WO1997031951A1

WO1997031951A1 - Process for oxidising starch

Info

Publication number: WO1997031951A1
Application number: PCT/NL1997/000098
Authority: WO
Inventors: Dogan Sahin Sivasligil; Theodoor Maximiliaan Slaghek; Dirk De Wit; Kornelis Fester Gotlieb; Reinier Jozef Johannes JANSEN; Hans Van Overeem
Original assignee: Kemira Chemicals B.V.
Priority date: 1996-02-29
Filing date: 1997-02-28
Publication date: 1997-09-04
Also published as: EP0883630A1; EA000896B1; NL1002494C2; PL328613A1; YU37298A; CN1212708A; IL125942A0; AU723782B2; AU2235497A; EA199800778A1; CZ266998A3; ZA971785B; HUP9901784A3; HUP9901784A2; CA2247109A1; BR9707771A; JP2000506197A; NZ331585A; BG102731A; KR19990087307A

Abstract

A process for reducing the viscosity of polysaccharides such as starch by treatment with hydrogen peroxide in the presence of an acylated polysaccharide as an activator is described. The acylated polysaccharide is used in particular in an amount which is equivalent to at least 1 acyl group per 100 monosaccharide units. The hydrogen peroxide is preferably used at 2-20 wt.%, with respect to the total of polysaccharide and acylated polysaccharide.

Description

Process for oxidising starch

The present invention relates to a process for reducing the viscosity of polysaccharides, especially starch, by oxidativc degradation without metal catalysts.

Starch is economically relevant both as a foodstuff and for non-food applications. Worldwide the annual starch production is about 26 million tons. One of the largest non-food users of starch derivatives is the paper industry. In this field starch solutions with high starch content (25 wt.%) and low viscosities are much desired. Important features of starch derivatives arc an improved initial wet strength of the paper sheet, a better printability, a better retention of cationic additives and applicability as glue. The demand for these starch derivatives is several million tons a year.

Starch has to be subjected to a viscosity-reducing treatment, before it can be used on an industrial scale. For this purpose, the starch is generally treated oxidatively in an alkaline medium at increased temperature (40-60°C). This treatment takes 4-15 hours at about 60°C, for example in case hypochlorite is used as an oxidising agent together with sulphuric acid or phosphoric acid. Disadvantages of this method are the amount of salt produced in addition to chlorine-containing derivatives, and a high proportion of short chain starch molecules. These disadvantages are becoming more and more important as there is an increasing demand for chlorine-free product.

According to another method, hydrogen peroxide is used together with a transition metal as a catalyst. However, it is difficult to eliminate the, often poisonous, catalyst afterwards, even when using compounds such as EDTA. Another disadvantage is the discoloration of the product.

US-A-5,362,868 discloses a process wherein the viscosity of (hydroxycthyl) starch is reduced by oxidation using a pcracid at pH's ranging from 1 to 6 and at 40- 50°C. For example, pcroxysulphuric acid (H₂SO₅) or pcracetic acid are used as peracid. The peroxysulphuric acid has to be prepared in situ from hydrogen peroxide and sulphuric acid, or else a salt of the acid has to be used, which has the disadvantage of generating salts as byproducts. The reactivity of pcracetic acid is lower and results in longer reaction times. It is also described in US-A-5,362,868, that the reaction with hydrogen perixide icsults in unacceptable reaction times French patent 878146 discloses the treatment of starch with pcracetic acid at 110°C, which gives a bettei viscosity reduction than treatment with sodium peroxide, but results in the starch to be simultaneously pasted.

According to DE-A-4035053, cellulosic materials such as cotton can be bleached by treating the materials with pciacctic acid without actu ators, and simultaneously subiccting them to an ultrasonic ticatment (35 kHz).

An object of the invention is to proxide a piocess for reducing the viscosity of starch and other polysaccharides with simple means, without the use of heavy metals and salt-producing agents, and without oiganic by -product It is also an obiect to provide a viscosity-ieducmg pioccss which allows the starch granule to remain intact, for easier working, drying and handling of the product .

This object is achieved by means of a process whcicin the polysacchande is oxidised using hydrogen peroxide in the presence of an acylatcd polysacchande as an activator

As a polysacchande which can be treated using the process of the invention, any wholh of partially water -soluble polysaccharide can be used. These comprise firstly starch (e.g. potato, corn. waxy maize, tapioca, wheat, rice and other starches) and fractions and derivatives thereof. such as amylose, floe gels, ethoxylated starch and carboxymethyl starch. Furthermore. the solubility of cellulose derivatives and inulm and derivatives thereof and pentosans such as xylans can be improved or their viscosity can be reduced with the process of the invention . Carbohydrate derivatives, such as N-acylated, carboxylatcd, carbow methylated, alkvlatcd, hydroxyalkylated, hydrogenated and dehydrogenated derivatives can also be treated according to the invention.

