The Science of Oxygen Absorbers in Heat-Sealed Mylar

heat-sealed-mylar-bags

If you have spent any time researching long-term food storage, you have likely seen the iconic image: a five-gallon bucket filled with Mylar bags of rice and beans, ready for an emergency. 

You know that sealing mylar bags is the first step, but here is a question that separates beginners from experts: If the seal is airtight, why can't you just seal the bag and call it good?

The answer lies in chemistry. A perfect heat seal stops the exchange of gases from the outside world, but it does nothing about the air trapped inside with your food.

That trapped air contains roughly 21% oxygen, enough to slowly degrade your food, hatch insect eggs, and allow aerobic bacteria to thrive.

This is where oxygen absorbers enter the equation. In this deep dive, we will explore the chemistry of why sealing alone isn't enough, how oxygen absorbers work at a molecular level, and how to calculate the correct CC size for your storage needs.

At myboxprinting.com, we know that successful long-term storage requires both high-quality barriers and the right scientific approach. Let's get into the science.

The Problem with Trapped Oxygen

When you heat-seal a Mylar bag, you create a physical barrier that is impermeable to gases and moisture. This is incredibly effective at keeping external contaminants out. However, the air that was inside the bag at the moment of sealing remains trapped.

The Composition of Trapped Air:

• Nitrogen: ~78%

• Oxygen: ~21%

• Trace Gases: ~1%

That 21% oxygen is the enemy of food preservation. Oxygen is a highly reactive element. It causes:

Oxidation

Fats and oils go rancid when they react with oxygen.

Insect Activity

Many insect eggs require oxygen to hatch. Without it, they remain dormant or die.

Microbial Growth

Aerobic (oxygen-loving) bacteria and mold cannot survive in oxygen-free environments.

Vitamin Degradation

Essential nutrients, particularly Vitamins A, C, and E, break down rapidly in the presence of oxygen.

Simply put, a heat seal without oxygen removal is like locking your doors but leaving the windows wide open. The threat is already inside.

The Chemistry of Oxygen Absorbers

Oxygen absorbers are not desiccants (which remove moisture). They are small packets containing iron powder, activated carbon, sodium chloride (salt), and water.

The Oxidation Reaction

The magic happens through a simple but effective chemical reaction: rusting.

When the absorber is exposed to oxygen, the iron (Fe) inside oxidizes to form iron oxide (Fe₂O₃), commonly known as rust. The chemical equation looks like this:

4Fe + 3O₂ → 2Fe₂O₃

Here is what happens step-by-step:

Permeation

The packet material is designed to be highly permeable to gases. Oxygen from the air inside the Mylar bag passes through the packet's membrane.

Hydration

The salt and moisture inside the packet create an electrolytic environment that catalyzes the reaction.

Oxidation

The iron particles begin to rust. This process consumes the oxygen molecules, pulling them out of the surrounding air.

Pressure Drop

As oxygen molecules are converted into solid iron oxide, the partial pressure of oxygen inside the bag drops to near zero (typically below 0.01%).

The result is an environment with effectively no free oxygen. The nitrogen remains, but nitrogen is inert and does not react with food.

Why This Matters for Heat-Sealed Mylar

When you are sealing Mylar bags for long-term storage, the combination of the barrier properties of Mylar and the chemical scavenging of oxygen absorbers creates a "modified atmosphere."

Mylar keeps new oxygen from getting in.

Absorbers remove the oxygen that was already in.

This one-two punch is what enables shelf lives of 10, 20, or even 30 years for dry goods. Without the absorber, oxygen would slowly react with the food, and while the Mylar prevents more from entering, the damage inside would continue until the oxygen was depleted naturally, by which time the food might already be spoiled.

Calculating the Correct CC Size

One of the most common mistakes in sealing Mylar bags is using the wrong size oxygen absorber. Absorbers are rated by their nominal oxygen absorption capacity in cubic centimeters (CC). A "100cc absorber" is designed to absorb 100cc of oxygen.

However, you cannot just guess. You need to calculate the volume of air in your bag and account for the fact that air is only 21% oxygen.

The Basic Formula

Step 1: Calculate the Bag Volume
Measure the length, width, and height of the space inside your bag after it is filled.

• Volume = Length × Width × Height (in inches or centimeters)

Step 2: Convert to Cubic Centimeters (if using inches)
If you measured in inches, multiply by 16.39 to convert cubic inches to cubic centimeters (cc).

• Volume (cc) = (Length × Width × Height in inches) × 16.39

Step 3: Account for Oxygen Percentage
Since air is only 21% oxygen, multiply the total volume by 0.21.

