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COATING OF CANS
R.Keerthana PriyaBTM 11008
Metal packaging plays an important role in the process of food preservation. The common expression used to describe such a process is “canning”.
The different types of metal packaging include:
Beer and soft drink cans
Food cans
Drums and pails
Aerosol containers
Tubes
Open trays
Caps and closures (e.g. lids on glass jars and bottle tops)
Lids (e.g. for yoghurt and butter containers).
INTRODUCTION
Lacquer is a resin coating of cans which protect the inside of the can from acids in food which would otherwise attack the metal causing it to corrode.
If the can is going to be stored in poor conditions the outside of the can, can be lacquered as well, to protect the metal from corrosion.
The lacquering seals and protects the tin from chemical reactions without affecting the food flavor, it also prevents the dissolution of tins into the products.
LACQUERING
INTERNAL (FOOD CONTACT) COATINGS
Protection of the contents from the metal – e.g. iron pick-up in beer or discolouration of some dark-coloured fruits, such as plums and strawberries, due to metal contact
Protection from the contents of the can – e.g. acidic soft drinks (which may corrode uncoated metal) or some fish,
meats and soups (which may cause
sulphur staining).
EXTERNAL (NON-FOOD CONTACT) COATINGS
Protection from the environment – e.g. atmospheric corrosion
Support decoration, labeling and consumer information
Influence mobility of the article during filling operations – e.g. beverage cans can only be filled with an external decoration, which provides the necessary friction (mobility) to pass through the filling head.
MAJOR CONSTITUENTS IN A CAN COATING
Resin(s)
Cross-linking agents (almost always present)
Additives
Solvents (not always present)
INTERNAL COATING MATERIALS
Epoxy phenol
Acrylic
Phenolic
Vinyl resin
Oleoresinous group
EXTERNAL COATING MATERIALS
White basecoat
Overprint Varnish
LACQUERING – METHOD OF APPLICATION
Lacquers can be applied in a following way
Electrophoretic Lacquers
Dip Applied Lacquers
Brush Lacquers
Spray Lacquers
EPOXY PHENOLIC
Widely used at present; resistant to acids, with good flexibility, adhesion and high heat stability.
A wide range epoxy-phenolic lacquers are applied in fruits, fruit juices, vegetables, soup, meats, and marinated fish.
ACRYLIC
Used when a good color retention and high resistance to heat are required. Formerly used only for exteriors.
Now they provide internal protection as hard white coatings with an attractive ceramic and stain-free appearance
PHENOLIC
Outstanding chemical stability and low permeability,especially against sulfide ions.
Contains meat and fish products.
VINYL
Good adhesion and flexibility. It cannot resist sterilization at high temperatures.
Free from odor and flavor.
Function as second lacquer layer for beer, wine and carbonated beverages and for cakes, sweets, dry foods and pharmaceutical products.
OLEORESINOUSIt is usually modified with the addition of zinc oxide to prevent blackening of product due to reaction of tin with sulfur compounds.
Subdivided to two groups : “R” or fruit enamels – protect the natural pigment of highly colored fruits such as dark berries, red beats and red cabbage.
“C” enamels – prevent the discoloration of foods containing proteins such as corn, peas, poultry and seafoods.
LOW-ACID FOODS LACQUERINGLow-acid foods have pH values higher than 4.6. They include red meats, seafood, poultry, milk, and all fresh vegetables except for most tomatoes.
Phenolic lacquers –Ham
Epoxy-Phenolic –Luncheon meats and fish
Phenolic lacquers prevents sulphur staining in products with polphosphates and other preservatives.
Phenolic lacquers have greater resistance than Epoxy-Phenolic.
Cont…Epoxy-Phenolic becomes normal ,when incorporated with
aluminium in the coating, in order to mask sulphur staining
that may take place on the surface of the tin plate.
During sterilization of foods, sulphur is released as
Hydrogen sulphide due to heating.
Due to this unpleasant sulphur odour is produced while
opening the can, so it is necessary to absorb or neutralize
the sulphur.
To prevent this lacquer is coated with Zinc, because Zinc
absorbs odours of sulphides.
HIGH-ACIDS FOODS LACQUERING
Acid foods have a pH of 4.6 or lower. They include
fruits, pickles, sauerkraut, jams, jellies,
marmalades, and fruit butters.
Generally steel cans are suitable.
Double lacquers are used for strong acidic foods
Eg: Pickles,tomato-concentrate
Cont…
In the case of coloured fruits that contain anthocyanin pigments, such as strawberries, raspberries, blackberries, black cherries and red plums, the fruit must be packed in a totally lacquered can because the juice will be quickly discolored by contact with tin or iron, thus every effort must be made to prevent all contact between the juices and the metal surface.
Epoxy-phenolic lacquers are normally used.
EXTERNAL COATINGINTERNAL COATING
Lacquers Flexibility Pack resistance Main end use
Epoxy phenol Good Very goodUniversal good lacquer for three piece cans
Organ sol Very good Very goodDrawn cansEasy open endsOften used over epoxy phenol base coat
Phenolic Very poor, but film quality is
weight dependent
Excellent for aggressive
foods
Drums and pails where flexibility is not a critical factor
Oleoresionus Variable Pack dependent Very limited uses
TYPES & PROPERTIES OF RESINS IN INTERNAL COATING
CONCLUSIONMost of the metal packaging has an internal food contact coating, which is essential to the performance of the packaging.
Failure of the coating could result in failure of the packaging and consequential spoilage of the foodstuff.
Consequently, a wide range of coatings based upon a limited range of approved chemicals is used to prevent toxicological concern in foodstuffs.
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