A ND M. J . C OV E NT RY . 1997. The effect of bacteriocin, piscicolin 126, on the growth of Listeria monocytogenes and cheese starter bacteria was investigated in milk and in Camembert cheese manufactured from milk challenged with 10 2 cfu ml −1 L. monocytogenes. In milk incubated at 30°C, piscicolin 126 added in the range of 512-2048 AU ml , and the viable count and acid production of these starter cultures in milk were not affected by the addition of 2048 AU ml −1 piscicolin 126. Camembert cheeses made from milk challenged with L. monocytogenes and with added piscicolin 126 showed a viable count of L. monocytogenes 3-4 log units lower than those without piscicolin 126. Inactivation of piscicolin 126 by proteolytic enzymes from cheese starter bacteria and mould together with the emergence of piscicolin 126-resistant isolates was responsible for the recovery of L. monocytogenes in the cheeses during ripening.

Lactobacillus acidophilus, L. bulgaricus, L. casei, L. delbrueckii, L. lactis and L. plantarum contained a pyruvate oxidase for the oxidation of pyruvate to acetyl phosphate and acetate. The presence of an acetate kinase converted the acetyl phosphate to acetate. L. casei and L. plantarum produced lactate and acetoin, in addition to acetate, under the conditions used while L. casei also produced diacetyL L. casei and L. plantarum were the only species to utilize citrate. L. helveticus and L. helveticus subsp. jugurti did not utilize pyruvate under the conditions used.

Mozzarella cheese was manufactured from milk containing either a low (olein) or a high (stearin) melting point fraction of milk fat or anhydrous milk fat. The fat was dispersed into skim milk by homogenization at 2.6 MPa before being manufactured into cheese. The melting point of the milk fat did not affect the size or shape of the fat globules, nor was there any effect of homogenization on the polymorphic state of the milk fat. There were no changes in milk fat globule size and shape concomitant with the amount of free oil formed. The polymorphic state of the milk fat did affect the amount of free oil formed and the apparent viscosity of the cheese. The lower melting point fraction yielded a larger amount of free oil. The higher melting point fraction yielded a higher viscosity of melted cheese at 60 degrees C. Mozzarella cheese was also manufactured from homogenized milk, nonhomogenized milk, and a 1:1 ratio of the two, without altering the milk fat composition. Increasing the proportion of homogenized milk yielded a lower free oil content and higher viscosity of the cheese.

A novel peptide bacteriocin produced by the lactic acid bacterium Carnobacterium piscicola JG126 isolated from spoiled ham was purified and characterized. This bacteriocin, designated piscicolin 126, inhibited the growth of several gram-positive bacteria, especially the food-borne pathogen Listeria monocytogenes, but had no effect on the growth of a number of yeasts and gram-negative bacteria. Bactericidal activity was not destroyed by exposure to elevated temperatures at low pH values; however, bactericidal activity was lost at high pH values, especially when high pH values were combined with an elevated temperature. Piscicolin 126 activity was not affected by catalase, lipase, or lysozyme but was destroyed by exposure to a range of proteolytic enzymes. Piscicolin 126 was purified to homogeneity and was found to be a peptide having a molecular weight of 4,416.6 ؎ 1.9. A sequence analysis revealed that this compound is a cystibiotic (class IIa) bacteriocin containing 44 amino acid residues and one intrapeptide disulfide ring. Piscicolin 126 has regions of homology with some other bacteriocins obtained from lactic acid bacteria and is most closely related to sakacin P and pediocin PA-1 (levels of identity, 75 and 55%, respectively). Addition of piscicolin 126 to a devilled ham paste test food system inhibited the growth of L. monocytogenes for at least 14 days. Piscicolin 126 was more effective than two commercially available bacteriocin preparations tested in the same system. MATERIALS AND METHODS Materials. Chemicals and other products were obtained from the following suppliers. Growth media and agar were obtained from Oxoid, Basingstoke, United Kingdom. Catalase was obtained from Boehringer, Mannheim, Germany. ␣-Chymotrypsin, ␤-chymotrypsin, lipase, lysozyme, pepsin, trypsin, protease type I, protease type XIV, protease type XXIII, and ␣-cyano-4-hydroxycinnamic acid were obtained from Sigma Chemical Co., St. Louis, Mo. Nisin (as Nisaplin) was obtained from Aplin and Barrett, Trowbridge, United Kingdom. ALTA 2431 was obtained from Quest International, Sydney, New South Wales, Australia. Devilled ham paste was obtained from Master Foods, Sydney, New South Wales, Australia. Plasmid pGEM-T was obtained from Promega Corp., Madison, Wis. CM-Sepharose Fast Flow was obtained from Pharmacia Biotech, Uppsala, Sweden. 2-Mercaptoethanol was obtained from BDH,

