventional yeast.

[0049] Most of the yeast used in fuel ethanol plants and other fermentation processes are purchased from manufacturers of specialty yeast. The yeast are manufactured through a propagation process and usually come in one of three forms: yeast slurry, compressed yeast or active dry yeast. Propagation involves growing a large quantity of yeast from a small lab culture of yeast. During propagation the yeast are provided with the oxygen, nitrogen, sugars, proteins, lipids and ions that are necessary or desirable for optimal growth through aerobic respiration.

[0050] Once at the distillery, the yeast may undergo conditioning. The objectives of both propagation and conditioning are to deliver a large volume of yeast to the fermentation tank with high viability, high budding and a low level of infection by other microorganisms. However, conditioning is unlike propagation in that it does not involve growing a large quantity from a small lab culture. During conditioning, conditions are provided to re-hydrate the yeast, bring them out of hibernation and allow for maximum anaerobic growth and reproduction.

[0051] Following propagation or conditioning, the yeast enter the fermentation process. The glucoamylase enzyme and yeast are often added into the fermentation tank through separate lines as the mash is filling the fermentation tank. This process is known as simultaneous saccharification and fermentation or SSF. The yeast produce energy by converting the sugars, such as glucose molecules, in the corn mash into carbon dioxide and ethanol.

[0052] The fermentation mash, now called "beer" is distilled. This process removes the 190 proof ethanol, a type of alcohol, from the solids, which are known as whole stillage. These solids are then centrifuged to get wet distillers grains and thin stillage. The distillers grains can be dried and are highly valued livestock feed ingredients known as dried distillers grains (DDGS). The thin stillage can be evaporated to leave a syrup. After distillation, the alcohol is passed through a dehydration system to remove remaining water. At this point the product is 200 proof ethanol. This ethanol is then denatured by adding a small amount of denaturant, such as gasoline, to make it unfit for human consumption.

[0053] The propagation, conditioning and fermentation processes can be carried out using batch and continuous methods. The batch process is used for small-scale production. Each batch is completed before a new one begins. The continuous fermentation method is used for large-scale production because it produces a continuous supply without restarting every time. The current method and apparatus are effective for both methods. [0054] During the propagation, conditioning or fermentation process the mash or the fermentation mixture can become contaminated with other microorganisms, such as spoilage bacteria, wild yeast or killer yeast. These microorganisms compete with the yeast for fermentable sugars and retard the desired bio-chemical reaction resulting in a lower product yield. They can also produce unwanted chemical by-products, which can cause spoilage of entire fermentation batches. Wild yeast are a primary concern in the beverage industry because they can cause taste and odor problems with the final product. Killer yeast produce a toxin that is lethal to the desired alcohol producing yeast.

[0055] Producers of ethanol attempt to increase the amount of ethanol produced from one bushel of cereal grains, which weigh approximately 56 pounds (25.4 kilograms). Contamination by microorganisms lowers the efficiency of yeast making it difficult to attain or exceed the desired levels of 2.8-2.9 gallons per bushel (.42- .44 liters per kilogram). Reducing the concentration of microorganisms will encourage yeast propagation and/or conditioning and increase yeast efficiency making it possible to attain and exceed these desired levels.

[0056] ClO ₂ solution has many uses in disinfection, bleaching and chemical oxidation. Yeast can withstand and indeed thrive in a ClO ₂ environment. However, bacteria, wild yeasts, killer yeasts and molds will succumb to the properties of ClO ₂ allowing the producing, desirable yeast to thrive and achieve higher production.

