Scientific Alert Note

New pre-slaughtering conditions and quality index of turbot

: Turbot

Pre-slaughtering is a multi-step operation consisting of preparing the fish by fasting it, crowding and netting or pumping it, transferring the animals into tanks for transport, the transport itself, unloading them at a fish processor, keeping the fish in holding tanks and then finally stunning them. Several studies have shown that stress during the pre-slaughter period may have a negative influence on product quality. Thus, for both ethical and economical reasons, it is of interest to minimize the stress in turbots (Psetta maxima) during the pre-slaughtering process. Therefore, transport followed by live chilling of the turbot was compared to a stunning and killing procedure, which was directly applied to fish in holding tanks.

Authors:

Hans van de Vis¹, Marie Champod¹, Stéphanie Druon¹, Melanie Poort¹, Henny Reimert², and Bert Lambooij²

¹ Wageningen IMARES BV, WUR, IJmuiden, The Netherlands

² Animal Sciences Group, Veehouderij BV, WUR, Lelystad, The Netherlands

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The overall objective of the study was to compare an experimental pre-slaughtering procedure to a current industrial procedure for farmed turbot with regard to product quality. In the experimental procedure transport was avoided by rendering the fish unconscious by applying electrical stunning and subsequently chilling the unconscious turbot as a killing method. The current industrial procedure consists of transport followed by live chilling for 1.5 h.

For our experiment 100 turbots were obtained from a commercial fish farm. After a resting period of 4 days one batch of 50 turbots was transported for 2.5 h and subjected to live chilling for 1.5 h after delivery to our laboratory (current industrial practice).

In order to avoid any handling stress, the non-transported batch of 50 turbots was stunned and killed in the holding tank (seawater 52 µS/cm). This was carried out by applying a voltage of 150 V_peak for 10 seconds across plate electrodes at 1.1 m distance, followed by chilling the unconscious fish for 1.5 h by adding flake ice to the water. Under these conditions the amperage was 150 A_peak.

The conditions to achieve an instantaneous stun by applying electricity, followed by killing the unconscious fish by chilling it has been established by registration of EEGs and ECGs in another project.

: Figure 1: Evolution of K values for turbot subjected to the experimental and the industrial pre-slaughter procedure

Effects on product quality were assessed by visual inspection of the fish after gutting and filleting for the presence of haemorrhages and broken bones. In order to establish effects of the studied pre-slaughter conditions on freshness of the product, the K value was determined. This ratio evaluates the accumulation of inosine and hypoxanthine compared to the total pool of ATP and related products. Increasing K values have been correlated with loss of freshness in many fish species.

Visual inspection of the fish flesh after gutting and filleting revealed no differences between the experimental and industrial procedure. Downgrading of the product was not observed.

Evolution of K values with time is depicted in Figure 1. Contrary to our expectations, K values in the range of 79 to 81% were observed after a storage period of 22 to 25 h. A linearly increasing relationship between K values and storage time of the whole fish was as expected was not obtained. Instead a decrease with time was observed. Analysis of inosine in the fish flesh revealed for both batches high initial values that decreased during storage.

: Figure 2: Evolution of H values for turbot subjected to the experimental and the industrial pre-slaughtering procedure

The obtained results clearly show that K values of turbot are not suitable as an indicator for freshness. Another ratio of nucleotides may therefore be more suitable for indicating freshness. In literature it is reported that an H value can be used as an indicator for freshness. The H value evaluates the accumulation of hypoxanthine compared to the pool of inosine monophosphate, inosine and hypoxanthine. The evolution of H value as function of storage time is shown in figure 2. This figure suggests that the H values may be used as an indicator of freshness for turbots. Statistical analysis did not reveal significant differences for H values between the experimental (NTE) and industrial (TI) procedure.

To conclude, transport followed by live chilling does not affect the freshness of the product, compared to electrical stunning and chilling applied directly to turbot in a holding tank, as determined by applying H value as freshness index. Visual inspection also revealed that the product obtained by the experimental procedure is similar to the industrial one. These results show that electrical stunning of turbot can be used in practice.

: Dutch turbot farm