Conservation en congélation et réfrigération de la viande hachée de boeuf et de porc: évolution de la flore microbienne et des propriétés physico-chimiques en cours de stockageFrozen and chilled minced meat (beef and pork): microbiol and physico-chemical evolution during storage

The minced meat market in Belgium is growing steadily; its share of the market has increased from 10 to 30% in three years. However, the mincing of meat can produce intense microbiol proliferation and thus cause food putrification.

Any changes occurring in meat, especially minced meat, are essentially of microbial origin, hence refrigeration is particularly important. With chilling, minimum temperatures must be maintained (0±1°C), since only temperatures below +3°C avoid any health risk linked with the growth of pathogens and production of toxins. Under chilled conditions the storage use of minced meat is four to five days. With freezing, temperatures of about - 18°C are needed to avoid the growth of bacteria, yeasts and mould. At this temperature micro-organisms are progressively rendered inactive, but destruction is never total. The kinetics of reducing micro-organism populations depends on the nature of the micro-organisms, the storage temperature (reduction is more intense at − 12°C that at − 18°C or at − 30°C) and the freezing rate (micro-organisms are better able to resist rapid freezing than slow freezing). One years storage at − 18°C will destroy 90 to 99% of the micro-organisms which were initially present in the product. Nevertheless, the chilled storage (+4°C) of thawed minced meat must not exceed three days.

The physico-chemical evolution of cold stored minced meat is explained. In chilled storage, the onset of flavour defects can be related to the measurement of volatile nitrogen. In frozen storage, the crystallisation of the water causes cellular breakdown. The role of packaging is important in avoiding superficial freeze-drying which is accompanied by browning. In addition, fatty matter can deteriorate by oxidation even at freezing temperatures. The degree of alteration of fats can be appreciated by the acidity index and peroxide index (showing that it is undesirable to store minced pork at − 18°C for more than nine months).

Use of a partial vacuum or modified atmosphere in combination with impermeable packaging will prolong the storage life of minced meat provided that the initial contamination is as low as possible and the maintenance of the cold chain is strictly respected.

Résumé

Le marché de la viande hachée progresse en Europe, du fait de plusieurs facteurs socio-économiques. Le hachage diminue considérablement la durée de conservation de la viande; sa consommation peut être dangereuse.

L'article analyse les points suivants: emballage, réfrigération et congélation (notamment vitesse de congélation), température et durée de conservation, atmosphères modifiées et leur influence sur les propriétés physico-chimiques de la viande et sur le comportement microbien.     

Effect of temperature fluctuation on energy consumption and quality changes of palletized foods in frozen storage

Pallet lots of frozen okra, peas, and strawberries were stored at: −24°C consstant, ±1°C (−11°F constant); at −24°C (−11°F), but power shutdown overnight, −24°C to −18°C (−11°F to 0°F); at −21°C to −18°C (−6°F to 0°F); and at −18°C to −15°C (0°F to +5°F). The temperature increases in these small rooms were estimated to be similar to the worst conditions that might exist in commercial freezer warehouses. Diurnal fluctuations were much smaller within the packages, particularly in the densely filled products located in the centre of the lots. All three time-temperature indicators provided an approximate history of storage conditions.

Compared to storage at a constant −24°C (−11°F), shutting down power at night yielded 8% saving in energy consumption, which increased to 23% when temperature was brought down to −21°C (−6°F), and to near 30% when temperatures were reduced to −18°C (0°F) only. Weight losses increased from 0.28% in the first chamber to 0.68% in the last chamber. Pouches lost much less weight than cartons, and internal packages less than those on the edge of the lots. Frost formation (in-package desiccation) increased from −24°C to −18°C (11°F to 0°F), and was more severe in the pouches than in the cartons. Clumping was reduced in all treatments with storage time. Sensory quality changes and ascorbic acid were reduced similarly, but the poorest treatment, the last chamber, lost at most half a grade score and up to 10% ascorbic acid. Total solids showed little relation to treatments.

It was tentatively concluded, depending on energy cost and availability, that −20°C to −18°C (−4°F to 0°F), overnight, might be the optimal storage condition for cartons, while pouches might be economically stored at −18°C to −16°C (0°F to 3°F) if for no longer than 6 months.     

