Despite thousands of years of winemaking history, we’re still refining our understanding of the myriad factors leading any given wine to taste and feel as it does. Arguably, no other consumer product has such variety or involves so many minute, often inscrutable, factors. A wine’s personality is influenced by geology, microbiology, chemistry, plant and human physiology, agronomy, entomology, geography, weather, and more.
Winemakers do their best to guide wines per their intent, based on their studies, their experiences, and expert advice. Yet there are gaps in our knowledge and many areas where effect cannot be neatly and fully tied to cause. With so many complex variables, fully controlled experimentation is not possible. Because of these complexities, rule of thumb often comes into play, and simplification is a must when communicating with consumers or even sommeliers. Unfortunately, overly broad conclusions can result. This article looks at five common assumptions about viticulture and winemaking and considers their accuracy.
Diurnal shift is the variation between the highest and lowest temperatures on any given day. Wineries in warm regions with large diurnal shifts tend to emphasize that factor in their marketing, as the shift can result in fresher wines than might otherwise be expected. The frequent mentions have led many people to assume a big diurnal shift is generally important. That’s not entirely true.
Grape vines are active (attempting to grow and to ripen grapes) at temperatures between the low 50s and low 90s degrees Fahrenheit, though of course the exact range for any given vine varies depending on multiple factors. The primary fuel for this growth and ripening is sugar created by photosynthesis—which doesn’t occur at night as it requires sunlight.
If nighttime temperatures are warm enough for growth and ripening to continue, the vines need an alternate fuel. That fuel is malic acid. So, in general, less ripening at night means more malic acid will remain in the grapes at harvest, thus the attraction of diurnal shift.
The other benefit of cold nights in hot areas is that warming in the vineyard occurs later in the day, so vines spend less time in the heat. This can retard sugar ripeness, allowing phenolic ripeness to better keep pace.
Big diurnal shift is, by definition, a matter of extremes. If daytime highs aren’t very high, there won’t be a big diurnal shift unless the lows are very low indeed. There’s no benefit for wine from very cold nights in cool and moderate regions, and if both day and night temperatures are moderate, malic acid consumption will be moderate as well.
In cold regions, growers are concerned with getting sufficient ripeness and harvesting before cold weather hits. Maintaining acidity is rarely an issue, and an excess of acidity isn’t uncommon. In this case, a minimal diurnal shift is best, because that means the area will warm up to suitable ripening temperature earlier in the day.
The rate of malic acid consumption at night depends on temperature, but that consumption is substantially reduced well above the temperature at which the vine ceases all activity. In other words, if the mercury drops to the low 60s, grapes aren’t losing too much more acid during that period than they would in the low 50s.
Since gentle ripening can take place at night in the low 60s, such temperatures can allow growers to harvest earlier than if the vines shut down completely most nights. Because malic acid consumption can be quite high during the day in warm regions, moderate nights and early harvest can mean more freshness in the wine than cold nights and a later harvest.
The temperature that really matters to a grape vine for ripening, and thus malic acid use, is the temperature of the vine and berries. Air temperature affects that, but so do wind, humidity, fog, water retention in the soil, vine height, and the degree to which the topsoil absorbs heat during the day and releases it at night.
A significant diurnal shift can be important to retaining freshness in warm-to-hot growing regions. It is less important, and may be counterproductive, in moderate-to-cool regions. Many other factors affect the nighttime behavior of vines, too. Don’t assume that greater diurnal shift produces a fresher—let alone better—wine. Look more carefully at the characteristics of the specific site.
Yeast selection is a key winemaking decision. Producers who inoculate with cultured yeast have a multitude of options from which to choose. Some wineries always use “wild yeast,” but this, too, is a decision.
A range of yeasts can bring about vinous fermentation. It’s rare for a single strain, or even species, to take the process from start to finish. Often, fermentation starts with a variety of yeasts, and then the strongest takes over to finish. In other cases, the starting yeasts die, and a completely different yeast completes the fermentation.
