Canning is a highly regarded method for packaging food, ensuring its long-term storage and preservation. Typically, food is packed into metal containers and subjected to appropriate heat treatments, the specifics of which vary depending on the type of food being canned. While the incidence of food spoilage in cans is generally low, there is always a possibility that microorganisms may survive if the heat treatment is not conducted properly, thereby leading to the spoilage of the food.

Spoilage in canned foods can be attributed to both biological and chemical factors, and sometimes a combination of the two. Biological spoilage primarily occurs as a result of microbial growth within the can. Microorganisms, such as bacteria, yeasts, and molds, can find a way to survive and proliferate if the canning process does not eliminate them effectively. These spoilage microorganisms can degrade the food, causing changes in texture, appearance, flavor, and overall quality.

Chemical spoilage, on the other hand, arises from the reaction between acids present in the food and the iron in the can. This reaction produces hydrogen gas, leading to the expansion or swelling of the can. If this chemical reaction goes unchecked, it can compromise the integrity of the can and potentially result in leaks or even explosions, thereby exposing the food to further contamination and spoilage.

In addition to biological and chemical factors, certain environmental conditions can also contribute to the spoilage of canned foods. High summer temperatures and high altitudes can exacerbate the degree of swelling in cans, making them more susceptible to spoilage. Furthermore, improper handling and storage practices, such as overfilling, buckling, denting, or sealing the can while it is still cool, can also compromise the hermetic seal of the can, allowing microorganisms to enter and spoil the contents.

To mitigate the risk of microbial spoilage in canned foods, it is crucial to adhere to strict quality control measures throughout the canning process. This includes ensuring proper heat treatment to eliminate or inactivate any potential spoilage microorganisms. Additionally, using high-quality cans with protective coatings and maintaining appropriate storage conditions, such as cool temperatures, can help minimize the risk of spoilage.

In conclusion, while canned foods generally have a low incidence of spoilage, the presence of microorganisms and the potential for chemical reactions necessitate careful attention to the canning process. By employing effective heat treatments, maintaining optimal storage conditions, and adhering to proper handling practices, the risk of microbial spoilage in canned foods can be significantly reduced, ensuring the safety and quality of the preserved food products.

Types of the food based on pH

The classification of canned food based on pH is crucial in understanding the susceptibility of food to microbial spoilage. By categorizing canned food according to its pH level, we can identify different groups with varying levels of acidity. The pH of a food product plays a significant role in determining its microbial stability and potential for spoilage. Let’s explore the three main types of canned food based on pH:

1. Low-acid canned food (pH > 5.2): This category comprises food products with a pH above 5.2. Examples of low-acid canned food include meat products, milk, dairy products, and seafood. Due to their higher pH levels, these foods provide an environment that is less acidic, which can promote the growth of certain spoilage microorganisms if proper precautions are not taken during the canning process.
2. Acid canned food (pH 4.5-3.7): Acid canned food falls within a pH range of 4.5 to 3.7. This group includes a variety of fruits and vegetables, such as tomatoes, pears, figs, oranges, apricots, and pineapples. The natural acidity of these foods helps inhibit the growth of many spoilage microorganisms, making them less prone to spoilage. However, it is still essential to maintain proper canning techniques to ensure the long-term preservation of these acidic foods.
3. High acid canned food (pH < 3.7): High acid canned food consists of products with a pH level below 3.7. This category encompasses pickled products, fermented foods, ketchup, jams, jellies, and similar items. The high acidity of these foods makes them less susceptible to microbial spoilage. The low pH creates an inhospitable environment for many spoilage microorganisms, helping to extend the shelf life of these products.

Understanding the pH levels of canned foods is critical for determining the appropriate processing methods and storage conditions required to prevent spoilage. It enables producers to apply specific preservation techniques, such as adjusting pH levels, adding preservatives, or employing thermal processing, to ensure the safety and quality of the canned products.

