Quantitative Evaluation of Thermal Inactivation Kinetics of Free-Floating Versus Surface-Attached Listeria innocua Cells

Surface pasteurization is one of the decontamination treatments that can contribute to better preservation of meat products retaining most of their quality characteristics relatively intact if compared with the raw products. The current research compares the kinetics of free-floating and surface attached Listeria innocua cells by using integrated microbial and heat transfer modelling approaches. Surface pasteurization treatments are applied on a (abiotic) Teflon® model system in a novel steam surface decontamination rig. The experimental set-up prevented following four technological aspects to occur, (1) cold purge migration to the surface during the heating process, (2) inactivation kinetics of a cocktail of microbes, (3) protective effect of food components, and (4) physical distribution of bacteria throughout the depth of the product skin. Microbial load predictions are performed based on the inactivation parameters obtained during free-floating cell experiments. These predictions, when compared with the microbial data of the surface treatments, prove that the surface attached cells were much more heat resistant, despite the experimental set-up preventing the aforementioned (technological) events to occur. Indeed, surface attached cells can have different physiological/phenotypical/genetical characteristics, such as cell aggregations, colony formations, presence of flagella. In a final step, three techniques are implemented to evaluate mathematically the kinetics of the surface attached cells. Overall, this research’s significance is lying in the quantitative assessment of microbial heat resistance. The technological reasons underlying the increased microbial heat resistance on biotic and abiotic surfaces should be reevaluated, taking into account possible physiological/phenotypical/genetical characteristics.     

High hydrostatic pressure inactivation of Lactobacillus viridescens and its effects on ultrastructure of cells

The effect of high hydrostatic pressure treatment on Lactobacillus viridescens, a major spoilage micro-organism of processed meat, was investigated. When pressure was applied at 400, 500 and 600 MPa for 5 min, cell survival in MRS broth was reduced approximately by 2, 7 and 8 log cycles, respectively. The combination of high pressure and temperature increase showed synergistic effects on microbial inactivation. Empty cavities in the cell and DNA denaturation were observed after pressure treatment above 400 MPa. The level of microbial inactivation when Lb. viridescens was pressured in protein-fortified MRS broth and ham were less than half compared with the inactivation achieved in peptone water. Increasing the temperature during high-pressure treatment solved these adverse effects in the ham.     

Riboflavin-Sensitized Photodynamic UV Spectrophotometry for Ascorbic Acid Assay in Beverages

Riboflavin-sensitized photodynamic ultraviolet spectrophotometric assay of various commercial beverages was carried out. The maximum concentration of ascorbic acid for obtaining a background correction (15 min illumination and pH 7.5) was 21 μg/mL ascorbic acid in the testing solution. The upper limit of the measurement range for a straight line in the calibration graph of standard ascorbic acid was 10 μg/mL, (working range 0–10 μg/mL). The results in repeatability of the method for ascorbic acid contents in commercial fruit juices and soft-drinks showed a maximum of 4% relative standard deviation. Recovery tests with known amounts of added ascorbic acid in different fruit juices and sports drink showed the recovery of added ascorbic acid was 97.5–102.3%. Indophenol, iodine and/or HPLC methods were used in parallel to ascertain the reliability of the proposed method. This type assay could be successfully applied to many commercial beverages for determination of ascorbic acid with good accuracy, precision and reliability.     

Effect of water activity on the inactivation kinetics of Escherichia coli O157:H7 by electron beam in ground beef, chicken breast meat, and trout fillets

Water activity (a_w) of ground beef, chicken breast meat, and trout fillets was modified to intermediate (a_w 0.98–0.99) and lowest (a_w 0.94–0.96) levels. The meat samples with modified and unaltered (native, a_w 1.00) a_w were inoculated with Escherichia coli O157:H7 and subjected to electron beam (e‐beam). Survivor curves were plotted and the D₁₀‐values were calculated. The D₁₀‐values ranged from 0.22 kGy for trout at native a_w to 0.33 kGy for beef at intermediate, and chicken and trout at lowest a_w. Regardless of the species, a_w reduction increased E. coli resistance to e‐beam, suggesting that even small depletion of unbound water from food increases survival. The difference of the D₁₀‐values between the samples at intermediate and lowest a_w was insignificant. E‐beam could be used before a_w‐reducing techniques are applied to food products. However, this would require stringent microbial control following e‐beam processing. The ‘tailing’ of survivors was observed for some samples with reduced a_w.     

