Influence of acetate and CO2 on the TMAO-reduction reaction by Shewanella baltica

In this work, the TMAO-reduction by Shewanella baltica, one of the represen tative spoilage organisms in modified atmosphere packaged marine fish fille ts, and the effect of acetate and CO, on this reduction were studied in vit ro. The growth of S. baltica and the corresponding evolution of some compou nds (acetate, lactate, pyruvate, glucose and trimethylamine (TMA)) were fol lowed during storage at 4 degreesC in two types of broths. The first medium was a defined medium (pH = 6.8) to which lactate or pyruvate was added as hydrogen donor. Pyruvate showed to be more efficient as H-donor for S. balt ica than lactate, as growth was much faster when equimolar amounts of pyruv ate instead of lactate were present. Although the growth of S. baltica, whe n pyruvate is used as H-donor and no acetate is added, was not much inhibit ed by the CO2-atniosphere, CO2 had a pronounced effect on the studied react ions as it partly inhibited the reduction of pyruvate to acetate. The effec t of acetate on this reaction was, on the other hand, not significant. To simulate the reactions occurring in situ, a buffered fish extract (pH = 6.8) was used. In spite of the neutral pH, the growth of S. baltica in this medium was highly inhibited by relatively small concentrations of acetate ( < 0.3 %). When 0.1% of acetate was added to the fish extract, less acetat e was formed and lactate was more slowly consumed in comparison to the expe riments without the addition of acetate. The consumption of lactate and the production of acetate were almost completely inhibited when the fish extra ct contained 0.25% of acetate. Apparently, the addition of acetate inhibite d the use of lactate as H-donor. After extended storage times (17 days at 4 degreesC) TMA production started. Most probably, alternative H-donors were used by S. baltica, from which the pathway seems to be less energy efficie nt. This can be deduced from the exceptional growth inhibition of S. baltic a by small amounts of acetate. However, when practical storage times for fi sh (e.g. 6 days at 4 degreesC after packaging) are considered, growth and T MAO-reduction by S. baltica was completely inhibited during this period by 0.25% of acetate. (C) 2001 Elsevier Science B.V. All rights reserved.