Material other than grapes, or MOG, is assiduously avoided by winemakers. Leaves, bugs, pebbles, sticks, and other debris are removed before grape clusters are processed. But before the prevalence of destemming machines, stems, which strictly speaking are MOG, also contributed to the finished product. Even with the availability of the crusher-destemmer, whole-cluster (WC) fermentation has continued in many regions in the world. This article looks at its increasing use for Pinot Noir in the Willamette Valley, in Oregon.
The grape stem comprises three parts. The rachis is the main branch, which connects to the vine shoot. The peduncle branches off the rachis; and the pedicel, to which each grape is attached, splits off the peduncle. Stems are about 55% to 80% water by weight, depending on the variety and other factors, such as stem maturity. The dry matter includes cellulose and hemicellulose, lignin, proteins, and small amounts of acids, ash, and sugars. As stems ripen, they become lignified, lessening the herbal and vegetal notes contributed by methoxypyrazine compounds. Note that lignification refers to the hardening of the stems; they can remain bright green.
When stems are included in the fermentation, either as whole clusters or dried and added separately from the grapes, the result typically is different from a wine made with destemmed grapes. Total titratable acidity is lower, while pH is higher because of the high amount of potassium. The high water content of stems results in a slight dilution of alcohol. Tannins increase proportional to the percentage of whole clusters and stems included. Anthocyanins are lower, lightening the color of the wine. Ironically, despite an increase in pH, trained sensory panels report that wines fermented with whole clusters taste fresher, even when they are chemically similar to ones made without stems.
Michael Davies, the executive winemaker at REX HILL, reported in 2016 on a trial he conducted of four treatments of Pinot Noir: completely destemmed, destemmed with stems put in the bottom of the tank, 100% whole cluster, and 100% whole cluster plus the stems from the first treatment (200% WC). Higher tannins resulted when stems were added back, but not in the case of whole-cluster fermentation. More stems led to higher pyrazines and pH. The higher the percentage of stems, the lower the total anthocyanin, which is diluted by the water content of stems. Alcohol was the same in all four treatments.
In 2019, teams led by L. Federico Casassa, a professor at California Polytechnic State University, in San Luis Obispo, published two studies assessing the impact of including stems in the fermentation of Pinot Noir. One looked at the effect of cold-soak and inclusion of 20% whole cluster and 3% stems in combination and separately for two vintages, 2014 and 2015, of Pinot Noir grown in Luján de Cuyo DOC, located in Mendoza, Argentina. The team concluded, “WC and stem addition applied to these Pinot Noir wines produced wines with specific sensory features. However, these sensory differences were detected by a panel of highly trained wine tasters and may not be apparent to the public at large.” While experts and some consumers might recognize these traits, the study notes, for most drinkers, “Descriptions of these unique enological processes featured on the back label may be sufficient to set their expectations, thereby increasing their positive hedonic experience.”
For the second study, published in 2021, only the influence of adding dried stems and 50% and 100% whole cluster was examined. The researchers used wines from two vintages, 2016 and 2017, of Pinot Noir from Edna Valley AVA, in California. Volatile acidity and increased pH were identified in the 100% whole-cluster inclusion. “Overall, tannin increases were generally proportional to the percentage of whole cluster and [dried] stems added. Concurrently, increases in perceived astringency were also noted by the trained panels during sensory evaluation, suggesting that WC and [dried stem] addition can be used to add mouthfeel and improve texture to an otherwise light-bodied wine such as Pinot Noir.” Though there is no chemical evidence, the wines were perceived as fresher. Perhaps this is because of the herbal and spice notes that the stems contribute.
In addition to a summary of Casassa’s findings, two studies involving whole-cluster Pinot Noir fermentation were reported at the 2020 Oregon Wine Symposium. Grant Coulter, the winemaker and vineyard director at Flâneur Wines, in the Willamette Valley, compared two versions of a 2019 Pinot Noir, one completely destemmed and the other 100% whole-cluster fermented. The latter, though higher in pH and lower in titratable acidity, tasted fresher, was lower in alcohol, and was higher in total phenolics.
Tom Gerrie, the winegrower and owner of Cristom Vineyards, in the Willamette Valley, compared two versions of a 2019 Pinot Noir from Eola Springs Vineyard, one fermented with 38% whole cluster and the other with 67% whole cluster. The latter had higher total tannins and total phenolics, slightly higher pH, and slightly lower titratable acidity, alcohol, and acetic acid. The tannins were chalky, with fine-grained texture—unlike the 38%. It was noted that the two ferments were chemically similar but sensorily very different.
