ALKYLIDENATION OF FRUCTOSE WITH PERFLUORINATED ACID CATALYSTS

BACKGROUIO OF THE INVENTION United States Patent Mb. 2,813,810 by Smith et al. discloses a method for separating glucose and fructose crystals from invert sugar or sucrose. In the Smith et al. method. Invert sugar solids were dissolved in water (20 ml) and acetone (400 ml) and then treated with 20 grams of a commercial sulfonated phenol-formaldehyde ion exchange resin. The patentees reported that, after 7 days, the dextrose precipitate and resin were separated from the liquid reactants to provide a supernatant solution of alpha-diisopropylidene-D-fructose. Crystalline fructose was recovered from the supernatant solution by concentrating the supernatant, hydrolyzing the concentrate with a mineral acid, neutralizing the hydrolyzate, dissolving the hydrolyzed concentrate in hot absolute ethanol and cooling the concentrate to provide a crystalline fructose product.

in a paper authored by K. Erne ("Studies of Glycosides and Isoppopylidene Derivatives" Acta Chemica Scandinavie a 9, (1955), pages 893-901) cation exchange resins (sulfonated acid type) were used to catalyze the condensation of acetone with glucose and fructose. Eme reported fructose more readily reacted with acetone to form 1,2-4,5- diisopropylIdene-beta-I>-fructopyranose than glucose under ambient conditions, but that glucose in boiling acetone produced a 1,2-mono- and alpha-1,2-5,6 diisopropylidene glucofUranose mixture. The sulfonic acid type ion exchangs resins were reported to be less effective than strong mineral acids. Substantial inactivation of the resin catalyst and the production of degradative by-products were also observed by Eme.

Eructose is obtainable from a variety of natural products and synthetic processes. The enzymatic modification of sugars provide a particularly attractive source for fructose. Within recent years fructose-containing syrups of about 30% to about 52% fructose content 3° (d.s.b.) have been conveniently prepared by isomerizing high dextrose syrup into high fructose syrups with glucose isomerase.

'«,'4» Hlgh fructose com syrups (HPCS) are comnerclally manufactured by enzymaticall y Isomerlzlng a high dextrose conversion syrup (which typically contains approximately i\%-8% dlsaccharides and higher oligosaccharides and 92-96 dextrose) to fructose. Compositionally a HFCS typically contains from 38%-H6% fructose, 48-54 dextrose, l%~h% disaccharide and frcm about 3%-Q% saccharides of a D.P._ or higher.

Fructose is sweeter than dextrose. It is conventional to enrich the syrup fructose content (e.g. 55% or higher) by chrcmatographie fractionation and separation techniques. The enrichment process must necessarily produce a syrup essentially free from organoleptically detectable by-products.

Although considerable research effort has been devoted towards enriching the fructose content of high fructose com syrups, the earlier work by Smith et al. and Erne is inapplicable to the manufacture of high fructose com syrups which contain at least 55% or higher fructose.

Smith et al. enployed dry invert sugar (approximately 50% dextrose and 50% fructose) while Erne utilized a dry monosaccharide (glucose or fructose) dissolved in acetone. The Smith et al. technique required several days (7) to precipitate the dextrose from the diacetone solution while Erne observed substantial degradation of the catalytic resin and reaction product within about 20 hours.

Within recent years, perfluorinated exchangp resins with functionally active ionic (e.g. sulfonated and/or carboxylate) groups have gained prominence for a variety of industrial applications. The commercially available perfluorinated ionic membranes are reportedly produced by a variety of chemical processes as disclosed in C&EN, March 15, 1982 "Electrolytic cell membrane development surges" by S. C. Stinson, pages 22-25.

Applicants' studies have shown that conventional sulphonated type exchange resins are ineffective for converting HFCS into enriched fructose syrups. This ineffectiveness arises mainly because of certain inherent deficiencies of these conventional sulfonated exchange resins.

A HFCS contains a substantial amount of water. In a ccmmercial opera¬ tion, it is impractical to dehydrate HFCS. Conventional sulfonated resins have a significantly greater affinity for water than acetone.

