FIELD OF THE INVENTION

The invention relates to a new process for the manufacture of starch nanoparticles and nanostarch. Further, the invention relates to starch nanoparticles and nanostarch obtainable by the process, as well as uses of said starch nanoparticles and nanostarch.

BACKGROUND

Starches are naturally occurring biodegradable polymers obtainable from renewable resources. Starches are traditionally used in several areas of industry, in applications such as pigments, barrier dispersions, flocculants, fixatives, films, additives in paper manufacture, adhesives, coatings, rheology stabilizers, pharmaceutical formulations, cosmetics, food etc. The amount of starch required for the above mentioned applications is typically high. The commercially most important sources of starch are corn, wheat, rice, potatoes, tapioca and peas.

By the chemical structure starch is a linear polymer consisting of p-l,4-linked D- glucose. Starch may contain pure amylopectin or it may be a mixture of amylose (typically 20-30%) and amylopectin (typically 70-80%). Amylose is a linear polysaccharide consisting of a-l,4-linked D-glucose. Amylopectin is an extremely high molecular weight polymer having the same backbone structure as amylose but with many a-l,6-linked branch points. Native starch occurs in the form of discrete, partially crystalline microscopic granules that are held together by an extended micellar network of associated molecules. The semi-crystalline structure of starch is attributed to the short-chain fraction of amylopectin, arranged as double helices and packed in crystallites. Typically the average particle size of native starch and modified starch is in the range of 1-40 μιη. Starch typically aggregates readily when drying, and it has limited film-forming ability, if any and high viscosity in aqueous solutions. These properties limit the use of native starches and require further chemicals in formulations whereby costs are increased . Starch nanoparticles have recently received interest because of their biocompatibility, biodegradability and nontoxicity in several fields of industry, particularly in nanotechnology, and in food and pharmaceutical applications.

Liu D et al, Journal of Colloid and Interphase Science 339 (2009) 117-124, describes a method for reducing particle size of high amylose corn starch to nano-size by subjecting a 5 w-% aqueous slurry of said corn starch to high-pressure homogenization, followed by cooling on ice bath and repeating the homogenization. The inventors of the present application were not able to repeat the method of Liu et al. for producing nano-sized starch.

Thus there is a need for a simple and reliable method, which is workable also on an industrial scale, for producing starch nanoparticles.

SUM MARY

An object of the present invention is to provide a new process for the manufacture of starch nanoparticles.

A further object is to provide starch nanoparticles obtainable by the method. A still further object is to provide uses of said starch nanoparticles.

Aspects of the invention are thus directed to a process for the manufacture of starch nanoparticles, said process comprising the steps of providing an aqueous dispersion comprising starch, homogenizing the dispersion in a homogenizer at a temperature between 30 and 80°C to yield a gelatinized slurry, diluting the gelatinized slurry with an aqueous medium to obtain starch content between 0.01 and 10 wt%, and homogenizing the diluted gelatinized slurry in a homogenizer at the temperature of 3- 20°C for 1 to 30 passes to obtain starch nanoparticles.

Aspects of the invention are also directed to a nanostarch and starch nanoparticles. Said starch nanoparticles are obtainable by the process described above.

Aspects of the invention are also directed to the use and method of use of the starch nanoparticles in applications, such as in barrier dispersions, pigments, films, flocculants, rheology modifiers, glues, adhesives, dispersion agents, thermoplastics, composites, water purification applications, cosmetics, pharmaceutical formulations, encapsulation agents, retention aids, sizing agents, fixatives, cleaning agents, and detergents, etc.

Particularly, the ease of manufacture of the starch nanoparticles, applicability of the starch nanoparticles to various uses, possibility to design the properties of starch nanoparticles according to the intended use are some examples of the desired benefits achieved by the present invention.

The characteristic features of the invention are presented in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows particle size distributions of native potato starch (Fig. la), and starch nanoparticles obtained in example 1 (Fig. lb). Figure 2 illustrates SEM images of the starch nanoparticles obtained in Example 1, in

Fig. 2a 2 passes and in Fig. 2b 12 passes.

Figure 3 shows the viscosity of the homogenized slurry as function of shear rate of starch nanoparticles manufactured from potato starch using the process of the invention after 2, and 6, 12, 15 and 18 passes, as manufactured in example 2.

DEFINITIONS

Unless otherwise specified, the terms, which are used in the specification and claims, have the meanings commonly used in the field of starch chemistry and processing, as well as in the various application fields of starch. Specifically, the following terms have the meanings indicated below.

The term "starch nanoparticles" or "nanostarch" refers here to nanoparticles of starch, having average particle size below 1 μιη.

The term "homogenization" refers here to disintegration or fluidization under high pressure and high shear. Any high-pressure and high-shear homogenizers, fluidizers, micro-fluidizers, and disintegrators can be used.

