Minimal inhibitory concentrations of undissociated lactic, acetic, citric and propionic acid for Listeria monocytogenes under conditions relevant to cheese

Enhancing safety and quality in the global cheese industry: A review of innovative preservation techniques

The global cheese industry faces challenges in adopting new preservation methods due to microbiological decay and health risks associated with chemical preservatives. Ensuring the safety and quality control of hard and semi-hard cheeses is crucial given their prolonged maturation and storage. Researchers are urged to create cheese products emphasizing safety, minimal processing, eco-labels, and clean labels to address consumer health and environmental worries. This review aims to explore effective strategies for ensuring the safety and quality of ripened cheeses, covering traditional techniques like aging, maturation, and salting, along with innovative methods such as modified and vacuum packaging, high-pressure processing, and active and intelligent packaging. Additionally, sustainable cheese preservation approaches, their impact on shelf life extension, and the physiochemical and quality attributes post-preservation are all analyzed. Overall, the cheese industry stands to benefit from this evaluation through enhanced market value, increased consumer satisfaction, and better environmental sustainability.The integration of novel preservation techniques in the cheese industry not only addresses current challenges but also paves the way for a more sustainable and consumer-oriented approach. By continually refining and implementing safety measures, quality control processes, and environmentally friendly practices, cheese producers can meet evolving consumer demands while ensuring the longevity and integrity of their products. Through a concerted effort to embrace innovation and adapt to changing market dynamics, the global cheese industry is poised to thrive in a competitive landscape where safety, quality, and sustainability are paramount.

Predictive Model for Listeria monocytogenes in RTE Meats Using Exclusive Food Matrix Data

Post-processing contamination of Listeria monocytogenes has remained a major concern for the safety of ready-to-eat (RTE) meat products that are not reheated before consumption. Mathematical models are rapid and cost-effective tools to predict pathogen behavior, product shelf life, and safety. The objective of this study was to develop and validate a comprehensive model to predict the Listeria growth rate in RTE meat products as a function of temperature, pH, water activity, nitrite, acetic, lactic, and propionic acids. The Listeria growth data in RTE food matrices, including RTE beef, pork, and poultry products (731 data sets), were collected from the literature and databases like ComBase. The growth parameters were estimated using the logistic-with-delay primary model. The good-quality growth rate data (n = 596, R2 > 0.9) were randomly divided into 80% training (n = 480) and 20% testing (n = 116) datasets. The training growth rates were used to develop a secondary gamma model, followed by validation in testing data. The growth model's performance was evaluated by comparing the predicted and observed growth rates. The goodness-of-fit parameter of the secondary model includes R2 of 0.86 and RMSE of 0.06 (μmax) during the development stage. During validation, the gamma model with interaction included an RMSE of 0.074 (μmax), bias, and accuracy factor of 0.95 and 1.50, respectively. Overall, about 81.03% of the relative errors (RE) of the model's predictions were within the acceptable simulation zone (RE ± 0.5 log CFU/h). In lag time model validation, predictions were 7% fail-dangerously biased, and the accuracy factor of 2.23 indicated that the lag time prediction is challenging. The model may be used to quantify the Listeria growth in naturally contaminated RTE meats. This model may be helpful in formulations, shelf-life assessment, and decision-making for the safety of RTE meat products.

Synergistic antifungal effects of the preservative ammonium propionate and medium chain fatty acids against dormant and germinating conidia, germ tubes and hyphae of Aspergillus chevalieri, a feed spoilage fungus

In feed, propionic acid is the weak organic acid of choice to prevent growth of spoilage fungi. For safe and easy industrial handling this antifungal agent is applied in the presence of neutralizing ammonium, which however has the disadvantage to negatively affect the efficacy of fungus-inhibiting properties of the formulation. In the present study we investigated the impact of medium chain fatty acids (MCFA) on the antifungal efficacy of an ammonium propionate formulation on dormant- and germinating conidia as well as germ tubes and hyphae of Aspergillus chevalieri, a xerophilic fungus predominant on moulded feed. Dormant conidia were not affected by 32 mM of ammonium propionate after a 28 h-treatment in demi water. Similar results were obtained with solely 0.52 mM MCFA. However, the combination of both components nearly eradicated formation of colonies from these conidia and was accompanied by distortion of the cellular structure as was visible with light- and transmission electron microscopy. Germination of conidia, characterised by swelling and germ tube formation, was significantly decreased in the presence of 16 mM ammonium propionate and 0.26 mM MCFA, while the latter component itself did not significantly decrease germination. We conclude that a combination of ammonium propionate and MCFA had a synergistic antifungal effect on dormant and germinating conidia. When the combination of ammonium propionate and MCFA was tested on hyphae for 30 min, we observed that cell death was significantly increased in comparison to components alone. Treatment of the hyphae with 16 mM of ammonium propionate caused aberrant mitochondria, as evidenced by irregularly shaped and enlarged mitochondria that contained electron-dense inclusions as observed by transmission electron microscopy. When the combination of ammonium propionate and MCFA was applied against the hyphae, more severe cell damage was observed, with signs of autophagy. Summarised, our results demonstrate synergistic antifungal effects of ammonium propionate and medium chain fatty acids on fungal survival structures, during their germination and after a short (sudden) treatment of growing cells. This is of potential importance for several areas of feed and food storage and shelf-life.

