Process for the production of bis-dialdehyde starch and bis-dialdehyde starch produced thereby

Abstract

This invention relates to a method for oxidizing starch to dialdehyde starch using ortho-periodic acid, the method comprising: a) reacting an aqueous solution of ortho-periodic acid with dry starch or a starch slurry having a starch content of 35-85 wt% under the following conditions: a molar ratio of periodic acid to starch (calculated as AGU) of 1:1-1:4, at a temperature below the starch gelatinization temperature, for a duration of less than 3 hours, and a pH < 1.0, wherein the starch concentration is typically 5-27% based on the weight of the mixture of ortho-periodic acid and starch. Within the range of wt%, the starch in contact with the ortho-periodic acid is provided in an amount conforming to the following formula: (i) starch concentration (wt%) ≤ -0.2DoO + 34%, and (ii) starch concentration (wt%) > -0.2DoO + 25%, wherein DoO is 25%, 50%, 75%, or 100%, b) dehydration and displacement washing, wherein the molar ratio of ortho-periodic acid to starch is selected to obtain a dialdehyde starch having a desired degree of oxidation [DoO] between 25% and 100%, wherein the molar ratio of ortho-periodic acid to starch is selected according to DoO% = 100 (periodic acid:AGU molar ratio). The invention also relates to a dialdehyde starch obtainable by the above method, wherein the degree of oxidation is at least 75% and the iodine content is less than 100 ppm.

Description

Technical Field

[0001] This invention relates to the production of dialdehyde starch and the dialdehyde starch produced therefrom. Background Technology

[0002] In this art, there are ways to modify carbohydrates, and particularly polysaccharides, by introducing reactive aldehyde groups into the polymer structure. In this case, the carbohydrates become chemically reactive and can form chemical bonds with amines or hydroxyl groups. In effect, this means that dialdehyde-carbohydrates can react with proteins and other polysaccharides. Depending on the carbohydrate chosen, this chemical modification opens up commercial prospects on an industrial scale, such as crosslinking cellulose to obtain waterproof paper, crosslinking leather proteins (=tanning) to prevent rot, enzyme stabilization, and membrane reactions with microorganisms to acquire antibacterial properties. These examples are non-limiting examples of applications.

[0003] However, the preparation of dialdehydes is not a simple carbohydrate modification like polysaccharide oxidation. The production of dialdehyde carbohydrates is based on a specific chemical mechanism called "oxidative cleavage of ortho-diols." In organic chemistry, the so-called Malaprad reaction or Malaprad oxidation has been known for nearly a century; this reaction is a diol cleavage reaction.

[0004] The ortho-diol is oxidized by periodic acid or periodate to give the corresponding carbonyl functional group.

[0005] However, while this mechanism has since been applied to glucose homopolymers (particularly cellulose and starch), it has never been developed into a method that makes it efficient and commercially viable. In most (if not all) cases, the oxidant in the art is periodate. However, in the case of periodate, cations are introduced into the system, which can negatively impact any regeneration of the used oxidant; moreover, periodate has limited solubility in water at ambient temperature, which is necessary when applying relatively high concentrations of oxidant. Furthermore, the choice of periodate requires additional acids (e.g., sulfuric acid and hydrochloric acid) to adjust the pH to an optimal level for efficient oxidation. Thus, inorganic acids introduce undesirable anions into the system. In short, this is not a favorable setup for oxidation. Even with suggestions of periodic acid and periodate as alternatives in such Malaprad reactions, periodate has in fact been used for over 80 years.

[0006] When it comes to the production of dialdehyde starch (DAS), some literature has also made efforts to study other parameters such as the oxidant-starch weight ratio, reaction temperature, starch concentration, and reaction time. The weight ratio varies from less than 0.1 to greater than 2, the reaction temperature ranges from 1°C to 55°C, the starch concentration is between 1% and 40%, and the reaction time is up to 100 hours. There is no strong indication in any particular direction for these parameters or combinations thereof, let alone whether the selection of these parameters in the art is supported by experiments. Most notably, these prior art methods typically involve reaction times of up to 100 hours, which makes these reactions unsuitable for industrial-scale DAS production. Examples of these unreasonable settings found in the literature and patents are, for example, the conditions used in [1], in which sodium periodate is used at a controlled pH of 3-5 (which is too high) at a constant temperature of 25°C for about one hour at different concentrations. In [2], the conditions applied were 4 hours of continuous use of 10% excess sodium metaperiodate at a controlled temperature of 32°C, with sulfuric acid added to pH 1.5. The method described in [3] also involves an excess (13%) of oxidant (also sodium metaperiodate) and a reaction at pH 4 and 4°C for 48 hours. The experiment reported in [4], with sodium periodate applied at pH 4 adjusted with sulfuric acid, produced an impossible aldehyde content (>36%) in four out of six experiments at the applied molar ratio. Although a wide range of starch concentrations are used in DAS production as described in the aforementioned prior art, in most cases, this starch concentration is typically lower than the conventional 40% concentration used in starch derivatization, likely due to physical limitations, particularly physical limitations on changing such slurry concentrations typically used in today's industry. Ernest et al. " Application of the Cleavage Type of Oxidation by Periodic Acid to Starch and Cellulose [Application of periodic acid in the pyrolytic oxidation of starch and cellulose]”, J. American Chemical Society, Vol. 59(10) 2049-2050. The reaction curves of 3.5 wt% starch with periodic acid are shown, where the ratio of periodic acid to starch is approximately 2:1, and the reaction time is 24 hours. The particles swell slightly during oxidation.

[0007] The prior art also fails to describe the interaction between starch slurry concentration and parameters such as oxidation degree and temperature. Sometimes, slurry concentration is based on factors such as viscosity, starch granule volume, and free water volume, but none of the papers induced these choices. Those skilled in the art are also unaware of the actual time required to complete the reaction under the existing reaction conditions. Even so, the corresponding yields are unsatisfactorily lower than expected. Given that the prior art suggests reaction times as high as 12 hours or longer (up to 144 hours in US6,620,928), it should be noted that extended reaction times do not necessarily provide better results.

[0008] Pfeifer et al. Two Stage Process for Dialdehyde Starch Using Electrolytic Regeneration of Periodic Acid [A Two-Stage Process for Electrolytic Regeneration of Dialdehyde Starch Using Periodic Acid]” Industrial and Engineering Chem. Vol. 52(3), 01-03-1960, pp. 201-206 also describes the use of an excess oxidant (periodic acid) at a ratio of 1.1:1, at a typical dry solids level of about 5% and at pH 1.2- Starch oxidation is carried out at 1.4°C. Excessive caustic alkali (used to increase pH) adversely affects the electrodes used in the regeneration process. Furthermore, the techniques and processing conditions described in Pfeifer (1960) mention a dry solids level of at least 50% in the final cake before drying. While the article suggests producing DoO between 50% and 100%, to the inventors' knowledge and further demonstrated in comparative examples 1a and 1b, it is impossible to achieve such a level with starch having maximum DoO, an aldehyde content >36%, and maintaining particle integrity. Although oxidized aldehyde starch with up to 98% is mentioned, the inventors have found that this can only be achieved with excessive iodine-based oxidants, and even under the conditions and iodine levels taught therein, the produced dialdehyde starch has an aldehyde content of at most 78%, and also shows an unfavorable increase in iodine concentration exceeding 300 ppm. A Pb concentration of 65 ppm is also reported. Therefore, there is a need to improve DAS to have 100% DoO, higher efficiency, and while still maintaining the quality of the final DAS.

