Survival and growth of Salmonella Enteritidis in liquid egg products varying by temperature, product composition, and carbon dioxide concentration.

Cryogenic cooling of shell eggs with carbon dioxide (CO(2)) is known to improve egg content quality through rapid cooling as well as by increasing internal CO(2) levels. A study was undertaken to determine the effects of variations in atmospheric CO(2) concentrations (aerobically stored, flushed with CO(2) and sealed, or bubbled with CO(2)) on the survival and growth of Salmonella Enteritidis in liquid egg products including whole egg, albumen, yolk, and albumen + 1% yolk. Egg products were inoculated with a three-strain composite of Salmonella Enteritidis at ca. 4 log colony-forming units (CFU)/mL and stored at 7 degrees C or 10 degrees C for 8 or 4 days, respectively, or at ca. 2 log CFU/mL and stored at 23 degrees C and 37 degrees C for 48 or 24 hours, respectively. Salmonella populations differed based on variations in liquid egg composition (p < 0.05). Manipulating the atmospheric concentrations of CO(2) in which liquid egg products were stored did not significantly inhibit the growth of Salmonella Enteritidis (p > 0.05) in yolk-containing egg products or affect the inhibitory activity of albumen-containing products. Populations of Salmonella were static at 7 degrees C over the entire storage period and significant growth occurred in whole egg and yolk stored at 10 degrees C. Populations in egg stored at 23 degrees C and 37 degrees C were greater in yolk than in whole egg, although whole egg had populations greater than in albumen or albumen +1% yolk (p < 0.05). Results of this investigation suggest that increasing atmospheric CO(2) to enhance egg quality should not promote the growth of Salmonella Enteritidis in eggs.

Effect of temperature and growth media on the attachment of Listeria monocytogenes to stainless steel.

Listeria monocytogenes ATCC 19111 cultivated in nutrient-rich medium (brain heart infusion, BHI) or starved in minimal medium (10% filter sterilized pond water and 90% sterilized distilled water) were investigated for their initial attachment to austenitic stainless steel No. 4 with satin finish at 4 degrees C, 20 degrees C, 30 degrees C, 37 degrees C, or 42 degrees C. A droplet (10 microl) containing approximately 10(7) CFU/ml of L. monocytogenes suspended in BHI or minimal medium was placed on the stainless steel surface. After holding in saturated humidity for 3 h at the desired temperature the surface was washed and prepared for scanning electron microscopy (SEM). Using SEM, attachment of L. monocytogenes was determined by counting cells remaining on the surface. When L. monocytogenes cultivated in BHI were used, with the exception of the number of attached cells being lower at 42 degrees C than at 37 degrees C and 30 degrees C, the number of attached cells increased with increasing temperature (P<0.05). When L. monocytogenes starved in minimal medium were used, the number of attached cells also increased with increasing attachment temperature (P<0.05), but the number of attached cells at 42 degrees C was lower than that at the other temperatures. The attachment of L. monocytogenes to stainless steel surface was greater when cultivated in rich medium of BHI vs starved in the minimal medium.

Attachment of Listeria monocytogenes to an austenitic stainless steel after welding and accelerated corrosion treatments.

Austenitic stainless steels, widely used in food processing, undergo microstructural changes during welding, resulting in three distinctive zones: weld metal, heat-affected zone, and base metal. This research was conducted to determine the attachment of Listeria monocytogenes in these three zones before and after exposure to a corrosive environment. All experiments were done with tungsten inert gas welding of type 304 stainless steel. The four welding treatments were large or small beads with high or low heat. After welding, all surfaces were polished to an equivalent surface finish. A 10-microl droplet of an L. monocytogenes suspension was placed on the test surfaces. After 3 h at 23 degrees C, the surfaces were washed and prepared for scanning electron microscopy, which was used to determine attachment of L. monocytogenes by counting cells remaining on each test surface. In general, bacteria were randomly distributed on each surface type. However, differences in surface area of inoculum due to differences in interfacial energy (as manifested by the contact angle) were apparent and required normalization of bacterial count data. There were no differences (P > 0.05) in numbers of bacteria on the three surface zones. However, after exposure to the corrosive medium, numbers of bacteria on the three zones were higher (P < 0.05) than those on the corresponding zones of noncorroded surfaces. For the corroded surfaces, bacterial counts on the base metal were lower (P < 0.05) than those on heat-affected and weld zones.

Bactericidal Activity of Organic Acids against Salmonella typhimurium Attached to Broiler Chicken Skint †.

The bactericidal activity of 0.5, 1,2,4, and 6% acetic, citric, lactic, malic, mandelic, propionic, or tartaric acid was determined against Salmonella typhimurium that were loosely or firmly attached to broiler chicken skin by using the skin-attachment model. Acid treatments were applied during a simulated chill (0°C/60 min), postprocess dip (23°C for 15 s), or scald (50°C for 2 min). For comparison, activity of the acid treatments when applied under these conditions were also determined against S. typhimurium in aqueous suspension. In general, bactericidal activity (mean reduction log CFU per skin) of all acids increased linearly with increasing concentration in all applications. The bactericidal activity of organic acids depended on concentration and method of application. When compared to freely suspended cells, it is clear that salmonellae both firmly and loosely attached to poultry skin have increased resistance to or are protected from organic acids. In general, concentrations of ≥4% of the acids were required to kill ≥2 log number of cells of S. typhimurium that were attached to broiler skin.

Heat Inactivation of Escherichia coli O157:H7 in Turkey Meat as Affected by Sodium Chloride, Sodium Lactate, Polyphosphate, and Fat Content †.