Inhibition of Clostridium perfringens and Bacillus cereus by Dry Vinegar and Cultured Sugar Vinegar During Extended Cooling of Uncured Beef and Poultry Products

The 2021 FSIS Stabilization Guidelines for Meat and Poultry Products (Appendix B) Option 1.2 limits Phase 1 cooling from 48.8 to 26.7 °C in uncured meats to 1 h. However, this time restriction is impractical to achieve in large-diameter whole-muscle products. The objective of this study was to compare the inhibitory effect of commercial dry vinegars (DVs) and cultured sugar-vinegar blends (CSVs) on Clostridium perfringens and Bacillus cereus in uncured beef and poultry products during extended cooling. Treatments (beef: 72-73% moisture, pH 6.2-6.3, 0.85-0.95% NaCl; turkey: 76-77% moisture, pH 6.5-6.7, 1.3-1.6% NaCl) included Controls without antimicrobials, and four DV and four CSV, each tested at 0.75 and 1.25%. Batches were inoculated with 2.5-log C. perfringens or B. cereus spores, vacuum-packaged, and cooked to 73 °C. Packages were cooled from 48.8 to 27 °C (Phase 1) in 3, 4, or 5 h; Phase 2 (27-12.8 °C) and Phase 3 (12.8-4 °C) were standardized for 5-h cooling each. Pathogens were enumerated on selective agar in triplicate samples assayed at precook, postcook, and at the end of Phase 1, 2, and 3 cooling. Experiments were conducted twice. B. cereus did not grow (<0.5-log increase) in any treatment when Phase 1 cooling was extended to 5 h. C. perfringens grew rapidly (2.5 to >4.5 log) in Control treatments when Phase 1 cooling was extended to ≥3 h. All 1.25% DV ingredients limited C. perfringens growth to ≤1-log when Phase 1 cooling was extended to 3 h but supported a >1-log increase when Phase 1 cooling was extended to 5 h. All 1.25% CSV inhibited growth under 3-h Phase 1 cooling; 1.25% CSV-A and ≥0.75% CSV-D inhibited growth in turkey during 5-h Phase 1 cooling, but inhibition with 1.25% CSV-C was inconsistent in beef. This study revealed that formulating uncured meats with 1.25% DV or certain CSV can extend Phase 1 cooling to 3 h. Although all ingredients inhibited growth when used at 0.75% or greater compared to a control, greater variability of inhibition was observed among CSV than for DV.

Microbiological and physicochemical profile of Italian steak tartare and predicting growth potential of Listeria monocytogenes

In the present study, growth potential of Listeria monocytogenes in steak tartare samples taken at retail and belonging to 13 brands marketed in Northern Italy was investigated. The samples were submitted to microbiological and chemical-physical characterization. The data obtained were used as inputs for the application of the predictive microbiology software FSSP that allows the estimation of the growth of L. monocytogenes during the shelf-life. Lactic acid bacteria, the main component of the microflora, gave variable counts among the brands (from 3.38 to 6.24 log CFU/g). pH and aw values were always higher than 5.3 and 0.96, respectively, thus they could not be considered as single efficient hurdles to prevent the growth of L. monocytogenes according to the EC Reg. 2073/2005; the same was observed for salt content (constantly <2 %) and nitrites (not quantifiable in all the samples, even if declared in some labels). Nevertheless, the combination of all the hurdles, evaluated by predictive microbiology using critical development factors, resulted in an estimated growth <0.5 log CFU/g throughout the shelf life; this output allowed us to consider all the steak tartare analysed as unfavourable substrate for L. monocytogenes growth. The information obtained could be useful for tartare producers as well as for competent authority to evaluate the effective risk concerning these typology of products.

