Risk factor-based clustering of Listeria monocytogenes in food processing environments using principal component analysis

Listeria monocytogenes has a range of strategies that allow it to persist as biofilms in food processing environments (FPE), making it a pathogen of concern to the food industry. The properties of these biofilms are highly variable among strains, and this significantly affects the risk of food contamination. The present study therefore aims to conduct a proof-of-concept study to cluster strains of L. monocytogenes by risk potential using principal component analysis, a multivariate approach. A set of 22 strains, isolated from food processing environments, were typed by serogrouping and pulsed-field gel electrophoresis, showing a relatively high diversity. They were characterized in terms of several biofilm properties that might pose a potential risk of food contamination. The properties studied were tolerance to benzalkonium chloride (BAC), the structural parameters of biofilms (biomass, surface area, maximum and average thickness, surface to biovolume ratio and roughness coefficient) measured by confocal laser scanning microscopy and (3) transfer of biofilm cells to smoked salmon. The PCA correlation circle revealed that the tolerance of biofilms to BAC was positively correlated with roughness, but negatively with biomass parameters. On the contrary, cell transfers were not related to three-dimensional structural parameters, which suggests the role of other variables yet unexplored. Additionally, hierarchical clustering grouped strains into three different clusters. One of them included the strains with high tolerance to BAC and roughness. Another one consisted of strains with enhanced transfer ability, whereas the third cluster contained those that stood out for the thickness of biofilms. The present study represents a novel and effective way to classify L. monocytogenes strains according to biofilm properties that condition the potential risk of reaching the consumer through food contamination. It would thus allow the selection of strains representative of different worst-case scenarios for future studies in support of QMRA and decision-making analysis.

Mechanisms of Listeria monocytogenes Disinfection with Benzalkonium Chloride: From Molecular Dynamics to Kinetics of Time-Kill Curves

Unravelling the mechanisms of action of disinfectants is essential to optimise dosing regimes and minimise the emergence of antimicrobial resistance. In this work, we examined the mechanisms of action of a commonly used disinfectant-benzalkonium chloride (BAC)-over a significant pathogen-L. monocytogenes-in the food industry. For that purpose, we used modelling at multiple scales, from the cell membrane to cell population inactivation. Molecular modelling revealed that the integration of the BAC into the membrane requires three phases: (1) the approaching of BAC to the cellular membrane, (2) the absorption of BAC to its surface, and (3) the integration of the compound into the lipid bilayer, where it remains at least for several nanoseconds, probably destabilising the membrane. We hypothesised that the equilibrium of adsorption, although fast, was limiting for sufficiently large BAC concentrations, and a kinetic model was derived to describe time-kill curves of a large population of cells. The model was tested and validated with time series data of free BAC decay and time-kill curves of L. monocytogenes at different inocula and BAC dose concentrations. The knowledge gained from the molecular simulation plus the proposed kinetic model offers the means to design novel disinfection processes rationally.

Architectural Features and Resistance to Food-Grade Disinfectants in Listeria monocytogenes-Pseudomonas spp. Dual-Species Biofilms

Listeria monocytogenes is considered a foodborne pathogen of serious concern capable of forming multispecies biofilms with other bacterial species, such as Pseudomonas spp., adhered onto stainless steel (SS) surfaces. In an attempt to link the biofilms’ morphology and resistance to biocides, dual-species biofilms of L. monocytogenes, in co-culture with either Pseudomonas aeruginosa, Pseudomonas fluorescens, or Pseudomonas putida, were assayed to ascertain their morphological characteristics and resistance toward benzalkonium chloride (BAC) and neutral electrolyzed water (NEW). Epifluorescence microscopy analysis revealed that each dual-species biofilm was distributed differently over the SS surface and that these differences were attributable to the presence of Pseudomonas spp. Confocal laser scanning microscopy (CLSM) assays demonstrated that despite these differences in distribution, all biofilms had similar maximum thicknesses. Along with this, colocalization analyses showed a strong trend of L. monocytogenes to share location within the biofilm with all Pseudomonas assayed whilst the latter distributed throughout the surface independently of the presence of L. monocytogenes, a fact that was especially evident in those biofilms in which cell clusters were present. Finally, a modified Gompertz equation was used to fit biofilms’ BAC and NEW dose-response data. Outcomes demonstrated that L. monocytogenes was less susceptible to BAC when co-cultured with P. aeruginosa or P. fluorescens, whereas susceptibility to NEW was reduced in all three dual-species biofilms, which can be attributable to both the mechanism of action of the biocide and the architectural features of each biofilm. Therefore, the results herein provided can be used to optimize already existing and develop novel target-specific sanitation treatments based on the mechanism of action of the biocide and the biofilms’ species composition and structure.

