Aroma volatility from aqueous sucrose solutions at low and subzero temperatures

The gas-liquid partition coefficients of ethyl acetate and ethyl hexanoate have been measured in water and aqueous sucrose solutions from 25 to -10 degrees C by dynamic headspace. Experiments were carried out on sucrose solutions at temperatures where no ice formation was possible. Results showed that when sucrose concentration increased, aroma volatility increased except for ethyl hexanoate and in the highest sucrose concentration solution (57.5%). A quasi-linear temperature decrease on aroma volatility was observed in sucrose solutions from 25 to around 4 and 0 degrees C. Then, from 0 to -10 degrees C, aroma volatility did not decrease: ethyl acetate volatility remained constant but that of ethyl hexanoate increased. Enthalpy of vaporization and activity coefficients of the aroma compounds were calculated.     

Enzyme activity in soils at subzero temperatures

Urease activity, phosphatase activity, and sulfatase activity were detected in soils at ?10 and ?20°C. The occurrence of enzyme activity in soils at subzero temperatures is attributed to enzyme-substrate interaction in unfrozen water at the surfaces of soil particles. Support for this explanation was obtained from experiments showing that hydrolysis of urea by jackbean urease occurs at ?10 or ?20°C in the presence, but not in the absence, of clay minerals or autoclaved soils. No enzyme activity could be detected in soils at ?30°C.     

Translational diffusion coefficients of volatile compounds in various aqueous solutions at low and subzero temperatures

Translational diffusion coefficients (D(12)) of volatile compounds were measured in model media with the profile concentration method. The influence of sample temperature (from 25 to -10 degrees C) was studied on translational diffusion in sucrose or maltodextrin solutions at various concentrations. Results show that diffusivity of volatile compounds in sucrose solutions is controlled by temperature, molecule size, and the viscosity of the liquid phase as expected with the Stokes-Einstein equation; moreover, physicochemical interactions between volatile compounds and the medium are determinant for diffusion estimation. At negative temperature, the winding path induced by an ice crystal content of >70% lowered volatile compound diffusion. On the contrary, no influence on translational diffusion coefficients was observed for lower ice content.     

Modeling bleaching of tomato derivatives at subzero temperatures

This work was addressed to obtain a predictive model of the rate of bleaching in tomato derivatives at subzero temperatures. To this aim, a tomato puree was freeze-dried and equilibrated at increasing solid fractions. The bleaching rate was assessed by measuring tomato color during storage for up to 18 months at temperatures from -30 to 0 degrees C. The temperature dependence of the tomato-bleaching rate was neither predictable using the Arrhenius equation nor simply related to tomato physical state. The lack of a clear Arrhenius relation was attributed to the occurrence of temperature-dependent phenomena, such as ice crystallization and oxygen solubility modifications, which strongly changed the local concentration of reactants. A modified Arrhenius equation predicting the tomato-bleaching rate in the entire temperature range was proposed. Tomato concentration, and hence its physical state, affected the temperature dependence of bleaching, modifying apparent activation energy and frequency factor of the modified Arrhenius equation. In light of these considerations, a mathematical model was set up and validated to accurately predict the tomato-bleaching rate on the basis of only its concentration and storage temperature.     

Soybean lipoxygenase inactivation by pressure at subzero and elevated temperatures

Soybean lipoxygenase (LOX) inactivation [0.4 mg/mL in Tris-HCl buffer (0.01 M, pH 9)] was studied quantitatively under constant pressure (up to 650 MPa) and temperature (-15 to 68 degrees C) conditions and kinetically characterized by rate constants, activation energies, and activation volumes. The irreversible LOX inactivation followed a first-order reaction at all pressure-temperature combinations tested. In the entire pressure-temperature area studied, LOX inactivation rate constants increased with increasing pressure at constant temperature. On the contrary, at constant pressure, the inactivation rate constants showed a minimum around 30 degrees C and could be increased by either a temperature increase or decrease. On the basis of the calculated rate constants at 102 pressure-temperature combinations, an iso-rate contour diagram was constructed as a function of pressure and temperature. The pressure-temperature dependence of the LOX inactivation rate constants was described successfully using a modified kinetic model of Hawley.     

