Nanofiltration process for making dextrose.

0001This invention relates to nanofiltration of a food processing feed stream - especially but not exclusively - for the production of dextrose.

0002Evaporation, freeze concentration, or freeze drying are common dewatering techniques used in the food, pharmaceutical and biological processing industries. Evaporation requires the input of about 1000 BTU for each pound of water that is evaporated (540 kcal/kg) while freezing requires about 144 BTU for each pound of water frozen, merely to effect the change in state of water from liquid to vapor or liquid to solid, respectively.

0003Since membrane filtration does not require a change in state to effect dewatering, it should result in considerable savings in energy. A less obvious advantage is the fact that no complicated heat transfer or heat-generating equipment is needed. Only electrical energy is required to drive a pump motor. Another advantage is that membrane filtration can be carried out at ambient or lower temperatures (e.g., to prevent microbial growth problems or denaturation of heat sensitive components) or at higher temperatures (e.g., to minimize microbial growth problems, to lower viscosity of the retentate thus lowering pumping costs, or to improve mass transfer). Since small molecules should normally pass freely through filtration membranes, their concentration on either side of the membrane should be about the same during processing and about equal to the original feed solution. Thus, membrane filtration offers many advantages over other dewatering processes.

0004A book entitled "Ultrafiltration Handbook" by Munir Cheryan, published by Technomics Publishing Co., Inc. 851 New Holland Ave., Lancaster, PA 17604 U.S.A. describes membrane filtration as a separation of two or more components from a fluid stream. A membrane is a selective barrier which prevents mass movement, but allows restricted or regulated passage, of one or more species through it. Membrane filtration includes the use of such a barrier to pass certain components while retaining certain other components of a mixture in order to separate dissolved solutes in liquid streams.

0005Membranes can be classified by their porous vs. nonporous structure. Osmosis involves a movement of a solvant from the dilute solution side through a semi-permeable membrane to the concentrated solution side of the membrane, responsive to the chemical potential difference between the water on either side of the membrane.

0006Five other major membrane separation processes are reverse osmosis, or ultrafiltration, microfiltration, dialysis and electrodialysis, which cover a wide range of particle sizes. Reverse osmosis or ultrafiltration permit a separatation of dissolved molecules down to the ionic range. Reverse osmosis or hyperfiltration relates to dewatering while ultrafiltration simultaneously purifies, by concentrating, and fractionating macromolecules or fine colloidal suspension. Reverse osmosis or hyperfiltration retains most/nearly all components other than the solvent (water) itself, while ultrafiltration retains only the macromolecules or particles larger than about 10-200 A. Ultrafiltration only needs a fairly low pressure for operation. Reverse osmosis, ultrafiltration, or hyperfiltration constitute continuous molecular separation processes which does not involve a phase change or interphase mass transfer, thus making these processes important for food, pharmaceutical and biological processing.

0007For these and other reasons, it is advantageous to use membrane filtration in the production of certain food products, such as dextrose. Heretofore, a drawback of using dextrose as a chemical feedstock centers about the difficulty encountered in obtaining a stream of dextrose with a sufficiently high purity because the dextrose molecules must be separated from molecules or other materials, which have almost the same characteristics, such as maltose and higher oligosaccharides. The conventional process for producing a high purity dextrose (i.e. greater than 99% purity) requires a costly and time consuming crystallization of a very highly concentrated syrup. Therefore, a non-crystallization alternative process is needed to provide an inexpensive high purity dextrose stream.

0008Heretofore, membranes have not been able to separate closely similar materials. Diffusion through a reverse osmosis membrane is able to concentrate a stream containing dextrose, maltose, and salts in order to provide a purified aqueous stream, but is does not purify the dextrose by removing the maltose and salts. While conventional ultrafiltration provides means for purifying or separating some fermentation and chemical products, it cannot do very much toward separating and purifying fairly similar compounds, such as maltose and dextrose.

0009Accordingly, an object of the invention is to provide new and novel means for and methods of treating and purifying chemical feed streams. Here, an object is to purify feed streams used in food processing. In this connection, an object is to provide means for and methods of separating and purifying dextrose feed streams.

0010Another object is to provide faster, less expensive, and more energy efficient means for and methods of producing dextrose. Here, an object is to separate dextrose from its closely related components in a food processing feed stream.

0011In keeping with an aspect of this invention, these and other objects are accomplished by providing a nanofiltration membrane at or near the output of a feed stream. The feed stream begins with a production of corn starch, proceeds through gelatinization, dextrinization, and saccharification steps to provide a feed stream of glucose syrup. The foregoing process may produce glucose syrup with a purity of about 95% dextrose, 5% di- and trisaccharides. The invention uses a nanofiltration process in order to further refine the syrup and remove most of the remaining 5 % of non-dextrose materials. After the nanofiltration, the material may be considerably more than 99 % pure dextrose.

