| In the recent decades, the high pressure technology has been widely applied in the food sterilization, storage and functional food development fields, and the technology of high-pressure low-temperature treatment of food has attracted lots of attentions due to its advantages of fresh-keeping, energy-saving and environmental Potection. But the development and application of this technology have been stalled by the lack of fundamental study and a mechanism study, as well as some unexplained complex phenomenons. Using a self-designed high-pressure low-temperature equipment, the freezing and thawing experiments were conducted with the representative plant food and animal food as model materials. The phase behavior of food during high-pressure freezing&thawing processes has also been simulated, aiming to get further understanding of the thermodynamic properties during the process. The main research work and conclusions are as follows:(1) Design and build a high-pressure low-temperature equipment with a pressure range of0～600MPa and a temperature range of-30℃～70℃. Different types of high-pressure freezing&thawing experiments were conducted on representative plant food and animal food such as potatoes, Chinese yam. white turnip, fish, mutton and pork(2) The freezing experimental studies of water are conducted including the atmospheric freezing, pressure-assisted freezing and pressure-shift freezing experiments. The results reveal the sub-cooled and metastable phenomenon of water at high pressure and provide basic data for designing proper high-pressure freezing&thawing processes for food.(3) The results of high-pressure freezing experiments of the food samplesrevearthe relationship of the phase change plateau time and the freezing effect. In the high pressure assited freezing process, the increase of the ice crystal size and the plateau time extension come along with the increase of operating pressure, resulting in more damage of the food structure. But in the high pressure shifted freezing process, the inverse effects are observed. The results indicate that the high pressure assited freezing process disagrees with the food storage, while the high pressure shifted freezing process shows unique advantages in the process of food storage. Analysis of the regular pattern of the metastable phenomenon of different samples through pressure-assisted freezing experiments is helpful in determing the sub-cooling degree. (4) The different effects of different freezing&thawing processes on foods are investigated through the analysis of the thawing time, the drip loss, histology and texture. The high-pressure thawing can save the phase change time, reduce the drip loss and protect food texture. The combination of the high pressure shifted freezing and the pressure-assisted thawing contributes to the protection of the food quality, decreases the drip loss and nutrition loss, and enhances the taste and flavor of food. The optimized freezing&thawing processes are also determined for different kinds of food.(5) Simulations of the pork thawing process at room pressure and high pressure are conducted respectively by the use of the sensible heat capacity method. The treatment effects of different impact factors and different thawing time with different sample size and pressure condition are also investigated. The results show that pressure-assisted process can obviously shorten the thawing time, and the phase-change temperature decreasing is the dominated factor influencing the thawing time. The increasing of sample size results in a dramatically drop of thawing time. Finally, thawing time is related with treatment pressure in a quadraticis way.(6) According to the continuity of entropy, the entropy calculation model and thermodynamic criterion are chosen to simulate the phase behavior during high-pressure shifted freezing process. The phase change plateau time, the freezing time and instantaneous ice ratio are calculated. The experimental data have confirmed the feasibility of the chosen calculation model. The freezing time and the phase change plateau time decrease with the increase of pressure during the high pressure shifted freezing process. The operation time is prolonged with the decrease of the precool temperature, and the instantaneous ice ratio increases with the increase of pressure. |
