Influence of water activity on microbial development in minced,cured pork,in relation to gas headspace composition

The water activity (a_w) of a cured minced meat mixture was reduced with NaCl or glycerol + NaCl and the products stored in air, N₂ or CO₂ at 4°C. The lower the a_w, the lower was the growth rate of the total microflora except when a_w was reduced below 0·98 with glycerol and products were stored in CO₂, when the growth rate was independent of a_w. At a_w 0·94 NaCl was more effective than glycerol + NaCl in reducing the growth rate. In air the final microflora of products stored at a_w ≤0·96 consisted primarily of yeasts while at 0.98 a_w, both Lactobacillus (62%) and yeasts (38%) were found. In CO₂ and N₂Lactobacillus predominated at 0·94–0·98 a_w when 2% NaCl or NaCl + glycerol were used. At 0·94 a_w, produced using 6% NaCl, a heterogenous final microflora was found, including Micrococcus and Staphylococcus. An increase in the microbial shelf-life of meat products may be obtained by reducing the a_w, particularly for use with storage in N₂- and CO₂-atmospheres. The type of solute chosen to regulate a_w may affect the solubility of CO₂, which in turn affects the shelf-life.     

Characterization of the Effect of Solutes on the Water-Binding and Gel Strength Properties of Carrageenan

The water activity of carrageenan gels with incorporated solutes was determined by cryoscopic osmometry. Kappa (k), kappalambda (kλ) and iota (κ) carrageenan were utilized at concentrations of from 0.1 2% solids producing a sol, viscous sol, and gel. The solutes used were sucrose, NaCl, Na₂SO₄, KCl, NH₄Cl and urea at concentrations ranging from 0.05–1.0 kinetic units per kg H₂O. All the solutes were found to give a_w values as measured cryoscopically withhin 0.0005 units of literature values. The water activity of pure carrageenans at 0.1–2% solids was greater than 0.999. Addition of some solutes increased the water binding of the carrageenan-solute-water system as was found by a lower a_w than expected. This interaction effect increased with increasing concentration of solute and carrageenan and was greater for solutes with lower activity coefficients. The interaction effect increased in the following order: Na₂SO₄, NaCl, KCl, and NH₄ Cl. The force required to rupture the gel was measured using the Instron Universal Testing Machine. Solutes were found to influence the gel strength of k and kλ carrageenans, but not ι carrageenan. The solutes increased kλ carrageenan's gel strength in increasing order: urea, sucrose, Na₂SO₄, NaCl, NH₄Cl, and KCl and k carrageenan's gel strength in increasing order: sucrose, Na₂SO₄ and NaCl. There appears to be no simple relationship between gel strength and water-binding when solutes are added to carrageenan.     

Baroprotective effect of increased solute concentrations on yeast and moulds during high pressure processing

The baroprotective effect of increasing solute concentrations on yeast cells and fungal conidia subjected to high pressure processing (HPP) was studied. Suspensions of yeast cells (Saccharomyces cerevisiae, Pichia anomala and Hanseniaspora uvarum) or fungal spores (Penicillium expansum, Fusarium oxysporum and Rhizopus stolonifer) in citrate phosphate buffer formulated with sucrose at 40, 50 and 60 °Brix, or with glycerol and NaCl at equivalent water activity (a_w) values (0.925, 0.903 and 0.866 a_w) were subjected to 600 MPa pressure for varying times, and then were enumerated by spread plate technique to assess survival. There was an increasing resistance to inactivation by high pressure with an increase in solute concentration. The two moulds with easily wetted spores, R. stolonifer and F. oxysporum, showed strongest resistance to HPP at 0.866 a_w. Differing responses to the three solutes were observed among the fungal species tested, indicating that the chemical nature of the solute may also be important in protecting yeasts and moulds during and after pressure treatment. Sucrose had a stronger baroprotective effect for S. cerevisiae than the other solutes, at two of the three investigated a_ws. For P. expansum at 0.903 a_w, NaCl gave the best protective effect. Scanning electron microscopy of HPP treated cells showed the protective effects of increased sucrose concentration. The results reported here have practical implications for the food industry in the application of HPP for production of fruit preparations or syrups, and should be taken in account in establishing efficient process design.

