Desorption isotherms of salted minced pork using K-lactate as a substitute for NaCl

The aim of this study was to obtain and compare water desorption isotherms of ground meat containing NaCl (0.107 kg NaCl/kg raw-meat dry matter) and/or K-lactate as NaCl substitute at two different levels of molar substitution (30% and 100%). A thin layer of salted ground meat was dried and sampled at pre-determined times. The moisture content of the samples and their water activities (a_w) were measured at 5 °C and 25 °C. Results showed that ground meat with NaCl and/or different K-lactate contents had a similar water desorption isotherm for a_w ranging from 0.7 to 1.0. Below 0.7, the water equilibrium content fell with small decreases in a_w faster for meat with NaCl than for meat with K-lactate. K-lactate could reduce the excessive hardening at the surface of salted meat products. Experimental desorption isotherms were compared to those estimated using two approaches of the Ross equation. Models provided a good fit for the experimental data.     

Moisture sorption isotherms of high pressure treated fruit peels used as dietary fiber sources

High hydrostatic pressure (HHP) treatments can improve the potential of orange, mango, and prickly pear peels as food formulation fiber sources. Akaike Information Criteria differences identified Peleg and GAB as the best model alternatives to describe experimental moisture isotherms. HHP (600 MPa/10 min/22 and 55 °C) effects on moisture isotherms expressed as relative water sorption content change with respect to controls (RWSC_aw) showed that in the 0.1–0.93 a_w range, HHP improved the adsorption water retention of orange peels. The same was true for the desorption water retention for all HHP-treated fruit peels except for prickly pear HHP-treated at 22 °C and > 0.35 a_w. The area under the hysteresis curve (A_H) in the 0.15–0.51 a_w range showed that HHP increased hysteresis for all fruit peels tested. All this illustrates the HHP potential to modify the hygroscopic properties of fruit peels at lower temperature and in less processing time than conventional processes.Industrial relevanceOrange, mango, and prickly pear peels are potential food fiber formulation sources with differentiated hygroscopic and functional properties. In this study, 600 MPa treatments at 22 and 55 °C for 10 min modified the adsorption and desorption moisture retention capacity of all fruit peels tested in this study. HHP technology can improve the potential of fruit peels as dietary fiber sources with the advantage of shorter processing times and lower temperatures than conventional technologies used to treat food fibers.     

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.     

Water activity change at elevated temperatures and thermal resistance of Salmonella in all purpose wheat flour and peanut butter

Water activity (a_w) is a major factor affecting pathogen heat resistance in low-moisture foods. However, there is a lack of data for a_w at elevated temperatures that occur during actual thermal processing conditions, and its influence on thermal tolerance of pathogens. The objective of this study was to gain an in-depth understanding of the relationship between temperature-induced changes in a_w and thermal resistance of Salmonella in all purpose flour and peanut butter at elevated temperatures (80 ^oC). Equilibrium water sorption isotherms (water content vs. water activity) for all purpose wheat flour and peanut butter over the range of 20 to 80 °C were generated using a vapor sorption analyzer and a newly developed thermal cell. The thermal resistance (D₈₀-values) of Salmonella in all purpose wheat flour and peanut butter with initial a_w of 0.45 (measured at room temperature, ~ 20 °C) was determined via isothermal treatment of small (< 1 g) samples. When increasing sample temperature from 20 to 80 °C in sealed cells, the a_w of all purpose flour increased from 0.45 to 0.80, but the a_w of peanut butter decreased from 0.45 to 0.04. The corresponding estimated D₈₀-values of Salmonella in all purpose flour and peanut butter with 20 ^oC a_w of 0.45 were 6.9 ± 0.7 min and 17.0 ± 0.9 min, respectively. The significantly (P < 0.05) higher D₈₀-value of Salmonella in peanut butter than in all purpose flour may be partially attributed to the reduced a_w in peanut butter in comparison to the increased a_w in all purpose flour at 80 °C. The improved understanding of temperature-induced changes in a_w of low-moisture products of different composition provides a new insight into seemly unpredictable results, when using heat treatments to control Salmonella in such food systems.     

