Kazutaka Yamamoto

Food processing by high hydrostatic pressure

High hydrostatic pressure (HHP) process, as a nonthermal process, can be used to inactivate microbes while minimizing chemical reactions in food. In this regard, a HHP level of 100 MPa (986.9 atm/1019.7 kgf/cm2) and more is applied to food. Conventional thermal process damages food components relating color, flavor, and nutrition via enhanced chemical reactions. However, HHP process minimizes the damages and inactivates microbes toward processing high quality safe foods. The first commercial HHP-processed foods were launched in 1990 as fruit products such as jams, and then some other products have been commercialized: retort rice products (enhanced water impregnation), cooked hams and sausages (shelf life extension), soy sauce with minimized salt (short-time fermentation owing to enhanced enzymatic reactions), and beverages (shelf life extension). The characteristics of HHP food processing are reviewed from viewpoints of nonthermal process, history, research and development, physical and biochemical changes, and processing equipment.

Injury and recovery of Escherichia coli ATCC25922 cells treated by high hydrostatic pressure at 400–600 MPa

Escherichia coli cells were inactivated by high hydrostatic pressure (HHP) at 400-600xa0MPa and their recovery under various conditions was evaluated by colony counting and flow cytometer (FCM) analyses. The lag time in colony formation and an improved recovery of cells under less oxidative conditions (pyruvate addition to the medium and incubation in anaerobic conditions) were observed for HHP treated cells, which indicated that a significant portion of cells were injured and recovered during incubation after HHP treatment. The lag time for colony formation varied, which suggested a wave of resuscitation and recovered cells may multiply before other injured cells complete resuscitation. The recovery process was monitored by FCM: The FCM profile of cells stained using propidium iodide and SYTO9 indicated that while the majority of cells died just after HHP treatment, the staining pattern of possibly injured cells displayed a specific spectrum that gradually became consistent with that of the dead cell population and a living cell population simultaneously appeared. Pyruvate addition to the medium not only enhanced viability of HHP treated cells, but also reduced the lethal effect of HHP. These observations suggested that the degree of damage by HHP may differ cell-by-cell, and oxidative stress may continue after HHP treatment. Pyruvate addition to the recovery medium enhanced viability of E.xa0coli cells inactivated by HHP treatment in tomato juice as well.

Membrane damage and viability loss of E. coli O157:H7 and Salmonella spp. in apple juice treated with high hydrostatic pressure and thermal death time disks.

Differences in membrane damage including leakage of intracellular UV-materials and loss of viability of Salmonella spp. and Escherichia coli O157:H7 bacteria in apple juice following thermal-death-time (TDT) disk and high hydrostatic pressure treatments were investigated. Salmonella spp. and E. coli O157H:H7 bacteria were inoculated in apple juice to a final 7.8 log10 CFU/ml and were thermally treated with TDT disks at 25, 35, 45, 50, 55 and 60°C for 4 min or pressurized at 350, 400 and 450 MPa at 25, 35, 45, 50, 55 and 60°C for 20 min. Sublethal injury, leakage of UV- materials and viability loss as a function of membrane damage of these bacterial pathogens was investigated by plating 0.1 ml of treated and untreated samples on non selective Trypticase Soy Agar (TSA) and selective Xylose Lysine Sodium Tetradecylsufate (XLT4) for Salmonella and Cefixime Potassium Tellurite Sorbitol-MacConkey (CT-SMACK) agar plates for E. coli bacteria with incubation at 36°C for 48 h. Sub-lethal injury occurred in Salmonella spp. and E. coli populations thermally treated with TDT disk at 55°C and above and at a pressure treatments of 25°C and above. Leakage of intracellular UV-materials and ATP of TDT disk injured cells was lower than the values determined from pressurized cells. Similarly, recovery of TDT injured cells occurred faster than pressurized cells during storage of treated samples at 22°C. The results of this study indicate that pressure treatment of 350 MPa at 35°C for 20 min and thermal treatments of 55 and 60°C and immediate storage of treated samples at 5°C will inhibit recovery and complete inactivation of injured bacteria in apple juice and therefore, will enhance the microbial safety of the treated juice.

Pressure Gelatinization of Starch

Starch, which is conventionally processed by heat, is a key food component and an industrial raw material. High hydrostatic pressure (HHP) can induce gelatinization of starch without heating. Spontaneous retrogradation can be observed immediately after HHP-induced gelatinization, depending on the starch content and temperature. In HHP treatment of starch systems, it should be differentiated whether it is anisotropic or isotropic compression, each of which results in different properties of the obtained starches. Physicochemical changes of various starches due to thermal or HHP treatment have been studied intensively by various methods, and the behavior of starch gelatinization under combinations of heat and HHP has been systematically revealed in recent years. In this paper, trends in the study of HHP-treated starch are reviewed from the viewpoints of fundamental and application approaches.

Metabolome analysis of Escherichia coli ATCC25922 cells treated with high hydrostatic pressure at 400 and 600 MPa

Escherichia coli cells were treated with high hydrostatic pressure (HHP) at 400 and 600xa0MPa. Metabolites (70-1027xa0m/z) extracted from HHP-treated cells were analyzed using capillary electrophoresis-time-of-flight mass spectrometry and were compared with those extracted from control cells (not treated with HHP). A total of 133 metabolites were identified and mapped to metabolic pathways, and many of these (42.1%) decreased due to the HHP treatment, including NAD+, NADP+, ATP, and substrates for DNA synthesis. Principal component analysis suggested that the central sugar and nucleic acid metabolic pathways were strongly influenced by HHP. A bottleneck in the central sugar metabolic pathway was observed in HHP-treated cells, which created a metabolic imbalance; metabolites mapped upstream (glucose 6-phosphate, fructose 6-phosphate, and fructose 1,6-diphosphate) were accumulated and those downstream (3-phosphoglycerate, 2-phosphoglycerate, and phosphoenolpyruvate) were depleted. Ribonucleotides were decreased, but the reduction was moderate compared with that of substrates for DNA synthesis; the exception was ATP, which also substantially decreased. The bottleneck in the glycolytic pathway partly explained the exhaustion of ATP. NAD+/NADH ratio of HHP treated cells was comparable with that of untreated control cells.

Characterization of high hydrostatic pressure-injured Bacillus subtilis cells

High hydrostatic pressure (HHP) affects various cellular processes. Using a sporulation-deficient Bacillus subtilis strain, we characterized the properties of vegetative cells subjected to HHP. When stationary-phase cells were exposed to 250 MPa of HHP for 10 min at 25 °C, approximately 50% of cells were viable, although they exhibited a prolonged growth lag. The HHP-injured cells autolyzed in the presence of NaCl or KCl (at concentrations ≥100 mM). Superoxide dismutase slightly protected the viability of HHP-treated cells, whereas vegetative catalases had no effect. Thus, unlike HHP-injured Escherichia coli, oxidative stress only slightly affected vegetative B. subtilis subjected to HHP. Graphical abstract Cell death and the recovery of HHP-injured Bacillus subtilis vegetative cell.