Design of an experimental viscoelastic food model system for studying Zygosaccharomyces bailii spoilage in acidic sauces

Unravelling the impact of fat content on the microbial dynamics and spatial distribution of foodborne bacteria in tri-phasic viscoelastic 3D models

The aim of the current study is to develop and characterise novel complex multi-phase in vitro 3D models, for advanced microbiological studies. More specifically, we enriched our previously developed bi-phasic polysaccharide (Xanthan Gum)/protein (Whey Protein) 3D model with a fat phase (Sunflower Oil) at various concentrations, i.e., 10%, 20%, 40% and 60% (v/v), for better mimicry of the structural and biochemical composition of real food products. Rheological, textural, and physicochemical analysis as well as advanced microscopy imaging (including spatial mapping of the fat droplet distribution) of the new tri-phasic 3D models revealed their similarity to industrial food products (especially cheese products). Furthermore, microbial growth experiments of foodborne bacteria, i.e., Listeria monocytogenes, Escherichia coli, Pseudomonas aeruginosa and Lactococcus lactis on the surface of the 3D models revealed very interesting results, regarding the growth dynamics and distribution of cells at colony level. More specifically, the size of the colonies formed on the surface of the 3D models, increased substantially for increasing fat concentrations, especially in mid- and late-exponential growth phases. Furthermore, colonies formed in proximity to fat were substantially larger as compared to the ones that were located far from the fat phase of the models. In terms of growth location, the majority of colonies were located on the protein/polysaccharide phase of the 3D models. All those differences at microscopic level, that can directly affect the bacterial response to decontamination treatments, were not captured by the macroscopic kinetics (growth dynamics), which were unaffected from changes in fat concentration. Our findings demonstrate the importance of developing structurally and biochemically complex 3D in vitro models (for closer proximity to industrial products), as well as the necessity of conducting multi-level microbial analyses, to better understand and predict the bacterial behaviour in relation to their biochemical and structural environment. Such studies in advanced 3D environments can assist a better/more accurate design of industrial antimicrobial processes, ultimately, improving food safety.

On the evaluation of the antimicrobial effect of grape seed extract and cold atmospheric plasma on the dynamics of Listeria monocytogenes in novel multiphase 3D viscoelastic models

The demand for products that are minimally processed and produced in a sustainable way, without the use of chemical preservatives or antibiotics have increased over the last years. Novel non-thermal technologies such as cold atmospheric plasma (CAP) and natural antimicrobials such as grape seed extract (GSE) are attractive alternatives to conventional food decontamination methods as they can meet the above demands. The aim of this study was to investigate the microbial inactivation potential of GSE, CAP (in this case, a remote air plasma with an ozone-dominated RONS output) and their combination against L. monocytogenes on five different 3D in vitro models of varying rheological, structural, and biochemical composition. More specifically, we studied the microbial dynamics, as affected by 1 % (w/v) GSE, CAP or their combination, in three monophasic Xanthan Gum (XG) based 3D models of relatively low viscosity (1.5 %, 2.5 % and 5 % w/v XG) and in a biphasic XG/Whey Protein (WPI) and a triphasic XG/WPI/fat model. A significant microbial inactivation (comparable to liquid broth) was achieved in presence of GSE on the surface of all monophasic models regardless of their viscosity. In contrast, the GSE antimicrobial effect was diminished in the multiphasic systems, resulting to only a slight disturbance of the microbial growth. In contrast, CAP showed better antimicrobial potential on the surface of the complex multiphasic models as compared to the monophasic models. When combined, in a hurdle approach, GSE/CAP showed promising microbial inactivation potential in all our 3D models, but less microbial inactivation in the structurally and biochemically complex multiphasic models, with respect to the monophasic models. The level of inactivation also depended on the duration of the exposure to GSE. Our results contribute towards understanding the antimicrobial efficacy of GSE, CAP and their combination as affected by robustly controlled changes of rheological and structural properties and of the biochemical composition of the environment in which bacteria grow. Therefore, our results contribute to the development of sustainable food safety strategies.

Effect of humectants and Zygosaccharomyces bailii inoculation on the physical stability of oil-in-water emulsions modeling salad dressings

BACKGROUND

Salad dressing formulations include a pH within 3.2-4.1, salt and other solutes to depress water activity. The interaction between hydrocolloids and other components such as humectants determines their physical and microbiological stability. To our knowledge, the effect of commonly used solutes on physical stability and rheological characteristics of oil-in-water emulsions stabilized by xanthan gum has not been reported, and neither has the effect of a spoilage yeast on physical stability been evaluated.

