Processing of Horticultural Crops (1+2)

Causes of spoilage of canned/bottled products

III Causes of spoilage of canned/bottled products
Food is mostly subjected to physical, chemical and biological changes which lead to quality deterioration and ultimately spoilage.
1. Chemical spoilage: Hydrogen swell is the important type of chemical spoilage of canned food. The hydrogen gas formed inside the can, by the action of food acid on the iron of the can causes the can to swell which is termed as Hydrogen swell. Major causes of hydrogen swell are:

• Presence of high acid in the can.
• Storage at high temperature.
• Imperfections in tinning and lacquering inside of the can.
• Insufficient exhausting during canning.
• Presence of soluble sulphur and phosphorus in the can contents.
• Interaction between steel base of can and contents of the food leading to chemical spoilage and may also cause following defects:
• discolouration of the food
• discolouration inside the can
• production of off flavour in the food
• cloudiness of liquors or syrups or brines
• corrosion or perforation of the metal and loss of nutritive quality.

2. Biological spoilage: Biological spoilage in the canned food is caused by either survival of organisms after heat treatment or entry of micro-organisms through leakage of the container after heat processing. The types of micro-organisms involved in spoilage of canned foods are thermophilic bacteria and mesophilic micro-organism and kind of spoilage brought about by these groups is characterized and discussed further in this chapter.

A) Spoilage by mesophilic organisms: This type of spoilage is caused by spore forming bacteria of genera Bacillus and Clostridium growing in the food as a result of under processing. Besides, spoilage of lightly heated food like acidic foods can also be caused by non-spore forming bacteria or even yeasts or moulds. Spoilage by mesophilic Clostridium & Bacillus sp. and sugar fermenting species of Clostridium like Clostridium butyricum and Clostridium pasteurianum cause the butyric acid type of fermentation in acid or medium-acid foods. This lead to swelling of container due to the production of hydrogen gas and carbon-dioxide gas. The mesophilic organisms responsible for spoilage are as under:

• Putrefactive anaerobes
• Butyric anaerobes
• Aciduric Flat sour and Lactobacilli
• Yeast
• Moulds

i) Putrefactive anaerobes: The species of Clostridium like C. sporogenes, C. putrefaciens and C. botulinum are proteolytic or putrefactive causing decomposition of proteins with the production of off odorous compounds such as hydrogen sulphide, and ammonia. Besides, putrefactive anaerobes also produce carbon dioxide and hydrogen gas, thus causing the can to swell. The spores of some putrefactive anaerobes are very heat resistant thus putrefaction along with flat sour and TA (thermophilic anaerobes) spoilage constitutes the major type of biological spoilage of canned foods resulting from under processing. Putrefactive anaerobes grow best in the low acid canned foods like peas, corn, meats, fish and poultry. C. botulinum is main putrifier causing food poisoning.

ii) Butyric anaerobes: The spores of saccharolytic Clostridia commonly called as butyrics having comparatively low heat resistance, cause spoilage of canned foods which have been processed at 100^oC or less such as commercially canned acid foods processed by hot water or steam. Canned acid foods such as pineapple, tomato and pears are generally spoiled by Clostridium pasteurianum. The spoilage by Saccharolytic bacteria is characterized by the production of butyric acid, carbon dioxide and hydrogen. Similarly, canned peas, asparagus, spinach, peach and tomatoes can be spoiled by aerobacilli or gas forming Bacillus species (B. polymyxa and B. macerans) by entering possibly through the leakage in the container. The heat resistance of Bacillus sp. is same as that of Clostridium pasteurianum.

