Custom Coffee Bags | Material, Seal Tech

Effective custom coffee packaging relies on engineering a 3-ply or 4-ply laminated structure, typically ranging from 100 to 140 microns in thickness, to balance physical durability with critical barrier performance.

For maximum freshness retention, incorporating a 7-micron aluminum foil layer is essential, reducing Oxygen Transmission Rates (OTR) to <0.01 cc/m²/24h and effectively extending shelf life up to 18 months, while modern sustainable alternatives utilize Mono-material PE with EVOH coatings to achieve recyclable high-barrier protection.

These advanced materials are complemented by one-way degassing valves that vent carbon dioxide without allowing air ingress, requiring precise heat sealing at 120°C–160°C to ensure hermetic integrity throughout the supply chain.

Material

Customized coffee bags generally adopt a multi-layer lamination process (Laminated Films), with a standard total thickness usually between 100-160 microns.

To effectively block oxygen and moisture that cause coffee flavor degradation, the industry standard structure is mostly PET (printing layer) + AL/VMPET (barrier layer) + LLDPE (heat seal layer).

Among them, a 7 microns thick pure aluminum foil layer (Alu Foil) performs best, controlling the Oxygen Transmission Rate (OTR) to <0.01 cc/m²/24h, supporting a shelf life of 18-24 months.

In contrast, Mono-material recyclable (Mono-PE) bags that meet the requirements of European and American retailers usually rely on EVOH high-barrier coatings, with an OTR of approximately 1.5-3.0 cc, which are more suitable for short-term sales products with a turnover period within 3-6 months.

Material Structure

Outer Layer

The main task is to carry ink (display patterns) and resist high temperatures (prevent damage during sealing).

• PET (Polyester Film)
  This is the most commonly used outer material, with a standard thickness of 12 microns.
  Its heat resistance is excellent, with a melting point around 260°C. When you use a heat-sealing knife at 160°C – 180°C to press the bag mouth, PET will not deform or stick to the knife head. Moreover, its surface tension after treatment can usually reach 42-50 Dyne, ensuring strong ink adhesion and the highest clarity for printed patterns.
• BOPP (Biaxially Oriented Polypropylene)
  Some products seeking a “soft touch” or lower cost will use BOPP, with a standard thickness usually between 18-20 microns.
  It feels softer than PET and is easier to tear. However, its disadvantage is poor heat resistance, with a melting point of only around 160°C. If the temperature is not controlled well during bag making, the edges of the bag mouth are prone to wrinkling or shrinkage.
• Matte Varnish and Soft-touch Varnish
  If the bag you see has a non-reflective matte effect, it is usually not a change of film, but a layer of 2-3 microns thick Matte Varnish coated on the PET surface. If it is the kind of “tactile film” that feels astringent like skin, it is achieved through a special coating process, but this will increase film costs by 15-20%.

Adhesives

During the lamination process, an adhesive (Adhesive) needs to be coated between the two layers of film. Industrially, there are mainly two types:

• Solvent-based adhesive:
  The adhesive contains solvents such as ethyl acetate. After coating, it needs to pass through an oven to volatilize the solvent. If drying is incomplete and residual solvent exceeds 5mg/m², the coffee will have a pungent chemical smell.
• Solvent-free adhesive:
  This is currently the mainstream requirement in the European and American markets. It is a mixture of two components (base agent + curing agent) that bond directly through a chemical reaction, without solvent volatilization issues, ensuring higher food safety.
  The standard adhesive weight is usually 1.8 – 2.5 gsm. If too little adhesive is applied, the bag is prone to delamination when dropped; if too much is applied, the bag becomes stiff and brittle.

  Note: The laminated rolls cannot be made into bags immediately; they must be placed in a curing room at 40°C – 50°C for 24-48 hours to let the adhesive fully cure (Curing), otherwise the sealing strength will be insufficient.

Middle Layer

This is the physical barrier that determines whether coffee beans can last for 3 months or 18 months.

