WO1997032902A1

WO1997032902A1 - Method for oxidizing dry starch using ozone

Info

Publication number: WO1997032902A1
Application number: PCT/NL1997/000105
Authority: WO
Inventors: Ronald Peter Wilhelmus Kesselmans; Ido Pieter Bleeker
Original assignee: Coöperatieve Verkoop- En Productievereniging Van Aardappelmeel En Derivaten Avebe B.A.
Priority date: 1996-03-04
Filing date: 1997-03-04
Publication date: 1997-09-12
Also published as: AU2236197A; ID16117A; NL1002525C2

Abstract

The invention relates to a method for the dry oxidation of starches or starch derivatives with ozone, wherein at least a part of the primary hydroxyl groups are selectively oxidized. The oxidation can be performed in the presence of a buffer and/or a halogenide-containing catalyst. The starches which are treated according to the invention are root starches, tuber starches and waxy cereal starches.

Description

Title: Method for oxidizing dry starch using ozone.

This invention relates to a method for oxidizing starch using ozone.

The oxidation of polysaccharides has been described in diverse variants. Known oxidizing agents for this purpose are periodic acid, peracetic acid, perborate, persulfate, potassium permanganate, lead(IV) salts, hydrogen peroxide, chlorite and hypochlorite.

The most conventional method for the oxidation of polysaccharides utilizes hypochlorites. This method has as a disadvantage that large amounts of salts are formed in the process, which causes problems in the processing of the waste water. Moreover, the yield of oxidized polysaccharides is often insufficient and long reaction times are necessary. Dutch patent specification 43493, European patent application 0 427 349 and international patent application 91/17189 disclose the oxidation of polysaccharides with a catalytic amount of bromide to form oxidized polysaccharides. The bromide is converted to hypobromite by hypochlorite or by electrochemical route. The method in which hypochlorite is used yields a great deal of salts again. The method that regenerates electrochemically is complicated; it requires a reservoir with at least two compartments and the anode and cathode become soiled rapidly. There is a great need for a method in which polysaccharides, and in particular starches, such as potato starch, can be oxidized in systems that contain as little solvents, and in particular water, as possible. The reason is that for many applications water and other solvents have to be removed again, which entails high (drying) costs. In addition, a dry or semi-dry process has the advantage that the product to be oxidized need not be suspended and need not be filtered off, and generally the reaction volumes can be greatly reduced. Further, no washing water needs to be used, so that purification costs and labor costs can be saved on.

It is known that ozone can be used as an oxidizing agent for starch, and sometimes even anhydrous oxidation seems to be suggested.

Szymanski, for instance, in the Journal of Applied Polymer Science 8. (1964), 1597-1606, describes a method for oxidizing maize starch using ozone. The oxidation with ozone is carried out both under dry conditions (10% water) and in aqueous suspension and in an organic solvent.

The oxidation reactions were carried out at decreasing pH. In particular, the final pH in the reaction mixture is found to be between 1 and 2.5.

The aim of the study which is described in this article by Szymanski was the introduction of a large amount of aldehyde groups into the maize starch. Because in addition to the intended aldehyde groups a considerable amount of ketone groups was formed as well, the objective was not achieved according to the author. It is further noted that the degree of oxidation is low when less than 65% by weight of water is present. It is explicitly stated that no tests were carried out with anhydrous starch. It is stated that the influence of water on the oxidation of maize starch is complex; water can function both as solvent, hydrating agent for hydroxyl groups and as catalyst.

In an article of Angibeaud et al. in "Cellulose, Its Derivatives" ed. J.F. Kennedy, published by Horwood Chichester U.K. (1985) entitled "Cellulose and Starch Reactivity with Ozone" it is concluded that the use of ozone as an oxidizing agent for carbohydrates, with starch being mentioned as an example, is impeded by the absence of selectivity. Both carbonyl and carboxyl groups are formed, while glycosidic bonds are broken. In order to gain some more insight into the mechanisms underlying the ozone oxidation, next the ozone oxidation of amylose is studied. In a suspension of amylose buffered at pH 7, in addition to depolymerization, the incorporation of carboxyl groups is reported. It is expressly stated that the results are not in agreement with the results Szymanski found for maize starch, viz. the incorporation of aldehyde groups. It is further noted that U.S. Patent 3,208,851 discloses a method for providing a batter for foods to be deep-fried, which batter adheres better to the food to be deep-fried. This batter is prepared through dry oxidation of starch using an oxidizing gas. One of the examples of such an oxidizing gas is ozone, in the examples, in the cases where ozone is used, only maize starch is treated with it. No specifics are given with regard to the starch product obtained.

