There are many kinds of canned food, which can be divided into low acid canned food, medium acid canned food, acid canned food and high acid canned food according to different pH values.
Canned food with animals as the main raw material belongs to low acidity canned food, while canned food with plants as the main raw material belongs to medium or high acidity canned food.
After the canned food is sealed, heated and sterilized, almost all the microorganisms inside are inactivated, but the microorganisms outside can't enter the canned food. At the same time, most of the air in the container has been removed, and many nutrients in the food have not been oxidized, so this food can be preserved for a long time without deterioration.
Causes of deterioration of canned food
1, chemical factors
For example, hydrogen expansion caused by the interaction between tinplate and the contents of medium acid canned containers;
2. Physical factors
Such as high storage temperature, poor exhaust, corrosion and perforation of metal containers, etc.
3. The most important factor is microbiology.
The contamination of microorganisms in canned food leads to the deterioration of canned food, and the microorganisms that lead to the deterioration of canned food are mainly some heat-resistant, thermophilic, anaerobic or facultative anaerobic microorganisms. The detection and control of these microorganisms is of great significance in the canned food industry.
Section 1 Microbial contamination of canned food
First, the source of microbial contamination of canned food
1. Incomplete sterilization leads to residual microorganisms in canned food.
In the processing of canned food, in order to maintain the normal sensory characteristics and nutritional value of the product, it is impossible to completely sterilize the canned food by heating sterilization, and only emphasize the killing of pathogenic bacteria and toxin-producing bacteria. In essence, it only reaches the level of commercial sterilization, that is, all pathogenic bacteria such as Clostridium botulinum spores in canned food and thermophilic bacteria that can cause deterioration of contents under normal storage and sales conditions are killed.
Some non-pathogenic microorganisms remaining in canned food will not grow and reproduce in a certain storage period, but if the conditions in canned food change and the storage conditions change, these microorganisms will grow and reproduce, leading to the deterioration of canned food.
Most of the microorganisms remaining in canned food sterilized by high pressure steam are heat-resistant spores. If the storage temperature of canned food does not exceed 43℃, the contents will not deteriorate.
2. After disinfection, the water tank will leak.
After sterilization, if the can is not tightly sealed, it is easy to leak and cause microbial pollution.
1) The important pollution source is cooling water, because the cans need to be cooled by cooling water after heat treatment, and microorganisms in the cooling water may enter the cans through the leaking place.
2) Air is also the pollution source of leaking tank pollution, but it is not very important.
3) Some thermotolerant bacteria, yeasts and molds have invaded from the outside.
4) The increase of oxygen content in the tank will lead to the vigorous growth of various microorganisms, thus reducing the pH value of the contents, which will seriously show sensory changes.
Second, the types of microorganisms that pollute canned food
(A) the main microorganisms that pollute low-acid canned food
1, thermophiles
This kind of bacteria has strong heat resistance and is easy to form spores. Most of the pollution caused by incomplete sterilization of canned food is caused by such bacteria. This kind of bacteria usually includes acidophilic spoilage bacteria (acidophilic bacteria), thermophilic anaerobic budding bacteria and so on.
(1) acidophilic bacteria:
Canned food with low acidity stored above 43℃ may deteriorate its contents due to the growth of thermophilic aerobic budding bacteria with strong heat resistance. However, because it can grow at the temperature above 43~C, the contents of the can become sour, which makes the can lose its edible value.
Because this kind of bacteria will not expand when moving in canned food, the pH value of the contents is obviously low, and this deterioration is usually called flat cover rancidity. The bacteria that cause flat-lid rancidity are collectively called flat-lid acidobacteria, that is, an aerobic or even facultative anaerobic bacteria, which can make some low-acid canned foods rancid and form spores.
According to the different thermophilic degree of acidophilic bacteria, they can be divided into obligate thermophilic bacteria and facultative thermophilic bacteria.
A. Bacillus stearothermophilus
It belongs to obligate thermophilic bacteria and only germinates at thermophilic temperature (45℃-50℃). In the process of storage or sale, if the ambient temperature is within the thermophilic growth range (above 43℃), flat cover rancidity may occur. In canned food processing, if it is not fully cooled after heat treatment, it is also the main reason for the rancidity of flat lid.
Another major acidophilic bacterium is Bacillus coagulans.
The strain is facultative thermophilic and can grow and reproduce at 37℃ and 55℃.