[0009] WO 2023 / 140734 describes the oxidation of carbohydrates such as starch using periodic acid and focuses on the regeneration of periodic acid used for carbohydrate oxidation. It mentions oxidation and regeneration cycles that take a total of 1-48 hours and use 5-100 g / h of periodic acid (HIO4 or H5IO6). Details regarding preferred starch oxidation process conditions and their results are not provided; in this example, a molar ratio of 0.5 mol periodic acid to 1.0 mol AGU is applied, and the starch concentration is 7.5 wt%. A reaction time of 24 hours and pH control are reported, but DoO or efficiency is not reported.

[0010] Moreover, those skilled in the art have always turned to periodate, which is readily available but not easily dissolved in water at high concentrations and is typically 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 [Research Progress on Preparation Conditions, Structural Characteristics, Physicochemical Properties, Functional Properties and Potential Applications of Dialdehyde Starch: A Review]” Int. J. Biological Macromolecules, Elsevier BV, Netherlands, Vol. 259 seems to be such an example. Regarding reaction temperature, existing techniques suggest fixing and controlling the temperature to achieve the desired oxidation.

[0011] CN 105646723 aims to provide an organic binder based on dialdehyde starch, and while it appears to suggest that periodate or periodic acid could be used alternatively to oxidize starch, these examples also systematically employ a combination of periodate and a strong acid, and this aldehyde formation step is integrated with a subsequent degradation step in which starch is hydrolyzed to reduce the molecular weight of the dialdehyde starch and its viscosity (a generally recognized problem in organic binder manufacturing processes). A hydrolysis step is needed to improve the degree of oxidation. However, the method in CN 105646723 produces a disappointing degree of oxidation between 72% and 92% (and the average molecular weight of the dialdehyde starch is 900-9600 Da).

[0012] Indeed, the above citations also observe that achieving a high degree of oxidation (DoO) remains a challenge. Theoretically, DoO is quite directly based on the idea that one dehydrated glucose unit (AGU) in a starch polymer requires one mole of periodic acid to oxidize. In the oxidation process, the M of the dehydrated glucose unit (AGU)... WThe concentration decreased from 162 to 160 g / mol. Since there are two aldehyde moieties at positions 2 and 3, 100% DoO corresponds to (58 / 160.15) at a 1:1 molar ratio of periodic acid to starch (in the form of AGU). 100% = 36%. Similarly, 50% of DoO means 18% ((29 / 161)). 100%. As such a specific chemical reaction should be achievable (for 100% DoO, one mole of periodic acid oxidizes one mole of AGU), but in this field, such a value is far from realistic.

[0013] Therefore, there is a need to produce dialdehyde starch in a resource-efficient manner and to improve the yield, quality, and DoO of dialdehyde starch. Summary of the Invention

[0014] The object of this invention is to provide an improved method for producing dialdehyde starch in 100% yield, wherein the molar ratio of periodic acid to AGU results in a corresponding DoO; under the conditions used by the inventors, periodic acid to AGU molar ratios of 1:4, 1:2, 3:4, and 1:1 result in DoO of 25%, 50%, 75%, and 100%, respectively. Therefore, based on the 100% efficiency or yield achievable using the method of this invention, according to the relationship DoO% = 100%, (Molar ratio), select the above molar ratio to achieve the desired DoO between 25% and 100%.

[0015] In the context of this invention, the term "periodic acid" refers to orthoperiodic acid (H5IO6) rather than metaperiodic acid (HIO4).

[0016] In this application, unless otherwise specified, the following values ​​are used: -AGU molecular weight = 162 g / mol; -H5IO6 molecular weight = 228 g / mol; -C=O (aldehyde group) Molecular weight = 29 g / mol; - Aldehyde percentage (at DoO = 100% and 100% efficiency) = 36%.

[0017] - Aldehyde percentage (at DoO = 50% and 100% efficiency) = 18%.

[0018] the term Oxidation degree As defined above; in connection with this, Oxidation efficiency(EoO) is defined as the actual percentage of AGU oxidized relative to DoO. In other words, if a 1:1 molar ratio of periodic acid to starch is applied, 100% DoO is produced, and the corresponding EoO is 100%. If, for example, a molar ratio of 1:25:1 is applied, DoO may still be 100%, but EoO will only be 80%; or if the molar ratio is 1:2, DoO will therefore be 50%, and in fact only 30% of AGU will be oxidized (only 10.87% instead of 18.12% of the aldehyde groups are measured), then EoO will be 60%.

[0019] Throughout this application, the term DoO 25-100 wt% actually means 25%, 50%, 75%, or 100% DoO and can be used interchangeably. It is not a continuous range of oxidation degrees.

[0020] In a first aspect, the present invention relates to a method for oxidizing starch to dialdehyde starch using orthoperiodic acid, the method comprising: a) React the original periodic acid aqueous solution with dry starch or a starch slurry with a starch content of 35-85 wt% under the following conditions: the molar ratio of original periodic acid to starch is 1:1-1:4, at a temperature below the starch gelatinization temperature, for a time of less than 3 hours, and the pH is <1.0; the starch concentration, based on the weight of the mixture of original periodic acid and starch, is about 5-27 wt%, preferably 7-25 wt%, more preferably 10-25 wt%. Preferably, no additional acid is added. Preferably, the molar ratio of periodic acid to starch in step a) is selected to obtain a desired degree of oxidation [DoO] between 25% and 100% (DoO% = 100%). (Molar ratio of periodic acid to AGU) of dialdehyde starch, i.e., starch in contact with the original periodic acid, is provided in an amount conforming to the following formula: (i) Starch concentration (wt%) ≤ -0.2 DoO + 34%, and (ii) Starch concentration (wt%) > -0.2 DoO + 25%, Where DoO is 25%, 50%, 75% or 100%, and b) Dehydration and displacement washing.

[0021] Particularly preferred is starch that has been in contact with extrinsic periodic acid, supplied in an amount conforming to the following formula: (i) Starch concentration (wt%) ≤ -0.2 DoO + 32%, and (ii) Starch concentration (wt%) > -0.2 DoO + 25%.

[0022] Because of the use of original periodic acid, solubility is improved and the pH remains low. This differs from existing technologies that claim operating conditions far exceeding pH>1 and require large amounts of alkali.

