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Digging for Gold

Winemaking is hard work. From hauling hoses around, pushing pumps from one end of the cellar to the other, moving barrels by hand, hefting cases of wine to and fro; it’s a decent way to stay in shape. One operation that is particularly labor-intensive is emptying red wine fermenters.


In Blog Post #1, I detailed a few of the ways that winemakers handle red wine cap management. In this installment, let’s investigate how winemakers separate the Marc (pomace) from the wine once it’s time to do so. There are a million different strategies to determine when that time is. Some winemakers “short-vat” their wine to limit aggressive tannins, or perhaps to minimize the time the wine might have in contact with compromised fruit. Others press off their reds once they’ve “gone dry” (aka all the sugars have been fermented into alcohol); while still others conduct extended maceration to build full-bodied, structured reds.

Regardless of how long wine is in contact with the Marc, at some point, the liquid must be separated from the solid fraction. In commercial wineries, this is done by transporting the liquid/solid mix to the press, where under pressure and through some sort of screen, the liquid is liberated from the clutches of the solids.

On the smaller scale, where the wine is fermented in picking bins (known commonly as macro-bins), they are most often dumped via a bin dumper into a press. Sometimes, the wine is sucked out using a “torpedo” device which consists of a screen and pump, then the solids are transferred into the press. Other times, they are punched down and the whole slurry is pumped to the press. In even smaller wineries, enter the 5-gallon bucket and a good workout.

When wines are fermented in a tank, the most common methods are “digging” and “sluicing.” There are some more elaborate feats of engineering where the tank itself accepts a hydraulic ram to squish the solids down, but that is quite rare and unbelievably expensive.

With the “dig” method, a hose is attached to the racking valve, where much of the wine is bled off, leaving a portion of the wine and all the solids behind. Then, a bin is placed under a bottom manway, which is ever so carefully opened and the wine/marc begins to fall into the bin. Once it is safe to open the door (i.e. the must won’t come gushing out in a huge mess), a worker uses a rake to start manually digging a channel into the Marc to reach the top of the pile. Once this channel has been dug, some sort of fan or ventilation device is used to evacuate the residual CO2 from the tank. Asphyxiation from CO2 is a very real concern in the industry. Ideally, the cellar worker uses a gas monitor to test the atmosphere to ensure safe levels of oxygen prior to getting inside the tank. The buddy system is absolutely required in this case, where the worker who enters the tank is tethered with a rope and safety harness to the outside, where another worker monitors their progress and health. If something goes wrong, they can pull them out and administer first aid. Once inside, the worker begins to push and shovel the Marc out the bottom door they entered and into the bin placed under the door. The worker outside monitors the fill level of the bin and removes/replaces this bin as necessary. Once the Marc has been removed, it is transferred to the press as previously mentioned.

There are a few negatives to this method. First, it is extraordinarily dangerous. I’ve personally done this, and in the process of digging out the Marc, have hit pockets of CO2 that have the potential of rendering someone unconscious. Secondly, there’s a contamination concern around sending a person inside the tank to walk over/through the pomace while they are digging. Usually, they are wearing rubber boots; ideally, those boots are sanitized prior to hopping in. Suffice to say, it’s certainly not the cleanest operation. Thirdly, it’s extremely labor-intensive. This cost is passed on to the consumer.

One could argue there are a couple of advantages to this method. Since no pump is involved, there is the potential that the fruit is handled more gently, less risk of oxidation, and the winemaker has more control of the phenolic extraction.

Over the years, I have come to believe that the wine industry tells itself a lot of ghost stories to frighten itself into believing this or that. Let’s take a winery that uses gravity flow to move its product, as opposed to using pumps. Gravity flow is often held as the pinnacle of gentle, high-quality wine production. The fundamental issue many have with pumping, in my opinion, is that it increases the oxygen pickup and negatively affects wine quality. 20 years ago, I might have agreed with that. Thankfully, pump technology and our understanding of oxidation and gas chemistry has improved. Those that know me will attest to my fanatic obsession with pump technology and Dissolved Oxygen (DO) management. If you want to go down that rabbit hole on DO, a simple google search will turn up quite a few articles and webinars I have put together on the topic. Pick up the most recent edition of Winemaker Magazine which has an article I’ve written on the topic; it’s great bedtime reading, no melatonin needed.

