Changes in psychrotrophic microbial populations during milk creaming to produce Grana Trentino cheese

The aim of this study was to study the psychrotrophic microbiota developing during milk creaming of Grana Trentino cheese-making. 138 isolates from raw whole milk, cream and skim milk samples were screened by Randomly amplified polymorphic DNA PCR biotyping and representative strains of each biotype were characterised by partial 16S rRNA gene sequencing and enzymatic activity. Pseudomonadaceae were commonly isolated in cream samples while Streptococcaceae and Enterobacteriaceae in milk samples. Moraxellaceae and Flavobacteriaceae were found in both cream and milk samples.More than 80% of psychrotrophic isolates could grow at 37 °C. All Flavobacteriaceae and half of Pseudomonadaceae biotypes displayed proteolytic activity on milk agar even at low temperatures such as 10 °C. All Streptococcaceae and some of Enterobacteriaceae displayed acidifying activity and almost all Acinetobacter spp. (Moraxellaceae) displayed lipolytic activity towards tributyrin.Even if psychrotrophic bacteria is not the dominant microbial group in raw milk, their total number increases during creaming and becomes one of the most present group together with Lactic Acid Bacteria. Their enzymatic activities may be key players in determining milk quality for cheese making.     

Hydrogen permeation studies of composite supported alumina-carbon molecular sieves membranes: Separation of diluted hydrogen from mixtures with methane

One alternative for the storage and transport of hydrogen is blending a low amount of hydrogen (up to 15 or 20%) into existing natural gas grids. When demanded, hydrogen can be then separated, close to the end users using membranes. In this work, composite alumina carbon molecular sieves membranes (Al-CMSM) supported on tubular porous alumina have been prepared and characterized. Single gas permeation studies showed that the H₂/CH₄ separation properties at 30 °C are well above the Robeson limit of polymeric membranes. H₂ permeation studies of the H₂–CH₄ mixture gases, containing 5–20% of H₂ show that the H₂ purity depends on the H₂ content in the feed and the operating temperature. In the best scenario investigated in this work, for samples containing 10% of H₂ with an inlet pressure of 7.5 bar and permeated pressure of 0.01 bar at 30 °C, the H₂ purity obtained was 99.4%.     

Multiobjective optimization under uncertainty in tunneling: application to the design of tunnel support/reinforcement with case histories

In this paper, two questions common in tunnel design are addressed: (i) how to choose an optimum solution when more than one conflicting objective must be achieved; (ii) how to deal with data affected both by imprecision and randomness. Fuzzy Set Theory and Random Set Theory are used to develop a general interactive multiobjective procedure, which is then applied to the design of tunnel support/reinforcement. A case history illustrates how the procedure was successfully used in the preliminary design of a total of 40 km of tunnels in Central Italy. It is shown that the procedure allows the designer to become a knowledgeable decision maker because his interaction is required at the key points of the process, and because the trade-offs among the objective functions can be easily assessed. The designer's personal input is valued and clearly defined in its impact on the solution. The case history demonstrates that, without an optimization procedure, it is extremely likely (probability of 99%) that a solution is chosen, which either increases the costs without increasing safety, or decreases the safety without decreasing the costs. Finally, it is shown that both imprecision and randomness can be easily taken into account in tunnel design.     

Effect of chemico-physical parameters on the histidine decarboxylase (HdcA) enzymatic activity in Streptococcus thermophilus PRI60

In this study the activity of the histidine decarboxylase (HdcA) of Streptococcus thermophilus PRI60 was determined during growth and in crude enzyme preparations to evaluate its hazardousness in dairy products. The effect of different pH values, lactose availability, NaCl concentration, and growth temperature on histamine production was evaluated in M17 medium during 168 h incubation. In each case, the production of histamine increased concomitantly with the cell number with a relatively small further rise during the stationary phase. In all cultures the maximum histamine levels were reached at the end of active growth. Histamine was detectable (10 to 55 mg/L) even when growth was strongly inhibited. The HdcA enzyme in crude cell-free extracts was mostly active at acidic pH values common in dairy products. NaCl concentrations lower than 5% did not affect its activity. The enzyme was quite resistant to heat treatments resembling low pasteurization, but was inactivated at 75 °C for 2 min. Given the features of the enzyme studied, efforts must be dedicated to a thorough risk analysis and development of strategies to contrast the presence of histaminogenic S. thermophilus strains in products from raw or mildly heat-treated milk. PRACTICAL APPLICATION: During its growth Streptococcus thermophilus can produce histamine over a wide range of conditions encountered in cheesemaking and cheese ripening. The histidine-decarboxylase is even more active in cell-free extract and histamine can be accumulated independently of cell viability.     

Separation of fish oils ethyl esters by means of supercritical carbon dioxide: Thermodynamic analysis and process modelling

A semicontinuous fractionation process of fish oil ethyl esters was carried out in this work by means of supercritical carbon dioxide. The process focused on the separation of ethyl esters on chain length basis. Experimental temperature ranged from 42 to , whereas pressure ranged from 10.1 to 17.2 MPa. Optimal operating conditions were found for solvent density in the range 570-. A thermodynamic model based on the Peng-Robinson equation of state, proposed to represent high-pressure phase equilibria for this complex system, was validated on the experimental data. The model assumes that the multicomponent natural mixture can be considered as composed of five major components, defined on chain length basis. The proposed thermodynamic model was then used in the modellization of a continuous multistage fractionation process. The continuous process simulations that were carried out allowed to investigate the effect of number of theoretical stages, reflux ratio and solvent to feed ratio on the distribution of the components between extract and raffinate. A case study was considered, in order to find out process operating conditions to attain 95% by weight of heavy components (acid chain length 20 and 22) in the raffinate together with a 95% recovery. Results show that the stated separation is possible with a solvent to feed ratio and a number of theoretical stages in the range 90-150 and 11-30, respectively. The simulations were also employed to study the effect of the operating conditions of the separator used for solvent recycling.     