The acylated carbohydiate used as a catalyst can be any oligo- or polysacchande which is wholh or partially acvlated It was found that only a small amount of acylated carbohy drate is necessary to cnsure an efficient reaction It is often sufficient if for each 700 anln diogkicosc (or other monosaccharide) units, one acylated monosaccharide unit is present. Prefceably at least one and especially at least four monoacylatcd monosaccharide units are present for each 100 units In particular an average of 0 06-0 2 acvlatcd monosacchandc units is present in the total of non-acylated and acylated polysaccharidc. The term "acylated" comprises alkanoylatcd (formylated, acetylatcd, propionylatcd, etc.), benzoylated, sulphatated, phosphory latcd etc..

The acylated carbohydrate can simply be obtained by treatment of a carbo- hydrate with an acylating agent such as acetic anhydride using standard methods.

Some acylated carbohydrates are commcrciallv available, such as acetyl starch having a DS of about 2.5% or 8%.

An advantage of the use of an acylated carbohydrate is that the oxidation reaction proceeds smoothly and no undesired by-products arc formed; the acylated polysaccharidc or its oxidation product can be part of the treated polysaccharidc product without any inconvenience.

The acylated carbohydrate can advantageously be a derivative of the same carbohydrate to be treated. Thus the treatment of starch can suitable be performed using acetylated starch as the activator. The acvlatcd carbohydrate can be prepared in situ, e.g. by acylation with the corresponding carboxylic anhydride at pH 8-9.5 in a concentrated solution or suspension of the carbohydrate. The acylation may then be followed by the oxidation, but oxidation may also be started during acylation.

The amount of hydrogen peroxide to be used is entirely dependent on the desired degree of oxidation. An amount of 1 wt.% is sufficient in general for achieving an effective viscosity degree. Advantageously, 1 -40 wt.%, in particular 2-

20 wt.% of hydrogen peroxide is used, with reference to the total of polysacchande and acylated carbohydrate. The hydrogen peroxide can be added at once, but it has been found that better products arc obtained when the hydrogen peroxide is added gradually or in portions, for example over a period from 5 minutes to 3 hours.

The reaction can be performed at room temperature, but preferably at increased temperature, generally from 20 to 90°C. in particular from 40°C to 5°C below the pasting temperature, which pasting temperature is at about 65°C for most starch types. This results in a viscosity-reduced product still having a granular structure. The reaction time is from several minutes to several hours at that temperature, depending on the particular polysaccharidc and the desired degree of viscosity reduction.

The polysaccharidc is preferably treated at a relatively high concentration. such as 10-55 wt.%, in particular 33-50 % by weight. The treatment is carried out under neutral to alkaline conditions, i.e. at a pH between 5 and 12, especially between 8 and 1 1, in particular between 9 and 1 1 . The treatment can advantageously be carried out in two or more stages, i.e. addition of peroxide at neutral pH (5-8.5) and subsequent reaction at alkaline pH (8.5- 1 1 ). In case of in situ acylation, the acylation can be performed under slightly alkaline conditions (pH 7.5-9.5), during or after acylation peroxide is added with some pH decrease (e.g. pH 6-8.5) and oxidation is completed at higher pH (8.5-1 1 ).

The product obtained by the process of the invention is essentially free of chlorine (as salt or covalcntly bound), i.e. not above natural abundance (< 20 ppm) and of transition metals. The carboxyl content is preferably between 0.2 and 5, especially between 0.5 and 3 wt.%. The viscosity is preferably below 4000 Brabender units (at 25 w.t% dry substance and at 40°C) and in particular below 1000 Brabender units.

Example 1

Oxidation of starch with H₂O₂ in the presence of πcetylafeci starch

To a stirred suspension of native potato starch (700 g) and Perfectamyl AC (commercially available acctylatcd starch, acctylation DS 8%/, 300 g) in water (1 1) at 45°C and pH 7 was added H₂O₂ (30 wt.% , 200 ml = about 0.3 mol.eq.). The pH was slowly raised to pH 10.5 using 0.5 M sodium hydroxide (within 30 minutes) and was then held constant for the remainder of the reaction time (total reaction time 3 hours). At the end of the reaction the pH of the reaction mixture was slowly adjusted to pH 6 with concentrated sulphuric acid (98 wt.% ). The reaction mixture was then filtered and the modified starch was washed with water (2 1, room temperature), after which it was dried. The Theological properties were determined with a Brabender

Viskograph E. The results arc given below in Brabender units at 40°C; V⁵ means viscosity at 5 wt.%.

