Determination of antimicrobial, antioxidant and prebiotic properties of banana and watermelon peels

In this study, the chemical composition, antioxidant, antimicrobial, and prebiotic properties of freeze-dried banana (Musa acuminata) and watermelon (Citrillus lanatus) peel powders were investigated. While ash, moisture, crude fat, crude protein, crude fiber, and carbohydrate contents of banana peel powder were 13.49%; 10.98%;6.27%; 8,19%; 7.28% and 53.79%, respectively; watermelon peel powders were found as 12.37%; 15.97%; 5.52%; 11.46%; 15.72%; 38.93%. The water activity values of banana and watermelon peel powders were measured as 0.12 and 0.19, respectively. The water holding capacities of banana and watermelon peel powders were 5.59 and 5.72g water/g, respectively; The oil holding capacities were determined as 2.78 and 2.60g oil/g. The DPPH radical scavenging activity of the banana peel and watermelon peel extracts were ranged from 34.78% to 50.00% and 30.53% to 50.89%, respectively. The highest total phenolic content was investigated in both banana and watermelon peel methanol extracts (p<0.05). Banana peel extracts illustrated antimicrobial activity against all tested bacteria, except Listeria monocytogenes. Likewise, watermelon peel extracts inhibited all tested bacteria except Salmonella Enteritidis and Salmonella Typhimurium. Acetone and ethanol extracts of watermelon peel inhibited Lactobacillus acidophilus, while acetone and methanol extracts inhibited Lactobacillus plantarum. However, only ethanol extract of banana peel showed the antimicrobial effect on both probiotic bacteria. The prebiotic effect of banana and watermelon peel powders (5%) in milk samples were tested on L. acidophilus and L. plantarum at 37°C for 40 hours incubation period. L. acidophilus,both in fortified and unfortified milk samples,showed similar growth kinetics (p>0.05). However, the generation time of L. plantarum was significantly shortened, exponential growth rate and maximum population density increased (p<0.05). The effect of banana and watermelon peel powders on the bile salt tolerance of the probiotic bacteria was tested in De Man Ragosa Sharp Broth medium with added 0.5-1% bile salt; the results showed that both peel powders increased the bile salt tolerance of L. acidophilus and L. plantarum. The addition of banana and watermelon peel powder (5%) to milk has supported the survival of L. acidophilus during 42 days of storage at 4°C. On the other hand, although the number of L. plantarum decreased in fortified milk samples, the total cell number remained above 7 log CFU/ml. Keywords: Banana peel, watermelon peel, antimicrobial activity, antioxidant activity, probiotic, prebiotic