Packaging Barrier Function Selection

Reference ID: MET-A283 | Process Engineering Reference Sheets Calculation Guide

Introduction & Context

The selection of packaging barrier materials is a critical process in food engineering, especially for oxygen‑sensitive dry snacks such as potato chips or nuts, and it often works hand‑in‑hand with the proper active packaging agent dosage to keep the internal headspace oxygen concentration below a critical threshold (C_max) throughout the product’s intended shelf life.

Methodology & Formulas

The calculation follows a mass-balance approach, determining the maximum allowable oxygen ingress and translating that into a required film permeability property. The following variables are utilized:

V_allow: Allowable oxygen volume (cm³)
C_max: Maximum tolerable oxygen concentration (% v/v)
C_init: Initial oxygen concentration after flushing (% v/v)
V_head: Headspace volume (cm³)
A: Package surface area (m²)
t: Target shelf life (days)
Δp: Partial pressure differential of oxygen (atm)
OTR_req: Required Oxygen Transmission Rate (cm³/(m² · day · atm))
OTR_base: Base Oxygen Transmission Rate at standard conditions (cm³/(m² · day · atm))
OTR_actual: Actual Oxygen Transmission Rate after humidity correction (cm³/(m² · day · atm))
f_RH: Humidity correction factor (dimensionless)

The allowable oxygen volume is calculated based on the concentration gradient:

\[ V_{allow} = \frac{(C_{max} - C_{init}) \cdot V_{head}}{100} \]

The required Oxygen Transmission Rate (OTR) is derived from the allowable volume, surface area, time, and pressure differential.

\[ OTR_{req} = \frac{V_{allow}}{A \cdot t \cdot \Delta p} \]

For multi-layer systems or materials sensitive to moisture, the actual performance of the film is adjusted by a humidity correction factor, f_RH, which accounts for the degradation of barrier properties in hydrophilic polymers:

\[ OTR_{actual} = OTR_{base} \cdot f_{RH} \]

Condition	Threshold / Logic	Impact
Humidity Sensitivity	RH_ext > 70%	Apply f_RH = 5.0 for hydrophilic films (e.g., EVOH)
Feasibility	OTR_actual ≤ OTR_req	Film is suitable for the application
Physical Validity	C_max > C_init	Required for positive oxygen ingress capacity

To establish the appropriate OTR for your packaging, you must evaluate the sensitivity of the product to oxidative degradation. Follow these steps:

Identify the critical oxygen concentration threshold (C_max) that triggers spoilage or chemical instability.
Calculate the shelf life requirement based on distribution and retail conditions.
Factor in the internal headspace volume (V_head) and the surface area (A) of the chosen packaging material.
Use the formula OTR_req = V_allow / (A · t · Δp) to compute the required transmission rate.
Perform accelerated aging studies to validate the calculated OTR requirement against actual product stability.

While both materials provide excellent barrier properties, process engineers must consider the following operational differences:

Flex-crack resistance: Foil laminates are prone to pinholing during mechanical stress, whereas metallized films offer superior durability.
Cost and sustainability: Metallized films are generally thinner and more cost-effective, though they may be harder to recycle than mono-material alternatives.
Detection: Metallized layers interfere with metal detection systems, necessitating the use of alternative inspection technologies like X-ray.

Polar polymers, such as EVOH or Polyamide, exhibit a significant dependency on relative humidity. As moisture levels increase, the polymer matrix undergoes plasticization, which leads to:

Increased free volume within the polymer chains, allowing oxygen molecules to permeate more readily.
A drastic increase in the effective OTR (i.e., a reduction in barrier performance), quantified by the humidity correction factor f_RH.
Potential loss of structural integrity if the material is not protected by a hydrophobic outer layer like PE or PP.

For hydrophilic barrier materials exposed to external RH > 70%, it is common practice to apply a correction factor of f_RH = 5.0 to the base OTR when determining actual barrier performance under humid conditions.

Worked Example: Packaging Barrier Function Selection for Oxygen-Sensitive Dry Snacks

Scenario: A snack manufacturer must select a barrier film for packaging potato chips. The product is oxygen-sensitive, with a maximum tolerable oxygen concentration of 2.5 % v/v at end of shelf life. After nitrogen flushing, the initial oxygen concentration in the headspace is 0.5 % v/v. The package geometry and storage conditions are known. A candidate EVOH-based barrier laminate (base OTR = 0.45 cm³/(m²·day·atm)) is considered, but the external storage humidity is high (75 % RH), which is known to severely degrade the oxygen barrier of hydrophilic polymers such as EVOH. The calculation below determines whether the candidate film meets the required oxygen barrier under actual humidity conditions.

Knowns:

Maximum tolerable oxygen concentration, C_max = 2.5 % v/v
Initial oxygen concentration after flushing, C_init = 0.5 % v/v
Headspace volume, V_head = 150.0 cm³
Package surface area, A = 0.15 m²
Target shelf life, t = 180.0 days
Partial pressure difference of oxygen, Δp = 0.21 atm
External relative humidity, RH = 75.0 %
Candidate film base OTR, OTR_{candidate, base} = 0.45 cm³/(m²·day·atm)
Humidity correction factor for RH > 70 %: f_RH = 5.0 (per standard literature for hydrophilic barrier layers such as EVOH)

Step-by-Step Calculation:

Calculate allowable oxygen ingress into the headspace.
\[ V_{allow} = \frac{(C_{max} - C_{init}) \cdot V_{head}}{100} \]

Substituting values:

\[ V_{allow} = \frac{(2.5 - 0.5) \cdot 150.0}{100} = 3.0 \; \text{cm}^3 \]
Determine the required film oxygen transmission rate (OTR).
\[ OTR_{req} = \frac{V_{allow}}{A \cdot t \cdot \Delta p} \]

\[ OTR_{req} = \frac{3.0}{0.15 \cdot 180.0 \cdot 0.21} \]

\[ OTR_{req} = 0.529 \; \text{cm}^3/(\text{m}^2 \cdot \text{day} \cdot \text{atm}) \]
Apply humidity correction to the candidate film.
Because external RH = 75.0 % > 70 %, the film's OTR increases by a factor of 5.0 (from published data for hydrophilic barrier layers such as EVOH):

\[ OTR_{candidate, actual} = OTR_{candidate, base} \cdot f_{RH} \]

\[ OTR_{candidate, actual} = 0.45 \cdot 5.0 = 2.25 \; \text{cm}^3/(\text{m}^2 \cdot \text{day} \cdot \text{atm}) \]
Validate the candidate film against the required OTR.
Feasibility condition: OTR_{candidate, actual} ≤ OTR_req.

Since 2.25 > 0.529, the condition is not satisfied.

\[ \text{is_feasible} = \text{False} \]

Final Answer: The required OTR for the packaging is 0.529 cm³/(m²·day·atm). Under high humidity (75 % RH), the candidate EVOH-based barrier laminate provides an actual OTR of 2.25 cm³/(m²·day·atm), which exceeds the requirement. Therefore, the film is not feasible for this application. A higher-barrier film or a moisture-protective laminated structure (e.g., adding an external polyolefin layer to shield the EVOH from humidity) must be selected to achieve the target shelf life.

"Un projet n'est jamais trop grand s'il est bien conçu." — André Citroën

"La difficulté attire l'homme de caractère, car c'est en l'étreignant qu'il se réalise." — Charles de Gaulle