Industrial relevance

As high concentrations of sugar, salt and glycerol provide protection for yeasts and moulds during high pressure processing, foods containing high levels of solutes may need longer processing times or higher pressures to achieve inactivation of these fungi.     

A World Survey of Water Activity of Selected Saturated Salt Solutions used as Standards at 25°C

This paper presents and discusses the results of a world survey of water activity (a_w) of selected saturated salt solutions used as standards by different researchers engaged in a_w determinations for food-related applications. Salt slurries cover the range of a_w about 0.57-0.97 at 25°C and are NaBr, NaCl (NH₄)₂SO, KCl, BaC1₂. 2H₂O, KNO₃ and K₂SO₄. The results indicate that there is a good agreement on the exact value to be assigned to NaBr, NaCl, KC1 and BaC1₂.2H₂O, and to a lesser extent, to K₂SO₄. However, a significant discrepancy exists between researchers on the a_w value assigned to (NH₄)₂SO₄ and KNO₃.     

On the selection of relevant environmental factors to predict microbial dynamics in solidified media

Several studies have shown that food structure causes slower growth rates and narrower growth boundaries of bacteria compared to laboratory media. In predictive microbiology, both a_w or corresponding solute concentration (mainly NaCl) have been used as a growth influencing factor for kinetic models or growth/no growth interface models. The majority of these models have been based on data generated in liquid broth media with NaCl as the predominant a_w influencing solute. However, in complex food systems, other a_w influencing components might be present, next to NaCl. In this study, the growth rate of Salmonella typhimurium was studied in the growth region and the growth/no growth response was tested in Tryptic Soy Broth at 20 °C at varying gelatin concentration (0, 10, 50 g L⁻¹ gelatin), pH (3.25–5.5) and water activity (a_w) (0.929–0.996). From the viewpoint of water activity, the results suggest that NaCl is the main a_w affecting compound. However, gelatin seemed to have an effect on medium a_w too. Moreover, there is also an interaction effect between NaCl and gelatin. From the microbial viewpoint, the results confirmed that the a_w decreasing effect of gelatin is less harmful to cells than the effect of Na⁺ ions. The unexpected shift of the growth/no growth interface to more severe conditions when going from a liquid medium to a medium with 10 g L⁻¹ gelatin is more pronounced when formulating the models in terms of a_w than in terms of NaCl concentrations. At 50 g L⁻¹ gelatin, the model factored with NaCl concentration shifts to milder conditions (concordant to literature results) while the model with a_w indicates a further shift to more severe conditions, which is due to the water activity lowering effect of gelatin and the interaction between gelatin and NaCl. The results suggest that solute concentration should be used instead of a_w, both for kinetic models in the growth region and for growth/no growth interface models, if the transferability of models to solid foods is to be increased.     

Microbial Growth Estimation in Liquid Media Exposed to Temperature Fluctuations

Comparisons of linear, square root and Arrhenius specific growth rate-temperature models showed no clear overall preference. At low a_w and refrigeration temperature, the linear model was satisfactory for Brochothrix thermosphacta and Pseudomonas fluorescens. A linear relationship between (specific growth rate)^?1 and lag phase was observed for these two psychrotrophs. The R² for B. thermosphacta in 3% (a_w= 0.968), 6% (a_w= 0.950), and 9% NaCl (a_w= 0.943) media was 0.865, 0.946 and 0.994, respectively. In 8% (a_w= 0.973) and 20% (a_w= 0.948) glycerol media, the R² was 0.998 and 0.999, respectively. R² for P. fluorescens growth in NaCl (3 or 4%) or glycerol (10 or 20%) media was >0.99. A cumulative growth adaptation function successfully estimated microbial growth at fluctuating temperatures.     