iii) Aciduric flat sour and Lactobacilli: It is also referred as spoilage by non-spore forming bacteria. The presence of viable non-spore forming bacteria in the canned food indicates that the product has received either a very mild heat treatment or the bacteria entered through a leakage in the container. Common micro-organisms found in under processed fruit products such as tomato and pear includes acid forming Lactobacillus and Leuconostoc species. Some thermoduric bacteria which can withstand pasteurization are Streptococcus thermophillus, some species of Micrococcus, Lactobacillus and Microbacterium. An important bacterium found in cooling water is coliform bacteria, which produce gas and cause the can to swell. However, spore forming bacteria can also enter the can through the leakage. Further non-spore forming and non-gas forming bacteria that may enter the can through leakage include those in the genera Pseudomonas, Alcaligenes, Micrococcus, Flavobacterium, Proteus etc.

iv) Spoilage by yeast: Detection of yeasts and their spores in the canned foods is the result of either gross under processing or leakage as the yeasts and their spores are readily killed by most heat processing methods. Canned fruits, jams, jellies, fruit juices, syrups etc are generally spoiled by fermentative yeasts, with swelling of the cans owing to the production of carbon dioxide. Presence and growth of film yeasts on the pickles, olives etc. indicates contamination, lack of heat processing and poor evacuation.

v) Spoilage by moulds: Growth of moulds is the common cause of spoilage of high sugar containing processed and canned foods as they enter through a leak in seal of the container. Though jams, jellies and marmalade having sugar concentration as high as 70% with normal acidity of 0.8 to 1.0%, practically removes the risk of mould spoilage yet sometimes mould growth can be seen on the surface of the product. Strains of Aspergillus, Penicillium and Citromyces found growing in jellies and canned fruits are able to grow in sugar concentration up to 67.5 percent but can be killed by heating the food at 90^oC for 1 minute. Some moulds are fairly resistant to heat like Byssochlamus fulva, a pectin fermenting moulds which resist the heat processing.

B) Spoilage by thermophilic organisms
Major cause of spoilage of heat processed foods by thermophilic spore is due to under processing as their spores are more heat resistant than those of mesophilic bacteria. Spoilage by thermophiles includes flat sour, TA spoilage and sulphide spoilage.

i) Flat sour spoilage: In this kind of spoilage, the ends of the can of food remain flat during souring or during the development of lactic acid in the food by the flat sour bacteria. Due to normal appearance of the can, this type of spoilage can not be detected by the examination of the unopened can. Flat sour spoilage occurs in low-acid foods such as peas, lima bean and corn etc and is caused by species of Bacillus which form acid without production of gas. They include mesophiles, facultative thermophiles or obligate thermophiles. In acidic foods like tomato and tomato juice, flat sour is caused by facultative thermophilic species such as Bacillus coagulans. The spores of mesophiles being least heat resistant are killed by heat processing and are therefore not involved in flat sour spoilage of low acid foods, but the spores of thermophiles are considerably more heat resistant and survive the heat process to cause flat sour spoilage.

ii) TA spoilage: TA is a nick name for the bacterium thermophillic anaerobe not producing hydrogen sulphide or for Clostridium thermo-saccharolyticum causing this type of spoilage. This bacterium is a thermophilic spore-forming anaerobe that forms acid and gas in foods. The gas (mixture of CO₂ and H2) developing inside the container cause the can to swell and when cans are stored for too long at high temperature they may result in bursting. The spoiled food has sour or cheesy odour. The source of bacteria for both flat sour and TA spoilage is starchy/sugary foods.

iii) Sulphide spoilage: TA spoilage producing H2S and cause sulphide spoilage. The micro-organism responsible for sulphide spoilage is Desulfotomaculum nigrificans and is found in low acid foods like peas and corn. The spores of this bacterium are less heat resistant than those of flat sour and TA bacteria; as such the appearance of sulphide spoilage in canned food is the indication of gross under-processing.

C). Classification of microbial spoilage on the basis of acidity
The low acid food with pH above 5.3 is subject to flat sour spoilage and putrefaction. Medium acid foods with pH between 5.3 and 4.5 are likely to undergo TA spoilage. Acid food with pH between 4.5 and 3.7 are spoiled by special flat sour bacterium or by saccharolytic anaerobe. However, high acid food with a pH below 3.7 generally does not undergo spoilage by bacteria, but in the cans it may result in hydrogen swell. The type of the spoilage in the canned food can be classified on the basis of acidity of the food (Table 14.1).

