Viscoelastic properties of high pressure and heat induced tofu gels

Tofu gels were rheologically examined to determine their storage or elastic (G') and loss or viscous (G '') moduli as a function of frequency within their linear viscoelastic limits. The tofu gels were made using either glucono-delta-lactone (GDL) or calcium sulphate (CaSO4 center dot 2H(2)O), followed by either heat treatment (heated soymilk at >= 97 degrees C prior to coagulation and subsequently held at 70 degrees C for 60 min, HT) or high pressure treatment (400 MPa at 20 degrees C for 10 min, HP). The overall moduli values of the GDL gels and CaSO4 center dot 2H(2)O gels of both physical treatments were similar, each gave frequency profiles expected for weak viscoelastic materials. However, although both temperature and high pressure treatments could be used to produce tofu gels, the final products were not the same. Pressure formed gels, despite having a higher overall "consistency" (increasing values of their moduli), had a proportionately higher contribution from the loss modulus (increased tan delta). Differences could also be observed using confocal scanning laser microscopy. While such treatment may give rise to differing systems/structures, with new or modified organoleptic properties, the more "open" structures obtained by pressure treatment may well cause processing difficulties if subsequent reworking or moulding is required. (c) 2007 Elsevier Ltd. All rights reserved.

Modeling the variability of single-cell lag times for Listeria innocua populations after sublethal and lethal heat treatments

Optical density measurements were used to estimate the effect of heat treatments on the single-cell lag times of Listeria innocua fitted to a shifted gamma distribution. The single-cell lag time was subdivided into repair time ( the shift of the distribution assumed to be uniform for all cells) and adjustment time (varying randomly from cell to cell). After heat treatments in which all of the cells recovered (sublethal), the repair time and the mean and the variance of the single-cell adjustment time increased with the severity of the treatment. When the heat treatments resulted in a loss of viability (lethal), the repair time of the survivors increased with the decimal reduction of the cell numbers independently of the temperature, while the mean and variance of the single-cell adjustment times remained the same irrespective of the heat treatment. Based on these observations and modeling of the effect of time and temperature of the heat treatment, we propose that the severity of a heat treatment can be characterized by the repair time of the cells whether the heat treatment is lethal or not, an extension of the F value concept for sublethal heat treatments. In addition, the repair time could be interpreted as the extent or degree of injury with a multiple-hit lethality model. Another implication of these results is that the distribution of the time for cells to reach unacceptable numbers in food is not affected by the time-temperature combination resulting in a given decimal reduction.

Interaction of heat-moisture conditions and physical properties in oat processing: II. Flake quality

Product quality is an important determinant of consumer acceptance. Consistent oat flake properties are thus necessary in the mill as well as in the marketplace. The effects of kilning and tempering conditions (30, 60 or 90 min at 80, 95 or 110 degrees C) on flake peroxidase activity, size, thickness, strength and water absorption were therefore determined. After kilning, some peroxidase activity remained but steaming and tempering effectively destroyed the activity of these enzymes. Thus the supposed protective effect of kilning or groat durability was not confirmed. Kilning resulted in an increase in flake specific weight, but no other significant effect on flake quality was observed. Tempering time and temperature interacted significantly to produce complex effects on flake specific weight, thickness and water absorption. Flake thickness and specific weight were significantly correlated (r = 0.808, n = 54). Longer tempering times resulted in an increased fines' fraction, from 1.45% at 30 min to 1.75% at 90 min. It is concluded that whilst kilning has little effect on flake quality, the heat treatment immediately prior to flaking, can be used to adjust flake quality independently of flake thickness.

