Mechanical, water vapor barrier and thermal properties of gelatin based edible films

Edible films are thin materials based on a biopolymer. The objectives of this work were to determine the water vapor permeability and the mechanical and thermal properties of edible films based on bovine hide and pigskin gelatins. These films were prepared with 1 g of gelatin/100 ml of water; 15–65 g sorbitol/100 g gelatin; and at natural pH. The samples were conditioned at 58% relative humidity and 22°C for 4 days before testing. The mechanical properties were determined by the puncture test and the water vapor permeability by gravimetric method at 22°C. For DSC analysis, samples were conditioned over silica gel for 3 weeks. Samples (10 mg) were heated at 5°C/min, between −150 and 150°C in a DSC TA 2010. A second scan was run after cell cooling with liquid nitrogen. As expected, the puncture force decreased and the puncture deformation and water vapor permeability increased with the sorbitol content. The origin of gelatin was important only above 25 g sorbitol/100 g gelatin. The DSC traces obtained in the first scan of samples with 15–35 g sorbitol/100 g gelatin, showed a well visible glass transition followed by a sol–gel transition. However, with the increase of sorbitol concentration, the glass transition became broader, typical of the system presenting a phase separation. The model of Couchman and Karazs for ternary system, was used to predict the Tg values as a function of sorbitol concentration.     

Thermomechanical properties of biodegradable films based on blends of gelatin and poly(vinyl alcohol)

The aim of this work was to study the effect of the poly(vinyl alcohol) (PVA) concentration on the thermal and viscoelastic properties of films based on blends of gelatin and PVA using differential scanning calorimetry (DSC) and dynamic-mechanical analysis (DMA). One glass transition was observed between 43 and 49 °C on the DSC curves obtained in the first scanning of the blended films, followed by fusion of the crystalline portion between 116 and 134 °C. However, the DMA results showed that only the films with 10% PVA had a single peak in the tan δ spectrum. However, when the PVA concentration was increased the dynamic mechanical spectra showed two peaks on the tan δ curves, indicating two T_gs. Despite this phase separation behavior the Gordon and Taylor model was successfully applied to correlate T_g as a function of film composition, thus determining k=7.47. In the DMA frequency tests, the DMA spectra showed that the storage modulus values decreased with increasing temperature. The master curves for the PVA–gelatin films were obtained applying the TTS principle (T_r=100 °C). The WLF model was thus applied allowing for the determination of the constants C₁ and C₂. The values of these constants increased with increasing PVA concentrations in the blend: C₁=49–66 and C₂=463–480. These values were used to calculate the fractional free volume of the films at the T_g and the thermal expansion coefficient of the films above the T_g.     

Associative and segregative interactions between gelatin and low-methoxy pectin: Part 2—co-gelation in the presence of Ca

The effect of segregative interactions with gelatin (type B; pI=4.9; 0–10 wt%) on the networks formed by low-methoxy pectin on cooling in the presence of stoichiometric Ca²⁺ at pH 3.9 has been investigated by rheological measurements under low-amplitude oscillatory shear. Samples were prepared and loaded at 85 °C, cooled (1 °C/min) to 5 °C, held for 100 min, and re-heated (1 °C/min) to 85 °C, with measurement of storage and loss moduli (G′ and G″) at 10 rad s⁻¹ and 2% strain. The final values of G′ at 5 °C for mixtures prepared at the same pH without Ca²⁺ were virtually identical to those observed for the same concentrations (0.5–10.0 wt%) of gelatin alone, consistent with the conclusion from the preceding paper that electrostatic (associative) interactions between the two polymers become significant only at pH values below 3.9. Increases in moduli on cooling in the presence of Ca²⁺ occurred in two discrete steps, the first coincident with gelation of calcium pectinate alone and the second with gelation of gelatin. Both processes were fully reversible on heating, but displaced to higher temperature (by 10 °C), as was also observed for the individual components. The magnitude of the changes occurring over the temperature range of the gelatin sol–gel and gel–sol transitions demonstrates that the gelatin component forms a continuous network; survival of gel structure after completion of gelatin melting shows that the calcium pectinate network is also continuous (i.e. that the co-gel is bicontinuous). On progressive incorporation of NaCl (to induce phase separation before, or during, pectin gelation) the second melting process, coincident with loss of calcium pectinate gel structure, was progressively abolished, indicating conversion to a gelatin-continuous network with dispersed particles of calcium pectinate. These qualitative conclusions are supported by quantitative analyses reported in the following paper.     