Modeling the influence of propionic acid concentration and pH on the kinetics of Salmonella Typhimurium

Salmonella Typhimurium is a foodborne pathogen often found in the poultry production chain. Antibiotics have been used to reduce S. Typhimurium contamination in poultry aviaries and improve chicken growth. However, antibiotics were banned in several countries. Alternatively, organic acids, such as propionic acid (PA), can control pathogens. This study determined the PA minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and mathematically modeled S. Typhimurium growth/inactivation kinetics under the influence of PA at different pH values (4.5, 5.5, and 6.5) which are within the pH range of the chicken gastrointestinal tract. The PA MIC against S. Typhimurium was pH-dependent, resulting in 5.0, 3.5 and 9.0 mM undissociated PA at pH 4.5, 5.5, and 6.5, respectively. The Baranyi and Roberts and the Weibull model fit growth and inactivation data well, respectively. Secondary models were proposed. The validated model predicted 3-log reduction of S. Typhimurium in 3 h at 68.2 mM of undissociated PA and pH 4.5. The models presented a good capacity to describe the kinetics of S. Typhimurium subjected to PA, representing a useful tool to predict PA antibacterial action depending on the pH.

Gut health benefits and associated systemic effects provided by functional components from the fermentation of natural matrices

Recently, the role of the gut microbiota in metabolic health, immunity, behavioral balance, longevity, and intestine comfort has been the object of several studies from scientific communities. They were encouraged by a growing interest from food industries and consumers toward novel fermented ingredients and formulations with powerful biological effects, such as pre, pro, and postbiotic products. Depending on the selected strains, the operating conditions, the addition of suitable reagents or enzymes, the equipment, and the reactor configurations, functional compounds with high bioactivity, such as short-chain fatty acids, gamma-aminobutyric acid, bioactive peptides, and serotonin, can be enhanced and/or produced through fermentation of several vegetable matrices. Otherwise, their formation can also be promoted directly in the gut after the dietary intake of fermented foods: In this case, fermentation will aim to increase the content of precursor substances, such as indigestible fibers, polyphenols, some amino acids, and resistant starch, which can be potentially metabolized by endogenous gut microorganisms and converted in healthy molecules. This review provides an overview of the main functional components currently investigated in literature and the associated gut health benefits. The current state of the art about fermentation technology as a promising functionalization tool to promote the direct or indirect formation of gut-health-enhancing components was deepened, highlighting the importance of optimizing microorganism selection, system setups, and process conditions according to the target compound of interest. The collected data suggested the possibility of gaining novel functional food ingredients or products rich in functional molecules through fermentation without performing additional extraction and purification stages, which are needed when conventional culture broths are used.

Sodium reduction in foods: Challenges and strategies for technical solutions

In many parts of the world, sodium consumption is higher than recommended levels, representing one of the most important food-related health challenges and leading to considerable economical costs for society. Therefore, there is a need to find technical solutions for sodium reduction that can be implemented by food producers and within food services. The aims of this review are to discuss the barriers related to sodium reduction and to highlight a variety of technical solutions. The barriers relate to consumer perception, microbiology, processing, and physicochemistry. Existing technical solutions include inhomogeneous salt distribution, coated salt particles, changing particle sizes and forms, surface coating, multisensory combinations, sodium replacements, double emulsions, adapted serum release by microstructure design, and adapted brittleness by microstructure design. These solutions, their implementation and the associated challenges, and applicable product categories are described. Some of these solutions are ready for use or are in their early development stages. Many solutions are promising, but in most cases, some form of adaptation or optimization is needed before application in specific products, and care must always be taken to ensure food safety. For instance, further research and innovation are required in the dynamic evolution of saltiness perception, consumer acceptance, the binding and migration of sodium, juiciness, microbiological safety, and the timing of salt addition during processing. Once implemented, these solutions will undoubtedly support food producers and food services in reducing sodium content and extend the application of the solutions to different foods.