Ascorbic acid and ascorbic acid 6-palmitate induced oxidation in egg yolk dispersions

The oxidation in aqueous dispersions of egg yolk powder and the influence of addition of the proposed antioxidants ascorbic acid and ascorbic acid 6-palmitate indicate that both ascorbic acid and ascorbic acid 6-palmitate propagated the oxidation of egg yolk powder dispersions. Ascorbic acid 6-palmitate was found to be more prooxidative than ascorbic acid. Moreover, it was found that addition of ascorbic acid or ascorbic acid 6-palmitate gave rise to an increase in the amount of free iron Fe(II) in the egg yolk dispersions. It is proposed that ascorbic acid and ascorbic acid 6-palmitate react with the phosvitin-Fe(III) complex found in egg yolk and release Fe(II), which subsequently propagates lipid oxidation. It appears that less oxidation occurs in egg yolk dispersions exposed to high concentrations of peroxy radicals with added ascorbic acid than egg yolk dispersions with added ascorbic acid without exposure to peroxy radicals.     

Identification of alpha-tocopherol oxidation products in triolein at elevated temperatures

The effect of high-temperature treatment on the stability of alpha-tocopherol (1) in triolein was assessed under a reduced-pressure atmosphere (4-40 mbar) simulating the deodorization step of the refining of vegetable oils. A marked degradation of 1 was observed, which increased with increasing temperature (180-260 degrees C) and heating time (20-80 min). The degradation of 1 in triolein at 240 degrees C was inhibited by the addition of the synthetic antioxidant TBHQ or when heating was performed under nitrogen atmosphere, indicating oxidative degradation. The oxidation products were isolated and identified as alpha-tocopherolquinone (2), 4a,5-epoxy-alpha-tocopherolquinone (3), and 7,8-epoxy-alpha-tocopherolquinone (4).     

Adsorption of Copper by Soils in aqueous media at different temperatures

Soils of Azamgarh, U.P. have been found to fix copper fairly in good amounts. The fixation is found to be endothermic and is accompanied by a decrease in the resultant pH. The fixation is more with loam soils and generally increases with the depth of soil. pH, sesquioxide, magnesium oxide, exchangeable magnesium and C/N ratio have been found to affect the fixation. Copper is fixed as Cu(OH)⁺ in the acid systems.     

Effect of high-pressure processing at elevated temperatures on thiamin and riboflavin in pork and model systems

High-pressure/high-temperature properties of vitamins in food are important with respect to the new pressure-assisted thermal sterilization method utilizing pressure-induced adiabatic temperature changes. Riboflavin, thiamin, and thiamin monophosphate (TMP) stabilities were assayed in the temperature range from 25 to 100 degrees C under normal pressure (0.1 MPa) and high pressure (600 MPa) in acetate-buffered (pH 5.5) model solutions, some with added fructose, hemoglobin, or ascorbic acid. Thiamin and riboflavin stabilities were also assayed in minced fresh pork fillet and in rehydrated pork reference material with and without pressure treatment at 600 MPa in the temperature range from 20 to 100 degrees C. In pork, the vitamins proved to be sufficiently stabile for high-pressure/high-temperature processing. Under similar conditions, vitamin decay in model solutions was up to 30 times faster, especially that of TMP. Thus, it appears that it may not be possible to draw conclusions for the pressure behavior of real food matrices from the results of investigations in food models. A further consequence is that caution is necessary when supplementing foods with synthetic B vitamins preceding high-pressure/high-temperature processing.     

Lipoxygenase inactivation in green beans (Phaseolus vulgaris L.) due to high pressure treatment at subzero and elevated temperatures

The kinetics of lipoxygenase (LOX) inactivation in green beans due to high-pressure treatment were studied in the pressure-temperature area of 0.1 up to 650 MPa and -10 up to 70 degrees C for systems with different levels of food complexity, i.e., in green bean juice and intact green beans (in situ study). For both systems, LOX was irreversibly inactivated by high-pressure treatment combined with subzero and elevated temperatures and the inactivation could be described as a first-order reaction. At ambient pressure, in situ LOX was less thermostable than in the juice at temperatures below 68 degrees C whereas the stability ranking was reverse at temperatures above 68 degrees C. At temperatures below 63 degrees C, sensitivity of the inactivation rate constants to temperature changes was on the same order of magnitude in the juice and in situ, while at higher temperature it was lower in situ. The pressure needed to obtain the same rate of LOX inactivation at a given temperature was lower in situ than in the juice. Application of high-pressure treatment at low/subzero temperature resulted in an antagonistic effect on LOX inactivation for both systems, whereas no such effect was found above room temperature. The pressure-temperature dependence of the LOX inactivation rate constants in green beans was successfully modeled.     

