Textural Optimization of Shelf‐Stable Bread: Effects of Glycerol Content and Dough‐Forming Technique

The effects of glycerol content and dough‐forming method on the physical, textural, and sensory characteristics of shelf‐stable bread were determined. Bread dough was produced with 0, 2, 4, and 6% nominal glycerol content, and formed into rolls by either dough‐dividing or extrusion‐forming methodologies. Baked products were evaluated by uniaxial compression and fitting of stress‐strain data to a three‐parameter mathematical model. A trained sensory panel quantified textural attributes using magnitude estimation methodology. Selected characteristics were also judged by an untrained consumer panel. Sensoryinstrumental relationships were determined. Products were tested instrumentally after different storage intervals to determine effects of glycerol level and dough‐forming process on degree of firming. Results showed that extrusion‐forming produced, on average, relatively more dense and less deformable products than did the dough‐dividing method; extrusionformed samples also had greater sensory firmness and were less similar to an ideal sensory texture. However, high glycerol concentrations in extrusion‐formed products gave sensory profiles that were substantially closer to the ideal. Sensory firmness and chewiness were closely correlated with parameters of power law functions that described compression behavior. Glycerol reduced ultimate firmness after storage.     

Content and Molecular Weight Distribution of Oat β‐Glucan in Oatrim,Nutrim, and C‐Trim Products

The soluble fiber, β‐glucan, in oat products is an active hypolipidemic component that is responsible for lowering plasma lipids. Quantitative analysis of β‐glucan in oat hydrocolloids such as Oatrim, Nutrim, and C‐Trim was performed to measure the total β‐glucan content and molecular weight distribution. For the measurement of total β‐glucan content, both modified flow‐injection analysis (FIA) method and the standard AACC enzymatic method were employed. FIA method uses the enhanced fluorescence produced when β‐glucan forms complexes with Calcofluor. Experimental results of both the modified FIA method and the standard AACC enzymatic method revealed very good coincidence with each other. This result confirms the applicability of either technique for the quantitative evaluation of β‐glucan in hydrocolloids. Molecular weight (MW) distribution of β‐glucan was determined by size‐exclusion chromatography with postcolumn detection. Experimental results revealed that the molecular weight of β‐glucan in the Trim products was decreased during the manufacturing process. This result was ascribed to the rigorous processing condition of jet‐cooking.     

Effect of Different Enzymes on the Textural Stability of Shelf‐Stable Bread

Three enzyme systems (2 amylase‐based and 1 protease‐based) were tested in shelf‐stable bread to determine effectiveness in preserving texture during storage for eight weeks. Each enzyme was tested in formulations without glycerol or with 6% glycerol. Bread samples were analyzed to determine physical properties (crumb density, crust‐to‐crumb ratio, rate of moisture distribution from crumb to crust), mechanical properties (modulus, and a parameter [C1] describing resistance to high levels of deformation obtained by fitting stress‐strain data to a three‐parameter function), and thermal properties (thermal stability and enthalpy of transitions) as a function of storage time. Mechanical properties were further analyzed to predict asymptotic firmness. Bread firmness after storage as evaluated in terms of modulus and C1 were lower in all enzyme‐added systems, the effect of protease being the most significant. Enzymes had less effect on glycerol‐containing systems with no apparent trend. The breads had complex thermal behavior and exhibited multiple transitions. Both amylase preparations in the presence of glycerol reduced the amount of starch recrystallization.     

Time‐resolved in‐situ pH measurement in differently treated,saturated and unsaturated soils