Mixed Saturated Salt Solutions as Standards for Water Activity Measurement in the Microbiological Growth Range

Present work explores the feasibility of using mixed saturated salt solutions to provide additional solutions of known water activity (a_w). The mixed saturated salt solutions studied were: K₂SO₄-KNO₃, K₂SO₄-MgSO₄, KCl-KNO₃, NaCl-KCl and NaCl-NaNO₃. The a_w of the ternary saturated salt mixtures was predicted using a theoretical model of the thermodynamic properties of mixed aqueous electrolyte solutions. Prediction of a_w compared very well with experimental determinations.     

Growth of Staphylococcus aureus as Related to 17O NMR Water Mobility and Water Activity

¹⁷O NMR data were determined for three solute-BHI (Brain Heart Infusion) mixtures. For each solute, NaCl, raffinose and glycerol, NMR data (linewidths and signal intensity) were determined as well as water activity (a_w), moisture contents, and effects on growth of Staphylococcus aureus in media of various moisture contents. Liquid (highly mobile) water content in all media tested was determined by NMR. This mobile water was important and had a stronger correlation with S. aureus growth rate than did a_w or moisture content, although solute effects limited empirical prediction. NMR linewidth also correlated with lag phase.     

Effect of selected Hofmeister anions on functional properties of protein isolate prepared from lablab seeds (Lablab purpureus)

Effects of selected Hofmeister anions, namely Na₂SO₄, NaCl, NaBr, NaI, NaClO₄ and NaSCN, on the functional properties of a protein isolate prepared from lablab seeds (Lablab purpureus) were investigated. The results of water absorption capacity indicated that highest water absorption was recorded in solutions of Na₂SO₄, and the lowest in NaSCN solutions. Reduction in water absorption capacity followed the Hofmeister series in the order Na₂SO₄ > NaCl > NaBr, NaI > NaClO₄ > NaSCN. Protein solutions prepared in chaotropic (NaI, NaClO₄, NaSCN) salts had better foam capacity, foam stability, emulsifying activity and emulsion stability than solutions prepared with kosmotropic salts (Na₂SO₄, NaCl, NaBr). The results also indicate that increase in foam capacity and stability followed the Hofmeister series in the order Na₂SO₄ < NaCl < NaBr, NaI < NaClO₄ < NaSCN. When least gelation concentration (LGC) was used as the index of gelation capacity, at various salts concentrations, the lowest LGC were observed in NaSCN and the highest LGC in protein solutions prepared with Na₂SO₄. Copyright © 2005 Society of Chemical Industry     

Prediction and Measurement of the Water Activity of Selected Saturated Salt Solutions at 5°C and 10°C

A theoretical approach, based on the thermodynamic properties of strong electrolyte aqueous solutions, was used to calculate the a_w at 5°C and 10°C of selected saturated salt solutions frequently used as standards in the range of microbial growth. The results agreed very well with experimental measurements at both temperatures for most salts studied, which included NaBr, NaCl, SrC1₂, (NH₄)₂SO₄, KCl, BaCl₂, and K₂SO₄. For KNO₃ the agreement was somewhat less satisfactory and the reasons are discussed.     

Vapor pressure and water activity measurements of saturated salt solutions made with D2O at 20°C

The water activity (a_w) values of 10 saturated salt solutions made with D₂O were measured in this study at 20°C: MgCl₂, K₂CO₃, Mg(NO₃)₂, KI, NaCl, KCl, ZnSO₄, BaCl₂, KNO₃, and K₂SO₄. These salts were selected because they represent a wide range of water activity values, from approximately 0.33 to 0.98. The a_w values were measured using two methods: (1) a vapor pressure manometer, and (2) an electronic chilled-mirror (AquaLab) instrument. The a_w values obtained by the manometer method were slightly, but consistently, higher than the a_w values obtained by the electronic chilled-mirror instrument. The a_w values of the saturated salt solutions made with D₂O (as measured by both methods) were also slightly, but consistently, higher than the a_w values of the same saturated salt solutions made with H₂O at 20°C. ©     