Perilla frutescens as potential antimicrobial modifier to against forage oat silage spoilage

The aim of this study was to investigate the influence of Perilla frutescens, alone or in combination with Lactobacillus plantarum a214 or citric acid, on forage oat silage quality, bacterial and fungal microbiological profile during ensiling and aerobic exposure. With the exception of Perilla frutescens, all additives could improve silage quality of forage oat based on lower ammonia-nitrogen content and higher residual of water soluble carbohydrates during anaerobic fermentation compared to control silage, especially in Perilla frutescens combined with citric acid (CAPF). Lactobacillus was the dominant bacteria in all silages, while CAPF group increased the relative abundance of Lactobacillus lindneri and Lactobacillus brevis compared to control silage. The application of Perilla frutescens suppressed the relative abundance of yeasts such as Pichia fermentans and Wickerhamomyces anomalus in response to aerobic exposure, especially in CAPF treatment, leading to high acetic acids and lower dry matter loss, as well as good aerobic stability. Therefore, Perilla frutescens, alone or in combination with citric acid, has potential to improve aerobic stability of forage oat silage by shifting bacterial and fungal community composition, and can be used as new additive to prepare high-quality silage for animal production.

Predicting the Growth of Listeria monocytogenes in Cooked, Sliced Deli Turkey Breast as a Function of Clean-Label Antimicrobials, pH, Moisture, and Salt

ABSTRACT

The use of antimicrobials in formulated ready-to-eat meat and poultry products has been identified as a major strategy to control Listeria monocytogenes. The U.S. Department of Agriculture's Food Safety and Inspection Service recommends no more than 2 log of Listeria outgrowth over the stated shelf life if antimicrobials are used as a control measure for a product with postlethality environmental exposure. This study was designed to understand the efficacy of a clean-label antimicrobial agents against the growth of L. monocytogenes as affected by the product attributes. A response surface method-central composite design was used to investigate the effects of product pH, moisture, salt content, and a commercial "clean-label" antimicrobial agent on the growth of L. monocytogenes in a model turkey deli meat formulation. Thirty treatment combinations of pH (6.3, 6.5, and 6.7), moisture (72, 75, and 78%), salt (1.0, 1.5, and 2.0%), and antimicrobials (0.75, 1.375, and 2.0%), with six replicated center points and eight design star points were evaluated. Treatments were surface inoculated with a 3-log CFU/g target of a five-strain L. monocytogenes cocktail, vacuum packaged, and stored at 5°C for up to 16 weeks. Populations of L. monocytogenes were enumerated from triplicate samples every week until the stationary growth phase was reached. The enumeration data was fitted to a Baranyi and Roberts growth curve to calculate the lag time and maximum growth rate for each treatment. Linear least-squares regression of the lag time and growth rate against the full quadratic, including the second-order interaction terms, design matrix was performed. Both lag time and maximum growth rate were significantly affected (P < 0.01) by the antimicrobial concentration and product pH. Product moisture and salt content affected (P < 0.05) lag phase and maximum growth rate, respectively. The availability of a validated growth model assists meat scientists and processors with faster product development and commercialization.

HIGHLIGHTS

A model to predict the fate of Listeria monocytogenes in different cheese types - A major role for undissociated lactic acid in addition to pH, water activity, and temperature

Undissociated lactic acid has been shown to play a major role in complete growth inhibition of Listeria monocytogenes in Gouda cheese. In addition, low water activity conditions may contribute to growth inhibition. In the current study, it was assessed whether the major factors that inhibit growth of L. monocytogenes in Gouda cheese are the factors that determine growth in other types of ready-to-eat cheese as well. Various types of cheeses were selected, some of which had been associated with listeriosis, while others had not. Based on the composition of the different cheese types, the concentrations of undissociated lactic acid were calculated for each type. The ability to support growth of L. monocytogenes was predicted using the Gamma model, based on literature data on total lactic acid content, moisture content, fat content, pH, Aw, and temperature, and optimal growth rates in milk at 30-37 °C. In addition, the actual specific growth rates of L. monocytogenes in the various cheeses were calculated based on available experimental growth data. In 9 out of the 10 RTE cheeses reviewed, the undissociated lactic acid concentrations and a_w determined growth/no growth of L. monocytogenes. No growth was correctly predicted for feta, Cheddar and Gouda, and growth was correctly predicted for ricotta, queso fresco, Camembert, high-moisture mozzarella, cottage and blue cheese. Growth of L. monocytogenes was not observed in practice upon inoculation of Emmental, whereas growth in this cheese type was predicted when including the above mentioned factors in the models. Other factors, presumably acetic and propionic acid, are thought to be important to inhibit growth of the pathogen in Emmental. The results from our study show that for cheeses in which lactic acid is a main acid, our model based on undissociated lactic acid, temperature, pH and a_w gives a good prediction of potential outgrowth of L. monocytogenes. Implications for L. monocytogenes legislation are discussed per type of RTE cheese reviewed.

