Stuck or sluggish fermentations can be a huge liability for today's winery. Protracted fermentations compromise wine quality, tie up valuable fermentation space, create time lags in the winemaking calendar and are more prone to spoilage, microbial infection and oxidation. Stuck fermentations, where the fermentation stops before the desired endpoint, are also prone to spoilage, can re-ferment later in its life and most painfully, may not ever be palatable or salable. Since winemakers can't bottle or even finalize a blend until a wine is microbially stable, stuck and sluggish fermentations become not only wine quality issues but also winery management issues.

Stuck and sluggish fermentations happen when the uptake of glucose and fructose is reduced or stopped as a response to various stresses. During a healthy fermentation, carbohydrates are transported by facilitated diffusion through various transport sites on the yeast cells outer bilipid membrane layer. Yeast membranes become increasingly challenged as alcohol levels increase, temperature increases, oxygen levels drop and nutrients are used up. Though proper yeast nutrition is critical for fermentation health, getting through a harvest stuck fermentation-free is impossible without considering other key processing parameters. Winemakers need to remember that the cleanliness of the raw material, the chemical composition of the must, the choice of yeast strain, the hydration and inoculation method and the fermentation parameters all have just as much to do with a successful fermentation outcome as does the nitrogen content. Going into harvest with a whole-process approach in mind, a winemaker can set up the best possible conditions for a successful fermentation outcome and avoid stuck or sluggish fermentations.

In the field is often where a winemaker will first spot harmful microbes that can compete with yeast for nutrients. Photo by Justin Beem

Clean and Balanced Juice and Must

Healthy fermentations start with healthy raw material. Vineyard residues can harm yeast and microbes can compete with Saccharomyces cerevisiae for resources in the fermentor and can produce inhibitory levels of VA. Grapes that come into the winery with high populations of bacteria, mold, yeast or other fungi can have high VA even before fermentation and, in the fermentor, can produce even more. So called "native" yeast, or non-Saccha­romyces yeast such as Hanseniaspora uvarum, Kloeckera apiculata and Candida krusei are all high VA producers, often producing well over the legal limit (1.2 g/L for reds, 1.1 g/L for whites). In a normal, healthy fermentation, S. cerevisiae will produce less than 0.5 g/L VA.

Besides VA, non-Saccharomyces microbes can compete for critical must nutrients and resources. Lactobacillus especially has complex nutrient requirements and can strip musts and juices of carbohydrates, amino acids, nucleic acids, vitamins and fatty acids. Another example, Kloeckera apiculata, is known to strip thiamine from grape juices. For this reason it's advised to pick grapes as clean as possible in the vineyard, to keep them cool during transport and to crush or press them promptly, before opportunistic microbes have a chance to gain a foothold. Adding SO₂ (50 ppm total is a standard rate for whites, 30 ppm for reds) at the hopper can help knock down non-Saccharomyces microbes while, in extreme cases, Lysozyme can be added or juice can be filtered to inhibit or exclude most bacteria (Lysozyme doesn't inhibit Acetobacter). Evaluating incoming vineyard lots and inoculating promptly can all help keep non-desirable microbial populations in check. Cold soaks can be especially risky with unsound fruit.

Keeping a tight handle on pH and TA is critical--acidity that is too high can inhibit yeast cells whereas acidity that is too low encourages bacterial growth. Photo by Chris Purdy

 Having balanced juices and musts are also important as high sugars and high pH can preclude a wine to stick. The higher the Brix, the higher the potential alcohol and the more likely a yeast cell will be poisoned by its own excretions before it can finish the fermentation. High pH juice or must can encourage competitive microbes and also may increase the relative speed of fermentation. If the pH is too low that can also be a problem; in a 1999 article in the American Journal of Enology and Viticulture, Dr. Linda Bisson found that a pH of 2.8 or below was inhibitory to a yeast fermentation (see further reading). Juice must also be balanced physiologically as well as chemically. Juice that is racked too clean, or under 150 NTUs, won't have enough turbidity to support a healthy population of yeast whereas a juice that's too turbid will have a higher likelihood of producing hydrogen sulfide and other off aromas. In today's era of increased "hangtime," with higher Brixes, higher pHs and unbalanced starting material, winemakers need to be more cognizant than ever of how the quality of the raw material will affect their fermentations.

