Extrusion technique for microencapsulation

Desai and Park (2005) described the extrusion process steps in a review. The core material is completely mixed with a molten carbohydrate mass. The carbohydrate mass may comprise of more than one compound. The adhered carbohydrate is then solidified by immersing into a dehydrating liquid and thereby entrapping the core material. The encapsulated matrix is then separated from the liquid bath, and dried using a suitable technology.

Extrusion is the oldest and most common technique for converting hydrocolloids into microcapsules (King, 1995). The property of certain biopolymers such as alginate, carrageenan and pectin to form gels in presence of minerals such as, calcium and potassium has been successfully applied to entrap probiotic bacteria using extrusion method. The chemical explanation behind such gel formation is the bonding of the multiple free carboxylic radicals by gelling ions and thereby formation of the gels (Champagne and Fustier, 2007). Various concentrations of sodium alginate solutions have been successfully used by several researchers to form tiny gel particles by spraying over a calcium chloride solution followed by filtering them out (Eikmeier and Rehm, 1987; Champagne et. al., 1992; Desai and Park, 2005). Sodium alginate is a heteropolysaccharide obtained from different species of sea algae. It is a polymer of L-

guluronic acid which gets bound with divalent cations and thereby bacterial cells suspended in alginate solutions get encapsulated into the resultant gel (Smidsrod and Skjakbraek, 1990).

Extrusion is a relatively new technique compared to some other popular forms such as spray drying and involves entrapping of core materials within a glassy carbohydrates mix (Madene et. al., 2006). The extrusion technique is highly preferred to coat volatile and hence unstable flavours and oils. This method has been reported to increase the shelf life of such compounds dramatically by prohibiting oxygen diffusion through the matrix (Gouin, 2004; Desai and Park, 2005; Madene et. al., 2006).

In a recent work, Li et. al. (2009) microencapsulated L. casei cells in a mix of sodium alginate and gelatin using extrusion process. This combination was reported to successfully protect the cells during gastro-intestinal transit but the beads were relatively larger in size with a mean diameter of 1.1± 0.2 mm. Starch was used in combination with alginate to encapsulate L. paracasei cells in another study by Babu et. al. (2009) and very good thermotolerance as well as salt, acid and bile tolerances have been reported. Another study using the extrusion technique was carried out where bacterial cell suspension was added into a mix of 20% non fat milk and 1.8% sodium alginate solution at a ratio of 1:1 (v/v) and found to be effective in protecting lactic acid bacteria against gastric environment (Ross et. al., 2008). Though not exactly for probiotics, chitosan in combination with alginate and extrusion technique was used to microencapsulate a bacteriophage (Felix O1), and very high acid resistance was reported

(Ma et. al., 2008). Free bacteriophages were completely destroyed at pH 3.7 within 5

min of exposure whereas only 0.67 log CFU reduction was observed for microencapsulated cells even when the medium pH was 2.4. The protective effect of microencapsulation against oxygen was studied by Talwalkar et. al. (2003). B. lactis and

L. acidophilus cells entrapped in calcium alginate-starch beads were grown aerobically

and then incubated into yogurt in presence of oxygen. All the 6 strains of tested L.

acidophilus were found nicely protected due to microencapsulation and 1.0 log higher

Some of the studies reported about microencapsulation of probiotic culture using extrusion technique followed by observing their behavior in different food products. Very recent ones worth mentioning are addition of L. acidophilus and B. bifidum into Kasar cheese by Ozer et. al. (2008) and evaluation of storage survibability of L. acidophilus and B. lactis into Iranian yogurt drink by Mortazavian et. al. (2008). The former one used both extrusion and emulsion techniques and found no significant difference between two in the result. The microencapsulated cells performed equally well with regard to count, proteolysis and sensory qualities than their counterparts in the free cell form. In the later case, the calcium alginate entrapped cells were extracted from the stored product and subjected to artificial gastric environment. The encapsulated cells showed 26.3 to 34.0% better survival rate in alleviated harsh conditions than the free ones. In another study, storage viability of probiotics microencapsulated into calcium alginate beads in presence of resistant starch was observed by incorporating into ice- cream (Homayouni et. al., 2008). After a period of 180 days at -20°C, encapsulated cells showed 30% better viability than the free cells. Shah and Ravula (2000) freeze dried calcium alginate beads prior to their incorporation into a frozen dessert. They concluded an improved viability of cells due to the encapsulation. In another study, alginate and Hi-Maize starch combination was used for microencapsulation and glycerol was added as a cryoprotectant before freeze drying by Sultana et. al. (2000). Eight weeks of storage study was performed after adding them in yogurt and only 0.5 log reduction in 8 weeks were reported. However, the control over bead size distribution was poor.

Bifidobacterium bifidum and B. lactis in encapsulated form were added in mayonnaise

having pH 4.42 and it was reported that the free cells died completely by 2 weeks but the encapsulated cells survived well up to 12 weeks and 8 weeks respectively (Khalil and Mansour, 1998).

A major advantage of the extrusion process is its shell-core character, where the encapsulated material is completely covered and protected by the wall material and residual or surface core material is removed by a dehydrating liquid, generally isopropyl alcohol (Gibbs et. al., 1999; Desai and Park, 2005). This is important when the presence

of residual core material may impart undesirable sensory properties to the product, (e.g. fish oils). The carbohydrate coating provides an excellent barrier property against oxygen and this fact is validated by some studies where flavor oils have been found to retain up to 5 years of shelf life (Gouin, 2004).

However, the main disadvantage has been reported as very poor payload of typically 8%. High payload i.e., higher ratio of core to shell material is important in economic as well as sensory perspectives. Attempts to increase the pay load had resulted into unstable microcapsules and leakage of core materials (Gouin, 2004). Other disadvantages worth mentioning are susceptibility of carbohydrates towards damage and structural defects, a larger particle size distribution, limited choice of wall materials etc. (Gouin, 2004; Madene et. al., 2006).