Centro de Biotecnologia e Química Fina (CBQF)
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- Optimization of heat transfer in thermal processing of conduction heated foodsPublication . Hendrickx, M.; Silva, Cristina L. M.; Oliveira, F.; Tobback, P.Optimal sterilization temperature was defined as the temperature resulting in a minimum surface cook-value product together with the desired degree of sterility. Sterilization of conduction heated foods, with simple shapes as infinite slab, infinite cylinder and sphere, was modelled. The model assumed first order inactivation kinetics for microorganisms and quality. The temperature distribution was solved by an explicit finite difference numerical method with a non-capacitance surface node. Optimal temperatures were calculated as a function of : i) Food Properties - thermal diffusivity, Z-value for the quality factor, Processing Conditions - dimensions and geometry of the food or container, surface heat transfer coefficient, initial product temperature, heating medium come-up-time and Processing Criteria - target Fovalue.Initial food temperature and heating medium come-up-time have a minor influence on the optimal conditions. Generalized (semi)-empirical formulas relating quantitatively optimal temperatures and all relevant variables were developed. The generalized equations are a valuable tool to calculate or estimate processing sterilization conditions for regularly and irregularly shaped products. They were successfully applied to predict the optimal sterilization temperatures for conduction heated foods, packaged in typical cylindrical cans and retortable pouches, with different surface resistances to hea, transfer.
- Rheological properties of high-methoxyl pectin and locust bean gum solutions in steady shearPublication . Silva, J. A.Lopes da; Gonçalves, M. P.; Rao, M. A.The Cross and Carreau flow models described well the shear rate-apparent viscosity data. The coil overlap parameter, c[η], correlated well with the zero-shear rate specific viscosities of these mixtures. Experimental values of intrinsic viscosities of the mixtures were in good agreement with the prediction based on an additive model of the individual contributions of the gums. From the results obtained for the mixtures of both polysaccharide solutions at similar viscosity, pH and ionic strength, no significant interactions were detected. Hypotheses for this lack of polymer interaction were developed. Results with such model systems are useful in understanding the relation of composition to the properties of complex food systems.
- Inducible thermotolerance in Lactobacillus bulgaricusPublication . Teixeira, P.; Castro, H.; Kirby, R.The effect of a sublethal heat challenge on the subsequent thermotolerance of Lactobacillus bulgaricus at different stages of growth was investigated along with the effect of heating menstrum on survival. The response of the cells to heat stress was shown to be dependent upon both cell age and heating menstrum. Heat-inducible thermotolerance could be provoked in cells which had been growing exponentially when they were subjected to the sublethal heat stress: pre-incubation at 10°C above the optimum growth temperature. The same effect could not, however, be reproduced in cells taken from the stationary phase. Cells from the stationary phase were shown to always be more thermotolerant as compared to exponential phase cells. Cells showed a greater thermotolerance when heated in milk as compared to buffer.
- Obtaining a well balanced product quality in thermally processed conduction heating foods by analyzing surface and volume average quality optimum processing conditionsPublication . Silva, Cristina L.M.; Oliveira, Fernanda A. R.; Hendrickx, MarkSterilization temperatures that maximize each of the two objective functions - surface or volume average quality - were calculated as a function of the relevant influential variables using computer modeling. The resulting volume average and surface quality retentions were both determined for each case study. The effect of using sterilization conditions defined, for optimal surface quality on volume average quality and the effect of using sterilization temperatures defined for optimum volume average quality on surface quality were qualitatively and systematically investigated using the statistical 21.1 factorial design method. If the degradation kinetics for the quality factors considered at the surface and in volume average terms are similar, the use of sterilization temperatures for maximizing surface quality is suggested, when the aim is to obtain a product with a well balanced maximum final quality. On the other hand, if the quality factors have different degradation kinetics, the use of optimum sterilization conditions maximizing the quality at the surface or in volume average terms has to be analyzed on a case by case basis.
- Mathematical design of continuous, isothermal crystallizers with homogeneous nucleation: a simplified approachPublication . Malcata, F. XavierA simplified, systematic approach to the mathematical simulation of crystallizers is attempted by using the fundamental principles of mass conservation, via a population balance to the solid phase and a solute balance to both solid and liquid phases. A continuous, isothermal and isochoric crystallizer is assumed to be described by the MSMPR model under transient operating conditions with complete micromixing. The birth and death functions are assumed nil. Homogeneous nucleation is considered at a rate which is independent of the solution supersaturation. The growth rate of the crystals is described by McCabe's law. The possibility of solving the population balance and the mass balance independently is explored, and the conditions of validity for such an approach are found. The maximum linear dimension of crystal and the liquor concentration profile as functions of time are obtained. The approximation is found to be generally good for a period of time right after start-up of the crystallizer. A much wider range of time ensuring a satisfactory approximation is possible provided that the system and operation-dependent parameter takes small values.
- Potencial biotecnológico das microalgasPublication . Carvalho, A. P.; Meireles, L. A.; Malcata, F. X.; Oliveira, G.; Raposo, M. F.; Morais, R.
