Nisin-resistant (Nisr) Listeria monocytogenes and Nisr Clostridium botulinum Are Not Resistant to Common Food Preservatives

Nisin‐resistant (Nisr) strains of Clostridium botulinum and Listeria monocytogenes may arise as nisin becomes more widely used as an additional safety barrier in minimally‐processed foods. The sensitivity of Nisr L. monocytogenes ATCC 700301 and ATCC 700302 and toxigenic Nisr C. botulinum 169B to low pH, salt, sodium nitrite, and potassium sorbate was assayed using discontinuous gradients in broth and compared to the parental wild‐type strains. The nisin‐resistant strains did not have intrinsic resistance to low pH, sodium chloride, potassium sorbate, or sodium nitrite. In no case were the Nisr L. monocytogenes and C. botulinum strains examined more resistant to inhibitors than the parental strains.     

Kinetic study of olefin polymerization with a supported metallocene catalyst. II. Ethylene/1‐hexene copolymerization in gas phase

A kinetic study of ethylene/1‐hexene copolymerization is conducted with a supported metallocene catalyst in a gas‐phase reactor. The investigation into the kinetics of ethylene/1‐hexene copolymerization includes the effects of operational parameters such as the reaction temperature, pressure, and comonomer concentration. The large variations in gas‐phase composition using only an initial charge of 1‐hexene are illustrated by experiment. To remedy this, the ability to control the comonomer composition of 1‐hexene online for the entire duration of the reaction is demonstrated. Online perturbation techniques are implemented to determine key kinetic parameters such as the activation energies for propagation and catalyst deactivation. From pressure perturbation results, a reaction rate order close to 1 is obtained for ethylene in the presence of 1‐hexene. Finally, all the parameters obtained from the study are compared to those determined from ethylene–propylene (E–P) copolymerization. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1096–1119, 2001     

Production of ultra-high temperature carbide (Ta,Zr)C by self-propagating high-temperature synthesis of mechanically activated mixtures

The combustion temperatures and rates of mechanically activated (MA) Ta–Zr–C mixtures depending on the initial temperature T₀ are determined. The self-heating phenomenon is observed in argon atmosphere at T₀>380 K due to oxidation of the surface of zirconium particles by adsorbed oxygen. Zirconium oxide is formed in the combustion zone at the initial stage of chemical interaction; it is subsequently transformed into zirconium carbide. In addition, tantalum carbide is formed in the combustion zone, while the binary tantalum–zirconium carbide (Ta,Zr)C is formed closer to the post-combustion zone. In order to maintain the layer by layer stationary combustion mode of SHS, the initial temperature T₀ needs to be 298 K, while the duration of mechanical activation needs to be less than 5 min. After longer mechanical activation, the mixtures are prone to bulk combustion even at low initial temperatures. Single-phase (Ta,Zr)C carbide with the lattice parameter of 0.4479 nm was synthesized by forced SHS compaction in a sand mold.     

自动电位滴定仪标定硝酸银标准溶液

采用配置DM141-SC型银电极的DL58型自动电位滴定仪标定硝酸银标准溶液,准确度及精密度均较手动滴定有很大程度的提高,且操作简单,仪器维护方便,滴定结果更准确,滴定过程采集信号为0.1mV,滴定最小进给量可达到0.001mL,是目前值得推荐的方法.     

共沉淀法制备ZrO2-Al2TiO5复合材料烧结性能研究

以平均粒径为5μm的二氧化锆微细粉为被覆体，分散在Al^3＋浓度0．02mol·L^-1可溶铝盐及Ti^3＋浓度0．01mol·L^-1的钛盐溶液中，以浓度为0．2mol·L^-1的氨水为沉淀剂，pH控制为5.3，获得Al2O3和TiO2纳米颗粒被覆ZrO2的悬浮体。将悬浮体脱水、干燥，经1350℃，1400℃，1450℃，1500℃烧结获得ZrO2-Al2TiO5复相陶瓷材料。结果表明：组成为95％ZrO2并经1500℃烧结的试样，其气孔率1．08％、体积密度4．57g/cm^3、抗折强度71．2MPa。     