• Pure Aluminum Foil (Alu Foil)
  This is the ceiling of barrier properties. It is an extremely thin sheet of metallic aluminum, with a standard thickness of 7 microns.
  Because it is solid metal, any light, oxygen molecules, and water molecules cannot pass through. In test data, its Oxygen Transmission Rate (OTR) and Water Vapor Transmission Rate (WVTR) are theoretically 0.
  However, aluminum foil has a physical characteristic called “Flex Cracking.” If you rub or fold the bag many times, the 7 microns aluminum foil layer may develop invisible pinholes (Pinholes), and the barrier property will decrease. Therefore, pure aluminum foil bags are usually paired with a thicker inner PE for support.
• Metalized Film (VMPET)
  This is formed by vaporizing high-purity aluminum wire at high temperatures in a vacuum and depositing it onto the surface of a PET film.
  Its aluminum layer thickness is only 300-500 Angstrom, which is about 0.03-0.05 microns, only a few hundredths of pure aluminum foil.
  Although thin, it can block more than 98% of ultraviolet rays and oxygen. For commercial coffee beans in supermarkets with fast turnover (consumed within 3-6 months), the barrier performance of VMPET is completely sufficient, and the cost is 20-30% lower than pure aluminum foil, with better flex resistance than pure aluminum foil.

Inner Layer

This layer has two functions: first, it directly contacts food and must be Food Grade; second, it is responsible for Heat Seal to seal the bag.

• LLDPE (Linear Low-Density Polyethylene)
  This is the most common inner layer material. It also has the greatest thickness, usually between 60 – 100 microns. The overall thickness and stiffness of the bag are mainly supported by this layer.
  Its Seal Initiation Temperature (SIT) is usually between 105°C – 115°C. Once heated to this temperature, it melts and softens, and after pressing the two sides together and cooling, they fuse into one.
• Metallocene PE (mPE)
  This is a high-performance PE. Its molecular structure is more regular, and its tensile strength is more than 20% higher than ordinary PE.
  More importantly, it has “Hot Tack through Contamination.” If coffee dust sticks to the bag mouth, ordinary PE may not seal firmly, causing leakage; whereas mPE can still seal very tightly even when contaminated with dust. For automated filling lines, this is a very critical specification.
• Easy Peel Film
  Some outer bags for drip coffee can be torn open neatly by hand without scissors. This is because Polybutene (PB-1) is mixed into the inner layer or a special co-extrusion process is used to reduce the transverse tear strength of the film, allowing the tear to follow a straight line instead of being messy.

Barrier Layer Differences

Pure Aluminum Foil

Thickness Specifications:
Standard aluminum foil thickness for coffee bags is usually 6.35 μm (0.00025 inch) or 7 μm. For very large packaging (such as over 2kg), it may increase to 9 μm to improve puncture resistance.

Absolute Barrier Theory and Pinholes:
Theoretically, a solid metal sheet of any thickness has zero transmission for gas and light. However, at an extremely thin thickness of 7 μm, aluminum foil will inevitably produce microscopic pinholes during the production rolling process.

• Data Reference: According to industry standards, the allowable pinhole density for 7 μm aluminum foil is usually < 200 units/m².
• Lamination Repair: This is why aluminum foil cannot be used alone. By laminating it with PET (outer layer) and PE (inner layer), the adhesive and plastic film will fill or cover these micropores, and the final product’s OTR can still be maintained at < 0.005 cc/m²/24h (ASTM D3985 standard), close to the lower limit of instrument detection.

Dead-fold Property:
Aluminum foil has metal memory. When you fold it, it stays folded and does not spring back. This property is known as “dead-fold.”

• Advantage: For Side Gusset Pouches or Block Bottom Pouches, the aluminum foil layer helps the bag maintain a square and upright shape, providing excellent shelf display.
• Disadvantage: Poor flex resistance. In the Gelbo Flex Test (ASTM F392), after 50 cycles, the pure aluminum foil layer is prone to fracture lines, leading to a sharp decline in barrier performance.

Metalized Film

Metalized film is not a metal sheet, but is formed by vaporizing high-purity aluminum wire in a vacuum environment so that its atoms deposit on the surface of a plastic film (usually PET).

Metal Layer Thickness:
Compared to 7 μm pure aluminum foil, the thickness of the metalized layer is only 300 – 500 Å (Angstrom), which is 0.03 – 0.05 μm. This means the metal consumption of pure aluminum foil is 150-200 times that of metalized film, which is the root cause of the huge cost difference between the two.

Optical Density (OD):
The OD value of high-quality VMPET should be controlled between 2.0 – 2.5.

• OD < 1.8: The metal layer is too thin, light shielding is insufficient, and ultraviolet rays easily penetrate, accelerating the oxidation of coffee bean oils.
• OD > 2.8: The metal layer is too thick, leading to a decrease in Bond Strength between the metal and the PET substrate, which can cause delamination after lamination.