RESULTS

The effect of different humectants (sodium chloride, glucose and xylitol) and Zygosaccharomyces bailii inoculation on the stability of oil-in-water emulsions containing different levels of xanthan gum to emulate different types of salad dressings was investigated by means of droplet size, zeta potential, rheological measurements and confocal laser scanning microscopy (CLSM). Generally, the addition of humectants strengthened the weak or strong emulsion structure, especially xylitol. Glucose or xylitol acted as stabilizers since they decreased Sauter and De Broucker diameter. On the contrary, NaCl destabilized the emulsions since it increased droplet size and exhibited the lowest absolute values of zeta potential. Inoculation with Z. bailii destabilized some emulsions since it increased Sauter and De Broucker diameter. Emulsion droplet size, polydispersity, and xanthan gum and yeast location were confirmed by CLSM imaging.

CONCLUSION

This study highlights the key role of humectants and xanthan gum level on physical characteristics and stability of oil-in-water emulsions. Addition of xylitol leads to an enhancement in emulsion rheology and physical stability, suggesting that it is a potential low-calorie multifunctional additive for salad dressings. © 2022 Society of Chemical Industry.

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.

The Inclusion of the Food Microstructural Influence in Predictive Microbiology: State-of-the-Art

Predictive microbiology has steadily evolved into one of the most important tools to assess and control the microbiological safety of food products. Predictive models were traditionally developed based on experiments in liquid laboratory media, meaning that food microstructural effects were not represented in these models. Since food microstructure is known to exert a significant effect on microbial growth and inactivation dynamics, the applicability of predictive models is limited if food microstructure is not taken into account. Over the last 10-20 years, researchers, therefore, developed a variety of models that do include certain food microstructural influences. This review provides an overview of the most notable microstructure-including models which were developed over the years, both for microbial growth and inactivation.

Effect of gellan gum, xylitol and natamycin on Zygosaccharomyces bailii growth and rheological characteristics in low sugar content model systems

This study evaluated the effect of some natural additives and the structure imparted by them on microbial growth and rheological characteristics in acidic model foods with reduced glycidic content. Systems were formulated using gellan gum, as gelling agent; xylitol, as a_w depressor; and natamycin, as antimicrobial. Additive-free control systems were prepared. The pH was adjusted to 3.50 or 5.50 as required. Systems were inoculated with Zygosaccharomyces bailii. The effect of additives alone and combined on Z. bailii growth was studied. In some cases, the possible use of additives as yeast nutrients was evaluated. Furthermore, systems rheological characterization was performed. Additives and the structure given by gellan gum significantly affected yeast growth. Gellan gum initially slowed Z. bailii development, but as storage progressed, it acted as yeast carbon source, promoting its growth. A similar trend was observed when xylitol effect was studied. Natamycin inhibited yeast growth in all systems assayed. Additives modified the rheological characteristics of the gels and this effect depended on gellan gum concentration and pH. Obtained results emphasize the importance of considering the different effects that additives and their combinations can exert on the growth of deteriorating microorganisms and on the physical characteristics of gels.

Modelling the microbial dynamics and antimicrobial resistance development of Listeria in viscoelastic food model systems of various structural complexities

Minimal processing for microbial decontamination, such as the use of natural antimicrobials, is gaining interest in the food industry as these methods are generally milder than conventional processing, therefore better maintaining the nutritional content and sensory characteristics of food products. The aim of this study was to quantify the impact of (i) structural composition and complexity, (ii) growth location and morphology, and (iii) the natural antimicrobial nisin, on the microbial dynamics of Listeria innocua. More specifically, viscoelastic food model systems of various compositions and internal structure were developed and characterised, i.e. monophasic Xanthan gum-based and biphasic Xanthan gum/Whey protein-based viscoelastic systems. The microbial dynamics of L. innocua at 10 °C, 30 °C and 37 °C were monitored and compared for planktonic growth in liquid, or in/on (immersed or surface colony growth) the developed viscoelastic systems, with or without a sublethal concentration of nisin. Microscopy imaging was used to determine the bacterial colony size and spatial organisation in/on the viscoelastic systems. Selective growth of L. innocua on the protein phase of the developed biphasic system was observed for the first time. Additionally, significant differences were observed in the colony size and distribution in the monophasic Xanthan gum-based systems depending on (i) the type of growth (surface/immersed) and (ii) the Xanthan gum concentration. Furthermore, the system viscosity in monophasic Xanthan gum-based systems had a protective role against the effects of nisin for immersed growth, and a further inhibitory effect for surface growth at a suboptimal temperature (10 °C). These findings give a systematic quantitative insight on the impact of nisin as an environmental challenge on the growth and spatial organisation of L. innocua, in viscoelastic food model systems of various structural compositions/complexities. This study highlights the importance of accounting for system structural composition/complexity when designing minimal food processing methods with natural antimicrobials.