Survival kinetics of Ephestia kuehniella eggs during 46–75 °C heat treatment

The effect of heat treatment on the survival of Ephestia kuehniella eggs was examined. Samples of 60 eggs were immersed in hot water at constant temperature in the 46–75 °C range for 5–1200 s. Following heat treatment and cooling, the eggs were stored at 24 ± 1 °C in a growth chamber for 7 days before survival evaluation. Statistical analysis of the data demonstrated that the thermal survival kinetics were best represented by a first-order reaction. The rate constant had an Arrhenius-type dependence over the 54–75 °C temperature range. Kinetic parameters were estimated by non-linear regression. The activation energy (E_a) and rate constant (k_ref) at the reference temperature (T_ref = 64.8 °C), were determined as 102.2 ± 6.2 kJ mol⁻¹ and 0.061 ± 0.003 s⁻¹, respectively, over the 54–75 °C temperature range. A 0.01% survival rate was obtained after 50 s at 75 °C. The data at temperatures below 50 °C were not in accordance with those at higher temperatures. Above this temperature, mortality was likely due to physiological disorders, as noted on a DSC thermogram.     

Effects of drying on the gel strength and cation mobility of furcellaran

Thermal stability, by means of air drying a furcellaran powder, and its impact on gel strength and cation mobility were studied. Halogen heating in the temperature range 90–115°C for 15 min resulted in loss on drying (L_D, %). These results can be described by polynom L_D=−9.583+2.989τ−0.249τ²+0.00729τ³+0.1034t (R²=0.9976), indicating a gradual decomposition of carbohydrates. Air-drying induced a decrease in gel strength and the partial removal of potassium, calcium and sodium ions from the matrix. Air drying above 115°C yielded a remarkable destruction of polysaccharides with a total collapse in gelling power.     

Characterization of gelation time and texture of gelatin and gelatin–polysaccharide mixed gels

The effects of cooling rate, holding temperature, pH and polysaccharide concentration on gelation characteristics of gelatin and gelatin–polysaccharide mixtures were investigated using a mechanical rheometer which monitored the evolution of G′ and G″. At low holding temperatures of 0 and 4 °C, elastic gelatin gels were formed whereas a higher holding temperature of 10 °C produced less elastic gels. At slow cooling rates of 1 and 2 °C/min, gelling was observed during the cooling phase in which the temperature was decreased from room temperature to the holding temperature. On the other hand, at higher cooling rates of 4 and 8 °C/min, no gelation was observed during the cooling phase. Good gelling behavior similar to that of commercial Strawberry Jell-O^® Gelatin Dessert was observed for mixtures of 1.5 and 15 g sucrose in 100 ml 0.01 M citrate buffer containing 0.0029–0.0066 g low-acyl gellan. Also, these mixed gels were stronger than Strawberry Jell-O^® Gelatin Desserts as evidenced by higher G′ and gel strength values. At a very low gellan content of 0.0029 g, increasing pH from 4.2 to 4.4 led to a decrease in the temperature at the onset of gelation, G′ at the end of cooling, holding and melting as well as an increase in gel strength. The gelation time was found to decrease to about 40 min for gelatin/sucrose dispersions in the presence of 0.0029 g gellan at pH 4.2 whereas the corresponding time at pH 4.4 was higher (79 min). In general, the gelation time of gelatin/sucrose dispersions decreased by a factor of 2 to 3 in the presence of low-acyl gellan. The addition of low-acyl gellan resulted in an increase in the gelation rate constant from 157.4 to 291 Pa. There was an optimum low-acyl gellan content for minimum gelation time, this optimum being pH dependent. Addition of guar gum also led to a decrease in gelation time to 73 min with a corresponding increase in the gelation rate constant to 211 Pa/min though these values were not sensitive to guar gum content in the range of 0.008–0.05 g. The melting temperature of gelatin/sucrose/gellan as well as gelatin/sucrose/guar mixtures did not differ significantly from that of pure gelatin or Strawberry Jell-O^® Gelatin Desserts. At pH 4.2, the melting rate constant was highest at a low-acyl gellan content of 0.0029 g whereas the rate constant was insensitive to low-acyl gellan content at pH 4.4. Addition of guar did not seem to affect the melting temperature or the melting rate constant.     

