Microbial Spoilage of Canned Food: Causes, Types, and Preservation Methods

Table of Contents

• Introduction to Microbial Spoilage of Canned Foods
• Spoilage of Canned food and its causes
• A. Spoilage by spore-forming thermophilic bacteria
• B. Spoilage by mesophilic spore-forming bacteria
• C. Spoilage by non-spore-forming bacteria
• D. Spoilage by yeast and mold
• Preservation of canned food from microbial spoilage
• What is Canning?
• Steps of canning
• References

Introduction to Microbial Spoilage of Canned Foods

• Microbial spoilage of canned foods is influenced by the pH level of the food.
• Based on pH, canned foods can be categorized as follows:
• Low-acid canned foods (pH > 5.2) include meat products, milk, dairy products, and seafood.
• Acid canned foods (pH 4.5–3.7) include tomatoes, pears, figs, oranges, apricots, pineapples, etc.
• High-acid canned foods (pH < 3.7) include pickled products, fermented products, ketchup, jams, jellies, etc.

Spoilage of Canned food and its causes

1. Physical damage leading to spoilage

• Physical damage can result in microbial or chemical spoilage due to the following reasons:
• Scheduled process not delivered because of mechanical or personnel failure.
• Improperly or inadequately formed seals/seams.
• Defective containers or defective lids of the containers.
• Overfilling of the food.

2. Chemical spoilages

Chemical spoilage may occur due to:

• Storage temperature.
• Acidity of food.
• Reaction between food and metal cans.
• Insufficient exhausting during canning.
• Presence of soluble sulphur and phosphorus in food.
• Enzymatic action of food.

These factors can lead to:

• Swelling of cans.
• Discoloration of food.
• Off-flavors in food.
• Corrosion of metal cans, leading to loss of nutritive quality.
• Cloudiness observed in syrups, brine, or gravy.

3. Microbial spoilages

Microbial spoilage occurs due to:

• Survival of spores of thermophilic bacteria.
• Growth of these bacteria due to inadequate cooling, insufficient heat treatment, or improper storage temperature.
• Recontamination of microorganisms due to can leakage.
• Microbial spoilage of canned foods and its preservation.

The microorganisms responsible for canned food spoilage are: 

A. Spoilage by spore-forming thermophilic bacteria

1. Flat sour spoilage

• Flat sour spoilage causes sourness in the product due to acid production from carbohydrates, without swelling of the can.
• It occurs only in low-acid foods and acid foods.
• Caused by the germination and growth of facultative anaerobic bacteria such as Geobacillus stearothermophilus, Bacillus coagulans, Bacillus thermoacidurans, and Bacillus stearothermophilus.
• Sources of these organisms include plant equipment, sugar, starch, and soil.

2. Thermophilic anaerobic spoilage

• Caused by anaerobic bacteria such as Thermoanaerobacter and Thermoanaerobacterium (e.g., Clostridium thermosaccharolyticum).
• Produces hydrogen (H₂) and carbon dioxide (CO₂) gases.
• Sour fermentation occurs at high temperatures in medium-acid canned foods.
• The gases cause can swelling, sometimes leading to bursting of the can.
• Often results in a butyric or "cheesy" odor in the food.
• Occurs due to slow cooling or storage at high temperatures (hot storage).

3. Thermophilic Anaerobic Sulfide Spoilage

• Caused by Gram-negative obligate anaerobic spore-formers such as Desulfotomaculum nigrificans, Clostridium bifermentans, and Clostridium sporogenes in low-acid canned foods.
• Produces hydrogen sulfide (H₂S), causing a "rotten egg" odor without can swelling.
• H₂S reacts with food to form iron sulfide, leading to black coloration of food and the interior of cans.
• Occurs due to under-processing, slow cooling, or hot storage.

B. Spoilage by mesophilic spore-forming bacteria

1. Spoilage by Mesophilic Clostridium Species 

• Several Clostridium species such as C. butyricum and C. pasteurianum can survive in low-acid canned foods and cause spoilage.
• These organisms ferment carbohydrates to produce volatile acids like butyric acid and generate H₂ and CO₂ gases, resulting in can swelling.
• Proteolytic species such as C. sporogenes, C. putrefaciens, and C. botulinum also spoil canned foods.
• These bacteria metabolize proteins and release foul-smelling compounds such as hydrogen sulfide (H₂S), mercaptans, indoles, and ammonia.
• Inadequate heat treatment allows the survival of mesophilic Clostridium spp.

