Effect of temperature of CO2 injection on the pH and freezing point of milks and creams

The objectives of this study were to measure the impact of CO2 injection temperature (0 degree C and 40 degrees C) on the pH and freezing point (FP) of (a) milks with different fat contents (i.e., 0, 15, 30%) and (b) creams with 15% fat but different fat characteristics. Skim milk and unhomogenized creams containing 15 and 30% fat were prepared from the same batch of whole milk and were carbonated at 0 and 40 degrees C in a continuous flow CO2 injection unit (230 ml/min). At 0 degree C, milk fat was mostly solid; at 40 degrees C, milk fat was liquid. At the same total CO2 concentration with CO2 injection at 0 degree C, milk with a higher fat content had a lower pH and FP, while with CO2 injection at 40 degrees C, milks with 0%, 15%, and 30% fat had the same pH. This indicated that less CO2 was dissolved in the fat portion of the milk when the CO2 was injected at 0 degree C than when it was injected at 40 degrees C. Three creams, 15% unhomogenized cream, 15% butter oil emulsion in skim milk, and 15% vegetable oil emulsion in skim milk were also carbonated and analyzed as described above. Vegetable oil was liquid at both 0 and 40 degrees C. At a CO2 injection temperature of 0 degree C, the 15% vegetable oil emulsion had a slightly higher pH than the 15% butter oil emulsion and the 15% unhomogenized cream, indicating that the liquid vegetable oil dissolved more CO2 than the mostly solid milk fat and butter oil. No difference in the pH or FP of the 15% unhomogenized cream and 15% butter oil emulsion was observed when CO2 was injected at 0 degree C, suggesting that homogenization or physical dispersion of milk fat globules did not influence the amount of CO2 dissolved in milk fat at a CO2 injection temperature of 0 degree C. At a CO2 injection temperature of 40 degrees C and at the same total CO2 concentration, the 15% unhomogenized cream, 15% vegetable oil emulsion, and 15% butter oil emulsion had similar pH. At the same total concentration of CO2 in cream, injection of CO2 at low temperature (i.e., < 4 degrees C) may produce a better antimicrobial effect during refrigerated shelf life due to the higher concentration of CO2 in the skim portion of the cream.     

Serum protein and casein concentration: effect on pH and freezing point of milk with added CO2

The objective of this study was to determine the effect of protein concentration and protein type [i.e., casein (CN) and serum protein (SP)] on pH (0 degree C) and freezing point (FP) of skim milk upon CO2 injection at 0 degree C. CN-free skim milks with increasing SP content (0, 3, and 6%) and skim milks with the same SP content (0.6%) but increasing CN content (2.4, 4.8, and 7.2%) were prepared using a combination of microfiltration and ultrafiltration processes. CO2 was injected into milks at 0 degree C using a continuous flow carbonation unit (230 ml/min). Increasing SP or CN increased milk buffering capacity and protein-bound mineral content. At the same CO2 concentration at 0 degree C, a milk with a higher SP or a higher CN concentration had more resistance to pH change and a greater extent of FP decrease. The buffering capacity provided by an increase of CN was contributed by both the CN itself and the colloidal salts solublized into the serum phase from CN upon carbonation. Skim milks with the same true protein content (3%), one with 2.4% CN plus 0.6% SP and one with 3% SP, were compared. At the same true protein content (3%), increasing the proportion of CN increased milk buffering capacity and protein-bound mineral content. Milk with a higher proportion of CN had more resistance to pH change and a greater extent of FP decrease at the same carbonation level at 0 degree C. Once CO2 was dissolved in the skim portion of a milk, the extent of pH reduction and FP depression depended on protein concentration and protein type (i.e., CN and SP).     

热交换器设计中两种设计方法的比较

分析比较了热交换器设计中常用的对数平均温差法和传热单元数法之间的联系和区别,并以增压柴油机的板翅式中冷器的设计为例,比较并验证了两种方法的设计结果.结果表明:两种方法设计出的换热器虽有一定偏差,但都可满足设计要求.     