The pH‐value is of utmost relevance for soil properties and functioning. Hence, a time‐resolved in‐situ measurement is mandatory but lacking. As an alternative, a two‐probe pH electrode with gel‐covered reference electrode was newly constructed and tested for a continuous, in‐situ pH recording in saturated and unsaturated soil. This was done using samples from a set of 14 soils with different composition and pH$ _{\rm CaCl_2} $ ranging from 3.5 to 7.5 in batch and repacked soil column experiments. In the latter, changes in pH and redox potential were monitored upon transport of citrate‐phosphate buffer and pig slurry through the soil columns. The pH measurements were largely stable even upon substantial shifts in soil moisture content down to air‐dry conditions. The results of the pH measurements agreed with standard methods using settled soil suspensions in electrolyte solutions and the conventional combination (single‐probe) pH electrode. Testing the suspension effect, it was found that measuring pH directly in the soil is recommended. The pH measured in‐situ was closest to pH values determined in 0.01 M CaCl₂ suspensions according to DIN ISO 10390 (DIN, 2005 ). The transport of citrate buffer and pig slurry as pH active substances through soil induced strong effects on the pH and in part on the redox potential; the reversible effects lasted over days, which may affect the mobility and speciation of nutrients and pollutants as well as microbial processes.     

Water Self‐Diffusion Coefficient and Staling of White Bread as Affected by Glycerol

Water self‐diffusion coefficient (D) was investigated in bread crumb during storage to determine the effect of moisture loss and glycerol on the staling mechanism. D increased with added glycerol in breads of the same moisture content. D remained unchanged after storage without crust (with no moisture loss from crumb to crust). When stored with crust (with moisture loss), more mobile water was lost (probably from glycerol), resulting in a more rapid initial decrease in D in glycerol‐added bread. Competition of water may be a key influencing factor. Glycerol and loss of moisture (according to crumb‐crust moisture gradient) triggered a shift in moisture redistribution from starch and gluten to glycerol. This could have contributed to the increased structural rigidity and more rapid firming of the glycerol‐added bread. As a result, a greater firming rate was observed in glycerol‐added bread even with less amylopectin recrystallization as compared with the control.     

Quantitative methods for estimating foliar uptake of zinc from suspension‐based Zn chemicals

The present study developed methods for quantifying foliar Zn uptake from suspension‐based Zn chemicals of low solubility, which were ZnO (particle size: 0.15^–1.34 μm) and a newly synthesized Zn hydroxide nitrate crystal (Zn‐HNC) (50^–100 nm thickness and 0.2^–1 μm lateral dimension). Recently matured leaves of citrus (Citrus aurantium L. cv. Valencia), capsicum (Capsicum annume L. cv. Giant Bell), and/or tomato (Solanum lycopersicum L. cv. Roma) were in vitro–treated with microdroplets (5 μL per droplet) of Zn‐HNC‐ and ZnO‐suspension solutions on the adaxial surface and incubated under controlled conditions for up to 72 h. Leaf‐washing protocols were compared, including: dilute ethanol (3%), dilute nitric acid (2%), and their combination. The methods for quantifying Zn uptake were: (1) whole‐leaf loading by which droplets of the Zn suspension solutions were loaded onto central regions of both left and right sides of leaf blades and (2) half‐leaf loading by which soluble‐Zn (ZnSO₄) droplets were loaded onto only one side of leaf blades while the other was used as the background Zn control. Foliar‐surface characteristics of the plant species affected the effectiveness of the washing methods. The dilute nitric acid (2%; ± 3% ethanol) was required to remove residue particles of the ZnO and Zn‐HNC suspensions from foliar surfaces of capsicum and tomato (highly trichomatic), but the residue Zn chemicals on citrus leaves (nontrichomatic and highly waxy) were similarly and effectively removed by the three washing methods. For quantifying Zn uptake by the leaves, the half‐leaf loading method showed its advantages over the whole‐leaf loading method, because it did not stringently require similar background Zn concentrations in the control and treated leaves at the start and had little risks of secondary absorption of soluble Zn in the washing solution.     

Impact of effective microorganisms and other biofertilizers on soil microbial characteristics,organic‐matter decomposition,and plant growth