Healthy Inoculations

Before you even start thinking about dumping a lot of yeast nutrients into your juice and must in the quest for the perfect, sound and problem-free fermentation, first think about selecting the right yeast strain for the job. Make sure it can accomplish what you want it to before you hydrate it and pitch it into the fermentor. Yeast strains differ widely in their temperature tolerance, alcohol tolerance, VA production, acid sensitivity, acetaldehyde sensitivity and nutrition needs. Read the supplier's information (most companies offer a handy yeast comparison chart--Scott Laboratories' chart is a good example) to make sure you are selecting a yeast that can motor through that 26 Brix old vine Zinfandel and won't die once the alcohol gets above 13 percent.

As you hydrate and inoculate, remember not to do anything to shock the yeast. Temperature of the hydration water (usually 100-104°F), initial juice or must temperature (55°F for whites and 65ºF for reds is usually a good target) and fermentation temperature (peaking above 85°F gets risky) are all critical control points. For specifics, check the data on your chosen yeast strain to find its ideal conditions. Additive shock is also to be avoided. I like to do any major juice or must adjustments (hydration, acidification, de-acidification, etc.) before pitching the yeast. This way the cells' recently awakened membranes don't have to adjust twice to new conditions, only once. Once they're hydrated in the warm water, I like to add juice from the fermentor at about 1/4 of the volume of the starter so the starter can get acclimated to the temperature and conditions in the juice gradually.

This custom-built pumpover pan lifts fermenting juice above the carbon dioxide layer in the sump, adding oxygen and cooling the fermentation. Photo by Chris Purdy

Always hydrate yeast first with warm water only, never with juice and especially not with wine. When a yeast's membrane is just coming out of its dehydrated stage, the hexose transporters can be disrupted by sugars and the membranes won't be fluid enough yet to deal with ethanol. Follow hydration procedures carefully and use higher doses for fermentations you know will be challenging. 1.5-2 #/K gallons seems standard for normal fermentations and up to 4#/K gallons is common for especially high Brix or infected juices and musts. Be careful about conducting "feral" or "native" fermentations (letting a fermentation start without inoculating) unless you know you've had success in the past with that specific fruit at your facility and the fruit is sound, clean and balanced. There's no doubt that non-inoculated ferments can and do go dry, especially later in the harvest season when there's a good natural inoculum built up in the winery and on equipment. When in doubt, however, inoculation is always a cheap insurance policy.

Make up for Nutritional Deficiencies

Yeast cells need both "macro" and "micro" nutrients to perform a healthy fermentation. "Macro" nutrients include nitrogen and phosphate, both of which are abundant in DAP (diammonium phosphate), sometimes called "junk food for yeast." Amino acids are a source of organic nitrogen and can be purchased in nutrient mixes made from autolyzed yeast cells or yeast extracts. "Micro" nutrients include vitamins like biotin, thiamine, calcium pantothenate, niacin, myo-inositol, vitamin B6 and minerals like zinc, magnesium and calcium. Long chain fatty acids and sterols are also important for strong membranes. All are important for healthy yeast growth, reproduction and survival as heat and alcohol increase in the fermentor.

Pressing off while warm, quickly settling and then racking can make a stuck ferment more manageable for a re-inoculation. Photo by Chris Purdy

There is a myriad of yeast nutrients on the market, so much so that it can be overwhelming. I always start my harvest-planning by asking my suppliers if they have any innovative products that are new for the year or are truly different than other products. Keep an eye out, however, as there is a lot of redundancy out there in the marketplace. See Curtis Phillips' "What to Feed Your Fermentations" in July's issue of Wine Business Monthly for a good schematic of who the suppliers are and what they're offering (see table on right). Most products fall into one of the following categories: yeast hydration nutrient, macronutrients (ammonium salts like diammonium phosphate/DAP), complex yeast nutrient (DAP plus vitamins, minerals and yeast hulls), or low-nitrogen nutrients (yeast extract, yeast hulls or "yeast ghosts"). Most of the complex yeast nutrients are in powdered form while additionally, some are designed to be added in two stages, like Lallemand's Actiferm 1 and Actiferm 2 and Gusmer Enterprises' new time-release tablets, Micro-Essentials Complete-TR^TM, that allow you to drop them in a barrel once and not have to come back for the second addition.

