Measurement of particle diameter of Lactobacillus acidophilus microcapsule by spray drying and analysis on its microstructure

Lactobacillus acidophilus, as a probiotic, is widely used in many functional food products. Microencapsulation not only increases the survival rate of L. acidophilus during storage and extends the shelf-life of its products, but also optimal size microcapsule makes L. acidophilus have an excellent dispersability in final products. In this paper, L. acidophilus was microencapsulated using spray drying (inlet air temperature of 170°C; outlet air temperature of 85'90°C). The wall materials used in this study were ß-cyclodextrin and acacia gum in the proportion of 9:1 (w/w), and microcapsules were prepared at four levels of wall materials (15, 20, 25 and 30% [w/v]) with a core material concentration of 6% (v/v). The microcapsule diameters were measured by Malvern's Mastersizer-2000 particle size analyzer. The results showed that the particle diameters of microcapsule were mostly within 6.607 'm and 60.256 'm and varied with 2.884'120.226 'm (the standard smaller microcapsule designated as <350 'm). Through comparison of microcapsule size and uniformity with different concentration of wall materials, we concluded that the optimal concentration of wall material was 20% (w/v), which gave microcapsule with a relatively uniform size (averaging 22.153 'm), and the number of surviving encapsulated L. acidophilus was 1.50 × 109 c.f.u./ml. After 8 weeks storage at 4°C, the live bacterial number was above 107 c.f.u./ml, compared with unencapsulated L. acidophilus, 104'105 c.f.u./ml. Through the observation of scanning electron microscopy, we found that the shapes of microcapsule were round and oval, and L. acidophilus cells located in the centre of microcapsule.