Wild yeast, also referred to as native yeast or, perhaps more correctly, ambient yeast, does not necessarily come from the vineyard. Yeast is everywhere. It occurs naturally on grape skins and is plentiful in the vineyard. There is also yeast in the air between the vineyard and winery and at the winery itself. While using ambient yeast is certainly in line with “low-intervention” winemaking, it doesn’t necessarily express the character of a vineyard.
The primary yeast associated with vinification is Saccharomyces cerevisiae. Other species occurring in vineyards and production facilities include Candida stellata, Kloeckera appiculata, Lachancea thermotolerans, Metschnikowia pulcherrima, and Zygosaccharomyces bailii, with the first two the most common. The yeasts referred to as wild are those other than Saccharomyces cerevisiae, as they are less ubiquitous and, in their variety, can create complexity and a unique personality in wine. Wild yeasts, however, are very sensitive to SO2 and rarely survive once alcohol levels above 5% are achieved. Thus, a “wild yeast fermentation” may begin with a unique mélange but will finish with ambient Saccharomyces cerevisiae.
Using ambient yeast doesn’t always yield satisfactory results. If there is not enough Saccharomyces cerevisiae available, the fermentation may be slow or stall entirely. That’s especially true when the juice has a very high potential alcohol. At best, this can lead to wines with residual sugar. At worst, slow or stuck fermentations can result in off-flavors and aromas and/or bacterial contamination.
There is also a risk, at least the first few times a vineyard or production facility is used, that the resident yeasts aren’t good for winemaking. If this proves to be an issue, or if pre-harvest analysis of the grapes shows contaminants or inappropriate yeast, the winemaker may neutralize everything with SO2 and then inoculate.
Cultured yeasts reduce risk. The behaviors of available yeasts are known—relative to potential alcohol, fermentation temperature, nutrient mix, pH, speed of fermentation, sensitivity to SO2, and more. So, too, are the flavors and aromas each tends to produce. Aside from the question of expressing place, the principle concern with cultured yeast is that it can lead to wines that “all taste the same.” That’s an overstatement, but it is true that inoculated yeast fermentation can limit complexity.
To capture complexity and terroir, while limiting risks of contamination, off-flavors, or unwanted residual sugar, some winemakers allow fermentation to begin with ambient yeast. Then, when alcohol levels reach a certain point or the fermentation begins to slow, the winemaker inoculates with cultured yeast. Some producers collect wild yeasts from their vineyards and have those cultured for inoculation.
Wild yeasts can yield complex wines of beauty or unattractive wines with residual sugar and off-aromas. Inoculated fermentations can make magnificent wines, clean but simple wines, or wines that taste like they are from somewhere else. It doesn’t make sense to pre-judge a wine based on fermentation choices—this is just a data point. The true measure of a wine is how it smells, tastes, and feels out of the glass.
Malolactic fermentation (MLF) is a secondary fermentation that converts tart malic acid, and then citric acid, into softer, rounder lactic acid. MLF takes place, whether naturally or induced, in most red wines and in some whites.
MLF also generates diacetyl, a natural by-product of many fermentations, including those used to make sour cream, buttermilk, cultured butter, and beer. Diacetyl has a pronounced buttery aroma and flavor that it contributes to “buttery” Chardonnay—and butter itself. It’s also used by food companies to flavor margarine, microwave popcorn, and more. Diacetyl usually exists in wine even before MLF takes place, as some is created during primary fermentation. The actual amount of diacetyl in a finished wine can be anywhere from zero to seven milligrams per liter. That said, typical levels are less than two milligrams per liter for whites and three milligrams per liter for reds.
The recognition threshold for diacetyl is much lower for white wines than for reds. A mere 0.2 milligrams per liter can be noticeable in whites, while five to ten times that much is necessary in a red. (The more intensely flavored a red wine, the higher the threshold.)
