TECHNICAL FIELD OF THE INVENTION
This invention is in the field of oligosaccharides, and in particular, the invention pertains to the preparation of a high molecular weight dextrin product.
BACKGROUND OF THE INVENTION
Certain enzymes, known as beta-amylase enzymes, are known to act on starch to produce low molecular weight species, typically maltose, and high molecular weight species, known as dextrins. With the exception of so-called waxy (corn) or glutinous (rice) starches, most starches found in nature are composed of a mixture of amylopectin and amylose. Amylose is a linear molecule which is substantially completely hydrolyzed by beta-amylase enzymes into maltose and glucose. Amylopectin, a branched molecule, is hydrolyzed into maltose and higher molecular weight dextrins, because the beta-amylase enzyme is unable to hydrolyze past the alpha 1-6- branch point in the amylodextrin molecule. If the enzymatic hydrolysis is allowed to proceed to its fullest extent, the remainder of the amylopectin molecule will exist as what is known as beta-limit dextrin.
Despite the potentially numerous commercial uses for such high molecular weight dextrins, it is believed that no such dextrins are sold commercially in bulk quantities. Present enzymatic processes yield a mixture of products from which it is difficult to resolve such dextrins. The present invention seeks to provide a process for preparing dextrins, such as beta-limit dextrin, in which this difficulty is overcome.
It has now been found that the treatment of starch with an enzyme that consists essentially of a beta-amylase enzyme, and which is to the substantial exclusion or complete exclusion of alpha-amylase enzymes and de-branching enzymes, will yield a product mixture that includes a dextrin and one or more low molecular weight sugars. The low molecular weight sugar or sugars may be readily separated from the product mixture thus formed via ultrafiltration to yield a dextrin in the retentate. If desired, diafiltration may be used to separate substantially all of the low molecular weight sugars from the dextrin in the retentate.
Retrograded amylose may be removed from the product mixture prior to ultrafiltration. In accordance with another embodiment, the invention provides a method for preparing retrograded amylose. It is contemplated that this material is useful as “resistant” starch, which is not as digestible as other starches and which therefore may be used as a low- or non-caloric bulking agent.
The dextrins thus prepared will have a number of desirable properties, including a high solubility and a high molecular weight, with low hazing in solution. Additionally, the dextrins have a very low dextrose equivalent value (DE), and thus are expected to be substantially more stable than carbohydrates of lower molecular weight. As such, it is contemplated that such dextrins may be used in applications such as viscosifiers or as spray drying aids for other carbohydrates (such as maltose). In accordance with another embodiment of the invention, the dextrin is added to maltose in an amount sufficient to assist in spray drying.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention contemplates the production of dextrins, such as beta-limit dextrin, from starch. Any suitable starch may be employed in connection with the invention, and thus, for instance, starches such as corn, rice, wheat, tapioca, maize, potato, barley, oat, and, more generally, any starch suitable for enzymatic hydrolysis may be used in connection with the invention. It is not necessary to use a so-called waxy or glutinous starch in connection with the invention, but to the contrary the starch can have any suitable amylose content, such as an amylose content of 10%, 15%, 20%, 25%, or a greater amylose content. It is contemplated that the starch may be a partially derivatized or otherwise modified starch, or may be a starch that has been thinned or enzymatically treated. For instance, a starch that has been lightly oxidized may be employed.
The starch should be liquefied via heat, enzymatic, or acid treatment prior to treatment with the beta-amylase enzyme. Preferably, the starch is liquefied via acid treatment, although low amylose starches may require liquefaction only with heat and may be suitably liquefied at the operating temperature of the enzymatic hydrolysis. As disclosed in more detail in copending application serial no. , filed Feb. 28, 2001 by Richard L. Antrim and Clark P. Lee and assigned attorney docket number 203946 and hereby incorporated by reference, it is desirable to recover maltose from the beta-amylase hydrolysis product. Thus, in general, the starch should be liquefied to an extent such that it would remain liquid at the operating temperature of the beta-amylase hydrolysis, but not liquefied to an extent such that the starch is converted to saccharides having so low a degree of polymerization that it is difficult to separate such saccharides from maltose via ultrafiltration. In other words, the degree of liquefaction should be such that, upon enzymatic hydrolysis with the beta-amylase enzyme, the combined content of glucose and oligosaccharides in the DP 3- 10 range does not exceed about 10%, and preferably does not exceed about 5%. It has been found acceptable to liquefy the starch to a dextrose equivalent (DE) value of about 2, as measured via conventional techniques. Generally, the DE of the starch should be kept below about 1, and thus the DE should range between 0 and about 1, although it may be difficult to measure the DE with precision in this range. For corn starch, it is preferable that the starch is liquefied in an aqueous solution at a liquefaction temperature ranging from about 220° F. to about 320° F., and for a time ranging from about 5 minutes to about 30 minutes.
The starch solids level preferably ranges initially from about 5% to about 30%, more preferably, from about 15% to about 30%. While it is not intended to limit the invention to a particular theory of operation, it is believed that a lower solids content requires a lesser degree of liquefaction to attain the desired viscosity range. In the case of dent corn starch, it has been found that a viscosity window of between 25 and 45 centipoise (Norcross Shell Cup) is optimal. In the case of waxy starches, viscosities outside this range may be acceptable. The pH of the starch slurry should be adjusted to a level sufficient to provide controlled acid hydrolysis of the starch in the presence or absence of catalyzing alpha amylase enzymes; most preferably, under a given set of conditions, the variability of the slurry pH should be no more than +/−0.1 pH, with the precise pH value depending upon the starch source, the slurry solids, and the operational conditions of the liquefaction equipment employed. As a practical matter, the pH can vary more widely while still resulting in a satisfactory product. Preferably, the starch liquefaction is monitored via viscosity and adjusted accordingly.