Now, I should state that not all pumps are created equally, nor are cellar operations always done correctly (to include gravity flow). The devil is in the details; without going into elaborate detail on pump design, I’ll simply state it is 100% possible to transfer wine and must with very little damage to fruit and with minimal, if any, oxygen pickup. Some of the finest wines in the world are 100% pumped.

This brings us to an interesting question when it comes to what and how we do things in the wine industry. That is; do we do a certain thing a certain way because it is the best or is it just the way it's always been done? One could argue such things till the end of time; but the end decision should be based on deliberate thought, intention, and backed by science… not romance or laziness. Examples of such arguments include barrels vs oak adjuncts, screwcaps vs corks, native vs commercial yeast, and so on. That’s not to say I want AI to make wine, I’ve actually been told by a winery owner I once consulted for that robots would someday replace a human winemaker.

On to the “Sluice” method. Without pictures, this may be difficult to explain, but here goes… step by step.

1. Pumpover the tank to homogenize the tank. You want a good liquid slurry.

2. Make sure you have a good slope to the tank floor, common thought is ¾” per foot

3. Have an adequate drain valve. The tanks I’ve selected for FermForge have a 4” drain and range from 1500-4000gal. I’ve used 6” drains on tanks from 9000-18,000gal without issue.

4. Hook a large diameter hose (shorter the better) to the aforementioned drain valve. Bigger the better. NPSH (net positive suction head) is extremely important for operations such as this; a long hose, with a bunch of turns in it, causes a significant “drag” and reduces the capability of the pump.

5. Use a pump capable of developing high pressure and pumping solids. Common pump designs suitable for this would be of the positive displacement variety to include Waukesha lobe pumps, progressive capacity, and perhaps peristaltic pumps. Again, the devil is in the details. Pumps must be sized according to the operation; a rabbit hole in and of itself. Winemaking is easy, remember? Who wouldn’t love to talk about NPSH, pumping efficiency, gear ratio, motor horsepower, slip, shear, pressure drop, motor RPM, displacement per revolution, etc?

6. Use this pump to transfer the slurry to the press. Once the press starts to drain liquid, use another pump (probably a centrifugal) to send the liquid back up and over the top of the tank with a “nozzle” that delivers a solid stream of wine at pressure to “Sluice” the skins towards the drain. This is most satisfying and can be quite the competition amongst cellar workers for the vaunted title of Sluice King or Queen.

7. Once all the solids have been sluiced out, shut off the pumps and push the wine to the press via a suitable method.

Notice, I never mentioned sending someone into a tank full of CO2.

With the sluice method, there isn’t a chance of someone sludging through the Marc with dirty boots, working up a sweat (is that where a wines salinity comes from?), and shoveling out a tank bit by bit. So, this method is safer, cleaner, more efficient, and (in my opinion) can produce wines of like quality as the “Dig” method. Less romantic? Perhaps, but it sure minimizes the sleeping beauty scenario where someone is called upon to perform CPR on their cellar worker colleague.

I do give a nod to tradition. FermForge’s tanks are predisposed for the sluice method, they are concurrently able to be “dug.” While there is always a safety risk associated with digging a tank, it’s done thousands and thousands of times each year without issue.

In conclusion, whether you are a winemaker or a consumer, one must consider what it takes to get that delicious product into the glass you are enjoying. Fairtrade and labor practices aren’t really a “thing” in wine production, yet. In some parts of the US, we have some very serious regulatory agencies that provide exogenous motivation to seek out safer methods of production. That doesn’t exist everywhere. Regardless of safety concerns, there’s still the looming question of whether or not digging a tank actually makes a quality difference, or if it is just “tradition.” Hock off the ham and all that.


Luke Holcombe

Partner, Director of Winemaking

IG: @fermforge @beardedwinewizard

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