A Review of the Preservation of Hard and Semi-Hard Cheeses: Quality and Safety

Cheese is a dairy product with potential health benefits. Cheese consumption has increased due to the significant diversity of varieties, versatility of product presentation, and changes in consumers’ lifestyles. Spoilage of hard and semi-hard cheeses can be promoted by their maturation period and/or by their long shelf-life. Therefore, preservation studies play a fundamental role in maintaining and/or increasing their shelf-life, and are of significant importance for the dairy sector. The aim of this review is to discuss the most effective methods to ensure the safety and sensory quality of ripened cheeses. We review traditional methods, such as freezing, and modern and innovative technologies, such as high hydrostatic pressures, chemical and natural vegetable origin preservatives, vacuum and modified atmosphere packaging, edible coatings and films, and other technologies applied at the end of storage and marketing stages, including light pulses and irradiation. For each technology, the main advantages and limitations for industrial application in the dairy sector are discussed. Each type of cheese requires a specific preservation treatment and optimal application conditions to ensure cheese quality and safety during storage. The environmental impact of the preservation technologies and their contribution to the sustainability of the food chain are discussed.

Dairy associations for the targeted control of opportunistic Candida

Antifungal and antibacterial activities of twenty-six combinations of lactic acid bacteria, propionibacteria, acetic acid bacteria and dairy yeasts inoculated in whey and milk were investigated. Associations including acetic acid bacteria were shown to suppress growth of the opportunistic yeast Candida albicans in well-diffusion assays. The protective effect of milk fermented with the two most promising consortia was confirmed in Caco-2 cell culture infected with C. albicans. Indeed, these fermented milks, after heat-treatment or not, suppressed lactate dehydrogenase release after 48 h while significant increase in LDH release was observed in the positive control (C. albicans alone) and with fermented milk obtained using commercial yogurt starter cultures. The analysis of volatile compounds in the cell-free supernatant using solid phase microextraction (SPME) coupled to gas chromatography-mass spectrometry (GC-MS) showed accumulation of significant amount of acetic acid by the consortium composed of Lactobacillus delbrueckii 5, Lactobacillus gallinarum 1, Lentilactobacillus parabuchneri 3, Lacticaseibacillus paracasei 33-4, Acetobacter syzygii 2 and Kluyveromyces marxianus 19, which corresponded to the zone of partial inhibition of C. albicans growth during well-diffusion assays. Interestingly, another part of anti-Candida activity, yielding small and transparent inhibition zones, was linked with the consortium cell fraction. This study showed a correlation between anti-Candida activity and the presence of acetic acid bacteria in dairy associations as well as a significant effect of two dairy associations against C. albicans in a Caco-2 cell model. These two associations may be promising consortia for developing functional dairy products with antagonistic action against candidiasis agents.

The occurrence, growth, and biocontrol of Listeria monocytogenes in fresh and surface-ripened soft and semisoft cheeses

Listeria monocytogenes continues to pose a food safety risk in ready-to-eat foods, including fresh and soft/semisoft cheeses. Despite L. monocytogenes being detected regularly along the cheese production continuum, variations in cheese style and intrinsic/extrinsic factors throughout the production process (e.g., pH, water activity, and temperature) affect the potential for L. monocytogenes survival and growth. As novel preservation strategies against the growth of L. monocytogenes in susceptible cheeses, researchers have investigated the use of various biocontrol strategies, including bacteriocins and bacteriocin-producing cultures, bacteriophages, and competition with native microbiota. Bacteriocins produced by lactic acid bacteria (LAB) are of particular interest to the dairy industry since they are often effective against Gram-positive organisms such as L. monocytogenes, and because many LAB are granted Generally Regarded as Safe (GRAS) status by global food safety authorities. Similarly, bacteriophages are also considered a safe form of biocontrol since they have high specificity for their target bacterium. Both bacteriocins and bacteriophages have shown success in reducing L. monocytogenes populations in cheeses in the short term, but regrowth of surviving cells can commonly occur in the finished cheeses. Competition with native microbiota, not mediated by bacteriocin production, has also shown potential to inhibit the growth of L. monocytogenes in cheeses, but the mechanisms are still unclear. Here, we have reviewed the current knowledge on the growth of L. monocytogenes in fresh and surface-ripened soft and semisoft cheeses, as well as the various methods used for biocontrol of this common foodborne pathogen.