The specific amount of diacetyl produced during MLF depends on various factors. Environment has a significant impact. Because the malolactic bacteria that do the conversion struggle at low temperatures, MLF and diacetyl generation may be limited or completely inhibited by temperature. Another key factor is pH. Ironically, the bacteria responsible for processing malic acid have a difficult time with very low pH (high acid) environments, but thrive when pH is above 3.6. This is one reason why relatively low-acid California Chardonnay may present loads of butter, while Burgundian examples—even after full malolactic fermentation—typically don’t.
The amount of malic and citric acid available for conversion make an impact, too. Greater citric acid concentration, especially, tends to result in more diacetyl production. Different yeasts produce different amounts of diacetyl, and Lactobacillus and Pediococcus damnosus bacteria, if present, will also create diacetyl.
If oxidation occurs, diacetyl production will increase. On the other hand, factors such as sulfur dioxide, lack of nutrients, fumaric acid, and fatty acids inhibit malolactic fermentation.
Winemakers have many tools for avoiding excessive buttery flavors when employing malolactic fermentation. Most simply, they can stop the process at any time by reducing temperature or adding SO2. MLF can also be limited (without being stopped) through cool temperature or the addition of fumaric acid. Another straightforward solution is to blend a wine that underwent MLF with one that did not.
Qualities in the initial wine matter, too. A wine that has low pH (whether due to grape variety, terroir, viticultural choices, or a combination of these) and does not contain any undesirable bacteria will not taste very buttery. The bacteria used for malolactic fermentation makes a difference as well; some lead to subtler buttery flavors.
Adding, or not adding, certain products can minimize these flavors, too. Winemakers can adjust nutrients or avoid acidulating with citric acid. After malolactic fermentation is complete, the addition of sulfur will make any buttery flavor less noticeable, as SO2 bonds with diacetyl. But the timing of sulfur additions matters—flavors will be subtler if the winemaker adds sulfur later, once the diacetyl has been entirely degraded by bacteria and yeast, which would be neutralized by an earlier addition.
There are many variables that affect the amount of diacetyl and buttery flavor in wine. Today, winemakers both understand and have significant control over many of those factors. Overly buttery wine is far from a given with malolactic fermentation.
Apart from Beaujolais and a few other exceptions, whole-cluster fermentation is polarizing. There are philosophical arguments, technical arguments, and aesthetic preferences on the topic.
Some people oppose whole cluster because they believe stem-derived aromas and flavors mask, rather than reveal, terroir. Others feel prominent flavors derived from anything but grapes should be avoided. Counterarguments are that the character of stems does speak to both vintage and vineyard and that these notes add beauty and complexity to wine.
The qualitative judgements are all about those aromas and flavors. Some people enjoy them, some hate them. Some like them in moderation, or only when used with certain varieties, such as Syrah. But there are many factors that affect how stemmy and green a wine will taste. And there are some wines that exhibit aromas suggesting whole-cluster fermentation when none was used.
One frequent response to those who argue against stems is that stems that are “fully lignified” or “ripe” do not create excessively stemmy wines. Unfortunately, that too is an oversimplification.
Lignification is the technical name for a stem becoming woody—brown, dry, and hard as opposed to green, sappy, and pliable. There are different parts to the stem, and they lignify at different rates. A peduncle is the stem that connects the entire bunch to the cane. Within the bunch are the rachis (the continuation of the peduncle that serves as the central stem within a bunch), lateral branches coming off the rachis within the bunch, and pedicels that connect individual grapes to the lateral branches.
Tyler Thomas, the managing director and director of winemaking for Santa Barbara’s Dierberg Vineyard and Star Lane Vineyard, has degrees in botany and plant molecular biology. He explains, “The peduncle will lignify. It usually happens pretty early in ripening. There’s a point after that when you get some lignification in the rachis, but I’ve never seen one fully brown, and your fruit is going to be very ripe if you wait for that.”
While “lignified stems” is an over-simplification, the degree to which lignification does occur can make a difference. Thomas continues, “The idea that we can’t use stems until they are fully lignified doesn’t make sense. But we may want to be below the radar on aromatics while still getting the tannins that we want.”