The term "passes" refer to how many times the slurry etc is run in/through the homogenizer. DETAILED DESCRIPTION OF THE INVENTION

It was surprisingly found that starch nanoparticles can be manufactured in a simple and efficient way, utilizing the process of the invention.

Process

The process according to the invention comprises the steps of providing an aqueous dispersion comprising starch, homogenizing said dispersion in a homogenizer at a temperature between 30 and 80°C to yield a gelatinized slurry, diluting the gelatinized slurry with an aqueous medium to obtain starch content between 0.01 and 10 wt%, and homogenizing the diluted gelatinized slurry in a homogenizer at the temperature of 3-20°C for 1 to 30 passes to obtain starch nanoparticles.

Particularly the process for the manufacture of starch nanoparticles comprises the steps, where in the first step starch is dispersed in an aqueous medium to obtain an aqueous dispersion, which is subjected to homogenization, suitably for 1-3 passes, preferably one pass, at the temperature of 30-80°C, preferably 40-75°C, particularly preferably 50-75°C to effect gelatinization whereby a gelatinized slurry is obtained. Optionally the gelatinized slurry may further be homogenized, suitably for 1-2 passes, at the temperature of 3-20°C, preferably 8-18°C.

In the second step an aqueous medium is added to the gelatinized slurry in a volumetric ratio of 1 : 1 or more to obtain diluted gelatinized slurry. Said diluted gelatinized slurry has starch content of 0.01-10 wt%, preferably 0.1-7 wt% and particularly preferably 0.1-6 wt%. The diluted gelatinized slurry is subjected to homogenization, which is carried out for 1-30 passes (times), preferably for 2-18 passes and particularly preferably for 2-9 passes. The diluted gelatinized slurry is subjected to homogenization at the temperature of 3-20°C, preferably 8-18°C, particularly preferably 10-15°C. Suitably after each pass the slurry is directed through a heat exchanger, which removes heat during the processing and keeps the diluted slurry at the temperature of 3-20°C, preferably 8-18°C, particularly preferably 10- 15°C. Nanostarch slurry comprising starch nanoparticles is obtained.

In the first step 0.1 - 20 % by weight, preferably 0.5 - 15 % by weight, particularly preferably 2 - 10 % by weight of starch is dispersed in an aqueous medium to obtain an aqueous dispersion. The starch is selected from native and modified starches, and any combinations thereof. Suitably starches with high amylopectin content and which starches can be gelatinized, are used.

The major sources of native starch are the cereals, the root vegetables and many kinds of beans. Examples of such starches are rice, wheat, barley, maize, potato and cassava, and favas, lentila, mung beans, peas, and chickpeas starches. However, there are many other starchy plants, which are grown, some only in specific climates, and thus acors, arrowroot, arracacha, bananas, beadfruit, buckwheat, canna, colacasia, katakuri kudzu, malanga, millet, oats, oca, Polynesian arrowroot, sago, sorghum, sweet potatoes, rye, taro, chesnut, water chesnut, yams starches may also be used. Preferably the native starch is selected from potato starch, corn starch, barley starch, wheat starch, tapioca starch, bean starch, rice starch, particularly preferably potato starch is used.

Modified starch is starch that has been chemically modified. Examples of modified starches are presented as follows: Dextrin, acid-treated starch, alkaline-treated starch, bleached starch, oxidized starch, enzyme-treated starch, monostarch phosphate, distarch phosphate, phosphated distarch phosphate, acetylated distarch phosphate, starch esters, starch ethers, acetylated distarch ad i pate, hydroxypropyl starch, hydroxypropyl distarch phosphate, hydroxypropyl distarch glycerol, starch sodium octenyl succinate, and acetylated oxidized starch. Typical modified starches for technical applications and useful in the presesnt invention are cationic (positively charged) starches, hydroxyethyl starches, carboxymethylated starches and starch acetate, starch propionate, starch butyrate, starch hexanoate and other starch esters.

Preferably the modified starch is selected from cationic (cationized) starches having DS (degree of substitution) of not more than 0.2, anionized starches having DS of not more than 0.3, thermoplastic starches, starch ethers and starch esters. Suitably cationized potato starch is used.

Water soluble charged starches may also be used, however they require, prior to the dispersing step, cross-linking with a suitable cross-linking agent, such as borax (boric acid), saccharin, citric acid, glyoxylates and the like whereby the water solubility is decreased, and the product can be ground.

The aqueous medium is selected from water and mixtures of water with a C1-C4 alcohol or acetone or methyl ethyl ketone or with combinations thereof. A preferable alcohol is ethanol. Water used in the process may be tap water or purified water, preferably purified water selected from RO (reverse osmosis) water, deionized water, distilled water and water purified with other means is used. The content of alcohol or ketone in said aqueous medium is suitably not more than 5 % by volume, preferably 0.1-2 % by volume. Alcohols and ketones reduce air bubble formation during homogenizing. The aqueous medium used in the first step may be the same or different from the one used in the second step. Preferably the same aqueous medium is used in both steps.