Binding of 2-nonanone and milk proteins in aqueous model systems

Interactions of the model flavor compound 2-nonanone with individual milk proteins, whey protein isolate (WPI), and sodium caseinate in aqueous solutions were investigated. A method to quantify the free 2-nonanone was developed using headspace solid-phase microextraction followed by gas chromatography with flame ionization detection. Binding constants (K) and numbers of binding sites (n) for 2-nonanone on the individual proteins were calculated. The 2-nonanone binding capacities decreased in the order bovine serum albumin > beta-lactoglobulin > alpha-lactalbumin > alpha s1-casein > beta-casein, and the binding to WPI was stronger than the binding to sodium caseinate. All proteins appeared to have one binding site for 2-nonanone per molecule of protein at the flavor concentrations investigated, except for bovine serum albumin, which possessed two classes of binding sites. The binding mechanism is believed to involve predominantly hydrophobic interactions.     

Contribution of lipid oxidation products to acrylamide formation in model systems

The reactions of asparagine with methyl linoleate ( 1), methyl 13-hydroperoxyoctadeca-9,11-dienoate ( 2), methyl 13-hydroxyoctadeca-9,11-dienoate ( 3), methyl 13-oxooctadeca-9,11-dienoate ( 4), methyl 9,10-epoxy-13-hydroxy-11-octadecenoate ( 5), methyl 9,10-epoxy-13-oxo-11-octadecenoate ( 6), 2,4-decadienal ( 7), 2-octenal ( 8), 4,5-epoxy-2-decenal ( 9), and benzaldehyde ( 10) were studied to determine the potential contribution of lipid derivatives to acrylamide formation in heated foodstuffs. Reaction mixtures were heated in sealed tubes for 10 min at 180 degrees C under nitrogen. The reactivity of the assayed compounds was 7 > 9 > 4 > 2 > 8 approximately 6 > 10 approximately 5. The presence of compounds 1 and 3 did not result in the formation of acrylamide. These results suggested that alpha,beta,gamma,delta-diunsaturated carbonyl compounds were the most reactive compounds for this reaction followed by lipid hydroperoxides, more likely as a consequence of the thermal decomposition of these last compounds to produce alpha,beta,gamma,delta-diunsaturated carbonyl compounds. However, in the presence of glucose this reactivity changed, and compound 1/glucose mixtures showed a positive synergism (synergism factor = 1.6), which was observed neither in methyl stearate/glucose mixtures nor in the presence of antioxidants. This synergism is proposed to be a consequence of the formation of free radicals during the asparagine/glucose Maillard reaction, which oxidized the lipid and facilitated its reaction with the amino acid. These results suggest that both unoxidized and oxidized lipids are able to contribute to the conversion of asparagine into acrylamide, but unoxidized lipids need to be oxidized as a preliminary step.     

Cadmium and arsenic adsorption in aqueous systems in the presence of monosilicic acid

The effect of different concentrations of monosilicic acid on the sorption capacity of quartz sand, diatomite, zeolite, and brown coal with respect to cadmium and arsenic has been studied in laboratory conditions. The applicability of different adsorption models, including exponential, semilogarithmic, and linear dependences, as well as the Langmuir and Freundlich equations, has been compared. An increase in monosilicic acid concentration from 0 to 2 mM increased the sorption capacity of all materials. It has been suggested that either the interaction of monosilicic acid with a pollutant takes place directly in the solution followed by sorption of the reaction products on the surface or in the pores of the sorbent, or first adsorption of the monosilicic acid by the sorbent occurs followed by interaction of the sorbed silicic acid with cadmium and arsenic; it is also possible that both process proceed in parallel.     