Spoilage of Canned food and its causes

1. Chemical spoilages

Chemical spoilage of canned food can occur due to various reasons, leading to undesirable changes in the product. Several factors contribute to chemical spoilage, including storage temperature, acidity of the food, reactions between the food and metal cans, insufficient exhausting during canning, presence of soluble sulfur and phosphorus in food, and enzymatic action. Let’s explore these causes and the resulting defects:

1. Storage temperature: The storage temperature of canned food plays a critical role in preventing chemical spoilage. If stored at elevated temperatures, especially in warm and humid conditions, the chemical reactions within the food can accelerate. This can lead to the breakdown of nutrients, flavor degradation, and the development of off-odors and off-flavors.
2. Acidity of food: The acidity level of the food can influence its susceptibility to chemical spoilage. Foods with a higher pH, indicating lower acidity, are more prone to spoilage. Acidic foods, on the other hand, have a natural protective mechanism against spoilage microorganisms. However, even acidic foods can undergo chemical changes if not properly processed or stored.
3. Reaction between food and metal cans: Interactions between the food and the metal cans can lead to chemical spoilage. This is particularly true when the food contains acids or reactive compounds that can corrode or react with the metal. Over time, such reactions can compromise the integrity of the can, causing leaks, off-flavors, and loss of nutritional quality in the food.
4. Insufficient exhausting during canning: Exhausting, which involves removing air from the can prior to sealing, is a crucial step in the canning process. Insufficient exhausting can result in the presence of residual air or gases in the can, leading to chemical spoilage. This can cause swelling of the can, discoloration of the food, off-flavors, and even the potential for can rupture or explosion.
5. Presence of soluble sulfur and phosphorus in food: Some foods naturally contain soluble sulfur and phosphorus compounds. These compounds can react with metal ions in the can, leading to discoloration, off-flavors, and the loss of nutritive quality in the food.
6. Enzymatic action of food: Enzymes present in certain foods can continue to function even after canning, causing chemical changes in the product. This enzymatic action can lead to degradation of nutrients, discoloration, and off-flavors.

The defects resulting from chemical spoilage include swelling of the can, discoloration of the food, off-flavors, corrosion of the metal cans leading to loss of nutritive quality, and cloudiness observed in syrups, brine, or gravy.

To prevent chemical spoilage, it is essential to store canned foods in appropriate conditions, including cool temperatures. Properly controlling pH levels during the canning process, ensuring thorough exhausting, selecting appropriate can materials, and minimizing the presence of reactive compounds can also help prevent chemical spoilage.

2. Physical damage

Physical damage to canned foods can result in both microbial and chemical spoilage, compromising the quality and safety of the products. Several factors contribute to these types of damage, including mechanical or personnel failures, improperly formed seals or seams, defective containers or lids, and overfilling of the food. Let’s delve into each of these causes:

1. Failure to deliver the scheduled process: Mechanical or personnel failures can disrupt the proper execution of the scheduled canning process. This may occur due to equipment malfunctions or errors during operation. When the scheduled process is not carried out as intended, it can lead to inadequate heat treatment or incomplete sterilization, allowing microorganisms to survive and spoil the food.
2. Improperly or inadequately formed seals/seams: The formation of a hermetic seal or seam is vital in preserving canned food. If the sealing process is not performed correctly or the seal is insufficient, it can result in leaks or compromised integrity of the can. These openings provide a pathway for microorganisms to enter, leading to microbial spoilage. Additionally, the loss of vacuum within the can can expose the food to the surrounding environment, accelerating chemical reactions and potential spoilage.
3. Defective containers or defective lids: Cans with manufacturing defects, such as dents, bulges, or compromised structural integrity, pose a significant risk to the preservation of canned food. These defects can weaken the can, making it more susceptible to damage during transportation or storage. Similarly, defective lids that fail to form a secure closure can expose the food to contaminants and increase the likelihood of spoilage.
4. Overfilling of the food: Overfilling a can beyond its intended capacity can exert excess pressure on the seal, resulting in a compromised seal or seam. This can lead to leakage or the distortion of the can’s shape, making it more susceptible to physical damage. Overfilled cans may also experience difficulties during the thermal processing stage, affecting the proper heat distribution and sterilization of the food.

To mitigate physical damage and prevent spoilage in canned foods, manufacturers must prioritize quality control measures throughout the entire production process. This includes regular inspection and maintenance of equipment to prevent mechanical failures, ensuring proper sealing techniques and adequate seal strength, rigorous assessment of containers and lids for any defects, and adhering to recommended fill levels to maintain the structural integrity of the cans.

