Nonenzymatic browning in food models in the vicinity of the glass transition: effects of fructose,glucose, and xylose as reducing sugar

Effects of a reducing sugar, fructose, glucose, or xylose, and glass transition on the nonenzymatic browning (NEB) rate in maltodextrin (MD), poly(vinylpyrrolidone) (PVP), and water systems were studied. Glass transition temperatures (T(g)) were determined using DSC. Water contents were determined gravimetrically, and NEB rates were followed at several temperatures spectrophotometrically at 280 and 420 nm. Reducing sugar did not affect water contents, but xylose reduced the T(g) of the solid models. Sugars showed decreasing NEB reactivity in the order xylose > fructose > glucose in every matrix material. The NEB reactivity and temperature dependence of the single sugars varied in different matrices. The NEB rates of the solid models increased at temperatures 10-20 degrees C above the T(g), and nonlinearity was observed in Arrhenius plots in the vicinity of T(g). The temperature dependence of nonenzymatic browning could also be modeled using the WLF equation.     

Color formation in dehydrated modified whey powder systems as affected by compression and T(g)

Whey powders have attracted attention for use in the food industry. The Maillard reaction is a major deteriorative factor in the storage of these and other dairy food products. The objective of the present work was to further study the Maillard reaction as related to the physical structure of the matrix, either porous or mechanically compressed, or to storage above the T(g) of anhydrous whey systems. Sweet whey (W), reduced minerals whey (WRM), whey protein isolate (WPI), and whey protein concentrate (WPC) were stored in ovens at selected temperatures. Colorimetric measurements were performed with a spectrocolorimeter, thermal analyses (TGA) by means of a thermobalance, and glass transition temperature studies by DSC. The browning order in the vials and in the compressed systems followed the order W > WRM> WPC > WPI. k(w2), the slope of the second linear segment of the TGA curve, was related to the loss of water due to nonenzymatic browning (NEB). Browning development was in good relationship with this loss of weight. In the glassy state, the compressed systems developed higher rates of browning and weight loss (assigned to NEB reactions) than the porous systems. Reaction rates in both (porous and compressed) systems became similar as (T - T(g)) increased.     

Changes in water content in and under frozen soil in Iowa,U.S.A.

Water contents were measured in a soil profile during the winter from the time of harvest until soil temperatures were greater than 0°C through this profile in the spring. Water movement between successive layers was predicted using the Darcy flow law and changes in water flux. These values were then compared with the changes in soil water content observed in field measurements. Results showed that water content changes (accumulation or loss) below the frozen layer were greater than predicted but that the differences between predicted and measured contents were small, much smaller than the amounts reported in previous experiments.     

Influence of thermal processing conditions on acrylamide generation and browning in a potato model system

Fried potato products such as French fries and chips may contain substantial amounts of acrylamide. Numerous efforts are undertaken to minimize the acrylamide content of these products while sensory properties such as color and flavor have to be respected as well. An optimization of the frying process can be achieved if the basic kinetic data of the browning and acrylamide formation are known. Therefore, heating experiments with potato powder were performed under controlled conditions (moisture, temperature, and time). Browning and acrylamide content both increased with heating time at all temperatures and moisture contents tested. The moisture content had a strong influence on the activation energy of browning and acrylamide formation. The activation energy strongly increased at moisture contents below 20%. At higher moisture contents, it was very similar for both parameters. At low moisture contents, the activation energy of acrylamide formation was larger as compared to the one for browning. This explains why the end of the frying process is very critical. Therefore, a lower temperature toward the end of frying reduces the acrylamide content of the product while color development is still good.     

Kinetics of freshly squeezed orange juice quality changes during ozone processing

Freshly squeezed orange juice samples were ozonated with control variables of gas flow rate (0-0.25 L min (-1)), ozone concentration (0.6-10.0%w/w), and treatment time (0-10 min). Effects of ozone processing variables on orange juice quality parameters of pH, degrees Brix, titratable acidity (TA), cloud value, nonenzymatic browning (NEB), color values ( L*, a*, and b*), and ascorbic acid content were determined. No significant changes in pH, degrees Brix, TA, cloud value, and NEB ( p < 0.05) were found. L*, a*, and b* color values were significantly affected by gas flow rate, ozone concentration, and treatment time. The changes in lightness ( L*) values and total color difference (TCD) values were fitted well to zero-order kinetics, whereas a*, b*, and ascorbic acid degradation followed first-order kinetics. The rate constants for a*, b*, and TCD were linearly correlated with ozone concentration ( R (2) = 0.88-0.99), whereas the rate constants for L* and ascorbic acid were exponentially correlated ( R (2) = 0.94-0.98).     