Co-current and counter-current modes for methanol steam reforming membrane reactor

In this simulation study, methanol steam reforming reaction to produce synthesis gas has been studied in a membrane reactor when shell side and lumen side streams are in co-current mode or in counter-current mode. The simulation results for both co-current and counter-current modes are presented in terms of methanol conversion and molar fraction versus temperature, pressure, _H2O/_CH3OH${\mathrm{H}}_2\mathrm{O}/{\mathrm{CH}}_3\mathrm{OH}$ molar feed flow rate ratio and axial co-ordinate.     

Defective structure of doped carbons obtained from preceramic polymers through Cl2- and NH3-assisted thermolysis

The processing parameters such as the heat treatment temperature, type of preceramic precursor, and post-synthesis treatments are key factors for the development the different microstructures in polymer-derived ceramics. Moreover, doping with different heteroatoms has increased the ability of the polymer-derived ceramics to produce tunable nanostructures with a controlled pore size and distributions. A preceramic precursor containing P has been prepared from a commercial polysiloxane polymer and a phosphate alkoxide. It has been subjected to thermal treatments in N₂, NH₃, and Cl₂ atmospheres in different order sequences to create differentiated microstructures either in the ceramic matrix and the carbon phase. The structural, textural, and spectroscopic characterization revealed that the P atoms play a key role in the evolution of the microstructure during the thermal treatments. If the chlorination is carried out before the treatment in NH₃, a silicophosphate matrix is formed and prevents from nitrogen incorporation into the free carbon phase. On contrary, if the NH₃ treatment is carried out before the chlorination, the carbon phase is predominantly modified by the incorporation of P atoms within the free carbon network.     

Turbine blade boundary layer separation suppression via synthetic jet: An experimental and numerical study

The present paper focuses on the analysis of a synthetic jet device (with a zero net massflow rate) on a separated boundary layer. Separation has been obtained on a flat plate installed within a converging-diverging test section specifically designed to attain a local velocity distribution typical of a high-lift LPT blade. Both experimental and numerical investigations have been carried out. Unsteady RANS results have been compared with experiments in terms of time-averaged velocity and turbulence intensity distributions. Two different Reynolds number cases have been investigated, namely Re = 200,000 and Re = 70,000, which characterize low-pressure turbine operating conditions during take-off/landing and cruise. A range of synthetic jet aerodynamic parameters (Strouhal number and blowing ratio) has been tested in order to analyze the features of control-separated boundary layer interaction for the aforementioned Reynolds numbers.     

PEMFC system simulation in MATLAB-Simulink environment

Modelling and simulation of PEM fuel cell stack operation is developed in Simulink^® environment and validated through experimental data. The present work is the starting point for the development of a user friendly and versatile tool aimed at controlling and optimizing the operation of a PEMFC stack; in addition, it could be of help in stack and BOP components design, for instance feeding and humidification systems, cooling circuit, temperature control logic and electrical interface. The constitutive equations used to model the FC stack operation are the fundamental equations of electrochemistry. First, the model is used to describe the behaviour of a single cell under steady-state conditions upon varying variables such as temperature, pressure and relative humidity of reactants; then, it is applied to simulate the operation of a stack configuration, including also fluid-dynamics aspects, thermal and kinetic behaviour of feed systems. In particular, thermal control modelling is based on a simplified approach where different heat removal mechanisms are accounted for in a separate way. In its present state, the simulation tool so developed allows a feasible investigation of some process variables influence on the FC stack performances. The stack modelling is tested against benchmark results obtained from a 300W 20-cell air-cooled stack under variable operative conditions. MEAs based on Nafion 112 and Carbon cloth GDLs developed ad hoc are assembled into each cell of the stack. Although the model is quite simple, these preliminary results point out that it may be an adequate tool to set design targets and support further steps of optimization.     

Modelling and simulation of the utilization of a PEM fuel cell in the rural sector of Venezuela

We studied the use of Proton Exchange Membrane (PEM) fuel cells in rural villages of Venezuela lacking a permanent and reliable energy supply. For this purpose, we formulated a semi-empirical mathematical model representing the main technical and economic features involved in the operation of the PEM cells. The simulation of the resulting non-linear model spans a 20-year time horizon, considering how costs are affected by the expected increase in the energy demand of the rural population, to which it is applied and the decrease in the unit costs of the cell on account of technological improvements and mass production of the cell. These villages are located in the parish of Trinidad de la Capilla, in the central-west part of the country. They were selected on the basis of various social and economic factors involving percentage of rural population and the Human Development Index. The results show that the main operating variables, current density, efficiency and generated voltage, show the typical behaviour of this type of cell, whereas, from the economic point of view, the cost of the electricity produced by the cell stack decreases to constant values, both for the same year and interanually, due to the economy of scale and because the investment costs and the costs of the hydrogen used offset one another. The use of PEM cells, besides meeting the energy requirements of this Venezuelan rural parish, is viable in principle, as it contributes in a large way to improving the quality of life and sustainable development of these isolated and depressed regions, which, due to their distance from the electrical grid and their surface area, are not covered by it, and probably will not be in the near future.