Experimental approach of micro-organism effect during cooperage wood drying
The ecological study of flora carried out in five cooperages in the Charentes has enabled the isolation of four fungi generally found in the internal layers of staves (Candida sp., Paecilomyces variotii, Phialemonium sp., and strain E). Several experiments were performed to establish how these four species affected the analytical and qualitative characteristics of the wood.
Their wood degradation ability is quantified by observing the mass loss of inoculated stick of wood. The mass loss is very small.
Next we measured the ability of these moulds to degrade cellulose, xylan and aesculin. In a synthetic liquid medium the four fungal species degrade aesculin and xylan, only Phalemonium sp. shows a measurable growth rate for the cellulose medium.
Afterwards, the effect of the moulds on the composition and sensorial qualities of the wood is studied by direct inoculation of wood shavings. After three months of incubation, shavings, eventually after heating, are soaking in white spirit of Cognac under standard conditions ; this enables, the production of extracts which may be analysed and tasted. When extracts are prepared from unheated shavings, development of the moulds results in a diminution of most of the dosed compounds : dry extract, ellagitanins, ellagic acid, sugars and volatile phenols. The action of heat erases differences previously observed in the sugars, volatile phenols and ellagic acid. However, we still notice a reduction, related to fungi action of the amounts of dry extract, ellagitanins, gallic acid and eugenol. The amounts of aromatic aldehydes including vanillin are not affected by the development of micro-organisms.
A few shavings are studied separately, under the transmission electron microscope. These observations do not show any enzymatic degradation of the wood’s cell walls.
Sensorial analysis of the extracts does not reveal any important differences between extracts coming from wood shavings and the blank.
Under practical conditions, where fungi biomass is much less than that obtained from shavings, the fungi’s contribution towards final sensorial qualities of the wood, is therefore most probably negligible.
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