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  • 1
    ISSN: 1432-0614
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Summary The microbial stability of water-in-oil emulsions is calculated as a function of the initial contamination, the concentration of carbon and energy sources, the size-distribution of the water droplets and of the storage time. The calculated values appear to be in good agreement with the experimental data.
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  • 2
    ISSN: 1432-0614
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Summary The heat resistance of Klebsiella pneumoniae, an organism of widespread occurrence in nature has been determined in media containing various amounts of sucrose at temperatures between 47° and 59°C. In the presence of sucrose and at all temperatures the inactivation curves show a fast initial drop (logarithmic phase) in the number of survivors followed by a less rapid one (tail phase). The influence of the sucrose concentration can be described with ln k s = ln k O − αT [sucrose] for media with more than 0.52 mol/l sucrose for the logarithmic as well as for the tail phase of inactivation. The heat-injured cells were recovered on various media to investigate the influence of the presence of small metabolites and nutrients on the shape of the inactivation curves and on the death rate. For cells heated in media without sucrose, the recovery on a rich medium was much better than on a poor one; for cells heated in media with more than 0.26 mol/l sucrose, no difference was observed between the various recovery media. The activation energies as determined on the various media are always nearly the same, which strongly suggests that the critical sites in the heat inactivation were not enzymes playing a key role in the synthesis of small molecules such as amino acids or nucleotides.
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  • 3
    ISSN: 1432-0614
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Summary In many cases, water-in-oil emulsions appear to be microbiologically more stable against the growth of non-lipolytic microorganisms than the isolated water phase itself. The two main reasons for this intrinsic stability are that only a small fraction of the droplets of the emulsion is occupied by microorganisms originating from the water phase and that the size of these droplets limits the outgrowth of microorganisms. It is possible to give a quantitative description of the intrinsic stability of a water-in-oil emulsion, using the yield coefficient of different microorganisms grown in different media and the size-distribution of the water droplets in the emulsion. Relationships are given between the amount and nature of growth compounds in a water droplet of an emulsion and the growth and fate of microorganisms as a function of storage time.
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  • 4
    ISSN: 1432-0614
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Summary The heat resistance of Salmonella senftenberg 775 W, NCTC 9959, has been determined in distilled water pH 6.5 at sucrose concentrations up to 2.20 mol l−1 at temperatures between 63 and 70°C. Surviving cells were counted on minimal and enriched agar media to investigate the influence of the various nutrients on the recovery of heat injured cells. At various sucrose concentrations and temperatures multiphasic exponential parts of inactivation curves were found. Systematic differences between the recovery media depended on sucrose concentration, temperature and phase of exponential inactivation. At 60°C and sucrose concentrations between 0.52 and 1.82 mol l−1 the relationship between inactivation rate and sucrose concentration could be described by the equation ln k5=ln k0-αT [sucrose]. The activation energy of thermal inactivation reactions, substantially decreased when sucrose (1.82 mol l−1) was added to the heating menstruum. The activation energies in different recovery agars were of the same order, which suggests that the critical sites in heat inactivation are not key enzymes of the synthetic pathways of amino-acids and nucleotides. The differences between activation energies, calculated for cells of the various exponential phases of inactivation in both non-sucrose and 1.82 mol sucrose per 1 heating media, were also small, further suggesting that these critical sites are the same in cells from the various phases. Compared to published data on the heat resistance of S. senftenberg 775 W, we found a decreased resistance in a non-sucrose medium but an equal or increased resistance, depending on the phase of exponential inactivation, at a sucrose concentration of 1.82 mol l−1.
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  • 5
    ISSN: 1432-0614
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Summary The heat resistance ofCitrobacter freundii NCTC 9750 between 45–65°C in media with various water activities has been determined. At a water activity of nearly 1.00, the Arrhenius plot of the death rate shows a sharp breakpoint at 56.5°C, suggesting the existence of at least two different thermal inactivation processes causing lethality of the bacterial cell. The activation energy below 56.5°C is 0.4186 MJ/mol (100 000 cal/mol), above 56.5°C it is 0.1863 MJ/mol (44 500 cal/mol). After addition of sucrose (1.8 mol/l) or NaCl (0.77 mol/l) to the heating medium, such a breakpoint is not observed. The activation energy for these processes are, for sucrose; 0.2097 MJ/mol, for NaCl; 0.3641 MJ/mol. However, at an NaCl concentration of 1.54 mol/l there is a breakpoint at 53.3°C. The influence of the sucrose concentration on the heat resistance can be described by the formula: ln kS=ln kO−a [sucrose]. Such a simple correlation does not exist for the influence of NaCl or glycerol. The heat inactivation of whole cells ofC. freundii was also measured with a differential scanning calorimeter. The first irreversible conformation change took place at 323 K, the main conformation change at 343 K.
