Abstract
The decomposition of nitrogenous compounds of extracts of cooked halibut meat due to the growth at 4°C and 17°C of Pseudomonas fragi, strain F 111, was followed with determinations of the total volatile nitrogen (TVN) and of trimethylamine (TMA). The steam-distillation method according to Bethea & Hillig (1965) and the Conway-microdiffusion-method according to Farber & Ferro (1956) were used for these determinations.
When fish extract was inoculated with the strain F 111 and stored at 4°C for 5 days or at 17°C for 3½ days an increase of TVN was started. This increase of TVN was slower at 4°C than at 17°C. It was shown that in the extract B, which was prepared from fish meat of poor but acceptable commercial quality, the initial TVN was higher, the increase of TVN caused by the action of the strain F 111 was slower, and the TVN maximum was lower than the corresponding values representing extract A. The last mentioned extract was prepared from halibut meat of good commercial quality.
The correlation between the increase of TVN and that of pH of the inoculated fish extract was poor. This indicates that the initial increase of pH was not caused by volatile basic compounds.
It was shown that the exclusion of air after 1 or more days of incubation at 17°C could delay the onset of the TVN increase but did not prevent it. The final TVN value of the sample, which was layered with paraffin oil 24 hrs. after the inoculation of the strain F 111, was approximately the same as that of the fish extract sample layered after 14 days of incubation at 17°C. In inoculated fish extract samples, which were sterile-filtered on the day when the extract was layered with paraffin oil, no further increase of TVN was observed.
It was confirmed that Pseudomonas fragi caused no increase of TMA in the extract of cooked halibut.
Keyword: volatile nitrogen compounds, Pseudomonas fragi; fish
Sammanfattning
Nedbrytningen av kvävehaltiga komponenter i extrakt av kokt hälleflundra förorsakad av tillväxten vid 4°C och vid 17°C av Pseudomonas fragi följdes genom bestämningar av flyktigt kväve (TVN) och av halten trimethylamin (TMA). Dessa nedbrytningsprodukter bestämdes enligt en metod beskriven av Bethea & Hillig (1965) och med en Conway-mikrodiffusionsmetod som har beskrivits av Farber & Ferro (1956).
En ökning av halten TVN i det inokulerade fiskextraktet började efter 5 dagars inkubering vid 4°C respektive efter 3½ dagars inkubering vid 17°C. Denna ökning av TVN var långsammare vid 4°C än vid 17°C. Det visades att i ett extrakt av fiskkött av dålig kommersiell kvalitet var initialmängden TVN större, ökningen av TVN förorsakad av tillväxten av Pseudomonas fragi var långsammere, och det uppnådda maximala värdet för TVN var lägre än i ett extrakt av fisk av god kommersiell kvalitet.
Korrelationen mellan ökningen av TVN och av pH i det inokulerade fiskextraktet var dålig. Detta innebär att den initiala pH-stegringen icke förorsakades av bildningen av flyktiga basiska ämnen.
Genom att pipettera ett lager av paraffinoma over det inokulerade fiskextraktet efter en viss tids inkubation bröts kontakten med omgivande luft. Detta medförde en försening av ökningen av TVN men förhindrade icke denna. Det slutliga TVN-värdet var ungefär lika högt i det prov som hade täckts efter 24 timmars inkubering som i det prov som täcktes först efter 14 dagars inkubering vid 17°C. Ingen ökning av TVN observerades i de infekterade prov av fiskextrakt, vilka sterilfiltrerades samma dag som paraffinoma skiktades över extraktet.
Det konfirmerades att Pseudomonas fragi icke förorsakade någon ökning av TMA i extrakt av kokt hälleflundra.
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