By addressing potential physical damage risks, producers can minimize the chances of microbial and chemical spoilage, safeguarding the quality, safety, and longevity of the canned food products.

3. Microbial spoilages

Microbial spoilage is a significant concern when it comes to canned food, and it can occur due to three primary reasons. These reasons include the survival of spores of thermophilic bacteria, the growth of surviving thermophilic bacteria, and recontamination of microorganisms due to can leakage. Let’s explore these causes in more detail:

1. Survival of spores of thermophilic bacteria: Thermophilic bacteria are heat-resistant microorganisms that can form spores, allowing them to survive extreme conditions, including the canning process. Despite the heat treatment applied during canning, some spores may remain viable. If these spores are not effectively eliminated, they can germinate and grow, leading to microbial spoilage of the canned food.
2. Growth of survived thermophilic bacteria: The survival of thermophilic bacteria in canned food can be attributed to various factors, such as inadequate cooling, inadequate heat treatment, and improper storage temperatures. If the cooling process after canning is not sufficient or if the heat treatment during canning is inadequate, it provides an opportunity for the surviving bacteria to grow. Similarly, improper storage temperatures, especially warmer conditions, can facilitate the growth of bacteria and contribute to spoilage.
3. Recontamination of microorganisms due to can leakage: Can leakage is another factor that can result in microbial spoilage of canned food. If the integrity of the can is compromised, either due to defects or damage, microorganisms from the surrounding environment can enter the can, leading to recontamination. These newly introduced microorganisms can multiply and cause spoilage, compromising the safety and quality of the food.

A. Spoilage by spore-forming thermophilic bacteria

Spoilage caused by spore-forming thermophilic bacteria is a significant concern in the preservation of canned foods. This type of spoilage can manifest in various forms, including flat sour spoilage, thermophilic anaerobic spoilage, and thermophilic anaerobic sulfide spoilage. Let’s explore each of these types in more detail:

1. Flat sour spoilage: Flat sour spoilage occurs when canned products become sour without any noticeable can swelling. It is observed in both low-acid and acid foods. This type of spoilage is caused by the germination and growth of facultative anaerobic bacteria, including Geobacillus stearothermophilus, Bacillus coagulans, Bacillus thermoacidurans, and Bacillus stearothermophilus. These bacteria can produce acid from carbohydrates present in the food. The source of these organisms is typically plant equipment, sugar, starch, or soil.
2. Thermophilic anaerobic spoilage: Thermophilic anaerobic spoilage is caused by anaerobic bacteria belonging to the genera Thermoanaerobacter and Thermoanaerobacterium, such as Clostridium thermosaccharolyticum. During this type of spoilage, these bacteria produce hydrogen (H2) and carbon dioxide (CO2) gases. The sour fermentation occurs in medium-acid canned foods at high temperatures. The gas production leads to can swelling and potentially even bursting. Thermophilic anaerobic spoilage bacteria often produce a butyric or “cheesy” odor in the affected food. This type of spoilage can occur due to factors such as slow cooling or hot storage.
3. Thermophilic anaerobic sulfide spoilage: Thermophilic anaerobic sulfide spoilage is caused by Gram-negative obligate anaerobic spore-forming bacteria, including Disulfotomacillum nigrificans, Clostridium bifermentans, and Clostridium sporogenes. This type of spoilage occurs in low-acid canned food. The bacteria produce hydrogen sulfide (H2S), resulting in a characteristic “rotten egg” odor in the canned food. The H2S gas can be absorbed by the food, leading to the production of iron sulfide and causing black discoloration in both the food and the inside of the cans. Thermophilic anaerobic sulfide spoilage can occur due to under-processing, slow cooling, or hot storage of the canned food.

