Development and validation of an extended predictive model for the effect of pH and water activity on the growth kinetics of Geobacillus stearothermophilus in plant-based milk alternatives

Detection of risk areas in dairy powder processes: The development of thermophilic spore forming bacteria taking into account their growth limits

Anoxybacillus flavithermus, Geobacillus stearothermophilus and Bacillus licheniformis are the main contaminants found in dairy powders. These spore-forming thermophilic bacteria, rarely detected in raw milk, persist, and grow during the milk powder manufacturing process. Moreover, in the form of spores, these species resist and concentrate in the powders during the processes. The aim of this study was to determine the stages of the dairy powder manufacturing processes that are favorable to the growth of such contaminants. A total of 5 strains were selected for each species as a natural contaminant of dairy pipelines in order to determine the minimum and maximum growth enabling values for temperature, pH, and aw and their optimum growth rates in milk. These growth limits were combined with the environmental conditions of temperature, pH and aw encountered at each step of the manufacture of whole milk, skim milk and milk protein concentrate powders to estimate growth capacities using cardinal models and the Gamma concept. These simulations were used to theoretically calculate the population sizes reached for the different strains studied at each stage in between two successive cleaning in place procedures. This approach highlights the stages at which risk occurs for the development of spore-forming thermophilic bacterial species. During the first stages of production, i.e. pre-treatment, pasteurization, standardization and pre-heating before concentration, physico-chemical conditions encountered are suitable for the development and growth of A. flavithermus, G. stearothermophilus and B. licheniformis. During the pre-heating stage and during the first effects in the evaporators, the temperature conditions appear to be the most favorable for the growth of G. stearothermophilus. The temperatures in the evaporator during the last evaporator effects are favorable for the growth of B. licheniformis. In the evaporation stage, low water activity severely limits the development of A. flavithermus.

Cardinal models to describe the effect of temperature and pH on the growth of Anoxybacillus flavithermus & Bacillus licheniformis

Anoxybacillus flavithermus and Bacillus licheniformis are among the predominant spore-formers of heat-processed foods. To our knowledge, no systematic analysis of growth kinetic data of A. flavithermus or B. licheniformis is currently available. In the present study, the growth kinetics of A. flavithermus and B. licheniformis in broth at various temperature and pH conditions were studied. Cardinal models were used to model the effect of the above-mentioned factors on the growth rates. The estimated values for the cardinal parameters T_min,T_opt,T_max,pH_min and pH_1/2 for A. flavithermus were 28.70 ± 0.26, 61.23 ± 0.16 and 71.52 ± 0.32 °C, 5.52 ± 0.01 and 5.73 ± 0.01, respectively, while for B. licheniformis they were 11.68 ± 0.03, 48.05 ± 0.15, 57.14 ± 0.01 °C, 4.71 ± 0.01 and 5.670 ± 0.08, respectively. The growth behaviour of these spoilers was also investigated in a pea beverage at 62 and 49 °C, respectively, to adjust the models to this product. The adjusted models were further validated at static and dynamic conditions and demonstrated good performance with 85.7 and 97.4% of predicted populations for A. flavithermus and B. licheniformis, respectively, being within the -10%-10% relative error (RE) zone. The developed models can be useful tools in assessing the potential of spoilage of heat-processed foods including plant-based milk alternatives.

Quantitative microbial spoilage risk assessment of plant-based milk alternatives by Geobacillus stearothermophilus in Europe

