A dialdehyde starch production method and dialdehyde starch produced therewith

Abstract

The invention pertains to a method for oxidizing a starch into a dialdehyde starch using orthoperiodic acid, comprising: a) reacting an aqueous solution of orthoperiodic acid with dry starch or a starch slurry, having a starch content of 35 - 85 wt%, in a molar ratio of periodic acid : starch (in AGU) of 1:1 - 1:4, at a temperature below the starch gelatinization temperature and for a time less than 3 hours and at a pH < 1.0, at a starch concentration generally in the range of 5 - 27 wt%, based on the weight of the mixture of orthoperiodic acid and starch, wherein the starch that is brought into contact with the orthoperiodic acid is provided in an amount which complies with the following formulas: (i) starch concentration (wt%) ≤ -0.2 * DoO + 34%, and (ii) starch concentration (wt%) > -0.2 * DoO + 25%, wherein DoO is 25, 50, 75 or 100%, b) dewatering and displacement washing, wherein the molar ratio of orthoperiodic acid : starch is selected to obtain a dialdehyde starch with a desired degree of oxidation [DoO] between 25 and 100%, wherein the molar ratio of orthoperiodic acid : starch is selected according to %DoO = 100*(molar ratio periodic acid:AGU). The invention also pertains to a dialdehyde starch obtainable by the above method, with a degree of oxidation of at least 75%, and with a iodine content of less than 100 ppm.