A master of science student at Lincoln University in New Zealand, Pradeep Wimalasiri, prepared five treatments of Pinot Noir harvested in 2019 from the university’s vineyards: completely destemmed, destemmed with stems added back, 30% whole cluster, 60% whole cluster, and 100% whole cluster. He found that anthocyanin concentrations dropped in the cases of stem inclusion and whole-cluster fermentation. There was no significant difference in most of the oenological parameters and color between the 30% and destemmed cases. Tannins and total phenolics increased with stem and whole-cluster inclusion. Interestingly, the wine made using destemmed grapes with stems added back had significantly lower tannins than the 100% whole-cluster case. Aromatics differed substantially between these two treatments as well.
My interest in whole-cluster fermentation of Pinot Noir was sparked when I began working in the tasting room of White Rose Estate, in Dundee Hills AVA, in the Willamette Valley, in 2012. Almost all the White Rose Estate Pinot Noirs are fermented with 100% whole clusters. I learned that some of the earliest experimentation in the valley was done by Steve Doerner at Cristom, starting in 1992. Previously, Doerner had worked at Calera Wine Company, in California, where, by the time he left, all Pinots were 100% whole cluster. The owner of Calera, Josh Jensen, had been influenced by Jacques Seysses, the founder of Domaine Dujac, where he had spent some time. Seysses represented the Burgundian school that advocated whole-cluster fermentation, in contrast to the late winemaker Henri Jayer, who hated the use of stems. Seysses’s son Jeremy continues the practice, as do an increasing number of Burgundians, particularly following the death of Jayer, in 2006, and with climate change, which is leading to riper stems.
In part the result of Doerner’s influence, a surprisingly large number of wineries in the Willamette Valley now routinely include at least some percentage of whole cluster in all or part of their Pinot Noir fermentations. None of the winemakers I contacted, however, add dried stems back to Pinot ferments. One was concerned about microbial contamination. Another, who tried it, did not like the elevated tannins, herbaceousness, and lack of carbonic perfume that resulted.
Below, Willamette Valley winemakers reflect on their use, or avoidance, of whole clusters in their Pinot Noir fermentations. Note that this is an unscientific sampling, not an exhaustive survey.
Nicolas-Jay: The Burgundian winemaker Jean-Nicolas Méo, an acolyte of Jayer, is a cofounder of Nicolas-Jay, with Jay Boberg. Not surprisingly, they do not do any whole-cluster fermentation. The assistant winemaker Tracy Kendall explains, “Although I believe there are beautiful whole-cluster wines out there, we prefer to destem our fruit. We want to really allow the terroir of the sites to show through, rather than making a ‘whole-cluster wine,’ [which] can be beautiful but can also muddy the purity of the site and the complexity of the wine.” She continues, “Whole cluster is also challenging to do well, as it robs the wine of acidity and can soften the wine, stripping tension and precision.”
De Ponte Cellars: Isabelle Dutarte, the French winemaker at De Ponte Cellars, has a more practical reason: “We don’t do whole cluster often due to a problem of space in the winery!”
The Eyrie Vineyards: Jason Lett, the winemaker at the Eyrie Vineyards, established in 1965 as one of the first producers in the Willamette Valley, originally experimented with whole-cluster fermentation in 2009. He explains, “I was interested in the idea of whole cluster as an extension of noninterventionist winemaking.” Describing the amount of whole cluster he employs, he says, “It really just depends on how much bandwidth I have in a given vintage. I always go 100% when I whole-cluster ferment but only do the process in 1 or 2 fermenters out of 80 or more.” This results in wines that are, once blended, less than 3% whole cluster.
Lett admits, “I don’t like the flavor of stems in wine when it becomes obvious. I value subtlety in any winemaking technique, because bombast can hide the expression of the vineyard—which is, after all, the whole point of making Pinot Noir.” He finds that stems, as compared with fruit, don’t vary significantly in flavor from one location to another; this, he says, can mask terroir. But, he continues, “I love it when it exists as a subconscious suggestion, supporting the cuvée rather than dominating it. In homeopathic doses, whole cluster can add an exciting spiciness and midpalate tannin structure without overwhelming the expression of the site.”