Identification of Emerging Hazards in Mussels by the Galician Emerging Food Safety Risks Network (RISEGAL). A First Approach

Emerging risk identification is a priority for the European Food Safety Authority (EFSA). The goal of the Galician Emerging Food Safety Risks Network (RISEGAL) is the identification of emerging risks in foods produced and commercialized in Galicia (northwest Spain) in order to propose prevention plans and mitigation strategies. In this work, RISEGAL applied a systematic approach for the identification of emerging food safety risks potentially affecting bivalve shellfish. First, a comprehensive review of scientific databases was carried out to identify hazards most quoted as emerging in bivalves in the period 2016–2018. Then, identified hazards were semiquantitatively assessed by a panel of food safety experts, who scored them accordingly with the five evaluation criteria proposed by EFSA: novelty, soundness, imminence, scale, and severity. Scores determined that perfluorinated compounds, antimicrobial resistance, Vibrio parahaemolyticus, hepatitis E virus (HEV), and antimicrobial residues are the emerging hazards that are considered most imminent and severe and that could cause safety problems of the highest scale in the bivalve value chain by the majority of the experts consulted (75%). Finally, in a preliminary way, an exploratory study carried out in the Galician Rías highlighted the presence of HEV in mussels cultivated in class B production areas.

Efficacy of Synthetic Furanones on Listeria monocytogenes Biofilm Formation

Furanones are analogues of acylated homoserine lactones with proven antifouling activity in both Gram-positive and Gram-negative bacteria though the interference of various quorum sensing pathways. In an attempt to find new strategies to prevent and control Listeria monocytogenes biofilm formation on stainless steel (SS) surfaces, different concentrations of six synthetic furanones were applied on biofilms formed by strains isolated from food, environmental, and clinical sources grown onto AISI 316 SS coupons. Among the furanones tested, (Z-)-4-Bromo-5-(bromomethylene)-2(5H)-furanone and 3,4-Dichloro-2(5H)-furanone significantly (p < 0.05) reduced the adhesion capacity (>1 log CFU cm-2) in 24 h treated biofilms. Moreover, individually conducted experiments demonstrated that (Z-)-4-Bromo-5-(bromomethylene)-2(5H)-furanone was able to not only significantly (p < 0.05) prevent L. monocytogenes adhesion but also to reduce the growth rate of planktonic cells up to 48 h in a dose-dependent manner. LIVE/DEAD staining followed by epifluorescence microscopy visualisation confirmed these results show an alteration of the structure of the biofilm in furanone-treated samples. Additionally, it was demonstrated that 20 µmol L-1 of 3,4-Dichloro-2(5H)-furanone dosed at 0, 24 and 96 h was able to maintain a lower level of adhered cells (>1 log CFU cm-2; p < 0.05). Since furanones do not pose a selective pressure on bacteria, these results represent an appealing novel strategy for the prevention of L. monocytogenes biofilm grown onto SS.

Tracking bacteriome variation over time in Listeria monocytogenes-positive foci in food industry

The variation in microbial composition over time was assessed in biofilms formed in situ on selected non-food and food contact surfaces of meat and fish industries, previously identified as Listeria monocytogenes-positive foci. First, all samples were analysed for the detection and quantification of L. monocytogenes using ISO 11290-1 and ISO 11290-2 norms, respectively. Although the pathogen was initially detected in all samples, direct quantification was not possible. Psychrotrophic bacteria counts were among resident microbiota in meat industry samples (Mean_max = 6.14 log CFU/cm²) compared to those form fish industry (Mean_max = 5.85 log CFU/cm²). Visual analysis of the biofilms using epifluorescence microscopy revealed a trend to form microcolonies in which damaged/dead cells would act as anchoring structures. 16S rRNA gene metagenetic analysis demonstrated that, although Proteobacteria (71.37%) initially dominated the bacterial communities at one meat industry location, there was a dramatic shift in composition as the biofilms matured, where Actinobacteria (79.72%) became the major phylum present in later samples. This change was largely due to an increase of Nocardiaceae, Micrococcaceae and Microbacteriaceae. Nevertheless, for the other sampling location, the relative abundance of the dominating phylum (Firmicutes) remained consistent over the entire sampling period (Mean = 63.02%). In fish industry samples, Proteobacteria also initially dominated early on (90.69%) but subsequent sampling showed a higher diversity in which Bacteroidetes and Proteobacteria were the most abundant phyla accounting for the 48.04 and 37.98%, respectively by the last sampling period. Regardless of the location, the community profiles of the endpoint samples were similar to those reported previously. This demonstrated that in a given industrial setting there is a trend to establish a determinate biofilm structure due to the environmental factors and the constant incoming microbiota. This information could be used to improve the existing sanitisation protocols or for the design of novel strategies.

