High hydrostatic pressure come-up time and yeast viability

Application of high hydrostatic pressure on Pacific white shrimp (Litopenaeus vannamei) pâté: Microbiological, physicochemical and consumer acceptance

The locally elaborated shrimp pâté is highly susceptible to microbial spoilage and deterioration during storage due to its content in nutrients. These conditions limit its commercialization in a larger scale. High hydrostatic pressure is an alternative to heat treatments technology used to inactivate microorganisms. The aim of this project was to evaluate the effect of high hydrostatic pressure on quality parameters and microbiological stability of Pacific white shrimp ( Litopenaeus vannamei) pâté during storage. Shrimp pâté was pressurized to 400, 500 and 600 MPa for 90 and 180 s. Samples were analysed for physicochemical, microbiological and flavour profile up to 21 days of storage at 4 ℃. A hedonic test was made to evaluate the acceptance of pâté. No microorganisms were detected at 600 MPa for 180 s and a shelf life of 14 days was reached. No relevant changes in pH or colour of pressurized samples were detected; flavour profile did not show any changes after being pressurized or during storage. Shrimp pâté treated with 600 MPa for 180 s presented good sensory acceptance. High hydrostatic pressure treatments could improve microbiological quality of shrimp pâté without a sensible modification of the physicochemical and sensorial qualities of this product.

An insight on the relationship between food compressibility and microbial inactivation during high pressure processing

This paper investigates the effect of high pressure liquid food compressibility on S. cerevisae inactivation. Honey with various adjusted sugar with different values of compressibility was selected as a model food. S. cerevisiae cells in different honey concentrations (0-80°Brix), 600 MPa (at ambient temperature) showed an increasing resistance to inactivation with increasing °Brix. D-values of S. cerevisiae at 200, 400 and 600 MPa, for 20 min/80°Brix were 136.99 ± 7.97, 29.24 ± 6.44 and 23.47 ± 0.86 min, respectively. These D-values resulted the Z _p -value of 526 ± 39 MPa. A significant correlation (p < 0.05) of cell reduction, °Brix and compressibility was found. Cell reduction in high pressure-treated samples varied linearly with °Brix suggesting that the baroprotective effect of the food was not solely due to sugar content, but also due to its compressibility. This research could have significant implications on the success of HPP (high pressure processing) preservation of foods containing high sugar content.

Synergistic and antagonistic effects of combined subzero temperature and high pressure on inactivation of Escherichia coli

The combined effects of subzero temperature and high pressure on the inactivation of Escherichia coli K12TG1 were investigated. Cells of this bacterial strain were exposed to high pressure (50 to 450 MPa, 10-min holding time) at two temperatures (-20 degrees C without freezing and 25 degrees C) and three water activity levels (a(w)) (0.850, 0.992, and ca. 1.000) achieved with the addition of glycerol. There was a synergistic interaction between subzero temperature and high pressure in their effects on microbial inactivation. Indeed, to achieve the same inactivation rate, the pressures required at -20 degrees C (in the liquid state) were more than 100 MPa less than those required at 25 degrees C, at pressures in the range of 100 to 300 MPa with an a(w) of 0.992. However, at pressures greater than 300 MPa, this trend was reversed, and subzero temperature counteracted the inactivation effect of pressure. When the amount of water in the bacterial suspension was increased, the synergistic effect was enhanced. Conversely, when the a(w) was decreased by the addition of solute to the bacterial suspension, the baroprotective effect of subzero temperature increased sharply. These results support the argument that water compression is involved in the antimicrobial effect of high pressure. From a thermodynamic point of view, the mechanical energy transferred to the cell during the pressure treatment can be characterized by the change in volume of the system. The amount of mechanical energy transferred to the cell system is strongly related to cell compressibility, which depends on the water quantity in the cytoplasm.

Strong static magnetic field effects on yeast proliferation and distribution

The present study focuses on the effects of gradient magnetic fields on the behavior of yeast, such as its proliferation and mass distribution, and evaluates the effects of magnetism on materials in the yeast culture system. Yeast, Saccharomyces cerevisiae, was incubated in a liquid medium under magnetic fields (flux density B = 14 T). When yeast in a tube was exposed to 9-14 T magnetic fields with a maximum flux density gradient of dB/dx = 94 T/m, where x is the space coordinate, the rate of yeast proliferation under the magnetic fields decreased after 16 h of incubation compared to that of the control group. The physical properties of the yeast culture system were investigated to discover the mechanism responsible for the observed deceleration in yeast proliferation under magnetic fields. Gas pressure inside the yeast culture flask was compared with and without exposure to a magnetic field. The results suggested that the gas pressure inside a flask with 6 T, 60 T/m slowly increased in comparison to the pressure inside a control tube. Due to the diamagnetism of water (medium solution) and yeast, the liquid surface distinctly inclined under gradient magnetic fields, and the hydrostatic force in suspension was strengthened by the diamagnetic forces. In addition, magnetophoresis of the yeast cells in the medium solution exhibited localization of the yeast sedimentation pattern. The roles of magnetically changed gas-transport processes, hydrostatic pressures acting on the yeast, and changes in the distribution of the yeast sedimentation, as well as the possible effects of magnetic fields on yeast respiratory systems in the observed disturbance of the proliferation are discussed.