Incubation and pot experiments were conducted to investigate the impact of commercially distributed biofertilizers (effective microorganisms [EM], BIOSTIMULATOR, BACTOFIL‐A, and BACTOFIL‐B) on soil microbial‐biomass content and activity, net N mineralization in soil, and growth of Lolium perenne. According to the manufacturers, the products tested are based on microbial inoculants or organic growth stimulants, and are supposed to influence soil microbial properties and improve soil conditions, organic‐matter decomposition, and plant growth. In the incubation experiment (40 d, 20.6°C, 50% maximum water‐holding capacity), EM was repeatedly applied to soil together with different organic amendments (nonamended, chopped straw, and lupine seed meal). Under the experimental conditions of this study, no or only marginal effects of EM on organic C, total N, and mineral N in soil could be observed. In soil treatments without any organic amendment, EM suspension slightly enhanced microbial activity measured as soil CO₂ evolution. In soil with easily degradable plant residues (lupine seed meal), EM suspension had a suppressive effect on microbial biomass. However, comparisons with sterilized EM and molasses as the main additive in EM suspension showed that any effect of EM could be explained as a pure substrate effect without the influence of added living organisms. In the pot experiment with Lolium perenne (air‐conditioned greenhouse cabin, 87 d, 16.8°C, 130 klxh d^–1 light quantity), the products EM, BIOSTIMULATOR, BACTOFIL‐A, and BACTOFIL‐B were tested in soil with growing plants. The products were repeatedly applied for a period of 42 d. Within this study, no effects of the different biofertilizers on mineral N in soil were detectable. There were clear suppressive effects of all tested biofertilizers on microbial‐biomass content and activity. Comparisons with sterilized suspensions showed that the effects were not due to living microorganisms in the suspensions, but could be traced back to substrate‐induced processes.     

Rheological Characterization of a New Oat Hydrocolloid and Its Application in Cake Baking

A new oat hydrocolloid containing 20% β‐glucan, called C‐trim20, was obtained from oat bran concentrate through steam jet‐cooking and fractionations. The rheological characterization of the C‐trim20 was conducted using steady and dynamic shear measurements. The C‐trim20 suspension exhibited a shear‐thinning behavior that was more pronounced at high shear rates and high concentrations. Its dynamic viscoelastic moduli increased with increasing concentration while the frequency at which G′ and G″ crossover decreased. The C‐trim20 suspension at various concentrations followed the Cox‐Merz rule. C‐trim20 was also evaluated for potential use in baked products, specifically cakes. The baking performance of C‐trim20 was tested by incorporating it into cake formulations. The inclusion of this hydrocolloid gave increased elastic properties to cake batters and produced cakes containing 1 g of β‐glucan per serving with volume and textural properties similar to those of the control cake.     

Soil‐phosphorus distribution and availability as affected by greenhouse subsurface irrigation

Vertical distribution and plant availability of soil P under subsurface irrigation were investigated in a 5‐year tomato‐grown‐greenhouse experiment. Irrigation was applied when soil water condition reached the predefined maximum allowable depletion (MAD) for different treatments, e.g., –10 kPa, –16 kPa, –25 kPa, –40 kPa, and –63 kPa. Results show that P distribution with soil depth was significantly affected by irrigation schedules. The general trend is that concentrations of soil total P and inorganic P were greater in topsoil than in subsoil, whereas the concentrations of soil organic P were larger at the depths of 0–10 cm, 30–40 cm, and 40–60 cm than at other soil depths. Comparison of different irrigation schedules indicates that more soil organic P was retained in the soils under the MAD of –25 kPa, –40 kPa, and –63 kPa, implying that irrigation of relatively low frequency and large water quantity of each irrigation event favored the accumulation of organic P in soils. In addition, we found that the concentrations of plant‐available P decreased with soil depth and were largest under the MAD of –16 kPa and –25 kPa. This result suggests that irrigation of relatively high frequency and low water quantity of each irrigation event led to greater P availability for plant uptake. Overall, this study suggests that the transformation and plant availability of soil P can be manipulated, to some degree, by soil‐water management. Maximum allowable depletion controlled between –16 kPa and –25 kPa could result in high availability of soil P in clay‐textured soils.     

