Abstract:
The invention relates to a starch composition containing a relatively high proportion of so called &#34;resistant starch&#34; structures. The compositions of the invention are obtrained by debranching an aqueous slurry of a gelatinized starch and retrograding the resulting product

Description:
This is a continuation of application Ser. No. 08/422,525, filed on Apr. 14, 1995, which was abandoned upon the filing hereof. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a starch composition containing a relatively high proportion of so-called &#34;resistant starch&#34; structures. 
     2. Background Information 
     It has been known for many years that starches, which commonly consist of varying proportions of amylose and amylopectin, may be subjected to an enzymatic treatment which reduces the content of the highly branched amylopectin and increases the content of short-chain amylose structures (see, for example, U.S. Pat. No. 3,729,380). It is also known that the products of such enzymatic treatment are more prone to retrogradation than normal starches and produce in this manner compositions containing increased amounts of resistant starch structures. Resistant starch (RS) is resistant to attack by alpha-amylase and may be used therefore as a low calorie bulking component in food compositions so contributing to the dietary fibre content of such compositions. 
     The treatment of the starch with the debranching enzyme takes place in an aqueous slurry which, for handling reasons, cannot be too concentrated. It would clearly be attractive therefore to be able to use a starchy composition at a higher concentration than normal while at the same time increasing the amount of RS formed as a proportion of the starchy material treated. EP 564 893 A describes and claims a process for the production of a RS product containing at least about 15% RS in which an aqueous slurry of starch containing at least 40% amylose is gelatinised and debranched by treatment with a debranching enzyme which hydrolyses 1,6-glucosidic bonds in the starch molecules, the debranched product subsequently being retrograded to produce RS. The optimum starch concentration in the aqueous slurry is said to be about 15% and the Examples of 564 893 A illustrate the process by using starch slurries containing 14% to 17% starch respectively. The starch starting material contains at least 40% amylose and for a corn (maize) starch containing 25% amylose it is shown that no RS is formed in the process. As the percentage of amylose in the starch starting material increases above 40% so does the RS in the product of the process until a starch containing 100% amylose is shown to produce a product containing 50.3% RS. 
     SUMMARY OF THE INVENTION 
     We have now found a method of producing a starchy composition containing a relatively large amount of RS structures from a starting material which contains less than 40% amylose and which can be debranched in slurries containing more than 15% solids. 
     Accordingly, the invention is a process for the production of a product containing RS structures in which an aqueous slurry of a gelatinised starch is debranched by a debranching enzyme and the resulting product retrograded and which is characterised in that the aqueous slurry contains at least 20% by weight solids and is a slurry of a partially degraded, root or tuber starch. 
     Preferably the product of the process comprises at least about 25% by weight RS, more preferably 30% by weight or more RS, particularly over 50% by weight RS. 
     By &#34;partially degraded starch&#34; is meant starch which has been treated to reduce the amylose/amylopectin chain length and hence the average molecular weight of the starch in question. The degradation treatment is conducted by known means and includes hydrolysis, either acid or enzyme catalysed, extrusion, oxidation or pyrolysis. One preferred partially degraded root or tuber starch is the so called &#34;acid thinned&#34; starch, particularly acid thinned potato starch, in which the potato starch granules are treated with acid in water suspension at sub-gelatinising temperature whereby the starch granules remain superficially unchanged. A second preferred partially degraded root or tuber starch is a maltodextrin, more preferably a potato starch maltodextrin which suitably has a DE (dextrose equivalent) of 1 to 19 preferably 2 to 10 more preferably 2 to 5. Such potato starch maltodextrins are produced from potato starch containing about 25% amylose. Other root or tuber starch maltodextrins which may be used in the process include tapioca maltodextrins. 
     The aqueous slurry of the maltodextrin used in the process according to the invention contains at least 20% by weight solids but may be more concentrated and may contain 30% by weight or more solids. 
     The debranching enzymes which may be used in the process are those known for this purpose ie pullulanases and isoamylases or mixtures thereof. Depending upon the nature of the specific enzyme chosen, the debranching may be carried out at a temperature in the range 25° to 75° C., preferably 50° to 60° C. , and at a pH in the range 3 to 7.5, preferably in the range 5 to 6. The treatment generally lasts up to 24 hours and at its conclusion the enzyme is deactivated by lowering the pH to 3 or less and/or by increasing the temperature to above that tolerated by the enzyme in question. 
     On completion of the enzymatic treatment the retrogradation of the amylose structures is effected by allowing the aqueous reaction product to stand, suitably at a temperature in the range 0 to 30° C. and for a period of up to three days, more preferably for a period of 1 to 36 hours. Following the completion of the retrogradation the composition is dried, eg. by spray drying, to give a powdered product containing up to 60% by weight RS particularly 25 to 55% by weight RS which is suitable for use as a component of various food compositions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be further described by reference to the following Examples in which the determinations of RS were made by the following method. 
     A 5% by weight suspension in water of the retrograded starch product is thoroughly homogenised in an acetate buffer solution. The acetate buffer is made by dissolving 8.2 g anhydrous sodium acetate in 250 ml of an saturated aqueous solution of benzoic acid, adding 4 ml of 1M calcium chloride and making up to 800 ml with distilled water before adjusting the pH to 5.2 with acetic acid and finally making up to 1000 ml with distilled water. 25 ml of the suspension are incubated with 1 ml pancreatic solution for 24 hours at 37° C. in a shaking water bath. The incubated suspension is next stirred into 119 ml of 95% alcohol, filtered and the filtration residue dried and its starch content determined acording to the relationship: ##EQU1## 
     The pancreatic solution is made by stirring 2 g pancreatin with 12 ml distilled water for 10 minutes, centrifuging and using the supernatant liquid as the pancreatic solution. 
     EXAMPLE 1 
     25 kg potato starch maltodextrin with a dextrose equivalent of about 3 was dissolved in water to give a concentration of 25% by weight. The suspension was heated to 50° C. and the pH adjusted to 5.0. After adding 25 ml isoamylase (product EN102 of Hayashibara Biochemical Laboratories Inc), the reaction was allowed to proceed for 20 hours after which time the temperature was increased to 80° C. and the pH reduced to 3.0 for 1 hour in order to deactivate the enzyme. The average Mol. wt. after debranching was 13680 as determined by GPC. The amount of oligomers with DP≦10 was 22%. 
     After readjustment of the pH to 5.0 the suspension was cooled to 4° C. and held at this temperature for 24 hours. The weak gel that formed was easily destroyed by stirring and the suspension finally brought to 15% by weight by dilution with water and spray dried. This product contained 28.9% RS compared with the maltodextrin before debranching which formed only 7.3% RS under comparable retrogradation conditions. 
     EXAMPLE 2 
     This example demonstrates the influence of the retrogradation conditions on the formation of resistant starch. The debranched maltodextrin used was essentially the same as that described in Example 1. 
     For 24 hours retrogradation the following conditions were compared: 
     concentration: 10%, 30% by weight 
     temperature: 4° C.; 25° C. 
     The following amounts of RS (% by weight) were detected 
     
