Pipette precisely 10 ml of filtered sample to a 25 ml volumetric flask and top-off with distilled water. 3. Pipette 10 ml of the diluted/filtered sample to a small beaker, insert a magnetic stir bar and pH probe. Add enough distilled water to ensure that the pH probe is sufficiently submerged. 4. Excessive levels of SO2 (previously added to the juice/must) can introduce error into the YAN determination. If SO2 > 150 PPM, add 1 ml of 3% hydrogen peroxide to neutralize the SO2. 5. Turn on the magnetic stirrer and titrate the sample with 0.05 N NaOH until neutralized (pH=8.0). 6. Add 2 ml (nominal) of previously neutralized formaldehyde solution to the sample using a small graduated cylinder. The formaldehyde will react with the YAN sources yielding a proportionate release of H+ ions (thus, lowering pH). 7. Record the current level of 0.05 N NaOH in your burette. 8. Titrate the sample with 0.05 N NaOH until pH = 8.0 to neutralize the H+ ions released in Step 6. 9. Record the new level of 0.05 N NaOH in your burette and calculate the amount needed to neutralize the sample following the addition of the formaldehyde solution. 10. Estimate yeast assimilable nitrogen (YAN) level as follows: