大肠杆菌射频加热杀菌效果及动力学模型拟合

为研究射频加热杀菌效果及微生物致死动力学。选取大肠杆菌为试验菌株,研究极板间距和菌悬液电导率对加热速度及杀菌效果的影响。选取4种常见的模型,即一级动力学、Weibull、Dose-response和Log-Logistic模型对大肠杆菌射频加热失活曲线进行动力学模型拟合。结果表明:极板间距和电导率影响射频加热速度,极板间距越大升温速度越慢,杀灭微生物所需时间越长,当电导率接近1 000μs/cm时,升温速度最快。在极板间距115 mm,电导率1 083μs/cm的条件下,加热90 s,大肠杆菌致死率即可达100%。大肠杆菌的失活动力学曲线基本呈倒"S"型,不符合一级反应动力学模型。Weibull、Log-Logistic和Dose-response模型都能较好地拟合射频杀菌失活曲线。通过对模型的拟合评价参数,即精确因子Af、偏差因子Bf、根平均方差RMSE和决定系数R2的比较,确定Log-Logistic模型拟合效果最好。     

猕猴桃浆欧姆加热特性及酶失活率数学模型的建立

利用欧姆加热技术对猕猴桃浆进行加热,探讨了加热过程中加热速率和电导率的变化规律,建立了多酚氧化酶和过氧化物酶失活率的数学模型。结果表明,加热速率随着电场强度的升高而增大,电场强度对试样的电导率影响不大,随着温度的升高,电导率呈线性关系增大;回归方程在α=0.05水平显著,可用于欧姆加热工艺参数对多酚氧化酶和过氧化物失活影响的预测。研究结果可为猕猴桃浆的深加工提供理论基础。     

百合中多酚氧化酶的部分性质

通过丙酮酚法从百合中提取出多酚氧化酶(PPO)的粗酶液。以儿茶酚为底物时它有两个最适pH,分别为4.0、7.0,在pH5.0-6.5之间酶活力可以在4℃保持至少10h。PPO的最适温度为40℃,从40℃开始酶出现热失活,热失活速度符合一级反应动力学。PPO除对L-酪氨酸没有活力外,对儿茶酚、儿茶素、没食子酸均有活力,其中对儿茶素具有最好的底物特异性。亚硫酸钠对PPO的抑制作用最强,高浓度的抗坏血酸、半胱氨酸、硫脲也有很好的抑制作用,氯化钠、氯化钙、柠檬酸的抑制作用较差。     

高压脉冲电场对酿酒酵母杀菌效果影响和模型分析研究

研究了高压脉冲电场(pulsed electric field,PEF)对接种于胡萝卜汁中酿酒酵母(Saccharomyces cerevisiae,CGMCC2.604)的杀菌效果并进行了模型分析。结果表明,随电场强度和脉冲时间增加,PEF对酿酒酵母的杀灭效果增强,25kV/cm、2173μs时酿酒酵母降低了4.5个对数。Weibull模型和Log-Logistic模型均能很好的拟合PEF处理酿酒酵母的失活曲线,模型评价参数(Af,Bf,SS,RMSE和R2)分析表明,Log-Logistic模型比Weibull模型能更好地拟合PEF处理下酿酒酵母失活动力学变化。     

西葫芦多酚氧化酶酶学特性研究

以邻苯二酚为底物,采用分光光度法对西葫芦多酚氧化酶(PPO)的酶学特性及不同抑制剂对多酚氧化酶活性的影响进行研究。结果表明,西葫芦多酚氧化酶最适pH为6.6,最适温度为35℃,90℃处理5min可使该酶失活。多酚氧化酶催化的酶促褐变反应动力学符合米氏方程,动力学参数为Km=0.0417mol/L,Vmax=208.33U。四种抑制剂对PPO的抑制效果由强到弱依次为:亚硫酸氢钠>L-半胱氨酸>抗坏血酸>柠檬酸。     

