Ultrasound treatment resulted also in alteration of a⁄ and b⁄
parameter, which corresponds to yellow and red colour, respectively
(Table 3). The application of ultrasound at a frequency of
21 and 35 kHz for 10 and 20 min did not cause significant changes
in parameter a⁄ and b⁄. However, elongation of the treatment to
30 min, regardless of the frequency of ultrasound, resulted in a significant
reduction of the values of both parameters.