福岡県農業総合試験場研究報告

［摘要］果汁のpH変化を観測しながら，ブドウ9品種の滴定酸含量を中和法および電位差法によって測定した。電位差法に比べて中和法では酸含量が常に高く測定され，両法間の偏差は0.02～0.05の範囲にあり，平均0.03であった。中和法では，濃着色した果汁試料の当量点の判定が極めて困難であった。電位差法による滴定曲線の一次微分解析から，当量点はpH7.61～7.98の範囲にあり，平均7.75であった。果実136単位（1単位当たり10果粒）について，電位差法（当量点pH7.8）による酸含量値（Y）と実測導電率値（X）との間には直線回帰関係（Y＝0.02998X－0.5583，r＝0.994）が認められ，1％水準で有意に適合した。ブドウ用酸含量測定器（富士平工業社製アシライザー・モデル5G）に内臓された回帰式を用いて，実測導電率値から酸含量を算出し，中和法および電位差法による酸含量値と比較した。中和法との偏差は0.03～0.05で平均0.04であったのに対し，電位差法との偏差は0～0.02で平均0.01であった。

［キーワード：ブドウ、中和法、導電率、電位差法、電気化学法、遊離酸］

　A Proposed Revision of Measurement of Free Organic Acid Content in Grape Juice. SHIRAISHI Mikio, Hiroyuki FUGISHIMA, Nobuyuki HIRAKAWA (Fukuoka Agricultural Research Center, Chikushino, Fukuoka 818-8549, Japan) Bull. Fukuoka Agric. Res. Cent. 21: 35 - 39(2002)

　Free acid contents of grape juice in 9 cultivars were measured by alkalimetry (phenolphthalein and potentiometric titration) together with monitoring changes in pH values. The deviation of free acid content by the difference between phenolphthalein- and potentiometric-method ranged from 0.02 to 0.05 with average of 0.03, and the acid content by the former method was always in higher than that by the latter one. In the well-colored juice sample, determination of equivalence point by phenolphthalein method was very difficult. From the primary derivative analysis on a titration curve, the equivalence point (pH value) varied from 7.61 to 7.98, averaging 7.75. From 136 units of juice sample (10 grape berries per unit),　linear regression equation was expressed in the following formula (Y= 0.02998X - 0.5583, r= 0.994**; Y: free acid content, X: specific conductance, **: significant at 1% level). Using an electrochemical method, free acid content was calculated by the built-in linear regression equation in Aclizer Model 5G (Fujihira Co. Ltd.). Deviation of free acid content by the difference between phenolphthalein- and electrochemical-method ranged from 0.03 to 0.05 with average of 0.04, whereas the deviation ranged from 0 to 0.02 with average of 0.01 between potentiometric- and electrochemical method.