Short communication: Urea hydrolysis in dairy cattle manure under different temperature,urea, and pH conditions

The objective of the study was to quantify the rate of urea hydrolysis in dairy cattle manure under different initial urea concentration, temperature, and pH conditions. In particular, by varying all 3 factors simultaneously, the interactions between them could also be determined. Fresh feces and artificial urine solutions were combined into a slurry to characterize the rate of urea hydrolysis under 2 temperatures (15°C and 35°C), 3 urea concentrations in urine solutions (500, 1,000, and 1,500 mg of urea-N/dL), and 3 pH levels (6, 7, and 8). Urea N concentration in slurry was analyzed at 0.0167, 1, 2, 4, 6, 8, 12, 16, 20, and 24 h after initial mixing. A nonlinear mixed effects model was used to determine the effects of urea concentration, pH, and temperature treatments on the exponential rate of urea hydrolysis and to predict the hydrolysis rate for each treatment combination. We detected a significant interaction between pH and initial urea level. Increasing urea concentration from 1,000 to 1,500 mg of urea-N/dL decreased the rate of urea hydrolysis across all pH levels. Across all pH and initial urea levels, the rate of urea hydrolysis increased with temperature, but the effect of pH was only observed for pH 6 versus pH 8 at the intermediate initial urea concentration. The fast rates of urea hydrolysis indicate that urea was almost completely hydrolyzed within a few hours of urine mixing with feces. The estimated urea hydrolysis rates from this study are likely maximum rates because of the thorough mixing before each sampling. Although considerable mixing of feces and urine occurs on the barn floor of commercial dairy operations from cattle walking through the manure, such mixing may be not as quick and thorough as in this study. Consequently, the urea hydrolysis rates from this study indicate the maximum loss of urea and should be accounted for in management aimed at mitigating ammonia emissions from dairy cattle manure under similar urea concentration, pH, and temperature conditions reported in this experiment.     

Mathematically modeling the repair of heat-injured Listeria monocytogenes as affected by temperature, pH, and salt concentration

Heat-injured cells of Listeria monocytogenes were inoculated into Listeria repair broth (LRB) adjusted to various pH levels (4.2, 5.0, 6.6, 8.0 and 9.6) and salt concentrations (0.5%, 2.5%, 5.0%, 7.5% and 10.0% w/v) at controlled temperatures (4, 10, 22, 37 and 43 °C) in a complete factorial manner (5³). Repair of the injured microorganisms was evaluated using selective and non-selective plating media. The Gompertz parameters, which were generated by fitting the equation with the bacterial counts, were used to calculate the repair percentage as a function of time from which the repair time was estimated. All growth curves fit the Gompertz equation well (R² ≥ 0.972). A first-order model described the repair trend closely (R² = 0.989 ± 0.011). Heat-injured Listeria could fully repair in LRB only under 63 of 125 conditions tested during 21 days of incubation. Refrigeration temperature was the most effective means to prevent the repair of heat-injured Listeria. The minimum temperature required for repair increased with an increase in NaCl concentration. The pH ranges at which the repair could occur were narrower at 4 and 10 °C than those at higher temperature. The repair was observed in media containing 10% NaCl between temperatures of 22 and 43 °C at pH 6.6.     

刮板式换热器研究现状及展望

文章阐述了刮板式换热器的结构、原理,对刮板式换热器的工艺应用、内部流动规律、剪切速率、传热机制、停留时间分布和功率消耗等进行了系统性归纳,并对刮板式换热器未来的发展趋势和研究方向进行了展望。     

非均布热管换热器的流动及其传热性能

为降低热管换热器的流动阻力并增强其换热效率,提出一种水滴形非均布热管换热器结构。使用计算流体力学软件ANSYS Fluent对传统错排热管阵列、水滴形热管阵列及非均布水滴形热管阵列的换热性能展开数值模拟,得到了其在不同雷诺数下的努塞尔数、阻力系数以及综合换热性能指标的变化曲线。对比分析结果表明:相对于传统错排热管阵列,水滴形热管阵列有效传热面积较大,内部流体流动无涡出现,流动摩擦较小,具有更大的努塞尔数以及更小的阻力系数,综合换热性能指标更佳;通过非均布设计,水滴形热管阵列内的流动紊流度增加,努塞尔数变大,换热性能增加。     

CO2超临界流体代替水作介质铬鞣机理的研究(3)--CO2超临界流体条件下铬鞣初始pH值和温度的优化

利用自制的CO2超临界流体制革设备，考察了CO2超临界流体代替水作介质条件下的铬鞣因素：初始pH值和温度的影响。通过对各实验条件下坯革的收缩温度、铬含量、铬的渗透以及革坯的状态等进行评价鞣制效果，得到了在CO2超临界流体条件下的铬鞣的最佳初始pH值为3．5，最佳鞣制温度为34℃。     