2. Spoilage by Mesophilic Bacillus Species

• Mesophilic Bacillus subtilis and Bacillus mesentericus cause spoilage in low-acid canned foods.
• Spoilage commonly occurs in poorly evacuated cans.
• Insufficient heat treatment permits survival of mesophilic Bacillus species.
• Bacillus polymyxa and Bacillus macerans also lead to spoilage due to can leakage.

C. Spoilage by non-spore-forming bacteria

• Non-spore-forming bacteria can survive in canned food due to mild heat treatments (such as pasteurization) or from can leakage.
• Thermoduric non-spore-formers include Enterococcus (E. faecalis and E. faecium), Microbacterium, Streptococcus thermophilus, Micrococcus, and Lactobacillus.
• Acid-forming Lactobacillus species may survive under-processed canned foods.
• These bacteria produce CO₂, causing can swelling.
• Other spoilage-causing bacteria in leaked cans include Pseudomonas, Alcaligenes, Micrococcus, Flavobacterium, Proteus, and others.

D. Spoilage by yeast and mold

• The presence of yeast and mold in canned foods indicates issues such as under-processing, leakage, recontamination, or poor evacuation.
• Fermentative yeasts produce CO₂, which leads to swelling of the can.
• Yeasts commonly grow on the surface of high-acid foods, including pickled products.
• Torulopsis lactis and T. globosa cause blowing or gaseous spoilage in sweetened condensed milk, especially when it is not heat-processed.
• Torulopsis stellata causes spoilage in canned lemon and can grow at a low pH of 2.5.
• Mold species such as Aspergillus, Byssochlamys, Penicillium, and Citromyces are frequently found in canned foods with high sugar content (70–72%).
• Mold spoilage of canned food is recognized by moldy taste, unpleasant odor, color fading, presence of mold mycelia, and occasionally slight swelling of the container.

Types of Canned Food Spoilage Based on Acidity: Causes and Resulting Defects in Cans and Food Products

Low-acid canned food (pH > 5.2):

• Examples: Meat and meat products, milk, dairy items, seafood.
• Cause: Thermophilic flat-sour bacteria (Geobacillus stearothermophilus, Bacillus coagulans).
• Can defects: Flat, no gas formation, altered vacuum.
• Product defects: Lowered pH, sour taste, off-odor and off-flavor, occasionally cloudy juice.
• Examples: Corn, peas.
• Cause: Sulfide-producing anaerobes (Clostridium nigrificans, Clostridium bifermentans, Desulfotomaculum nigrificans).
• Can defects: Flat, H₂S absorbed by the food.
• Product defects: Blackening of food, “rotten egg” odor, iron sulfide precipitate.
• Examples: Corn, spinach.
• Cause: Mesophilic putrefactive anaerobic bacteria (Clostridium botulinum).
• Can defects: Swelling, possible bursting due to gas production.
• Product defects: Partial digestion, increased pH, putrid odor, production of ammonia (NH₃), indoles, CO₂, H₂, and H₂S.

Acid canned food (pH 4.5–3.7):

• Examples: Tomatoes, pears, figs, oranges, apricots, pineapples.
• Cause: Mesophilic spore-formers (Bacillus polymyxa, Bacillus macerans, Clostridium pasteurianum).
• Can defects: Flat or swollen.
• Product defects: Coagulation, gas and acid production.
• Cause: Butyric anaerobes (Clostridium butyricum, Clostridium tertium).
• Can defects: Swollen, may burst.
• Product defects: Fermentation, gas production (CO₂, H₂), butyric (rancid) odor.
• Cause: Aciduric bacteria (Geobacillus stearothermophilus, Bacillus coagulans).
• Can defects: Flat, no gas formation, altered vacuum.
• Product defects: Lowered pH, souring, off-odor and flavor, occasionally cloudy juice.

