Method of preparing modified starch granules

Abstract: Disclosed is a method of producing starch granules having modified characteristics, by treating starch granules with an amylase. Where cereal starch granules are lightly decomposed with α-amylase, the viscosity characteristic of them greatly varies so that the viscosity of the decomposed granules decreases to such a degree that they are only slightly viscous. The decomposed starch granules may be used in preparing instant liquid foods, and the use of them is expected to be extremely broad. By appropriately blending the decomposed starch granules and other non-treated starch granules or decomposed starch granules, blends of different starch granules having various viscosity values can be obtained. Since the paste or liquid to be prepared from the enzyme-treated starch granules is smooth and soft to the touch, they may be expected to be useful as base materials for producing various foods. Starch granules with holes may adsorb aromatic components. By combining them with enzyme-treated low-viscosity starch granules, various foods can be produced. Since the enzyme-treated low-viscosity starch granules of the invention may be produced extremely simply, they are useful as a raw material in the starch sugar industry.

Description: The present invention relates to a method of preparing starch granules having modified characteristics. More precisely, it relates to a method of preparing modified starch granules, in which starch granules are treated with various amylases to modify the characteristics of them. BACKGROUND OF THE INVENTION For improving and modifying physical properties of starch, heretofore, there is known a method of adding an α-amylase or acid to starch, when starch is blended with water for gelatinization, so as to lower the viscosity of the blend; or a method of adding various substances of a metal ion, an algal polysaccharide or a water-soluble gum to starch so as to elevate or lower the physical properties of the starch gel. For modifying the characteristics of starch granules, there is known a method of crosslinking starch granules to decrease their solubility. EP-A-0182296 discloses a dusting powder composition containing porous starch granules, which is prepared by treating granular starch with an α-amylase enzyme. WO 89/04842 describes amylase treated granular starches which provide a microporous matrix material adapted for absorption and releasable containment of functional compositions. The porous starch granules can be used as a carrier for different substances. This is, however, only applicable after a treatment with surface modifying agents to enable absorption of lipophil substances. H. Fuwa et al., starch / Stärke 30 (1978), number 6, page 186 - 191 gives a general description of the possibility to treat starch with α-amylase and/or glucoamylase. SUMMARY OF THE INVENTION The present inventors variously investigated so as to modify the characteristics of starch granules by directly treating them with an enzyme and, as a result, have found that the intended object can be attained by treating starch granules with an α-amylase. Specifically, they treated starch granules with various amylases such as α-amylase, β-amylase or glucoamylase, took out the enzyme-treated starch granules and investigated the physical properties of them. They have found that the enzyme-treated starch granules display various physical properties when dissolved, depending upon the kind of the enzyme used for treatment and the kind of the starch granules to be treated therewith. On the basis of the finding, they have completed the present invention. Accordingly, the present invention relates to a method of preparing starch granules with modified characteristics, in which starch granules are treated with an α-amylase under specific conditions to have a specific decomposition percentage. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows photographs of α-amylase-treated rice starch granules "Wakaho-no-minori", as taken with a scanning electromicroscope;in which A is a control sample (decomposition percentage, 0 %), B is a sample with a decomposition percentage of 2.0 %, and C is a sample with a decomposition percentage of 5.0 %. Fig. 2 shows a viscograph of α-amylase-treated rice starch granules "Nihon-bare", in which A is a control sample (decomposition percentage, 0 %), B is a sample with a decomposition percentage of 0.5 %,C is a sample with a decomposition percentage of 0.9 %, D is a sample with a decomposition percentage of 1.9 %, and E is a sample with a decomposition percentage of 3.1 %. DETAILED DESCRIPTION OF THE