High pressure (1 to 10 kbars, i.e. 100-1000 MPa) affects biological constituents and systems. Several physicochemical properties of water are modified, such as the density, the ionic dissociation (...

Review : High-pressure, microbial inactivation and food preservation:

Ultrahigh Pressure (UHP) offers interesting possibilities for food processing ranging from extraction of plant compounds, restructuring foods and rapid formation of small ice crystals. Pressures between 300 and 600 MPa can inactivate yeasts, moulds and most vegetative bacteria including most infectious food-borne pathogens. Thus, pressure is a potential alternative to heat pasteurization as pressure leaves small molecules such as many flavour compounds and vitamins intact. In general, bacterial spores can only be killed by very high pressures (>1000 MPa). Bacterial spores, however, can often be stimulated to germinate by pressures of 50–300 MPa. Germinated spores can then be killed by relatively mild heat treatments or mild pressure treatments. However, in most cases a small fraction of spores can survive this treatment. Consequently, there is still no practical application of UHP treatment as a sterilization process, but it is still an interesting area for further exploration.

Recent advances in the microbiology of high pressure processing

Summary
AtHKT1 is a sodium (Na+) transporter that functions in mediating tolerance to salt stress. To investigate the membrane targeting of AtHKT1 and its expression at the translational level, antibodies were generated against peptides corresponding to the first pore of AtHKT1. Immunoelectron microscopy studies using anti-AtHKT1 antibodies demonstrate that AtHKT1 is targeted to the plasma membrane in xylem parenchyma cells in leaves. AtHKT1 expression in xylem parenchyma cells was also confirmed by AtHKT1 promoter–GUS reporter gene analyses. Interestingly, AtHKT1 disruption alleles caused large increases in the Na+ content of the xylem sap and conversely reduced the Na+ content of the phloem sap. The athkt1 mutant alleles had a smaller and inverse influence on the potassium (K+) content compared with the Na+ content of the xylem, suggesting that K+ transport may be indirectly affected. The expression of AtHKT1 was modulated not only by the concentrations of Na+ and K+ but also by the osmolality of non-ionic compounds. These findings show that AtHKT1 selectively unloads sodium directly from xylem vessels to xylem parenchyma cells. AtHKT1 mediates osmolality balance between xylem vessels and xylem parenchyma cells under saline conditions. Thus AtHKT1 reduces the sodium content in xylem vessels and leaves, thereby playing a central role in protecting plant leaves from salinity stress.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.2005.02595.x

Enhanced salt tolerance mediated by AtHKT1 transporter-induced Na unloading from xylem vessels to xylem parenchyma cells.

Publisher Summary The chapter focuses on mycological aspects of the genus Metarhizium . The genus includes several species, varieties within species, and individual isolates with broad ranges of physiological traits—including host range. The interactions of these fungi with their hosts, and the large literature on their use for pest control, largely define the scientific and popular concepts of Metarhizium spp. Fungi of the hyphomycete genus Metarhizium have been isolated from infected insects and soil. Although some isolates of these fungi have rather restricted host ranges, the group is better known for its ability to kill a wide spectrum of insects, including insects in at least seven orders. The common name for Metarhizium -induced disease is “green muscardine,” based on the encrustation of insect cadavers with green conidia. The rapid increase in research on Metarhizium , followed by sustained high scientific output, can be explained by several important worldwide attitude changes and the initiation of several promising Metarhizium -based pest-control and molecular-biology efforts.

Metarhizium spp., cosmopolitan insect-pathogenic fungi: mycological aspects.

High pressure processing is a food processing method which has shown great potential in the food industry. Similar to heat treatment, high pressure processing inactivates microorganisms, denatures proteins and extends the shelf life of food products. But in the meantime, unlike heat treatments, high pressure treatment can also maintain the quality of fresh foods, with little effects on flavour and nutritional value. Furthermore, the technique is independent of the size, shape or composition of products. In this paper, many aspects associated with applying high pressure as a processing method in the food industry are reviewed, including operating principles, effects on food quality and safety and most recent commercial and research applications. It is hoped that this review will promote more widespread applications of the technology to the food industry.

