Inter- and intra-species variability in heat resistance and the effect of heat treatment intensity on subsequent growth of Byssochlamys fulva and Byssochlamys nivea

Modeling the Thermal Inactivation of Monascus ruber Ascospores Isolated from Green Olive (Arauco Cultivar) Storage Brine: An Alternative Strategy to Reduce Antifungal Chemical Agents

Monascus ruber is an important fungus that causes spoilage in table olives, resulting in the darkening of the brine, the softening of the fruit, increased pH, and apparent mycelial growth. This study aimed to evaluate this resistance, providing a model to determine the optimal processing conditions for mitigating fungal contamination and prolonging shelf life without antifungal agents while optimizing pasteurization to reduce energy consumption. The resistance in brine (3.5% NaCl; pH 3.5) from Arauco cultivar green olives imported from Argentina was assessed. Four predictive models (log linear, log linear + shoulder, log linear + tail, log linear + shoulder + tail) estimated kinetic parameters for each survival curve. Log linear + shoulder + tail provided the best fit for 70 °C and 75 °C, with low RMSE (0.171 and 0.112) and high R2 values (0.98 and 0.99), respectively, while the log linear model was used for 80 °C. Decimal reduction times at 70, 75, and 80 °C were 24.8, 5.4, and 1.6 min, respectively, with a z-value of 8.2 °C. The current regulatory processes are insufficient to eliminate M. ruber at requisite levels, considering reduced antifungal agents.

Modeling the combined effect of temperature, pH, acetic and lactic acid concentrations on the growth/no growth interface of acid-tolerant Bacillus spores

The application of minimal processing technologies has led to increased spoilage incidents in low-acid pasteurized sauces due to the outgrowth of acid-tolerant spore-forming spoilage bacteria (ATSSB). Controlling the germination and subsequent growth of ATSSB spores is vital to enhance the ambient storage stability of pasteurized sauces. This study developed and validated a set of growth/no growth (G/NG) models for spores of two ATSSB strains (Bacillus velezensis and Bacillus subtilis subsp. subtilis) isolated from pasteurized sauces. The G/NG data at two levels of temperature (22 and 30 °C) were collected in Nutrient Broth (a_w = 0.98 adjusted with NaCl) by a full factorial design with five equidistant levels of pH (4.4-5.6), four concentrations of total acetic acid (0.0-0.3% (w/w)), and four concentrations of total lactic acid (0.00-1.00% (w/w)). The growth, starting from heat-treated (10 min 80 °C) spores, of each strain was assessed under 160 combinations by regular optical density measurements during three months. Twelve replicates were made for each combination. The developed models demonstrate that without organic acids even the lowest pH (4.4) allows a high growth possibility of the ATSSB spores, while acetic and lactic acids exhibit a significant antibacterial activity, which can be enhanced at decreased pH. The growth starting from B. subtilis spores can be inhibited for at least three months with 1.0% (w/w) total lactic acid in the water phase at both temperatures, which was not the case for B. velezensis, while 0.3% acetic acid achieves a full inhibition on both strains at 22 °C. With a combination of 0.3% acetic acid and 0.7% lactic acid, no growth should occur in the investigated range. This research is one of the first studies exploring the feasibility of ambient storage for low-acid pasteurized sauces eliminating preservatives such as benzoic and sorbic acids, and proves the synergistic effect of decreased pH and the presence of acetic and lactic acids on inhibiting bacterial growth from ATSSB spores.

Intraspecific variability in cardinal growth temperatures and water activities within a large diversity of Penicillium roqueforti strains

Different strains of a given fungal species may display heterogeneous growth behavior in response to environmental factors. In predictive mycology, the consideration of such variability during data collection could improve the robustness of predictive models. Among food-borne fungi, Penicillium roqueforti is a major food spoiler species which is also used as a ripening culture for blue cheese manufacturing. In the present study, we investigated the intraspecific variability of cardinal temperatures and water activities (a_w), namely, minimal (T_min and a_wmin), optimal (T_opt and a_wopt) and maximal (T_max) temperatures and/or a_w estimated with the cardinal model for radial growth, of 29 Penicillium roqueforti strains belonging to 3 genetically distinct populations. The mean values of cardinal temperatures and a_w for radial growth varied significantly across the tested strains, except for T_max which was constant. In addition, the relationship between the intraspecific variability of the biological response to temperature and a_w and putative genetic populations (based on microsatellite markers) within the selected P. roqueforti strains was investigated. Even though no clear relationship was identified between growth parameters and ecological characteristics, PCA confirmed that certain strains had marginal growth response to temperature or a_w. Overall, the present data support the idea that a better knowledge of the response to abiotic factors such as temperature and a_w at an intraspecific level would be useful to model fungal growth in predictive mycology approaches.