This higher affinity for water tends to load the porous interstices and reactive sites of the resin with a disproportionate concentration of water. The presence of water excesses within the catalytically active sites complicates the efficacy of the acetonation. The catalytic sites are capable of hydrolyzlng dlacetone fructose to fructose in the presence of excess water. This reversible reaction leads to Incomplete catalysis 1° of the fructose to diacetone fructose. High fructose com syrups also contain significant amounts of saccharide canponents other than dextrose and fructose. Conventional cation exchange resins tend to absorb and retain an enriched content of the disaccharide and oligosaccharide can¬ ponents of the HFCS. This creates an excessive viscosity for effective mobility of fructose to the catalytic sites and transfer of the diacetone fructose therefrom. This excessive disaccharide and higher saccharide concentration also substantially reduces the level of available fructose.

This significantly reduces the amount of fructose and acetone available for catalysis to diacetone fructose. In addition, a substantial portion of the dextrose, fructose and converted diacetone fructose will remain within the porous Interstices of the resin, which in turn, reduces the amount of recoverable dextrose precipitate and diacetone fructose.

The inventors unexpectedly discovered that the problems related to the use of HFCS and conventional sulfonated resins could be effectively overcome by conducting the catalysis with a perfluorinated exchange resin.

The mobility of the desired (i.e. ftuctose and acetone) reactants and reaction product trom the catalytic active sites was significantly enhanced through the use of the perfluorinated exchange resins. Brploying HFCS as a fructose source, these perfluorinated resins provide an exceptionally high interchange rate. The efficacy of the perfluorosulphonated polymeric resins more conpletely converts the fïmctose of HFCS into diacetone fructose. By this mode of catalysis, one achieves a higher degree of solution supersaturation. This leads to quicker and more complete precipitation of dextrose from, the reaction mixture. The perfluorinated sulfonate resins are very potent catalysts and significantly accelerate the rate of catalysis. The preferential absorbtion of water and oligo¬ saccharides by conventional resins is essentially eliminated through use of the perfluorinated catalyst system. The catalysis substantially reduces the level of undesirable by-products. The perfluorosulphonic acid resins also provide a catalyst system which can be more easily segregated from the processed materials.

DESCRIPTION OF THE INVEMTION According to the present invention there Is provided a method for converting an aqueous solution of a high dextrose and fructose content Into a syrup of enriched fructose, said method comprising:

(a) admixing an aqueous fructose and dextrose solution with an effective amount of at least one member selected from the group consisting of aldehye, ketone and acetal to permit the catalytic conversion of a substantial portion of the fructose into alkylidene fructose; (b) converting a substantial portion of the fructose within the mixture to a alkylidene fructose by catalysis with perfluorinated acid catalyst; (c) allowing a substantial portion of the dextrose to precipitate from the converted mixture; and (d) partitioning the precipitated dextrose fron the converted mixture to provide a liquid portion of an enriched, alkylidene fructose content.

It has been discovered that perfluorinated cation exchange resins are highly effective catalysts for converting fructose and an aldehyde, ketone and/or acetal into alkylidene fructose. These water-insoluble resins readily catalyze the fructose of high fructose com syrups (HFCS) and acetone into diacetone fructose. They may also be used to hydrolyze aqueous diacetone fructose solutions to fructose.

The perfluorinated cation exchange resins are typically com¬ prised of a perfluorinated polymeric backbone chain which contains a plurality of appendant acid groups. Illustrative perfluorinated exchange resins may be generally depicted by the polymeric structural formula:

—E4CP2CF2—fepCgPoQ] wherein "Q" represents an appendant perfluorinated acid group, "n" is an integer representing the number of tetrafluoro- ethylene units which intervene between the Q-containing trifluoroethylene units and "x" the number of polymeric acid groups. The appendant Q groups are typically comprised of a perfluorinated ionic moiety represented by in ' + ' + - the formula -Q -A , wherein Q represents a perfluorinated group and A t represents an acid group. The Q group will typically consist essentially of a perfluoro-organo moiety connected to the polymeric carbon atom via an oxy or difluoranethylene radical which forms a bridging linkage between the polymeric chain and the acid moiety. The commercially available exchange resins reportedly contain3 as Q , either the —f-CP.'ir and/or -O-CF2-CF(CFj>-0(CF2-::f- linking moiety. The acid groups will typically be comprised of hydrogen3 oxygen and at least one other atom selected from the group consisting of a Period II element of an atomic weight range of about 10 to about 14 (e.g. boron, carbon, nitrogen) and Period m element of an atomic weight ranging frcm about 28 to about 32 (e.g.

