GB2297734A - Process for producing microcapsules - Google Patents

Abstract

A process for the production of food grade microcapsules comprises forming a primary water-in-oil emulsion from an aqueous medium, a liquid lipid medium and a primary emulsifier, the droplet size of the primary emulsion being less than 500 microns, adding the primary emulsion to an aqueous mixture to form a secondary water-in-oil-in-water emulsion which is then cooled to below the melting point of the lipid medium to cause the lipid to solidify and form microcapsules. The product is particularly useful in foods to incorporate functional components or small amounts of water into fatty foods.

Description

Process for Produclng Microcapsules This invention relates to a process for the production of microcapsules of fatty material which enclose an aqueous medium. The invention also relates to microcapsules so produced. The microcapsules may be used, for example, to impart desirable properties to foods.

Microcapsules are small capsules, usually of size less than a few hundred pm, which enclose a substance to protect the substance or simply contain it.

Microcapsules have many applications; for example they are used in the printing industry to encapsulate dyes for the production of carbonless copy paper, in the food industry to encapsulate flavours, additives, aromas, stabilizers etc, in the pharmaceutical industry, in the cosmetic industry, and the like.

Given the numerous applications for microcapsules, it is not surprising that the techniques for producing microcapsules are also numerous. For example, microcapsules may be produced by coacervation, spray drying, air suspension coating, extrusion, spray cooling or chilling, centrifugal extrusion, rotational suspension separation, inclusion complexing, and the like. Each of these techniques has advantages and disadvantages and have certain restricted areas of application. Jackson et al; 1991; Lebensm. - Wiss. u. -Technol, 24, 289-297 gives a review of these techniques as applied in the food industry.

In the food industry, when it is wished to encapsulate an aqueous medium in a capsule of fatty material, it is common to first form a water-in-oil emulsion from the aqueous medium and the fatty material. Suitable emulsifiers are used to stabilize the emulsion. The emulsion is then usually atomized and the aerosol subjected to spray drying to produce microcapsules having a capsule of a fatty material and an aqueous core. A disadvantage with this technique is that spray drying is expensive and, in certain cases, may result in thermal degrading of heat sensitive products.

Therefore a need for a simple procedure for producing microcapsules, which requires little in the way of heating, still exists.

Accordingly this invention provides a process for the production of microcapsules; the process comprising: (i) forming a primary, water-in-oil emulsion from an aqueous medium, a liquid lipid medium, and a primary emulsifier; (ii) reducing the droplet size of the primary emulsion to less than 500 microns; (iii) adding the primary emulsion to an aqueous mixture to form a secondary (water-in-oil)-in-water emulsion; (iv) simultaneously or subsequently cooling the secondary emulsion to below the melting point of the lipid medium to cause the lipid medium to solidify and form microcapsules; and (v) separating the microcapsules from the aqueous mixture.

The invention has the advantage of providing microcapsules in a simple manner which does not require significant thermal processing of the ingredients.

Also the uptake of the initial aqueous medium in the microcapsules is good; for example uptakes of up to 96% by weight have been achieved. Achieving a high degree of uptake is very important if the aqueous medium contains expensive ingredients; although less so if it does not. Further the process is sufficiently flexible such that adjustments to meet desired functional requirements of the capsules; for example high water uptake, high water content, capsule morphology, melting point and the like, are readily feasible.

Preferably, the weight ratio of aqueous medium to lipid medium in the primary emulsion is in the range of about 20:80 to about 70:30; for example 25:75 to 60:40. The amount of the primary emulsifier used is preferably less than 10% by weight of the primary emulsion; more preferably less than 5% by weight. For example, the amount of the primary emulsifier used is from about 0.5% to 2.5% by weight of the primary emulsion. Preferably the primary emulsifier has a HLB in the range of 1 to 8; more preferably 1 to 5.

The lipid medium preferably melts at a temperature above 20"C; for example between 25 and 70"C. Particularly preferred is a lipid medium which melts at between 30 to 400C. However for chilled or frozen foods such as ice creams, the lipid medium may melt at temperatures lower than 20"C.

