Winetech Technical Yearbook 2022

JANUARY/FEBRUARY

EFFECTIVE MICROBIOLOGICAL MONITORING IN WINE CELLARS

BY CHARL THERON

THE MAKING OF WINE is a microbiological process with yeasts and bacteria playing a decisive role. All yeasts and bacteria are however not necessarily beneficial for wine, but can spoil it. It is consequently important that cellars monitor the microflora during winemaking. Microbes like Saccharomyces cerevisiae and Oenococcus oeni play a basic role in the making of quality wines. The management of all micro-organisms during the winemaking process from the grapes to the bottle is essential to ensure that quality wines are produced. The removal of micro-organisms or the sterilisation of wines are usually only done just before bottling. Winemakers thus accept that a variety of micro-organisms occur in wine and must therefore apply practices to promote the desirable micro organisms, but limit the undesirable ones. It is consequently important to know which species occur in order to apply the practices accordingly. Cellars must therefore have a microbial monitoring programme in place as part of their overall quality plan. When such a monitor programme is compiled, a cellar must decide beforehand what its aim is and how it will be utilised. Critical control points must be identified, where monitoring will occur. It will differ between different cellars and wine types. The availability of internal or external laboratories for the monitoring will also differ between cellars and influence the application of the programme. The first critical control point is after the juice recovery from the grapes. The microbe population at that stage will together with the chemical analyses give an indication of certain required actions. It is especially the pH that is important, because it influences the micro-organisms’ growth and efficiency of sulphur dioxide (SO 2 ). At a pH below 3,4 in comparison with a high pH above 3,6, the possibility of spoilage during cold maceration will be much less. At lower pHs, the effective molecular percentage of SO 2 is also higher. Cold maceration at temperatures above 10°C will also benefit the growth of micro-organisms. Information regarding the microbe population at this stage can for example also influence the practices after alcoholic fermentation. A high population of Acetobacter will for example not cause any problems during alcoholic fermentation, but after that it will be essential to limit its further multiplication. This can for instance entail a review of the topping policy and SO 2 management. The second critical control point is after completion of the alcoholic fermentation, but before malolactic fermentation (MLF). At that stage wine can be exposed to different potential microbial spoilages. The wine temperature can as result of the alcoholic fermentation still be relatively high and also contain a low free SO 2 concentration. This can boost the growth of Brettanomyces and lactic acid bacteria. Knowledge of the micropopulation at this point can influence the MLF inoculation resolutions, storage temperature and whether MLF should take place in tanks or barrels. The micropopulation after MLF completion, before and during barrel maturation can also give certain indications. Regular chemical analysis during maturation may indicate potential spoilage and confirm the necessary actions. A quick decrease of the free SO 2 can for instance be an indication of acetaldehyde formation by Acetobacter and/or film yeasts. A quick increase in

The use of pure culture yeast is one of the methods to obtain a favourable microflora.

the volatile acidity concentration can be attributed to spoilage by oxidative micro-organisms and/or lactic acid bacteria. The third critical control is prior to bottling. After bottling it is difficult to solve spoilage problems. The most serious scenario is when consumers observe spoilage in bottles. Microbial monitoring together with chemical analyses like pH, residual sugar (RS) concentration, malic acid concentration, free SO 2 concentration and the microbial history of a wine will determine which actions need to be implemented to obtain microbial stability in wine. In most cases a sterile membrane filtration is required. The microbial monitoring during winemaking and just prior to bottling however requires different interpretations. A low Brettanomyces bruxellensis population (10 - 50 colonies per mL [ cfu/mL ]) at the start of barrel maturation will for instance require different attention than the same population prior to bottling. During the early stage of winemaking, winemakers have more corrective actions. The spoilage risk of a wine with a low pH (<3,5), low storage temperature (<30°C) and high molecular SO 2 (0,5 to 0,8 mg/L) is for example low and may perhaps require less attention, but prior to bottling microbial stability should be ensured. The compiling and implementation of a microbial monitoring programme will be senseless if the sampling is not executed correctly and effectively. The two major problems are samples not being representative and the contamination of samples. Representative samples can be ensured by mixing the juice or wine, or composing it from different heights of the container. If barrel samples are drawn, the same barrels must be sampled to observe a trend over time. Sample contamination can create a wrong impression of the reality and wrong reactive actions may be implemented due to it. One example is when a wine thief is exposed to the bunghole of the barrel, because high populations of Acetobacter occur there. If the sample valves of tanks are used for sampling, it must be ensured that the valve is properly cleaned and sanitised. This can be done with a 70% alcohol solution or a citric acid SO 2 solution. The valve must also be rinsed with juice or wine before sampling (Osborne, 2021). REFERENCE https://www.wineland.co.za/effective-microbiological monitoring-in-wine-cellars/

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WINETECH TECHNICAL YEARBOOK 2022