Synergistic effect of Sodium Chloride,temperature and pH on growth of a cocktail of spoilage yeasts: a research note

The effect of sodium chloride (0·5–10% w/v), pH (2·6–6·3) and temperature (1–22°C) were studied on the growth of a cocktail of food spoilage yeasts. The length of the lag phase and the time taken to reach the level of 10⁶cfu ml⁻¹were calculated for each set of conditions. It was found that the lag phase constituted as little as 21% of the total time to reach 10⁶cfu ml⁻¹when the yeasts were grown in favourable conditions and as much as 62% of the total time when more extreme conditions were used. It was concluded that the lag phase was the most important factor affecting the spoilage potential of chilled foods with low pH and high salt values. The single most effective factor in reducing the growth rate of yeasts was temperature. The lag phase was 15, 38, 270, 630 and 875 h when the temperature was 15, 8, 4, 2 and 1°C respectively. At any single temperature, there appeared to be a synergistic effect of NaCl and pH and under the most extreme conditions tested (1°C; pH 5·8; 6% NaCl), the lag phase was over 1000 h. These data have implications for the spoilage potential of high salt, reduced pH foods stored at chill temperatures.     

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.     

Tuning the properties of β-lactoglobulin nanofibrils with pH,NaCl and CaCl2

We investigated the effects of pH (1.6–2.4), NaCl and CaCl₂ (0–100 mm) on the kinetics of β-lactoglobulin fibril formation during heating at 80 °C. The morphology of fibrils was also examined. At pH 1.8–2.4 fibril formation occurred slightly faster with decreasing pH. At pH 1.6 fibril formation during the growth phase occurred much faster than at any other pH. Fibril morphology was unchanged between pH 1.6 and pH 2.0. Addition of NaCl or CaCl₂ accelerated fibril formation during the growth phase, and CaCl₂ shortened the lag phase as well. Worm-like fibrils were seen at ≥60 mm NaCl or ≥33 mm CaCl₂, and these had a persistence length which was much shorter than the long semi-flexible fibrils formed without salts. The efficiency of fibril formation can be substantially enhanced by varying pH and salt concentration.     

Modelling the survival of Enterobacter cloacae in a model olive cover brine solution

The main goal of this research was the evaluation of the survival of Enterobacter cloacae in a model olive brine. Two different assays were run; the first experiment assessed the viability of the target in brines containing NaCl (6–12%) and p‐coumaric acid (0.0–0.05%), adjusted to different pHs (4–10) and stored at 10–30 °C for 9 days. The death rate and cell levels at selected times were modelled with a polynomial equation to highlight the individual and interactive effects of NaCl/p‐coumaric acid/pH/temperature. Then, a second experiment was run for 3 months (temperature, 10 °C; pH, 4.5–5.5; NaCl, 6–8%). The survival of E. cloacae was affected mainly by pH, then by salt and temperature; however, the significance of the variables changed within the time, as salt and temperature acted in a significant way only after 1 day.     

Growth of Listeria monocytogenes as a function of dynamic environment at 10°C and accuracy of growth predictions with available models

A combination of a factorial design and two central composite designs was used to assess quantitatively the effects and interactions of water activity (1–0·95) and pH (5·6–9·5) variations on the growth of Listeria monocytogenes in a meat broth at 10°C. At inoculation or at the beginning of the exponential phase, the cells were exposed to the addition of NaCl and acetic acid or NaCl and NaOH. The effects of abrupt fluctuating conditions on the generation and lag times were analysed using turbidity measurements. The data indicated that the cells exposed to osmotic and acid or alkaline variable conditions from the time of inoculation were less affected than cells exposed at the beginning of the exponential phase. In this last case, a lag phase could be induced and the growth recovery was different from those expected in the new environment. Generation time values were estimated by three available predictive models which describe the effects of temperature, salt concentration and pH on L. monocytogenes growth to highlight the potential problems of variable conditions.     