Many winemakers create wines that are not 100% whole cluster by blending different batches, using some made with stems and some without. A combined fermentation keeps the stems in juice much longer, and a wine made that way may be greener than one with the same proportions but made with separate fermentations.
It’s logical that grapes with great intensity can stand up to more stem inclusion. Syrah is more powerful than Pinot Noir. Young vines are more intensely fruity but less nuanced than old vines. So, young Syrah might be harmonious with a larger proportion of stems, which will add complexity and structure without becoming overwhelming. Old-vine Pinot Noir might have the intensity to stand up to stems but, in some instances, there are so many nuances from fruit alone that stems can be a distraction.
Vintage and climate can make a difference, too. In warm years or regions, the ripeness of the fruit might want stems for added structure yet have enough intensity to not be dominated by those flavors. Clone, climate change, vigor, and viticultural techniques also impact intensity and phenolic ripeness, affecting the impact stems can have. Some vineyards never seem to make stemmy wines, even with substantial whole cluster. According to Thomas, the Syrah from Walker Vine Hill Vineyard in Russian River Valley is a prime example.
It is true that some wines featuring whole clusters smell and taste less stemmy than others. However, it’s not correct to say this is due to fully lignified stems, as that rarely, if ever, occurs. The degree to which stems have lignified does have an impact, but so do site, vintage, fruit character, winemaking, and viticultural techniques.
There are many assumptions about punchdown and pumpover that ought to be reexamined. Generally, keeping the cap (grape skins and other solids) moist and somewhat immersed in the juice during fermentation enhances extraction of color, tannins, and some flavors. There are two primary ways to achieve this in large fermentation vessels: punchdown and pumpover.
Punchdown (pigeage in French) is pushing the cap down into the liquid. This can be done manually with various types of poles and paddles, or mechanically using a motorized or hydraulic pressing system.
Instead of moving the cap, pumpover (remontage in French) moves the juice. Liquid is removed from the bottom of the tank and poured over the top of the cap, either manually or with a pump. Pumpover may or may not involve aeration. That depends on the winemaker’s choice of fermentation vessels, open versus closed top, and the specific technique employed.
The effect of either technique on the cap depends on the shape of the fermentation vessel, the grape variety, ripeness at harvest, stem inclusion, how long fermentation has been going on, and the specific method used.
If the fermentation tank tapers toward the top, punchdown is less violent than if the tank has straight sides. The taper yields a cap of smaller diameter than the lower portions of the tank. That means the juice is more easily displaced as the cap is submerged. Less force is necessary, and the grape solids aren’t mangled as much.
Likewise, pumpover can be gentle or violent; it’s a choice for winemakers. The juice can be delicately sprinkled on top, slowly wetting the cap. It can also be shot out at high pressure, as if from a firehose. In that case, the goal is to break up the cap and redistribute the solids within it. But gentle pumpover is also possible, using low-pressure pumps or avoiding pumps entirely.
There are also systems that employ a closed tank and thus don’t expose the juice to much, if any, oxygen. That could be appropriate for easily oxidized grapes, such as Pinot Noir. Of course, some grape varieties, like Cabernet Sauvignon, thrive on a bit of aeration. Since yeast needs oxygen, aeration can also optimize fermentation.
Early in the fermentation process, the grape solids and cap are firmer, with more structural integrity, than they are later in the process. This can increase the force needed for punchdown but might also mean the solids are less susceptible to damage from that force.
Thin-skinned grapes, such as Pinot Noir and Grenache, are more susceptible to being torn up and having their seeds exposed by either pumpover or punchdown than tough, thick-skinned grapes such as Cabernet Sauvignon and Syrah. Ripeness is also a factor. The riper the grapes at harvest, the softer and more fragile their skins.
