1970 Volume 36 Issue 6 Pages 606-611
It has been known that the picrate colour reaction is not specific for trimethylamine (TMA) because the reaction occurs also with many types of amines formed during the spoilage of fish. Thus, the TMA level in the muscle of gadoid fishes are considered to be overestimated by the DYER's picrate method, since the muscle often contains a considerable amount of dimethylamine (DMA) even in a good grade of quality.
By chance the authors observed that the picrate value of cod muscle, obtained by HASHIMOTO's method using KOH instead of K2C03, was too low for the expected level from the amount of DMA contained, and therefore, examined the colour development of DMA picrate in relation to the reagents employed in the picrate method.
Optical density of DMA picrate was much smaller with the combined use of KOH and 1ml of 10% formaldehyde (FA), than the O. D. obtained with K2C03 and FA. The ratio of the O. D. of DMA picrate to the O. D. of the equivalent TMA picrate (ratio of colour development) was approximately 4% for the KOH method, while the ratio was about 50% in the case of using K2CO3 as the reagent.
The amount of FA showed markedly different effects in the two methods: with K2C03 the colour intensified in proportion to the increase of FA, while with KOH, a maximum value occurred in the absence of FA and abruptly dropped with the addition of 0.5 to 1.0ml of FA, followed by a gradual intensification in colours as FA increased.
The use of KOH as well as K2C03 was examined to determine their effect on the picrate values of fish samples. Cod and Alaska pollock fillets, including irradiated or salted, were analyzed after chilled or frozen storage. The picrate values with the use of KOH were found close to the “real” TMA values measured by gas-liquid chromatography (GLC), except one sample which contained a larger amount of DMA. However, the picrate values obtained with K2CO3 were much higher than the GLC values.
With regard to the results mentioned above, the authors propose that KOH be used instead of K2C03 as the alkali reagent, and the amount of 10% FA added be kept to 1ml, when gadoid fishes are subjected to TMA analysis by the picrate method.