Preservation effect of oyster by sono-photodynamic sterilization technology

Oyster is one of the four favorite cultured shellfish in China, particularly in the high content of mineral elements rich in vitamins and high-quality proteins. However, the meat of oysters was easy to be infected by external microorganisms, due to the direct exposure to the air after opening. In addition, the fat oxidation, yellowing, and juice loss of oyster meat can also occur during storage, leading to destroy the sensory quality and the economic value of oysters. Therefore, it is necessary for new and efficient sterilization to inhibit microbial contamination for oyster preservation. Traditional thermal sterilization technology mainly transmits a large amount of heat energy to the food via heat conduction, in order to achieve the purpose of killing microorganisms and inactivating enzymes at high temperatures. However, some damage can often occur to the food texture and flavor during thermal sterilization. Fortunately, the Sono-photodynamic sterilization technology (SPDT) is a new type of sterilization to combine sonodynamic and photodynamic technology. The sterilization can be expected to activate the Sono-photosensitizer molecules for a series of active oxygen bactericidal substances under the action of a specific wavelength of light and ultrasound. The purpose of this study is to explore the preservation effect of sono-photodynamic sterilization technology on oysters in view of the shortcomings of photodynamic sterilization and sonodynamic sterilization technology alone. Firstly, the growth curve was drawn for the total colonies number of microorganisms in oysters at 4℃. The initial colony number of oysters was (4.02±0.02) lg (CFU/g), and the shelf life was only 6 days. Then, orthogonal experiments were carried out based on the single factor experiments of curcumin concentration, sono-photodynamic processing time and ultrasonic power. The results showed that the fresh oysters treated with 50 μmol/L curcumin, 600 W ultrasonic power for 60 min LED exposure presented the best preservation effects, and the total number of bacterial colonies was 4.52 lg (CFU/g). Finally, the quality changes of oyster products were evaluated to determine the total bacterial count and volatile base nitrogen under the optimal preservation parameters of Sono-photodynamic sterilization. The results showed that the Sono-photodynamic sterilization was used to effectively kill the microorganisms in the oysters for the extending shelf life of oysters to 12 days. The thiobarbituric acid value determination showed that the light in the Sono-photodynamic sterilization accelerated the oxidation of oyster fat. Therefore, the sterilization effect and exposure time were considered under oxidation reaction conditions. The color difference analysis, pH value, and hardness measurement showed that the Sono-photodynamic sterilization can be expected to effectively delay the color change of oysters, and then control the spoilage rate of oysters, finally preventing the autolysis of oysters. The sensory evaluation showed that the Sono-photodynamic sterilization can maintain the product quality for the longer shelf life of oysters. In general, comparison of sono-photodynamic group with control group, curcumin group, illumination group and ultrasonic group，sono-photodynamic group can keep the pH value, thiobarbituric acid and volatile base nitrogen of oysters at low values, maintain the sensory quality of oysters such as appearance, hardness and color, and prolong the shelf life of oysters from 6 days to 12 days at 4 ℃. In summary, Sono-photodynamic sterilization can be used to obtain better bactericidal activity for the preservation of oysters.