0012In the attached drawings: <ul id="ul0001" list-style="none"><li>Fig. 1 shows the steps in a process which incorporates the invention;</li><li>Fig. 2 is a flow diagram which shows a system designated "Osmo 19T" by its manufacturer Osmonics Inc. of Minnetonka, Minnesota that was used to conduct tests leading to some working examples; and</li><li>Fig. 3 is a flow a pilot system used in a plant which practices the invention in order to produce other working examples.</li></ul>

0013The initial steps in the particular feed stream shown in the attached Fig. 1 are taken from the <u style="single">Novo Handbook of Practical Biotechnology</u>, which is published by Novo Industri A/S Enzyme Division, Baajsuaerd, Denmark. The feed stream begins with a starch slurry 20 which is produced from processed corn. The slurry is exposed to an a-amylase enzyme at high temperatures (100° C) which gelatinizes and liquefies the starch as part of a liquefaction step. The starch in the presence of a-amylese is cooked in two steps to produce first a gelatinization and then a dextrinization, as shown at 22, 24, in order to provide a dextrin syrup which is exposed to a glucoamylase enzyme. As part of and following this step, there is a saccharification, as shown at 26, resulting in a glucose syrup.

0014This process leads to a glucose syrup 27 which is approximately 95 % dextrose and 5 % di- or trisaccharides. Heretofore, there has been no easy way to eliminate the remaining 5 % di- and trisaccharides. The manufacturer either sold the glucose syrup with the saccharides in it or performed a further processing that used yet another enzyme, which escalates costs.

0015According to the invention, the need for a further enzyme step may be eliminated by a nanofiltration process. More particularly, the glucose syrup 27 is passed through a nanofiltration membrane 28. This filtration separates the dextrose from the di- and trisaccharides, and produces a purer dextrose stream, much faster and at less cost than the previously added steps which required further enzyme processing.

0016In greater detail, nanofiltration uses a pressure driven membrane that is between reverse osmosis and ultrafiltration membranes, which is called a "nanofilter" membrane. The nanofiltration rejection is low for salts with monovalent anion and nonionized organics with molecular weight below 150. Rejection is high for salts with di-and multivalent anions and organics with molecular weight about 300. When working with dilute streams of salts and sugars, these nanomembranes retain sugars and divalent ions versus monovalent ions.

0017Surprisingly, we have found that when used with a highly concentrated dextrose feed stream, these nanomembranes yield an initial permeate dextrose feed stream which has a much higher purity than the original feed stream. Further work has also shown that when used in a downstream processing step these nanomembranes not only remove disaccharides and higher saccharides but also remove, to some extent, divalent salts thus providing a highly purified product.

0018Presently FilmTec, a company located at 7200 Ohms Lane, Minneapolis, MN 55435, has two commercial nanofiltration membranes, NF70 and NF40 (NF stands for nanofiltration). Each membrane has a negative surface charge which has not been quantified.

0019The filter membrane NF70 is crosslinked aromatic polyamides. The filter membranes NF40 and NF70 are similar; however, the membrane NF40 has a slightly lower NaCl rejection, which indicates that its pores are slightly larger than the pores of the NF70 membrane. By way of example, FilmTec describes their nanofiltration membrane NF70, as follows: <tables id="tabl0001" num="0001"><img file="EP0452238A2_D0001.tif" /></tables> ELEMENT SPECIFICATIONS: Specifications are based on 1,000 mg/l solute feed solution at 70 PSI net pressure, 25 DEG. C, 10 % recovery, pH 5-8. <tables id="tabl0002" num="0002"><img file="EP0452238A2_D0002.tif" /></tables> PERFORMANCE DATA: INORGANICS: The following data is based on 70PSI net pressure, 25 DEG. X, 10 % recovery, pH7-8, inorganic rejections may vary with concentration. <tables id="tabl0003" num="0003"><img file="EP0452238A2_D0003.tif" /></tables> ORGANICS: The following data is based on 70 PSI net pressure 25 DEG. C and 10 % recovery. <tables id="tabl0004" num="0004"><img file="EP0452238A2_D0004.tif" /></tables> Some of the operating conditions and performances of the FilmTec nanofilter membranes are shown in Table 1. <tables id="tabl0005" num="0005"><img file="EP0452238A2_D0005.tif" /></tables>

0020Another source of nanofiltration membranes is Filtration Engineering Co. inc. 4974 County Road 18 North, New Hope, Minnesota 55428. Filtration Engineering describes its FE-700-002 membrane as a cross-linked polyamide, having a rejection characteristics which enables it to discriminate among low molecular weight species. This membrane has rejection characteristics which are between those common in reverse osmosis and ultrafiltration. The pore structure of the membrane enables a separation between sodium chloride and calcium sulfate. The utility of the membrane is said to be further enhanced by the simultaneous ability to concentrate the retained species. This membrane gives the users considerable latitude in process stream parameters, such as variations of pH, ionic strength, and temperature.