Processing-induced changes in frozen storage life of lamb

Quality deterioration of lamb under frozen storage was assessed in terms of rancid flavour development. Twenty-five lamb carcasses were each given one of eight processing treatments and the fifty loins from these lambs were each assigned to one of three storage periods (10, 15, 20 weeks) at −5°C. Minced lamb samples from a separate carcass were also stored at −5°C. Rancid flavour was assessed by a fiften member trained taste panel.

Pre-storage freezing to a high nadir (−5°C) or electrical stimulation both appear to prolong the subsequent storage life at −5°C. Mincing, freezing to a low nadir (−35°C), or a 24 h pre-freezing chill (6°C) period appear to significantly reduce subsequent storage life.     

Modelling colour and chlorophyll losses of frozen green beans (Phaseolus vulgaris, L.)

Colour changes and chlorophyll degradation of frozen green beans (Phaseolus vulgaris, L., variety bencanta) were studied during 250 days of storage at −7, −15 and −30 °C. Chlorophyll a and b losses and colour Hunter a and b co-ordinates and total colour difference (TCD_H) changes were successfully described by first order and reversible first order models, respectively. The temperature effect was described by the Arrhenius law. Disagreement between the colour co-ordinates and chlorophyll content was obtained. Therefore, chlorophyll content is not a good colour index of frozen green beans. The results emphasise that colour is a more important parameter to assess frozen green beans visual quality.     

Influence of cryogenic temperatures on food products and packaging during rail transport

Six commercially packaged frozen food products were subjected to simulated railway shock and vibration conditions and manual handling abuse at both cryogenic (−70°C) and non-cryogenic (−18°C) temperatures. The amount of package and food product damage under different temperatures and levels of abuse were determined. The effects of cryogenic temperatures upon sensory and various analytical quality attributes of the food products were examined. The test results indicated that frozen food can be shipped at cryogenic temperatures in railcars equipped with cushioned draft gears with minimal product or packaging damage, provided there is not excessive manhandling abuse.     

Freezing of a parallelepiped food product. Part 1. Experimental determination

A number of procedures are used to predict the freezing time of food. The objective of the project reported here was to test the adequacy and applicability of the various mathematical models and methods used to predict the freezing time of a small, parallelepiped food product. The approach was to compare experimental and predicted freezing times. In this paper the experimental methods and results are described; the freezing time and the thermophysical properties of both the frozen and non-frozen food were determined. Comparison between experimental and predicted freezing times will be given in a subsequent paper. The experimental conditions consisted of individually freezing the product in an air blast; the food product was french fries. The time required to lower the temperature of a french fry from 31 to −18°C in the −29°C air blast freezer used was approximately 1200 s. The mean moisture content of the fries was 73.7%. The average densities of the non-frozen and frozen fries were 1069 and 1012 kg m⁻³, respectively; the average thermal conductivities were 0.50 and 1.0 W m⁻¹ °C⁻¹; and the heat capaciti were 3420 and 1870 J kg⁻¹ °C⁻¹. The overall surface heat transfer coefficient of a parallelepiped object in the air blast freezer used for these experiments was 21.0 W m⁻² °C⁻¹. The low surface heat transfer coefficient resulted in a flat temperature profile within the fries.     

Reactions in Au/Nb bilayer thin films

The interdiffusion and intermetallic compound formation of Au/Nb bilayer thin films annealed at 200–400 °C have been investigated. The bilayer thin films were prepared by electron beam deposition. The Nb film was 50 nm thick and the Au film was 50–200 nm thick. The interdiffusion of annealed specimens was examined by measuring the electrical resistance and depth-composition profile and by transmission electron microscopy. Interdiffusion between the thin films was detected at temperatures above 325 °C in a vacuum of 10^-4 Pa. The intermetallic compound Au₂Nb₃ and other unknown phases form during annealing at over 400 °C. The apparent diffusion constants, determined from the penetration depth for annealing at 350 °C, are 3.5 × 10⁻¹⁵ m² s⁻¹ for Nb in Au and 8.6 × 110^7minus;15 m² s⁻¹ for Au in Nb. The Au surface of the bilayer films becomes uneven after annealing at over 400 °C due to the reaction.     