phosphorous, sulfur, etc.). Illustrative acid groups Include the sulfonic, carboxylic, phosphonic, phosphorous, phosphoric acids, mixtures thereof and the like. The perfluorinated cation exchange resins containing strong acid moieties (e.g. sulfonic acids) are highly functional catalysts for converting fructose and acetone into l,2:il,5-di-0-isopropylidene-beta- D-fructopyranose.

A sufficient amount perfluorinated acid resin should be provided to the reactor for the catalysis of fructose and acetone into diacetone fructose. Relatively small amounts of catalyst (e.g. 50 meq. or 3° lower/fructose mole) to levels in excess of 1,000 meq. or higher may be used for this purpose. Normally a catalyst level of about 100 meq. to about 800 meq. will be sufficient to effectively convert the reactants into diacetone fructose. Advantageously the amount of catalyst will range from about 150 meq. to about 700 meq. with the preferred catalytic usage level ranging from about 200 meq. to about 400 meq.

-fV.-»" 1I948B7 Ihe perfluordnated exchange resins can be provided in a fos v/hich ma.y be easily segregated from the processing streams and products.

If desired, the reactor and/or stirring equipiKent rnay be coated -with the perfluorinated exchange resin to provide the catalytic source for the acetonation reaction. Alternatively they may be used sheeted, fragriented, granulated or beaded catalyst of a dimensional size to permit their segregation from either the diacetone fructose solids or dextrose precipitate. The catalytic reaction nay be adapted to batch, serni- continuous and continuous reactor systems. The catalyst system is particularly suitable for a continuous process. In a continuous operation, the acetone and HFUS solution may be continuously fed and passed through a single or plurality of fixed beds containing the itranobilized perfluorosulfonic acid resin or reactors inpregnated or coated with the resin. The solution flow rate and reaction terrperature may be suitably monitored to optimize the conversion to diacetone fructose. Cooling means, crystallizers or holding tanks, may be adapted to the operation to facilitate the precipitation of dextrose from the concentrated diacetone fructose solutions. The continuous operation may also be equipped with means to recycle the acetone and dextrose to the catalytic converter.

A variety of fructose-containing syrups may serve as a fructose source. The extent of fructose enrichment will depend primarily upon the fructose and dextrose content of the fructose-containing syrup. Dextrose and fructose should ordinarily comprise the major dry solids constituents (d.s. by weight basis) of the fructose source material. The dextrose and fructose will advantageously comprise at least 8055 and preferably at least 90% by welgjit of the total dry syrup solids weight. The weight ratio of fructose and dextrose will typically range from about 1:2 to about 2:1 and advantageously within about 2:3 to about 3:2 range.

3° Fructose-containing syrups obtained through enzymatic modification of sugars (particularly the Isornerizaticn of dextrose syrups with glucose isor/Brase), will normally contain an appropriate dextrose and frmctose content for use herein. Froictose syrups of about 305? to about 55%, ummw -8~ especially commercially available high fructose com syrups of about 38$~46$ fructose content (d.s.b.)j are an excellent fructose source material for the aeetonation.