Table 14.1: Classification of microbial spoilage based on acidity of the food

Type of food	Foods involved	Type of spoilage
Low acid pH > 5.4 Medium acid pH 5.3-4.6	Meat and fish products milk, vegetables like corn, lima beans, peas, meat and vegetable mixers.	i) Thermophilic flat sour group. (Bacillus sterothermophilus, B. coagulans). ii) Sulphide spoilage (Clostridium nigrificans, C. bifermentans). iii) Gas formers (Clostridium thermosaccharolyticum). iv) Mesophilic spoilage like putrefactive anaerobe. v) Spoilage and toxin production by Clostridium botulinum.
Acid food pH 3.7-4.6	Fruits, pears, figs, tomato etc.	i) Thermophilic spoiler Bacillus coagulans ii) Mesophilic spoiler B. polymyxa, Clostridium pasteurianum, C. butyricum, Lactobacilli etc.
High acid pH<3.7	Fruits like grape fruit, citrus, rhubarb etc and products like sauerkraut, pickles etc.	Non-spore forming mesophiles, yeast, mould and/or lactic acid bacteria.

Discolouration of fruit products
Besides microbial spoilage, the processed products may experience discolouration, which may be caused by various reactions brought by the action of enzymes, metallic contamination or through the reaction between different components.

i) Enzymatic browning: Browning of cut and peeled apples, potatoes and pears is caused by the oxidation of phenolic compounds brought about by the action of oxidase enzyme (Polyphenol oxidase) in the presence of air. The browning can be checked by placing the cut and peeled fruit in 2-3% NaCl solution until used for canning.

ii) Non-enzymatic browning: Browning of fruit products brought out by the reactions other than enzymes is called as non-enzymatic browning. The changes in colour of fruit products may be caused by reactions between i) nitrogenous matter and sugar ii) nitrogenous matter and organic acids iii) sugar and organic acids and iv) organic acids among themselves like ascorbic acid degradation and sugar degradation. The browning reactions between nitrogenous matter and sugar are known as Maillard reactions.

iii) Metallic contamination: The browning of canned fruit products is generally caused by the presence of iron and copper salts. Important metallic contaminations in fruit products include ferric tannate, iron sulphide, copper sulphide etc.

a) Ferric tannate: The natural tannins present in fruit and vegetables react with the iron of the tinplate of can to form ferric tannate which make the product black and spoils the appearance of the canned product.

b) Iron sulphide: Sulphur dioxide may be formed inside the can due to decomposition of protein in the product or it may come from the sulphited sugar used in canning. The SO₂ may react with hydrogen formed by the fruit acid acting on the tin plate and get reduced to H₂S, which in turn may react with the iron of the can and form the black iron sulphide. Ferrous sulphide releases obnoxious smell of H₂S, besides spoiling the appearance of product.

c) Copper sulphide: The copper from the plant and equipment made of copper or brass may find its entry into the product and such product when comes in contact with H₂S formed inside the can may form black copper sulphide, which causes discolouration of the product.

d) Black deposit in canned pumpkin. The amino compounds present in the pumpkin react with the iron of the can forming deposits in the canned product.

e) Discoloration in canned corn. Canned corn turns grey in colour. This is due to the formation of sulphides of iron and copper as a result of corrosion of the tin plate and tarnishing of the metal of the equipment, respectively. To prevent it only ‘C-enamel’ cans should be used.

f) Black deposit in canned fruits. In the case of fruits canned in syrups prepared from sugar, which sometimes contain sulphur dioxide, cause blackening of the tin plate due to the formation of iron sulphide.

g) Pink discoloration in canned pears, guava and peaches. Pears, peaches and guava turn pink, if the cans are not cooled properly after sterilization.

Thus major cause of spoilage of canned products are under processing, cooling of cans in contaminated water, defects in seaming operation, use of non-lacquered cans in some products etc which may be avoided during processing of canned products.