The relative nutritional needs of wine yeast are strain dependent and can vary widely. Strains like BA11, VL3 and W15 are known "high maintenance" strains that can produce high levels of hydrogen sulfide and can become sluggish or stuck unless fed appropriately. This is why it's critical to read the manufacturer's information about the yeast you choose but also to measure the nitrogen content in your juices. Even more important is that you must measure not just the ammonia (NH3+) ion content but the nitrogen available in the amino acids as well. This combined nitrogen measure is called YAN (yeast available nitrogen) and is a measure of the ammonia nitrogen plus the alpha-amino acids in ppm. Most winemakers I know calculate their nutrient additions in the following way: Subtract the measured initial YAN from the desired YAN (shoot for 275 ppm desired but go with 325 ppm for high-requirement yeast or if the Brix is over 25.5). Then add that difference in ppms to your juice or must using a combination of DAP, a complex yeast nutrient like Superfood or Fermaid K and possibly even a hydrolyzed yeast extract. Yeast nutrient supply houses, even though they keep their nutrient mix's exact recipe a proprietary secret, should always tell you how many ppms of nitrogen you'll be adding per unit of nutrient mix added so you can calculate your nutrient "cocktails" accurately. It's critical to know how much you need to add so you don't add too much; un-eaten yeast food will just become food for possible spoilage organisms later on in your wine's life and excess ammonia can turn into ethyl carbamate.

Smart Nutrient Timing

Most people add yeast nutrients during hydration, after the first Brix drop, about 1/3 of the way through Brix drop, and then perhaps again once the Brix gets under 10. This last addition, if done, only should consist of "survival factor" nutrients like yeast hulls or non-DAP products. It's best to divide your total nutrient addition (desired YAN-initial YAN) in two or three parts--most people choose two because it allows them to spread out the additions over time while not having to touch a barrel group or tank more than is necessary. Here's a basic nutrient addition timeline:

1.   Yeast hydration to 6 hours post inoculation (first part of lag phase):

•   Don't use any nitrogen-containing compound, especially ammonium salts, in the yeast hydration water
as that's toxic to developing
membranes.

•   Hydration nutrients, usually proprietary mixes of vitamins and minerals, can be helpful here as they do seem to help yeast cells develop stronger cell membranes as well as to store and utilize nutrients before they hit the challenging juice
environment.

•   Don't ever add nutrients of any kind to must that you are trying to cold soak or you'll encourage spoilage organisms.

2. End of lag phase (6-12 hours after inoculation, first Brix drop noticed) through exponential phase:

•   This is when you add the first hit of complex yeast nutrients, usually 1/3-1/2 of the total nutrients you plan on using.

•   Use a complex yeast nutrient, not just DAP, because ammonium salts don't provide the vitamins, minerals, biotin, pantothenate, etc. that yeast need. In fact, using DAP alone can actually inhibit the efficient uptake of nitrogen as well as the formation of enough yeast biomass in the growth phase. Yeast cells can actually take up nutrients into their vacuoles for later use so it's important to give them what they need early so they can finish strongly.

•   Oxygen is critical in this stage for healthy membrane development, especially for reds but also for whites. In-line venturis, sparging stones and even old micro-oxygenation stones can be a big help here.

3. End of exponential growth and beginning of stationary phase, about 1/3 sugar depletion:

•   This is when you add your second hit of complex yeast nutrient.

•   If you've divided your nutrient additions into thirds, you want to make your last nutrient addition before Brix gets much below 10.

•   Yeast still need N and oxygen
during stationary phase so keep oxygenating.

•   You need nitrogen during stationary phase because carbohydrate transporters in the cell walls have a high turnover and need to replenish themselves. Certain amino acids also prolong maximal fermentation activity, like glycine which, even though it's a poor nitrogen source, has been shown to be critical during this time.

•   "Survival factors," like long chain saturated and unsaturated fatty acids, sterols and ergosterols can be added at this stage to help set up the end of fermentation for a strong finish.

4. End of stationary, going into death phase:

•   Hopefully, you've already set up the fermentation for a strong finish and won't have to add anything at this point and can just concentrate on keeping the temperatures warm enough to encourage finishing but not too hot as to fry the yeast. However, if you smell, hear or see problems (creeping VA, slowing fermentation, hydrogen sulfide production) you may want to try to remedy the situation.

•   At this stage it's OK to add sterols, long chain saturated and unsaturated fatty acids, (yeast hulls) during last part of fermentation, but watch the 3#/K gallon legal limit on the yeast hulls as many of your prior nutrient additions may already
contain them.

