hydroxypropyl starch acetate is prepared by reacting propylene oxide with starch dispersed in an organic solvent , preferably toluene . toluene is preferred because it is inert , water immiscible , high boiling , and a good solvent for the starch ether - ester . the starch is made alkaline by addition of an aqueous solution of sodium hydroxide . the reaction is catalyzed by means of the sodium hydroxide . the reaction mixture is maintained at a temperature ranging from 100 to about 110 ° c . the amount of time required for the reaction ranges from about 2 to about 4 hours . the hydroxypropyl starch is then acetylated by reaction with acetic anhydride . after the acetylation reaction is complete , hydroxypropyl starch acetate gum can be recovered from the reaction mixture by any of several alternative methods . in one method , a toluene - water emulsion is formed by adding 1 part by weight of warm water ( 40 ° c .) to 3 parts by weight of the reaction mixture . the water layer is then drained off . the toluene - gum solution is washed with water , neutralized with a 20 % aqueous sodium hydroxide solution , and again washed with water . the latex is dissolved in 95 % ethanol and then precipitated with water . the precipitate is heated in boiling water until all traces of solvent are removed . the remaining gum coagulates into a tough elastic mass . in another method , the reaction product is neutralized with 25 % aqueous sodium hydroxide . the resulting mixture is then washed with warm water . the toluene is thereupon removed by decantation and distillation . the latex is then dispersed in cold water and reprecipitated by heating or treating in boiling water . the coagulated gum is then dried in a forced air oven . in a preferred embodiment of the invention , 3 to 4 parts of propylene exide is reacted with 1 part of starch ( w / w ) in toluene . aqueous sodium hydroxide is added to the starch slurry for the purpose of accelerating the rate of reaction . the reactants are maintained at 100 ° to 110 ° c . for 2 to 4 hours . 1 to 2 parts acetic anhydride are added to the mixture after hydroxypropylation is completed . about 2 to 3 parts of crude hydroxypropyl starch acetate is recovered from the mixture . the crude gum is dissolved in 1 to 10 parts cold water ( 20 ° c .) and the hydroxypropyl starch acetate gum is precipitated from the cold water by heating to 35 ° c . or above . the hydroxypropyl starch acetate precipitate is dried at 60 ° to 80 ° c . in a forced air oven for about 4 to about 8 hours . 800 grams of pearl starch is dispersed in 9 , 000 m1 toluene in a pfaudler reactor . 320 m1 of 25 % aqueous sodium hydroxide is added with rapid agitation . 4 , 000 ml propylene oxide is then added . the reactor is then sealed and purged with nitrogen four times at 70 psi . the vessel is slowly heated from room temperature ( 25 ° c .) to 110 ° c . over a one - hour period . the mixture is held at 110 ° c . for 2 hours , and then cooled to 50 ° c . 1 , 600 g acetic anhydride is added to the mixture and the resulting solution is stirred for 2 hours at 50 °- 55 ° c . the mixture is then cooled to 25 ° c . five liters of warm water ( 40 ° c .) are added to the mixture . the resulting mixture is then stirred for 1 hour . the toluene - water emulsion which forms is allowed to separate into a toluene layer and a water layer , and the water layer is then drawn off . the toluene - gum solution is washed with five liters of warm water three times , then neutralized with a 20 % aqueous sodium hydroxide solution , and then washed once again to remove water soluble salts . the toluene gum solution is concentrated by steam distillation . the resulting concentrate ( 2 liters ) is dissolved in 2 liters of a 95 % ethanol solution and precipitated with water . the precipitate is a white fluffy material . the precipitate is heated in boiling water until all traces of solvent are removed , and the gum is coagulated into a tough elastic mass . the steamed gum contains 30 % moisture , and has a smooth satiny mouth - feel , both before and after drying . the material has an acetyl d . s . of 0 . 92 . it is easily dispersed in water at 20 ° c . and has a milky appearance . 50 g of pearl starch is dispersed in 400 ml toluene in a two - liter parr pressure reactor . 2 . 5 ml of 25 % aqueous sodium hydroxide is added along with 200 ml of propylene oxide . the mixture is heated to 110 ° c . the pressure will rise to 45 psi as the mixture is held at 110 ° c . as the propylene oxide is consumed , the pressure will fall to 20 psi . 50 ml acetic anhydride is added and the mixture is held at 55 ° c . for 2 hours , then cooled , and then neutralized with 25 % aqueous sodium hydroxide . the mixture is washed with warm water ( 50 ° c ). the toluene is then removed by distillation . the resulting gum is dispersed in 500 ml cold water ( 20 ° c ). then the dispersion is slowly poured into one gallon of boiling water . the gum coagulates and is thereupon skimmed from the water surface , and then dried in a forced air oven at 70 ° c . the gum has a smooth satiny mouth - feel with good elasticity . the gum remains an elastic cohesive mass in water at 40 ° c ., but is readily dispersible in water at 20 ° c . the acetyl d . s . is 0 . 