Stress Relaxation of Fruit Gels. Evaluation of Models and Effects of Composition

Stress relaxation behavior of kappa carrageenan (1%) gels (C) and kappa carrageenan (0.5%) plus locust bean gum (0.5%) gels (G) was studied. Effects of addition of sugar (18.3, 36.6 and 55° Brix) and fruit pulp (15%) on relaxation response were analyzed. Curves were fitted to Maxwell (M), Peleg (P) and simplified Maxwell (SM) equations. All three equations were valid to quantify relaxation behavior of fruit gels and to evaluate the effects of changes in composition, with P and SM equations being simpler. Locust bean gum increased initial stress decay rate and decreased proportion of unrelaxed force. Sucrose produced the opposite effect. Fruit pulp effects were weak and dependent on gelling system.     

无量山乌骨鸡腿肌营养成分分析和评价

通过对无量山乌骨鸡腿肌的营养成分进行测定,对比不同性别无量山乌骨鸡腿肌的营养成分差 异,评价其营养价值。结果表明：无量山乌骨鸡腿肌的粗蛋白含量为（23.02±0.69）%、粗脂肪含量为 （2.00±0.97）%、灰分含量为（1.25±0.06）%、水分含量为（73.63±1.29）%;腿肌冻干粉中17 种氨基酸的总量 达（82.59±4.30）%,鲜味氨基酸总量为（28.78±1.56）%,必需氨基酸含量为（34.08±1.83）%,必需氨基酸/ 总氨基酸的值为（41.26±0.52）%,必需氨基酸/非必需氨基酸的值为（70.26±1.52）%,氨基酸组成比例均衡; 腿肌冻干粉中18 种脂肪酸的总量为（33.88±9.41） mg/g,必需脂肪酸含量为（10.21±2.89） mg/g,饱和脂肪酸 含量为（14.13±4.41） mg/g,不饱和脂肪酸含量为（19.75±5.22） mg/g,不饱和脂肪酸占比高于50%;腿肌中 磷元素含量为（721.25±294.75） mg/kg,镁、钙、铁、锌、铜含量分别为（240.03±63.06）、（24.28±5.56）、 （14.94±2.76）、（11.40±1.94）、（1.51±0.85） mg/kg。无量山乌骨鸡腿肌的营养成分具有性别差异性,公鸡 腿肌的水分含量、17 种氨基酸总量、鲜味氨基酸、必需氨基酸及磷元素含量显著（P＜0.05）或极显著（P＜0.01） 高于母鸡,且公鸡腿肌的氨基酸比值系数分高于母鸡;母鸡腿肌的粗脂肪、灰分、饱和脂肪酸、单不饱和脂肪酸及 锌元素含量显著（P＜0.05）或极显著（P＜0.01）高于公鸡。     

Effect of carbon dioxide on autoxidation and photooxidation of nitrosylmyoglobin

Autoxidation and photooxidation of nitrosylmyoglobin, MbFe(II)NO, the pigment of nitrite-cured meat, was investigated in aqueous solution saturated with gas mixtures containing varying levels of CO(2) (0%, 20% and 90%) in the presence of either 1.5 or 10% O(2) and balanced with N(2) in order to mimic modified atmosphere (MA) packaging of meat products. Quantum yields for photooxidation at 1.5% O(2) for monochromatic UV-light (366 nm) were slightly higher than for visible light (436 nm) in agreement with previous findings, while the quantum yields showed no dependence on the CO(2) levels. Autoxidation of MbFe(II)NO was evaluated by global analysis of spectral data applying a kinetic model with two consecutive reaction steps, and the second rate constant was significantly reduced at the highest CO(2) level investigated, while the rate constant of the initial reaction step was found independent of the CO(2) levels. The varying dependence observed for autoxidation and photooxidation of MbFe(II)NO in relation to CO(2) level confirms the differing reaction mechanisms for the two types of MbFe(II)NO degradation. Photooxidation of nitrite-cured meat products packed in MA is accordingly expected to be independent of the presence of varying CO(2) levels, while thermal oxidation (autoxidation) of MbFe(II)NO is reduced at elevated levels of CO(2), which may be of some relevance during product storage and retail display.     

