CA1062581A - Method of making gels based on biologically produced saccharides - Google Patents

Abstract

METHOD OF MAKING GELS BASED ON
BIOLOGICALLY PRODUCED SACCHARIDES

Abstract of the Disclosure Gels are prepared from a biologically produced beta-1,3-glucan-type polysaccharide by lowering the pH of a basic aqueous solution of said polysaccharide. The pH adjustment is accomplished via an encapsulated particulate organic acid.

Description

~62581 Towle Case 3 This invention relates to the preparation of gels of certain polysaccharide materials. In particular, it relates to the prepa-ration of gels suitable for a variety of uses from a biologically produced beta-1,3-glucan-type polysaccharide.
In U.S. patent 3,822,250, there is ~sclosed a method of preparing a beta-1,3-glucan-type polysaccharide material by culti-vation of certain microorganisms. The microorganisms of interest in this connection are:
(a) Agrobacterium radiobacter - ATCC 6466: This strain is 10 available from American Type Culture Collection under the acces-sion number of ATCC-6466;
(b) Agrobacterium radiobacter - Strain U-l9: This strain is a mutant derived from the parent strain ATCC-6466 by irradia-tion with ultraviolet rays in a conventional manner,and has a unique property in that it produces substantially no other poly-saccharide. A subculture of this strain has been deposited with Institute for Fermentation, Osaka, Japan, under the accession number of 'iIFO-13126";
(c) Alcaligenes faecalis var. myogenes, Strain K, with 20 N-methyl-N'-nitro-N-nitrosoguanidine.
Inasmuch as these microorganisms are known entities, further de-scription of them is not deemed necessary here. For a more de-tailed description of the microorganisms, their cultivation, and the polysaccharide produced thereby, reference can be had to the aforesaid U.S. patent 3,822,250.
The polysaccharide prepared by cultivation of the specified microorganisms is, as stated above, of the beta-1,3-glucan type.
Hereinafter, reference to "beta-1,3-glucan" or to "the polysac-charide" can be taken to mean such a compound prepared by the ac-30 tion of these microorganisms.
The polysaccharide is substantially insoluble in neutralwater at temperatures below about 50C. although it is swellable.

In water at acid pH levels, it forms gels and at pH levels above about 10.5 it is soluble.

