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M D Larsen

Publications and source records attributed to M D Larsen.

5 recordsLinked to original sources
PubMed1999

The effects of environmental conditions on the lipolytic activity of strains of Penicillium roqueforti.

The lipolytic activity of 30 strains of Penicillium roqueforti was investigated by agar diffusion tests on tributyrin (esterase activity) and olive oil agar (lipase activity), by titration of the free fatty acids (FFA) produced and by gas chromatographic analysis of the individual FFA released after growth at 25 or 10 degrees C in butterfat emulsions containing 0, 2 or 7% NaCl. All strains investigated by the agar diffusion tests possessed esterase activity and 23 strains were also able to hydrolyse olive oil, but differences in esterase activity were seen. The agar diffusion tests and the titration of FFA showed that the amount of FFA released by a strain of P. roqueforti is determined by both esterase and lipase activity. A large release of FFA was only seen for strains with the ability to hydrolyse both short- and long-chained fatty acids, while strains with esterase activity produced smaller amounts of FFA. Between 7 and 14 days of incubation a steep increase in the release of FFA was observed both by the titration and by GC analysis, and then a decline from 14 to 21 days, probably caused by conversion of FFA to methyl ketones. Identical FFA profiles were found for two strains with different lipolytic activity. Long-chained fatty acids dominated the profile, while the short-chained fatty acids only were detected in small amounts and mainly in the end of incubation. Both strains were stimulated by NaCl in the emulsions.

Butter↗
PubMed1998

Characterization of the proteolytic activity of starter cultures of Penicillium roqueforti for production of blue veined cheeses.

Thirty strains of Penicillium roqueforti used for the production of blue cheeses were examined for proteolytic activity by agar diffusion on casein agar, by the azocasein test and by capillary zone electrophoresis (CE). Distinct differences were seen between the strains by all three methods applied and the 30 strains could be subdivided in three groups being significantly different in their proteolytic activity as measured by the agar diffusion test. The quantitative differences seen in the agar diffusion test were confirmed by the azocasein test. However, a negative result on casein agar, i.e., no clearing of the agar was observed for one strain while it showed low proteolytic activity in the azocasein test. CE proved to be a valuable method for revealing qualitative differences between strains of P. roqueforti in the breakdown of casein. Three strongly proteolytic strains broke down the specific casein fractions differently: one strain broke down betaA1-casein faster than betaA2-casein, the second preferred alpha s1-casein while the last strain broke down the casein fractions at equal rates. For a strain with medium proteolytic activity, the degradation of casein was seen by the appearance of a peak with migration time similar to alpha s1-I casein, a peptide normally produced by chymosin.

Bacteriological Techniques↗
PubMed1998

A simple method of sample size calculation for linear and logistic regression.

A sample size calculation for logistic regression involves complicated formulae. This paper suggests use of sample size formulae for comparing means or for comparing proportions in order to calculate the required sample size for a simple logistic regression model. One can then adjust the required sample size for a multiple logistic regression model by a variance inflation factor. This method requires no assumption of low response probability in the logistic model as in a previous publication. One can similarly calculate the sample size for linear regression models. This paper also compares the accuracy of some existing sample-size software for logistic regression with computer power simulations. An example illustrates the methods.

Humans↗