Molecular cloning and sequence analysis of the Zygosaccharomyces bailii HIS3 gene encoding the imidazole glycerolphosphate dehydratase

Zygosaccharomyces bailii is a spoilage yeast belonging to the Zygosaccharomyces genus. In recent years these yeasts, due to their exceptional resistance to several stresses, have become more and more interesting as model organisms to study the molecular basis of the said resistance. A Z. bailii cDNA library has been built and the 672 bp nucleotide sequence coding for the HIS3 gene was cloned by complementation of a Saccharomyces cerevisiae his3 mutant strain. The deduced 223 amino acid sequence shares a high degree of homology with His3p homologues in other non-conventional yeast species. The GeneBank Accession No. is AY050224.

Physiological and mathematical aspects in setting criteria for decontamination of foods by physical means

In heat processing, microbial inactivation is traditionally described as log-linear. As a general rule, the relation between rate of inactivation and temperature is also described as a log-linear relation. The model is also sometimes applied in pressure and in pulsed electric field (PEF) processing. The model has proven its value by the excellent safety record of the last 80 years, but there are many deviations from log-linearity. This could lead to either over-processing or under-processing resulting in safety problems or, more likely, spoilage problems. As there is a need for minimal processing, accurate information of the inactivation kinetics is badly needed. To predict inactivation more precisely, models have been developed that can cope with deviations of linearity. As extremely low probabilities of survival must be predicted, extrapolation is almost always necessary. However, extrapolation is hardly possible without knowledge of the nature of nonlinearity. Therefore, knowledge of the physiology of inactivation is necessary. This paper discusses the physiology of denaturation by heat, high pressure and pulse electric field. After discussion of the physiological aspects, the various aspects of the development of inactivation models will be addressed. Both general and more specific aspects are discussed such as choice of test strains, effect of the culture conditions, conditions during processing and recovery conditions and mathematical modelling of inactivation. In addition to lethal inactivation, attention will be paid to sublethal inactivation because of its relevance to food preservation. Finally, the principles of quantitative microbiological risk assessment are briefly mentioned to show how appropriate inactivation criteria can be set.

Aspergillus flavus dose-response curves to selected natural and synthetic antimicrobials

The effects of selected concentrations of antimicrobials from natural (vanillin, thymol, eugenol, carvacrol or citral) or synthetic (potassium sorbate or sodium benzoate) origin on Aspergillus flavus lag time inoculated in laboratory media formulated at water activity (a(w)) 0.99 and pH 4.5 or 3.5, were evaluated. Time to detect a colony with a diameter > 0.5 mm was determined. Mold response was modeled using the Fermi function. Antimicrobial minimal inhibitory concentration (MIC) was defined as the minimal required inhibiting mold growth for 2 months. Fermi function successfully captured A. flavus dose-response curves to the tested antimicrobials with a highly satisfactory fit. Fermi equation coefficients, Pc and k, were used to compare antimicrobials and assess the effect of pH. Important differences in Pc and k were observed among antimicrobials, being natural antimicrobials less pH dependent than synthetic antimicrobials. A large Pc value represents a small antimicrobial effect on A. flavus lag time; thus, high concentrations are needed to delay growth. A. flavus exhibited higher sensitivity to thymol, eugenol, carvacrol, potassium sorbate (at pH 3.5), and sodium benzoate (at pH 3.5) than to vanillin or citral. MICs varied from 200 ppm of sodium bcnzoate at pH 3.5 to 1800 ppm of citral at both evaluated pHs.

Kinetic analysis and modelling of combined high-pressure-temperature inactivation of the yeast Zygosaccharomyces bailii

Eight foodborne yeasts were screened for sensitivity to high-pressure (HP) inactivation under a limited number of pressure-temperature combinations. The most resistant strains were Zygoascus hellenicus and Zygosaccharomyces bailii. The latter was taken for a detailed study of inactivation kinetics over a wide range of pressures (120-320 MPa) and temperatures (-5 to 45 degrees C). Isobaric and isothermal inactivation experiments were conducted in Tris-HCl buffer pH 6.5 for 48 different combinations of pressure and temperature. Inactivation was biphasic, with a first phase encompassing four to six decades and being described by first-order kinetics, followed by a tailing phase. Decimal reduction times (D) were calculated for the first-order inactivation phase and their temperature and pressure dependence was described. At constant temperature, D decreased with increasing pressure as expected. At constant pressure, D showed a maximum at around 20 degrees C, and decreased both at lower and at higher temperatures. A mathematical expression was developed to describe accurately the inactivation of Z. bailii as a function of pressure and temperature under the experimental conditions employed. A limited number of experiments in buffer at low pH (3-6) suggest that the model is, in principle, applicable at low pH. In apple and orange juice however, higher inactivation than predicted by the model was achieved.