Predictive model of Vibrio parahaemolyticus growth and survival on salmon meat as a function of temperature

The growth and survival curves of a strain of pandemic Vibrio parahaemolyticus TGqx01 (serotype O3:K6) on salmon meat at different storage temperatures (range from 0 °C to 35 °C) were determined. In order to model the growth or inactivation kinetics of this pathogen during storage, the modified Gompertz and Weibull equations were chosen to regress growth and survival curves, respectively, and both equations produced good fit to the observed data (the average R² value equals to 0.990 for modified Gompertz and 0.920 for Weibull equation). The effect of storage temperature on the specific growth rate (μ) was modeled by square root type equation, and the relationship between μ and lag time (λ) was described by a rule of μ × λ = constant. The shape factor (n) and scale factor (b) values of the Weibull equations versus the temperature (°C) were plotted and the temperature effects on these parameters were described by two linear empirical equations. The predicted growth and survival curves from the model were compared to real enumeration results, using the correlation coefficient (R²), bias factor (B_f) and accuracy factor (A_f), to assess the performance of the established model. The results showed that the overall predictions for V. parahaemolyticus TGqx01 growth or inactivation on salmon at tested temperatures agreed well with observed plate counts, and the average R², B_f and A_f values were 0.958, 1.019 and 1.035, respectively.     

Microbiological and physicochemical characteristics of Cameros cheese

Eighteen samples of Cameros cheese, a fresh Spanish goat's cheese, were collected from the four different producers of this regional kind of cheese. Physicochemical and microbiological analyses were carried out to evaluate the sanitary quality and the physicochemical characteristics of the product. The influence of the season of the year and the elaboration conditions were also studied. Cameros cheese is a fat cheese (54·2±6·5% of total solids; TS), with a high pH close to the neutrality (6·35±0·14), high moisture (TS value of 42·5±4·7%) and low salt content (0·78±0·30% of TS). Listeria monocytogenes was found in 5·6% of samples. Staphylococcus aureus counts above 2 log cfu g⁻¹were found in 55% of the cheeses studied. Thirty-three percent of the April samples and 67% of the July samples reached microbiological levels above the legally established standards. None of the samples yieldedSalmonella spp. The dairy had a significant effect on salt content (P<0·001) because different salt application methods were applied. Also significant differences (P<0·01) of S. aureus counts were detected among the different dairies. Significant seasonal differences of non-protein nitrogen (P<0·01) were detected. The season significantly affected the counts of Enterobacteriaceae (P<0·01), mesophilic micro-organisms and psycrotrophs (P<0·05) and yeast and mould counts (P<0·05).     

Effect of heat and thermosonication treatments on watercress (Nasturtium officinale) vitamin C degradation kinetics

The use of ultrasound in food processing creates novel and interesting methodologies, which are often complementary to classical techniques. In this work, the effect of heat and the combined treatment heat/ultrasound (thermosonication) on the thermal degradation kinetics of vitamin C in watercress (Nasturtium officinale) was studied in the temperature range of 82.5 to 92.5 °C. First order reaction kinetics adequately described the vitamin C losses during both blanching processes.The activation energies and the reaction rates at 87.5 °C for heat (H) and thermosonication (Ts) treatments were, respectively, E_avitCH = 150.47 ± 42.81 kJ mol^− 1 and E_avitCTs = 136.20 ± 60.97 kJ mol^− 1, and k_{87.5 °CvitCH} = 0.75 ± 0.10 min^− 1 and k_{87.5 °CvitCTs} = 0.58 ± 0.11 min^− 1. No significant differences (P > 0.05) were detected between both treatments. The thermosonication treatment was found to be a better blanching process, since it inactivates watercress peroxidase at less severe blanching conditions and consequently retains vitamin C content at higher levels. The present findings will help to optimise the blanching conditions for the production of a new and healthy frozen product, watercress, with heat and a new blanching process methodology.

