Fermentation for Distilling

1. Fermentation in a Distillery Context: What It Is and Why It Matters

Fermentation is the controlled conversion of fermentable sugars into ethanol, carbon dioxide, heat, and a wide spectrum of minor compounds.  In distilling, those “minor compounds” matter disproportionately because the still concentrates them, and the barrel transforms them.  A ferment is therefore both a manufacturing step and a design step: it determines not only how much alcohol is produced, but the shape of the spirit’s eventual aroma, texture, and aging trajectory.

The central products of yeast fermentation are ethanol and CO₂.  But yeast also produce organic acids, aldehydes, esters, higher alcohols (fusel alcohols), sulfur compounds, and countless trace metabolites.  Some of these are desirable in the right amounts.  Many are undesirable when yeast are stressed, underfed, overheated, infected, or forced to ferment in a hostile chemical environment.  Because distillation is selective rather than corrective, most of the distiller’s quality control is really fermentation control.

For grain-based spirits, fermentation sits downstream of conversion. Grain begins as starch; yeast cannot ferment starch.  The mash stage must convert starch into fermentable sugars, primarily via enzymes.  For molasses-based spirits, the sugars are already present, but nutrition and inhibitors often become the limiting factors.
In both cases, the distiller’s job is to deliver a liquid (or slurry) that yeast can ferment quickly, completely, and cleanly.

2. The Big Process Fork: Lautered Liquid vs On-Grain Fermentation

A distillery can ferment a clarified or semi-clarified liquid (lautered wort/beer) or ferment with solids present (on-grain).  This single choice reshapes fermentation through three mechanisms:  chemistry (what gets extracted into the liquid or bound by solids), physics (mixing, viscosity, heat transfer, foam and CO₂ behavior),  and biology (nutrient availability, yeast health, and microbial ecology).

2.1 Lautered Fermentation: What It Optimizes

Lautering removes most solids before the fermenter.  For distilling, this tends to optimize consistency and measurement.  Gravity readings are cleaner, pH readings are more representative, and temperature control behaves more predictably because the liquid is comparatively uniform.  Oxygenation is easier to deliver evenly, and nutrient additions disperse reliably.  Transfer lines and fermenters are easier to clean, and the risk of persistent biofilms anchored in grain solids is reduced.

Lautered ferments also tend to reduce certain harsh extraction outcomes associated with prolonged contact between liquid and husk material.  While the majority of husk-derived polyphenol extraction occurs during hot-side processing (mash and lauter),  keeping solids out of the fermenter shortens total contact time and reduces further extraction of woody, drying phenolics during the multi-day fermentation window.

2.2 On-Grain Fermentation: What It Unlocks (and What It Complicates)

On-grain fermentation keeps solids in the fermenter.  This can increase yield potential by reducing the sugar left behind in spent grain.  It can also intensify grain character: solids carry lipids, proteins, and flavor precursors that influence yeast metabolism and congener formation.  However, on-grain fermentation introduces operational complexity: viscosity increases, caps form, foam can become aggressive, and sampling becomes less reliable.  Temperature gradients are more likely, especially under a thick cap that insulates the fermenting mass from cooling surfaces.

On-grain fermentation also changes microbial dynamics.  Solids provide surface area and microenvironments where bacteria can persist.  If yeast do not dominate quickly, bacteria can create acids and off-aromas that reduce yield and alter flavor.  This does not mean on-grain fermentation is inherently “dirty”; it means it rewards fast starts, strong sanitation, and deliberate cap management.

2.3 The Decision Framework

If the primary goal is clean, repeatable, high-confidence fermentation—especially for lighter styles or neutral bases—lautered fermentation typically offers the most control.
If the goal is intensified grain expression, certain traditional textures, or maximum extract utilization, on-grain fermentation can deliver, but it demands tighter operational discipline.

3. Grain Anatomy, Husks, and Phenolic Extraction: Why Barley and Rye Behave Differently Than Corn

When distillers talk about “tannins,” “huskiness,” or “woody dryness,” they are usually describing the sensory effect of polyphenols and related compounds extracted from grain outer layers.
The magnitude and character of this extraction depends on the grain.  Corn behaves differently than barley; wheat behaves differently than rye.  The reason is anatomy.