Thermodynamic analysis of the sorption isotherms of pure and blended carbohydrate polymers

The adsorption isotherms of gum Arabic (GA), mesquite gum (MG), and maltodextrin DE 10 (MD), and a blend of the three gums (17%GA–66%MG–17%MD) were determined at 25, 35, and 40 °C. All isotherms were fitted using the GAB model and the thermodynamic properties (enthalpies and entropies, differential and integral) were estimated by the Clausius–Clapeyron method. The minimum integral entropy was considered as the point of maximum stability where strong bonds between the adsorbate and adsorbent occurred, and water is less available and likely to participate in spoilage reactions. The point of maximum stability was found between 12.24 and 14.68 kg H₂O/100 kg d.s. (corresponding to water activity, a_w, of 0.32–0.57) for GA, 12.12–14.27 kg H₂O/100 kg d.s. (a_w = 0.33–0.55) for MG, and 11.37–13.84 kg H₂O/100 kg d.s. (a_w = 0.28–0.55) for the biopolymer blend, in the temperature range studied.     

Development of a device and method for the time-course estimation of low water fluxes and mean surface water activity of food products during ripening and storage

Accurate measurement of water activity (a_w) is an important goal for the food industry because a_w is a key parameter in microbial growth, biological reaction rates and physical properties. An experimental device was setup using air-product water balance to non-destructively estimate the time-course of mean a_w at the food product surface under well-controlled airflow conditions. The device is especially suited for studying the ripening of cheeses and fermented meat products, where water fluxes exchanged between products and air are very low. The validation tests performed with a_w-known model products showed that water fluxes of 10^?7 kg s^?1 can be estimated with an accuracy better than 2% over very short periods of time, and that surface a_w can be estimated with an absolute uncertainty of less than 0.01 a_w units. A handful of cheese ripening trials illustrate the potential of the method, highlighting the effects of a low air velocity and high air RH on the water exchanges occurring at a cheese surface, thus demonstrating the strong surface sensitivity to external air conditions.     

Growth of Listeria Monocytogenes in Traditional Austrian Meat Jelly Products

Individual components of a traditional meat jelly (cooked meat chunks, gelatin and preboiled vegetable) with differences in pH and a_w can constitute a niche for the multiplication of Listeria. Listeria monocytogenes counts remained stable in jelly over 21 days at 2 and 8 °C, whereas in meat and vegetables, a >1 log₁₀ unit increase was observed after 7 days at 2 °C (or >5 log₁₀ at 8 °C). In the composed product, Listeria numbers remained stable at 2 °C (21 days), but increased more than 1 log₁₀ during 7 days at 8 °C. Improving safety of jellied meat by lowering pH is discussed.     

Modeling the effect of natamycin,pine-resin and environmental factors on the growth and OTA production by Aspergillus carbonarius using response surface methodology

The effect of two antifungal compounds (natamycin, pine-resin), temperature and water activity, on the growth rate, lag phase duration and Ochratoxin A (OTA) production by three Aspergillus carbonarius isolates (Ac-28, Ac-29, and Ac-33), was studied by means of Response Surface Methodology (RSM) based on a Central Composite Design (CCD). Two different experimental designs were performed as a function of temperature (16.6–33.4 °C), water activity (0.90–0.97 a_w), natamycin (0–1000 ng ml^− 1) or pine-resin (0–2.61%, w/v) on a Synthetic Grape-juice Medium (SGM). OTA production was analyzed after 5, 10 and 15 days of incubation. A second-order polynomial model was fitted to each response parameter to assess the growth and OTA potential of all fungal isolates. Results showed that natamycin, a_w and temperature had significant effects on the lag phase duration of all isolates, as well as on OTA accumulation after 10 days of incubation for Ac-29 and 15 days for Ac-28 and Ac-33 isolates. The same results were obtained for OTA production after treatment with pine-resin. However, fungal growth rates were not statistically significant in both experiments, with the exception of Ac-29 and Ac-33 after treatment with pine-resin. Overall, high natamycin concentrations (800 and 1000 ng ml^− 1) delayed fungal growth depending on the environmental factors assayed. Moreover, treatment with pine-resin at 16.6 °C/0.94 a_w/1.1% w/v, as well as at 25 °C/0.90 a_w/1.1% w/v, completely inhibited fungal growth up to 15 days of incubation.     