Industrial relevance

Thermosonication blanching can be useful since it reduces processing times, and consequently minimizes the adverse effects of heating on watercress quality. This new application will provide good material, in terms of vitamin C, for further processes, and can be an excellent alternative to the traditional heat treatment.     

Effect of food matrix and heat treatment on the rheological properties of salmon-based baby food

Shelf stable baby foods from Alaskan salmon were developed from red and pink salmon with and without bones. The effect of salmon type, presence of bones and thermal treatment (121 °C for 55 min) on the dynamic (viscoelastic) and flow models were evaluated. Rheological behaviors of all samples were also tested over a temperature range of 25–55 °C. All samples had a higher viscoelastic behavior with consistently higher storage modulus (G′) than loss modulus (G″) over the entire frequency range used (0.5–100 rads/s at 25 °C). Thermal treatment had a significant effect (p < 0.0001) on viscoelastic behavior of baby foods when the exponential model (G′ or G″ = A(ω)^b) was used. A values were higher for the processed food for G′ and G″, and b values. The Casson model was found to be the best for the shear rate – shear stress for all types of tested samples. Retorted samples exhibited lower yield stress than their unretorted counterparts. Retorted samples were susceptible to temperature change more than unretorted samples as shown by energy of activation values (E_a) when the effect of temperature (25–55 °C) was studied with an Arrhenius type model. Red salmon without bone had higher E_a values among all samples and with pink salmon with bone recording a lowest among all of them.     

Enumeration, isolation and characterization of Bacillus cereus strains from Spanish raw rice

Bacillus cereus was present in 61 samples of raw rice analysed representing unhusked, husked and commercial origins. B. cereus in husked and white rice samples did not reach 10² cfu g⁻¹, while in the unhusked rice B. cereus densities exceeded 10³ cfu g⁻¹. Processing steps such as drying, husking and polishing reduced the number of B. cereus in the final product. Eight strains with typical morphology of B. cereus on Polymyxin–Mannitol–Egg Yolk–Phenol Red Agar (PMYPA) were isolated. According to ISO confirmatory tests, the API System tests and supplementary tests of motility, oxidase activity and enterotoxin production, these isolates were characterized and identified as B. cereus. All strains were motile, oxidase-negative and produced diarrheal enterotoxin in TSB. D and z -values were used to characterize heat resistance of spores obtained from the eight strains ofB. cereus characterized. A large diversity in heat resistance was observed among the isolates. At 90°C, D -values ranged from 2·23 to 23·26 min, with five groups of D -value means significantly different at the 95% confidence level. D₉₅- and D₁₀₀ values calculated for the eight strains ranged from 0·69 to 5·17 min and from 0·43 to 1·09 min, respectively. Statistical analysis revealed that there was significant difference between the D -value means obtained for the strains at each temperature. The z -values for the eight strains of B. cereus tested in this study ranged from 7·42°C to 8·20°C with an average of 7·7°C.     

Contamination of infant powdered milk in use with enterotoxigenic Staphylococcus aureus

One-hundred and ten of the 114 samples of infant milk formulae collected from nursing mothers contained viable staphylococci, with the highest mean count of 1·0 × 10² cfu g⁻¹, from samples collected the day the tin was opened. The highest mean total aerobic plate count was 2·6 × 10³ cfu g⁻¹. Titratable acidity and pH of the reconstituted milk ranged from 0·06 to 0·8 and 6·40 to 6·52 respectively. 52·0% of the 123 isolates were S. aureus, 49·6% produced β-hemolysin and 17·1% produced α-hemolysin. Enterotoxin A was produced by 6·5%, B by 1·6%, C by 2·4%, D by 1·6% and E by 0·8% of the isolates. The total staphylococcal and aerobic plate counts were not affected by either the period elapsed from the opening of tin to sampling or the brand of milk.     