•   Autolyzed yeast and yeast extract products with no added inorganic nitrogen can be helpful here, like Lallemand's SIY33 extract as an example. The important thing is just not to add anything containing inorganic nitrogen (DAP or ammonium salts) as it won't get used by the yeast cell and will stick around to be used by spoilage organisms later or will be turned into excess urea and ultimately into ethyl carbamate, a carcinogen which some countries are starting to regulate.       

Avoid Heat Spikes and Temperature Swings

Once the fermentation is healthily inoculated and has had its first complex nutrient addition at the end of the lag phase, your job is to keep things in the tank from going to extremes. Moderating the temperature to avoid wild heat spikes or cold spots is critical. A fermentation that's too hot (peaking over 85°F is risky) will make the yeast cells more sensitive to alcohol toxicity while temperatures that are too cold (below 55 is usually risky) will slow them down. Big temperature swings, even within the "safe" range, can damage yeast because they won't have enough time to adapt their membranes to the new conditions. This is where delestage (rack and return), as long as you don't hold the liquid portion out too long, can really help with reds to prevent "hot spots" and keep the cap from heating up too much. Heat exchangers in the pump over line, warm glycol circulators or even warm water trickled over stainless tanks can help keep a fermentor from getting too cold. Always try to ferment the last wines of the season in indoor tanks so they won't be affected by cold nighttime temperatures.

Oxygen is Key During Fermentation

Active fermentation is the time to prevent alcohol toxicity and to build up those cellular membranes. Adding oxygen during fermentation can help build up sterols for strong membranes, either in the form of aerative pump overs for reds or sparging stones or conservative venturi "pump throughs" (closed pump over) for whites. Reds typically need more air than whites (whites might only need a conservative amount once a day during the most active fermentation days); make sure you're really incorporating air into your fermentation rather than splashing. Venturis in the pump over lines, rack and returns and fans blowing hard on pump over sumps with screens can all help. Some people use sparging stones in line with filtered compressed air or oxygen for really big tanks that have a hard time getting enough oxygen.

Non-Nitrogen Additives Only Below 10 Brix

Yeast hulls (keep in mind 3#/K gal legal limit) and other non-ammonium salt nutrients can be helpful during active fermentation and stationary phase. This is when you really need sterols and fatty acids, not nitrogen alone. As alcohol content goes up, yeast cells have a harder and harder time utilizing nitrogen and so at this point you'd just be throwing nitrogen after whatever ML or spoilage organisms might come along later. Yeast hulls added after 10 Brix can contribute sterols and can absorb some of the toxins produced by yeast during the latter part of the fermentation. Don't rely on yeast hulls to contribute too much nitrogen early on in a fermentation--they won't really break down much and don't contribute that much nitrogen. However, adding a true hydrolyzed yeast extract before 6-7 Brix may help the fermentation get through stationary phase strongly.

Salvage Maneuvers for When the Going Gets Slow

Jump on it early. By the time you notice a fermentation slowdown it could be too late to rescue it. If your yeast viability is still greater than 25 percent (check with a methylene blue stain under a microscope), you are still producing CO₂ and the temperature is still above 55 for whites and 60 for reds, resuscitation (as opposed to re-inoculation) is probably your best bet. Try to incorporate oxygen if it won't damage your wine quality at this stage. Reds obviously can take more oxygen than whites. Using microbullage cylinders at high O₂ dose or other kinds of sparging stones inline or in tank can be great for this purpose. Sometimes just stirring up the tank will help re-suspend the yeast cells and get them out of the glucose-depleted environment that can sometimes occur on the bottom of fermentors. Yeast hulls can often be added at this stage in 0.5-1 #/K gallon increments but just be sure you watch the legal limit as nutrients you've already added may have a significant proportion of yeast hulls. Inorganic nitrogen (ammonium salts, DAP) will not help at this point though a yeast extract rich in survival factors (sterols, fatty acids) might.

Regulate the tank temperature to keep it in that struggling fermentation sweet spot, 65-75°F but not higher than 80°F. Yeast cells become more sensitive to alcohol as temperatures go up but they also get sluggish if they are too cold (under 60°F). As your fermentation loses CO₂, take care to blanket the surface of the fermentor with argon or carbon dioxide gas in order to protect the wine from Acetobacter or other aerophilic microorganisms. Check for Lactobacillus and try to add Lysozyme if you've got an infection. Keep an eye on your alcohols and VA levels. As VAs approach legal limit (1.2 g/L for reds, 1.1 g/L for whites) and as alcohols approach 15 percent, re-inoculation gets harder. If you have the equipment available (reverse osmosis) or can justify the expense, sometimes taking the alcohol or VA down is enough to get a sluggish ferment to finish out. Sometimes blending other wine in with your stuck lot will lower the offending compound and can be enough to encourage yeast to finish out. Sometimes, if your program volumes, style, appellation and varietals work out, you can blend down a stuck or sluggish lot with enough wine to "disappear" the high sugar percentage.