Effects of a Dual‐Frequency Frequency‐Sweeping Ultrasound Treatment on the Properties and Structure of the Zein Protein

We investigated the effects of a dual‐frequency frequency‐sweeping ultrasound (DFFSU) treatment on the functional properties and structure of zein. The solubility of ultrasound‐treated zein proteins increased slightly but significantly as the treatment time increased. The results showed that the DFFSU treatment had an obvious influence on the mean particle size and the size distribution. A significant (P < 0.05) increase in the size of the particles with respect to time was observed after a sonication time of more than 20 min in zein solutions. Differential scanning calorimetry results showed that sonication alters the thermal behavior of zein. Circular dichroism spectra showed a small increase in the percentage of ordered structure elements within the protein molecule. After 60 min of ultrasonication, the percentage of α‐helix structures increased by 0.9%, whereas the percentage of β‐sheets and β‐turns decreased by 0.5%. Microstructural analyses by scanning electron microscopy showed that several microholes appeared in the zein following ultrasonic pretreatment. Under the conditions investigated in this study, DFFSU treatment was found to affect the studied functional properties of the zein protein. This technology could be used to obtain improved functional properties in some protein samples.     

Microbial uptake of low‐molecular‐weight organic substances out‐competes sorption in soil

Low‐molecular‐weight organic substances (LMWOS) such as amino acids, sugars and carboxylates, are rapidly turned over in soil. Despite their importance, it remains unknown how the competition between microbial uptake and sorption to the soil matrix affects the LMWOS turnover in soil solution. This study describes the dynamics of LMWOS fluxes (10 µm ) in various pools (dissolved, sorbed, decomposed to CO₂ and incorporated into microbial biomass) and also assesses the LMWOS distribution in these pools over a very wide concentration range (0.01–1000 µm ). Representatives of each LMWOS group (glucose for sugars, alanine for amino acids, acetate for carboxylates), uniformly ¹⁴C‐labelled, were added to sterilized or non‐sterilized soil and analysed in different pools between 1 minute and 5.6 hours after addition. LMWOS were almost completely taken up by microorganisms within the first 30 minutes. Surprisingly, microbial uptake was much faster than the physicochemical sorption (estimated in sterilized soil), which needed 60 minutes to reach quasi‐equilibrium for alanine and about 400 minutes for glucose. Only acetate sorption was instantaneous. At a concentration of 100 µm , microbial decomposition after 4.5 hours was greater for alanine (76.7 ± 1.1%) than for acetate (55.2 ± 0.9%) or glucose (28.5 ± 1.5%). In contrast, incorporation into microbial biomass was greater for glucose (59.8 ± 1.2%) than for acetate (23.4 ± 5.9%) or alanine (5.2 ± 2.8%). Between 10 and 500 µm , the pathways of the three LMWOS changed: at 500 µm , alanine and acetate were less mineralized and more was incorporated into microbial biomass than at 10 µm , while glucose incorporation decreased. Despite the fact that the LMWOS concentrations in soil solution were important for competition between sorption and microbial uptake, their fate in soil is mainly determined by microbial uptake and further microbial transformations. For these substances, which represent the three main groups of LMWOS in soil, the microbial uptake out‐competes sorption.     

Long‐term no‐tillage effects on particulate and mineral‐associated soil organic matter under rainfed Mediterranean conditions

Soil organic carbon (SOC) plays an essential role in the sustainability of natural and agricultural systems. The identification of sensitive SOC fractions can be crucial for an understanding of SOC dynamics and stabilization. The objective of this study was to assess the effect of long‐term no‐tillage (NT) on SOC content and its distribution between particulate organic matter (POM) and mineral‐associated organic matter (Min) fractions in five different cereal production areas of Aragon (north‐east Spain). The study was conducted under on‐farm conditions where pairs of adjacent fields under NT and conventional tillage (CT) were compared. An undisturbed soil nearby under native vegetation (NAT) was included. The results indicate that SOC was significantly affected by tillage in the first 5 cm with the greatest concentrations found in NT (1.5–43% more than in CT). Below 40 cm, SOC under NT decreased (20–40%) to values similar or less than those under CT. However, the stratification ratio (SR) never reached the threshold value of 2. The POM‐C fraction, disproportionate to its small contribution to total SOC (10–30%), was greatly affected by soil management. The pronounced stratification in this fraction (SR>2 in NT) and its usefulness for differentiating the study sites in terms of response to NT make POM‐C a good indicator of changes in soil management under the study conditions. Results from this on‐farm study indicate that NT can be recommended as an alternative strategy to increase organic carbon at the soil surface in the cereal production areas of Aragon and in other analogous areas.     