Rheological characterizations of texturized whey protein concentrate-based powders produced by reactive supercritical fluid extrusion

A powder blend comprising (by weight) 94% whey protein concentrate (WPC80), 6% pre-gelatinized corn starch, 0.6% CaCl₂, and 0.6% NaCl was texturized using a supercritical fluid extrusion (SCFX) process. The blend was extruded at 90 °C in a pH range of 2.89 to 8.16 with 1% (db) supercritical carbon dioxide (SC-CO₂) and 60% moisture content. The texturized WPC-based (TWPC) samples were dried, grounded into powder, reconstituted in water, and evaluated using a range of rheological studies. Most TWPC samples exhibited shear thinning behavior and their mechanical spectra were typical of weak gel characteristics. The TWPC produced under extremely acidic condition of pH 2.89 with SC-CO₂ yielded the highest η^* (10,049 Pa s) and G′ (9,885 Pa) compared to the unprocessed WPC (η^* = 0.083 Pa s and G’ = 0.036 Pa). The SCFX process rendered WPC into a product with cold-setting gel characteristics that may be suitable for use as a food texturizer over a wide range of temperatures.     

The incidence and antibiotic resistance profiles of Salmonella spp. on Irish retail meat products

Irish retail meat (n=74) and poultry samples (n=106) were tested for the presence of naturally occurring Salmonella spp. The pathogen was detected in 28 poultry (n=106), two pork (n=22) and one cooked meat samples (n=20) examined. Salmonella was not isolated from minced beef or lamb samples tested. Initial counts on samples ranged from 0 to log₁₀2·5 cfu g⁻¹. The most commonly isolated serotype was S. bredeney accounting for 48·4%, followed by S. kentucky (35·5%) and S. enteritidis (6·5%). Salmonella spp. (n=31) isolated from food products were also examined for antibiotic resistance. A total of 155 strains (five strains from each isolate) were tested for resistance to 26 antibiotics using the Bauer method. The percentage of samples showing antibiotic resistance amongstSalmonella isolates were as follows: Riampicin (100%); Tetracycline (92·92%); Oxytetracycline (86·26%); Sulphamethoxazole (86·25%) and Streptomycin (80·92%).     

Inhibitory effect of clove oil on Listeria monocytogenes in meat and cheese

The antilisteric activity of clove oil was examined in meat and cheese at both 30°C and 7°C. At concentrations of 0·5% and 1% clove oil restricted the growth of Listeria monocytogenes in the food items at both temperatures. The inhibitory activity of clove oil was more pronounced at a concentration of 1%.Listeria counts in treated samples were 1–3 log₁₀cfu g⁻¹less compared to controls at different intervals during storage. The results revealed the potential of clove oil as a natural preservative in meat and cheese.     

Influence of freeze-drying conditions on survival of Oenococcus oeni for malolactic fermentation

Malolactic fermentation (MLF) is an important process in wine production. Oenococcus oeni is most often responsible for MLF. Starter culture technology, involving the inoculation of O. oeni into wines, has been developed for inducing MLF. In this study, the effects of cell washing, pH of suspension medium, preincubation in sodium glutamate, initial cell concentration and freezing temperature on viability of freeze-dried O. oeni H-2 were investigated. The cell viability of samples without washing with potassium phosphate buffer was significantly lower than those samples undergone washing. When pH of suspension medium was 7.0 the cell survival was the highest. The cell viability was enhanced when the cells were preincubated at 25 °C before freezing. When 2.5% sodium glutamate was used as protective agent in suspension medium, the optimal initial cell concentration was 10⁹ CFU/ml. The cell viability increased by 21.6% as freezing temperature decreased from − 20 °C to − 65 °C. However, when the cells were frozen in liquid nitrogen (− 196 °C), the cell survival significantly decreased.     

Kinetic studies of the thermal inactivation of plasmin in acid or sweet whey

The thermal behaviour of the milk alkaline proteinase, plasmin, was studied in acid and sweet (rennet) whey; indigenous (bovine) plasmin was studied in the former system, but endogenous porcine plasmin was added in the latter, due to the very low levels of residual plasmin. The inactivation of plasmin in both systems followed first-order inactivation kinetics, which was consistent with previous observations of plasmin inactivation in milk and model milk systems. The thermal inactivation of plasmin in acid whey (D_{90 °C}=108±29 min, z=24.5±1.2 °C) was much slower than in the sweet whey system (D_{90 °C}=0.021±0.006 min, z=7.3±0.3 °C). Similarly, denaturation of β-lactoglobulin (β-lg) followed a first-order inactivation profile and this protein was also more heat stable in acid whey (D_{90 °C}=86±76 min, z=13.7±1.5 °C) than sweet whey (D_{90 °C}=0.81±0.29 min, z=9.1±0.5 °C). While it is possible that the increased heat stability of plasmin in acid whey is due to reduced sulphydryl/disulphide interchange reactions between plasmin and β-lg, it also appears that structural changes in the plasmin molecule were a significant contributory effect on the thermal stability of plasmin in this system. Increasing the pH of acid whey decreased the heat stability of plasmin. However, adjusting the pH of sweet whey had little effect on the heat stability of plasmin. Overall, severe heat treatments may be required to ensure inactivation of the enzyme in acid whey, but a balance is required between reducing the activity of plasmin and maintaining the functionality of whey proteins as food ingredients.     