Description

A dialdehyde starch production method and dialdehyde starch produced therewithField of the inventionThe invention relates to the production for dialdehyde starch, and to dialdehyde starch produced therewith.Background of the inventionIn the art there exists a way of modifying carbohydrates and particularly polysaccharides by introducing reactive aldehyde groups in the polymer structure. In doing so, carbohydrates become chemically reactive and can form chemical bonds with amines or hydroxyl groups. Practically, that means that a dialdehyde-carbohydrate can react with proteins and other polysaccharides. Dependent on the carbohydrate of choice, such a chemical modification if possible on industrial scale opens commercial perspectives, such as crosslinking cellulose to obtain water-resistant paper, crosslinking leather proteins (= tanning process) to prevent decay, stabilisation of enzymes and reaction with the membrane of micro-organisms with bacteriostatic properties as a result. This list is a non-limiting list of applications.However, making a dialdehyde is not a straightforward carbohydrate modification, like polysaccharide oxidation is. Production of a dialdehyde carbohydrate is based on a particular chemical mechanism named ‘oxidative cleavage of vicinal diols’. In organic chemistry the socalled Malaprade reaction or Malaprade oxidation is already known for almost a century; this reaction is aglycol cleavage reaction in which a vicinal diol is oxidized by periodic acid or a periodate salt to give the corresponding carbonyl functional groups.Still, while this mechanism has been applied to glucose homopolymers cellulose and starch in particular ever since, it has never been developed in a way to make it efficient and commercially viable. In most if not all cases the oxidation agent in the art is a periodate salt. However, with periodate salt, a cation is introduced to the system which can negatively impact any form of regeneration of the spent oxidant; also, periodate salts are limited in their solubility in water of ambient temperature, necessary when applying relative high concentrations of the oxidant. Furthermore, the choice for periodate salt requires additional acid (e.g. sulphuric acid and hydrochloric acid) to adjust the pH to an optimized level for an efficient oxidation reaction. Therewith, the inorganic acids introduce undesired anions into the system. Altogether this is not a favorable set-up for the oxidation. Even in those cases where periodic acid and periodate salts have been suggested being alternatives in such Malaprade reactions, in reality is it periodate that has been used for more than 80 years.When it comes to dialdehyde starch (DAS) production, the literature also struggles with other parameters like the oxidant - starch weight ratio, reaction temperature, starch concentrations and reaction time. The weight ratio varies from below 0.1 to more than 2, reaction temperatures are everywhere in the range of 1 to 55 °C, starch concentrations are between 1 and 40%, and reaction times up to 100 hours. There is no strong indication in a certain direction with or combination of those parameters, let alone have the choices for these parameters in the art been backed up by experiments. Most strikingly, these prior art processes typically involve reaction times of up to 100 hours, and this makes these reactions unsuited for DAS production on industrial scale. Examples of these incoherent set-ups found in literature and patents are for example the conditions used in [1] in which sodium metaperiodate is used in various concentrations at controlled pH of 3 - 5, which is too high, for mainly one hour at a constant temperature of 25 °C. In [2] conditions are applied where sodium paraperiodate is used in 10% excess with sulphuric acid addition to pH 1.5 at a controlled temperature of 32 C throughout for 4 hours. The process described in [3] also involvesan excess (13%) of oxidant, also sodium metaperiodate, and a reaction at pH 4 for 48 hours at 4 °C. The experiments reported in [4] apply sodium periodate at an adjusted pH of 4 (with sulphuric acid) result in impossible aldehyde contents (> 36%) in four out of six experiments at applied mole to mole ratios. While the concentration of the starch is applied in DAS production described in the aforementioned prior art in a wide range, in most cases it is typically lower than the traditional 40% used in starch derivatization, presumably because of physical limitations especially to modify in such slurry concentrations typically being used in the industry today. Ernest et al. "Application of the Cleavage Type of Oxidation by Periodic Acid to Starch and Cellulose", J. American Chemical Society, vol 59(10) 2049-2050 shows the reaction profile of a 3.5 wt% starch concentration reacting with periodic acid with a periodic acid to starch ratio of about 2:1 , and with a 24 hour reaction time. The granules swell somewhat during oxidation.The prior art also does not describe the interplay between starch slurry concentrations and parameters like degree of oxidation, temperature, etc.. Sometimes the slurry concentration is motivated based on viscosity, starch granule volume, free water volume, but in none of the papers these choices are motivated. The skilled person is also left in the dark about the actual time required for the reaction with prior art reaction conditions to be completed. And even then, the corresponding yields are still unsatisfactorily lower than one would expect. Where the prior art suggests reaction times of up to 12 hours or a multitude thereof (up to 144 hours in US6,620,928), it is noted that reaction time extension does not necessarily provide better results.Pfeifer et al. "Two Stage Process for Dialdehyde Starch Using Electrolytic Regeneration of Periodic Acid" , Industrial and Engineering Chem. vol 52(3), 01-03- 1960, pages 201-206 describes without exception to carry out starch oxidation with an excess of oxidant (periodic acid), at 1.1 :1 , and at typical dry solid levels of about 5% and at pH 1 .2 - 1 .4. The excess amounts of caustic (to increase the pH) disadvantageously affects the electrodes used in the regeneration process. Also, with the technology and processing conditions described in Pfeifer(1960), it mentions that the dry solid levels of the final cake before drying is at least 50%. While the article may suggest that it produces a DoO between 50 and 100%, to the inventors’ best knowledge, and as further demonstrated in comparative examples 1 a and 1 b, it is impossible to achieve such levels with starches having a maximum DoO and an aldehyde content > 36%, and retaining granular integrity. While there is mention of up to 98% oxidized aldehyde starch, the inventors found that this could only be achieved with an excess of iodine-based oxidizing agent, and even at the conditions and iodine levels taught there, the dialdehyde starch produced had an aldehyde content of 78% at best, and also showed disadvantageously elevated iodine concentrations of more than 300 ppm. It also reports Pb concentrations of 65 ppm. Hence, there is a need to improve DAS with 100% DoO, with high efficiency, and yet maintain the quality of the DAS ultimately formed.WO2023 / 140734 describes oxidizing carbohydrates such as starch using periodic acid and focuses on regenerating the periodic acid used in the carbohydrate oxidation. It mentions oxidation and regeneration cycles which together take 1-48 hours, and the use of 5-100 g / h periodic acid either HIO4 or H5IO6. There is no detail on the preferred starch oxidation processing conditions and consequences thereof, in the example a molar ratio of 0.5 mol periodic acid vs. 1.0 mol AGU was applied, and the starch concentration was 7.5 wt%. There is report of reaction times of 24 hours, pH control, and no DoO or efficiency is reported.Also, in the art the skilled person always falls back to periodate salt which is conveniently available, but which does not readily dissolve in high concentrations in water, and which is often added to starch slurries. Huimin et al. "Recent advances on the preparation conditions, structural characteristics, physicochemical properties, functional properties and potential applications of dialdehyde starch: A review, Int. J. Biological Macromolecules, Elsevier BV, NL, vol 259 seems to be such an example. With respect to the reaction temperature, the prior art suggests fixing and controlling temperature in order to achieve the desired oxidation.CN105646723 aims to provide an organic adhesive based on dialdehyde starch, and while it appears to suggest that starch can be oxidized using periodate or periodic acid alternatively, the examples are again all systematically carried out with periodate salt in combination with a strongacid, and this aldehyde formation step is integrated with a subsequent degradation step in which the starch is hydrolyzed in order to reduce molecular weight of the dialdehyde starch and reduce its viscosity, often a perceived issue in the organic adhesive production process. The hydrolysis step is required to improve the degree