Theoretical Prediction of the Water Activity of Standard Saturated Salt Solutions at Various Temperatures

A theoretical approach was used to predict the water activity (a_w) at 15°C and 35°C of selected saturated salt solutions used as standards in the range of microbial growth (～0.57–0.97). For this purpose a rigorous thermodynamic model to predict osmotic coefficient was used. The results agreed very well with experimental measurements at 15°C and 35°C for most salts studied, which included NaBr, NaCl, (NH₄)₂SO₄, KCl, BaCl₂, and K₂SO₄. For KNO₃ the agreement at the higher temperature was somewhat less satisfactory and the reasons for this behavior are discussed.     

Influence of sporulation awon heat resistance and germination ofPenicillium roquefortispores

Spores ofPenicillium roquefortiwere produced using potato dextrose agar which was solute agjusted using either NaCl, sucrose, or glycerol to water activity (a_w) levels of: 0·99 (control), 0·97, 0.·90 and 0·88. All spore crops, regardless of the sporulation a_won which they were produced, exhibited near total germination within 24 h at 22°C in sterile water. However, when spores were initially heated either in water (42°C, 2 h or 55°C, 10 min) or in air (60°C, 20 min) and then germinated, levels were observed to decrease particularly for spores produced on 0·88 a_wadjusted media. Germination levels were also observed to decrease with time when harvested spores were initially suspended in sterile distilled water (0 to 24 h, 2°C) and then heated. Control spores (0·99 a_w) showed no effects. Heat studies of spores suspended in water were used to calculate thermal decimal reduction times (D). Control spores and spores produced at 0·88 a_wwith glycerol, exhibitedD_55°Cvalues of 10·3and 2·3 min, respectively. Specific effects attributable to the nature of the solute appeared to influenceDvalues; however, no clear trend was observed. The results of this study indicated that spores that were produced at reduced a_wbecame heat-sensitized perhaps owing to injury and/or damage incurred during sporogenesis.     

Predicting Water Activity in Solutions of Mixed Solutes

The Ross equation is a multiplicative procedure for estimating the water activity (a_w) of aqueous unprocessed food preparations that contain only solutes, only food polymers, or mixtures of solutes and polymers. Using accurate a_w values of single solute solutions in the Ross equation resulted in prediction accuracies to ± 0.01 a_w for 118 of 120 multiple-solute solutions. The exceptions at ±0.02 a_w occurred within a range of concentrated solutions containing NaCl, KCl, and CaCl₂.     

Sorption Behavior of Mechanically Mixed and Freeze-Dried NaCl/Casein Mixtures

Adsorption isotherms of mechanically mixed and freeze-dried mixtures of NaCl/casein at solute percentages of 1, 5, 10, 15, and 20 (wet basis) were obtained over the a_w range 0.23–0.927 at 20°C. Experimental isotherm values were compared to those calculated by a mass balance equation. The mechanically mixed samples sorbed additively as predicted by the mass balance equation below 0.755 a_w, whereas the freeze-dried mixtures sorbed more water than predicted (positive interaction) below 0.755 a_w, Both types of mixtures sorbed less water than predicted by the mass balance equation (negative interaction) above the saturation a_w of the NaCl, 0.755.     

Effect of Temperature, Solute and pH on the Tolerance of Clostridium perfringens to Reduced Water Activities

Two strains of C. perfringens type A (FDI and S45) were grown in Thioglycolate medium adjusted to a_ws of 0.995, 0.975, or 0.965 by the addition of NaCl, KCL or LiCl. Combinations of controlled and uncontrolled pHs (7.0, 6.5 and 6.0) and incubation temperatures of 45°C, 37°C, or 30°C were observed at each a_w. Maximal numbers of cells and shortest lag times occurred at a_w of 0.995, 45°C and pH 7.0. No growth occurred at a_w of 0.965. At 0.985, growth did not occur when KCl was used as the solute and the temperature and pH were 30°C and 6.0, respectively. NaCl was less inhibiting than KCl. LiCl inhibited growth completely.     

