Colorimetric Assay for Free Fatty Acids in Butter Using Flow‐Injection and Immobilized Enzymes

Butyrate kinase (BK) was extracted from Escherichia coli DH5 pJc7*, purified, immobilized on porous glass beads and packed in a glass tube as a biosensor coupled with a flow‐injection analysis system (FIA). The limits of detection were: FIA (25ppm) and batch method (BM, 5ppm). BK is highly specific for butyric, propionic and valeric acids. Regression coefficient values were: FIA r=0.992 and free enzyme r=0.987. The FIA and BM were tested on butter samples with high correlation (r= 1.000) between the methods. Correlation coefficients were: r=0.994 and r=0.988 between acid degree value and FIA/BM.