B. Spoilage by mesophilic spore-forming bacteria

Spoilage caused by mesophilic spore-forming bacteria is a significant concern in the preservation of canned foods. This type of spoilage can occur due to different bacterial species, including Clostridium and Bacillus, as well as non-spore-forming bacteria. Let’s explore each of these categories in more detail:

1. Spoilage by Mesophilic Clostridium Species: Several species of Clostridium, such as C. butyricum, C. pasteurianum, C. sporogenes, C. putrefaciens, and C. botulinum, can cause spoilage in low-acid canned foods. These organisms have the ability to survive and proliferate under anaerobic conditions. They ferment carbohydrates present in the food, leading to the production of volatile acids, including butyric acid. Additionally, they generate hydrogen (H2) and carbon dioxide (CO2) gases, resulting in can swelling. Proteolytic Clostridium species metabolize proteins, producing foul-smelling compounds such as hydrogen sulfide (H2S), mercaptans, indoles, and ammonia. Insufficient heat treatment during canning can allow the survival of these mesophilic Clostridium species, increasing the risk of spoilage.
2. Spoilage by Mesophilic Bacillus Species: Mesophilic Bacillus species, such as B. subtilis, B. mesentericus, B. polymyxa, and B. macerans, can cause spoilage in low-acid canned foods. Inadequate evacuation of the can can create an environment conducive to the growth of these bacteria. Insufficient heat treatment can also contribute to the survival of mesophilic Bacillus species, increasing the risk of spoilage. Furthermore, B. polymyxa and B. macerans can cause spoilage in canned foods due to can leakage.
3. Spoilage by Non-Spore-Forming Bacteria: Certain non-spore-forming bacteria can survive in cans, either due to mild heat treatment or can leakage. Thermoduric non-spore-forming bacteria, including Enterococcus (E. faecalis and E. faecium), Microbacterium, Streptococcus thermophilus, Micrococcus, and Lactobacillus, can survive in canned foods. Acid-forming Lactobacillus species can produce carbon dioxide (CO2), leading to can swelling. Additionally, bacteria such as Pseudomonas, Alcaligenes, Micrococcus, Flavobacterium, and Proteus, among others, can cause spoilage in leaked canned foods.

C. Spoilage by non-spore-forming bacteria

• In addition to spore-forming bacteria, non-spore-forming bacteria can also contribute to the spoilage of canned foods. These bacteria have the ability to survive in cans due to mild heat treatments, such as pasteurization, or as a result of can leakage. Let’s explore this type of spoilage in more detail:
• Thermoduric non-spore-forming bacteria are a group of bacteria that can survive in cans despite mild heat treatments. This group includes Enterococcus species (such as E. faecalis and E. faecium), Microbacterium, Streptococcus thermophilus, Micrococcus, and Lactobacillus. These bacteria are capable of withstanding moderate temperatures and can persist in canned foods.
• Lactic acid bacteria, particularly Lactobacillus, are among the non-spore-forming bacteria that can survive in under-processed canned food products. These bacteria have the ability to produce CO2 during fermentation, which can result in can swelling.
• Aside from lactic acid bacteria, other non-spore-forming bacteria such as Pseudomonas, Alcaligenes, Micrococcus, Flavobacterium, and Proteus can also cause spoilage in leaked canned foods. These bacteria can contaminate the food when can leakage occurs, leading to deterioration of the product.
• The presence of non-spore-forming bacteria in canned foods highlights the importance of proper heat treatment and can integrity in preventing spoilage. Adequate processing and sealing measures are essential to eliminate or minimize the survival of these bacteria and maintain the quality and safety of canned food products.
• Manufacturers must adhere to rigorous quality control protocols and follow recommended guidelines to ensure proper canning practices. Regular testing, inspection, and monitoring can help detect and prevent the growth of non-spore-forming bacteria, mitigating the risk of spoilage and ensuring the longevity of canned foods.

D. Spoilage by yeast and mold

Yeast and mold are common causes of spoilage in canned foods, indicating issues such as underprocessing, leakage, recontamination, and poor evacuation. The presence of these microorganisms can lead to various defects and changes in the canned products. Let’s explore the spoilage caused by yeast and mold in more detail:

1. Yeast Spoilage: Yeasts can ferment carbohydrates, leading to the production of carbon dioxide (CO2) gas. This gas generation can result in can swelling. Yeasts are commonly found on the surface of high-acid foods, such as pickled products. For instance, Torulopsis lactis and T. globosa can cause blowing or gaseous spoilage in sweetened condensed milk that has not undergone heat processing. Another yeast, T. stellata, can cause spoilage in canned lemon products, growing at a pH as low as 2.5.
2. Mold Spoilage: Mold species, including Aspergillus, Byssochlamys, Penicillium, and Citromyces, are often found in canned foods with high sugar content, reaching levels of 70-72%. Mold spoilage in canned food is characterized by a moldy taste, odor, color fading, and the presence of mold mycelia. In some cases, slight swelling of the container may also occur.