Although the catalysis of aqueous fructose solutions into aUcylidene fructose generally applies to ketones (e.g. acetone, methyl ethyl ketone, diethyl ketone, methyl propyl ketone, cyclohexanone, mixtures thereof and the like), aldhydes (e.g. propionaldehyde, butyraldehyde, mixtures thereof and the like) and acetals (e.g. 2,2- dimethoxypropane, 2,2-diethoxypropane, acetaldehyde diethyl acetal and acetaldehyde dijnethyl acetal, mixtures thereof and the like) it is particularly well suited for the catalytic conversion of acetone and fructose into diacetone fructose. Tne acetone level is monitored for effective conversion of the fructose to diacetone fructose. Sufficient acetone should be added to the converter to compensate for water contributed by the HFCS and to prevent hydrolysis of the diacetone fructose to fructose. The aeetonation nomally requires at least 2 iroles of acetone for each fructose mole. In a conmercial operation, the reactor will most appropriately be provided with at least 5 moles of acetone and advantageously at least 10 moles of acetone for each mole of fructose. Substantially greater molar excesses of acetone (e.g. 200 rnoles or higher) may be used but are generally avoided due to added equipment and acetone recovery expenses. Advantageously the acetone level will range from about 15 moles to less than about 100 moles for each fructose mole and preferably from about 20 moles to about 25 moles of acetone/fnactose mole.

The catalytic efficacy of the perfluorinated acid resins Is not adversely affected by the presence of substantial amounts of water.

Fructose syrops v/hich contain from about 10% to about 80% by weight water or higher ma,y be adapted to the process. When high fructose com syrups are utilized as the fructose source material, the feed syrup v/ater content will advantageously range fron about 20% to about 60% of the total syrup weight and most advantageously about 25$ to about 35% by v/elght water. These ranges typify the v/ater content of syrups normally discharged from the isorrerization columns. Within these operational parameters, the reactor will generally be provided with a reaction media which contains less than about 1 mole water for each acetone mole.

Advantageously the molar proportion of acetone to water will range from about 1 mole to about 10 moles acetone for each water mole with particularly effective results being obtained at about a 3:2 to about 4:1 (most preferably at about 2:1) acetone to water molar ratio.

!he reaction temperature and time intervals are suitably regu¬ lated so as to convert the reactants into diacetone fructose. The most appropriate temperature and contact time will depend to a large extent upon the type of reactor used for the conversion. In a continuous opera¬ tion, shorter reaction times may be combined with higher tenperatures so as to inhibit the formation of fructose degradative by-products. The conversion time interval may be appropriately controlled by regulating the flow rate of reactants through the reactor. Longer reaction periods may be effectively used in batch or continuous reactors by reducing the reaction temperature.

For most operations, the catalytic conversion will be conducted at a temperature of less than about 50oC. with farther improvements in product quality and production rates being accomplished at conversion temperatures of less than 350C. These reduced operational temperatures minimize the formation of mono- and diacetone glucoses and other undesirable by-products. The catalytic con- version of the reactants into diacetone fructose will proceed more rapidly than the rate of dextrose precipitation. However, the lower conversion températures will reduce the total time needed for effective recovery of crystalline dextrose from the diacetone fructose solution. The catalytic converting temperature is advantageously maintained within about 10oC. to about 250C. range and most preferably fran about 150C. to about 20oC.

Subtantially complete catalysis of the reactants into diacetone fructose can generally be effectuated within less than about a day (e.g. about half-hour to about 24 hours). More typically, the catalysis can be completed within about 2 hours to about 20 hours and most typically within about 5 hours to about 15 hours. The reaction tenperatures may be controlled to optimize the crystallization of the dextrose. Ibr example.

iBore elevated converting temperatures may be used to accelerate the rate of catalysis with lower converting temperatures "oeing enployed in the later stages of the catalysis to facilitate the dextrose crystallization.

The present method provides a means for more effectively converting fructose to diacetone fructose. Vore highly concentrated diacetone fructose solutions are achievable under the present invention.

Œhe more highly concentrated diacetone solutions permit more dextrose to be precipitated from the reaction mixture which, in turn, significantly enriches the diacetone fructose solution content. Œhe method avoids the water dilution requirements, water removal and auxiliary equipment costs of conventional chrcmatographic separation processes. The catalytic reaction minimizes the production of adverse by-products and organoleptically objectionable bodies. Unlike conventional exchange resins which absorbtively and tenaciously retain a substantial amount of water and reaction product, the polymeric resins used herein retain only a trace amount of the reactants and desired reaction product. The acetonation proceeds at a substantially faster rate and at significantly lower temperatures than conventional catalysis. The ability to catalytically convert the reactants at low operative temperatures permits dextrose crystallites to simultaneously develop and form during the catalysis of the fr-uctose and acetone to diacetone fructose. îhis should be contrasted with the prior methods which typically require the catalysis to be conducted at temperatures substantially above the dextrose crystallization ter/perature and a separate cooling step to precipitate the dextrose from the diacetone fructose solution.

