Canned food is food or food raw materials that have been pre-processed, put into containers, sealed and sterilized.
There are many types of canned food. According to different pH values, they can be divided into low-acid canned food, medium-acidic canned food, acidic canned food and high-acidic canned food.
Cans that are mainly made of animal food raw materials are low-acid cans, while cans that are mainly made of plant food raw materials are medium-acidic or high-acidic cans.
After canned food is sealed, heated and sterilized, almost all the microorganisms in it are inactivated, and external microorganisms cannot enter the can. At the same time, most of the air in the container has been removed, and the food in the food Various nutrients will not be oxidized, so this food can be stored for a long time without deterioration.
Causes that cause canned food to deteriorate
1. Chemical factors
For example, hydrogen expansion caused by the interaction between the tinplate of a medium-acidic can container and its contents;
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2. Physical factors
Such as excessive storage temperature, poor exhaust, corrosion and perforation of metal containers, etc.
3. The more important factor is microbiological factors
The contamination of microorganisms in the can causes the deterioration of the canned food. The microorganisms that cause the spoilage of canned food are mainly some heat-resistant, thermophilic and anaerobic or facultative anaerobic microorganisms. The inspection and control of these microorganisms are important in the canning industry. It is of considerable significance.
Section 1 Microbial contamination of canned food
1. Sources of microbial contamination of canned food
1. Incomplete sterilization results in residual microorganisms in the canned food
During the processing of canned food, in order to maintain the normal sensory properties and nutritional value of the product, it is impossible to make the canned food completely sterile during heat sterilization. The emphasis is on killing pathogenic bacteria and toxin-producing bacteria. In essence, it only reaches the level of commercial sterilization, that is, all Clostridium botulinum spores and other pathogenic bacteria in the can, as well as thermophilic bacteria that can cause the content to deteriorate under normal storage and sales conditions. Bacteria are killed.
Some non-pathogenic microorganisms remaining in the tank will generally not grow and reproduce within a certain storage period. However, if the conditions in the tank change and the storage conditions change, these microorganisms will grow and reproduce. , causing cans to deteriorate.
Most of the microorganisms remaining in cans that have been sterilized by high-pressure steam are heat-resistant spores. If the storage temperature of the can does not exceed 43°C, it will usually not cause the contents to deteriorate.
2． Can leakage occurs after sterilization
After sterilization, if the can is not sealed tightly, it will easily cause the can to leak and cause microbial contamination.
1) An important source of pollution is cooling water. This is because cans need to be cooled by cooling water after heat treatment, and microorganisms in the cooling water may enter the can through leaks.
2) Air is also a source of pollution caused by leaky cans, but it is less important.
3) Some heat-resistant bacteria, yeasts and molds invade from the outside
4) The oxygen content in the tank increases, causing various microorganisms to grow vigorously, thus affecting the pH value of the contents. Decrease, severe sensory changes will occur.
2. Types of microorganisms that contaminate canned food
(1) Main microorganisms that contaminate low-acid cans
1. Thermophilic bacteria
This type of bacteria has strong heat resistance and is easy to form spores. Most of the contamination of canned food caused by incomplete sterilization is caused by this type of bacteria. Such bacteria usually include acid-flattening putrefactive bacteria (flat acid bacteria), thermophilic anaerobic spore bacteria, etc.
(1) Flat acid bacteria:
Low-acidic canned foods stored above 43°C can be affected by the residual thermophilic properties that are highly resistant to heat. The growth of aerobic spore bacteria will cause the content to deteriorate, but because it can grow at temperatures above 43°C, the canned content will become sour, making the canned food lose its edible value.
Because when this type of bacteria is active in the can, the can does not expand and the pH value of the content is significantly low. Therefore, this kind of deterioration is usually called flat lid rancidity, causing flat lid rancidity. The causative bacteria are collectively called flat acid bacteria, which are a type of aerobic and even facultative anaerobic bacteria that can cause some low-acid canned foods to become rancid and form spores.