Mechanical Properties of Ovalbumin Gels Formed at Different Conditions of Concentration,Ionic Strength,pH, and Aging Time

Ovalbumin gels were prepared by heat treatment at constant pH and ionic forces. Ovalbumins are widely utilized as emulsifying or binding agents. However, due to their protein origin, mechanical properties of ovalbumins are enclosed in a wide range of rheological responses depending on concentration, ionic strength, pH, and aging time. The objective of this work was to study the effect of processing conditions (pH, ionic strength, and protein content) on the textural attributes of an ovalbumin protein system by means of uniaxial compression. Gels were prepared by dispersing proteins (purity 98%) (8.3–12.5% w/w) until complete dissolution in deionized water at 90°C by 45 min, pH (6.3–9.1) was adjusted using citric acid, and the ionic strength (0–100 mM of NaCl) was adjusted using NaCl. The storage of gels was done at 63°C (24–168 h). The rheological tests of gels were done by uniaxial compression. A rupture force peak was observed at high protein content together with an increase in the Young’s modulus. At fixed conditions of ion content (NaCl 50 mM) and pH of 7, the gels presented a maximum in fracture force and Young's modulus after 7 days of storage. The addition of minimum amount of citric acid increases the stability of ovalbumin gels. This information is useful to ensure that the final product will remain stable during storage time at longer shelf lives.     

Influence of Temperature, NaCI, and pH on the Growth of Aeromonas Hydrophila

Growth of clinical isolates of Aeromonas hydrophila at various temperatures, pH values, and salt levels was studied in BHI broth. A majority of the isolates grew at 4-5°C and 42°C, and all grew equally well over the range 20-35°C. At 28°C, most isolates could tolerate 4% NaCl, while at 4°C only a limited number grew in 3% NaCl. Similarly, isolates could better tolerate acidic conditions when cultured at 28°C as compared to 4°C. These data suggest that it is likely that A. hydrophila strains associated with human gastroenteritis are capable of growing in foods at refrigeration temperatures currently considered adequate for preventing the growth of foodborne pathogens.     

Purification and biochemical properties of pepsins from the stomach of skipjack tuna (Katsuwonus pelamis)

Pepsins 1 and 2 from the stomach of skipjack tuna (Katsuwonus pelamis) were purified to homogeneity by using a series of chromatographic purification involving DEAE-cellulose, Sephadex G-50 and Sephadex G-75 with increase in purity of 246-fold and 213-fold, respectively. Molecular weights of pepsins 1 and 2 were estimated by SDS–PAGE to be 33.9 and 33.7 kDa, respectively. The N-terminal amino acid sequences of the first 20 amino acids of both isoenzymes were YQDGTEPMTNDADLSYYGVI. The optimal pH and temperature for pepsin 1 were 2.5 and 50 °C, respectively, while pepsin 2 showed optimal activity at pH 2.0 and 45 °C. The activity of two pepsins was stable in the pH range of 2–5 and at temperatures up to 50 °C. The activity of purified pepsins was strongly inhibited by pepstatin A in a dose-dependent manner. SDS and cysteine showed inhibitory effects toward both pepsins. Activity of pepsin 2 was slightly activated by NaCl, but NaCl had no effect on pepsin 1. Pepsins 1 and 2 had high affinity and hydrolytic activity toward hemoglobin with K _m of 54 and 71 μM, respectively. k _cat of pepsins 1 and 2 were 38.1 and 44.3 s⁻¹, respectively. Both pepsins effectively hydrolyzed bovine serum albumin, egg white, natural actomyosin from brownstripe red snapper muscle and acid-solubilized collagen from arabesque greenling skin. Nevertheless, the hydrolytic activity was slightly less than that of pepsin from porcine stomach.     

Behaviour of Listeria monocytogenes in Lighvan cheese following artificial contamination during making,ripening and storage in different conditions

This study investigated the behaviour and fate of Listeria monocytogenes at different ripening temperatures and NaCl concentrations in traditional Lighvan cheese. L. monocytogenes was added to raw sheep's milk. After producing the cheese, they were stored in 8%, 12% and 15% NaCl at 4, 9 and 14 °C. Sampling was performed for 150 days. Different temperature and NaCl concentrations had a significant effect on the survival of L. monocytogenes (P < 0.001). The lowest growth and survival rates of L. monocytogenes were in 15% NaCl at 14 °C and 12% NaCl at 14 °C, respectively. Also, the highest growth and survival rates of the bacterium were in 8% NaCl at 4 °C.     

