Functional properties of aqueous fruit extracts towards probiotic and pathogenic bacteria
Functional foods and ingredients provide health-promoting benefits to consumers in addition to their nourishing role. Exploitation of fruit ingredients that stimulate the growth of probiotic bacteria and suppress that of pathogenic bacteria could contribute to the success of health-promoting probiotic foods. This study investigated the functional properties of aqueous extracts of four New Zealand-grown fruits, namely, blueberry, strawberry, green kiwifruit, and feijoa, towards probiotic bacteria and food-borne pathogenic bacteria. The fruit extracts were prepared by homogenizing dried fruit pulp in sodium phosphate buffer (pH 7.4), followed by centrifugation and filtration. Prior to assay, the fruit extracts were sterilized by a two-step membrane filtration using 0.45 µm and 0.22 µm membrane filters. Before mixing the fruit extract with the bacteria, the bacteria were allowed to adapt to the growth medium using two sub-cultures and the cell concentration was standardized at103 cells/mL. Fruit extracts at 10%, 20%, and 30% were each used in a bioassay using a high-throughput method in a sterile 96-well microplate. A series of two-fold dilutions of the fruit extract was prepared resulting in an overall concentration range of 0.01 g/L to 30 g/L. The change in growth was calculated as ∆Growth values after reading the optical density (620 nm) in a microplate reader. The fruit extracts were used in the assay as single extract and as combined extracts. Results showed that the effect of the fruit extracts were dose- and species-dependent. At high concentrations, the four fruit extracts exerted growth-stimulating effect on the probiotic species of Bifidobacterium and Lactobacilli, except with Bifidobacterium longum. However, a biphasic effect was observed with strawberry, green kiwifruit and feijoa extracts, where at low concentrations, the extracts had growth-inhibiting effect on the probiotic bacteria while high concentrations had growth-stimulating effect. In contrast, all fruit extracts were found to inhibit the pathogens used in this study which included both Gram-positive and Gram-negative bacteria. A biphasic effect was exerted on all the pathogens, except with feijoa extract towards Listeria monocytogenes. Feijoa extract exhibited significant inhibition towards L. monocytogenes even at low concentrations. Based on functionality of a single extract, green kiwifruit seems to be the most effective extract. At high concentration, it exerted the greatest stimulatory effect on probiotic bacteria and inhibitory effect on pathogenic bacteria. The combination of blueberry and feijoa extracts, similar to that of strawberry and kiwifruit extracts, exerted no beneficial stimulatory effect on the probiotic bacteria, except on B. longum. The growth of B. longum was enhanced in the presence of combined strawberry and green kiwifruit extracts. The two sets of combined extracts did not have any functional growth-inhibiting effect towards the pathogenic bacteria. The combined extracts of strawberry and feijoa were identified as undesirable due to their growth-stimulating effect on Yersinia enterocolitica. These results are relevant for potential application of fruit extracts not only as functional ingredients in foods but also for cell propagation of probiotic bacteria using appropriate concentrations of the fruit extracts.