Supercritical carbon dioxide drying of tuna fillet: optimization and quality study

Food drying is an effective and well-known preservation technique that ensures a prolonged shelf life by reducing the amount of water in the food. Currently available drying technologies showed some limitations, such as quality degradation, limited inactivation against microorganisms, together with high production cost and energy consumption. In this regard, the use of supercritical carbon dioxide (scCO2) has been recently investigated as an alternative low temperature drying technique able to dry and inactivate microorganisms simultaneously, allowing microbiologically safe and high-quality products. The present Thesis explores the feasibility to apply scCO2 drying on tuna fish fillets. Experiments were carried out with a lab-scale reactor with recirculation. A central composite design was used to investigate the impact of temperature (35-45 °C), treatment time (2-6 h) and CO2 flowrate (15-25 kg/h) on the drying performance and the final product characteristics. Specifically, the response surface methodology aimed to identify an optimal domain of process conditions to obtain a sufficiently dried product of good quality. The drying efficiency was evaluated in terms of the final moisture content, weight loss, moisture ratio and water activity. The microbial inactivation capacity was evaluated toward the total mesophilic bacteria naturally present in the tuna. The sample was also characterized for the rehydration capacity and the colour and pH change before and after the process was measured. The results showed that time and flowrate had the most significant effect on the drying performance achieving the minimum value of moisture content (18.4 %) and water activity (0.679) at 7 h and 20 minutes, 40 °C, 20 kg/h and 6 h , 45°C, 25 kg/h respectively. The samples with lower moisture content demonstrated a higher capacity of rehydration. Furthermore, from the physical-chemical analysis, no change of the pH in the treated sample was measured. The process was able to completely inactivate the total mesophilic bacteria in 60 % of the experimental trials, but a trend with the process variable was not deducted. The colour of the samples changed after the process and it was observed that longer was the time, the higher the change on the final colour of the sample with respect to the raw one. The results demonstrated the capacity to obtain a dried product (≤ 30 % of moisture content) using a wide range of experimental conditions. For this reason, the process optimization continued considering also the effect on physical and structural attributes, comparing products dried at three optimal conditions. These properties were analyzed at the Federal University of Rio de Janeiro (UFRJ), by the research group of Professor Carlos Adam Conte Junior. Similar water holding capacity and textural properties, with exception for hardness and the related microstructure, were obtained. Consequently, the optimal condition might be selected considering an economic perspective or further studies on product stability over time. In conclusion, the results achieved in the present work confirm the potentiality of scCO2 drying technique as an alternative to conventional processes.