Oak Barrels for Wine – Part Four

Oak barrels are most often used for aging wines after the initial fermentation that is usually done in stainless steel tanks or plastic tanks/bins.  However, actual fermentation in barrels is also a time-honored process.  Chardonnay is the varietal most often fermented in oak.  Common characteristics for barrel fermented chardonnay include coconut, cinnamon and cloves, and an overall toasted, silky texture with notes of bread dough, caramel and butter cream.  Because of the toasted inner surface of the barrel, the wine will usually be darker gold in color than similar wine fermented in tank.  Fermentation of red wines in barrel will bring out a toasty, smokiness with notes of mocha and dark toffee. 

Although it may seem counter-intuitive, aging wine in oak introduces more oak flavors than will result in wine fermented in oak.  After fermentation in oak barrels, dead yeast cells, or lees, are separated as the wine is racked to another container (tank or barrel).  When these lees are removed, they take with them a measurable amount of the oak character that resulted from fermentation in oak.  On the other hand, when a wine that has finished fermentation in tank is transferred to a barrel for aging, it is usually clear of most lees, thus the full impact of oak character ends up in the wine. 

The time a wine spends in oak barrel usually varies from a few months to a few years – typically 4-12 months for whites and 12-24 months for reds.  Wine takes up most of its oak flavoring in the first few months.  Longer periods in barrel impact the wine through aeration (oxygen contact) which helps to precipitate phenolic compounds (tannins) and hasten the maturation and aging process. 

Malolactic, or secondary, fermentation is another important process that usually occurs during barrel aging.  This is a bacterial conversion of malic acid, the crisp acid found in green apples and many other fruits, into lactic acid, such as that found in milk, yogurt and soft cheeses.  Malolactic fermentation (malo) softens the acidic character of a wine and also allows the conversion of lactic acid into diacetyl, a compound that smells like butter and introduces buttery notes into wine.  Most white wines that spend time in oak barrels undergo malo, but the process can be blocked if more acidity is desired in the finished wine.  Almost all red wines that are barrel aged undergo complete malo. 

It was previously noted that oak alternatives, i.e., chips, pellets, staves, etc., can be used to introduce oak aromas and flavors to wine aging in stainless steel or plastic tanks.  This is certainly less costly than oak barrels, but stronger oak flavors can result that may not integrate as well with the wine.  Limited or no air contact in tanks can exacerbate this situation.  Over the past decade, new techniques for controlled introduction of oxygen, called micro-oxidation (micro-ox), have been developed to mitigate this problem and, together with judicious use of oak alternatives, to more closely mimic barrel aging.