Functional foods containing probiotic bacteria (PB) and polyphenol (PP) antioxidants are gaining increased market leverage. Many probiotic strains, however, exhibit low viability, because of exposure to high acidity and oxidative stress during industrial processing, storage and gastric transit. This subsequently affects their adhesion to the gastrointestinal (GI) tract. The survival of PB in foods and their attachment to the GI tract are vital for conferring the desired health benefits of PB. The health-benefits of PPs, as well as their ability to reduce oxidative stress in foods, are well recognised. PPs are also sensitive to environmental factors, including heat, oxygen and light. It is of great interest to establish a combination of PPs and PB for delivering their maximised health-promoting properties.

The main objective of this study was to investigate the efficacy of apple skin PPs on the adhesion and survival of PB in a model milk drink system. Apple skin PP extracts, prepared using either ethanolic or aqueous extraction methods, were used as sources of PPs. These two apple skin PP extracts were analysed in terms of their total extractable polyphenol content (TEPC), total antioxidant activity (TAA), pectin fibre and vitamin C contents. To elucidate the effect of individual PP compounds on the viability of PB, both the crude apple skin PP extracts and commercially available purified PP compounds were added to milk drinks containing PB Lactobacillus acidophilus, which were then subjected to 50-day storage at 4° C. The influence of the two apple skin extracts and purified PPs on the adhesion of L. acidophilus was evaluated using a crystal violet attachment assay that utilised hydrophilic tissue culture treated microtitre plates as the attachment surface. Co-extrusion technology was also used to microencapsulate PB alone and co-encapsulate PB and either of the two apple skin PP extracts, aiming to preserve these bioactive components in the final milk drinks. Finally, the effect of an acidic condition (pH 2 for 120 minutes, simulating the exposure of foods to the stomach environment) on the integrity of the encapsulated beads containing PB alone or the combination of PB and PPs, was investigated. Results indicated that the aqueous apple skin extract had significantly higher TEPC (21.44 mg catechin eq./g extract) and TAA (25.69 mg Trolox eq./g extract) compared with ethanolic extract (with TEPC, 20.66 mg catechin eq./g extract; TAA, 23.98 mg Trolox eq./g extract). The pectin fibre and vitamin C contents of the two extracts were statistically similar. The two apple skin PP extracts (at a concentration of 1% or 2%) and the six purified PP compounds were remarkably effective in maintaining the viability of PB in the milk drinks above the required minimum PB level (106 CFU/mL) after 50-day storage. In contrast, the viability of L. acidophilus gradually declined in the control milk (in the absence of added PPs) to 5.91 Log CFU/mL on Day 50 of the refrigerated storage. The apple skin PP extracts and purified PPs were found to be effective in improving the adhesion of PB to the surface of the hydrophilic microtitre plate. The aqueous apple skin extract promoted significant adhesion of PB under both aerobic and anaerobic conditions and at both incubation temperatures (37 and 4 °C). Microencapsulation of PB and especially co-encapsulation of PB with an apple skin PP extracts exerted remarkable ability to prolong the survival of PB in the milk. Cell loss of 0.13-Log CFU/mL and 0.16-Log CFU/mL was detected when PB was co-encapsulated with the aqueous and ethanolic apple skin extracts, respectively. In comparison, a decrease of 1.1-Log CFU/mL in cell count was detected for the unencapsulated PB.

The knowledge obtained from thesis not only contributes to scientific advances, such as the positive influence of apple skin PP extracts on the viability and adhesion of PB, but also demonstrates the commercial potential of a concept for a stable milk drink containing sufficient and desired PB and apple PPs.