[0057] Recently, it was determined that ClO ₂ can be added at various points in the propagation, conditioning and/or fermentation processes to kill unwanted microorganisms and promote growth and survival of the desirable microorganisms. This ClO ₂ can be added as an aqueous solution or a gas. The ClO ₂ can be added during propagation, conditioning and/or fermentation. The ClO ₂ solution can be added to cook vessels, fermentation tanks, propagation tanks, conditioning tanks, starter tanks or during liquefaction. The ClO ₂ solution can also be added to the interstage heat exchange system or heat exchangers. In one embodiment the ClO ₂ has an efficiency as ClO ₂ in the stream of at least about 90%. Adding ClO ₂ having a known purity allows for addition of a controlled amount of ClO ₂ .

[0058] Similarly, antibiotics are useful for killing bacteria, wild yeasts, killer yeasts and molds while allowing yeast or other producing microorganisms to survive and thrive. Fermentation industries typically apply antibiotics to conditioning, propagation and fermentation. Typically, antibiotic dosage rates range between 0.1 to 3.0 mg/L and do not typically exceed 6 mg/L.

[0059] In evaluation of chlorine dioxide for use in conditioning, propagation and fermentation, it was determined that not only is chlorine dioxide compatible with antibiotics, such as Erythromycin, Penicillin and Virginiamycin, but is synergistic when applying both technologies simultaneously. The chlorine dioxide and antibiotics do not compete or decrease the effectiveness of the other. Rather, applying chlorine dioxide and antibiotics simultaneously increases ethanol yield over that achieved by adding either by itself.

[0060] Plant scale evaluations have determined that adding a small amount of antibiotic, for example about .1 to about 1.5 mg/L, in addition to and simultaneously with chlorine dioxide results in a synergistic effect. The addition of chlorine dioxide and antibiotic simultaneously results in improved microbiology efficacy, enhanced ethanol production and reduced glycerol formation. This effect has been document in both low and high infection plants.

[0061] Evaluations were conducted at two fuel ethanol facilities utilizing chlorine dioxide and the antibiotics Virginiamycin, Penicillin and Erythromycin. One facility had low microbial loading. The other facility was an infected plant with high microbial loading. Both facilities dosed chlorine dioxide to the propagator, fermentor and mash cooler heat exchanger at a dosage rate of between about 1 and about 50 mg/L. Antibiotics were simultaneously applied to the propagator and fermentor at a rate of between about 0.1 and about 1.5 mg/L.

[0062] The fermentation batches were evaluated using high performance liquid chromatography (HPLC). The HPLC analysis was calibrated to ethanol, acetic acid, lactic acid and glycerol.

[0063] The following tables detail HPLC analysis for the infected plant. Three fermentation batches were evaluated in each example.

TABLE 1

High Microbiological Loading Plant Treated with Chlorine Dioxide Only

TABLE 2

High Microbiological Loading Plant Treated with Antibiotics Only

TABLE 3

High Microbiological Loading Plant Treated with Chlorine Dioxide and Antibiotics

[0064] As shown in Tables 1-3, ethanol efficiency in an infected, high microbiological loading, high organic acid plant is increased by addition of chlorine dioxide and antibiotics simultaneously. Chlorine dioxide alone produces an ethanol efficiency of 14.43% by volume. Antibiotics alone produce an ethanol efficiency of 12.0% by volume. Chlorine dioxide and antibiotics used simultaneously produce an ethanol efficiency of 15.2% by volume. An unexpected result was that no detrimental or inhibitory effect was noted between chlorine dioxide and antibiotics.

[0065] Another plant scale evaluation determined that adding a small amount of chlorine dioxide to antibiotics results in a synergistic effect. The addition of chlorine dioxide and antibiotic simultaneously results in improved microbiology efficacy and enhanced ethanol production.

[0066] Evaluations were conducted at a fuel ethanol facility utilizing chlorine dioxide and the antibiotics Virginiamycin, Penicillin and Erythromycin. The facility was an infected plant with high microbial loading. The facility dosed chlorine dioxide to the 78,000 gallon fermentation tanks at a dosage rate of about 22 lb/hr for 24 hours per day. Approximately, 2-3 lbs of the antibiotics were also added to the fermentation tanks each day. The plant was previously using 20-22 lbs of the antibiotics per day with no chlorine dioxide. The plant was treated using only antibiotics for approximately four months and then using chlorine dioxide and antibiotics for three months.

