Cap Management, Punch downs and Pump Overs

Cap management of red wine fermentations is a time consuming but crucial. As CO₂ is released during fermentation of red grapes, the solid mass of skins rises so that 1/3 of the skins may be out of contact with liquid. Extraction of phenolics, aromatics and polysaccharides from pomace requires contact with liquid, so immersing these solids is an important element in flavor development. Extraction is also governed by diffusion, and as more compounds are extracted from the solids in the cap, they can experience local saturation (Lerno et al 2017). Cap management helps to homogenize these elements throughout the fermentation vessel, encouraging further extraction (Ribereau-Gayon et al 2006). However, when used excessively, over-extraction of phenolics can lead to dry tannins or bitterness (Razungles 2010). Decisions as to the type, duration, and frequency of cap management all contribute to the extraction of these compounds into the final wine.

Cap management also plays a role in managing the microbes in the fermentation. During cap management, aerobic microbes such as Acetobacter are submerged into an environment lacking in oxygen, thus limiting production of acetic acid and ethyl acetate. Bathing the cap in fermenting liquid exposes lurking microbes to alcohol, many of which are sensitive or intolerant to alcohol levels higher than 4%. Oxygen introduction into the fermenting liquid also helps increase yeast cell production of cell membrane components that will help them tolerate rising alcohol levels later in fermentation. Oxygen introduced during cap management is also important in tannin evolution and stabilization of anthocyanins (Ribereau-Gayon et al 2006).

Another important element of cap management is the effect on temperature. Heat accelerates the enzymatic break down of cell walls, loosens cell membranes and makes them more permeable, and increases diffusion rates of chemicals in liquid, all of which contribute to greater extraction of compounds from fermenting grapes. Most notably in red wine fermentations, heat during fermentation increases extraction of phenolics but also increases the rate at which anthocyanins are lost, leading to more tannin and less color (Razungles 2010, Ribereau-Gayon et al 2006). Heat can also lead to the loss of volatile aromas, and stress yeast if it builds up too much (Ribereau-Gayon et al 2006).

In a study looking at the formation of temperature gradients in red wine fermentations, Schmid et al (2009) measured the temperature in the cap at several locations during an active fermentation as well as at several depths of the fermentation before, during, and after cap management. In a 6-ton fermentation of Pinot Noir, there was up to 12°C difference in temperature between the cap and the liquid and a 6-8°C difference between the center and sides of the cap. The gradient was not always fully dissipated during routine cap management (Schmidt et al 2009). The figure shown below (from Schmid et al 2009) indicates heat at various depths in the tank over time. Panel (a) shows heat built up 3 hours after a 10-minute pump over, (b) shows heat immediately after punch down, and (d) shows the heat built up after 3 hours. Here, the heat in the cap 3 hours after pump over (a) is higher than 3 hours after punch down (d), indicating the punch down did a better job dissipating heat. However, panel (b) shows that this was incomplete homogenization. It is important to note that a temperature gradient was also found in three other fermentations studied, but to a lesser extent. The authors hypothesize the magnitude of the temperature gradient is affected by grape variety and processing, however do not elaborate on those differences.

In addition to managing heat, cap management helps to homogenize phenolics and presumably other flavor and aroma compounds extracted from the cap. In a study of chemical gradients during fermentation, Lerno et al (2017) found that gradients of anthocyanins, skin phenolics and seed phenolics all develop during red wine fermentations. Pumping over of one tank volume each day homogenized these gradients. When pumping over was not done, anthocyanins reached a saturation point after 24 hours on at least one day of the three monitored (days 2-4 of fermentation). In addition, extraction kinetics were different for different types of phenolics, with anthocyanins reaching maximum extraction after 24-72 hours of fermentation, gallic acid (a skin phenolic) continually increasing throughout the 10 days of monitoring, and catechin (a seed phenolic) beginning its extraction later in the fermentation. Cap management, therefore, is also phenolic management.

Cap management is most often done as a pump over or punch down. Pumping over involves the pumping of fermenting liquid from the bottom of the tank over the cap, sometimes with intentional aeration. The duration of pump over varies considerably from cellar to cellar, and often reflects the stage of fermentation, with aeration in the beginning of the fermentation and longer pumping over in the active stages of fermentation (Razungus et al 2010). It is thought that to obtain full extraction, pumping over the full volume of the tank each day (usually in two events) is sufficient in the first few days of fermentation and half the tank volume is sufficient in the second half of fermentation (Razungus et al 2010, Ribereau-Gayon et al 2006). At any stage, pump over should thoroughly wet the cap, including sides and center of the tank.

