a variety of different starches may be derivatized in accordance with the invention , and indeed essentially any starch can be so modified . preferable however , the starches are selected from the group consisting of wheat , waxy wheat , corn , waxy corn , high amylose corn , oat , rice , tapioca , mung bean , and potato starches and mixtures thereof . the starches useful in the invention can be used in their native form by substitution with a polyvalent metal selected from the group consisting of groups 2 - 10 , inclusive . however , it is preferred that the starches be initially cross - linked and substituted with hydrophobic moieties before reaction with polyvalent metal . cross - linking may be effected in a number of ways using an assortment of different cross - linking agents , such as those selected from the group consisting of sodium , trimetaphosphate ( stmp ), sodium tripolyphosphate ( stpp ), phosphoryl chloride , epichlorohydrin and mixtures thereof . however , it is particularly preferred that the cross - linking reaction be carried out by preswelling the starch granules in the presences of an alkali ( e . g ., alkali metal hydroxide ) and a salt ( e . g ., alkali or alkaline earth metal chloride , sulfate or carbonate ). the alkali is present to promote swelling of the starch , whereas the salt is added to prevent excess swelling leading to complete destruction of the granular structure of the salt ( i . e ., gelatinization ). preferred preswelling / cross - linking conditions and parameters are set forth in u . s . pat . no . 6 , 299 , 907 which is expressly incorporated by reference herein . in more detail , the most preferred initial cross - linking reaction involves a process of first forming a dispersion of starch granules in water where the granules undergo swelling in the dispersion and have a crystalline phase . a cross - linking agent is added to the dispersion while the granules are swelled in order to cross - link the swelled granules , the cross - linking being carried out under conditions to avoid complete gelatinization of the swelled granules . thereafter , the cross - linked starch granules are heated in excess water in order to melt the crystalline phase of the granules . in a preferred procedure , the starch granules are preswelled by first forming a starch / water dispersion and heating the latter in order to swell the granules prior to the addition of the cross - linking agent ; the preswelling step is preferably carried out in the presence of a base ( such as an alkali metal hydroxide which promotes swelling ) and a salt ( such as an alkali or alkaline earth metal chloride , sulfate or carbonate ). again , it is important that the preswelling and cross - linking steps be carried out so as to avoid complete gelatinization of the starch granules . accordingly , the temperature of the starch dispersion during preswelling is generally 5 - 10 ° c . below the starch gelatinization temperature . it is also possible to preswell the starch at elevated temperatures , for example 70 - 80 ° c . if high concentrations ( greater than about 20 % based on starch ) of salt are used with reduced amounts of base . the hydroxide is normally present at a level of about 1 - 3 % by weight based upon starch , while the salt is used at a level of from about 0 . 1 % by weight on the same basis . the ph of the preswelling system is generally from about 10 - 12 . during the cross - linking step , the dispersion should have from about 20 - 40 % by weight of starch solids therein . the cross - linking step generally involves heating to a temperature of from about 30 - 75 ° c . for a period of from about 0 . 1 - 12 hrs , more preferably from about 0 . 5 - 2 hr . when the preferred stmp cross - linking agent is used the level of use is typically from about 0 . 1 % by weight , on a dry starch basis . during cross - linking , if an inadequate level of stmp is employed , the starch will eventually gelatinize and cause the reaction mixture to gel . when this occurs , swelling has not been counterbalanced by sufficient inhibition from cross - linking . increasing the temperature of the cross - linking reaction is a compromise between accelerating the swelling and accelerating the cross - linking reaction , such that gelling of the reaction mixture does not occur prior to sufficient cross - linking in a reasonable period of reaction time . after reacting at a warm temperature usually for several hours , the mixture is neutralized and the starch isolated from the salts to give quantitative product yields . the product exhibits an x - ray diffraction pattern very similar to the starting starch , and a gelatinization temperature somewhat elevated as compared with the parent starch . in the next preferred preparative step , the partially crystalline , swollen / cross - linked starches are heated in excess water followed by drying in order to melt the crystalline phase . for example , a 10 % aqueous slurry of the partially crystalline product may be heated to boiling with stirring for about 5 min . to achieve this end . the boiled product is then cooled and centrifuged . the liquid fraction contains at most 1 - 2 % of the original weight of the partially crystalline modified starch in the form of soluble and damaged starch . if the starches are merely tray dried without removal of the soluble and damaged starch fraction , the product may form a cake - like structure comprised of granules that cling together . in lieu of centrifugation , the starch products may be spray dried . although preferred , the above - described preswelling / cross - linking procedure is not essential and suitable starches can be prepared using straightforward cross - linking strategies without preswelling . these cross - linking reactions are entirely conventional and need not be described in detail . the initially cross - linked starches are next preferably subjected to a substitution reaction with a hydrophobic agent so that hydrophobic moieties or residues become chemically bonded or attached to the surfaces of the starch granules . the preferred substitution reaction is carried out in a aqueous solvent or system , normally using an acid or acid anhydride hydrophobe , especially those of the formula where r is a di - or trimethylene group and r ′ is a straight or branched chain hydrocarbon group having from about 2 - 20 carbon atoms therein ; preferred agents have from about 6 - 12 carbon atoms and are selected from the group consisting of straight or branched chain alkyl , alkenyl or alkynl groups . the single most preferred hydrophobic agent is octenyl succinic and hydride ( osa ). the hydrophobic agent is used at a level so that the moieties thereof are present in the modified starch at a level of from about 0 . 