Potential roles of polyphenol oxidase in apple tissues: effects on microbial growth and antioxidant capacity

Polyphenol oxidase (PPO) is a typical food enzyme considered very important in the food industry mainly for its damaging effects. It is responsible for tissue darkening after fruit cuts or bruises, resulting in a nutritional value loss and a general decrease in food acceptability. However, this colour development is desirable, too, for certain products like coffee or apple cider. Despite all these considerations, it is not completely clear which roles this compound may have in cell metabolism. In this project, its possible antimicrobial and antioxidant effects were examined. Analyses were carried out in two distinct ways: properties of standard polyphenolic substances oxidized by PPO were assayed alongside tests on Golden delicious apple extract, whose phenolics were previously isolated. Toxicity investigations were performed against three bacteria belonging to Lactobacillus spp. (L. brevis, L. casei and L. plantarum) and three strains of Saccharomyces cerevisiae, all isolated from fruits. For these organisms, Minimum Inhibitory Concentration (MIC) test was executed and results between oxidized and not oxidized solutions were compared. Simultaneously, Total Antioxidant Capacity (TAC) was researched, for the same samples. In this case, data were collected with DPPH test and crocin bleaching assay and results were compared to a Trolox C® standard solution. From the MIC analyses of four different standard polyphenols (catechol, chlorogenic acid, epicatechin and quercetin) antimicrobial capacity seemed to be affected by PPO only for the first, and just against L. brevis and L. plantarum. Besides that, quercetin promoted microbial growth, despite the inconsistent outcomes of PPO influence on that: the enzyme was able to reduce such promotional effect for one strain (S. cerevisiae SC/D/1P) while in another case it was accentuated (S. cerevisiae SC/B/4A). On the other hand, results obtained from apple extracts showed how fruit phenolics, either treated or not with PPO, were able to accentuate microbial and yeast growth. S cerevisiae SC/B/4A and SC/D/1P increased their metabolism, effect confirmed for L. brevis, even if in a much lower manner. Examining TAC data, an increase in free radical scavenging activity for the action of PPO arose for catechol and chlorogenic acid, while quercetin exerted an opposite behaviour. Despite the lower TAC values of apple extracts, they appeared to confirm results collected for the first two phenolics, especially when measured with DPPH test.