According to the different degrees of thermophilicity of Acidobacteria, they can be divided into two categories: obligate thermophiles and facultative thermophiles.
A. Bacillus stearothermophilus
It is an obligate thermophilic bacterium. The spores of this bacterium can only germinate under thermophilic temperature (45'C-50'C). During storage or sales, if the ambient temperature is in the thermophilic growth range (above 43°C), flat cover rancidity may occur. During the processing of canned food, if it is not followed by sufficient cooling after heat treatment, it is also the main cause of rancidity of flat lids.
B. Another major flat acid bacterium is Bacillus coagulans
This bacterium is a facultative thermophile and can grow at two temperatures: 37'C and 55'C. Breed.
(2) Thermophilic Anaerobic Bacillus
Low-acidic canned foods stored above 43'C may also be affected by the growth of residual thermophilic anaerobic saccharomyces. The deterioration of canned food can be divided into the following two types due to different bacterial causes:
A. Clostridium thermosaccharolytica
A gas-producing type of deterioration , usually refers to the expansion and deterioration of cans (fat cans). This deterioration is caused by the obligate thermophilic spore-forming anaerobic bacterium Clostridium thermophila. This bacterium is an obligate anaerobic bacterium with an optimal growth temperature of 55°C. It has a strong ability to decompose sugar. It can decompose glucose, lactose, sucrose, salicin and starch, produce acid and a large amount of gas, and does not decompose protein. It cannot reduce nitrate and does not produce toxins.
B. Clostridium nigricans (sulfide odor deterioration)
It is rare for canned food to be contaminated by sulfide spoilage bacteria. This type of deterioration is characterized by the flat contents of the can. It is dark and smells like rotten eggs. It is usually caused by the obligate thermophilic spore-forming anaerobic bacterium Clostridium niger. It is not very capable of decomposing sugar, but it can decompose protein to produce hydrogen sulfide. Hydrogen sulfide is closely related to the tinplate of canned containers. The compound produces black sulfide, which turns the food black. The hydrogen sulfide generated in the can is absorbed by the food in the can. Therefore the can will not expand.
C. Other types of thermophilic bacteria have been isolated from spoiled cans, but they are few in number.
2． Mesophilic anaerobic bacteria
The suitable growth temperature is about 37°C, and some can grow at 50°C. It can be divided into two categories:
1) One category has a strong ability to decompose protein and can also decompose some sugars. It mainly includes Clostridium botulinum, Clostridium sporogenes, Clostridium bienzyme, and Clostridium putrificans. Bacteria etc.
2) Another type that decomposes sugars, such as Clostridium butyricum, Clostridium pasteurianum, Clostridium weldenii, etc.
Mesophilic anaerobic bacteria cause spoilage, the cans swell, and the contents have a putrid smell.
Clostridium botulinum is particularly important. Clostridium botulinum decomposes proteins to produce hydrogen sulfide, ammonia, skatole, etc., which leads to obesity. The contents are putrid and spoiled, and toxins are produced and a foul smell is released. It is worth noting that because botulinum toxin is highly toxic, if spore-bearing bacilli are found in the contents, regardless of the degree of corruption of the can, mice must be inoculated with the contents to detect botulinum toxin.
3. Mesophilic aerobic bacteria
This type of bacteria belongs to the genus Bacillus and is a mesophilic bacterium that can produce spores. Its heat resistance is poor, and the spores of many bacteria are At a temperature of 100°C or lower, it can be killed in a short time. Common mesophilic aerobic bacteria that cause canned spoilage include: Bacillus subtilis, Bacillus megaterium, and Bacillus cereus.
The almost vacuum state inside the can inhibits their activity. This type of bacteria can decompose protein and sugar. After the sugar is decomposed, most of them produce acid but not gas, so they are also prone to acid spoilage. , but Bacillus polymyxa and Bacillus maculans can decompose sugars, produce acid and gas, causing obesity.
