Effects of heat treatment conditions on fresh taste and on pectinmethylesterase activity of chilled mandarin and orange juices

Juices from oranges, mandarins and hybrids were thermally treated in a plate exchanger at different conditions to evaluate the effects of treatment on fresh taste and on residual pectinmethylesterase (PME) activity. Freshness was significantly higher in fresh juices than in samples treated at 70 degrees C or higher temperatures for 10 seconds of retention time, whereas no differences were found among samples heated at temperatures from 70 to 90 degrees C for the same time, however at 95 degrees C fresh taste decreased again. Residual PME activity was about 20% in samples treated at 70 degrees C for 5, 10 and 20 seconds and in those heated at 80 degrees C for 5 and 10 seconds, decreasing to 15%, also at 80 degrees C, when retention time increased to 20 seconds. A drastic reduction to about 3% of residual activity was observed at 85 degrees C for 10 seconds. Minimum activities of 0-1% corresponded to samples treated at 95 degrees C. Considering the results of sensory and residual enzyme analyses, the treatment at 85 degrees C for 10 seconds can be considered suitable. In these conditions fresh taste did not differ from that of juices treated at lower temperatures but residual enzyme activity was clearly smaller and acceptable for chilled juices, products of high quality but short shelf life. On the other hand, a deeper reduction of PME activity increasing the temperature to 95 degrees C does not seem advisable since fresh taste decreases. Mandarin juices pasteurised at 85 degrees C for 10 seconds and pasteurised again at the same conditions did not show a further decrease of fresh taste. Two heat treatments were usually applied when packing plants receive the juice from other factories.