食品接触表面对金黄色葡萄球菌生物被膜形成的影响

在食品工业中,如果金黄色葡萄球菌粘附在食品接触表面并形成生物被膜,不但会造成设备的腐蚀,更重要的是会污染食品,影响了食品的安全性.生物被膜形成的第一步是微生物在食品接触表面的粘附.本文研究了4种不同食品接触表面对金黄色葡萄球菌生物被膜形成的影响.结果表明：与玻璃和PE塑料相比,不锈钢表面粘附形成生物被膜的活菌数较多.不同的食品接触表面影响了生物被膜的形成.     

ATTACHMENT AND REMOVAL OF SALMONELLA SPP. ON MEAT AND POULTRY TISSUES

Muscle pieces from beef, pork, and chicken were exposed to Salmonella strains in various aqueous solutions to determine mechanisms of microbial attachment and release. Binding was measured by scanning electron microscopy and by bacteriological methods. Bacteria appear to attach preferentially to connective tissue fibers, rather than to myofibrils. Muscle fiber swelling and shrinkage during processing permits some microbial entrapment between muscle bundles. Mannose and salt solutions were examined as potential inhibitors of attachment or as removal agents. Mannose inhibited attachment slightly and isotonic saline rinses removed some attached cells, but either method effected only about a one log reduction (90%). Application of 41 rinsings only effected a 4 log reduction. The apparent variety of attachment mechanisms by Salmonella hinders complete removal from meat tissues by simple rinsing procedures.     

THERMAL RESISTANCE OF ANTIBIOTIC-RESISTANT AND ANTIBIOTIC-SENSITIVE SALMONELLA SPP. ON CHICKEN MEAT

The aim of this research was to investigate the potential relationship, if any, between the acquisition/possession of antibiotic resistance genes in strains of Salmonella and its resistance to heat stress. Chicken pieces were inoculated with antibiotic sensitive (AS) strains of S. Enteritidis and S. Typhimurium, its laboratory‐acquired antibiotic‐resistant (AR) mutant strains (nalidixic acid and streptomycin), or a multiresistant strain of S. Typhimurium DT104. Half of these samples were heat‐shocked (48C for 30 min) and all were heat‐challenged at 55C for up to 30 min. Samples were then plated on xylose lysine desoxycholate (XLD) and tryptone soya agar (TSA) overpoured with XLD. Heat‐shocked cultures of S. Typhimurium DT104 had significantly higher D‐values (the time required for a 1 log reduction in the number of bacteria) than their non‐heat‐shocked counterparts (P < 0.05). No significant differences were observed between AR and their AS. However, the D‐values for S. Typhimurium DT104 were significantly higher than the D‐values for S. Typhimurium (AS) and S. Enteritidis (AS) (P < 0.05). This study concluded that laboratory‐acquired antibiotic‐resistant mutation did not affect heat resistance of the Salmonella strains studied and suggested a potential link between multiantibiotic resistance and heat resistance in S. Typhimurium DT104.     

FATE OF LISTERIA MONOCYTOGENES AND SALMONELLA SPP. IN THERMALLY PROCESSED COLOR-MODIFIED TURKEY MEAT

The growth/survival of Listeria monocytogenes and Salmonella spp. was evaluated in two thermally-processed turkey raw materials. Flaked turkey (FT) was prepared from boneless, skinless thighs. Portions were washed with sodium phosphate buffers to lighten tissue color and produce color-modified turkey (CMT). Raw materials were cooked, inoculated with pathogens, and held at 4 and 20C. Salmonella numbers declined in cooked samples stored at 4C for 21 days, but increased by approximately 6 logs in 2 days for the FT and CMT held at 20C. L. monocytogenes increased by approximately 5 logs in samples held for 14 days at 4C. By 2 days at 20C, L. monocytogenes increased more than 5 logs in both materials. Growth/survival of these pathogens did not differ (p > 0.05) between FT and CMT. The data suggest that CMT does not present any more health hazards than conventional turkey raw materials.     