Current Knowledge on Listeria monocytogenes Biofilms in Food-Related Environments: Incidence, Resistance to Biocides, Ecology and Biocontrol

Although many efforts have been made to control Listeria monocytogenes in the food industry, growing pervasiveness amongst the population over the last decades has made this bacterium considered to be one of the most hazardous foodborne pathogens. Its outstanding biocide tolerance capacity and ability to promiscuously associate with other bacterial species forming multispecies communities have permitted this microorganism to survive and persist within the industrial environment. This review is designed to give the reader an overall picture of the current state-of-the-art in L. monocytogenes sessile communities in terms of food safety and legislation, ecological aspects and biocontrol strategies.

Tolerance development in Listeria monocytogenes-Escherichia coli dual-species biofilms after sublethal exposures to pronase-benzalkonium chloride combined treatments

This study was designed to assess the effects that sublethal exposures to pronase (PRN) and benzalkonium chloride (BAC) combined treatments have on Listeria monocytogenes-Escherichia coli dual-species biofilms grown on stainless steel in terms of tolerance development (TD) to these compounds. Additionally, fluorescence microscopy was used to observe the changes of the biofilm structure. PRN-BAC exposure was carried out using three different approaches and TD was evaluated treating biofilms with a final 100 μg/ml PRN followed by 50 μg/ml BAC combined treatment. Results showed that exposure to PRN-BAC significantly decreased the number of adhered L. monocytogenes (P < 0.05), while E. coli counts remained generally unaltered. It was also demonstrated that the incorporation of recovery periods during sublethal exposures increased the tolerance of both species of the mixed biofilm to the final PRN-BAC treatment. Moreover, control biofilms became more resistant to PRN-BAC if longer incubation periods were used. Regardless of the treatment used, log reduction values were generally lower in L. monocytogenes compared to E. coli. Additionally, microscopy images showed an altered morphology produced by sublethal PRN-BAC in exposed L. monocytogenes-E. coli dual-species biofilms compared to control samples. Results also demonstrated that L. monocytogenes-E. coli dual-species biofilms are able to develop tolerance to PRN-BAC combined treatments depending on way they have been previously exposed. Moreover, they suggest that the generation of bacterial tolerance should be included as a parameter for sanitation procedures design.

Numerical spatio-temporal characterization of Listeria monocytogenes biofilms

As the structure of biofilms plays a key role in their resistance and persistence, this work presents for the first time the numerical characterization of the temporal evolution of biofilm structures formed by three Listeria monocytogenes strains on two types of stainless-steel supports, AISI 304 SS No. 2B and AISI 316 SS No. 2R. Counting methods, motility tests, fluorescence microscopy and image analysis were combined to study the dynamic evolution of biofilm formation and structure. Image analysis was performed with several well-known parameters as well as a newly defined parameter to quantify spatio-temporal distribution. The results confirm the interstrain variability of L. monocytogenes species regarding biofilm structure and structure evolution. Two types of biofilm were observed: homogeneous or flat and heterogeneous or clustered. Differences in clusters and in attachment and detachment processes were due mainly to the topography and composition of the two surfaces although an effect due to motility was also found.