Changes of glycinin conformation due to pH, heat and salt determined by differential scanning calorimetry and circular dichroism

Conformational changes of glycinin caused by either pH, heat or salt were investigated using differential scanning calorimetry (DSC) and circular dichroism (CD). Glycinin of more than 97% purity was used for the experiments. Without the presence of salt (NaCl), glycinin (protein concentration of 16%) was most stable at pH 4.5; the temperature for midpoint of transition (T_m) and enthalpy change (ΔH) were 93.5 °C and 18.1 J g^?1 protein, respectively. Both parameters decreased gradually as the pH of the protein solution was increased to 9.5. The endothermic peak of glycinin disappeared when the pH was >11.5 or <3.0. DSC results showed that with increasing of salt concentration, glycinin was substantially stabilized even in acidic (pH 3.0) and alkaline (pH 11.5) conditions; the degree of stabilization was more significantly affected at the acidic pH. The CD profiles of soy glycinin were, however, not significantly altered even in acidic and alkaline conditions. Thermal denaturation was suppressed by NaCl even in 90 °C, pH 8.0, when the salt concentration was 0.45 m . The stabilizing effects of NaCl coincided with an increase in the percentage of α‐helix in glycinin. Therefore, soy glycinin appeared to have a well‐ordered structure under the pH conditions studied, especially in the presence of NaCl. The λ_min signal at 222 nm remained unchanged up to 80 °C, but the negative intensity increased substantially when the temperature was >80 °C.     

Effect of Salt Reduction on Growth of Listeria monocytogenes in Meat and Poultry Systems

Abstract: Reducing sodium in food could have an effect on food safety. The objective was to determine differences in growth of Listeria monocytogenes in meat and poultry systems with salt substitutes. For phase 1, fresh ground beef, pork, and turkey with NaCl, KCl, CaCl₂, MgCl₂, sea salt, or replacement salt added at 2.0% were inoculated with L. monocytogenes to determine growth/survival during 5 d at 4 °C to simulate a pre‐blend process. L. monocytogenes populations significantly decreased (0.41 log CFU/g) during the storage time in beef, but no differences (P > 0.05) were observed over time in pork or turkey. Salt type did not affect (P > 0.05) L. monocytogenes populations during pre‐blend storage. MgCl₂ and NaCl allowed significant growth of aerobic populations during storage. For phase 2, emulsified beef and pork products were processed with 2% NaCl, KCl, sea salt, or a NaCl/KCl blend and post‐process surface‐inoculated with L. monocytogenes to determine growth/survival at 4 °C for 28 d. Pork products showed significantly greater L. monocytogenes population growth at all sampling times (0, 7, 14, 21, and 28 d) than beef products, but salt type had no effect on L. monocytogenes populations with sampling times pooled for data analysis. Although salt types had no impact on L. monocytogenes populations in preblend and emulsified meat products, pork and turkey preblends and emulsified pork had greater L. monocytogenes populations compared with beef products. These studies demonstrate that sodium may not affect the safety of preblends and emulsified meat and poultry products. Practical Application: odium reduction in food is an important topic because of sodium's unfavorable health effects. This research shows that reducing sodium in pre‐blends and emulsified meat and poultry products would have no effect on Listeria monocytogenes populations, but replacement of NaCl with MgCl₂ may affect growth of aerobic populations.     

Effects of encapsulation on the viability of probiotic strains exposed to lethal conditions

The effect of microencapsulation on the viability of Lactobacillus casei, L. paracasei, L. acidophilus Ki and Bifidobacterium animalis BB‐12 during exposure to lethal conditions (25% NaCl, pH 3.0 and 55–60 °C) was evaluated. Results demonstrated that survival of probiotic strains to the imposed lethal stress conditions was strain dependent. With the exception of exposure to 25% (w/v) NaCl, L. acidophilus Ki (free and encapsulated cells) demonstrated the highest survival rates through exposure to lethal conditions of temperature and pH. For this probiotic strain exposed to heat, microencapsulated cells expressed a higher heat tolerance at 55 °C than free cells. For the other tested bacteria, in general, encapsulation had no positive effect on survival through the tested lethal conditions.     

