YEAST ASSIMILABLE NITROGEN (YAN) - SÖRENSEN METHOD
MATERIALS AND REAGENTS:
The growth and development of yeasts during fermentation of a must needs the supply of nutrients. These nutrients are especially those that provide carbon (sugars) and nitrogen (ammonium, aminoacids) among others.
Nitrogen can be found in must and wine in mineral form as ammonium cation NH4+ and as organic nitrogen in the form of free aminoacids. Yeasts can quickly assimilate and use both.
During the harvest it is important to determine assimilable nitrogen in musts that need to be fermented in order to know whether musts need nitrogen nutrition (ammonium phosphate or organic nitrogen). These nitrogenous forms can be determined by adding formaldehyde by excess blocking NH4+ group and in aminoacids, amino groups, leaving the carboxyl groups, which lowers the pH in proportion to the amount of these nitrogen compounds.
1- In a 100ml beaker, place a volume of about 50ml of formaldehyde and with the help of a pH-meter neutralize it slowly untill approximately pH 6, using the 50ml bottle, and then continue to pH 8.00 with the burette filled with titration liquid. Reserve the reagent for the analysis and keep the rest for further applications.
2- Pour in a 50ml beaker 25ml of wine or must, measured with a pipette of 2 marks, and add 2-3 drops of hydrogen peroxide 33% (10ml bottle).
3- With the 50ml bottle neutralize slowly the wine or must untill approximately pH 6 and keep going untill pH 8.00 with the burette wich contains titration liquid.
4- Once neutralized the wine or must add, measured in the measuring cylinder, 10ml of formaldehyde from point 1. The pH of the mixture will slightly decrease, wait about 1 minute or untill the mixture becomes stable.
5- Fill an top burette with titration liquid and neutralize again the mixture untill pH 8.00 and multiply the spent milliliters by 56 getting milligrams per liter (mg/l) of Yeast Assimilable Nitrogen (YAN):
Make neutralizations slowly.
Hydrogen peroxide 33% is used to eliminate SO2 from the sample so it doesn't interfere with the results. Handle with care.
Formaldehyde is relatively unstable and polymerizes, lowering the pH, which should be neutralized to pH 8.00 only the amount needed for the analysis of the day and storage it over 20ºC (68ºF). Presence of turbidity or precipitates in the bottle doesn't interfere with the results. Avoid inhalation.
The content of Yeast Assimilable Nitrogen (YAN) in the must is very variable, because it depends on many factors, such as: soil, climate, nutrition of the vineyard, variety etc. It is considered that the musts with values between 200 < YAN < 300 mg/l are in good conditions for the yeasts to develope.