The 240° Myth: Botulism, Heat Resistance, and the Misunderstood Pressure-Only Mandate

Most educators emphasize that Clostridium botulinum spores require 240°F to be “killed.” But this oversimplified message creates confusion and ignores a critical scientific truth: thermal lethality is a combination of temperature and time—not just hitting a magic number.

In fact, spores can be inactivated at lower temperatures, including 212°F, if sustained for long enough under the right conditions. While pressure canning is the most efficient method, it is not the only scientifically valid method. This myth—when left unchecked—has caused confusion, unnecessary fear, and international misunderstandings.

A few years back I wrote an article, Understanding Botulism in Home Canning, which cited multiple sources and gave tangible proof with actual numbers demonstrating how infrequently humans are affected by botulism toxin, especially from home canned foods. Sadly, over the course of time since I published the article, government bodies have rerouted the links, canceled or buried the articles, or rewrote the article in its entirety.

That was the case with the CDC’s PDF on botulism (gone from the Internet) and the revised article from the CDC (revised and sterilized on April 25, 2024) which showed 18 years’ worth of data and broke down the number of cases and their causes. Why did the CDC remove this important collected data from their website? Likely because when we read it, critically think, and compare the numbers to the 300 million Americans, the fear dissipates and home canners grow confident in the art and craft of home canning food.

So, I am shedding light on this very important scientific data using the ORIGINAL research adopted by the USDA, which is the science and math we still use to this day when processing low-acid foods for long term storage. Click here so you too may read along and comprehend the data provided to us in Technical Bulletin No. 903 titled Home Canning Processes for Low-Acid Foods, Developed on the Basis of Heat Penetration and Inoculated Packs.

Here’s a breakdown of the valid science and math so you, my fellow home canner, have the facts…

ORIGIN OF THE MATH: THE 1940s WHITE PAPERS

In the mid-1940s, several independent food scientists and bacteriologists were conducting experiments on the thermal destruction of C. botulinum.

Many home canners mistakenly believe that “the government” created all canning guidelines in a vacuum, that some infallible panel of experts crafted the rules from a place of absolute authority. In truth, these standards came from the work of citizens: bacteriologists, food scientists, and researchers working in university labs (like UC Berkeley and Oregon State) and private facilities.

These were real people—Americans passionate about food safety—who ran rigorous tests, challenged assumptions, and shared their findings to protect families. The USDA didn’t invent the science; they adopted and disseminated it. That tradition of discovery and testing still happens today, though it’s often funded privately and less visible to the public.

These experiments conducted back in 1946 inoculated canning jars with known concentrations of botulinum spores, then processed them at various temperatures (often 240°F) for different lengths of time.

• After processing, they monitored:
  • Whether germination occurred
  • Whether toxin was produced post-storage
  • Whether the food remained organoleptically acceptable, that is, appealing in taste, smell, sight, and texture to evaluate the quality or characteristics of a food, substance, or product.
• These studies are where the concept of the 12-log reduction (botulinum cook) and F₀ values originated.

The scientists applied:

• D-values to measure how long it took to reduce the spore population by 90% at 250°F.
• Z-values to calculate how the required D-value changed when temperature varied.
• F-values to establish the cumulative heat exposure required for commercial sterility.
• All of this culminated in what we now call the botulinum cook standard:
  → 12D reduction at 121°C (250°F) = commercial sterility in low-acid foods.

HOW THE USDA GOT INVOLVED

• In 1946–1947, as home canning became a wartime and post-war necessity, the USDA began assembling these private studies into a federal guidance model. Much of this research was prompted by a rise in foodborne illnesses linked to advancements in commercial canning, which introduced new technologies and processing times without adequate safety validation. Home canners, observing and mimicking these commercial methods without access to scientific data, also began experiencing cases of spoilage and botulism. While the scientific community had focused almost exclusively on commercial safety protocols throughout the 1930s, no one had yet investigated what was truly safe for the individual canner. Bulletin No. 930 changed that by finally applying rigorous, spore-focused testing to household preservation methods.
• The USDA did not conduct most of the original thermal destruction research themselves, they adopted these white papers, standardized the data, and published guidelines based on the math. Hence bodies like the National Center for Home Food Preservation, created decades later, were established to archive and distribute the data, not to conduct new scientific studies.
• Contrary to popular belief, the NCHFP is not a research facility. They do not currently operate laboratories for testing new recipes or conduct original scientific research for home canning. Instead, their role is focused on maintaining access to historically validated guidance and promoting safe preservation practices based on prior research.
• The NCHFP is a collaborative effort between the federal government and land-grant universities, designed to preserve past findings, not necessarily to further the science of home canning.