[0023] The above method allows for increased oxidation levels to be achieved quickly and at mild temperatures. Furthermore, the granular structure of the DAS facilitates step b) of dehydration and displacement washing, thereby reducing residual concentrations of iodine and heavy metals to levels beyond what is possible in the art. As explained below, the granular structure is not the normal integrity of granular starch (no Maltese cross was observed, and starch containing amylose does not turn blue with I2), which is not surprising, as two-thirds of the hydrogen bonds have "disappeared" with oxidation; it is known in the art that the loss of approximately 1 / 15 of hydrogen bonds is typically sufficient to lose the granular structure. Surprisingly, the DAS obtained under the conditions of step a) possesses a granular structure, which may be related to the (semi)acetal bonds formed under the selected conditions. This new granular structure allows step b) to be performed without loss of starch. This granular structure is also unexpected given the observed swelling properties of the DAS thus obtained in step a).

[0024] The method of the present invention has proven efficiency of 100%, which means that in step a), the molar ratio of periodic acid to starch is based on DoO% = 100. The molar ratio of periodic acid to AGU is selected. At a 1:1 molar ratio, 100% DoO is practically obtained. Any lower but desired DoO can be achieved simply by adjusting the molar ratio proportionally according to the aforementioned formula. The molar ratio is preferably between 1:1 and 1:2, thus producing dialdehyde starch with a corresponding DoO content of 50%–100%. DoO represents the maximum or theoretically achievable functionality based on the number of oxidized groups relative to the number of dehydrated glucose units.

[0025] Under those conditions, and in conjunction with the particle structure of the DAS obtained under the conditions applied in this method, iodine-free and lead-free DAS with selective DoO (including DoO 100%) and 100% efficiency was produced. As presented above, the particles swell but still exhibit a “particle structure,” allowing the DAS to be washed without leaching starch. Using the method of the present invention, even at DoO 100%, the dry matter content of the cake is less than 35% before and after step b). As explained below, surprisingly, cakes still form at these lower solids levels and no non-Newtonian fluid behavior is observed. It has been found that the 50% dry solids reported in Pfeifer (1960) cannot be achieved without sacrificing particle integrity. Starch is insoluble in cold water. Starch molecules are held together in the particle structure via hydrogen bonds (hydroxyl groups from AGU). This can be achieved by heating, breaking hydrogen bonds, or by sodium substitution of the hydroxyl group (-OH group) with NaOH. Furthermore, starch loses hydrogen bonds when chemical groups are substituted; when a large fraction of these hydrogen bonds break, particle integrity is indeed compromised, and the particles disintegrate in water at ambient temperature. DAS with 100% DoO and 100% EoO lost 67% (two-thirds) of its hydrogen bonds, thus losing particle integrity. In contrast to the method of the invention, no particle structure was obtained in Pfeifer's (1960) experiments. However, not wanting to be bound by any theory, the inventors believe that a particle "structure" (rather than original integrity) is obtained due to the rapid and / or immediate formation of (semi)acetals between molecules in the particles, and that this particle structure allows for step b) to be performed to a certain extent, thereby facilitating the elution of iodine and heavy metals.

[0026] More preferably, the present invention relates to a method for oxidizing starch to dialdehyde starch using orthoperiodic acid, the method comprising: a) React the original periodic acid aqueous solution with dry starch or starch slurry under the following conditions: the molar ratio of periodic acid to starch is 1:1 to 1:4, at a temperature below the starch gelatinization temperature, for a time of less than 3 hours, and at a pH < 1.0. Preferably, no additional acid is added. The step a) involves selecting a molar ratio of periodic acid to starch to obtain a desired degree of oxidation [DoO] between 25% and 100% (DoO% = 100%). (Periodic acid:AGU molar ratio) of dialdehyde starch, This yields a cake-like product with a dry matter content of less than 35 wt%. The starch that comes into contact with the original periodic acid is provided in an amount conforming to the following formula: (i) Starch concentration (wt%) ≤ -0.2 DoO + 34%, and (ii) Starch concentration (wt%) > -0.2 DoO + 25%, The DoO is 25% - 100% (i.e., 25%, 50%, 75% or 100%). as well as b) Dehydration and displacement washing.

[0027] Particularly preferred is starch that has been in contact with extrinsic periodic acid, supplied in an amount conforming to the following formula: (i) Starch concentration (wt%) ≤ -0.2 DoO + 32%, and (ii) Starch concentration (wt%) > -0.2 DoO + 25%.

[0028] More preferably, the concentration of starch used in step a) conforms to: (i) Starch concentration (wt%) ≤ -0.2 DoO + 30%, where DoO is 25% - 100%. And more preferably, the starch concentration also meets the following requirements: (ii) Starch concentration (wt%) > -0.2 DoO + 27% (where DoO is 25% - 100%).

[0029] In the above text, the starch concentration is based on the weight of the mixture of original periodic acid and starch.

[0030] In the above method, the conditions in step a) maintain the granular structure of the dialdehyde starch. Furthermore, the iodine concentration in the DAS obtained after step b) is less than 100 ppm, preferably less than 80 ppm, and most preferably less than 50 ppm. The Pb level is below the detection limit (preferably less than 1 ppm).

[0031] Relatedly, in a second aspect, the present invention relates to a dialdehyde starch having an oxidation degree of at least 75%, more preferably 100%, and an iodine content of less than 100 ppm, preferably less than 80 ppm, and most preferably less than 50 ppm. The lower iodine concentration prevents undesirable decolorization of the product. This represents a significant improvement over the lower DoO (<50%) DAS obtained in the prior art (typically combined with high iodine content, high heavy metal concentration, and high ash content).

[0032] As a result of the method of the present invention, dialdehyde starch with reduced heavy metal content (including lead (Pb)) can be produced. Preferably, the total heavy metal content of the starch is less than 1 mg / kg starch, more preferably up to 0.3 mg / kg starch.

[0033] As is associated with a method carried out at low pH and without the need for additional pH adjusters, the ash content of dialdehyde starch is preferably at most 3 mg / g.

[0034] DAS can preferably be obtained by the method described above. DAS preferably has a pH of 3-6.

[0035] DAS preferably has a swollen granular structure, and preferably wherein the swelling is characterized by the average granule volume being at least 80% larger than the average granule volume of unreacted starch.

[0036] Unlike methods described in the prior art, the above methods can be applied on an industrial scale.

[0037] A key parameter for success is using virgin periodic acid instead of periodate, and the associated low pH.

[0038] Relatedly, the inventors discovered that a concentrated aqueous solution of orthoperiodic acid can be prepared, to which starch in dry or slurry form is added, wherein the molar ratio of orthoperiodic acid to starch is balanced. As detailed in the examples, the order can also be reversed, adding the concentrated aqueous solution of orthoperiodic acid to starch in slurry form, provided that the same molar ratio is balanced. In either case, this contrasts sharply with existing techniques that involve adding periodate and additional acids (such as sulfuric or hydrochloric acid) to a 40% starch slurry; these strong acids can be advantageously avoided. Given the relatively high solubility of periodic acid, particularly compared to periodate, the use of periodic acid allows operation at moderate starch concentrations, which in turn has been found to affect the degree of oxidation. Excessive iodine concentration and the accumulation of heavy metal concentrations can be avoided, and without the need for pH adjustment, any increase in ash content can be minimized (preferably up to 0.3 ppm heavy metals and preferably up to 3 mg / g ash content).

