Picking decisions and pre-fermentation adjustments rely on accurate analysis of grape maturity indices such as Brix, TA, pH and yeast assimilable nitrogen (YAN). But, no matter how precise and careful the analysis is, the information provided may be misleading if a representative sample has not been collected. When taking a vineyard sample, sampling should be based on a standardized protocol to ensure consistency. When sampling from a uniform block a basic grid system could be used to collect samples. If there are significant site differences in the vineyard block, such as soil variability, water availability, and aspect/slope, then design a sampling protocol that accounts for these variables. Sample size should be relative to the size of the block and the uniformity. Berry sampling is more accurate than cluster sampling if performed correctly. 200-400 berries per sample should be taken from at least 30-40 clusters. Take berries from various locations on the cluster in a systematic manner, as there can be large variability between berries located on different parts of the cluster. Store berry/cluster samples in sealed bag in fridge until needed and prepare samples in a consistent manner. For whites, press the juice away from the skins and use for analysis. For reds, gently crushing the berries and having the juice in contact with the skins for a time will give a better representation of must composition as skin contact can affect pH and YAN in particular. Leave the berries crushed in a sealed bag in a refrigerator overnight if possible or at least for a few hours before taking a sample for analysis. When taking tank samples for juice analysis try to take as representative sample as possible. Sample after mixing where possible. For example, post-racking after setting a white juice or post pump over of juice during a cold soak. Taking a representative sample of grape must can be difficult due to the challenge of homogenizing a vessel of processed grapes. Combining multiple samples from various depths of the tank will help minimize some of the variability that exists within the container.
Once accurate juice/must analysis has been performed, picking decisions and pre-fermentation adjustments can be made. Sugar adjustments to achieve a desired final alcohol content should be based on calculated potential alcohol given a certain starting fermentable sugar content. When calculating potential alcohol the sum of glucose & fructose is a more accurate measure of fermentable sugar than °Brix. °Brix is a measurement of soluble solids rather than a direct measurement of fermentable sugar and is affected by other non-fermentable soluble solids in the juice. Alcohol conversion is also impacted by yeast strain and fermentation conditions so calculations of potential alcohol should be viewed as approximations. Multiplying °Brix by a conversion factor of 0.6 is a common way to determine potential alcohol. A more accurate calculation is to add glucose & fructose (g/L) and divide by 17. Remember to take into account any volume changes when making adjustments. While it is straight forward to calculate additions per volume for whites, for reds the calculations should be based on press volumes due to the presence of grape solids during fermentation.
Base nutrient adjustments on YAN analysis and stagger over the first few days of fermentation. If performing a cold soak, measure YAN at the end of cold soak and make amendments once fermentation has begun to ensure you are feeding a growing population of Saccharomyces. Growth of microorganisms during cold soaking can deplete YAN and other micronutrients so adding nutrients at the beginning of cold soak and/or making adjustments based on YAN analysis pre-cold soak could result in insufficient nutrients for fermentation. If large adjustments are needed then a mix of diammonium phosphate (DAP) and an organic nitrogen product will provide a boost in YAN as well as micronutrients. For smaller adjustments, an addition of an organic nitrogen product is often sufficient. For further information on nutrients please read: https://extension.oregonstate.edu/food/wine-beer/yeast-nutrients-stuck- fermentations
Article By Dr. James Osborne, Associate Professor & Enology Extension Specialist, Dept. of Food Science and Technology, Oregon State University