Ultrasonic monitoring of fish thawing process optimal time of thawing and effect of freezing/thawing

International audience; Overseas fi sh is either directly treated on boats before being frozen or directly frozen, then treated in the factory after being unloaded. At the factory, the thawing of that fi sh is an essential step in its transformation and treatment; the quality of the texture is directly related to the quality of thawing. During this ABSTRACT Introduction. Fish quality is traditionally controlled by chemical and microbiological analysis. The non-destructive control presents an enormous professional interest thanks to the technical contribution and precision of the analysis to which it leads. This paper presents the results obtained from a characterisation of fi sh thawing process by the ultrasonic technique, with monitoring thermal processing from frozen to defrosted states. Material and methods. The study was carried out on fi sh type red drum and salmon cut into fi llets of 15 mm thickness. After being frozen at-20°C, the sample is enclosed in a plexiglas vessel with parallel walls at the ambient temperature 30°C and excited in perpendicular incidence at 0.5 MHz by an ultrasonic pulser-receiver Sofranel 5052PR. the technique of measurement consists to study the signals refl ected by fi sh during its thawing , the specifi c techniques of signal processing are implemented to deduce informations characterizing the state of fi sh and its thawing process by examining the evolution of the position echoes refl ected by the sample and the viscoelastic parameters of fi sh during its thawing. Results. The obtained results show a relationship between the thermal state of fi sh and its acoustic properties , which allowed to deduce the optimal time of the fi rst thawing in order to restrict the growth of microbial fl ora. For salmon, the results show a decrease of 36% of the time of the second thawing and an increase of 10.88% of the phase velocity, with a decrease of 65.5% of the peak-to-peak voltage of the signal refl ected, thus a decrease of the acoustic impedance. Conclusions. This study shows an optimal time and an evolution rate of thawing specifi c to each type of fi sh and a correlation between the acoustic behavior of fi sh and its thermal state which approves that this technique of ultrasonic monitoring can substitute the control using the destructive chemical analysis in order to monitor the thawing process and to know whether a fi sh has suffered an accidental thawing.