Quantification of fish partial volatile basic nitrogen by SPME-GC/MS
Résumé
Freshness is a key parameter in the evaluation of the fish quality. As this matrix is highly perishable, objective tools for the freshness estimation are required. By the past, numerous methods including sensorial, chemical, microbiological and physical analysis were developed to follow evolution of odour, sight or texture throughout the spoiling process.
Spoiling of fish is a complex process combining autolysis and exogenous degradations. One of the most famous examples is the degradation of trimethylamine-N-oxide (TMAO) in Gadidae by both TMAO reductases of specific spoiling organisms and TMAO demethylase, an endogenic enzyme. These reactions lead respectively to the production of trimethylamine (TMA) and dimethylamine (DMA). To date, TMA and DMA are routinely non-specifically measured among other volatile amines thanks to total volatile basic nitrogen (TVB-N) analysis; TMA can be nonetheless measured more specifically by TVB-N when formaldehyde is added. The concept of partial volatile base nitrogen (PVB-N) was implemented using SPME-GC/MS to quantify both TMA and DMA. Further, this technique allows specifically detecting and measuring TMA and DMA.
Extraction (temperature, time, sample preparation) and separation (temperature, column phase, split type) parameters were optimized to obtain best chromatographic profiles. Both fresh and spoiled fish were analyzed by TVB-N and PVB-N techniques. First results show consistent conclusions. Moreover, comparison of TVB-N and PVB-N leads to the conclusion that the current TMA measurement, with addition of formaldehyde, is overestimating TMA content in the flesh of fish.