Effect of pre-incubation conditions on growth and survival of Staphylococcus aureus in sliced cooked chicken breast

In this work, the effect of pre-incubation conditions (temperature: 10, 15, 37 °C; pH 5.5, 6.5 and water activity, a_w: 0.997, 0.960) was evaluated on the subsequent growth, survival and enterotoxin production (SE) of Staphylococcus aureus in cooked chicken breast incubated at 10 and 20 °C. Results showed the ability of S. aureus to survive at 10 °C when pre-incubated at low a_w (0.960) what could constitute a food risk if osmotic stressed cells of S. aureus which form biofilms survive on dried surfaces, and they are transferred to cooked meat products by cross-contamination. Regarding growth at 20 °C, cells pre-incubated at pH 5.5 and a_w 0.960 had a longer lag phase and a slower maximum growth rate. On the contrary, it was highlighted that pre-incubation at optimal conditions (37 °C/pH 6.5/a_w 0.997) produced a better adaptation and a faster growth in meat products what would lead to a higher SE production. These findings can support the adoption of management strategies and preventive measures in food industries leading to avoid growth and SE production in meat products.     

Growth of Aeromonas hydrophila K144 as Affected by Organic Acids

The influence of different acids on the aerobic growth kinetics of Aeromonas hydrophila was studied in BHI broth with 0.5 and 2.0% NaCl incubated at 5 and 19°C. Growth curve data were analyzed by the Gompertz equation and a nonlinear regression program; generation and lag times were calculated from the Gompertz parameters. Type of acid, pH, NaCl level and temperature influenced lag and generation times. The organic acids (acetic, lactic, citric and tartaric) inhibited growth at higher pH values than inorganic acids (HCl and H₂SO₄). The high NaCl level interacted with type of acid and pH to restrict growth of the organism at the lower temperature of incubation. Acetic and lactic acids were effective in controlling the growth of A. hydrophila and could readily be combined with low holding temperature to render foods free of the organism.     

Mass Transfer Modelling During Osmotic Dehydration of Jumbo Squid (Dosidicus gigas): Influence of Temperature on Diffusion Coefficients and Kinetic Parameters

Mathematical modelling was used to study the effect of process temperature on moisture and salt mass transfer during osmotic dehydration (OD) of jumbo squid with 6% (w v ⁻¹) NaCl at 75, 85 and 95 °C. The diffusion coefficients for moisture and salt increased with temperature. Based on an Arrhenius-type equation, activation energy values of 62.45 kJ mol⁻¹ and 52.14 kJ mol⁻¹ for moisture and salt, respectively, were estimated. Simulations of mass transfer for both components were performed according to Newton, Henderson and Pabis, Page, Weibull and logarithmic mathematical expressions. The influence of drying temperature on the kinetic parameters was also studied. Based on statistical tests, the Weibull and logarithmic models were the most suitable to describe the mass transfer phenomena during OD of jumbo squid.     

Quantitative assessment of the shelf life of ozonated apple juice

Sterile apple juice inoculated with S. cerevisiae ATCC 9763 (10³ CFU/mL) was processed in a bubble column with gaseous ozone of flow rate of 0.12 L/min and concentration of 33–40 μg/mL for 8 min. The growth kinetics of S. cerevisiae as an indicator of juice spoilage was monitored at 4, 8, 12 and 16 °C for up to 30 days. The kinetics was quantitatively described by the primary model of Baranyi and Roberts, and the maximum specific growth rate was further modeled as a function of temperature by the Ratkowsky type model. The developed model was successfully validated for the microbial growth of control and ozonated samples during dynamic storage temperature of periodic changes from 4 to 16 °C. Two more characteristic parameters were also evaluated, the time of spoilage of the product under static temperature conditions and the temperature quotient, Q ₁₀. At lower static storage temperature (4 °C), no spoilage occurred either for unprocessed or ozone-processed apple juice. In the case of ozone-processed apple juice, the shelf life was increased when compared with the controls, and the Q ₁₀ was found to be 7.17, which appear much higher than that of the controls, indicating the effectiveness of ozonation for the extension of shelf life of apple juice.     

Effect of vanillin concentration,pH and incubation temperature onAspergillus flavus,Aspergillus niger,Aspergillus ochraceusandAspergillus parasiticusgrowth

The effects of incubation temperature (10–30°C), pH (3.0–4.0) and vanillin concentration (350–1200ppm) on the growth ofAspergillus flavus, Aspergillus niger, Aspergillus ochraceusandAspergillus parasiticuswere evaluated using potato–dextrose agar adjusted to water activity (a_w) 0.98. The radial growth rates after a lag period followed zero-order kinetics with constants that varied from 0 (no growth) to 0.63mmh⁻¹. The lag period depended on vanillin concentration, pH and incubation temperature. The germination time and the radial growth rates were significantly affected by the three studied variables (P<0.001). The inhibitory conditions (no growth after 30 days) depend on the type of mold. A niger, the most resistant species, was inhibited at 15°C, pH 3.0 and 1000ppm. ForA. ochraceus, the most sensitive, the inhibitory conditions in presence of 500ppm vanillin were pH 3.0 and temperature ≤25°C or pH 4.0 with temperature ≤15°C.     

