Effect of honey on in vitro probiotic efficacy of Lactobacillus reuteri DPC16

Abstract

Consumer acceptance of synbiotics, which are synergistic combinations of probiotics and their prebiotic substrates, continues to expand in the functional food category. Manuka honey is well known for its potent antibacterial activity against pathogenic gut microbes. This doctoral research aimed to study the effect of Manuka honey on probiotic growth, survival, and activity in potentially synbiotic food matrix and media. In-vitro probiotic survivability trials were conducted in three distinct experimental set-ups with Manuka honey as a substrate for probiotic Lactobacillus reuteri DPC16 in pure and mixed culture systems: fermented milk, probiotic yogurt, and a batch fermenter with MRS broth. In pure culture fermented milk systems, the probiotic viable counts (greater than 8 log cfu/mL) with graded Manuka honey (UMF18+ and MGO550+ ) were significantly (p < 0.05) higher than with 5% w/v Manuka Honey Blend, Invert Syrup, and the unsweetened control. The results were significant, given the enhanced survivability of the probiotic strain in a sub-lethally antibacterial food matrix, which was reported for the first time. Furthermore, a novel NMR technique was optimised for identifying the major metabolites that contribute towards the acidity development and simple sugars and oligosaccharide present in honey. The research also evaluated the effect of Manuka honey on the probiotic growth and sensory characteristics (n = 102) of potentially synbiotic yogurts manufactured with L. reuteri DPC16, and the combination with AMF15+ Manuka honey that can be further developed as a synbiotic functional food. In the anaerobic fermentation trials, however, no statistically significant difference was observed in the probiotic growth-promoting effect of NZ Manuka Honey labelled by its Active Manuka Factor or AMF (05+ , 10+ , 15+ and 20+ ), signifying different levels of antimicrobial activity. Nevertheless, after 24 hours, the probiotic biomass growth was pronounced (greater than 3.0 mg/mL), and the pH readings touched the lowest levels (below 4) with the addition of Manuka honey, but not the unsweetened and invert syrup control samples. Moreover, the enhancement of probiotic growth and survival in in vitro stress conditions (acid, bile and refrigerated storage), and the production of short chain fatty acids metabolites, in both food matrix and bacterial growth media, contribute to establishing the prebiotic potential of Manuka honey. The results can thus be applied to formulate functional foods with the desired probiotic counts by incorporating antibacterially-graded Manuka honey at 5% levels.