[0039] When starch is contacted with periodic acid in step a), it is preferred that the starch be provided in an amount of 5-25 wt%, preferably 10-25 wt%, of the reaction mixture (preferably according to either formula (i) or (ii) as described above). At higher concentrations, the water absorption and swelling of starch can lead to excessively high viscosity of the starch slurry, which adversely affects DoO and further processing. Viscosity increases proportionally with starch concentration (and DoO), and processing these higher concentrations of viscous slurries requires additional resources. This applies to both steps a) and b) of the method. At lower concentrations, cake formation in step b) can be challenging, and the water balance used for washing and dehydration can be severely disrupted. To control viscosity and produce the desired cake, it is preferred to use starch of a concentration conforming to formula (i) in step a): (i) Starch concentration (wt%) ≤ -0.2 DoO + 34%, of which DoO is 25% - 100%. Furthermore, the starch concentration preferably conforms to the following formula (ii): (ii) Starch concentration (wt%) > -0.2 DoO + 25% (where DoO is 25% - 100%).

[0040] Particularly preferred is starch that has been in contact with extrinsic periodic acid, supplied in an amount conforming to the following formula: (i) Starch concentration (wt%) ≤ -0.2 DoO + 32%, and (ii) Starch concentration (wt%) > -0.2 DoO + 25%. All of the above violates the conventional wisdom of using 40 wt% starch slurry.

[0041] As described in more detail below, the difficulty lies in the fact that with the formation of DAS, a large number (2 / 3 in the case of DoO = 100%) of hydroxyl groups are converted into aldehyde groups, thereby eliminating the possibility of hydrogen bonding. This usually leads to the complete destruction of particle integrity, but surprisingly, these problems do not occur with the method according to the invention. Not wishing to be bound by any theory, the inventors believe that instead, hemiacetal bonds are formed, resulting in starch granules even when swollen to up to twice their initial volume, thus enabling b) washing and dehydration without loss of starch.

[0042] In the most preferred embodiment, the molar ratio of periodic acid to AGU is about 1:1 (and the corresponding DoO is about 100%), and the starch concentration used in step a) is 5-10 wt%.

[0043] Surprisingly, using the starting materials and conditions of the method of the present invention, even during Malaprander oxidation, an initial starch volume increase of up to 100% was found, but at these swelling rates, the starch did not dissolve or disintegrate while remaining at these concentrations, contrary to what a person skilled in the art would reasonably expect; the method of the present invention allows for an irreversible doubling of the initial starch volume while still retaining the desired “starch cake” or “starch filter cake” [5][6], making it possible to process the starch using filtration and dehydration steps. This is key to further processing on an industrial scale, but also unexpected: after all, it is known that when starch begins to swell, it can only swell in a reversible manner to a limited extent due to water absorption at elevated temperatures during starch modification and, depending on the source and the amylopectin:amylose ratio, until the swelling reaches a threshold where the internal structure of the starch molecules becomes irreversibly unbalanced, and typically the swollen starch disintegrates. This would pose a challenge to the physical separation of external water using processes such as filtration and centrifugation, forcing a person skilled in the art to find other cumbersome methods (e.g., using energy-intensive spray drying) to remove water. Based on typical starch oxidation (not according to the invention), the expected increase in oxidation leads to increased starch solubility, swelling, and undesirable loss of particle integrity. In the case of the method of the invention, it has been found that relatively high concentrations of starch can be used, and significant swelling occurs, but without impairing the reaction and subsequent dehydration steps; particle integrity is enhanced. In fact, even at the high swelling volumes observed using the method of the invention, the desired starch cake structure is preserved, and any remaining water can be easily removed from the starch granules using a simple dehydration step. With a dry matter content of 30%–33% (at 100% DoO) and irreversible starch swelling exceeding 100%, the inventors still obtained a slurry that can be further processed, from which cakes can be formed and from which water can be removed. This allows for the complete washing away of iodide ions, a crucial step, as obtaining iodine-free (preferably less than 100 ppm, more preferably less than 80 ppm, most preferably less than 50 ppm) dialdehyde starch is critical. In this context, reference is made to an example in which the method of Pfeifer (1960) is reproduced.

[0044] Without being bound by any theory, it is believed that the reason the cake-like structure retains its characteristic shape despite significant swelling is due to the formation of hemiacetal bonds within and between starch molecules, even though in fact two-thirds of the hydroxyl groups capable of forming hydrogen bridges have been converted to aldehyde groups. These swollen particles do not disintegrate and are essentially filled with water. Because, despite this volume, the integrity of the starch granules is maintained (through hemiacetal bonds), and the dialdehyde starch suspension can still be dehydrated to produce the so-called starch cake.

[0045] List of preferred embodiments 1. A method for oxidizing starch to dialdehyde starch using periodic acid, the method comprising: a) React an aqueous periodic acid solution with dry starch or starch slurry under the following conditions: a molar ratio of periodic acid to starch (based on AGU) of 1:1 to 1:4, at a temperature below the starch gelatinization temperature, for a time of less than 3 hours, with a starch concentration based on the weight of the mixture of periodic acid and starch of approximately 5-25 wt%. b) Dehydration and displacement washing, The periodic acid:starch molar ratio is selected to obtain dialdehyde starch with a desired degree of oxidation [DoO] between 25% and 100%, wherein the periodic acid:starch molar ratio is based on DoO% = 100. (Periodic acid: AGU molar ratio) is used for selection.

[0046] 2. The method according to Example 1, wherein the molar ratio of periodic acid to starch is in the range of 1:1 to 1:2.

[0047] 3. The method according to any one of the foregoing embodiments, wherein the starch in contact with the periodic acid in step a) is provided in an amount conforming to formula (i): (i) Starch concentration (wt%) ≤ -0.2 DoO + 30%, where DoO is 25% - 100%. Furthermore, the starch concentration preferably conforms to the following formula (ii): (ii) Starch concentration (wt%) > -0.2 DoO + 25%, where DoO is 25% - 100%.

[0048] 4. The method according to any one of the foregoing embodiments, wherein the starch in contact with the periodic acid in step a) is provided in an amount of 10-25 wt% starch, preferably 15-25 wt% starch, in the reaction mixture.

[0049] 5. The method according to any one of the foregoing embodiments, wherein in step a), periodic acid of 5 to 25 wt%, preferably 5 to 22 wt%, more preferably 10 to 20 wt%, and most preferably 13 to 16 wt% is contacted with the starch.

[0050] 6. The method according to any one of the foregoing embodiments, wherein the reaction time is less than 2 hours.

[0051] 7. The method according to any one of the foregoing embodiments, wherein the temperature in step a) is below 50°C, preferably between 5°C and 45°C, and particularly between 10°C and 40°C.

[0052] 8. The method according to any one of the foregoing embodiments, wherein no acid other than periodic acid is added to reaction step a).