INVENTION In the present invention, any starch granules from various sources may be used, such as those from rice, wheat, corn, potato and sweet potato. As amylases for decomposition of such starch granules in accordance with the present invention, any α-amylase derived from various sources can be used. In general commercial products are used, such as a purified α-amylase (produced by Sigma Co., derived from Bacillus amyloliquefaciens , with 930 IU(international unit)/mg). In treatment of starch granules with the enzyme, where the pH variation in the reaction system is small, use of any particular buffer is unnecessary. As the case may be, various buffers can be used such as a phosphoric acid buffer(pH 6.9), an acetic acid buffer (pH 5.2), and an acetic acid buffer (pH 4.8). The enzymatic treatment of starch granules is to be effected in such a way that the starch granules are not over-decomposed. It is effected in the manner that starch granules are decomposed to have a decomposition percentage of from 0.1 to 15.0 %. The concentration of the starch granules which are the substrate for the enzymatic treatment may be such that the granules are suspended or are in the form of a cake. Preferably, it is from 5 to 40 %. The amount of the α-amylase to be used for the enzymatic treatment varies, depending upon the kind of the starch granules to be treated therewith and the kind of the enzyme itself. For instance, where the above-mentioned purified α-amylase (produced by Sigma Co., derived from Bacillus amyloliquefaciens) is used,it is added to the reaction system in an amount of from 0.4 to 400 IU, preferably from 4 to 40 IU, per g of starch granules to be treated therewith. The enzymatic reaction is effected in the above-mentioned buffer or in water under the condition, at a temperature of from 10 to 65° C, preferably from 25 to 40° C, for 1 to 4 hours. Where the enzymatic reaction needs a long period of time, the amount of the enzyme to be added thereto may be reduced. The enzyme titer is herein identified by the Somogyi-Nelson method; and the decomposition percentage of the starch granules treated is herein determined by sampling the decomposed starch suspension at regular intervals, centrifuging it at 3,000 rpm for 10 minutes, and measuring the free saccharide as liberated in the resulting supernatant by a phenol-sulfuric acid method. On the basis of the thus measured value, the intended decomposition percentage of all the starch granules treated is calculated. The present invention will be explained in more detail by way of the following examples, which, however, are not intended to restrict the scope of the present invention. In the following examples, the scanning electromicroscopic (SEM) observation was effected in the manner mentioned below. The precipitates to be used for measurement of the decomposition percentage of the decomposed starch suspension, which is sampled at regular intervals, are isolated by centrifugation (3,000 rpm, 10 min), washed twice with a distilled water and once with acetone and dried. The dried powder is sprinkled over a sample stand having a duplicate-adhesive tape as attached thereto, and this is coated with gold (200 Å thick) by gold vapor deposition with an ion-sputtering device (JFC-1500 Model, manufactured by Nippon Electronic Co.). The sample preparation is then observed with SEM of JSM-880 Model (manufactured by Nippon Electronic Co.) under the condition of an accelerated voltage of from 3 to 10 KV and a magnification of from 500 to 2,000 times. Measurement of the viscosity of each sample is effected with a microviscograph (manufactured by Toyoh Seiki Seisakusho KK). Briefly, enzyme-treated rice starch grains as previously determined with respect to the water content thereof are adjusted to be 8 % dried ones, and the variation of the viscosity of 10 g of an aqueous suspension of the rice starch grains is measured. The conditions for the measurement are such that the start temperature and the final temperature are both 30°C, the start temperature retention time is 5 minutes, the heating and cooling rate is 1.5°C/min,the higher most critical temperature is 99°C (whereupon the temperature of the sample is 95°C), the higher most critical temperature retention time is 10 minutes, the rotation number with torque detection blade A type is 75 rpm, and the torque range is 20 and 50 g.cm. Display of the viscosity curve is effected with a TS viscograph software. As rice samples, five kinds of polished rice of "Wakaho-no-megumi", "Wakaho-no-minori" and "Nihon-bare" as produced in Saitama-ken in 1990; and "Koshihikari (trade name: Pearl Rice Ten-keimai)" and "Mochihikari (trade name: Pearl Rice Mochi-gome)" as produced in Nagano-ken in 1990 are used. These are purified by an alkaline method (with 0.25 % sodium hydroxide) and dried under pressure at 40°C to prepare starch granules, which are used. As corn starch, potato starch, wheat starch and sweet potato starch samples, commercial products of them are used. EXAMPLE 1 Regarding the enzyme decomposability of rice starch grains, enzyme decomposition conditions of rice samples of the above-mentioned five kinds with α-amylase, glucoamylase (comparison and β-amylase (comparison) are shown in Table 1 below. Enzyme Decomposition Conditions of Rice Starch Grains Name of Enzyme Determined Decomposition Percentage(%) Amount of Starch(g) Amount of Enzyme(mg) Amount of Buffer(ml) Decomposition Time(hr) α-amylase 0.5 25 0.01 100 0.5∼2.5 1.0 25 0.10 100 1.0 2.0 25 1.00 100 1.0∼3.5 3.0 25 5.00 100 2.0∼4.0 5.0 25 5.00 100 2.0∼4.0 0.5 25 1.00 100 1.0∼5.5 glucoamylase (comp.) 2.0 25 1.00 100 1.0∼5.5 2.5 25 2.00 100 2.0∼8.0 5.0 25 2.0 100 2.0∼8.0 30.0 25 10ml 100 9. 0.4 10 0.01 50 4. (comp.) β-amylase 0.8 10 1ml 25 4 The difference in the enzyme decomposability with α-amylase between rice starch granules of different kinds was investigated with respect to the time-dependent variation thereof in a low decomposition percentage range (1 to 9 %) and in a high decomposition percentage range (8 to 27 %). As a result it was observed that the enzyme decomposability of "Mochihikari" was high and that of "Nihon-bare" was low in both the low decomposition percentage range and the high decomposition percentage range. Regarding the difference in the decomposability between the other kinds, "Wakaho-no-megumi" and "Wakaho-no-minori" had the highest decomposability, then "Koshihikari" in this order, in the low decomposition percentage range, while there was almost no difference between "Wakaho-no-megumi" and "Wakaho-no-minori" in this respect. In the high decomposition range, "Wakaho-no-megumi" had the highest decomposability, then "wakaho-no-minori" and then "koshihikari" in this order, but the difference between them was not so large. With respect to glucoamylase (comparison), there was almost no difference in the decomposability between rice starch granules of different kinds. With respect to β-amylase (comparison), a large amount of concentrated pure enzyme liquid thereof was added to the reaction system but the enzyme decomposability thereof of decomposing rice starch grains was weak. In every enzyme tested, there was admitted no relationship between the protein and amylase content therein and the enzyme decomposability. Rice starch granules as decomposed by α-amylase in a decomposition percentage of 0 %, 2 % and 5 % were observed by SEM, and the photographs taken are shown in Fig. 1. The outward appearance of the rice starch grains as decomposed with the enzyme in a decomposition percentage of 1 to 2 % was not specifically different from that of the non-treated rice starch granules. However, the rice starch grains as decomposed with α-amylase in a decomposition percentage of 5 % gave definite holes. With elevation of the decomposition percentage, the number of the holes on the surface of the decomposed starch granules and the size of them increased. The starch granules were admitted to be destroyed in a decomposition percentage of 20 %; and a fairly large number of starch granules were destroyed in a decomposition percentage of 25 %. Also in the case of glucoamylase (comparison), the number of the holes and the size of them on the surface of the decomposed starch granules increased with elevation of the decomposition percentage. However, when glucoamylase was compared with α-amylase with respect to the decomposability in the same decomposition percentage, the size of the holes as made with glucoamylase was smaller than that with α-amylase. Even in the decomposition percentage of 30 %, the number of the holes and the size of them increased with glucoamylase, but the shape of the decomposed starch granules was not destroyed but was maintained as it was. However, decomposition with β-amylase (comparison) was weak, and both the number of the holes and the size of them made in rice starch granules of all the kinds as decomposed with it were small. As mentioned above, the enzyme decomposability varies, depending upon the kind of the enzyme used and the kind and sort of the starch granules decomposed therewith. However, the difference between the decomposability and the physical properties was not clarified up to now. The physical properties of the enzyme-treated starch granules were investigated with a