Microbial spoilage of canned foods and its preservation (Canning)
Food preservation is the process of preventing food spoilage and extending its shelf life. Food spoilage is the deterioration of food quality and safety due to physical, chemical, and microbial factors. Food preservation methods aim to inhibit or eliminate these factors by applying various techniques such as drying, freezing, salting, smoking, fermenting, pickling, and canning.
Canning is one of the most widely used food preservation methods in the world. It involves processing and sealing food in airtight containers that can withstand high temperatures and pressures. Canning was invented in the early 19th century by Nicolas Appert, a French confectioner who experimented with preserving food in glass bottles. He discovered that heating food in sealed containers could kill harmful microorganisms and prevent spoilage. He also found that adding salt, sugar, vinegar, or spices could enhance the flavor and shelf life of canned food.
Canning has many advantages as a food preservation method. It can preserve almost any type of food, such as fruits, vegetables, meats, seafood, dairy products, soups, sauces, jams, and juices. It can retain the nutritional value, color, texture, and taste of food for a long time. It can also reduce food waste and transportation costs by making food available all year round and in remote areas. Canning is also a safe and convenient way of storing and consuming food, as it does not require refrigeration or special preparation before eating.
However, canning also has some disadvantages and challenges. It requires specialized equipment and skills to ensure proper processing and sealing of containers. It also consumes a lot of energy and water during the heating and cooling stages. Canning may also affect the quality of food by causing changes in texture, flavor, color, and nutrient content due to high temperatures and pressures. Moreover, canning may pose health risks if not done correctly or if the containers are damaged or contaminated. Some of the common problems associated with canned food are botulism, flat sour spoilage, thermophilic anaerobic spoilage, sulfide spoilage, and mold spoilage.
Therefore, canning is a complex and delicate process that requires careful attention to details such as the type of food, the pH level, the container material and size, the filling method, the exhausting method, the sealing method, the processing time and temperature, the cooling method, the labeling method, and the storage conditions. In this article, we will discuss these aspects in detail and explain how canning can prevent microbial spoilage of canned food. We will also classify canned food based on pH and describe the causes and types of microbial spoilage that may occur in canned food. Finally, we will suggest some alternative or complementary preservation methods that can enhance the safety and quality of canned food.
The pH of food is a measure of its acidity or alkalinity. It affects the growth and survival of microorganisms that can cause spoilage or foodborne illness. Therefore, based on pH, canned food can be divided into three categories:
- Low-acid canned food (pH > 5.2): These foods have a high pH and can support the growth of spore-forming bacteria, such as Clostridium botulinum, which can produce a deadly toxin. Examples of low-acid canned foods include meat products, milk, dairy products, and seafood. These foods require a high temperature and pressure treatment (such as retorting) to ensure their safety and shelf life.
- Acid canned food (pH 4.5-3.7): These foods have a moderate pH and can inhibit the growth of most bacteria, but may allow the growth of acid-tolerant bacteria, such as Bacillus coagulans and Lactobacillus. Examples of acid canned foods include tomatoes, pears, figs, oranges, apricots, pineapples, etc. These foods require a lower temperature and pressure treatment (such as boiling water bath) than low-acid foods to ensure their safety and shelf life.
- High-acid canned food (pH < 3.7): These foods have a low pH and can prevent the growth of almost all bacteria, except for some acidophilic molds and yeasts. Examples of high-acid canned foods include pickled products, fermented products, ketchup, jams, jellies, etc. These foods require the least temperature and pressure treatment (such as pasteurization) to ensure their safety and shelf life.
The classification of canned food based on pH is important for determining the appropriate heat treatment and storage conditions to prevent microbial spoilage and ensure food quality and safety.
Canned food spoilage can occur due to various factors that compromise the quality and safety of the product. The main causes of spoilage are:
- Physical damage: This refers to any defect or damage in the container that allows the entry of air, water, or microorganisms into the food. Physical damage can be caused by improper sealing, defective cans or lids, overfilling, rough handling, punctures, or mechanical failures. Physical damage can lead to microbial or chemical spoilage of the food.
- Chemical spoilage: This refers to any deterioration of the food due to chemical reactions between the food and the can, or within the food itself. Chemical spoilage can be caused by storage temperature, acidity of food, insufficient exhausting during canning, presence of soluble sulfur and phosphorus in food, or enzymatic action of food. Chemical spoilage can result in swelling of can, discoloration of food, off-flavors, corrosion of metal cans, loss of nutritive quality, or cloudiness of syrups or brines.
