Interference by formaldehyde in the 2‐thiobarbituric acid test for rancidity

The 2‐thiobarbituric acid (TBA) test is often used to detect oxidative rancidity, although certain limitations of the technique have been described. This study was of formaldehyde (FA) interference in the TBA test in model systems involving various concentrations of malonaldehyde (MA), FA and TBA. At a TBA concentration of 20 μmol ml−1 it was found that FA could give rise to turbidity or precipitation. However, as the amount of MA in the reaction mixture increased (10 to 400 times higher than the upper limit of the standard curve), higher FA concentrations were required. Turbidity or precipitation resulted in fluctuating readings, and centrifugation or filtration lowered the readings (532 nm) to below those obtained when no FA was present. The ratio of TBA to MA must be ⩾1000 for full colour development, and hence lack of colour may be due to the TBA acting as a limiting factor in the reaction. The presence of a peak at 323 nm and/or turbidity or precipitate confirmed of the reaction between FA and TBA. FA competed with MA for TBA thereby lowering the effective TBA:MA ratio. Other TBA‐reacting substances (TBARS) present in the medium may augment this effect.