3.1 Corn: Low Husk Influence, High Conversion Consequence

Corn does not contribute a barley-like husk tannin load.  Its fermentation outcomes are dominated more by conversion efficiency, oil fraction, and process-derived aromatics.
If corn ferments show harshness, it is more likely to be linked to yeast stress, excessive temperature, bacterial acids, or fusel alcohol production than to classic husk tannins.
This is why corn fermentation is often “forgiving” in the tannin sense but unforgiving in conversion: starch left unconverted will not ferment, and that lost potential becomes lost yield.

3.2 Wheat: Softer Phenolic Signature, Higher Foam and Protein Effects

Wheat is often perceived as soft and bready in the distillate.  It typically brings less husk-driven astringency than barley.  Operationally, wheat can contribute high protein levels and foam potential, especially in on-grain ferments.  Wheat proteins can also stabilize foam, which matters in sealed tanks where headspace is limited.  The “wheat problem” is usually not tannin; it is foam, cap behavior, and fermentation management under vigorous CO₂ release.

3.3 Barley: The Classic Husk Grain and a Major Polyphenol Source

Barley, especially malted barley, carries a substantial husk that can contribute polyphenols, phenolic bitterness, and astringency under certain conditions.
Most extraction occurs on the hot side—mash and lauter—where pH, temperature, and sparging determine how much husk chemistry enters the wort.
However, if husk solids remain in the fermenter (on-grain or high carryover), fermentation extends the contact window and can continue extracting husk character.
Fermentation’s gradually acidifying environment can change the extraction profile, but prolonged contact time, agitation, and fine particulate load still increase the chance of drying, woody notes.

3.4 Rye: Viscosity, Gums, and Spicy Precursors

Rye’s most notorious contribution is viscosity driven by beta-glucans and other gums.
This affects lauter efficiency, pumpability, and fermentation mixing.
Rye can also contribute spicy, earthy grain character and certain phenolic impressions.
In on-grain ferments, rye’s viscosity makes cap formation and stratification more severe, which can indirectly increase yeast stress and congener roughness if temperature and nutrient delivery are uneven.
Rye is less about “tea tannin” and more about a thick ferment environment that demands active management.

3.5 Practical Implication: Tannins Are Mostly Decided Before the Fermenter

For a distillery that lauters, the strongest controls over husk-derived astringency are mash pH, lauter temperature, sparge strategy, milling, and how much husk particulate is carried into the fermenter.  On-grain fermentation adds another extraction window.
Therefore, when comparing on-grain to lautered fermentations, it is useful to separate “hot-side extraction” from “fermentation contact.”  If hot-side extraction is already high, on-grain fermentation can amplify harshness.  If hot-side extraction is well controlled, on-grain fermentation may add complexity without excessive dryness.

4. Closed (Sealed) Fermentation vs Open Fermentation: Oxygen, Microbes, and Aroma Retention

Fermentation vessel design changes the fermentation environment.  A sealed, temperature-controlled fermenter does not necessarily operate under pressure,
but it does limit oxygen ingress, reduce exposure to airborne microbes, and provide the operator control over temperature curves.  An open fermenter, even if temperature-controlled, exposes the surface to ambient oxygen, ambient microbes, and more aggressive stripping of volatile compounds.  These differences change both risk and flavor.

4.1 Oxygen Exposure: Helpful Early, Harmful Late

Yeast require oxygen early in fermentation to synthesize sterols and unsaturated fatty acids that strengthen cell membranes.  Strong membranes improve yeast tolerance to ethanol and help the ferment finish completely.  Oxygen introduced at the wrong time, however, can increase oxidative chemistry and push yeast into stress pathways.
Open fermenters allow continuous surface oxygen exposure, while sealed systems encourage a deliberate approach:  oxygenate once at the start, then let CO₂ blanket the ferment.

4.2 Microbial Ecology: The Open Ferment Advantage and Cost

Open fermentation can introduce microbial complexity, sometimes sought in certain rum traditions or in specific whiskey styles where a slight bacterial contribution is part of the profile.  The cost is inconsistency and higher contamination risk.
In sealed fermenters, microbial exposure is reduced, and the distiller can rely on yeast dominance rather than hoping environmental conditions favor a desirable mixed culture.  For distilleries prioritizing repeatability and clean profiles, sealed fermentation is usually the rational default.

4.3 Aroma Retention and CO₂ Stripping

Open fermenters tend to lose more volatile aromatics to the atmosphere.
That can be beneficial if the goal is to vent sulfur compounds and reduce certain harsh volatile intermediates.  It can also reduce desirable esters that contribute fruity and floral notes.  Sealed fermenters retain more volatiles within the headspace and liquid, though CO₂ still strips aromatics through the blow-off system.  The practical implication is that open fermentation often produces a “lighter, more vented” aromatic profile, while sealed fermentation produces a “tighter, more retained” profile.

