Thermosonication applied to kiwi peel: a healthy source of nutrients

The peels of many fruits are not commonly consumed being, however, important sources of nutrients. Finding approaches to add value to such non-edible parts and preventing them from being discarded is interesting. This may include the development of processes that allow the retention of nutrients and guarantee the product´s safety from a microbiological point of view. The objective was to apply thermosonication processes to kiwi peel and evaluate the impact on several key nutrients and Listeria innocua survival, which was used as a non- pathogenic surrogate of L. monocytogenes. Kiwi (Actinidia deliciosa cv. Hayward) peels were manually removed and cut into small pieces, which were artificially inoculated with L. innocua by immersion in a suspension of about 107 cfu/mL. Thermosonication treatments were performed at 55 °C (US+T55) and 60 °C (US+T60) with three sampling times till 30 and 15 min maximum, respectively. Thermal treatments were performed for control using the same temperatures and times (T55 and T60). All treatments were repeated three times independently. L. innocua on kiwi peel was enumerated before and after each treatment. Proteins, fibers, and minerals (Ca, K, Mg, Na, and P) were analyzed in fresh and treated kiwi peels. A Weibull model with a decimal reduction time (D-value) was successfully used in L. innocua survival data fitting by regression analysis procedures. D-values obtained for thermal treatments were 1.60 ± 1.71 min (T55) and 2.82 ± 0.90 min (T60). When thermosonication was used, those values decreased respectively to 0.97 ± 0.54 min and 0.73 ± 0.29 min, showing that temperature coupled to ultrasound is more effective in L.innocua inactivation. All treatments significantly increased protein content, which was 9.22 ± 1.20 mg/g (on a dry basis, d.b.) in the fresh peel. The most effective was US+T60, which allowed an 84% increase. The total fiber content in kiwi peel was 305.57 ± 10.18 mg/g d.b., of each 297.85 ± 4.63 mg/g d.b. were insoluble. After thermosonication those values increased significantly, up to 75% after US+T60 and doubling after US+T55. Minerals were not significantly affected by thermosonication. Applying thermosonication to kiwi peel is more effective than thermal treatments in reducing L. innocua loads while allowing retention and even increasing essential healthy nutrients.