Re-Inoculation: If nothing else works

Re-starting a stuck fermentation, as any winemaker that has lived through it knows, is one of the most time-consuming and tricky tasks in the cellar. Sometimes it's the only thing you can do, however, especially if you don't have enough winery volume to blend away your stuck lot or it's a particularly high-value program. Before you start, it's always a good idea to make sure you've got the staff and equipment lined up well before hand. Restarting a stuck fermentation (see procedure next page) involves creating a high-concentration yeast starter with a very strong yeast and then adding a little (1-5 percent volume of total stuck volume of wine) juice to get them turning the juice to alcohol. This acclimates their membranes to being in an environment where alcohol is present. Then you add incremental amounts of your stuck wine to the newly fermenting starter, all the while monitoring the starter itself for fermentation rate and only adding stuck wine while the starter itself is fermenting strongly. The first part of the process is attention-intensive and can take 24 hours to complete. Once you've added the stuck wine to your starter, finishing out the fermentation can take weeks. I find that well-trained cellar crew or even visiting enology interns are ideal re-inoculation "babysitters" because they know how to monitor Brixes, temperatures and can use their senses to critically analyze a troubled fermentation.

Restarting is most successful when you have less than 25 percent yeast viability, (check with methylene blue under a microscope--above 25 percent and resuscitation techniques alone may work) your Delle Units are under 65 ([percent alcohol x 4.5] + percent RS), and you don't detect an infection of Lactobacillus. Pressing off early and racking (or even filtering or centrifuging) to get rid of dead yeast cells can cut down on the amount of toxic inter-cellular signals the new yeast will receive. Lactobacillus, if present, can be dealt with by filtering or by using Lysozyme. Check your VA and alcohol levels as well--some wineries have the capability to de-alc and de-VA with reverse osmosis if levels dictate (over 14.5 percent alcohol, over 0.9 g/L VA), which can help the new yeast get a better start in life.

The re-start procedure I mention below is one that's worked for me over the years, across many different appellations and varietals but do keep in mind that every fermentation, cellar and stuck wine are different. Besides the step-wise addition of your stuck wine to your starter mentioned above, the general idea is to give a high inoculum of strong yeast the best possible start and fermentation conditions you can. You will be essentially introducing a new population of yeast (your starter culture) into a very hostile environment (the stuck wine, replete with alcohol, VA and those autotoxic inter-cellular messages) so control all of the aspects of the meeting you possibly can, like nutrition, temperature and timing of the stuck wine additions. If you've never done a re-inoculation before, you may want to call in expert help. The wine consultants at companies like Gusmer Enterprises (www.gusmerenterprises.com) and Vinquiry (www.vinquiry.com) can perform pre-inoculation panels (VA, Alcohol, Glucose:Fructose, yeast viability), provide advice for your particular situation and can help walk you through the re-inoculation process.

Avoiding stuck and sluggish fermentations goes well beyond just throwing a bunch of nutrients at a blend and seeing what sticks. Rather than hoping for miracles, even if it is in the form of the latest "magic powder" sold by industry supply companies, winemakers must pro-actively manage a multitude of aspects of their fermentations, from the field to the fermentor. Only by applying a whole-process approach--by paying attention to competing microbes, must composition, yeast characteristics, specific nutrient deficiencies and by employing sane fermentation temperatures--can winemakers once and for all keep stuck and sluggish fermentations from haunting their harvest. wbm

Sidebar: How to Re-Start a Stuck Fermentation

It's critical to avoid temperature shocking the yeast in the starter. Never drop the temperature of the starter by more than 10°F at any given addition. Add the stuck wine (usually much colder than the starter) to the starter gradually enough to keep the post-addition starter temperature within 10°F of where it was before the addition.

INGREDIENTS:

1. Your stuck wine

• Measure volume if juice, or estimate volume if must. Depending on how much water you may have added, usually 165-170 gallons/ton is a good ballpark red yield. Measure Brix (or RS), VA and alcohol of stuck wine if possible.