Interaction of heat-moisture conditions and physical properties in oat processing: I. Mechanical properties of steamed oat groats

Research interest in oats has focussed on their nutritional value, but there have been few studies of their food processing. Heat treatment is characteristic of oat processing, as it is needed to inactivate lipase and to facilitate flaking. A Texture Analyser was used to characterise the mechanical properties of unkilned and kilned oat groats after steaming and tempering in an oven for 30, 60 and 90 min at 80, 95 and 110 degrees C. Maximum force, number of peaks before maximum and final force after 5s hold were used to characterise the behaviour of the groats during compression. Kilned groats were larger and softer before steaming. After steaming and tempering, the moisture content of the kilned groats was higher than for unkilned groats. Hot, steamed oats were softer than cold, unsteamed groats, indicated by a decrease in maximum force from 59 to 55 N, and there was no significant difference between kilned and unkilned groats. However, higher temperatures during tempering increased maximum force. These results suggest that mild steam treatment yields softer oat groats, whereas cold or over-treated groats tend to be harder. (c) 2007 Elsevier Ltd. All rights reserved.

A novel technique for differentiation of proteins in the development of acid gel structure from control and heat treated milk using confocal scanning laser microscopy

The incorporation of caseins and whey proteins into acid gels produced from unheated and heat treated skimmed milk was studied by confocal scanning laser microscopy (CSLM) using fluorescent labelled proteins. Bovine casein micelles were labelled using Alexa Fluor 594, while whey proteins were labelled using Alexa Fluor 488. Samples of the labelled protein solutions were introduced into aliquots of pasteurised skim milk, and skim milk heated to 90 degrees C for 2 min and 95 degrees C for 8 min. The milk was acidified at 40 degrees C to a final pH of 4.4 using 20 g gluconodelta-lactone/l (GDL). The formation of gels was observed with CSLM at two wavelengths (488 nm and 594 nm), and also by visual and rheological methods. In the control milk, as pH decreased distinct casein aggregates appeared, and as further pH reduction occurred, the whey proteins could be seen to coat the casein aggregates. With the heated milks, the gel structure was formed of continuous strands consisting of both casein and whey protein. The formation of the gel network was correlated with an increase in the elastic modulus for all three treatments, in relation to the severity of heat treatment. This model system allows the separate observation of the caseins and whey proteins, and the study of the interactions between the two protein fractions during the formation of the acid gel structure, on a real-time basis. The system could therefore be a valuable tool in the study of structure formation in yoghurt and other dairy protein systems.

Heat-inactivated peroxidases and the role of calcium abstraction as a cause of their enhanced lipid oxidation activity: potential effects on the flavour quality of heat-processed vegetables

Cationic swede and anionic turnip peroxidases were partially purified by ion-exchange and gel-filtration chromatography, respectively. Heat treatment of these enzymes and of a commercial high purity horseradish peroxidase (HRP) caused a loss of enzyme activity and a corresponding increase in linoleic acid hydroperoxide formation activity. The hydroperoxide levels in model systems increased only in the early stages of the oxidation reaction and then declined as degradation became more significant. The presence of a dialysed blend of cooked swede markedly lowered the hydroperoxide level formed. Analysis of volatile compounds formed showed that hexanal predominated in a buffer system and in a blend of cooked turnip. In dialysed blends of cooked swede, hexanol was the primary volatile compound generated. After inactivation under mild conditions in the presence of EDTA, the peroxidases showed hydroperoxide formation activity and patterns of volatile compounds from linoleic acid that were similar to those found on heat-inactivation. This suggested that calcium abstraction from the peroxidases was critical for the enhancement of lipid oxidation activity. (C) 2002 Elsevier Science Ltd. All rights reserved.

Structural Evaluation of Mechanically Alloyed and Heat-Treated Ti-25at-%Si Powders

This work discusses on the structural evaluation of mechanically alloyed and heat-treated Ti-25at%Si powders. The milling process was conducted in a planetary ball mill using stainless steel balls/vials, 200 rpm and ball-to-powder weight ratio of 5:1, whereas the heat treatment was conducted under Ar atmosphere at 1100 C for 4 h. Samples were characterized by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy and energy dispersive spectrometry. The Si peaks disappeared after milling for 30h, indicating that the Si atoms were dissolved into the Ti lattice in order to form an extended solid solution. The Ti peaks were broadened and their intensities reduced for longer milling times whereas a halo was formed in Ti-25Si powders milled for 200h suggesting that an amorphous structure was achieved. The crystallite size was decreased with increasing milling times. A large Ti3Si amount was found in mechanically alloyed Ti-25at%Si powders after heating at 1100 degrees C for 4h.