Inactivation of Oxidative Enzymes by High-Intensity Pulsed Electric Field for Retention of Color in Carrot Juice

The effects of high-intensity pulsed electric fields (HIPEF) on oxidative enzymes and color of fresh carrot juice were studied. A response surface methodology (RSM) was used to evaluate the effect of pulse polarity (mono or bipolar mode), pulse width (from 1 to 7 μs), and pulse frequency (from 50 to 250 Hz) on color and peroxidase (POD) inactivation of carrot juice treated by HIPEF. The total treatment time and the electric field strength were set at 1,000 μs and 35 kV/cm, respectively, at a temperature below 35°C. The physicochemical characteristics of carrot juice were measured. There was a linear relationship between electrical conductivity and temperature of the carrot juice. The results showed that HIPEF-treated carrot juice at 35 kV/cm for 1,000 μs applying 6 μs pulse width at 200 Hz in bipolar mode led to 73.0% inactivation of POD. The color coordinates did not change significantly. Therefore, HIPEF was effective in POD inactivation and carrot juice color preservation.     

超高压加工生鲜牡蛎的研究

研究了超高压处理对牡蛎的杀菌效果及其对产品感官性质的影响。研究结果表明,当超高压达到500MPa后,牡蛎达到商业无菌。600MPa处理的牡蛎在4℃冷藏21d后,总菌落数小于6个对数,仍然符合生鲜牡蛎细菌总数的要求。超高压对牡蛎中不同蛋白酶具有不同的影响。超高压处理使牡蛎的硬度、弹性、咀嚼度、粘结性和恢复力均表现出不同程度的下降,而且超高压处理对牡蛎的色泽产生了一定影响。超高压处理可引起TBA值显著升高,表明超高压引起了牡蛎中脂肪的氧化。     

大豆胰蛋白酶抑制剂研究概况

该文介绍国内外研究大豆胰蛋白酶抑制剂概况,并对其失活和测定方法进行初步探讨。     

米根霉原生质体电融合条件研究

以米根霉突变株mut-A、mut-B为亲本,采用双亲灭活标记筛选融合子,对亲本灭活条件、原生质体电融合条件进行了研究。实验结果表明,0.6mol/L山梨醇溶液为最适的渗透压缓冲液,mut-A、mut-B的灭活条件分别为50℃加热6min及紫外照射180s。原生质体在电场中排队所需的细胞浓度为1×107/mL,交变电场电压为8V;最佳的电融合参数为直流脉冲电压260V,脉冲持续时间40μs,脉冲次数2次;最高融合率可达7.29±0.71%。     

Effect of electric and flow parameters on PEF treatment efficiency

The effects of both the electric and flow parameters on the lethality and energy efficiency of a pulsed electric fields (PEF) treatment were studied. An experimental plan was designed in order to study the microbial inactivation of Saccharomyces cerevisiae and Escherichia coli cells inoculated in a buffer solution. The following process parameters were taken into consideration: electric field strength (13-30 kV/cm), total specific energy input (20-110 J/mL), flow rate of the processed stream (1-4 L/h) and number of passes through the chamber (up to 5).The results showed that, at a fixed flow rate (2 L/h), microbial inactivation of both microbial strains increased with increasing field strength and applied energy input. The maximum inactivation level (5.9 Log-cycles for S. cerevisiae and 7.0 Log-cycles for E. coli) corresponded to the more intensive PEF treatment (30 kV/cm and 110 J/mL). However, for any given field strength applied, the inactivation rate decreased by increasing the energy input. This behavior was attributed to the presence of heterogeneous treatment conditions due, for example, to a different morphology (size and shape) or cell membrane (composition, structure), a local variation of the electric field strength in the treatment chamber, the tendency of microbial cells to form clusters, or a non-uniform distribution of the residence time of the product in the PEF chamber.A more effective stirring of the microbial suspensions which was achieved, at a fixed field strength (18 kV/cm), either by increasing the flow rate with a single pass operation through the PEF chamber, or by operating in re-circulating mode at a constant flow rate, provided a significant increase in the effectiveness and energy efficiency of the pulse treatment.A mathematical model based on the Weibull distribution adequately described the inactivation kinetics of both microbial strains under different flow dynamic conditions.     

绿茶的微波杀青

以多酚氧化酶 (PPO)活性为指标对微波杀青工艺进行了初步研究 ,确定了微波杀青的最佳工艺条件 :投叶量 40g ,功率 72 9W ,杀青时间 180s。在该条件下多酚氧化酶几乎完全被钝化 ,且杀青叶呈浅黄绿色 ,柔软卷缩 ,有利于绿茶外形美观 ,香气的保持 ,达到了绿茶杀青的品质要求。