of oxidation. Still, the method of CN105646723 results in a disappointing degree of oxidation between 72 and 92 % (and the mean molecule quantity of the dialdehyde starch is 900 - 9600 Da).In fact, the above citation is no exception to the observation that it remains a challenge to get to high degrees of oxidation (DoO). In theory, the DoO is rather straightforwardly based on the idea that one mole periodic acid is needed for one anhydro-glucose unit (AGU) in the starch polymer to oxidize. In the oxidation process, the Mw of the anhydroglucose unit (AGU) is reduced from 162 to 160 g / mol. As there are two aldehyde moieties at the 2 and 3 positions, a DoO of 100% at a 1 :1 molar ratio periodic acid to starch (in the form of AGU) corresponds to (58 1 160.15) * 100% = 36%). In analogy, a DoO of 50% means 18% ((29 1 161) * 100%). Being such a specific chemical reaction, a DoO of 100% should be within reach (for 100% DoO one mole of periodic acid has oxidized one mole AGU), but in the art such numbers are far from reality.Hence, there is a need for producing dialdehyde starch in a resources-effective way, and improving yields and the quality and DoO of the dialdehyde starch.Summary of the inventionIt is the aim of the invention to provide an improved method for producing dialdehyde starch with a 100% yield i.e. wherein the molar ratio periodic acid:AGU results in a corresponding DoO; at the conditions used by the inventors, using a molar ratio of periodic acid:AGU of 1 :4, 1 :2, 3:4 and 1 :1 results in a DoO of 25%, 50%, 75% and 100%, respectively. Hence, based on the 100% efficiency or yield that is possible using the method of the invention, the above molar ratio is selected to arrive at the desired DoO between 25 - 100 % according to the relation %DoO = 100*(molar ratio).In the context of the invention, the term ‘periodic acid’ refers to orthoperiodic acid (HsIOe) and not metaperiodic acid (HIO4).In the application, unless indicated otherwise, the following values are used:Molecular weight AGU = 162 g / mol; molecular weight H5IO6 = 228 g / mol; molecular weight C=O (aldehyde group) = 29 g / mol; percentage aldehyde groups (at DoO = 100% and 100% efficiency) = 36%. percentage aldehyde groups (at DoO = 50% and 100% efficiency) = 18%.The term degree of oxidation is as defined here above; associated therewith, the oxidation efficiency (EoO) is defined as the actual percentage of AGUs which is oxidized, relative to the DoO. In other words, if a molar ratio orthoperiodic acid:starch of 1 :1 is applied, a DoO of 100% is yielded, with a corresponding EoO of 100%. If for example a molar ratio of 1 :25:1 is applied, the DoO can still be 100% but the EoO is only 80%; or if the molar ratio is 1 :2 and the DoO is thus 50%, and in reality only 30% of the AGUs is oxidized (only 10.87% instead of 18.12% aldehyde groups are measured), EoO is 60%.Throughout the application, the terminology DoO 25 - 100 wt% means in fact DoO 25%, 50%, 75% or 100%, and used interchangeably. It is not a continuous range of degree of oxidation.In a first aspect, the invention pertains to a method for oxidizing a starch into a dialdehyde starch using orthoperiodic acid, comprising: a) reacting an aqueous solution of orthoperiodic acid with dry starch or a starch slurry, having astarch content of 35 - 85 wt%, in a molar ratio of orthoperiodic acid : starch of 1 :1 - 1 :4, at a temperature below the starch gelatinization temperature and for a time less than 3 hours, and at a pH < 1 .0, at a starch concentration of about 5 - 27 wt%, preferably 7 - 25%, more preferably 10 - 25% based on the weight of the mixture of orthoperiodic acid and starch, wherein preferably no additional acid is added, wherein the molar ratio of periodic acid : starch in step a) is preferably selected to obtain a dialdehyde starch with a desired degree of oxidation [DoO] between 25 and 100% (%DoO = 100*(molar ratio periodic acid:AGU)), i.e. the starch that is brought into contact with the orthoperiodic acid is provided in an amount which complies with the following formulas:(i) starch concentration (wt%) < -0.2 * DoO + 34%, and(ii) starch concentration (wt%) > -0.2 * DoO + 25%, wherein DoO is 25, 50%, 75% or 100%, and b) dewatering and displacement washing.It is particularly preferred that the starch that is brought into contact with the orthoperiodic acid is provided in an amount which complies with the following formulas:(i) starch concentration (wt%) < -0.2 * DoO + 32%, and(ii) starch concentration (wt%) > -0.2 * DoO + 25%.As a consequence of the use or orthoperiodic acid, solubility is improved and the pH is kept low. This is a difference with the prior art advocating working conditions well above pH > 1 and requiring significant amounts of alkali.With the above method it is possible to reach increased degree of oxidation in a short time and at mild temperatures, and because it is possible to obtain a DAS with granular structure it is much easier to realize step b) dewatering and displacement washing, in turn making it possible to reduce the concentrations residual content of iodine and heavy metals beyond what is possible in the art. As will be explained below, the granular structure is not the normal granular starch integrity (there is no Maltese cross observed, and the amylose-containing starch does not turn blue with h) which is not surprisingly as two-thirds of the hydrogen bonds have ‘disappeared’ with oxidation; it is known from the art that the loss of about 1 / 15thof the hydrogen bonds would typically already be sufficient to lose the granular structure. Surprisingly, the DAS obtained with the conditions of step a) has a granular structure, which is presumably associated with the (hemi)acetal bonds that form under the selected conditions. The new granular structure make it possible to carry out step b) without losing starch. This granular structure is surprising also given the observed swollen nature of the DAS thus obtained in step a).The method of the invention has a proven 100% efficiency, meaning that in step a) the molar ratio of periodic acid : starch is selected according to %DoO = 100*(molar ratio periodic acid:AGU); at a molar ratio of 1 :1 , a DoO of 100% is in fact obtained. Any lower but desired DoO can be achieved merely by adjusting the molar ratio proportionally, according to the aforementioned formula. The molar ratio is preferably between 1 :1 and 1 :2, thus resulting a dialdehyde starch with a corresponding DoO of 50-100%. The DoO is the maximum or theoretical achievable functionality, based on the number of oxidated groups vs anhydroglucose units.Under those conditions, and associated with the granular structure of the DAS obtained with the conditions applied in the process, there is produced a iodine-free and Pb-free DAS with DoO of choice, including DoO 100%, with a 100% efficiency. As addressed above, the granules are swollen yet show a ‘granular structure’ making it possible to wash the DAS without leaching starch. With the process of the invention, the dry matter content of the cake before and after step b) is less than 35%, even at DoO 100%. As explained below, it is surprising that at these lower solid levels there is still cake formation and there is no non-Newtonian fluid behavior observed. It was found that the reported 50% dry solids in Pfeifer(1960) cannot be reached without losing the granular integrity. Starch is insoluble in cold water. The starch molecules are kept together in the granule structure bythe hydrogen bonds (from the hydroxyl groups at the AGU). By heating, the hydrogen bonds break or the sodium of NaOH takes the place of the H of the hydroxyl (-OH group). Moreover, when substituting starch with chemical groups it will lose hydrogen bonds too; when larger fractions of hydrogen bonds are broken, it does destroy granule integrity and the granule falls apart in water at ambient temperature. DAS with a DoO of 100% and and EoO of 100% has 67% (two-third) of its hydrogen bonds lost, so the granule integrity is gone. In the experiments of Pfeifer(1960) no granular structure was obtained, in contrast with the process of the invention. However, without wishing to being tied down to any theory, the inventors believe that because of the quick and / or immediate (hemi-)acetal formation between the molecules in the granule, a granule ‘structure’ (not the original integrity) is obtained, and this granular structure makes it possible to carry out step b) to an extent that iodine and heavy metals can be readily washed out.More preferably, the invention pertains to a method for oxidizing a starch into a dialdehyde starch using orthoperiodic acid, comprising: a) reacting an aqueous solution of orthoperiodic acid with dry starch, or a starch slurry, in a molar ratio of periodic acid : starch of 1 :1 - 1 :4, at a temperature below the starch gelatinization temperature and for a time less than 3 hours, and at a pH < 1 .0, wherein preferably no additional acid is added, wherein the molar ratio of orthoperiodic acid : starch in step a) is selected to obtain a dialdehyde starch with a desired degree of oxidation [DoO] between 