Intake and risk assessment of nitrate and nitrite from New Zealand foods and drinking water

Exposure to excess nitrite is a potential health risk for humans. One hundred meat and processed foods and 100 vegetable samples purchased from New Zealand retail outlets were prepared as for consumption and analysed for nitrite and nitrate concentration using a standard, validated methodology. Nitrate concentrations ranged from less than the limit of detection (LOD = 5 mg kg^-1) in cheddar cheese and cream cheese-based dips to 3420 mg kg^-1 in lettuce. Nitrite was detected in half the processed foods and meats analysed (levels up to 119 mg kg^-1), but detected in only one vegetable sample above the LOD (broccoli at 27 mg kg^-1 nitrite). Concentration data were combined with 24 h dietary recall information to generate 4398 individual adult daily exposure scenarios for exogenous nitrite and nitrate including a contribution from water assessed from 1021 drinking water samples. The mean adult daily intake of exogenous nitrate and nitrite from food and water combined was 16 and 13% of the Acceptable Daily Intake (ADI), respectively, and therefore should not pose a health risk for the average consumer. A maximally exposed New Zealand adult is estimated to have an intake of up to seven times the ADI for nitrate. When the endogenous conversion of nitrate to nitrite is taken into account, approximately 10% of people with an average rate of conversion and half of all people with a high rate of conversion are estimated to exceed the ADI. Either the ADI is inappropriate and needs to be re-evaluated, or those individuals who have a high rate of conversion of nitrate to nitrite are at risk to adverse effects of nitrite exposure.     

装饰薄膜ZrN的中频反应磁控溅射沉积工艺研究

利用中频反应磁控溅射技术在1Cr18Ni9Ti不锈钢基底上沉积ZrN薄膜。通过控制N/Ar、溅射功率和基体偏压等参数,得到不同实验条件的ZrN膜层。通过对膜层颜色测量和AES分析,研究N分压强对ZrN膜层质量的影响。实验结果表明：工作气压0.3Pa,溅射功率5kW,基体偏压-150V、占空比50%等工艺参数一定的前提下,N分压强在不同的范围内,可以分别制备出视觉效果类似于18K、23K和纯金的氮化锆膜层。     

滴灌系统运行方式和施肥频率对番茄根区土壤氮素动态的影响

在日光温室内开展田间试验，研究了滴灌施肥灌溉系统运行方式和施肥频率对番茄根区硝态氮在生育期内随时间的变化动态、空间分布特征以及氮肥利用效率和回收率的影响。系统运行方式包括不同灌水和施肥次序组成的3种方案。结果表明，在整个生育期内，在灌溉过程的前期施肥，硝态氮在各土层中的分配较为均匀；随着施肥次序向后推移，硝态氮在0～20cm土层的累积逐渐明显。随着施肥频率降低，硝态氮在生育期内随时间的变化逐渐加剧。在滴头的周围，硝态氮易随水流运动，并在湿润土体的横向边缘产生累积。随着施肥次序向前推移，硝态氮向湿润土体边缘运移的趋势愈加明显。剖面内的硝态氮总量随着施肥频率的降低而降低。随着施肥次序向后推移，氮回收率逐渐增大，氮肥利用效率逐渐减小。氮肥利用效率随施肥频率提高而增加。在滴灌施肥灌溉条件下，运行方式选用最初灌溉总时间的1／4时间灌水，再用1／2时间施肥，最后用1／4时间冲洗管道，施肥频率取每周一次较为适宜。     

一种推进剂分解机理的量子化学方法分析

运用量子化学方法,优化计算得到了硝酸异丙酯的稳定几何构型,结合该构型对其静态性质进行了分析.使用两种不同的算法,分别计算了气相状态下硝酸异丙酯热分解和碱性环境下水解反应的路径及其活化能,并与实验值进行了对比.结果显示,量化方法能够比较准确地预测和计算分解路径,但对活化能的准确计算有待进一步研究.