In the first step the aqueous medium is dispersed with starch suitably at the temperature of 1-50°C, preferably at a temperature of 4-30°C to obtain an aqueous dispersion.

If necessary, the pH of the dispersion is adjusted to 4-8, preferably to 6-7.5 with an acid or base, such as HCI, citric acid, sodium hydroxide etc. Typically there is no need for pH adjustment when native starches are used, but when modified starches are used the pH adjustment may be necessary.

The homogenization (fluidizing, disintegration) is carried out under the pressure of 100-2200 bar, in the first and subsequent homogenization steps. The homogenization may be carried out using any high-shear homogenizers, fluidizers, microfluidizers, or ultrasonic homogenizers etc.

After the homogenization steps are completed the obtained nanostarch slurry may be subjected to drying. Said drying may be carried out as spray-drying, drying under vacuum, with heat, or as freeze-drying (lyophilization). Suitably the starch nanoparticles obtained from native grade starches are freeze-dried or dried under vacuum at temperatures below 40°C. The starch nanoparticles obtained from modified grades of starches may also be dried at elevated temperatures, suitable at temperatures not more than 80°C. Typically finely divided nanostarch powder is obtained.

The process of the invention can be up-scaled to larger industrial scale without problems.

Product

The obtained starch nanoparticles have average particle size of less than 1 μιη, preferably not more than 200 nm, particularly preferably 20-100 nm. The average particle size and particle size distribution of starch nanoparticles may be measured using any apparatus for measuring particle size, such as Coulter counter, methods based on laser diffraction, TEM (transmission electron microscopy), SEM (scanning electron microscopy, back-scattered quasi-elastic light scattering device etc.

The dry starch nanoparticles are typically in the form of a free flowing powder.

The starch nanoparticles have specific surface are of 10-70 m ² /g, preferably 10-60 m ² /g- The determination of the specific surface area is based on laser diffraction using samples in wet state. In the laser diffraction method the particle size distributions of the dispersions are characterized by laser diffraction (such as Coulter LS230, Beckman Coulter, CA) using refractive indices 1.53 and 1.333 for particles and media (water), respectively. At least two replicate samples are analysed in two consecutive measurements, of which the average geometric volume distributions, the standard deviation for mean value and the specific surface area are calculated.

The viscosity of aqueous dispersions of said nanostarch particles is typically 1-100 mPa*s, preferably 2-80 mPa*s, and particularly preferably 2-50 mPa*s, even more preferably 2-10 mPa*s, measured using a cone and plate rheometer in shear rate range of 0.2-500 1/s, using 5 wt% sample content and the temperature of 21°C.

The obtained starch nanoparticles have improved properties when compared with traditional starches. Particularly the obtained starch nanoparticles have improved reaction efficiency and reactivity in further chemical modifications. This results in that less chemicals are needed for the modifications due to the increased amount of active surface. No visible aggregation takes place during drying of the obtained starch nanoparticles. Further, they have improved film-forming ability and clear, transparent and homogenous films with even surface are obtained. Thin films may be obtained with for example solvent casting methods from unmodified and modified nanostarches. The viscosity of aqueous dispersions formed of said nanostarch particles is decreased when compared to the ones obtained with native starch. The performance of the obtained starch nanoparticles is improved when compared with native starch. The costs are reduced in various applications, because lower amounts of other chemicals are needed due to the increased active surface area provided by the starch nanoparticles, and thus the loading of the particles can be altogether decreased.

Particularly the excellent film-forming is a significant advantage, as well as drying of the starch nanoparticles without aggregation or other problems. Further, the ease of manufacture of the starch nanoparticles, applicability of the starch nanoparticles to various uses, possibility to design the properties of starch nanoparticles according to the intended use are some examples of the desired benefits achieved by the present invention. The obtained nanostarch and starch nanoparticles can be used for example in applications where starch is traditionally used, such as in barrier dispersions, pigments, films, flocculants, rheology modifiers, glues, adhesives, dispersion agents, thermoplastics, composites, water purification applications, cosmetics, pharmaceutical formulations, encapsulation agents, retention aids, sizing agents, fixatives, cleaning agents, and detergents.

