Malo-Lactic Fermentations: A Need-to-Know Primer
We, here in Oregon, are blessed with some of the world's prime grape-growing real estate. But unlike California's Napa or Sonoma Valleys, the Willamette Valley has short summers, cool nights and early autumn rainfalls. Besides limiting the variety of grapes that can be successfully grown here (think Pinot Noir), these environmental conditions also have another effect on our grapes.
Each year it is with crossed fingers and baited breath that we wait and see what the sugar levels of our grapes will be. The past several years, we have been extremely blessed: 24 Brix (meaning 24 percent sugar levels) have been widespread and commonplace. This is about as high as one can expect Pinot Noir (or Chardonnay) to get and it is the sign of a fully mature, ripe, ready-to-go grape.
But as a grape ripens, not only does its sugar levels increase, but its acid levels decrease. Think of blackberries: a green, unripe blackberry sucks your jaws back and is as sour as a lemon. A ripe blackberry is lush and sweet and gooey. Thus is the same for grapes. Unripe grapes are tart and sour and high in acid. As a grape ripens, its acid levels will decrease and the juice will become less tart.
So, in Oregon, as harvest time nears and the grapes are ripening, acid levels are decreasing. Unfortunately, we usually cannot leave the grapes on the vine long enough for the acid levels to drop as low as we want. The grapes need to be picked, sulphited and started on the long road to wine. But what is a winemaker to do with these higher-than-desired acid levels?
Luckily, there is a natural organism that lives out on the grapes themselves that can help the winemaker during this time of trouble. Various species of bacteria - collectively known as Malo-Lactic Bacteria - are almost always used in Oregon winemaking to help lower this tart acid taste, and transform it into something more palatable. These bacteria "eat" malic acid - the harshest and most biting of the acids - and change it into lactic acid - a soft, gentle refined acid that is much smoother to the taste. ML Bacteria can usually lower an overall acid level by 0.1 to 0.2 parts per thousand. (We carry acid test kits at the store for $9.95.)
But out in the field there are three different varieties of malo-lactic bacteria. Only one of them is desired and prized for its superior taste. If one were to let nature take over, one of the three, or none of the three, might make it into the wine. For someone who has invested hundreds of dollars - and a year or two of waiting - into their grapes, this is a risky proposition. Therefore, most home winemakers, and every commercial winery, will sulphite the must in order to inhibit the natural organisms, and then add in their own strain of ML bacteria at the desired time.
Since ML bacteria do NOT like high alcohol, do NOT like sulphites, DO like pulp and skins, and DO like warmth, the best time to add ML bacteria into red wines is about two or three days into the primary fermentation, after the initial sulphite/Campden dosage has been wafted away. This gives them the best of all worlds. If kept warm for a few weeks into the secondary, ML fermentation should be done by the time you rack into the third fermenter, if not well before.
For Chardonnay grapes, the addition is done at a slightly different time. ML bacteria produce a buttery-flavored compound called diacetyl which - in red grape fermentations - is absorbed by the yeast cells and reduced to nothing more than a silky back flavor: this is as we want it. But in Chardonnay, we WANT a strong buttery flavor. Therefore, the ML bacteria are usually added into the fermenters after the fermentation is complete, and the wine is racked off of the primary lees (i.e., in the secondary fermenter). This helps to accentuate the buttery flavors. If you do not want the butteriness, add the ML bacteria into the primary as per the red wine.
OK - I hear a question in the back row. What about if my acid levels are normal? Do I still want to add ML? Yes and no. In Oregon, it is very unusual to have a low acid level. If it is moderate - 0.7 to 0.8 parts per thousand - I would still add ML bacteria just for the "softening up" that it accomplishes, as well as to prevent a spontaneous ML fermentation from attacking my wine months later, right before it's about to get bottled. Getting it done ASAP gets it out of the way - you never have to worry about it again later on down the line. If, for some strange reason, you have low acid levels - 0.5 or less parts per thousand - you probably do NOT want to initiate ML fermentation, but you DO need to keep a continuous, moderately high sulphite level in your wine to prevent the natural organisms from starting up.
The signs of ML fermentation are faint. Usually you will see very, very small bubbles rising from the lees and slowly drifting to the surface of the wine in the carboys. (You can often see them in the angled shoulders of the carboy as the race along up towards the neck.) If you sharply spin your carboys you may see a significant release of bubbles. These are signs of ML bacteria at work. If kept warm, all ML activity should be done within four weeks after the racking from the primary. This is the time when you should be considering racking into a fresh glass fermenter (your tertiary) - topping it up - and putting the wine to sleep for the winter. If ML is NOT finished, and you put it out in the cold too early, be very sure to allow it to fully warm up for a few weeks/months next spring/summer to make sure that it does not spontaneously re-start. Under no circumstances should you bottle a wine that is half-completed with its ML fermentation - it will continue in the bottle and your wine will be spritzy, sickeningly buttery and not very palatable.
We carry two different forms of ML bacteria,White labs liquid culture, and a commercial winery freeze-dried culture. For volumes over 20 gallons, the freeze dried is more economical and faster acting. For up to 20 gallons, Wyeast's packs are better-suited.