Effect of different pretreatments on the quality of deep frozen green beans and carrots

The effect of pretreatment on the quality of samples of green beans and carrots frozen and stored at - 21°C is reported. The processes tested for green beans are given in Fig. 1 and those for carrots in Fig. 2. Included min both cases are blanching, evacuation, vacuum packaging and heat shock.The tests ,involved the retention of dry matter, texture change and peroxydase activity, colour our change and residual ascorbic acid content. Results are presented for measurements over 24 weeks storage.For green beans heat shock was better nthan blanching for retaining colour. Evacuation at 20°C before heat shock gave the best results with more than 760% retention of ascrobic acid and peroxydase activity. Carrots were also better without blanching but were improved by vacuum opacking. 90% peroxydase was retained after 9 monthes storage.     

Effect of different pretreatments on the quality of deep frozen green beans and carrotsInfluence de différents traitements préalables sur la qualité de haricots verts et de carottes surgelés

The effect of pretreatment on the quality of samples of green beans and carrots frozen and stored at - 21°C is reported. The processes tested for green beans are given in Fig. 1 and those for carrots in Fig. 2. Included min both cases are blanching, evacuation, vacuum packaging and heat shock.The tests ,involved the retention of dry matter, texture change and peroxydase activity, colour our change and residual ascorbic acid content. Results are presented for measurements over 24 weeks storage.For green beans heat shock was better nthan blanching for retaining colour. Evacuation at 20°C before heat shock gave the best results with more than 760% retention of ascrobic acid and peroxydase activity. Carrots were also better without blanching but were improved by vacuum opacking. 90% peroxydase was retained after 9 monthes storage.     

Studies of thermally stimulated currents and surface charge in polystyrene thin films

A careful study was undertaken of the initial charge developed on the non-metallized surface and of short-circuit thermally stimulated currents (TSCs) in unilaterally metallized polystyrene (PS) films 25 microm thick. The films had been negatively charged by Townsend breakdown at a voltage of −5 kV at various temperatures ranging from 90 to 130°C. The surface charge densities observed were of the order of 10^-8 C cm^-2, corresponding to full trap densities of the order of 10¹⁴ cm⁻³. The short-circuit TSC spectra of newly charged samples show a single peak at a temperature between 98 and 102°C. The thermal activation energy associated with this peak was found to be 1.2 eV. An analysis of TSCs indicates that electrons are subject to fast retrapping in PS. The ratio of the mean charge depth to the sample thickness is found to increase from 0.069 to 0.12 with the increase in the temperature of polarization, indicating a higher charge accommodation at higher temperatures of polarization. The trap-modulated mobility values for electrons at the TSC peak temperatures are the order of 10⁻¹² cm² V⁻¹s⁻¹. Such low mobility values at peak temperatures are considered to indicate the excellent charge storage properties of PS.     

Thermal evolution of TiO2---ZrO2 composites prepared by chemical coating processing

A xZrO₂ (100−x)TiO₂ composite gel powder (x = 50 mol%) has been prepared by chemical processing of anatase as inner core coated with in situ precipitated amorphous zirconia as outer core, which was obtained from controlled hydrolysis of zirconyl chloride using ammonia at pH=9–10. The thermal evolution of this composite is reported, showing the crystallization of metastable cubic/tetragonal zirconia at 437°C, as detected by DTA at 10°C min⁻¹ and XRD, with activation energy of 407 ± 10 kJ mol⁻¹, and the feasibility of preparing zirconium titanate powders by reaction of TiO₂ and ZrO₂ by heating at higher temperatures is discussed.     

Effect of strain rate on high-temperature low-cycle fatigue of 17-4 PH stainless steels

The effect of strain rate (10⁻², 10⁻³ and 10⁻⁴ s⁻¹) on the low-cycle fatigue (LCF) behavior was investigated for 17-4 PH stainless steels in three different conditions at temperatures of 300–500 °C. The cyclic stress response (CSR) for Condition A tested at 300 and 400 °C showed cyclic hardening due to an influence of dynamic strain aging (DSA). An in situ precipitation-hardening effect was found to be partially responsible for the cyclic hardening in Condition A at 400 °C. For H900 and H1150 conditions tested at 300 and 400 °C, the CSR exhibited a stable stress level before a fast drop in load indicating no cyclic hardening or softening. At 500 °C, cyclic softening was observed for all given material conditions because of a thermal dislocation recovery mechanism. The cyclic softening behavior in Conditions A and H900 tested at 500 °C is attributed partially to coarsening of the Cu-rich precipitates. The LCF life for each material condition, tested at a given temperature, decreased with decreasing strain rate as a result of an enhanced DSA effect. At all given testing conditions, transgranular cracking was the common fatigue fracture mode.     

