Freezing and storage of orange and orange/melon juices: effects on pectinesterase activity and quality

The pectinesterase (PE) enzyme is responSible for the citrus juices clarification.Usually, to prevent this undesirable effect, 90 to 100% of the PE is thermally inactivated by a pasteurization process.However, this processing causes also the degradation of quality parameters, such as flavor and color.Therefore, if freezing and frozen storage inactivates part of the PE, this unit operation could be used as a pre-treatment before pasteurization.The objective of this work is to study the freezing and frozen storage effects on PE activity and quality of fresh pure orange and mixed orange/melon juices.Samples of fresh orange 'Valencia' and orange/melon 'Cantaloupe' juices (ratio 1:1) were frozen and stored at -20°C during 120 days.The parameters analyzed along storage were: PE activity, pH, °Brix, acidity, Brix/acid ratio, color, cloud stability and sensory evaluation.The PE activity was determined at two temperatures (30°C and 55°C) using the method described by K6ner and adaptations suggested by Amstalden and Montgomery.The freezing process increased the initial PE activity of the juices.After 120 days of storage, it was verified a reduction on PE activity of approximately 28% and 9% for orange and orange/melon juices, respectively.This enzyme did not reactivate after 24 hours at 5°C.As observed by other authors, two main enzyme fractions, a thermolabile and another thermostable, were identified.The frozen storage inactivates the less resistant fraction.The freezing and frozen storage did not change significantly the pH, Brix, acidity and total color difference (TCD) of the two juices.The freezing and frozen storage caused the lost of the cloud stability in the orange juice.However, this effect was not observed in the orange/melon mixture.For the sensory evaluation the juices were thawed and pasteurized (85°C/2min for orange juice and 90°C/2min for orange/melon mixture).The fresh samples used in the sensory tests were also thermally treated during 2 min at 90°C and 95°C for orange and orange/melon juices, respectively.Using a Triangular test it was concluded that there is a significant difference between the samples (within 99% confidence).The Hedonic test showed that there is a greater preference for the fresh samples.