25 and 100% (%DoO = 100*(molar ratio periodic acid:AGU)), to obtain a cake with a dry matter content less than 35 wt%, wherein the starch that is brought into contact with the orthoperiodic acid is provided in an amount which complies with the following formulas:(i) starch concentration (wt%) < -0.2 * DoO + 34%, and(ii) starch concentration (wt%) > -0.2 * DoO + 25%, wherein DoO is 25 - 100% (i.e. 25, 50, 75 or 100%), and b) dewatering and displacement washing.It is particularly preferred that the starch that is brought into contact with the orthoperiodic acid is provided in an amount which complies with the following formulas:(i) starch concentration (wt%) < -0.2 * DoO + 32%, and(ii) starch concentration (wt%) > -0.2 * DoO + 25%.More preferably, the starch is used in a concentration in step a) which complies with:(i) starch concentration (wt%) < -0.2 * DoO + 30%, wherein DoO is 25 - 100%, and more preferably the starch concentration also complies with:(ii) starch concentration (wt%) > -0.2 * DoO + 27% (wherein DoO is 25 - 100%).In the above, the starch concentration is based on the weight of the mixture of orthoperiodic acid and starch.In the above methods, the conditions in step a) maintain a granular structure of the dialdehyde starch. Also, iodine concentrations in the DAS obtained after step b) are below 100 ppm, preferably below 80 ppm, most preferably below 50 ppm. Pb levels are below detection limits (preferably less than 1 ppm).Related therewith, in a second aspect the invention relates to a dialdehyde starch with a degree of oxidation of at least 75%, more preferably 100%, and with a iodine content of less than 100 ppm, preferably less than 80 ppm, most preferably less than 50 ppm. The low iodine concentrations prevent disadvantageous decolourization of the product. This is a major step up from the lower DoO (<50%) DAS obtained in the art, typically combined with high iodine contents and high heavy metal concentrations and high ash content.As a consequence of the method of the invention, it is possible to produce a dialdehyde starch with reduced heavy metal content including lead (Pb). It is preferred that the total heavy metal content of the starch is less than 1 mg / kg starch, more preferably at most 0.3 mg / kg starch.Associated with the method being carried out low pH and there is no need for additional pH adjustment agents, the ash content of the dialdehyde starch is preferably at most 3 mg / g.The DAS is preferably obtainable by the method as described above. The DAS preferably has a pH 3 - 6.The DAS preferably has a swollen granular structure, preferably with a swelling that is characterized in that the average granule has a volume that is at least 80% larger than the average granules of the unreacted starch.Unlike the processes described in the art, the above method can be applied on industrial scale.One key parameter to success is the use of orthoperiodic acid rather than periodate, and the low pH associated therewith.Related therewith, the inventors found that it is possible to prepare a concentrated aqueous orthoperiodic acid solution to which starch is added in dry or slurry form, and where the molar ratio of the orthoperiodic acid and starch is balanced. As detailed in the examples, it was also possible to reverse the order, and add a concentrated aqueous periodic acid solution to the starch in slurry form, provided that the same molar ratio is balanced. Either way it is in sharp contrast with the prior art, where 40% starch slurries to which periodate salts and additional acids such as sulfuric of hydrochloric acid are added; these strong acids can advantageously be refrained from. The use of periodic acid makes it possible to work at intermediate starch concentrations, given its relatively high solubility particularly compared to periodate salt, which in turn is found to have its effect on the degree of oxidation. Excess iodine concentrations can be avoided, as well as the build-up of concentrations of heavy metals and without the need for pH adjustment any increase in ash content can be controlled to a minimum, preferably at most 0.3 ppm heavy metals and preferably at most 3 mg / g ash content.When brought into contact with the periodic acid in step a), it is preferred that the starch is provided in an amount of 5 - 25 wt%, preferably 10 - 25 wt% of the reaction mixture, preferably according to any of the formulas (i) and (ii) as described above. At higher concentrations, water absorption and swelling of the starch results in too high starch slurry viscosities, and this adversely affects the DoO and further processing. Viscosity increases proportionally with starch concentration (and with DoO), and processing these viscous slurries at higher concentrations requires additional resources. This applies to both steps a) and b) in the method. At lower concentrations, cake formation in step b) can be challenging and balancing the water household for washing and dewatering is seriously disturbed. In order to control the viscosity and yield the desired cake, it is preferred to use starch in a concentration in step a) which complies with formula (i):(i) starch concentration (wt%) < -0.2 * DoO + 34%, wherein DoO is 25 - 100%, and wherein the starch concentration preferably also complies with formula (ii):(ii) starch concentration (wt%) > -0.2 * DoO + 25% (wherein DoO is 25 - 100%).It is particularly preferred that the starch that is brought into contact with the orthoperiodic acid is provided in an amount which complies with the following formulas:(i) starch concentration (wt%) < -0.2 * DoO + 32%, and(ii) starch concentration (wt%) > -0.2 * DoO + 25%.AII of the above goes against the conventional idea of using 40 wt% starch slurries.- 1-As is described in more detail below, the difficulty rests in the fact that with DAS production a significant (in case of DoO = 100%, 2 / 3rd) hydroxyl groups are turned into aldehyde groups and therewith the possibility of hydrogen bonds is cancelled. This would normally lead to a complete destruction of the granular integrity, but surprisingly with the method according to the invention these problems do not arise. Without wishing to be tied down to any theory, the inventors believe that the hemiacetal linkages which form instead, a starch granule is obtained even while swollen up to twice its initial volume, thus rendering it possible to b) wash and dewater without starch leaking out.In a most preferred embodiment, the molar ratio periodic acid:AGU is about 1 :1 (and the corresponding DoO is about 100%), and the starch concentration applied in step a) is 5-10 wt%.Surprisingly, using the starting materials and conditions of the method of the invention, even though during the Malaprade oxidation the initial starch volume was found to increase with up to 100%, still at these swelling rates the starch when kept within these concentrations did not solubilize or disintegrate, which would be in contrast to the skilled person’s reasonable expectations; with the method of the invention it is possible to double the initial starch volume irreversibly yet retain the desired 'starch cake’ or ‘starch filter cake’ [5][6], rendering it possible to process the starch using filtration and dewatering steps. This is key for further processing on industrial scale, but at the same time surprising: After all, it is known that when starch starts to swell - due to water uptake at increased temperature, during starch modification and dependent on the source and amylopectin:amylose ratio - it can swell in a reversible manner only to a limited extent, until swelling reaches a threshold value at which the internal organization of the starch molecules becomes irreversibly imbalanced, and typically the swollen starch disintegrates. This would pose a challenge on physical separation of the outside water using processes like filtration and centrifugation, forcing the skilled person to find other tedious ways of taking out water using for example energyconsuming spray-drying. Based on typical starch oxidation (not according to the invention) it would be expected that an increase in oxidation results in an increase in starch solubility, swelling and an undesired loss of granule integrity. While the process of the invention, it was in fact found that relatively high concentrations of starch can be used and significant swelling occurs but without compromising the reaction and following dewatering steps; granule integrity was consolidated. In fact, even at the high swelling volumes observed using the method of the invention, the desired starch cake structure is retained, and any remaining water could readily be removed from the starch granules using simple dewatering steps. With a dry matter of 30 - 33% (at DoO 100%) and irreversible starch swelling of more than 100%, the inventors still obtained a slurry which can be further processed and from which cake could be formed and from which water could be removed. This allows for the crucial step of washing out the iodine ions completely, as it is key that a dialdehyde starch is obtained that is free from iodine, preferably