viscograph and, as a result, it was clarified for the first time that there was a great difference in the physical properties even though the decomposition ratio is the same. A viscogram of each of the enzyme-treated rice starch granules of all the above-mentioned kinds was taken. As one typical example, the viscogram of "Nihon-bare" as decomposed with α-amylase is shown in Fig.2. As is obvious from it, extreme change is admitted even in the starch granules which are same as the non-treated ones with respect to the outward appearance. Regarding the characteristic of the variation of the viscosity of α-amylase-decomposed starch granules, the maximum viscosity and the minimum viscosity as well as the final viscosity lower even by a weak enzymatic decomposition of 0.5 % or so to often cause noticeable decrease of break-down, consistency and setback. In the decomposition percentage of 2 %, the characteristic values greatly decrease; and in the decomposition percentage of 3 % and 5 %, the rice starch granules of almost all the kinds lost gelatinization and aging. With increase of the decomposition percentage, the tendency was admitted towards elevation of the gelatinization temperature. With respect to the difference in the kind of rice, "Wakaho-no-megumi" had noticeable lowering of the characteristic values in the decomposition percentage of 0.5 % and 0.9 %, and it had extreme lowering of them in the decomposition percentage of 2.2 % with detection of almost neither gelatinization and aging. "Wakaho-no-minori" had noticeable lowering of the characteristic values in the decomposition percentage of 0.5 %, and the characteristic values of it gradually lowered with elevation of the decomposition percentage of from 1.1 % to 2.2 %, while slight gelatinization and aging was found even in the decomposition percentage of 2.7 % and 4.5 %. In the case of "Nihon-bare", it had noticeably lowered characteristic values in the decomposition percentage of 0.5 %. With elevation of the decomposition percentage from 0.9 % to 1.9 % to 3.1 %, the characteristic values gradually lowered, but the difference in the characteristic values between the decomposition percentage of 1.9 % and that of 3.1 % was small. In the decomposition percentage of 5.1 %, both gelatinization and aging disappeared. In the case of "Koshihikari", it had noticeable lowering of the characteristic values in the decomposition percentage of 0.4 %, and the characteristic values of it greatly lowered in the decomposition percentage of 1.1 %. With elevation of the decomposition percentage of from 1.9 % to 2.9 %, the characteristic values gradually lowered, and almost no gelatinization or aging was detected in the decomposition percentage of 5.2 %. On the other hand, in the case of "Mochihikari", it had noticeable lowering of the characteristic values in the decomposition percentage of 0.5 %. The characteristic values of it remarkably lowered in the decomposition percentage of 1.0 %; and almost no gelatinization or aging was found in the decomposition percentage of 2.0 %, 3.0 % and 5.6 %. As will be understood from the above, the variation of the characteristics in the enzyme-decomposed rice starch granules is in total large in a low decomposition percentage range (2 % or less), though somewhat varying in accordance with the kinds of rice. The glucoamylase decomposition and β-amylase decomposition were found to have no particular influence on the variation of the viscosity of the decomposed products, irrespective of the degree of the decomposition percentage and the kind of rice used. Only with respect to the glucoamylase decomposition, slight delay of the gelatinization temperature was found with increase of the decomposition percentage. For corn starch, the decomposability with three enzymes of α-amylase, β-amylase (comparison) and glucoamylase (comparison) was investigated; and for potato, sweet potato and wheat, the decomposability with α-amylase was investigated. As shown in Table 2, the decomposability of corn and wheat with α-amylase was high, while that of potato and sweet potato with the same was low. The decomposability with glucoamylase and β-amylase was low. Particularly, they were decomposed only slightly with β-amylase. The thus enzyme-decomposed four kinds of starch granules were observed by SEM. As a result, it was found that all the four kinds of starch granules were decomposed with α-amylase with increase of the number of the holes and the size of them along with elevation of the decomposition percentage. In addition, decomposition of corn starch granules with glucoamylase and β-amylase was also found. In determination