4. Non-spore-producing bacteria
There are two major groups of non-spore-producing bacteria contaminated in cans:
1) One type is intestinal bacteria
For example, E. coli, which can cause obesity when grown in tanks;
2) Another type of non-spore-producing bacteria is mainly Streptococcus
Specially They are Streptococcus thermophilus and Streptococcus faecalis, etc. These bacteria have strong heat resistance. They are often found in canned vegetables and fruits. Their growth and reproduction will produce acid and gas, causing obesity. Non-spore-forming bacteria such as Streptococcus faecalis and Streptococcus urinaire can often be detected in canned ham.
5. Yeast and mold
Yeast contamination of low-acid cans is rare and only occasionally occurs in canned sweetened condensed milk.
(2) Main microorganisms that contaminate acidic cans
1. Spore-producing bacteria
This type of bacteria is more common in spoiled canned fruits, such as Bacillus coagulans, Clostridium butyricum, Clostridium pasteurianum, Bacillus polymyxa, Bacillus macerans, etc. .
Bacillus coagulans is a common flat-acid bacterium in acidic canned foods. It often appears in canned tomato juice. It has strong resistance to heat and has facultative anaerobic characteristics. It can adapt to higher acidity and can Decomposes sugars to produce acid but not gas.
Clostridium butyricum and Clostridium pasteurianum can decompose sugars in cans and produce butyric acid, carbon dioxide and hydrogen, giving the product a sour smell.
Bacillus polymyxa and Bacillus maculans can also cause acid and gas production in canned fruits.
2． Bacteria that do not produce spores
This type of bacteria is mainly lactic acid bacteria, such as Lactobacillus and Leuconostoc, which can cause rancidity of fruits and fruit products: and heterozygous fermentation strains of Lactobacillus can cause tomato products to become rancid. of rancidity and gasogenic spoilage of canned fruits.
3． Heat-resistant mold and yeast
The common yellow silk mold has stronger heat resistance than other molds. It can survive at 85C for 30 minutes and can survive, grow and reproduce in an environment with insufficient oxygen. It has a strong effect of destroying pectin. If it remains and multiplies in canned fruits, it can soften and disintegrate the fruits. It can decompose sugar to produce carbon dioxide and cause the canned fruits to be bloated;
Secondly, it is white Silkworm mold is also heat-resistant and can survive at a temperature of 76.6°C for 30 minutes. It can also cause canned food to deteriorate. This type of heat-resistant mold causes the deterioration of canned food through moldy odor, food fading or tissue structure. Structural changes, the presence of mold hyphae in the contents and sometimes mild swelling of the jar lid are confirmed. ,
Other molds such as Penicillium, Aspergillus, etc. can also cause the spoilage of canned jams and sugar water fruits.
The heat resistance of yeast is very low. Except for insufficient sterilization or leaking cans, canned food usually does not suffer from yeast contamination through normal sterilization treatment.
Section 2 Microbial Testing of Canned Food
Different types of cans have different causative bacteria that cause spoilage, and these causative bacteria are sometimes not single, but often multiple. Bacterial contamination at the same time. In order to ensure the safety and hygiene of canned food, canned products must be subjected to biological inspection to eliminate substandard products.
1. Collection of samples
1. When inspecting a large number of canned foods
Sampling is carried out according to the factory, trademark, variety, source and manufacturing time. .
2. For canned food in the production process
Sampling can be carried out according to production shifts, and the sampling base of each variety in each shift shall not be less than 3 cans. You can also take samples from sterilizing pots, taking 1 can from each pot. However, each batch shall not be less than 3 cans of each variety.
3. In batches of cans stored in warehouses or stores
When there are deformations, expansions, dents, cracks in the can wall, rust and damage, etc., sampling can be decided according to the situation. quantity.
2. Sterility test of canned food
Before the sterility test of canned food, a sealing test should generally be conducted first, and then an expansion test should be conducted on well-sealed cans before opening. Take the contents of the can for sterility testing.
