The Winemaker’s Blind Spots

Imagine that first set of grape lugs that hit the crush pad floor. Employees stack them up, one lug at a time, bursting with the most pristine looking fruit you could have imagined for this season. 

You can smell the aromatics wafting in the air. 

The sound of the crush equipment is getting warmed up. Your cellar hand is throwing in the last bit of acidulated sulfur dioxide sanitizer … all ready to receive the grapes. 

One lug goes into the crusher/destemmer. Now two, three… ten lugs! Your team starts working in synchronized movements, maintaining good efficiency. 

The grapes get thoroughly crushed. They look beautiful! 

You taste a small portion of that sweet, sweet juice…

But wait!

What is this?

Something lingers in the back of your throat. 

What is that? You think

You take another taste.

Your mind races as the grapes continue to pour through the crusher/destemmer. Maybe it’s nothing. You walk back over to the lugs. Everything looks okay. 

And then it hits you.

The grapes aren’t perspiring. Your hand reaches out to touch the top level of grapes floating at the surface of the lugs. 

They’re warm. 

Actually, not just room temperature warm. Warm-warm. 

Is that okay? You wonder. 

I guess that is the question: is it okay that the grapes are warm by the time they hit your crusher/destemmer? Or did you just detect a grape flaw that is about to turn into a wine flaw which will ultimately become… a problem wine?

THIS is a common scenario I face with winemakers every year. And often, the questions I get posed back at me are related to how could something have gone so wrong? And surely, it must not have been the winemaker’s fault.

Often “fault” for an issue is a bit more complex than pointing fingers. Most often, it’s linked to a specific habit that has felt “standard” for years, but is usually related to poor winemaking habits that no one has corrected for far, far too long. Let’s take a look at some of the most common scenarios:

The Fundamentals of Grape Storage

The age-old saying in the wine industry is to pick grapes in the dead of night when temperatures are at their coldest. This holds especially true in regions that experience significant diurnal temperature swings during harvest — those cool nights naturally slow spoilage microorganisms from taking over the fruit, preserve delicate flavor and aroma compounds from natural degradation processes, and reduce oxidation before the wine is even made.

But the reality for most U.S. growers and wineries is far less ideal. 

Many regions lack meaningful day-to-night temperature shifts, and an even greater challenge is the shortage of labor needed to hand-pick grapes quickly and efficiently. Add in harvest-season heat, high humidity, and grapes that may travel several hours before reaching the crush pad, and you have a recipe for rapid fruit deterioration.

So what can a winery do? 

Get back to the ideal: store grapes cold. Yes, refrigeration cold. Yes, that means cold storage rooms and refrigerated trucks, and yes, those come at a cost. This is exactly why grape logistics need to be planned well before harvest begins. It’s also worth remembering that the larger the grape holding container — think one-ton macrobins versus standard lugs — the harder it is to cool the fruit to its core, leaving a warm center vulnerable to all of the risks above.

Leaving grapes sitting out on an open crush pad or processing space at ambient temperatures is simply not acceptable commercial practice. It accelerates spoilage, degradation, and oxidation before fermentation even begins, and it can attract pests and rodents. Will you commonly see wineries or growers do this anyway? Perhaps. But what’s at risk? Spoiled fruit before you even add the yeast. Is it worth it? From my perspective: watching all the work that went into keeping that fruit disease-free and ripe, I would say no. It is not worth losing quality at the point of harvest. 

Juice Handling and Temperature Requirements

During juice settling, the recommended storage temperature is refrigeration (40–45°F). This isn’t arbitrary: above 45°F enters spoilage territory from a food safety standpoint for any perishable product. For white and rosé grape juice, 55-65°F is actually an optimal growth temperature for many microorganisms that naturally occur on grapes, including acetic acid bacteria which are all over your fruit and throughout the winery. Despite what some winemakers assume, adding sulfur dioxide to juice during settling provides little protection — most of it will bind quickly, rendering its antimicrobial properties ineffective.

Once juice is inoculated for primary fermentation, 65°F is generally appropriate for many wine styles. But during juice settling and clarification, that temperature becomes a liability, particularly if the process is taking longer than 4–6 hours.

What can you do if 65°F or higher is your normal settling temperature?

• Plan before harvest begins. Identify gaps in your production before grapes hit the crush pad. Most harvest problems are foreseeable. Address them early.
• Don’t fear cold inoculations. Many winemakers hesitate to inoculate juice or must at 50–60°F if it falls outside their yeast strain’s marketed ideal range. In practice, many inoculations succeed within that range. When in doubt, check with your yeast supplier before inoculating as they’re typically generous with technical guidance.
• Consider a quick-settling enzyme. Some pectinase enzymes can clarify juice faster at warmer temperatures. Keep in mind that if the enzyme goes into warm juice in the morning, you’ll likely need to rack and inoculate by early evening the same day so timing matters. Again, planning is your friend.
• Rent cold storage. This option is becoming increasingly popular as grape delivery logistics grow more unpredictable and winery labor remains limited. A rented cold storage room at 40–45°F gives you flexibility, reduces extended hours, and takes some of the stress out of an unpredictable harvest season.

Volatile Acidity Can Accumulate at All Processing Stages

Many winemakers take volatile acidity (VA) for granted, assuming they’ll detect it sensorially before it becomes a problem. The flaw in that logic? The recognition threshold for acetic acid — the point at which 50% of the population can detect and identify it — sits above the TTB allowable limit in wine. By the time you smell it, you’re already over the allowable limit.

This may explain why so many winemakers come to me with wines well above TTB allowances shortly after primary or malolactic fermentation, puzzled about where it came from. In most cases, they also haven’t been measuring VA throughout production.