- 2 -A highly interesting property of this polysaccharide is its capacity to form gels possessing excellent water-holding and flavor-binding abilities. The polysaccharide is also non-toxic and pharmacologically and nutritionally inert. Gels prepared therefrom can be taken into the human body safely, affording to them a variety of applications in the food industry.
The above-referenced U.S. patent 3,822,250 discusses at great length the formation ofgels from the polysaccharides contem-plated by this invention and the utilization of such gels in food-10 stuffs. The technique taught in that reference for gelling thepolysacchaxide is by heating. The reference teaches that if the polysaccharide is heated to a temperature between about 50 and 100C., a gel is formed very readily which has excellent gel strength and freeze-thaw stability, is thermally irreversible and retains its favorable properties over a wide pH range from about 1 to 11.5.
British patent 1,379,406 teaches the preparation of gels from beta-1,3-glucan by a procedure which involves dissolving the polysaccharide in basic aqueous medium and removing the base by 20 diffusion, e.g.,dialyzing or by neutralization with an acid. Gels can be prepared by this technique in the form of films, thin-walled tubes, filaments or globules. In the gelling process, the basic polysaccharide solution is brought into contact with the acid, whereupon neutralization and gelling take place substantially immediately.
Both of the techniques taught by the prior art are subject to certain objections. There are many instances when heating to effect gelling is an impractical nuisance which it is desirable to avoid if possible. The acid gelling technique is subject to the 30 objection that, except for very thin configurations, it is not useful for forming continuous bodies of gel of any significant size. This is due to the difficulty encountered in dispersing the acid throughout the solution uniformly before neutralization of the base begins. To prepare a continuous body of gel, it is necessary 106258~
for neutralization and gelling to take place substantially simul-taneously and uniformly throughout the solution.
It is an object of this invention to provide a technique for gelling the beta-1,3-glucan polysaccharide which overcomes some of the objections just cited. It is a further object to prepare gels having the same favorable combination of properties as those taught by the prior art as well as other properties which are im-provements over those possessed by prior art gels.
In accordance with this invention a technique has been de-10 veloped whereby a solution of the beta-1,3-glucan can readily be gelled to form a continuous, solid, shaped mass. Briefly, the technique comprises preparing a solution of beta-1,3-glucan in basic aqueous solution having pH greater than about 10.5 and in-corporating into this solution a sufficient amount of a water-soluble, normally solid organic acid to effect gelling of the poly-saccharide, said acid, at the time of incorporation, being encap-sulated in a polymer matrix which is water-soluble but which dis-solves at a rate slow enough to permit uniform dispersion of the encapsulated particles throughout the polysaccharide solution be-20 fore the polymer matrix dissolves.
When proceeding according to the method of this invention,the gel can be formed while the polysaccharide solution is in a quiescent state. The entire solution is neutralized substantially simultaneously, leading to a smooth, continuous, uniform body of gel. By contrast, if an uncoated acid is stirred into a solution of the polysaccharide, it begins to dissolve and neut-alize the base immediately, creating a situation where the pH is not uniform throughout the solution and gelling is therefore not simultaneous throughout. In this latter situation a continuous, uniform body 30 of gel is not formed; rather, the gel forms in strings or in dis-crete particles such as the synthetic fish roe type of product de-scribed in the British patent cited above.
As mentioned hereinabove, the polysaccharide dissolves in water only at a pH of about 10.5 or higher. To raise the pH to this level, any base can be used, provided the salt which it forms wi~h the acid is compatible with the end use envisioned for the gel. Alkalies such as sodium, potassium or lithium hydroxide;
basic salts such as trisodium phosphate, tripotassium phosphate, - sodium carbonate and potassium carbonate; and ammonium hydroxide can be used. For food uses, the tri-substituted phosphates are pre-ferred.
Gelling begins substantially immediately when the pH of the system drops below about 10.5. Organoleptically, however, it is 10 preferable for food uses to have a pH of about 3 to 5. To accomp-lish pH reduction to this point very rapidly, an acid having an ionization constant of about 1 x 10-2 to 1 x 10-6 is required.
~oreover, in order to be readily encapsulated, it is preferrea that the acid be normally solid and it should have a particle size no greater than about 40 mesh. Any normally solid, water-soluble organic acid meeting these qualifications can be employed, subject again to the limitation that the salt formed by the acid when neu-tralized with the base used to solubilize the polysaccharide must be compatible with the intended end use of the gel. Exemplary 20 acids which can be employed include citric acid, tartaric acid, adipic acid, succinic acid, maleic acid and fumaric acid.
- In order to be operative in the process of this invention, the acid must be encapsulated within a water-soluble polymer matrix. It is not satisfactory simply to blend the acid and ti~e water-soluble polymer as, e.g., by a coprecipitation technique.
Coprecipitation results in the acid being distributed throughout the polymer. This type of product releases the acid too rapidly, allowing it to begin its neutralization and gelling action before it is uniformly distributed throughout the polymer solution. In 30 such a case, gelling does not take place simultaneously throughout the solution and a continuous, coherent gel body does not result.
