Inquiring Winemaker


Sift Through Facts In Solids and Wine

August 2011
by Tim Patterson
Wine is, ideally, all liquid; that is, we don’t generally want to see things floating or swimming in the glass. This is one of the many reasons wine is considered a more noble beverage than, say, bubble tea, with its scads of tapioca pearls in the mix.

In the quest for great liquids, winemakers rarely give solids much thought, except for making sure to get rid of all of them before bottling. But that 100% liquid wine starts out full of all manner of solid stuff, and the amount and composition of this indissoluble matter can matter, big time. A quick review of the relevant literature unearths an astonishing number of things solids can, might or will do, especially for white wine fermentations, including some contradictory findings and some things that show up in the lab but rarely in commercial-scale winemaking.

It would be tempting to just say—as the great majority of winemakers do—that the plan is to get rid of most but not all of the juice solids before fermentation and leave it at that. But there is enough evidence to suggest that managing solids more carefully could be a way to fine-tune wine style—if only we knew how to do it with certainty.

Sludge composition 101
The composition of solids in white grape juice—juice lees—is all over the map; it’s a “them,” not an “it.” There are sure to be itsy bits of skin, pulp, seeds and stem jacks in some proportion or other. There may be mold, or mold byproducts, as well as dirt and perhaps residues from vineyard spraying. Figure in some stray feral microbes, plus an insect part or two (there are, in truth, no strictly vegetarian wines) and the occasional shredded gum wrapper. Finally, there may be some solids intentionally added in the cellar: yeast hulls, powdered oak tannins and so on. Every batch of wine is complicated, and every batch is different, depending on the variety, the vintage, vineyard practices and the equipment and protocols used for crushing and pressing.

The grape-derived solids break down further into many components: several flavors of polysaccharides, mannoproteins, mineral salts, nitrogen, tannins and on and on. Plus multiple forms of fatty acids—although, fortunately for the health-conscious, more of these are unsaturated than saturated.

We know this long roster of likely contents from lab experiments focused on parsing the sludge into its component parts. In winery practice, however, all we usually know is how much we have—the overall volume of solids compared to the volume of liquid—and we can observe that the heavier particles settle first, leading to a stratified sediment that changes particle size (bigger to smaller) and color (browner to greener) from bottom to top. It is a rare winery indeed that performs analysis to determine what exactly is on hand.

Contemporary conventional white winemaking wisdom says that white juices need some solids (since over-clarified juice can lead to stuck fermentation), but not too much, which can result in reduced aromas and other off-elements. There is no consensus about how much is just right. Most winemakers, seeking fruitiness above all else in white wines, aim for somewhere between 1% and 2% of solids by volume and do some form of settling, draining, racking or centrifuging to get there. Whole-cluster pressing generally achieves that target range without further need for clarification.

This practice more or less works, with only juices at the extremes—water-clear or highly sludgy—causing obvious problems. If it ain’t broke, don’t fix it. But there are indications that the details of juice lees composition matter, and that if we had ways to get better analysis and control of solids—plus additional research on this unglamorous topic—we might be able to use solids management as a tool to fine-tune wine style.

Cornucopia of effects
Solids have been linked with an amazing number of biochemical reactions. The idea that white fermentations need “some solids but not too much” has been demonstrated repeatedly in controlled experiments. Juice that has been clarified ferments more quickly and more efficiently than wine with high solids, but over-clarified juice ferments slowest of all and can simply stick. Exactly why this pattern holds is not entirely clear, though researchers from half a dozen countries have offered suggestions. Nor is there agreement about an optimum level of solids, which leaves the discussion framed by “too much” and “too little,” not by hard numbers.

Solids may be helpful in supporting yeast multiplication, and they may make nutrients more available to yeast. Fatty acids are themselves a possible source of nutrition. Solids may remove potentially toxic substances through adsorption (stuff sticking to the solids), and in a similar fashion they may serve as magnets for carbon dioxide, getting it out of the yeast’s way. Knowing that solids can help fermentation somehow, many winemakers add fresh solids (often yeast-based products) after removing most of the old ones, getting a cleaner set of solids. These additions are the one major example of conscious alteration of juice lees content.

Popular literature has some mixed findings about the role of solids in malolactic fermentation, with indications that under some conditions solids help, and in others solids delay and slow malolactic fermentation.

Too low a level of solids, resulting in slow fermentations, leads to increased production of acetic acid. Too high a level can increase lactic acid bacterial activity—both the good kind (Oenococcus oeni, the driver of successful malolactic) and some bad actors. Too high a level of solids leads to green aromas and flavors, reductive smells, bitterness and oxidation. High solids yield elevated levels of higher alcohols (isoamyl alcohol and some relatives), which are the kinds of double-edged components that improve some wines and mess with others.

