In this study

the effects of quick-freezing through sprayingof liquid nitrogen (-40

-60

-80

and -100 °C) and slow- freezing (-18 °C) on microorganism biochemical activity and fruit quality were studied with pitaya (Stenocereusgriseus L) blocks as raw materials. The rate of freezing of the sample accelerated with decreasing freezing temperature

and the freezing time was reduced from 1.25 to 0.367 min as freezing temperature declined. However

the sample exhibited low temperature fracturing after quick-freezing at -100°C. The liquid nitrogen quick-freezing treatment inhibited the activity of polyphenol oxidase

and the residual activity of polyphenol oxidase decreased to 47.27%after -80°C freezing

while it increased to 301.09% after the slow-freezing treatment. The texture of the sample after liquid nitrogen quick-freezing was better than that of the slow-freezing treatment and the rate of juice loss was between 8.88% and 10.50%. In contrast

the rate of juice loss was as high as 38.29% after the slow-freezing treatment. The content of titratable acid

soluble solids

total phenol

anthocyanin

and ascorbic acid increased with the decreased freezing temperature of liquid nitrogen. The -80°C liquid nitrogen quick-freezing treatment maintained the best quality of the sample.