Abstract: A new method of ultrasonic-formaldehyde extraction was applied to extract acid materials from fruit. By using the parameters of total acidity with fixed pH titration method, the extraction conditions were optimized with single factor and orthogonal tests. The results showed that the mass ratio of fruit to water , processing time of ultrasonic and temperature of water have significant effects on the extraction. And the optimizing conditions are: power 500 W, ratio of material to water 16:1, 2.00 ml neutral formaldehyde, processing fime 15 min and ultrasonic treatment at normal temperature, for 7 min at 80 ℃. The conclusion is that the ultrasonic extraction- formaldehyde method is superior to the traditional water extraction method in determining the fruit acidity comparably.

Key words: ultrasonic-formaldehyde extraction method, total acidity, fruit, fixed pH method