Development and validation of a regression model for Listeria monocytogenes growth in roast beefs

Food business operators are responsible for food safety and assessment of shelf lives for their ready-to-eat products. For assisting them, a customized software based on predictive models, ListWare, is being developed. The aim of this study was to develop and validate a predictive model for the growth of Listeria monocytogenes in sliced roast beef. A challenge study was performed comprising 51 different combinations of variables. The growth curves followed the Baranyi and Roberts model with no clear lag phase and specific growth rates in the range <0.005-0.110 hr^-1. A linear regression model was developed based on 528 observations and had an adjusted R-square of 0.80. The significant predictors were storage temperature, sodium lactate, interactions between sodium acetate and temperature, and MAP packaging and temperature. The model was validated in four laboratories in three countries. For conditions where the model predicted up to + log 2 cfu/g Listeria concentration, the observed concentrations were true or below the predicted concentration in 90% of the cases. For the remaining 10%, the roast beef was coated with spices and therefore different from the others. The model will be implemented in ListWare web-application for calculation of "Listeria shelf life".

Synergistic activity of weak organic acids against uropathogens

BACKGROUND

Urinary tract infections (UTIs) are among the most common hospital-acquired infections, with an estimated 75% of UTIs caused by urinary catheters. In addition to the significant healthcare costs and patient morbidity, the escalating antimicrobial resistance reported among common uropathogens make the investigation of efficacious new antimicrobial strategies of urgent importance.

AIM

To examine the antibacterial activity of a suite of weak organic acids (WOAs) (citric acid, malic acid, propionic acid, mandelic acid, lactic acid, benzoic acid, pyruvic acid and hippuric acid), alone and in combination, against common nosocomial uropathogens (Proteus mirabilis, Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa).

METHODS

Minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), minimum biofilm eradication concentration (MBEC), fractional inhibitory concentration index (FICI) values and kinetics of bactericidal activity of WOAs were determined by microdilution and time-kill assays.

FINDINGS

All tested WOAs displayed bactericidal activities against uropathogens in their planktonic and biofilm modes of growth when used individually. Moreover, WOAs in combination displayed synergistic activity against P. mirabilis, S. aureus and E. coli, with reductions in MIC values of up to 250-fold and significant reductions in biofilm formation.

CONCLUSION

The synergistic multi-mechanistic combinations identified herein are anticipated to play an important role in the treatment and prevention of catheter-associated UTIs.

Propionic acid production from apple pomace in bioreactor using Propionibacterium freudenreichii: an economic analysis of the process

Propionic acid and its salts are widely used as food and feed preservative. Currently, these compounds are chemically produced, which is more profitable compared to biotechnological production using bacteria of the Propionibacterium genus. Appropriate steps can enable reducing the production costs; for example, cheap industrial byproducts can be used as culture media. One such cost-effective raw material is apple pomace, a low-value byproduct from the food industry. It contains sugars such as glucose and fructose which can serve as potential carbon sources for microorganisms. This paper discusses the possibility of using apple pomace in the production of propionic acid and presents an economic analysis of the production process. The tested strain produced 8.01 g/L of propionic acid (yield 0.40 g/g) and 2.29 g/L of acetic acid (yield 0.11 g/g) from apple pomace extract. The economic analysis showed that the production of 1 kg of propionic acid (considering only waste) from 1000 kg of apple pomace would cost approximately 1.25 USD. The manufacturing cost (consumables, including feedstock, labor, and utilities) would be approximately 2.35 USD/kg, and the total cost including taxes would be approximately 3.05 USD/kg. From the economic point of view, it is necessary to improve the production of propionic acid from apple pomace, to increase the yield of fermentation and thus decrease the total production costs. This can be achieved, for example, using industrial byproducts as nitrogen and vitamin sources, instead of high-cost substrates such as yeast extract or peptone.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-020-02582-x.

Modelling viability of Listeria monocytogenes in paneer

Paneer is a fresh, soft ready-to-eat cheese that is susceptible to Listeria monocytogenes contamination, exemplified by product recalls in Australia, Canada, and the USA. Previous research demonstrates that L. monocytogenes grows in paneer, however there are no paneer-specific predictive models that quantify the effect of environmental conditions on L. monocytogenes viability. This study measured the viability of a five-strain cocktail of L. monocytogenes in freshly prepared paneer incubated at 4-40 °C. Growth rates were fitted with the extended Ratkowsky square root model, with growth rates ranging from 0.014 to 0.352 log₁₀ CFU/h. In comparison with published models, only the ComBase L. monocytogenes broth model acceptably predicted growth (B_f = 1.01, A_f = 1.12) versus the developed model. The influence of paneer pH (5.0-6.0) and storage temperature (41-45 °C) on L. monocytogenes growth at the upper temperature growth boundary was described using a logistic model. These models provide quantitative tools to improve the safety of paneer processing conditions, shelf-life estimation, food safety management plans, and risk assessment.