红外与漂烫处理下马铃薯多酚氧化酶的失活机理研究

测定了两种灭酶处理后马铃薯片的多酚氧化酶(PPO)相对活性,利用圆二色谱分析处理后多酚氧化酶结构变化,结合酶活性的变化进行二者的比较,进而从蛋白质分子二级结构的角度探究红外灭酶下多酚氧化酶的失活机理。红外处理下马铃薯PPO失活从热效应方面来看,是由α-螺旋向β-转角和无规则卷曲转化的转化而造成的。从非热效应方面来看,红外辐射可以造成β-折叠相对含量下降,从而加速PPO的失活。     

荠菜过氧化物酶热失活动力学的研究

目的 研究热烫处理引起荠菜过氧化酶(POD)失活的速率常数和动力学规律。方法 对荠菜在80～100℃的热水处理一定时间条件,对荠菜过氧化酶的残余活性进行测定,并用一级动力学模型对过氧化物酶失活动力学进行拟合,分析失活速率常数。结果 热烫处理钝化新鲜荠菜POD活性效果明显,在80～100℃处理范围内,随着处理温度的上升,荠菜POD 失活速率常数k 值从0.01702增加到0.14260,反应活化能为114 kJ/mol。一级动力学模型拟合决定系数R²＞0.99。结论 荠菜POD 的热失活动力学符合一级动力学模型。     

苹果浓缩汁加工中热灭酶工艺研究

通过在不同温度和时间下热处理苹果汁,研究了使果汁中多酚氧化酶(PPO)部分或完全失活的条件,并考察了与果汁自然pH值相近的模拟体系对处理后的果汁酶活的抑制情况。表明:75℃、30s虽不能完全灭活PPO,但对其活性破坏较大,处理后的PPO酶活可以较好地被接近果汁自身pH值的低pH(3.8)所抑制,为最佳处理条件。     

低频超声对丰水梨多酚氧化酶酶活的影响研究

研究低频超声对丰水梨多酚氧化酶(PPO)的影响,探讨超声功率、超声时间、超声温度、pH等因素对其酶活的影响,同时对多酚氧化酶的酶反应动力学进行研究。研究表明,多酚氧化酶经超声波处理后,酶活性均比未经超声波处理的酶活力有所下降,而超声对过氧化物酶酶活的最适温度和最适pH影响不大。经过超声处理后PPO的K_m值变大,可见超声处理使PPO和底物的亲和力有所降低。     

马铃薯中多酚氧化酶的酶学特性研究

采用分光光度法对马铃薯中多酚氧化酶的酶学特性进行了较深入系统的研究,其中包括酶活力、酶热稳定性以及pH、温度、抑制剂对酶活性的影响。结果表明:多酚氧化酶的最适pH为6.6,最适温度为30℃;酶热失活遵循一级动力学原理,二巯基丙醇、亚硫酸钠、半胱氨酸、抗坏血酸对酶活性有较强的抑制作用。     

Inactivation of Salmonella typhimurium DT 104 in UHT whole milk by high hydrostatic pressure

Cell suspensions of Salmonella typhimurium DT 104 in ultra-high temperature (UHT) whole milk were exposed to high hydrostatic pressure at 350, 400, 450, 500, 550, and 600 MPa at ambient temperature (ca. 21 degrees C). Tailing was observed in all survival curves, and sigmoidal survival curves were observed at relatively high pressure (500-600 MPa). Four modeling methods (linear and nonlinear including Weibull, modified Gompertz, and log-logistic models) were fitted to these data at 500, 550, and 600 MPa. Performances of the modeling methods were compared using mean square error (MSE). The linear regression model at these three pressure levels had a mean square error (MSE) of 1.260-2.263. Nonlinear regressions using Weibull, modified Gompertz, and log-logistic models had MSE values in the range of 0.334-0.764, 0.601-1.479, and 0.359-0.523, respectively. Modeling results indicated that first-order kinetics could not accurately describe pressure inactivation of S. typhimurium DT 104 in UHT milk; the log-logistic model produced the best fit to data.     

