Maurice Bensoussan

A temperature-type model for describing the relationship between fungal growth and water activity

Growth of Penicillium chrysogenum, Aspergillus flavus, Cladosporium cladosporioides and Alternaria alternata at their respective optimum temperatures was studied in Potato Dextrose Agar (PDA) medium at different water activities (a(w)) adjusted with glycerol. The growth rate (mu) was expressed as the increase in colony radius per unit of time. This paper extends the model that showed the relationship between temperature and bacterial growth rate developed by Rosso et al. [J. Theor. Biol. 162 (1993) 447] to describe the influence of a(w) on fungal development. An excellent correlation between the experimental data and the model predictions was obtained, the regression coefficients (r2) were greater than 0.990, with the exception of that for A. flavus (r2 = 0.982). In addition, the use of such a model allows predictions of the cardinal water activities: a(wmin), a(wopt) and a(wmax). The estimation of the minimum water activity (a(wmin)) was in accordance with data literature for all the moulds considered here, but seemed to be slightly underestimated for P. chrysogenum and A. flavus when compared to our experimental values. The estimations of the optimal water activity (a(wopt)) and the optimal growth rate (muopt) were in excellent agreement to the experimental results for the four moulds. Through this example, it is suggested that the same approach for modelling can be used for various microorganisms (e.g. bacteria and moulds), and different environmental parameters (e.g. temperature and water activity).

Production of volatile compounds in model milk and cheese media by eight strains of Geotrichum candidum Link

Eight strains of Geotrichum candidum isolated from cheese were compared for aroma production. Twenty-four components, including alcohols, methyl ketones, fatty acids and esters, were identified by gas–liquid chromatography and gas chromatography–mass spectrometry of the volatile compounds produced by G. candidum cultures. Strains could be differentiated by their ability to produce dimethyldisulphide, phenol, phenylethanol and methyl ketones.

Prediction of conidial germination of Penicillium chrysogenum as influenced by temperature, water activity and pH

M. SAUTOUR, A. ROUGET, P. DANTIGNY, C. DIVIES AND M. BENSOUSSAN. 2001

Distributions of the growth rate of the germ tubes and germination time of Penicillium chrysogenum conidia depend on water activity

The effects of water activities for sporulation (a(wsp)) and germination (a(wge)) on the distributions of the growth rate of the germ tubes (mu) and the germination time (t(G)) of Penicillium chrysogenum conidia were determined by monitoring the length of the same germ tubes throughout the experiments automatically. No relationship between the individual t(G)s and mus could be established. Irrespective of the water activity for germination, mu was greater and t(G) was less for conidia produced at 0.95a(wsp) than that at 0.99a(wsp). At 0.99 a(wge) the mean and the standard deviation of t(G) were smaller than those obtained at 0.95a(wge). At 0.99a(wge), normal distributions for mu and t(G) were exhibited, but not at 0.95a(wge). The cumulative frequencies were used to reconstruct the germination curves. Great differences in the percentage of spores capable of germination (P(G)) and in the mean germination times between conidia produced at 0.95a(wsp) and at 0.99a(wsp) were clearly exhibited at 0.95a(wge), thus demonstrating the paramount influence of sporulation conditions on germination kinetics.

Impact of water activity of diverse media on spore germination of Aspergillus and Penicillium species

The effects of water activity (a(w)) of diverse media i/ culture medium for sporogenesis, a(w sp) ii/ liquid spore suspension medium, a(w su) and iii/ medium for germination, a(w ge), on the germination time t(G) of Aspergillus carbonarius, Aspergillus flavus, Penicillium chrysogenum and Penicillium expansum were assessed according to a screening matrix at 0.95 and 0.99 a(w). It was shown that i/ reduced t(G)s were obtained at 0.95 a(w sp) except for P. expansum ii/ a significant effect of a(w su) on t(G) was demonstrated for A. carbonarius, P. chrysogenum and P. expansum iii/ the most important factor for controlling the germination time was the medium for germination except for A. carbonarius (a(w su)). In accordance with the fact that fungal spores can swell as soon as they are suspended in an aqueous solution it is recommended to re-suspend fungal spores in a solution at the same water activity as that of subsequent germination studies.