Growth Potential of Listeria monocytogenes in Chef-Crafted Ready-to-Eat Fresh Cheese-Filled Pasta Meal Stored in Modified Atmosphere Packaging

This study evaluated the growth of lactic acid bacteria (LAB) in a fresh, filled-pasta meal, stored in modified atmosphere packaging and the influence of lactic acid (LA) and pH on the growth of Listeria monocytogenes (Lm). Samples were taken from three lots manufactured by a local catering company and stored at both 6 and 14°C. LAB numbers, LA concentration, pH, and the presence of Lm were evaluated at 1, 4, 6, 8, 10, 12, and 14 days of shelf life and the undissociated LA concentration ([LA]) was calculated. The LAB maximum cell density was greater in the products stored at 14°C than those stored at 6°C (10.1 ± 1.1 versus 5.6 ± 1.5 log CFU/g) and [LA] at 14 days was 9 to 21 ppm at 6°C and 509 to 1,887 ppm at 14°C. Challenge tests were made to evaluate the interference of LAB and [LA] on Lm growth. Aliquots of the samples (25 g) were inoculated at 1 to 10 days of shelf life and incubated at 9°C for 7 days, and the difference between Lm numbers at the end and at the beginning of the test (δ) was calculated. Logistic regression was used to model the probability of growth of Lm as a function of LAB and [LA]. The products inoculated at 1 day of shelf life had δ values between 4.2 and 5.6 log CFU/g, but the growth potential was progressively reduced during the shelf life. Lm growth was never observed in the products stored at 14°C. In those stored at 6°C, it grew only in the samples with LAB <5.7 log CFU/g. LAB interaction might thus inhibit the growth of Lm in temperature-abused products and limit its growth in refrigerated products. Logistic regression estimated that the probability of Lm growth was <10% if LAB was >6.6 log CFU/g or log[LA] was >2.2 ppm. The growth or inactivation kinetic of Lm was investigated with a homogenate of three samples with LAB numbers close to the maximum population density. After an initial growth, a subsequent reduction in the number of Lm was observed. This means that the maximum numbers of Lm might not be detected at the end of the product shelf life.

Evolutionary engineering in Saccharomyces cerevisiae reveals a TRK1-dependent potassium influx mechanism for propionic acid tolerance

BACKGROUND

Propionic acid (PA), a key platform chemical produced as a by-product during petroleum refining, has been widely used as a food preservative and an important chemical intermediate in many industries. Microbial PA production through engineering yeast as a cell factory is a potentially sustainable alternative to replace petroleum refining. However, PA inhibits yeast growth at concentrations well below the titers typically required for a commercial bioprocess.

RESULTS

Adaptive laboratory evolution (ALE) with PA concentrations ranging from 15 to 45 mM enabled the isolation of yeast strains with more than threefold improved tolerance to PA. Through whole genome sequencing and CRISPR-Cas9-mediated reverse engineering, unique mutations in TRK1, which encodes a high-affinity potassium transporter, were revealed as the cause of increased propionic acid tolerance. Potassium supplementation growth assays showed that mutated TRK1 alleles and extracellular potassium supplementation not only conferred tolerance to PA stress but also to multiple organic acids.

CONCLUSION

Our study has demonstrated the use of ALE as a powerful tool to improve yeast tolerance to PA. Potassium transport and maintenance is not only critical in yeast tolerance to PA but also boosts tolerance to multiple organic acids. These results demonstrate high-affinity potassium transport as a new principle for improving organic acid tolerance in strain engineering.