In step (ii), the droplet size of the primary emulsion is preferably reduced to below 1 opt; for example from about 1 Fm to about 5 pm.

In steps (iii and iv), the secondary emulsion may be cooled simultaneously with its formation by providing the aqueous mixture at a temperature below the melting point of the lipid medium. For example, the aqueous mixture may be at a temperature at least 10"C less than the melting point of the lipid medium; preferably 15 to 200C less.

In step (iv), the secondary emulsion may be cooled subsequent to its formation by lowering the temperature to at least 100C below the melting point of the lipid medium. The temperature may be lowered gradually or rapidly.

The process may also comprise the further step of incorporating a functional component into the aqueous medium which forms the primary emulsion.

A secondary emulsifier may also be incorporated in the aqueous mixture in step (iii) to stabilize the secondary emulsion. Preferably the secondary emulsifier is a protein-based emulsifier having a HLB in the range of 8 to 18.

The invention also provides microcapsules produced by the process defined above. The microcapsules are particularly useful in foods to incorporate a functional component into fatty food and to incorporate small amounts of water into fatty foods. This permits the controlled release of water, or of the functional components, into lipid systems, the reduction of calories in fatty and confectionery products, and the alteration of the texture of fatty foods in desired ways.

Embodiments of the invention are now described, by way of example only.

As a first overall step in the process, a primary emulsion, which is a waterin-oil emulsion, is formed from a lipid medium, an aqueous medium, and a primary emulsifier.

The lipid medium may be any suitable fat; preferably a fat which has a major melting point above room temperature; for example above about 15 to 20"C, such that the lipid medium is substantially solid at room temperature. For chilled and frozen foods, the fat may have a lower melting point. Otherwise, in general, the particular fat selected is not critical. The fat may be of natural origin (for example vegetable, animal, marine, or mineral), a hydrogenated fat or oil of natural origin, an interesterified fat or oil of natural origin, or of synthetic origin (for example synthetically prepared triglycerides, polyol polyesters of fatty acids and alcohols, polycarboxylic acid polyesters and the like). If the microcapsules are intended for use in food, the fat should be edible and non-toxic.Specific examples of edible fats are cocoa butter stearine, hardened bread fat, high melting butter fat, butterfat, coconut fat, palm kernel fat, cocoa butter, cocoa butter substitutes or cocoa butter improvers, lard, and fat blends, or such vegetable oils when hydrogenated or such fats when interesterified. Otherwise the fat should be selected to best accord with the desired purpose to which the microcapsules are intended. For example, if the microcapsules are intended for use in chocolate, cocoa butter stearine or cocoa butter would be a suitable fat.

The aqueous medium may be water, a polar solvent, mixtures of water and polar solvents, and the like; in fact any aqueous substance or water miscible substance which it is intended to encapsulate. The aqueous medium may contain functional substances such as flavours, aromas, antioxidants, minerals, salts, colourants, texture modifiers, and the like. Alternatively, the aqueous medium may be a substance which imparts a desired property. Again, if the microcapsules are intended for use in food, the aqueous medium, including any functional substances, should be food grade. For example, the aqueous medium may be water, glycerol, a mixture of water and glycerol, to improve final quality, reduce fat content, and to aid processing of fat-containing foods; for example, in confectionery having a high fat content.

Of particular importance as functional components are natural antioxidants.

Owing to the toxicological and nutritional misgivings concerning the use of synthetic antioxidants, natural antioxidants are being used more and more.

However, natural antioxidants tend to be consumed very rapidly during their activity and it is necessary to use them in high concentrations. Also, they are often water soluble and lipid insoluble; for example ascorbic acid. However when natural antioxidants are microencapsulated, they are held in an aqueous phase and are released at a slower and more prolonged rate into fat containing foods.

