Green synthesis of fluorescent carbon dots with antibacterial activity and their application in Atlantic mackerel (Scomber scombrus) storage.

Antimicrobial materials prepared from natural products could provide new ways to preserve seafood and extend the shelf life. Herein, four kinds of fluorescent carbon dots were prepared using onion, ginger, garlic, and fish through one-step hydrothermal synthesis. The four prepared carbon dots were nearly spherical and nanosized, with amorphous structure, neutral charge and good water dispersibility. The onion and garlic carbon dots contained more sulfur elements than the ginger and fish carbon dots. Interestingly, the onion carbon dots exhibited the best antibacterial activity against Pseudomonas fragi with good stability over a wide pH range. In addition, the onion carbon dots also exhibited antimicrobial activity against representative Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria. The minimum inhibitory concentration (MIC) and the minimal bactericidal concentration (MBC) of onion carbon dots against Pseudomonas fragi were 2 mg mL-1 and 4 mg mL-1, respectively. The integrity of the cell wall and the cell membrane were damaged for Pseudomonas fragi, and the extracellular alkaline phosphatase (AKP) and ATP activity also increased after exposure to the onion carbon dots, thus leading to a decrease in the cell viability and alteration of the cellular morphology for Pseudomonas fragi. Furthermore, the preservation effect of onion carbon dots on Atlantic mackerel evaluated by storage at 4 °C revealed that the onion carbon dots significantly reduced drip loss, total volatile basic nitrogen (TVB-N) value and total viable counts (TVC) value, and extended the shelf life of Atlantic mackerel by 2 days. This finding suggests that onion carbon dots have potential to be applied as a bacteriostatic agent for aquatic products.

Lactose oxidase: A novel activator of the lactoperoxidase system in milk for improved shelf life.

The lactoperoxidase system (LS), an antimicrobial system naturally present in milk that is activated by H2O2, has been used to inhibit microbial outgrowth in raw milk in areas where refrigeration is not viable. This study evaluated lactose oxidase (LO) as a novel activator of the LS. Lactose oxidase oxidizes lactose and produces H2O2 needed for the activation of the LS. The antimicrobial effect of different concentrations of LO with and without components of the LS, thiocyanate (TCN) and lactoperoxidase (LP), was evaluated in model systems and then applied in pasteurized milk and raw milk. In general, an increase in LO caused greater reductions of Pseudomonas fragi in the model systems and treatments were more effective at 6°C than at 21°C. At 6°C, the LO solution at 0.12 and 1.2 g/L showed significantly higher microbial reduction than the control when both added alone and combined with LS components. At 21°C, treatments with 1.2 g/L of LO solution achieved a reduction of >2.93 log cfu/mL in 24 h, but at lower levels there was not a significant reduction from the control. Higher concentrations of TCN led to a greater P. fragi reduction at both temperatures when LO was added alone but not when combined with LP. In pasteurized milk, the LO solution at 0.12 g/L caused a reduction of approximately 1.4 log of P. fragi within 24 h when added alone and a reduction of approximately 2.7 log when combined with LP and TCN. Bacterial counts remained at significantly lower levels than the control during storage, and the TCN-supplemented milk exhibited an approximately 6-log difference from the control by d 7. In raw milk, the total bacterial growth curve showed a longer lag phase when the LS was activated by LO (11.3 ± 1.4 h) compared with the control (4.0 ± 1.0 h), but it was not different from the recommended method (9.4 ± 1.0 h). However, the total bacterial count after 24 h for the sample treated with LO and TCN (5.3 log cfu/mL) was significantly lower compared with the control (7.2 log cfu/mL) and the recommended method (6.1 log cfu/mL). Results from this study suggest that LO is an alternative source of H2O2 that enhances the microbial inhibition achieved by the LS. Lactose oxidase could be used to develop enzyme-based preservation technologies for applications where cold chain access is limited. This enzymatic approach to improving the shelf life of dairy products also represents a novel option for clean label spoilage control.

Modified atmosphere packaging decreased Pseudomonas fragi cell metabolism and extracellular proteolytic activities on meat.

