Changes in the quality of apple tissue subjected to different freezing rates during long-term frozen storage at different temperatures

Abstract

In this manuscript, the impact of various parameters including freezing rate, storage temperature and duration on the quality of apples is reported. Freezing rate significantly affected the quality attributes (firmness, drip loss, and morphology of ice crystals). Significant difference in the firmness loss was seen between the static freezing (SF at -18 °C) and rapid freezing (RF at -40 °C and 2 m/s air velocity) and ultra rapid freezing (URF at -72 °C and 1 m/s air velocity) samples on day 0. Regarding drip loss, significant difference could only be seen between SF and URF conditions. Nevertheless, microscopy results showed that ice crystal size reduced with the increase in the freezing rates. Storage of frozen products at two different temperatures (i.e. -12 °C and -18 °C) for 90 days caused serious damage to the product. Whatever the freezing condition and storage temperature, the maximum reduction in firmness happened during the first 30 days of storage. A maximum reduction (85%) in the texture was seen for SF samples stored for 30 days at -12 °C compared to fresh samples. On the other hand, drip loss increased gradually with the storage period for all freezing methods and storage temperatures. After 90 days, SF samples stored at -12 °C recorded the highest drip loss (6.12%), while RF samples stored at -18 °C exhibited the lowest value (5.05%). MANOVA analysis revealed that all three independent variables (i.e. freezing rate, storage time, and storage temperature) significantly affected the drip loss and texture. Lastly, microscopy after 90 days showed a high degree of tissue damage, making it difficult to differentiate the freezing method and storage temperature.