High-acid canned food (pH < 3.7):

• Examples: Pickled and fermented products, ketchup, jams, jellies, marmalades, fruits in syrup, fruit butters.
• Cause: Yeasts (Saccharomyces, Zygosaccharomyces).
• Can defects: Swelling, bursting, or leakage due to gas production.
• Product defects: Fermentation, CO₂ formation, and yeasty odor.
• Cause: Molds (Byssochlamys fulva, Aspergillus, Penicillium, Citromyces).
• Can defects: Swelling, bursting, or leakage due to gas formation.
• Product defects: Fermentation, CO₂ production, and moldy/yeasty odor.
• Cause: Lactic acid bacteria (Lactobacillus brevis, L. mesentericus, L. dextranicum, L. mobilis).
• Can defects: Swelling, possible bursting, or halted swelling.
• Product defects: Acidic odor, ropiness, and formation of CO₂ and acids.

Preservation of canned food from microbial spoilage

• Canned foods undergo thermal processing to achieve “commercial sterility,” allowing for extended shelf life under storage conditions.
• Although canning is a heat-based preservation method, it does not guarantee complete protection from microbial spoilage.
• Recontamination may still occur due to various factors post-processing.
• To effectively prevent microbial spoilage and maintain the nutritional quality of canned foods, additional preservation methods can be used in combination with canning to further extend shelf life.

1. Radiation

• Radiation sterilization, also known as radappertization, involves exposing sealed canned foods to ionizing radiation to eliminate spoilage microorganisms.
• This method is particularly effective for disinfecting the surfaces of jars and lids used in canning.

2. Preservatives agents

• Preservatives are compounds that inhibit or slow down the growth of microbes and are often classified as “Generally Recognized As Safe (GRAS).”
• Examples include organic acids, sulfites, ethylene oxide, sodium nitrite, ethyl formate, lactic acid bacteria (LAB), and bacteriocins.

3. Chilling storage

• Storing canned foods at low temperatures (0–5 °C) helps reduce microbial growth and enzymatic activity.
• This cooling significantly extends the shelf life and maintains the quality of canned food products.

What is Canning?

• Canning is a food preservation technique in which food is processed and sealed in an airtight container (such as metal cans, glass jars, plastic containers, thermostable plastics, or multilayered flexible pouches).
• This method is also known as Appertization, named after its inventor, Nicolas Appert.
• Canning is an effective way to extend the shelf life of food products for a prolonged period.
• The primary goal of applying heat during canning is to destroy harmful pathogens and spoilage microorganisms. The airtight seal of the container prevents any new microorganisms from contaminating the food.

Steps of canning

1. Preparations

• The containers intended for filling are preheated for sterilization.
• The preparation of food depends on its type and typically includes processes like washing, trimming, sorting, grading, peeling, cutting, dicing, slicing, segmenting, pitting, and blanching.

2. Filling in the containers

• The prepared food is filled into sterilized containers.
• The container is then filled with hot solutions such as sugar syrup or fruit juices, brine with spices for vegetables, or gravy for meats.
• This step adds flavor, preservatives, assists in further sterilization, and helps remove air spaces.

3. Exhausting

• Exhausting refers to the process of removing air from the container before sealing.
• This step helps prevent excessive pressure and strain during sterilization. It prevents the bulging of cans and minimizes the chemical reactions between the container and food.

4. Sealing

• After exhausting, the containers are immediately sealed.
• Different types of sealing methods are used depending on the product, such as screw-type or crown capping machines for glass jars and bottles, or dielectric or conduction heating for plastic pouches.
• It is crucial that the temperature of the canned food is not below 74°C during sealing.

5. Processing of the can

• The sealed cans are heat-treated immediately after sealing at the appropriate temperature and time.
• The goal is to eliminate microorganisms and their spores without affecting the food’s nutritional quality.
• Heat is typically applied at temperatures above 100°C (vegetative bacteria are killed at 80°C for 30 minutes, spore-forming microorganisms at 110°C for 30 minutes, and spores are destroyed at 121°C for 3 minutes).

6. Container cooling

• After processing, the cans are rapidly cooled to room temperature.
• This can be done by immersing the hot cans in cold water or by spraying them with jets of cold water.

7. Labeling and storage

• After cooling, the outer surface of the cans is dried for labeling.
• The label includes product identification, brand name, net content by weight or volume, code number, processor’s name and address, ingredients, nutritional information, manufacture date, and expiry date.
• After labeling, the cans are stored in cool and dry places until they are ready for dispatch.

References

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