Effect of Continuous Flow Microwave and Conventional Heating on the Bioactive Compounds, Colour, Enzymes Activity, Microbial and Sensory Quality of Strawberry Purée

The impact of continuous microwave heating (MV) at 90 and 120 °C for 10 s and at 80 and 90 °C for 7 s, as well as conventional thermal processing (CTP) at 90 °C for 15 min, on the quality of strawberry purée was studied. Product quality was determined, covering total polyphenols, anthocyanins, phenolic acids, flavonols and vitamin C content, as well as colour parameters, enzymatic activity, and finally, the sensory and microbiological quality. CTP caused higher degradation of polyphenols (ca. 7 %), anthocyanins (ca. 20 %) and vitamin C (ca. 48 %) compared to MV at 120 °C. Changes of the colour of MV-preserved purée were insignificant (dE <2), whereas CTP caused significant changes (dE >3). Only MV at 120 °C and CTP effectively decreased the level of total microbial count (<1 log cfu/g), but considering enzymes activity, only CTP was effective, inactivating ca. 98 and 100 % of polyphenol oxidase (PPO) and peroxidase (POD), respectively. The most effective inactivation of PPO (82 %) and POD (88 %) after MV was noted at 120 °C, but the best compromise between colour and nutrient preservation and enzyme inactivation was achieved at the MV at 90 °C. Considering the obtained results, it can be concluded that the microwave heating better preserved the quality of strawberry puree than conventional heat pasteurization.