Flex Resistance Advantage:
Although the initial barrier property of VMPET (OTR ≈ 0.8-1.5 cc) is not as good as pure aluminum foil, it has excellent flexibility. When squeezed or dropped during long-distance transportation, the metalized layer deforms along with the PET film and is less likely to undergo brittle fracture. For commercial coffee beans with a turnover period within 6 months, VMPET is often a more cost-effective choice.

Transparent High Barrier

Chemical Nature:

AlOx (Aluminum Oxide): Essentially a transparent sapphire component.

SiOx (Silicon Oxide): Essentially a glass component.
These two materials are also coated onto PET via Physical Vapor Deposition (PVD).

Performance Parameters:

OTR: High-quality AlOx-PET can reach 1.0 cc/m²/24h, comparable to metalized film.

Surface Tension Challenge: As it is a ceramic/glass coating, the surface energy is very high (> 50 Dynes) and the texture is hard and brittle. During printing and lamination, if tension is not properly controlled, the coating is very prone to micro-cracks.

Top Coating: To prevent damage during processing, top material suppliers will add a Top Coating layer on the AlOx layer, which makes the material cost 30%-50% higher than ordinary VMPET.

Eco-friendly Barrier Layer

With the rise of the Recyclable Ready trend, metal layers are rejected because they are difficult to separate. EVOH (Ethylene-Vinyl Alcohol Copolymer) has become the core of all-plastic barrier solutions.

Co-extrusion Process:
EVOH usually does not exist as a single-layer film but is sandwiched in the middle of a PE film. For example, in a 5-layer co-extrusion blown film:
PE / Tie / EVOH / Tie / PE

Humidity Sensitivity:

Dry Environment (0% RH): The barrier property of EVOH is extremely strong, with OTR as low as 0.5 cc.

High Humidity Environment (90% RH): As humidity increases, EVOH molecules absorb water and swell, and the barrier capacity will decrease exponentially.

Solution: In coffee packaging, EVOH must be sandwiched between two layers of high-density PE, utilizing the excellent moisture resistance of PE (low WVTR) to protect the intermediate EVOH layer, ensuring it remains effective in high-humidity climates.

Eco-friendly Material

All-plastic bags can be directly recycled

This structure is called Mono-material PE. It unifies all the originally different types of plastics into PE. To make such bags as upright and heat-resistant as traditional bags, factories use MDO-PE (Machine Direction Oriented Polyethylene) technology.

Physical Performance Parameters:

• Total Thickness: Usually between 100 microns to 135 microns.
• Stiffness Control: Through the stretching process, the tensile strength of MDO-PE can reach 3 to 5 times that of ordinary PE, and the modulus (Stiffness) is close to PET, ensuring that coffee bags can stand on the shelf without collapsing.
• Recycling Logo: The PE content of the entire bag exceeds 95%, which complies with international #4 LDPE recycling streams and can directly enter existing curbside recycling bins.

Heat Seal Window:

The difficulty with this type of bag is that both the outer and inner layers are PE. To prevent the sealing machine from melting the entire bag, the outer layer uses high-melting-point PE (about 130°C) and the inner layer uses low-melting-point PE (about 105°C), creating a processing window of about 25°C to ensure a firm seal and flat appearance.

Barrier layer added to this plastic

Ordinary PE plastic has a very poor ability to block oxygen, with OTR usually as high as 2000 cc or more. To protect coffee flavor, engineers add an extremely thin layer of EVOH (Ethylene-Vinyl Alcohol Copolymer) during the multi-layer co-extrusion process.

Barrier Data Comparison:

Non-barrier PE: OTR is approximately 2500 – 3000 cc / m² / 24h.

PE with EVOH: When EVOH thickness reaches 5 microns, the OTR can instantly drop to 1.5 – 2.0 cc / m² / 24h.

Thickness Distribution (taking 120 microns 5-layer co-extruded film as an example):

1. Outer layer: PE (30 microns)
2. Tie layer: Tie (5 microns)
3. Barrier layer: EVOH (5 microns)
4. Tie layer: Tie (5 microns)
5. Inner layer: PE (75 microns)

Recycling Compatibility: Even with EVOH added, as long as its weight percentage is below 5% of the total packaging, it is still defined as 100% recyclable under North American (APR) and European (RecyClass) recycling standards and will not contaminate the quality of recycled plastic granules.