Comparison between the resistance of benzalkonium chloride-adapted and -nonadapted biofilms of Listeria monocyogenes to modified atmosphere packaging and nisin once transferred to mussels

Benzalkonium chloride-adapted and -nonadapted Listeria monocytogenes biofilm cells were transferred by contact to cooked or live mussels and packed in rich CO(2) and O(2), respectively. The viabilities of transferred cells during storage of these packed samples at 2.5 °C were compared. In addition, in cooked mussels the combined effect of CO(2) and nisin against the survival of L. monocytogenes was also studied by using a first-order factorial design. The results obtained demonstrated that biofilms formed by benzalkonium chloride-adapted L. monocytogenes cells could be more resistant to the application of modified atmospheres rich in CO(2) and nisin once they have been transferred to cooked mussels by contact (simulating cross-contamination). This implies an increase in the risk associated with the presence of these cells in food processing plants. Significant empirical equations obtained after 7, 11, and 20 days showed an inhibitory effect of CO(2) and nisin against L. monocytogenes. However, a significant positive interaction between both variables highlights an incompatibility between CO(2) and nisin at high concentrations. Results also demonstrated that L. monocytogenes could persist after cross-contamination during the processing of live mussels, so L. monocytogenes is of concern as a contaminant in live mussels packaged in high-O(2) atmospheres.

Resistance to benzalkonium chloride, peracetic acid and nisin during formation of mature biofilms by Listeria monocytogenes

Increase of resistance to the application of benzalkonium chloride (BAC), peracetic acid (PA) and nisin during biofilm formation at 25 °C by three strains of Listeria monocytogenes (CECT 911, CECT 4032, CECT 5873 and BAC-adapted CECT 5873) in different scenarios was compared. For this purpose, resistance after 4 and 11-days of biofilm formation was quantified in terms of lethal dose 90% values (LD(90)), determined according with a dose-response logistic mathematical model. Microscopic analyses after 4 and 11-days of L. monocytogenes biofilm formation were also carried out. Results demonstrated a relation between the microscopic structure and the resistance to the assayed biocides in matured biofilms. The worst cases being biofilms formed by the strain 4032 (in both stainless steel and polypropylene), which showed a complex "cloud-type" structure that correlates with the highest resistance of this strain against the three biocides during biofilm maturation. However, that increase in resistance and complexity appeared not to be dependent on initial bacterial adherence, thus indicating mature biofilms rather than planctonic cells or early-stage biofilms must be considered when disinfection protocols have to be optimized. PA seemed to be the most effective of the three disinfectants used for biofilms. We hypothesized both its high oxidizing capacity and low molecular size could suppose an advantage for its penetration inside the biofilm. We also demonstrated that organic material counteract with the biocides, thus indicating the importance of improving cleaning protocols. Finally, by comparing strains 5873 and 5873 adapted to BAC, several adaptative cross-responses between BAC and nisin or peracetic acid were identified.

Effects of mussel processing soils on the adherence of Listeria monocytogenes to polypropylene and stainless steel

A comparative study of adhesion kinetics of L. monocytogenes (strains CECT 5873, CECT 936, CECT 911, and CECT 4032, representing serotypes 1/2a, 1/2b, 1/2c, and 4b, respectively) to polypropylene (PP) and stainless steel (SS) under two surface contamination conditions in plants processing cooked mussel was carried out. The conditions were either (i) contamination of clean surfaces with mussel cooking juice carrying L. monocytogenes or (ii) contamination with L. monocytogenes after soiling with mussel cooking juice, i.e., conditioning film (CF). The kinetics of adhesion were successfully described by a modified logistic model. Adhesion to PP was higher than to SS in all strains, except CECT 5873. Adhesion was initially higher in the presence of CF, but numbers of adherent cells decreased sharply in the late phase of study in three of eight cases as a result of cell detachment. Combinations of strain, surface material, and surface conditioning where adhesion was most enhanced were defined as the worst-case scenarios (CECT 911-PP, 4032-PP-CF, 5873-SS, and 4032-SS-CF). Subsequently, adhesion in worst-case scenarios was compared with a similar contamination event taking place in plants processing live mussels, using intervalvar water of mussel as a food residue matrix. Adhesion levels were higher in intervalvar water than in cooking juice, especially in both cases with no CF; this was attributed to more space available for adhesion or to physicochemical conditions enhancing cells to adhere.