[0067] The fermentation batches were evaluated using high performance liquid chromatography (HPLC). The HPLC analysis was calibrated to ethanol, acetic acid and lactic acid.

[0068] The following table averages the HPLC analysis for the infected plant for the three months with antibiotics alone and the three months with reduced antibiotics and chlorine dioxide.

TABLE 4

High Microbiological Loading Plant Treated with Antibiotics Alone and with Chlorine Dioxide and Antibiotics

[0069] As shown in Tables 4-5, ethanol efficiency in an infected, high microbiological loading, high organic acid plant is increased by addition of chlorine dioxide and antibiotics simultaneously. Antibiotics alone produce an ethanol efficiency of 12.980% by volume. Chlorine dioxide and antibiotics used simultaneously produce an ethanol efficiency of 13.274% by volume. An unexpected result was that no detrimental or inhibitory effect was noted between chlorine dioxide and antibiotics.

[0070] The antibiotics can be added simultaneously with the chlorine dioxide at the various points in the propagation, conditioning and/or fermentation processes where chlorine dioxide was previously added. The antibiotics can be added to cook vessels, fermentation tanks, propagation tanks, conditioning tanks, starter tanks or during liquefaction. The antibiotics solution can also be added to the interstage heat exchange system or heat exchangers.

[0071] As mentioned above, ClO ₂ and antibiotics can be added directly into the fermentation mixture. This can be done by adding the ClO ₂ and antibiotics in conjunction with the yeast and glucoamylase, for example during the SSF stage. Chlorine dioxide dosages of less than about 15 mg/L, preferably less than about 10 mg/L and most preferably less than about 7.5 mg/L used with antibiotic dosages of between .5 and 6 mg/L, preferably between 0.1 and 1.5 mg/L showed greater ethanol production than the controls containing only chlorine dioxide or only antibiotics.

[0072] The ClO ₂ can also be added to the mash prior to the fermentation process, for example before the SSF stage. Chlorine dioxide dosages of between about 10 and about 75 mg/L, preferably between about 10 and about 50 mg/L and most preferable between about 20 and about 50 mg/L used with antibiotic added to the fermentor or propagator at dosages of between .5 and 6 mg/L, preferably between 0.1 and 1.5 mg/L showed greater ethanol production than the controls containing only chlorine dioxide or only antibiotics.

[0073] Chlorine dioxide and antibiotics can also be added during propagation and/or conditioning. For example ClO ₂ can be added to the yeast slurry before SSF replacing the acid washing step. Chlorine dioxide dosages of less than about 50 mg/L used with antibiotic dosages of between .5 and 6 mg/L, preferably between 0.1 and 1.5 mg/L showed greater ethanol production than the controls containing only chlorine dioxide or only antibiotics.

[0074] Since ClO ₂ gas can decompose explosively, it is typically produced on-site. There are a number of methods of producing ClO ₂ gas having a known purity, which are known to persons familiar with the technology involved here. One or more of these methods can be used. ClO ₂ gas can be produced using electrochemical cells and a sodium chlorite or sodium chlorate solution. An equipment based sodium chlorate/hydrogen peroxide method also exists. Alternatively, non-equipment based binary, multiple precursor dry or liquid precursor technologies can be used. Examples of non-equipment based methods of ClO ₂ generation include dry mix chlorine dioxide packets that include both a chlorite precursor packet and an acid activator packet. Other such processes include, but are not limited to, acidification of sodium chlorite, oxidation of chlorite by chlorine, oxidation of chlorite by persulfate, use of acetic anhydride on chlorite, use of sodium hypochlorite and sodium chlorite, use of dry chlorine/chlorite, reduction of chlorates by acidification in the presence of oxalic acid, reduction of chlorates by sulfur dioxide, and the ERCO R-2 ^® , R-3 ^® , R- 5 ^® , R-8 ^® , R- 10 ^® and R-11 ^® processes, from which ClO ₂ is generated from NaClO ₃ in the presence of NaCl and H ₂ SO ₄ (R-2 and R-3 processes), from NaClO ₃ in the presence of HCl (R-5 process), from NaClO ₃ in the presence Of H ₂ SO ₄ and CH ₃ OH (R-8 and R-IO processes), and from NaClO ₃ in the presence of H ₂ O ₂ and H ₂ SO ₄ (R- 11 process).