Effect of processing conditions on the structure of monostearin–oil–water gels

The effect of processing conditions on the formation and stability of a gel comprised of oil, water, monostearin and stearic acid was studied. Processing conditions (homogenization rate (10,000–30,000 rpm), cooling rate (0.05–20 °C/min), homogenization temperature (70–95 °C)) had no effect on the initial storage modulus and melting temperature of the gel. Salt (NaCl) addition to the aqueous phase in the range 0–0.25% did not affect these parameters either; however, 0.5% salt addition lead to a ∼2 °C decrease in melting temperature and a doubling of the storage modulus. Optimal preparation conditions for the gel were mainly determined from its microstructure by polarized light microscopy – a smaller globule size and a more homogenous size distribution were considered a desirable feature. Optimum conditions included homogenization rates greater than 20,000 rpm, cooling rates above 6 °C/min, homogenization temperatures above, but close to, the Krafft temperature of monostearin (72 °C), and 0% salt concentration. The gelation temperature decreased by 2 °C with each 1 °C/min increase in cooling rate, from 1 °C/min to 5 °C/min.     

Use of Poultry Byproduct for Production of Keratinolytic Enzymes

The production of keratinolytic enzymes by Chryseobacterium sp. isolated from the poultry industry was tested on different growth substrates: casein, peptone, yeast extract, gelatin, soybean meal, fish meal, feather meal, raw feathers, and cheese whey. Raw feather, an important byproduct from the poultry industry, was the selected growth substrate to test the effect of three variables (temperature, initial pH, and feather concentration) on keratinase production by response surface methodology. A 2³ central composite design was performed with the central point chosen as: temperature 30 °C, initial pH 8.0, and feather concentration 20 g l⁻¹. Statistical analysis of results showed that, in the range studied, temperature had a strong effect on keratinase production. The interaction between temperature and feather concentration and between temperature and initial pH had a significant effect on enzyme production. Response surface data showed maximum keratinase production at 23 °C, initial pH 9.0, and 30 g l⁻¹ of raw feathers. Under these conditions, the model predicted a keratinase activity of 1,559 U ml⁻¹.     

Presence and growth prediction of Staphylococcus spp. and Staphylococcus aureus in Minas Frescal cheese,a soft fresh cheese produced in Brazil

Physical-chemical characteristics of Minas Frescal cheese (MFC) favor the growth of Staphylococcus spp. and allow the production of enterotoxins by specific strains. Here, we aimed to characterize the physical-chemical aspects (pH, storage temperature, and salt content) and the presence of Staphylococcus spp. in MFC samples (n = 50) to support a modeling study for the growth by this microorganism. Coagulase-positive staphylococci isolates were obtained and subjected to PCR assays to identify them as Staphylococcus aureus (nuc) and to detect staphylococcal enterotoxin-related genes (sea, seb, sec, sed, see). Staphylococcus aureus growth kinetics (maximum growth rate, Grmax, and lag time) were predicted based on ComBase model and MFC physical-chemical aspects. Mean counts of Staphylococcus spp. ranged from 3.3 to 6.7 log cfu/g, indicating poor hygiene practices during production. Selected isolates (n = 10) were identified as S. aureus, but none presented classical enterotoxin-related genes. pH, temperature, and salt content ranged from 5.80 to 6.62, 5°C to 12°C, and 0.85% to 1.70%, respectively. The Grmax values ranged from 0.012 to 0.419 log cfu/g per h. Independent of the storage temperature, the lowest Grmax values (0.012 to 0.372 log cfu/h) were obtained at pH 5.80 associated with salt content of 1.7%; independent of the pH and salt content, the best temperature to avoid staphylococcal growth was 7.5°C. Hygienic conditions during MFC production must be adopted to avoid staphylococcal contamination, and storage at temperatures lower than 7.5°C can prevent staphylococcal growth and the potential production of enterotoxins.