Don’t believe me? The NCHFP website states so themselves…

“The National Center for Home Food Preservation is your source for current research-based recommendations for most methods of home food preservation.”

Their mission is to disseminate science-based guidance, but no mention of conducting new lab research.

“As part of the NCHFP mission a website was created to locate, review and disseminate research-based home food preservation information.” Confirming their role is curating and sharing, not creating original research.

Even their “About Us” page of their website outlines their timeline, where they state projects between 2000-2005 included applied research (refrigerator dill pickles, tomato salsa), not new research.

The National Center for Home Food Preservation serves as a repository and translator of existing science, not a developer of new home-canning methods. Its work ensures historical and peer-reviewed research is accessible—but it doesn’t test or validate new recipes in a lab. This critical differentiation should inform how we talk about the authority of “official guidelines.”

Which leads me back to this…

If the role of the NCFHP is to curate and share the research, why are they failing to share with us the full original thermal lethality research for low-acid food preservation?

THE SCIENCE OF THERMAL DESTRUCTION

While pressure canning is promoted as the gold standard for low-acid foods, the underlying science is this:

Thermal destruction is a function of both time and temperature—not temperature alone.

Remember the “Three Pillars” I often write and speak about? The three main pillars of home canning are time, temperature, and acidic value. They all play an interconnective role in keeping our food safe for long-term storage, especially when home canning. The research shows us that it is not one pillar alone which keeps us safe when preserving low-acid foods, it is the synergy between all three – time, temperature, and the overall acidic value of the recipe or food type.

What Does That Mean?

• Spores like those from Clostridium botulinum don’t require 240°F to be destroyed—they require a sufficient thermal dose, which can be achieved at lower temperatures if the time is extended or at 240°F for a shorter exposure of time.
• This principle is well established in food microbiology using D-values (time to reduce a spore population by 90% at a specific temp) and Z-values (how temperature changes impact the D-value).
• While pressure canning reaches 240°F, boiling water sits at 212°F, so to match the necessary spore inactivation, you must dramatically increase the processing time.

THE LETHALITY OF THE COOLING CURVE

In thermal processing, most people focus on the target processing temperature—like 240°F for pressure canning—as the moment of greatest microbial destruction. But the science laid out in Technical Bulletin No. 930 reveals something critical: the cooling period after processing is just as important in achieving total lethality.

After the canner is turned off and begins to depressurize, jars don’t instantly cool to room temperature. Instead, they go through a gradual decline in internal temperature that can last 30 minutes or more, depending on food density, jar size, and ambient conditions. This prolonged exposure to sub-boiling temperatures continues the work of microbial inactivation.

On pages 5–8 of the bulletin, the authors emphasize:

• That thermal lethality doesn’t stop at 240°F but continues well into the cooling phase.
• In fact, “lethal effects continue while the temperature is descending, particularly above 190°F.”
• The research measured heat retention curves in jars and confirmed that meaningful microbial death still occurs during this phase, especially for organisms that are more heat-sensitive than botulinum spores.

This means the entire curve of heating and cooling contributes to the F-value, or the total amount of lethality delivered.

Why It Matters for Home Canners

• Don’t rush the cool-down phase. Removing jars too early from the pressure canner or trying to speed cooling by placing them in cold drafts or water, undermines the intended heat exposure. It could also shock your glass jars causing them to break due to the vast temperature swing. Rushing the cool-down phase cuts short a vital part of the kill zone.
• Respect natural heat retention. Jars act as thermal batteries. The slow and steady descent in temperature is a silent guardian of safety, especially when dealing with dense foods where heat takes time to reach the center.

Think of the cooling phase as the second half of the race, not a cool-down lap, but the part where safety is sealed. The researchers showed that jars, even after the pressure drops, remain hot enough for long enough to extend the microbial death curve which is a crucial yet often overlooked part of thermal processing. This continuation ensures that the cumulative heat exposure, not just the peak, contributes to achieving commercial sterility.

SHELF STABILITY VS. SPORE DESTRUCTION

Shelf stability means food won’t spoil or support dangerous microbial growth under storage conditions. Home canning creates shelf-stable conditions by removing free oxygen from the jar and raising the temperature of the jar contents high enough for long enough to prevent germination and spoilage.

But pressure canning does not guarantee 100% spore inactivation. That’s why public health agencies advise reheating your low acid home canned food before consumption. They are accounting for human error, not scientific impossibility.

The research tells us pressure canning is a method to reduce risk, not a guarantee of sterilization.

Here’s how these facts are supported by science:

• USDA Technical Bulletin No. 930 demonstrates that spores are challenged under worst-case conditions—meaning that if home canning procedures aren’t followed, there’s still a chance surviving spores may germinate (TB 930, pp. 13–15).
• The authors utilized extended incubation during storage at high temperatures (e.g., 98.6°F for months) to ensure the jar contents are truly safe (TB 930, pp. 14–15).
• Modern USDA guidance itself clarifies that reheating before consumption is still recommended, even for pressure-canned low-acid foods, highlighting that sterilization is never 100% guaranteed outside of industrial retorts. And even then, commercial canneries are not impervious to mishaps. It is why we commonly see recalls on our purchased foods, both human and pet food.

EDUCATORS MUST DO BETTER

Teaching “240°F or die” is lazy science. It bypasses the principles of thermal kinetics. It excludes legitimate cultural methods used globally. Worse, it creates distrust when people discover valid alternatives (like 3-hour boiling methods). This distrust then fuels online hostility toward innovators and researchers as well as home canners in countries outside of America. I should know, I have been attacked for educating people about the science and math used to create these safe canning guidelines, and for using this applied science when creating new recipes.

“We should be teaching why 240°F works—not just that it does. And we should empower home canners to understand the full science, not weaponize it.” – Diane Devereaux, The Canning Diva®.

Real-World Global Example:

Pressure Canning vs. Water Bath Processing Times

Note: These longer boiling times have been successfully used in many cultures where pressure canners are not common and where newer canning resources are limited.

KEY TERMS & EQUATIONS FOR THERMAL PROCESSING

When reading through Technical Bulletin No. 930, you will notice the very math used during the research. For many of us, the math looks like Greek and may be hard to decipher. But not to worry, I have broken down the key terms and mathematical equations into digestible bites.

D-value (Decimal Reduction Time)

• Definition: The time (in minutes) at a specific temperature required to reduce a microbial population by 90% (1 log cycle).
• Symbol: DTD_TDT​
• Units: Minutes at a given temperature (e.g., D₁₂₁°C = 0.21 minutes for C. botulinum spores at 121°C)

The D-value, or Decimal Reduction Time, represents the number of minutes at a given temperature required to reduce a microbial population by 90% (or 1 log). For example, if a jar contains 1,000 spores of C. botulinum and is processed at a certain temperature for the D-value length of time, only 100 spores would remain. If the D-value is repeated, that number drops to 10, then 1, and so on. This logarithmic reduction explains why longer processing is required for foods with high initial loads or dense ingredients that slow heat penetration. In the bulletin, this concept is demonstrated clearly when over-inoculated pork jars were processed for different times. At 60 minutes, spoilage still occurred. At 90 minutes and above, no spoilage was detected even under incubated conditions of 98.6°F—validating that enough decimal reductions had occurred to reach commercial sterility (PDF pg. 14).

🔹 Z-value

• Definition: The temperature change (in °C or °F) required to change the D-value by a factor of 10 (i.e., reduce it by one log).
• Indicates the thermal resistance of the organism.
• For C. botulinum, the Z-value is typically 10°C (or ~18°F)

Equation:

The Z-value is the number of degrees Fahrenheit (or Celsius) required to change the D-value by a factor of 10. This shows how sensitive a pathogen is to increases or decreases in temperature. A low Z-value means the organism is highly sensitive to temperature shifts; a high Z-value indicates greater heat resistance. The scientists behind Bulletin 930 implicitly acknowledged the role of Z-values when they explored the impact of increasing the temperature from 220°F to 240°F and monitored how spoilage decreased sharply as the temperature rose—but also how longer times at lower temperatures (e.g., boiling water at 212°F) could still result in complete lethality, provided the jars were held long enough and the heat penetrated to the geometric center (PDF pg. 3).