The end viscosities could be lowered somewhat by a) increasing the pH to 11 ; b) increasing the amount of H₂O₂; c) adding the H₂O₂ gradually in 7- 10 minutes at pH = 8. A strong further viscosity reduction was obtained by increasing the ratio of acetylated starch to native starch.

By way of comparison, the experiment was repeated, using 85 g of galactose-penta- acetate instead of Perfectamyl, which resulted in and end viscosity V of 1120.

Example 2

To a stirred suspension of Perfectamyl AC (acetylated starch, acetylation DS 8%, 1000 g) in water (1 1) at 50°C and pH 10.5 was added H₂O₂ (30 wt.%, 400 ml = about 0.6 mol.eq.) in 20 portions over 1 hour using 1 M sodium hydroxide. After 3 hours, another 0.2 mol.eq was added at constant pH of 10.5 (total reaction time 4 hours). The reaction mixture was worked-up as in Example 1 . The carboxyl content was determined bv titration. The viscositv data arc eiven below:

Example 3

Oxidation of starch with H₂O₂ and in situ acety lated starch

To a stirred suspension of native potato starch (250 g) in water (250 ml) at 25°C and pH 8-9.5, acetic anhydride (AA, 42 g = 27 molc%, 0.216 ml.min^-1) was slowly added in 3 hours. 25 ml of 30 wt.% H₂O₂ was added during acetylation. The DS for acetylation was 11 %. Subsequently the temperature was raised to 50°C at pH 7. Three portions of 25 ml H₂O₂ were then added in 1 hour (total amount about 0.6 mol eq.). The pH was slowly raised to pH 10.5 using 1 .0 M of sodium hydroxide (within 30 minutes) and was then held constant for the remainder of the reaction time (total reaction time 3 hours). The reaction mixture was worked up as in Example 1. The results are given below.

Comparative Example

To a slurry of starch (70 g dry weight in 130 g water: 35 wt.%), 2 g of glucose penta-acctate is added at 50°C. Then a solution of 10 ml hydrogen peroxide (30 wt.%) is added at once. The pH is lowered to 1.4 using 1 N aqueous hydrochloric acid. After a reaction time between 15 minutes and 6 hours, the reaction mixture is neutralised, and the oxidised starch is separated by filtration, washed with water en dried with acetone. The viscosity of a suspension of 20 g of product in 380 ml of water is determined using a Brabender viscosimcter.

Table 1 summarises the results of the product after 3 hours and after 6 hours. The result of the treatment using only hydrogen peroxide after 3 hours is also given for comparison.

¹ Initial temperature of Brabender curve

2 Temperature of the top of the Brabender curve

³ Brabender units at top of Brabender curve: 75 rpm. temp, gradient:

1.5°C/min., tstarl 40°C, thold 90°C, hold: 30 min.: AVEBE standard 019142

⁴ Brabender units in valley of Brabender curve (as ³ )

Claims

1. A process for reducing the viscosity of a poly saccharide by treatment with hydrogen peroxide in the presence of an activator, characterised by using an acylated polysaccharidc as the activator.

2. A process according to claim 1 , wherein an amount of acylated polysacchande is used which is such that at least 1 acyl group is present for each 700 monosaccharide units of the total of polysaccharidc and acylated polysaccharidc.

3. A process according to claim 2. wherein an amount of acylated polysacchande is used which is such that at least 1 acyl group, preferably at least 4 acyl groups are present for each 100 monosaccharide units of the total of polysaccharidc and acylated polysaccharidc.

4. A process according any one of claims to claim 1 -3, wherein 2-20 wt.% (0.1-1 mol eq.) of hydrogen peroxide, with respect to the total of polysacchande and acylated polysaccharidc, is used.

5. A process according to any one of claims 1 -4, wherein the polysaccharidc is starch, especially potato, corn, waxy maize, tapioca, wheat or rice starch.

6. A process according to any one of claims 1 -5, wherein the acylated polysacchande is a derivative of the same polysaccharidc to be treated.

7. A process according to claim 6. wherein the acylated carbohydrate is prepared in situ.

8. A process according to any one of claims 1-7. wherein an average of 0.06-0.2 acylated monosaccharide units is present in the total of polysacchande and acylated polysaccharidc.

9. A process according to any one of claims 1 -8, wherein the polysacchande is treated at a concentration of 10-50 % by weight.

10. A process according to any one of claims 1 -9, wherein the treatment is carried out at a pH between 5 and 1 1.

11. A process according to claim 10. wherein the treatment comprises a stage wherein a pH between 5 and 8.5 is used, and a subsequent stage wherein a pH between 8.5 and 11 is used.

12. A process according to any one of claims 1 -11 , wherein the polysaccharidc is starch and the treatment is carried out at a temperature between 40°C and 5°C below the starch pasting tempersture. in particular between 40 and 60°C.