检索结果-南通市图书馆

Advances in Chemical Engineering and Science 2012年第1期2卷 9-14页

作者： Yolanda Cocotle-Ronzon marisol Zendejas-Zaldo m.cloth lópez del castillo-lozano maGuadalupe Aguilar-Uscanga 不详

Xylose reductase (EC 1.***.1.21) of Candida tropicalis IEC5-ITV, an indigenous xylitol-producing strain, was partially purified by reversed micelles and characterized, an 8.1 fold purification factor being obtained. The XR present in the crude extract exhibited its highest specific activity at pH 6.0 and 40℃, while in that obtained by reverse micelles, this occurs at pH 6.0 and 30℃. XR before and after extraction is stable within a range of 30 to 40℃, pH 7 after one hour of incubation under these conditions. After two months’storage at –18℃, the enzyme obtained by reverse micelles lost 76.60% specific activity. The estimated molecular weight by PAGE-SDS was 32.42 kD. Km for xylose was higher for the XR extracted by reverse micelles (0.026 m) than that obtained for the enzyme before extraction (0.0059 m), while Km for cofactor NADPH was lower after than before extraction (1.85 mm to 12.0 mm respectively). There was no activity with NADH as a cofactor. Variations in pH and temperature optima, as well as kinetic parameters before and after partial XR purification by reverse micelles are probably due to an alteration in enzyme molecule structure caused by the solvents used during extraction.

关键词： Xylose Reductase Xylitol Reversed micelles Candida Tropicalis

Shear Rate Effect on the Viscosity of Homemade and Commercial Tomato Sauce

在线全文

维普期刊数据库

学校读者我要写书评

暂无评论

Journal of Food Science and Engineering 2017年第10期7卷 479-496页

作者： Virues-delgadillo Jorge-Octavio lozada-Santillan Claudia-Karina Bulbarela-Sampieri Carmen Galán-méndez Frixia lópez-del-castillo-lozano m.cloth Facultad de Ciencias Quimicas Universidad Veracruzana Lomas del Estadio S/N Xalapa Veracruz 91000 México

Rheological characterization of tomato products is important not only for design of unit operations, but also to optimizeprocesses and guarantee high quality of food products. Time dependence is related with structural changes due to shear rate. Thus,rheological characterization as a function of time is extremely important to understand the changes that occurred in food productsduring commercial processes. However, these characterizations are rare in the literature for tomato products. Those rheologicalproperties depend of several parameters, such as agronomic, structural and process parameters. In this study, the effect of shear rateon the viscosity at room temperature （22 ~C） of two kinds of tomato sauce （homemade and commercial） is analyzed; using aBrookfield viscometer model 4535, lab-line Instruments. Homemade sauce was prepared with fresh saladette tomatoes from a localmarket. Rheological analysis was performed at shear rates from 0.05 to 10.47 sl; using all the spindles available. Statistical analysiswas made using lSD and Duncan tests with a confidence interval of 95% （p-value of 0.05）. It was demonstrated that homemade andcommercial tomato sauce behaved as a thixotropic fluid, due to the typical decrease on viscosity samples observed when testing timewas increased. When tomato sauce is at rest, it has a gelled tridimensional structure; and, as soon as a shear stress is applied, themovement generates an alignment of tomato constituent chains in flow direction, breaking physic entanglements and thus causing adecrease in viscosity as a function of time. Furthermore, it was discovered that the viscosity of homemade sauce was higher than theviscosity of commercial sauce. The latter may be due to the pre-thermal treatment. During this unit operation, polymeric chains oflycopene most likely break into smaller chains of isoprene, causing the observed decrease in viscosity. Besides, commercial saucecontains additives, seasonings, preservatives and thickeners that ar

关键词： Food rheology, viscosity, shear rate, tomato sauce.

