Packaging foods in glass
Practical Action
Sealing
Most caps for bottles and jars have a ring of plastic material (sometimes waxed card or cork)
which forms a tight seal against the glass. During hot filling and heat processing this plastic
softens and beds itself around the glass to make an hermetic seal. However, before this
happens there is a risk that small amounts of air can be sucked into a container and cause
contamination of the product. The risk of contamination can be reduced by laying a filled
container on its side for about 10 minutes to ensure that the seal is perfectly formed.
Specific types of equipment are used for sealing the different caps that are used for glass
containers.
For bottles the main types are:
crown caps
roll-on-pilfer-proof (ROPP) caps
snap-on caps
corks
For jars the main types are:
twist-on-twist-off (TOTO) lids
push on lids
All lids and caps should neither affect the product nor be affected by it and they should seal
the container for its expected shelf life. This is usually found by testing trial containers with
the product to be packaged to make sure that there is no interaction between the pack and
the product. Expert advice should also be sought from the packaging suppliers when
selecting the type of closure to be used.
Bottles
Crown caps are commonly used for beer bottles and fruit juices. Hand-operated equipment is
available in a number of sizes from a simple former that is placed over the cap and hit with a
mallet, to the hand-held lever type shown in Figure 9 and table mounted model shown in
Figure 10.
Figure 9: Hand held bottle capping
Figure 10: Table mounted bottle capping
Roll-on-pilfer-proof (ROPP) caps are fitted by placing a blank cap on the bottle and then
pressing the metal into the screw thread of the glass. Finally, a ring of perforated metal is
formed at the base of the cap that shows evidence of tampering or pilfering. Hand operated
ROPP machines can be constructed locally (Figure 11) and small motorised version are
available commercially (Figure 12). A simpler cap which does not incorporate the pilferproof
feature is known as a 'Roll-on (RO) cap and this can be fitted by similar types of equipment.
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