The type of spoilage and the defects caused by yeast and mold can vary based on the acidity of the canned food. Let’s explore these differences:

• Low-acid canned food (pH > 5.2): Thermophilic flat-sour spoilage bacteria, such as Geobacillus stearothermophilus and Bacillus coagulans, can cause defects such as reducing pH, souring, off-odor, and flavor changes. Sulfide-producing anaerobic bacteria, like Clostridium nigrificans and Desulfotomaculum nigrificans, can lead to blackening, a “rotten egg” odor, and the formation of iron sulfide precipitate. Mesophilic putrefactive anaerobic bacteria, including Clostridium botulinum, can cause can swelling, putrid odor, and the production of various compounds.
• Acid canned food (pH 4.5-3.7): Mesophilic spore-forming bacteria, such as Bacillus polymyxa, Bacillus macerans, and Clostridium pasteurianum, can lead to flat or swollen cans, coagulation, gas formation, and acid production. Butyric anaerobic bacteria, like Clostridium butyricum and Clostridium tertium, can cause can swelling, fermentation, and a butyric odor. Aciduric bacteria, including G. stearothermophilus and B. coagulans, can result in changes in vacuum, pH reduction, souring, off-odors, flavors, and sometimes cloudy juice.
• High acid canned food (pH < 3.7): Yeasts such as Saccharomyces and Zygosaccharomyces can cause can swelling, leakage, fermentation, CO2 production, and yeasty odor. Molds, including B. fulva, Aspergillus, Penicillium, and Citromyces, can also result in can swelling, leakage, fermentation, CO2 production, and yeasty odor. Lactic acid bacteria such as L. brevis, L. mesentericus, L. dextranicum, and L. mobilis can cause can swelling, acid odor, ropiness, and the formation of acid and CO2.

Preservation of canned food from microbial spoilage

Canned food undergoes thermal processing to achieve “commercial sterility” and ensure long-term storage. However, it’s important to note that canning alone does not guarantee freedom from microbial spoilage. Recontamination of canned food can occur due to various factors. To prevent microbial spoilage and extend the shelf life of canned food while maintaining its nutritive quality, additional preservation methods can be applied in combination with canning. Let’s explore some of these methods:

1. Radiation: Radiation sterilization, also known as radappertization, involves exposing food in sealed containers to ionizing radiation. This process effectively kills all spoilage-causing organisms. Radiation is particularly useful for eliminating microorganisms on the surfaces of canned jars and their lids, further reducing the risk of spoilage.
2. Preservative agents: Preservatives are substances that inhibit or retard the growth of microorganisms and are considered “Generally Regarded As Safe (GRAS).” They can be used in conjunction with canning to enhance the preservation of canned food. Some examples of preservatives include simple organic acids, sulfite, ethylene oxide, sodium nitrite, ethyl formate, lactic acid bacteria (LAB), and bacteriocins. These agents help to inhibit the growth of spoilage microorganisms and extend the shelf life of canned food.
3. Chilling storage: Canned foods can be stored at temperatures between 0–5 °C. Chilling storage is employed to reduce the rate of microbial growth and enzymatic activities. Lower temperatures inhibit the proliferation of microorganisms, helping to extend the shelf life of canned food. Chilling storage also slows down enzymatic reactions, preserving the quality and freshness of the product.

Implementing a combination of these preservation methods with canning can effectively safeguard canned food from microbial spoilage. Each method has its own advantages and considerations, and the selection of preservation techniques may depend on factors such as the specific food product, desired shelf life, and regulatory requirements.

It is crucial for manufacturers to adhere to good manufacturing practices, maintain strict quality control measures, and conduct regular inspections and testing to ensure the efficacy of preservation methods and prevent recontamination. By employing a multi-faceted approach to preservation, producers can maintain the safety, quality, and longevity of canned food, meeting consumer expectations and reducing the risk of spoilage.

What is Canning?