For most applications the acid is preferably coated with about 10 to 50% of its own weight of the coating polymer. Higher add-on levels can retard the dissolution of the acid excessively, ~2581 thereby increasing the time required to achieve gelation. Lower add-on levels do not always give sufficient retardation time to promote uniform gel formation.
Encapsulation of the acid can be accomplished by several methods. A presently preferred technique is to dissolve the water-soluble polymer in an organic solvent which is a non-solvent for the acid and to spray this onto the dry particulate salt using a fluidized bed coater. Any o~her method which completely and uni-formly coats the acid particles can also be used.
The acid can be encapsulated in substantially any water-soluble polymer which is compatible with the end-use contemplated.
Gelatin, polyvinyl alcohol, gum arabic, starch and hydroxypropyl cellulose are typical polymers which can be used to advantage. A11 are essentially inert to most other materials and do not greatly affect the properties of gels for most purposes.
Gel preparation can be carried out by adding the encapsu-lated acid to a solution of the polysaccharide in aqueous base and stirring to disperse it substantially uniformly therethrough.
Stirring is discontinued as soon as the acid is completely dis-20 persed and the solution is allowed to sit while the acid dis-solves and neutralizes the base and forms the gel. The viscosity o the solution maintains the encapsulated particles in suspension so that the neutralization proceeds substantially simultaneously throughout the entire body of liquid.
In an attractive alternative technique, the polysaccharide, base and encapsulated acid can be added simultaneously to water.
This technique can be used to make powdered instant dessert mixes, e.g., which can be prepared for table use by simply mixing a powder with a specified amount of water. In this case, it is pref-30 erable to have the encapsulant coating weight on the high side ofthe specified range to assure that the base and the polysaccharide dissolve first.
If large quantities of sugar or other polyol are to be incor-porated into the gel, the polysaccharide must be dissolved in the 1~62S81 basic water prior to the addition of the sugar or polyol. Alterna-tively, the sugar can be encapsulated in the same manner as is the acid. It is found that the polysaccharide will not dissolve in the presence of these materials. After the gel is dissolved, the sugar and the encapsulated acid can be added simultaneously as soon as the polysaccharide is dissolved. If the latter ingredi-ents are added simultaneously, care must be taken to assure that the sugar dissolves prior to the acid in order to assure uniform-ity of taste throughout the gel. For this reason the encapsulant 10 polymer coating will be on tne high side of the range when pro-ceeding in this manner. If the sugar is also encapsulated, the coating will be on the low side so that it dissolves more quickly than the acid.
As with most gelling processes, the strength of gels prepared according to the process of this invention is related to the con-centration of the polysaccharide in solution. The strength or quality of gel desired similarly depends upon the use for which it is intended. For food uses, the gel strength and the related tex-ture of the gel is an important factor in organoleptic acceptance 20 of the gel. For other uses, a gel that is stronger or weaker than the optimum for food use may be needed. The concentration of poly-saccharide in the gels can vary between about 0.1 and 10%, prefer-ably between about 0.2 and 5%. The practical minimum concentration of the polysaccharide is the minimum that will yield a useful gel while creating a solution of sufficiently high viscosity to main-tain the encapsulated acid particles in suspension under quiescent conditions until the encapsulant polymer dissolves.
In the following examples, the coated asidulant was prepared by suspending the solid, particulate acid powder, screened to 60 30 mesh or less, in a stream of warm air in a fluidized bed coater.
A 5% solution of the polymeric encapsulant was sprayed into con-tact with the acid. The rate of spraying was regulated to strike the proper balance required to effect uniform coating and drying while avoiding agglomeration of the coated particles. Portions 1~6Z58:1 were withdrawn at appropriate intervals to achieve add-on levels of 20, 30 and 40% based on the weight of the acid.
Gel strength was determined with a conventional Bloom gelom-eter (Precision Scientific Co., Chicago, Ill.). A one-inch diam-eter plunger set for 4 mm. travel is forced into the gel surface.
The force in grams required to depress the plunger 4 mm. into the gel is taken as the gel strength and is expressed as "grams Bloom".
If large quantities of sugar or other polyols are to be in-corporated in the gel, the polysaccharide must be dissolved in the 10 basic water first, then the sugar or polyol added, the encapsulated acid being added with it or subsequently.
The invention is exemplified in the following examples. Parts and percentages are by weight unless otherwise specified.
Example 1 A blend consisting of 1 part polysaccharide, 0.5 part tri-sodium phosphate (less than 40 mesh particle size) and 0.5 part of citric acid, coated with hydroxypropyl cellulose to an add-on level between 30 and 40~ was added to 100 parts of water at room temper-ature and stirred for about one-half minute to disperse the par-20 ticles throughout. The resulting viscous mixture was poured intoan appropriate mold, set aside and allowed to gel undisturbed. Gel strengths and pH were measured periodically. Pertinent data are recorded in Table I.
Table I
Time of Standing (Min.) Gel Strength (g. Bloom) pH

