Spoilage in canned foods
Canned foods are liable to spoilage in storage for various reasons. The two important kinds of spoilages are: (1) Spoilage by micro-organisms (2) spoilage due to physical and chemical changes. Various indications (Symptoms) of spoilage in cans are –
In swelled can, the ends are tightly bulged due to the formation of CO2 or other gas inside the can as a result of decomposition of the contents by micro-organisms. The decomposed food in the can has an offensive and sour odour and is discoloured. It is not fit for consumption and even may contain toxins produced by bacilli like Clostridium botulinum.
Hydrogen formed by the action of the acids present in the fruit on the tin plate causes the can to bulge at both the ends. In such cases, the food remains free from harmful micro-organisms and is still fit for consumption.
A mild swell at one or both ends of a can is called a ‘Springer’. It may be an initial stage of hydrogen swell or may be caused by Insufficient exhausting or by over filling of the can. Food generally remains fit for consumption.
The cans appear normal, but when struck against a table top one or both ends become convex and spring or flip out, but can be pushed back to normal condition by a little pressure. Such a can is termed as ‘flipper’. It may be an initial stage of swell or hydrogen swell, but does more frequently, owe to over filling or under exhausting.
it is caused mostly in non-acid foods like vegetables by micro-organisms like thermophyllic bacteria (B.coagulens and B.sterothermophilus ) which produce acid with out formation of gas. The thermophyllic bacteria thrive at a high temperature of 100OC
.If cans are stored without adequate cooling, the contents remain at a temperature favourable for incubation of such bacteria for fairly long time, which results in their multiplication and spoil the product. It is therefore difficult to detect from them external appearance.
It may be due to by under sterilization (processing).Therefore the thermophillic bacteria would be of significance. The product has a sour odour, and its acidity much be higher than that of the normal product. It is not fit for consumption.
A leaking can is known as leaker. A very small leak may appear in the can owing to (1) defective seaming, (2) nail holes caused by faulty nailing of cases while packing (3) excessive internal pressure due to microbial spoilage sufficient to burst the can (4) internal or external corrosion (5) mechanical damage during handling.
There may be a very tiny leak in the can through which air may pass in and destroy the vacuum but not the micro-organisms. Consequently the vacuum in the can is always nil, and the pressure inside the can is equal to that of atmosphere. The damage to food is usually owing to rusting of the can caused by oxygen in the air but still the food remains fit for consumption.
Bursting of the can:
Cans may some times burst. This may be due to excess of pressure caused by the gas inside, produced by the decomposition of the food by micro-organisms, or by hydrogen gas formed by chemical action of acids of the food on the tin plate. Thus the canned product becomes a total loss.
Discolouration of the fruit products:
Discoloration of the canned foods may be owing to bio-logical causes or metallic contamination or both.
Cut and peeled apples and pears when kept in air, turn brown due to oxidation. This change is induced by co-oxidases (enzymes) present in the fruit and can be avoided by placing them in 2 or 3 per cent common salt solution.
Poly phenol oxidase
Poly phenols (in cells) + Oxygen (air) Brown (in cells)
Brown discoloration of the fruit products may also be caused by reactions other than enzymatic.
The colour changes may be caused by reactions between:
- Nitrogenous matter and sugars
- Nitrogenous matter and organic acids
- Sugar and organic acids and
- Organic acids among themselves.
These reactions, generally known as ’Millard’ reactions (also known as Millard browning). Due to these reactions colour, flavour, odour and some times texture changes occur, which results from a chemical reaction between the above) are of great importance to food preservation. It is named after the French man Millard who discovered it.
Metallic contamination: This type of discoloration is caused by iron and copper salts mostly. The following are important among them.
- Ferric tannate: Some fruits and vegetables contain tannins which on contact with the exposed iron of the tin plate form ferric tannate.This is black and spoil the appearance of the product.
- Iron sulphide: Sulpher dioxide either from the source of sugar or formed owing to the decomposition of the proteins in the product may react with the hydrogen formed by fruit acid acting on the tin plate and gets reduced to H2This H2S, in turn will react with the iron of the can and form black iron sulphide. Therefore, the product appearance is spoiled and the obnoxious smell of H2S will also persist.
- Copper sulphide:
When the plant is shut down for some time, even for a few days, a thin film of copper oxide will be formed on the surface of the equipment made of copper or brass.
Fruit acids, reacting on the metal can produce hydrogen. The hydrogen thus formed will react with the red or purple pigments of fruits like straw berries, red plums, damsons etc. and bleach them. The remedy for this is to ensure that lacquering is complete and perfect.