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 animal as the main raw material belongs to low acidity canned food, while canned food with plant as the main raw material belongs to medium acidity or high acidity canned food.

After the canned food is sealed, heated and sterilized, almost all the microorganisms in it are inactivated, but the external microorganisms cannot enter the can. At the same time, most of the air in the container has been removed, and many nutrients in the food are not 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 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 cans leads to the deterioration of canned food, and the microorganisms that lead to the deterioration of canned food are mainly some heat-resistant, thermophilic and anaerobic or facultative anaerobic microorganisms. The inspection and control of these microorganisms is of great significance in the canned 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 make canned food completely sterile when heating sterilization, and only emphasize the killing of pathogenic bacteria and toxin-producing bacteria, which in essence only reaches the level of commercial sterilization, that is, all Clostridium botulinum spores and other pathogenic bacteria 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 the can generally do not grow and reproduce within a certain storage period, but if the conditions in the can change and the storage conditions change, these microorganisms will grow and reproduce, resulting in the deterioration of the can.

Most of the microorganisms remaining in the canned food sterilized by high-pressure steam are heat-resistant spores. If the storage temperature of the canned food does not exceed 43℃, the contents will not deteriorate.

2. After sterilization, the tank leaks.

After sterilization, if the can is not tightly sealed, it is easy to leak the can 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 leakage.

2) Air is also the pollution source of leaking tank pollution, but it is less 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 the pH value of the contents will decrease, 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 thermophilic bacteria

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 this kind of bacteria. This kind of bacteria usually includes acidophilic putrefying bacteria (acidophilic bacteria), thermophilic anaerobic budding bacteria, etc.

(1) Acinetobacter:

Low-acidity canned food stored above 43~C may cause the contents to deteriorate due to the growth of thermophilic aerobic bud bacteria with strong resistance to heat. However, because it can grow at the temperature above 43~C, the contents of canned food become sour, which makes the canned food lose its edible value.

Because this kind of bacteria does not swell when they move in canned food, and the pH value of the contents is obviously low, this kind of deterioration is usually called flat-cover rancidity. The bacteria that cause flat-cover rancidity are collectively called flat-cover acidobacteria, that is, a kind of aerobic and even facultative anaerobic bacteria that can make some low-acid canned foods rancid and form spores.

According to the different thermophilic degree of acidophilus, it can be divided into obligate thermophilic bacteria and facultative thermophilic bacteria.

A. Bacillus stearothermophilus

It belongs to obligate thermophilic bacteria, which germinate only at thermophilic temperature (45'C-50'C). During storage or sale, if the ambient temperature is in the thermophilic growth range (above 43℃), flat cover rancidity may occur. In the processing of canned food, if it is not fully cooled after heat treatment, it is also the main reason for the rancidity of flat cover.

B, another major acidophilic bacteria is Bacillus coagulans.

The strain is facultative thermophilic and can grow and reproduce at 37'C and 55' C.

(2) thermophilic anaerobic blastomyces

Low-acidity canned food stored above 43'C can also be deteriorated due to the growth of residual thermophilic anaerobic spores. This deterioration can be divided into the following two types due to different pathogenic bacteria:

A. Clostridium thermophilus

A kind of gas-producing metamorphism, usually refers to the metamorphism in which cans swell (fat cans), which is caused by obligate thermophilic spore-producing anaerobic bacteria? Caused by Clostridium thermophilus. The strain is obligate anaerobic, and its optimum growth temperature is 55℃. It has a strong ability to decompose sugar, which can decompose glucose, lactose, sucrose, salicin and starch, produce acid and a lot of gas, and does not decompose protein, reduce nitrate and produce toxins.

B, Clostridium nigricans (sulfide odor deterioration)

It is rare for canned food to be contaminated by sulfide spoilage bacteria. This deterioration is characterized by the dark contents of canned food and the smell of rotten eggs, which are usually caused by obligate thermophilic spore-producing anaerobic bacteria. It is caused by Clostridium nigricans, and its ability to decompose sugar is not strong, but it can decompose protein to produce hydrogen sulfide, which combines with tinplate of canned containers to produce black sulfide, which makes food black, and the hydrogen sulfide produced in canned food is absorbed by the canned food. So that the can does not swell.

C, other types of thermophilic bacteria were isolated from spoiled canned food, but only a few of them.