1 - 50 % by weight , based upon th total weight of the starch taken as 100 % by weight , more preferably from about 1 - 30 %, and most preferably from about 5 - 25 % by weight . the substitution reaction is usually carried out at ph of from about 7 - 11 , more preferably from about 8 - 9 . the temperature should be from about 10 - 50 ° c . and more preferably from about 25 - 45 ° c . reaction times are variable depending upon the degree of substitution desired , but generally range from about 1 - 12 hours , more preferably from about 2 - 6 hours . it is normally preferred that the substitution reaction be conducted with continuous agitation . at the end of the reaction , the reaction mixture maybe neutralized with acid to a ph of from about 5 - 7 , more preferably of about ph 6 . thereafter , the starch products maybe water washed and then gelatinized in a hot aqueous system at a temperature of from about 50 - 150 ° c ., more preferably from about 50 - 85 ° c . the final step involves recovery and drying of the modified starch , which is preferably accomplished by spray drying . this step can be done after the metal substitution reaction to facilitate washing . because fully gelatinized starch takes up more water and takes more water to wash residual salts . the metal substitution reaction using the expanded or preswelled and hydrophobic moiety - substituted starches is relatively straightforward and generally involves merely contacting the starches with an aqueous dispersion of a selected polyvalent metal salt . this is followed by an appropriate degree of mixing or agitation to achieve the final metal ion substitution . it is not required nor preferred to heat the starch during the final metal substitution reaction , i . e ., the reaction is normally carried out at room temperature and pressure . as indicated above , a wide variety of polyvalent metals may be reacted with the preferred cross - linked starch products . these metals are generally selected from the group consisting of the polyvalent metals of groups 2 - 13 , inclusive . any suitable salt of these metals can be employed so long as the salt has sufficient dispersability in aqueous media . typical salts are the sulphates , chlorides , fluorides , bromides , oxides , carbonates , nitrates and phosphates , although this list is in no way limiting . the following examples set forth preferred methods for producing the starch derivatives of the invention . it is to be understood , however , that these examples are provided by way of illustration only , and nothing therein should be taken as a limitation upon the overall scope of the invention . this example describes the production of cold water swellable granular wheat starch cross - linked with stmp , substituted with osa and complexed with aluminum sulfate . in the first step , 100 parts ( db ) wheat starch was dispersed with 233 parts of water and 2 parts of sodium sulfate . the dispersion was warmed to 45 ° c ., whereupon 3 parts of 1m sodium hydroxide solution were slowly added with continued heating and stirring for 1 h . next , 3 parts of sodium sulfate and 2 parts stmp were added , followed by continued heating and continuous stirring for 16 h . the starch slurry was then adjusted to ph 9 . 0 with sodium hydroxide and then cooled to room temperature . ( 25 ° c .). 5 parts of osa were next added to the slurry . the slurry was heated for 4 h with continuous mixing and ph maintenance at between 8 . 0 - 9 . 0 . the reaction mixture was then neutralized with 1 m hcl , and cooled to 25 ° c . 5 parts aluminum sulfate were added to the neutralized mixture with mixing for 4 h . the starch was washed 4 times using 4000 parts of water per wash . the recovered starch was then completely gelatinized by heating at 75 ° c . for 30 min in 4000 parts of water . final product was recovered by spray drying . in this example , modified wheat starch products were prepared as set forth in example 1 , except that osa level was varied , namely , 1 , 2 , 10 , 25 % by weight respectively , as compared with 25 % in example 1 . in this example , modified wheat starch products were prepared as set forth in example 1 , except that the level of stmp was varied , namely , 0 . 1 , 0 . 5 , 1 . 0 % by weight respectively , as compared with 2 % in example 1 . in this example , modified wheat starch products were prepared as set forth in example 1 , except that level of aluminum sulfate was varied , namely , 0 . 1 , 1 , 2 , 10 % by weight respectively , as compared with 5 % in example 1 . in this example , modified wheat starch products were prepared as set forth in example 1 , except that the level of titanium oxide was 5 % by weight was used in lieu of aluminum sulfate . in this case a modified corn starch product was prepared . 100 parts corn starch were dispersed in 233 parts water with the addition of 10 parts sodium sulfate . the dispersion was warmed to 55 ° c . and 3 parts of an aqueous 1 m sodium hydroxide solution were slowly added with stirring for 1 hour at temperature . thereafter , 5 parts sodium sulfate and 2 parts stmp were added with continuous stirring over 16 hours at temperature . the starch slurry was adjusted to ph 9 with sodium hydroxide , and cooled to room temperature ( 25 ° c .). 5 parts osa were then added to the slurry and the ph was maintained between 8 . 0 - 9 . 0 for 4 hours with continuous stirring . the reaction mixture was then neutralized with 1 m hcl , and cooled to 25 ° c . 5 parts aluminum sulfate were added and mixed for 4 h . the starch was washed 4 times using 4000 parts of water per wash . the recovered starch was then completely gelatinized by heating at 75 ° c . for 30 min in 4000 parts of water . final product was recovered by spray drying .
improved starch - metal derivatives are provided which have excellent dry flow characteristics and ready dispersability in hot or cold water . the preferred derivatives comprise granules of starch which have been expanded or preswelled and cross - linked , followed by reaction with a polyvalent metal salt , especially salts of ca , mg , zn cu and al .