Temperature-dependent behavior of Langmuir monolayers of an amphiphilic spiropyran

The effect of subphase temperature on behaviors and morphologies of an amphiphilic spiropyran, 1′,3′-dihydro-3′,3′-dimethyl-6-nitro-1′-octadecyl-8-docosanoyloxymethylspiro[2H-1-benzopyran-2,2′[2H]indol] onto pure water was investigated by measurements of surface pressure–area isotherms and Brewster angle microscopy, respectively. Both depend strongly on the subphase temperature. The isotherms at 30 °C show a distinct phase transition at 7.5 mN m⁻¹, while no such phase transition is observed at 7 °C. The morphology changes drastically accompanying the phase transition at 7.5 mN m⁻¹, at which many large circular domains are formed. Furthermore, we have demonstrated that the morphology changes reversibly accompanying appreciable hysteresis in the isotherms upon continuous compression and expansion cycles.     

Properties of tantalum oxide thin films grown by atomic layer deposition

Thin tantalum oxide films were deposited using atomic layer deposition from TaCl₅ and H₂O at temperatures in the range 80–500 °C. The films deposited at temperatures below 300 °C were predominantly amorphous, whereas those grown at higher temperatures were polycrystalline containing the phases TaO₂ and Ta₂O₅. The oxygen to tantalum mass concentration ratio corresponded to that of TaO₂ at all growth temperatures. The optical band gap was close to 4.2 eV for amorphous films and ranged from 3.9 to 4.5 eV for polycrystalline films. The refractive index measured at λ = 550 nm increased from 1.97 to 2.20 with an increase in growth temperature from 80 to 300 °C. The films deposited at 80 °C showed low absorption with absorption coefficients of less than 100 cm⁻¹ in the visible region.     

Sintering kinetics of 8Y–cubic zirconia: Cation diffusion coefficient

Zirconia ceramics, mainly of cubic phase, are used in different applications because of their particular electrical and structural properties.

After the forming stage, sintering leads to a material with suitable microstructural characteristics. The sintering process mainly depends on thermal cycle and on starting particle size and its distribution; it also depends on density and the microstructure of green material. Cubic zirconia has a high (2680 °C) melting temperature; however, effective sintering could be observed for temperatures higher than 900 °C (nanoparticles), and it may reach a final density of 96–98% the theoretical value at relative low temperatures.

The objective of this paper is to study the sintering kinetics of stabilized zirconia in its cubic phase with 8% molar of Y₂O₃ under fast firing rates up to nearly isothermal conditions. Samples were shaped from suspensions dispersed with ammonium polyacrylate by slip casting. Sintering was performed in the temperature range between 1200 °C and 1400 °C. The sintering kinetic process was followed by measuring density as a function of time. A sintering model was applied to fit the experimental data of the first steps of densification. It was observed that sintering obeys the same mechanism in the temperature and time ranges under study, which results in an activation energy of 170 kJ mol⁻¹. Sintering is controlled by Zr cation diffusion, for which a lattice diffusion coefficient of D_l = 8 × 10⁻¹² cm² s⁻¹ at 1400 °C was found, and the activation energy of the diffusion process was 223 kJ mol⁻¹.     

Structure and conducting properties of La0.5Sr0.5CoO3−δ films on YSZ

La_0.5Sr_0.5CoO_3−δ (LSCO) thin films were deposited on yttria stabilized zirconia (YSZ) substrates by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell electrodes. During the deposition, the substrate temperature was varied from 450 to 750°C, and the oxygen pressure in the chamber was varied from 80 to 310 mTorr. Films deposited at 650°C and an oxygen background pressure of 150 mTorr were mostly (100) oriented. Deposition at higher temperatures or under lower oxygen pressures lead to mostly (110) oriented films. Films with low electrical resistivity of 10⁻³ Ω·cm were obtained.     