Thermal stability of 5-o-caffeoylquinic acid in aqueous solutions at different heating conditions

Chlorogenic acid is a naturally occurring phenolic compound found in all higher plants. This component, being the ester of caffeic acid with quinic acid, is an important biosynthetic intermediate and plays an important role in the plant's response to stress. Potential uses of chlorogenic acid are suggested in pharmaceuticals, foodstuffs, feed additives, and cosmetics due to its recently discovered biomedical activity. This finding caused new interest in chlorogenic acid properties, its isomers, and its natural occurrence. It has been found that as many as nine compounds (chlorogenic acid derivatives and its reaction product with water) can be formed from 5-o-caffeoylquinic acid during the heating of its water solution. Three of them, two hydroxylated 5-o-caffeoylquinic acid derivatives and 4,5-dicaffeoylquinic acid, have been not reported, yet. The amount of each formed component depends on the heating time and temperature. The presented results are important for researchers investigating plant metabolism and looking for new plant components. The transformation product can be mistakenly treated as a new component, not found before in the examined plant, or can be a cause of erroneous quantitative estimations of plant composition.     

鲜带鱼不同贮藏温度的货架期预测模型（简报）

为了研究鲜带鱼在冷链流通中的品质变化与货架期,通过不同温度下的贮藏试验研究了鲜带鱼的货架期预测模型。将鲜带鱼贮藏在268、273、278、283和293 K条件下,测定了鲜带鱼的总菌落数、总挥发性盐基氮（T-VBN）、鲜度指标（K值）与感官品质指标的变化。在Arrhenius动力学方程基础之上,建立了菌落总数、总挥发性盐基氮和鲜度指标（K值）与贮藏时间及贮藏温度之间的动力学模型。试验表明一级化学反应动力学模型和Arrhenius方程对总菌落数、总挥发性盐基氮（T-VBN）及鲜度指标（K值）的变化具有较高的拟合精度。菌落总数变化预测模型中的活化能（EA）及速率常数（k0）分别为：71.26 kJ/mol和3.987×1013,挥发性盐基氮变化的活化能（EA）及速率常数（k0）分别为：68.86 kJ/mol和2.159×1012,鲜度指标（K值）变化的活化能（EA）及速率常数（k0）分别为：41.26 kJ/mol和2.539×107。结果表明,鲜带鱼的总菌落数、总挥发性盐基氮（T-VBN）、鲜度指标（K值）随着贮藏时间的延长而增加,且随着贮藏温度的升高而增加迅速,其感官品质指标随着贮藏时间的延长而下降,且随着贮藏温度的升高而下降迅速。该试验建立的带鱼货架期预测模型所获得货架期预测值准确率达到±10%以内,可根据菌落总数、T-VBN值及K值在268～293 K范围内,对带鱼的剩余货架期进行预测。     

冷却猪肉不同贮藏温度的货架期预测模型

为了建立冷却猪肉货架期的预测模型,把特定腐败菌接种到无污染的冷却猪肉表面,托盘包装分别置于0℃,4℃,7℃,10℃,14℃和20℃的温度下贮藏,分别测定不同贮藏时间的细菌总数,同时对4℃贮藏的不同企业冷却猪肉进行品质分析,确定腐败限控量.结果表明,冷却猪肉腐败限控量为7.23 lg(cfu/g).应用修正的Gompertz函数能很好的描述特定腐败菌在不同温度下的生长动态,建立了6种温度下其在猪肉中的生长模型.温度对最大比生长速率和延滞时间等动力学参数的影响,采用平方根模型呈现良好的线性关系,模型残差值的绝对值均小于0.1,上下浮动于零左右,表明该模型描述的温度与比生长速率和延滞时间是可信的,由此建立了0～20℃范围内冷却猪肉贮藏过程中货架期的预测模型.     

