Measurement of free organic acid concentration in tomato and strawberry fruit juice by the conductometric method

An acid and sugar content analyzer (NH-1000, HORIBA Ltd.), for citrus fruit based on the conductometric method (acid) and on the refractometric method (Brix value), is put to practical use. To develop a simple method for meas uring free organic acid concentration in tomato and strawberry juice, the a ccuracy of this equipment and the effects of some major compounds in these juice on analyzed values were studied. The values measured for tomato and s trawberry juice by this equipment were higher than the titratable acid. Esp ecially tomatoes with blossom-end rot showed very high measured values of a cid content. It is considered that the acid content of tomato and strawberr y juice obtained by the acid and sugar content analyzer need to be correcte d using calibration equations, and that tomatoes with blossom-end rot shoul d be removed before analysis. The organic acid composition of fruit juice v aried slightly among tomato, strawberry and Satsuma mandarin samples, but t he differences in conductivity of organic acids or the errors in the conver sion factor for titratable acidity were not considered to be the main cause s for the difference between titratable acid and the measured acid value by the acid and sugar content analyzer. The main positive element in all frui t juice samples was potassium, whereas the main inorganic anions were phosp hate and chloride ions in tomato and Satsuma mandarin, and phosphate chlori de and nitrate ions in the strawberry sample, The difference between the me asured acid values by the acid and sugar content analyzer and those by alka line titration is highly correlative to both the positive element and anion concentrations in the fruit juice.