1. Field of the Invention
This invention relates to the preparation of vinegar by the aerobic fermentation of ethanol. More particularly, it relates to the use of semi-continuous and batch fermentation in the production of vinegar. This invention especially relates to the utilization of membrane filtration and the combination of semi-continuous and batch aerobic conversion of ethanol by Acetobacter bacteria in the production of vinegar.
2. Description of the Prior Art
Vinegar has been produced for centuries by the aerobic bacterial fermentation of ethanol. In the presence of bacteria of the genus Acetobacter, oxygen oxidizes ethanol to acetic acid, a dilute solution of which is vinegar. The bacteria utilized is referred to in the art as vinegar bacteria.
As employed herein, the concentration of alcohol is expressed in volume by volume and the concentration of acetic acid in weight by volume. The sum of percent weight by volume of acetic acid and percent volume by volume of alcohol is called total concentration. Since 1% volume by volume of alcohol is oxidized during fermentation to almost exactly 1% weight by volume of acetic acid, the total concentration thus means the maximum acidity to which a mash can theoretically be fermented.
Improvements have been made over the years in the conventional batch fermentation process so that today vinegar is produced by semi-continuous and continuous processes. Since the fermentation conversion of ethanol is an exothermic process, the temperature of a fermentation mixture will slowly increase with time unless cooling is provided. Also, as a batch of alcohol is converted to vinegar the concentration of alcohol decreases while the concentration of acetic acid increases. Both high temperature and high acid concentration suppress the growth of vinegar bacteria and are therefore considered undesirable in the commercial production of vinegar. Cooling of the batch mixture has been employed to maintain a fermentation temperature of below 30.degree. C. and the acetic acid concentration has been limited in some cases to no more than 7%. This has achieved satisfactory bacteria growth but has resulted in lowering the rate of production of vinegar. In another approach to the problem, attempts have been made to develop strains of vinegar bacteria which will grow at higher rates in high acetic acid concentrations and at high fermentation temperatures. The efforts to date have been time consuming and the results have not been as good as expected.
Other efforts have been directed to improving the production rate of vinegar by increasing the concentration of the bacteria in the fermentation mash by other means, such as recovering the bacteria when the fermented mash is removed from the fermentor and returning the bacteria to the fermentor. Such methods as centrifugation and precipitation have been suggested to recover the bacteria. Limited success has been achieved by these approaches because the mechanical forces often injure the bacteria and the period of time during which the bacteria is denied oxygen results in the death of a high percentage of vinegar bacteria. Methods of separating the mash from the bacteria within the fermentor keep the bacteria in an oxygen environment but maintenance of the equipment has proven troublesome.
Membrane filtration is employed when small particles and/or macromolecules are to be recovered or removed from a liquid. Membranes of varying porosity are employed and the nature of the retained materials may be employed in classifying the types of membrane filtration: microfiltration, ultrafiltration and reverse osmosis. Ambiguity and overlapping of the size of particulates or molecules retained by the membranes employed in these several processes has occurred in the field. In general the pores of the membranes employed in microfiltration are too large to retain macromolecules in solution but will retain particulates, organisms, colloids and viruses. Ultrafiltration membranes will retain macromolecules in the 300 to 300,000 molecular weight range, e.g., proteins, carbohydrates and polymers. Reverse osmosis membranes will retain ionic species, e.g., NaCl, MgSO.sub.4, which generally pass through microfiltration and ultrafiltration membranes. Since ionic species have relatively high osmotic pressures, reverse osmosis is a relatively high pressure (400-1000 psi) process while microfiltration and ultrafiltration are practiced at much lower pressures (1-100 psi). (See, Porter, M. C., Selecting the Right Membrane and Making It Work, presented at the 79th Annual Meeting of A.I.Ch.E.).
Ultrafiltration membranes are classified by their nominal molecular weight cutoff or fractionation, i.e., a 10,000 mw cutoff membrane will retain materials of 10,000 molecular weight and above. Microfiltration membranes are described by their porosity, usually in terms of micrometers. Tangential flow is conventionally employed in membrane filtration to prevent a build-up of retained material at the filtering surface. This type of flow is inherent in micro and ultrafiltration. As used herein the term membrane filtration, will include only relatively low pressure membrane filtration, micro and ultrafiltration, i.e. reverse osmosis will not be included.
