One of the varietal aromatic compounds of Sauvignon Blanc is 4-MMP. In terms of its sensory descriptors it is almost like a chameleon: sometimes it smells like a box tree and on other occasions it will remind you of gooseberries. Some people indeed dare to say cat pee.
Flavour scientists have identified 4-MMP (full name: 4-mercapto-4-methyl-2-pentanone) as a constituent of the aroma of Sauvignon Blanc in as early as 1995 . It is a varietal flavour classified as a primary flavour. This compound can be traced back to the grape, where it is present in a bound form. Bound flavour compounds are too heavy to volatilize and therefore cannot reach our nasal cavity, which implies they have no aroma or flavour. The bound form of 4-MMP is also referred to as the precursor. 4-MMP gets released during fermentation when yeast is able to break up the precursor.
Influencing factors – vineyard
Riper fruit has been associated with higher levels of the precursor of 4-MMP . Moderate water insufficiency reduces 4-MMP. A high copper content in the soil, for example due to the use of Bordeaux mixture, may reduce levels of 4-MMP as well. It has been implied the copper binds to thiols.
Influencing factors – winery
Increased skin contact results in more extraction of the precursor to 4-MMP in the grape must. This technique is indeed sometimes applied in Sauvignon Blanc vinification. Fermentation is a key step to producing more 4-MMP, as the aromatic compound is released from its conjugate compound. Some commercial yeast strain are very efficient at breaking up the link between 4-MMP and the conjugate, resulting in higher levels of 4-MMP. See the illustration below of a commercial yeast strain.
4-MMP is not the compound that gives cat urine is typical smell. A different yet very similarly structured compound is responsible for the smell of cat urine: 3-mercapto-3-methylbutan-1-ol. Both compounds are thiols.
Occasionally 4-MMP is found in other white grape varieties, such as Verdejo. 4-MMP can also be found in some red wines and is thought to contribute to black fruit characters in e.g. Cabernet Sauvignon. 4-MMP has also been identified in several varieties of American hops.
Box tree, cat urine, broom, underarm sweat, gooseberry, blackcurrant, elderflower.
The detection threshold is the minimum quantity of the flavour compound required to be present to elicit a flavour sensation. For 4-MMP, this is about 1 nanogram / litre in wine. A nanogram (ng) is a million of a milligram (mg). One milligram is about the weight of sugar granule. We are looking at incredibly small quantities of flavour material here!
Less than 1% of the population is known to have specific anosmia towards this compound. Specific anosmia means someone is unable to smell a specific aromatic compound, while otherwise having an intact sense of smell.
To the best of my knowledge, there are no other routes of 4-MMP to find its way into wine other than being a varietal character.
4-MMP is both a thiol and a ketone.
Thiols are sulphur-containing compounds that can give pleasant characteristics to wine in the case of 4-MMP, 3-mercaptohexanol (grapefruit), or coffee thiol (coffee). Other thiols may also play a role in minerality in wine.
Some thiols are associated with reduction and give undesirable flavour characters. An example is methanethiol (drains).
Ketones often give fruity or floral characters. Other examples of ketones in wine are β-ionone (violet) and β-damascenone (black tea)
The precursor to 4-MMP is cys – 4-MMP. Cys is short for cysteine, the amino acid conjugate to which 4-MMP is bound. Yeast is primarily interested in the nutritional value of cysteine and therefore is equipped to break up the link between 4-MMP and cysteine.
Being a sulphur-containing compound, 4-MMP is rather volatile. It can be detected in wine by swirling the glass and taking short sniffs. 4-MMP is also quite strong on the palate. It may be more detectable by pinching your nose, taking some wine in your mouth, swallow/spit the wine then release your nose.
Some of the precursor to 4-MMP may have been left untouched by yeast during fermentation and is therefore present in the final wine. There is some evidence to suggests the precursor can break down in the mouth to release 4-MMP, due to enzymatic activity in your saliva . This can result in a more pronounced sensation of 4-MMP on the palate.
|||“. T. T. V. L. J.-N. B. a. D. D. Philippe Darriet, “Identification of a Powerful Aromatic Component of V vinifera L. var. Sauvignon Wines: 4-Mercapto-4-methylpentan-2-one,” 1995.|
|||W. J. d. T. Carien Coetzee, “A comprehensive review on Sauvignon blanc aroma with a focus on certain positive volatile thiols,” 2011.|
|||M. M. F. C. Sarah Ployon, “The role of saliva in aroma release and perception,” 2017.|