Effect of total replacement of pork backfat with walnut on the nutritional profile of frankfurters

The nutritional profile of a frankfurter with walnut (25%) (WF) was compared with that of low fat (6%) (LF) and normal fat (16%) (NF) sausages. LF frankfurters did not improve the fatty acid profile as compared with NF (p>0.05), or the indices of atherogenicity (IA) (0.50) or thrombogenicity (IT) (1.12), indicating that the reduction of animal fat produced no cardioprotective effect. However, frankfurter with walnut had a healthy polyunsaturated fatty acid profile, a lower ω-6/ω-3 ratio and lower IA (0.10) and IT (0.15). In addition, WF could be a good source of manganese, iron, copper, potassium and magnesium, supplying 39%, 35%, 34%, 16% and 15% of the respective daily recommended intakes, respectively. Also, there were significant amounts of other bioactive components such as α-,γ- and δ-tocopherol, total dietary fibre, polyphenols and tannins. The amino acid composition of WF indicated a healthier lysine/arginine ratio (0.83) than in LF (1.23) and NF (1.19).     

Impact of interactions between metal oxides to oxidative reactivity of manganese dioxide

Manganese oxides typically exist as mixtures with other metal oxides in soil-water environments; however, information is only available on their redox activity as single oxides. To bridge this gap, we examined three binary oxide mixtures containing MnO(2) and a secondary metal oxide (Al(2)O(3), SiO(2) or TiO(2)). The goal was to understand how these secondary oxides affect the oxidative reactivity of MnO(2). SEM images suggest significant heteroaggregation between Al(2)O(3) and MnO(2) and to a lesser extent between SiO(2)/TiO(2) and MnO(2). Using triclosan and chlorophene as probe compounds, pseudofirst-order kinetic results showed that Al(2)O(3) had the strongest inhibitory effect on MnO(2) reactivity, followed by SiO(2) and then TiO(2). Al(3+) ion or soluble SiO(2) had comparable inhibitory effects as Al(2)O(3) or SiO(2), indicating the dominant inhibitory mechanism was surface complexation/precipitation of Al/Si species on MnO(2) surfaces. TiO(2) inhibited MnO(2) reactivity only when a limited amount of triclosan was present. Due to strong adsorption and slow desorption of triclosan by TiO(2), precursor-complex formation between triclosan and MnO(2) was much slower and likely became the new rate-limiting step (as opposed to electron transfer in all other cases). These mechanisms can also explain the observed adsorption behavior of triclosan by the binary oxide mixtures and single oxides.     

Kinetics and mechanisms of heterogeneous reaction of gaseous hydrogen peroxide on mineral oxide particles

Recent studies have shown that heterogeneous reactions of hydrogen peroxide (H(2)O(2)) on aerosol surfaces may play an important role in tropospheric chemistry. The data concerning the kinetics and mechanisms of these reactions, however, are quite scarce so far. Here, we investigated, for the first time, the heterogeneous reactions of gaseous H(2)O(2) on SiO(2) and α-Al(2)O(3) particles, two major components of mineral dust aerosol, using transmission-Fourier Transform Infrared (T-FTIR) spectroscopy, and high-performance liquid chromatography (HPLC). It is found that H(2)O(2) molecularly adsorbs on SiO(2), and a small amount of molecularly adsorbed H(2)O(2) decomposes due to its thermal instability. For α-Al(2)O(3), catalytic decomposition of H(2)O(2) evidently occurs, but there is also a small amount of H(2)O(2) molecularly adsorbed on the particle surface. The BET uptake coefficients of H(2)O(2) on both particles appear to be independent of gaseous H(2)O(2) concentration (1.27-13.8 ppmv) and particle sample mass (2.8-6.5 mg for SiO(2) and 8.6-18.9 mg for α-Al(2)O(3)), but are strongly dependent on relative humidity with the values ranging from (1.55 ± 0.14) × 10(-8) and (1.21 ± 0.04) × 10(-7) at 2% RH to (0.61 ± 0.06) × 10(-8) and (0.76 ± 0.09) × 10(-7) at 76% RH for SiO(2) and α-Al(2)O(3), respectively. On the basis of the experimental results and literature data, the potential mechanisms for heterogeneous decomposition of H(2)O(2) were proposed, and the atmospheric implications of these reactions were discussed. It is found that heterogeneous reaction of H(2)O(2) on both mineral oxides plays a significant role in processing mineral aerosols, although its role as a sink for ambient H(2)O(2) is probably limited.     