Protein insolubilization and thiol oxidation in sulfite-treated wheat gluten films during aging at various temperatures and relative humidities

Gluten films were prepared by casting an acidic and ethanolic solution of gluten previously treated with sodium sulfite. The effects of sulfitolysis on proteins were investigated by SE-HPLC and thiol/disulfide content measurements. During sulfitolysis, insoluble glutenin macropolymer was converted into its constitutive subunits. About 10% of gluten disulfide bonds were cleaved, of which three-fourths originated from interchain disulfide bonds. Oxidation of thiol groups released during sulfitolysis was followed for various temperatures (T) and relative humidities. Oxidation was shown to be a second-order rate process occurring below the glass transition temperature (T(g)) and related to T - T(g). Thiol oxidation ensured the formation of interchain bonds between specific classes of gluten proteins according to an ordered process. Intrachain bonds were also formed and through thiol/disulfide-exchange reactions were finally converted to interchain bonds. Thus, fully oxidized gluten films had more insoluble glutenin macropolymers than native gluten.     

Ascorbic acid-induced browning of (+)-catechin in a model wine system

The ability of ascorbic acid to induce browning of (+)-catechin in a model wine system has been studied. A significant increase in absorbance at 440 nm was observed over 14 days when ascorbic acid was incubated at 45 degrees C with (+)-catechin in a model wine base. The onset of browning was delayed for about 2 days, although the length of the lag period was dependent on the amount of molecular oxygen in the headspace of the reaction system. The lag period was not observed when a preoxidized solution of ascorbic acid was used, suggesting that a product of ascorbic acid oxidation is responsible for the onset of browning. Hydrogen peroxide, when added directly to (+)-catechin in the model system, was not capable of producing the same degree of browning as that generated by ascorbic acid. Liquid chromotography evidence is presented to show that different reaction products are produced by ascorbic acid and hydrogen peroxide.     

Structure-reactivity relationships of flavan-3-ols on product generation in aqueous glucose/glycine model systems

Ring structure-reactivity relationships of three flavan-3-ols [epicatechin (EC), epicatechin gallate (ECG), and epigallocatechin gallate (EGCG)] and three simple phenolic compounds (1,3,5-trihydroxybenzene, 1,2,3-trihydroxybenzene, and methylgallate as the analogous individual A, B, and C benzene rings of EGCG) on product generation in an aqueous glucose-glycine reaction model system (125 degrees C and 30 min) were investigated. The addition of EC, ECG, or EGCG to a glucose-glycine model was reported to similarly significantly reduce the formation of pyrazine, methyl-substituted pyrazines, and cyclotene. All three flavan-3-ols were also reported to generate phenolic-C2, C3, C4, and C6 sugar fragment adducts and to statistically reduce the concentration of glyoxal, glycolaldehyde, methylglyoxal, hydroxyacetone, diacetyl, acetoin, and 3-deoxyglucosone during the reaction time course, except for the EGCG reaction where 3-deoxyglucosone was not statistically different from the control after 20 min. For the simple phenolic compounds, methylgallate followed by 1,2,3-trihydroxybenzene was the least reactive, while 1,3,5-trihydroxybenzene was reported as the most reactive phenolic structure for quenching or reducing the concentration of the alpha-hydroxy- and alpha-dicarbonyl sugar fragments during the reaction time course. These results imply that the main mechanism flavan-3-ols reduced product generation was phenolic-sugar fragment carbonyl trapping reactions primarily on the A ring (the meta-polyhydroxylated benzene ring) or not due to the alteration of the reaction reduction potential.     

Ultraviolet-induced oxidation of ascorbic acid in a model juice system: identification of degradation products

Degradation products of ultraviolet (UV-C, 254 nm) treated ascorbic acid (AA) are reported. Analysis by high-performance liquid chromatography-mass spectroscopy (HPLC-MS) conducted in a 0.5% malic acid model juice system (pH 3.3) demonstrated increased degradation of AA above untreated controls with concomitant increases in dehydroascorbic acid (DHA) and 2,3-diketogulonic acid (DKGA) levels. Electron spin resonance (ESR) spectroscopy studies, conducted in phosphate buffer (pH 7.0) to increase detection sensitivity, demonstrated that ascorbyl radical (AA?) formation occurs simultaneously with AA degradation. Consistent with a previous study in which UV treatments were shown to accelerate dark storage degradation, AA? radicals continued to form for up to 200 min after an initial UV treatment. Results from this study suggest that the mechanism for UV-induced degradation is the same as the general mechanism for metal-catalyzed oxidation of AA in juice.