Yongzhi Zhu

The level of heat shock protein 90 in pig Longissimus dorsi muscle and its relationship with meat pH and quality.

The 90 kDa heat shock protein (HSP90) is a molecular chaperone that participates in various cellular processes, the role and significance of HSP90 in postmortem muscle though remains unclear. In the present study, pig Longissimus dorsi muscles, categorized into three pH groups, were tested for HSP90 levels and meat quality parameters (i.e. water holding capacity, colour, tenderness and lipid oxidation). The muscles with a high initial pH (pHi) group (pH>6.4) possessing the greatest water holding capacity and lightness, contained the highest HSP90 level, followed by intermediate (6.0-6.4) and low pHi groups (pH<6.0). Statistical analysis indicated HSP90 level was significantly and negatively correlated with cooking loss, drip loss, and lightness (r=-0.797, -0.785, -0.604, respectively, P<0.01). The results suggest that HSP90 may play a crucial role in water retention of meat and may be involved in postmortem meat quality development.

Changes in actomyosin dissociation and endogenous enzyme activities during heating and their relationship with duck meat tenderness.

The objectives of this study, were to examine the relationship between duck meat tenderness, actomyosin dissociation and endogenous enzyme activities when heating the duck breast muscle, to the internal temperature of 30, 40, 50, 60, 70, 80, 90°C. The shear force increased in the temperature range of 30-50°C and 70-90°C and decreased from 50 to 70°C, which was negatively related with liberated actin (P<0.05). The activity of cathepsin B and L was stable while heating the meat to a temperature below 50°C, then it decreased rapidly with temperature increase. The calpain activity kept decreasing with the temperature increase. There was no significant change in the cathepsin D activity below 70°C but it declined rapidly thereafter, and its activity was strongly correlated with actomyosin dissociation (P<0.05). The results suggest that actomyosin dissociation and cathepsin D, could contribute to the tenderness of duck meat during the cooking process.

Composition of intramuscular phospholipids and free fatty acids in three kinds of traditional Chinese duck meat products

Composition of intramuscular phospholipids and free fatty acids in 3 kinds of traditional Chinese duck meat products (dry-cured duck, roasted duck, and water-boiled salted duck) was investigated. Phospholipids were identified and quantified by HPLC combined with ultraviolet and evaporative light scattering detectors. The types and quantities of free fatty acids and fatty acids derived from phospholipids were analyzed by capillary gas chromatography. The results showed that raw duck meat had high quantities of phosphatidyl-ethanolamine and phosphatidylcholine (28.1 and 64.7% of total phospholipids, respectively), which contained high percentages of polyunsaturated fatty acids. The percentages of total phospholipids, phosphatidylethanolamine, and phosphatidylcholine decreased during processing, with a concomitant increase in quantities of free fatty acids. A selective degradation of fatty acids distributions of phospholipids was found through the dry-cured duck processing, but was not found during the roasted and water-boiled duck processing.

Apoptosis during postmortem conditioning and its relationship to duck meat quality

The aim of this work was to examine the relationship of skeletal muscle apoptosis and postmortem development of meat quality. Colour, cooking loss, myofibril fragmentation index (MFI) and shear force of duck breast and thigh meat postmortem were measured, and changes of positive nuclei were assessed with Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphophate nick end-labelling method (TUNEL). Correlation analysis revealed that apoptosis were positively correlated with colour (L(*), a(*) and b(*)), cooking loss and MFI (P<0.05), while it is negatively correlated with shear force (P<0.05). Our results indicate the growing level of duck skeletal muscle cell apoptosis was associated with the postmortem development of meat quality traits such as meat colour, water holding capacity and tenderness.

Diversity of the Predominant Spoilage Bacteria in Water-Boiled Salted Duck during Storage

The spoilage microbiota in water-boiled salted duck during storage at 4 degrees C was determined using culture-dependent and independent methods. Analysis of the denaturing gradient gel electrophoresis (DGGE) patterns of PCR amplicons targeting the V3 region of the 16S rDNA and sequencing of the bands allowed profiling of the microbiota present in the duck. Community DNA extracts were prepared directly from water-boiled salted duck and from culturable bacterial fractions harvested from both MRS and PCA media. The spoilage bacteria mainly consisted of Staphylococcus saprophyticus, Macrococcus caseolyticus, Weissella, Halomonas sp. or Cobetia sp., and Exiguobacterium sp. based on sequencing and homology search of the DGGE bands. It appeared that both the bacterial counts and diversity increased during storage time. By plating method, bacterial counts in MRS agar increased from 10(4) to 10(8) CFU/g from day 1 to 10, while total bacterial counts in PCA agar reached 10(9) CFU/g after 10 d. Total of 14 strains isolated from PCA and MRS agar were identified as M. caseolyticus (2), S. saprophyticus (7), S. sciuri (1), W. paramesenteroides (2), and W. confusa (2) by 16S rDNA sequencing. The identification of the spoilage-related microbiota is helpful to better understand the bacteria ecology in water-boiled salted duck and may lead to the discovery of appropriate preservation strategies.