A new model for germination of fungi.

The objective of this study was to design a germination model dedicated to fungi. The percentage of germinated spores, P(%), depended on the maximum percentage of germination P(max) (%), the germination time, τ (h) and a design parameter, d (-) according to : [formula in text]. The model was capable to fit satisfactorily either apparent symmetric and asymmetric shapes of germination curves. The accuracy of τ determined by using the logistic or the present model was at least twice that obtained by the Gompertz equation. In contrast to the logistic model, the new model is by essence asymmetric. Therefore, its use is consistent with skewed distributions of the individual germination times that were observed experimentally in many cases.

Production of 6-pentyl-α-pyrone byTrichoderma harzianum from 18∶n fatty acid methyl esters

Biosynthesis of 6-pentyl-α-pyrone byTrichoderma harzianum in two different media was evaluated. Best yields were found in nitrogen deficient medium (C/N=60). Limited growth seems to favour the production of this lactone. When fungal cells, precultured in low nitrogen medium, were incubated on methyl ricinoleate (10 g/l, C/N=60) an increase in 6-pentyl-α-pyrone production was observed in comparison with the media containing methyl oleate or methyl linoleate.

Modeling the effect of ethanol vapor on the germination time of Penicillium chrysogenum.

The influence of ethanol vapor on germination of Penicillium chrysogenum was determined on yeast nitrogen base plus glucose agar medium at 25 degrees C. Ethanol vapors were generated by 0 to 6% (wt/wt) ethanol solutions at the bottom of hermetically closed petri dishes. The logistic equation was used to describe the data as the percentage of germination versus time and to estimate the germination time. The effect of ethanol concentration on germination time was described by a new reparameterized equation, resulting in an estimated limiting ethanol concentration of 4.3%. Up to 3% ethanol, all spores germinated, and the germination time increased with increasing ethanol concentration. At 3.5 and 4%, some spores formed abnormal germ tubes and others were inhibited at the swelling stage. The inhibiting effect of ethanol was reversible under these experimental conditions.

Modelling the inhibitory effect of copper sulfate on the growth of Penicillium expansum and Botrytis cinerea

Aims:  This study aimed to investigate the effect of copper sulfate (from 0 to 8 mmol kg−1) on radial growth rate and lag time of two moulds responsible for vine grapes spoilage: Penicillium expansum strain 25·03 and Botrytis cinerea, strains BC1 and BC2.

Effect of inoculum size and water activity on the time to visible growth of Penicillium chrysogenum colony

In order to assess the effect of the inoculum size on the time to visible growth for Penicillium chrysogenum, the correlation described by González et al. (González, H.H.L., Resnik, S.L., Vaamonde, G., 1987. Influence of inoculum size on growth rate and lag phase of fungi isolate from Argentine corn. International Journal of Food Microbiology 4, 111-117) was compared to the model introduced by Gougouli et al. (Gougouli, M., Kalantzi, K., Beletsiotis, E., Koutsoumanis, K.P., 2011. Development and application of predictive models for fungal growth as tools to improve quality control in yogurt production. Food Microbiology 28, 1453-1462). Based on the regression coefficient, the latter model performed better than the former one to fit the data obtained for P. chrysogenum grown on Potato Dextrose Agar at 25 °C. Inoculum sizes in the range 10(1)-10(5) spores were tested at 0.930, 0.950, 0.970, and 0.995 aw. By extrapolation of the straight line, the model of Gougouli et al. (2011) provided accurate estimations of the time to visible growth for a single spore inoculum, tvg (N=1). In order to avoid experiments at reduced water activities, the influence of water activity on the model parameters, and on the ratio tvg (N=1) over the germination time was assessed.