Materials that can rot in soil

For some specialty coffee brands advocating zero waste, Industrial Compostable materials are the first choice. This type of packaging is mainly made of PLA (Polylactic Acid) or cellulose extracted from plant starches (such as corn, cassava).

Standards Met:

• EN 13432 (European Standard)
• ASTM D6400 (American Standard)

Degradation Conditions and Data:

• Environmental Requirements: Must be in industrial composting facilities, with temperature maintained at 58°C to 60°C and relative humidity around 90%.
• Degradation Speed: Under the above conditions, the bag must physically break into fragments smaller than 2mm within 84 days (12 weeks) and achieve more than 90% biological conversion (becoming water, carbon dioxide, and minerals) within 180 days.

Performance Limitations:

• Weak Barrier Property: The Water Vapor Transmission Rate (WVTR) of PLA material is high, usually 20 – 30 g / m² / 24h, thousands of times that of aluminum foil structures.
• Recommended Solution: This type of bag is suggested for coffee beans with very fast turnover (consumed within 3 months), or by laminating a high-barrier cellulose film (such as NatureFlex) on the inner layer to barely extend the shelf life to 6 months.

Old Plastic

PCR (Post-Consumer Recycled) refers to packaging materials made by recycling, cleaning, and granulating plastic waste used by consumers. This is very effective in reducing the use of virgin plastic.

Content Standards: Currently, mainstream Western brands tend to use packaging with 30% PCR content. This is because when the PCR ratio exceeds 50%, the transparency and physical strength (such as tensile strength and puncture resistance) of the film will significantly decrease.

Food Safety Assurance:

• Since coffee directly contacts the inner layer of the packaging, PCR granules must pass the harmless treatment certification of the FDA or EFSA (European Food Safety Authority).
• Factories usually adopt a “three-layer co-extrusion” structure, sandwiching PCR plastic between two layers of brand-new virgin PE, forming an A-B-A structure. This way, PCR plastic does not directly contact the coffee beans, preventing the migration of any potential harmful substances.

Physical Performance:

After adding 30% PCR, tiny black spots or gels (Gels) may appear on the film surface, which are regarded in industrial production as a characteristic of eco-friendly materials rather than a quality defect.

Plastic Packaging Made from Plants

Bio-based PE differs from recycled plastic in that its source is renewable plants (such as sugarcane) rather than petroleum.

• Performance Consistency: The molecular structure of bio-based PE is completely identical to that of ordinary petroleum-based PE. This means that when producing coffee bags, there is no need to adjust heat sealing temperature or sealing pressure, and the processing performance is completely equivalent.
• Carbon Reduction Data: For every 1 ton of bio-based PE produced, approximately 3 tons of carbon dioxide can be captured and sequestered from the atmosphere. For coffee roasters with carbon neutrality goals, this is a solution that reduces emissions without sacrificing packaging quality.

Seal Tech

Ordinary physical folding seals cannot block gas exchange, leading to flavor loss within 7 days. Adopting ISO-compliant one-way degassing valves (opening pressure usually set at 2-6 mbar), combined with 10mm wide heat-pressed seals, can reduce the oxygen content in the bag to below 1% within 24 hours after roasting, thereby extending the optimal taste period to 12-18 months.

For consumer use, the Pocket Zipper system can withstand more than 50 repeated opening and closing tests while maintaining a low water vapor transmission rate (MVTR < 1.0 g/m²/day), which is currently the best solution for balancing filling efficiency and long-term storage.

Degassing Valve Pressure Control

Data shows that every 100 grams of freshly roasted dark roast coffee beans will contain about 1000 ml to 1200 ml of gas. In the first 24 hours after roasting, about 40% to 50% of the gas is released rapidly.

Without intervention, this internal pressure is enough to make a standard composite aluminum foil bag swell like a balloon or even burst the heat-sealed edges.

To solve this problem, the One-Way Degassing Valve we install on the packaging is not a simple hole, but a sophisticated micro-mechanical system.

Valve Internal Structure

A standard industrial degassing valve is usually assembled from five independent components, with an overall diameter usually between 20 mm and 23 mm and a thickness of about 3 mm to 6 mm.