Belle Pente Vineyard & Winery: The proprietor and winemaker of Belle Pente, Brian O’Donnell, says, “We are not big into whole cluster but dabble a bit.” Influenced by Burgundian producers and Cristom, he experimented in 1993, then avoided the practice until 2014. He now typically uses about 15%.
Ken Wright Cellars: Ken Wright, the eponymous proprietor and winemaker of Ken Wright Cellars, is a close friend of Doerner and uses whole cluster situationally. He most appreciates its impact in warm years, when the wines are more muscular, and whole cluster, he says, “brings freshness to what would be a plodding wine.” In particularly warm years, he goes up to 20% to 25% whole cluster. In contrast, in a colder year, he uses no stems. In 2019, which is considered a vintage that harks back to those before the more drastic effects of climate change set in, he used between 5% and 15% whole cluster, depending on the site. He also “aims to make wines that don’t require laying down for years like whole-cluster wines do.”
Celestial Hill Vineyard: A newcomer to the valley, Chris Thomas, the owner of Celestial Hill Vineyard, first experimented with whole-cluster fermentation in 2021. He currently uses some amount of whole cluster for about half of his Pinot Noir wines. For his cooler sites, he uses up to 25% whole cluster, and, he explains, “The warmer the site, the more I add. I always have some that is completely destemmed so that I can taste [the] difference and then blend appropriately.”
Abbott Claim: Alban Debeaulieu, the French winemaker at Abbott Claim, says he “always was attracted to Pinot Noir vinified using high amounts of whole clusters since my years in Burgundy. And [it] has always made more sense to me.” While the amount varies, he uses some whole cluster for most of his Pinot Noir wines. In 2019, he used 10% and 25%, respectively, in two different Pinot Noir wines.
Domaine Divio: Bruno Corneaux, the winemaker and co-owner of Domaine Divio, believes that, in the Willamette Valley, where there is a much shorter winemaking history than in Burgundy, winemakers must pay particular attention to the quality and ripeness of the stems and to the sanitary conditions in the vineyard when they consider making wines with a high proportion of whole clusters. He typically uses about 20% whole cluster but might go up to 100%. Corneaux explains, “Whole-cluster fermentation, with good-quality and lignified rachis, always, to my taste, bring[s] an additional dimension to your Pinot Noir, and an elegance you will not find in destemmed grapes.” In 2019, he made two pairs of wines from the same site, picked the same day, one destemmed and the other with 50% whole cluster.
Archery Summit: On average, Ian Burch, the winemaker at Archery Summit, uses about 30% whole cluster, but he makes one 100% whole-cluster cuvée. “Knowing that we are aiming for about 30% whole cluster, it usually depends on how much we need to fill a fermenter. Having a fermenter with the ideal quantity of grapes and whole cluster trumps making a wine with whole cluster. Sometimes we run out of space.” He continues, “Since we add whole cluster to the top of the fermentation and primarily pump over the wine until about half the sugar is gone, we don’t pull in lots of greenness. When half of the sugar is gone, then we pump over and punch down at the same time. At this stage, the clusters are no longer green and have been protected by CO2. By punching and pumping at the same time, we allow for a slow process of the skin closing around the berry and are still able to treat the stems gently. With browner stems, we have more of the brown spice spectrum than the green: allspice, cardamom, cinnamon, nutmeg.”
Domaine Drouhin Oregon: Véronique Boss-Drouhin, the head winemaker at Domaine Drouhin Oregon, began experimenting with whole-cluster fermentation with the 1988 vintage. She reports, “In Burgundy, we have always used whole clusters in some, not all, of our wines and like it very much. It was evidence for me to try it in Oregon.” She typically uses about 15% whole cluster for the Édition Limitée Pinot Noir. “When doing this, I am more thinking of the future of the wine than the immediate taste. Stems allow the wines to age with incredible grace,” she explains.
Dominio IV Wines: Patrick Reuter, the winemaker and owner of Dominio IV, first experimented with whole-cluster fermentation after visiting Burgundy in 2004. Today, less than 15% of his Pinot production is fermented whole cluster. “We either do 100% whole cluster or I will layer it in a two-thirds destemmed to one-third whole-cluster fermentation sandwich. I wanted to add a high-C note to the wines, a little flutter of the piccolo, and, in some years, a structure from the stem.”
