Xuepeng Li

Quality enhancement in refrigerated red drum (Sciaenops ocellatus) fillets using chitosan coatings containing natural preservatives.

The present work was undertaken to examine the influence of grape seed extract (GS) and tea polyphenols (TP), as natural preservatives, combined with chitosan (Ch), on the quality of red drum (Sciaenops ocellatus) fillets during refrigerated storage. Two different treatments (Ch+GS and Ch+TP) and a control were prepared. The samples were stored (4 ± 1°C) for 20 days and the sampling was done at 0, 4, 8, 12, 16, 20 days. Microbiological, physicochemical and sensory attributes were periodically assessed. The results indicated that the two pretreatments could more effectively maintain quality and could extend the shelf life by 6-8 days compared with the control group during refrigerated storage.

Physicochemical responses and quality changes of red sea bream (Pagrosomus major) to gum arabic coating enriched with ergothioneine treatment during refrigerated storage.

The combined effects of gum arabic coating (GA) and ergothioneine (ER) treatment on the sensory and physicochemical characteristics of red sea bream (Pagrosomus major) stored at 4 ± 1 °C for 16 days were investigated. Fish proximate composition, pH value, total volatile basic nitrogen (TVB-N), thiobarbituric acid (TBA), K-value, TCA-soluble peptides, colour, texture profile analyses (TPA), microbiological properties and sensory quality were measured. The results indicate that treatment with gum arabic and ergothioneine (GAER) retarded nucleotide breakdown, lipid oxidation, protein degradation, and reduced microbial growth compare with the control. The efficiency was better than that of GA or ER treatment. Sensory evaluation proved the efficacy of GAER coating by maintaining the overall quality of red sea bream during the storage period. Furthermore, GAER maintained better colour and textural characteristics. Our study suggests that GAER treatment has potential to improve the quality of red sea bream and extend its storage life.

Antimicrobial effect and membrane-active mechanism of tea polyphenols against Serratia marcescens

In this study, we investigated the antimicrobial effect of tea polyphenols (TP) against Serratia marcescens and examined the related mechanism. Morphology changes of S. marcescens were first observed by transmission electron microscopy after treatment with TP, which indicated that the primary inhibition action of TP was to damage the bacterial cell membranes. The permeability of the outer and inner membrane of S. marcescens dramatically increased after TP treatment, which caused severe disruption of cell membrane, followed by the release of small cellular molecules. Furthermore, a proteomics approach based on two-dimensional gel electrophoresis and MALDI-TOF/TOF MS analysis was used to study the difference of membrane protein expression in the control and TP treatment S. marcescens. The results showed that the expression of some metabolism enzymes and chaperones in TP-treated S. marcescens significantly increased compared to the untreated group, which might result in the metabolic disorder of this bacteria. Taken together, our results first demonstrated that TP had a significant growth inhibition effect on S. marcescens through cell membrane damage.

Preparation and antibacterial properties of titanium-doped ZnO from different zinc salts

To research the relationship of micro-structures and antibacterial properties of the titanium-doped ZnO powders and probe their antibacterial mechanism, titanium-doped ZnO powders with different shapes and sizes were prepared from different zinc salts by alcohothermal method. The ZnO powders were characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible spectroscopy (UV-vis), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED), and the antibacterial activities of titanium-doped ZnO powders on Escherichia coli and Staphylococcus aureus were evaluated. Furthermore, the tested strains were characterized by SEM, and the electrical conductance variation trend of the bacterial suspension was characterized. The results indicate that the morphologies of the powders are different due to preparation from different zinc salts. The XRD results manifest that the samples synthesized from zinc acetate, zinc nitrate, and zinc chloride are zincite ZnO, and the sample synthesized from zinc sulfate is the mixture of ZnO, ZnTiO3, and ZnSO4 · 3Zn (OH)2 crystal. UV-vis spectra show that the absorption edges of the titanium-doped ZnO powders are red shifted to more than 400 nm which are prepared from zinc acetate, zinc nitrate, and zinc chloride. The antibacterial activity of titanium-doped ZnO powders synthesized from zinc chloride is optimal, and its minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) are lower than 0.25 g L−1. Likewise, when the bacteria are treated by ZnO powders synthesized from zinc chloride, the bacterial cells are damaged most seriously, and the electrical conductance increment of bacterial suspension is slightly high. It can be inferred that the antibacterial properties of the titanium-doped ZnO powders are relevant to the microstructure, particle size, and the crystal. The powders can damage the cell walls; thus, the electrolyte is leaked from cells.

