Lesson- 7 Glass
7.1. Definition of glass
The American Society for Testing Materials defined glass as ‘an inorganic product of fusion which has cooled to a rigid state without crystallizing’ (ASTM, 1965). The atoms and molecules in glass have an amorphous random distribution. Scientifically this means that it has failed to crystallize from the molten state, and maintains a liquid-type structure at all temperatures. In appearance it is usually transparent but, by varying the components, this can be changed-as also can important properties such as thermal expansion, colour and the pH of aqueous extracts. Glass is hard and brittle, with a chonchoidal (shell-like) fracture.
7.2. Glass Composition
Glass is primarily formed from oxides of metals, with the most common being dioxide which is common sand. Glass is made by mixing several naturally-occurring inorganic compounds at a temperature above their melting points. The molten mixture is then cooled to produce a noncrystalline, amorphous solid. The main ingredient is silica (sand) (SiO2) that serves as the network-forming backbone of the glass. However, silica has a very high melting temperature, and molten silica has high viscosity that makes it difficult to form into shapes. Adding soda (Na2O) modifies the silica network by disrupting some of the Si-O bonds, with resulting lower melting temperature and viscosity but reduced resistance to dissolving in water. Thus, lime (CaO) is added as a network stabilizer, with the result that durability is increased but tendency to crystallize is also increased. Finally, alumina (Al2O3) is added as an intermediate to resist crystallization. Minor amounts of colorants are added to produce colored glass, including chromium oxide for green, cobalt oxide for blue, nickel oxide for violet, selenium for red, and iron plus sulfur and carbon for amber. Amber provides the best protection for light-sensitive foods and beverages, transmitting very little light with wavelength shorter than 450 nm.
7.3 Types of glass
7.3.1 White flint (clear glass)
Colorless glass, known as white flint, is derived from soda, lime and silica. This composition also forms the basis for all other glass colors. A typical composition would be: silica (SiO2) 72%, from high purity sand; lime (CaO) 12%, from limestone (calcium carbonate); soda (Na2O) 12%, from soda ash alumina (Al2O3), present in some of the other raw materials or in feldspar-type aluminous material; magnesia (MgO) and potash (K2O), ingredients not normally added but present in the other materials. Cullet, recycled broken glass, when added to the batch reduces the use of these materials.
7.3.2 Pale green (half white)
Where slightly less pure materials are used, the iron content (Fe2O3) rises and a pale green glass is produced. Chromium oxide (Cr2O3) can be added to produce a slightly denser blue green colour.
7.3.3 Dark green
This colour is also obtained by the addition of chromium oxide and iron oxide.
7.3.4 Amber (brown in various colour densities)
Amber is usually obtained by melting a composition containing iron oxide under strongly reduced conditions. Carbon is also added. Amber glass has UV protection properties and could well be suited for use with light-sensitive products.
Blue glass is usually obtained by the addition of cobalt to a low-iron glass. Almost any colored glass can be produced either by furnace operation or by glass colouring in the conditioning forehearth. The latter operation is an expensive way of producing glass and commands a premium product price. Forehearth colors would generally be outside the target price of most carbonated soft drinks.
7.4 Attributes of food packaged in glass containers
The glass package has a modern profile with distinct advantages, including:
7.4.1 Quality image
Consumer research by brand owners has consistently indicated that consumers attach a high quality perception to glass packaged products and they are prepared to pay a premium for them, for specific products such as spirits and liqueurs.
It is a distinct advantage for the purchaser to be able to see the product in many cases, e.g. processed fruit and vegetables.
7.4.3 Surface texture
Most glass is produced with a smooth surface, other possibilities also exist, for example, for an overall roughened ice-like effect or specific surface designs on the surface, such as text or coats of arms. These effects emanate from the moulding but subsequent acid etch treatment is another option.
A range of colors are possible based on choice of raw materials. Facilities exist for producing smaller quantities of nonmainstream colors.
7.4.5 Decorative possibilities
Decorative possibilities including ceramic printing, powder coating, colored and plain printed plastic sleeving and a range of labeling options.
All practical purposes in connection with the packaging of food, glass is impermeable.
7.4.7 Chemical integrity
Glass is chemically resistant to all food products, both liquid and solid. It is odorless.
7.4.8 Design potential
Distinctive shapes are often used to enhance product and brand recognition.
7.4.9 Heat processable
Glass is thermally stable, which makes it suitable for the hot-filling and the in-container heat sterilization and pasteurization of food products.
Glass is open to microwave penetration and food can be reheated in the container. Removal of the closures is recommended, as a safety measure, before heating commences, although the closure can be left loosely applied to prevent splashing in the microwave oven. Developments are in hand to ensure that the closure releases even when not initially slackened.
7.4.11 Tamper evident
Glass is resistant to penetration by syringes. Container closures can be readily tamper-evidenced by the application of shrinkable plastic sleeves or in-built tamper evident bands. Glass can quite readily accept preformed metal and roll-on metal closures, which also provide enhanced tamper evidence.
7.4.12 Ease of opening
The rigidity of the container offers improved ease of opening and reduces the risk of closure misalignment compared with plastic containers, although it is recognized that vacuum packed food products can be difficult to open. Technology in the development of lubricants in closure seals, improved application of glass surface treatments together with improved control of filling and retorting all combine to reduce the difficulty of closure removal. However, it is essential in order to maintain shelf life that sufficient closure torque is retained, to ensure vacuum retention with no closure back-off during processing and distribution.
7.4.13 UV protection
Amber glass offers UV protection to the product and, in some cases, green glass can offer partial UV protection.
Although glass is a brittle material glass containers have high top load strength making them easy to handle during filling and distribution. While the weight factor of glass is unfavorable compared with plastics, considerable savings are to be made in warehousing and distribution costs. Glass containers can withstand high top loading with minimal secondary packaging. Glass is an elastic material and will absorb energy.