Effect of heat treatment intensity on the survival, activation and subsequent outgrowth of Byssochlamys nivea ascospores

Development Control and Inactivation of Byssochlamys nivea Ascospores by Hyperbaric Storage at Room Temperature

This study tested hyperbaric storage (25-150 MPa, for 30 days) at room-temperature (HS/RT, 18-23 °C) in order to control the development of Byssochlamys nivea ascospores in apple juice. In order to mimic commercially pasteurized juice contaminated with ascospores, thermal pasteurization (70 and 80 °C for 30 s) and nonthermal high pressure pasteurization (600 MPa for 3 min at 17 °C, HPP) took place, and the juice was afterwards placed under HS/RT conditions. Control samples were also placed in atmospheric pressure (AP) conditions at RT and were refrigerated (4 °C). The results showed that HS/RT, in samples without a pasteurization step and those pasteurized at 70 °C/30 s, was able to inhibit ascospore development, contrarily to samples at AP/RT and refrigeration. HS/RT for samples pasteurized at 80 °C/30 s evidenced ascospore inactivation, especially at 150 MPa, wherein an overall reduction of at least 4.73 log units of ascospores was observed to below detection limits (1.00 Log CFU/mL); meanwhile, for HPP samples, especially at 75 and 150 MPa, an overall reduction of 3 log units (to below quantification limits, 2.00 Log CFU/mL) was observed. Phase-contrast microscopy revealed that the ascospores do not complete the germination process under HS/RT, hence avoiding hyphae formation, which is important for food safety since mycotoxin development occurs only after hyphae formation. These findings suggest that HS/RT is a safe food preservation methodology, as it prevents ascospore development and inactivates them following commercial-like thermal or nonthermal HPP pasteurization, preventing mycotoxin production and enhancing ascospore inactivation.

Effect of storage temperature, water activity, oxygen headspace concentration and pasteurization intensity on the time to growth of Aspergillus fischerianus (teleomorph Neosartorya fischeri)

This study aims to assess, by means of a full factorial design, the effect of storage temperature (10-30 °C), water activity (a_w, 0.87-0.89), headspace oxygen (O₂) level (0.15-0.80%) and pasteurization intensity (95 °C-105 °C/15sec) on the time to visible growth (t_v, days) of Aspergillus fischerianus on acidified Potato Dextrose Agar (aPDA, pH 3.6) for up to 90 days. Moreover, in order to validate the results obtained on aPDA, 12 conditions were selected and assessed in concentrate strawberry-puree based medium. Overall, storage temperature had the greatest effect on the t_v of A. fischerianus on the evaluated conditions. At 10 °C, no visible growth was observed over the 90 day incubation period, whilst visible mycelia (diameter ≥ 2 mm) were present in 37% and 89% of the conditions at 22 °C and 30 °C, respectively. Pasteurization intensity had only a minor effect on the outgrowth of A. fischerianus. Growth inhibition was observed when a_w was reduced to 0.870 ± 0.005 in combination with very low headspace O₂ levels (0.15% ± 0.10) in both, aPDA and concentrate strawberry-based media, regardless of the incubation temperature and heat pasteurization intensity. Overall, longer t_v's were required when incubation was done at 22 °C compared to 30 °C. Ultimately, the effect of O₂ (0.05 and 1%) and pasteurization intensity (95 °C and 105 °C/15sec) were evaluated on totally 22 fruit purees (un-concentrates and concentrates) over a 60 day storage period. None of the concentrates purees (a_w ≤0.860) evaluated in this study supported the growth of A. fischerianus. On the other hand, A. fischerianus growth inhibition was only observed when the O₂ levels were ≤0.05% on un-concentrates fruit purees (a_w ≥ 0.980) stored at ambient temperature (22 °C). Combination of multiple stress factors effectively inhibited growth of A. fischerianus. In general, storage of fruit purees at low temperatures (<10 °C) or distribution in the form of concentrates can be considered as important strategies to prevent the growth of spoilage associated heat-resistant moulds.