INHIBITION OF SALMONELLA TYPHIMURIUM ON POULTRY BY THE LACTOPEROXIDASE SYSTEM1

The antibacterial activity of the lactoperoxidase (LP) system on the growth and survival of a naladixic acid resistant strain of Salmonella typhimurium on poultry was determined. The LP system treatment, which consisted of lactoperoxidase (1 μg/ml), potassium thiocyanate (5.9 mM), and hydrogen peroxide (2.5 mM) (final concentrations) in water, reduced the level of Salmonella on inoculated chicken legs. The magnitude of reduction was dependent on temperature and time. For a water temperature of 25C and a time of 30 min, a 13.2% reduction was seen, as compared to a water temperature of 60C and a time of 15 min which showed a 80.6% reduction. In a 48 h shelf-life study, the LP system controlled growth of psychrotrophic bacteria on chicken legs. Hunterlab color values (L, a, b) for chicken thigh skin and oxidative deterioration as measured by TBA values for chicken thigh meat did not significantly differ (p < 0.01 and p < 0.05, respectively) between control and treated thighs.     

PRODUCTION OF ALTERNARIOL AND ALTERNARIOL METHYL ETHER BY ALTERNARIA SPP.

Five isolates of Alternaria alternata, five isolates of A. tenuis, one isolate of A. hemicota and three isolates of unidentified Alternaria spp. were examined for their production of alternariol methyl ether (AME) and alternariol (AOH) in complex liquid media. The data were compared with the production of these two toxins on solid substrate rice cultures. A new high-performance liquid chromatography method with minimum detection levels for AME and AOH of 20 ng and 40 ng, respectively, was used to quantitate the toxins. All isolates produced AME and AOH, but the amount varied considerably with the isolates and media tested. Of the three different liquid media tested, the highest quantity of AME and AOH was produced in a modified Czapek-Dox broth. The highest yield of toxins was obtained in the solid rice cultures. As much as 125 mg of AME and 16 mg of AOH were produced by one A. tenuis isolate on 300 g of rice after 14 days of incubation at 25°C in darkness.     

EFFECT OF A BACTERIOCIN-LIKE SUBSTANCE (BLS) PRODUCED BY STREPTOCOCCUS THERMOPHILUS STRAINS ON LISTERIA SPP. STRAINS

In this study, fifty-two strains of Streptococcus, isolated from Turkish yogurt were tested for their ability to inhibit Listeria strains. Only two isolates exhibited antimicrobial activity (CFF; cell-free filtrate) against eight strains of Listeria innocua, four strains of Listeria monocytogenes and one strain of Listeria welshimeri by agar diffusion. However, bacteriocin-like substance (BLS) producing strains were determined under conditions with the effects of organic acids and hydrogen peroxide eliminated. Inhibitory activities of Streptococcus thermophilus ET5 and S. thermophilus H22 were due to BLS. BLS of S. thermophilus ET5 was observed to exhibit more significant inhibition to the Listeria strains than BLS of S. thermophilus H22.
Inhibitory effect of nisin in 50, 100, 150, 200, 250 and 300 ppm concentrations was compared with BLS producing S. thermophilus strains against Listeria spp. strains. While all Listeria strains were resistant to 50 and 100 ppm concentrations of nisin, some strains of Listeria spp. were only sensitive to 150, 200, 250 and 300 ppm concentrations. The inhibitory activity of S. thermophilus ET5 was more effective against Listeria strains than the inhibitory activity of nisin. It appears that BLS of S. thermophilus ET5 could provide greater potential and usefulness as bacteriocin than nisin. These results indicate that the potential application of antimicrobial-production of S. thermophilus ET5 and H22 as bio-preservatives against Listeria contamination on dairy products could be suggested.     

LEVAN FORMATION BY ACETOMONAS

The mucilage formed by growth of Acetomonas on a medium containing sucrose has been isolated and purified. Partial hydrolysis, methylation followed by hydrolysis, periodate oxidation and optical rotation of the product and its methylated derivative showed it to be a 2β,6′-linked levan having branch points with 2β,1′ linkage and an average unit chain of 12 anhydrofructofuranose residues; the molecular weight was of the order of several hundred thousand. In this property of levan formation, Acetomonas resembles the related genus Pseudomonas and differs from the dextran-forming genus Acetobacter.     