Comparison of different reducing agents for enhanced detection of heat-injured Listeria monocytogenes

The effect of different reducing agents (L-cysteine, Oxyrase, and Enterococcus faecium) and their combinations on the detection of heat-injured (62.8 degrees C, 7.5 min or 10 min) Listeria monocytogenes was examined. The incorporation of L-Cysteine (0.5 g/liter) yielded higher percentage detection than any of the other reducing agents (P < 0.05). The combination of Oxyrase (10 U/ml) and E. faecium (10(3) CFU/ml) synergistically enhanced the detection of L. monocytogenes heat-injured for 10 min at 62.8 degrees C (P < 0.05). Simultaneous addition of L-cysteine (0.5 g/liter) and Oxyrase (10 U/ml) significantly lowered the recovery of heat-injured L. monocytogenes (P < 0.05). Higher activities of Oxyrase (50 U/ml) inhibited the detection of heat-injured L. monocytogenes (P < 0.05). The rates of depletion of relative percentage O2 were in the order: L-cysteine (0.5 g/liter; 6.63%/ min) > Oxyrase (10 U/ml; 5.00%/min) > E. faecium (10(8) CFU/ml; 1.66%/min) > E. faecium (10(3) CFU/ml; 0.20%/min). The final levels of redox potential (Eh) achieved were -110.5 mV, -100 mV, -83.5 mV, and -25 mV for E. faecium (10(8) CFU/ml), L-cysteine, Oxyrase, and E. faecium (10(3) CFU/ml), respectively.     

Influence of high hydrostatic pressure on the proteolysis of beta-lactoglobulin A by trypsin

This work describes the effect of the hydrolysis time and pressure (0.1-400 MPa) on the proteolysis of beta-lactoglobulin A (beta-lg A) with trypsin, either conducting hydrolysis of beta-lg under pressure or hydrolysing beta-lg that was previously pressure treated. Pressurisation, before or during enzyme treatments, enhanced tryptic hydrolysis of beta-lg. Trypsin degraded pressure-modified beta-lg and pressure-induced beta-lg aggregates, favouring proteolysis to the intermediate degradation products: (Val(15)-Arg(40)), (Val(41)-Lys(69))S-S(Leu(149)-Ile(162)) and (Val(41)-Lys(70))S-S(Leu(149)-Ile(162)). These were further cleaved at the later stages of proteolysis to yield: (Val(15)-Tyr(20)), (Ser(21)-Arg(40)), (Val(41)-Tyr(60)), (Trp(61)-Lys(69))S-S(Leu(149)-Ile(162)) and (Trp(61)-Lys(70))S-S(Leu(149)-Ile(162)). Particularly, in the tryptic hydrolysates of pre-pressurized beta-lg, two other fragments linked by disulphide bonds: (Lys(101)-Arg(124))S-S(Leu(149)-Ile(162)) and (Tyr(102)-Arg(124))S-S(Leu(149)-Ile(162)), were found. These corresponded to rearrangement products induced by SH/SS exchange between the free thiol group of Cys(121) and Cys(160), that normally forms the disulphide bond Cys(66)-Cys(160). In the light of these results, structural modifications of beta-lg under high pressure are discussed.     

