Micro-encapsulation of the probiotic micro-organism Bifidobacterium lactis isolated from a bio-yoghurt starter culture, was carried out using a mixture of hydrated gellan and xanthan gums. Rheological studies showed that the gum mix was suitable for encapsulation of B. lactis, for incorporation into soft foods/beverages. The gel behaved as a non-Newtonian material, and the flow curve fitted well to the Herschel-Bulkley model. The average yield stress of the gum was 1.515 Pa, indicating gum stability, and the yield stress range was 1 Pa over a temperature range of 35-50°C. Almost constant minimum gum viscosity occurred between 46 and 61°C. Oval/round capsules were synthesized manually using a monoaxial gum flow through a 27.5 G bevelled needle, together with a superposed air stream (air knife technique). The diameter of the capsules, measured using laser diffractometry, varied from 20 to 2200lm, with 50% of the capsules having a diameter of £637 lm. Numbers of viable B. lactis in the capsules were estimated using high power ultrasound (20 kHz). By using a concentrated inoculum of B. lactis, microcapsules containing log 10 11-12 c.f.u. g )1 were synthesized. Apart from the anaerobic culturing of B. lactis, all other work was done in the presence of atmospheric oxygen. The organism exhibited a high degree of oxygen tolerance. A 21-day survival study of immobilized cells in 1 M sodium phosphate buffer (pH 7) stored at either 4 or 22°C indicated that B. lactis survived in excess of log 10 11 c.f.u. g )1 microcapsule. This technique represents a suitable means of supplying viable probiotics to the food and/or pharmaceutical industries.
Mathematical symbolsHerschel-Bulkley model: s 0 ¼ s y À j Àn y s 0 yield stress s y shear stress j y fluid consistency index n flow behaviour index