The influence of immersion and contact ultrasound treatment on selected properties of the apple tissue

Ultrasound (US) treatment is considered to be one of the most promising non-thermal technology used in the food processing. The food-related applications of this technique are linked to the analytical and technological purposes. The ultrasound waves in the food can cause the formation of micro-channels due to the systematic and alternating compression and decompression of the material (so called sponge effect). Additionally, in liquids the ultrasound application can cause the cavitation which can modify the food properties as well. Hence, due to its mechanism, the ultrasound treatment can also improve the extraction of pigments, aromas or antioxidants from the food matrices. The aim of this study was to analyze the influence of ultrasound application on electrical conductivity, color, total polyphenols and antioxidant activity (determined in the ethanol extracts) of the apple tissue. The plant tissue samples were either treated by immersion method at 21 or 40 kHz or by contact ultrasound (24 kHz). The sonication lasted for 5, 10, 20, 30 min. Ultrasound generally did not change the electrical conductivity of the analyzed material. However, the changes caused by the contact method were slightly higher. The application of ultrasound regardless the utilized method generally increased the total polyphenolic concentration up to 145.3% and the antioxidant activity by 64.5% in comparison to the intact material. Among the ultrasound treated samples, the apples sonicated by immersion method at 40 kHz were characterized by the highest total polyphenolic content regardless the sonication time. Ultrasound treatment influenced the color of the investigated material. The total color difference of sonicated samples ranged from 1.2 to 9.8, whereas the changes were most prompt in the case of contact US treatment. (C) 2015 Elsevier Ltd. All rights reserved.