Isolation,Fractionation, and Structural Characteristics of Alkali‐Extractable β‐Glucan from Rye Whole Meal

The main nonstarch polysaccharide of rye is arabinoxylan (AX), but rye contains significant levels of (1→3)(1→4)‐β‐d ‐glucan, which unlike oat and barley β‐glucan, is not readily extracted by water, possibly because of entrapment within a matrix of AX cross‐linked by phenolics. This study continues objectives to improve understanding of factors controlling the physicochemical behavior of the cereal β‐glucans. Rye β‐glucan was extracted by 1.0N NaOH and increasing concentrations of ammonium sulfate were used to separate the β‐glucan from AX and prepare a series of eight narrow molecular weight (MW) distribution fractions. Composition and structural characteristics of the isolated β‐glucan and the eight fractions were determined. High‐performance size‐exclusion chromatography (HPSEC) with both specific calcofluor binding and a triple detection (light scattering, viscometry, and refractive index) system was used for MW determination. Lichenase digestion followed by high‐performance anion exchange chromatography of released oligosaccharides, was used for structural evaluation. The overall structure of all fractions was similar to that of barley β‐glucan.     

Textural Stability of Intermediate‐Moisture Extrudates: Effects of Formulation

Effects of formulation on the textural stability of intermediate‐moisture, flour‐based, “jerky”‐type extrudates were assessed. Potato‐based extrudates containing various particulate‐meat concentrations and different plasticizers (sucrose, fructose, glycerol, and glucose) were produced and subjected to accelerated storage for three weeks. The elastic modulus of the samples was measured before storage and then weekly. The relative fluidity and moisture mobility of the specimens were assessed by dynamic mechanical spectrometry (DMS), electron spin resonance (ESR), and nuclear magnetic resonance (NMR). Samples were also evaluated by fluorometry and X‐ray diffraction to determine the extent of browning reaction and degree of molecular ordering, respectively. While elastic modulus increased appreciably during storage, firming was progressively reduced by entrained meat content and also by plasticizers, especially glycerol; plasticized and meat‐containing samples had correspondingly lower tan δ peak temperatures as measured by DMS. Textural results were also in keeping with fluidity and local viscosity as assessed by ESR measurements. NMR T1 relaxation values, reflecting moisture mobility, increased during storage. Diffraction spectra were consistent with published observations of hydrated starch, suggesting that water may have been released due to increased association of proteinaceous constituents. Fluorescence measurements confirmed moderate Maillard browning in all samples and significant chlorogenic browning in glucose‐containing samples, although these effects were unrelated to degree of firming. It was concluded that textural stability was optimized by interruption of the matrix by dispersed meat or by plasticization by low molecular weight constituents.     

Rheological‐stiffness analysis of K+‐treated and CaCO3‐rich soils

Rheological methods are applied whenever flow behavior of substances needs to be investigated on a particle‐to‐particle scale executed by a parallel‐plate rheometer. Under oscillation, mechanical effects due to trafficking or vibrations caused by agricultural and forest machinery can be simulated by conducting amplitude‐sweep tests. Hooke's law of elasticity, Newton's law for ideal fluids (viscosity), Mohr‐Coulomb's equation, and, finally, Bingham's yielding are well‐known relationships and parameters in the field of rheology. This paper aims to introduce rheometry as a suitable method to determine the mechanical behavior of salt‐affected soils when subjected to external stresses. Potassium‐treated loamy sand from Halle and loamy silt from Kassel, both sites located in Germany, as well as loess from Israel, saturated with NaCl solutions in several concentrations were analyzed. From the stress‐strain–relationship parameters like the storage modulus G′ and the loss modulus G″, yield stress τ_y and the linear viscoelastic (LVE)–deformation range including the deformation limit γ_L, i.e., the transition from an elastic to a viscous state, were determined and calculated, respectively. With respect to salt effects, amplitude‐sweep tests on originally CaCO₃‐rich Avdat Loess show an increasing stability if saturated with higher NaCl concentrations. Comparable tests with K⁺‐rich substrates from Halle and Kassel evinced similar tendencies including the phenomenon of a critical K⁺ content, which becomes more obvious in case of the drained (–60h Pa) loamy‐silt samples from Kassel. Nevertheless, a higher microstructural stability is given in both substrates from Halle and Kassel, affected by different water contents, in general, which influence the exchange and availability of cations. The results verify that oscillatory tests are applicable for retracing salt‐induced effects, beside those ones, which are influenced by texture, current water content, and/or further chemical parameters.     