of the physical properties, noticeable variation of the physical properties was found in the four kinds of starch granules as decomposed with α-amylase. Precisely, the variation was remarkable in potato and sweet potato having a decomposition percentage of 2.0 % and in wheat having a decomposition percentage of 0.5 %; or that is, both the maximum viscosity and the minimum viscosity and the final viscosity lowered and the break-down, consistency and set-back were reduced, but disappearance of gelatinization and aging was not detected. Also in corn having a decomposition percentage of 2.0 %, these characteristic values decreased, and the decrease of them was especially large in the decomposition percentage of 9.4 % with disappearance of both gelatinization and aging. However, in the decomposition percentage of from 0.5 to 1.0 % or so, increase of the consistency and the final viscosity was found. Where corn starch granules were decomposed with glucoamylase or β-amylase, no noticeable variation of the viscosity of the decomposed products was found. From the above-mentioned results, it is understood that since the α-amylase-treated rice and corn starch granules in a decomposition percentage of 2 % rapidly liquefy and uniformly dissolve when a hot water is added thereto, they are widely utilizable in instant soup and broth dishes. On the other hand, the corn starch granules as lightly decomposed may well be blended with wheat flour, and they are effective for improving the texture of noodle and bread. Since the paste as prepared from the α-amylase-treated starch granules is smooth and soft to the tongue, it may be blended with drinks and other foods to improve the taste of them. Improvement of the physical properties of raw starch by acid-treatment could be anticipated. Any and every starch granules may be used, irrespective of the kind of them, provided that they have a starch granular structure. The method of the present invention may therefore be applicable to any and every of raw starch, dry heat-treated starch, alkali-treated starch and others treated by any other means. The enzyme to be used for the purpose is α-amylase, in order to effectively lower the viscosity of the decomposed starch granules. The action and effect of the α-amylase used may also vary, depending upon the kind of the starch granules to be treated therewith. The α-amylase usable in the present invention broadly includes various kinds such as α-amylase and raw starch decomposing α-amylase. The enzyme which is suitable to the intended characteristics is used, and two or more different enzymes may be used in combination. EXAMPLE 2 10 mg of α-amylase (commercial product) and 100 ml of phosphate buffer (200 mM, pH 6.9) were added to 25 g of rice starch and stirred at 30°C to effect enzymatic reaction therebetween. After the reaction, the reaction mixture was centrifuged. The total saccharide content in the resulting supernatant was measured, and the decomposition percentage was calculated out. The centrifuged starch was washed twice with 40 ml of a pure water, and 40 ml of acetone was added thereto. The whole was dehydrated and dried under reduced pressure at 40°C overnight, and the various characteristic values of the product were measured by viscography. The results obtained are shown in Table 3 below. As is obvious from the results in Table 3, the maximum viscosity suddenly decreased in the decomposition percentage of about 2 %, but any particular deformation of the outward appearance of the decomposed product was not admitted by observation with SEM. As a whole, glutinous rice starch granules are easily decomposable with amylase, and the amylase decomposability of nonglutinous rice starch granules varies in accordance with the kind and sort of them. In particular, a so-called high-quality rice displayed amylase decomposition resistance. EXAMPLE 3 (comparison) Enzyme-treated dry starch granules were obtained in the same manner as in Example 2, except that from 1 to 2 mg of glucoamylase of a commercial product and acetate acid buffer (200 mM, pH 5.2) were used. By observation with SEM, definite holes were found in the decomposed product with a decomposition percentage of 1 %. The size of each hole was different from that in the a -amylase-decomposed product and was sharp. Various viscographic characteristic values of the decomposed starch granules are shown in Table 4 below. As is obvious therefrom, any particular change of the viscosity characteristic is not admitted in the decomposition percentage of about 3 % or so. With further progress of the decomposition of starch granules, elevation of the