Effect of stabilizers, oil level and structure on the growth of Zygosaccharomyces bailii and on physical stability of model systems simulating acid sauces

The effect of xanthan gum, guar gum, oil and the structure promoted by these compounds on the growth of Zygosaccharomyces bailii and on physical stability of emulsified systems simulating acid sauces was studied. Furthermore, the effect of yeast growth on physical stability of emulsions was also evaluated. Yeast growth was evaluated by plate count and modeled by the modified Gompertz equation. Emulsions characteristics and their stability were determined by droplet size, zeta potential and rheological measurements. The latter was also used to evaluate structure's effect on yeast growth. Physical characteristics of emulsions depended on system composition. Yeasts slightly affected droplet size. Z. bailii growth was satisfactorily modeled by the modified Gompertz equation. The specific growth rate (μ_m) and the asymptotic value (A) obtained depended on xanthan gum, guar gum and oil content. Furthermore, the structure promoted by these compounds exerted a significant effect on growth. In general, an increase in the solid character and yield stress through the addition of xanthan gum promoted a decrease in A parameter. On the contrary, a decrease in the solid character through the addition of guar gum promoted an increase in the A parameter. The results obtained stressed that stabilizers, oil and their structuring ability play an important role on Z. bailii growth.

Recent trends in non-invasive in situ techniques to monitor bacterial colonies in solid (model) food

Planktonic cells typically found in liquid systems, are routinely used for building predictive models or assessing the efficacy of food preserving technologies. However, freely suspended cells often show different susceptibility to environmental hurdles than colony cells in solid matrices. Limited oxygen, water and nutrient availability, metabolite accumulation and physical constraints due to cell immobilization in the matrix, are main factors affecting cell growth. Moreover, intra- and inter-colony interactions, as a consequence of the initial microbial load in solid systems, may affect microbial physiology. Predictive food microbiology approaches are moving toward a more realistic resemblance to food products, performing studies in structured solid systems instead of liquids. Since structured systems promote microbial cells to become immobilized and grow as colonies, it is essential to study the colony behavior, not only for food safety assurance systems, but also for understanding cell physiology and optimizing food production processes in solid matrices. Traditionally, microbial dynamics in solid systems have been assessed with a macroscopic approach by applying invasive analytical techniques; for instance, viable plate counting, which yield information about overall population. In the last years, this approach is being substituted by more mechanistically inspired ones at mesoscopic (colony) and microscopic (cell) levels. Therefore, non-invasive and in situ monitoring is mandatory for a deeper insight into bacterial colony dynamics. Several methodologies that enable high-throughput data collection have been developed, such as microscopy-based techniques coupled with image analysis and OD-based measurements in microplate readers. This research paper provides an overview of non-invasive in situ techniques to monitor bacterial colonies in solid (model) food and emphasizes their advantages and inconveniences in terms of accuracy, performance and output information.

Role of growth morphology in the formulation of NaCl-based selective media for injury detection of Escherichia coli, Salmonella Typhimurium and Listeria innocua

Sublethal injury (SI) poses major public health concerns since injured cells are responsible for serious limitations in food diagnostics and are susceptible to recovery, often developing adaptive stress responses. Detection of SI is based on the difference in plate counts between non-selective media, which represent the total cell population, and selective media, to which injured cells become sensitive. Selective media for detection of sublethal membrane damage are often based on NaCl supplement, although there is a lack of consensus in the literature about appropriate levels. Planktonic cells are generally used to investigate SI mechanisms, although they often exhibit different stress tolerance than cell colonies in/on solid food (model) systems. In this work, the effect of growth morphology, colony size and concentration of the gelling agent in the growth media, on the maximum non-inhibitory NaCl concentration in the plating medium was assessed for Escherichia coli, Salmonella Typhimurium and Listeria innocua. Stationary phase cultures of planktonic cells and large and small colonies grown in either 1.5% (w/v) xanthan gum-based system or 2.5% (w/v) xanthan gum-based system exhibited significantly different viable counts and osmotolerance. The effect of cell arrangement and xanthan gum percentage in the growth media depended on the microorganism under investigation. Additionally, differences in the maximum non-inhibitory concentration were evident, with 5.0% (w/v) NaCl for the Gram-negative bacteria and 6.5% (w/v), for L. innocua. Different extent of colony shrinkage and morphological damage was observed as NaCl concentration in the plating medium increased. This information will contribute to define NaCl-based selective media for accurate SI detection under realistic scenarios.