Migration of α-tocopherol from an active multilayer film into whole milk powder

Antioxidant active packaging is a promising technology for whole milk powder (WMP) protection. In this study, the migration of α-tocopherol from a multilayer active packaging (made of high density polyethylene, ethylene vinyl alcohol and a layer of low density polyethylene containing the antioxidant) to WMP was studied. A model based on the Fick’s diffusion equation was used to calculate the diffusion coefficients (D) of α-tocopherol as 2.34 × 10⁻¹¹, 3.06 × 10⁻¹¹, and 3.14 × 10⁻¹¹ cm² s⁻¹ at 20, 30 and 40 °C, respectively. The D at 20 °C was different from those at 30 and 40 °C (P < 0.05); but it was similar at 30 and 40 °C. This low influence of temperature on the migration of α-tocopherol from 20 to 40 °C assures the release at real storage and commercialization conditions in regions with warm/hot climate. The antioxidant delivering system delayed the lipid oxidation of WMP and it was more effective at 30 and 40 °C since the rate of oxidative reactions was higher at these temperatures than at 20 °C.     

Maturation effects in fish gelatin and HPMC composite gels

This study assessed the effectiveness of using hydroxylpropylmethylcellulose (HPMC) to enhance mechanical strength and thermal stability in fish skin gelatin (FG). The significant increase in absorbance (A₄₀₀) observed after HPMC had been added to FG and then matured indicated successful formation of a composite gel. Increased gel strength and storage modulus (G′) indicated the enhanced gelation ability of the matured composite gel, while increased melting temperature (T_m) and enthalpy (ΔH) indicated its improved thermal stability. Maturation-related rheological property improvements were more noticeable at 4 °C than 10 °C, but no apparent differences in T_m improvement were observed between 4 °C and 10 °C maturation. Nevertheless, the composite gel exhibited reversible cold and thermal gelation properties.     

Inhibition of Clostridium sporogenes growth in mascarpone cheese by co-inoculation with Streptococcus thermophilus under conditions of temperature abuse

The ability of a biological control system to inhibit the outgrowth of Clostridium sporogenes spores during storage of mascarpone cheese under temperature-abuse conditions was investigated. Challenge studies were carried out on mascarpone cheese artificially contaminated with spores of C. sporogenes (10 cfu g⁻¹), and with or without the coinoculum of a Streptococcus thermophilus strain (10⁵cfu g⁻¹). During storage at 4, 12, and 25°C, the outgrowth of clostridia spores, the growth of S. thermophilus, and the pH changes were evaluated at 10, 20, 30, and 40 days. In mascarpone cheese stored at 4° and 12°C, S. thermophilus and C. sporogenes did not show any growth. The initial pH (6·14) of the product also remained unchanged. During storage at 25°C S. thermophilus grew up to about 10⁷cfu g⁻¹after 10 days, resulting in a pH decrease of mascarpone cheese to values close to 4·5. The cell number decreased progressively during storage reaching values near to 10¹cfu g⁻¹after 40 days, whereas product acidity remained constant. C. sporogenes, when inoculated alone, also grew at 25°C. The cell number increased to levels of about 10⁷cfu g⁻¹after 20–40 days of storage according to the different mascarpone cheese lots used. No growth was found when C. sporogenes was co-inoculated in mascarpone cheese with S. thermophilus and stored at 25°C. The study on the behaviour of C. sporogenes, known as a non-toxigenic variant of Clostridium botulinum, allowed us to obtain useful information for setting up an effective biological control system to inhibit growth of the toxigenic species as well. The use of an additional barrier, besides refrigerated storage, may help to maintain the safety of mascarpone cheese in the event it was exposed to elevated temperatures.