Inactivation kinetics of polyphenol oxidase and peroxidase in pineapple juice by dielectric barrier discharge plasma technology

The non-thermal plasma technique is an innovative non-thermal approach for food preservation and decontamination. Polyphenol oxidase (PPO) and peroxidase (POD), the principal food degrading enzymes in pineapple juice, were inactivated by the Dielectric Barrier Discharge (DBD) plasma. The inactivation kinetics of PPO and POD in pineapple juice were examined at three different voltages 25, 35, and 45 kV for up to 10 min of plasma treatment. The present study's findings revealed that plasma parameters such as voltage and treatment time substantially reduced enzyme activity of both enzymes, with the former parameter having a more pronounced influence on enzyme inactivity. To mathematically represent the model parameters, established kinetic models viz. log-linear, Weibull, and logistic were fitted to observe experimental data. The Weibull model was discovered to be the best fit for the enzyme inactivation kinetics. The scale factor for PPO was 15.95, 10.87, and 5.73 min at 25, 35, and 45 kV, respectively which was lower than POD with scale factor 40.74, 19.76, and 7.28, respectively. This shows that POD was more resistant to inactivation by DBD plasma than PPO.     

Pressure inactivation kinetics of Yersinia enterocolitica ATCC 35669

The survival curves of Yersinia enterocolitica ATCC 35669 inactivated by high hydrostatic pressure were obtained at four pressure levels (300, 350, 400, and 450 MPa) in sodium phosphate buffer (0.1 M, pH 7.0) and four pressure levels (350, 400, 450, 500 MPa) in UHT whole milk. Tailing was observed in all the survival curves. A linear model and three nonlinear models were fitted to these data and the performances of these models were compared. The linear regression model for survival curves at four pressure levels had regression coefficients (R2) values of 0.785-0.962 and mean square error (MSE) of 0.265-0.893. A residual plot strongly suggested that a linear regression function was not appropriate as there was strong curvature in the plotted data. The nonlinear regression model using the log-logistic had R2 values of 0.946-0.982 and MSE values of 0.110-0.320. The Weibull model had R2 values of 0.944-0.975 and MSE values of 0.153-0.349. These results indicated that both were better models to describe the pressure inactivation kinetics of Y. enterocolitica in milk and buffer. Among the three nonlinear models studied, the modified Gompertz model produced the poorest fit to data. The number of parameters of the log-logistic model was reduced from four to two so that the model was greatly simplified. The reduced log-logistic model still produced a fit comparable to the full model. Since pressure had no significant effect on the shape factors of the Weibull model at the pressure levels of 300-400 MPa for buffer and 400-500 MPa for milk, models were developed to predict survival curves of Y. enterocolitica at pressures different from the experimental pressures.     

Pressure inactivation kinetics of Enterobacter sakazakii in infant formula milk

Survival curves of Enterobacter sakazakii inactivated by high hydrostatic pressure were obtained at four pressure levels (250, 300, 350, and 400 MPa), at temperatures below 30 degrees C, in buffered peptone water (BPW; 0.3%, wt/vol) and infant formula milk (IFM; 16%, wt/vol). A linear model and four nonlinear models (Weibull, log-logistic, modified Gompertz, and Baranyi) were fitted to the data, and the performances of the models were compared. The linear regression model for the survival curves in BPW and IFM at 250 MPa has fitted regression coefficient (R2) values of 0.940 to 0.700, respectively, and root mean square errors (RMSEs) of 0.770 to 0.370. For the other pressure levels, the linear regression function was not appropriate, as there was a strong curvature in the plotted data. The nonlinear regression models with the log-logistic and modified Gompertz equations had R2 values of 0.960 to 0.992 and RMSE values of 0.020 to 0.130 within pressure levels of 250 to 400 MPa, respectively. These results indicate that they are both better models for describing the pressure inactivation kinetics of E. sakazakii in IFM and BPW than the Weibull distribution function, which has an R2 minimum value of 0.832 and an RMSE maximum value of 0.650 at 400 MPa. On the other hand, the parameters for the Weibull distribution function, log-logistic model, and modified Gompertz equation did not have a clear dependence on pressure. The Baranyi model was also analyzed, and it was concluded that this model provided a reasonably good fit and could be used to develop predictions of survival data at pressures other than the experimental pressure levels in the range studied. The results provide accurate predictions of survival curves at different pressure levels and will be beneficial to the food industry in selecting optimum combinations of pressure and time to obtain desired target levels of E. sakazakii inactivation in IFM.     

