FOOD PACKAGING: PACKAGING MATERIALS
Diagram
Glass and plastic: two materials for holding food
Glass packaging
A rigid container made from melted sand and minerals. It is inert, clear and impermeable, and it can be heated and reused.
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Plastic packaging
A container made from polymers. It is light, tough and flexible in form, and it can be moulded into many shapes.
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The choice
Glass protects best and suits foods that need a perfect barrier. Plastic is lighter and cheaper to carry, and suits many everyday packs.
Figure 1: An inert rigid material compared with a light mouldable material. Drawn in our own style from Lee, Yam and Piergiovanni, Food Packaging Science and Technology, CRC Press, and Robertson, Food Packaging: Principles and Practice, 2nd ed., CRC Press, 2005. Note: the comparison is drawn from the glass and polymers chapters, not from a single head to head passage.
Introduction
Glass packaging and plastic packaging are the two most common rigid materials for holding food, and they differ in almost every property that matters to a packer. Glass is made from sand and minerals melted at high heat, and it is inert, which means it does not react with the food, and impermeable, which means almost no gas or water vapour passes through it. Plastic is made from polymers, and it is light, tough, and can be moulded into bottles, tubs, trays and films in many shapes.
The core difference is barrier against weight and toughness: glass gives the best barrier and the cleanest taste, while plastic gives lightness, low breakage and design freedom. The choice depends on the food, the shelf life needed, the cost of carrying the pack, and whether the pack is returned and reused.
Quick comparison
Table 1: Glass packaging compared with plastic packaging
| Aspect | Glass packaging | Plastic packaging |
|---|---|---|
| Material | Melted sand and minerals | Polymers, such as PET and HDPE |
| Barrier to oxygen | Almost perfect, very little gas passes | Good to moderate, some gas passes depending on the polymer |
| Barrier to water vapour | Almost perfect | Good to moderate, varies with the polymer |
| Reaction with food | Inert, does not react with the food | Mostly inert, but some substances can migrate at low levels |
| Weight | Heavy | Light |
| Breakage | Breaks if dropped, with sharp pieces | Tough, rarely breaks, and does not shatter |
| Transparency | Clear, and stays clear | Clear or opaque, depending on the polymer |
| Heat | Takes high heat, suits hot filling and retort | Softens with heat, only some plastics take hot filling |
| Reuse and recycling | Can be washed and reused, and recycled | Mostly single use, recycled by polymer type |
| Food example | Jam jars, sauce bottles, baby food jars | Water bottles, milk bottles, yogurt tubs |
Scientific explanation
Definition
Glass packaging is packaging made of glass, a hard, brittle material formed from melted silica sand and minerals. Plastic packaging is packaging made of plastic, a polymer material that can be moulded when warm into bottles, tubs, trays and films.
Working principle
Glass protects food because its structure lets almost no oxygen or water vapour through, and because it does not react with acids, oils or flavours in the food. Plastic protects food through the polymer itself and through layers: each polymer lets through a different amount of gas and vapour, so the barrier depends on which polymer is used and how thick it is. A plastic bottle can be made with layers of different polymers to improve the barrier.
Apparatus
Glass containers are formed in a glass furnace and moulding machine, and closed with metal or plastic caps. Plastic containers are formed by blow moulding, injection moulding or thermoforming. The books name no single apparatus that tests glass against plastic.
Measurement procedure
The books name no single standard test that compares the two materials. In practice the barrier is checked by measuring how fast oxygen and water vapour pass through the material, the seal is checked for leaks, and packs are stored and checked for changes in the food over time.
Factors affecting readings
For glass, the result depends on the thickness of the wall and on the seal, because the glass itself is almost impermeable. For plastic, the result depends on the polymer, the thickness, the temperature, and whether the pack has layers. Warm storage lets more gas through a plastic pack, while glass is hardly affected by temperature.
Formulas and calculations
The books read give no formula for glass against plastic packaging. The comparison is made from material properties: glass is almost impermeable to gases and vapours, while the permeability of a plastic depends on the polymer and the thickness. No permeability number is given here, because the number changes with the polymer, the thickness and the temperature.
Industrial applications
- Glass packaging: jam, sauces and pickles in jars, where acid foods need an inert container and a long life.
- Glass packaging: baby food jars, where an inert, clean container matters most.
- Glass packaging: sauces and drinks that are hot filled or retorted, where the pack must take heat.
- Plastic packaging: water and soft drinks in PET bottles, where lightness and toughness matter for carrying.
- Plastic packaging: milk bottles, yogurt tubs and trays, where a light, cheap, unbreakable pack suits daily use.
Advantages and limitations
- Glass packaging, advantage: the best barrier and an inert surface, so taste and quality are protected for a long time.
- Glass packaging, limitation: heavy and breakable, so carrying costs more and broken glass is a hazard.
- Plastic packaging, advantage: light, tough and cheap to shape into many forms, with no shattering.
- Plastic packaging, limitation: a weaker barrier than glass, and some substances can migrate from the plastic at low levels.
FAQ
Why is jam sold in glass jars?
Because jam is acid and kept for a long time, and glass is inert and gives an almost perfect barrier.
Why is water sold in plastic bottles?
Because plastic is light and tough, so bottles are cheap to carry and do not break if dropped.
Does plastic let air through?
Yes, a little. The amount depends on the polymer and the thickness, and it is more than glass lets through.
Can glass be reused?
Yes. Glass bottles and jars can be washed and refilled, and broken glass can be melted and made into new glass.
Can plastic take hot filling?
Some plastics can, and some cannot. The choice depends on the polymer, because plastic softens with heat.
References and further reading
- Fellows, P., Food Processing Technology: Principles and Practice, Woodhead Publishing (FTN book on disk).
- Lee, D. S., Yam, K. L. and Piergiovanni, L., Food Packaging Science and Technology, CRC Press (Google Books page read: Ch. 6, Polymers, and Ch. 7, Glass).
- Robertson, G. L., Food Packaging: Principles and Practice, 2nd ed., CRC Press, 2005 (Google Books page read: glass, metal, paper and polymer material chapters).
