1. Field of the Invention
This invention relates to a process for isomerizing glucose in a glucose-containing liquor to fructose. More particularly, this invention relates to a process for treating a glucose-containing liquor with an ion exchange material and then contacting the treated liquor with immobilized glucose isomerase to convert a portion of the glucose to fructose.
The use of microbial and fungal enzymes adsorbed onto or bonded to inert carriers to provide immobilized biological catalysts has largely superseded older methods whereby soluble enzymes or whole cells of microorganisms were utilized. In general, immobilized enzymes provide a number of significant advantages over soluble or cell-bound enzymes particularly in commercial systems for carrying out continuous conversion processes. Because of the economics of these systems, it is of the utmost importance that the enzymes not be substantially inactivated or denatured by the process used to affect immobilization. It is equally important that the conditions under which the immobilized enzymes are utilized are such that the stability of the enzymes is maintained over a period sufficient to permit conversion of large quantities of substrate. Thus, for example, the presence in the substrate of materials which in some manner interfere with or inactivate glucose isomerase may have a deleterious effect on the stability of the immobilized enzyme and shorten its effective life to a significant degree.
Generally, prior to isomerization, glucose-containing liquors are refined by conventional means, e.g., by treating the liquors with carbon and ion exchange materials in order to remove interferring metals and carbohydrate by-products which might inactivate or denature glucose isomerase in an uneconomically short period. It has been found, however, that although such treatments provide some prolongation of the effective life of immobilized glucose isomerase, the stability of the enzyme is not as great as is desirable in continuous processes for isomerizing glucose to fructose.
2. Discussion of the Prior Art
U.S. Pat. No. Re. 28,885 to Cotter et al. relates to an enzymatic method for isomerizing glucose syrups utilizing soluble glucose isomerase or cellular material containing this enzyme. Incorporation of a source of SO.sub.2 into glucose-containing liquors during isomerization is taught to reduce denaturation of the glucose isomerase and to inhibit undesirable color formation in the finished product. Cotter et al. disclose that it is preferred to provide soluble salts of sulfurous acids in the glucose-containing liquor before the isomerization process is initiated in order to obtain the full benefit of their presence. It is also disclosed in U.S. Pat. No. Re. 28,885 that levels of SO.sub.2 effective to inhibit undesirable color formation in the finished product may be provided by passing the liquor through ion exchange resins in the sulfite form.
German Pat. No. 2,160,919 to Takasaki teaches a process for the separation of a mixture of carbohydrates by treating the mixture with an anion exchanger in the sulfite or bisulfite form.
Anet in Adv. Carbohydrate Chem., Vol. 19, pp. 212-213 (1964) discloses that the inhibition of non-enzymatic browning in foods by sulfite depends, in part, on the capture of various reactive intermediates and their conversion to carbonyl-bisulfite addition compounds of sulfonic acids.
In Die Starke, 26 Jahrg., 1976/Nr.10, pp. 350-356, Oestergaard et al. recommend that glucose-containing substrates be filtered and treated with carbon and ion-exchange materials prior to carrying out continuous isomerizations with glucose isomerase to remove impurities which may adversely affect the activity of the enzyme. They further disclose that possibly harmful enzyme contaminants in the syrup, which apparently are formed during isomerization, may be protected against by utilizing a particular arrangement of a plurality of columns containing the immobilized glucose isomerase.