Dr. Linda Bisson from the University of California, Davis, said growers whose vineyards are prone to stuck fermentations are welcome to send in juice samples for analysis.
They are some of the questions that can keep a winemaker up at night during harvest.
• Is that tank starting to smell a bit funky?
• That lot was at 10º Brix yesterday and it was still there this afternoon, are things stalling?
• We inoculated those barrels days ago, what’s the hold up?
The questions stem from nagging concerns about fermentation brought on by the stress of harvest and the inherent uncertainties of putting one’s trust in microbes. In a session appropriate for the pre-harvest season, a panel of University of California, Davis
, professors and yeast experts offered insights into wine fermentation research, yeast biology and best practices to ensure an efficient and healthy fermentation during a daylong session earlier this month.
Dr. Linda Bisson, a yeast expert with UC Davis, organized the seminar and provided an overview of problematic fermentations. The causes of stuck or sluggish fermentations are insufficient or imbalanced nutrients, ethanol toxicity, the presence of toxic substances, a yeast strain that’s ill suited for the particular must or juice, low pH and temperature shock.
Toxic substances include acetic acids produced by stressed yeast or other microbes. The acids may have occurred in the vineyard but don’t cause problems until the fermentation has produced a certain level of ethanol. Sluggish or slow fermentations indicate there’s something out of balance with the fermentation while a complete stop is likely the result of a misguided step such as making a pH adjustment in the middle of fermentation. “Usually if there is an abrupt stop it’s because someone did something to cause it,” Bisson said.
She also touched on some of her new research regarding juices that regularly exhibit difficult fermentations. “By difficult I mean you’re doing everything right and the yeast are still struggling with these juices,” she said.
The “difficult to ferment” juice in question was compared to other juices that fermented normally. The difficult juice showed a low ratio of assimilable to non-assimilable nitrogen, but the issue did not appear to simply be a lack of nutrients because additions had no effect. The juice was from Chardonnay Clone 5 on 5C stock, but it’s not clear that was part of the problem. “We are not sure what’s wrong,” Bisson said.
Bisson said that if anyone encounters fermentation issues this harvest—or if they have a particular vineyard lot that always is a tricky ferment—they are welcome to contact her or send in samples for analysis.
Problems can start in the soil
Dr. Jean Jacques Lambert, a research soil scientist at UC Davis, discussed how excessive or deficient amounts of nitrogen and other key nutrients in the vineyard could cause problems during fermentation.
Excess nitrogen results in too-vigorous vines that produce smaller yields of berries with reduced levels of phenolics, anthocyanins, skin tannins and color. Too much potassium can lead to higher Brix, but without corresponding amounts of aromatic and phenolic compounds as well as a lack of acidity. A vine suffering from a lack of potassium can produce grapes resulting in acidic, low color musts with rough tannins and green pepper and vegetal tastes. “I think we could gain a lot of insights of what is going on in the vineyard if we measured these elements in the must and later the wine,” he said.
Nutrient additions may be necessary to ensure vines produce grapes with the nutrients yeast need, but he stressed too much can be just as deleterious. He recommended following the “4R”s of nutrient additions; use the right fertilizer at the right location at the right time and in the right place. “You can’t manage your vineyard uniformly for every block,” he said.
Similarly, not every level of YAN is ideal for different strains of yeast. The general recommendation is a YAN of 125 to 225 ppm for a moderate level to power a fermentation that’s not too slow or too fast. That moderate range, however, is a bit of a sliding scale depending on the vineyard, variety, vintage and other factors.
Dr. Nichola Hall, a technical representative with Scott Laboratories
, said the nitrogen requirements for one strain of yeast to consume 1 gram of sugar can be twice that of a different yeast. Hall said yeast also require several nutrients working in tandem so that they can process wine sugars effectively.
The host of other microbes usually present in cold soak can rapidly deplete the amount of nutrients. Hall said winemakers should run a complete nutrition check after cold soak, going so far as to say a YAN analysis prior to cold soak “is almost a waste of analysis.”
Hall noted nutritional needs change during the different phases of the fermentation curve. Having a good understanding of your must’s nutrient levels will better inform your addition schedule so as not to leave any nutrients in the finished wine to feed unhelpful microbes. “If your Saccharomyces isn’t going to use them, who will?”
Picking the right yeast
Darren Michaels, technical representative for Laffort USA
, said while there are more than 130 distinct yeast strains to chose from, a winemakers’ options are limited by his or her budget, the winery’s technical capabilities and the alcohol tolerance of the yeast.
Michaels said specialty strains offer winemakers nuances in aspects such as mouthfeel, aromas, color and other more specialized considerations such as less foam production (good for barrel-fermented Chardonnay), VA production and the ability to mask green flavors. “It’s actually kind of a cheap way to change things without having to change everything.”
But the yeast can only impart those characteristics if it has the nutrients it needs and is within its pH, temperature and alcohol comfort zones. “If you can’t control fermentation temperature, it would be hard to say that strain is going to give you what you want,” he said.
And in addition to temperature control, all the experts during the session urged those in the audience to simply follow the supplier recommendations. “There is a reason the expiration date is on there, and it’s not to make you buy more yeast,” Bisson said.
Several members of the panel shared stories of problem fermentations that turned out to be the result of people failing to follow the instructions. There was the winemaker who tried pitching dried yeast into cold water and then putting the water and yeast in a microwave on high to get it to the right rehydration temperature.
And Hall recalled a winery with a problematic tank of juice that would just not start. The winemaker eventually decided to drain the tank, give the juice some oxygen, warm it up and see if that would get things moving. As the juice drained out, there—lying on the bottom of the tank—was an unopened package of yeast. She said the winemaker had obviously just told someone to toss this yeast in the tank and the worker had done just that.