🔹 F-value

• Definition: The total time (in minutes) required to destroy a specific number of organisms at a reference temperature, accounting for the entire process.
• For commercial sterilization, this is typically expressed as F₀, which uses 250°F (121.1°C) as the reference.

F₀ Equation (Lethality Integration):

Finally, the F-value is the total accumulated lethality delivered during processing. Think of it as the mathematical “receipt” showing how much bacterial destruction has occurred. F-value combines time, temperature, and Z-value to determine whether a process meets the threshold for safety. This is where the bulletin’s emphasis on post-processing cool-down and incubation becomes critical: the scientists continued recording temperatures until they dropped below 190°F (PDF pg. 15), recognizing that lethality continues during cool-down and contributes to the overall F-value.

KEY TAKEAWAY

The key takeaway is 240°F is not the requirement—thermal lethality is. Water bathing can achieve this if the time is correctly extended, and the conditions (pH, density, water activity) are managed properly.

Let’s dive deeper…

The white paper, USDA Technical Bulletin No. 930, reinforces that the lethality of a canning process is a function of both time and temperature, not just temperature alone. This concept is described using three fundamental microbiological measurements: D-value, Z-value, and F-value.

By using inoculated jars and simulating poor storage conditions, the authors of Bulletin 930 created a clear, measurable way to verify the effectiveness of their proposed processing times. They didn’t just guess, they applied the same scientific principles used today in commercial food processing and thermal validation.

Their work proves that lethality can be achieved at lower temperatures with longer times, and that safety is derived from a careful balance of temperature, exposure duration, and post-process handling—not a single magic number.

UNDERSTANDING THE CLOSTRIDIUM BOTULINUM

Now while I realize these facts might be upsetting to some of you, especially those who have become very familiar with parroting what the National Center for Home Food Preservation posts on their website, let’s not waste energy on being upset.

If anything, get excited because you are learning! And knowledge is power, transforming us forward free from innovative constraints. The best part, now that you know the facts, we can take a deeper, more scientific look at Clostridium botulinum and its three stages relevant to home canning safety:

1. the bacterium (vegetative cell),
2. the spore (dormant, heat-resistant form), and
3. the toxin (produced only under specific conditions by the bacterium).

Understanding how each of these behaves, survives, and is destroyed—or rendered inert—is crucial to mastering safe food preservation at home.

THE THREE STAGES OF C. BOTULINUM

Clostridium botulinum is not just a single threat. It operates in a cycle, transforming between forms depending on environmental conditions. To preserve food safely, especially low-acid foods like meats, beans, and vegetables, we must understand and interrupt this cycle at each critical stage. From the fragile vegetative cell to the dormant spore and finally the potent neurotoxin, each phase requires a different defensive strategy in home canning. Let’s break down what each stage is, how it behaves, and what measures we take to control it.

1. The Vegetative Cell – Easily destroyed by heat

This is the active, living form of the bacterium that can multiply and produce toxin if the environment is right (anaerobic, low-acid, moist, low-sugar, and stored above 70°F).
GOOD NEWS: This stage is easily destroyed by temperatures as low as 185°F for just a few minutes. In home canning, all vegetative cells are destroyed during the pressure canning process or even during water bathing (provided time and temp are correct).

In short: You destroy the living bacterium during processing.

2. The Spore – Heat-resistant and built to survive

A C. botulinum spore is not alive; it’s a dormant survival structure. It cannot be “killed” in the traditional sense; rather, it can be inactivated, meaning the protective proteins break down (denature), the core is compromised, and the bacterium inside can no longer germinate or produce toxin.

When environmental conditions are harsh (too hot, too cold, dry, acidic, etc.), C. botulinum protects itself by forming a spore. A spore a dormant, shell-like capsule that can survive boiling, dehydration, and more. These spores are not harmful on their own, but if they survive and later find the right environment, they can “wake up,” grow, and produce toxin.

What we aim to do in canning is to:

• Denature the spore’s protective coating,
• Prevent it from germinating by removing favorable conditions (via acid, sugar, oxygen, or cold storage), or
• Hold it at a high enough temperature for a long enough time to break down its structure (thermal destruction).

In pressure canning, this means exposing the spore to 240°F for a tested, recipe-specific time, or alternatively, to lower temperatures like 212°F for longer periods, as detailed in Technical Bulletin No. 930.