Geobacillus stearothermophilus is one of the predominant spoilers of UHT-treated food products, due to its extremely heat-resistant spores. However, the surviving spores should be exposed to temperature higher than their minimum growth temperature for a certain time to germinate and grow to spoilage levels. Considering the projected temperature increase due to climate change, the events of non-sterility during distribution and transportation are expected to escalate. Hence, the aim of this study was to build a quantitative microbial spoilage risk assessment (QMRSA) model to quantify the risk of spoilage of plant-based milk alternatives within Europe. The model consists of four main steps: 1. Initial contamination of raw materials 2. Heat inactivation of spores during UHT treatment 3. Partitioning 4. Germination and outgrowth of spores during distribution and storage. The risk of spoilage was defined as the probability of G. stearothermophilus to reach its maximum concentration (N_max = 10^7.5 CFU/mL) at the time of consumption. The assessment was performed for North (Poland) and South (Greece) Europe, and the risk of spoilage was estimated for the current climatic conditions and a climate change scenario. Based on the results, the risk of spoilage was negligible for the North European region, while the risk of spoilage in South Europe was 6.2 × 10^-3 95% CI (2.3 × 10^-3;1.1 × 10^-2) under the current climatic conditions. The risk of spoilage was increased for both tested countries under climate change scenario; from zero to 1.0 × 10^-4 in North Europe, risk multiplied 2 or 3 in South Europe depending on air conditioning implementation at consumer's place. Therefore, the heat treatment intensity and the use of insulated trucks during distribution were investigated as mitigation strategies and led to significant reduction of the risk. Overall, the QMRSA model developed in this study can support risk management decisions of these products by quantify the potential risk under current climatic conditions and climate change scenarios.

Improved growth and metabolite accumulation in Codonopsis pilosula (Franch.) Nannf. by inoculation with the endophytic Geobacillu sp. RHBA19 and Pseudomonas fluorescens RHBA17

In this study, we focused on the plant-growth-promoting properties of two strains isolated from Codonopsis pilosula, and the effect of inoculation with different strain treatments on physiological and metabolite accumulation of C. pilosula. The strains RHBA19 and RHBA17 were isolated and identified as Geobacillu sp. and Pseudomonas fluorescens, respectively. The two strains produced indole acetic acid (IAA), siderophore, biofilm, and various exoenzymes. Based on the pot experiments, inoculation of RHBA19 (G group) and the two mixed bacteria (M group) significantly improved the growth, root development, and photosynthesis of C. pilosula. Compared with the no-inoculation group (CK), the total polysaccharide content of root in the G and M groups was dramatically enhanced by 59.27% and 96.07%, and the lobetyolin (root) improved by 58.4% and 66.0%, respectively. After inoculation with bacteria agents, the activities of antioxidant enzymes (CAT, POD, SOD) of C. pilosula increased differentially. Inoculation with two types of bacterial agents significantly increased the activities of sucrose synthase (SS) and sucrose phosphate synthase (SPS) in root, and phenylalanine ammonia lyase (PAL) in leaf of C. pilosula. In addition, the content of signaling molecules (NO and H₂O₂) in three types of tissue increased significantly. The magnitude of these results was higher with mixtures than with individual strains. These results imply that the two types of bacterial agents induce physiological metabolism changes to accumulate polysaccharides and lobetyolin by regulating stress resistance enzymes and signal molecules, especially NO and H₂O₂.

Development and validation of a predictive model for the effect of temperature, pH and water activity on the growth kinetics of Bacillus coagulans in non-refrigerated ready-to-eat food products

A cardinal model (CM) for the effects of temperature (range: 32-59 °C), pH (range: 5.0-8.5) and water activity (a_w) (range: 0.980-0.995) on Bacillus coagulans DSM 1 growth rate was developed in brain heart infusion broth (BHI), using the Bioscreen C method and further validated in selected food products. The estimated values for the cardinal parameters T_min, T_opt, T_max, pH_min, pH_opt, pH_max, [Formula: see text] and [Formula: see text] were 23.77 ± 0.19 °C, 52.89 ± 0.01 °C, 59.37 ± 0.07 °C, 4.70 ± 0.02, 6.43 ± 0.02, 8.56 ± 0.01, 0.969 ± 0.0007 and 0.998 ± 0.0011, respectively. The growth behaviour of B. coagulans was studied in five commercial non-refrigerated ready-to-eat food products under static conditions at 53 °C in order to estimate the optimum specific growth rate for each tested food product. The developed models were validated in the five selected food products under four different dynamic temperature profiles by comparing predicted and observed growth behaviour of B. coagulans. The validation results indicated a good performance of the model for all tested products with the overall Bias factor (B_f) and Accuracy factor (A_f) estimated at 1.00 and 1.12, respectively. The developed model can be considered an effective tool in predicting B. coagulans growth and spoilage risks of non-refrigerated ready-to-eat food products during distribution and storage.