Effect of water activity on the crispiness of a biscuit (Crackerbread): Mechanical and acoustic evaluation

The objective of this study was to use mechanical and acoustic methods developed in our laboratory to monitor the changes in crispiness of a Crackerbread biscuit following equilibration at different water activities (a_w). Crackerbread samples were equilibrated at a_w of 0.113–0.98. The equilibrated Crackerbread samples were punctured with a cylindrical probe (5 mm) using an Instron Universal Testing Machine. The force and acoustic signals were simultaneously captured using Labview software. Changes in parameters associated with crispiness were modelled using Fermi’s equation. The number of sound peaks, force and sound curve lengths and area under sound amplitude–time curve decreased with increasing a_w. The sound frequency decreased linearly (R² = 0.926) with increasing a_w. The critical water activity (a_wc) of Crackerbread ranged between 0.51 and 0.59. Acoustic parameters were more sensitive to a_w-induced changes in Crackerbread occurring at a_wc than mechanical parameters.     

X-ray diffraction analysis of lactose crystallization in freeze-dried lactose–whey protein systems

Water sorption, time-dependent crystallization and XRD patterns of lactose and lactose–WPI mixtures were studied with glass transition data. The results indicated that the sorbed water of lactose–WPI mixtures was fractional and water content of individual amorphous components in lactose–WPI mixtures at each a_w from 25 °C to 45 °C could be calculated. Crystallization occurred in pure lactose whereas partial crystallization was typical of lactose–WPI mixtures (protein content ≤ 50%) at intermediate and high a_w (> 0.44 a_w) from 25 °C to 45 °C. The extents of crystallization were significantly delayed by WPI. The T_g values of lactose–WPI systems showed the composition-dependent property in systems and might indicate the occurrence of phase separation phenomena during 240 h storage. XRD showed no anhydrous β-lactose and mixed α-/β-lactose with molar ratios of 4:1 crystals in crystallized lactose–WPI systems (70:30 and 50:50 solids ratios). Reduced crystallization in the presence of WPI was more pronounced possibly because of reduced nucleation and diffusion during crystal-growth. The present study showed that WPI could present an important role in preventing sugar crystallization.     

Edible methylcellulose-based films containing fructo-oligosaccharides as vehicles for lactic acid bacteria

The goal of this work was to investigate the physicochemical properties of methylcellulose (MC) based films as stabilizers of two strains of lactobacilli: Lactobacillus delbrueckii subsp. bulgaricus CIDCA 333 and Lactobacillus plantarum CIDCA 83114. The incorporation of 3% w/v fructo-oligosaccharides (FOS) into the MC film formulation improved the viability of L. delbrueckii subsp. bulgaricus CIDCA 333 after film preparation. L. plantarum CIDCA 83114 was intrinsically more resistant as no viability loss was observed upon preparation of the films in the absence of FOS.Scanning electronic microscopy images also showed a good incorporation of microorganisms without affecting the homogeneity of the films. FTIR spectroscopy provided structural information about the bacteria-loaded films. Water sorption isotherms showed an impervious behavior at low a_w but on exceeding 0.7 of a_w the film started to dissolve and form syrup, causing a drastic drop of bacterial viability (log N/N₀ ≤ − 5). Dynamic mechanical analysis (DMA) demonstrated that the incorporation of microorganisms into the MC films had no effect on vitreous transition temperatures. FOS incorporated into the MC films had a plasticizing effect.Microorganism-loaded films were stored at relative humidities (RH) ranging from 11 to 75%. Both strains could be stored at 11% RH for 90 days. At 33 and 44% RH L. delbrueckii subsp. bulgaricus CIDCA 333 could be stored up to 15 days and L. plantarum CIDCA 83114 up to 45 days. At 75% RH only L. plantarum CIDCA 83114 could be equilibrated (log N/N₀: − 2.05 ± 0.25), but CFU/g films were undetectable after 15 days of storage.The results obtained in this work support the use of MC films containing FOS as a good strategy to immobilize lactic acid bacteria, with potential applications in the development of functional foods.     