The following examples are illustrative of embodiments of the present invention, as described above, and they are not meant to limit the invention in any way. EXAMPLES

Example 1. Manufacture of starch nanoparticles

Potato starch was dispersed in RO water to obtain a 10 wt% aqueous dispersion, calculated by dry substance. The dispersion was mixed in a Waring blender to obtain homogenous slurry. The pH of the dispersion was 6.1, thus no pH adjustment was needed. The slurry was poured in to a microfluidizer M 110EH (equipped with an auxiliary processing module (APM, channel 200 μιη), and diamond interactions chambers (channel 100 μιη, channel 87 μιη, or channel 75 μιη), for effecting high- pressure homogenization. The slurry was in the first step fluidized under high shear, under a pressure of 1850 bar or 2050 bar and at the temperature above 30°C to effect gelatinization whereby gelatinized slurry was obtained. A heat exchanger was used for controlling the temperature of the output slurry in the range of 10-15°C. Said slurry was diluted with RO water to a consistency of 5 wt%, calculated based on dry matter. In the second step said diluted gelatinized slurry was subjected to homogenization (in the same apparatus as in the first step) under a pressure of 1850 bar or 2050 bar and at the temperature of 10-15°C, where practically no gelatinization took place. Under the constant pressure the slurry passed through the interaction chambers and nano- sized material was obtained. The homogenization at 10-15°C was repeated and total of passes was 18. The homogenized slurry was transparent. The material was subjected to freeze-drying to obtain a powdery product. The nanostructure of the particles of the product was confirmed by particle size analysis (Beckman Coulter LS230 Laser Diffraction Particle Size Analyzer or Beckman Coulter N5 Laser Diffraction Particle Size Analyzer) and SEM .

The obtained starch nanoparticles had a specific surface area of 15-30 m ² /g, and is based on laser diffraction using samples in wet state. In the laser diffraction method the particle size distributions of the dispersions are characterized by laser diffraction

(Coulter LS230, Beckman Coulter, CA) using refractive indices 1.53 and 1.333 for particles and water, respectively. Two replicate samples were analysed in two consecutive measurements, of which the average geometric volume distributions, the standard deviation for mean value and the specific surface area were calculated.

Specific surface area was also measured from dry samples using BET determination based on physical absorption of gas molecules on solid surface, giving the value 0.04085 m ² /g- It was noticed that agglomeration of the particles took place, which affected the results obtained with BET.

In Figure 1 particle size distributions are shown, of native potato starch (Fig. la), and nanostarch manufactured from native potato starch diluted to 10 wt% consistency with respect to dry matter and gelatinized, followed by diluting the gelatinized slurry to 5 % consistency calculated based on dry matter and subjecting to 18 passes in the fluidizer (Fig. lb), as instructed in Example 1.

SEM images of the starch nanoparticles obtained in Example 1 are presented in Figures 2a (2 passes) and 2b (12 passes). Example 2. Manufacture of starch nanoparticles

Potato starch was dispersed in an aqueous mixture of ion-exchanged water containing 1 % by volume of ethanol to obtain a 10 wt% aqueous dispersion, calculated by dry substance. The dispersion was mixed in a blender to obtain homogenous slurry. No pH adjustment was needed. The slurry was poured in to a microfluidizer M l lOEH for effecting high-pressure homogenization. The slurry was in the first step fluidized for two passes under high shear, under a pressure of 1850 bar or 2050 bar and at the temperature above 30°C to effect gelatinization whereby gelatinized slurry was obtained. A heat exchanger was used for controlling the temperature of the output slurry in the range of 40-45°C after the first pass and 19°C after the second pass. Said slurry was diluted with ion-exchanged water to a consistency of 5 wt%, calculated based on dry matter. In the second step said diluted slurry was subjected to second fluidization (in the same apparatus as in the first step) under a pressure of 1850 bar or 2050 bar and at the temperature of 10-15°C, where practically no gelatinization took place. A heat exchanger was used for controlling the temperature of the output slurry in the range of 13-15°C after each following pass. Under the constant pressure the slurry passed through the interaction chambers and nano-sized material was obtained, said slurry having pH of 6.8-6.9. The fluidization at 13-15°C was repeated and total of passes was 18. The obtained homogenized slurry was transparent. No significant change in the viscosity of the samples was observed after 3-18 passes, the viscosity was between 3 and 8 mPa*s when measured at the temperature of 21°C. The material was subjected to freeze-drying to obtain a powdery product. Viscosity of the homogenized slurry after 2, 6, 12, 15 and 18 passes is presented in Figure 3.

Example 3. Film formation of starch nanoparticles

A transparent and clear dispersion containing 1 wt% of native starch nanoparticles in water was evenly cast on a glass support and another dispersion containing 5 wt% of modified starch nanoparticles in acetone was evenly cast on a glass support, followed by drying at RT (approx. 20°C) for 24 hours. A solvent casting method was used. The films were formed readily and evenly. The films were removed from the supports and assessed visually and by microscope.

While the invention has been described with respect to specific examples presented in the figures, including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described embodiments that fall within the spirit and scope of the invention. It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. Variations and modifications of the foregoing are within the scope of the present invention.