[0075] Here, three methods will illustrate some possibilities. In the first method, chlorine reacts with water to form hypochlorous acid and hydrochloric acid. These acids then react with sodium chlorite to form chlorine dioxide, water and sodium chloride. In a second method, sodium hypochlorite is combined with hydrochloric or other acid to form hypochlorous acid. Sodium chlorite is then added to this reaction mixture to produce chlorine dioxide. The third method combines sodium chlorite and sufficient hydrochloric acid. In one embodiment the ClO ₂ gas produced is between 0.0005 and 5.0 % by weight in air.

[0076] The ClO ₂ gas is dissolved in a solvent in order to create a ClO ₂ solution. ClO ₂ gas is readily soluble in water. In one embodiment the water and ClO ₂ gas are combined in quantities that create a solution for application directly to the fermentation mixture, with a concentration of less than about 15 mg/L, preferably less than about 10 mg/L, and most preferably less than about 7.5 mg/L. In another embodiment the water and ClO ₂ gas are combined in quantities that create a solution for application to the corn mash prior to fermentation, with a concentration of between about 10 and about 75 mg/L, preferably between about 10 and about 50 mg/L, and most preferable between about 20 and about 50 mg/L. In yet another embodiment the water and ClO ₂ gas are combined in quantities that create a solution for application to the yeast during propagation with a concentration of less than about 50 mg/L. In the solution of one embodiment the ClO ₂ solution has an efficiency as ClO ₂ in the stream of at least about 90%.

[0077] Pure or substantially pure ClO ₂ is desirable because it allows the user to precisely maintain the amount of ClO ₂ added to the yeast. (The single term "pure" will be used hereinafter to mean either pure or substantially pure.) If too little ClO ₂ is added the dosage will not be effective in killing undesirable microorganisms. If too much ClO ₂ is added it can kill the desired yeast. If either of these situations occurs, the addition of ClO ₂ will not result in more efficient ethanol production. Addition of pure ClO ₂ allows the user to carefully monitor and adjust the amount of ClO ₂ added to the yeast. This enables the user to add adequate ClO ₂ to assure microbial efficacy without killing the yeast. [0078] Pure ClO ₂ is also desirable for another reason. Glucoamylase enzyme is important in ethanol production to convert short chain starches (or dextrins) into fermentable glucose molecules. ClO ₂ does not exhibit a significant reaction with glucoamylase. However, ClO ₂ can reduce to form chlorite ion. The chlorite ion can inhibit the glucoamylase enzyme at approximately 14 mg/L and above. Inhibition of glucoamylase enzyme can lower ethanol production. A chlorite ion concentration of 14 mg/L can be produced by a ClO ₂ dosage rate of about 50 to 60 mg/L. Addition of pure ClO ₂ allows the user to add dosage rates below the level where glucoamylase inhibition can occur.

[0079] The ClO ₂ solution and antibiotic is introduced at some point during the production of ethanol. The ClO ₂ solution and antibiotic can be added during propagation, conditioning and/or fermentation. The ClO ₂ solution can also be added directly to the corn mash while antibiotic is added to the fermentor or propagator. The ClO ₂ solution and antibiotic can be added to cook vessels, fermentation tanks, propagation tanks, conditioning tanks, starter tanks or during liquefaction. The ClO ₂ solution and antibiotic can also be added to the piping between these units or heat exchangers.