Modified atmosphere packaging (MAP) is considered an effective method for extending the shelf life of meat. The use of optimal mixture of gases (CO2 and N2) in food packaging containers has been proved to effectively inhibit the growth of microorganisms in poultry meat. In general, a minimum CO2 concentration range of 20%-30% is required for the inhibitory effect. The aim of this study was to investigate the mechanism by which MAP (CO2/N2 30%/70%) inhibits Pseudomonas fragi, a dominant spoilage microorganism in aerobically stored chilled meat. The cell physiological changes were determined by measuring membrane integrity, membrane potential, ATP level, and extracellular proteolytic activity. The results showed that samples stored under MA retained cell membrane integrity, but lost significant membrane potential and ATP synthesis activity. Furthermore, the peptides issued from 2 structural proteins (myosin and actin) were mainly identified in air samples, indicating that these fragments result from bacterial proteolytic activity while MAP inhibited this activity. Overall, the study found that cell metabolism and extracellular protease activity decreased under MAP conditions. This study showed that MAP is an effective food preservation strategy and revealed mechanisms by which MAP inhibits spoilage.

The effect of reduced sodium chloride content on the microbiological and biochemical properties of a soft surface-ripened cheese.

Many health authorities have targeted salt reduction in food products as a means to reduce dietary sodium intake due to the harmful effects associated with its excessive consumption. In the present work, we evaluated the effect of reducing sodium chloride (NaCl) content on the microbiological and biochemical characteristics of an experimental surface-ripened cheese. A control cheese (1.8% NaCl) and a cheese with a reduced NaCl content (1.3% NaCl) were sampled weekly over a period of 27d. Reducing NaCl content induced microbial perturbations such as the lesser development of the yeast Debaryomyces hansenii and the greater development of the gram-negative bacterium Hafnia alvei. This was accompanied by changes in proteolytic kinetics and in profiles of volatile aroma compounds and biogenic amine production. Finally, the development of the spoilage microorganism Pseudomonas fragi was significantly higher in the cheese with a reduced salt content.

Microencapsulation, chemical characterization, and antimicrobial activity of Mexican (Lippia graveolens H.B.K.) and European (Origanum vulgare L.) oregano essential oils.

The effect of solvent polarity (methanol and pentane) on the chemical composition of hydrodistilled essential oils (EO's) of Lippia graveolens H.B.K. (MXO) and Origanum vulgare L. (EUO) was studied by GC-MS. Composition of modified starch microencapsulated EO's was conducted by headspace-solid-phase microextraction (HS-SPME). The antimicrobial activity of free and microencapsulated EO's was evaluated. They were tested against Salmonella sp., Brochothrix thermosphacta, Pseudomonas fragi, Lactobacillus plantarum, and Micrococcus luteus. Thymol and carvacrol were among the main components of EO's and their free and microencapsulated inhibitory activity was tested against M. luteus, showing an additive combined effect. Chemical composition of EO's varied according to the solvent used for GC analysis and to volatile fraction as evaluated by HS-SPME. Thymol (both solvents) was the main component in essential oil of MXO, while carvacrol was the main component of the volatile fraction. EUO showed α-pinene (methanol) and γ-terpinene (pentane) as major constituents, the latter being the main component of the volatile fraction. EO's showed good stability after 3 months storage at 4°C, where antimicrobial activity of microencapsulated EO's remained the same, while free EO's decreased 41% (MXO) and 67% (EUO) from initial activity. Microencapsulation retains most antimicrobial activity and improves stability of EO's from oregano.

Effect of industrial and natural biocides on spoilage, pathogenic and technological strains grown in biofilm.

This study aimed at investigating bactericide solutions effective on spoilage and pathogenic bacteria while preserving technological bacteria. Two compounds of essential oil (thymol and eugenol), one essential oil of Satureja thymbra and two industrial biocides (PE 270-30, Brillo) were tested on technological strains (Staphylococcus equorum, Staphylococcus succinus and Lactobacillus sakei) grown in monoculture biofilm and on a mixed biofilm of pathogenic bacteria (Staphylococcus aureus, Listeria monocytogenes) and spoilage bacteria (Pseudomonas fragi, Escherichia coli). Biofilm cultures were performed in glass fibre filters for 24h at 20 degrees C before application of biocides. Thymol and eugenol had no effect on the mixed biofilm. S. thymbra (2%) was highly effective on spoilage strains (5 log reduction), and S. equorum (4 log reduction) was moderately effective on pathogens (2.3 log reduction) and not effective on S. succinus and L. sakei (0.5 log reduction). PE-270-30 with 10% Na((2))SO((4)) decreased spoilage bacteria (5.1 log reduction), maintained the technological bacteria, but did not reduce the pathogens. The disinfectant Brillo (3%) killed all the strains. These results showed the difficulty in obtaining a biocide that is effective in destroying spoilage and pathogenic bacteria while preserving technological bacteria. Essential oils could be a good alternative for eradicating spoilage bacteria in the food environment where they are often found at high levels.