Influence of drying temperature, water content, and heating rate on gelatinization of corn starches

The gelatinization properties of starch extracted from corn and waxy corn dried at different temperatures were determined at various water contents and heating rates by differential scanning calorimetry. All gelatinization transition temperatures increased with drying temperature and heating rate. Onset and peak temperatures remained relatively constant, whereas end temperature decreased in the presence of excess water. The gelatinization enthalpy (deltaH(g)) of corn starch decreased with drying temperature at 50% water; however, it remained constant for waxy corn starch. The effects of water content and heating rate on deltaH(g) were dependent on each other. The minimum water levels required for gelatinization of starch extracted from corn dried at 20 and 100 degrees C are 21 and 29%, respectively. The activation energy (E(a)) was calculated using an Arrhenius-type equation and two first-order models; the degree of conversion (alpha) was predicted using a newly proposed model that produced good results for both E(a) and alpha.     

Role of water upon the formation of acrylamide in a potato model system

The moisture sorption isotherms of a commercial potato powder were investigated at 20 degrees C for water activities ranging from 0.11 to 0.97. The sorption isotherms were typical type-II sigmoidal curves, with a steep increase in moisture content for water activities above 0.9 and exhibiting hysteresis over the whole water activity range. On the basis of the isotherms, the influence of the initial water activity and moisture content on both Maillard browning and acrylamide formation was determined by heating oil containing potato powder mixtures in a closed stainless-steel tubular reactor. The Maillard browning, as determined spectrophotometrically, showed an optimum at intermediate water activities. The yields of acrylamide, expressed relatively to the molar amount of asparagine, remained constant below 0.8 aw and below moisture contents of about 20% (on a dry basis). For the more intense heat treatments, an increased acrylamide yield was however observed at higher moisture contents, with an optimum at water contents of about 100% (on a dry basis). However, this increase and optimum was not observed at less intense heat treatments. At moisture contents above 100%, a significant decrease in acrylamide yields was assessed, although the water activity increased only marginally in this area of the sorption isotherms. It was thus observed that the acrylamide content was rather dependent upon the moisture content than upon the water activity in the high-moisture potato powder model system.     

Measuring and Modeling Grain Sorption Equilibria of Amaranth Grains

Water activity (a_w)of Amaranthus cruentus were measured at 35, 45, and 65°C at different moisture contents by means of an electronic water activity meter. Experimental curves were fitted to one three-parameter equation (GAB equation). The equilibrium values and temperature shifts were modeled with a three-parameter equation in the approximate a_w range of 0.03–0.88. An analytical expression to calculate the isosteric heat of sorption and its moisture content dependence was used, and the results were compared with the isosteric heat calculated from the experimental equilibrium data by means of the Clausius-Clapeyron equation.     

Effect of the pyrrole polymerization mechanism on the antioxidative activity of nonenzymatic browning reactions

The present investigation was undertaken to study how the antioxidative activity (AA) of nonenzymatic browning reactions is changing at the same time that the browning (by the pyrrole polymerization mechanism) is being produced. The antioxidative activities of eight model pyrroles (pyrrole, 1-methylpyrrole, 2,5-dimethylpyrrole, 1,2,5-trimethylpyrrole, 2-acetylpyrrole, 2-acetyl-1-methylpyrrole, pyrrole-2-carboxaldehyde, and 1-methyl-2-pyrrolecarboxaldehyde) as well as the browning reaction of 2-(1-hydroxyethyl)-1-methylpyrrole (HMP) and the dimer (DIM) produced during HMP browning were determined. The results obtained suggest that the AAs observed in nonenzymatic browning reactions are the result of the AAs of the different oxidized lipid/amino acid reaction products formed. Thus, the different pyrrole derivatives produced in these reactions had different AAs, and the highest AAs were observed for alkyl-substituted pyrroles without free alpha-positions. Because some of these pyrrole derivatives are implicated in nonenzymatic browning production and this browning production implies the loss of hydroxyl groups and the transformation of some pyrroles with one type of substitution into others, changes in AA during browning production were observed, and the resulting DIM derivative was more antioxidant than HMP or higher polymers. These results explain the AA observed in fatty acid/protein mixtures after slight oxidation and suggest that, when the pyrrole polymerization mechanism is predominant, slightly browned samples may be more antioxidant than samples in which nonenzymatic browning has been highly developed.     