1. Base Plate: Usually made of food-grade polyethylene (PE), it is directly welded to the inner film of the coffee bag via heat pressing. There are multiple tiny vent holes on the base plate to allow gas from inside the bag to enter the valve cavity.
2. Rubber Disc: This is the physical barrier that controls airflow. It covers the vent holes of the base plate.
3. Sealant Fluid: A thin layer of food-grade silicone oil is applied between the rubber disc and the base plate. This layer of oil not only lubricates but also utilizes the surface tension of the liquid to form an airtight layer, preventing air molecules from penetrating.
4. Filter: Some high-end valves are equipped with nylon mesh or non-woven layers to block coffee powder (especially espresso powder) from entering the valve body, preventing dust from jamming the disc and causing leakage.
5. Cap: Protects internal components and provides an exit channel for gas.

Opening Pressure Setting

The set opening pressure (Opening Pressure) is usually between 2 mbar and 8 mbar.

• Exhaust Process: When the coffee beans inside the bag release carbon dioxide, causing the internal pressure to rise and exceed atmospheric pressure plus the elastic force of the rubber disc, the disc will be pushed open. Gas breaks through the surface tension of the silicone oil and escapes through the side vent holes.
• Closing Process: Once the internal pressure in the bag drops near the equilibrium point (usually maintaining a slight positive pressure), the rubber disc will bounce back under its own elasticity, pressing tightly against the base plate. The silicone oil refills the gap, instantly cutting off the channel.

This process is completely passive and requires no electricity or manual intervention. This micro-positive pressure environment (internal pressure slightly higher than external pressure) is very conducive to preservation, as it prevents external air from backflowing during air pressure fluctuations.

Blocking External Oxygen

The concentration of oxygen in the air is about 21%, and oxygen is the main cause of oxidation of coffee flavor compounds (such as aldehydes and ketones).

Standard one-way valves must pass a reverse leakage test. In a laboratory environment, the valve is placed in a vacuum chamber to simulate a high-pressure external environment.

For a qualified valve, when the external pressure is higher than the internal pressure, the rubber disc will fit more tightly against the base plate, and the theoretical oxygen transmission should be close to zero.

Through the physical displacement effect, the heavier carbon dioxide (molecular weight 44) will gradually deposit at the bottom of the bag and act like a piston to squeeze out residual lighter oxygen (molecular weight 32) and nitrogen (molecular weight 28) through the valve.

Test data shows that within 72 hours after bagging, the oxygen content in bags equipped with high-quality degassing valves can naturally drop to 0.5% to 1%, which preserves the integrity of the beans better than vacuum packaging and does not crush the brittle bean bodies.

Zipper Sealing

For coffee packaging, the zipper must not only provide physical closure but also maintain sufficient water vapor transmission rate (MVTR) and oxygen transmission rate (OTR) after multiple openings and closings.

If there are defects in the design or material of the zipper track, even if the barrier performance of the bag material is high, air will seep in through the micron-level gaps between the zipper teeth, causing the coffee to oxidize rapidly within 3 to 5 days after opening.

Track Engagement Structure

Standard coffee bag zippers are usually made of low-density polyethylene (LDPE), which has good flexibility and heat sealing properties. The physical structure is mainly divided into two parts: the raised Male Track and the recessed Female Track.

• Asymmetric Design: To ensure tight sealing, the inner side of the female track of a high-quality zipper is usually designed with hook-like fine ribs. When the male track is pressed in, these ribs will undergo elastic deformation and wrap around the head of the male track.
• Click Sound Feedback: This mechanical interlocking process produces a tactile vibration and an audible “click” sound. This is not a decorative design but to give users a clear signal that the track is fully engaged.
• Tolerance Control: The manufacturing tolerance of industrial-grade zippers is usually controlled between 0.02 mm to 0.05 mm. If the gap is too large, the gas leakage rate will increase exponentially; if the gap is too small, the opening force (Opening Force) will exceed 30 N, causing users to damage the composite layer of the bag when pulling forcefully.

Pocket Zipper System

In flat-bottom bags (Flat Bottom Pouch) and some gusset bags, we widely use Pocket Zipper or Pull Tab Zipper.

This design pre-seals the zipper into the inner single side of the packaging film, without needing to run through the entire bag top like traditional zippers.

1. Filling Efficiency Improvement: Since the zipper is hidden in the film interlayer, the top of the packaging bag is completely open on the filling line. For automated filling equipment, this means the nozzle can penetrate deep into the bottom of the bag without obstruction, and the filling speed can be increased by more than 20% compared to traditional zipper bags.
2. Anti-counterfeiting and Protection: Before the consumer tears open the tear strip (Tear Strip) on the front for the first time, the zipper is in a completely closed, sterile environment. This eliminates the risk of dust entering the track during transport and causing sealing failure.
3. Operation Logic: After tearing open the seal, the user directly sees the already closed zipper. This design avoids the trouble of traditional bags needing to be cut at the top and then laboriously searching for the zipper head.