30 Gels were also prepared using hydroxypropyl cellulose coated tar-taric acid and adipic acid with similar results.
Example 2 A blend consisting of one part polysaccharide and 0.2 part sodium hydroxide was added to 100 parts water and stirred for about 30 seconds.

10625~1 To this solution was added with stirring 0.5 part tartaric acid coated with hydroxypropyl cellulose to an add-on level between 10 and 20%. Stirring was continued for 5-10 seconds after which the mixture was set aside to gel undisturbed. Gel strength and pH
were measured periodically. Pertinent data are recorded in Table II.
Table II
Time of Standin~a (Min.) Gel Strength (g. Bloom) pH

Example 3 A blend consisting of one part polysaccharide and 0.5 part trisodium phosphate was added to 100 parts water and stirred for 30 seconds. To this solution was added with stirring a mixture consisting of 0.5 part hydroxypropyl cellulose coated citric acid (10-20% add-on), 5 parts sugar and about 0.10 part of fruit flavor along with several drops of appropriate food coloring. After stir-20 ring for 5-10 seconds this mixture was set aside to gel. The firm gel resulting after about one hour was easily unmolded to give a free standing dessert gel. This gel was easily spoonable and had a pleasing consistency, taste and mouth-feel.
Example 4 A dessert gel formulation was prepared by dry blending the following ingredients:

Polysaccharide 13140 1 part Hydroxypropyl cellulose coated citric acid (30-40% add-on) 0.5 part 30 Trisodium phosphate (~ 20 mesh)0.5 part Artificial sweetener 0.02 part Fruit flavor 0.01 part Food color 1 drop One part of this blend was added to 50 parts water at room temper-ature, stirred for 30 seconds, and set aside to gel. After one hour the gel was found to have a strength of 90 g. Bloom. It could be easily unmolded to give a free standing gel of pleasing appearance. The gel had a pleasant spoonable consistency and good taste and mouth-feel.
_ g _ ~062~;B1 Example 5 A mixture of one part polysaccharide, 3 parts trisodium phos-phate, and 300 parts sugar was added to 200 parts water and warmed while stirring to a temperature between 70 and 80~F. The solution was removed from the heat and 5 parts citric acid coated with hy-droxypropyl cellulose (20-30% add-on) along with fruit flavor and coloring was added with stirring. The mixture was then poured into suitable molds and allowed to cool. After cooling the mixture had assumed the consistency of a firm gel which was easily unmolded 10 from the container. The gel had excellent clarity and resembled a standard 150 Sag grade high methoxyl pectin gel in texture, mouth-feel, and spreadability.
Example 6 One part of polysaccharide was dispersed in 200 parts water with stirring and to this mixture was added 3 parts trisodium phos-phate. After stirring to dissolve the salt and polysaccharide, 300 - parts sugar was added and the mixture was warmed to between 70 and 80F. Four parts of citric acid coated with hydroxypropyl cellu-lose to an add-on of 20-30% was then added along with fruit flavor 20 and color and the mixture was poured into molds. After cooling, a gel had formed which could be easily unmolded from the container to give a free-standing gel having excellent clarity and resembl-ing a standard 150 grade high methoxyl pectin gel in texture, mouth-feel, and spreadability.

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a process for preparing a gel from a beta-1,3-glucan-type poly-saccharide wherein said beta-1,3-glucan-type polysaccharide is dissolved in an aqueous solution having a pH greater than about 10.5, and water-soluble, normally solid organic acid is added thereto to lower the pH of said solution to effect gel formation, the improvement which comprises incorporating the acid into said aqueous solution in the form of particles which are encapsul-ated in a matrix, which matrix dissolves in water at a rate slow enough to permit the acid to be substantially uniformly dispersed throughout the solu-tion before said acid can dissolve in said aqueous solution.

2. A process according to claim 1 wherein the acid is a normally solid organic acid having an ionization constant of about 1 x 10-2 to 1 x 10-6 and being of a particle size no greater than about 40 mesh.

3. A process according to claim 1 wherein the acid is selected from the class consisting of citric acid, tartaric acid and adipic acid.

4. A process according to claim 3 wherein the encapsulating matrix is hydroxypropyl cellulose.

5. A process according to claim 1 in which sugar is dissolved in the aqueous solution after the dissolution of the polysaccharide.