No abstract

Microbial species present on the surface of grape berries at harvest play an important role in winemaking, thus counting and identifying them is of great importance. The use of conventional microbial techniques and molecular methods allowed a quantitative and qualitative inventory of the different microbial species present on the grape berries. These experiments were carried out in several areas of the Bordeaux region on the red grape varieties Merlot, Cabernet Sauvignon and Cabernet Franc. Populations and species clearly varied according to berry development stage. The most widespread yeast species at berry set, Aureobasidium pullulans was never detected at harvest. Fermentative yeasts were detected at harvest and not in the first stage of grape growth. Oenoccocus oeni was detected on immature as well as on mature berries. Gluconobacter oxydans was detected mainly at harvest. Detection of Pediococcus parvulus, was dependent on the vineyard. Veraison appeared to be a key stage for yeast colonisation and the increase in population involved a change in the proportion of each species. The number of A. pullulans fell significantly at veraison as it was superseded by fermentative yeasts. Microbial populations peaked at harvest when the berry surface available for adhesion was largest and no agrochemical treatments had been applied for some weeks. Soil, grape variety and grapegrowing practices may also influence this microbial ecosystem. Based on these and published data, we formulated hypotheses to describe this microbial ecosystem, thus enabling us to develop the concept of a microbial biofilm.

V . M IL L ET AN D A. LO N VA UD -FU NE L . 2000. Colony counting and DEFT did not give the same results when wine micro-organisms were enumerated. Both methods were used to monitor the population of acetic acid bacteria (AAB) and lactic acid bacteria (LAB) during wine storage. Results suggest that part of the populations had reached a viable but non-culturable (VBNC) state. These cells were unable to produce colonies but could hydrolyse fluorescent esters and could be counted by DEFT. For AAB, O 2 deprivation quickly induced this state. Recovery from this state was very rapid as soon as O 2 was available. The response was not so clear for LAB during wine storage. However, a similar state was induced by sulfiting. Moreover, filtration of wine stored in barrels and contaminated by Brettanomyces, AAB and LAB demonstrated that cell size was not homogeneous. Cells which remained in wine after several weeks could pass through a 0·45-mm membrane. However, when they re-entered a growing phase, they were again retained by membrane filtration. During and after the decline phase, wine micro-organisms might survive as smaller cells in a VBNC state.

Biogenic amines have undesirable physiological effects when absorbed at too high a concentration. Several kinds of food and beverages contain biogenic amines. Lactic acid bacteria can decarboxylate amino acids. Since winemaking involves the growth of lactic acid bacteria for malolactic fermentation, biogenic amines may occur. However, not all bacterial strains carry these activities. In the same wine-producing area, some wines may contain very low amounts of biogenic amines while others may have relatively large quantities. It is now possible to detect the presence of undesirable histamine-producing strains by PCR test or DNA probe based on the presence of the gene encoding histidine decarboxylase. Other strains have the ornithine and/or tyrosine decarboxylase. When biogenic amine-producing strains are present, the winemaker is encouraged to inoculate selected malolactic starters to replace the indigenous microflora.

An enzymatic complex able to transform L-malate to L-lactate was obtained from a Leuconostoc mesenteroides strain isolated from grapes. The molecular weight was about 235,000, the isoelectric point was at pH 4.35, and the optimal pH for activity was 5.75. The malolactic activity followed a sequential pattern concerning the involved substrates. At pH values substantially different from the optimum, a positive cooperativity between malate molecules was observed. Oxamate, fructose-i,6-diphosphate, and L-lactate acted as noncompetitive inhibitors, whereas succinate, citrate, and tartrate isomers produced a competitive inhibition.