Coronatine‐induced lateral‐root formation in cotton (Gossypium hirsutum) seedlings under potassium‐sufficient and ‐deficient conditions in relation to auxin

A large root system plays a decisive role in potassium (K)‐acquisition efficiency of cotton. Coronatine (COR), a non‐host‐specific phytotoxin, may affect the auxin level in plants and might therefore be useful in regulating lateral‐root (LR) development. Our objectives were (1) to examine the effects of COR on root development, especially the LR formation in hydroponically grown cotton seedlings, and (2) to explore possible mechanisms involved. The results showed that K deficiency (0.05 mM) significantly reduced LR formation in cotton seedlings, possibly due to the decrease of endogenous indole acetic acid (IAA) in roots by more than half. Following the application of 10 nM COR, the LRs significantly increased by 26% in K‐sufficient (0.5 mM) solution and by 95% in K‐deficient solution. Although COR did not increase the free IAA level in the primary root, the polar auxin‐transport inhibitor N‐1‐naphthylphthalamic acid (NPA) decreased its stimulating effects on LR formation by 25%–30%, suggesting that the COR‐induced LR formation was independent of increased auxin level but likely associated with auxin transport. Treatment of plants with 1‐naphthalene acetic acid (NAA) increased LR formation at NAA concentrations of 100 nM, but had no effect at 10 nM. In the presence of 1 nM COR, however, NAA increased LR formation at 10 nM concentrations. This indicates that LR formation due to COR possibly involves changes in auxin sensitivity. In addition, the shorter LRs of COR‐treated seedlings were clearly restored when COR was removed from solutions for 12 d, and the total root length, total root surface area as well as K uptake increased significantly, suggesting that COR may be potentially useful for enhancing the K‐acquisition efficiency of cotton seedlings.     

Effect of air‐drying on phosphorus fractions in clay soil

Despite the publication of a number of papers dealing with the effect of drying on the soil labile P pool, less attention has been paid to the possible drying‐evoked changes in the more stable P pools. We applied Hedley's sequential fractionation procedure that aims at quantifying soil P reserves according to their decreasing plant availability to examine the effects of drying on soil P fractions in clayey soil samples of different cultivation history. To further investigate the contribution of organic matter disruption to the solubility of soil P, the P extracted in each fractionation step was divided into two size classes by filtering the suspension through a 0.2 μm membrane filter. There were no air‐drying‐induced changes in the total amount of P extracted in each fractionation step. However, air‐drying changed the distribution of water‐extractable P in size fractions; increase in the small‐sized P took place at the expense of large‐sized P. Air‐drying increased also small‐sized molybdate‐unreactive P (MUP) in the NaOH fraction giving evidence that drying‐induced alterations take place also in less labile P forms. The results revealed that air‐drying alters the extractability and distribution of P in various pools rather than the total amount of extracted P and that a large proportion of H₂O‐ and NaOH‐extractable large‐sized MUP may remain undetected if only filtered samples are analyzed.     

Cross‐Linking of Wheat Gluten Using a Water‐Soluble Carbodiimide

Wheat gluten was cross‐linked using water‐soluble 1‐ethyl‐3‐(3‐dimethylaminopropyl) carbodiimide HCl (EDC). To enhance cross‐linking, N‐hydroxysuccinimide (NHS) was added to the reaction mixture. The cross‐linking efficiency was evaluated by the decrease in the amount of amino groups, the solubility of the protein in aqueous solutions with different pH levels, and by the change in the molecular weight distribution of the cross‐linked compounds. Cross‐linking was dependent on the reaction time, the molar ratio of added reactants, and the pH level of the reaction mixture. If the reaction was carried out at pH 3, no decrease in the amount of amino groups or solubility was observed. At pH 5–7, the amount of amino groups decreased from 15 to 10 mmol/100 g of protein. This was accompanied by a large decrease in the water solubility of the protein (<10%, w/v). Finally, reaction at pH 11 decreased the amount of amino groups from 15 to 8 mmol/100 g of protein. However, hardly any decrease in the water solubility was observed. Based on these results and SDS‐PAGE experiments, two cross‐link mechanisms are suggested: one resulting in inter‐ and the other resulting in intramolecular cross‐links.     

