Effect of Freeze-Thaw Cycles on the Freshness of Prepackaged Penaeus vannamei

The effect of temperature fluctuations on the freshness of shrimp in simulated trays was investigated by setting a freeze-thaw (F-T) cycle of 12 h after freezing at -20 °C and thawing at 1 °C under refrigeration. The results showed that the shrimp's physicochemical properties deteriorated to different extents with the increase in F-T cycles. The total colony count of shrimp was 6.07 lg CFU/g after 21 cycles, and the volatile saline nitrogen content reached 30.36 mg/100 g, which exceeded the edible standard. In addition, the sensory quality and textural properties (hardness, elasticity, chewiness, and adhesion) declined to different degrees with increased F-T cycles. LF-NMR and protein property measurements showed that F-T cycles resulted in reduced water holding capacity and protein denaturation, which were the main factors leading to the deterioration of shrimp quality. Furthermore, flavor changes were analyzed using an electronic nose sensor to establish a freshness model. The W1W, W1S, W2S, and W5S sensors were correlated with the quality changes in shrimp and used as the main sensors for detecting the freshness of Penaeus vannamei. As a result, to better maintain the overall freshness, temperature fluctuations should be minimized in sales and storage, and fewer than 8 F-T cycles should be performed.

[Xylem anatomical characteristics of Fraxinus mandshurica and relationship with climate in different slope positions]

With global climate warming, warming and drying in Northeast China are becoming more and more serious. To understand the effects of drought on the radial growth of Fraxinus mandshurica at different slope positions in Xiaoxing'an Mountains, we analyzed the responses of xylem anatomical characteristics of F. mandshurica to moisture change at different slope positions by wood anatomi-cal method. The results showed that F. mandshurica on the upper and lower slope positions were the same in the changes of xylem anatomical parameters. The abundance of vessels showed a linear distribution, the maximum vessel area and total vessel area showed a logarithmic distribution, while the average vessel area showed a partial normal distribution. There were significant differences in many anatomical parameters among different slope positions. The abundance and area of vessels on the lower slope positions were larger than those on the upper slope positions. Most of the anatomical parameters were significantly positively correlated with daily accumulated temperature, though there were some differences between them. The total vessel area of F. mandshurica on the upper slope positions was positively correlated with the daily mean temperature, while the number and total area of vessels were positively correlated with the relative humidity. The correlation on the upper slope positions was stronger than that on the lower slope positions. The minimum vessel area was negatively correlated with daily accumulated temperature and relative humidity on the upper slope positions, while the total vessel area was positively correlated with relative humidity on lower slope positions. The warming and drying climate would reduce the area and number of vessels in F. mandshurica, but would not affect the distribution of vessels, while the distribution of vessels on the upper and lower slope positions was generally the same. The current climate warming and drying do not limit but promote the radial growth of F. mandshurica.