Proteolysis and cathepsin activities in the processing of dry-cured duck

Changes in sarcoplasmic and myofibrillar proteins, free amino acids, and cathepsin activities were measured to evaluate the contribution of cathepsins to the proteolysis of muscle proteins in dry-cured duck processing. Thirty-six dry-cured ducks were processed with the traditional method, and samples were collected at different stages. Sarcoplasmic and myofibrillar proteins were found to be degraded to some degree at different stages, whereas content of free amino acids increased from 43.9 to 133.97 mg/100 g during the whole process. Cathepsin B, D, and L activities decreased significantly, and the activities in the end product were 22.4, 26.2, and 40.5% of those in the raw material, respectively. Statistical analysis showed there were significant correlations among changes in proteins, free amino acids, and cathepsin activities. The results indicated that cathepsins are involved in the proteolysis of muscle proteins in dry-cured duck processing.

Heterologous expression and characterization of tyrosine decarboxylase from Enterococcus faecalis R612Z1 and Enterococcus faecium R615Z1.

Tyrosine decarboxylase (TDC) is responsible for tyramine production and can catalyze phenylalanine to produce β-phenylethylamine. Enterococcus strains are a group of bacteria predominantly producing tyramine and β-phenylethylamine in water-boiled salted duck. In this study, the heterologous expression and characterization of two TDCs from Enterococcus faecalis R612Z1 (612TDC) and Enterococcus faecium R615Z1 (615TDC) were studied. The recombinant putative proteins of 612TDC and 615TDC were heterologously expressed in Escherichia coli. 612TDC is a 620-amino-acid protein with a molecular mass of 70.0 kDa, whereas 615TDC is a 625-amino-acid protein with a molecular mass of 70.3 kDa. Both 612TDC and 615TDC showed an optimum temperature of 25 °C for the tyrosine and phenylalanine substrates. However, 612TDC revealed maximal activity at pH 5.5, whereas 615TDC revealed maximal activity at pH 6.0. Kinetic studies showed that 612TDC and 615TDC exhibited higher specificity for tyrosine than for phenylalanine. The catalysis abilities of both 612TDC and 615TDC for phenylalanine were restrained significantly with the increase in NaCl concentration, but this was not the case for tyrosine. This study revealed that the enzyme properties of the purified recombinant 612TDC and 615TDC were similar, although their amino acid sequences had 84% identity.

Production of tyramine by Enterococcus faecalis strains in water-boiled salted duck.

The potential to produce biogenic amines was investigated with 15 Lactococcus lactis and 15 Enterococcus faecalis strains isolated from water-boiled salted duck. The production of biogenic amines from the isolated strains grown in de Man Rogosa Sharpe broth containing precursor amino acids was determined by thin-layer chromatography and high-performance liquid chromatography. None of the L. lactis strains produced any biogenic amines, whereas 12 strains of E. faecalis produced tyramine and b -phenylethylamine. PCR assays were used to detect the presence of tyrosine decarboxylase genes in all of the isolated strains. Only the 12 biogenic amine-producing Enterococcus strains had a 924-bp fragment characteristic for the tyrosine decarboxylase gene. The comparison of the amplified partial tyrDC gene sequences of the 12 positive Enterococcus strains revealed 99% similarity within the same species. The tyramine production of the sterilized water-boiled salted duck inoculated with E. faecalis R612Z1 increased significantly during storage. This study reveals that the isolated E. faecalis strains can produce tyramine and β-phenylethylamine in the medium; however, they can only produce tyramine in water-boiled salted duck.

Synergistic Antibacterial Effect of the Combination of ε-Polylysine and Nisin against Enterococcus faecalis.

This study evaluated the antibacterial effect of the combination of ε-polylysine (ε-PL) and nisin against Enterococcus faecalis strains. The combination of ε-PL and nisin showed synergistic antibacterial activity against three Enterococcus strains. Scanning electron microscopy and a membrane permeability assay revealed that the combined treatment with ε-PL and nisin synergistically damaged the cell morphology of E. faecalis strain R612Z1 cells. Both ε-PL and nisin can dissipate the transmembrane electric potential of E. faecalis R612Z1 cells, but these peptides did not affect the transmembrane pH gradient. The combination of ε-PL and nisin can produce a high reactive oxygen species level in E. faecalis R612Z1 cells. The results indicated that the uptake of ε-PL into cells was promoted through nisin and that the combination of ε-PL and nisin could produce a high reactive oxygen species level in E. faecalis R612Z1 cells, leading to cell growth inhibition.

Changes of phospholipase A2 and C activities during dry-cured duck processing and their relationship with intramuscular phospholipid degradation

Phospholipid hydrolysis, as the main stage and cause of phopholipid degradation, is generally attributed to phospholipases. In this study, the changes of phospholipase A₂ (PLA₂) and C (PLC) activities, neutral lipid, free fatty acids and phospholipids in dry-cured duck leg muscles during processing, were examined. The composition of free fatty acids and neutral lipids increased significantly (P<0.05) with extension of processing time while the phospholipids composition decreased. The PLA₂ and PLC activity decreased in the final product, but retained 83.70% and 86.78% of their initial activities, respectively. The relative activities of both PLA₂ and PLC highly correlated with the decline of phospholipids and the increase of free fatty acids. High correlations were also obtained between the relative activities of PLC and the increase of neutral lipid (P<0.01). All these results suggest that PLA₂ and PLC contribute to the degradation of intramuscular phospholipids during the processing of dry-cured duck.