2. A grape juice source, 1-5 percent of stuck wine volume (more is better)

• Chilled hold-back juice, pressed second crop juice, organic purchased
grape juice or grape juice concentrate (though juice is better)

3. Yeast: 5-8 lb/K gal calculated for stuck wine volume

• Fermichamp, K1, PDM/EC1118, Uvaferm 43 or BCS 103 are good strain choices

4. Yeast nutrients

• Hydration water nutrient (NOT DAP) like Go-Ferm

• Vitamin mix for starter volume only (like Vitamix) can help

• Complex yeast nutrient (like Superfood) for 1 g/L for starter volume only, 1.5 g/L if brix is stuck over 4 Brix

• Yeast hulls: Add 1-2#/K gallons per stuck wine volume (watch legal limit if you've already added some)

PROCEDURE:

1. Prepare your new yeast culture.

• Add enough water to the unfermented grape juice to get it under
14 percent final alcohol

• Warm unfermented grape juice juice to 65-70°F

• Hydrate yeast in warm water (100-104°F) with hydration nutrient, stir gently after a few minutes, allow to stand for 20 minutes

2. Create the starter.

• Acclimate hydrated yeast to the unfermented juice by following the below procedure:

• Add small amounts of the warmed juice (65-70°F) to the hydrated yeast (should be ~90-95°F), dropping the temperature of the hydrated yeast solution no more than 10 F at a time, waiting ten minutes between additions, until the unfermented juice/hydrated yeast mixture are combined

• Measure the initial Brix of your starter and monitor every 30 minutes

3. Double volume of the starter with stuck wine.

• As soon as you notice a Brix drop in your starter, or as soon as you hear or see the starter actively fermenting, start adding stuck wine to your starter by either 1) doubling the volume of the starter once with stuck wine if the temperature of the starter and the stuck wine are within 10°F of each other or 2) just adding enough stuck wine to the starter to drop the temperature of the yeast mixture by 5-10°F. Usually the stuck wine is colder than the warm hydration mixture- this is when it helps to warm your stuck wine to 65-75 F if it's below that) and therefore you don't want to shock the warm hydrated yeast with cold wine. It's important to not add more stuck wine than double the volume at this stage.

• Add 0.1 g/L of Superfood or other complex yeast nutrient for the stuck wine volume added

• Measure Brix and monitor every 30 minutes

4. Double volume of the starter again.

• As soon as a new Brix drop is measured, double volume again with the stuck wine (or use less, if temp between stuck wine and starter is more than 10°F to avoid shocking the starter)

• Add 0.1 g/L of Superfood or other complex yeast nutrient for the volume of new stuck wine added

• Measure Brix and monitor every 30-60 minutes to get an idea of Brix drop and fermentation activity. As fermentation gets slower, you can take Brixes less often

5. Double volume again until starter is 30-60 percent of the entire volume of stuck wine.

• Maintain temps between 60-75°F

• If fermentation activity of the starter slows, try adding smaller portions of the stuck wine at a time

• Don't add any nutrients once the starter gets below 7 Brix

• Keep monitoring starter Brix for fermentation activity

6. Keep adding stuck wine to starter, all the while monitoring the Brix of the starter/stuck wine combination.

• You want the starter to keep actively fermenting. Usually you'll have to wait longer between additions towards the end as the starter will have a harder time responding to each new addition

7. Hope for the best, prepare for the worst.

• Some sources recommend blending the starter with the remaining stuck wine when the starter volume reaches 25-50 percent of the stuck wine volume, but I have had the most success keeping on with the fractional blending described above- adding portions of the stuck wine in gradual intervals, dictated by the activity of the starter and the respective Brixes of the starter and the stuck wine. You want to time it such that the starter Brix doesn't drop below that of your stuck wine. For example, if your wine is stuck at 5 percent RS you want to make sure you time your last addition to the starter before the starter gets much below 5 percent. However, if your starter is very slow and sluggish itself at 5 percent, it's best to let the starter keep going without adding more stuck wine and let the starter finish out as strongly as possible. It's not ideal, but you can always start up a second starter batch to ferment the remainder of the original stuck lot. At the very least you'll have fermented at least a portion of the stuck wine to dryness, even if you have to do a second batch. wbm

Further Reading

L.F. Bisson, "Stuck and Sluggish Fermentation" (AJEV 50:1, 107-199)

J. Eglinton and P. Henschke, "The Effect of High Concentration of Acetic Acid on the Restarting of a Stuck Ferment" (Australian and New Zealand Wine Industry Journal, 1999).