Differentiation of dietary regimene of Iberian swine by means of isotopic analysis of carbon and sulphur in hepatic tissue

The aim of this work was to differentiate the feed received by Iberian swine during fattening (acorns, feed) and their breed (Iberian or White) using analysis of the stable isotopes of carbon (δ(13)C) and sulphur (δ(34)S) in liver tissue samples. The results obtained in the determination of δ(34)S, using a procedure in which organic and inorganic sulphur are converted into BaSO(4) and the procedure that measures δ(34)S in samples of dried ground liver tissue were compared. Joint analysis of carbon (δ(13)C) and sulphur (δ(34)S) permits the differentiation of swine of different breeds receiving different diets (acorns or feed).     

果寡糖对肉鸡消化机能的影响

本试验研究了饲粮中添加0.2％、0.4％和0.6％果寡糖对艾维茵商品代肉鸡消化机能的影响。结果表明:肉鸡日粮中添加0.2％、O.4和0.6％果寡糖使粗纤维的表观消化率分别提高了21.09％(P<0.05)、33.28％(P<0.01)和29.90％(P<0.01),添加O.2％和0.4％果寡糖使粗灰分的表观消化率分别提高了21.83％(P<0.05)和31.94％(P<0.05),添加0.4％果寡糖使钙和磷的表观消化率分别提高了17.39％(P相似文献   

酶解鸡蛋清寡肽混合物对小鼠体外免疫功能的影响

考察了用Alcalase（AL）、Flourezyme（FL）、木瓜蛋白酶（PA）、胃蛋白酶（PE）和胰蛋白酶（TR）酶解鸡蛋清制得的5种酶解鸡蛋清寡肽混合物（MOEE）对小鼠体外免疫功能的影响．采用体外培养，细胞吞噬试验和细胞增殖试验，观察同一MOEE不同剂量以及同一剂量不同MOEE对小鼠腹腔巨噬细胞吞噬功能和脾细胞增殖能力的影响,结果发现，所有MOEE均能显著（P〈0．05）或极显著（P〈0．01）提高小鼠腹腔巨噬细胞的吞噬能力，促进小鼠脾淋巴细胞增殖，且表现出一定的剂量效应关系．其中，1．0mg／mL不同MOEE促进巨噬细胞吞噬能力（PI）大小顺序为：FL1-5〉PA1-55〉TR1-55〉AL1-5〉PE1-5〉对照；促脾淋巴细胞增殖能力（SI）大小顺序为：FL1-5〉AL1-5〉PA1-5〉TR1-5〉PE1-5〉对照（无丝裂原），FL1-5〉PA1-5〉TR1-5〉AL1-5〉PE1-5〉对照（LPS刺激），FL1-5〉TR1-55〉PE1-5〉PA1-5〉AL1-5〉对照（ConA刺激）．结果表明，所有MOEE都具有免疫增强活性，且以FL1-5的活性最强．     

Absence of fractionation of mercury isotopes during trophic transfer of methylmercury to freshwater fish in captivity

We performed two controlled experiments to determine the amount of mass-dependent and mass-independent fractionation (MDF and MIF) of methylmercury (MeHg) during trophic transfer into fish. In experiment 1, juvenile yellow perch (Perca flavescens) were raised in captivity on commercial food pellets and then their diet was either maintained on unamended food pellets (0.1 μg/g MeHg) or was switched to food pellets with 1.0 μg/g or 4.0 μg/g of added MeHg, for a period of 2 months. The difference in δ(202)Hg (MDF) and Δ(199)Hg (MIF) between fish tissues and food pellets with added MeHg was within the analytical uncertainty (δ(202)Hg, 0.07 ‰; Δ(199)Hg, 0.06 ‰), indicating no isotope fractionation. In experiment 2, lake trout (Salvelinus namaycush) were raised in captivity on food pellets and then shifted to a diet of bloater (Coregonus hoyi) for 6 months. The δ(202)Hg and Δ(199)Hg of the lake trout equaled the isotopic composition of the bloater after 6 months, reflecting reequilibration of the Hg isotopic composition of the fish to new food sources and a lack of isotope fractionation during trophic transfer. We suggest that the stable Hg isotope ratios in fish can be used to trace environmental sources of Hg in aquatic ecosystems.     

