Mara Lucia Stecchini

Changes of physicochemical, microbiological, and textural properties during ripening of Italian low-acid sausages. Proteolysis, sensory and volatile profiles.

In this study low-acid sausages were studied to characterize their physicochemical, microbiological, and textural properties during ripening. The final a(w) was 0.87-0.88, whereas pH values stayed around their initial values during processing. Lactic acid bacteria increased very slowly in number and a small increase of Micrococcaceae was also noticed. Low-acid sausages showed low hardness and cohesiveness, and were easily distinguishable by sensory analysis from other industrial and artisan sausages. Under the conditions of the study, observed volatile compounds were mainly from spices and wine. The respective contribution of muscle and indigenous bacterial enzymes to proteolysis was determined by comparing changes in low-acid sausages to those containing an antibiotic-antimycotic mixture or sugar. A large part of the degradation of myofibrillar proteins appeared due to endogenous enzymes, although bacterial proteinases contributed to the degradation of these proteins. The role of microorganisms in proteolysis was more evident in the degradation of sarcoplasmic proteins.

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.

Prevalence and behaviour of Bacillus cereus in a REPFED of Italian origin

This paper reports on the prevalence and behaviour of Bacillus cereus in gnocchi, a REPFED of Italian origin. A survey of gnocchi under varying storage conditions, revealed that, although B. cereus was found in 33% of the samples, the contamination level was lower than 10(2) CFU/g for the unstored and the refrigerated (8 degrees C) samples. Counts increased with increasing storage temperatures and prolonged storage times in samples prepared without sorbic acid. The effect of different formulations (sorbic, citric and lactic acid) and storage conditions (8, 12 and 20 degrees C) have been evaluated in a challenge testing with spores of B. cereus. Results indicate that the use of sorbic acid in association with citric or lactic acid to pH 5.0 is effective in inhibiting growth of B. cereus and the anticipated shelf life of the product is safe even if temperature abuse occurs. If sorbic acid is omitted, lactic acid can inhibit B. cereus growth during storage at 8 degrees C. On the contrary, when temperature abuse occurs (12 and 20 degrees C), lactic or citric acid are not able to prevent growth of B. cereus.

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.

Investigation of the ability of proteolytic Clostridium botulinum to multiply and produce toxin in fresh Italian pasta

The ability of proteolytic Clostridium botulinum (types A, B, and F) to produce toxin in filled fresh Italian pasta (tortelli) packed under a modified atmosphere was investigated. Four types of tortelli (filled with artichoke, meat, ricotta-spinach, or salmon) were inoculated with a suspension of heat-shocked spores to give an initial concentration of approximately 10(3) spores per piece. Samples were incubated at both 12 and 20 degrees C for up to 50 days and examined at selected time intervals for the presence of toxin by an ELISA and the mouse test. Toxin was not detected in any tortelli stored at 12 degrees C. When storage was at 20 degrees C, toxin was detected in the salmon-filled tortelli at day 30, in the meat and ricotta-spinach tortelli at day 50, but not in the artichoke-filled tortelli at day 50.

Incidence of Aeromonas species in influent and effluent of urban wastewater purification plants

Influent and effluent samples from urban wastewater plants were examined for presumptive Aeromonas. Removal efficiency was comparatively low for both presumptive Aeromonas (96.5%) and faecal coliforms (95.8%). Starch ampicillin agar (SA) medium was superior to mA medium for selectivity, with 32.3% of typical colonies confirmed as Aeromonas spp., and for the recovery of Aer. veronii biotype veronii.

A simplified approach for modelling the bacterial growth/no growth boundary.

A simplified growth/no growth (G/NG) model, conceptually derived from the Gamma model and making direct and explicit use of growth limits of bacteria through a normalization constant (η), was proposed. The η value, which quantifies the product of the cardinal optimal distances for growth probability, is a species-independent constant. This is of importance when experimental data is missing or insufficient. The simplified G/NG model was developed including the effect of temperature, pH and water activity, and was expanded incorporating the preservative effects. As a practical application, the model was investigated for its ability to describe published data. The successful validation of the simplified G/NG model is discussed in regard to its potential applicability as a first estimate method for the development of safe food products.

Bacillus cereus cell and spore properties as influenced by the micro-structure of the medium.

Aim:  To investigate the effect of different growth conditions on Bacillus cereus cell and spore properties.