Abstract: In order to study the effect of ultrasonic on the freezing characteristics of fruits with different porosities, apple, pear, and Hami melon (Cucumis melo var. saccharinus) were allowed to undergo ultrasonic-assisted immersion freezing at the same ultrasonic frequency and different ultrasonic intensities. A temperature recorder was used to measure temperature changes at different freezing time points during the freezing process. The drip loss, texture characteristics, color difference, moisture distribution, and ascorbic acid content of frozen-thawed samples were determined. The results showed that at an ultrasonic intensity of 0.145 W/cm2, the phase-changing times of apple, pear and Hami melon samples were 21.67, 21.33, and 16.67 s, respectively, which were significantly shorter than those of immersion frozen samples (P < 0.05). For each ultrasonic intensity, ultrasonic-assisted immersion freezing led to a decrease in drip loss during thawing compared to direct immersion freezing for all three fruits, especially for apple, which showed a significant decrease due to its low porosity (P < 0.05). Moreover, the hardness of the fruits was increased. Compared with the fresh samples, ascorbic acid contents in the treated samples were decreased significantly (P < 0.05). Compared with the samples without ultrasonic treatment, the fruit samples with smaller porosity maintained better quality under ultrasonic treatment.

Key words: porosity; fruit; ultrasonic strength; freezing characteristics; immersion freezing