Search results

Whey protein isolate and glycerol were mixed to form a matrix to incorporate antimicrobial agents and produce edible films with antimicrobial activity against Listeria monocytogenes strains isolated from cheeses. Various organic acids were used to decrease pH down to approximately 3. In a preliminary assay without nisin, the effect of each organic acid was evaluated with respect to the rheological properties of the film solutions and the inhibitory and mechanical properties of the films. Lactic, malic, and citric acids (3%, wt/vol), which were used in a subsequent study of their combined inhibitory effect with nisin (50 IU/ml), had significantly higher antilisterial activity (P , 0.05) compared with the control (2 N HCl, 3% [wt/vol], with nisin). The largest mean zone of inhibition was 4.00 6 0.92 mm for malic acid with nisin. Under small-amplitude oscillatory stress, the proteinglycerol-acid film solutions exhibited a predominantly viscous behavior or a weak gel behavior, with the storage modulus (G9) slightly higher than the loss modulus (G0). The malic acid–based solution was the only one whose viscosity was not influenced by the addition of nisin. The addition of nisin resulted in a nonsignificant (P . 0.05) increase in the percentage of elongation at break. Results from tensile and puncture stress were variable, but in general no significant differences were found after the incorporation of nisin. The overall results support the use of malic acid with nisin to produce effective antimicrobial films to control L. monocytogenes growth.

The inhibitory effects of nisin, natamycin and malic acid, incorporated in whey protein films with pH 3, were investigated alone or with addition of sucrose esters, Tween80 or EDTA. Water vapour permeability measurements and mechanical and rheological tests were also assessed. EDTA and Tween80 did not significantly (P < 0.05) influence the inhibitory activity of films against Pseudomonas aeruginosa and Yarrowia lipolytica in contrast with the improved effect against Listeria monocytogenes, Penicillium commune and Penicillium chrysogenum. Sucrose esters reduced significantly (P < 0.05) the inhibitory effect for Y. lipolytica and Penicillium spp. The present work provides an antimicrobial film formulation with potential to be a hurdle against spoilage and pathogenic microorganisms isolated from cheese surface.

A survey was made in 1995–1996 for Listeria spp. in 63 soft cheeses, made from raw ewe’s milk using traditional methods, in the Province of Beira Baixa (Portugal). Listeria spp. were isolated from 47 (75%) of the cheeses, L.monocytogenes was isolated from 29 (46%), and L.innocua but not L.monocytogenes from 18 (29%). Of 24 isolates of L.monocytogenes that were serotyped, 20 were serotype 4b, three were serotype 1/2b and one was serotype 1/2a. Phage typing of isolates of L.monocytogenes and L.innocua showed that in some cases a particular phage type was associated with cheese from a particular source. Twenty four strains of L.monocytogenes tested were able to grow at 30 Cin culture medium adjusted with HCl to a pH in the range from 4.4 to 6.0 within 3 days; in the pH range 4.4–6.8 a representative strain grew most rapidly at pH 6.8. The pH range in the cheeses during maturation was between about 5.2–6.4. Whether L.monocytogenes could multiply in the cheeses would depend on factors such as concentration of organic acids and of salt, and storage temperature.

For 5 months, the udders of milking ewes, raw ewe’s milk, cheese, and the plant and environment of a cheese manufacturer in Portugal were investigated using standard methods for the presence of Listeria spp. An association between subclinical mastitis and Listeria monocytogenes in a single lactating sheep was investigated by visual inspection of udders for signs of inflammation, application of somatic cell counts, the California mastitis test, pH measurement to milk, and culture of L. monocytogenes and Staphylococcus spp. To track the routes of contamination by L. monocytogenes, 103 isolates were characterized by molecular serotyping and amplified fragment length polymorphism, and a selection was further tested by pulsed-field gel electrophoresis. This study provides molecular and epidemiological evidence tracking the persistence of a single L. monocytogenes strain causing a subclinical udder infection without obvious inflammation in a single ewe. This infection was the likely source of contamination of raw milk that was subsequently used to produce unpasteurised milk cheese and resulted in a single strain of this bacterium colonizing the processing environment and the final cheese product.