Diversity of virulence profiles of Shiga toxin-producing Escherichia coli serotypes in food-producing animals in Brazil

The prevalence, serotypes and virulence profiles of Shiga toxin-producing Escherichia coli (STEC) were investigated in 205 healthy beef and dairy cattle, and 106 goats reared in the southeastern region of Minas Gerais State, Brazil. The prevalence of STEC was 57.5% (61/106) in goats, 39.2%, (40/102) in beef cattle and 17.5% (18/103) in dairy cattle. Among the 514 STEC isolates, 40 different serotypes were found and some of them were identified in a specific host. STEC isolates harboring stx(1) corresponded to 15.6% (28/180), 26.7% (16/60) and 24.1% (66/274) in beef cattle, dairy cattle and goats, respectively. stx(2) was found in 30% (54/180), 53.3% (32/60) and 34.7% (95/274) of beef and dairy cattle, and goats. stx(1) plus stx(2) sequences were harbored by 54.4% (98/180), 20% (12/60) and 41.2% (113/274) of beef cattle, dairy cattle and goats, respectively. The eae sequence was found in 15% (9/60) and 0.6% (1/180) of STEC isolates from dairy and beef cattle, respectively, and the toxB gene was found only in one O157:H7 strain isolated from beef cattle. Strains with the genetic profiles stx(2) ehxA iha saa and stx(1) stx(2) ehxA iha saa were the most prevalent among STEC isolates from cattle. Profiles stx(1) stx(2) ehxA iha, stx(2), and stx(1) iha accounted for 75.5% (207 /274) of the STEC isolates from goats. While STEC strains carrying either stx(2) alone or associated with stx(1) were found more frequently in cattle, those harboring sequences stx(1c) and stx(2d) alone or associated with stx(1c) predominated in goats. Our data show a diversity of STEC strains in food-producing animals, most of them carrying genes linked to severe forms of human diseases.     

Relationship of USDA Maturity Groups to Palatability of Cooked Beef

In comparison to carcasses of B, C or E maturity, carcasses of A maturity produced broiled steaks which: (a) had higher (P < 0.05) palatability ratings in 62 to 86% of comparisons, (b) were decidedly less variable (C.V.), (c) were more likely to be assigned high (≥ 6.00) and less likely to be assigned low (≤ 2.99) sensory panel ratings, and (d) were more likely to have low (≤ 3.63 kg) and less likely to have high (≥ 6.35 kg) shear force values. Position within the A or A + B maturity group(s) explained 0–4% (loin steaks) and 10–18% (round steaks) of the observed variation in overall palatability ratings and/or shear force values.     

Control and comparison of the antioxidant capacity of beers

The purpose of the present work is to determine the antioxidant capacity (AC) of 27 commercial beers. The AC indicates the degree of protection of a certain organism against oxidative damage provoked by reactive oxygen and nitrogen species.Assays were carried out by the following methods: (i) total radical trapping antioxidant parameter (TRAP); (ii) trolox equivalent antioxidant capacity (TEAC); (iii) trolox equivalent antioxidant capacity (DPPH); (iv) ferric-ion reducing antioxidant parameter (FRAP); (v) cupric reducing antioxidant capacity (CUPRAC); (vi) oxygen radical absorbance capacity (ORAC). Ascorbic acid (AA), gallic acid (GA) and trolox (TR) were used as standards.All beers showed antioxidant power, but a wide range of ACs was observed. The effect of several factors upon these differences was studied. Statistical differences were found between ACs of beers of different colours. ORAC method provided always higher experimental ACs, of significant statistical differences to other assays.     