[0053] 9. A dialdehyde starch obtained by the method according to any one of the foregoing embodiments, having an oxidation degree of at least 75%, more preferably 100%. Detailed Implementation

[0054] This invention relates to a method for oxidizing starch to dialdehyde starch using periodic acid. Starch + H5IO6 → Dialdehyde starch + HIO3 + 3 H2O

[0055] The method comprises: a) reacting an aqueous periodic acid solution with dry starch or starch in slurry form with a starch content of 35-85 wt% under the following conditions: a molar ratio of periodic acid to starch (AGU) of 1:1 to 1:4, at a temperature below the gelatinization temperature, for a time of less than 3 hours, a starch concentration of about 10-25 wt%, and a pH < 1.0, wherein substantially no additional acid is added (i.e., apart from periodic acid); followed by a dehydration step to remove free water from a starch filter cake containing less than 40 wt%, preferably less than 35 wt% DAS; and subsequent dehydration and displacement washing, suitable for removing at least 99%, more preferably at least 99.9%, and most preferably at least 99.99% of the iodine-containing components, to obtain dialdehyde starch with a DoO of 50%-100% (depending on actual requirements). The actual DoO is set by the initial molar ratio of periodic acid to AGU, considering a typical yield of 100%. This method has 100% efficiency, meaning that the DoO corresponds to the theoretical DoO, which is directly derived from the periodic acid:starch molar ratio used in this method. The actual starch concentration in step a) that comes into contact with the original periodic acid is set by the desired DoO as follows: (i) Starch concentration (wt%) ≤ -0.2 DoO + 34%, and (ii) Starch concentration (wt%) > -0.2 DoO + 25%, The DoO ranges from 25% to 100%.

[0056] Particularly preferred is starch that has been in contact with extrinsic periodic acid, supplied in an amount conforming to the following formula: (i) Starch concentration (wt%) ≤ -0.2 DoO + 32%, and (ii) Starch concentration (wt%) > -0.2 DoO + 25%.

[0057] The DoO content is preferably 75% or 100%, more preferably 100%. The iodine concentration in the DAS is less than 100 ppm, preferably less than 80 ppm, and most preferably less than 50 ppm. The heavy metal (including Pb) level is below the detection limit (preferably less than 0.3 ppm). The ash content is preferably less than 3 mg / g, which is a direct result of the fact that the method is carried out at pH < 1.0, and differs from methods described as in Pfeifer (1960) that require alkaline materials to operate at pH > 1.0.

[0058] There are no restrictions on the source of starch, which can be any of the following non-restrictive lists: potato starch, wheat starch, tapioca starch, corn starch, pea starch, rice starch, and glutinous corn starch, or combinations thereof. Starch can be in its natural form, or chemically, physically, or enzymatically modified starch, or combinations thereof.

[0059] The method and its success are specific to the oxidation of starch to dialdehyde starch using periodic acid, and also take into account the unique starch swelling behavior. Using the method of this invention, starch with any desired dialdehyde functional group content per monosaccharide unit can be obtained in the most efficient manner (i.e., DoO between 25% and 100%). 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%), and more preferably between 3:4 and 1:1 (DoO 75%–100%).

[0060] Unlike existing techniques that use periodate, the method according to the present invention does not require the use of any additional acid other than periodic acid. The molar ratio of periodic acid to starch (represented by dehydrated glucose units (AGU)) is 1:1 to 1:4, preferably 1:1 to 1:2.

[0061] This method preferably uses a concentrated aqueous solution of periodic acid (H5IO6) at 5 to 25 wt%, more preferably 5 to 22 wt%, even more preferably 10 to 20 wt%, and most preferably 13-16 wt%, with the actual amount chosen taking into account the amount of starch in contact with it. The combination of high concentrations of starch and periodic acid accelerates the reaction, and simultaneously sets the DoO according to the calculated theoretical DoO. These high concentrations are unattainable when using periodates as used in the art. Therefore, the reaction time is less than 3 hours, preferably less than 2.5 hours, and more preferably less than 2 hours.

[0062] In one embodiment, concentrated periodic acid is regenerated by subjecting iodic acid, generated during the reaction of starch and periodic acid, to a) a two-step electrolytic process. The iodic acid is first electrolytically regenerated to periodate under alkaline conditions, and then the resulting periodate is electrolytically acidified back to periodic acid, which is then returned to a) starch oxidation. Different subsequent regeneration steps can be characterized as follows: • Neutralization with iodic acid: HIO3 + OH- - ->IO3 - + H2O •Periodate regeneration: IO3 - + H2O + 2OH - ->H2IO6 3- + H2 • Periodate acidification: H₂IO₆ 3- + 3H₂O -> H₅IO₆ + 3OH⁻ - In the case of regeneration, reverse osmosis, membrane or nanofiltration, evaporation, freeze drying, sedimentation or centrifugation are preferably used to concentrate periodic acid.

[0063] The DAS obtained by the method of the present invention preferably has an oxidation degree [DoO] between 50% and 100%, more preferably between 75% and 100%, and most preferably 100%.

[0064] The reaction conditions ensure that the starch does not undergo hydrolysis; strong acids are absent (periodic acid is not a strong acid), and the temperature is preferably below 50°C. The reaction temperature is controlled below the starch gelatinization temperature, preferably below 50°C, most preferably between 5°C and 45°C, and particularly between 10°C and 40°C.

[0065] To achieve a DAS with a DoO of 25%–100%, preferably 50%–100%, the method of the present invention works effectively with 10–25 wt% starch, preferably 10–20 wt% (which is significantly lower than the typical 40 wt% dry solids starch slurry used in the prior art). Preferably, the actual amount of starch is selected in combination with the molar ratio of periodic acid to starch [in the range of 1:1–1:4] according to formula (i), and more preferably according to formulas (i) and (ii) as described herein, to produce a DAS with an acceptable starch viscosity at the selected starch wt% at the end of step a). At higher DoOs, starch granules absorb a large amount of water without losing granule integrity, but the resulting granule volume is twice that of the original. This also results in a much lower overall dry solids level in the cake and a higher risk of non-Newtonian fluid behavior. Applying a higher concentration increases the viscosity of the starch slurry, due to the extensive swelling of the particles as explained above, and also depending on DoO; the higher the DoO, the more water the starch particles absorb, and the higher the viscosity of the starch slurry. This is also why the starch wt% in the prepared slurry is adjusted according to the aforementioned equations (i) and (ii). It goes without saying that the more swollen the particles, the more difficult it is to dehydrate the reacted starch slurry into a solid starch cake, thus avoiding the opportunity to produce a “pseudo” cake with so-called non-Newtonian fluid characteristics, which makes further processing difficult and uneconomical. In this context, refer to Comparative Example 2, in which the method of Pfeifer (1960) is reproduced.

