Modeling growth of a Ochratoxin A producing Penicillium nordicum strain on a dry-cured pork-medium

Introduction. Ochratoxin A was recently reported in dry cured pork meat as caused by Penicillim nordicum contamination. The ecological needs of the fungus are limitedly known. This study wishes to assess the role of temperature, aw and incubation time on P. nordicum growth in a dry-cured pork-like medium (DPM). Methods. A Penicillium nordicum strain isolated from dry cured pork during ripening, able to produce OTA in vitro (1, 2), was selected. Thirty g of lyophilised dry-cured pork (4% moisture, 11% NaCl, 67% proteins, 18% fat) were added to 1 l of bidistilled water, maintained 1 h in a thermostatic bath, shaken, filtered and added with 15 g of agar to prepare DPM. The original aw (0.97) was modified by glycerol. Then the DPM was poured into 9 cm  Petri dishes (10 ml/ plate) for inoculation. A blocked Central Composite Design (CCD) was organized on three independent factors at 5 levels: temperature (7, 10, 15, 20, 23°C), aw (0.83, 0.86, 0.90, 0.94, 0.97), and incubation time (2, 7, 14, 21, 26 days). Each condition was quadruplicated (a total of 112 Petri dishes). Fungal growth rate (mm/d) was the measured response. The CCD was generated by TRIAL RUN (TRIAL RUN 1.0, SPSS Inc. Chicago, IL), and a fitting model was calculated according to a polynomial multiple regression (MLR). Results. Mycelial growth occurred in all the assayed experimental region. The diameter of P. nordicum colonies was maximum at 20°C and 0.94 aw, after 3 weeks ( = 67.9 mm). In the CCD central point (15°C, 0.90 aw and 14 days of incubation time), the average  = 22.1 mm was measured. The model calculated to explain the measured response (fungal growth) was (sdz coeff.): mm/day = 0.645 temperature + 0.073incubation time + 0.771aw - 0.087temperature×incubation time + 0.363 temperature×aw - 0.037incubation time×aw - 0.136temperature2 + 0.051incubation time2 - 0.006aw2 (R2 = 0.913). The coefficients of the MLR model showed temperature, aw and their two-way interaction as positively affecting the growth rate of P. nordicum, (P < 0.001); the quadratic temperature coefficient (P < 0.05) was negative, thus flattening the consistency of temperature on fungal growth rate.; in the central point of CCD (15°C, 0.90 aw, 14 d), growth rate was in the range 1.2 – 1.9 mm/day. The highest values of growth rate were observed at 20°C - 0.90 aw (3.2 mm/day) and 15°C – 0.97 aw (3.1 mm/day). Discussion. The assayed aw range (0.83 – 0.97) include the surface values of dry-cured pork derivatives during maturation and the temperature range (7 – 23°C) is in agreement with those used in dry-curing practices (3). The optimal fungal growth conditions include rather low temperature (15°C) and aw (0.90). The CCD model allows to describe P. nordicum growth in the investigated ecological factors, yielding a good prediction of fungal behavior in plants for meat dry-curing and storage at different temperature conditions.