less than 100 ppm, more preferably less than 80 ppm, most preferably less than 50 ppm. In this context, reference is made to the example in which the process of Pfeifer(1960) is reproduced.Without wishing to be tied down to any theory, the reason for the cake-retaining character in spite of large swelling is believed to rest in the formation of hemiacetal linkages in and between the starch molecules, despite the fact that two-third of the hydroxyl groups enabling hydrogen bridge formation have turned into aldehyde groups. These swollen granules do not fall apart and are basically filled with water. Because and despite its volume, the starch granule integrity is maintained (through the hemiacetal linkages), and the dialdehyde starch suspension could still be dewatered to yield a so called starch cake.List of preferred embodiments1 . A method for oxidizing a starch into a dialdehyde starch using periodic acid, comprising: a) reacting an aqueous solution of periodic acid with dry starch, or a starch slurry, in a molarratio of periodic acid : starch (in AGU) of 1 :1 - 1 :4, at a temperature below the starch gelatinization temperature and for a time less than 3 hours, at a starch concentration of about 5 - 25 wt%, based on the weight of the mixture of periodic acid and starch, b) dewatering and displacement washing, wherein the molar ratio of periodic acid : starch is selected to obtain a dialdehyde starch with a desired degree of oxidation [DoO] between 25 and 100%, wherein the molar ratio of periodic acid : starch is selected according to %DoO = 100*(molar ratio periodic acid:AGU).2. The method according to embodiment 1 , wherein the molar ratio periodic acid:starch is in the range of 1 :1 - 1 :2.3. The method according to any one of the preceding embodiments, wherein the starch that is brought into contact with the periodic acid in step a) is provided in an amount which complies with the formula (i):(i) starch concentration (wt%) < -0.2 * DoO + 30%, wherein DoO is 25 - 100%, and wherein the starch concentration preferably further complies with formula (ii):(ii) starch concentration (wt%) > -0.2 * DoO + 25%, wherein DoO is 25 - 100%.4. The method according to any one of the preceding embodiments, wherein the starch that is brought into contact with the periodic acid in step a) is provided in an amount to yield 10 - 25 wt% starch, preferably 15 - 25 wt% starch, in the reaction mixture.5. The method according to any one of the preceding embodiments, wherein in step a) between 5 and 25 wt%, preferably between 5 and 22 wt%, more preferably between 10 and 20 wt%, most preferably 13 - 16 wt% periodic acid is brought into contact with the starch.6. The method according to any of the preceding embodiments, wherein the reaction time is less than 2 hours.7. The method according to any of the preceding embodiments, wherein the temperature in step a) is below 50 °C, preferably between 5 and 45 °C, particularly between 10 and 40 °C.8. The method according to any of the preceding embodiments, wherein no acid other than periodic acid is added to reaction step a).9. A dialdehyde starch obtainable by the method according to any one of the preceding embodiments, with a degree of oxidation of at least 75%, more preferably 100%.Detailed description of the inventionThe invention pertains to a method for oxidizing a starch into a dialdehyde starch using periodic acid,Starch + H5IO6 Dialdehyde starch + HIO3 + 3 H2Othe method comprising: a) reacting an aqueous solution of orthoperiodic acid with starch in dry or slurry form, having a starch content of 35-85 wt%, in a molar ratio periodic acid : starch (AGU) of 1 :1 - 1 :4, at a temperature below the gelatinization temperature and for a time less than 3 hours, at a starch concentration of about 10 - 25 wt%, and at a pH < 1 .0, wherein substantially no additional acid (i.e. other than the periodic acid) is added, followed by a dewatering step to remove the free water from the starch filter cake comprising DAS with a dry solid content of less than 40 wt%, preferably less than 35 wt%, and a subsequent dewatering and displacement washing, suited for removing at least 99%, more preferably at least 99.9%, most preferably at least 99.99% of the iodine-containing components, to obtain a dialdehyde starch with a DoO of 50 - 100%, dependent on the actual need. Given the typical yield of 100%, the actual DoO is set by the initial molar ratio of periodic acid:AGU. The method has a 100% efficiency, i.e. the DoO corresponds to the theoretical DoO which is directly derived from the molar ratio of periodic acid:starch applied in the process. The actual starch concentration brought into contact with the orthoperiodic acid in step a) is set by the desired DoO:(i) starch concentration (wt%) < -0.2 * DoO + 34%, and(ii) starch concentration (wt%) > -0.2 * DoO + 25%, wherein DoO is 25 - 100%.It is particularly preferred that the starch that is brought into contact with the orthoperiodic acid is provided in an amount which complies with the following formulas:(i) starch concentration (wt%) < -0.2 * DoO + 32%, and(ii) starch concentration (wt%) > -0.2 * DoO + 25%.The DoO is preferably 75 or 100%, more preferably DoO 100%. The iodine concentrations in the DAS are below 100 ppm, preferably below 80 ppm, most preferably below 50 ppm. Heavy metal (including Pb) levels are below detection limits (preferably less than 0.3 ppm). The ash content is preferably below 3 mg / g, which is a direct consequence of the fact that the process is caried out at pH < 1 .0 and does - unlike the process such as described in Pfeifer(1960) require alkali materials to work at pH > 1 .0.There is no restriction to the origin of the starch, which could be any one of the non-limiting list of potato, wheat, tapioca, maize, pea, rice, and waxy maize starch, or combinations thereof. The starch could be in native form or a chemically, physically or enzymatically modified starch, or combinations thereof.The method conditions and its success are specific to starch oxidation with periodic acid into a dialdehyde starch, and also taking into account the unique starch swelling behaviour. With the method of the invention, a starch with any desirable content of dialdehyde functional groups per monosaccharide unit (i.e. with a DoO between 25 and 100%) can be obtained in the most effective manner. The molar ratio of periodic acid to starch (AGU) is between 1 :1 and 1 :4, preferably between 1 :2 and 1 :1 (DoO between 50 and 100%), more preferably between 3:4 and 1 :1 (DoO 75 - 100%).Unlike in the prior art working with periodate, the method according to the invention does not require the use of additional acids other than the periodic acid. The molar ratio of periodic acid to the starch (expressed in terms of anhydroglucose units (AGU)) of 1 :1 - 1 :4, preferably 1 :1 - 1 :2.The method preferably uses concentrated aqueous periodic acid solutions of between 5 and 25 %, more preferably between 5 and 22 wt%, more preferably between 10 and 20 wt%, most preferably 13 - 16 wt% periodic acid (HsIOe) and the actual amount is selected in consideration with the amount of starch it is brought into contact with. It is a combination of high concentrations of starch and periodic acid that accelerates the reaction and simultaneously sets the DoO according to the calculated theoretical DoO. These high concentrations are out of reach when using periodate as isused in the art. Consequently, the reaction time is less than 3 hours, preferably less than 2.5 hours, more preferably less than 2 hours.In one embodiment, the concentrated periodic acid is regenerated by subjecting the iodic acid that is generated when a) reacting the starch and the periodic acid to a two-step electrolytic process, wherein the iodic acid is first electrolytically regenerated into periodate under alkaline conditions, and the periodate thus formed is then electrolytically acidified to periodic acid, which periodic acid is returned to a) the starch oxidation. The different, subsequent regeneration steps can be characterized as follows:• Iodic acid neutralisation: HIO3 + OH- -> ICh- + H2O• Periodate salt regeneration: ICh- + H2O + 2OH- -> H2IO63" + H2• Periodate salt acidification: H2IO63' + 3H2O -> H5IO6 + 3OH-In case of regeneration, the periodic acid is preferably concentrated using reverse osmosis, membrane or nanofiltration, evaporation, freeze drying, sedimentation or centrifugation.The DAS obtainable by the method of the invention preferably has a degree of oxidation [DoO] between 50 and 100%, more preferably between 75 - 100%, most preferably 100%.The reaction conditions are such that the starch is not subjected to hydrolysis; there are no strong acids (periodic acid is not a strong acid) and the temperatures are preferably below 50 °C. The reaction temperature is controlled below the starch gelatinization temperature, preferably below 50 °C, most preferably between 5 and 45 °C, particularly between 10 and 40 °C.In order to arrive at a DAS with a DoO of 25 - 100%, preferably a DoO of 50 - 100%, the method of the invention works efficiently between 10 and 25 wt%, preferably 