(2) Thermophilic anaerobic yeast
Canned food with low acidity stored above 43℃ will also deteriorate due to the growth of residual thermophilic anaerobic spores. Due to different pathogenic bacteria, this deterioration can be divided into the following two types:
A. Clostridium thermophilus
A kind of gas-producing metamorphism, usually refers to the metamorphism caused by the expansion of cans (fat cans), which is caused by Clostridium thermophilum, an obligate thermophilic anaerobic bacterium that produces thermophilic Bacillus. The strain is obligate anaerobic, and the optimum growth temperature is 55℃. The ability to decompose sugar is very strong, and it can decompose glucose, lactose, sucrose, salicin and starch, producing acid and a lot of gas, without decomposing protein, reducing nitrate and producing toxins.
B. Clostridium nigrum (sulfide odor deterioration)
It is rare for canned food to be contaminated by sulfide-spoilage bacteria. This deterioration is characterized by the dark flat contents of canned food and the smell of rotten eggs, which is usually caused by Clostridium nigrum caused by obligate thermophilic spore-producing anaerobic bacteria. Its ability to decompose sugar is not strong, but it can decompose protein to produce hydrogen sulfide, which combines with tinplate of canned container to produce black sulfide, which makes food black, and hydrogen sulfide produced in canned food is absorbed by canned food. So that the tank does not expand.
Other types of thermophilic bacteria were isolated from spoiled canned food, but only a few.
2. Thermophilic anaerobic bacteria
The suitable growth temperature is about 37~C, and some can grow at 50℃. Can be divided into two categories:
1), which has a strong ability to decompose protein, and can also decompose some sugars, mainly Clostridium botulinum, Clostridium sporogenes, Clostridium bifidum and Clostridium putrescens.
2) another kind of decomposed sugar, such as Clostridium butyricum, Clostridium pasteureum and Clostridium welchii.
Thermophilic anaerobic bacteria lead to corruption, the cans swell, and the contents have a rotten smell.
Clostridium botulinum is particularly important. The decomposition of protein by Botox will produce hydrogen sulfide, ammonia gas, fecal odor and so on. , leading to fat hearing, content rot, produce toxins, release foul smell. It is worth noting that due to the strong toxicity of botulinum toxin, if Bacillus with spores is found in the contents, no matter how rotten the can is, mice must be inoculated with the contents for botulinum toxin detection.
3. Thermophilic aerobic bacteria
This kind of bacteria belongs to the genus Bacillus, which can produce psychrophiles spores with poor heat resistance. At the temperature of 100℃ or lower, spores of many bacteria can be killed in a short time. The common thermophilic aerobic spores causing canned food spoilage are Bacillus subtilis, Bacillus megaterium and Bacillus cereus.
The vacuum in canned food almost inhibits their activities. These bacteria can decompose protein and sugar. After sugar decomposition, most of them produce acid but not gas, so it is also flat acid corruption. However, Bacillus polymyxa and Bacillus macerans can decompose sugar, produce acid and gas, and produce lipid hearing.
4. Bacteria that do not produce spores
There are two groups of bacteria that cannot produce spores:
1) One is intestinal bacteria.
Such as Escherichia coli, growing in the tank will cause fatty hearing;
2) Another kind of bacteria that does not produce spores is mainly streptococcus.
Especially Streptococcus thermophilus and Streptococcus faecalis, these bacteria have strong heat resistance. More common in canned vegetables and fruits. Their growth and reproduction will produce acid and gas, which will cause fat hearing. Non-spore-producing bacteria, such as Streptococcus faecalis and Streptococcus urine, can often be detected in canned ham.
5. Yeast and mold
Yeast rarely pollutes low-acid canned food, and only occasionally appears in canned sweet condensed milk.
(2) Main microorganisms that pollute acidic canned food.
1. Spore-producing bacteria
This kind of bacteria is commonly found in rotten canned fruits, such as Bacillus coagulans, Clostridium butyricum, Clostridium pasteureum, Bacillus polymyxa, Bacillus Macedonia and so on.
Bacillus coagulans is a common acidophilic bacterium in acidic canned food, which often appears in canned tomato juice. It has strong heat resistance and facultative anaerobic characteristics, can adapt to high acidity, can decompose sugar to produce acid, but does not produce gas.
Clostridium butyricum and Clostridium pasteureum can decompose the sugar in canned food, produce butyric acid, carbon dioxide and hydrogen, and make the product sour and smelly.
Bacillus polymyxa and Bacillus macerans can also cause acid production and gas production in canned fruits.
2. Bacteria that do not produce spores
This kind of bacteria is mainly lactic acid bacteria, such as lactobacillus, Leuconostoc, etc., which can cause rancidity of fruits and fruit products; Another example is the heterotypic fermentation strain of Lactobacillus, which can lead to rancidity of tomato products and gas production deterioration of canned fruits.
