Freezing and storage of orange juice: 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 pasteurisation process.However, this processing causes also the degradation of quality parameters, such as flavour and colour.Therefore, if freezing and frozen storage inactivates part of the PE, this unit operation could be used as a pre-treatment before pasteurisation.The objective of this work is to study the freezing and frozen storage effects on PE activity and quality of fresh orange juice.Samples of fresh orange "Valencia Late" juice were frozen and stored at -20 °C during 120 days.The parameters analysed during storage were: PE activity, pH, °Brix, acidity, °Brix/acid ratio, colour, cloud stability and sensory evaluation The PE activity was determined at two temperatures (30 °C and 55 °C) using the method described by Köner and adaptations suggested by Amstalden and Montgomery.The freezing process increased the initial PE activity of the juice.After 120 days of storage, a reduction on PE activity of approximately 28 % was verified.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 thermal resistant fraction.The freezing and frozen storage did not change significantly the pH, °Brix, acidity and total colour difference (TCD) of the juice.However, it caused the loss of its cloud stability.For the sensory evaluation the juice was thawed and pasteurised (85 °C/2 min).The fresh samples used in the sensory tests were also thermally treated during 2 min at 90 °C.Using a Triangular test it was concluded that there was a significant difference between the samples (within 99 % confidence).The Hedonic test showed that there was a greater preference for the fresh samples.