Canned foods:
- Heated canned foods are microbiologically more stable than most other foods.
- Better keeping quality of heated canned food is due to:
- Heat treatment applied during canning kills most of the microorganisms present in original food.
- Recontamination of food from outside is prevented by sealed can.
Spoilage of heated canned food:
- Spoilage of heated canned foods occurs by two ways:
- Survival of spores of some thermophilic bacteria during heating of canned foods.
- Recontamination of microorganisms from outside through leak in container.
Various types of spoilage of heated canned food are:
i. Spoilage of canned food by spore forming thermophilic bacteria:
- Spores of thermophilic bacteria are not killed easily by heat treatment applied during canning.
- Later, spore germinates in food and cause spoilage.
- Three important types of spoilage caused by thermophilic spore formers are:
- Flat sour spoilage:
- This type of spoilage occurs mainly in low acid foods and is caused by species of Bacillus, such as Bacillus coagulase and Bacillus stearothermophilus.
- Later, spore germinate when canned food is kept hot for sometimes as in case of slow cooling of can.
- They produce acid without gas in food.
- Therefore, the can remains flat during spoilage and hence spoilage cannot be detected by examination of can from outside.
- Thermophile anaerobe spoilage:
- A spoilage is caused by Clostridium thermosaccharolyticum, which is a thermophilic anaerobe.
- It produces acid and gas in food. Gas is a mixture of CO2 and H2.
- Due to gas production, can swells and finally burst if it is kept for long time.
- The spoiled food has usually sour odor or taste.
- Sulfides or sulfur stinker spoilage:
- It is caused by Desulfotomaculum nigrificans.
- Spoilage usually occur in low acid food.
- Spore of these organisms are usually less heat resistant than those of flat sour and thermophile anaerobes bacteria.
- Therefore, spoilage by this organism indicates inadequate heat treatment during canning.
- The organism is also obligate thermophile. Therefore, spoilage by this organism occur in case of poor cooling or hot storage of can.
- The organism produces H2S that react with tin of can to form black spots of FeS in food and on inner wall of can.
- H2S produced give putrid odor which is widened when can is opened.
ii. Spoilage of canned foods by spore forming mesophilic bacteria:
- Spoilage by mesophilic spore formers result from inadequate heat treatment during canning.
- Spoilage by mesophilic Clostridium species:
- Several mesophilic Clostridium causes spoilage of canned food.
- Sugar fermenting species such as Clostridium butyrium and C. pasteurianum produced butyric acid by fermentation of sugar.
- They also produce hydrogen and carbon-dioxide that sweet the can other species such as Clostridium botulinum, C. sporogens and C. putrifaciens are proteolytic or putrefactive.
- They decompose protein to produce H2S, endole, skatable and mercaptans that give bad odor to the food.
- Spoilage by mesophilic Bacillus species:
- Many species of Bacillus are aerobic and therefore cannot grow in well evacuated can.
- They cause spoilage especially in poorly evacuated can.
- Bacillus subtilis, B. mesentericus and B. polymyxa cause spoilage of many canned foods such as corn, tomato etc.
- Since, spores of these organisms are less heat resistant, they usually spoiled food by contaminating through leakage container.
iii. Spoilage of canned foods by non-spore forming bacteria:
- Non spore forming bacteria cause spoilage of canned food if mild heat treatment is applied during canning or, when they enter into the can through leakage during cooling by contaminated water.
- Many thermoduric bacteria such as Enterococcus, Streptococcus thermophilus, Micrococcus, Lactobacillus and Leuconostoc species can survive pasteurization temperature and spoil the canned food.
- However, in most of the cases, spoilage by non-spore formers indicates leakage of container.
- Type of spoilage depends on type of microorganisms. For example: Coliforms and heterofermentative lactic acid bacteria swell the can by production of gas.
- Non gas forming bacteria such as Pseudomonas, Alcaligenes, Micrococcus, Proteus etc. also spoiled canned food.
iv. Spoilage of canned foods by yeast and mold:
- When yeast grow in canned food, they cause swelling of can by production of CO2.
- Spoilage by yeast indicates recontamination through leakage or lack of heat processing plus poor evacuation.
- Spoilage by mold:
- Some species of mold such as Sclerotia and Byssochlamys fulva are somewhat heat resistant and they survive mild heat treatment.
- Other molds enter through leakage in can.
- Mold cause spoilage of high acid and high sugar foods such as jam strain of Aspergillus penicillium and Citromyces are found commonly in canned food.