Application of nisin and pediocin against resistance and germination of Bacillus spores in sous vide products

Sous vide and other mild preservation techniques are increasingly demanded by consumers. However, spores often will survive in minimally processed foods, causing both spoilage and safety problems. The main objective of the present work was to solve an industrial spoilage problem associated with two sous vide products: mushrooms and shellfish salad. Bacillus subtilis and Bacillus licheniformis predominated as the most heat-resistant organisms isolated from mushrooms and shellfish salad, respectively. The combined effects of nisin and pediocin against resistance and germination of both Bacillus species were described by empirical equations. Whereas nisin was more effective for decreasing thermal resistance of B. subtilis spores, pediocin was more effective against B. licheniformis. However, a significant positive interaction between both biopeptides for decreasing the proportion of vegetative cells resulting from thermoresistant spores was demonstrated in later experiments, thus indicating the increased efficacy of applying high concentrations of both bacteriocins. This efficacy was further demonstrated in additional challenge studies carried out at 15 degrees C in the two sous vide products: mushrooms and shellfish salad. Whereas no vegetative cells were detected after 90 days in the presence of bacteriocins, almost 100% of the population in nontreated samples of mushrooms and shellfish salad was in the vegetative state after 17 and 43 days of storage at 15 degrees C, respectively.

Adhesion and detachment kinetics of several strains of Staphylococcus aureus subsp. aureus under three different experimental conditions

The kinetics of adhesion of five Staphylococcus aureus subsp. aureus strains (CECT 976, 4459, 4465, 4466 and 5191) to polypropylene at 25 degrees C in the absence of nutrients (PBS medium) were initially compared. Those strains with the highest (CECT 4459) and the lowest (CECT 976) adhesion levels were selected for further studying the effects of a nutrient-rich adhesion-promoting medium (TSB plus 1% glucose-TSBG) as well as of a conditioning film consisting of dried mussel cooking juices (MCJ) on adhesion to and detachment from polypropylene surfaces. Adhesion kinetics were properly described by an empirical model in the absence of conditioning film. The maximum adhesion level was much higher in the presence of TSBG than in PBS, decreasing sharply in both cases after 10-15 h. In contrast, adhesion increased exponentially during 25 h in the presence of dried MCJ. Clear differences were thus found in different media, and it suggests that cleaning strategies should vary under different conditions. The comparison of the adhesion strengths under the different experimental conditions showed that the persistence was highest when biofilms were formed on MCJ, which indicates that cells would remain longer as a source of cross-contamination. Some biofilms were examined by electronic microscopy, and different structures were observed under the different experimental conditions. It is concluded that the study of biofilm formation by S. aureus is necessary to establish efficient control systems in the food industry.

Two mathematical models for the correction of carbohydrate and protein interference in the determination of uronic acids by the m-hydroxydiphenyl method

The most common method in the routine determination of uronic acids, the m-hydroxydiphenyl reaction, recently adapted to rapid microplate analysis, has as a main inconvenience, in any one of their modalities, interferences due to the frequent presence of proteins and neutral carbohydrates in the samples. Corresponding corrections in the literature are unsatisfactory when applied to complex matrices, and further adaptation to the microplate analysis is not free from additional problems. With particular reference to hyaluronic acid, the interactions between the principal reactants and the interfering materials are studied kinetically under realistic conditions, and simple mathematical models are proposed which satisfactorily describe the experimental results and allow adequate corrections to be made.

Survival of Lactic Acid Bacteria in Seawater: A Factorial Study

A feasibility study of lactic bacteria as potential probiotics in larval cultures of marine fish was performed by investigating the survival of five strains of lactic bacteria in seawater by readily standardized procedures at different temperatures and salinities. These conditions were chosen in such a way that their combinations define a complete first-order factorial design. Depending on the strain and the ambient conditions, the survival adhered to first-order kinetics in some cases, and to the Gompertz equation in others. The half lives (t0.5) calculated from these models were subsequently introduced as responses to the factorial designs, estimating the coefficients of empirical equations that describe the group effect of temperature and salinity on t0.5. Simply additive effects were found in two cases, a negative first-order interaction in another case, while another two required second-order models.

Apparent antifungal activity of several lactic acid bacteria against Penicillium discolor is due to acetic acid in the medium

Fifty-six dairy bacteria belonging to the genera Lactococcus, Lactobacillus, Pediococcus, Propionibacterium, Streptococcus, Enterococcus, Leuconostoc, and Brevibacterium were screened for antifungal activity against four species of fungi relevant to the cheese industry (Penicillium discolor, Penicillium commune, Penicillium roqueforti, and Aspergillus vesicolor). Most of the active strains belonged to the genus Lactobacillus, whereas Penicillium discolor was found to be the most sensitive of the four fungi investigated. Further studies on P. discolor showed antifungal activity only below pH 5. This effect of pH suggests that organic acids present in the culture could be involved in the detected activity. Determination of acid composition revealed lactic acid production for active dairy strains and the presence of acetic acid in active as well as inactive strains. It was demonstrated that the undissociated acetic acid originates from the bacterial growth medium. The synergistic effect of the acetic acid present and the lactic acid produced was likely the main factor responsible for the antifungal properties of the selected bacteria. These results could explain some discrepancies in reports of the antifungal properties of lactic acid bacteria, since the role of acetic acid has not been considered in previous studies.