Mechanism of molybdenum sorption to iron oxides using pressure‐jump relaxation

It is widely accepted that the fixation of oxyanions is due to diffusion of the ions into the pores and interdomains of iron oxides. Most studies have used batch techniques, which do not allow to clearly differentiate chemisorption from mass transport phenomena. Thus, it is not yet clear, whether strengthening of chemical Mo bonding occurs along with residence time, in addition to diffusion processes. In this study we used pressure jump relaxation (p‐jump), a very fast kinetic technique, to (1) elucidate the Mo/goethite interaction and to (2) analyze the effects of aging the Mo/goethite complex on Mo chemisorption. A synthetic goethite was incubated with Mo solution (1 mM Mo) for 12, 24, and 72 hours at pH 4. At the end of the incubations p‐jump experiments were performed on the suspensions at temperatures ranging from 283 to 303 K. Relaxation kinetics were modelled using a combination of two first order terms. In addition, the amount of Mo sorbed to the goethite after different incubation times was determined by graphite furnace atomic absorption spectroscopy. The MoO₄/goethite systems revealed a fast relaxation time (= reciprocal of rate constant, about 4 ms), that decreased with increasing temperature and a slow one (about 60 ms) that did not depend on temperature. Activation energy of the fast process was 76 kJ mol^—1. We did not observe any effects of incubation time on the fast process. However, the amount of Mo sorbed to the iron oxide increased with increasing incubation time. We conclude that the fast relaxation represents Mo chemisorption to the goethite. Slow relaxation seems to be due to Mo transport within the suspension. The pressure jump results indicate, that the dominant surface species of Mo sorbed to goethite do not change along with residence time.     

Effect of soil microorganisms on the sorption of zinc and lead compounds by goethite

The objectives of this study were (1) to determine the effect of microorganisms during in‐vitro incubation on the amount of Zn and Pb from solution retained on goethite precipitated as coatings on a sand matrix and (2) to evaluate accumulation of heavy metals in the biomass of soil microorganisms in the fresh soil samples using an extractive approach. A mixture of colonies of cultivated microorganisms extracted from a Haplic Luvisol (Russia) and an Antropi‐urbic Regosol (Germany) were used to prepare the cell and the microbial‐debris suspensions. The concentrations of Zn and Pb in the studied solutions supplied with microbial suspensions and/or goethite coated sand were 0.1 mM (130.8 and 414 mg kg^–1 of sand, respectively). Exchangeable forms of metals were determined by extraction with 10 mL of 1.0 M KNO₃. Nonexchangeable forms of Zn and Pb were recovered using 40 mL of 0.3 M NH₂OH‐HCl in 1 M HNO₃. Concentrations of Pb increased in the solutions and decreased on the surface of the Fe‐mineral due to living microorganisms. In comparison to incubation of heavy‐metal solutions with goethite only, the absolute concentrations of nonexchangeable forms of metal were reduced by microbial suspension to a greater extent than those of the exchangeable forms, whereas the relative content of both fractions decreased by a factor of almost two. Sorption of Pb by goethite was inversely correlated with the concentration of organic C in the solution. Microorganisms clearly influenced the Zn sorption by goethite at concentrations of C_org > 400 mg L^–1. The amount of Zn retained was decreased primarily due to decreasing Zn portions in the exchangeable fraction. Microbial debris prepared by autoclaving reduced the Pb sorption by goethite similar to the results for living cells. Living microorganisms accumulated more Zn than did microbial debris. The data of this paper show that a direct determination of heavy‐metal accumulation in soil microorganisms by extraction with 2.0 M KCl as well as by extraction with 1 M CH₃COONH₄ at the natural pH of the soils after chloroform fumigation of fresh soils samples with different concentrations of organic C was not possible.