Physicochemical Responses and Quality Changes of Turbot (Psetta maxima) During Refrigerated Storage

The sensory, physicochemical, microbiological, and volatiles changes of postmortem turbot during 20 days of chilled storage at 4°C were studied. No significant changes were found in pH value and whiteness during the storage, while total viable counts, total volatile base nitrogen, and K-values increased significantly with storage time. The textures profile, springiness and resilience, and sensory attributes changed significantly during storage. A total of 50 volatile compounds including aldehydes, ketones, esters, alcohols, alkenes, alkanes, aromatics, amines, and others were identified in turbot muscle during storage by the solid-phase microextraction and gas chromatography–mass spectrometry methods. Among these volatile components, the relative content of some decreased significantly with time, while others increased significantly. The results suggested that hardness, springiness, resilience, sensory attributes, total volatile base nitrogen, K-value, and total viable counts combined with some volatile compounds could more perfectly reflect the quality changes of turbot during storage. The study indicated that turbot fillets stored at 4°C maintain better quality for 16 days during storage.

ESR studies on the thermal decomposition of trimethylamine oxide to formaldehyde and dimethylamine in jumbo squid (Dosidicus gigas) extract

The effects of ferrous iron, heating temperature and different additives on the decomposition of trimethylamine oxide (TMAO) to formaldehyde (FA) and dimethylamine (DMA) and generation of free radicals in jumbo squid (Dosidicus gigas) extract during heating were evaluated by electron spin resonance (ESR). The thermal decomposition of TMAO to TMA, DMA and FA and free radical signals was observed in squid extract, whereas no DMA, FA and free radical signals were detected in cod extract or in aqueous TMAO solution in vitro at high temperatures. Significant increase in levels of DMA, FA and radicals intensity were observed in squid extract and TMAO solution in the presence of ferrous iron with increasing temperature. Hydrogen peroxide stimulated the production of DMA, FA and ESR signals in squid extract, while citric acid, trisodium citrate, calcium chloride, tea polyphenols and resveratrol had the opposite effect. Similar ESR spectra of six peaks regarded as amminium radical were detected in the squid extract and TMAO-iron(II) solution, suggesting that the amminium radical was involved in the decomposition of TMAO.

Quorum sensing system and influence on food spoilage in Pseudomonas fluorescens from turbot

The spoilage of aquatic products is mainly caused by the bacterial growth, and the specific spoilage organism (SSO) plays an important role. Quorum sensing (QS) is a microbial cell–cell communication system which is coordinated with the population density, and is controlled by N-acyl-homoserine lactone (AHLs) as the Gram-negative bacteria communication signals. In this study, the SSO was Pseudomonas fluorescens (PF-04), isolated from the turbot (Scophthalmus maximus L.) during aerobically refrigerated storage. The supernatant extract of PF-04 tested the AHLs activities utilizing biosensor Chromobacterium violaceum CV026. AHL production was influenced by the environment temperature, and AHL production reduced obviously at 10 °C compare with 25 °C. In Luria-Bertani (LB) supplemented with 0.5–1.0% NaCl, AHL production reached the maximum. The AHL production was also regulated by pH of culture medium, acidic condition was conducive to persistent existence of the AHL molecules, but the alkaline environment would cause chemically unstable of AHL molecules. QS system in P. flurosecens played an imperative role in biofilm formation, protease and siderophore production. AHLs could regulate above three factors in PF-04. In summary, this study showed that (1) the influence of different environmental conditions (temperature, NaCl and pH) on AHL production revealed the correlation of QS in foods and (2) that proved the effect of external AHLs to regulate the biofilm formation, protease and siderophore production in PF-04.

Protein degradation of olive flounder (Paralichthys olivaceus) muscle after postmortem superchilled and refrigerated storage

ABSTRACT The aim of this study was to investigate the effect of superchilling at −2°C in comparison with refrigerated storage at 4°C on the protein degradation of olive flounder (Paralichthys olivaceus) muscle. Flounder muscle softened and shear force value decreased markedly (P < 0.05) with prolonged storage time, while values of electrical conductivity, TCA-soluble peptide, free amino acids, and proteolysis index increased (P < 0.05). The changes were slowed down significantly in samples superchilled at −2°C (P < 0.05). The fracture of muscle fibre and formation of cracks were accelerated in the samples refrigerated at 4°C, and intercellular spaces were observed after 9 days of storage. Moreover, protein bands of myosin heavy chain (MHC), actin, tropomyosin, 97 kDa, 50 ~ 60 kDa and 35 ~ 36 kDa occurred in varying degrees of degradation with storage time. The results demonstrated that significant postmortem degradation of muscle proteins occurred with extending storage time, while the changes were retarded obviously in samples during superchilled storage.