WO1996006866A1

WO1996006866A1 - Method of modifying starch

Info

Publication number: WO1996006866A1
Application number: PCT/NL1995/000283
Authority: WO
Inventors: Gerard Lammers; Luuk Balt; Eize Jacob Stamhuis; Antonie Albertus Cornelis Maria Beenackers
Original assignee: Rijksuniversiteit Te Groningen
Priority date: 1994-08-31
Filing date: 1995-08-25
Publication date: 1996-03-07
Also published as: AU3231995A; NL9401412A; EP0778850A1

Abstract

The invention relates to a method of continuously modifying starch, wherein a suspension of starch granules (slurry) is heated in an aqueous liquid to a temperature at which still no gelatinization occurs, then lye and reagent are continuously fed to the suspension, the lye and optionally also the reagent being distributed homogeneously in the slurry, whereafter by means of electromagnetic radiation the mixture is further heated to a temperature that is desired for complete gelatinization and the modification and the mixture is fed to a reactor section provided with static mixing elements.

Description

Title: Method of modifying starch

This invention relates to a method of modifying starch, in which a suspension of starch granules in an aqueous liquid is gelatinized with lye and modified with a reagent.

The thus obtained modified starch products, such as starch ethers and starch esters, are often used as additive in the food and paper industry. Examples of applications of inter alia hydroxypropyl starch are the use as additive in textile printing and as size for corrugated board. The production of modified starch with a low molar substitution is done on an industrial scale by reaction of a suitable reagent, for instance ethylene oxide, propylene oxide or vinyl acetate, in an aqueous slurry of starch.

The preparation of products with a high molar substitution in an aqueous suspension (slurry) is rather difficult because the gelatinizing temperature lowers with increasing molar substitution. Because in that case gelatinization already occurs at a low temperature, there is a risk of the starch granules dissolving to form a viscous mass in which a uniform substitution is no longer possible, which may lead to poorly reproducible and inhomogeneous products.

It has already been proposed to perform the modification of starch, such as the preparation of starch ethers, in equipment where increased shearing forces occur, as in extruders and in static mixers. In this connection, see, for instance, Carbohydrate Polymers 24 (1994), pp. 55-59. When such equipment is used, however, the problems outlined above arise substantially unalleviated. Finally, it is of importance that the lye content in the end product be kept as low as possible, so that no or only a minor aftertreatment is necessary for the neutralization and/or the removal of the lye used. However, the problem of using small amounts of lye is that the reproducibility and the homogeneity of the product leave to be desired, because gelatinization then does not proceed uniformly in the mixture. If a part of the material gelatinizes faster than the remainder, this gives rise to a considerable spread in residence time, since the viscosity a gelatinizing starch suspension increases markedly, viz. t value that may be a factor 10⁶ higher than that of the ungelatinized material. Then the viscosity diminishes again, viz. to a value which is approximately halfway between the t extremes. It will be clear that this is undesired from the point of view of stable process control and reproducible product quality. It is an object of the invention to provide a method of modifying starch, for instance preparing gelatinized starch ethers, whereby in a simple manner a reproducible product is obtained. Another object of the invention is to provide a method for modifying starch, whereby a homogeneous product i obtained in a continuous manner. It is also an object of the invention to perform the modification, which is catalyzed by lye, with a smallest possible amount of lye, without the occurrence of problems due to differences in viscosity.

The invention contemplates providing a solution to one o more of these problems and is partly based on the insight th the problem of poor reproducibility and homogeneity of the product can be obviated at least partly by distributing the added lye homogeneously in the slurry.

The invention accordingly relates to a method for continuously modifying starch, wherein a suspension of starc granules (slurry) in an aqueous liquid is heated to a temperature at which still no gelatinization occurs, then ly and reagent are continuously fed to the suspension, with the lye and optionally also the reagent being distributed homogeneously in the slurry, whereafter by means of electromagnetic radiation the mixture is further heated to temperature desired for complete gelatinization and the modification and the mixture is fed to a reactor section provided with static mixing elements. The invention is based on the surprising insight that b special combination of measures the objects of the inventio can be realized. Of importance in this combination is first the preheating of the suspension of starch particles to a temperature at which still no gelatinization occurs. Preferably, this preheating is effected to a temperature which is not more than 15, preferably not more than 10 and more particularly not more than 5°C below the temperature at which gelatinization will occur.

A second aspect of the invention is concerned with the homogeneous feed of lye, which means that the lye is uniformly distributed throughout the entire slurry in a shortest possible period of time, while in the liquid substantially no concentration gradient of the supplied component over the cross section of the space arises. Because the gelatinization occurs virtually instantaneously, whereby the viscosity increases dramatically, it is preferred that the modifying reagent, often an alkene oxide or vinyl acetate, preferably ethylene oxide and/or propylene oxide, is added simultaneously with the lye or prior thereto. Preferably, this addition is also effected homogeneously, although this seems to be less critical. Owing to the often longer reaction time in the static mixing section, where radial mixing occurs as well, the modification is normally sufficiently homogeneous, even if the feed of the modifying reagent is not distributed entirely uniformly over the cross section of the slurry. It is preferred that the modifying reagent be fed homogeneously, however, because this has a positive effect on the reproducibility of the process and the homogeneity of the final product.

