Effect of Lactic Acid Bacteria Addition on the Microbiological Safety of Pasta-Filata Types of Cheeses.

In this work, the effects of different combinations of lactic acid bacteria (LAB) on the growth of coagulase-positive staphylococci (CPS) and Escherichia coli were evaluated during ripening of 23 curd cheeses, and their subsequent behavior during the manufacture and storage of pasta-filata cheeses was characterized. Three groups of cheeses were prepared in total: first, control cheeses from raw milk without LAB addition; further pasteurized milk cheeses with LAB, CPS and E. coli intentional inoculation; and finally, raw milk cheeses with LAB added. The aim was to compare the effect of LAB from starter culture, and also in combination with native LAB, and to evaluate the LAB effect as a group, and further to suggest the culture with the best inhibitory potential. Based on the results, counts of CPS increased over 24 h in control curd cheese by 1.76 ± 0.56 log CFU/g. On the other hand, in raw milk cheeses with the addition of starter culture, the increase in CPS counts by 0.76 ± 0.87 log CFU/g was noticed. Counts of E. coli increased during the first 24 h of curd manufacture by 3.56 ± 0.41 log CFU/g in cheeses without LAB addition. Contrary to this, using of LAB cultures resulted in an increase in E. coli counts by only 1.40 ± 1.07 log CFU/g. After steaming at 63.6 ± 1.9°C for 7.2 ± 2.1 min (temperature of heated acidified curd was 54.9 ± 1.7°C), CPS decreased by 0.58 ± 1.12 log CFU/g, and E. coli decreased by 1.23 ± 0.97 log CFU/g in all cheeses, regardless of LAB addition. Finally, during storage of cheeses at 6 ± 0.5°C for 28 days, the levels of E. coli in control cheeses and in raw milk LAB-enriched cheeses reached levels of 2.07 ± 2.28 log CFU/g and 1.20 ± 0.85 log CFU/g, respectively. In addition, the counts of CPS at the end of storage met the criteria of EU Commission Regulation (EC) No. 1441/2007 (2007) (less than 4 log CFU/g) in all manufactured cheeses with added LAB culture, while in the control raw milk cheeses, a level of 3.80 ± 1.22 log CFU/g was observed. Regarding the culture used, the best microbiological inhibitory effect in 28-day-old cheeses was reached by the combination of Fresco culture with Lacticaseibacillus rhamnosus GG, and the best sensory properties were judged to be those for cheeses manufactured with Culture A. A moderate negative effect of storage on overall sensory acceptance was noted, according to the final evaluation of overall acceptability of pasta-filata cheeses. The most satisfactory overall acceptability after 28 days of storage at 6°C was reached for cheese with the addition of culture A.

Growth prediction of two bacterial populations in co-culture with lactic acid bacteria.

The co-culture growth of Staphylococcus aureus, Escherichia coli and lactic acid bacteria starter culture in milk was quantitatively evaluated and modelled with a set of coupled differential equations originally proposed by Baranyi and Roberts and by Gimenez and Dalgaard (BR-GD model). The lactic acid bacteria starter culture showed the ability to induce an early stationary phase of both E. coli and S. aureus populations at different combination of temperature (ranging from 12 to 37 ℃) and lactic acid bacteria inocula (from approx. 103 to 106 CFU/ml). First, the prediction ability was performed only with parameters estimated from individual growth curves of E. coli, S. aureus and the lactic acid bacteria in milk (Dataset 1, 21 experiments). Subsequently, the model was extended with the average competition coefficients (E-BR-GD model) that represented quantitative relations among the populations. The prediction ability of this model was validated with the second dataset consisting of seven experiments. Results and also their statistical indices (accuracy and bias factors) showed that the E-BR-GD model improved growth prediction of all involved populations. Thus, the total root mean square error decreased from 0.457, 0.840 and 0.322 log CFU/ml (BR-GD model) to 0.290, 0.245 and 0.333 log CFU/ml (E-BR-GD) for S. aureus, E. coli and lactic acid bacteria, respectively. This approach in growth prediction of multiple competing microbial populations can be used in assessment of S. aureus and E. coli exposure from raw milk cheeses consumption and contribute to decision making in prevention of staphylococcal enterotoxin production.

Modelling the effects of lactic acid, sodium benzoate and temperature on the growth of Candida maltosa.

The growth of the oxidatively imperfect yeast Candida maltosa Komagata, Nakase et Katsuya was studied experimentally and modelled mathematically in relation to sodium benzoate and lactic acid concentrations at different temperatures. Application of gamma models for the growth rate resulted in determination of cardinal temperature parameters for the growth environment containing lactic acid or sodium benzoate (Tmin  = 0·7/1·3°C, Tmax  = 45·3/45·0°C, Topt  = 36·1/37·0°C, µopt  = 0·88/0·96 h-1 ) as well as the maximal lactic acid concentration for growth (1·9%) or sodium benzoate (1397 mg kg-1 ). Based on the model, the times to reach the density of C. maltosa at the level of 105  CFU per ml can be determined at each combination of storage temperature and preservative concentration. The approach used in this study can broaden knowledge of the microbiological quality of fermented milk products during storage as well as the preservation efficacy of mayonnaise dressing for storage and consumption. SIGNIFICANCE AND IMPACT OF THE STUDY: The strain of Candida maltosaYP1 was originally isolated from air filters that ensured clean air overpressure in yoghurt fermentation tanks. Its growth in contaminated yoghurts manifested outwardly through surface growth, assimilation lactic acid and slight production of carbon dioxide. This was the opportunity to model the effects of lactic acid and sodium benzoate on growth and predict its behaviour in foods. The approach used in this study provides knowledge about microbiological quality development during storage of the fermented milk products as well as some preserved foods for storage and consumption.