Powder Interference Resistance

Coffee powder, especially espresso grind (Espresso Grind), has extremely fine particles that are very easily adsorbed onto plastic surfaces with strong static electricity. When users use a spoon to take powder, dust will inevitably fall into the zipper track.

• Limitations of Single Track: Once coffee particles larger than 50 microns are caught in a standard single-track zipper (Single Track), its airtightness will drop by more than 60%. Particles will prop the track open, creating invisible micro-channels.
• Multiple Lines of Defense: We recommend using double-track (Double Track) or multi-rib (Multi-rib) designs. Even if the first track is contaminated by powder, the second track can still remain clean and maintain a seal.
• Velcro-style System: For extremely fine powder products, hook and loop (Hook and Loop) closure systems (similar to Velcro) are a better choice. It does not rely on precise track alignment but closes through hundreds of tiny plastic hooks entangling with each other. Even if a large amount of coffee powder is sandwiched in between, as long as the pressing force is sufficient, these micro-hooks can still penetrate the powder layer and lock together. Although its airtightness is slightly lower than track zippers, the reliability of physical closure is extremely high.

Tin Tie Physical Seal

Material Composition and Specifications

The so-called “Tin Tie” is actually a composite material accessory. It is not made of pure tin; the core usually consists of two parallel metal wires.

• Wire Material: To ensure repeated folding without breaking, the interior uses low-carbon steel wire or galvanized iron wire, with the diameter strictly controlled between 0.6 mm to 0.8 mm. For all-plastic tin ties (used in microwave heating scenarios), high-strength polymer strips are used instead of metal.
• Outer Wrapping: The outside of the wires is wrapped in a flat protective sleeve, with a width generally of 8 mm and lengths ranging from 120 mm to 160 mm depending on the bag width. The wrapping material is usually Kraft Paper or Polyethylene (PE) plastic.
  • Kraft Paper Layer: Provides a natural paper touch with greater friction, making it less likely to slip off after folding, making it very suitable for gusset bags with kraft paper outer layers.
  • Plastic Layer: The surface is smooth, waterproof, and oil-proof, suitable for bags with glossy or matte films, and the color choices are extremely rich, allowing for precise alignment with brand colors (Pantone).

Adhesive Bond Strength

The tin tie is usually pasted as a post-process on the top of the back of the packaging bag. The performance of the adhesive directly determines whether the tin tie will fall off after a few uses.

Industrial-grade Pressure Sensitive Adhesive is typically used. This type of adhesive is activated by finger pressure at room temperature.

• Initial Tack: The test standard usually requires reaching above 15 N / 25 mm. This means that as soon as it is pasted, it takes a lot of effort to tear it off.
• Holding Power: In environments with high temperature fluctuations such as coffee shops or kitchens (e.g., placed next to an espresso machine), the adhesive must not soften or flow. A qualified adhesive must maintain no displacement for 24 hours under a vertical 1 kg load in a high-temperature environment of 60°C.
• Curing Time: For optimal strength, it is recommended to let the tin tie sit for 24 hours after pasting before folding, allowing the adhesive molecules to fully cross-link with the bag surface.

Sealing Logic

Unlike zippers that rely on track engagement, tin tie sealing relies on the physical principle of “curling and compaction.”

1. Compressed Space: As coffee beans are gradually consumed, the space inside the bag becomes empty. Users can physically expel excess air from the bag by rolling down the bag mouth. This “volume compression” function is something zipper bags cannot do (the volume of zipper bags is fixed). Reducing the amount of headspace air directly reduces the absolute amount of residual oxygen.
2. Dead-fold Effect: Metal wire has excellent plastic deformation ability, i.e., “dead-fold” property. When you roll the bag mouth two to three times and bend the two wings of the tin tie inward, the metal wire locks this state, creating a continuous clamping force.
3. Leakage Path: Although rolled tightly, gas can still permeate through the side gaps of the folded layers. Lab data shows that the oxygen transmission rate (OTR) of a tin tie folded seal is about 10 to 50 times that of a heat-sealed zipper. Therefore, this method is more suitable for coffee beans with very fast turnover (consumed within two weeks after roasting).