No abstract

Bacterial biofilms have been observed and reported on food and food-processing surfaces and can contribute to increased risks for product quality and food safety. The colonization of fruit and vegetables by pectynolitic bacteria like Pseudonomas fluorescens attributable to conditions such as soft rot, can also manifest as biofilms. A developed biofilm structure can provide a protective environment for pathogens such as Listeria monocytogenes reducing the effectiveness of sanitisers and other inhibitory agents. Understanding the colonization of bacteria on leaf surfaces is essential to the development of a better understanding of the leaf ecology of vegetable products. Studies of microbial colonization of leaf surfaces have been conducted using SEM and more recently using confocal microsocpy techniques. In the current study, a Leica TCS NT laser scanning confocal microscope was used to investigate biofilm formation using vital fluorescence staining on intact vegetable leaves. Reflection contrast and fluorescence three-dimensional imaging successfully delineated bacterial and biofilm morphology without disturbing the bacterial or leaf surface structure. The results demonstrate the presence and development of biofilm on the surface of lettuce. The biofilms appeared to originate on the cuticle in distinct micro-environments such as in the natural depression of the stomata, or in the intercellular junction. Bacteria also adhered to and developed biofilm colonies within an hour of contact and with clean stainless steel surfaces. Our study investigates the progression of biofilm formation from leaf colonization, and will assist in characterising the critical mechanisms of plant/host interaction and facilitate the development of improved preservation, sanitising and packaging strategies for minimally processed vegetable products.

The effect of the extent and rate of compression and stretching on free oil formation in Mozzarella cheese curd was investigated at 55, 65, and 75 degrees C. Confocal laser scanning microscopy was used to determine the maximum cross-sectional diameter, cross-sectional area, elongation factor (maximum divided by minimum cross-sectional diameter), and circularity of fat globules in the cheese curd at the different temperatures, and after stretching or compression. Free oil was not significantly affected by the rate of biaxial compression from 50 to 2000 mm/min at 65 degrees C, the rate of tensile stretching from 1000 to 2500 mm/min at 60 degrees C, or the extent of biaxial compression from 40 to 80% of the original height at 1000 mm/min and 65 degrees C. Increasing the rate of stretching from 1000 to 2500 mm/min increased the elongation factor from 1.91 to 2.61. Cross-sectional area, maximum diameter, and circularity were not affected by the rate of biaxial compression. The extent of curd compression had no effect on the milk fat globule size and shape. Increasing the extent of stretching at 60 degrees C and at 1000 mm/min increased the free oil content (on a fat basis) from 23.8% (curd stretched by 1.4x) to 32.3% (stretched by 4.6x) and the elongation factor of the globules, but did not affect any of the other globule parameters. Increasing the temperature of the cooking-stretching water increased the cross-sectional area, diameter of the globules, and free oil content from 24.1% at 55 degrees C to 34.5% at 75 degrees C for curd compressed to 50% height at 1000 mm/min.

A .J . H I LL IE R AN D J . WA N. 1997. A total of 663 533 colonies from 72 dairy and meat sources showed a detection rate of 0·2% for bacteriocin producers using direct plating techniques. A further 83 000 colonies from 40 fish and vegetable sources showed a detection rate of 3·4% for bacteriocin producers using selective enrichment procedures. A collection of seven purified isolates showing a different host spectrum of bacteriocin activity and with the ability to produce bacteriocins in broth culture were compared with nisin and pediocin (with respect to their inhibitory activity, determined by the critical dilution method), against various indicator bacteria in agar and broth. The sensitivity of Listeria species to various bacteriocins was influenced by the agar and broth test systems used. A Lactobacillus curvatus strain was found to be the most suitable indicator for quantitating antimicrobial effects of all the bacteriocins investigated in both agar and broth test systems. The bacteriocin-producing isolates were characterized by biochemical reactions and DNA restriction enzyme profiles and taxonomic identification revealed species of Lactobacillus, Carnobacterium and Lactococcus assigned on the basis of 16S rDNA sequences.