Modelling and predicting the simultaneous growth of Escherichia coli and lactic acid bacteria in milk.

Modelling and predicting the simultaneous competitive growth of Escherichia coli and starter culture of lactic acid bacteria (Fresco 1010, Chr. Hansen, Hørsholm, Denmark) was studied in milk at different temperatures and Fresco inoculum concentrations. The lactic acid bacteria (LAB) were able to induce an early stationary state in E. coli The developed model described and tested the growth inhibition of E. coli (with initial inoculum concentration 10(3) CFU/mL) when LAB have reached maximum density in different conditions of temperature (ranging from 12 ℃ to 30 ℃) and for various inoculum sizes of LAB (ranging from approximately 10(3) to 10(7) CFU/mL). The prediction ability of the microbial competition model (the Baranyi and Roberts model coupled with the Gimenez and Dalgaard model) was first performed only with parameters estimated from individual growth of E. coli and the LAB and then with the introduced competition coefficients evaluated from co-culture growth of E. coli and LAB in milk. Both the results and their statistical indices showed that the model with incorporated average values of competition coefficients improved the prediction of E. coli behaviour in co-culture with LAB.

Genotoxic and antimutagenic activities of extracts from pseudocereals in the Salmonella mutagenicity assay.

Extracts of amaranth (Amaranthus L.), sorghum (Sorghum bicolor L.) and Japanese millet (Echinochloa frumentacea L.) were evaluated for mutagenicity in Salmonella typhimurium strains TA98, TA100 and TA102. All three pseudocereal extracts were also assessed for their antimutagenic properties against the direct mutagens 2-nitrofluorene (2NF) for strain TA98, 3-(5-nitro-2-furyl)acrylic acid (5NFAA) for TA100 and H(2)O(2) for TA102 strain and against the indirect mutagen aflatoxin B(1) (AFB(1)). No mutagenicity was induced by any of the pseudocereal extracts when tested at concentrations as high as 50mg/ml. All three extracts showed similar antimutagenicity against 5NFAA and no antimutagenicity against 2NF. The number of revertants induced by H(2)O(2) extract was inhibited in order amaranth>Japanese milet>sorghum. All extracts were effective in the inhibition of mutagenic activity of aflatoxin B(1). The total polyphenol content as well as the amount of the flavonoids and phenolic acids as main component of polyphenolics were also determined.

Modelling the effect of the starter culture on the growth of Staphylococcus aureus in milk.

The competitive growth of a starter culture of lactic acid bacteria (Fresco 1010, Chr. Hansen, Hørsholm, Denmark) and Staphylococcus aureus was studied in milk. The lactic bacteria (LAB) were able to induce an early stationary state in S. aureus. The developed model highlights that the growth of S. aureus is inhibited when the LAB have reached a critical density. The model was tested in different conditions of temperature (from 12 degrees to 25 degrees C), for various inoculum sizes of LAB and S. aureus. The results show that the model accurately quantifies the kinetics of S. aureus as a function of the starter culture.

Growth modelling of heat-resistant fungi: the effect of water activity.

Growth of three strains of heat-resistant fungi, Byssochlamys fulva, Neosartorya fischeri and Talaromyces avellaneus, was studied at 25 degrees C at eight or nine water activities (a(w)) adjusted with sucrose between 0.995 and 0.85. The flexible growth model of Baranyi describing the change in colony diameter (mm) with respect to time was first fitted to measured growth data and from the fitted curves the maximum colony growth rates were estimated. These values were fitted with respect to a(w) to predict colony growth rates at any a(w) within the range tested. Optimum a(w) values of 0.986, 0.985, and 0.990 for growth, and maximum colony growth rates of 12.6, 13.2 and 20.2 mm d(-1), respectively, were calculated. The time to reach a colony diameter of 3 mm and the optimum a(w) for each strain were also calculated. The results can be applied to the prevention of spoilage of canned fruit or juices and other heat processed fruit based products.

Predicting fungal growth: the effect of water activity on Penicillium roqueforti.

The effect of water activity on the colony growth of Penicillium roqueforti is studied by predictive modelling techniques. Measured colony diameter growth curves are fitted to estimate the growth rate and lag phase of the curves. The colony growth rate was modelled by a quadratic function of transformed water activity (a(w)) values, as suggested by Baranyi et al. (Food Microbiol. 10 (1993) 43-59). The lag time was modelled as a function of water activity, by means of the sum of a constant term and a hyperboloid function of a(w) raised to the second power. The lag-phase of Penicillium roqueforti was found insensitive to the water activity in the range of its higher (a(w) > 0.92) values.

Growth of Staphylococcus aureus in pasta in relation to its water activity.

Water activity (aw) of pasta decreased linearly with time during drying. A strain of Staphylococcus aureus isolated from commercial pasta began multiplying immediately after inoculation but decreased in number when aw was reduced to below 0.93. Another strain of S. aureus appeared to multiply until the aw was below 0.86. The results are discussed in relation to practices for pasta production.