4.4 Temperature Control as the Real Differentiator

In many modern distilleries, temperature control has a larger impact than vessel openness.  Yeast metabolism is temperature-sensitive.  Higher temperatures accelerate fermentation and can increase ester production, but they also increase the risk of fusel alcohol formation and yeast stress.  Lower temperatures tend to produce cleaner profiles and lower fusel load but can extend fermentation time.  With temperature control, a sealed fermenter can be tuned to express more character without accepting open fermentation’s contamination risk.

5. The Fermentation Timeline: From Cooling and Transfer to “Ready to Distill”

Although specific timings vary by substrate, gravity, and yeast strain, most ferments follow a recognizable arc:  a lag phase, an exponential growth and fermentation phase, a deceleration phase, and a finish phase.  The distiller’s job is to set up the initial conditions so lag is short, exponential fermentation is healthy, and the finish is complete.  The still prefers a wash that is fully fermented, stable, and not actively foaming.

5.1 Cooling the Mash or Wash: Minimizing the Warm Exposure Window

After conversion (for grains) or dilution and preparation (for molasses), the liquid must be cooled to pitching temperature.  Cooling is a quality step, not a convenience step.
Warm sugar-rich liquid is an ideal environment for bacteria and wild yeast.
The longer the liquid remains warm, the greater the chance of infection, pH drift, and yield loss.  Efficient cooling reduces microbial opportunity and increases fermentation predictability.

5.2 Transfer Into the Fermenter: Sanitation and Uniformity

Transfer is where sanitation either holds or fails.  Pumps, hoses, gaskets, valves, sample ports, and manways must be clean and sanitary.  In sealed systems, the benefit of reduced exposure is only real if the entire transfer path is also clean.  Uniformity matters as well.  If the fermenter receives stratified liquid (hot at the top, cooler at the bottom), yeast performance becomes uneven.  For on-grain fermentations, uniformity is harder to achieve and usually requires deliberate mixing during and after fill.

5.3 Oxygenation: Building Yeast Cell Membranes for a Strong Finish

Yeast need oxygen early to build healthy cell membranes.  Strong membranes increase ethanol tolerance and improve finishing behavior.  In sealed fermenters, oxygenation is typically delivered at the start—inline aeration or oxygen sparging—followed by a CO₂ blanket that limits further oxygen ingress.  In open fermentation, oxygen enters continuously at the surface, which can support yeast growth but can also encourage oxidation and microbial competition.  The core concept remains: early oxygen is beneficial; late oxygen is usually harmful.

5.4 Pitching Yeast: Lag Time Is Risk Time

After oxygenation, yeast are pitched.  The pitch must be healthy and sufficient.
Underpitching increases lag time, and lag time is when bacteria can gain a foothold.
Underpitching also forces yeast to reproduce more before fermentation, which can increase stress and congener load if nutrients and oxygen are insufficient.
Overpitching can reduce yeast growth and sometimes reduce ester formation, but in distilling, robust pitching is often preferred because it increases reliability and suppresses infection.

5.5 The Lag Phase: Preparing the Cellular Machinery

In the lag phase, yeast acclimate to the wash.  They adjust membrane composition, begin nutrient uptake, and prepare enzymes for glycolysis and fermentation.  A short lag phase is desirable.  A long lag phase is often a symptom: low pitch rate, poor yeast health, temperature mismatch, insufficient oxygen, or an inhospitable pH.  For molasses washes, lag can extend if nutrients are inadequate or if inhibitors are present. We use Diammonium Phosphate (DAP).  For grain worts, lag is usually short when oxygenation and pitching are correct.

5.6 Exponential Fermentation: Sugar Uptake, CO₂ Release, and Heat

Once yeast begin active fermentation, CO₂ production becomes visible and measurable.
Yeast import sugars through membrane transporters.  Glucose and fructose are consumed readily.  In grain worts, maltose and maltotriose consumption depends on yeast strain.  In molasses, sucrose is often split into glucose and fructose by yeast invertase before uptake.

Inside the cell, sugars are metabolized through glycolysis into pyruvate, producing ATP and NADH.  Without oxygen, yeast regenerate NAD⁺ by converting pyruvate into acetaldehyde and then ethanol, releasing CO₂ in the process.  This biochemical pathway is why fermentation produces both ethanol and CO₂, and why fermentation also produces heat.  Fermentation is exothermic.  Temperature control must anticipate rising heat during peak activity.