- Microbial spoilage: This refers to any spoilage caused by the growth and activity of microorganisms in the food. Microbial spoilage can be caused by survival of spores of thermophilic bacteria, growth of survived thermophilic bacteria due to inadequate cooling, inadequate heat treatment, or improper storage temperature, or recontamination of microorganisms due to can leakage. Microbial spoilage can be classified into different types based on the type of microorganism and the type of canned food involved. These types are:
- Spoilage by spore-forming thermophilic bacteria: These are bacteria that form heat-resistant spores and grow at high temperatures (above 45°C). They cause spoilage in low-acid and medium-acid canned foods (pH above 3.7). The main types are:
- Flat sour spoilage: This is caused by facultative anaerobic bacteria such as Geobacillus stearothermophilus, Bacillus coagulans, Bacillus thermoacidurans, and Bacillus stearothermophilus. They produce acid from carbohydrates and make the product sour with no can swelling. They usually originate from plant equipment, sugar, starch, or soil.
- Thermophilic anaerobic spoilage: This is caused by obligate anaerobic bacteria such as Thermoanaerobacter and Thermoanaerobacterium (e.g., Clostridium thermosaccharolyticum). They produce hydrogen and carbon dioxide gases that swell and burst the can. They also produce a butyric or "cheesy" odor on food. They occur due to slow cooling or hot storage.
- Thermophilic anaerobic sulfide spoilage: This is caused by gram-negative obligate anaerobic spore formers such as Disulfotomaculum nigrificans, Clostridium bifermentans, and Clostridium sporogenes. They produce hydrogen sulfide gas that gives a "rotten egg" odor to the food with no swelling. The hydrogen sulfide reacts with iron in the food or can to form iron sulfide that causes blackening of food and inside of cans. They occur due to under-processing, slow cooling, or hot storage.
- Spoilage by mesophilic spore-forming bacteria: These are bacteria that form heat-resistant spores and grow at moderate temperatures (between 20°C and 45°C). They cause spoilage in low-acid canned foods (pH above 5.2). The main types are:
- Spoilage by mesophilic Clostridium species: These are anaerobic bacteria such as C. butyricum and C. pasteurianum that ferment carbohydrates to produce volatile acids such as butyric acid and gases such as hydrogen and carbon dioxide that swell the can. They also include proteolytic species such as C. sporogenes, C. putrefaciens, and C. botulinum that metabolize proteins to produce foul-smelling compounds such as hydrogen sulfide, mercaptans, indoles, and ammonia. They survive due to insufficient heat treatment.
- Spoilage by mesophilic Bacillus species: These are aerobic or facultative anaerobic bacteria such as B. subtilis and B. mesentericus that cause spoilage in poorly evacuated cans. They also survive due to insufficient heat treatment. B. polymyxa and B. macerans can also cause spoilage due to can leakage.
- Spoilage by non-spore-forming bacteria: These are bacteria that do not form heat-resistant spores and survive in cans because of mild heat treatment (such as pasteurization) or due to leakage. They include thermoduric species such as Enterococcus (E. faecalis and E. faecium), Microbacterium, Streptococcus thermophilus, Micrococcus, and Lactobacillus. Acid-forming Lactobacillus can survive in under-processed canned food and produce carbon dioxide that swells the can. Pseudomonas, Alcaligenes, Micrococcus, Flavobacterium, Proteus, and others can also cause spoilage in leaked canned foods.
- Spoilage by yeast and mold: These are fungi that grow in canned foods due to under-processing, leakage, recontamination, or poor evacuation. Fermentative yeast produce carbon dioxide that swells the can. Commonly yeasts are found to grow on the surface of high-acid foods such as pickled products. Torulopsis lactis and T. globosa cause blowing or gaseous spoilage on sweetened condensed milk that is not heat processed. T. stellata causes spoilage in canned lemon and grows at a pH of 2.5. Mold species such as Aspergillus, Byssochlamys, Penicillium, and Citromyces are commonly found in canned food with high sugar content of up to 70-72%. Mold spoilage is characterized by moldy taste, odor, color fading, presence of mold mycelia, and sometimes by slight swelling of the container.
