Ileana Sarais

Effect of essential oils on Aeromonas hydrophila in a culture medium and in cooked pork

The inhibitory effects of essential oils of coriander, clove, nutmeg, and pepper towards Aeromonas hydrophila were studied in tryptic soy agar. Essential oils of clove, coriander, and nutmeg at concentrations of 500, 1,250, and 10,000 μg/ml, respectively, were effective in inhibiting the growth of A. hydrophila on tryptic soy agar. Pepper was the least active oil of which 15,000 μg/ml appeared to inhibit only a diluted culture of A. hydrophila . Behavior of A. hydrophila was also evaluated in samples of noncured cooked pork treated either with coriander or clove oil. Meat samples were packaged either under vacuum or air and stored at 2 and 10°C. Both essential oils caused a marked reduction of A. hydrophila number. In the meat samples treated with clove oil and stored at 10°C, the lethal effect was significantly enhanced by the vacuum packaging. These data suggest that essential oils of coriander and clove could be used to control the hazard of A. hydrophila in noncured cooked meat at low and increased temperatures, especially in combination with vacuum packaging.

Behavior of Listeria monocytogenes in Mozzarella cheese in presence of Lactococcus lactis

The behavior of Listeria monocytogenes (Scott A) on fully processed Italian Mozzarella cheese was examined in presence and in absence of bacteriocins produced by Lactococcus lactis ssp. lactis strains (DIP 15 and DIP 16). These strains, isolated from raw milk, produced heat stable bacteriocins that were inactivated by pronase, alpha- chymotrypsin and proteinase K, but not by pepsin, trypsin and catalase. The addition of crude bacteriocins to the growing culture of Listeria monocytogenes resulted in a significant reduction in cell number at 5 degrees C, but not at 30 degrees C. Mozzarella cheese was inoculated with the Listeria culture to obtain an initial level of approximately 30 CFU/cm2 surface of Mozzarella and approximately 10(3) CFU/ml of the surrounding fluid and then packaged in bags containing the heat-treated neutralized-cultures of Lactococcus lactis ssp. lactis in skim milk (in Italy, Mozzarella is sold in small size pieces, individually packaged in bags containing some fluid). Bags were stored at 5 degrees C up to 21 days. The presence of bacteriocins resulted in apparent death of Listeria monocytogenes after 24 h storage. After 7 days of storage, a revival of Listeria monocytogenes was observed, followed by an increase in number. However, for a storage period of 2-3 weeks the number of Listeria monocytogenes remained significantly below the number observed for Mozzarella cheese packaged in absence of the heat-treated cultures of Lactococcus lactis.

The influence of Lactobacillus plantarum culture inoculation on the fate of Staphylococcus aureus and Salmonella typhimurium in Montasio cheese

The growth and survival of Staphylococcus aureus and Salmonella typhimurium were investigated during the manufacturing and ripening of raw milk Montasio cheese. Initial inoculated populations in the cheese milk were about 10(5) cfu/ml for S. aureus and 10(6) cfu/ml for S. typhimurium. Samples of curds and cheeses were taken during manufacturing and storage and analysed for pH and microbial populations. S. aureus increased slightly in number during the early period of ripening and attained a population of about 10(6) cfu/ml during the remaining period of storage. S. typhimurium decreased during cheesemaking and storage but persisted through 90 days. The addition of Lactobacillus plantarum culture (0.2% v/v) produced a marked reduction in populations of the test strains in 10 days of storage. Enterotoxin A was not detected in Montasio cheese even with a S. aureus population of 1.1 X 10(7) cfu/ml. L. plantarum strains were also tested by the spot method and the associative growth approach for their antagonistic activity against S. aureus and S. typhimurium. The compound excreted by L. plantarum was active only toward S. aureus. Furthermore, its activity was destroyed by protease treatment. These results indicated that while the growth of S. typhimurium is reduced by the acid production, S. aureus inhibition can be ascribed to bacteriocin production.

The Behavior of Yeast Populations in Stracchino Cheese Packaged under Various Conditions

The growth of yeasts, lactic acid bacteria, pseudomonads, and enterobacteria during the refrigerated storage of an Italian soft cheese (Stracchino) in various packaging conditions (under air, under vacuum, and in the presence of ethanol) was studied. Yeasts were found to play a significant role in cheese spoilage, producing unplesant flavors and odors and causing significant reductions in the shelf life of the paper-wrapped cheeses. Packaging under vacuum decreased the yeast growth rates and reduced the populations attained at the end of the exponential phase of growth, resulting in a shelf-life extension of the Stracchino cheese of over 28 days. A total of 129 yeasts isolates were identified according to conventional methods. The most frequently isolated yeasts were Candida colliculosa , Debaryomyces hansenii , and Candida famata . Other species encountered were Tolurospora delbrueckii , Kluyveromyces marxianus , and Saccharomyces cerevisiae.

ADAPTATION OF A SACCHAROMYCES-CEREVISIAE STRAIN TO HIGH COPPER CONCENTRATIONS

A strain of Saccharomyces cerevisiae has been adapted to increasing concentrations of copper at two different pH values. The growth curve at pH 5.5 is characterized by a time generation increasing with the amount of added copper. A significant decrease of cell volume as compared with the control is also observed. At pH 3 the cells grow faster than at pH 5.5 and resist higher copper concentrations (3.8 against 1.2 mm). Experimental evidence indicates that, after copper treatment, the metal is not bound to the cell wall, but is localized intracellularly. A significant precipitation of copper salts in the medium was observed only at pH 5.5. Increased levels of superoxide dismutase (SOD) activity were observed in copper-treated cells and which persisted after 20 subsequent inocula in a medium without added metal. On the contrary, catalase activity was not stimulated by copper treatment and, hence, not correlated with SOD levels. The mechanism of copper resistance, therefore, probably involves a persistent induction of SOD, but not of catalase, and it is strongly pH-dependent.

Antibiotic and disinfectant susceptibility in Enterobacteriaceae isolated from minced meat.

The resistance of Enterobacteriaceae isolated from minced meat to antibiotics and to an amphoteric surfactant, was examined. Despite the wide range of antibiotic resistance occurring in the strains tested, resistance to the disinfectant was not observed.

Effect of electron irradiation and packaging atmosphere on the survival of aeromonas hydrophila in minced poultry meat

Abstract Resistance to electron irradiation of Aeromonas hydrophila inoculated in minced poultry meat packed in presence of air or under vacuum was examined. Surviving bacteria were counted on starch ampicillin agar containing 100 μg/ml of ampicillin. Radiation resistance, expressed as D10 values, was calculated from the survival curves and found to be 0.12 and 0.12 kGy in poultry meat packed in air or under vacuum respectively. Storage at 2°C of meat samples irradiated at 0.5 kGy further reduced the number of A. hydrophila.

The effect of incubation temperature, sodium chloride and ascorbic acid on the growth kinetics of Aeromonas hydrophila

The effects of temperature, sodium chloride and ascorbic acid on the aerobic growth kinetics of a clinical strain of Aeromonas hydrophila were evaluated. At 5°C, ascorbic acid (1 mmol l‐1) and sodium chloride (3% w/v) inhibited the growth of the organism. At 10°C, ascorbic acid depressed only the maximum population densities (A) by approximately 2 log cycles, but not maximum specific growth rate (μm) or the lag time (Λ). On the contrary, NaCl caused A to increase, with the effect being greatest when the NaCl content was 1.5%. Temperature increase from 10 to 15°C resulted in an approximate doubling of μm and unexpectedly an apparent increase in Λ However, this apparent increase resulted from the particular manner in which the lag phase was mathematically calculated.