5.7 CO₂ Behavior: Foam, Caps, and Gas Management

CO₂ is not just a byproduct; it shapes fermentation physically.  In liquid ferments, CO₂ bubbles provide mixing and strip volatile compounds.  In on-grain ferments, CO₂ lifts solids into a cap.  Caps can insulate, trap heat, and create localized environments that differ from the bulk liquid.  Rye and wheat ferments, especially on-grain, can form persistent caps and foam.  Sealed fermenters must have appropriate blow-off systems and headspace planning.  Open fermenters must manage foam-over risk.

5.8 pH Trajectory: Yeast Comfort vs Bacterial Suppression

pH affects yeast performance and microbial competition.  Yeast generally ferment well in mildly acidic environments.  As fermentation progresses, pH tends to drop due to organic acid production and dissolved CO₂ forming carbonic acid.  A moderate pH drop is normal.  A sudden crash can indicate bacterial activity or a stressed ferment. A high pH environment can encourage bacterial growth and increase husk polyphenol extraction during hot-side processing.  Therefore, pH is both a process control and a diagnostic signal.

5.9 Nutrients: The Difference Between “It Ferments” and “It Finishes Clean”

Yeast require nitrogen, vitamins (especially B vitamins), minerals (notably magnesium and zinc), and phosphate.  Grain worts often contain a reasonable nutrient package, especially when malted barley contributes free amino nitrogen.  High-adjunct mashes, however, can be less nutritionally complete.  Molasses washes are often nutritionally inconsistent: they may contain high minerals but insufficient usable nitrogen and vitamins.  Nutrient deficiency increases lag time, increases stress compounds, and increases the risk of incomplete fermentation.

The timing of nutrient addition matters.  Yeast need nutrients early for growth and for building the biomass required to finish fermentation.  Late nutrient additions can sometimes rescue a sluggish ferment, but they can also feed bacteria if yeast dominance has already weakened.  In sealed, temperature-controlled systems, nutrient strategy can be consistent and repeatable, which is one reason these systems often outperform open ferments in reliability.

5.10 Congeners: How Fermentation Conditions Create the Spirit’s “DNA”

Congener formation is not random; it is responsive to conditions.  Higher fermentation temperatures, rapid fermentation under stress, and nutrient imbalance increase fusel alcohol production.  Moderate temperatures, healthy yeast, and balanced nutrition produce a cleaner profile.  Esters—often perceived as fruity or floral—are formed from alcohols and acids and are influenced by yeast strain, temperature, oxygenation, and the availability of fatty acids.  On-grain ferments can increase fatty acid availability, sometimes increasing ester potential, but they can also increase stress if mixing and temperature control are insufficient.

Sulfur compounds often indicate stress.  In open fermentations, certain sulfur volatiles may vent more readily.  In sealed fermentations, sulfur management relies more on correct nutrition, correct yeast health, and allowing the ferment to finish rather than forcing a premature stop.  For distillers, the goal is not “zero congeners”; it is congeners that fit the style and remain controllable through distillation cuts and maturation.

5.11 The Finish: When Fermentation Stops Being Productive

As sugars diminish, fermentation slows.  Yeast transition from rapid growth into a maintenance mode.  Temperature often stabilizes and then begins to fall toward ambient as metabolic heat output declines.  CO₂ production slows.  At this stage, yeast may reduce certain intermediates such as acetaldehyde.  If the wash is held too long under warm conditions, however, bacteria can grow, and yeast can autolyze, creating off-notes.
The distiller’s goal is to identify completion accurately and proceed toward distillation or appropriate holding conditions.

5.12 “Ready to Distill”: A Practical Definition

A wash is ready to distill when fermentation has reached its intended endpoint and stabilized.  Practical indicators include:  a stable gravity reading over a meaningful interval,  a stable or declining temperature profile,  a pH that has stabilized rather than crashing,  and sensory evidence that sugar is depleted and the aroma is clean for the intended style.  For on-grain ferments, sampling must be standardized because solids distort readings.  The wash should not be actively foaming, and gas management should be stable.  The objective is not simply “bubbles slowed”; it is confirmation that the ferment has finished and is not developing faults during a hold.

