水溶性大米淀粉的研磨动力学研究

以籼米为材料，采用机械球磨的方法制备水溶性淀粉，研究不同研磨程度的淀粉的水溶性和黏性，建立基于溶解率和运动黏度的研磨动力学模型。结果表明，淀粉溶解率和运动黏度的变化规律分别用化学反应动力学模型和二次模型描述可以达到较高的拟合精度。在25℃溶解时淀粉的运动黏度随球磨时间呈先上升后下降的趋势，100℃溶解时呈指数规律下降。淀粉的含水率对研磨后的淀粉溶解率和运动黏度有较大影响。较低含水率时，溶解率的变化速度常数较高，淀粉的运动黏度可更快地达到最大值。动力学模型的建立对研磨效果的预测和指导研磨参数的确定具有较大的意义。     

Deactivation of triplet-excited riboflavin by purine derivatives: important role of uric acid in light-induced oxidation of milk sensitized by riboflavin

The reactivity of purine derivatives (uric acid, xanthine, hypoxanthine, and purine) toward triplet-excited riboflavin in aqueous solution at pH 6.4 is described on the basis of kinetic (laser flash photolysis), electrochemical (square-wave voltammetry), and theoretical data (density functional theory, DFT). Direct deactivation of triplet-excited riboflavin in aqueous solution, pH 6.4 at 24 degrees C, in the presence of uric acid, xanthine, and hypoxanthine strongly suggests a direct electron transfer from the purine to the triplet-excited riboflavin with k = 2.9 x 10(9) M(-1) s(-1) (DeltaH(++) = 14.7 kJ mol(-1), DeltaS(++) = -15.6 J mol(-1) K(-1)), 1.2 x 10(9) M(-1) s(-1) (DeltaH(++) = 34.3 kJ mol(-1), DeltaS(++) = +45.3 J mol(-1) K(-1)), and 1.7 x10(8) M(-1) s(-1) (DeltaH(++) = 122 kJ mol(-1), DeltaS(++) = +319 J mol(-1) K(-1)), respectively. From the respective one-electron oxidation potentials collected in aqueous solution at pH 6.4 for uric acid (E = +0.686 vs normal hydrogen electrode, NHE), xanthine (E = +1.106 vs NHE), and hypoxanthine (E = +1.654 vs NHE), the overall free energy changes for electron transfer from the quencher to the triplet-excited riboflavin are as follows: uric acid (DeltaG(o) = -114 kJ mol(-1)), xanthine (DeltaG(o) = -73.5 kJ mol(-1)), hypoxanthine (DeltaG(o) = -20.6 kJ mol(-1)), and purine (DeltaG(o) > 0). The inertness observed for purine toward triplet-excited riboflavin corroborates with its electrochemical inactivity in the potential range from 0 up to 2 V vs NHE. These data are in agreement with the DFT results, which show that the energy of the purine highest occupied molecular orbital (HOMO) (-0.2685 arbitrary unit) is lower than the energy of the semioccupied molecular orbital (SOMO) (-0.2557 a.u.) of triplet-excited riboflavin, indicating an endergonic process for the electron-transfer process. The rate-determining step for deactivation by purine derivatives can be assigned to an electron transfer from the purine derivative to the SOMO orbital of the triplet-excited riboflavin. The results show that uric acid may compete with oxygen and other antioxidants to deactivate triplet-excited riboflavin in milk serum and other biological fluids leading to a free radical process.     

Effects of riboflavin and fatty acid methyl esters on cholesterol oxidation during illumination

The effect of riboflavin or fatty acid methyl esters on cholesterol photooxidation was studied. Samples containing cholesterol, either alone or in combination with riboflavin or fatty acid methyl esters, were illuminated at 25 degrees C in an incubator for 28 days. The various cholesterol oxidation products (COPs) and cholesterol were analyzed by gas chromatography-mass spectrometry (GC-MS), and riboflavin was determined by HPLC. Results showed that the presence of riboflavin or fatty acid methyl esters facilitated production of COPs and degradation of cholesterol, and the degradation fits a first-order model. The COPs formed during light storage included 7 alpha-OH, 7 beta-OH, 7-keto, 3,5-cholestadien-7-one, 5,6alpha-EP, and 5,6beta-EP. The addition of riboflavin caused formation of 3,5-cholestadien-7-one through dehydration of 7-keto, whereas in the presence of docosahexaenoic acid methyl ester, the formation of 5,6alpha-EP or 5,6beta-EP was favored. Riboflavin was more effective for generation of COPs than fatty acid methyl esters.     