         ______________________________________          10%  30%______________________________________ 4° C.    31.8   28.525° C.    27.2   26.4______________________________________ 
    
     This Example shows that retrogradation at 25° C. gives only a slightly lower figure for resistant starch compared with retrogradation at 4° C. The lower concentration of debranched starch (10% by weight) in the slurry leads to the formation of a slightly increased amount of resistant starch compared with the slurry containing 30% by weight debranched starch. 
     EXAMPLES 3 to 5 
     In Examples 3 to 5 the process used was the same as that described in Example 1 replacing the potato starch maltodextrin by other starting materials within the scope of the present invention. These starting materials and the experimental results obtained are given in the following table 
     
         __________________________________________________________________________       M. wt (Mw)             M. wt      R.S. %Starting       before             (Mw) after                   Oligomers                        before                              afterExamplematerial       debrancing             debranching                   DP ≦ 10                        debranching                              debranching__________________________________________________________________________3    Acid thinned       52,180             9340  4.5% 20.0  51.8potato starch4    Potato starch       106,500             11,530                   5.8% 4.9   28.0pyrodextrin5    Tapioca starch       129,600             19,770                    13% 2.4   25.6maltodextrin(DE approx 2)__________________________________________________________________________ 
    
     EXAMPLE 6 
     A regular corn starch maltodextrin with a dextrose equivalent of 3 was treated in the same way as that described for potato maltodextrin in Example 1. 
     After debranching the product had a average Mol. Wt. of 14390 as determined by GPC and contained 20% oligosaccharides with DP≧10. The product was retrograded at respectively 4° C. and 25° C. for 24 hours. The following amounts of resistant starch were determined: 
     
         4° C.=10.0% 
    
     
         25° C.=8.3% 
    
     This example shows that a debranched corn maltodextrin forms much less RS than a debranched potato maltodextrin under the same conditions of treatment. Because the amylose contents of corn starch and potato starch differ only marginally the amylose content cannot be the reason for the large difference in resistant starch. It seems probable therefore that debranching of the partially hydrolysed potato amylopectin liberates more linear fragments which are capable of being retrograded to resistant structures. 
     EXAMPLE 7 
     A waxy maize starch maltodextrin with a DE of about 4 was treated by the process described in Example 1. The debranched product was found to have a Mw of 12010 but after holding at 4° C. for 24 hours the amount of resistant starch (RS) which was formed amounted to only 0.3%.