To: Virginia Vintners
From: Bruce Zoecklein
Subject: Bleeding and Fermentation
As a result of the wet weather this season, many of you will or have conducted tank bleeding in order to help concentrate and increase the body and structural depth of your red wines. In the past, several have expressed concern that some juices produced by bleeding do not ferment to dryness, have a tendency toward reductive notes and/or are difficult to get to complete MLF. Some very recent research results may provide answers as to why wines produced by bleeding can have problems and may suggest solutions.
Amino acids are not equally distributed in the grape berry. For example, with mature Cabernet Sauvignon about 8.5% of the total are in the seeds, 15% in the skins and 77% in the pulp. It would seem that the separation of the pulp juice from the skins as occurs with bleeding would not have a large quantitative effect on juice that you remove. However, there is a significant qualitative influence.
The two amino acids present in the greatest concentration are proline and arginine. Proline can not be used by the yeast, while arginine can. Indeed, because it has four atoms of N per molecule, arginine is a very good source of fermentable N.
In the case of Cabernet Sauvignon (and likely most other red varieties) the ratio of arginine to proline is much greater in the skins than the pulp. In other words, the pulp juice which you take off in bleeding has a relatively high concentration of proline (approximately 55%) which cannot be used by the yeast and a small concentration of the more potent amino acid arginine and others needed to carry out a healthy fermentation. The lower incidence of incomplete fermentation in red compared with white musts supports the concept that the slow release of nitrogen from grape skins during fermentation is important.
Wines produced by bleeding should be given a higher concentration of supplemental fermentable N-higher than is required to ferment the juice remaining in contact with the skins. This will help assure clean fermentations which go to completion. If you desire to have your blush or rose wines undergo malolactic fermentation , I would also suggest that you add a commercial MLF supplement at dryness and before bacterial inoculation.
Call or write if you desire assistance or additional information-BZ