Winter’s Hill Estate: Russell Gladhart, the winemaker at Winter’s Hill Estate, started experimenting in 2016. He says, “The decision to include stems is determined by maturity and health of the fruit. If we decide that a whole-cluster fermentation is a good idea for a particular lot of Pinot Noir, we usually include 30% to 50% whole clusters in the fermenter.”
Raptor Ridge Winery: Scott Shull, the founder of Raptor Ridge Winery, has been making a whole-cluster Pinot Noir since 2014. The percentage depends on the vintage. The winemaker Shannon Gustafson says, “We love the interesting spice component the stems lend to the wine.”
Flâneur Wines: Grant Coulter began experimenting with whole cluster in 2008, inspired by wines from producers such as Cristom, Calera, Domaine Dujac, Brewer-Clifton, Domaine de Montille, and Big Table Farm. Coulter uses anywhere from 10% to 100% whole cluster for his Pinot Noir. He says, “When the grapes arrive at the winery, I tear all the berries off the clusters and start munching on the stems. At this point, I have been tasting berries for weeks in the vineyard, so I know where I stand with those flavors. I have a mental register of [stem] flavors that [include] grass, cilantro, green chili, banana skins. I have learned over the years what combinations seem to work for my style.” He finds that using 30% to 70% whole cluster often yields the richest, most tannic wines.
Ribbon Ridge Winery: The winemaker and owner of Ribbon Ridge Winery, Wynne Peterson-Nedry, and her father, Harry, have experimented with whole cluster for as long as she can remember. She says, “Quantity of whole cluster in a year depends on the vintage. Riper vintages are best, when stems are lignified, so [those are] typically the vintages we will play with whole cluster. Some years, we might have some whole cluster in 50% to 75% of our fermenters.”
Cristom Vineyards: Steve Doerner says, “When I arrived at Cristom in 1992, I repeated many whole-cluster experiments that I had done at Calera, but in the end decided on roughly 50% most of the time.” He continues, “Usually, I only cut it back when I feel the stems’ surface area would be detrimental, like 2013, when there was a lot of disease pressure, or 2020, when we had to deal with smoke. Sometimes, the whole-cluster percentage has been reduced due to practical reasons, like not having enough fermentation room near the end of the harvest in an abundant year.” The first commercial release of the Whole Cluster Series was the 2021 vintage. Three versions—0%, 50%, and 100% whole cluster—were made.
REX HILL: Michael Davies typically uses whole cluster in the same vineyards where he has previously been successful with the method. He explains, “I would say, most years, approximately 75% of our REX HILL ferments include at least some percentage of whole cluster. This can vary from as little as 10% to 100%.”
David Hill Vineyards & Winery: Chad Stock, David Hill’s winemaker since 2019, first experimented with whole-cluster fermentation in 2007. “My motivation for doing this was to produce a more complex smelling and tasting wine, with more tertiary compounds, to intentionally make the wine taste less fruity. With Pinot Noir, I try to do as much as I can, because I would much rather have nonfruit complexities from stems than from new oak, which I don’t care for.” He thinks that the wines are most successful when the amount of whole cluster in the fermenter is either under 33% or over 66%. He explains, “There is a dangerous middle zone that produces an overly extracted wine.”
David Hill releases a whole-cluster wine under the Discovery Series label that is typically about 70% to 80% whole cluster. Stock says, “It is only made in colder vintages, when we can get a long enough hang time on the fruit to get lignified stems without excessive ripeness.”
Brooks Wine: Claire Jarreau, the associate winemaker at Brooks Wine, began experimenting with whole-cluster fermentation in the cool 2019 vintage, when pH levels were low enough that she didn’t need to add acid to account for the change in pH that resulted from including stems. She says, “We typically look at the chemistry of the incoming fruit, as well as stem ripeness, to determine whether or not the fruit could be a good fit for whole cluster incorporation, and, if so, how much. We also ferment tanks of 100% destemmed fruit from the same blocks to compare.”
Brick House Vineyards: Doug Tunnell, the owner and winemaker of Brick House, says, “We started in 1993 when we were custom crushing at Cristom. We really liked the results Steve Doerner achieved with his fermentations.” Today, Tunnell uses nearly 100% whole cluster, with different proportions in each tank. He describes his goal as “a completely different aromatic profile and mouthfeel, one which improves with time.”
