| The principle of vacuum cooling is based on rapid evaporation of part of the moisture of the product under vacuum. The latent heat of product itself is moved and cooling is achieved. Vacuum cooling is a key link of the implementation of seamless joint of post harvest horticultural products, the loss reduction of storage and transportation and the shelf-life extension.One of the most important factors affecting the quality of horticultural crops is temperature. In previous papers, thermocouples were secured by tapes to measure the temperature. There are three major disadvantages to thermocouple temperature measurement:①causing a restriction in the escape of water vapor, which certainly affects the microclimate around the thermocouple.②It is very difficult to measure the petal temperature without damaging the thin fragile membrane with the thermocouple.③The inherent limitation of thermocouple is that each thermocouple can measure temperature only at the point on a surface that it is contacting. It is impractical to use more than a small number of thermocouples.Moreover, there has been little information about biological study, especially for cell membrane system and microstructure changes during vacuum cooling. There is a complex binding relationship between the moisture, constituents and microstructure of materials. The water evaporation during vacuum cooling is not the theoretical evaporation of free water. There are several practical productive problems such as how do the states and participation level in cooling of intracellular moisture, how does the effect on water vaporization affected by the changes of cell membrane system and microstructure. Especially to membrane system, which was subject to rapid pressure reduce in a short time and influenced by water evaporation and a long time vacuum environment. It was not clear whether the changes of pressure and forces could damage the cell membrane system.Cut flower is a very popular horticultural product, with high economic value, good social and ecological benefits. Different physical properties of petals, leaves and stems of flowers affect the temperature changes during vacuum cooling and the main quality during vase period. Onion epidermis composed by a single layer cells with large size and typical plant cell structure is an ideal material to study the changes in cell morphology. The reliable temperature measurement is beneficial to the judgment of pre-cooling end-temperature, control of cooling processes, and improvement of pre-cooling effects. The understanding and mastering of the mechanism of vacuum cooling is helpful for the scientific and deep development of vacuum cooling.The main research part can be divided into four parts as follows:1. Thermocouples were applied to measure the temperatures of cut flowers during vacuum cooling, and study the effects of loading and pressure reduction rates on the pre-cooling effects of cut roses.①The local environment made by different amounts of flowers had obviously influence （P<0.05） on the temperature especially for the inner parts of cut flowers. Though the temperature changes and average pre-cooling end-temperatures for different parts of 10, 50 and 120 flowers, the end-temperatures of external parts were higher than internal parts, the end-temperature of the internal petals were the lowest and the temperature reduction rates were the fastest. There were big differences between the individual flower and other three treatments. The fresh weight loss of 50 flowers treatment was the smallest and its vase life was the longest.②The activities of SOD, POD and CAT were improved. Consequently, the active oxygen produced during the aging process of cut flower was scavenged effectively, and the balance of active oxygen in vivo and the stability of cell membrane were maintained.③Different pressure reduction rates (3.11×104 Pa min–1, 1.78×104 Pa min–1, 0.62×104 Pa min–1) had significant effects （P<0.05） on the pre-cooling quality of cut roses. The moderate pressure reduction rate had the smallest influence on relative electrical conductivity, weight loss, MDA and pH.2. Thermal infrared imaging was used to realize the real-time measurement of the distributions and changes of temperatures of cut flowers, and compared with the results made by thermocouples.①Thermal infrared imaging showed that the average cooling rates of different parts of the rose were different （the slowest was the inner petals, the fastest was the outer petals, and there were no remarkable differences between the leaf and the stem） , the final temperature of cooling of the inner petal was 3.1 times higher than that of the outer petal. For cut gerberas, the fastest was the ligule-shape petal, and the slowest was the stem, the final temperature of cooling of the stem was 9.9 times higher than that of the ligule-shape petal. The sequences of cooling rates of different parts of rose and gerbera flowers during different cooling stages were different, and the temperature distributions of different parts measured with thermal infrared camera were non-uniform.②Under the same pre-cooling conditions, the temperatures detected by the thermal infrared imaging were ac. 2.46 to 4.50°C and 4.55 to 10.4°C lower than the results measured by thermocouples of cut rose and gerbera flowers, respectively. The results showed that thermocouple temperature measurement was affected by fixation method, location and water vapor.③The parts which chilling injury occurred easily were the outside edges of outer petals of cut roses and ligule-shape petal of gerbera flowers. In order to obtain better pre-cooling effects without chilling injury, temperatures of the different parts of cut flowers especially for the outside edges of outer petals must be monitored closely during vacuum cooling.④The changes of relative fresh weight, stomatal conductivity and transpiration of the individual flowers were bigger than bunches of flowers for both cut roses and gerberas. The average pre-cooling end-temperatures of bunches of flowers were higher than the individual flowers. Not only direct but also delay pre-cooling of cut roses and gerberas, the vase life of bunches of flowers were shorter than the individual flowers. Furthermore, the vase life of individual and bunches of flowers were decreased obviously （P<0.05） with time during vase period. In this experiment, two kinds of flowers lost their ornamental values after ten hours storage at room temperature.3. Effects of antitranspirant and vase solution on the quality of cut flowers were studied. Spray application of antitranspirant decreased the fresh weight loss, delayed the process of flowering, slowed down the decrease rate of stomatal conductivity, decreased transpiration and the accumulation of MDA and maintained the integrity of cell membrane. The vase life was extended by 32% compared with the control flowers. The combined use of antitranspirant and vacuum cooling could effectively improve the pre-cooling quality. Furthermore, the experimental effects of spraying antitranspirant followed by vacuum cooling were better than the treatment of vacuum cooling followed by the application of antitranspirant （P<0.05）, and the vase life was extended by 12% compared with the treatment of vacuum cooling followed by the application of antitranspirant.4. Effect of vacuum treatment on microstructure and physical properties of tissue.①The present study has shown that vacuum treatment induced the changes of onion epidermis microstructure.②For cellular membrane system, which was subject to rapid pressure reduce in a short time and influenced by microstress caused by water evaporation. Surface morphology and that cell rupture occurred result from the joint effects of water loss and pressure gradient. The vacuole and cell membranes were damaged which led to loss of intracellular water, which explained the reason for excessive loss of tissue water at micro-level. It presented preliminary proof of the excessive water loss induced by vacuum treatment.③Disruption and damage to cell membranes altered permeability led to the loss of electrolyte and anthocyanin, resulting in fade in cellular color and increase of electrical conductivity. The spaces between the membrane and cell wall decreased first and then increased with the decreasing of pressure in vacuum chamber.④SEM micrographs indicated that vacuum treatment induced significant changes on cell surface morphology, cell wall and intercellular layers of onion epidermis, which led to the decrease of tensile strengths of epidermal tissue.⑤Changed the pressure reduction rate had obvious effects on the surface morphology, spaces between the cell membrane and cell wall, mass, REC, color, and mechanical properties. Of the 3 pressure reduction rates, treatments at the slow and fast pressure reduction rates led to remarkable effects on the cell membrane integrity of the onion epidermis, while treatment at the medium pressure reduction rate caused less damage.This paper adopts the method of combination of macro-and microscopic, and the combination of real-time image analysis and detection of physiochemical indexes. The effects of temperature changes measured by thermal infrared imaging and thermocouples during vacuum cooling on the storing properties of cut flowers were analyzed and compared, so as to overcome some shortcomings such as the difficulties of temperature detection and pre-cooling end-temperature judgment of cut flowers during vacuum cooling. Furthermore, the impacts of vacuum cooling on the microstructure, especially for cell membrane system were studied. |
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