There are two isomeric forms of diacetone fructose. The kinetlcally controlled reaction conditions herein favor the formation of the IjajJ-dl-O-iscpropylidene-beta-D-fructopyranose isomer instead of the thermally stable 2,3:4,5 beta-D-firxictopyranose isomeric form. The reduced conversion température coupled with catalytic activity of the perfluorinated sulfonic acid resins of this invention affords a neans for providing a diacetone fructose reaction product in the 1,2:4,5 isomeric form and substantially free from the 2,3:4,5 isoner. This will provide a -ii- dlacetone fructose solution which can be easily hydrolyzed Into fructose while minimizing the formation of objectionable organoleptically flavor, color and odorous by-products.

After converting the fructose to the desired solution concentration of diacetone fructose, the dextrose is allowed to precipitate from the solution. In general, catalytic conversion and precipitation of the dextrose may be accomplished within a total time interval ranging from about a half-day or less to about 4 days. Although recoverable dextrose precipitate may be obtained at temperatures in excess of 30oC.3 it is advantageous to cool the reaction product to a temperature of less than about 250C. (e.g. 5oC-20oC.) and preferably about 10oC. to about 180C. Cooling provides a supersaturated dextrose solution and accelerates the rate at which dextrose precipitates from the diacetone fructose reaction solution.

The dextrose precipitate may conveniently be recovered from the diacetone fructose solution by conventional nsans such as by filtration, centrlfugation, décantation, etc. If desired, the dextrose recoveiy may be facilitated by progressively decreasing temperatures with or without intermittent removal of dextrose precipitate therefrcm. Similarly, the diacetone fructose solution may be concentrated by conventional techniques to further accelerate the rate of dextrose precipitation.

Through effective cooling, the total time interval for dextrose recovery from initial start-up may be reduced to less than about 2 days (e.g.

about 15 hours to about 50 hours) and most typically within about hours to about 35 hours interval.

In general, the partitioning step will rénove more than about >i0% by weight of the total dextrose content of the diacetone fructose solution and advantageously a major weight portion. In a typical operation, from about 60 to about 95 weight percent of the total dextrose content of the diacetone fructose solution and advantagsously from about 70 to about 90 weight percent will be recovered by the partitioning step herein.

The catalytic hydrolysis of dlacetone fructose to fructose Is a reversible reaction. The hydrolysis reaction is generally favored by providing a sufficient amount of water to the hydrolyzing msdium to shift the equilibrium towards fructose production. Catalytic reconversion of the fructose and acetone into dlacetone fructose will generally be inhibited by maintaining the molar ratio of water to total acetone (free and chemically corribrmed) during the hydrolysis reaction at a level in excess of about 11:10 and advantageously at a level of more than about 2:1 moles water for each mole acetone. Further excesses of water (e.g.

moles or higher) may be used but are generally unnecessary and undesir¬ able due to additional processing and equipment needed to reirove excess water and place the recovered fructose in a marketable form. In a commer¬ cial operation, more effective and complete hydrolysis of the dlacetone fructose will be obtained by generally maintaining the water level at iS least about 3 moles and preferably in excess of 10 moles for each acetone mole. During the hydrolysis, the free acetone is advantageously removed from the hydrolyzing medium (e.g. evaporating under a vacuum) as it is formed by the hydrolyzing reaction. Removal of the free acetone during the hydrolysis contributes towards more conplete hydrolysis of the dlacetone fructose into fructose.

In the manufacture of enriched fructose syrups of a 55$ or higher fructose content, the hydrolyzing medium will more typically contain about 2 to about 50 moles water per fructose mole and advan¬ tageously from about H to about 30 moles water for each fructose mole.

If desired, the water content of the reaction product may be adjusted to more closely approxmate that of the desired syrup end- product. By maintaining the free acetone level during the hydrolysis at less than about 2 moles acetone (preferably less than 1 mole) for each five moles of water, substantially complete catalytic conversion of the dlacetone fructose into fructose may be accomplished.