Recent Advances in the Use of High Pressure as an Effective Processing Technique in the Food Industry

The structural damage to and leakage of internal substances from Saccharomyces cerevisiae 0–39 cells induced by hydrostatic pressure were investigated. By scanning electron microscopy, yeast cells treated at room temperature with pressuresbellw 400 MPa for 10 min showed a slight alteration in outer shape. Transmission electron microscopy, however, showed that the inner structure of the cell began to be affected, especially the nuclear membrane, when treated with hydrostatic pressure around 100 MPa at room temperature for 10 min; at more than 400–600 MPa, further alterations appeared in the mitochondria and cytoplasm. Furthermore, when high pressure treatment was carried out at — 20° C, the inner structure of the cells was severely damaged even at 200 MPa, and almost all of the nuclear membrane disappeared, although the fluorescent nucleus in the cytoplasm was visible by 4,6-diamidino-2-phenylindole (DAPI) staining. The structural damage of pressure-treated cells was accompanied by the leakage of internal substances. The efflux of UV-absorbing substances including amino acid pools, peptides, and metal ions increased with increase in pressure up to 600 MPa. In particular, amounts of individual metal ion release varied with the magnitude of hydrostatic pressures over 300 MPa, which suggests that the ions can be removed from the yeast cells separately by hydrostatic pressure treatment.

Effects of hydrostatic pressure on the ultrastructure and leakage of internal substances in the yeast Saccharomyces cerevisiae

The ultrastructure of regenerating cell wall in Schizosaccharomyces pombe protoplasts was studied with a high resolution, low voltage scanning electron microscope (LVSEM). In contrast to the transmission electron microscopy, the LVSEM images give three-dimensional information on the cell wall regeneration in yeast protoplasts. We found that, after only a few minutes of incubation, the protoplasts began to show protuberances in a unipolar manner, and a fibrilar network was formed asymmetrically which covered the whole surface of the protoplasts after 5 hr. The network consisted of microfibrils about 8 to 10 nm wide, forming flat and wavy bundles of various widths and lengths, up to about 200 nm wide and 1 micron long, mainly made of yeast glucan. Free ends of microfibrils were seldom found. Interfibrillar spaces were progressively filled with granular particles and finally the complete cell wall was formed after 12 hr. The fibrillar network was destroyed by the digestion with beta (1----3)-glucanase. When protoplasts were regenerating in the presence of aculeacin A, the fibrillar networks were not formed, resulting in incomplete cell wall formation. These observations suggest that beta-glucan is the main component of the microfibrils and that it plays an important role in the formation of the cell wall in S. pombe.

Cell Wall Formation in Regenerating Protoplasts of Schizosaccharomyces pombe: Study by High Resolution, Low Voltage Scanning Electron Microscopy

When an n-alkane-utilizable yeast, Candida tropicalis pK233, was cultivated on butyrate, the fatty acid of shortest chain-length for beta-oxidation, as the sole source of carbon and energy, catalase and the enzymes of the fatty acid beta-oxidation system were inducibly synthesized at high levels. As in the alkane-grown cells, the proliferation of peroxisomes was harmonized with the induction of peroxisomal enzymes. The results of subcellular fractionation and immunoelectronmicroscopy indicated the localization of these enzymes in peroxisomes, not in mitochondria. It was suggested that only peroxisomes have a role in fatty acid beta-oxidation in the yeast cells, unlike in mammalian cells, in which cooperation between peroxisomes and mitochondria is essential.

Beta-oxidation of butyrate, the short-chain-length fatty acid, occurs in peroxisomes in the yeast Candida tropicalis.

High Resolution, Low Voltage Scanning Electron Microscopy of Uncoated Yeast Cells Fixed by the Freeze-Substitution Method

The time course of BMY-28864-dependent morphological changes in Candida albicans A9540 was studied by electron microscopy. Scanning electron microscopy revealed that BMY-28864 often induced cell surface deformations such as abnormal swelling and bulging around budding sites and bud scars. The cell membrane damage was visualized by freeze-fracturing technique as deep pit-like invaginations. Transmission electron microscopy with thin-sectioned specimens also demonstrated that BMY-28864 induced cell membrane invaginations together with cell membrane detachment from the cell wall, nuclear membrane fragmentation and mitochondrial aberration. Statistical sequence analysis of the prominent BMY-28864-dependent morphological changes of the yeast cells led to the conclusion that BMY-28864 first attacked the cell membrane and then caused disintegration of other intracytoplasmic organelles, resulting in the lethal effect.

Nobuko Naito

Papers

Effects of hydrostatic pressure on the ultrastructure and leakage of internal substances in the yeast Saccharomyces cerevisiae

Cell Wall Formation in Regenerating Protoplasts of Schizosaccharomyces pombe: Study by High Resolution, Low Voltage Scanning Electron Microscopy

Beta-oxidation of butyrate, the short-chain-length fatty acid, occurs in peroxisomes in the yeast Candida tropicalis.

High Resolution, Low Voltage Scanning Electron Microscopy of Uncoated Yeast Cells Fixed by the Freeze-Substitution Method

Morphological changes of Candida albicans induced by BMY-28864, a highly water-soluble pradimicin derivative.