Resistance of Enterobacter sakazakii to pulsed electric fields

The influence of various environmental factors on Enterobacter sakazakii inactivation by pulsed electric fields was studied and the mechanisms underlying the changes in resistance were also explored. E. sakazakii PEF resistance was higher upon entering the stationary growth phase, but it did not significantly change with growth temperature. E. sakazakii cells were also more resistant to PEF in both acidified and low water activity media. Thus, for stationary-phase cells grown at 30 °C a treatment of 50 pulses at 31 kV/cm led to 5.1 log₁₀ cycles of inactivation in media of pH 7.0 (a_w > 0.99), 1.4 log₁₀ cycles in media of pH 4.0 (a_w > 0.99) and 0.3 log₁₀ cycles in media of a_w = 0.98 (pH 7.0). However, whereas the higher PEF tolerance in acid media was coincident with an increased number of cells capable of repairing their sublethally-injured cytoplasmic membranes, the higher resistance in media of lower water activity was not. To the best of our knowledge, this is the first time that sublethal injuries in outer membrane after PEF treatments have been found.Industrial relevanceThis work provides data about PEF inactivation kinetics and PEF resistance of E. sakazakii under several conditions that might be useful for designing food pasteurization processes by PEF technology. The occurrence of sublethal injuries in cytoplasmic and outer membranes under the most protective treatment conditions, gives the chance to develop combined processes that might increase the effectiveness of the PEF process.     

Water vapor transport properties during staling of bread crumb and crust as affected by heating rate

In order to develop a mathematical model to simulate mass transfer occurring between the crumb and the crust during bread staling, water vapor sorption properties, i.e., moisture diffusivity, WVP and sorption of bread crumb and crust were investigated at 15 °C. Two types of bread baked with two heating rates (7.39 °C/min and 6.32 °C/min) were considered. Sorption and desorption isotherms were determined using Dynamic Vapor Sorption (DVS) and FF and GAB models were applied in the range of 0–0.95 a_w, to fit isotherm curves. Diffusivity was determined from sorption isotherms by using Fick's law and WVP was measured by two methods (gravimetric and from sorption data). Results exhibited maximum values of D_eff in the range of 0.1 and 0.14 g/g d.b. moisture contents. They varied between 0.88 × 10^? 10 and 0.92 × 10^? 10 m²/s for the crust and between 2.24 × 10^? 10 and 2.64 × 10^? 10 m²/s for the crumb, baked respectively at 220 °C and 240 °C. Results of WVP showed that the crust baked at 240 °C was significantly more permeable than the crust baked at 220 °C. This fact was attributed to the difference in porosity and the molecular structure due to heating effects. Also, the presence of steam in the oven atmosphere enhanced the development of higher porosity in the crust, leading to different structures and properties. Moreover, SEM images showed that starch granules were intact and less swelled in the upper crust when baking at 240 °C, resulting in higher WVP.     