The aqueous dlacetone fructose is hydrolyzed by acid catalysis. Although conventional strong mineral or organic acids may be used to hydrolyze the dlacetone fructose to fructose, acid ion exchange resins have been found to yield an unexpectedly superior food-grade hydrolyzate product. The perfluorinated exchange resins mentioned hereinbefore are especially well suited for this purpose. They may be utilized in the hydrolysis of dlacetone fructose to fructose at substantially lower temperatures and catalytic concentrations than conventional acids. Enriched fructose products which rely upon perfluorinated acid catalysts to hydrolyze the dlacetone fructose to fructose have been found to be essentially free from organoleptic and other objectionable by-products (e.g. color, flavor, degradative, etc.

bodies).

Relatively small amounts of catalyst (e.g. 1 meq./fructose mole or less) to levels in excess of 1,000 meq. or higher may be used to hydrolyze the dlacetone fructose to fructose. The most appropriate catalytic amount will primarily depend upon the efficacy of the particular catalyst which is used for the hydrolysis. Normally a catalyst level ranging frcm about 2 meq. to about 800 ireq. will suffice for this purpose. Advantageously, the amount of catalyst will range from about 5 meq. to about 700 meq. with the preferred catalytic amount level ranging from about 10 meq. to H00 meq. Fbr the perfluorosulfonic acid resin, it is advantageous to use less than 20 meq. in the hydrolysis.

Although relatively higi tenperatures (e.g. 80oC. or more) for short time intervals (e.g. one hour or more) may be used to hydrolyze the dlacetone fructose to fructose for those applications in which by-product residues are unimportant, it is advantageous for food grade syrups to conduct the hydrolysis at a temperature of less than about TO0!:. A hydrolyzing temperature of about 20oC. to about 650C. (preferably from about 50oC. to about 60oC.) for about 2 hours to about 10 hours (preferably from about 2 hours to about 3 hours) are particularly effective for converting dlacetone fructose solution into food-grade, enriched fructose syrups.

3° Upon ccmpletion of the dlacetone fructose hydrolysis, the immobilized catalyst (if present) may be removed from the hydrolyzed solution. The hydrolyzing acid exchange resins can be easily separated frcm the hydrolyzed product by providing the catalyst in a form similar -in¬ to that used for the catalysis of fructose and acetone Into dlacetone fructose. Ohe hydrolyzate may be adjusted. If necessary, with an appropriate acid to about pH 3 to about pH 5 (preferably from about pH 3 to about pH 4). Any acetone residue may be removed from the hydrolyzate by conventional techniques such as by distillation, evaporation, etc.

Pursuant to the present process, syrups of an enriched fructose content of at least 55$ (e.g. 551-95% or higher fructose content) and preferably of a fructose content of about 60% to about 90% may be easily prepared from 38%-H6% HPCS. Notwithstanding the high fructose content, the acid hydrolysis produces an enriched fructose syrup product substantially free fran objectionable flavoring and coloring bodies. This substantially reduces the carbon, cationic and anionic exchange resin requirements for placing these syrups in a marketable condition for food applications.

The inmobilized catalysts provide syrups essentially free from ash residues which norroally arise from salts formed by neutraliz;ing water-soluble acid catalyst with a base.

The following exanples are illustrative of the invention.