Rates of detection of Salmonella and Campylobacter in meats in response to the sample size and the infection level of each species

Pork, beef and chicken meat samples were collected from slaughter houses, poultry-processing plants and meat shops. Rates of incidence of Salmonella spp., Campylobacter jejuni and C. coli with respect to the sample size were compared and the most probable number for these species were determined. Salmonella spp. were detected in 69 (24.1%) of 286 chicken meat samples, in three (3.2%) of 94 pork samples, and in one (1.9%) of 52 beef samples. With chicken meat, the rates of detection were: 19.9% in 25-g, 15.7% in 10-g, and 12.2% in 1-g samples. The populations in most probable numbers, that gave positive results in 31 (20.8%) of 149 samples, ranged from 30 to 10(4) per 100 g, the majority (93.5%) being between 30 and 10(3) per 100 g. C. jejuni and C. coli were detected in 106 (67.9%) of 156 chicken meat samples, in two (2.1%) of 94 pork samples, and none of 52 beef samples. The results obtained with different sample sizes of chicken were compared. Positive rates were 55.8%, 39.7%, 27.6% in 10 g, 1 g, and 0.1 g, respectively. The most probable numbers in 107 (68.6%) positives out of 156 chicken samples examined ranged from 30 to 10(6) per 100 g: 46 (29.5%) contained between 10(2) and 10(3) per 100 g, 22 (14.1%) between 10(3) and 10(4) per 100 g, and the other 19 samples (12.2%) between 10(4) and 10(5) per 100 g.     

Microbial modification of the texture of grated cassava during fermentation into akyeke

The traditional akyeke inoculum and fermenting akyeke, an indigenous cassava product, were investigated to identify microbial species responsible for the modification of cassava texture during fermentation. Both field and laboratory samples were examined and only some cultures isolated on Plate Count Agar and Malt Extract Agar were found to be capable of causing a softening of cassava tissue when plated directly on sterile cassava slices. The cassava tissue softening isolates on PCA were tentatively identified as Bacillus subtilis and isolates on MEA as Candida tropicalis and Zygosacchromyces florentinus. The population of B. subtilis in the laboratory sample of inoculum was found to be 2.4 x 10(9) cfu g(-1) and increased during dough fermentation from 1.1 x 10(7) to 3.5 x 10(9) cfu g(-1) after 96 h. C. tropicalis was present in the inoclum at 3.0 x 10(9) cfu g(-1) and increased during dough fermentation from 3.2 x 10(6) to 6.9 x 10(7) cfu g(-1) whilst Z. florentinus was present in the inoclum at 9.1 x 10(8) cfu g(-1) and increased from 8.1 x 10(5) to 7.5 x 10(6) cfu g(-1) during dough fermentation.     

Influence of nitrate on microbial reduction of pertechnetate

Factors influencing microbial reduction of Tc(VII) in nitrate and radionuclide contaminated aquifer sediments were investigated using sediment microcosms containing organic electron donor, nitrate and Tc(VII). Microcosms underwent nitrate reduction followed by Tc(VII) reduction. During this transition, the microbial community changed from being dominated by bacteria affiliated with the genus Paenibacillus during the nitrate reduction phase, to those affiliated with genera Agrobacterium, Geothrix, and Desulfosporosinus during the Tc(VII) reduction phase. To investigate the mechanism of Tc(VII) reduction, the nitrate reducing strains Agrobacterium FRC-A2, Azoarcus FRC-B1, and a fermentative Clostridium FRC-C11 were isolated from sediment microcosms undergoing Tc(VII) reduction. Nitrate reducing bacteria reduced Tc(VII) effectively only in the presence of Fe(lll) and after nitrate was reduced, implying a major role for Fe(ll) as an electron shuttle in Tc(VII) reduction. It is likely that accumulation of nitrite blocks Fe(ll) production and hence Tc(VII) reduction during the active nitrate reduction phase. The pure culture of Clostridium FRC-C11 is able to reduce Tc(VII) enzymatically with H2 or glucose as electron donor and deposits insoluble Tc compounds within the cells in a manner that is not significantly influenced by the presence of nitrate. These results provided a possible mechanism for Tc(VII) reduction independent of Fe(III) and not influenced by nitrate.     