10 - 20 wt% starch which is much lower than the typical 40 wt% dry solid starch slurries applied in the prior art. The actual amount of starch is selected in combination with the periodic acid : starch molar ratio [within the range of 1 :1 - 1 :4], preferably according to formula (i) and more preferably according to formulas (i) and (ii) as described herein, to yield a DAS at the end of step a) which has an acceptable starch viscosity at the chosen starch wt%. At higher DoO the starch granules pick up much water without losing the granules integrity but yielding granule volumes twice as high. This also leads to a much lower overall dry solids level of the cake and a higher risk for non-Newtonian fluid behaviour. Applying higher concentrations increase the viscosity of the starch slurry due to extensive swelling of the granules as explained and which is also dependent on the DoO; the higher the DoO the more water is taken up into the starch granule and the higher the starch slurry viscosity. This is also the reason to adjust the wt% starch in the prepared slurry according to the aforementioned formulas (i) and (ii). It goes without saying that the more swollen the granules the harder it is to dewater the reacted starch slurry towards a solid starch cake avoiding the chance of yielding a ‘pseudo’ cake with so called non-Newtonian fluid characteristics which make it difficult and uneconomical to further process. In this context, reference is made to comparative example 2 in which the process of Pfeifer(1960) is reproduced.In the art there is a belief that lower starch slurry concentrations have a negative impact on the overall water balance in the process, have a negative impact when modifying starch (chemically, physically and enzymatically) and have a negative effect when removing the excess of water in following dewatering and drying steps. It goes without saying that all negative effects result in bad economics for processes run that way. Under conventional conditions 40 wt% starch in water slurries are dewatered (by vacuum or pressure filters, or centrifuges) to yield a (solid) starch cake with a dry solid concentration of 55 - 65% by weight through partial loss of the free water surrounding the starch granules. In the art, lower dry solid levels of the cake are found to yield nonNewtonian fluid behaviour with associated problems for further processing. Unlike in the prior art, with the method of the invention the inventors have made it possible to make DAS granules which can be swollen yet retain their granule integrity with water up to 100% of the initial unreacted starchvolume, and these DAS granules can be dewatered to yield a solid starch cake with much lower solid content, preferably 30 - 40 wt%, more preferably 30 - 35 wt% dry solids, with a DoO between 50 and 100 %. The cake thus obtained with the method according to the invention does not behave as a non-Newtonian fluid, which is basically caused by the fact that 25 - 30% of the residual water in the cake is water present inside the swollen starch granules and not available as ‘free’ water between the starch granules. The inventors estimate that the above numbers do result in free water between the starch granules of 35 - 45%, still about the same as in conventional native and modified starches. It is surprising that at a processable cake can still be obtained at these lower solids levels, which makes it also easier in terms of processing and easier to wash out any remaining iodine-containing compounds following the use of periodic acid. For comparison sake, in the prior art it is believed that at least 55 wt% dry solids are necessary to get a solid and processable cake. With the method of the invention it is possible to refrain from the use of alcohol or acetone, centrifugation and extensive drying techniques applied in the starch-processing industry.In step a), the pH is below 1 .0, which is a consequence of the inventors’ use of orthoperiodic acid which because of its acidity does not require additional acidification, and because of its high water solubility (well above 10%). In fact, up to 1 :1 ratios may require 14.1 % orthoperiodic acid solution. There is no need for further acidification, but also any addition of alkali materials such as NaOH can be readily refrained from. With the orthoperiodic acid and amounts thereof needed in the process, the pH in step a) remains below 1.0. With the process of the invention, it is possible to avoid the need for alkali salts such as NaOH or KOH, ultimately producing DAS with low ash content (preferably at most 3 mg / g). The process at this low pH results in swollen starch granules in spite of the lack of hydrogen bonds.After completion of the reaction in step a), the starch filter cake comprising the DAS is formed by subjecting the mixture of step a) to a dewatering step where free water is removed from the DAS without applying heat. A non-limiting example of such dewatering step to remove free water is vacuum filtration or pressure filtration. A thick layer is formed by sucking or pressing the free water out. Upon completion, the dewatered starch cake is subjected to displacement washing which is a conventional concept known to the skilled person. Typically fresh water is carefully dosed onto the cake, which is then passed through the cake in a plug-flow manner, thus extracting any solubles such as the iodine-containing components from the cake using osmotic pressure. At this stage, the dry solid level is typically between 30 and 35 %. Non-limiting examples of displacement washing of starch filter cake on industrial scale are vacuum filtration or pressure filtration, and all kinds of techniques that are nowadays mostly applied for removal of soluble high-value protein materials from starch.These dewatering and displacement washing steps are different from the conventional 40 wt% slurry-based process where washing is achieved by applying a 10-12 step hydrocyclone washing battery setup; the cake is washed and freed from iodine-containing compounds by dilution washing in which the cake is reslurried in washing liquid followed by filtration repeatedly until the desired washing result is achieved; this method requires high amounts of washing liquid and personnel effort. The method according to the invention makes it possible to wash more efficiently and without wasting any valuable resources. In fact, traditional washing risks destroying the starch granule integrity of the severely swollen particles by applying high shear and temperatures which are associated with a hydrocyclone setup. Instead, it is preferred to apply displacement washing, wherein the washing liquid is forced through the cake after the filtration step at the end of reaction step a). The mother liquor is removed from the filter cake in a plug flow, which reduces the consumption of washing liquid and is carried out without great effort.Hence, in view of foregoing, the method of the invention preferably comprises a) reacting an aqueous solution of orthoperiodic acid with starch in dry or slurry form, in a molar ratio periodic acid: starch (AGU) of 1 :1 - 1 :4, at a temperature, pH and reaction time as described here above, at an initial starch concentration of about 5 - 25 wt%, more preferably 10 - 25 wt%, and wherein the starch concentration is selected such that there is obtained at the end of step a) and after dewatering and displacement washing a solid starch composition (aka cake) with 30 - 40 wt%, more preferably 30 - 35 wt% dry solids, and a DAS with a DoO between 50 and 100 %. The composition does not show non-Newtonian fluid behaviour. The DAS has reduced iodine and Pb levels as described here above.Finally, the cake can optionally be dried using traditional means such as belt, flash or ring dryers as these are also used at the end of the conventional starch slurry processes.The invention also pertains to the use of the DAS obtainable by the method of the invention in the non-limiting list of catalysis, packaging, environmental remediation, biomedical applications, food, resin modification, antimicrobial activity, enzyme immobilization, phytoremediation, lipase immobilization, thermoplastics, drug delivery, paper industry, erodible medical polymers, biodegradable plastics, photochemical applications, orthogonal methods and the like. DAS can be used as such, but also further modified. The invention thus extends to DAS obtainable by the method of the invention as described herein, wherein the DAS has been cationized to cationic DAS, for example for (tissue) paper applications, or wherein the DAS has been submitted to additional oxidation followed by crosslinking to yield carboxylic distarch polymer, for example for superabsorbent properties in personal hygiene products. More specifically, the invention also relates to dialdehyde starch according to the invention, wherein it has been subsequently modified to cationic DAS or to cross-linked carboxylic distarch polymer. However, other modifications making use of the advantageous properties of the DAS according to the invention are also envisaged.List of references