[0066] In the art, lower starch slurry concentrations are believed to negatively impact the overall water balance in the process, negatively impacting modified starch (chemical, physical, and enzymatic methods) and negatively impacting the removal of excess water in subsequent dehydration and drying steps. Needless to say, all these negative impacts lead to poor economics for processes operated in this manner. Under normal conditions, an aqueous slurry containing 40 wt% starch is dehydrated (by vacuum or pressure filtration, or centrifuge) to produce a (solid) starch cake with a dry solids concentration of 55%–65% by weight, due to partial loss of free water around the starch granules. In the art, it has been found that lower dry solids levels in the cake result in non-Newtonian fluid behavior and introduce related problems for further processing. Unlike existing technologies, the inventors, using the method of this invention, have enabled the preparation of DAS particles that swell but retain their granular integrity even when water reaches 100% of the initial unreacted starch volume. These DAS particles can be dehydrated to produce solid starch cakes with a much lower solids content (preferably 30-40 wt%, more preferably 30-35 wt% dry solids) and a DoO between 50% and 100%. The cakes thus obtained using the method according to the invention do not exhibit non-Newtonian fluid behavior, primarily due to the fact that 25%-30% of the residual water in the cakes is water present within the swollen starch granules, rather than “free” water between the starch granules. The inventors estimate that this value does indeed result in 35%-45% free water between the starch granules, still roughly the same as conventional natural and modified starches. Surprisingly, processable cakes can still be obtained at these lower solids levels, which also makes processing easier and facilitates the washing out of any residual iodine compounds after the use of periodic acid. For comparison, in the prior art, it is believed that at least 55 wt% dry solids are necessary to obtain a solid and processable cake. The method of this invention avoids the use of ethanol or acetone, centrifugation, and extensive drying techniques employed in the starch processing industry.

[0067] In step a), the pH is below 1.0, a result of the inventors using orthoperiodic acid due to its acidity (requiring no additional acidification) and its high water solubility (well above 10%). In fact, a 1:1 ratio could require a 14.1% orthoperiodic acid solution. Not only is further acidification unnecessary, but the addition of any alkaline materials (such as NaOH) can also be easily avoided. Using the orthoperiodic acid and its amount required in this method, the pH in step a) remains below 1.0. The method of this invention avoids the need for alkali metal salts (such as NaOH or KOH), ultimately producing DAS with a low ash content (preferably up to 3 mg / g). Despite the lack of hydrogen bonds, the process at this low pH still produces swollen starch granules.

[0068] After the reaction in step a) is complete, a starch cake containing DAS is formed by subjecting the mixture of step a) to a dehydration step (in which free water is removed from the DAS without the application of heat). Non-limiting examples of such a dehydration step to remove free water are vacuum filtration or pressure filtration. A thick layer is formed by suctioning or pressing out the free water. After completion, the dehydrated starch cake is subjected to displacement washing (a conventional concept known to those skilled in the art). Typically, fresh water is carefully added to the cake and then passed through it in a plug flow manner, thereby using osmotic pressure to extract any soluble components, such as iodine-containing components, from the cake. At this stage, the dry solids level is typically between 30% and 35%. Non-limiting examples of displacement washing of starch cakes on an industrial scale are vacuum filtration or pressure filtration, and a wide variety of techniques currently used primarily for removing soluble high-value protein materials from starch.

[0069] These dehydration and displacement washing steps differ from conventional processes based on 40 wt% slurry, where washing is achieved using a 10-12 stage hydrocyclone washing setup; the cake is washed and iodine-containing compounds are removed through a dilution wash in which the cake is re-slurryed in the washing liquid and then repeatedly filtered until the desired washing result is achieved; this method requires a large amount of washing liquid and manpower. The method according to the invention allows for more efficient washing without wasting any valuable resources. In fact, conventional washing carries the risk of damaging the integrity of severely swollen starch granules due to the high shear forces and temperatures associated with the hydrocyclone setup. Instead, displacement washing is preferred, where 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 less labor-intensive.

[0070] Therefore, in view of the foregoing, the method of the present invention preferably comprises a) reacting an aqueous periodic acid solution with starch in dry or slurry form under the following conditions: a molar ratio of periodic acid to starch (AGU) of 1:1 to 1:4, at the temperature, pH, and reaction time as described above, an initial starch concentration of about 5-25 wt%, more preferably 10-25 wt%, and wherein the starch concentration is selected such that at the end of step a) and after dehydration and displacement washing, a solid starch composition (also known as a cake) having 30-40 wt%, more preferably 30-35 wt% dry solids and DAS with DoO between 50% and 100%. This composition does not exhibit non-Newtonian fluid behavior. As described above, the DAS has reduced iodine and Pb levels.

[0071] Finally, conventional methods such as belt dryers, flash dryers, or ring dryers can be used to dry the cakes, as these are also used at the end of conventional starch slurry processes.

[0072] This invention also relates to the use of DAS obtainable by the methods of this invention in catalysis, packaging, environmental remediation, biomedical applications, food, resin modification, antimicrobial activity, enzyme immobilization, phytoremediation, lipase immobilization, thermoplastics, drug delivery, the paper industry, easily corrosive medical polymers, biodegradable plastics, photochemical applications, orthogonal methods, and other non-limiting applications. DAS can be used as is, but can also be further modified. Therefore, this invention extends to DAS obtainable by the methods of this invention as described herein, wherein the DAS has been cationicized to cationic DAS (e.g., for (thin) paper applications), or wherein the DAS has undergone additional oxidation and subsequent crosslinking to produce carboxymethyl distarch polymers (e.g., for superabsorbency properties in personal hygiene products). More particularly, this invention also relates to dialdehyde starch according to the invention, wherein it is subsequently modified to cationic DAS or crosslinked carboxymethyl distarch polymers. However, other improvements utilizing the advantageous properties of DAS according to the invention are also contemplated.

[0073] Reference List [1] An improved kinetic model for the periodate oxidation of starch [An improved kinetic model for the oxidation of starch periodate] Veelaert, 1994. [2] Chemical Process for Making Dialdehyde Starch method] McGuire, 1971. [3] The Oxidation of the Aldehyde Groups in Dialdehyde Starch [Dialdehyde Starch [Oxidation of aldehyde groups in powder] , Haaksman, 2006. [4] The preparation and properties of dialdehyde starch and Thermoplastic dialdehyde starch [Preparation and properties of dialdehyde starch and thermoplastic dialdehyde starch] Yu, 2010. [5] Starch Production Technology , JA Radley, ISBN-13: 978-0853346623, Page 38, Page 129, Page 147, etc. [6] Starch Chemistry Technology , RL Whistler JNBeMiller EF Paschall, ISBN 978-0-12-746270-7, p. 458. Example 1 A solution containing 285 g (1.25 mol) of orthoperiodic acid (H₅IO₆) in water was charged into a reaction vessel, yielding a 13.8 wt% solution at ambient temperature. 247.2 g of commercially available, dried, natural (untreated) potato starch (82% dry solids, 1.25 mol) was rapidly added to this solution and mixed at a moderate rate to form a homogeneous (10% starch) suspension. Mixing was continued throughout the reaction.

[0074] When starch is suspended in solution, the exothermic oxidation reaction begins immediately. The temperature rises rapidly; within thirty minutes, the temperature of the starch suspension reaches a maximum of approximately 40°C. After about 30 minutes, the temperature decreases and eventually returns to RT. The oxidation reaction is completed within two hours of the addition of starch. During this period, the pH is <1.0.

