Gluconic acid, an oxidation product of glucose, has been extensively used in applications as varied as metal cleaning operations in the dairy industry; alkaline bottle washing operations; alkaline derusting operations in the metallurgic industry; and iron deposition prevention in the textile industry. Furthermore, the sodium salt of gluconic acid is used as an additive in cement mixtures.
The production of gluconic acid from glucose may be achieved by the use of processes which may be broadly classified as being either microbial fermentation, electrochemical, chemical and enzymatic (wherein enzymatic systems are employed separately from their source microorganism(s)). While microbial fermentation has perhaps been the most widely employed of these methods, it nonetheless suffers many drawbacks, including those associated with process conditions required for the fermentation microorganisms used, which has limited its commercial applicability.
The enzymatic conversion of glucose to gluconic acid involves treating a glucose bearing material with an enzyme preparation having glucose oxidase and catalase activity. This reaction is performed in the presence of a free oxygen source, such as hydrogen peroxide. Generally, the glucose-bearing material is in the form of an aqueous solution.
To insure that the glucose oxidase functions at its most effective level, during enzymatic conversion the pH of the reaction media is controlled so as to favor the desired reaction. In the glucose oxidase conversion of glucose, acid (gluconic) is continuously formed. Thus, it is necessary to continuously regulate the pH of the reaction media throughout the enzymatic conversion. Generally, if the pH is maintained between about 4.2 and about 7 (preferably, between about 5 and about 6), the conversion proceeds satisfactorily. A common method of regulating the pH involves the continuous addition of an alkali, such as sodium hydroxide. The alkali neutralizes the gluconic acid to a corresponding gluconate, e.g., sodium gluconate.
Examples of enzymatic processes for the production of gluconic acid from glucose using a glucose oxidase/catalase enzyme system can be found in, for example, U.S. Pat. No. 2,651,592 and Romanian Patent No. 92,739.
While being useful for their particular purposes, these enzymatic processes suffer from several drawbacks.
A primary drawback associated with fermentation processes is that the reaction process generally results in the crude reaction broth containing gluconic acid along with other impurities including biomass. This reaction broth must then be purified by multi-step processes including biomass separation (filtration), carbon treatment (decolorization), evaporation (concentration) and crystallization (purification) to provide a final product with high purity.
Another drawback is the presence of residual mother liquid in the reaction broth which must be either recycled, further purified and/or disposed of, thereby adding to the problems and costs of such fermentation conversion processes.
A further drawback associated with enzymatic processes is that they have a low conversion efficiency. This feature results in incomplete conversion of glucose to gluconic acid leaving residual unconverted glucose as a contaminant in the gluconic acid solution produced thereby. In order to reduce or eliminate such unconverted glucose from the final product, costly downstream, separation, recovery and purification steps must be employed.
To alleviate problems associated with incomplete conversion, resort has been had to limiting the glucose concentrations of the starting material to less than 30% weight to weight (w/w) dissolved solids (ds.). Unfortunately, such low glucose concentrations in the starting material are unsatisfactory in that they greatly reduce the efficiency of the process, negatively impacting on its commercially desirability.
Alternatively, resort has been made to interrupting the process prior to the completion thereof. For example, in the processes disclosed in the aforesaid United States patent, the reaction was stopped after converting only 50% of the glucose to gluconic acid. Nonetheless, the reaction mixture still needs to be subjected to electrodialysis to separate and recover the gluconic acid from the residual unconverted glucose and purification of the gluconic acid produced by such processes remains difficult and costly, especially where the glucose is still present.
Another drawback associated with enzymatic processes is the use of hydrogen peroxide (H.sub.2 O.sub.2) as a source of oxygen. As an acid, the presence of H.sub.2 O.sub.2 necessitates constant monitoring of the pH of the reaction solution, as well as the employment of a buffer (such as calcium carbonate in the form of lime) to maintain the solution in a pH range which is acceptable (about 5-6) for the conversion reaction. Furthermore, the use of hydrogen peroxide as the oxygen source results in the by-product formation of large quantities of yet more hydrogen peroxide and acids, necessitating the use of yet more catalases (to convert the hydrogen proxides formed into water and oxygen) and pH neutralizers.
Accordingly, it can be seen that there remains a need for the provision of enzymatic processes for the production of gluconic acid from glucose wherein dissolved glucose solid concentrations of 30% (w/w) ds. (dissolved solids) and higher may be used while still obtaining high conversion rates, wherein reduced quantities of buffers, such as sodium hydroxide, need to be employed, wherein the use of extensive and/or expensive downstream, separation, recovery and/or purification processes and/or apparatuses need not be employed and which permits the production of spray-dried and essentially pure gluconic acid granules without employing crystallization processes.