0021The manufacturer describes the Thin Film FE-700-002 membrane characteristics, as follows: Composition: Crosslinked Polyamide Permeability: (Nominal) <dl id="dl0001"><dt>Na Cl</dt><dd>95 %</dd><dt>Lactose</dt><dd>0-4 %</dd><dt>Magnesium Sulfate</dt><dd>5 %</dd><dt>Calcium Chloride</dt><dd>70 %</dd><dt>Calcium Phosphate</dt><dd>20-60 % (pH Dependent)</dd><dt>Citric Acid</dt><dd>10-95 % (pH Dependent)</dd><dt>Acetic Acid</dt><dd>10-95 % (pH Dependent)</dd></dl> Molecule Weight Rejections: <dl id="dl0002"><dt>Rejection above 500</dt><dd>95 %</dd><dt>Rejection Below 200</dt><dd>5 %</dd></dl> Flux Rate: 20 l/m²/h nominal design flux rate 40° C Membrane size: 4˝ x 30˝ spiral with 6 m² membrane area per element <dl id="dl0003"><dt>Operating Pressure:</dt><dd>41 Bar (600 PSIG) Max.</dd><dt /><dd>30-40 Bar (450-600 PSIG)</dd><dt /><dd>recommended.</dd><dt>Temperature limitations:</dt><dd>57°C. Maximum, 10-50°C.</dd><dt /><dd>recommended.</dd><dt>pH tolerance</dt><dd>2.3 minimum</dd><dt /><dd>11.0 maximum short term exposure</dd><dt /><dd>2.3 to 10.0 recommended</dd><dt>Oxidizer tolerance:</dt><dd>NONE</dd><dt>Rejection rate:</dt><dd>> 99 % True Protein (TRP)</dd><dt>Flux rate:</dt><dd>27 l/m²/h nominal design flux rate (50°C).</dd></dl>

0022A company Osmonics Inc. manufactures an experimental membrane designated "Osmo MX-06" which is a thin film, membrane similar to the Filtration Engineering membranes. However, the manufacturer has not published any specifications on this membrane.

0023For all the nanofilter membranes, the rejection of magnesium sulfate is fairly high (90-98 %), while the rejection of sodium chloride is in the 50 percent range or lower. Since these membranes are negatively charged, it is the anion repulsion which mainly determines the solute rejection. For example, the rejection of calcium chloride is about the same (can even be lower), than that of sodium chloride while rejection of sodium sulfate is about the same as that for magnesium sulfate. Di- and multivalent anions are highly rejected. So far, no known case has evolved where highly charged cations have interacted with the nanofiltration membranes to give a positive net surface charge.

0024In general, according to the invention, a 5.0 to 50 % solution of the desired low molecular weight compound or molecule is fed to a nanofilter under approximately 600 PSI. A low molecular weight has less than 500 MW. The product passes through the membrane while varying degrees of the larger molecules do not pass through and are retained by the membrane. The amount of any given molecule passing through the membrane depends on the molecular weight, ionic charge, and concentration of the molecule in the feed stream.

0025During an experimental practice of the invention, dextrose was retained by the membrane in a low concentration; however, when a 28 % dextrose is used, the dextrose permeates to some extent while virtually all of the higher oligosaccharides are retained.

0026In the case of an organic acid salt, such as lactic acid more or less of the acid appears in the permeate stream depending on whether it is present as salt or as a free acid. The stream permeates the membrane faster as the free acid than it does as the salt.

0027Fig. 2 shows a laboratory instrument which has been designated "Osmo 19T" by its manufacturer. This instrument was used in the laboratory to make experimental runs leading to some of the following working examples. It has an open tank 50 for holding glucose syrup 27, the tank being coupled through a feed pump 52 and a pressure pump 54, to a membrane vessel 56. A pressure gauge 58 maintains about 450 PSI at 4-5 gallons per minute. Suitable valve means 60 passes a limited flow which creates a feed back loop represented by arrow A in order to mix some of the feed stream which has gone through the turbulence of pump 54 back into the fresh, incoming feed stream. The limited flow also buffer stores some material to adjust the line pressure to the 450 PSI.

0028The membrane vessel 56 may be thought of as a strainless steel tube having a membrane stretched diametrically across its interior to divide the interior into entrance and exit chambers with the only passage between them being via the membrane. The membrane may be thought of as a strainer which does not pass any molecules which are larger than a dextrose molecule. In reality, the membrane is a complex spiral shape; in any event, the material enters vessel 56 on an entrance side of the membrane, passes through the membrane, and leaves from an exit side, as a permeate at 62.