High solids also promote astringency, another characteristic that may or may not be desirable. In the modern, hyper-fruity style of white winemaking, phenolic extraction is a no-no, since it can be jarring to consumers. But for wines that hope to age and develop in the bottle, a little phenolic boost can surely help. That can be gained with several hours of skin contact, but the tannin-bearing skin shards in the juice lees may also contribute to the same end. In other words, a particular winemaker may or may not want p erceptible astringency.

Fatty acids in the solids can provide some nutrition, and the good ones can adsorb some of the bad ones that may be toxic to yeast, thus providing a yeast survival factor toward the end of fermentation. But too many lipids can yield noticeable amounts of methionol, a compound that smells like broth or meat and potatoes. Generally, we want to drink our wines with the pot roast, not to have them smell like it.

In yielding fruitier wines, juice clarification seems to promote the retention of all those nifty volatile esters that white wines exude. Excess solids depress the fruit, hide those esters and let the sulfur-related aromas move front and center.

But wait, there’s more.

The volume and type of solids at fermentation can strongly affect the later clarification of the finished wine. A small amount of botrytis can yield certain polysaccharides that stubbornly resist clarification. (But, weirdly enough, a very, very small amount of botrytis can generate a large amount of pectinase in response, which is highly effective in advancing clarification.)

The Australian Wine Research Institute has reported that during fermentation, yeast can produce haze-protective mannoprotein material (HPM) that reduces the level of protein haze later on, and that this HPM is produced in greater volume during fermentation than later through the slow disintegration of yeast cells during aging. The implication here is that if you could collect a bunch of HPM, you could add it as a bonus solid to your next fermentation and have less haze to worry about later.

Mannoproteins—proteins found in yeast cell walls that contain mannose, a form of sugar—show up in more and more wine analysis and research. In the winery, they are a boon to mouthfeel and are an important part of what gets pulled out from spent yeast cells through sur lie aging. In fermentations conducted with too high a level of solids, however, the mannoproteins may be coaxed out prematurely, at which point they adsorb onto the solids, fall out of solution and aren’t around later when it’s sur lie time.

Is it just me, or is all this making your head hurt, too? The plethora of findings and semi-well-founded speculations mainly come from small-scale lab work, microvinifications that don’t always parallel commercial realities. (Kind of like the relationship between rat studies and human trials for pharmaceuticals.) It’s also clear that the endless variation and complexity of juice composition means that what solid X does in one juice may not be repeated in the next one. The volume and composition of solids clearly have a significant effect on fermentation and the resulting wine; what exactly is happening in a specific fermentor is anybody’s guess.

Measurement methods
While we wait for the magic instrument or additive that can size up the sludge and pluck out just the bad fatty acids (for example) and leave the good ones in, there remains the issue of knowing the sheer volume of solids. This much at least is essential to conscious winemaking: measuring the solids in each batch of juice, keeping records, evaluating the results, adjusting practices over time. When grape sources change or reception equipment gets updated, solids measurement needs to be revisited.

Some winemakers “measure” solids simply by eyeballing the juice after 12-24 hours of settling. The standard here is that the juice be “opalescent”—letting light through but slightly milky, like the appearance of an opal. Opalescent is a fine and fancy descriptor, but the accuracy of purely visual methods is in doubt. One step up is putting a juice sample in a clear, calibrated cylinder, letting it settle and doing the math. While hardly ultra-precise, this method used consistently over time can more or less do the job. 

Several more steps up is the use of a centrifuge, either at the winery or through a testing service. A small volume of juice gets spun for a few minutes, then the plug of solids is measured and the proportion calculated. Many French enologists prefer to use a turbidity meter (nephelometer), which measures the scattering of a beam of light as it gets reflected off the suspended particles, resulting in an NTU (nephelometric turbidity unit) rating rather than a percentage of volume to volume. In California and the rest of the United States, solids tend to get measured with centrifuges, with the turbidity meter reserved for measuring protein haze later on. Either of these machines can cost between $2,000 and $3,000.

One final option is inline monitoring, using an alternative method of measuring turbidity, looking at light absorption, not scattering. This method has the potential to measure not simply the overall volume of solids but to discriminate particle size—one more clue to the content of the solids. While mainly the province of bigger players in the U.S. wine industry, this approach has made bigger inroads into craft brewing. Equipment costs range from $3,000 to $10,000.

All of these methods, of course, depend on careful collection of samples to be tested. Juice from the top of the settling tank will have fewer solids than juice at the bottom; but since the point is to settle the lees, you don’t want to mix it up all over again. And so on. If the volume of solids is the one factor you can determine and alter, might as well get it right.

As to all those intriguing issues of the composition of any given layer of lees and what they might do to your wine, you may have to wait to find out. Fermentation product suppliers will be happy to offer you packaged solids to add to the brew, and they may very well help in many situations. But for the most part, what we know about solids and winemaking is a list of things we don’t exactly know.


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