6. Substrate-Specific Fermentation Considerations

The underlying biochemistry of yeast fermentation is consistent across substrates.
What changes is the composition of sugars, nutrients, inhibitors, viscosity, and precursor chemistry.  Corn, wheat, barley, rye, and molasses each present a different fermentation environment.  Understanding these differences allows a distiller to anticipate problems and deliberately shape character.

6.1 Corn Ferments

Corn ferments are often defined by conversion success.  If starch conversion is complete and the wort is nutritionally sufficient, corn ferments can be clean, fast, and high-yield.
Corn does not usually contribute husk tannins, so harshness is more often tied to temperature stress, nutrient deficiency, or bacterial acids than to polyphenol extraction.
Corn can ferment vigorously; temperature control helps prevent runaway peaks that increase fusel production.

6.2 Wheat Ferments

Wheat ferments can be vigorous and can foam heavily due to protein.  Wheat often produces a softer, rounder grain impression in the distillate.  On-grain wheat ferments demand headspace planning and foam management.  The husk-derived tannin risk is generally lower than barley, but cap behavior can still create temperature gradients that stress yeast if not managed.

6.3 Barley Ferments

Barley—especially malted barley—often provides the most complete nutrient  environment for yeast, which can support healthy, complete ferments.  The tradeoff is husk chemistry.  If mash and lauter conditions extract high levels of polyphenols, the wash can carry a drying, woody character.  Lautered fermentation limits additional extraction during fermentation.  On-grain fermentation can extend contact time and increase the chance of husk-driven astringency, especially if fine particulate carryover is high.

6.4 Rye Ferments

Rye ferments are dominated by viscosity.  Even with lautering, rye can create a challenging fermentation environment if the wort retains gums and fine solids.
On-grain rye ferments can form thick caps and trap heat and CO₂.  This increases the risk of uneven fermentation, local overheating, and yeast stress.  With strong mixing and temperature control, rye can produce distinctive spicy character without roughness.
Without those controls, rye can produce harsh fusel-heavy profiles that require aggressive cuts.

6.5 Molasses Ferments

Molasses contains fermentable sugars but often requires deliberate preparation.
High starting gravity increases osmotic stress on yeast.  Nutrient availability can be inconsistent; usable nitrogen and vitamins can be limiting even when mineral content is high.  Molasses can also contain inhibitory compounds depending on its source and processing.  As a result, molasses ferments benefit from a consistent dilution strategy, a consistent nutrient program, and temperature management.  The sealed vs open decision becomes a style lever: open fermentation can invite microbial complexity associated with “funk” in some rum traditions,  while sealed fermentation favors clean, repeatable profiles.

7. How Choices Drive Yield

Yield is not only a function of sugar concentration.  It is the fraction of fermentable sugar that becomes ethanol rather than being lost to incomplete conversion, incomplete fermentation, bacterial consumption, foam-over, or operational loss.
Process choices—lautered vs on-grain, sealed vs open, temperature-controlled vs uncontrolled—change yield by changing yeast health and the probability of failure modes.

7.1 Lautered vs On-Grain Yield Tradeoffs

Lautered fermentation tends to increase measurement accuracy and fermentation completion, improving yield consistency.  On-grain fermentation can improve extract utilization by keeping sugars with the ferment, but it can also increase losses via foam-over and incomplete fermentation in stratified environments.  In practice, the yield winner is whichever method the distillery can run more consistently with fewer deviations.  Control often beats theoretical potential.

7.2 Sealed vs Open Yield Tradeoffs

Open fermentation can increase variability and infection risk, both of which can reduce yield through bacterial sugar consumption and acid production.  Sealed fermentation reduces exposure and often produces more repeatable attenuation.  However, sealed fermentation also demands competent gas management and foam planning; a sealed tank with poor blow-off design can lose yield through foaming events.  Temperature control, in either system, is one of the highest-yield interventions because it reduces yeast stress and stuck ferment risk.

8. How Choices Drive Flavor

Flavor in distilling begins with fermentation-derived congeners and precursor compounds.  Decisions that influence yeast stress, oxygen exposure, nutrient status, and grain contact change the congener spectrum.  Distillation then selects and concentrates those compounds.  Maturation transforms them.  Therefore, fermentation decisions are not merely about “getting alcohol”; they are about defining the starting chemistry that the still and barrel will sculpt.