Improved liquid chromatographic determination of riboflavin in milk and dairy products

Reported here is a simple liquid chromatographic (LC) method for the determination of riboflavin in milk (liquid, evaporated, and dry), yogurt, and cheese. The method involves passing liquid samples or filtrates of semisolid and solid samples through a C18 cartridge. Retained riboflavin is then eluted with an aliquot of 50% methanol in 0.02M acetate buffer of pH 4. A volume of the eluate is injected into the LC system consisting of a C18 column, a solvent of water-methanol-acetic acid (65 + 35 + 0.1, v/v) with a flow rate of 1 mL/min, and a UV detector set at 270 nm. The method is precise and accurate and compares favorably with the present AOAC method. Moreover, it involves fewer sample preparation steps and has a total analysis time of less than 1 h.     

Photosensitizing effect of riboflavin, lumiflavin, and lumichrome on the generation of volatiles in soy milk

Lumichrome and lumiflavin were formed from riboflavin under light. pH had a significant influence on the formation of lumichrome and lumiflavin from riboflavin. Lumichrome was the only major product from riboflavin under neutral or acidic pH values. Lumiflavin was also formed from riboflavin in basic pH. The maximum concentration of lumiflavin from 100 microM riboflavin at pH 8.5 was 30.9 microM, and it was reached after 2 h of exposure at 1500 lux. The maximum concentration of lumichrome formed from 100 microM riboflavin at pH 4.5, 6.5, or 8.5 was 79.9, 58.7, and 73.1 microM, respectively, after 8, 6, or 2 h of light exposure. The formation of lumichrome and lumiflavin from riboflavin was due to the type I mechanism of the riboflavin photosensitized reaction. Singlet oxygen was also involved in the photosensitized degradation of lumiflavin and lumichrome. The reaction rates of riboflavin, lumiflavin, and lumichrome with singlet oxygen were 9.66 x 10(8), 8.58 x 10(8), and 8.21 x 10(8) M(-1) s(-1), respectively. The headspace oxygen depletion and headspace volatile formation were significant in soy milk containing lumichrome or lumiflavin under light (p < 0.05) and were insignificant (p > 0.05) in the dark. Ascorbic acid could inhibit the total volatile changes of soy milk under light. Soy milk should be protected from light to prevent the photodegradation of riboflavin and the oxidation of soy milk.     

Biochemical fluorometric method for the determination of riboflavin in milk

Methods of analysis of vitamin B2 in foods generally consist of the extraction of the sample, followed by enzymatic hydrolysis and quantitative measurement of the analyte, typically through RP-HPLC. The scope of our work here is to present a soft method to measure the free riboflavin content of a nontransparent and nonhomogeneous matrix such as milk, avoiding any extraction and separation of phases that are required in any published method for determination of the free RBF content in foods. We combine the front-face (FF) measurement of the light emission of milk with the ability of the apo-form of the riboflavin-binding protein (RBP) from chicken egg white to quench the riboflavin fluorescence. Thus, we titrate the RBF present in milk by gradually adding a solution of RBP to the milk sample and measuring, upon each addition, the FF residual emission due to uncomplexed RBF. The RBP binding capability has been measured in the same matrix of the analyte. Our results indicate a concentration of free RBF practically co-incident with the certified value for total B2 vitamin content in reference milk CRM 421. Keywords: Front-face fluorescence; riboflavin; apo-riboflavin-binding protein; milk fluorescence.     

Quenching mechanisms and kinetics of Trolox and ascorbic acid on the riboflavin-photosensitized oxidation of tryptophan and tyrosine

The effects of 0, 0.3, 0.6, and 0.9 mM Trolox and ascorbic acid on the singlet oxygen oxidation of tryptophan and tyrosine containing 25 ppm of riboflavin were determined by measuring tryptophan and tyrosine concentration by high-performance liquid chromatography analysis. The samples were stored in the a 1000 lx light storage box for 4 h at 30 degrees C. As the concentration of Trolox and ascorbic acid increased, the degradation of tryptophan and tyrosine decreased significantly at p < 0.05. Trolox reduced tryptophan and tyrosine degradation by quenching both singlet oxygen and excited triplet riboflavin, whereas ascorbic acid quenched singlet oxygen only. The total singlet oxygen quenchings of Trolox in the presence of tryptophan and tyrosine were 1.55 x 10(7) and 1.32 x 10(7) M(-1) s(-1), respectively. The total singlet oxygen quenchings of ascorbic acid in the presence of tryptophan and tyrosine were 1.16 x 10(7) and 1.10 x 10(7) M(-1) s(-1), respectively. Trolox was more effective than ascorbic acid in preventing the degradation of tryptophan and tyrosine.     