Influence of thermal annealing treatment in oxygen atmosphere on grain boundary chemistry and non-ohmic properties of SnO2 center dot MnO polycrystalline semiconductors

The present work studied the influence of thermal treatment in oxygen rich atmosphere on heterogenous junctions in Mn-doped SnO2 polycrystalline system presenting varistor behavior. The samples were prepared by conventional oxide mixture methodology, and were submitted to heat treatment in oxygen rich atmosphere at 900 degrees C for 2h. The samples were characterized by X-ray diffraction, scanning electron microscopy, dc and ac electrical measurements. The results showed that there is an evident relationship between the microstructure heterogeneity and non-ohmic electrical properties. It was found that for this SnO2 center dot MnO-based varistor system the heat treatment in oxygen rich atmosphere does not necessarily increase the varistors properties, which was related to the decrease in the grain boundary resistance. The results are compared with Co-doped SnO2 varistors and ZnO based varistors. (C) 2008 WILEY-VCH Verlay GmbH & Co. KGaA, Weinheim.

Electrochemical and structural characterization of heat-treated Cr3C2-NiCr coatings

The influence of heat-treatments on the electrochemical behavior of thermal spray Cr3C2-NiCr coatings prepared by high velocity oxygen fuel (HVOF) was studied in NaCl solution, at 25 degrees C, using open-circuit potential (E-OC) and electrochemical impedance spectroscopy (EIS) measurements. Coating characterization were performed before and after the heat-treatments and electrochemical tests by scanning electron microscopy, X-ray diffraction, and Auger electron spectroscopy. In addition to the changes in the original powder composition occurring during HVOF process, heat-treatment performed at 450 degrees C caused no significant changes in electrochemical response compared with untreated sample, and at 760 degrees C the main difference was the formation of a thin and defective layer of Cr2O3 at the coating surface, which increased the total impedance at the first day of immersion. Higher influence on the electrochemical was noted for samples treated at 880 degrees C, which also showed higher E-OC and total impedance, and lower corrosion current. This behavior was interpreted considering the formation of a chromium oxide layer on the coating surface, dissolution and decomposition of smaller carbide particles and their surface enrichment with Cr due to C diffusion and dissolution into the matrix, and possible Ni, Cr, and Fe diffusion to coating/substrate interface. (c) 2006 the Electrochemical Society.

EFFECT OF THE ADDITION OF CR AND NB ON THE MICROSTRUCTURE AND ELECTROCHEMICAL CORROSION OF HEAT-TREATABLE AL-ZN-MG ALLOYS

The effect of the addition of Cr and Nb on the microstructure and the electrochemical corrosion of the weldable, high-strength and stress corrosion cracking (SCC) resistant Al-5%Zn-1.67%Mg-0.23%Cu alloy (H) has been studied. Combined additions of the alloying elements, J (with Nb), L (with Cr) and O (with Cr and Nb) and different heat treatments, ST (cold-rolled), A (annealed), F (quenched), B (quenched and aged) and C (quenched in two steps and aged), to obtain different microstructures and hardness have been performed. To correlate the electrochemical corrosion with the microstructure of the specimens, corrosion potential (E(cor)) measurements in different chloride solutions were performed and optical microscopy, SEM, TEM and EDX were applied. In chloride solutions containing dissolved O-2 or H2O2, the present alloys were polarized up to the pitting attack. It was shown that the E(cor) measurements were very sensitive to the alloy composition and heat treatment, increasing in the order H < J < L < O < Al (for a given heat treatment) and F < A approximate to ST < B < C (for a given alloy). The MgZn2 precipitates of the annealed (A) and cold-rolled (ST) specimens were dissolved in chloride solutions containing oxidizing agents and pitting attack was shown to develop in the cavities where the precipitates were present. In the specimens B and C, the compositions of the precipitate free zones was found to be equal to that of the matrix solid solution and preferential intergranular attack was not evident, this being in agreement with their SCC resistance. The addition of Cr and Nb increased the pitting corrosion resistance. The effects of Cr and Nb were additive, that of Cr being predominant, either, in the E(cor) shift or in the increase in the pitting corrosion resistance.

