Enology Notes

Enology Notes #24 August 8, 2001

To: Virginia Vintners

From: Bruce Zoecklein

Subject: HACCP-Like Plan, Rot and the 2001 Season

HACCP-like plan. The most recent edition of my hard copy subscription newsjournal, the Vintner's Corner, outlines some red wine production considerations of importance in formulating a HACCP-like plan. All premium wine producers should have developed a HACCP plan. For additional information look at my web site under newsjournal index.

Rot and the 2001 Season. So far the year looks very good. As Virginia vintners well know, however, things can change quickly. A frequently asked question is "How much rot is acceptable going into the fermenter?" Answer - zero sour rot. Every premium wine producer should have a fruit sorting tray to cull-out rot-degraded fruit.

In both sour rot and Botrytis cinerea, several aroma modifications may occur. Generally, fruitiness disappears, sometimes with the formation of unpleasant odors described as "phenol" and "iodine." Noble rot often imparts a "honey" or "roasted" component to the aromatic character of wine, which may or may not be desirable.

Botrytis secretes esterases that hydrolyze esters produced during fermentation. Botrytis also destroys monoterpenes, which are, in part, responsible for the varietal aromas of Muscat, Riesling, and Gewurtztraminer. Botrytis and sour rot use a lot of the fermentable N of the fruit along with thiamine. These two rots can significantly reduce the fermentable or assimilable nitrogen concentrations.

The incidence and severity of Botrytis cinerea degradation can be estimated by visual inspection or analysis of laccase (see Zoecklein et al. 1995). Test kits estimating laccase content are available. Laccase activity above 3 units/mL indicate significant activity of Botrytis.

Processing Considerations for Botrytis and Sour Rot

Methods of handling grapes degraded by these rots include removal of deteriorated fruit, thermo-vinification, light whole cluster pressing, segregation of press fractions, cryoextraction, and post-fermentation heat treatment.

When the rot is not homogeneous, pressing preferentially extracts juice from berries that are least affected by rot and lowest in sugar. Cryoextraction has been used to freeze grapes lowest in sugar, followed by immediate pressing, thus allowing only the ripest grapes to release juice. Mechanical processes that grind skins can cause glucans to diffuse into the must, posing clarification problems.
Fermentation difficulties may arise from native yeast and bacteria, low levels of assimilable nitrogen and, in the case of Botrytis high must sugar, and the presence of botryticine. Botryticine, a heteropolysaccharide, has been demonstrated to stimulate production of acetic acid by Saccharomyces towards the end of fermentation.

Be prepared for rot. Have a sorting table. Use whole cluster pressing of cool fruit. Segragate press fractions, have PVP on hand to help bind flavonoid phenols which are hard/harsh (see Zoecklein et al., 1995). Use sulfur dioxide, but know that it will bind with the already reduced concentration of thiamine. Run the formol analysis for assimilable/fermentable nitrogen and supplement with both a nutrient lock-tail and DAP. Note, the formol analysis has been made easier - see my web site under Formol titration. Also note that I have a review of issues for conducting a healthy fermentation in Vol. 14, No. 2 (Marcy/April, 1999) of the Vintner's Corner. This is posted on my web site at www.fst.vt.edu/zoecklein/index.html under newsjournal.

Dr. Bruce Zoecklein
Associate Professor and Enology Specialist Head,
Enology-Grape Chemistry Group
Department of Food Science and Technology
Virginia Tech, Blacksburg, VA 24061

Enology-Grape Chemistry Group Web address:
Phone: (540) 231-5325
Fax: (540) 231-9293
E-mail: bzoeckle@vt.edu