# Research and Development Status of Prepared Foods in China: A Review
## 6 Sterilization and Packaging of Prepared Foods
### Sterilization Technology  
Sterilization is the most important method to keep prepared foods from being contaminated by microorganisms that can spoil them [85]. Thermal sterilization is currently the most widely used method in the prepared foods industry because it is stable, effective, and low-cost. Traditional thermal sterilization technology is mainly through high-temperature heating to achieve sterilization (commercial sterilization) [86], which can kill most of the microorganisms and spores in prepared foods, greatly extending the shelf life of prepared foods, and methods include pasteurization [87] and ultra-high-temperature instant sterilization. However, the limitation of traditional thermal sterilization technology is that it will destroy the nutritional value of prepared foods and change the sensory quality of prepared foods [88]. Companies or laboratories in the industry are currently developing non-traditional food sterilized technologies in order to overcome the limitations of thermal sterilization technology [76]. Non-conventional food sterilization technologies can be divided into two main categories: the first refers to the use of high-pressure treatment, UV,  
Figure 4: The processing of prepared foods. pulsed light, ultrasound, and pulsed electric field combined with heat treatment; the other category is purely non-thermal technology, including cold plasma technology [89] and irradiation technology [76]. This review focuses on high hydrostatic pressure (HHP) processing technology, ultrasonic sterilization technology [90], and irradiation technology [91]. High hydrostatic pressure processing technology is a new type of non-thermal food processing technology [92]. Prepared foods placed in specific containers will be subjected to extremely high pressures (usually over 600 MPa), and this pressure is then transmitted to the microorganisms through the medium (usually water), which induces damage to the cell membrane, leading to denaturation of cellular proteins and a decrease in intracellular pH, resulting in microbial inactivation. HHP is able to inactivate most of the bacterial microorganisms [93] in prepared foods without causing changes in the sensory and nutritional value of the foods [94]. Ultrasonic sterilization technology [95] is used to inactivate microorganisms through the cavitation phenomenon generated by ultrasonic waves [96]. Ultrasound is a pressure wave with a frequency equal to 20 kHz or higher. Ultrasound will cause bubbles to form in the liquid medium, and then the liquid bubbles will continue to grow until the energy of ultrasound is not enough to support the gas phase in the bubbles, which will cause the bubbles to collapse rapidly and produce a shock wave. The shock wave will produce extremely high temperature and pressure in the local area in a short time (less than 100 ns), and the high temperature and pressure will kill microorganisms and achieve the purpose of sterilization. Ultrasonic sterilization is usually combined with thermal sterilization to achieve the best sterilization results [97]. Ultrasound can accelerate the thermal sterilization of prepared foods and reduce processing time and nutritional loss, and its ultrasonic frequency for sterilization is generally 20 to 100 kHz [77]. Prepared food irradiation [98] is a non-thermal process that achieves microbial inactivation by exposing the food to a certain amount of ionizing radiation generated artificially or by natural sources [99]. The lethal effects of radiation on microorganisms are classified as direct or indirect. The direct effect is that the radiation damages the DNA of the target microorganism, inhibiting DNA synthesis to prevent cell division; the indirect effect is due to irradiation interacting with water molecules, leading to the production of reactive molecules, the production of which eventually leads to microbial cleavage. There are three main food irradiation technologies, namely, electron beam, X-ray, and gamma rays.