Yeast hulls

Presence of medium chain fatty acids The presence of MCFA can decrease yeast viability and even stop alcoholic fermentation. This problem is more prevalent in white winemaking because fermentation is usually carried out at low temperatures and without any aeration. Yeast hulls have been very useful for avoiding this problem. Yeast hulls adsorb MCFA from the media and provide sterols and UFA to the yeasts. Yeast hulls can be used as preventives (20 g/hl) or as curatives (40-50 g/hl) of stuck and sluggish fermentations. 

Volatile fatty acids are by-products in the formation of long-chain fatty acids, which are required for cell membrane phospholipid biosynthesis. The biosynthesis of volatile fatty acids is generally controlled by the same factors that control the formation of ethyl fatty acid esters, that is, oxygen, ergosterol and various insoluble solids (grape solids, clarification solids, yeast hulls) tends to suppress production whereas sugar concentration and clarification are stimulatory (Bardi et al. 1999 Delfini et al. 1992, 1993 Edwards et al. 1990 Houtman et al. 1980). 

To avoid long periods of maceration and to use enological adsorbents, such as activated charcoal or yeast hulls, in red wine, and bentonite, in white wine, when the crop has a relevant percentage of rotten grapes ... 

Table 8.8 Average depletions (statistically significant, p < 0.05) in wine as a consequence of yeast hulls addition.

Several proprietary mixtures of yeast and LAB superfoods are available as supplements during alcoholic and ML-fermentations. Yeast supplements consist largely of DAP, yeast extract, and yeast hulls (or ghosts). Yeast hulls are cell-wall by-products of commercial laboratory media manufac-... 

Yeast hull preparations may have variable and significant amounts of cell membrane lipids. Oxidation of the lipid component may yield a rancid character that can be transferred to the treated wine. To minimize the potential for this problem, it is recommended that hulls be used upon receipt and not stored for periods of time. 

Munoz, E. and W.M. Ingledew. 1989. Effect of yeast hulls on stuck and sluggish wine fermentations Importance of the lipid component./. Appl. Environ. Microbiol. 55 1560-1564. 

Wahlstrom, V.L. and K.C. Fugelsang. 1988. Utilization of yeast hulls in winemaking. Calif Agric. Tech. Inst. Bull. 880103. 

Munoz, E. and W.M. Ingledew. 1989b. An additional explanation for the promotion of more rapid, complete fermentation by yeast hulls. Am.J. Enol. Vitic. 40 61-64. 

In white winemaking, suspended solids activate fermentation (Section 3.7.3). Certain constituents, probably sterols and fatty acids, are involved in this phenomenon (Ribereau-Gayon et al., 1975b). Although these substances are not very soluble, yeasts are capable of using them to improve fermentation kinetics. They probably act in conjunction with other factors, such as oxygenation and possibly nitrogen additions. Yeast hulls have a similar effect independent of their ability to eliminate inhibition (Section 3.6.2). 

Table 3.7. Stimulation of wine fomentation by the addition of (NH4)2S04 or yeast hulls to grape must before fermentation (results at the end of fermentation) (Lafon-Lafourcade et al, 1984)...

Yeast hulls have proven to be effective in musts that are difficult to ferment for example, those containing high sugar concentrations or containing pesticide residues. Yeasts are also more temperature resistant in their presence (Table 3.8). They may be used, although less effectively, in cases of stuck fermentation (Section 3.8.3). 

The involvement of yeast hulls in fermentation processes is also accompanied by variation in the concentration of secondary products (higher alcohols, fatty acids and their esters). As a result, wine aromas and tastes can be modified. All operations that affect fermentation kinetics affect the wine—temperature, oxygenation, addition of ammonium salts, etc.—and yeast hulls have no more of an impact on the fermentation than these other factors, and certainly less than temperature. 

Must lees particles and even glucidic macromolecules, making up part of the colloidal turbidity of musts such as yeast hulls, can adsorb short-chain fatty acids (C8 and CIO) (Section 3.6.2) (Ollivier et al., 1987). In consequence, the level of must clarification should be controlled for each type of white winemaking by measuring must cloudiness or turbidity, expressed in NTU... 

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. 

A massive addition of yeast hulls combined with an inoculation of active yeast can result in olfactive modifications of light wines such as whites and roses. Doses between 20 and 30 g/hl (maximum) are therefore recommended. 

In wines, the first step in preventing lactic disease is the proper sulfiting of grapes, especially when they are very ripe. The corresponding mnsts are more subject to stuck fermentations than others (Section 3.8.1). The winemaker mnst react accordingly and, if need be, use additives snch as nitrogen, vitamins and yeast hulls whose effectiveness is clearly established. Of conrse, elemental operations, notably aeration and temperatnre control, must also be scrupulously respected. 

Inoculation of the juice is strongly recommended. The chosen yeast strain should be highly ethanol tolerant and should produce little volatile acidity in difficult fermentation conditions. The dry yeasts should not be introduced directly into the juice to start the fermentation for this type of winemaking. A yeast starter should be prepared in diluted must, supplemented with NH4+ and yeast hulls, then seeded with dried active yeast at a dose of 2.5 g/hl of the total volume to be inoculated. The starter is added to the must on the second day of fermentation, at a rate of 2% of the total volume. This increases the maximum yeast population, which controls fermentation rate and volatile acidity production (Section 2.3.4). In one experiment, adding yeast in this way reduced the final volatile acidity content by 20%. 

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