3. Thermotolerant mold and yeast
The common Trichoderma spp. has stronger heat resistance than other molds, can survive at 85℃ for 30 minutes, can survive and grow and reproduce in an anoxic environment, and has a strong destructive effect on pectin. If it remains in the canned fruit, it can soften and disintegrate the fruit, decompose sugar to produce carbon dioxide, and cause the canned fruit to smell.
Secondly, Trichoderma spp., which also has heat resistance, can survive for 30 minutes at 76.6℃ and can also rot canned food. This kind of heat-resistant mold can cause the deterioration of canned food, which can be confirmed by musty smell, food fading or organizational structure change, mold hyphae in the contents and sometimes slight expansion of the can lid. ,
Other molds, such as Penicillium and Aspergillus, can also destroy jam and canned fruit in syrup.
Yeast has low heat tolerance. Canned food is usually not contaminated by yeast after normal sterilization except for insufficient sterilization or leaking cans.
Section 2 Microbiological Inspection of Canned Food
Different kinds of cans lead to different bacteria, and sometimes these bacteria are not single, and often multiple bacteria are polluted at the same time. In order to ensure the safety and hygiene of canned food, it is necessary to carry out biological testing on canned products to prevent unqualified products.
I. Collection of samples
1, testing a large number of canned foods.
According to the factory and trademark, samples are classified by variety, source and manufacturing time.
2. For the production process of canned food.
Sampling can be carried out according to production shifts, and the sampling base of each variety per shift shall not be less than 3 cans. Sampling can also be carried out according to the sterilization pot, and each pot takes 1 can, but each variety shall not be less than 3 cans in each batch.
3. Batch cans are stored in warehouses or shops.
In case of deformation, expansion, depression, crack, corrosion and damage of the tank wall, the number of samples can be determined according to the situation.
Second, the aseptic inspection of canned food
Generally, before the aseptic inspection of canned food, the air tightness test is carried out first, then the well-sealed canned food is tested for expansion, and then the contents are opened for aseptic test.
1. Closed test
Put the inspected jar in a water bath of 86+ 1℃, let the jar sink 5 cm below the water surface, and then observe for 5 minutes. If small bubbles are found to be rising, it indicates air leakage. When the glass jar is tested, it should be immersed in warm water first, and then put into the water at the above temperature to avoid sudden bursting.
2. Expansion test
For fresh cans, it is usually stored at 36+ 1 degree for 7 days, while canned fruits and vegetables are stored at 20 ~ C ~ 25 ~ C for 7 days, and then the top and bottom of the cans are observed for swelling.
3. Sterility inspection
The cans to be inspected must be cooled to room temperature, and the cans that have expanded after the expansion test should be put into the refrigerator to cool first.
(1) can opening and sampling
Before opening cans, the cans to be inspected shall be numbered and recorded. It is carried out in a sterile environment.
First, for the fat man's hearing
It can be disinfected with 70% alcohol solution containing 4% iodine and dried with sterile towel. Do not cauterize with a lighted alcohol cotton ball, in case the internal gas is heated, the expansion of the can is intensified, cracks appear, and the contents are ejected.
Piercing the top of a can with a sterilized can opener can try to capture some gas in the can, and then identify the nature of the gas by chemical methods, whether it is carbon dioxide, hydrogen or other gases.
Samples should be taken from the central part of the can under aseptic conditions, and the sampling amount should be enough for reinspection.
B, for cans with normal appearance
You can wipe off the dirt and oil stains that may exist at the open end with an alcohol cotton ball, then dry it with a clean towel, and then burn the open end with a flame until all the attached water evaporates.
Pierce the top of the can with a sterilized can opener, and take aseptic samples from the center of the can. The sampling amount should be enough for re-inspection.
② check
Take two tubes of broth (or bromocresol purple glucose broth) and two tubes of liver slice broth (or just boiled and quickly cooled bubble meat medium), and inoculate samples at the same time. The inoculation amount of liquid sample is 1 ~ 2 ml, and that of solid sample is 1? 2g, both of them. Sometimes, you have to take half each. After inoculation, aerobic culture and anaerobic culture were examined at 37 ~ C, and the samples were smeared, Gram (or other dyes) stained and examined by microscope.
③ Result analysis
If all aerobic medium tubes and anaerobic medium tubes have no bacterial growth, the sterility test is qualified and further pathogen inspection is not needed.
If bacteria grow in 2 tubes of aerobic medium and are also found in smear, it is necessary to detect pathogenic cocci and intestinal pathogens in the sample.
If bacteria grow in two tubes of anaerobic medium, and bacteria are also found in the smear, the samples should be tested for botulinum and Clostridium welchii. If the swelling test is positive, the deflation test is hydrogen, but the culture does not grow. This expansion is mostly due to the hydrogen generated by the chemical reaction of the contents in the tank on the tank wall, that is, hydrogen expansion.
