Oxygen Transmission Rate (OTR) Comparison
Let’s get straight to the point: yes, for the vast majority of products sensitive to air, light, and moisture, custom resealable mylar bags are an exceptionally effective solution for preserving freshness. Their superiority isn’t just marketing hype; it’s rooted in the fundamental physical and chemical properties of the material and the design of the packaging itself. To understand why, we need to look at what actually causes products to degrade.
The primary enemies of freshness are oxygen, moisture, light, and contaminants. Oxygen leads to oxidation, which causes fats to become rancid and flavors to fade. Moisture can lead to mold growth, sogginess, or clumping. Light, especially UV light, can break down chemical compounds, degrading potency and color. Mylar, which is a brand name for a specific type of BoPET (Biaxially-Oriented Polyethylene Terephthalate), is engineered to be a formidable barrier against these elements.
The key metric for any barrier material is its Oxygen Transmission Rate (OTR), measured in cubic centimeters per square meter per 24 hours (cc/m²/24hrs). A lower number means a better barrier. Standard low-density polyethylene (LDPE) bags, like common plastic bags, might have an OTR of over 2,000 cc/m²/24hrs. In stark contrast, a typical 5.5 mil (140 micron) thick custom resealable mylar bags can have an OTR of less than 1.0 cc/m²/24hrs. This isn’t just a little better; it’s a difference of several orders of magnitude. The same principle applies to moisture vapor transmission rates (MVTR), where mylar also excels.
| Packaging Material | Typical Oxygen Transmission Rate (OTR) cc/m²/24hrs | Moisture Barrier | Light Barrier |
|---|---|---|---|
| Paper Bag | Extremely High (porous) | Very Poor | Poor |
| Standard LDPE Plastic Bag | > 2,000 | Fair | Transparent |
| Polypropylene (PP) Bag | 500 – 1,500 | Good | Transparent |
| Glass Jar (with seal) | ~0 (impermeable) | Excellent | Varies (opaque/clear) |
| Mylar Bag (5.5 mil) | < 1.0 | Excellent | Excellent (opaque) |
The Science Behind the Seal and the Reseal
The initial seal is critical. High-quality mylar bags are heat-sealed, creating a permanent, airtight fusion along the top. This is far more reliable than a simple fold-over or a twist-tie closure, which always allows microscopic amounts of air to seep in over time. However, the “resealable” feature is what truly sets these bags apart for products that are consumed incrementally, like coffee, snacks, or supplements. A robust zipper track, often made from polyethylene, is integrated into the bag. When this zipper is properly pressed closed, it re-establishes a barrier that, while not as perfect as the original heat seal, is dramatically more effective than rolling the top of the bag down and clipping it.
This combination addresses the two phases of a product’s life: long-term storage and daily use. The heat seal protects the product from the moment it’s packaged until the first time a customer opens it, ensuring it arrives fresh. The resealable zipper then becomes the first line of defense, significantly slowing down the degradation process each time the bag is opened and closed. For context, after opening a bag of roasted coffee beans, they can go stale in a matter of days if left in a paper bag. In a resealable mylar bag with the air pressed out, they can retain their peak freshness for several weeks.
Material Composition and Layering
It’s a common misconception that mylar bags are just a single layer of foil. In reality, they are sophisticated laminates, typically consisting of three or more layers, each serving a distinct purpose. This multi-layer construction is the secret to their performance.
- Polyester (Mylar®) Layer: This is the outer layer. It provides incredible tensile strength, making the bag puncture-resistant and durable. It’s also this layer that gives the bag its characteristic metallic sheen and acts as an absolute barrier against light and gases.
- Aluminum Layer: Sandwiched in the middle is a microscopically thin layer of aluminum. This is the primary barrier. It is completely impermeable to oxygen, moisture, and light. The vacuum deposition process used to apply it ensures there are no pinholes, creating a continuous shield.
- Polyethylene (PE) or Cast Polypropylene (CPP) Layer: This is the food-grade inner layer that makes contact with your product. It has two key functions. First, it provides a sealable surface—this is the layer that melts during the heat-sealing process to create the hermetic closure. Second, it is chemically inert and safe for direct food contact.
This “triple-ply” structure combines the strength of polyester, the impermeability of aluminum, and the sealability of polyethylene into one superior material. The thickness of these layers, measured in mils (thousandths of an inch) or microns, directly impacts durability and barrier properties. A 3.5 mil bag is good for lightweight items like tea, while a 7.0 mil bag is better suited for sharp, heavy products like coffee beans or granola.
Applications Across Industries: A Data-Driven Look
The effectiveness of mylar bags isn’t theoretical; it’s proven across diverse sectors. The data on shelf-life extension is particularly compelling.
Food & Beverage: For roasted coffee, exposure to oxygen is the fastest way to stale. Studies show that coffee stored in barrier packaging with an OTR of less than 1-2 cc/m²/24hrs can maintain its freshness for 6-12 months. In contrast, coffee in a simple valve bag (which only allows CO2 to escape but is not a strong oxygen barrier) may see significant quality loss in just a few weeks. Similarly, for dehydrated foods like jerky or dried fruits, moisture is the enemy. Mylar bags with an MVTR of less than 0.1 g/m²/24hrs prevent rehydration, preventing mold and preserving texture for up to several years when combined with oxygen absorbers.
Pharmaceuticals and Nutraceuticals: Many active ingredients in medicines and supplements are highly sensitive to humidity and oxygen. For example, the potency of certain vitamins can degrade rapidly in poor conditions. Packaging in a high-barrier mylar pouch is a standard practice to ensure the product delivers the promised dose throughout its entire shelf life, which is often 2-3 years.
Cannabis and Hemp: This industry provides a clear case study. The key compounds, THC and CBD, along with the aromatic terpenes, are highly volatile and degrade with exposure to light and air. Properly cured and stored in mylar bags, cannabis can preserve its potency, flavor, and aroma for over a year. Glass jars are also used, but mylar offers advantages in cost, weight, and durability.
The “Custom” Advantage: Beyond Just Preservation
While the preservation science is critical, the “custom” aspect of these bags delivers the business and marketing benefits. A generic, clear plastic bag signals a commodity. A custom-printed mylar bag with your logo, brand colors, and compelling product messaging transforms the package into a powerful marketing tool on the shelf and in the customer’s home.
Customization allows for:
- Brand Recognition: Consistent, high-quality packaging builds brand equity.
- Consumer Trust: A professional package signals a quality product. It includes essential elements like UPC barcodes, ingredient panels, and usage instructions directly printed on the bag, which are legally required for many products.
- Functionality: You can choose the exact size, shape, and zipper type (e.g., single-track, double-track for extra security) that best suits your product. You can also integrate one-way degassing valves for coffee beans, which allow carbon dioxide to escape without letting oxygen in.
When evaluating if custom resealable mylar bags are the right solution, you must weigh the initial per-unit cost against the tangible value they create: extended shelf life (reducing waste and returns), enhanced brand perception, and the ability to command a higher price point for a premium-preserved product. For any business serious about quality and longevity, the investment is not just in a bag, but in the integrity of the product inside. The data on barrier protection and shelf-life extension makes a compelling, fact-based case for their use.