Introduction
Cold plasma is an emerging sanitizing technology that has been implemented in recent years to extend the shelf-life of food product (Andoni et al., 2022.) (Olatunde et al., 2021). Seafood products are among the main constituents of food diets, important for their content of vitamins, proteins and other essential constituents such as polyunsaturated fatty acids (PUFA) (FAO, 2020). The main concern associated with this matrix is the rapid deterioration of freshness and quality. The aim of this study was to evaluate the application of cold gas plasma (CP) as a non-thermal preservation technology to prolong the shelf life of sea bream (Sparus aurata) fillets, stored in modified atmosphere packaging (MAP).
Materials and methods
The samples were divided into three experimental groups, two of which were respectively treated in air (18 kV for 20 min), or argon (80%) and oxygen (20%) mixture (18 kV for 20 min), while the third one was not treated (control). A plasma source with an SDBD (surface dielectric barrier discharge) configuration was used for plasma generation. The samples were following packed in conventional MAP (80% N2 and 20% CO2), and subjected to refrigerated storage (4±1 °C) for 14 days. During this period, at different storage time, possible differences among the samples were investigated in terms of quality characteristics (e.g. O2 and CO2 in the package headspace, pH, water content, t-Bars, texture, colour and sensorial properties, microbes load). Experimental data were subjected to two-way analysis of variance (ANOVA) to determine the significant differences among samples during the storage and in each sampling time. Tukey HSD (Honestly Significant Difference) multiple range test, at a significance level of p < 0.05 was applied.
Results
Considering the physico-chemical characteristics, with the application of cold air plasma for 20 min, a reduced pH value of the product was recorded during storage, probably caused by the formation of acids (HNO3, HNO2, H2O2 and O3) in the aqueous phase of the fish flesh, and a simultaneous slowing down of the natural increase in pH during shelf-life, promoted by the reduction of enzymatic activities. All the samples did not show significant differences in terms of texture parameters, evidencing that the applied treatments did not promote protein oxidation and/or evident structural modification, This aspect was also confirmed by the constant maintenance of the water content during storage for all the considered samples.
The major limitation of the application of cold plasma to fish matrices, or those characterized by a high proportion of unsaturated fatty acids, can be lipid oxidation, considering the ‘oxidative nature’ of this technology for food applications. Actually cold plasma is constituted by various reactive species, including free radicals, positive and negative ions. These species responsible of its potential sanitizing effect can also stimulate the oxidation process in food products with a high fat content, having a negative impact on their sensory and nutritional quality.
The present study showed that already by the sixth day of storage, the samples treated by argon plasma, reached the quality limit of t-Bars > 4 mg MDA/kg of fresh product compared to the samples treated by air plasma (2.4mg MDA/kg) and to control samples (1.20 mg MDA/kg). Color measurement following the application of both treatments also revealed significant changes in the red-green index colour (a*), the reduction of which can be correlated with lipid oxidation of the matrix.
The microbiological analyses conducted were evaluated for an extended shelf life (20 days). The presence of total mesophiles was evaluated, setting a limit of 6 Log10 CFU/g. Results showed that especially at day 10 and day 20 of storage, the treated samples had a lower total load than the control; and in particular at day 10, the control and argon group exceeded the set limit, while the data obtained from the analysis of the air-treated samples showed that the value was still below the limit. At day 20, the differences were significant, but all experimental groups were considered to be microbiologically unacceptable.
Although the sensory analysis showed that the overall visual and olfactory acceptability by the consumer did not result in significant variations, this technology seemed to accelerate oxidative processes. For this reason, it could be useful to conduct further studies to investigate the application of the ’hurdles theory’, which envisages the application of cold plasma in conjunction with other stabilizing factors, such as the use of antioxidants etc., in order to minimize the negative side effects on the food matrix of the CP treatment (e.g. lipid oxidation).
References
Andoni, E., Ozuni, E., Bijo, B., Shehu, F., Branciari, R., Miraglia, D., & Ranucci, D. (2022). Efficacy of Non-thermal Processing Methods to Prevent Fish Spoilage. Journal of Aquatic Food Product Technology., 30(2), 228–245.
FAO. (2020). The State of World Fisheries and Aquaculture 2020. Sustainability in action. Fao. doi:https://doi.org/10.4060/ca9229en
Olatunde, O. O., Shiekh, K. A., & Benjakul, S. (2021, May 1). Pros and cons of cold plasma technology as an alternative non-thermal processing technology in seafood industry. Trends in Food Science and Technology. Elsevier Ltd. doi:10.1016/j.tifs.2021.03.026