Relating physicochemical and microbiological safety indicators during processing of linguiça,a Portuguese traditional dry-fermented sausage

Linguiça is a Portuguese traditional fermented sausage whose microbiological quality and safety can be highly variable. In order to elucidate risk factors and the particularities of the manufacturing technology that explain the between-batch variability in total viable counts (TVC), Enterobacteriaceae, Staphylococcus aureus and Listeria monocytogenes in the product; microbiological and physicochemical characterisation of linguiça at five stages of production (i.e., raw pork meat, mixed with ingredients, macerated, smoked and ripened) was carried out. A total of six production batches were surveyed from two factories; one utilised curing salts and polyphosphate in their formulation (Factory II). The delayed fermentation in the nitrite-formulated sausages was partly responsible for the increase (p < 0.01) in Enterobacteriaceae, S. aureus and L. monocytogenes from raw meat (3.21 log CFU/g, 1.30 log CFU/g and 22.2 CFU/g, respectively) to the end of maceration (4.14 log CFU/g, 2.10 log CFU/g and 140 CFU/g, respectively) while the better acidification process in the nitrite-free sausages (Factory I) led to lower counts of S. aureus (2.64 log CFU/g) and L. monocytogenes (10 CFU/g) in the finished products. In Factory II, although L. monocytogenes entered the chain at the point of mixing, it became steadily inactivated during smoking and ripening (< 50 CFU/g), despite the initially-delayed fermentation. Nitrite had a strong effect on reducing Enterobacteriaceae throughout smoking (r = − 0.73) and ripening (r = − 0.59), while it failed to control the growth of S. aureus. The main hurdle preventing the development of S. aureus in linguiça is the pH, and other factors contributing to its control are: longer ripening days (p = 0.019), low S. aureus in raw meat (p = 0.098), properly-washed casings (p = 0.094), and less contamination during mixing (p = 0.199). In the case of L. monocytogenes, at least three hurdles hinder its development in linguiça: low a_w (p = 0.004), low pH (p = 0.040) and nitrite (p = 0.060), and other factors contributing to its control are: longer ripening (p = 0.072) and maceration (p = 0.106) periods, lower a_w at the end of smoking (p = 0.076) and properly-washed casings (p = 0.099). Results have shown that there is a need to standardise the productive process of linguiça, to optimise the initial acidification process, and to reinforce proper programmes of quality control of ingredients and good hygiene practices, so as to minimise the introduction of Enterobacteriaceae and pathogens from external sources.     

Influence of storage period and packaging method on sliced dry cured beef “Cecina de Leon”: Effects on microbiological,physicochemical and sensory quality

Quality aspects of sliced dry-cured beef “Cecina de León” preserved in vacuum and gas mixtures (20%/80% CO₂/N₂ and 80%/20% CO₂/N₂) were studied. The evolution of microbiological, physicochemical and sensory parameters were analysed during storage (210 days) at 6 °C. Although microbial counts at 60 days of the gas-packaged samples were lower than the vacuum-packed ones, they were never higher than the spoilage limit (7 log ufc/g). A slight increase (p < 0.05) in pH was observed throughout storage of “Cecina de León” packaged under vacuum and in gas mixtures. However, a decrease (p < 0.05) in a_w was observed during storage of “Cecina de León” packaged under vacuum but a_w did not vary (p > 0.05) during storage in the gas-packaged samples. No changes were observed (p > 0.05) in lightness (L¹), redness (a¹) and yellowness (b¹) in vacuum and gas packaged samples during storage. However, sensorially evaluated colour showed lower values in gas packaged samples during 30 days storage. This difference was decisive in establishing the shelf-life of “Cecina de León” slices preserved in gas mixtures (20%/80% CO₂/N₂ and 80%/20% CO₂/N₂). Therefore, from a microbiological point of view, gas mixtures are more effective in extending the shelf-life of “Cecina de León” slices. It is concluded that vacuum packaging allows longer storage than gas-packaging as it maintains a good visual appearance of “Cecina de León”, the main parameter in consumers’ perception of meat quality.     