A third aspect of the invention is concerned with the homogeneous heating of the continuously gelatinizing mixture by means of electromagnetic radiation, in short, with a magnetron. Heating the continuously gelatinizing mixture with a magnetron leads to a highly uniform rise in temperature, so that the viscosity of the mixture over a cross section of the reactor remains the same. This prevents any substantial spread in the residence time due to differences in viscosity. Other methods of heating entail the problem that the non-uniform distribution of the heat, for instance due to a gradient over the cross section of the reactor, gives rise to large viscosity differences, as a consequence of which parts of t reactor content "shoot through" while other parts linger fo long time. The result is a marked difference in product quality in the reactor output. Another resultant effect is that of plugs of material "shooting through". In this connection it is noted that the above-mentioned article in Carbohydrate Polymers reports the use of electromagnetic radiation for heating the feed of a reactor. Finally, the heated material is to be reacted further i reactor suitable for the purpose, in this case a reactor comprising static mixing elements, to form a modified and completely gelatinized starch product. The term 'completely gelatinized' means that a homogeneous and substantially transparent solution of (modified) starch has been obtained This does not preclude the micro-scale presence of some fractions of non-gelatinized starch granules.

By the combination of the above aspects of the method, modified starch is obtained which satisfies the conditions in the introduction. More particularly, a homogeneous and reproducible product of a low sodium hydroxide (lye) conten is obtained.

In the practise of the method according to the inventio the various process conditions, as far as they have not already been laid down hereinabove, can be chosen by the skilled person, depending on the final products actually desired. This involves inter alia the molar substitution, which is preferably between 0.05 and 1.0, more particularly between 0.1 and 0.6. The nature of the substitution and the degree of substitution partly determine the process conditions, such as the amount of lye, the preheat temperature, and so forth. Of course, these variables also depend on the amount of starch in the suspension, which may for instance be between 10 and 45% by weight. As indicated hereinabove, the lye solution, preferably with a concentration of from 0.05 to 5 molar, is to be adde homogeneously, it being sufficient to use a concentration o maximally 300 mol/m³ slurry. This can be effected with advantage by feeding the lye and optionally also the alkene oxide to the slurry in a tubular reactor segment using one or more supply tubes which are arranged axially in the tubular reaction segment, and which supply tubes are provided with radial outlet openings.

In this manner a homogeneous distribution of the lye in the starch suspension is obtained, so that also during and after the uniform heating step a continuous and homogeneous gelatinization occurs, which is followed by the modification with the reagent.

Heating of the mixture is effected to a temperature which will be dependent on the degree of substitution and on the nature of the substituent, as well as on the amount of lye. It is noted that the supplied lye not only (partly) effects the gelatinization, but can also catalyze the modification reaction, for instance the etherification or the esterification reaction. Preferably, heating is effected to a temperature between 60 and 120°C, whereafter the heated mixture reacts further in the section of static mixing elements.

In the drawing a reactor is depicted which is suitable for the practise of the method according to the invention. In this drawing the reactor 1.is provided with a supply for the starch slurry 2, which slurry is mixed in mixing chamber 3 with reagent and lye. These are supplied via feed lines 4 and 5. It is of great importance in this connection that the feed of lye via line 5 takes place uniformly throughout the liquid. According to the invention, this can be achieved by providing the feed line 5 at the end thereof with radial openings, indicated by arrow 6. After passing the mixing chamber, the gelatinizing slurry enters reactor section 7, which is provided with static mixing elements 8, for instance SMX^® elements. In this section the slurry is further heated to the temperature desired for the complete gelatinization and modification, by means of electromagnetic radiation generated with microwave unit 9. Finally, the reaction can be continued further in react segments (not shown) provided with static mixing elements, whereafter finally the modified starch can be discharged fr the reactor and be processed further into a form that is suitable for sale and/or be stored.

The invention is further elucidated in and by an exampl but is not limited thereto.

EXAMPLE

A slurry consisting of 34.8% by weight of starch granul in water was fed at a flow rate of 3.07xl0"³ kg/s to a reac as depicted in the drawing. This slurry had been heated in preheating unit (not shown in the drawing) to a temperature 57°C. In the mixing chamber 0.21 g/s propylene oxide and 0. g/s of a 4 molar lye solution were continuously added to th slurry. The resultant highly viscous liquid was then heated a temperature of 90°C in the mixing section 8 by means of t microwave unit 9. After a residence time of 300 s in the reactor section of the reactor, which was also maintained a

90°C, 1.1 g/s dry modified starch with a molar substitution 0.30 was obtained.

Claims

C LA IMS

1. A method of continuously modifying starch, wherein a suspension of starch granules (slurry) in an aqueous liquid is heated to a temperature at which still no gelatinization occurs, then lye and reagent are continuously fed to the suspension, with the lye and optionally also the reagent being distributed homogeneously in the slurry, whereafter by means of electromagnetic radiation the mixture is further heated to a temperature desired for complete gelatinization and the modification and the mixture is fed to a reactor section provided with static mixing elements.

2. A method according to claim 1, wherein the temperature in the first heating step is not more than 58°C.

3. A method according to claim 1 or 2, wherein the lye and the reagent are fed to the slurry in a tubular reactor segment using one or more supply tubes which are axially arranged in said tubular reactor segment and which supply tubes are provided with radial outlet openings.

4. A method according to claims 1-3, wherein as reagent an alkene oxide, for instance ethylene oxide and/or propylene oxide, or vinyl acetate is used.

5. A method according to claim 4, wherein as reagent propylene oxide is used, preferably in such an amount that a molar substitution of 0.05 to 1.0, preferably 0.1 to 0.6 is achieved.

6. A method according to claim 5, wherein said heating is effected by means of electromagnetic radiation to a temperature above the gelatinizing temperature, preferably between 60 and 120°C.

7. A reactor system suitable for practising the method according to claims 1-6, comprising a preheating unit, means for homogeneously feeding two or more substances to a slurry, a unit for heating by means of electromagnetic radiation, an after-reaction unit in which static mixing elements are disposed.