Shelf life of sonicated apple juice: effects on microbial population and volatile compounds

Power ultrasound (US) has been investigated as an alternative novel technology for the microorganisms’ inactivation in fruit juices. However, there are few information about its impact on chemical properties and quality traits. This study aimed to evaluate the microbial shelf-life of apple juice after US treatment combined with a semiquantitative determination of volatile compounds. Two different storage tests (60 days) were carried out on cloudy apple juice: the first substrate was raw juice, while the other one was inoculated with two spoilage yeasts, Rodotorula glutinis and Candida parapsilosis. The apple juice was processed with US (6’/100ml, 100% Amplitude in ice bath; final temperature of 35±7°C) and then stored at 4°C. The intensity of the treatment did not affect sensory traits. Microbiological analysis included mesophilic, psychrophilic, yeast and mould counts (3 replicates, 5 times). Volatile compounds were determined after a solid-phase extraction (SPE) with C18 Silica-based cartridges followed by GC/MS analysis. Titratable acidity, pH, temperature and Brix values were determined during the storage of samples. The dominant microflora of the control juice (C) was mainly composed of psychrophiles (5.5±0.04 log cfu/ml; 14 days: T14). The US treatment affected the growth of all microbial targets, providing its most relevant effect on psychrophilic counts especially at T14. At this point, the logarithmic difference between C and US-juice (S) was 5.2 Δlog cfu/ml. Considering the inoculated juice, S and C yeast curves diverged at T14 (2.9 Δlog cfu/ml) and the Δlog decreased to 1.6 at 60 days. Flavours typical of raw juice (e.g. Hexanal, 2-hexenal) dramatically decreased in control (T14) when compared to S, while alcohols (e.g. 2-Hexenol; 1-Hexanol) presented the opposite trend. Aldehydes and alcohols were considered as chemical markers of the oxidative process that occurred on juices, and the results suggested higher flavor stability provided by US treatment.