ELIMINATION OF SALMONELLA FROM FROZEN SHRIMP BY GAMMA RADIATION

Shrimp samples were found to be contaminated with Salmonella. The D₁₀ values of S. typhimurium and S. enteritidis were between 0.225 to 0.250 kGy when cells were irradiated in frozen 0.1 M phosphate buffer. The radiation resistance of these Salmonella spp. increased marginally when cells were irradiated in shrimp homogenate. A radiation dose of 4.0 kGy completely eliminate Salmonella from frozen prepackaged shrimp, which was confirmed by inoculated pack studies .     

INACTIVATION KINETICS OF SALMONELLA DUBLIN BY PULSED ELECTRIC FIELD

Microbial inactivation kinetic models are needed to predict treatment dosage in food pasteurization processes. In this study, we determined inactivation kinetic models of Salmonella dublin in skim milk with a co-field flow high voltage pulsed electric field (PEF) treatment system. Electric field strength of 15–40 kV/cm, treatment time of 12–127 μs, medium temperatures of 10–50C were tested. A new inactivation kinetic model that combines the effect of treatment time to electric field strength or medium temperature was developed.     

BIOSYNTHETIC FORMATION OF HIGHER ALCOHOLS BY YEAST. DEPENDENCE ON THE NITROGENOUS NUTRIENT LEVEL OF THE MEDIUM

In previous investigations it has been shown that in batch fermentations the yield of higher alcohols formed biosynthetically (in the sense that yeast synthesizes the carbon skeletons needed) is mainly negatively correlated with the intial level of nitrogenous nutrients in the medium. The validity of this relationship has been established with different nitrogen sources and sugars, and at different sugar concentrations. Its causes have been studied in simple systems, using single, rapidly-absorbed nitrogen sources, usually ammonium salts. Under such conditions it is the amount, rather than the concentration, of nitrogen source that is of importance. Yields of higher alcohols per unit weight sugar consumed are practically identical at varying nitrogen levels as long as nitrogen is available in the medium, but they increase sharply after nitrogen exhaustion, by a factor of about 2–3. However, if allowance is made for the production of valine, leucine, and isoleucine during the period when nitrogen is present, there is no rise in the total production of carbon skeletons along these pathways after the nitrogen is exhausted. On the contrary, the yields of valine-isobutanol and isoleucine-2-methylbutanol skeletons per unit weight of sugar are appreciably reduced in the absence of nitrogen, whereas the yields of leucine-3-methylbutanol skeletons are nearly unchanged.     

EVALUATION OF CELLS OF LACTOBACILLUS DELBRUECKII SPP. LACTIS RM2–5 AND PEDIOCOCCUS ACIDILACTICI D3 SUSPENDED IN LACTIC OR ACETIC ACID AS BIOPRESERVATIVES ON FRESH CUT CANTALOUPE AT 7C1

Abstract The objective of this study was to evaluate the potential antibacterial effect of selected cultures of lactic acid bacteria toward Salmonella choleraesuis and spoilage organisms on fresh cut cantaloupe during storage at 7C. A small although statistically significant (P < 0.05) inactivation of S. choleraesuis was observed when cantaloupe cubes were additionally inoculated with cells P. acidilactici suspended in 5.5 mM lactic acid or 5.5 mM glacial acetic acid solutions. However, the numbers of S. choleraesuis were not significantly lower than in the control on day 10. The amount of inactivation likely would have no practical importance. No significant decline in numbers of S. choleraesuis was attained when the cells of L. delbrueckii spp. lactis were suspended in 5.5 mM lactic acid. None of the treatments contributed to an effective (P > 0.05) inhibition of the spoilage flora on the melons.     

珍稀真菌桑黄的研究进展

综述近年来国内外学者对桑黄的名称、化学成分、深层发酵、药理作用等方面的研究进展.