[0001] An improved kinetic model for the periodate oxidation of starch by Veelaert, 1994.[2] Chemical Process for Making Dialdehyde Starch by McGuire, 1971 .[3] The Oxidation of the Aldehyde Groups in Dialdehyde Starch by Haaksman, 2006.[4] The preparation and properties of dialdehyde starch and thermoplastic dialdehyde starch by Yu, 2010.[5] Starch Production Technology, by J.A. Radley, ISBN-13: 978-0853346623, pages 38, 129, 147, etc.[6] Starch Chemistry Technology, R.L. Whistler J. N. BeMiller E.F. Paschall, ISBN 978-0-12- 746270-7, page 458.Example 1We filled a reactor vessel with 2064.8 g of a solution containing 285 g (1 .25 mole) orthoperiodic acid (H5IO6) in water giving a 13.8 wt% solution) and having ambient temperature. To this solution, 247.2 g commercially dry native (untreated) potato starch (82% dry solids, 1 .25 mole) was added quickly and mixed to a homogeneous (10% starch) suspension at a moderate speed. Mixing was continued throughout the reaction.The exothermic oxidation reaction started immediately when the starch was suspended in the solution. The temperature rose immediately and quickly; within thirty minutes the starch suspension temperature increased to a maximum of about 40 °C. After about 30 minutes, the temperature decreased, ultimately back to RT. The oxidation reaction was completed within two hours after starch addition. All this time, the pH < 1 .0.The starch slurry was dewatered by vacuum or pressurized filtration and subsequently washed with an additional amount of (less than) 5000 g of water; typically an amount of 4000 - 5000 g, using a Buchner funnel. The surface area was chosen in such a way that the resulting thickness of the cake was typically between 2 and 5 cm, and the applied vacuum or pressure was such that: compressionof the cake was balanced in a way that washing still could be applied at a reasonable pace, no channeling appeared, the cake did not crack; hindering plug flow evenly distributed over the entire cake volume for optimal washing. In this way we applied displacement washing (unlike dilution washing). At least 99.99% of the iodic acid (this product is a result of periodic acid oxidation) was washed out, resulting in a iodine content in the dialdehyde starch being far less than 50 ppm. No Pb was detected.The washed starch cake could be applied directly as such, or the cake could be subsequently dried with hot air in an oven or by any other proven and commercially available technology for drying starch. Whatever route, the resulting starch was a dialdehyde starch with an aldehyde level of 36% on dry weight, meaning that the efficiency was 100% and the degree of oxidation (DoO) was 100%.Example 2Example 1 was repeated with 1298.5 g of a solution containing 285 g (1 .25 mole) orthoperiodic acid (H5IO6) giving a 21 .9% wt solution, to which 1013.5 g 20% (dry solids, 1 .25 mole) native (untreated) potato starch slurry was added. Similar exothermic behaviour was observed, with a maximum temperature of 40 °C, over a time period of 30 minutes. After that, the temperature decreased ultimately back to RT again.After 2 hours reaction time (all this time, the pH was measured below 1.0), the starch slurry was dewatered by vacuum or pressure filtration and subsequently washed with an additional amount of (less than) 5000 g of water; typically an amount of 4000 - 5000 g, by using a Buchner funnel and in this way applying so called displacement washing (unlike dilution washing). At least 99.99% of the iodic acid (this product as a result of the periodic acid oxidation) was being washed out, yielding a iodine content in the dialdehyde starch being far below 50 ppm. No Pb was detected.The washed starch cake can be applied directly as such, or the cake can be subsequently dried with hot air in an oven or by any other proven and commercially available technology for drying starch. In any case will the resulting starch be a dialdehyde starch with an aldehyde level of 36% on dry basis, meaning that the efficiency was 100% and the degree of oxidation (DoO) was 100%.Example 3Same as examples 1 and 2, but with 1805.2 g of a solution containing 285 g (1 ,25 mole) orthoperiodic acid (H5IO6) giving a 15.8% solution, to which was added 506.8 g of a 40% wt dry solids (1 .25 mole) native (untreated) potato starch slurry. The same exothermic behaviour was observed, oxidation reaction was completed within two hours after the starch addition. All this time, the pH < 1 .0.The starch slurry was then dewatered as in examples 1 and 2, with a iodine content in the dialdehyde starch below 50 ppm; no Pb was detected. The DoO was 36%, the oxidation efficiency was 100%.Example 4A reactor vessel was filled with 506.8 g of 40% (1 .25 mole) native (untreated) potato starch slurry suspension at an ambient temperature. While mixing, a total of 1805.2 g of an aqueous solution containing 285 g (1.25 mole) orthoperiodic acid (H5IO6) was added, still at ambient temperature resulting in a 10% starch suspension. In doing so, the temperature development and swelling behaviour of the starch were controlled, for example by adding the periodic acid solution in four or ten parts with intervals ranging up to 15 minutes. Applying dosage of the periodic acid in parts as described here above, the reaction was typically finished two hours after the last dosage of H5IO6.Similar exothermic conditions were observed. Eventually rather quickly after the last oxidant wasdosed to the starch slurry, the temperature decreased from 40 °C to ambient temperature again. The oxidation reaction was completed within two hours after the starch addition. All this time, pH was below 1 .0.The starch slurry was dewatered as described above. At least 99.99% of the iodic acid (this product as a result of the periodic acid oxidation) was washed out, and a iodine content in the dialdehyde starch was obtained below 50 ppm; no Pb was detected. The resulting starch was a dialdehyde starch with an aldehyde level of 36% on dry basis, meaning that the efficiency was 100% and the degree of oxidation (DoO) was 100%.Example 5.As in examples 1 - 3, with 2081.7 g of a solution containing 285 g (1.25 mole) orthoperiodic acid (H5IO6) giving a 13.7% solution, to which 230.3 g commercially dry native (untreated) pea starch (88% dry solids, 1.25 mole) was added. All this time, pH < 1.0. Following the same recipe as in examples 1 - 3, an aldehyde level of 36% (on dry weight) was obtained, meaning that the efficiency was 100% and the degree of oxidation (DoO) was 100%. A iodine content in the dialdehyde starch was obtained below 50 ppm; no Pb was detected.Example 6.As in examples 1-3, with 1922.3 g of an aqueous solution containing 142.5 g (0.625 mol) orthoperiodic acid (H5IO6) giving a 7.4% wt solution in water, to which 247.2 g commercially dry native (untreated) potato starch (82% dry solids, 1.25 mole) was added quickly, while mixing. A 10% starch suspension was obtained. The pH was below 1.0. After 2 hours of reaction time, showing exothermic behaviour as described in the foregoing examples, the starch slurry was dewatered by vacuum or pressurized filtration and subsequently washed with an additional amount of (less than) 5000 g of water; typically an amount of 4000 - 5000 g, by using a Buchner funnel. At least 99.99% of the iodic acid (this product as a result of the periodic acid oxidation) was washed out, resulting in a iodine content in the dialdehyde starch below 50 ppm. No Pb was detected.The washed starch cake could be applied directly as such, or the cake could be subsequently dried with hot air in an oven or by any other proven and commercially available technology for drying starch. In any case, the resulting starch was a dialdehyde starch with an aldehyde level of 18% on dry base, indicating that with a 50% applied degree of oxidation (DoO). The oxidation efficiency was 100%.Comparative example 1 aWe filled a reactor vessel with 1051.3 g of a solution containing 285 g (1.25 mole) orthoperiodic acid (H5IO6) in water giving a 27.1 % wt) and having ambient temperature. To this solution, 247.2 g commercially dry native (untreated) potato starch (82% dry solids, 1 .25 mole) was added quickly and mixed to a homogeneous (20% starch) suspension at a moderate speed.This did not work because the solubilization of periodic acid was at its limits (maximum is about 30% in water at ambient temperature), but even worse was trying to mix in the amount of starch. While the aim was a DoO of 100%, it was found that the 20 % starch was too high concentration. This immediately thickened the slurry in such a way that it was impossible to add and mix in all of the starch in a proper manner and we were not able to obtain a homogeneous suspension that we could continue mixing at moderate speed.Comparative example 1 bWe filled a reactor vessel with 1665.7 g of a solution containing 342 g (1 .5 mole) orthoperiodic acid (H5IO6) in water giving a 20.5 wt% solution) and having ambient temperature. To this solution, 296.3 g commercially dry native (untreated) potato starch (82% dry solids, 1 .5 mole) was added quickly and mixed to a homogeneous (15% starch) suspension at a moderate speed.While the aim was a DoO of 100%, it was found that the 15 % starch was too high concentration. It did not work since very quickly, due to the high reaction rate of this exothermic reaction, and due to its high efficiency, the viscosity increased rapidly to such an extent that proper mixing was not possible anymore and spontaneous dissipation (taking out) of the generated heat through the jacketed reactor or its surface was seriously hindered. The resulting “yoghurt like” reaction mixture could not be dewatered and washed properly anymore.Example 7We filled a reactor vessel with 5105.3 g of a solution containing 285 g (1.25 mole) orthoperiodic acid (H5IO6) in water giving a 5.6% wt% solution) and having ambient temperature. To this solution, 247.2 g commercially dry native (untreated) potato starch (82% dry solids, 1 .25 mole) was added quickly and mixed to a homogeneous (4% starch) suspension at a moderate speed. Mixing was continued throughout the reaction.The exothermic oxidation reaction started immediately when the starch was suspended in the solution. The temperature rose immediately and quickly; within thirty minutes the starch suspension temperature increased to a maximum of about 40 °C. After about 30 minutes, the temperature decreased, ultimately back to RT. The oxidation reaction was completed within two hours after starch addition.The starch slurry was dewatered by vacuum or pressurized filtration and subsequently washed with an additional amount of (less than) 5000 g of water; typically an amount of 4000 - 5000 g, using a Buchner funnel. In this way we applied displacement washing (unlike dilution washing). At least 99.99% of the iodic acid (this product is a result of periodic acid oxidation) was washed out, resulting in a iodine content in the dialdehyde starch of less than 50 ppm. There was no Pb detected. A disadvantageously high amount of water was used as part of the ‘washing’ was done as dilution washing. This led to a too diluted (with all the additional wash water taking into account) iodine- containing component stream which required additional processing steps in order to be regenerated and led back to a next reaction.Comparative example 2The recipe of Pfeifer(1960) with 5% starch slurry at 38 °C and pH 1.2 was reproduced (twice) as follows:A starch suspension was made by mixing 14,2 grams native (untreated) maize starch as is (12,5 gram on dry solids) with 235.8 grams of tap water in a glass beaker. It results in a 5% maize starch slurry. The beaker was placed in a water bath and the starch slurry was heated to 38 °C. When the temperature was reached, 19.33 grams of ortho-periodic acid (H5IO6) was added, but also 2.39 grams of iodic acid (HIO3). The temperature was maintained at 38 °C. After five minutes, the pH was adjusted to 1 .2 - 1 .4 with 1 M NaOH. Once the appropriate pH and temperature were reached, the reaction time of three hours started. After three hours the slurry was dewatered and washed. A Buchner funnel was used to dewater the slurry first up to the point that the top of the cake just got dry and then 187.5 grams of tap water was evenly poured onto the cake, to wash the starch (at awash water ratio of 1 :15). Then the cake was ‘sucked’ as dry as possible by applying the silicon sheet on top, to prevent the cake from cracking and to stop false air lowering the vacuum. After that, the cake was crumbled and dried with hot air.Initially the pH was about 0.5, after addition of periodic and iodic acid. A significant amount of caustic was needed to bring the pH up to 1 .2 - 1 .4 (meaning that additional ions from the caustic (sodium in this case) was accumulating in the system). When re-working Pfeifer(1960), it was about 60 ml 1 M NaOH (more than 1380 mg of sodium)). The temperature was found to rise to about 40 °C after periodic acid addition (exothermic oxidation reaction) and it required a significant amount of time to cool to 38 °C again.The first filtrate was very hazy and reworking the filtrate by pouring it back on top of the cake did not help. This was expected to result in a loss of starch, protein or fat. The surface of the cake felt smooth but was slightly yellow. After crumbling of the cake the yellowness seemed to have disappeared, but after a few days the filtrate turned orange / brown and also the wash water turned pale yellow. The color formations at this stage of the process were caused by iodine (-containing components). Most of the granules were clearly swollen and most of the swollen granules had lost their original integrity. In these trials, the granules did not swell to twice their original volume like reported in the examples according to the invention. The granules were not able to express the ‘polarized light effect’ (“Maltezer Cross”) nor did the granules turn blue upon ‘iodine staining’.The dry solid level of the cake was just below 40%. This clearly indicates the take-up of water inside the starch granules, but not as much as expected based on the applied periodic acid : starch ratio applied in the experiment. It was concluded that the efficiency of oxidation (EoO) was not met, because an even lower dry solid content of about 35% would have been expected. The EoO was estimated at 78%. The residual iodine contents in the DAS was more than 300 ppm. The relatively low amounts of dry solids in the beginning of the experiment lead to a greater part of dilution washing as compared to the replacement / permeation part of the washing.