Influence of thermal treatments on the elastic parameters in iron rich metallic glasses

The change of magnetoelastic properties after thermal treatments has been investigated for two groups of metallic glasses. (Fe₇₉Co₂₁)_75+xSi_15−1.4xB_10+0.4x (x (at.%)=0, 2, 4, 6, 8, 10) has been studied both in the as-prepared state and after thermal annealing in an applied magnetic field, to achieve a particular domain structure, at temperatures well below the crystallization temperatures. Changes in the ΔE effect, magnetomechanical coupling (k) and internal friction coefficient (Q⁻¹) are reported, reaching values of about 60% of the saturation value E_S. Fe₆₄Ni₁₀Nb₃Cu₁Si₁₃B₉ alloys annealed in vacuum for 1 h in the temperature range 350–550 °C showed maximum values of the ΔE effect and k of 61% and 0.85, respectively, accompanied by a minimum value of Q of around 2 for the sample annealed at 460 °C. These variations are related to the progress of nanocrystalization. The properties achieved are among the best reported for magnetomechanical applications.     

Fabrication and characterization of (1−x)SrBi2Ta2O9–xBi3TaTiO9 layered structure solid solution thin films for ferroelectric random access memory (FRAM) applications

We report on the properties of (1−x)SrBi₂Ta₂O₉–xBi₃TaTiO₉ solid solution thin films for ferroelectric non-volatile memory applications. The solid solution thin films fabricated by modified metalorganic solution deposition technique showed much improved properties compared to SrBi₂Ta₂O₉. A pyrochlore free crystalline phase was obtained at a low annealing temperature of 600°C and grain size was found to be considerably increased for the solid solution compositions. The film properties were found to be strongly dependent on the composition and annealing temperatures. The measured dielectric constant of the solid solution thin films was in the range 180–225 for films with 10–50% of Bi₃TaTiO₉ content in the solid solution. Ferroelectric properties of (1−x)SrBi₂Ta₂O₉–xBi₃TaTiO₉ thin films were significantly improved compared to SrBi₂Ta₂O₉. For example, the observed remanent polarization (2P_r) and coercive field (E_c) values for films with 0.7SrBi₂Ta₂O₉–0.3Bi₃TaTiO₉ composition, annealed at 650°C, were 12.4 μC/cm² and 80 kV/cm, respectively. The solid solution thin films showed less than 5% decay of the polarization charge after 10¹⁰ switching cycles and good memory retention characteristics after about 10⁶ s of memory retention. The improved microstructural and ferroelectric properties of (1−x)SrBi₂Ta₂O₉–xBi₃TaTiO₉ thin films compared to SrBi₂Ta₂O₉, especially at lower annealing temperatures, suggest their suitability for high density FRAM applications.     

Equilibrium, kinetic and thermodynamic study of the biosorption of uranium onto Cystoseria indica algae

Biosorption equilibrium, kinetics and thermodynamics of binding of uranium ions to Cystoseria indica were studied in a batch system with respect to temperature and initial metal ion concentration. Algae biomass exhibited the highest uranium uptake capacity at 15 °C at an initial uranium ion concentration of 500 mg l⁻¹ and an initial pH of 4. Biosorption capacity increased from 198 to 233 mg g⁻¹ with an decrease in temperature from 45 to 15 °C at this initial uranium concentration. The Langmuir isotherm model were applied to experimental equilibrium data of uranium biosorption depending on temperature. Equilibrium data fitted very well to the Langmuir model C. indica algae in the studied concentration range of Uranium ions at all the temperatures studied. The saturation type kinetic model was applied to experimental data at different temperatures changing from 15 to 45 °C to describe the batch biosorption kinetics assuming that the external mass transfer limitations in the system can be neglected and biosorption is chemical sorption controlled. The activation energy of biosorption (E_A) was determined as −6.15 using the Arrhenius equation. Using the thermodynamic equilibrium coefficients obtained at different temperatures, the thermodynamic constants of biosorption (ΔG°, ΔH° and ΔS°) were also evaluated.     

Effects of refrigerated storage conditions on microbial activity of selected perishables

The effects of storage temperature and high relative humidity on the microbial activity of mature green tomato (‘Dombito’), mushroom (‘Sylvan Hybrid White’) and apple (‘Red Delicious’) were investigated at three temperatures (5, 10 and 15°C), and four different relative humidity levels (91, 94, 97 and 100%). Relative humidity effects were not found to be statistically significant for the ranges of variables investigated. The microbial counts for tomatoes were not very high during storage (<650 colony forming units per cm²), due in part to low initial counts. In mushrooms, the bacterial population was much higher (about 4 log scales) than the yeast and mould population throughout the storage period. These data also showed some evidence of a competitive relationship between bacterial and yeast and mould populations at 5°C. The final microbial count for apples at each temperature was less than 300 colony forming units per cm².