[0080] ClO ₂ and antibiotics can also be used simultaneously to achieve improved results in the production of cellulosic ethanol. Cellulosic ethanol is a type of ethanol that is produced from cellulose, as opposed to the sugars and starches used in producing carbohydrate based ethanol. Cellulose is present in non-traditional biomass sources such as switch grass, corn stover and forestry. This type of ethanol production is particularly attractive because of the large availability of cellulose sources. Cellulosic ethanol, by the very nature of the raw material, introduces higher levels of contaminants and competing microorganism into the fermentation process. ClO ₂ and antibiotics used simultaneously could be particularly helpful in cellulosic ethanol production as an antimicrobial agent.

[0081] There are two primary processes of producing alcohol from cellulose. One process is a hydrolysis process that utilizes a fungi such as Trichoderma reesei and Trichoderma viride. The other is a gasification process using a bacteria such as Clostridium Ijungdahlii. ClO ₂ and antibiotics could be utilized in either process.

[0082] In the hydrolysis process the cellulose chains are broken down into five carbon and six carbon sugars before the fermentation process. This is either done chemically and enzymatically.

[0083] In the chemical hydrolysis method the cellulose can be treated with dilute acid at high temperature and pressure or concentrated acid at lower temperature and atmospheric pressure. In the chemical hydrolysis process the cellulose reacts with the acid and water to form individual sugar molecules. These sugar molecules are then neutralized and yeast fermentation is used to produce ethanol. ClO ₂ and antibiotics could be used during the yeast fermentation portion of this method as outlined above.

[0084] Enzymatic hydrolysis can be carried out using two methods. The first is known as direct microbial conversion (DMC). This method uses a single microorganism to convert the cellulosic biomass to ethanol. The ethanol and required enzymes are produced by the same microorganism. ClO ₂ and antibiotics could be used during the propagation/conditioning or fermentation steps with this specialized organism.

[0085] The second method is known as the enzymatic hydrolysis method. In this method cellulose chains are broken down using cellulase enzymes. These enzymes are typically present in the stomachs of ruminants, such as cows and sheep, to break down the cellulose that they eat. In this process the cellulose is made via fermentation by cellulolytic fungi such as Trichoderma reesei and Trichoderma viride.

[0086] The enzymatic method is typically carried out in four or five stages. The cellulose is pretreated to make the raw material, such as wood or straw, more amenable to hydrolysis. Next the cellulase enzymes are used to break the cellulose molecules into fermentable sugars. Following hydrolysis, the sugars are separated from residual materials and added to the yeast. The hydrolyzate sugars are fermented to ethanol using yeast. Finally, the ethanol is recovered by distillation. Alternatively, the hydrolysis and fermentation can be carried out together by using special bacteria or fungi that accomplish both processes. When both steps are carried out together the process is called sequential hydrolysis and fermentation (SHF).

[0087] ClO ₂ and antibiotics can be introduced for microbiological efficacy at various points in the enzymatic method of hydrolysis. ClO ₂ and antibiotics could be used in the production, manufacture and fermentation of cellulase enzymes made by Trichoderma and other fungi strains. The ClO ₂ and antibiotics can be added in the cellulosic simultaneous saccharification and fermentation phase (SSF). The ClO ₂ and antibiotics can be introduced in the sequential hydrolysis and fermentation (SHF) phase. They could also be introduced at a point before, during or after the fermentation by cellulolytic fungi that create the cellulase enzymes. Alternatively the ClO ₂ and antibiotics could be added during the yeast fermentation phase, as discussed above.

[0088] The gasification process does not break the cellulose chain into sugar molecules. First, the carbon in the cellulose is converted to carbon monoxide, carbon dioxide and hydrogen in a partial combustion reaction. Then, the carbon monoxide, carbon dioxide and hydrogen are fed into a special fermenter that uses a microorganism such as Clostridium Ijungdahlii that is capable of consuming the carbon monoxide, carbon dioxide and hydrogen to produce ethanol and water.