UV-visible absorption characteristics of TEOS-derived and Cr-doped silica sonogels aged in different pH solutions and heat treated up to 600 degrees C

Cr-doped xerogels were obtained by sol-gel process from the acid-catalyzed and ultrasound-stimulated hydrolysis of tetraethoxysilane (TEOS) with addition of CrCl3.6H(2)O in water solution during the liquid step of the process. The gels were aged immersed in different pH solutions for about 30 days, after that they were allowed to dry. The samples were annealed at temperatures ranging from 40 to 600degreesC and analyzed by UV-visible absorption spectroscopy. Cr3+ is the preferable oxidation state of the chromium ion in the gels annealed up to 250-300degreesC, in the case of aging in solutions of pH=5 and 11. A high UV absorption below similar to320 nm, due to the host gel, and different absorption bands, depending on the temperature, due to the chromium ion were observed in the xerogels at temperatures below similar to250degreesC, in the case of aging in solutions of pH=1 and 2. These absorption bands have not been assigned. Above 300degreesC up to 600degreesC, Cr5+, and possibly Cr6+, are the preferable oxidation states of the chromium ion independent of the pH of the aging solution, so the xerogels turn to a yellowish appearance in all cases.

Hydrothermal treatment of gadolinium oxide in presence of silica

Hydrothermal and solvothermal (isopropanol) treatments of gadolinium oxide and silica were investigated under different pressure and temperature conditions. Products were examined by infrared vibrational spectroscopy (IR), x-ray powder diffraction (XRD) and thermal analysis (DTA, TG). Hexagonal gadolinium hydroxide was obtained in hydrothermal conditions, even in presence of silica, while no change was observed from isopropanolic medium treatment. Hydrothermally treated samples are more reactive as precursors for solid state reactions in inorganic synthesis.

Preparation and characterization of glass-ceramic materials in the BaO-Li2O-ZrO2-SiO2 system and their dependence on treatment temperatures

The preparation and characterization of transparent glass-ceramics in the composition of 30Li2O:5ZrO2:xBaO:(100-x) SiO2 with x = 0, 5, 10, 15, and 20 mol% are described. Glasses were melted in a platinum crucible at 1100°C for 2 h and then heat-treated at 900°C for 3 h. The characterizations were performed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman and infrared spectroscopy, and scanning electron microscopy (SEM). The experimental results indicate that there was a structural change in the glass-ceramics as the BaO concentration in the mixture increased. In the XRD patterns of samples without heat treatment, only the halo was observed. After heat treatment, the appearance of the materials was verified by X-ray diffraction peaks. The reorganization of the amorphous solid was confirmed by Raman and IR spectroscopy along with XPS and SEM, with a more homogeneous phase formation being observed.

Thermal treatment effects on phase transitions in P(VDF-TrFE)

Differential scanning calorimetry (DSC) studies were performed for 60/40 P(VDF-TrFE). The results not only confirm the importance of thermal history but also show that the samples with various configurations in terms of ferroelectric phases can be obtained via thermal treatment.

Effects of thermal treatment on phase transitions and on the mechanical relaxation in poly(vinylidene fluoride)

This work reports on the effects from thermal treatment in poly(vinylidene fluoride), PVDF, obtained with differential scanning calorimetry (DSC) and dynamic thermal analysis (DMA) measurements. It is shown that in successive DMA measurements performed with one sample the α relaxation peak almost disappears while the γ′ peak appears. The α relaxation peak, at ∼100°C in DMA measurements, is attributed to the preferential orientation of chains in the amorphous phase while the γ′ relaxation peak, at ∼50°C, is related to the thermal treatment to which the sample was submitted.