8.1 Lautered Fermentation Flavor Tendencies

Lautered ferments tend to produce cleaner profiles and more repeatable ester and fusel balance.  Grain character often presents as cereal sweetness rather than husk dryness.
Because the matrix is simpler, yeast are easier to keep within a narrow comfort zone.
For many whiskey styles and for neutral bases, this is an advantage.

8.2 On-Grain Fermentation Flavor Tendencies

On-grain ferments often increase perceived grain intensity and can support richer congener formation due to altered fatty acid availability and a more complex fermentation environment.  They can also increase phenolic dryness when husk contact is high, especially with barley.  Rye on-grain ferments can heighten spicy character but require strong control to avoid fusel roughness.  When managed well, on-grain fermentation can produce a robust, characterful distillate that feels “built” rather than merely “clean.”

8.3 Sealed vs Open Flavor Tendencies

Open fermentation often vents certain volatiles more aggressively and can introduce microbial contributions that increase acidity and ester potential.  Sealed fermentation retains more volatiles and limits microbial variability, producing a more controlled profile.  With temperature control and deliberate oxygenation at the start, sealed fermentation can be tuned to express style without relying on environmental randomness.

9. Common Failure Modes and the Controls That Prevent Them

9.1 Stuck or Sluggish Fermentation

Stuck fermentation usually arises from yeast stress or an unfavorable environment.
Common causes include insufficient pitch, insufficient early oxygen, nutrient deficiency, temperature mismatch, pH drift into an inhibitory zone, excessive starting gravity, or infection.  Lautered fermentation reduces certain physical causes (stratification and cap insulation), while on-grain fermentation increases them.  Temperature control reduces the risk of thermal stress and supports completion.

9.2 Excessive Foam and Cap Events

Foam events are both operational hazards and yield hazards.  Wheat and rye are frequent foam drivers.  On-grain fermentations amplify cap and foam risk.
Controls include headspace planning, antifoam readiness, temperature management to prevent runaway peaks, and mixing strategies that prevent CO₂ retention under caps.
In sealed systems, blow-off design must be robust enough to handle peak activity.

9.3 Harshness and Solventy Notes (Fusel Load)

Harshness is often a fermentation artifact: high fermentation temperature, nutrient imbalance, or rapid stress fermentation increases fusel alcohol production.
The simplest preventive control is managing the fermentation temperature curve, followed by ensuring yeast health through correct oxygenation and nutrition.
On-grain fermentation can increase fusel risk indirectly by creating temperature gradients and microenvironments; it does not inherently cause fusels, but it makes them easier to produce if control is weak.

9.4 Astringency and Husk Dryness

Astringency often begins on the hot side with husk polyphenol extraction.
Barley is the major driver.  Controls include mash pH management, careful sparging, and limiting fine husk particulate carryover.  On-grain fermentation extends contact and can amplify woody impressions.  Lautered fermentation shortens contact and reduces this risk, especially when combined with a gentle lauter strategy.

9.5 Sour, Rancid, or Vinegar Notes (Bacterial Acids)

Infection reduces yield and can distort flavor.  The most reliable defense is a fast yeast start, strong sanitation, and limiting exposure.  Sealed fermenters reduce exposure; open fermenters increase it.  On-grain fermentations increase bacterial footholds unless yeast dominance is established quickly.  pH monitoring helps detect abnormal trajectories early.

10. Putting It Together: A Distiller’s Practical Operating Philosophy

Fermentation choices are style choices and business choices.
A distillery that prioritizes high throughput, low variance, and consistent quality will naturally favor lautered, sealed, temperature-controlled fermentation with robust yeast health practices.
A distillery that prioritizes traditional textures or maximum grain intensity may choose on-grain fermentation, perhaps even open fermentation, and accept greater operational complexity in exchange for character.  Neither choice is automatically superior; the superior choice is the one that the distillery can execute repeatably at its quality standard.

In modern distilling, temperature control is the single most powerful stabilizer.
It allows the distiller to run yeast in their comfort zone and avoid stress pathways that generate harsh congeners.  Sealed fermentation increases repeatability and reduces infection risk, which is especially valuable when producing a consistent house profile.
On-grain fermentation can still be run cleanly in sealed, controlled tanks, but it requires intentional cap management and sampling discipline.

The broader truth is that every fermenter is a design studio.  The distiller chooses how much grain contact remains, how much oxygen yeast receive early, how stable the temperature curve remains, and how the microbial environment is controlled.
Those decisions determine not only the yield in the tank but the flavor in the glass years later.  Fermentation is where distillation becomes craftsmanship rather than mere separation.