Quantitative microbial spoilage risk assessment (QMSRA) of pasteurized strawberry purees by Aspergillus fischeri (teleomorph Neosartorya fischeri)

Aspergillus fischeri ascospores are known as potential spoilage microorganisms of pasteurized fruit products due to their high incidence in fruits, the ability to survive pasteurization and to grow in acidic conditions. This study aimed to develop a quantitative microbial spoilage risk assessment (QMSRA) model approach to estimate the spoilage risk of packaged strawberry purees due to A. fischeri under various scenarios regarding product formulation, processing and storage conditions. The development of the risk assessment comprised three steps: (1) initial contamination level of raw material by ascospores (N₀), (2) inactivation of ascospores during thermal processing (N_p) and (3) determination of the number of ascospores which are able to survive thermal processing and develop visible mycelia (D = 2 mm) during storage (N_f). Data of visible growth (t_v, days) comprised distributions previously obtained as function of water activity (a_w) (0.860-0.985), oxygen (0-21%), temperature (8-30 °C) and pasteurization (95-105 °C/15 s). The simulations were performed in triplicate with 100,000 iterations using the software R. The outcome "spoilage risk" was defined as the probability of having at least one ascospore (N_f) capable of forming visible colonies in 100 g-pack strawberry puree within the typical use-by dates. Overall, high probabilities of spoilage were estimated for purees pasteurized at milder treatments at 85 °C/15-60 s (67%) and 90 °C/15-60 s (≥40%) stored at ambient temperature (22 °C). The spoilage risk was only effectively reduced (0.02%) by increasing pasteurization conditions to 95 °C for at least 45 s. Moreover, the microbial stability of such purees, i.e., spoilage risk <0.001% (=less than 1 spoilage pack out of 10⁵ produced units) was predicted to occur for purees treated at 100 °C/15 s or stored at chilled conditions (≤8 °C) or at strict anaerobic conditions or produced as concentrates (a_w ≤ 0.860). Based on the outcomes obtained, a set of specifications for Heat-Resistant Moulds (HRMs) in raw material and pasteurized purees aimed to be used as an ingredient was suggested. Furthermore, the results can be used to support risk management decisions in identifying and quantifying the impact of possible interventions during formulation, processing and storage conditions of fruit purees to effectively reduce this risk.

Multilevel modelling as a tool to include variability and uncertainty in quantitative microbiology and risk assessment. Thermal inactivation of Listeria monocytogenes as proof of concept

Variability is inherent in biology and also substantial for microbial populations. In the context of food safety risk assessment, it refers to differences in the response of different bacterial strains (between-strain variability) and different cells (within-strain variability) to the same condition (e.g. inactivation treatment). However, its quantification based on empirical observations and its incorporation in predictive models is a challenge for both experimental design and (statistical) analysis. In this article we propose the use of multilevel models to quantify (different levels of) variability and uncertainty and include them in the predictions. As proof of concept, we analyse the microbial inactivation of Listeria monocytogenes to thermal treatments including different levels of variability (between-strain and within-strain) and uncertainty. The relationship between the microbial count and time was expressed using a (non-linear) Weibullian model. Moreover, we defined stochastic hypotheses to describe the different types of variation at the level of the kinetic parameters, as well as in the observations (microbial counts). The model parameters (kinetic parameters and variances) are estimated using Bayesian statistics. The multilevel approach was compared against an analogous, single-level model. The multilevel methodology shrinks extreme parameter estimates towards the mean according to uncertainty, thus mitigating overfitting. In addition, this approach enables to easily incorporate different levels of variation (between-strain and/or within-strain variability and/or uncertainty) in the predictions. On the other hand, multilevel (Bayesian) models are more complex to define, implement, analyse and communicate than single-level models. Nevertheless, their ability to incorporate different sources of variability in predictions make them very suitable for Quantitative Microbial Risk Assessment.

Comparing thermal inactivation to a combined process of moderate heat and high pressure: Effect on ascospores in strawberry puree

High pressure processing is a mild preservation process that inactivates pathogenic and spoilage micro-organisms in food products, but preserves the fresh characteristics of a product. Compared to untreated product, an enhanced shelf life is obtained during refrigerated storage. Knowledge on the use of high pressure pasteurisation aimed for ambient storage is limited. The aim of this research was to investigate if a combination of high pressure and moderate heat could be used to produce a shelf-stable high-acid fruit product. Ascospores of the heat resistant fungi Talaromyces macrosporus and Aspergillus fischeri were added to fresh strawberry puree that served as a model system. The effect of the processing steps and storage at ambient temperature for 2 weeks was studied on viability of the ascospores. A preheating step at 69 °C/2 min resulted in full or partial activation of A. fischeri and T. macrosporus spores, respectively. The pressure build-up by the process without any holding time resulted in additional activation of spores. A combination of moderate heat (maximum 85-90 °C) and high pressure (500-700 MPa) for holding times up to 13 min inactivated these highly resistant spores much faster than a heat treatment alone. At T_max = 85 °C and 600 MPa the spores of T. macrosporus and A. fischeri were inactivated by 5.0 and 5.5 log₁₀ after 13 and 7 min, respectively. At T_max = 85 °C the heat treatment alone did not reduce the viability of these spores up to 60 min of treatment. At T_max = 90 °C the holding time of the combined pressure-heat treatment could be reduced to obtain the same degree of inactivation of the heat resistant fungi. In addition, treated and untreated ascospores in strawberry puree were stored for 14 days at room temperature to evaluate delayed outgrowth of spores. Untreated ascospores of A. fischeri were activated by storage in the puree. However, at conditions combining high pressure ≥ 600 MPa with T_max ≥ 85 °C for 13 min, heat resistant fungi were successfully inactivated. This research showed that a combination of moderate heat and pressure can drastically improve the effectiveness to inactivate heat-resistant ascospores in a high-acid fruit product compared to a heat treatment, potentially resulting in a better product quality.

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.