On the use of the Weibull model to describe thermal inactivation of microbial vegetative cells

This paper evaluates the applicability of the Weibull model to describe thermal inactivation of microbial vegetative cells as an alternative for the classical Bigelow model of first-order kinetics; spores are excluded in this article because of the complications arising due to the activation of dormant spores. The Weibull model takes biological variation, with respect to thermal inactivation, into account and is basically a statistical model of distribution of inactivation times. The model used has two parameters, the scale parameter alpha (time) and the dimensionless shape parameter beta. The model conveniently accounts for the frequently observed nonlinearity of semilogarithmic survivor curves, and the classical first-order approach is a special case of the Weibull model. The shape parameter accounts for upward concavity of a survival curve (beta < 1), a linear survival curve (beta = 1), and downward concavity (beta > 1). Although the Weibull model is of an empirical nature, a link can be made with physiological effects. Beta < 1 indicates that the remaining cells have the ability to adapt to the applied stress, whereas beta > 1 indicates that the remaining cells become increasingly damaged. Fifty-five case studies taken from the literature were analyzed to study the temperature dependence of the two parameters. The logarithm of the scale parameter alpha depended linearly on temperature, analogous to the classical D value. However, the temperature dependence of the shape parameter beta was not so clear. In only seven cases, the shape parameter seemed to depend on temperature, in a linear way. In all other cases, no statistically significant (linear) relation with temperature could be found. In 39 cases, the shape parameter beta was larger than 1, and in 14 cases, smaller than 1. Only in two cases was the shape parameter beta = 1 over the temperature range studied, indicating that the classical first-order kinetics approach is the exception rather than the rule. The conclusion is that the Weibull model can be used to model nonlinear survival curves, and may be helpful to pinpoint relevant physiological effects caused by heating. Most importantly, process calculations show that large discrepancies can be found between the classical first-order approach and the Weibull model. This case study suggests that the Weibull model performs much better than the classical inactivation model and can be of much value in modelling thermal inactivation more realistically, and therefore, in improving food safety and quality.     

INACTIVATION KINETICS AND REDUCTION OF BACILLUS COAGULANS SPORE BY THE COMBINATION OF HIGH PRESSURE AND MODERATE HEAT

The combination effect of high pressure (400, 500 and 600 MPa) and moderate heat (70 and 80C) on the inactivation kinetics and reduction of Bacillus coagulans spore in phosphate buffer and ultra-high temperature (UHT) whole milk was investigated. The pressure come-up time and corresponding logarithmic reduction of spore inactivation were considered during pressure-thermal treatment. B. coagulans spore had a much higher resistance to pressure in UHT whole milk than in phosphate buffer. Survival data were modeled using the linear, Weibull and log-logistic models to obtain relevant kinetic parameters. The tailing phenomenon occurred in all survival curves, indicating the linear model was not adequate for describing these curves. The mean square error and regression coefficient suggested that the log-logistic model produced best fits to all survival curves, followed by the Weibull model.

PRACTICAL APPLICATIONS

It becomes increasingly apparent that high-pressure treatment combined with moderate heat treatment for low acid and acid products is often required for effective bacterial spores' inactivation. Consequently, the prediction model of microbial survival curves is essential. Bacillus coagulans is a slightly pressure-resistant and relatively heat-resistant spoilage bacterium of considerable concern during the processing of acid foods. Spore inactivation effect during the pressure come-up time is sometimes considerable and should not beignored. The use of mathematical models to predict inactivation for spores could help the food industry further to develop optimum process conditions.     