The primary emulsifier is selected from emulsifiers which are able to provide reasonably stable water-in-oil emulsions. The primary emulsifier used will depend upon the lipid medium used and the aqueous medium used and may be selected by the skilled person accordingly. Usually, the primary emulsifier has an HLB in the range of about 1 to about 8. Examples of suitable emulsifiers are fatty acid esters of high molecular weight alcohols; lecithin; partial esters of polyalcohols such as glycerol mono- and di- stearates and oleates, sorbitol monostearates and oleates; sucrose fatty acid esters; and the like, Again, if the microcapsules are intended for use in food, the primary emulsifier should be food grade. For example, suitable food grade emulsifiers are glycerol monostearate and polyglycerol polyricinoleate.

The primary emulsifier may also be a mixture of emulsifiers if this leads to the formation of a better emulsion or provides certain desired properties. For example, a mixture of glycerol monostearate and polyglycerol polyricinoleate enables greater amounts of the aqueous phase to be incorporated in the primary emulsion than glycerol monostearate alone. The total amount of emulsifier used may be selected to achieve the desired properties of the primary emulsion but is preferably kept to a minimum; particularly if the microcapsules are to be used as food stuffs. Up to 10% by weight, based on the weight of the primary emulsion, of primary emulsifier is acceptable.

The amount of lipid medium and aqueous medium used to produce the primary emulsion may vary as desired within the constraints of the ability of the primary emulsifier to form and maintain a water-in-oil emulsion. Weight ratios of aqueous medium to lipid medium of 70:30 to 20:80 are possible.

The primary emulsion is formed by heating the lipid medium until it is liquid. The primary emulsifier is then added; or it may be added prior to heating of the lipid medium. The aqueous medium is then added to the lipid medium under mixing; for example vortex mixing. Suitably, the aqueous medium is heated to the temperature of the lipid medium to avoid cooling of the lipid medium upon addition of the aqueous medium. The emulsion is then subjected to procedures, for example sonification, high speed mixing, microfluidizing or homogenization, conventional in the art to reduce the droplet size to the micron range and to further stabilize the primary emulsion.

As a second overall step in the process, a secondary emulsion, which is an (water-in-oil)-in-water emulsion, is formed from the primary emulsion and an aqueous mixture. Although not necessary, the aqueous mixture may include a secondary emulsifier. This helps stabilize the secondary emulsion and reduces the loss of water from the microcapsules.

The secondary emulsifier is selected from emulsifiers which are able to provide reasonably stable oil-in-water emulsions. The secondary emulsifier used will depend upon the lipid medium used, the aqueous medium used, and the primary emulsifier used. In any event, the secondary emulsifier should be different from the primary emulsifier so as not to destabilize the primary emulsion.

Usually, the secondary emulsifier has an HLB in the range of 8 to 18 and is water soluble. Suitable secondary emulsifiers may be selected by the skilled person as necessary from, for example, polysaccharides, cellulose derivatives, protein based emulsifiers and the like. Again, if the microcapsules are intended for use in food, the secondary should be food grade. For example, protein based emulsifiers such as caseinates, gelatine, albumins are suitable food grade emulsifiers. Sodium caseinate is particularly suitable.

The secondary emulsifier may also be a mixture of emulsifiers if this leads to the formation of a better emulsion or provides certain desired properties. Again the total amount of secondary emulsifier used may be selected to achieve the desired properties of the secondary emulsion but is preferably kept to a minimum; particularly if the microcapsules are to be used as food stuffs. Up to 10% by weight, based on the weight of the secondary emulsion, of secondary emulsifier is acceptable. If obtaining a high water uptake is not essential, the secondary emulsifier may be omitted entirely.

To form the secondary emulsion, the primary emulsion is added to the aqueous mixture of the water and optionally the secondary emulsifier under stirring. If desired, the mixture of water and the secondary emulsion may contain other aqueous components. The amount of water used is not critical except it should be sufficient to form the secondary emulsion. The use of an excess of water has the advantage of providing a safety factor. Also, if rapid cooling of the primary emulsion is desired, an excess of cold water is necessary.