If the outgassing gas is not hydrogen or carbon dioxide, and the culture test is positive, the expansion is caused by carbon monoxide and nitrogen produced by aerobic Bacillus decomposing the additive nitrate in some canned meat.
Three, canned food food poisoning bacteria inspection
1. In the aseptic detection of canned food, if cocci are found, pathogenic staphylococci and pathogenic streptococci must be detected.
2. If gram-negative bacilli are found, intestinal pathogens such as Salmonella and Escherichia coli should be detected.
3. If gram-positive bacilli are found, botulinum, Clostridium perfringens and botulinum toxin should be detected.
4. If the sterility test of canned food is negative or its pH value is lower than 4.6, there is no need to detect food poisoning bacteria.
Four, canned food acid bacteria inspection
Canned food suspected of acid-fast corruption should be tested for acid-fast bacteria.
Specific methods:
Randomly take a certain number of samples, put them in an incubator at 55℃ for 3 days, take them out, perform aseptic operation, absorb 65438±0g(mL) of canned contents, inoculate them into bromocresol purple glucose broth, and cultivate them at 55℃ for 5 days.
Typical acidophilic bacteria are characterized by uniform and turbid culture solution, acidic reaction and no alkaline reaction.
On bromocresol purple glucose agar plate, the typical colony of Acinetobacter is milky yellow, with deep center, slightly prominent flat and neat or irregular edge.
In addition, if there is no obvious acid reaction in 55 degree bromocresol purple glucose broth medium, or there is acid reaction, but there is alkali reverse reaction and bacterial membrane, this kind of acidophilic bacteria is atypical, such as Bacillus subtilis and Bacillus licheniformis.
All atypical acidophilic bacteria detected should be confirmed by rancidity test.
Five, canned food anaerobic bacteria inspection
1. Thermophilic anaerobic bacteria test
A certain number of canned samples were randomly selected and the contents were aseptically inoculated into the broth medium of liver slices. After 5 days of anaerobic culture at 55℃, the culture solution was streaked and inoculated on egg yolk agar plate containing 0. 1% thioglycolic acid, and anaerobic culture was carried out at 50℃ for 24 hours. After 48 hours, Gram-positive colored colonies were selected for pure culture, and according to the table 1 1? 3 for further identification.
Clostridium thermophilus, Clostridium nigrum.
2. Thermophilic anaerobic bacteria test
Section 3 Control of Micro-harrow Pollution in Canned Food
Microbial contamination of canned food is the main reason for reducing the quality of canned food and deteriorating most canned food. Therefore, effectively controlling the microbial contamination of canned food is an important method to prevent the deterioration of canned food and reduce the occurrence of Eucommia ulmoides poisoning. The control of microbial contamination of canned food is a complex systematic project, and a set of practical comprehensive measures must be taken in production to minimize the pollution.
First, the control of microbial contamination in canned food pretreatment.
The raw materials used in canned food must be fresh, clean and hygienic. For meat food, raw materials must come from healthy animals and avoid pollution as much as possible. For fruit and vegetable products, fruits and vegetables that are crushed, scratched, cracked and dehydrated due to mechanical friction and oppression should be eliminated.
Cleaning is an important process in the pretreatment of canned food. Cleaning should not only remove dirt and dirt on the surface of raw materials, but also reduce microorganisms on the surface, so the cleaning water must be clean and hygienic, otherwise it will aggravate food pollution if it is contaminated by microorganisms. Canned food, like other foods, processing environment, machinery and equipment, processing water, shaking materials and operators may all become microbial pollution sources.
In particular, processing equipment may become an important pollution source of thermophilic microorganisms, so special attention should be paid to these aspects of health management. For details, please refer to the relevant chapters on the control of food microbial contamination in this book.
Second, the control of microbial contamination in canned food processing
As mentioned above, the main source of micro-pollution of canned food is incomplete sterilization and leakage of canned food. Therefore, the most effective way to control the pollution of canned food is to cut off these two pollution sources, which involves the manufacturing process and sterilization procedure of canned food. Under the premise of maintaining the nutritional value and sensory characteristics of canned food, try to kill the microorganisms remaining in canned food as much as possible.
Minimize the amount of oxygen remaining in the tank. The heat treatment tank must be fully cooled, and the cooling water used must be clean and hygienic. In addition, the sealing of cans must be strict, so as not to leak the cans.
Three, the control of microbial contamination in the process of canned food storage and sales
Canned food should be stored in a clean, dry, ventilated and cool place. The unreliable heat source is too close, and the storage temperature should be controlled below 20℃. If possible, it can be kept in the refrigerator. In the process of storage and sale, it is found that the wall of cans is rusted, deformed and cracked. And it is not allowed to be sold or eaten.