In short: We aim to neutralize the spore before it has a chance to germinate.

3. The Toxin – Deadly but fragile

The neurotoxin produced by C. botulinum is one of the most potent toxins known to man, but here’s the key:
It’s extremely heat-sensitive.
Heating food to 185°F for 5–10 minutes will destroy the toxin completely. That’s why the contents of a home-canned jar should always be boiled before eating, especially if you’re unsure of the storage conditions.

In short: If a toxin has formed, it can still be destroyed before consumption—if you reheat properly.

What About High-Acid Foods?

High-acid foods, including most fruits, pickles, jams, and naturally acidic tomatoes, play by different rules because Clostridium botulinum cannot grow or produce toxin in environments with a pH below 4.6. That acidity is your primary line of defense.

Why This Matters:

• Spores may still be present, but they cannot germinate in a properly acidified jar.
• The vegetative cells die during processing (often 10–20 minutes in a boiling water bath).
• Toxin cannot form in acidic conditions—so even if a spore survives, the chain is broken.

Because spores may survive but remain inert in these acidic environments, high-acid canning focuses more on preserving flavor and texture, while the acidity itself does most of the heavy lifting when it comes to food safety. We often use the phrase, “acid kills the botulism” but what we are really trying to convey is acidity prevents the growth of botulism toxin.

What Happens to C. botulinum Spores in High-Acid, Water-Bathed Foods?

Spores of Clostridium botulinum are incredibly resilient and can survive boiling water temperatures (212°F) for extended periods. However, in high-acid foods, survival of the spore is not the central safety concern—because the acid prevents it from germinating.

Key fact: C. botulinum spores do not grow or produce toxin in environments with a pH of *4.6 or lower.

Do Spores Survive Water Bath Canning?

Yes, some may. Water bath canning of high-acid foods is not intended to destroy the spores themselves—it doesn’t need to. Instead, it:

• Destroys vegetative bacteria, yeasts, and molds that could spoil the food.
• Creates a vacuum seal and heat-sterilized environment.
• Ensures shelf stability by denying the spores what they need to activate: a low-acid, low-oxygen, warm, moist environment.

As long as the food maintains its proper acidity, the spores that do survive water bath processing will remain inactive or dormant, unable to germinate or pose a threat.

But What If a Recipe Isn’t Acidic Enough?

That’s where problems arise. If a food, or recipe, is on the borderline of safe pH (like tomatoes or figs), and not enough acid is added (via lemon juice or vinegar), it may provide just enough wiggle room for a spore to germinate and produce toxin during storage.

I have been questioned repeatedly regarding my Basil Diced Tomatoes canning recipe. I get countless emails and messages from canners perplexed as to why I add lemon juice to this recipe despite the fact we use a pressure canner to process it.

Canning recipes are a synergy of time, temperature and the recipe’s OVERALL acidic value. By adding additional acid, we can decrease the processing time to 15/20 minutes in a pressure canner or 30/40 minutes in a water bather, without degrading the integrity of the foods keeping its texture palatable.

This is why:

• Tested recipes matter.
• Adding acid (like lemon juice in tomatoes) is often warranted.
• Storage conditions (cool, dark, under 75°F) are still important—even for high-acid foods.

What This Means for Home Canners Across the Globe

You don’t need to destroy every spore to stay safe. Instead, home canning success means:

• You’ve destroyed all vegetative cells (during processing),
• You’ve prevented spores from germinating (via acid, time/temp, sugar, or storage),
• And if all else fails, you’ve rendered any potential toxin harmless by reheating the food to at least 185°F.

This is why time, temperature, acid level, and proper storage conditions matter. These are the four walls of safety that prevent any stage of C. botulinum from completing its deadly cycle.

I am no longer speaking in terms thrown about online to match non-scientific language. Terms like “kill” are truly an improper understanding of why we are able to do what we do in our home kitchens. From here on out, I will only speak in proper terms outlined by the research, science and math; and you should too! No longer should we dumb down this important terminology. Let’s all get into the habit of using words like denature, vegetative cells and germination.

So, you too may elevate your comprehension of this research and grow comfortable using the proper terms, here is a guide I created to help us all grow together.

Click here for my downloadable PDF, A Modern Guide to Reading USDA Bulletin No. 930.