0029Since smaller molecules, if any, were removed earlier in the process, the permeate at 62 is almost pure dextrose. Therefore, on the entrance side of the membrane, material which does not pass through the membrane builds up and could accumulate to clog the membrane. To avoid this clogging, some of the material ("retentate") is returned from the entrance side, through a pipe 64, to the tank. A pressure gauge 66 is set at about 430 PSI which establishes a net difference of 20 PSI across the membrane. The valve 68 is set to adjust the volume of the fed back retentate.

0030A pilot system (Fig. 1) was set up in a factory to test larger scale production. In this example, the glucose syrup 27 enters via a feed pipe 70, passed through a pump 72, and flow meter 74 to a membrane tank vessel 76, which is constructed approximately the same as the vessel 56. A pressure gauge 78 controls the input pressure to the membrane. A recirculation loop (shown by arrow B) has a flow which is controlled by valve 80.

0031The outflow product of the purified dextrose product appears at 82. A pressure gauge 84 maintains the back pressure on the membrane in about the same manner that gauge 66 maintains it. Pressure control valve 86 is adjusted to maintain the desired pressure reading at gauge 84. A portion of the retentate is recycled at 88 to the input of pump 72. Another valve 90 is set so that a percentage of the retentate is bled off. This bleed material may be utilized in any suitable manner, as by returning to some appropriate upstream point in the process of Fig. 1 or by using it to produce pruducts other than substantially pure dextrose.

EXAMPLE 1

0032Three membranes were tested for dextrose purification of a feed stream derived from saccharified corn starch, using an Osmo 19T pilot system. <tables id="tabl0006" num="0006"><img file="EP0452238A2_D0006.tif" /></tables>

EXAMPLE 2

0033Larger scale runs were carried out using a Filtration Engineering UO membrane. An around the clock system was set up to determine filtration during a production scale of operations. The membrane had 1000 square feet of filtration area. The results are shown below: <tables id="tabl0007" num="0007"><img file="EP0452238A2_D0007.tif" /></tables> The daily average results for the product and feed properties are shown below. <tables id="tabl0008" num="0008"><img file="EP0452238A2_D0008.tif" /></tables>

EXAMPLE 3

0034Twenty gallon batches of dextrose liquor having different percentages of dry solids were processed in an Osmo 19T pilot system, using a Filmtech NF-40 membrane. The process conditions, fluxes and purity of feed and product streams are shown below. <tables id="tabl0009" num="0009"><img file="EP0452238A2_D0009.tif" /></tables>

EXAMPLE 4

0035Results from the same system that was used as in Example 2 after five days. <tables id="tabl0010" num="0010"><img file="EP0452238A2_D0010.tif" /></tables> Average results for the day. <tables id="tabl0011" num="0011"><img file="EP0452238A2_D0011.tif" /></tables>

EXAMPLE 5

0036A concentrated 500,000 MW ultrafiltered <u style="single">Lactobacillus casei</u> fermentation broth which contained approximately 36 % lactate ion was diluted to approximately 10 % lactate, ultrafiltered (50,000 insert, MW) and nanofiltered at pH 6.0 and at pH 2.3 after pH adjustment with sulfuric acid. The fermentation broth used in this Example 5 was taken from a 48- hour fermentation of a solution containing 140 grams of dextrose per liter, 5 grams/liter of yeast extract, 30 grams steepwater dry solids/liter, and 1.0 grams of (NH₄)₂PO₄ per liter. This mash was fermented with <u style="single">Lactobacillus casei</u> subspecies <u style="single">rhamnosus</u> with ammonia added for pH control at pH 6.0 and 110° C. When all of the dextrose was fermented, the broth was ultrafiltered and concentrated to 36 % lactate ion.

0037For this example, testing was conducted on an Osmo 19T pilot system with an Osmo MX06 membrane at approximately 400 PSI and 45° C. Samples of the ultrafiltered material (A), nanofilrered at pH 6 (B), and nanofiltered at pH 2.3 (C) were all adjusted to pH 2.4 and concentrated to between 20-30 % lactic acid for further processing. An HPLC (high pressure liquid chromatography) analysis was carried out on these samples with the results which are shown below. <tables id="tabl0012" num="0012"><img file="EP0452238A2_D0012.tif" /></tables><dl id="dl0004"><dt>DP₃+</dt><dd>= Trisaccharides and higher polymers.</dd><dt>DP₂</dt><dd>= Disaccharides</dd><dt>DP₁</dt><dd>= Monosaccharides</dd></dl>

0038As can be seen the nanofilter removed most of the disaccharides and some of the monosaccharides. Also, at pH's where the lactic acid is not ionized, the ratio of lactic acid to inorganic salts in the permeate increased, thereby providing a higher purification factor.

0039Those who are skilled in the art will readily perceive how to modify the invention. Therefore, the appended claims are to be construed to cover all equivalent structures which fall within the true scope and spirit of the invention.