gelatinization temperature is admitted. The glucoamylase-treated starch granules displayed a property of well adsorbing oil substances. EXAMPLE 4 Corn starch granules were treated and decomposed with α-amylase in a decomposition percentage of from 0.5 to 1.0 % to obtain decomposed starch granules having a larger final viscosity and a larger consistency value than those of the original starch granules. Because of the properties of the decomposed starch granules, they can be added to various starch foods so as to tighten the taste of them. As has been explained in detail in the above, starch granules are treated with various amylases in accordance with the present invention to modify the characteristics of them. In particular, where cereal starch granules are lightly decomposed with α-amylase, the viscosity characteristic of them greatly varies so that the viscosity of the decomposed granules decreases to such a degree that they are only slightly viscous. Therefore, the decomposed starch granules may be used in preparing instant liquid foods, and the use of them is expected to be extremely broad. By appropriately blending the decomposed starch granules and other non-treated starch granules or decomposed starch granules, blends of different starch granules having various viscosity values can be obtained. Since the paste or liquid to be prepared from enzyme-treated starch granules is smooth and soft to the touch, they may be expected to be useful as base materials for producing various foods. Starch granules with holes have an interesting characteristic of adsorbing aromatic components. Amylases are expected to decompose preferentially the hydrophilic parts of starch granules, whereupon the hydrophobic parts of them remains to give a result that the decomposed starch granules adsorb hydrophobic substances such as aromatic components. Enzyme-treated starch granules into which aromatic components have been introduced may be combined with other α-amylase treated low-viscosity starch granules to produce various foods. Since α-amylase-treated low-viscosity starch granules may be produced extremely simply, they are useful as a raw material in the starch sugar industry. The concentration of a substrate of α-amylase-treated starch granules having a low viscosity can be elevated, and even though the temperature thereof is lowered to a suitable temperature for the enzymatic reaction, the liquefied starch is not gelatinized. Therefore,continuous addition of the substrate is possible and the α-amylase-treated low-viscosity starch granules may be utilized in the process of continuous production of starch sugar.

Claims: A method of producing starch granules having characteristics of low viscosity at a time of solubilization, which comprises treating raw starch granules with an α-amylase at a temperature of from 10 to 65° C, whereby the starch granules are decomposed to have a decomposition percentage of from 0.1 to 15.0 %. The method as claimed in claim 1, wherein the raw starch granules are treated with the α-amylase in a water or buffer solution. The method as claimed in claim 1, wherein the starch granules are selected from the group consisting of rice, wheat, corn, potato and sweet potato. The method as claimed in claim 1, wherein the α-amylase is used in an amount of from 0.4 to 400 IU per gram of the starch granules. The method as claimed in claim 1, wherein the concentration of the starch granules in the water or buffer solution is from 5 to 40 %. A method of producing starch granules having characteristics of absorbing hydrophobic substance, which comprises treating raw starch granules with a glucoamylase at a temperature of from 10 to 65° C, whereby the starch granules are decomposed to have a decomposition percentage of from 0.1 to 15.0 %. The method as claimed in claim 6, wherein the raw starch granules are treated with the glucoamylase in a water or buffer solution, The method as claimed in claim 6, wherein the starch granules are selected from the group consisting of rice, wheat, corn, potato and sweet potato. The method as claimed in claim 6, wherein the glucoamylase is used in an amount of from 0.4 to 400 IU per gram of the starch granules. The method as claimed in claim 6, wherein the concentration of the starch granules in the water or buffer solution is from 5 to 40 %.

Inventors: KOBAYASHI SHOICHI, MIWA SHOJI, TSUZUKI WAKAKO, KOBAYASHI, SHOICHI, MIWA, SHOJI, TSUZUKI, WAKAKO
Applicants: NAT FOOD RES, DIRECTOR OF NATIONAL FOOD RESEARCH INSTITUTE MINISTRY OF AGRICULTURE FORESTRY AND FISHERIES
Application year: 1992
Main classification: C12P19
IPC codes: C08B30, C12P19, A23L1