Stability of dried and intermediate moisture tomato pulp during storage

Commercial tomato pulp was air-dried to two final moisture contents in order to obtain intermediate moisture pulp (IMP, 23% moisture) and dried pulp (DP, 9% moisture). IMP and DP were stored at 4, 20, and 37 degrees C for approximately 5 months; periodically samples were analyzed to evaluate heat and oxidative damage by measurement of color changes, total phenolics, rutin, lycopene and furosine concentrations, and antioxidant activity of the lipophilic extract. IMP and DP, despite different drying degree, had similar antioxidant activity; in fact, whereas lycopene was stable to drying treatments, ascorbic acid was totally degraded in both products. During storage, IMP and DP showed different behavior: IMP was more sensitive to degradation than DP, especially with regard to lycopene, rutin, and antioxidant activity degradation and to nonenzymatic browning. Effects of storage temperature varied with regard to different parameters: variations in rutin, furosine, and color indices were higher in products stored at higher temperatures, while lycopene and antioxidant activity of the lipophilic fraction were maximally degraded in products stored at 4 degrees C.     

Antioxidant properties of water extracts from Cassia tora L. in relation to the degree of roasting

The antioxidant properties of water extracts from Cassia tora L. (WECT) prepared under different degrees of roasting were investigated. The water extracts of unroasted C. tora L. (WEUCT) showed 94% inhibition of peroxidation of linoleic acid at a dose of 0.2 mg/mL, which was higher than that of alpha-tocopherol (82%). Water extracts prepared from C. tora L. roasted at 175 degrees C for 5 min and at 200 degrees C for 5 min exhibited 83% and 82%, respectively, inhibition of linoleic acid peroxidation. This result indicated that the antioxidant activities of WECT decreased with longer roasting time or higher roasting temperature. The IC(50) of WEUCT in liposome oxidation induced by the Fenton reaction was 0.41 mg/mL, which was higher than that of alpha-tocopherol (IC(50) = 0.55 mg/mL). WEUCT also exhibited good antioxidant activity in enzymatic and nonenzymatic microsome oxidative systems. The water extracts of roasted C. tora L. increased in the degree of browning and produced chemiluminescence when compared with the unroasted sample. However, the total polyphenolic compounds of WECT decreased after the roasting process finished. In conclusion, the decrease in the antioxidant activity of water extracts from roasted C. tora L. might have been due to the degradation of Maillard reaction products and the decrease of polyphenolic compounds.     

植烟黄壤氮素矿化动态模拟研究

研究不同温度和水分条件下植烟土壤有机氮的矿化动态,为田间土壤氮素矿化的预测提供依据。采用好气间歇淋洗方法,探求不同温度培养模式[恒温20℃,恒温35℃,变温(5、10、15、20、25、30、35、30、25、20℃)],不同温度(5~40℃)与不同土壤含水量(风干土~53%)交互作用下的土壤有机氮矿化动态,并建立回归方程。运用田间土壤氮素矿化数据,进行模型验证。结果显示,变温培养下土壤氮素矿化动态与恒温培养显著不同,变温下土壤矿化氮的累积动态以积温模型的拟合效果最好;指数模型能够较好描述土壤有机氮矿化对土壤水分含量的反应。在土壤氮素矿化积温模型和水分函数的基础上,建立了变化温度与水分条件下的土壤氮素矿化模型。田间实测矿化数据验证了此模型的可行性。因此,可以利用有效积温和土壤含水量来估测田间土壤氮素矿化量。     

The interdependent effects of soil temperature and water content on soil respiration rate and plant root decomposition in arid grassland soils

Soil respiration (CO₂ evolution), soil temperature (1 dm) and water content (0–1dm) were determined over a 2 yr period in a grassland soil of the arid shrub-steppe. Respiration was due primarily to decomposition of plant roots by soil organisms. Although respiration rate was generally limited by soil temperature in the fall, winter and early spring and by soil water content in the late spring and summer, temperature and water content were interdependent in their effects on soil respiration rate. Soil organisms responded to changes in soil temperatures at water contents as low as 1–2 per cent (106-88 bar suction). Above approximately 6° C, increased soil water content resulted in increased soil respiration rate. but the extent of the increase was non-linear and dependent upon soil temperature. Respiration rate approached a maximum at soil water contents of 6–10 per cent (35-13 bar suction) depending upon soil temperature and was generally optimum at temperatures above 15° C. The mutual regulation of soil respiration rate by temperature and moisture during this study was best described by a soil temperature-water interaction or multiplicative term, and regression equations which included this term served to accurately predict seasonal changes in soil respiration rate. Using a simple regression equation which included only the interaction term, it was possible to account for 70 per cent of the total variation in soil respiration rate during the monitoring period.     