Phenol and terpene quenching of singlet- and triplet-excited states of riboflavin in relation to light-struck flavor formation in beer

Phenolic compounds present in beer were shown by fluorescence spectroscopy and laser flash photolysis to deactivate both singlet- and triplet-excited states of riboflavin with bimolecular rate constants close to the diffusion control ranging from 2.8x10(9) to 1.1x10(10) M-1 s-1 and from 1.1x10(9) to 2.6x10(9) M-1 s-1, respectively. Enthalpies of activation were low (up to 33.2 kJ mol-1), and entropies of activation were positive, ranging from 17 to 92 J mol-1 K-1, as derived from temperature dependence, indicating a compensation effect. From a Stern-Volmer analysis of the singlet-excited riboflavin quenching by phenols it was found that high amounts of phenolic compounds (>0.3 M) would be needed to hinder triplet-excited riboflavin generation. On the other hand, a phenolic content of 0.36 mM is likely to quench 90% of the triplet-excited state. Phenol photodegradation was found to be complex, and using ESI-MS analysis it was not possible to identify specific photooxidation products of the phenolic compounds; only the photoproducts of riboflavin could be detected and structurally assigned. The rate of reaction of triplet-excited riboflavin with phenolic compounds in acetonitrile/citrate buffer (pH 4.6, 10 mM) is 550 times faster than the reaction with iso-alpha-acids from hops, indicating that triplet-excited quenchers such as phenols may be involved in the early steps in light-struck flavor formation in beer through radical formation. Terpenes present in herb-flavored beers were found to be nonreactive toward singlet- and triplet-excited-state riboflavin, and any protection depends on other mechanisms.     

Formation of soluble and micelle-bound protein aggregates in heated milk

The formation of heat-induced aggregates of kappa-casein and denatured whey proteins was investigated in milk-based dairy mixtures containing casein micelles and serum proteins in different ratios. Both soluble and micelle-bound aggregates were isolated from the mixtures heated at 95 degrees C for 10 min, using size exclusion chromatography. Quantitative analysis of the protein composition of the aggregates by reverse phase high-performance liquid chromatography strongly suggested that primary aggregates of beta-lactoglobulin and alpha-lactalbumin in a 3 to 1 ratio were involved as well as kappa-casein, and alpha(s2)-casein in micellar aggregates. The results gave evidence that heat-induced dissociation of micellar kappa-casein was implicated in the formation of the soluble aggregates and indicated that a significant amount of kappa-casein was left unreacted after heating. The average size of the aggregates was 3.5-5.5 million Da, depending on the available kappa-casein or the casein:whey protein ratio in the mixtures. The size and density of these aggregates relative to those of casein micelles were discussed.     

含寡糖水溶肥对菠菜生长、品质和生理生化指标的影响

探讨以牛蒡提取液为溶剂,以牛蒡寡糖(水溶肥1)或壳寡糖(水溶肥2)为主成分的水溶肥在菠菜上的应用效果。采用盆栽试验,研究了水溶肥和寡糖对菠菜生长、品质、生理生化指标及地上部组织显微结构的影响。结果表明,与喷清水处理相比,市购叶面肥、水溶肥1、水溶肥2、牛蒡寡糖、壳寡糖处理的菠菜鲜重增加8.70%~16.77%;叶绿素含量增加5.18%~29.60%;硝酸盐含量降低10.82%~60.04%;草酸含量降低11.99%~67.61%;丙二醛含量降低11.94%~30.91%;氧自由基产生速率降低43.58%~72.99%。与市购叶面肥处理相比,水溶肥1处理的菠菜鲜重增加4.41%;水溶肥1、水溶肥2处理的菠菜硝酸盐和草酸含量分别降低49.03%、42.80%和11.32%、52.20%;水溶肥1处理的丙二醛含量和氧自由基产生速率分别降低4.09%和6.26%。总之,喷施水溶肥、牛蒡寡糖和壳寡糖处理均可增加菠菜生物量,提高叶绿素含量,降低硝酸盐及草酸含量,从而提高了菠菜品质。而且喷施牛蒡寡糖和壳寡糖处理的菠菜叶片丙二醛含量和氧自由基产生速率均降低,表示菠菜抗氧化能力提高,细胞膜完整性好。叶细胞排列紧密、规则,表皮整齐。综合菠菜各项指标,在本试验中水溶肥1处理的效果最好。     

