MODELLING OF SALAMI PROCESS PARAMETERS TO OPTIMIZE NITRITE DEPLETION BY THE STARTER STAPHYLOCOCCUS XYLOSUS ITEM 18282 USING BOX-BEHNKEN DESIGN WITH DESIRABILITY FUNCTION

Staphylococcus xylosus is a QPS species frequently used as microbial starter in industrial production of dry-cured salami mainly due its ability to utilize nitrate and nitrite added addressing to both consumer compliance and safety. This microbial skill could be also exploited in the large Italian traditional cured dry-salami with a long ripening and low acidity profile. Indeed, in these products a complete nitrite depletion is appreciated at the end of ripening in spite of its useful antimicrobial activity at the starting steps of production. To this aim, nitrite reductase (Nir) activity of the salami starter S. xylosus ITEM 18282 (Quintieri et al., 2018; http://www.ispa.cnr.it/Collection/) was in vitro evaluated in response the process parameters mostly affecting the dry-cured salami production. The interactive effects on the microbial Nir activity between pH (5.0, 5.5, and 6.0) and oxidative reduction potential (ORP) at 0, 100 and 200 mV of the cultures incubated at 3 different temperatures (5, 15, and 25 °C) in a defined medium containing 150 mg/ml of NaNitrite was calculate by a Box-Behnken incomplete randomized factorial design. The empirical-mathematical models developed were further studied by Response Surfaces methodology in order to evaluate the main, interaction and quadratic effects of the process parameters (temperature, pH and ORP) on the activity of nitrite depletion at the conditions closer to those generally used in salami processing (desirability). Overall, the results showed at 15 °C a high depletion rate (ca. 75%) of nitrite at values of pH and ORP higher than 6.0 and 200 mV, respectively. By contrast, very low experimental temperature and ORP values depressed microbial Nir activity. Statistical analyses put in evidence the adequacy of the second-order polynomial model suggesting a good fit with the experimental data (R2 = 0.92). The estimation of the regression coefficients also allowed to establish that the nitrite depletion rate mediated by the strains is significantly correlated to the single and quadratic effects of the main factors temperature, pH and ORP and their interactions (P < 0.004), except for those due to the interaction temperature x ORP (P < 0.500). The effect of ORP is predominant compared to that of pH, nevertheless their interaction significantly increases the Nir activity, in contrast, the response decreased as temperature increased. An optimization study using desirability (D) function methodology was performed, and the optimal conditions for nitrite depletion activity were found to be temperature of 10.3 °C, pH of 6.0 and ORP of 200 mV. The predicted Nir activity for these conditions was 38.9 UI/ml with the highest value of D (0.968). However, a lower Nir activity (20 U/ml), enabling to drop ca. 50% of starting nitrite concentration within 16-hour incubation, could allow to reach the same results fixing temperature at 15 °C and modulating ORP and pH within values > 170 mV and < 5.3, respectively. Likewise, the same nitrite rate depletion activity is guaranteed holding temperature and ORP lower than 15 °C and at 100 mV, respectively but considering pH values higher than 6.0. Therefore, the technological conditions permitting the maximum nitrite depletion rate of the S. xylosus ITEM 18282 strain are compatible with those adopted during the early stages of salami ripening. Moreover, the data reported here have shown that the optimal temperature of the Nir activity is consistent with that observed in the seasoning cells, suggesting an interesting industrial application for this selected strain.