Canning itself is a heat-treated method of food preservation that ensures the "commercial sterility" of the food product for long-term storage. However, canned food may still be susceptible to microbial spoilage due to various factors, such as inadequate heat treatment, recontamination, leakage, or poor evacuation. To prevent microbial spoilage of canned food, several other food preservation methods can be applied in combination with canning to extend its shelf life without deteriorating the nutritive quality of food. Some of these methods are:
- Radiation: Radiation sterilization of food (radappertization) requires exposing food in sealed containers to ionizing radiation to kill all spoilage-causing organisms. It is used to eliminate microorganisms on the surfaces of canned jars and their lids. Radiation can also reduce the need for high-temperature processing and preserve the sensory and nutritional qualities of food.
- Preservative agents: Preservatives are substances that are capable of inhibiting or retarding the growth of microorganisms and are categorized as "Generally Regarded As Safe (GRAS)". They include simple organic acids (such as citric, acetic, lactic, and sorbic acids), sulfite (such as sodium or potassium metabisulfite), ethylene oxide (a gas that can penetrate packaging materials), sodium nitrite (used for curing meat products), ethyl formate (a fumigant that can kill insects and molds), and lactic acid bacteria (LAB) and bacteriocin (natural antimicrobial compounds produced by LAB). Preservatives can be added to the food before or after canning or applied to the containers or lids.
- Chilling storage: The canned foods are stored at low temperatures (0–5 °C) to reduce the rate of microbial growth and enzymatic activities. Chilling storage can extend the shelf life of canned food by slowing down chemical reactions, such as oxidation and browning, that can affect the quality and safety of food. Chilling storage also helps to prevent the growth of thermophilic bacteria that can survive heat treatment.
The canning process involves several steps to ensure the safety and quality of the canned food. Here are the main steps of the canning process:
- Preparation: The containers and the food are prepared for canning. The containers are sterilized by heating or chemical treatment to kill any microorganisms that may contaminate the food. The food is washed, trimmed, sorted, graded, peeled, cut, diced, sliced, segmented, pitted, and blanched according to its type and size. The preparation of food helps to remove dirt, spoilage organisms, enzymes, and pesticides from the food.
- Filling: The prepared food is filled into the sterilized containers. The food is packed tightly to minimize air space and maximize heat penetration during processing. The food is also covered with a liquid such as sugar syrup, fruit juice, brine, or gravy to add flavor, prevent oxidation, and assist in heat transfer. The liquid should fill the container completely to avoid air pockets that may harbor microorganisms.
- Exhausting: The process of removing air from the container before sealing is called exhausting. Exhausting helps to prevent excessive pressure and strain on the container during processing. It also reduces the chemical reaction between the food and the container that may cause discoloration, corrosion, or off-flavors. Exhausting can be done by heating the filled containers in steam or hot water, or by vacuum sealing.
- Sealing: After exhausting, the containers are sealed immediately to prevent recontamination by microorganisms. The sealing method depends on the type of container and the product. Glass jars and bottles are sealed by screw caps or crown caps that have a rubber gasket to create an airtight seal. Metal cans are sealed by double seaming that crimps the lid and the body of the can together. Plastic pouches are sealed by dielectric or conduction heating that melts and fuses the edges of the pouch.
- Processing: The sealed containers are heat-treated to destroy any remaining microorganisms and their spores that may cause spoilage or illness. The processing time and temperature depend on the type and acidity of the food, the size and shape of the container, and the method of heating. Low-acid foods (pH > 5.2) such as meat, dairy products, and vegetables require higher temperatures (above 100°C) and longer times (up to several hours) than high-acid foods (pH < 4.5) such as fruits, pickles, and jams that require lower temperatures (below 100°C) and shorter times (a few minutes). The processing methods include boiling water bath, pressure canning, steam-air retort, water spray retort, water immersion retort, and microwave heating.
- Container cooling: After processing, the containers are rapidly cooled to room temperature to stop further cooking and prevent spoilage by thermophilic bacteria. Cooling can be done by immersing the hot containers in cold water or by spraying them with jets of cold water. Cooling also helps to create a vacuum inside the container that improves the seal and prevents leakage.
- Labeling and storage: The outer surface of the containers is dried for labeling after cooling. The label should provide information such as product name, brand name, net weight or volume, ingredients list, nutritional information, manufacturer`s name and address, code number, date of manufacture and expiry, and instructions for use and storage. After labeling, the containers are stored in a cool and dry place away from direct sunlight and heat sources until they are ready for distribution or consumption.
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