Manothermosonication of foods and food-resembling systems: effect on nutrient content and nonenzymatic browning

The effect of manothermosonication (MTS), an emergent technology for food preservation, on thiamin, riboflavin, carotenoids, and ascorbic acid was evaluated in milk and orange juice. The effect of both heat treatment and MTS on several compounds produced in nonenzymatic browning in model systems was also studied. MTS does not affect significantly the nutrient content studied. However, it changes the behavior of nonenzymatic browning. No formation of 5-(hydroxymethyl)-2-furfuraldehyde (HMF) was detected in fruit juice model systems after heat and MTS treatments at the experimental conditions used. In a milk-resembling system, free HMF formation by MTS is higher compared to that by heat treatment. As the MTS temperature increases, free HMF production by both treatments equaled on another. For bound HMF the production rate is lower by MTS than by heat treatment under the experimental conditions used. Formation kinetics of brown pigments and that of fluorescent compounds are different for both treatments. Fluorescence and brown pigment production are faster in MTS.     

Modeling the effect of water activity and storage temperature on chemical stability of coffee brews

This work was addressed to study the chemical stability of coffee brew derivatives as a function of water activity (aw) and storage temperature. To this purpose, coffee brew was freeze-dried, equilibrated at increasing aw values, and stored for up to 10 months at different temperatures from -30 to 60 degrees C. The chemical stability of the samples was assessed by measuring H3O+ formation during storage. Independently of storage temperature, the rate of H3O+ formation was considerably low only when aw was reduced below 0.5 (94% w/w). Beyond this critical boundary, the rate increased, reaching a maximum value at ca. 0.8 aw (78% w/w). Further hydration up to the aw of the freshly prepared beverage significantly increased chemical stability. It was suggested that mechanisms other than lactones' hydrolysis, probably related to nonenzymatic browning pathways, could contribute to the observed increase in acidity during coffee staling. The temperature dependence of H3O+ formation was well-described by the Arrhenius equation in the entire aw range considered. However, aw affected the apparent activation energy and frequency factor. These effects were described by simple equations that were used to set up a modified Arrhenius equation. This model was validated by comparing experimental values, not used to generate the model, with those estimated by the model itself. The model allowed efficient prediction of the chemical stability of coffee derivatives on the basis of only the aw value and storage temperature.     

黏弹性材料动态力学性能的分数阶时温等效模型与验证

为构造一种更加精准的黏弹性时温等效模型,基于黏弹性阻尼材料的分数阶本构关系和Vogel-Fulcher-Tammann黏度方程,结合WLF(Williams-Landel-Ferry)方程,提出了黏弹性阻尼材料动态力学性能的分数阶时温等效模型(fractional time-temperature superposition model,FTTSM),导出频率转化因子,并给出参数识别方法。在DMA(dynamic thermomechanical analysis)试验数据的基础上,对比研究了FTTSM和WLF两种模型,并应用分数阶Kelvin-Voigt模型对二者在参考温度5℃的主曲线进行了验证。结果表明,FTTSM和WLF所表征的频率转化因子在温度范围(–80~80℃)内最大相对误差为0.984 4%,FTTSM主曲线和WLF主曲线相对于Kelvin-Voigt模型理论预测值的均方根误差(RMSE)分别为1.291和1.834 MPa,验证了FTTSM的精确性。另外FTTSM主曲线扩展的最低频域低于WLF主曲线2个数量级,证明FTTSM对黏弹性动态力学特性具有更广泛的预测能力。为黏弹性材料动态性能预测、物理老化、蠕变损伤演化机理等研究提供理论参考。     

Comparison of the ion exclusion chromatographic method with the Monier-Williams method for determination of total sulfite in foods

