Geranium tone

Sorbic acid is a short-chained unsaturated fatty acid widely used as a chemical preservative in sweetened wines at bottling. Issues regarding its use as well as merits or deficiencies are discussed elsewhere (see Chapter 6 and Zoecklein et al. 1995). In the context of the present discussion, it should be noted that whereas sorbic acid is generally effective in the inhibition of Saccharomyces it has little activity toward oxidative film-forming yeasts, Bret-tanomyces and Zygosaccharomyces or LAB or acetic acid bacteria (Zoecklein et al., 1995). With the exception of AAB, all the above represent a postbot-tiing threat. 

Winemakers wishing to stabilize juice with sorbate should also consider the potential presence and activity of LAB. In the absence of alcoholic fermentation and other countermeasures (SO2, filtration, etc.), bacterial reduction of the sorbic acid to sorbyl alcohol may occur. Upon blending 

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Some LAB are known to reduce sorbic acid to sorbyl alcohol, the precursor to geranium tone . As alcohol is not present, the end product is not produced until the juice is added to sweeten the wine at bottling. Subsequendy, geranium tone is produced in the bottled wine chemically and in the absence of viable LAB. Thus, where sorbic acid is used to stabilize stored juice against fermentative yeast activity, SO2 should also be considered to prevent growth of LAB (see section 1.43). 

Sorbic acid should be added to wine just prior to bottling. Additions should be carried out in stainless steel or other containers that can be cleaned and sanitized. Use of wood tanks for prebotding mixing and storage should be avoided. Residual sorbic acid trapped in the wood may be utilized by resident LAB in the production of geranium tone which continually leech into wines processed through that tank. 

Lactic acid bacteria are able to utilize sorbic acid in the formation of 2-ethoxyhexa-3,5-diene. Commonly referred to as geranium tone, the compound is sensorially detectable at parts-per-billion ( Llg/L) levels in wine. Details regarding formation of geranium tone may be found in Chapter 1 (see section 1.4.3). 

Bacteria are not affected by sorbic acid, and, in fact, several species can metabolize the acid to eventually yield 2-ethoxyhexa-3,5-diene, a compound that imparts a distinctive geranium odor/tone to wines (Section 11.3.5). Other odor/flavor-active compounds detected in spoiled wines treated with sorbic acid include l-ethoxyhexa-2,4-diene and ethyl sorbate (Chisholm and Samuels, 1992), the latter of which has been associated with off-flavors in sparkling wines (De Rosa et al., 1983). Whereas Chisholm and Samuels (1992) described ethyl sorbate as possessing a honey or apple aroma, De Rosa et al. (1983) thought the compound imparted a very unpleasant pineapple-celery odor upon short-term (6 month) storage. Based on this observation, De Rosa et al. (1983) recommended that sorbates should not be used in sparkling wine production. 

Figure 11.5. Microbiological formation of geranium odor or tone.

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