Finally, the ethanol is separated from the water in a distillation step. ClO ₂ and antibiotics could be used as an antimicrobial agent in the fermentation step involving microorganisms such as Clostridium Ijungdahlii that are capable of consuming carbon monoxide, carbon dioxide and hydrogen to produce ethanol and water.

[0089] Another embodiment of the current technology is an apparatus for carrying out the fermentation process with an integrated ClO ₂ and antibiotic system.

[0090] The apparatus has a ClO ₂ generator. The ClO ₂ generator has an input for electricity. There is also an inlet for at least one chlorine containing chemical. There are three different types of chemical feed systems: a vacuum system, a pressure system and a combination system. Many types of feed systems can be employed to deliver chemicals in a fluid state. Chlorine gas, for example, can be added by a vacuum or combination feed system. The ClO ₂ generator should also have an outlet for exhausting a ClO ₂ gas stream from the generator. In one embodiment the ClO ₂ gas stream exiting the generator is between 0.0005 and 5.0 % by weight in air.

[0091] A batch tank that receives the ClO ₂ gas stream is fluidly connected to the ClO ₂ generator outlet. In the batch tank the ClO ₂ gas is dissolved in water to form a ClO? solution. The batch tank has an inlet for introducing a water stream. The water stream and the ClO ₂ gas stream are combined to form a ClO ₂ solution. The concentration of the ClO ₂ solution in the batch tank can vary across a wide range. Concentrations of up to about 5,000 mg/L can be achieved and concentrations of up to about 8,000 mg/L can be achieved with additional equipment. The ClO ₂ solution is then exhausted from the batch tank through an outlet at a specified dosage rate to create a solution of the desired concentration. In one embodiment the dosed ClO ₂ solution, for application directly to the fermentation mixture, has a concentration of less than about 15 mg/L, preferably less than about 10 mg/L, and most preferable less than about 7.5 mg/L. In another embodiment the dosed ClO ₂ solution, for application to the corn mash prior to fermentation, has a concentration of between about 10 and about 75 mg/L, preferably between about 10 and about 50 mg/L, and most preferable between about 20 and about 50 mg/L. In yet another embodiment the dosed ClO ₂ solution, for use in propagation has a concentration of less than about 50 mg/L. In one embodiment, the exiting ClO ₂ solution has an efficiency as ClO ₂ in the stream of at least about 90%.

[0092] A process vessel containing an aqueous microorganism solution is fluidly connected to the batch tank and the antibiotic tank via outlets on the batch tank and antibiotic tank. The process vessel could be a cook vessel, fermentation tank, conditioning tank, starter tank, propagation tank, liquefaction vessel and/or piping or heat exchanger between these units. Introducing the ClO ₂ solution and antibiotic into the process vessel is capable of promoting propagation of producing microorganism present while simultaneously decreasing the concentration of undesirable microorganisms.

[0093] The process vessel has a ClO ₂ solution inlet and an antibiotic inlet. Antibiotic such as Virginiamycin, Penicillin or Erythromycin is introduced through the antibiotic inlet. The antibiotic is dissolved in the process water. The concentration of the antibiotic in the process vessel can vary across a wide range. In one embodiment the dosed antibiotic has a concentration between about .5 and 6 mg/L, preferably between about 0.1 and about 1.5 mg/L. The antibiotic can be introduced via packets containing measured amounts of antibiotic.

[0094] For smaller scale production of fermentation products, skid- mounted equipment is ideal. Skid mounting allows the equipment to be manufactured off site, shipped to the desired location and easily installed. This ensures ease in transportation, faster erection and commissioning. The ClO ₂ generator, batch tank, process vessel and connecting equipment could be made in a skid-mounted fashion.

[0095] While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.