Processing and quality characteristics of apple slices processed under simultaneous infrared dry-blanching and dehydration with intermittent heating

This study investigated the effects of three processing parameters, e.g. product surface temperature, slice thickness and processing time, on blanching and dehydration characteristics of apple slices exposed to simultaneous infrared dry-blanching and dehydration (SIRDBD) with intermittent heating. A three-factor factorial experiment design was conducted to determine the influence of processing parameters on product temperature, moisture reduction, drying rate, residual polyphenol oxidase (PPO) and peroxidase (POD) activities and surface color change. Slice thickness had a significant effect on product quality and processing characteristics, as faster inactivation of enzymes and quicker moisture reduction took place in thinner slices. A Page model performed well for describing drying behavior during the treatment, and first-order kinetics and a biphasic model fit well for PPO and POD inactivation, respectively. Surface color changes (ΔE) of apple slices during prolonged heating resulted from non-enzymatic browning with an increase in b value was observed. In order to achieve a 1 log reduction in POD activity, the process resulted in a reduction in moisture from 20% to 59% and in ΔE from 2.27 to 5.59. It is suggested that SIRDBD with intermittent heating could be used as an alternative to manufacture high quality blanched and partially dehydrated fruits and vegetables.     

超高压对泡萝卜的杀菌效果及其动力学研究

研究300～600 MPa超高压条件下处理四川泡萝卜5～25 min,对其细菌总数的影响、对霉菌、酵母菌及大肠菌群的杀灭效果的影响。并采用3 种模型对不同压力条件下杀菌动力学过程进行分析比较。结果表明：随处理压力和时间的增加,超高压对泡萝卜的杀菌效果增强;霉菌和酵母菌对压力较为敏感,500 MPa处理5 min可被全部杀死;Weibull模型能很好地拟合泡萝卜超高压杀菌的动力学过程（决定系数R2＞0.99）,且相较Log-logistic模型更简洁、灵活实用。尺度参数b随处理压力的增加而增大,形状参数n则随压力的增加而减小。     

催化式红外杀青对绿茶热风干燥的影响

以镇江金山翠芽绿茶鲜叶为原料,以蒸汽杀青为对照,研究红外杀青对茶叶品质的影响,对杀青后茶叶进行不同温度热风干燥得到茶叶成品。考察茶叶中多酚氧化酶(polyphenol oxidase,PPO)含量、VC保留率、茶多酚含量以及感官审评分等指标的变化,建立一种催化式红外杀青联合热风干燥的绿茶加工技术。最优杀青干燥工艺条件为:红外辐照距离20 cm杀青150 s,经揉捻作形后,热风干燥温度70℃、干燥40 min。实验结果建立了红外杀青过程中PPO钝化动力学模型和干燥过程中水分干燥动力学模型,可为杀青干燥过程预测提供理论参考。     

聚乙烯醇薄膜中茶多酚向水中释放的不同动力学模型比较分析

研究聚乙烯醇薄膜中茶多酚向水中释放的不同动力学模型表征。通过释放实验研究了薄膜中茶多酚向水中的释放规律,基于4?种动力学模型对实验数据进行拟合表征,比较分析了不同模型的拟合效果。结果表明：茶多酚释放率随薄膜中茶多酚质量浓度的增加而降低。对比不同模型的拟合效果,Fickian模型和Fickian＋一级动力学模型可以很好地模拟茶多酚的释放（R2≥97.27）,后者更优;Weibull模型的拟合效果较前两个模型差（R2≥85.21）,但是该模型可以很好地模拟从释放开始到平衡初期茶多酚的释放（R2≥92.20）;短期释放模型的拟合效果较好（R2≥91.32）,但其扩散系数与由Fickian模型和Fickian＋一级动力学模型得到的扩散系数相差较大。总的来看,Fickian＋一级动力学模型模拟效果最好,Fickian模型次之,Weibull模型模拟从释放开始到平衡初期过程的效果较好,短期释放模型不适合用于模拟评估聚乙烯醇薄膜中茶多酚整个释放过程中的释放性能。