Pumping over may be done in an aerative fashion to supply oxygen to fermenting yeast and help degas CO₂ that can inhibit yeast action (Ribereau-Gayon et al 2006). This may be simply by “fireshosing” the liquid or by sending liquid through a sump cart prior to pumping over the cap. It is most important to add oxygen at the beginning of fermentation when yeast can make sterols that act as survival factors to stabilize their cell membranes once the alcohol levels rise. Ribereau-Gayon et al (2006) recommend aeration on the second and third day of fermentation for the best effect. Once fermentation slows, it is good to limit oxygenation during cap management to avoid oxidation of the wine. Punching down includes the displacement of solids into the fermenting liquid by a punching tool. This is usually done until the cap is physically broken up and the floating solids are well wet by fermentation liquid. Oxygen is introduced as liquid from the surface is forced down into the tank.

Due to physical breakup of the cap, punching down is thought to increase skin extraction, promote extraction of seed tannin, and increase the tannic structure of the wine. It is often recommended for lighter varieties like Pinot Noir but not for more tannic varieties like Cabernet Sauvignon or Merlot (Ribereau-Gayon et al 2006, Razungus 2010). Pumping over is not believed to affect tissue integrity, and is thought to be more gentle, limiting vegetal or bitter extraction (Ribereau-Gayon et al 2006). In a review of the effect of winemaking techniques on phenolic extraction in red wines, Sacchi et al (2005) found the comparison of effects of punch downs vs. pump overs varied by variety. Early work shows that more color and tannin are present in wines managed with pump overs. More recent studies show that quercetin, a skin tannin, increased in pump over vs. mechanical or manual punch down while differential extraction of other phenolic components is variety dependent. In Pinot Noir especially, mechanical punch down or pump over greatly increased extraction (100-200%) over manual punch down with mechanical punch down extracting more, but this effect was not seen as strongly in Dornfelder. In another study, anthocyanins, catechin (a seed phenolic) and total phenolics were greater in wines managed with pump overs vs. punch downs for Negramano and Primitivo but for Sangiovese there was little difference, and in Pinotage, there were lower levels of phenolics with pump overs vs. punch downs. Sacchi et al (2005) point out that the effect of pump over is dependent on timing during fermentation and is influenced by both temperature and whether skins circulate through the pump.

The purpose of this study is to compare the effects of gentle punch downs with short pump overs on Cabernet Franc. The winemaking goal for this wine was to produce an early bottling fruit forward style. The goal of the study was to understand the influence of cap management strategies on the style of the final wine including basic chemistry, phenolic composition and sensory differences.

References

• Lerno, L., M. Reichwage, S. Panprivech, R. Ponangi, L. Hearne, A. Oberholster, and D. E. Block. (2017) Chemical Gradients in Pilot-Scale Cabernet Sauvignon Fermentations and Their Effect on Phenolic Extraction. American Journal of Enology and Viticulture 68, no. 4 pp 401–11. https://doi.org/10.5344/ajev.2017.16104.
• Razungles, A. (2010) Chapter 20: Extraction technologies and wine quality. In A.G. Reynolds (Ed.) Managing Wine Quality Volume 2: Oenology and Wine Quality (pp 589-625). Cambridge, UK: Woodhead Publishing.
• Ribereau-Gayon,P., D. Dubourdieu, B. Done`che and A. Lonvaud (2006) Handbook of Enology Volume 1 The Microbiology of Wine and Vinifications 2nd Edition, Chapyer 12. West Sussex, England: John Wiley & Sons, Ltd
• Sacchi, K.L., L.F. Bisson, and D.O. Adams. (2005) A Review of the Effect of Winemaking Techniques on Phenolic Extraction in Red Wines. American Journal of Enology and Viticulture 56, no. 3 pp 401–11.
• Schmid, F, J. Schadt, V. Jiranek, and D.E. Block. (2009) Formation of temperature gradients in large and small-scale red wine fermentations during cap management. Australian Journal of Grape and Wine Research, 15 pp 249-255.