74 . 1 , 000 g of pearl starch is dispersed in 10 liters of toluene in a 10 gallon pfaudler reactor . 400 ml of 25 % aqueous sodium hydroxide is added with rapid agitation . the reactor is sealed and purged with nitrogen three times at 45 psi . 5 , 000 ml propylene oxide is added under vacuum . the reactor is slowly heated from room temperature ( 25 ° c .) to 110 ° c . over a period of 1 hour . the mixture is held at 110 ° c . for 2 hours . the mixture is then cooled to 25 ° c . 1 , 200 g acetic anhydride is added and the mixture is stirred at 55 ° c . for 2 hours , then cooled to 25 ° c . the mixture is neutralized with 30 % aqueous sodium hydroxide , washed six times with warm water ( 60 ° c . ), and allowed to stand for 6 hours . upon standing , the mixture separates into three distinct layers - clear toluene , gum - toluene emulsion , and water . about six liters of clear toluene are decanted from the surface of the gum emulsion . the gum - toluene emulsion layer is then steamed for two hours to yield a stiff latex . the latex is dispersed in cold water ( 20 ° c .) and reprecipitated by heating to 60 ° c . the gum is then dried for 8 hours in a forced air oven at 70 ° c . five pounds of elastic off - white material is obtained . this material has a d . s . of 0 . 50 . this example shows the preparation of chewing gum from the hydroxypropyl starch acetate base described in examples , i , ii , and iii . the above ingredients are blended into a smooth tacky mass at 80 ° c ., and divided into individual gum balls while still warm . the chewing gum ball became firm and brittle upon cooling , softened readily in the mouth and had excellent chewing properties . the chewing test rating performed in accordance with the method generally accepted in the trade for the chewing gum of example iv is shown in the following table and compared to commercial gum : table i__________________________________________________________________________gum chewing testsample no . 1 2 3 4__________________________________________________________________________ commercial hydroxypropyl hydroxypropyl hydroxypropyl gum , starch acetate starch acetate starch acetate sticks m . s . 4 . 6 m . s . 4 . 4 m . s . 4 . 8 d . s . 0 . 92 d . s . 0 . 74 d . s . 0 . 50 initial stagestiffness 13 16 16 16adhesion 8 9 9 9 intermediateconsistency 10 10 8 6 final stagesmoothness 9 11 11 8stiffness 12 9 8 6lift 11 11 10 7freeness to teeth 10 9 9 9cohesion 11 12 12 13stretch 12 12 11 10taste 10 10 10 10__________________________________________________________________________ the numeral 10 is optimum in the foregoing table and both above and below 10 are less than optimum . table ii______________________________________numerical evaluation 0 10 20______________________________________initial flexibility brittle good floppy stiffness raglike medium leathery adhesion crumbly gummy tackyintermediate firmness of sloppy firm as low point finalfinal smoothness rough silky too slippery stiffness soupy medium leathery lift plastic round rubbery freeness tacky free cohesion waxy slight wet too wet paper papery stretch stringy full and none wide ( tears ) object - taste ionable none______________________________________ the term &# 34 ; medium &# 34 ; as used above defines a chew which is between softness and firmness . consists of the first few bites before the gum is hydrated or warmed . flexibility describes the condition of the piece even before chewing and is self - explanatory . adhesion when too high becomes tacky due to glucose rather than base and its low extreme is crumbly . is a temporary softness due to hydration before all sugar is removed and the cud approaches straight base . firmness at low point is the consistency of the base at maximum softness as compared to its consistency at the final stage of the chew . smoothness describes the feel of the cud ( when flattened against the roof of the mouth ) to the tongue . slippery or slimy is excessive smoothness . a satiny or sily feeling is desirable . stiffness is the opposition of the cud to the teeth at the start of the bite . lift is the capacity to obliterate or round out indentations in a cud caused by the tooth . it may be described as resilience or spring - back . the absence of lift is plasticity or waxiness . cohesion is the opposition offered to the teeth at the end of the bite as compared to the start . ( the increment of increase in stiffness from the starch of the bite to the end .) it is desirable to have the increase in stiffness graduate throughout the bite rather than a sharp increase at the end of the bite . stretch is that property which enables a sample to be pulled out in a ribbon to arm &# 39 ; s length . taste -- a measure of the desirable lack of this quality in the base .
hydroxypropyl starch acetate having a degree of molecular substitution of hydroxypropyl radicals of about 3 to about 6 and a degree of substitution of acetyl groups of about 0 . 5 to about 0 . 9 . the gum is dispersible in cold water , has good elasticity , excellent mouth feel , and excellent chewing quality .