Goodfellow Family Cellars: The winemaker Marcus Goodfellow first employed whole cluster in his fermentations in 2003, influenced by the wines of Cristom, Domaine Dujac, Domaine de l’Arlot, and Domaine de la Romanée-Conti. Today, all his fermentations include some whole clusters, most at 100%. He says, “Whole-cluster wines, in my opinion, take longer to develop but have an extra layer of flavors and nuance and tend to age for longer periods of time—15 to 20 years pretty easily.” He has worked without a destemmer since 2019.
Bergström Wines: The winemaker Josh Bergström began using whole-cluster fermentation in 1999. Currently, 90% to 100% of his Pinots are made that way, and usually with 100% whole-cluster inclusion. His motivation is “to introduce savory, saline, and umami flavors; carbonic perfume; and a different tannin structure to our wines.”
White Rose Estate: Greg Sanders, the owner of White Rose Estate and its first winemaker, started experimenting with whole-cluster fermentation with the 2004 vintage. “[A]t the time, my impression of wine quality and wine personality were still somewhat conflated,” he says. “I imagined that [whole cluster] would potentially make a better wine by giving me ‘more.’ I did not have a clear sense of what ‘more’ meant at the time but thought that would be a good thing.”
Sanders hired Gary Andrus, the late founder of Archery Summit, as a consultant, and Andrus advised Jesús Guillén, who would become winemaker in 2008, on the process. Sanders adopted 100% whole-cluster fermentation for almost all Pinot Noir in 2011, with the support of Guillén, who died in 2018, and then general manager, Gavin Joll.
Maysara Winery: Tahmiene Momtazi, the winemaker at Maysara, started making whole-cluster Pinots for her wine club. “Due to our holistic farming practices, the stems turn brownish around harvest time, and making the wine with whole cluster [makes it] more ageworthy, [with] better flavor and more structure,” she explains. “We look at each block from our vineyard and see the blocks that have more mature stems and delicious, tasty fruit. We have noticed that the tannins of the [wines made with] whole clusters are more refined, and the aromatics have a more floral bouquet.” Momtazi makes one 50% and one or two 100% whole-cluster Pinot Noir wines, depending on the vintage.
Big Table Farm: Brian Marcy, the winemaker and cofounder of Big Table Farm, with his wife, Clare Carver, began doing whole-cluster fermentation while working at Sterling Vineyards in Napa. Marcy and Carver continued the practice when they relocated to Oregon, in 2006. Beginning with the 2014 vintage, all their Pinot has been fermented with 100% whole cluster. It is a “very nuanced part of the process [and] not a defining aspect of our wine,” Carver stresses. They, too, work without a destemmer.
I was pleasantly surprised that all but two of the wineries I contacted use at least some whole cluster in at least one of their Pinot Noirs, with many regularly including a moderate to high percentage. Though whole-cluster fermentation is neither necessary nor sufficient to produce excellent Pinot Noir, it is increasingly recognized as a means of adding an attractive seasoning, more nuanced complexity, and enhanced aging potential to the final product. Climate change has led to warmer, even hot, vintages in the Willamette Valley, which allow stems to ripen more consistently, eliminating the off-putting green vegetal impact and encouraging more experimentation.
While all other MOG would make unwanted contributions to the finished wine, stems, under the right circumstances, do not. In fact, a growing number of Pinot Noir producers in the Willamette Valley are demonstrating that stems can enhance a wine’s appeal and add freshness and aromatic complexity in a way that destemming can’t. After all, shouldn’t Pinot Noir be a symphony rather than a concerto for fruit? Whole-cluster fermentation can advance that goal by contributing spicy, floral, earthy, and herbal notes. So perhaps a more appropriate acronym for the unsuitable additives would be MOGS: material other than grapes and stems.