1． Sealing test
Put the tested can in a water bath at 86 1℃, let the can sink 5cm below the water surface, and then observe it for 5 minutes. If small bubbles are found to rise continuously, it indicates air leakage. When testing glass cans, they should first be immersed in warm water and then placed in water at the above temperature to avoid sudden bursting.
2． Expansion test
For fresh cans, it is generally placed in an environment of 36 1 degrees for 7 days, while canned fruits and vegetables are placed in an environment of 20~25~C for 7 days, and then the top and bottom of the can are observed. Is there any expansion phenomenon?
3． Sterility test
All cans to be inspected must be cooled to room temperature. Cans that become fat after the expansion test should be placed in the refrigerator to cool down.
① Can opening and sampling
Before opening the can, the cans to be inspected should be numbered for easy recording. Performed in a sterile environment.
A. For fat patients,
can be disinfected with 70% alcohol solution containing 4% iodine, and dried with a sterile towel. Do not use lit alcohol cotton balls to burn them to prevent internal The heated gas causes the tank to expand more rapidly, causing cracks to appear and the contents to erupt.
Use a sterilized can opener to puncture the top of the can to try to capture some of the gas in the can, and then chemically identify the nature of the gas to see whether it is carbon dioxide, hydrogen or other gases.
Aseptically take a sample from the center of the can, and the sample size should be sufficient for re-inspection.
B. For cans with normal appearance
Use alcohol cotton balls to wipe off possible dirt and oil stains on the open end, then wipe dry with a clean towel, and then burn the open end with a flame Until all the attached water evaporates.
Use a sterilized can opener to puncture the top of the can and aseptically take a sample from the center of the can. The sample size should be sufficient for re-inspection.
②Inspection
Take 2 tubes of broth (or bromocresol purple glucose broth) and 2 tubes of liver slices broth (or just boil and quickly cool down the blister meat) culture medium), and inoculate the test sample at the same time. The inoculation amount is 1 to 2 ml for liquid samples and 1 to 2 g for solid samples. When both are used, half of each should be taken. After inoculation, perform aerobic bacterial culture examination and anaerobic bacterial culture examination at 37°C. At the same time, smear the test sample and perform microscopic examination after Gram staining (or other staining).
③Result analysis
If there is no bacterial growth in all aerobic culture medium tubes and anaerobic culture medium tubes, the sterility test is qualified and no further pathogenic bacteria testing is required.
If there is bacterial growth in the two tubes of aerobic culture medium and bacteria are also found in the smear, the sample needs to be tested for pathogenic cocci and enteropathogenic bacteria.
If there is bacterial growth in the two tubes of anaerobic culture medium and bacteria are also found in the smear, the sample will be tested for Clostridium botulinum and Clostridium welchenii. If the expansion test is positive and the outgassing test is hydrogen, but the culture does not grow, this expansion is mostly caused by the hydrogen generated by the chemical reaction of the can content on the can wall, that is, hydrogen expansion.
If the outgassing gas is neither hydrogen nor carbon dioxide, and the culture test is positive, the expansion is caused by aerobic spore bacteria decomposing the additive nitrate in some canned meats to produce carbon monoxide and nitrogen.
3. Inspection of toxic bacteria in canned food
1. In the sterility test of canned food, if cocci are found, pathogenic Staphylococcus and pathogenic bacteria must be tested. Testing for streptococci.
2. If Gram-negative bacilli are found, tests for intestinal pathogenic bacteria such as Salmonella and Escherichia coli are required.
3. If Gram-positive bacilli are found, tests for Clostridium botulinum, Clostridium perfringens and botulinum toxin must be carried out.
4. If the sterility test of canned food is negative or its pH is below 4.6, there is no need to test for toxic bacteria in the food.
IV. Inspection of canned foods for acid-resistant bacteria
Canned foods suspected of being sour-resistant should be tested for acid-resistant bacteria.
