Inactivation and Damage of Histamine-Forming Bacteria by Treatment with High Hydrostatic Pressure

Elucidation of high-pressure processing toward microbial inhibition, physicochemical properties, collagen fiber and muscle structure of blood clam edible portion

Impact of high-pressure processing (HP-P) on microbial inactivation, protein oxidation, collagen fiber, and muscle structure of the edible portion (EP) of blood clams (BC) was investigated. Aerobic plate count, Vibrio parahaemolyticus, V. vulnificus, other Vibrio spp. and Shewanella algae counts were not detectable when HP-P pressure of ≥300 MPa was applied. Carbonyl, disulphide bond content, and surface hydrophobicity upsurged as HP-P with augmenting pressure was employed. Protein with ∼53 kDa appeared when HP-P at 100 and 200 MPa was implemented. Increased pressure enhanced gap formation and abnormal muscle cell structure arrangements. HP-P also affected connective tissue, causing size reduction and disruption of the collagen filament fibers. However, firmness and toughness of BC-EP with HP-P ≤ 300 MPa were comparable to those of the control. HP-P at 300 MPa was therefore appropriate for treatment of BC with maintained textural properties, while less protein oxidation, collagen fiber and muscle structure disruption occurred.

Bacterial Changes in Boiled Crayfish between Different Storage Periods and Characterizations of the Specific Spoilage Bacteria

This study investigated changes in the microbial compositions of crayfish tails during storage at 4 °C (for 0-12 days) as measured using high-throughput sequencing (HTS). The specific spoilage organisms (SSOs) in the crayfish tails were isolated using culture-dependent cultivation methods, and they were identified by 16S rRNA and characterized for their enzymatic spoilage potentials (e.g., protease, lipase, phospholipase, and amylase). The spoilage abilities of the selected strains in the crayfish tails were assessed by inoculating them into real food. Moreover, the microbial growth and the volatile basic nitrogen (TVB-N) changes were monitored during the storage period. The results from the HTS showed that the dominant genus of shrimp tails evolved from Streptococcus (D0) to Pseudomonas (D4) and, finally, to Paenisporosarcina (D12) during storage. Seven bacterial species (Acinetobacter lwoffii, Aeromonas veronii, Kurthia gibsonii, Pseudomonas sp., Exiguobacterium aurantiacum, Lelliottia amnigena, and Citrobacter freundii) were screened from the spoiled shrimp tails by the culture-dependent method, among which Aeromonas veronii had the strongest spoilage ability.

Sublethally injured microorganisms in food processing and preservation: Quantification, formation, detection, resuscitation and adaption

Sublethally injured state has been recognized as a survival strategy for microorganisms suffering from stressful environments. Injured cells fail to grow on selective media but can normally grow on nonselective media. Numerous microorganism species can form sublethal injury in various food matrices during processing and preservation with different techniques. Injury rate was commonly used to evaluate sublethal injury, but mathematical models for the quantification and interpretation of sublethally injured microbial cells still require further study. Injured cells can repair themselves and regain viability on selective media under favorable conditions when stress is removed. Conventional culture methods might underestimate microbial counts or present a false negative result due to the presence of injured cells. Although the structural and functional components may be affected, the injured cells pose a great threat to food safety. This work comprehensively reviewed the quantification, formation, detection, resuscitation and adaption of sublethally injured microbial cells. Food processing techniques, microbial species, strains and food matrix all significantly affect the formation of sublethally injured cells. Culture-based methods, molecular biological methods, fluorescent staining and infrared spectroscopy have been developed to detect the injured cells. Cell membrane is often repaired first during resuscitation of injured cells, meanwhile, temperature, pH, media and additives remarkably influence the resuscitation. The adaption of injured cells negatively affects the microbial inactivation during food processing.

Inhibitory Effects of High-Hydrostatic-Pressure Processing on Growth and Histamine Formation of Histamine-Forming Bacteria in Yellowfin Tuna Meat during Storage

In the research, we evaluated the effects of high-pressure processing (HPP) on the growth and histamine formation of histamine-forming bacteria (HFB) in yellowfin tuna meat during storage. Tuna meat samples inoculated with the individual HFB species Morganella morganii and Photobacterium phosphoreum were subjected to HPP treatment at 250, 350, 450, and 550 MPa for 5 min, and the changes in bacterial count, total volatile basic nitrogen (TVBN) content, pH, and histamine content during storage at 4 and 15 °C were analyzed. The results indicate that the bacterial counts of the HFB species decreased significantly with increasing pressure, and HFB became undetectable in the samples treated at 450 and 550 MPa. At a storage temperature of 15 °C, the bacterial counts of both HFB species in the control group and samples subjected to HPP treatment at 250 and 350 MPa increased significantly with storage time. The bacterial counts of M. morganii in the samples stored at 4 °C decreased, whereas those of P. phosphoreum increased gradually owing to its psychrophilic nature. HPP treatment (>250 MPa) inhibited the increases in pH and TVBN content of the samples stored at 15 °C and delayed histamine formation in the samples during storage; these effects were more significant as the pressure during HPP treatment was increased.

