Physical Properties Associated with Packaging Materials

Stored-product insects have been shown to produce different types of damage to various types of packaging materials (Riudavets et al., 2007). Common physical properties of polymer films tested include elongation percentage and tensile strength. Packaging films with high elongation (%) and low tensile strength (MPa) are generally more resistant to insect penetration (Chung et al., 2011). The hole shape and size insects create in a packaging material is related to the shape of the mouthparts of a particular species, the head capsule width, and the mechanical properties of that particular film (Chung et al., 2011). Chung et al. (2011) demonstrated that P.

interpunctella was capable of creating holes with clear cut edges in PET films, but linear low-density polyethylene (LLDPE) films contained scratches and tears around entry holes. The difference between the materials is PET films have a low elongation and high tensile strength compared to LLDPE films. High elongation in polyethylene (PE), 0.05 mm thick, produced a hole by L. serricorne with large amounts of filaments or fraying of the material (Riudavets et al., 2007). Harder materials such as PE and PP are less resistant to fraying and insects create holes

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which are clean cut or contain scratches near the entrance (Riudavets et al., 2007). Packaging materials can be ranked in many different ways, but ultimately the resistance of a particular film is related to the films thickness, and thicker films are more resistant to insect penetration.

However, among packaging types Kraft paper and cellophane are the least resistant to insect penetration among all packaging materials. Polyvinyl chloride (PVC) is less resistant to insect infestation than PP films (Bowditch, 1997a). Among packaging films tested by Chung et al.

(2011), LLDPE, PET, oriented polypropylene (OPP), and cast polypropylene (CPP) were all resistant to penetration by P. interpunctella, which is classified as a penetrator and a serious pest of packaged products. When pinholes are introduced into the packaging material, stored-product insects are able to invade packages more readily. Between the packaging materials tested by Chung et al. (2011), 0.05 mm thick LLDPE was the most resistant to invasion by P.

interpunctella. The resistance of polymer films to penetration or invasion by stored-product insects, varies by resins from which the films are made and their physical properties (Highland and Wilson, 1981). The multilayer or laminate films, appear to be the most resistant to insect penetration among all packaging types.

1.4.1. Methods used to Test Packaging Integrity

Several methods for testing packaging integrity against stored-product insects have be developed over the years. Generally, there are two types of testing methods employed. The first method consists of constructing “bags” containing food from the packaging materials under test, and introducing the bags into a room containing stored-product insects (Wohlgemuth, 1979).

This test requires extensive room capacity and long experimental periods, and determining if the packaging material was penetrated versus invaded through packaging defects, can be difficult to

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assess (Wohlgemuth, 1979). This test is suitable for quality control testing of mass produced packages (Wohlgemuth, 1979).

The second method employed, is creating a barrier from the packaging material under test and challenging the material with the test insect species. In this situation, a piece of material is placed between two metal holders, the material is held taut, and insects are placed on top of the material being tested. In this scenario, the insect can be held with or without food, and

penetration of the material can be determined because once the insect penetrates the material they fall through and cannot escape. Therefore, this eliminates the possibility of insects invading the material through a defect. This test is suitable for small sample sizes, but not for large

quality control type studies, which mimic retail environments (Wohlgemuth, 1979). In this testing method, the introduction of a pinhole can also be included and invasion properties of the material can be determined. The presence or absence of food, will also affect the penetration or invasion ability of stored-product insects (Newton, 1988). Insect that are starved may penetrate packages to obtain food, but in the presence of a food source they may not necessarily invade those packages. In addition, pricking packaging materials to provide access through pinholes increases the likelihood that insects will penetrate/invade the product. In instances where insects normally fail to penetrate packages, the pinhole is utilized for assessment.

1.4.2. Insect Boring Direction Determination

On a manufacturer’s standpoint, it is important to determine whether insects bore through the packaging from the outside to inside and vice-versa (Wohlgemuth, 1979). This enables manufactures to determine if an infestation occurred in the product before or after final packaging. If there are no holes, tears, seam failures, or insect-bored holes in the packaging material, it is reasonable to assume that infestation originated during processing (Brickey et al.,

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1973). However, if insect-bored holes are present the direction of penetration is key to

determining the origin of the infestation (Brickey et al., 1973). Brickey et al. (1973) developed a methodology for determining the direction of insect penetration. One common characteristics in materials in which insects bore through is a tapered hole in which the diameter of the hole is greater on the entrance side than then exit side (Brickey et al., 1973). In foil, cellophane, and polyethylene plastics, upturned edges around the perimeter are commonly found (Brickey et al., 1973). The mandibles of insects also caused roughening to the surface or surface fraying on packaging materials formed by the pincer like action of the mandibles (Brickey et al., 1973).

Though it is important to determine whether an insect bored through the packaging materials or were present inside prior to packaging, ultimately any type of flaw can easily negate the effects of a strong package design.