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  • 6
    ISSN: 1432-0614
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Abstract  A synthetic derivative of the cutinase cDNA of Fusarium solani pisi was expressed in Aspergillus awamori using the A. awamori endoxylanase II (exlA) promoter and terminator. The influence of the origin of the pre-sequence and the presence of a pro-sequence on the efficiency of extracellular cutinase production was analysed in single-copy transformants containing an expression cassette integrated at the pyrG locus. Transformants containing a construct encoding a direct, in-frame fusion of the xylanase pre-peptide to the mature cutinase showed a 2-fold higher cutinase production level compared to strains containing constructs with an additional cutinase pro-peptide. The effect of multicopy integration of the expression cassette on cutinase production was analysed in strains with different numbers of a cutinase construct containing its own pre-prosequence. The multicopy strains showed a 6- to 12-fold increased production of extracellular cutinase relative to the single-copy strains. No linear dose response relation to the number of expression cassettes present in the strains was observed. The amount of active enzyme produced by the strains correlated with the amount of cutinase-specific mRNA, suggesting that cutinase overproduction is not limited at the level of translation or secretion.
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  • 7
    ISSN: 1432-0614
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Abstract The function of the endoplasmic-reticulum-localized chaperone binding protein (BiP) in relation to protein secretion in filamentous fungi was studied. It was shown that the overproduction of several homologous and heterologous recombinant proteins by Aspergillus strains induces the expression of bipA, the BiP-encoding gene from Aspergillus niger and Aspergillus awamori. As this result could imply that BiP plays a role in protein overproduction, the effect of modulation of bipA gene expression on protein secretion was studied in several recombinant strains expressing glucoamylase (glaA) fusion genes. For overproduction of BiPA in these strains, extra copies of the bipA gene under the control of an inducible promoter were introduced. To allow analysis of the effect of a decreased bipA expression level on protein secretion, replacement of the wild-type gene for a bipA gene driven by the glaA promoter was attempted. However, this endeavour failed because of the lethality of this replacement. Although the final amount of secreted recombinant protein did not change significantly in strains with increased BiPA levels, increased levels of unprocessed fusion protein were detected in the total protein extracts of these strains.
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  • 8
    ISSN: 0749-503X
    Keywords: Hansenula polymorpha ; guar α-galactosidase ; continuous cultures ; Life and Medical Sciences ; Genetics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology
    Notes: A strain of the methylotrophic yeast Hansenula polymorpha, A16 has been developed that expresses the guar α-galactosidase gene to 22.4 mg/g dry cell weight in chemostat cultures at a dilution rate of 0.1 h-1. This corresponds to more than 13.1% of solube cell protein, of which 56-62% is secreted into the medium. The α-galactosidase gene was flanked by the promoter and terminator sequences of the H. polymorpha mox gene, which can direct expression of the mox gene itself more than 30% of total cell protein under methanol growth. The expression cassette (pUR3510) based on the Saccharomyces cerevisiae plasmid, YEp13, was integrated into the genome. Such transformants were stable in chemostat cultures and exhibited 100% stability for both α-galactosidase+ and leu+ phenotypes. Chemostat cultures produced higher levels of α-galactosidase with higher specific productive expressed as mg α-galactosidase g-1 h-1 compared to batch cultures.
    Additional Material: 3 Ill.
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  • 9
    ISSN: 1572-9699
    Keywords: lactic acid bacteria ; rDNA technology ; risk assessment and consumer acceptance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract To increase the acceptability of food products containing genetically modified microorganisms it is necessary to provide in an early stage to the consumers that the product is safe and that the product provide a clear benefit to the consumer. To comply with the first requirement a systematic approach to analyze the probability that genetically modified lactic acid bacteria will transform other inhabitants of the gastro-intestinal (G/I) tract or that these lactic acid bacteria will pick up genetic information of these inhabitants has been proposed and worked out to some degree. From this analysis it is clear that reliable data are still missing to carry out complete risk assessment. However, on the basis of present knowledge, lactic acid bacteria containing conjugative plasmids should be avoided. Various studies show that consumers in developed countries will accept these products when they offer to them health or taste benefits or a better keepability. For the developing countries the biggest challenge for scientists is most likely to make indigenous fermented food products with strongly improved microbiological stability due to broad spectra bacteriocins produced by lactic acid bacteria. Moreover, these lactic acid bacteria may contribute to health.
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  • 10
    ISSN: 1572-9699
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The influence of the growth medium and the growth temperature on the heat resistance of Citrobacter freundii has been established. Logarithmic growth phase cells grown on rich media have a higher heat resistance than cells of the same phase grown on minimal media. This finding was independent of type of carbon source in the growth medium, but the kind of carbon source has a definite influence on the heat resistance. Logarithmic phase cells grown at 37°C are much more heat stable than cells grown at 20 or 41°C. Stationary growth phase cells are much more heat resistant than logarithmic phase cells, whereas Mg2+-or glucose-starved cells are even slightly more heat stable than stationary phase cells.
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