Specific method:
Randomly select a certain number of samples, place them in a 55°C incubator for 3 days, take them out, perform aseptic operations, and inoculate 1g (mL) of the contents of the can into In bromocresol purple glucose broth medium, culture at 55°C for 5 days.
The culture solution is uniformly turbid, shows acidic reaction, and has no alkaline reaction, which are the main characteristics of typical flat acid bacteria.
On the Bromocresol Purple Glucose Agar plate, the typical bacterial colonies are milky yellow, with a deep center, flat and slightly protruding, and neat or uneven edges.
In addition, if there is no obvious acidic reaction after culturing in bromocresol purple glucose broth medium at 55 degrees, or although there is an acidic reaction, there is an alkaline reverse reaction and a bacterial film, this category Flat acid bacteria are atypical flat acid bacteria, such as Bacillus subtilis, Bacillus licheniformis, etc.
Any atypical acid-producing bacteria detected should be tested for rancidity confirmation.
5. Anaerobic bacteria testing of canned food
1. Thermophilic Anaerobic Bacteria Test
Randomly select a certain number of canned samples, inoculate the contents into the liver slice broth culture medium aseptically, and then incubate anaerobically at 55°C for 5 days. Lines were inoculated on egg yolk agar plates containing 0.1% thioglycolate, and then cultured anaerobically at 50°C for 24 to 48 hours. Gram-positive stained colonies were picked for pure culture, and biochemical tests were performed as listed in Table 11.3 Characteristics for further identification.
Clostridium thermosaccharolyticum and Clostridium nigricans.
2． Mesophilic Anaerobic Bacteria Test
Section 3. Control of Microorganism Contamination of Canned Food
Microbiological contamination of canned food is the main reason for reducing the quality of canned food and causing damage to the canned food. , therefore, effectively controlling the microbial contamination of canned food is an important method to prevent canned food from deteriorating and reducing the occurrence of poisonous phenomena. The control of microbial contamination of canned food is a complex system project, and a complete set of measures must be taken during production. Only feasible comprehensive measures can reduce pollution to a minimum.
1. Control of microbial contamination during the pre-processing of canned food
The raw materials used for canned food must be fresh, clean and hygienic, and the raw materials for meat food must come from healthy animals. To avoid contamination, for the raw materials of fruit and vegetable products, fruits and vegetables that are crushed, scratched, cracked, or dehydrated due to mechanical friction and compression should be removed.
Cleaning is an important step in the canning process. Cleaning not only removes soil and dirt on the surface of raw materials, but also reduces microorganisms on the surface. Therefore, the cleaning water must be clean and hygienic. Otherwise, if it is contaminated by microorganisms, it will aggravate the contamination of the food. Canned food is the same as other foods. The processing environment, mechanical equipment, processing water, materials and operators may become sources of microbial contamination.
In particular, processing equipment may become an important source of contamination by thermophilic microorganisms, so special attention must be paid to hygienic management in these aspects. For details, please refer to the relevant chapters of this book on the control of microbial contamination of food.
2. Control of microbial contamination during canned food processing
As mentioned before, the main sources of microbial contamination in canned food are incomplete sterilization and leaky cans. Therefore, the most effective way to control canned food contamination is to cut off these two sources of pollution, which involves the production process and sterilization procedures of canned food. On the premise of maintaining the normal nutritional value and sensory properties of canned food, it should be killed as much as possible Microorganisms retained in the tank.
Reduce the residual amount of oxygen in the tank as much as possible. Cans after heat treatment must be fully cooled, and the cooling water used must be clean and fresh. In addition, the cans must be sealed tightly to avoid can leakage.
3. Control of microbial contamination during the storage and sale of canned food
During the storage and sale of canned food, avoid rough handling. Canned food should be stored in a clean, dry, ventilated, In a cool place, unreliable heat source is too close, the storage temperature should be controlled below 20℃, and it can be stored in a freezer if possible. Cans found to be rusty, deformed, or have cracks in the wall during storage and sales must not be sold or eaten.