EXMPLE 1 This example illustrates the use of a perfluorinated sulfonic acid resin to prepare a 75% fructose syrup from H0.li% HPCS . The reaction medium was prepared by adding 900 ml. of acetone to 68.2 grams of 4o.il% high fructose syrup. The reaction medium and solid catalyst (112 square inches of the perfluorinated sulfonic acid membrane - 14 meq.) were stirred at 2i|-250C. for 45 hours. Under these acetonation conditions, the fructose was converted to l,2:4,5-ài-0-isopropylidene- beta-D-fructopyranose with a substantial portion of the dextrose being precipitated from the liquid solution. The membrane was then removed and the dextrose precipitate was filtered (Whatman No. 2 paper) from the 1 - Nafion 125 - A copolymer of tetrachloroethylene perfluoro-3:6-dioxa 4-*nethyl-7-octensulfonic acid membrane manufactured and distributed by E.I. du Pont de Nemours & Co., Wilmington, Delaware 19898 2 - 40.45? fructose, 55.3$ dextrose, 2.6% maltose and iscmaltose and 1.7% by weight saccharide of D.P._ and higher single liquid phase. Saccharide analysis by high pressure liquid chromatography (HPLC) of the unwashed precipitate revealed that the precipitate consisted of T-S dextrose, 18.6% fructose and a balance (7.1%) primarily of D.P.? and higher sugars. Analysis of the liquid phase by HPLC indicated it contained 2H.0% dextrose, 29.9$ fructose, M.7% diacetone fructose (Q6.H% by weight being l,2:iJ,5-dl-0-isopropylidene- beta-D-fructopyranose) and the balance (IM) being canprised of D.P.

and higher saccharides. Substantially all of the unreacted acetone was then removed from the liquid phase by aspirating with water vacuum in a rotary evaporator in a 60oC. water bath for about 20 minutes. The evaporated syrup (28$ solids) was then heated to 650C. for 2 hours in the presence of 6.3 sq. in. of the perfluorinated sulfonic acid catalyst (0.8 meq.) to hydrolyze the diacetone fructose to fructose. The acetone generated by the hydrolysis was continuously removed by the above rotary evaporating conditions to provide a 50% by weight dry solids syrup product. Analysis of the hydrolyzed product by high pressure liquid chromatography indicated it contained 7.3% fructose and 22.5% dextrose with the balance (3.2%) being primarily comprised of di~ and higher HFCS saccharide conponents.

The unrefined, enriched fructose syrup (23.2 grams), which contained 50% by weight dry solids, was blended with sufficient 0% high fructose com syrup2 (50$ d. s.) to provide an enriched fructose syrup containing 55% by weight (d.s.b.) fructose content. The blended 55$ high fructose com syrup (pH 3.5) was then treated with powdered carbon (3% by weight of 55% HFCS dry solids weight) for 30 minutes at 60oC. and filtered through Whatman No. 2 filter paper. The carbon-treated filtrate was then ion excbangsd (i(0oC.) through a pair of cation and anion exchange columns. The syrup effluent of a dry substance in excess of 20% by weight solids was collected, adjusted to a pH 3.5, and con¬ centrated under aspirating vacuum (water) in a rotary evaporator immersed in a 60oC. water bath to a 78% dry solids syrup. When evaluated by an expert syrup flavor panel, the 55% high fructose com syrup received an average flavor grade rating of 8.5 +0.5 (1-10 basis). The syrup was characterized as being a colorless, bland, sweet-tasting syrup essentlally free rrom other flavor principles. Die flavor rating exceeded those typically obtained from conventional 55% HPCS.

EXAMPLE 2 Example 1 was generally repeated except for the replacement of the perfluorlnated sulfonic acid catalyst with conventional acid ion exchange resin. Œhe acetonation reaction was conducted by mixing 100 grams (d. s. ) of the styrene/dlvinylbenzene sulfonic acid exchange resin (Itowex 50WXi-100 (50-100 mesh) ) with 1187 grams of acetone and 6 grams of Mgi fructose com syrup for 21 hours at 260C. and an additional 22 hours at 180C.' The decrease in catalytic temperature to 180C. was designed to optimize the rate of dextrose precipitation fron the diacetone fïi:ctose solution. The dextrose precipitate was separated by filtrating (Whatman No. 2 filter paper) at 180C. 5he catalytic resin was removed by sieving through a 200 mesh screen (U.S. Series), washed with water and the washings combined with the liquid diacetcne fructose filtrate. Due to attrition of the resin and the substantial amount of absorbed material retained by the resin considerable more difficulty was encountered m attenpting to separate the resin flxan the dextrose precipitate and absorbed substances.

ie acetone was evaporated from the filtrate by aspirating with a rotary evaporator maintained at iJ0oC. The HPLC analysis of the dextrose precipitate for saccharf.de revealed that the dextrose precipitate contained 91.2% dextrose, 7.2% fructose and 1.65? D.P.p and higher saccharides. An HPLC analysis of the nitrate revealed (on a total dry solids weigit basis) 10.7? diacetone fructose, 35.0$ dextrose and HH.9% fructose. ïhe level of ftmctose by-products was also higher than that obtained for the perfluorlnated sulfonic acid membrane used for the acetonation.