[0075] The starch slurry was dehydrated by vacuum or pressure filtration, followed by washing with an additional (less than) 5000 g of water using a Buchner funnel; typically 4000-5000 g of water was used. The surface area was selected such that the resulting cake thickness was typically between 2 and 5 cm, and the applied vacuum or pressure was such that the pressure on the cake was balanced in such a way that washing could still be applied at a reasonable rate without channeling or cracking; and the obstructive flow was evenly distributed throughout the cake volume for optimal washing. In this way, displacement washing (as opposed to dilution washing) was applied. At least 99.99% of the iodic acid (this product is a result of periodic acid oxidation) was washed out, resulting in an iodine content in the dialdehyde starch well below 50 ppm. Pb was not detected.

[0076] The washed starch cakes can be applied directly as is, or the cakes can be subsequently dried in an oven with hot air, or dried using any other proven and commercially available starch drying technique. Regardless of the method, the resulting starch is a dialdehyde starch with an aldehyde level of 36% on a dry weight basis, implying 100% efficiency and 100% degree of oxidation (DoO).

[0077] Example 2 Example 1 was repeated using 1298.5 g of a solution containing 285 g (1.25 mol) of orthoperiodic acid (H5IO6), yielding a 21.9% wt solution, to which 1013.5 g of 20% (dry solids, 1.25 mol) natural (untreated) potato starch slurry was added. Similar exothermic behavior was observed over a 30-minute timeframe, with a maximum temperature of 40°C. Afterward, the temperature decreased and eventually returned to RT.

[0078] After a 2-hour reaction time (during which the pH is measured to be below 1.0), the starch slurry is dehydrated by vacuum or pressure filtration, followed by washing with an additional amount (less than) 5000 g of water; typically 4000-5000 g of water is used, applied via a Buchner funnel in this manner as a so-called displacement wash (different from a dilution wash). At least 99.99% of the iodic acid (this product is a result of periodic acid oxidation) is washed out, resulting in an iodine content in the dialdehyde starch well below 50 ppm. Pb was not detected.

[0079] The washed starch cakes can be applied directly as is, or the cakes can be subsequently dried in an oven with hot air, or dried using any other proven and commercially available starch drying technique. In any case, the resulting starch is a dialdehyde starch with an aldehyde level of 36% on a dry basis, implying 100% efficiency and 100% degree of oxidation (DoO).

[0080] Example 3 Similar to Examples 1 and 2, but using 1805.2 g of a solution containing 285 g (1.25 mol) of orthoperiodic acid (H5IO6) to produce a 15.8% solution, 506.8 g of 40% wt dry solids (1.25 mol) natural (untreated) potato starch slurry was added. The same exothermic behavior was observed, with the oxidation reaction completed within two hours of starch addition. During this period, pH < 1.0.

[0081] The starch slurry was then dehydrated as in Examples 1 and 2, resulting in an iodine content of less than 50 ppm in the dialdehyde starch; no Pb was detected. DoO was 36%, and the oxidation efficiency was 100%.

[0082] Example 4 At ambient temperature, 506.8 g of a 40% (1.25 mol) suspension of natural (untreated) potato starch slurry was charged into the reaction vessel. During mixing, an aqueous solution containing 285 g (1.25 mol) of ortho-periodic acid (H5IO6) was added at ambient temperature, resulting in a 10% starch suspension. In this case, the temperature development and swelling behavior of the starch were controlled, for example, by adding the periodic acid solution in quadruplicates or ten portions at intervals up to 15 minutes. With the periodic acid dosage applied in portions as described above, the reaction typically completed two hours after the last H5IO6 dose.

[0083] Similar exothermic behavior was observed. Ultimately, shortly after the final addition of the oxidant to the starch slurry, the temperature dropped again from 40°C to ambient temperature. The oxidation reaction was completed within two hours of starch addition. During this period, the pH remained below 1.0.

[0084] The starch slurry was dehydrated as described above. At least 99.99% of the iodic acid (this product is the result of periodic acid oxidation) was washed out, yielding a dialdehyde starch with an iodine content of less than 50 ppm; Pb was not detected. The resulting starch was a dialdehyde starch with an aldehyde level of 36% on a dry basis, implying an efficiency of 100% and a degree of oxidation (DoO) of 100%.

[0085] Example 5. As in Examples 1-3, a 13.7% solution was obtained by using 2081.7 g of a solution containing 285 g (1.25 mol) of orthoperiodic acid (H5IO6), to which 230.3 g of commercially available, dried, natural (untreated) pea starch (88% dry solids, 1.25 mol) was added. During this period, the pH was <1.0. Following the same formulation as in Examples 1-3, an aldehyde level of 36% (on a dry weight basis) was obtained, implying 100% efficiency and 100% degree of oxidation (DoO). The iodine content in the dialdehyde starch obtained was less than 50 ppm; Pb was not detected.

[0086] Example 6. As in Examples 1-3, 1922.3 g of an aqueous solution containing 142.5 g (0.625 mol) of orthoperiodic acid (H5IO6) was used to give a 7.4% wt aqueous solution. 247.2 g of commercially available, dried, natural (untreated) potato starch (82% dry solids, 1.25 mol) was rapidly added to this solution and mixed. A 10% starch suspension was obtained. The pH was below 1.0. After a reaction time of 2 hours (exhibiting exothermic behavior as described in the preceding examples), the starch slurry was dehydrated by vacuum or pressure filtration, followed by washing with an additional amount (less than) 5000 g of water using a Buchner funnel; typically 4000–5000 g of water was used for washing. At least 99.99% of the iodic acid (this product is a result of periodic acid oxidation) was washed out, resulting in an iodine content of less than 50 ppm in the dialdehyde starch. Pb was not detected.

[0087] The washed starch cakes can be applied directly as is, or the cakes can be subsequently dried in an oven with hot air, or dried using any other proven and commercially available starch drying technique. In either case, the resulting starch is a dialdehyde starch with an aldehyde level of 18% on a dry basis, indicating a degree of oxidation (DoO) of 50% for application. The oxidation efficiency is 100%.

[0088] Comparison Example 1a 1051.3 g of a solution containing 285 g (1.25 mol) of orthoperiodic acid (H5IO6) in water was charged into a reaction vessel, giving 27.1% wt and at ambient temperature. 247.2 g of commercially available, dried, natural (untreated) potato starch (82% dry solids, 1.25 mol) was rapidly added to this solution and mixed at a moderate rate to form a homogeneous (20% starch) suspension.

[0089] This didn't work because the solubility of periodic acid reached its limit (maximum of about 30% in water at ambient temperature), but worse still, attempts to incorporate a certain amount of starch were made. While the goal was 100% DoO, a starch concentration of 20% was found to be too high. This immediately thickened the slurry to the point that it was impossible to add and mix all the starch in properly, and it was impossible to obtain a homogeneous suspension that could continue to be mixed at a moderate rate.

[0090] Comparison Example 1b A solution containing 342 g (1.5 mol) of orthoperiodic acid (H5IO6) in water was charged into a reaction vessel, yielding a 20.5 wt% solution at ambient temperature. 296.3 g of commercially available, dried, natural (untreated) potato starch (82% dry solids, 1.5 mol) was rapidly added to this solution and mixed at a moderate rate to form a homogeneous (15% starch) suspension.