• Canning is a highly effective method of food preservation that involves processing and sealing food contents in a hermetically sealed container, such as metal cans, glass jars, plastic cans, thermostable plastic, or multilayered flexible pouches.
• This preservation technique, also known as Appertization after its inventor Nicolas Appert, is widely used to extend the shelf life of food for an extended period. The primary objective of canning is twofold: to eliminate pathogenic and spoilage microorganisms through heat application and to prevent contamination by new microorganisms through the hermetic sealing of the container.
• The process begins with the preparation of the food, which may involve cleaning, cutting, cooking, or other treatments depending on the specific product. The prepared food is then packed into the selected containers, and the containers are sealed to create a tight, impermeable seal.
• The next crucial step is the application of heat. The sealed containers are subjected to heat treatment, often using methods such as steam, boiling water, or pressure canning. The specific heat treatment parameters, including temperature and duration, are carefully determined based on the type of food being processed. The goal is to raise the temperature sufficiently to destroy any existing microorganisms, including harmful pathogens, while also inactivating enzymes that can cause spoilage.
• Once the heat treatment is complete, the containers are rapidly cooled to prevent any potential recontamination. This cooling process helps maintain the integrity of the food and ensures its quality and safety.
• The hermetically sealed containers play a crucial role in preserving the food. They create a barrier that prevents the entry of air, moisture, and microorganisms from the surrounding environment. This sealing process helps safeguard the food, maintaining its freshness, flavor, and nutritional value over an extended period.
• Canning is widely employed in the food industry to preserve a wide range of products, including fruits, vegetables, meats, seafood, soups, sauces, and more. It allows for safe, convenient, and long-term storage of food, reducing waste and ensuring food availability even when fresh produce is not in season.
• By employing the canning process, food manufacturers can provide consumers with high-quality, shelf-stable products that can be stored for extended periods without compromising safety or taste. Canning continues to be a popular and reliable method of food preservation, contributing to food security and reducing food waste worldwide.

Steps of canning

Canning involves a series of carefully executed steps to ensure the preservation and safety of food. Let’s explore the various stages involved in the canning process:

1. Preparations: The containers that will hold the food are preheated to sterilize them. This step helps ensure the cleanliness and safety of the containers. The food preparation varies depending on the type of food being canned. It may involve washing, trimming, sorting, grading, peeling, cutting, dicing, slicing, segmenting, pitting, or blanching, among other processes specific to the food item.
2. Filling in the containers: The prepared food is then filled into the sterilized containers. Depending on the specific product, the canned goods may be filled with hot solutions such as sugar syrup or fruit juices, brine with spices for vegetables, or gravy for meats. This step adds flavor, preservative agents, and helps with further sterilization while minimizing the presence of air space within the container.
3. Exhausting: Exhausting is the process of removing air from the can before sealing. This step is crucial to prevent excessive pressure and strain during sterilization. By eliminating excess air, exhausting reduces the likelihood of can bulging and minimizes chemical reactions between the container and the food contents.
4. Sealing: Once exhausting is complete, the can is immediately sealed. The sealing method varies depending on the type of product. Glass jars and bottles may be sealed using screw-type or crown capping machines, while plastic pouches are sealed using dielectric or conduction heating. It is essential to maintain the temperature of the canned food during sealing, ensuring it remains above 74°C.
5. Processing of the can: The sealed cans undergo heat treatment immediately after sealing. The primary purpose of can processing is to eliminate microorganisms and their spores while preserving the nutritional quality of the food. Heat is applied at temperatures typically above 100°C. Vegetative bacteria are killed at 80°C for 30 minutes, spore-forming microorganisms are killed at 110°C for 30 minutes, and spores are destroyed at 121°C for 3 minutes.
6. Container cooling: Following processing, the cans are rapidly cooled to room temperature. This cooling process can be achieved by immersing the hot cans in cold water or by spraying them with jets of cold water. Rapid cooling helps maintain the quality and safety of the canned food.
7. Labeling and storage: After cooling, the outer surface of the cans is dried for labeling. The labels provide important information such as product identification, brand name, net content by weight or volume, code number, name and address of the processor, list of ingredients, nutritional information, and manufacturing and expiry dates. Once labeled, the cans are stored in cool and dry places before being dispatched.

Following these steps ensures that canned food is properly processed, sealed, and labeled, maintaining its quality, safety, and shelf life. Canning is a reliable method of preserving food, allowing for long-term storage and availability of various food items throughout the year.

FAQ

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