Participation of soluble and oxidizable soil organic compounds in denitrification

Summary The role of soluble organic carbon (SOC) in denitrification in four mineral soils and one organic soil was evaluated in laboratory studies. Denitrification capacities and SOC concentrations were determined by nitrate loss from air-dried flooded soil treated with a solution containing 100 g/ml N0₃ ^–-N, while the rate of consumption was measured by Warburg manometry on 20 g air-dried soils to which 10 ml water had been added. High correlation coefficients (r > 0.93) were obtained between denitrification capacities, SOC, and oxygen consumption in the five soils. A mineral soil was amended with extracts of an organic soil. After incubating for 1 week, denitrification capacity was enhanced and SOC concentrations decreased in that soil. Extracted mineral soil had a lower denitrification capacity than an unextracted one. Decreases in concentrations of SOC were related to color change. Infrared spectra of precipitates from soil extracts indicated that absorption at wave number 1420–1440 cm -1 was also related to the color changes. It was implied that low molecular weight fulvic acid like compounds represented the SOC mineralized in denitrification, and that their supply to soil solution by solubilization of organic matter influenced the denitrification rate in the soil.     

Effects of ultra-high-pressure homogenization treatment on the lipolysis and lipid oxidation of milk during refrigerated storage

Free fatty acid (FFA) release and quantification and lipid oxidation extent of ultra-high-pressure homogenized (UHPH) milk samples were evaluated to assess the effect of UHPH on the susceptibility of milk lipids to lipolysis and oxidation. Milk was UHPH-treated at 200 and 300 MPa with inlet temperatures of 30 and 40 degrees C. UHPH-treated samples were compared to high-pasteurized milk (PA; 90 degrees C, 15 s). Results showed that all FFA increased significantly during storage only in 200 MPa samples. Lipid oxidation was measured as an accumulation of lipid hydroperoxides as the primary oxidation product and malondialdehyde and hexanal as the secondary oxidation products. Samples treated at 300 MPa presented higher malondialdehyde and hexanal content compared to 200 MPa treated-samples and to PA milk.     

Oxidation of diazinon by aqueous chlorine: kinetics, mechanisms, and product studies

The oxidation kinetics and mechanisms of diazinon, an organophosphorus pesticide, by aqueous chlorine were studied under different conditions. The oxidation is of first order with respect to both diazinon and chlorine. The oxidation rate is found to increase with decreasing pH. The second-order rate constants at pH 9. 5, 10.0, 10.5, and 11.0 are determined to be 1.6, 0.64, 0.43, and 0. 32 M(-)(1) s(-)(1), respectively. Based on the rate constants at different temperatures, the activation energy is calculated to be 30 kJ/mol at pH 10.0 with a chlorine-to-diazinon ratio of 11:1, 33 kJ/mol at pH 11.0 with a 11:1 ratio, and 36 kJ/mol at pH 11.0 with a 5:1 ratio, respectively. Diazoxon is identified as the oxidation product by GC-MS. Ion chromatography analysis shows an increase of sulfate concentration as the reaction proceeds, indicating that sulfur is being oxidized to sulfate. This study indicates that oxidation by aqueous chlorine can significantly affect the fate of diazinon in the environment.     

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.     

水分状况与供氮水平对土壤可溶性氮素形态变化的影响

采用通气培养试验,研究比较了两种水稻土在不同水分和供氮水平下的矿质氮(TMN)和可溶性有机氮(SON)的变化特征。结果表明,加氮处理及淹水培养均显著提高青紫泥的NH4+-N含量;除加氮处理淹水培养第7 d外,潮土NH4+-N含量并未因加氮处理或淹水培养而明显升高。无论加氮与否,控水处理显著提高两种土壤的NO3--N含量,其中潮土始见于培养第7 d,青紫泥则始于培养后21 d;加氮处理可显著提高淹水培养潮土NO3--N含量,却未能提高淹水培养青紫泥NO3--N含量。两种土壤的SON含量从开始培养即逐步升高,至培养21～35 d达高峰期,随后急剧下降并回落至基础土样的水平;SON含量高峰期,潮土SON/TSN最高达80%以上,青紫泥也达60%。综上所述,潮土不仅在控水条件下具有很强硝化作用,在淹水条件下的硝化作用也不容忽视,因此氮肥在潮土中以硝态氮的形式流失的风险比青紫泥更值得关注;在SON含量高峰期,两种土壤的可溶性有机氮的流失风险也应予以重视。