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- Interesterification and acidolysis of butterfat with oleic acid by Mucor javanicus lipase: changes in the pool of fatty acid residuesPublication . Balcão, Victor M.; Malcata, F. X.Lipases have become powerful tools in the manufacture of structured fats either via randomization of their glyceride composition or incorporation of externally supplied fatty acid residues in such glycerides. The present communication reports on changes that occurred in the fatty acid pool of anhydrous butterfat subject to interesterification and to acidolysis with oleic acid catalyzed by a commercial lipase immobilized by plain physical adsorption onto hydrophobic hollow fibers at 40°C under controlled water activity. The main goal of this research effort was to engineer butterfat so as to increase its level of unsaturated fatty acid residues and concomitantly decrease its level of medium- and long-chain saturated fatty acid residues (viz. lauric and myristic acids). Although a certain degree of net hydrolysis of butterfat was observed, the triacylglycerols of butterfat subject to acidolysis were found to possess more (approximately 30% w/w) oleic acid and significantly less (8% w/w) lauric acid and less (2% w/w) myristic acid than those of the original butterfat.
- Alternative formulation of the problem of mass transfer during bubble formation in the presence of an interfacial chemical reactionPublication . Malcata, F. XavierThe removal of a dilute solute from a gas mixture being injected through a submerged single nozzle with an instantaneous chemical reaction occurring on the gas/liquid interface is theoretically studied. An improved version of the double surface-renewal model for the molecular transport of solute is reported. The reasoning underlying this improvement arises from the decoupling of the bulk and overall concentration of solute in the bubble, and requires an extra parameter for full description. An analytical formula pertaining to the overall depletion of solute by reaction with time is obtained. Simplifications of this result are presented and discussed f or asymptotic behaviors. The propriety of the analysis for the modeling of experimental evidence is included.
- Enhancement of product selectivity via enzyme immobilization in sequential degradation reactions of polymeric substratesPublication . Varga, S.; Malcata, F. XavierThe balance equations pertaining to the modelling of a slap-shaped bead containing immobilized enzyme uniformly distributed which catalyzes the sequential reactions of degradation of a polymeric substrate were written and analytically solved in dimensionless form. The effect of the Thiele modulus on the selectivity of consumption of each multimeric product was studied for a simple case. Whereas plain diffusional regime leads to lower selectivities than plain kinetic regime, improvements in selectivity of species A i relative to species Ai+1 may be obtained at the expense of higher Thiele moduli within a limited range when the diffusivity of A i is larger than that of A i +1, or when the pseudo first order kinetic constant describing the rate of consumption of A i is lower than that of Ai+1.
- Microbiological profile in Serra ewes' cheese during ripeningPublication . Macedo, A. C.; Malcata, F. X.; Hogg, T. A.The microflora of Serra cheese was monitored during a 35 d ripening period at three different periods within the ewe's lactation season. After 7 d ripening, the numbers of micro-organisms reached their maximum, and lactic acid bacteria (LAB) and coliforms were the predominant groups. Pseudomonads were not detected after 1 week of ripening. At all stages of ripening, cheeses manufactured in spring exhibited the lowest numbers of LAB and yeasts, whereas cheeses manufactured in winter showed the lowest numbers of coliforms and staphylococci. Leuconostoc lactis was the most abundant LAB found in Serra cheese whereas Enterococcus faecium and Lactococcus lactis spp. lactis exhibited the highest decrease in percentage composition. Numbers of both Leuc. mesenteroides and Lactobacillus paracasei tended to increase throughout ripening. The most abundant coliform was Hafnia alvei. Klebsiella oxytoca was found in curd but declined in number during ripening. Staphylococcal flora of curd was mainly composed of Staphylococcus xylosus, Staph. aureus and Staph. epidermidis. Staphylococcus xylosus was the major species found at the end of ripening. Pseudomonas fluorescens, was the only Pseudomonas species isolated from the curd. Although a broad spectrum of yeasts were found in Serra cheese, Sporobolomyces roseus was the most abundant yeast isolated.
- Rapid spectrophotometric determination of nitrates and nitrites in marine aqueous culture mediaPublication . Carvalho, A. P.; Meireles, L. A.; Malcata, F. XavierThe spectrophotometric determination of nitrate in sea water broths for cultivation of, say, microalgae is complicated by the frequent presence of nitrite. Two methods - sulphamic/perchloric acid method (also known as Cawse method) and sulphamic acid method - both claimed to be able to eliminate nitrite interference, were tested using a set of standards, but statistical treatment of the results proved their limitations in nitrate quantification. An improved method, based on former published methods for quantification of nitrite and coupled determination of nitrate and nitrite, was designed and tested. This improved method was compared with the reference method (based on use of a cadmium column) using several standards and biological samples of two culture media for microalgae, in different phases of their growth curve. The results thus obtained have demonstrated that there is no statistically significant difference between them at the 5% level. The precision of the method was tested by repeating determinations with three sets of standard mixtures containing nitrate and nitrite. The method proposed has advantages over conventional methods in reduced time of analysis, as well as high precision and accuracy, so it may be a good alternative for determination of nitrite and nitrate in marine aqueous media.