The microcapsules may be produced from the secondary emulsion by at least two methods. In the first method, the aqueous mixture is at approximately the same temperature as the primary emulsion upon addition of the primary emulsion. Then, once the secondary emulsion has formed, the secondary emulsion is cooled to below the melting point of the lipid medium. This causes the lipid medium to harden and form capsules around the aqueous phase of the primary emulsion.

In the second method, the aqueous mixture is at a temperature below the melting point of the lipid medium. Then, the primary emulsion is rapidly cooled upon being added to the aqueous mixture; again causing the lipid medium to harden and form capsules around the aqueous medium of the primary emulsion.

Once the lipid medium has hardened to form the microcapsules, the microcapsules are separated from the aqueous mixture; for example by filtration.

Any suitable filtration technique which is able to separate particles of size of a few microns from a liquid may be used. The filter cake is conveniently washed to remove excess secondary emulsifier and collected. The filter cake is made up of the microcapsules which may then be dried and stored.

The first method offers the advantage that small, spherical microcapsules form. However, it is found in some systems that the uptake of water in the microcapsules may be lower. In the second method, the uptake of water may be higher but the manner in which the emulsion is mixed may need to be more carefully controlled. In particular, if the secondary emulsion is not stirred sufficiently, large aggregates may form; as opposed to microcapsules. On the other hand, if the stirring is too intense, very small capsules with a low overall water content may result. For any particular system, the optimum stirring parameters may be rapidly determined.

The microcapsules produced by the process have an average diameter of from several tenths of a micron to a few thousand microns; for example from 0.5 to 5000 microns, preferably from 10 to 1000 and especially from 20 to 750 microns.

The following examples further illustrate the invention. Parts, ratios and percentages are given by weight unless otherwise stated.

Example 1 0.2 g of glycerol monostearate are dissolved in 5.9 g of cocoa butter stearine at 50"C. 3.9 g of water are added slowly while vortexing followed immediately afterwards by sonification using an MSE soniprobe (obtained from Fisons Instruments Ltd, Gatwick Road, Crawley, West Sussex, England) which has a 2.5 cm probe. 10 g of a primary water-in-oil emulsion is obtained.

A secondary (water-in-oil)-in-water emulsion is prepared by adding the primary emulsion dropwise to 200 ml of a 2% solution of sodium caseinate in water at 50"C under stirring. A fine dispersion of water-in-oil droplets in the water results. There is minimal coalescence.

The temperature is then lowered to 4"C over 10 minutes to cause the cocoa butter stearine to solidify. The emulsion is then filtered under vacuum through a glass fibre filter in a precooled filtration unit and the filter cake is washed with ice cold distilled water to remove excess caseinate. The microcapsules so produced are dried in a desiccator over silica gel at 40C. The size of the microcapsule ranges from 25 to 500 pm diameter with the majority having a diameter from 60 to 200 ,um. The water content of the microcapsules is determined by Karl Fisher titration to be 27% of the initial water added to the primary emulsion.

Example 2 The procedure of example 1 is repeated except that a mixture of 0.04 g of glycerol monostearate and 0.02 g of polyglycerol polyricinoleate (obtained from Croda Surfactants Ltd, Leek, Staffordshire, UK) is used as the primary emulsifier.

The size of the microcapsule ranges from 25 to 500 pm diameter with the majority having a diameter from 60 to 200 pm. The water content of the microcapsules is determined by Karl Fisher titration to be 57% of the initial water added to the primary emulsion.

Example 3 The procedure of example 2 is repeated for the formation of the primary emulsion. A secondary emulsion is prepared by adding the primary emulsion dropwise to 200 ml of a 2% solution of sodium caseinate in water at 1 00C under stirring. Stirring is carried out using (a) a magnetic stirrer, (b) a high speed shear mixer, or (c) a variable speed paddle stirrer at medium speed (all obtained from IKA Werke, Janke & Kunkel GmbH, Staufen, Germany). The morphology of the capsules produced in each case is determined using confocal scanning laser microscopy.

