Technological optimisation of Picante cheese using microbiological, chemical and physical criteria

In order to optimise the final quality of Picante cheese, several processing parameters (i.e., volumetric fraction of caprine milk, ripening time and percentage of salt added to fresh cheese) were manipulated and gave rise to a number of experimental cheeses produced according to a second-order, composite factorial design. Microbiological, physicochemical, biochemical and textural analyses were carried out in samples from all experimental cheeses. The most important effect in microbiological terms was produced by ripening time, in both linear and quadratic forms; caprine milk fraction and amount of salt added were also found to be significant factors, again in both linear and quadratic forms, especially with respect to total viable mesophilic microorganisms, Enterobacteriaceae and staphylococci. In terms of proteolysis and lipolysis, ripening time was the dominant factor, but caprine milk fraction and NaCl content were also significant at the 5% level. A higher content of caprine milk was associated with higher extents of proteolysis and lipolysis, but the reverse held for NaCl content. It was concluded that 50–80% (v/v) caprine milk, ripening for 195 days and 15% (w/wTS) NaCl provide safe counts of Enterobacteriaceae and staphylococci, while maximising proteolysis and lipolysis in Picante cheese

How performance of integrated systems of reaction and separation relates to that of parallel and sequential configurations

Given the thermodynamic and kinetic limitations which often constrain the extent of chemical reactions and post-reactional separation processes, and therefore constrain the yield and the degree of purity of the resulting products, integration of reaction and separation in a single unit has been under the scope of several bioengineering researchers in recent years. It is the aim of this work to compare the performance of a cascade of N reactor/separator sets, either in series or in parallel, with that of an integrated reaction/separation unit. In order to do so, a Michaelis-Menten reaction in dilute substrate solutions (i.e. a pseudo ®rst order reaction) was considered to take place in either con®guration and, under the same reaction and separation conditions, comparison of the performance and ef®ciency of these con®gurations was made in terms of fractional recovery of pure product, total time required to achieve such recovery and rate of recovery. It was concluded that: (i) the series combination of reactor/separator sets yields better results, both in terms of fractional amount of product recovered and time required to do so, than the parallel combination; and (ii) the integrated approach is much more time- and cost-effective than plain cascading, thus making it very attractive from an economic point of view

Michaelis-menten kinetics: explicit dependence of substrate concentration on reaction time

The nonlinear dependence of the rate expressions associated with enzymecatalysed reactions on the concentration of substrate implies that the corresponding integrated form of the substrate mass balance in a batch reactor cannot be expressed as an explicit function of time..This paper addresses this problem for the classical case of Michaelis—Menten kinetics by providing a self-pacing exploration of the characteristics of a Taylor expansion of the substrate concentration on time. The accuracy of such an approximation is discussed. The procedure presented is appropriate to model situations of technological and practical interest

Influence of milk type, coagulant, salting procedure and ripening time on the final characteristics of picante cheese

Picante da Beira Baixa cheese is a hurd, spicy, salty traditional cheese with a minimum ripening time of 120 d that is manufactured in Portugal at a farm level 0nly. The purpose of this work was to study the influence of several manufacturing conditions (viz. mixture of ovine and caprine milks, source of coagulant, level of NaCl addition, and duration of ripening period) on the final characteristics of this cheese following a (replicated) factorial design. Milk type proved to be a statistically significant technological parameter in terms of numbers of viable microorganisms of various genera and the extent of proteolysis, probably us u consequence of higher initial contamination of caprine milk. The type of coagulant ad a major effect on proteolysis: values of water-soluble nitrogen for cheeses coagulated with animal rennet were in general lower than those for cheeses coagulated with plant rennet, but much smaller differences were detected between values of non-protein nitrogen; and breakdown of a3 and b-caseins was more extensive in Picante cheeses manufactured with plant rennet. The level of NaCl was a statistically significant parameter ,for all microbiological, physicochemical and chemical Characteristics measured, an observation that is probably due to its capacity to reduce water activity. Ripening time did not have a significant effect on the numbers of viable staphylococci

Optimal temperature and concentration profiles in a cascade of CSTR's performing Michaelis-Menten reactions with first order enzyme deactivation

A necessary condition is found for the intermediate temperatures and substrate concentrations in a series of CSTR's performing an enzyme-catalyzed reaction which leads to the minimum overall volume of the cascade for given initial and final temperatures and substrate concentrations. The reaction is assumed to occur in a single phase under steady state conditions. The common case of Michaelis-Menten kinetics coupled with first order deactivation of the enzyme is considered. This analysis shows that intermediate stream temperatures play as important a'role as intermediate substrate concentrations when optimizing in the presence of nonisothermal conditions. The general procedure is applied to a practical example involving a series of two reactors with reasonable values for the relevant five operating parameters. These parameters are defined as dimensionless ratios involving activation energies (or enthalpy changes of reaction), preexponential factors, and initial temperature and substrate concentration. For negligible rate of deactivation, the Qptimality condition corresponds to having the ratio of any two consecutive concentrations as a single-parameter increasing function of the previous ratio of consecutive concentrations

Rapid spectrophotometric determination of nitrates and nitrites in marine aqueous culture media

The 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

Increase of the yields of eicosapentaenoic and docosahexaenoic acids by the microalga Pavlova lutheri following random mutagenesis

The high commercial values of eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids have driven a strain-improvement program, aimed at increasing the content of those fatty acids in the microalga Pavlova lutheri (SMBA 60) as parent strain. After a round of mutation using UV-light as mutagenic agent, an isolate strain (tentatively called II#2) was obtained, the EPA and DHA contents of which (in % dry biomass) were 32.8% and 32.9% higher than those of the control, native strain. The final EPA yields, when the cultures were maintained under appropriate conditions, were 17.4 and 23.1 mg · g−1 dry biomass, for the wild-type and the II#2 strain, respectively, whereas the final DHA yields were 8.0 and 10.6 mg · g−1 dry biomass, respectively. These results suggest that random mutagenesis can successfully be applied to increase the yield of n-3 fatty acids by microalgae

Microbiological profile in Serra ewes' cheese during ripening

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