- Investigação aplicada sobre o pão de milho PortuguêsPublication . Rocha, João M.; Malcata, F. Xavier
- Determinação de nitratos em culturas de microalgasPublication . Carvalho, Ana P.; Meireles, Luís A.; Malcata, F. Xavier
- Production of lactan using plain whey, whey permeate and synthetic medium as feedstockPublication . Pintado, Manuela E.; Pintado, Ana I. E.; Malcata, F. XavierWhey (or whey permeate), a by-product of cheese manufacture, has created a worldwide problem of waste disposal owing to its high biological oxygen demand. Production of lactan has been previously described based on a semidefined medium rich in lactose using Rahnella aquatilis. This research was aimed at obtaining lactan directly from whole whey without additional nutrients, as well as and whey permeate obtained after ultrafiltration, using a similar type of strain, and the fermentation process was compared with that using the synthetic medium previously tested. The growth of biomass growth rate, the polysaccharide production rate and the viscosity of the broth were monitored. Organic acids, lactose, peptides and free amino acids were also determined. The growth curves were similar for the three media, showing a maximum specific growth rate of 0.61 h1, 0.65 h-1 and 0.63 h-1 for whey, whey permeate and synthetic medium, respectively. The major increase in polysaccharide production was observed between 12 h (beginning of stationary phase) and 24 h for whey and the synthetic medium; however, the increase in the case of whey permeate is less pronounced and occurs essentially after 24 h. The yield of polysaccharide was 0.59 g/glactose, 0.56 g/glactose and 0.37 g/glactose for synthetic medium, plain whey and whey permeate, respectively. The larger amount of citrate present in whey was used by Rahnella aquatilis with significant formation of acetic acid in the first 12 h and acetoine thereafter; whey permeate and synthetic media did not lead to acetoine formation. The final yields of the various organic acids for the synthetic medium, whey and whey permeate, respectively, were: 0.08, 0.07 and 0.03 (g/glactose) for acetic acid; 0.02, 0.06 and 0,00 (g/glactose) for lactic acid; 0.08, 0.08 and 0.02 (g/glactose) for formic acid; 0.04, 0.01 and 0.00 (g/glactose) for succinic acid; and 0.00, 0.11 and 0.00 (g/glactose) for acetoine. Lactose was almost completely depleted by 48 h of fermentation in the case of whey and synthetic medium, but only part of lactose was consumed in the whey permeate (final yield of 0.43 g/glactose). Small peptides (< 4,000 Da) and most free amino acids were consumed by 24 h in whey and synthetic medium. The whey permeate possessed low amounts of peptides (virtually consumed by 12 h) and very low concentrations of free amino acids, which increased slightly between 12 and 24 h.
- Influence of salt content, degree of proteolysis and aeration on the production of a polymer via fermentation of whey-related media by Rahnella aquatilisPublication . Pintado, Manuela E.; Pintado, Ana I. E.; Malcata, F. XavierUtilization of whey as fermentation feedstock has been attempted widely by the dairy industry. Production of lactan, a polysaccharide composed of mannose, galactose and galacturonic acid (at the molar ratio 5:3:2), starting from a semi-defined medium containing lactose via fermentation under aerobic conditions with Rahnella aquatilis was described previously. In this communication, the effect of salt, previous hydrolysis and aeration were studied during the polysaccharide production from whey in alternative fermentation media: hydrolyzed whey (under (i) aerobic and (ii) anaerobic conditions), hydrolyzed whey with 2.0% NaCl (w/v) (iii) and 0.5% NaCl (w/v) (iv),.and plain whey (v). The growth of biomass and the variation in concentration of organic acids, lactose, peptides and free amino acids were monitored. The polysaccharide production and the variation of viscosity of were also followed throughout 48 h of fermentation. Under the different conditions tested, Rahnella aquatillis showed a maximum specific growth rate of 0.61 h-1, 0.60 h-1, 0.61 h-1, 0.64 h-1, and 0.46 h-1 for hydrolyzed whey under aerobiosis and under anaerobiosis, hydrolyzed whey with 2.0% NaCl (w/v) and 0.5% NaCl, and plain whey, respectively; the final yields of the various organic acids were: 0.07, 0.18, 0.07, 0.04 and 0.05 (g/glactose) for acetic acid; 0.06, 0.07, 0.00, 0.04 and 0.02 (g/glactose) for lactic acid; 0.08, 0.09, 0.03, 0.04 and 0.04 (g/glactose) for formic acid; 0.01, 0.04, 0.01, 0.01 and 0.02 (g/glactose) for succinic acid; and 0.11, 0.09, 0.14, 0.19 and 0.00 (g/glactose) for acetoine. Lactose was almost completely depleted during the 48 h of fermentation for hydrolyzed whey; however, lactose was only partly consumed in plain whey (final yield of 0.48 g/glactose).Small peptides (< 2,000 Da) and most free amino acids were consumed by 24 h in hydrolyzed whey fermented under anaerobiosis and plain whey, but these peptides were present until the end of fermentation in the remaining media. R. aquatilis showed similar behavior in free amino acid consumption in hydrolyzed whey with NaCl and hydrolyzed whey fermented under aerobiosis. Plain whey yielded very low concentrations of free amino acids throughout the whole fermentation. The yield of polysaccharide was 0.56, 0.26, 0.39, 0.40 and 0.44 g/glactose for hydrolyzed whey fermented under aerobiosis and under anaerobiosis, hydrolyzed whey with 2.0% NaCl (w/v) and 0.5% NaCl, and plain whey, respectively.