The purpose of this research was to determine the survival of Escherichia coli O157:H7 when heated in ground turkey containing various additives and fat levels. D values and z values were determined for low (3%)- and high (11 %)- fat ground turkey with or without one of three additives: 8% NaCl, 4% sodium lactate, or a mixture of 8% NaCl, 4% sodium lactate, and 0.5% polyphosphate. Products inoculated with E. coli O157:H7 strain 204P were mixed, aseptically placed into thermal-death-time (TDT) tubes which were sealed and heated at 52, 55, 57 and 60°C. Survival was determined by enumeration on phenol red sorbitol agar, and D values were calculated by two methods. Mean D52 values ranged from 46.8 to 104.8 min; mean D55 values ranged from 7.7 to 27.2 min; mean D57 values ranged from 2.7 to 13.0 min; and mean D60 values ranged from 0.7 to 4.8 min. The greatest survival, as evidenced by higher (P < 0.001) D values, occurred in turkey containing the mixture of additives. The z values ranged from 6.09 to 4.08°C, and higher z values were obtained in turkey meat containing the additive mixture versus other turkey additive formulations. The additives evaluated enhanced survival of E. coli O157:H7 in cooked turkey meat as compared to turkey meat with no additives. In contrast to earlier reports, added fat did not enhance survival (P > 0.05). Product formulation should be a critical consideration when safe cooking processes are developed for ready-to-eat turkey products.

Fructooligosaccharide Utilization by Salmonellae † and Potential Direct-Fed-Microbial Bacteria for Poultry.

Experiments were done to characterize potential direct-fed-microbial (DFM) bacteria for poultry and Salmonella spp. with respect to their abilities to metabolize fructooligosaccharide substrates (FOS-50® or pure FOS). Oxygen uptake (QO2) by these bacteria in media containing either glucose, FOS-50®, or FOS was determined with a Warburg respirometer. QO2 values for Salmonella spp. In media containing glucose or FOS-50® were similar(P >0.05); however, QO2 values in medium with FOS were significantly lower (P <0.05).The QO2 values for Enterococcus faecium , Lactococcus lactis , and Pediococcus sp. were considerably lower, reflecting the inability of these bacteria to oxidatively utilize these carbohydrates. The ability of E. faecium , L. lactis , and Pediococcus sp. to ferment glucose, FOS-50®, or FOS was determined by measuring pH changes of the media. All carbohydrate sources were fermented by these bacteria, but at different rates. The lowest pH values (<4.6) were obtained in inoculated media supplemented with glucose. The highest fermentation rate was achieved by Pediococcus sp. (pH< 5.2 at 7h), while L. lactis showed the slowest fermentation rate (pH > 6.4 at 10 h). To test the ability of Pediococcus sp. to hydrolyze FOS substrates, a cell-free extract was spectrophotometrically analyzed for the presence of active enzymes capable of hydrolyzing FOS or sucrose (a component of FOS). Hydrolysis of FOS (release of glucose) but not of sucrose was evident. However, equal activity was found in aqueous FOS without the cell-free extract, which suggests that free glucose was a component of the FOS solution tested.

Evaluation of Various Media for Recovery of Thermally-Injured Escherichia coli O157:H7 1.

Efficacies of plating media for recovering heated Escherichia coli O157:H7 were determined and compared. To compare populations of recovered cells, suspensions of cells (three isolates, four replications/isolate) were heated at 50, 55, or 60°C, and then inoculated onto eight media: PCA-PA (plate count agar with 1% pyruvic acid [PA]), MSA (MacConkey sorbitol agar), MSA-Mg (MSA with 0.025% MgSO4), MSA-PA (MSA with 1% PA), MSA-MUG (MSA with 0.005% 4-methylumbelliferyl-ß-d-glucuronide (MUG), PRSA-MUG (phenol red sorbitol agar [PSRA] with 0.005% MUG), PRSA-PA (PRSA with 1% PA), and TSA-PA (tryptic soy agar with 1% PA). Recovery was consistently higher (P < 0.05) with PRSA-MUG and PRSA-PA. At 50, 55, and 60°C, mean numbers (log10 CFU/ml) of recovered cells on PRSA-MUG were 4.42, 4.62, and 3.32, respectively, as compared to 2.78, 2.08, and 1.63, respectively, on MSA. PCA-PA and TSA-PA were less effective than PRSA media, but better than MSA media. Thus, PRSA with MUG or PA was an effective medium for recovering heated cells of E. coli O157:H7; whereas MSA failed to detect sublethally injured cells. Furthermore, addition of Mg, PA, or MUG to MSA further compromised this medium.

Growth, Inhibition, and Survival of Listeria monocytogenes as Affected by Acidic Conditions.

Growth and survival of four strains of Listeria monocytogenes under acidic conditions were investigated. Tryptic soy broth with yeast extract (TSBYE) was acidified with acetic, citric, hydrochloric, lactic, or propionic acid to pH 4.0-6.0, inoculated with L. monocytogenes and incubated at 30 or 4°C. The minimum test pH at which L. monocytogenes did not grow (inhibitory pH) was determined for each acid. In the pH range tested, this inhibitory pH was 5.0 for propionic acid, 4.5 for acetic and lactic acids, and 4.0 for citric and hydrochloric acids. All four strains gave similar results. Subsequent studies were conducted at 10 and 30°C to determine changes in cell populations in TSBYE adjusted to each inhibitory pH. Initial populations of viable cells (104 CFU/ml) were reduced to <10 CFU/ml within 1-3 weeks at 30°C, whereas at 10°C, L. monocytogenes survived for 11-12 weeks in acetic, citric, or propionic acid-adjusted media and for 6 weeks in media adjusted with hydrochloric or lactic acid. The concentration of undissociated lactic acid was 0.002 M at pH 4.5.