Survival and sensory assessment of probiotic bacteria on non-dairy foods : A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Lincoln University

Abstract

This research provides new understanding of the survival of probiotic bacteria on non-dairy foods, and the role of bacteria strains, non-dairy food matrices, storage temperature and relative humidity on bacteria viability and stability. This research also provides further understanding of effects of probiotic bacteria and storage on consumers’ acceptance and sensory characteristics of probiotic muesli. Survival of probiotic bacteria at room temperature for a period of storage is believed to be a critically technological challenge in many applications of non-dairy foods containing probiotic bacteria. Technological attempts using encapsulated cells to stabilise their viability in a food system during storage have showed limited improvements of bacterial survival. Indeed, limitations of this technique are exacerbated by apparent specific bacteria strain requirements. Strain-dependent properties of bacteria may induce complex reactions between bacteria, food matrix and various stresses existing in processing and storage, thus leading to the current challenges of maintaining bacterial viability. The first goal of this PhD research was to extend the knowledge of characteristics of potential probiotic bacteria (Bifidobacterium longum ATCC 15707, Lactobacillus acidophilus ATCC 4356 and Lactobacillus plantarum RC 30) that were grown in De Man, Rogosa and Sharpe broth containing 0.05% L-cysteine (MRSc broth) with or without 0.3% bile salt at different pH and temperatures. Cell surface properties of the three bacteria were also investigated by studying hydrophobicity and auto-aggregation (CHAPTER 3). Results showed variability of the three bacteria growing under different conditions and their cell surface properties. In order to understand the effects of recognized characteristics of the three bacteria on their survival on non-dairy foods, stability studies of these bacteria incorporated onto rice collet, peanut, coconut, raisin, oat, and wheat bran were conducted at room temperatures for four weeks (CHAPTER 4). Generally, B. longum ATCC 15707 showed a better survival than the other bacteria on the six food matrices at 20 °C and 20% relative humidity (RH) and 30 °C and 50% RH during the storage. While survival of L. acidophilus ATCC 4356 was poor compared to B. longum ATCC 15707 or L. plantarum RC 30. These stability observations were consistent with their growth characteristics at different temperatures. Additionally, coating onto oat and peanut maintained survival of the three bacteria was better than the other food matrices, whereas coating onto raisin gave the poorest survival under both storage conditions. High survival of B. longum ATCC 15707 led its selection to study cell membrane integrity following coating onto a food matrix (oat, peanut and raisin) and drying (at 20 or 50 °C) for 24 h (CHAPTER 5). Percoll Buoyant Gradient Density Centrifugation (PBDC) and Quantitative Fluorescence Microscopy (QFM) methods were developed and validated for this purpose and compared with typical plate counting. Increasing drying temperature caused cell membrane damage to some extent, with some cells being dormant and alive but not cultivable. Environmental Scanning Electronic Microscopy (ESEM) showed different food surfaces may play an important role in bacterial survival. In CHAPTER 6, consumers’ acceptance of muesli coated with B. longum ATCC 15707 was investigated to assess effects of probiotic bacteria and storage on sensory characteristics, thus providing useful information that will lead to the transfer of this research to the development of probiotic muesli or similar food products.