Fermentations restarting

In spite of all precautions, a stuck fermentation may still occur. In this case, white wines must be treated differently from reds which undergo malolactic fermentation. At the time of the stuck fermentation, the red wine tank contains must and pomace rich in bacteria. The wine should be drained rapidly, even if the skin and seed maceration is not complete. Draining eliminates part of the bacterial contamination and introduces oxygen, which favors the restarting of fermentation and decreases the temperature. The wine can be sulfited at the same time, to inhibit bacterial development. In some cases, the fermentation restarts spontaneously. 

Some winemakers and coimnentators believe that the ambient yeast population in the vineyard and winery constitnte part of the characteristics of a natural wine (Lewin, 2010). They believe that the uniqne contributions of diverse yeast species -including non-Saccharomyces species - confer a complexity upon wine not seen in inoculated ferments. This might be tme (Varela et al., 2009), but it comes with the risk of spoilage. There is also an increased risk that the fermentation will become stuck (Martini, 1993) - i.e. the ferment will stop and be difficult to restart. In addition. 

Ability to restart a fermentation by backstopping with the same technological aim, if it is harvested at the end of fermentation No Yes No Sometimes... 

Technological effects Leavening, overaU bread quality Leavening, acidification, overall bread quality Acidification, overall bread quality, base for dried sourdoughs as flavour enhancers Flavour enhancer with the possibility to restart a fermentation, if required from legal aspects... 

Batch versus Continuous Fermentation A process can be run in batch or continuous mode. In continuous mode, there is a constant flow of fermented sugar out of the reactor that is equal to a continuous flow of fermentation medium into the reactor. During batch fermentation, there can be an inflow of medium, but there is no outflow [58]. Batch fermentation needs to be inoculated with a starter culture every time, whereas this is not needed in a continuous fermentation setup. However, in case of problems, the continuous fermentation needs to be restarted, so an infrastructure for starter cultures is needed anyway. A high volumetric production rate can be achieved when combining continuous... 

Gavazza, a., E. Poznanski, and G. Trioli. 2004. Restart of fermentation of simulated stuck wines by direct inoculation of active dry yeasts. Am.J. Enol. Vitic. 55 160-167. 

The presence of ethanol at the time of inoculation prolongs the latent phase and reduces cellular multiplication. An elevated temperature increases this inhibitory action. This effect of ethanol on yeast growth and fermentation speed occurs even at low concentrations from the start of fermentation. The difficulty of restarting a stuck fermentation is, therefore, understandable. 

Residual sugar is not acceptable in dry white and most red wines. A stuck fermentation therefore requires the restarting of yeast activity in a hostile medium. Evidently, if the alcohol content is already elevated (13% voL), the chances of restarting the fermentation are sUm. 

A density that remains stable during 24 or 48 hours confirms a stuck fermentation. In this case, different procedures exist to restart the fermentation while avoiding bacterial spoilage. 

If the fermentation does not restart on its own, an inoculation with active yeast is required. At present, commercial dry yeasts are inactive in media containing more that 8-9% vol. of alcohol, due to manufacturing conditions. In the future, industrially prepared yeast capable of developing in a medium containing alcohol would be desirable. Bacteria with this property have now been developed for malolactic fermentation. 

Effectively, the volatile acidity of the wine tends to increase during the restarting of a stuck fermentation. This generally occurs when the yeasts encounter unfavorable conditions. Certain yeast strains are more predisposed to forming it than others. The addition of 50 mg of pantothenic acid per hectoliter (not authorized by EU legislation) not... 

The temperature for restarting the fermentation must be considered. A slightly elevated temperature favors cellular multiplication but the antiseptic properties of ethanol increase with temperature. The risk of an increase in volatile acidity also seems to be in function of temperature. For these reasons, the restarting of the fermentation should be carried out at a temperature between 20 and 25°C. 

In white winemaking, at least when malolactic fermentation is not sought, the wine with a stuck fermentation should be lightly sulfited to protect against bacterial development. The fermentation can then be restarted using a yeast starter prepared according to the preceding instructions. 

Many possible adjuvants helping to restart a stuck fermentation have been proposed. The addition of ammonium salts does not raise any counterindications, but no appreciable improvement of the second fermentation has been observed. The addition of ammonium sulfate should be limited to 5 g/hl due to the limited use of nitrogen by yeasts. 

The addition of yeast hulls is certainly the most effective way of restarting a stnck fermentation, although less so than in preventing fermentation from stopping in the first place. (Section 3.6.2). They can be added to the yeast starter preparation or directly to the medinm with the stuck fermentation. 

Table 3.16. Restarting fermentation (after a spontaneous stuck fermentation) by addition of yeast hnUs (Lafon-Lafourcade et al., 1984)...

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