Stimulating effect of sorbitol and xylitol on germination and growth of some xerophilic fungi

Sorbitol and xylitol are polyols often used in foods as naturally occurring sugar substitutes. They provide sweet taste and reduced calories in products of intermediate moisture. This type of food is susceptible to spoilage by xerophilic molds which affect shelf life of foods and produce significant losses. The aim of the present work was to study the effect of glycerol, sorbitol and xylitol on the germination and growth of four xerophilic fungi at different temperatures and water activity levels. Penicillium chrysogenum, Wallemia sebi, Eurotium chevalieri and Eurotium repens were cultivated on malt extract agar with the addition of the respective polyols and a_w adjusted to 0.85, 0.88, 0.90 and 0.93. Incubation was made at 25, 30 and 35 °C. Results of the present study demonstrated that sorbitol and xylitol affect the growth kinetics of the four fungal species. The observed tendency was that these solutes shortened the germination times and increased the growth rates. The effect of each solute depended on the fungal species and the a_w/temperature combinations. At lower a_w the influence was more evident on the germination times while the effect on growth rates was more pronounced at higher a_w levels.     

Solute Infusion Effects on Texture of Minimally Processed Kiwifruit

Samples of kiwifruit infused with glucose, glycerol or sucrose solutions at atmospheric pressure to a_w=0.97 showed smaller failure forces, under compression, than fresh fruit. Calcium lactate in the solution increased failure forces 30–60% depending on the humectant used. Glucose infusion under vacuum to the same a_w produced failure forces >300% greater than those observed after atmospheric infusion. Solute infusion reduced greatly the elastic component of the rheological behavior. Microscopic observations showed atmospheric solute infusion caused contraction of cellular membranes, degradation of cell walls and intercell contact decrease. Calcium helped maintain cell cohesion. Vacuum infused tissues showed cell wall optical density similar to fresh cells.     

Interaction of Solutes with Raw Starch during Desorption as shown by Water Retention

The objective was to study interaction of salt and sucrose with raw starch as determined from water retention at a given a_w. NaCI-starch mixtures were 5:95, 10:90, 15:85, and 25:75. Sucrose-starch mixtures were 1:9, 2:8, 1:1, and 9:1. Equilibration was against salt slushes over the water activity range 0.33 to 0.97. At each a_w, as the solute concentration increased above zero, the amount of interacted solute rose to a maximum and then, in contrast to previous findings with protein, declined to zero. For each solute, saturation a_w showed the highest amount of interaction at any solute concentration; the interaction decreased with increasing a_w. At maximum, 87.2% of total NaCl and 56.3% of the total sucrose interacted with starch. These desorption data showed less interaction than previously found for absorption.     

Effect of solute and matric potential on growth rate of fungal species isolated from cheese

The effect of water potential (Ψ_w) on the growth of 15 fungal species isolated from cheeses was analysed. The species, identified mainly by analysis of DNA sequences, belonged to genera Penicillium, Geotrichum, Mucor, Aspergillus, Microascus and Talaromyces. Particularly, the effect of matric potential (Ψ_m), and ionic (NaCl) and non-ionic (glycerol) solute potentials (Ψ_s) on growth rate was studied. The response of strains was highly dependent on the type of Ψ_w. For Ψ_s, clear profiles for optimal, permissive and marginal conditions for growth were obtained, and differences in growth rate were achieved comparing NaCl and glycerol for most of the species. Conversely, a sustained growth was obtained for Ψ_m in all the strains, with the exception of Aspergillus pseudoglaucus, whose growth increased proportionally to the level of water stress. Our results might help to understand the impact of environmental factors on the ecophysiology and dynamics of fungal populations associated to cheeses.