The reality is that VA can accumulate at virtually every stage of the wine making process, from the vineyard to the bottle. Measuring it monthly isn’t just good practice — it provides an ongoing window into whether spoilage is occurring and what may be driving it.

VA can accumulate at the following stages:

• In the fruit before harvest, due to grape diseases, molds, or mildews such as sour rot. Aggressive cases are detectable on the vine before picking.
• During grape storage, if temperatures are too warm or grapes are held too long before fermentation begins. Cold soaks carry this same risk. This scenario is what the introductory story features and I see it happen every year.
• During primary fermentation, particularly if the incoming fruit already carried elevated VA, if fermentation struggled, or if the yeast strain selected is a higher acetic acid producer.
• During malolactic fermentation (MLF), where the warm temperatures required for MLB growth also create ideal conditions for acetic acid bacteria. Controlling exogenous factors to favor MLB over acetic acid bacteria is critical during this stage. (DGW Insider and Elite Members can access The Dot Races Winemaking Lesson for a deeper dive on best MLF practices.)
• During wine storage, typically driven by oxygen exposure, ambient or warm storage temperatures, and insufficient free SO2 levels.

Consistent VA monitoring does two things: it catches problems early and it reveals patterns that help you refine your practices over time. Without it, you’re reacting rather than preventing. DGW Insider and Elite Members can access the Wine Analysis Expected Results Production Guide — a thorough reference for what your enological numbers should look like from harvest to bottle so you know when you are safe and when you may need extra guidance.

Sanitation Is Primarily Cleaning

There’s a saying in the wine industry that good winemaking is 90% cleaning/sanitation, 8% paperwork, and 2% actual winemaking. It’s only a slight exaggeration.

Effective sanitation starts with one non-negotiable: a visually clean surface. You cannot properly sanitize something that isn’t clean first — whether it’s a tank interior, a hose line, or a press. If that distinction isn’t immediately clear, the Winemaking Lesson Winery Sanitation Steps that Work! is worth revisiting. It includes visuals that drive home exactly why surface cleanliness is the foundation of everything else.

The hardest part of improving sanitation habits is convincing people that their version of “clean” often isn’t clean enough. It’s one of the most consistent issues I see across the industry:

• Harvest equipment with visible residue.
• Press interiors and hose lines that remain sticky to the touch.
• Old rice hulls left to mold inside presses.
• Drains crusted with dead fruit flies.
• Juice and wine dripping down the exterior of tanks.
• Outdoor debris scattered across winery floors.
• Spilled wine ingredients (i.e., sugar, fining agents, yeast hulls… you name it!) spilled onto the cellar floor and never cleaned up until a more convenient time.
• Opened bags that are not sealed shut.
• Piled plasticware left to soil until there is “more time to clean it thoroughly.”
• Dirty walls, ceilings, and windows.
• Dusty benches and fixtures.

Sound familiar? If any of these hit close to home, the fix isn’t a better sanitizer — it’s more time spent cleaning. While this isn’t the most romantic part of winemaking, it is one of the most essential, and likely the one that will help reduce some wine contamination risks that circle through your cellar every day without you thinking about it.

Not sure if your facility meets the mark? Contact Denise at [email protected] to schedule a virtual winery audit and get an honest assessment of where your sanitation practices stand.

Malolactic Fermentation (MLF) is Trickier Than You Think

It’s easy to assume MLF is as straightforward as primary fermentation, but the two are fundamentally different. Lactic acid bacteria (LAB), the microorganisms responsible for MLF, must thrive in a high-alcohol, high-acid environment that is inherently hostile to their growth. That makes MLF one of the more delicate and unforgiving fermentations a winemaker manages.

Success comes down to optimizing the wine environment for bacterial growth: minimizing oxygen exposure, maintaining ideal temperatures for your selected bacteria strain, and eliminating competing spoilage microorganisms. These strategies are covered in depth in the October 2024 Winemakers’ Blogpost: 5 Tips for MLF Success — worth a read before your next MLF inoculation. 😉

By the time most winemakers reach out to me about MLF problems, the window for an easy fix has already closed. VA may have climbed significantly, the wine may show signs of spoilage, or a film has developed that makes it nearly impossible for LAB to outcompete other microorganisms. And anyone who has attempted an MLF restart knows just how frustrating and unreliable that process can be.

The lesson here is simple: don’t get lazy with monitoring. Harvest season gets busy, but skipping MLF checks is a costly shortcut. Since MLF can take a month or more to complete, weekly monitoring is the minimum standard. Three analytical tools worth mentioning for MLF monitoring:

• Paper chromatography is the workhorse option — affordable, easy to use, and capable of tracking multiple wines simultaneously. For a primer on how to use it, refer to the DGW Winemaking Lesson: The Dot Races.
• The Sentia offers a more precise read on malic acid degradation over time, provided your initial malic acid concentration falls within its specs. It’s user-friendly, though the cost may be a consideration for smaller operations.
• Enzymatic assays offer a full picture for the winemaker. Enzymatic assays can measure the exact amount of malic acid that is in the wine at the start of MLF and slowly show it’s depletion over time. The difficult part here is having a skilled cellar staff that can effectively execute the analysis and measure results.

Have you been caught short cutting any of these steps? You aren’t alone, but you can get help to improve!

Ready to have that expert voice in your corner? DGW Elite Membership connects you directly with seasoned winemaking expertise twice monthly—whether you’re wrestling with a specific fermentation crisis or want to dive deep into the broader principles that will elevate your entire program. Because the difference between a good vintage and a great one often comes down to having the right guidance at exactly the right moment.

The views and opinions expressed through dgwinemaking.com are intended for general informational purposes only. Denise Gardner Winemaking does not assume any responsibility or liability for those winery, cidery, or alcohol-producing operations that choose to use any of the information seen here or within dgwinemaking.com.