Power Ultrasound and High Pressure Processing Inactivation of Specific Microbial Spores in Foods

Abstract

Bacterial and fungal spores are a great concern in food industries due to their extreme resistance to physical and chemical treatments. Thermal processing at high temperatures often diminishes food quality. Therefore in this study, high pressure processing (HPP) and power ultrasound alone and in combination with heat (HPP-thermal) and thermosonication (TS) were investigated for their abilities to inactivate the spores of Clostridium perfringens and psychrotrophic Bacillus cereus in low-acid (pH>4.6) foods, and Alicyclobacillus acidoterrestris, Neosartorya fischeri, and Byssochlamys nivea spores in high-acid (pH<4.6) foods. The spore inactivation was compared with thermal processing alone and the inactivation kinetics was modeled. The 600 MPa HPP-thermal and TS treatments were better than thermal processing alone for the microbial spore inactivation, requiring between 8−30°C lower temperatures to obtain the same lethality. The 600 MPa HPP-75°C was the best technique to inactivate C. perfringens in beef slurry, and N. fischeri, and B. nivea moulds in juice/puree. With respect to C. perfringens, spore reductions of ≥2 log after 20 min were obtained for HPP-thermal vs. almost no inactivation for TS and thermal processes. Regarding the moulds, the 600 MPa HPP-75°C inactivated ≥4 log after 20 min, while the TS and thermal treatments increased the spore numbers by up to 2.5 log. Regarding B. cereus spores, TS was the most effective method to inactivate them in skim milk and beef slurry. Over 15 min, TS caused ≥5 log in milk vs 3 log after HPP-thermal and 2 log with thermal process. In beef slurry, TS was actually able to increase the thermal spore inactivation in beef slurry by more than 6 fold. TS and thermal processing alone at 78°C had no effect on A. acidoterrestris spores. However, TS treatment (78°C) of HPP pretreated spores suspended in orange juice increased the spore inactivation by ≥1.6 fold. Lower D-values were obtained at higher acoustic power densities. In addition, heat shock (HS) and ultrasonication pretreatment of the spores doubled the spore thermal inactivation of C. perfringens and A. acidoterrestris: pretreated C. perfringens spores D95°C = 9.8 min vs 22 min in beef slurry; pretreated A. acidoterrestris spores D95°C = 0.8 min vs 1.5 min in orange juice. With respect to overall spore resistance to different technologies at 70-75°C, psychrotrophic B. cereus spores were the least resistant. The spores of B. nivea and N. fischeri showed the highest resistance to thermal treatment over 30 min. A. acidoterrestris and C. perfringens were more difficult to inactivate with TS processing and C. perfringens was more difficult to inactivate with HPP-thermal treatment. The mould ascospore resistance to HPP-thermal and TS processes increased with increasing spore age. Regarding 4 week old N. fischeri spores at 75°C TS or HPP-thermal, 27 min were required for 1 log reduction, whereas 74 min was required to obtain the same spore inactivation for 12 week old spores. With respect to 4 week old B. nivea spores, the results were closer. While 13 min were required for 1 log reduction, 29 min were required to obtain the same spore inactivation for 12 week old spores. The HPP-thermal inactivation for all the microbial spores was well described with the Weibull model, whereas the inactivation kinetics for TS treatment was species/strain/food dependent. The TS inactivation of psychrotrophic B. cereus spores in skim milk and A. acidoterrestris spores in orange juice followed simple first order kinetics, whereas log logistic and Weibull models described the TS inactivation of B. cereus and C. perfringens spores in beef slurry, respectively. Lorentzian distribution modeled the 4-10 week old mould spore inactivation with TS treatment. With the exception of thermal inactivation of mould spores at T ≤ 85°C, all the spore thermal inactivations followed the simple first order kinetic model.