含芯棒水平换热管冷凝传热数值模拟

为提高蒸汽换热器换热效率,提出了一种在水平换热管中插入芯棒的含芯棒蒸汽换热管结构,并通过采用Fluent中两相流模型分析了该结构中芯棒直径对换热管管内冷凝换热特性的影响规律。研究结果表明,由于芯棒的插入使得管内两相流流体的平均流速与管道壁面附近速度梯度增大,故换热管的换热效率得到有效提升;定量而言,换热管的换热效率在棒径比值处于0.52~0.57区间范围内时,换热管内蒸汽冷凝率最高及通流截面内速度梯度最大,此时换热效率达到最高,可确保较低的换热管出口温度和稳定的换热效率。此外,场协同性能分析结果还表明,协同角与棒径比值?呈正相关性,且当棒径比值?>0.52后,芯棒对水平换热管的强化传热效果明显。     

Feasibility of using dialysis for determining calcium ion concentration and pH in calcium-fortified soymilk at high temperature

Abstract: Dialysis was performed to examine some of the properties of the soluble phase of calcium (Ca) fortified soymilk at high temperatures. Dialysates were obtained while heating soymilk at temperatures of 80 and 100 °C for 1 h and 121 °C for 15 min. It was found that the pH, total Ca, and ionic Ca of dialysates obtained at high temperature were all lower than in their corresponding nonheated Ca‐fortified soymilk. Increasing temperature from 80 to 100 °C hardly affected Ca ion concentration ([Ca²⁺]) of dialysate obtained from Ca chloride‐fortified soymilk, but it increased [Ca²⁺] in dialysates of Ca gluconate‐fortified soymilk and Ca lactate‐fortified soymilk fortified with 5 to 6 mM Ca. Dialysates obtained at 100 °C had lower pH than dialysate prepared at 80 °C. Higher Ca additions to soymilk caused a significant (P≤ 0.05) reduction in pH and an increase in [Ca²⁺] of these dialysates. When soymilk was dialyzed at 121 °C, pH, total Ca, and ionic Ca were further reduced. Freezing point depression (FPD) of dialysates increased as temperature increased but were lower than corresponding soymilk samples. This approach provides a means of estimating pH and ionic Ca in soymilks at high temperatures, in order to better understand their combined role on soymilk coagulation.     

植物乳杆菌对pH、酒精浓度和SO2浓度耐受性的研究

通过调整MRS培养基的pH、酒精浓度及SO2浓度,以菌株培养前后的菌密度差(△OD600nm)为指标研究了6株植物乳杆菌(Lactobacillus plantarum)对低pH、高SO2、酒精浓度及三者相互作用的耐受性.结果表明:试验所选取的6株菌都可耐受pH 3.0、75mg/LSO2浓度,其中有5株菌可耐受16％vol酒精浓度,3个因素对菌株生长影响的次序是SO2浓度＞pH＞酒精浓度.     

高压CO2处理对香蕉果肉中多酚氧化酶活性的影响

比较了CO2压力、温度和处理时间对香蕉果肉中多酚氧化酶活性的影响,并且采用二次回归正交旋转组合设计优化了高压CO2处理对香蕉果肉中多酚氧化酶的钝化条件.结果表明,影响高压CO2处理钝化香蕉果肉中多酚氧化酶活性的主次因素顺序为温度＞时间＞ CO2压力;高压CO2处理钝化香蕉果肉中多酚氧化酶的最佳条件为温度60℃、CO2压力19MPa、处理时间50min,在此条件下,香蕉果肉中多酚氧化酶的残活力仅为0.9％.     

Thermal properties of fruit juices as a function of concentration and temperature determined using the photopyroelectric (PPE) method

The photopyroelectric (PPE) method is proposed as a new technique for thermal characterisation of fruit juices. By combining different geometries and experimental conditions, the PPE method was shown capable of providing temperature-dependent (between 0 and 70 °C) values of thermophysical parameters (thermal diffusivity, thermal effusivity, thermal conductivity and volume specific heat) for juices having different Brix index. The computer-controlled PPE cell allowed stable temperature variation rates and continuous signal acquisition. The experimental data were used to develop empirical models and hence prediction equations for all the thermal parameters. © 1999 Society of Chemical Industry     

Dielectric properties of soybean protein isolate dispersions as a function of concentration, temperature and pH