- Viability of bifidobacterium la ctis and lactobacill us acidophilus in milk: sodium chloride concentration and storage temperaturePublication . Gomes, Ana M. P.; Teixeira, M. G. M.; Malcata, F. XavierThe growth and viability of Bifidobacterium lactis and Lactobacillus acidophilus per se and for both strains as a coculture were studied in reconstituted skim milk so as to assess their dependence upon temperature (5, I0 and 15C), NaCl concentration (0, 0.51 and 1.03 mol L-'), and storage time (0–8 weeks). Cultures of B. lads exhibited no signijkant loss of viability either with increasing NaCl concentration or increasing storage temperature within the ranges studied On the other hand, viability of L. acidophilus decreased with increasing temperature and increasing NaCl concentration under similar conditions. When cocultured with L. acidophilus, B. lactis was signiJicantly less tolerant to higher NaCl levels and higher temperatures than when in pure culture, although rial numbers were still above the threshold required for commercial application. Coculturing with B. lactis had no detrimental effect on viability of L. acidophilus. A mechanistic model, which considers the behavior of the pure and mixed microbial populations is described by specijic death rates which vary with temperature following Arrhenius relationships) and NaCl levels following simple inhibition kinetics) in the milk medium. Activation energies for the death rates of B. lactis and L. acidophilus were 14 kcal mot' and 9–15 kcal mol-I, respectively.
- Potencial biotecnológico das microalgasPublication . Carvalho, A. P.; Meireles, L. A.; Malcata, F. X.; Oliveira, G.; Raposo, M. F.; Morais, R.
- Effects of different ripening procedures on the final characteristicsPublication . Freitas, A. C.; Malcata, F. X.Picante da Beira Baixa (or Picante) cheese is a hard, piquant, salted traditional cheese manufactured in Portugal from raw sheep’s and goat’s milks. The purpose of this work was to quantitatively assess the influence of various ripening procedures on the final characteristics of Picante cheese. Two alternative ripening protocols were considered, the traditional one and another with controlled environmental conditions via use of maturation chambers set at different preselected temperatures. The experimental cheeses were characterised in terms of microbiological, physicochemical, biochemical, sensorial and textural properties. Ripening time and temperature were statistically significant parameters for all microflora. The two ripening methods led to statistically significant differences in all physicochemical and biochemical parameters, especially the moisture content and the soluble nitrogen fractions (i.e. water loss was slower and proteolysis was faster in cheeses ripened via the traditional method). Differences in microbiological, physicochemical and biochemical properties were probables implicated in differences in textural and sensorial properties, especially cheese hardness and flavour. It was concluded that the standard ripening method was closest to the traditional one in terms of final cheese characteristics when the ripening temperature was above 11.5 7C.
- Microbiological characterization of Serra da Estrela cheese throughout its appellation d'origine protégée regionPublication . Tavaria, Freni K.; Malcata, F. X.The purpose of this study was to assess the typical microbiological quality of the most famous Portuguese traditional cheese, Serra da Estrela, and to assess its ripening time and geographical dependence. Ninety-six experimental cheeses manufactured from sixteen batches of milk on eight dairy farms scattered over the Appellation d' Origine Protégée (AOP) region were qualitatively and quantitatively evaluated microbiologically at various ripening times. Viable counts were performed after inoculation on appropriate selective media for aerobic mesophiles and proteolytic and lipolytic microflora, as well as lactococci, lactobacilli, species of Enterobacteriaceae, lactic streptococci, staphylococci, and yeasts. Members of the Enterobacteriaceae and lactic acid bacteria were the predominant microbial groups on all dairy farms throughout maturation; the latter are probably the microbial group responsible for most proteolytic and lipolytic breakdown in Serra da Estrela cheese. The microbial groups whose numbers were most affected by dairy-to-dairy variation were species of Enterobacteriaceae staphylococci, and enterococci, which are the most critical groups in terms of health hazards. It is therefore suggested that tighter control should be implemented at the level of choice of raw materials, in milk-handling practices, and in general throughout the manufacturing process in attempts to standardize production and consistently reduce microbiological risks (even though the distinctiveness of a few final organoleptic characteristics may somehow be reduced.
- Proteolysis of ovine and caprine Caseins in solution by enzymatic extracts from flowers of Cynara cardunculusoPublication . Sousa, M. José; Malcata, F. XavierPrimary proteolysis of ovine and caprine Na-caseinate at 30°C in phosphate buffer at pH 6.5 or 5.5 in the absence of NaCl and at pH 5.2 with 5% (w/v) NaCl by cardosins in aqueous extracts of Cynara cardunculus flowers was investigated using urea-polyacrylamide gel electrophoresis and reversed-phase high performance liquid chromatography. Caprine caseinate underwent more extensive degradation than ovine caseinate under the same conditions (pH 6.5 and pH 5.5); proteolysis of b- and as-caseins in ovine and, to a lesser extent, in caprine caseinates was reduced in the presence of 5% (w/v) NaCl. Peptide profiles of the pH 4.6-soluble extract had different patterns throughout ripening arising from the different specificity of cardosins toward ovine and caprine Na-caseinates. The major cleavage sites in ovine (caprine) caseinate were Phe105-Met106 (Lys116-Thr117) for k-casein, Leu127-Thr128 and Leu190-Tyr191 (Glu100-Thr101, Leu127-Thr128, Leu136-Pro137 and Leu190- Tyr191) for b-casein, Phe23-Val24 (Phe23-Val24, Trp164-Tyr165 and Tyr173-Thr174) for as1-casein and Phe88-Tyr89 (Ser9-Ser10, Phe88-Tyr89 and Tyr179-Leu180) for as2-casein.