Comparison of five methods for measuring sediment toxicity of hydrophobic contaminants

Sediment toxicity from hydrophobic organic compounds (HOCs) is complicated by chemical partitioning among multiple phases and sediment-specific bioavailability. In this study, we used three hydrophobic pyrethroid insecticides as test compounds and derived 10-d median lethal concentrations (LC50s) for Chironomus tentans in three different sediments. The LC50s were expressed using HOC concentrations on a bulk sediment basis (C(S)), organic carbon (OC)-normalized sediment basis (C(S-OC)), porewater basis (C(PW)), dissolved organic carbon (DOC)-normalized porewater basis (C(PW-DOC)), and freely dissolved porewater basis (C(free)). The bulk phase C(S) and C(PW) yielded highly variable LC50s across sediment types, whereas the use of normalized concentrations C(S-OC) and C(PW-DOC) generally reduced variability due to sediment type but not that due to aging. In contrast, LC50s based on C(free) were essentially independent of sediment conditions. The sediment pore water samples contained approximately 20-90 mg L(-1) DOC, and the C(free) expressed as a percentage of the total bulk pore water concentration ranged from 9 to 28% for fenpropathrin (mean = 19%), 8 to 18% for bifenthrin (mean = 13%), and 3 to 8% for cyfluthrin (mean = 6%) across the different sediments. These results indicate thatthe use of C(free) reduces uncertainties caused by sediment variables such as OC properties and aging effects.     

Interaction of uranyl with calcite in the presence of EDTA

Adsorption of uranyl at the surface of calcite was investigated by using batch sorption experiments and synchrotron X-ray standing wave (XSW) measurements. Aqueous solutions containing 236U(VI) (4.5 x 10(-7) to 1.0 x 10(-4) M) and EDTA (5.0 x 10(-7) to 1.1 x 10(-4) M) were reacted for 90 s to 60 min with freshly cleaved calcite (104) surfaces and calcite powders. Surface exchange coefficients, sorption kinetics, and influence of powder surface area/solution volume (SA/V) ratio were investigated by alpha-counting of 236U. Powder sorption results at SA/V = 870 cm2/mL fit a Freundlich isotherm [log [U]surface (in monolayers) = log K + n log [U]aq (in moles/L)], where K = 1.9+/-0.5 and n = 0.9+/-0.1, consistent with uptake of U(VI) by a specific surface reaction where the availability of sorption sites is nonlimiting in the U concentration range measured. Measured U(VI) coverages along this isotherm, based on the calcite (104) surface Ca site density, ranged from 0.04% to 5.4% of a monolayer. Steady state surface coverages were obtained within 90 s. Sorption of U(VI) on calcite (104) single-crystal cleavage surfaces using identical solutions yielded higher coverages, because of increased step density induced by dissolution at the relatively low SA/V ratio (approximately 1) of these measurements. The crystallographic location of the sorbed U(VI) was examined with the synchrotron XSW technique. Measurements were performed at the Advanced Photon Source on fresh calcite (104) cleavage surfaces reacted for 90 s with U(VI) solutions. Coherent fractions for sorbed U ranged from 0.14 to 0.62, and the mean value of the U coherent position was 0.84+/-0.02. This position was independent of dissolved U(VI) concentration and corresponds to a distance between the U atom and the calcite (104) plane of 2.55+/-0.06 A. These results are consistent with U(VI) adsorption atthe calcite surface as an inner-sphere uranyl-carbonate surface complex bonded with the outer oxygen atom(s) of a single surface carbonate group. Steric considerations allow this observed U(VI) surface complex to occur both at step sites ((441)_ and (481)_) and on terrace areas adjacent to Ca vacancies.