Evaluation of Structural Changes and Molecular Mechanism Induced by High Hydrostatic Pressure in Enterobacter sakazakii

The contamination of infant milk and powder with Enterobacter sakazakii poses a risk to human health and frequently caused recalls of affected products. This study aims to explore the inactivation mechanism of E. sakazakii induced by high hydrostatic pressure (HHP), which, unlike conventional heat treatment, is a nonthermal technique for pasteurization and sterilization of dairy food without deleterious effects. The mortality of E. sakazakii under minimum reaction conditions (50 MPa) was 1.42%, which was increased to 33.12% under significant reaction conditions (400 MPa). Scanning electron microscopy (SEM) and fluorescent staining results showed that 400 MPa led to a loss of physical integrity of cell membranes as manifested by more intracellular leakage of nucleic acid, intracellular protein and K⁺. Real-time quantitative PCR (RT-qPCR) analysis presents a downregulation of three functional genes (glpK, pbpC, and ompR), which were involved in cell membrane formation, indicating a lower level of glycerol utilization, outer membrane protein assembly, and environmental tolerance. In addition, the exposure of E. sakazakii to HHP modified oxidative stress, as reflected by the high activity of catalase and super oxide dismutase. The HHP treatment lowered down the gene expression of flagellar proteins (fliC, flgI, fliH, and flgK) and inhibited biofilm formation. These results determined the association of genotype to phenotype in E. sakazakii induced by HHP, which was used for the control of food-borne pathogens.

Microbial community changes induced by Pediococcus pentosaceus improve the physicochemical properties and safety in fermented tilapia sausage

Amine-negative lactic acid bacteria can prevent excess biogenic amines from accumulating in sausage. In this study, the amine-negative Pediococcus pentosaceus 30-7 and 30-15 with good fermentation properties and biogenic amine removal ability were isolated for tilapia sausage production. P. pentosaceus 30-7 improved the physical characteristics such as gel strength and hardness in tilapia sausage, while P. pentosaceus 30-15 significantly enhanced the contents of umami and sweet free amino acids. The microbial metabolic network revealed that the dominant microbial community in the fermentation process including Pediococcus and Lactococcus contributed to the physicochemical formation of sausage. The significant decrease of biogenic amine contents after addition of P. pentosaceus strains mainly resulted from their ability to remove biogenic amines and to inhibit the growth of amine-producing Enterobacter, Citrobacter, and Streptococcus. This study provides an effective method for directionally improving the physicochemical properties and safety in fermented tilapia sausage.

Novel insight into physicochemical and flavor formation in naturally fermented tilapia sausage based on microbial metabolic network

The quality and flavor formation in fermented fish sausages are based on the complex metabolism of microbial community. In this study, the dynamic changes of physicochemical characteristics, volatile compounds, and microbial communities in the naturally fermented tilapia sausage were studied during the fermentation process. The main physical indexes (gel strength, whiteness, and hardness), dominant flavor free amino acids (glycine, alanine, and glutamic acid) and characteristic volatile flavor compounds (hexanal, heptanal, octanal, benzaldehyde, (E)-2-octenal, 4-ethylbenzaldehyde, (E)-2-heptenal, (E,E)-2,4-decadienal, 1-octen-3-ol, 2-pentylfuran, and 2-ethyl-furan) were significantly enhanced after fermentation, and were positively correlated with Lactococcus, Pediococcus, Enterococcus, and Lactobacillus. The microbial metabolic network showed that Lactococcus, Pediococcus, and Enterococcus played a significant role in the formation of physicochemical and flavor characteristics, while the accumulation of biogenic amines might result from the metabolism of Enterococcus, Enterobacter, and Citrobacter. Isolation of lactic acid bacteria in Lactococcus and Pediococcus might be suitable to improve the fermented tilapia sausage. Microbial metabolic network has revealed the physicochemical and flavor formation of tilapia sausage and can provide guidance for future research on screening of starters.

Effect of Low-Temperature-High-Pressure Treatment on the Reduction of Escherichia coli in Milk

Non-thermal processing of milk can potentially reduce nutrient loss, and a low-temperature-high-pressure (LTHP) treatment is considered as a promising alternative to thermal treatment, attracting considerable attention in recent years. The effect of LTHP treatment (−25 °C, 100–400 MPa) on the phase transition behavior of frozen milk was evaluated. The lethal and injured effects of different pressures and cycle numbers on E. coli in frozen milk were studied by using selective and non-selective enumeration media. Results from the gathered transient time–temperature–pressure data showed that pressures over 300 MPa could induce a phase transition from Ice I to Ice III. The treatment at −25 °C and 300 MPa could achieve a lethal effect similar to the two-cycle treatment of 400 MPa at room temperature. This meant that LTHP conditions can lower the operating pressure by at least 100 MPa or reduce the operation from two cycle to one cycle. Increasing the number of pressure cycles enhanced the lethal effects, which was not additive, but resulted in a transformation of part of the injured cells into dead cells. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) provided direct evidence for the breakdown of cell membrane and cell walls by phase transitions. Combined with a designed internal cooling device, the LTHP process can be expected to be a more attractive alternative to non-thermal processing for the dairy industry.