Coacervation technique for microencapsulation

In a coacervation process, colloidal particles are separated from a solution and deposited around a targeted core material. It’s a popular process in encapsulating flavor oil but has also been used in fish oils, vitamins, enzymes, nutrients and preservatives. The process can be termed as simple where only one hydrocolloid is involved whereas complex coacervations involve two or more polymers.

The coacervation process has been described as a three step method comprising of phase separation, deposition and solidification (Desai and Park, 2005). In the first step, the coating material consisting of usually one or more polymers goes through a phase separation process and forms a coacervate. Core materials remain in the suspended or emulsified form and as soon as the wall material particles coalesce, it causes a decrease in surface area and total free interfacial energy of the system. This process favors the coacervate nuclei adsorption to the surface of the core material and a uniform layer or coating forms around the core particles. The final step causes the solidification of coating material, generally by a cross linking reaction using chemical, thermal or enzymatic methods. The formed microparticles are then collected by filtration or mild centrifugation followed by drying (Desai and Park, 2005; Madene et. al., 2006).

Microencapsulation of probiotic bacteria Lactobacillus E1 using the coacervation technique was investigated by Sun et. al. (2009). A double emulsion (w/o/w type) was made using diatomite, sodium alginate, dextrin and gelatin. Good storage stability at 10ºC over a period of 37 days was reported. A coacervation process followed by spray drying to encapsulate B. lactis and L. acidophilus was tried by Oliveira et. al. (2007).

Casein and pectin complex was used as the wall material. A higher shelf life of spray dried culture mass and very good in vitro acid tolerance was reported in this study.

The coacervation technique is advantageous for microencapsulation purpose because of its very high pay load of up to 99% and total control over the release of core materials (Gouin, 2004). The process can be carried out at room temperature making it particularly suitable for heat sensitive objects such as probiotic bacteria (Desai and Park, 2005).

A major disadvantage of using the coacervation technique is its high costing particle isolation procedure in the last step and the complexity of the technique. But it was suggested that the optimization of the last step and use of a spray dryer instead of fluidized or freeze dryer can reduce the overall cost dramatically (Gouin, 2004). Glutareldehyde is used often as the cross linking agent but it is not applicable in the food industry due to toxicity issues (Desai and Park, 2005). Instead of glutareldehyde, cross linking enzyme transglutaminase is used to address this problem (Truong et. al., 2004).

2.3.4.6Co-crystalization method

Co-crystalization is one of the simplest forms of encapsulation used mainly for fruit juices, essential oils, flavors and brown sugar (Madane et. al., 2006). The process is carried out by crystallization of a super saturated sucrose solution with the desired core materials dispersed into it.

The supersaturated sucrose solution is maintained at a high temperature to prevent crystallization. The heat is gradually released allowing the solution to crystallize with the core material already added into it. Following encapsulation into the crystal matrix, the product is dried and sieved as per the particle size requirements (Bhandari et. al., 1998).

No scientific publication related to microencapsulation of probiotic bacteria using co- crystalization technique was found, probably due to the high temperature required to maintain the supersaturated sucrose solution.

The process has advantages of being economic, with high pay load of up to 90%. This process has found maximum utility in the confectionary and pharmaceutical industries as because liquid products can be readily formed into tablets with simple processing. However, the co-crystallization process demands a tighter control rate of nucleation and crystallization and also a strict thermal balance during various stages of operations (Desai and Park, 2005).

2.3.4.7Molecular inclusion

This method is also known as inclusion complexation, which involves entrapment of smaller molecules inside the hollow cavity of a larger molecule (Hedges and McBride, 1999; Madane et. al., 2006). A very few such molecules exist which are appropriate for food applications. Cyclodextrins are commonly used for this type of encapsulation process. However, use of cyclodextrins in food formulations is restricted to certain countries only. Therefore this is not a popular method to encapsulate probiotic bacteria.

Cyclodextrins are formed from the enzymatic hydrolysis of starch molecules and during hydrolysis; linear fragments are created, which are finally joined to form circular structures with hollow cavities inside. Therefore, cyclodextrins can form inclusion complexes with small enough compounds which can fit inside these hollow cavities (Gouin, 2004; Madane et. al., 2006).

Molecular inclusion in β-cyclodextrin has found its popularity in encapsulating flavors,

aromas, vitamins and minerals. Hydrophobic vitamins (A, E and K) are good substrates for this type of encapsulation. The control release mechanism is also very interesting where the core materials are released when displaced by more favorable substrates. An example is the flavor burst in the mouth from cyclodextrin complex because compounds

found in the mouth are more favorable substrates for cyclodextrin (Gouin, 2004). It has

been reported that β-cyclodextrin molecules containing core compounds are highly heat

stable, can tolerate up to 200ºC temperature and highly resistant to chemical degradation (Hedges and McBride, 1999; Gouin, 2004).

No research publication was found which involved the molecular inclusion property of cyclodextrin to encapsulate probiotic bacteria.

Some of the major limitations of this molecular inclusion technology are very low payload (Gouin, 2004) and very high cost of raw materials (Madane et. al., 2006).

2.3.4.8Centrifugal extrusion technique

The centrifugal extrusion technique involves the pumping of the core and coating materials through separate tubes to the surface of a rotating cylinder. With the rotational motion of the cylinder, both materials are mixed and extruded as a fluid rod which is broken by the centrifugal force. The coating over the core material to form capsules is caused by the difference in surface tensions. Finally, the formed capsules are placed on a moving bed of starch, which absorbs excess moisture and cushions the impact (Desai and Park, 2005).

This is a popular method used in the food industry to encapsulate a wide range of ingredients such as flavors and seasonings (Desai and Park, 2005), aspartame, vitamins, methionine etc. (Gibbs et. al., 1999). Compared to extrusion, smaller particles are produced in this technique and a wide range of coating materials such as gelatin, alginate, carrageenan, starches, fatty acids, waxes etc. can be used in centrifugal extrusion (Gibbs et. al., 1999; Desai and Park, 2005).

Major advantages of this system as mentioned by Gouin (2004) are slower release properties of the capsules and higher throughput rate in comparison to the spray drying process.