Understanding the stabilisation of lactobacillus plantarum by drying

Abstract

In the context of the benefits of probiotics and the interest in incorporating these into foods, this thesis reports on research to address the challenges in drying, storage and particularly involving viability as well as protection of the bacteria. Two strains of Lactobacillus plantarum (A17 and B21) were chosen for study due to their potential as probiotics. The aim has been to investigate drying by convective heating using single droplet drying (SDD) and spray drying in conjunction with strategies to enhance viability. Protective effects of encapsulants were investigated using SDD and whey protein isolate (WPI) and skim milk provided the highest protection. This was attributed to formation of an outer capsular layer of WPI, with reduced rates of temperature increase, enhancing bacterial viability during drying. Higher survival in a matrix of native WPI at pH 7 corresponded to a more compact and globular protein structure. Exposure to pH 4 reduced survival of A17 where the proteins had a less compact structure: partial unfolding was seen with an increase in b-sheets and a complete loss of random coils. A lower degree of protein denaturation before spray drying, increases the ability of L. plantarum A17 to survive during spray drying. It is hypothesised that a unique layer-by-layer electrostatic mechanism is involved during encapsulation of L. plantarum A17 at pH 7. Cell survival during microencapsulation of bacteria with selected formulations was compared and the cells were effectively embedded in the whey protein layer during drying. The hydrophobic bacterial cells appear to be protected by attaching to hydrophobic regions of the proteins, minimising interactions of the proteins with each other. Effects of WPI were concentration dependant with the more hydrophobic B21 requiring only half the amount of the encapsulant to provide similar protection. The retention of viability during storage was evaluated in relation to the glass transition temperature (Tg) of the matrix. This was determined by dynamic mechanical analysis to be approximately 34 °C. Bacteria of both strains retained viability during storage for 8 weeks at 4 and 20°C with final cell counts of approximately 10 log CFU/g. Storing at 30 °C, in the vicinity of Tg resulted in reduced survival of both strains after two weeks of storage and at 50°C there was a large decline in numbers from the first week of storage. Capsules stored at 20 and 30°C showed virtually no deterioration in colour and the high cell counts meet the minimum requirement recently established by the International Scientific Association for Probiotics and Prebiotics. In conclusion, whey protein isolate, a natural dairy-based material provided adequate protection to L. plantarum A17 and B21 cells against the adverse conditions during spray drying and also demonstrated promising storage stability. It is proposed that WPI affords protection to A17 and B21 by a combination of mechanisms involving electrostatic and hydrophobic interactions as well as the glassy state. The use of WPI as structural elements and probiotic carriers provides effective protection and viability for the strains of L. plantarum during storage