One vinegar recovery method disclosed in Japanese Patent Publication No. 8150-1980, published Mar. 1, 1980, involves the use of ultrafiltration in either a semi-continuous or a continuous vinegar process. A hollow fiber ultrafiltration unit having a nominal molecular weight fractionation of 13,000 is employed. In the subject process it is used to retain the vinegar bacteria. The mash, removed from a fermentation vessel employing an aerobic bacterial vinegar fermentation procedure, is subjected to ultrafiltration producing a clear mash containing no vinegar bacteria and a mash containing the bacteria in a concentration greater than that in the mash removed from the fermentor. The clear mash is discharged for recovery of the vinegar while the bacteria-rich mash is returned to the fermentation vessel to raise the bacteria concentration level of the mash in the fermentor. In the disclosed process the bacteria-rich mash must be returned to the fermentor within one minute after the mash is removed from the fermentor because of the physiology of the vinegar bacteria.
In commonly assigned and copending patent application Ser. No. 437,425, filed Oct. 28, 1982, incorporated herein by reference, quantities of gas, such as, air, or mixtures of air and carbon dioxide or nitrogen, are added to a vinegar mash prior to micro or ultrafiltration. Substantially more of the liquid passes through the filter membrane than if the mixture were subjected to substantially the same membrane filtration without having been mixed with gas. The retained concentrate of the vinegar bacteria and the remaining liquid containing ethanol and acetic acid are returned to the fermentation vessel so as to increase the bacteria level therein thereby causing a concomitant increase in the rate of fermentation.
U.S. Pat. No. 4,076,844 of Ebner et al. discloses a two stage serial fermentation process for producing vinegar with an acetic acid concentration of more than 15%. In the first stage, the alcohol-containing mixture is fermented whereby both bacteria multiplication and acidification take place at a temperature of 27.degree.-34.degree. C. to provide an acetic acid concentration which does not exceed 15%. In the second stage the fermentation is continued at the same temperature as in the first stage. Acidification takes place in this second stage but bacteria multiplication decreases and then stops to provide an acetic acid concentration above 15%. The fermentation may be conducted in two vessels with a portion of the mash transferred from the first to the second fermentor. While the fermentation is concluded in the second fermentor, the first fermentor is re-supplied and the initial fermentation is continued in a semi-continuous fashion.
U.S. Pat. No. 4,282,257 of Kunimatsu et al. also discloses a two stage serial fermentation process for producing vinegar. In the first stage, the fermentation is conducted at a temperature of 27.degree.-32.degree. C. until the acetic acid concentration reaches 12-15%. In the second stage, the fermentation is continued at a temperature of 18.degree.-24.degree. C. until an acetic acid concentration higher than 18% is obtained. The fermentation may be conducted in a single fermentor or in two fermentors in a fashion similar to that employed by Ebner et al.
Japanese Patent Publication No. 55,193-1981, published May 15, 1981 discloses a process which enables the production of vinegar containing more than 20% acetic acid whereby a vinegar mash is fermented at 27.degree.-32.degree. C. until the acetic acid concentration reaches 12-15% and then the temperature is decreased by less than 2.degree. C. for every 1-2% increase in acetic acid concentration.
Japanese Patent Publication No. 61,987-1981, published May 27, 1981 discloses a semi-continuous vinegar fermentation process. A vinegar mash is aerobically incubated at 27.degree.-31.degree. C. until the acetic acid concentration is 12-15%. A major portion of the mash is transferred to a second fermentator, mixed with additional ethanol and incubated at above 18.degree. C. to obtain an acetic acid concentration of at least 20%. In the meantime, fresh mash is added to the first fermentor to continue the fermentation.
It is an object of this invention to provide an improved process for producing vinegar by the aerobic fermentation of ethanol.
It is another object of this invention to provide a process for producing vinegar at concentrations greater than about 15% acetic acid by the aerobic fermentation of ethanol.
It is a further object of this invention to provide a process which utilizes membrane filtration for producing vinegar at acetic acid concentrations greater than about 15%.
The achievement of these and other objects will be apparent from the following description of the subject invention.