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.