Inactivation of Bacillus subtilis spores by pulsed electric fields (PEF) in combination with thermal energy II. Modeling thermal inactivation of B. subtilis spores during PEF processing in combination with thermal energy

The application of pulsed electric fields (PEF) can be used to inactivate bacterial endospores if combined with thermal energy. A model was developed in the second part of the study aiming to separate thermal and PEF induced spore inactivation. Therefore thermal inactivation data of B. subtilis spores was obtained by glass capillary method and combined with temperature time profile of the PEF process. The PEF process can be separated in pre heating, PEF treatment and cooling phase. The temperature development in pre heating and cooling phase was simulated based on a theoretical numerical approach, the temperatures in the PEF unit were measured by fiber optic sensors. To evaluate the effect of pH on the inactivation the PEF treatment was performed in Ringer solution (4 mS/cm) at pH 4 and 7. The results of the developed model indicate a thermal and a PEF induced inactivation, where the temperature induced inactivation is slightly higher in acid Ringer solution. The impact of specific energy inputs up to 195 kJ/kg was evaluated, showing an increasing total inactivation of the spores, but also shift towards PEF induced inactivation. Applying an energy input of 195 kJ/kg an inactivation of 4.4 log was obtained in Ringer solution at pH 7, which could be separated in 1.15 log of thermal and 3.25 log of PEF related inactivation. The F-value, typically used to describe thermal spore inactivation processes was calculated for the combined PEF process based on the temperature time profile. The highest F-value determined for the maximum applied energy was 2.43 s, which confirms the low product heat load.     

Pilot study of countercurrent cold and mild heat extraction of sugar from sugar beets, assisted by pulsed electric fields

This study describes experiments on pulsed electric field (PEF)-assisted water extraction of sugar from sugar beet using a pilot countercurrent extractor with 14 extraction sections. Cossettes were prepared from sugar beet by industrial knives and PEF treatment of the cossettes was done with electric field strength E varied between 100 and 600 V/cm. The total time of PEF treatment was t_PEF = 50 ms. The effects of the main extraction parameters (temperature of extracting water and draft) on the extraction kinetics, as well as on the juice and cossette (pulp) characteristics, were investigated. The temperature of extraction from PEF-treated cossettes was varied from 30 to 70 °C; the draft was varied from 120 to 90%. The principal possibility of cold (at 30 °C) and moderate thermal (50-60 °C) extraction of sucrose from sugar beet cossettes treated by PEF was confirmed on the pilot countercurrent extractor. The purity of the juice obtained by cold and moderate thermal extraction was not lower than the purity of juice obtained by conventional hot water diffusion at 70 °C. The sugar beet pulp can be well exhausted by cold or mild thermal extraction of cossettes treated by PEF. Decrease of draft to 100-90% permitted increasing of the extracted juice concentration, but the cossettes were worse exhausted. The pulp obtained by cold extraction of PEF-treated cossettes had dryness >30%, which was noticeably higher than dryness of the pulp obtained by conventional hot water extraction technique.     

Optimization of pulsed electric field pretreatment parameters for preserving the quality of Raphanus sativus

Pulsed electric field (PEF) pretreatment can increase the drying rate of produce, but preserving product quality while minimizing energy consumption and maintaining food quality is a significant challenge. The goal of this study was to determine optimal PEF parameters for pretreatment of Raphanus sativus (radish) prior to the drying process. The effects of pulse intensity, treatment time, and pulse number on the drying rate, vitamin C (Vc) content, and ascorbic acid oxidase activity of R. sativus were characterized. Optimal PEF pulse parameter values were determined through quadratic orthogonal regression tests followed by multi-objective nonlinear optimization. The optimal PEF pulse parameters for pretreatment of R. sativus were: pulse intensity, 1446 V · cm^?1; reaction time, 28 μs; and pulse number, 87. This study provides reference values to guide application of PEF pretreatment in R. sativus processing.     

Pulsed electric fields and their impact on the diffusion characteristics of potato slices

Mass transfer in potato slices and strips after Pulsed Electric Fields (PEF) treatment was examined to evaluate potential application of PEF in potato processing. PEF treatment on cell material leads to pore formation in cell membrane and thus modifies diffusion of intra- and extracellular media. Results showed enhanced release of intracellular molecules from permeabilized tissue as well as improved uptake of low molecular substances into the sample. Sugar, one substrate for the Maillard reaction, was decreased in PEF treated potatoes, while conductivity increased after electroporation and soaking in sodium chloride solution, indicating the improved diffusion of salt caused by PEF. Higher release of cell liquid during drying of PEF treated potatoes was noticed in comparison to untreated potato slices. This effect increased with the treatment intensity. Furthermore, it was revealed that PEF application leads to a distinct reduction of fat content after deep fat frying and thus provides a potential for the production of low-fat French fries. It can be presumed that PEF is a capable assistance to thermal treatments in the processing of potato chips or French fries for the achievement of structural modifications and improved process conditions.     