Experimental data comparing the alkali extraction/ion exclusion chromatographic method with the Monier-Williams method for determination of total sulfite are presented in (a) enzymatic and nonenzymatic browning systems, (b) vegetables containing naturally occurring sulfite, and (c) a carbohydrate-type food additive, erythorbic acid. Excellent agreement, with a linear correlation coefficient of 0.99, was observed in fresh potato samples homogenized with sulfite and allowed to react for different time intervals (enzymatic browning system). A good overall correlation was observed in dehydrated, sulfited apple samples heated for different times (nonenzymatic browning system); however, as heating time increased, higher results were obtained by the Monier-Williams method than by the alkali extraction/ion exclusion chromatographic method. The results of determining sulfite in the alkali trapping solution following acid distillation or acid treatment without heat suggested that this deviation was due to a fraction of sulfite bound to the browning reaction products in such a way that it was released by acid distillation but not by alkali extraction or acid treatment without heat. Similar behavior was demonstrated in cabbage with naturally occurring sulfite, which was released by acid distillation but not by alkali extraction or acid treatment without heat. The ion exclusion chromatographic method could overcome interference by the volatile caramelization reaction products in the Monier-Williams determination of erythorbic acid.     

Influence of processing parameters on acrylamide formation during frying of potatoes

Consistent evidence suggests that the probable human carcinogen acrylamide is formed in starch-rich foodstuffs through heat-induced interaction of asparagine and reducing sugars during Maillard browning. However, information regarding the influence of processing parameters on acrylamide formation is scarce. We investigated the impact of temperature, heating time, browning level, and surface-to-volume ratio (SVR) on acrylamide generation in fried potatoes. Acrylamide content was determined by liquid chromatography (LC) and electrospray ionization tandem mass spectrometry (ESI-MS/MS). In potato shapes with low SVR, acrylamide content consistently increased with increasing temperature and processing times. By contrast, in shapes with intermediate to high SVR, maximal acrylamide formation occurred at 160-180 degrees C, while higher temperatures or prolonged processing times caused a decrease of acrylamide levels. Moreover, browning levels were not a reliable measure of acrylamide content in large-surface products.     

Temperature effect on lactose crystallization, maillard reactions, and lipid oxidation in whole milk powder

Whole milk powder with an initial water content of 4.4% (w/w) and a water activity of 0.23 stored in hermetically sealed vials for up to 147 days below (37 and 45 degrees C) and above (55 degrees C) the glass transition temperature (T(g) determined to have the value 48 degrees C) showed a strong temperature dependence for quality deterioration corresponding to energies of activation close to 200 kJ/mol for most deteriorative processes. The glass transition was found not to cause any deviation from Arrhenius temperature dependence. Lactose crystallization, which occurred as a gradual process as monitored by isothermal calorimetry, is concluded to liberate bound water (a(w) increase to 0.46) with a modest time delay (approximately 2 days at 55 degrees C) and with concomitant surface browning as evidenced by an increasing Hunter b-value. Browning and formation of bound hydroxymethyl-furfural determined by HPLC seem to be coupled, while formation of another Maillard reaction product, furosine, occurred gradually and was initiated prior to crystallization. Initiation of lipid oxidation, as detected by lipid-derived radicals (high g-value ESR spectra), and progression of lipid oxidation, as detected by headspace GC, seem not to be affected by lactose crystallization and browning, and no indication of browning products acting as antioxidants could be determined.     

Predicting diurnal variations in water content along with temperature at the soil surface

Under field conditions, much of the uncertainty regarding the magnitude of nitrogen loss as ammonia from surface-applied urea fertilizers arises from the interaction of short-term fluctuations in soil water and temperature with chemical processes. A mechanistic model to combine present knowledge of individual components of the volatilization process may provide insight into the variability. Thus, the ability to adequately predict fluctuations in surface water content and temperature is desirable.Diurnal fluctuations in soil surface water contents and temperatures were compared with predictions from a model based on the finite difference solution of the coupled soil water and heat flow equations and on simulated mass and energy balances at the soil surface. The measured values were taken over several weeks on Muir silt loam (fine-silty, mixed, mesic, Pachic Haplustolls) and Haynie very fine sandy loam (coarse-silty, mixed, mesic, Typic Udifluvents). Small diurnal fluctuations in the surface soil water content were simulated closely by the model for both soils. The fluctuations of soil temperature were simulated well by the model for both soils, even under overcast skies. Generally, there was good agreement (< 1.7°C average difference) between modeled and measured temperatures.