Abbott Claim Abbott Claim Vineyard 2019, 25% WCArchery Summit Arcus Vineyard 2018, 33% WCBelle Pente Vineyard & Winery Estate Reserve 2018, 12% WCBergström Wines Silice 2020, 100% WCBig Table Farm Willamette Valley 2019, 100% WCBrick House Vineyards Select 2021, 20% WCBrooks Wine Muska 2019, about 35% WCCristom Vineyards Mt. Jefferson Cuvée 2021, 46% WCDomaine Divio Côte à Côte 2019, 50% WCDominio IV Wines Tapis Reserve 2018, about 20% WCGoodfellow Family Cellars Heritage No. 12 2018, 100% WCMaysara Winery Mahtaub 2018, 100% WCREX HILL Jacob-Hart Estate Vineyard 2018, 39% WCWhite Rose Estate White Rose Vineyard 2018, 100% WCWinderlea Vineyard and Winery Murto Vineyard 2018, 100% WCWinter’s Hill Estate Single Block Series Block 10 2018, about 60% WC
Interview: Perspectives on Whole-Cluster Fermentation, by Kelli WhiteResearch Paper: Whole-Cluster Fermentation in the Côte d'Or, by Robin Kick [Members only]Viticulture Expert Guide [Members only]
Blackford, Marie, Montaine Comby, Liming Zeng; Ágnes Dienes-Nagy, Gilles Bourdin, Fabrice Lorenzini, and Benoit Bach. “A Review on Stems Composition and Their Impact on Wine Quality.” Molecules 26, no. 5 (2021): 1240. https://www.mdpi.com/1420-3049/26/5/1240.
Casassa, L. Federico, Niclas P. Dermutz, Paul F. W. Mawdsley, Margaret Thompson, Aníbal A. Catania, Thomas S. Collins, P. Layton Ashmore, Fintan du Fresne, Gregory Gasic, Jean C. Dodson Peterson, “Whole Cluster and Dried Stem Additions’ Effects on Chemical and Sensory Properties of Pinot Noir Wines over Two Vintages.” American Journal of Enology and Viticulture 72 (2021): 21-35. https://www.ajevonline.org/content/72/1/21.
Casassa, Federico L., Santiago E. Sari, Esteban A. Bolcato, Mariela A. Diaz-Sambueza, Aníbal A. Catania, Martin L. Fanzone, Fernando Raco, Nora Barda. “Chemical and Sensory Effects of Cold Soak, Whole Cluster Fermentation, and Stem Additions in Pinot Noir Wines.” American Journal of Enology and Viticulture 70 (January 2019): 19-33. https://www.ajevonline.org/content/70/1/19.
Goode, Jamie. “Stemming the Tide.” The World of Fine Wine 37 (2016): 90–97. https://worldoffinewine.com/uncategorized/stemming-the-tide-4869650.
Lasky, Michael S. “Winemaker Trials: Discovering the Potentials of Stem Inclusions at Fermentation.” Wine Business Monthly 23, no. 3 (March 2016): 52–55. https://www.winebusiness.com/wbm/?go=getArticle&dataId=165495.
2020 Oregon Wine Symposium. Whole Cluster Fermentation: Intent, Execution, Expression and Analytics. https://industry.oregonwine.org/resources/workshops/2020-oregon-wine-symposium-whole-cluster-fermentation-intent-execution-expression-and-analytics/.
White, Kelli. “Interview: Perspectives on Whole-Cluster Fermentation.” GuildSomm, January 18, 2018. https://www.guildsomm.com/public_content/features/articles/b/kelli-white/posts/whole-cluster-interview.
Wimalasiri, Pradeep. "The effect of grape stem inclusion fermentation on Pinot Noir wine composition.” Master’s Thesis, Lincoln University, Christ Church, New Zealand, 2020. https://www.academia.edu/79625855/The_effect_of_grape_stem_inclusion_fermentation_on_Pinot_noir_wine_composition_A_thesis_submitted_in_partial_fulfilment_of_the_requirements_for_the_Degree_of_Master_of_Science_at_Lincoln_University.
Carbonic Maceration involves sealing the fermenter so that no oxygen gets in. All fermentation happens within the grape and is enzymatic. Willamette Valley Vineyards does a whole cluster Pinot noir that way and it has a distinctive palate much like a beaujolais. None of the wineries in the article seal the fermenters so the CO2 escapes. That's the difference.
When we're talking about whole cluster, how is this different (if at all) From carbonic fermentation a la beaujolais? Are the clusters generally, or sometimes, crushed with stems to circumvent carbonic maceration?
At White Rose, the fermented clusters are gently pressed in a basket press. The juice is sampled and pressing is stopped when unwanted flavors are detected. I have no information specifically on what the rachi contribute. I’m not sure that anyone has done a before and after pressing comparison on the free run and pressed juices.
A lot of talk regarding the impact of stems upon the fermentation but I wonder what, if any, of this impact stems from the pressing of the wine. All those rachi provide channels for the juice so I would imagine the wine can be pressed at an overall lower pressure. Surely that must have an impact as well?