Furthermore, it is described in U.S. Patent 3,361,741 that starch in an aqueous suspension can be oxidized by tetravalent lead ions, these lead ions being obtained by oxidation of bi- or trivalent lead ions with ozone.

German Offenlegungsschrift 22 33 977 describes the oxidation of carbohydrates using an aqueous basic solution of a silver oxide system, in the presence of this silver oxide system the carbohydrates can be oxidized with a large number of agents, including ozone.

The object of the present invention is to provide methods in which starch in dry or semi-dry condition can be selectively oxidized. More particularly, the invention relates to a method in which starch in dry or semi-dry condition is treated with an oxidizing agent, whereby the efficiency of the reaction is greatly increased.

This object is achieved for particular starches by oxidizing those starches by passing ozone through.

The invention accordingly relates to a method for oxidizing root starch, tuber starch or waxy cereal starch as well as derivatives thereof, wherein ozone is passed through an amount of dry or semi-dry starch. Root starches, such as tapioca starch, tuber starches, such as potato starch, and waxy cereal starches (derived from amylopectin-rich grains such as waxy maize and waxy rice) are used because these starches are characterized by a low content of lipids and proteins.

The derivatives of the above-mentioned starches which are used in the method according to the invention must be compatible with ozone under the reaction conditions used. Examples of such derivatives are cross-linked starches, starches with esterified or etherified groups attached; or physically modified starches such as roller-dried or extruded starch. Often it is simpler first to oxidize the starch and only then to (chemically) derivatize or modify it.

Very good results are achieved when the starch, before being treated with ozone, is subjected to a roller-drying treatment. Possibly, roller-drying enlarges the active surface of the starch granules, so that the carbon centres to be oxidized become more available or more accessible to the reagent ozone.

The term "dry or semi-dry starch", in the case of tuber starch, for instance potato starch, is understood to refer to starch containing a maximum of 25% by weight of water, and in the case of root starch, such as tapioca starch, and waxy cereal starches, is understood to refer to starch containing a maximum of 15% by weight of water. The term "starch" is understood to include the derivatives of these starches.

In a preferred embodiment, the dry or semi-dry starch is treated in a fluidized bed with ozone. In this fluid bed optionally a known fluidizing agent, for instance Aerosil®, can be included. At least a part of the primary hydroxyl groups is selectively oxidized.

It has been found that in particular C6 oxidation occurs, whereby carbonyl groups are introduced. This can be demonstrated through gas chromatography/mass spectrometry techniques (Hewlett Packard, gas chromatograph GC 5890 series II and mass spectrometer MS 5989A). The products with a high content of carbonyl groups obtained by the method of the invention can be used in adhesives, in paper (e.g. coating, surface-sizing), in textile (e.g. sizes). Carbonyl groups formed according to the invention can moreover be converted to e.g. amines, imines, acetals, and the like, by the conventional procedures.

The advantage of the above-mentioned method is that a starch with a high content of aldehyde groups is obtained, while the glucosation remains intact; as contrasted with known oxidation methods which lead to dialdehyde starch (DAS), such as described, for instance, in WO-A-93/01905. This has a favorable effect on the biodegradability and application properties of the product. A further advantage of this method is that an oxidation product can be obtained without the intrinsic viscosity being greatly reduced and hence without the starch chains being broken down to a large extent. The intrinsic viscosity of a substance is understood to mean the extent to which one molecule of that substance in solution can increase the viscosity and is determined by measuring the hydrodynamic volume of that substance. The intrinsic viscosity is expressed in dl/g.

In a preferred embodiment of the invention, a buffer is added to the starch and homogeneously distributed through it, so as to increase the efficiency of the reaction. As buffer can be used the conventional buffer systems such as citric acid/phosphate buffers, phosphate buffers, acetate buffers, boric acid buffers in amounts of up to a maximum of 10% by weight of dry buffer based on the starch present. The pH during the reaction may not be higher than 10.5 because ozone decomposes at higher pH values, and not lower than 1 in connection with hydrolysis of the starch. According to the invention, the pH is preferably between 5 and 8 because then the reaction efficiency is highest. Further, in a preferred embodiment, bromine and iodine compounds can be used as catalyst to increase the efficiency of the reaction. Preferably, the bromine compounds are bromides or oxy bromides, while the iodine compounds are iodides, oxy iodides and iodic acid compounds. These catalysts are employed in amounts of up to a maximum of 10% by weight of dry catalyst based on the starch present. The catalyst can be used in combination with a buffer. It is noted here that in this way salts are introduced into the product contemplated, which is not desired in all cases. Instead of adding a buffer, it is also possible to continuously apply lye to the product so as to keep the pH constant.