In accordance with one embodiment of the invention, the starch is liquefied with an alpha-amylase enzyme to reduce the molecular weight of the starch, thereby reducing the viscosity of the starch and thereby permitting processing at a higher solids level. Suitable commercial liquefying enzymes may be obtained from Genencor International, Inc. or from Novozymes A/S. The dosing level of the alpha-amylase enzyme depends upon the desired solids level and, when maltose is recovered as a co-product, on the desired maltose purity. Desirably, the dosing level ranges from about 0.005% to about 0.02% of a commercial strength enzyme by dry solids basis starch. In this embodiment, the alpha-amylase enzyme preferably is quenched prior to saccharification via any suitable quenching procedure. For instance, when the starch is liquefied at a temperature less then 250° F. and 5 minutes residence, the alpha-amylase enzyme is quenched by reducing the liqefact pH to less than 4.0 and holding at a temperature of from 180 to 190° F. for at least about 15 minutes.
Upon liquefaction, the liquefact is immediately cooled and the pH is adjusted to the optimum conditions for beta-amylase activity. The starch then is treated with the enzyme under any conditions suitable to result in the hydrolysis of this liquefied starch to form dextrin, and preferably, to form beta-limit dextrin. A preferred enzyme is OPTIMALT BBA, available from Genencor International, Inc. The enzyme may be added in any amount sufficient to achieve this result, but generally, the dosing of the enzyme should be in excess of the minimum viscosity limited conversion of approximately two Genencor OPTIMALT BBA Diastatic Power units per kilogram of starch, the Diastatic Power units being defined as being the amount of enzyme contained in 0.1 ml of a 5% solution of the sample enzyme preparation that will provide sufficient reducing power to reduce 5 ml of Fehling's solution when the sample is incubated with 100 ml of substrate for one hour at 20° C.
The enzymes should be allowed to act on the starch for any amount of time suitable to form the desired dextrin. Under the preferred reaction conditions discussed hereinabove, the enzymatic action generally is 90% complete within 4 hours. The optimum temperature and pH of the starch hydrolysis will vary depending on the particular beta-amylase enzyme employed, but typically the temperature will range from about 55° C. to about 65° C. and the pH will range from about 5.0 to about 6.0. Optionally, but preferably, the product mixture thus formed is clarified and decolored by any suitable procedure, such as carbon treatment, filtration, centrifugation, and or precipitation, before it is further processed. If the enzyme is allowed to act under optimum conditions for an optimum reaction time, the dextrin content may be greater than about 20%, most of which will comprise beta-limit dextrin. The combined content of glucose and of oligosaccharides in the DP 3-10 range is below about 10%, and preferably is below 5%.
Retrograded amylose may be found as a by-product of the enzymatic hydrolysis. In accordance with one embodiment of the invention, at least some of the retrograded amylose is separated from the product mixture. For instance, the saccharified solution may be maintained at a temperature below about 14° F. to allow at least a portion of the retrograded amylose to crystallize. The crystallized amylose then may be separated from the saccharified starch mixture by any suitable technique, such as via microfiltration, by which is contemplated separation at a resolution sufficient to separate the retrograded amylose but not sufficient to separate dextrins from low molecular weight sugars in the product mixture. Alternatively, the retrograded amylose may be separated via centrifugation, using any technique known in the art or otherwise found to be suitable. Preferably, the solution prepared upon enzymatic hydrolysis is centrifugated for at least 15 minutes at a relative g force of 3000. The amylose crystals will form a pellet, and the low molecular weight sugars and limit dextrin will remain in the clarified supernate.
In accordance with one embodiment of the invention, a dextrin is separated from the product mixture. Most preferably, a dextrin product is separated from the product mixture via ultrafiltration of the product mixture, by which is contemplated separation of the beta-limit dextrin from lower molecular weight carbohydrates using a membrane or other suitable separation medium that is effective for this purpose. Generally, a membrane having a molecular weight cut off (MWCO) of 10,000 or less, preferably a MWCO of 5000 or less, is suitable. Suitable commercially available membranes are available from Syndar Filtration and from Osmonics De Sal. Upon ultrafiltration, the retentate typically will include the desired dextrin and some retained low molecular weight sugar (typically maltose). If desired, the retentate may be diafiltered to recover additional maltose by flushing the filter with excess water.
The product thus formed has numerous desirable properties, including a high molecular weight, for instance, a molecular weight of at least 30,000 Daltons and ranging up to 600,000 Daltons, in some cases higher, depending on the starch used as a starting material. The product further has a low DE (and hence a low reactivity and susceptibility to color change), and, surprisingly, a high degree of solubility with very low hazing, even at high molecular weights. Numerous commercial uses are contemplated, including use as a viscosifier. In such applications, the limit dextrin may be added to a product to be made more viscous, in any amount effective for this purpose. It is further contemplated that the dextrin prepared in accordance with the invention can be added to a solution of maltose or of another carbohydrate, or to a dry maltose or other carbohydrate product in an amount sufficient to enhance spray drying of the solution or dry product. In this embodiment, the dextrin preferably is added in an amount ranging from about 5% to about 70% dry solids basis per dry weight of the maltose or other carbohydrate to form a mixture. The mixture may contain other ingredients besides the carbohydrate to be spray dried and limit dextrin, some of which ingredients also may function to enhance spray drying of the maltose.
Carbohydrate percentages given herein are expressed on a dry solids basis per total carbohydrate weight.
The following examples are provided to illustrate the invention, but should not be construed as limiting in scope.