Dielectric properties of commercial soy protein isolate (SPI) dispersions were measured over the frequency range of 200-2500 MHz by the open-ended coaxial probe method using a network analyzer as a function of concentration (5, 10 and 15 g/100 g water), temperature (20-90 °C) and pH (4.5, 6.6 and 10). Results indicated that the dielectric constant (ε′) decreased with temperature (except at 90 °C) and frequency while increased with concentration. The loss factor (ε″) increased with frequency and concentration; however, temperature showed mixed effect. Both ε′ and ε″ data were related to frequency using a polynomial model. The significant change in ε′ and ε″ at 90 °C was a result of protein denaturation which was identified by differential scanning calorimetry (DSC). Change in the association/dissociation behavior of SPI dispersions due to electrostatic attraction/repulsion among protein molecules on heating as a function of pH was assumed to be responsible for the significant increase in dielectric parameters. Penetration depth (D_p) was estimated under various conditions and it decreased with an increase in frequency, concentration, temperature and pH. The minimum D_p was found at alkaline pH at a temperature of 90 °C and frequency of 2450 MHz.     

Milk pasteurization at low temperature under N2O pressure

Non-thermal inactivation techniques in food industry are a major research issue, driven by an increased consumer demand for fresh and nutritious food products that are microbiologically safe, with a long shelf-life. Here we demonstrate for the first time the potential of pressurized N₂O as a non-thermal inactivation processing aid for bacterial reduction in the pasteurization of raw fresh milk at mild operating conditions.Treatments were performed in a multi-batch apparatus at 12 MPa and at different temperature conditions (40-50 °C). Chemical parameters (pH, titratable acidity (TA)) of the untreated and treated samples were monitored and compared. Four independent experiments have been carried out at each operative condition (temperature, time, pressure).The study clearly demonstrates the great potentiality of the emerging technology: further development of the N₂O process may open the doors to a new low temperature method for milk pasteurization, to be exploited on an industrial scale.     

Modeling Combined Effects of Temperature and pH on Heat Resistance of Spores by a Linear-Bigelow Equation

A generalized linear-Bigelow model was proposed to describe the effects of both temperature and pH on spore heat resistance. The model required only 3 parameters, each having a physicochemical significance. In addition to the conventional z value, the effect of pH on thermal resistance of spores was characterized by a z_pH value. Although the model neglected interactions between temperature and pH, its goodness of fit and its robustness enable it to be applied for optimization of heat treatments. Further experiments need to be undertaken to validate the model under industrial conditions.     

Low pressure CO2 storage of raw milk: microbiological effects

The effects of holding raw milk under carbon dioxide pressures of 68 to 689 kPa at temperatures of 5, 6.1, 10, and 20°C on the indigenous microbiota were investigated. These pressure-temperature combinations did not cause precipitation of proteins from the milk. Standard plate counts from treated milks demonstrated significantly lower growth rate compared with untreated controls at all temperatures, and in some cases, the treatment was microcidal. Raw milk treated with CO₂ and held at 6.1°C for 4 d exhibited reduced bacterial growth rates at pressures of 68, 172, 344, and 516 kPa; and at 689 kPa, demonstrated a significant loss of viability in standard plate count assays. The 689-kPa treatment also reduced gram-negative bacteria and total Lactobacillus spp. The time required for raw milk treated at 689 kPa and held at 4°C to reach 4.30 log₁₀ cfu/mL increased by 4 d compared with untreated controls. Total coliform counts in the treated milk were maintained at 1.95 log₁₀ cfu/mL by d 9 of treatment, whereas counts in the control significantly increased to 2.61 log₁₀ cfu/mL by d 4 and 2.89 log₁₀ cfu/mL by d 9. At d 8, Escherichia coli counts had not significantly changed in treated milk, but significantly increased in the control milk. Thermoduric bacteria counts after 8 d were 1.32 log₁₀ cfu/mL in treated milk and 1.98 log₁₀ cfu/mL in control milk. These data indicated that holding raw milk at low CO₂ pressure reduces bacterial growth rates without causing milk protein precipitation. Combining low CO₂ pressure and refrigeration would improve the microbiological quality and safety of raw milk and may be an effective strategy for shipping raw single strength or concentrated milk over long distances.     

超临界CO2萃馏浓缩天然生育酚高压相平衡的研究

采用气液相交替循环装置,在温度为313.15 K,压力为9～20 MPa条件下分别测定了复杂油脂体系中生育酚和纯DL-α-生育酚与超临界CO2的气液高压相平衡数据.复杂体系中气液相样品的生育酚含量分析采用高效液相色谱来进行,纯二元体系生育酚的含量采用重量法.结果表明,复杂体系中生育酚在超临界CO2中的溶解度比纯二元体系的高,生育酚的分配系数也比二元体系高得多.实验数据可为超临界CO2逆流萃馏浓缩天然生育酚提供重要的基础理论依据.