Effect of cell immobilization on heat-induced sublethal injury of Escherichia coli, Salmonella Typhimurium and Listeria innocua

The occurrence of sublethally injured cells in foods poses major public health concerns and is an essential aspect when assessing the microbial response to food preservation strategies, yet there is limited research dealing with its specific implications for mild heating. All available studies so far have been performed in broths colonized by planktonic cells, although their susceptibility to lethal agents has often been reported to be markedly different to the stress tolerance of cell colonies developed in solid foods. In this work, the effect of planktonic and colony growth, as well as the influence of colony density on sublethal injury induced by mild heating of Escherichia coli, Salmonella Typhimurium and Listeria innocua were assessed in food model systems. Detection of injured survivors relied on their inability to form visible colonies on salt-based selective media, which do not affect the growth of healthy cells. Sublethal injury (SI) increased rapidly with shorter exposure times and afterwards, decreased progressively, suggesting a mechanism of cumulative damage triggering lethal instead of SI. Cell arrangement affected the degree of SI, higher values being generally found for gelified systems, although the effect of colony density depended on the target microorganism. This information is essential for optimizing the design of food safety assurance systems.

Development of a model describing the effect of temperature, water activity and (gel) structure on growth and ochratoxin A production by Aspergillus carbonarius in vitro and evaluation in food matrices of different viscosity

The present study aimed: (i) to develop models for the combined effect of water activity (0.99, 0.94 and 0.90), microstructure expressed as 0, 5, 10 and 20% w/v gelatin, and temperature (15, 20 and 25 °C), on growth and OTA production rates by Aspergillus carbonarius; and (ii) to evaluate the performance of the developed models on food matrices (jelly, custard and marmalade) of different viscosity at pH 5.5. The square root of biomass increase rate (fungal growth rate) and OTA production rate were determined by the Baranyi model and were further modeled as a function of temperature, gelatin concentration and a(w) by applying polynomial models. Time for visible growth and the upper asymptote of the OTA production curve were also determined by the Baranyi model. Increase in gelatin concentration resulted in a significant delay in all parameters describing fungal growth and OTA production rates, at all temperatures and a(w). The effect of microstructure on fungal growth and OTA production rates was less evident at stress conditions of a(w) and temperature. Detection time for visible fungal growth was markedly influenced by a(w) and temperature. Coefficients of determination were 0.899 and 0.887 for the models predicting the square root (√μ(max)) of growth and OTA production rate, respectively. Predictions of growth rate agreed well with the recorded data of custard and marmalade, while observations of OTA production rate indicated low agreement with model predictions, in all food matrices except for marmalade. The present findings may provide a basis for reliable assessment of the risk of fungal growth and OTA production in foods of different structural and rheological properties.

On the critical evaluation of growth/no growth assessment of Zygosaccharomyces bailii with optical density measurements: liquid versus structured media

Growth/no growth (G/NG) studies that include the effect of medium structure have typically been performed for (pathogenic) bacteria and on the basis of gelatin/agar as a gelling agent. In this study, the growth potential of the spoilage yeast Zygosaccharomyces bailii was investigated in two model systems that resemble the macroscopic physicochemical and rheological properties of acidic sauces. In a Carbopol model system, the effect of pH (3.5-4.5), glycerol concentration (17-32%), acetic acid concentration (1.5-2.0%) and medium structure (3 levels) was investigated. In xanthan gum, the behavior of the yeast was studied at different levels of pH (3.5-4.5), NaCl concentration (0.5-13.5%), acetic acid concentration (0-2.0%) and medium structure (2 levels). Rheologically, viscoelastic moduli failed to discriminate between different forms of microbial growth, whereas yield stress data appeared to provide a better indication. In general, G/NG results revealed an unexpected increase of growth probability as a function of medium structure, both at 22 and 30 °C. Whether this behavior is the result of an underlying growth-promoting mechanism could not be explained from a macroscopic point of view (e.g., macrorheology, a(w)), but may be more related to the local microscopic properties of the gels. In a second part of this study, the potential use and information content of optical density measurements for G/NG data collection in structured media were critically evaluated and confronted with their practical relevance to the food industry.