Inactivation of Penicillum expansum in sour cherry juice, peach and apricot nectars by pulsed electric fields

Inhibitory effects of pulsed electric fields (PEF) on Penicillum expansum inoculated into sour cherry juice, apricot and peach nectars were determined based on germination tube elongation, spore germination rate, and light and scanning electron microscopy (SEM) observations in this study. After inoculation of juice/nectar samples with P. expansum spores at the level of 10(5)-10(6)cfu/mL, the samples were processed by bench scale PEF pulse generator as a function of differing electric field strengths (0, 13, 17, 20, 23, 27, 30 and 34kV/cm) and processing times (0, 62, 94, 123, 163, 198 and 218mus). Results revealed that with an increase in electric field strength and processing time, germination tube elongation and spore germination rate were completely inhibited. Light and SEM observations revealed considerable morphological alterations in fungal conidia such as cytoplasmic coagulation, vacuolations, shrinkage and protoplast leakage. PEF processing of juice/nectars was demonstrated to be effective in inactivating P. expansum. To our knowledge, this is the first study confirming the inhibitory effects of PEF on germination tube elongation and spore germination rate of P. expansum in fruit juice/nectars.     

保香低温速溶红茶粉的研发

为了解决速溶茶粉的滋味淡、香气低、冷溶性差等问题,该试验以红茶为原料,采用高压脉冲电场（Pulsed Electric Field,PEF）技术辅助提取和杀菌,并结合冷冻浓缩和真空冷冻干燥联合真空吸附干燥法生产速溶红茶粉,使得整个加工工艺都在低温状态下完成。试验结果表明,高压脉冲电场技术有助于提高红茶的提取率,冷冻浓缩和高压脉冲电场杀菌能够有效保持茶汤的色、香、味;真空冷冻干燥联合真空吸附干燥后所得的速溶红茶粉的得率为22.7%,且其冷溶性高,滋味鲜浓而醇厚,香气高而纯正,汤色红艳明亮,无沉淀。     

Degradation behaviour of methamidophos and chlorpyrifos in apple juice treated with pulsed electric fields

Apple juice (13 °Brix) spiked with methamidophos and chlorpyrifos (2–3 mg/l of each compound) was treated by pulsed electric fields (PEF), and pesticide residues were quantified by gas chromatography with flame photometric detection (GC-FPD). Results showed that electric field strength (8–20 kV/cm) and pulse number (6–26 pulses) have significant effects on the degradation of methamidophos and chlorpyrifos. PEF treatment is effective for the degradation of methamidophos and chlorpyrifos residues in apple juice, and chlorpyrifos is much more labile to PEF than methamidophos. An increase in either pulse number or electric field strength could speed the degradation of methamidophos and chlorpyrifos, and the kinetics equations and related parameters quantitatively characterized the degradation behavior of the pesticides. The exponential model better fits the experimental data for all treatments than the linear model.     

Shelf life of whole milk processed by pulsed electric fields in combination with PEF-generated heat

Application of pulsed electric fields (PEF) in combination with mild thermal treatment was studied to extend the shelf life of whole milk. Five pulses with peak electric field strength of 35 kV/cm and pulse width of around 2.3 μs were applied to milk at 65 °C and sustained for less than 10 s. Shelf life of the milk was extended by a minimum of 24 days. A synergistic interaction between PEF and mild thermal treatment was found. Neither the severe PEF treatment applied at lower temperatures, nor the mild thermal treatment equivalent, including longer treatment times than used in this study, could significantly extend the shelf life of milk. However, the combination of both PEF and mild temperature extended milk's shelf life adequately. The use of a thermal regeneration system improved the energy efficiency of the studied preservation process making it highly competitive with pasteurization.     

Inactivation kinetics of pulsed electric field-resistant strains of Listeria monocytogenes and Staphylococcus aureus in media of different pH

A study of the effect of pulsed electric fields (PEF) on the inactivation of Listeria monocytogenes STCC 5672 and Staphylococcus aureus STCC 4459 in McIlvaine buffer covering a range from pH 3.5 to 7.0 was conducted. Mathematical models based on the Weibull distribution were developed to describe the influence of the electric field strength, treatment time and pH of the treatment medium on the lethality of both Gram positive pathogenic bacteria after PEF treatments. Both microorganisms were more sensitive to PEF in media of low pH, although the influence of the pH on the PEF resistance was more significant in S. aureus. In the best cases scenario, the highest inactivation levels achieved were 3.3 and 6.1 log₁₀ cycles for L. monocytogenes and S. aureus respectively in pH 3.5 after 500 μs of 35 kV/cm. Based on these results and those observed in literature, L. monocytogenes STCC 5672 at any pH investigated has been shown as one of the most PEF resistant microorganism. Therefore, this microorganism should be considered as a possible target microorganism to define process criterion for PEF pasteurization.     

高压脉冲电场辅助乙醇连续化提取栀子黄色素工艺条件的研究

探讨高压脉冲电场(PEF)辅助乙醇连续提取栀子黄色素的工艺条件.首先考察脉冲电场强度、脉冲频率、脉冲时间、温度及溶提时间对栀子黄色素溶出率的影响,在此基础上采用正交试验法优化提取条件.实验结果表明,较佳提取条件为脉冲电场强度25kV·cm<'-1>,脉冲频率6 Hz,脉冲时间4 s,提取温度40℃,提取时间15 min...