• Oxygen Volume = Total Bag Volume (cc) × 0.21

Step 4: Add a Safety Margin
Absorbers are rated under ideal conditions (77°F / 25°C and high humidity). In reality, you should add a buffer of 50-100% to account for the air trapped between food particles and less-than-ideal conditions.

Recommended Absorber Size = Oxygen Volume × 2 (or more)

Practical Examples

Example A: Sealing a 1-Gallon Bag of Rice

A 1-gallon bag is approximately 10" × 14" when flat. When filled, the internal volume might be roughly 10" × 7" × 2" = 140 cubic inches.

Convert to cc: 140 × 16.39 = 2,294 cc total air volume.

Oxygen content: 2,294 × 0.21 = 482 cc of oxygen.

With a safety margin (×2): You need a 1000cc oxygen absorber.

Example B: Sealing a 5-Gallon Bag of Beans

A 5-gallon Mylar bag liner has an internal volume of roughly 18" × 14" × 5" = 1,260 cubic inches.

Convert to cc: 1,260 × 16.39 = 20,651 cc total air.

Oxygen content: 20,651 × 0.21 = 4,336 cc of oxygen.

With a safety margin, you need a 5000cc to 7500cc oxygen absorber (often achieved by using multiple 2000cc packets).

Common Absorber Sizes and Their Uses

Absorber Size (cc)	Typical Application
100cc	Spice jars, small herb bags, vacuum-sealed coffee
300cc	Quart-sized bags, 1-pound coffee bags
500cc	Gallon-sized bags (flour, sugar, rice)
1000cc	1 to 2-gallon bags, large bulk items
2000cc+	5-gallon buckets, 25+ pound bulk storage

Timing is Everything

Once you open a package of oxygen absorbers, they immediately begin working. They do not care if they are inside a Mylar bag or sitting on your counter; they will absorb oxygen from the air around them until they are saturated.

The Golden Rule: Only expose oxygen absorbers to air when you are ready to use them. Work quickly when sealing Mylar bags. Seal at least 90% of your bags before opening the absorber package, then add the absorbers and seal the remaining bags immediately.

If you have leftover absorbers, store them in a mason jar with a tight-sealing lid. You can even throw a visual indicator (like a piece of steel wool) in the jar to monitor if oxygen is present.

The Nitrogen Flush Alternative

For commercial applications, some manufacturers use nitrogen flushing instead of oxygen absorbers. This involves injecting nitrogen gas into the bag just before sealing to displace the oxygen-rich air.

However, for home preppers and small businesses, oxygen absorbers are superior because:

They require no special equipment.

They continue to scavenge oxygen released from the food itself over time.

They are significantly cheaper than nitrogen tanks.

Common Myths About Oxygen Absorbers

Myth 1: Oxygen absorbers remove all air.
False. They remove oxygen, leaving nitrogen behind. The bag may not look vacuum-sealed because the nitrogen remains, taking up space.

Myth 2: You don't need absorbers if you vacuum seal.
False. Vacuum sealing removes most air, but it cannot remove oxygen molecules trapped within the food's structure. Absorbers provide an extra layer of protection.

Myth 3: Bigger is always better.
False. While using a slightly larger absorber is fine (the excess iron just remains unused), using an absorber that is drastically too large is wasteful and expensive. More importantly, using an absorber that is too small leaves oxygen behind, defeating the purpose.

The Role of Bag Quality

The science of oxygen absorption relies entirely on the bag maintaining its integrity. If the Mylar bag has pinholes, weak seals, or is made from low-quality materials that delaminate, the oxygen-free environment will be short-lived.

Oxygen absorbers work hard to remove oxygen, but if the barrier is compromised, they will constantly be fighting against new oxygen seeping in, and they will eventually become saturated and fail.

This is why starting with high-quality materials is essential. When you purchase Mylar bags from a trusted supplier like myboxprinting.com, you are investing in a consistent barrier thickness and reliable sealant layers. This ensures that the low-oxygen environment you worked so hard to create stays that way for the long haul.

Conclusion: The Perfect Partnership

Custom Sealing mylar bags create the fortress, but oxygen absorbers remove the enemy within. The heat seal provides the physical barrier, while the chemical reaction of iron oxidation provides the atmospheric control.

Understanding the science behind this partnership allows you to

Choose the correct absorber size using proper calculations.

Avoid wasting money on undersized or oversized packets.

Achieve true long-term storage measured in decades, not months.

By combining high-barrier Mylar bags with precisely calculated oxygen absorbers, you create a preservation system that halts the clock on oxidation, insect activity, and microbial growth. For premium Mylar bags designed to support this scientific approach to storage, explore the custom options available at myboxprinting.com. Your food and your future self will thank you.