[0091] Although the target was 100% DoO, a starch concentration of 15% was found to be too high. Due to the high reaction rate and efficiency of this exothermic reaction, the viscosity increased rapidly to the point that proper mixing was no longer possible, and the generated heat was severely hampered by spontaneous dissipation (carry-off) through the jacketed reactor or its surface. Therefore, this method quickly became unworkable. The resulting "yogurt-like" reaction mixture could no longer be properly dehydrated and washed.

[0092] Example 7 A solution containing 285 g (1.25 mol) of orthoperiodic acid (H₅IO₆) in water was charged into a reaction vessel, yielding a 5.6% wt solution at ambient temperature. 247.2 g of commercially available, dried, natural (untreated) potato starch (82% dry solids, 1.25 mol) was rapidly added to this solution and mixed at a moderate rate to form a homogeneous (4% starch) suspension. Mixing was continued throughout the reaction.

[0093] When starch is suspended in solution, the exothermic oxidation reaction begins immediately. The temperature rises rapidly; within thirty minutes, the temperature of the starch suspension reaches a maximum of approximately 40°C. After about 30 minutes, the temperature decreases and eventually returns to RT. The oxidation reaction is completed within two hours of the addition of starch.

[0094] The starch slurry is dehydrated by vacuum or pressure filtration, followed by washing with an additional (less than) 5000 g of water using a Buchner funnel; typically 4000–5000 g of water is used. In this manner, a displacement washing (different from dilution washing) is applied. At least 99.99% of the iodic acid (this product is a result of periodic acid oxidation) is washed out, resulting in an iodine content of less than 50 ppm in the dialdehyde starch. Pb was not detected. Disadvantageously, because a portion of the “washing” is performed as a dilution wash, a large amount of water is used. This results in the iodine-containing component stream being overly diluted (considering all the additional wash water), thus requiring additional processing steps to regenerate and guide it back to the next reaction.

[0095] Comparison Example 2 The following is a reproduction (twice) of Pfeifer's (1960) formulation containing 5% starch slurry at 38°C and pH 1.2: A starch suspension was prepared by mixing 14.2 g of natural (untreated) corn starch (12.5 g, on a dry solids basis) with 235.8 g of tap water in a glass beaker. This produced a 5% corn starch slurry. The beaker was placed in a water bath, and the starch slurry was heated to 38°C. When this temperature was reached, 19.33 g of orthoperiodic acid (H₅IO₆) and 2.39 g of iodic acid (HIO₃) were added. 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, a three-hour reaction time was initiated. After three hours, the slurry was dehydrated and washed. First, the slurry was dehydrated using a Buchner funnel until the top of the cake was just dry. Then, 187.5 g of tap water was poured evenly onto the cake to wash the starch (wash water ratio 1:15). Next, the disc is "vacuumed" as much as possible by attaching a silicone sheet to the top of it, preventing the disc from cracking and stopping any leaking air that could reduce the vacuum level. Afterward, the disc is broken up and dried with hot air.

[0096] After the addition of periodic acid and iodic acid, the initial pH is approximately 0.5. Achieving a pH as high as 1.2–1.4 requires a large amount of caustic alkali (meaning additional ions from the caustic alkali, in this case sodium, accumulate in the system). When Pfeifer (1960) reproduced this, it was approximately 60 ml of 1 M NaOH (over 1380 mg of sodium). It was found that the temperature rose to approximately 40°C after the addition of periodic acid (an exothermic oxidation reaction), and it required a considerable amount of time to cool back to 38°C.

[0097] The first filtrate was very turbid, and reprocessing it by pouring it back onto the top of the cake was ineffective. This was expected to result in the loss of starch, protein, or fat. The surface of the cake felt smooth but was slightly yellowish. After the cake broke, the yellow color seemed to disappear, but after a few days the filtrate turned orange / brown, and the wash water also turned pale yellow. The color formation at this stage of the process was caused by iodine (the iodine-containing component). Most of the particles swelled noticeably, and most of the swollen particles had lost their original integrity. In these tests, the particles did not swell to twice their original volume as reported in the examples according to the invention. The particles did not exhibit the "polarized light effect" ("Maltese cross"), and the particles did not turn blue after "iodine staining."

[0098] The dry solids level of the cake was slightly below 40%. This clearly indicates water absorption within the starch granules, but not as much as expected based on the periodic acid:starch ratio applied in the experiment. The conclusion is that the oxidation efficiency (EoO) was not met, as an even lower dry solids content of approximately 35% was expected. The EoO was estimated at 78%. The residual iodine content in the DAS exceeded 300 ppm. The relatively low dry solids level at the start of the experiment resulted in a larger dilution wash compared to the displacement / permeation portion of the wash.

Claims

1. A method for oxidizing starch to dialdehyde starch using ortho-periodic acid, the method comprising: a) React the original periodic acid aqueous solution with dry starch or a starch slurry with a starch content of 35-85 wt% under the following conditions: a molar ratio of periodic acid to starch (based on AGU) of 1:1 to 1:4, at a temperature below the starch gelatinization temperature, for a duration of less than 3 hours, and a pH < 1.

0. The starch concentration, based on the weight of the mixture of original periodic acid and starch, is typically in the range of 5-27 wt%, wherein the starch in contact with the original periodic acid is provided in an amount conforming to the following formula: (i) Starch concentration (wt%) ≤ -0.2 DoO + 34%, and (ii) Starch concentration (wt%) > -0.2 DoO + 25%, The DoO values ​​are 25%, 50%, 75%, or 100%. b) Dehydration and displacement washing, The molar ratio of proto-periodic acid to starch is selected to obtain dialdehyde starch with a desired degree of oxidation [DoO] between 25% and 100%, wherein the molar ratio of proto-periodic acid to starch is based on DoO% = 100%. (Periodic acid: AGU molar ratio) is used for selection.

2. The method according to claim 1, wherein, The original periodic acid: starch molar ratio was in the range of 1:1 to 1:

2.

3. The method according to any one of the preceding claims, wherein, In step a), the starch that comes into contact with the original periodic acid is provided in an amount conforming to formula (i): (i) Starch concentration (wt%) ≤ -0.2 DoO + 32%, of which DoO is 25% - 100%. (ii) Starch concentration (wt%) > -0.2 DoO + 27%, of which DoO is 25% - 100%.

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

5. The method according to any one of the preceding claims, wherein, In step a), the starch is contacted with 5 to 25 wt%, preferably 5 to 22 wt%, more preferably 10 to 20 wt%, and most preferably 13 to 16 wt%.

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

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

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

9. The method according to any one of the preceding claims, wherein, No acid other than the original periodic acid is added to reaction step a).

10. A dialdehyde starch obtainable by the method according to any one of the preceding claims, having an oxidation degree of at least 75%, more preferably 100%, and an iodine content of less than 100 ppm, preferably less than 80 ppm, and most preferably less than 50 ppm.

11. The dialdehyde starch according to claim 10, having a heavy metal concentration of up to 0.3 ppm and / or an ash content of up to 3 mg / g.

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

13. The dialdehyde starch according to any one of claims 10-12, wherein, The dialdehyde starch was then modified into cationic DAS or cross-linked carboxymethyl distarch polymer.

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