In the regeneration of the diacetone fructose to fructose, 650 ml. of the concentrated filtrate syrup (3% dry solids) was treated with 3 grams (diy substance basis) tbwex 50WXiJ-200 for 3 hours at 1}5-850C.

* Trade Mark After the catalyst was separated by filtration, the acetone generated by the diacetone fructose hydrolysis was removed fran the filtrate (as described hereinbefore) to provide a H6.5% dry substance fructose syrup which, upon HPLC analysis, revealed a saccharide distribution (on a total dry substance weight basis) of 57.8% fructose, 35.8% dextrose and 6.4% D.P.2 and higher saccharides.

By comparing the Example 1 and 2 results, it will be observed that the perfluorinated sulfonic acid catalyst was substantially more effective in catalyzing a higi fructose com syrup into diacetone fructose. This is evident by conparing the enrichment of the kO.H% fructose content of the high fructose com syrup to 75% by weigit fructose as opposed to the 57.8% fructose content for the Dowex 50WX1-100 catalysis. The perfluorinated sulfonic acid membrane which was removed upon completion of the fructose to diacetone fructose was essentially free from absorbed materials. In contrast, the Eowex WX1-100 contained substantial amounts of absorbed water and other reaction media contaminants which is believed to significantly reduce its efficacy as an acid catalyst. The perfluorinated sulfonic acid catalyst was easily separated frcm the dextrose precipitate while considerable difficulty was encountered in the separation of the Dowex 50WX1-100 acid catalyst frcm the dextrose precipitate. The level of perfluorinated sulfonic acid J.

(i.e. 200 meq.H /fructose mole) to catalyze the fructose to diacetone fructose conversion was substantially less than the 450 meq. level of Dowex 50WX1-100 used in Example 2.

Complete hydrolysis with the perfluorinated sulfonic acid resin of Example 1 was accomplished with about 0.01-0.02 meq. H+/DAF mole within about 2 hours. In contrast, diacetone fructose hydrolysis with the styrene/vinylbenzene acid exchange resin will typically require more catalyst (e.g. about 0.3 - 1.4 meq. H+/irDle diacetone fructose) and time 3° (about 2-4 hours) to conpletely hydrolyze the diacetone fructose to fructose. Ihe hydrolyzate of this example with the styrene/vinylbenzene acid exchange yields a slightly discolored solution in contrast to the clear solution obtained in Example 1 with the perfluorinated sulfonic acid resin.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1 1. A method for converting an aqueous solution of a high dextrose and fructose content into a syrup of enriched fructose, said method comprising:

4 (a) admixing an aqueous fructose and dextrose solution with an effective amount of at least one member ° selected from the group consisting of aldehyde, 7 ketone and acetal to permit the catalytic conversion o of a substantial portion of the fructose into an " alkylidene fructose; (b) converting a substantial portion of the fructose within the mixture to an alkylidene fructose by catalysis with perfluorinated acid catalyst; '-3-' (c) allowing a substantial portion of the dextrose to - precipitate from the converted mixture ; and iS (d) partitioning the precipitated dextrose from the 3-6 converted mixture to provide a liquid portion of an -'-7 enriched, alkylidene fructose content.

1 2. The method according to claim 1 wherein the member consists p essentially of acetone, the fructose and acetone are catalytically 3 converted into diacetone fructose, the diacetone fructose of the liquid portion is hydrolyzed into fructose and a syrup of an enriched fructose content is recovered from the partitioned liquid portion.

1 3» The method according to claim 2 wherein the catalysis with 2 the perfluorinated acid catalyst is conducted at a temperature of less 3 than 350C.

1 H. The method according to claim 2 wherein the perfluorinated 2 acid catalyst conprises a perfluorinated sulfonic acid catalyst.