Enthalpy–entropy compensation in sorption phenomena of starch materials

The enthalpy–entropy compensation theory was applied to the experimental moisture adsorption and desorption isotherm data (water activity (a_w) range 0.006–0.982) of raw potato, potato starch gel, potato starch powder, highly amylopectin corn starch powder and highly amylose corn starch powder in the temperature range 30–60 °C. A linear relation existed between differential enthalpy (ΔH) and differential entropy (ΔS) for all the experimental data considered, thus satisfying the enthalpy–entropy compensation theory. Further analysis of the data indicated an enthalpy-controlled (isokinetic temperature (T_β) > harmonic mean temperature (T_hm)) and spontaneous (−ΔG) sorption process.     

Destruction of Salmonella Enteriditis inoculated onto raw almonds by high hydrostatic pressure

The effects of continuous (50,000, 60,000 and 70,000 psi with holding times of 5 and 10 min) and discontinuous (oscillatory) (six cycles at 60,000 psi with a holding time of 20 s) high hydrostatic pressure (HHP) treatments on the viability of two Salmonella Enteriditis strains (FDA and PT30) inoculated onto raw almonds were evaluated at 25, 50, and 55 °C. Complete inactivation of the S. Enteriditis was achieved in 0.1% peptone water after continuous pressurization at 60,000 psi and 25 °C for 5 min. Continuous pressurization of raw almonds inoculated with S. Enteriditis at 60,000 psi and 50 °C for 5 min resulted in less than a log reduction (log₁₀ 0.83) of vegetative cells. The decimal reduction time using the continuous pressurization parameters was determined to be 9.78 min. A discontinuous process consisting of six cycles of pressurization at 60,000 psi and 50 °C for 20 s provided greater than a one log reduction (log₁₀ 1.27 for FDA and log₁₀ 1.16 for PT30) of the S. Enteriditis concentration. The low water activity (a_w) of the almonds was found to impart baroprotective attributes on the S. Enteriditis cells. When the almonds were directly suspended in water and then pressurized, a log₁₀ reduction of 3.37 was achieved. HHP of certain dry foods appears to be feasible if the food is directly suspended in the pressurizing medium (water).     

Effect of reducing nitrate and nitrite added to dry fermented sausages on the survival of Salmonella Typhimurium

Salmonella is one of the pathogens that most frequently contaminate pork processing lines. Several hurdles can control this organism in dry fermented sausages, among them is nitrite. However, the traditional use of nitrate/nitrite in the meat industry is being questioned due to their involvement in nitrosamine formation. In this study, minced pork and sausages inoculated with Salmonella Typhimurium were prepared with 150 ppm NaNO₃ and 150 ppm NaNO₂ (maximum amounts allowed by EU), and with a reduction of 25% and 50%. The absence of nitrate/nitrite favored Salmonella growth, with 2–2.5 log cfu/g higher counts at the end of ripening, compared to nitrate/nitrite added batches. The 50% reduction showed the same inhibitory effect as the maximum amounts. Nitrate/nitrite represented an essential hurdle to control Salmonella even when pH and a_w were below the values considered as minimum for its growth. The effect of this reduction on other pathogens should be considered.     

Effect of osmotic dehydration on the quality of air-dried Porphyra

Osmosis with a solution of NaCl and sucrose to depress the water activity (a_w) to 0.970 and the pH to 3.0 was assayed as a pretreatment to air-drying of red seaweeds (Porphyra columbina Montagne). Air-drying was performed at 30°C and 14% relative humidity. Process characteristics, color, flow pattern and rehydration ability of the milled product, were evaluated to determine the usefulness of osmosis. The a_w attained after 2 h of drying was similar for both osmosed and nonosmosed Porphyra and higher than monolayer water activity for both products. Consequently, no additional benefits concerning stability during storage were observed, after applying osmosis. Moreover, severe changes in color occurred. Powder flowability evaluated through the measurement of compressibility and cohesiveness decreased with pretreatment, which also determined a more plastic behavior. In summary, osmotic dehydration as a pretreatment for air-dried seaweeds did not seem to improve the final product quality.