Temperature Effect on the Viscosity of Commercial Carrot Puree

在线全文

维普期刊数据库

学校读者我要写书评

暂无评论

Journal of Food Science and Engineering 2017年第10期7卷 505-513页

作者： Vimes-delgadillo Jorge-Octavio, Dominguez-Rendón Ana-Cristina lópez-del-castillo-lozano m.cloth Dominguez-Cafiedo Irma-liliana Galán-méndez Frixia Facultad de Ciencias Quimicas Universidad Veracruzana Lomas del Estadio S/N Xalapa Veracruz 91000 México

Temperature effect was evaluated over the viscosity of commercial carrot puree in the range 30 to 60 ℃, using aBrookfield viscometer, model 4535, lab-line instruments. In order to perform the viscosity tests, 28 specimens of carrot puree froma local market were used. Totally, 72 tests （each one with a couple replicas） were performed during five minutes, using fourtemperatures （30, 40, 50 and 60 ℃）, six shear rates （0.26, 0.52, 1.05, 2.09, 5.24 and 10.47 s1） and three spindles （4, 5 and 6）.Statistical analysis was performed using lSD and Duncan tests with a confidence interval of 95% （p value of 0.05）. In the currentstudy, it was observed that an increase in the temperature triggers a reduction in the viscosity of the carrot specimens analyzed,although the samples tested at 60℃ were deviated from this trend, which may be due to denaturation of the starch containing carrotsamples at such temperature. multiple biochemical and micro-structural changes inside the food specimens may begin to occur belowand over the physiological range （20-45 ℃）. Experimental data were analyzed with Power law model in order to obtain the bestfitted parameters as a function of shear rate： It was observed that the consistent index （k） decreased with an increment in thetemperature; while all the values of parameter ＂n＂ lesser than one, which means that the puree samples behaved as a pseudo-plasticfluid. Besides that, an Arrhenius model was used to fit the parameters as a function of temperature： Parameter A was lower usingtheexperimental data at shear rates of 10.47 s-1. The modeling performed may be helpful for the appropriate selection of some processvariables, as the shear rate and operation temperature, used for the production of carrot puree; all this is in order to obtain the finishedproduct with the desired consistency.

关键词： Food rheology, viscosity, temperature, carrot puree.

Physicochemical and Rheological Characterization of an Acidic milk Product: Kefir Concentration Effect

在线全文

维普期刊数据库

学校读者我要写书评

暂无评论

Journal of Food Science and Engineering 2017年第2期7卷 86-92页

作者： Jorge Octavio Virues delgadillo miguel de Jesús luna lara Claudia Karina lozada Santillan Carmen Bulbarela Sampieri lópez del castillo lozano m.cloth Facultad de Ciencias Químicas Universidad Veracruzana Lomas del Estadio S/N Xalapa Veracruz 91000 México

The main objective of this study was the analysis of physicochemical and rheological behavior of a drinkable beverage prepared with different concentrations (5%, 10% and 20% w/w) of Kefir. Several rheological models were also evaluated to determine the model that better fit experimental data. Apparent viscosity was measured with a Brookfield Viscometer within shear rates of 0.26 rad/s to 2.09 rad/s; and with an Ar-2000ex TA Instruments Rheometer within shear rates of 0.01 rad/s to 1,000 rad/s. Physicochemical tests, such as titratable acidity, chloride percent, fat concentration, color, humidity, syneresis, and total solids, were determined using laboratory equipment. The main result observed in all samples tested with Brookfield viscometer was that Kefir drink behaved as a Newtonian Fluid within shorter strain rates (0.26 rad/s to 2.09 rad/s). However, when the same samples were tested with Ar-2000ex Rheometer, a different behavior was observed: Over a wider range of shear rates, the fully non-Newtonian behavior of Kefir samples was discovered. Besides, additional variables such as shear and normal stresses, and loss and storage modulus, were analyzed with Ar-2000ex Rheometer. The rheological model that better fitted the experimental data was Cross model, followed by Power law model. Statistical analysis of viscosity data acquired with Brookfield Viscometer from all Kefir concentrations was not significant (p values greater than 0.05); which demonstrated that the effect of Kefir concentration over shorter strain rates was not significant, and thus, the samples behave as a Newtonian fluid. On the other hand, p values lower than 0.05 were observed in most of the different Kefir concentration samples tested with Ar-2000ex Rheometer; which indicates that a statistical significant effect was observed and thus a non-Newtonian behavior of Kefir samples. All statistical analysis was performed with SPSS v.16 software, selecting a Duncan Test with a confident interval of 95%

关键词： Food rheology viscosity shear rate kefir concentration