Characterization and comparative analysis of toxin-antitoxin systems in Acetobacter pasteurianus

Bacterial toxin-antitoxin (TA) systems play important roles in diverse cellular regulatory processes. Here, we characterize three putative type II TA candidates from Acetobacter pasteurianus and investigate the profile of type II TA systems in the genus Acetobacter. Based on the gene structure and activity detection, two-pairs loci were identified as the canonical hicAB and higAB TA systems, respectively, and DB34_01190-DB34_01195 as a putative new one without a canonical TA architecture. Physiologically, the expression of the three pairs conferred E. coli with additional plasmid maintenance and survival when under acetic acid stress. Chromosomal TA systems can be horizontally transferred within an ecological vinegar microbiota by co-option, and there was a tendency for toxin module loss. The antitoxin retention in the genome is suggested to have a broad role in bacterial physiology. Furthermore, A. pasteurianus strains, universally domesticated and used for industrial vinegar fermentation, showed a higher number of type II TA loci compared to the host-associated ones. The amount of TA loci per genome showed little positive relationship to insertion sequences, although its prevalence was species-associated, to the extent of even being strain-associated. The TA system is a candidate of studying the resistant mechanistic network, the TAs-dependent translatome affords a real-time profile to explore stress adaptation of A. pasteurianus, promoting industrial development.

Hurdle factors minimizing growth of Listeria monocytogenes while counteracting in situ antilisterial effects of a novel nisin A-producing Lactococcus lactis subsp. cremoris costarter in thermized cheese milks

The capacity of growth, survival, and adaptive responses of an artificial contamination of a three-strain L. monocytogenes cocktail in factory-scale thermized (65 °C, 30 s) Graviera cheese milk (TGCM) was evaluated. Bulk TGCM samples for inoculation were sequentially taken from the cheese making vat before process initiation (CN-LM) and after addition of a commercial starter culture (CSC), the CSC plus the nisin A-producing (NisA+) costarter strain Lactococcus lactis subsp. cremoris M78 (CSC + M78), and all ingredients with the rennet last (CSC + M78-RT). Additional treatments included Listeria-inoculated TGCM samples coinoculated with the NisA+ costarter strain M78 in the absence of the CSC or with the CSC in previously sterilized TGCM to inactivate the background microbiota (CSC-SM). All cultures were incubated at 37 to 42 °C for 6 h, followed by additional 66 h at 22 °C, and 48 h at 12 °C after addition of 2% edible salt. L. monocytogenes failed to grow and declined in all CSC-inoculated treatments after 24 h. In contrast, the pathogen increased by 3.34 and 1.46 log units in the CN-LM and the CSC-SM treatments, respectively, indicating that the background microbiota or the CSC alone failed to suppress it, but they did so synergistically. Supplementation of the CSC with the NisA+ strain M78 did not deliver additional antilisterial effects, because the CSC Streptococcus thermophilus reduced the growth prevalence rates and counteracted the in situ NisA+ activity of the costarter. In the absence of the CSC, however, strain M78 predominated and caused the strongest in situ nisin-A mediated effects, which resulted in the highest listerial inactivation rates after 24 to 72 h at 22 °C. In all TGCM treatments, however, L. monocytogenes displayed a "tailing" survival (1.63 to 1.96 log CFU/mL), confirming that this pathogen is exceptionally tolerant to cheese-related stresses, and thus, can't be easily eliminated.

Listeria monocytogenes contamination of ready-to-eat foods and the risk for human health in the EU

Food safety criteria for Listeria monocytogenes in ready-to-eat (RTE) foods have been applied from 2006 onwards (Commission Regulation (EC) 2073/2005). Still, human invasive listeriosis was reported to increase over the period 2009-2013 in the European Union and European Economic Area (EU/EEA). Time series analysis for the 2008-2015 period in the EU/EEA indicated an increasing trend of the monthly notified incidence rate of confirmed human invasive listeriosis of the over 75 age groups and female age group between 25 and 44 years old (probably related to pregnancies). A conceptual model was used to identify factors in the food chain as potential drivers for L. monocytogenes contamination of RTE foods and listeriosis. Factors were related to the host (i. population size of the elderly and/or susceptible people; ii. underlying condition rate), the food (iii. L. monocytogenes prevalence in RTE food at retail; iv. L. monocytogenes concentration in RTE food at retail; v. storage conditions after retail; vi. consumption), the national surveillance systems (vii. improved surveillance), and/or the bacterium (viii. virulence). Factors considered likely to be responsible for the increasing trend in cases are the increased population size of the elderly and susceptible population except for the 25-44 female age group. For the increased incidence rates and cases, the likely factor is the increased proportion of susceptible persons in the age groups over 45 years old for both genders. Quantitative modelling suggests that more than 90% of invasive listeriosis is caused by ingestion of RTE food containing > 2,000 colony forming units (CFU)/g, and that one-third of cases are due to growth in the consumer phase. Awareness should be increased among stakeholders, especially in relation to susceptible risk groups. Innovative methodologies including whole genome sequencing (WGS) for strain identification and monitoring of trends are recommended.