Stem inclusion can make grape solids less vulnerable to damage from cap submersion. On the other hand, they make both punchdown and pumpover more difficult. Early on in a fermentation, manipulating a cap with 100% whole cluster may require walking around on it to break up the berries, because the cap just won’t move. It could be two weeks before the cap can be moved.
It is worth looking at assumptions around the frequency of pumpover and punchdown as well. Conventional wisdom is that high frequency—two to three times per day—is good because it yields significantly more color and tannin. But considerable tannin extraction may not be desirable. For example, Petite Sirah is a very pigmented and tannic grape. Deep color can be achieved with less frequent cap submersion and that, in turn, can limit the quantity and harshness of the tannins.
Some winemakers believe frequent cap submersion is also important for temperature control. Fermentation generates a lot of heat. That heat rises, and the solid cap is less impacted by a tank’s cooling jacket than is the liquid. Other winemakers are less concerned about that heat, though—and again, different grapes and wine styles call for different temperatures.
Many winemakers believe higher temperatures substantially increase extraction. But studies have shown the effect of temperature on extraction is different for skins, for which extraction occurs early in fermentation, than for seeds, which give up their tannins later. Research has also indicated that must temperature is more important to extraction than cap temperature.
Similarly, the effect of frequent submersion on tannins, flavor, and color will vary depending on grape variety, ripeness, and how long fermentation has been going on. If the juice has captured most of the available extract from skins and pulp during the first part of fermentation, there will be diminishing returns for continuing frequent punchdowns or pumpovers. If tannin extraction in that final phase is mostly from seeds, then a winemaker will want to modulate that extraction depending on the style of wine and how brown and crunchy the seeds are.
The “correct” method depends on the goal and the specific situation. Today, winemakers often use both techniques, and each in a variety of ways and frequencies, during a single fermentation. There are times to be gentle, times to be rough, times to keep cool, and times for heat. General comments about technique and frequency don’t tell us enough to make qualitative judgements.
The seemingly infinite complexity of wine almost necessitates simplification, especially when talking to consumers. However, we need to be careful not to become dogmatic about how we speak of certain techniques and concepts. The key to understanding a wine, and enjoying it, comes from reveling in its specific details and nuances—and, of course, tasting it.
Hi Eric, Thank you for the kind words. I'm glad you enjoyed the article.
There are many variables that can effect the amount of diacetyl produced by MLF, even beyond those I mentioned in the article. They include the speed and temperature of MLF, lees stirring during MLF, the amount of bacteria added during inoculation for MLF (more bacteria results in lower diacetyl), the particular strain used, longer lees contact, etc. As far as timing goes, doing MLF more or less at the same time as alcoholic fermentation tends to produce less diacetyl as well. That would be counter to the theory you were given recently.
However, if the fermentations are both non-inoculated, it's possible both are occurring simultaneously and both finish late. That, along with the extended lees/bacteria contact that suggests could result in less diacetyl. The downside is that slow fermentations (and the associated delay in applying SO2) present considerable risk of contamination. And any benefits in lower diacetyl due to simultaneity and extended contact might be be balanced out by higher diacetyl production due to slow, low temperature MLF.
In short, it's an interesting theory but I suspect any correlation between the timing you suggest and the results are most likely coincidental rather than causative.
Hi Fred, late actually getting to read this fantastic article. Hoping to see more. I was recently told that diacetyl production levels were more pronounced if the MLF occurred relatively close to the primary fermentation and this was the reason why you saw more of the character in warmer climate Chardonnay as opposed to Burgundy as the malos there typically occur in spring following harvest. It would seem to follow in with your explanation but does the timing have really anything to do with it? I think sometimes we get a piece of information and if it seems logical it becomes a fact when in truth it is only a piece of the overall picture.
Thanks again for the great article!
Big "thank you" everyone for the comments and support. Stephen, thank you for discussing your experiences as well. That's great info.
Fred this is such a great article, thank you for writing this. Definitely gave me more to think about when asking winemakers about their practices.
A terrific article, Fred! Great information, I learned a lot!