Preparing for the Upcoming Harvest
Dr. James Osborne, Enology Extension Specialist and Associate Professor
Harvest is here and in the winery there are many things to prepare for before the fruit starts arriving. One key area to prepare for is yeast and nutrient management. While yeast and nutrient management are always key factors in conducting successful fermentations, extra care is needed in years like this where grape composition may lend itself to more problematic fermentations. Because of the warm and dry growing season fruit may contain high °Brix and lower than optimal nutrients. This fruit chemistry can cause problem alcoholic fermentations as yeast need to metabolize a greater amount of sugar with a lower amount of nutrients in an increasingly high alcohol environment. The end result is often a very slow/sluggish fermentation or fermentations that do not complete fermentation but rather stall out with a few Brix still remaining. One key factor in preventing stuck/sluggish fermentations is ensuring there is sufficient yeast nutrients present during the fermentation. Yeast assimilable nitrogen (YAN) is one of these key nutrients and insufficient amounts can result in stuck fermentations as well as increased production of hydrogen sulfide. YAN is composed of nitrogen from ammonia (inorganic nitrogen) and nitrogen from primary amino acids (organic nitrogen). Luckily, we have a number of tools at our disposal to supplement YAN but how and when to perform this supplementation is a little more complicate.
The first step is measuring how much YAN is present in the juice/must. While you want to add enough YAN for a complete and clean fermentation, you do not want to add excessive nutrients as this can also cause problems. Large additions of YAN early in the ferment may lead to over vigorous fermentations and alter the aroma compounds produced by the yeast. In addition, residual nutrients in the wine may contribute to microbial spoilage during aging. So how much YAN do you need? Well, it depends. The general recommendation is between 150-250 mg/L for a 21-23°Brix must. If you have higher °Brix must or are using a high nutrient demand yeast strain then you may want to consider higher YAN levels. These are not hard and fast rules as many people may have no problems fermenting juice with much lower YAN levels then these. But these YAN levels have been found by researchers to result in fermentations with good kinetics. Aside from nitrogen, the other nutrients that are essential factors for yeast growth are the micronutrients such as the vitamins biotin, pantothenic acid, and thiamin. A simple method for analyzing these compounds does not exist so the general rule is that if your grapes are low in nitrogen they are probably also low in micronutrients. If you just want to increase YAN then DAP is an efficient way to do this. However, DAP does not contain any micronutrients so in addition to DAP you also should use a complex yeast nutrient that contains a blend of organic nitrogen (amino acids, peptides) and micronutrients. A balanced approach of both DAP and complex nutrients works best if you need to significantly increase your YAN levels.
The timing of nutrient additions is important for successful fermentations. Yeast preferentially up-take ammonia (DAP) before amino acids. Therefore, one large addition of DAP at the beginning of fermentation may delay/inhibit uptake of amino acids and cause problems later in fermentation. It is therefore recommended that you do multiple additions of nutrients during the early to mid-fermentation stage. For example, add half the nutrients 12-24 hours after inoculation followed by the remainder of the nutrients around 1/3 sugar depletion. Adding nutrient supplements all at once can lead to too fast a fermentation rate, and an imbalance in uptake and usage of nitrogen compounds. Alternatively, supplements added too late in the fermentation (after mid-fermentation) may not be utilized by the yeasts. This is because as the fermentation proceeds ethanol concentrations reach a point it impacts the yeast membrane and reduces the ability of the yeast to uptake nutrients. In addition, adding nutrients to a stuck fermentation seldom does any good at all and may add to the problem by ‘feeding’ spoilage bacteria that may have caused the issue.
Aside from YAN, the other yeast nutrient that can play a critical role in conducting successful alcoholic fermentations is oxygen. During the early stages of alcoholic fermentation Saccharomyces can use O2 for the production of sterols. These sterols are a key component of the yeast cell membrane and will help the yeast resist osmotic stress at the beginning of fermentation and ethanol toxicity near the end of fermentation. There are two times during the fermentation where oxygen addition has been shown to be beneficial. First, once the fermentation has become active and a 1-3 0Brix drop has occurred. The second time is at about 1/3 0Brix drop. Addition of oxygen to the ferment after this time is not recommended. Pumping over or racking and returning can supply some oxygen to the ferment but using a macro-oxygenator or micro-oxygenator at a high rate is a more reliable way to provide air to the ferment.
Warmer years also typically present us with fruit containing lower acidity. If acid additions are to be performed it is important to measure a few different components of acidity. The typical measurements of acidity are pH and titratable acidity (TA). These two measurements are not interchangeable and provide different information about acidity. pH is a measure of acid strength (hydrogen ions concentration) while TA is a measure of titratable groups. Often we consider pH a parameter for microbial stability and SO2 effectiveness while TA is often related more to taste. The pH and TA of your juice/grapes will be impacted primarily by the concentrations of tartaric and malic acid. These acids have different strengths and so a different ratio of these acids will impact both pH and TA independently. For example, you can have two juices with very similar TAs but quite different pH values if their tartaric and malic acid concentrations differ. pH is also impacted by the buffering capacity of the juice/must. The major component of grapes that impacts buffering capacity is potassium. Grapes with high potassium concentrations can be resistant to pH change from acid additions because of this buffering capacity. For example, you may make an acid addition to a set TA level but not see the expected decrease in pH if the juice/must contains high amounts of potassium. Bench-top trials for acid adjustments are recommended so that you can an accurately determine how much acid will be needed to achieve a certain pH and what the resulting TA will be. Knowing your malic acid concentration can also impact pre-fermentation acid additions. Malic acid will be converted to lactic acid during malolactic fermentation (MLF) and so this needs to be considered when making pre-fermentation acid additions. If your juice/must is high in malic acid then a larger pre-fermentation acid addition may be required than your juice/must TA values would initially indicate. On the other hand, if your juice/must contains very little malic acid (as is typically the case in warmer seasons) then your TA values pre-fermentation will be a little more predictive of what the wines TA will be post-MLF.
One additional note when it comes to calculating pre-fermentation adjustments (YAN and acid). When taking grape and juice samples for analysis, the more closely these samples represent the grape/juice in the tank the better. When assessing red grapes I would recommend taking the grape samples and crushing them by hand in a zip lock bag and letting the juice soak on the skins for a few hours (in the fridge). This will give you a more accurate pH value because the grape skins contain a significant amount of potassium that will soak out during this time. If you analyze the juice immediately after crushing the grapes then you will not account for this potassium. Grape skins also contain some YAN and so soaking the grapes will give a better estimation of the YAN content of the grapes.
I wish you all many clean and complete fermentations this coming harvest. If you have additional questions please contact me at 541-737-6494 or email james.osborne@oregonstate.edu