Mixer Water Uptake, Morphology Magnetic stirrer 96 Large irregularly shaped aggregates High speed, high shear 52 Small spherical microcapsules; mixer some very fine Variable speed paddle 79 Uniform microcapsules, no mixer aggregates Hence, although a high water uptake is achieved, the magnetic mixer did not sufficiently disperse the primary emulsion in the aqueous mixture. The high speed, high shear mixer tended to over mix the emulsion resulting in small droplets and a lower water uptake.

Example 4 Example 3 is repeated using hardened bread fat, cocoa butter stearine, high melting point butterfat fraction (obtained from St Ivel Ltd, Wootton Bassett, Swindon, Wiltshire, UK), and a low melting fat blend which has the following NMR profile:- N10:28.8, N20:1 1.0, N25:6.5, N30:2.8, N35:0.7, N40:0.0 (also obtained from St Ivel Ltd) and varying amounts of the primary emulsifier polyglycerol polyricinoleate and the water. The primary emulsion is prepared at 72"C for the hardened bread fat, 650C for the cocoa butter stearine, 60"C for the high melting point butterfat fraction, and at 50"C for the low melting fat blend.

Varying amounts of the secondary emulsifier are used and the secondary emulsion is mixed using a variable speed paddle mixer at medium speed. The results obtained are as follows:

Fat Water polyglycerol Sodium Water Water uptake added % polyricinoleate caseinate content % % % HBF 40 0.2 1.0 34.8 86.9 HBF 60 0.5 1.0 25.5 42.1 HBF 40 0.2 0.0 33.2 83.0 HBF 60 0.5 0.0 20.0 34.6 CBS 40 0.2 1.0 23.1 57.8 CBS 60 0.2 1.0 42.7 71.1 CBS 60 0.5 1.0 57.8 96.3 CBS 40 0.2 0.0 28.3 70.8 CBS 60 0.5 0.0 33.2 55.4 BFF 60 0.2 1.0 51.4 85.6 BFF 60 0.2 0.0 46.6 78.2 LMF 60 0.2 1.0 46.2 76.9 LMF 60 0.2 0.0 41.8 69.6 In the table, HBF is hardened bread fat, CBS is cocoa butter stearine, BFF is high melting point butterfat fraction, and LMF is low melting fat blend.

Claims (11)

1. A process for the production of microcapsules; the process comprising: (i) forming a primary, water-in-oil emulsion from an aqueous medium, a liquid lipid medium, and a primary emulsifier; (ii) reducing the droplet size of the primary emulsion to less than 500 microns; (iii) adding the primary emulsion to an aqueous mixture to form a secondary (water-in-oil)-in-water emulsion; (iv) simultaneously or subsequently cooling the secondary emulsion to below the melting point of the lipid medium to cause the lipid medium to solidify and form microcapsules; and (v) separating the microcapsules from the aqueous mixture.

2. A process according to claim 1 in which the weight ratio of aqueous medium to lipid medium in the primary emulsion is in the range of about 20:80 to about 70:30

3. A process according to any claim 1 or claim 2 in which the amount of the primary emulsifier comprises less than 5% by weight of the primary emulsion.

4. A process according to any one of claims 1 to 3 in which the primary emulsifier has a HLB of 1 to 8.

5. A process according to claim 4 in which the primary emulsifier is glycerol monostearate, polyglycerol polyricinoleate, or both.

6. A process according to any one of claims 1 to 5 in which, in step (ii), the droplet size of the primary emulsion is reduced to below 10cm.

7. A process according to any one of claims 1 to 6 in which, in steps (iii and iv), the secondary emulsion is cooled simultaneously upon its formation by providing the aqueous mixture at a temperature below the melting point of the lipid medium.

8. A process according to any one of claims 1 to 6 in which, in step (iv), the secondary emulsion is cooled subsequent to its formation by lowering the temperature to at least 100C below the melting point ofthe lipid medium.

9. A process according to any one of claims 1 to 8 in which a secondary emulsifier is incorporated in the aqueous mixture in step (iii) to stabilize the secondary emulsion.

10. A process according to claim 9 in which the secondary emulsifier has a HLBof8to 18.

11. A process for the production of microcapsules substantially as hereinbefore described with reference to the Examples.