The reaction can be carried out at temperatures up to the point where ozone decomposes too fast. Preferably, oxidation occurs at a temperature of 0 to 60°C and most preferably between 5 and 40°C. The reaction time is between 10 minutes and 20 hours and depends on the desired degree of oxidation.

The invention will be further illustrated in and by the following non-limiting examples. The oxidation products are analyzed in the following manner.

The content of carboxyl groups (DS_COOh) is expressed in the number of moles carboxyl per mole anhydroglucose units and is determined titrimetrically. To that end, the sample material is brought into the H⁺ form with IN HC1 and titrated to pH 8.6 with 0.1 M NaOH. The titration is carried out in 0.5 M NaCl.

The content of carbonyl groups (DS_C=₀) is expressed in the number of moles carbonyl per mole anhydroglucose units and is determined by determining the dextrose equivalent DE (in mg/g) of the sample material with the Luff-Schoorl method, followed by conversion as follows: (DS_C=0) = DE/(1000-DE) .

The intrinsic viscosity (IV) is expressed in g/dl. It is determined in a conventional manner with a Viscotek Y501B in 1 M NaOH as solvent. Example 1

Semi-dry ozone oxidation of potato starch at 20 and

30°C

1.0 kg (800 g dry matter) potato starch was introduced into a 15 1 rotor reactor with blade. Then for 6 hours at 30°C ozone (100 1/h) was passed through the rotating reactor. The reaction was repeated at 20°C.

After the reaction a part of the product (400 g based on dry matter) was suspended in 500 ml demineralized water. This suspension was neutralized to pH 5.5 with 4.4% sodium hydroxide solution. After filtration the product was washed with 2.0 1 demineralized water. Finally, the product was dried and this dried material was analyzed.

Table 1

Oxidation of potato starch with ozone at 20 and 30°C

temp. DScooh DSc=o IV

20°C 0.006 0.024 1.01

30°C 0.006 0.023 0.90

The intrinsic viscosity of native potato starch is about 2.60. The product has an IV of about 1 and yet a high degree of oxidation. This points to less breakdown of the starch chain than occurs upon ozone oxidation in suspension or solution.

Example 2

Semi-dry ozone oxidation of potato starch in the presence of different catalysts and buffers

The catalyst (or buffer) was dissolved in 51 ml demineralized water. This solution was added dropwise to 949 g (742 g dry matter) potato starch in a Hobart mixer. After 1 week the oxidation reaction was carried out with the mixtures at 30°C as described in Example 1. The following catalysts and buffers were used:

- NaBr (10 g)

- HIO3 (5 g, pH of the solution adjusted to 3.0) - Acetate buffer pH 4.5 (8.0 g NaOAc and 2.0 HOAc)

- Acetate buffer pH 6.0 (10.0 g NaOAc)

- Phosphate buffer pH 6.0 (1.0 g Na₂HP0 and 9.0 g KH₂P0₄)

- Phosphate buffer pH 7.5 (8.0 g Na₂HP0₄ and 2.0 g KH2PO4)

After the reaction, a part of the product was washed and dried as described in Example 1. The suspension was neutralized to pH 5.5 with 4.4% sodium hydroxide solution or 6 M hydrochloric acid.

The washing, neutralization and drying were carried out because the presence of catalysts and buffers can interfere with the determinations.

Table 2

Oxidation of potato starch with ozone in the presence of catalysts or buffers

catalyst/buffer DScooh DSc=o IV

0.006 0.023 0.90

NaBr 0.008 0.041 0.37

HIO3 0.008 0.035 0.51

Ac buffer pH 4.5 0.002 0.010 0.91

Ac buffer pH 6.0 0.008 0.044 0.71

Phosph. buffer pH 6.0 0.010 0.039 0.70

Phosph. buffer pH 7.5 0.010 0.043 0.70

At a pH of around 6 an increased degree of oxidation is clearly observable.

Claims

1. A method for oxidizing root starch, tuber starch or waxy cereal starch as well as derivatives thereof, wherein the starch is contacted in dry or semi-dry condition with ozone.

2. A method according to claim 1, wherein the starting material is potato starch and/or tapioca.

3. A method according to claim 1 or 2, wherein use is made of a buffer.

4. A method according to claim 3, wherein use is made of a reaction system buffered at a pH between 5 and 8.

5. A method according to claims 1-4, wherein use is made of a catalyst from the group consisting of bromine and iodine compounds.