Joint effect of nisin, CO2, and EDTA on the survival of Pseudomonas aeruginosa and Enterococcus faecium in a food model system

A study on the joint effect of either nisin or Nisaplin, headspace CO2 levels, and EDTA on the survival of Pseudomonas aeruginosa and Enterococcus faecium was carried out in a water-soluble fish muscle extract at 3 degrees C using a second-order rotatable factorial design. E. faecium was completely deactivated by all processing after 2 days of storage. In contrast, P. aeruginosa was much less susceptible to treatments, and cell death was satisfactorily described by two models. Nisin increased cell death, whereas Nisaplin (commercial form of nisin) was not suitable, as it caused undesirable interference, presumably due to its co-compounds. Interactions between Nisaplin or nisin and either EDTA or CO2 were found to be nonstatistically significant. Factors that could account for this unexpected lack of synergism are discussed. However, a statistically significant positive interaction was found between CO2 and EDTA. This finding could allow CO2 levels to be decreased and hence to reduce the main disadvantages of CO2 application, namely, exudation and acidification.

An empirical model for describing the effects of nitrogen sources on nisin production

AIMS

A study on the effects of tryptone and yeast extract on nisin production by Lactococcus lactis was carried out using a second order rotatable factorial design.

METHODS AND RESULTS

The results show that both ingredients increased nisin production, although a small decrease in nisin levels was noted at high tryptone concentrations. In view of the low vitamin content of tryptone, the amino acids present in both tryptone and yeast extract may be responsible for these observations.

CONCLUSIONS, SIGNIFICANCE AND IMPACT OF THE STUDY

The observed response suggests that maintaining tryptone concentrations in the centre of the domain and increasing the concentration of yeast extract would be ideal conditions for nisin production. However, when the economic aspects are considered, it appears that low concentrations of yeast extract and reasonably high concentrations of tryptone are optimal.

Dose-response relationships. A model for describing interactions, and its application to the combined effect of nisin and lactic acid on Leuconostoc mesenteroides

As well as producing bacteriocins, many lactic bacteria produce other potentially toxic compounds or growth inhibitors, especially lactic acid, which may interfere in the assays commonly used to quantify these peptides. A systematic set of modifications is proposed which, when applied to the logistical equation, enable it to describe the combined (but not additive) effects of two or more active principles. The general model thus derived is applied to the interaction of nisin and lactic acid on Leuconostoc mesenteroides.

A method for bacteriocin quantification

Different aspects of the most commonly used assay methods in the study of bacteriocins were examined. The conditions under which extraction and incubation (including exposure time) take place were analysed, and several different formal models that are usually employed to calculate ID50 were compared. As an alternative designed to overcome the problems which characterize the response of micro-organisms that are sensitive to bacteriocins, an operative procedure in a liquid medium and a modified re-parameterized logistic equation is proposed. When applied to the inhibition of Leuconostoc mesenteroides by nisin, the model allows an optimal experimental procedure to be defined.

Effect of various cryostabilizers on the production and reactivity of formaldehyde in frozen-stored minced blue whiting muscle

The production of formaldehyde in frozen-stored minced blue whiting muscle was described by a rectangular hyperbolic model, and the effectiveness of each cryostabilizer is discussed in terms of its parameters. The maltodextrins assayed noticeably inhibited formaldehyde production, this effect being greater at -20 degrees C than at -10 degrees C. Sucrose was only effective at -20 degrees C. It seems that these compounds act by restricting molecular diffusion. The effect of each cryostabilizer on formaldehyde binding was closely regulated by its effect on production. This is discussed in terms of the binding equation parameters. The binding of formaldehyde during frozen storage was dependent on protein rearrangements leading to reactive groups becoming available. The constraints of cryostabilizers on molecular diffusion reduced the exposure of these groups. Consequently, the interpretation of formaldehyde reactivity was biased, leading to conclusions different from those that would be obtained from a study done under standard conditions.