Claims

Claims1 . A method for oxidizing a starch into a dialdehyde starch using orthoperiodic acid, comprising: a) reacting an aqueous solution of orthoperiodic acid with dry starch or a starch slurry, having a starch content of 35 - 85 wt%, in a molar ratio of periodic acid : starch (in AGU) of 1 :1 - 1 :4, at a temperature below the starch gelatinization temperature and for a time less than 3 hours and at a pH < 1 .0, at a starch concentration generally in the range of 5 - 27 wt%, based on the weight of the mixture of orthoperiodic acid and starch, wherein the starch that is brought into contact with the orthoperiodic acid is provided in an amount which complies with the following formulas:(i) starch concentration (wt%) < -0.2 * DoO + 34%, and(ii) starch concentration (wt%) > -0.2 * DoO + 25%, wherein DoO is 25, 50, 75 or 100%, b) dewatering and displacement washing, wherein the molar ratio of orthoperiodic acid : starch is selected to obtain a dialdehyde starch with a desired degree of oxidation [DoO] between 25 and 100%, wherein the molar ratio of orthoperiodic acid : starch is selected according to %DoO = 100*(molar ratio periodic acid:AGU).

2. The method according to claim 1 , wherein the molar ratio orthoperiodic acid:starch is in the range of 1 :1 - 1 :2.

3. The method according to any one of the preceding claims, wherein the starch that is brought into contact with the orthoperiodic acid in step a) is provided in an amount which complies with the formula (i):(i) starch concentration (wt%) < -0.2 * DoO + 32%, wherein DoO is 25 - 100%,(ii) starch concentration (wt%) > -0.2 * DoO + 27%, wherein DoO is 25 - 100%.

4. The method according to any one of the preceding claims, wherein the starch that is brought into contact with the orthoperiodic acid in step a) is provided in an amount to yield 15 - 25 wt% starch, in the reaction mixture.

5. The method according to any one of the preceding claims, wherein in step a) between 5 and 25 wt%, preferably between 5 and 22 wt%, more preferably between 10 and 20 wt%, most preferably 13 - 16 wt% orthoperiodic acid is brought into contact with the starch.

6. The method according to any of the preceding claims, wherein the reaction time is less than 2 hours.

7. The method according to any of the preceding claims, wherein the granular structure of the dialdehyde starch in step a) is preserved.

8. The method according to any of the preceding claims, wherein the temperature in step a) is below 50 °C, preferably between 5 and 45 °C, particularly between 10 and 40 °C.

9. The method according to any of the preceding claims, wherein no acid other than orthoperiodic acid is added to reaction step a).

10. A dialdehyde starch obtainable by the method according to any one of the preceding claims, with a degree of oxidation of at least 75%, more preferably 100%, and with a iodine content of less than 100 ppm, preferably less than 80 ppm, most preferably less than 50 ppm.11 . The dialdehyde starch according to claim 10, having a heavy metal concentration of at most 0.3 ppm and / or having an ash content of at most 3 mg / g.

12. The dialdehyde starch according to claim 10 or 11 , wherein the starch has a granular structure and wherein the average granule volume exhibits at least 80% swelling compared to the average granule volume of unreacted starch.

13. The dialdehyde starch according to any one of claims 10 - 12, wherein it has been subsequently modified to cationic DAS or to cross-linked carboxylic distarch polymer.