2. Mesophilic anaerobic bacteria

The suitable growth temperature is about 37~C, and some of them can grow at 50℃. Can be divided into two categories:

1), a kind of strong ability to decompose protein, but also can decompose some sugars, which mainly include Clostridium botulinum, Clostridium sporogenes, Clostridium bispecium, Clostridium putrescens and so on.

2) another kind of decomposed sugar, such as Clostridium butyricum, Clostridium pasteureum, Clostridium welchii, etc.

Mesophilic anaerobic bacteria cause spoilage, the cans swell, and the contents smell rotten.

Clostridium botulinum is particularly important. The decomposition of protein by Clostridium botulinum produces hydrogen sulfide, ammonia, fecal odor, etc., which leads to fat hearing, and the contents are putrefying and rotten, and toxins are produced and malodorous odor is released. It is worth noting that because of the strong toxicity of botulinum toxin, if Bacillus with spores is found in the contents, mice must be inoculated with the contents to detect botulinum toxin, no matter how corrupt the cans are.

3, mesophilic aerobic bacteria

This kind of bacteria belongs to the genus Bacillus, which can produce spores of mesophilic bacteria, and its heat resistance is poor. Many bacteria' spores can be killed in a short time at the temperature of 100~C or lower. Common mesophilic aerobic spores that cause can spoilage are Bacillus subtilis, Bacillus megaterium and Bacillus cereus.

The almost vacuum state in the canned food inhibits their activities. These bacteria can decompose protein and sugar, and most of them produce acid without gas after sugar decomposition, so it is also flat acid corruption, but Bacillus polymyxa and Bacillus macerans can decompose sugar, produce acid and gas, resulting in fat hearing.

4. Bacteria that do not produce spores

There are two groups of bacteria that can not produce spores:

1) One is intestinal bacteria.

Such as Escherichia coli, which can cause fat hearing when growing in tanks;

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. It is 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

The contamination of low-acid canned food by yeast is rare, and it only occasionally appears in canned sweet condensed milk.

(2) Main microorganisms that pollute acidic canned food

1.Bacteria producing spores

Such bacteria are common in spoiled canned fruits, such as Bacillus coagulans, Clostridium butyricum, Clostridium pasteureum, Bacillus polymyxa, Bacillus macedonis 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, and can decompose sugar to produce acid, but does not produce gas.

Clostridium butyricum and Clostridium pasteureum can decompose the sugar in canned food, producing butyric acid, carbon dioxide and hydrogen, which makes 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 are mainly lactic acid bacteria, such as Lactobacillus and Leuconostoc, which can cause rancidity of fruits and fruit products; another example is the heterotype fermentation strain of Lactobacillus, which can cause rancidity of tomato products and gas production deterioration of canned fruits.

3. Heat-resistant mold and yeast

The common yellow silk mold has stronger heat resistance than other molds, and can survive for 85 c and 30 minutes, and can survive and grow and reproduce in the oxygen-deficient environment, which has a strong role in destroying pectin. If it remains and reproduces in canned fruits, it can soften and disintegrate fruits, and it can decompose sugar to produce carbon dioxide and cause canned fruits to be heard.

Followed by white silk mold, which also has heat resistance, can survive for 30 minutes at the temperature of 76.6℃, and can also spoil canned food. This kind of heat-resistant mold can cause the deterioration of canned food, which can be confirmed by mildew 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 spoil jam and canned fruit in syrup.

The heat resistance of yeast is very low. Except for insufficient sterilization or can leakage, canned food is usually not contaminated by yeast after normal sterilization.

Section 2 Microbiological Inspection of Canned Food

Different types of canned food 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 tests on canned products to prevent unqualified products.

I. Collection of samples

1, when inspecting a large number of canned foods

According to the factory and trademark, the samples are classified by 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. Sampling can also be carried out according to the sterilization pot, and each pot takes 1 can, but each batch of each variety shall not be less than 3 cans.

3. In batches of cans stored in warehouses or shops.

In case of deformation, expansion, depression, cracks in the tank wall, rust and damage, 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 airtight test should be made first, then the well-sealed canned food should be tested for expansion, and then the contents should be opened for aseptic test.

1.Closed test

Put the canned food under inspection in a water bath of 86+ 1℃, let the canned food sink 5cm below the water surface, and then observe for 5 minutes. If small bubbles are found to rise continuously, it indicates air leakage. When the glass can 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 canned food, it is usually stored in the environment of 36+ 1 degree for 7 days, while for fruits and vegetables, it is stored in the environment of 20 ~ c ~ 25 ~ c for 7 days, and then whether the top and bottom of the can are swollen or not is observed.

3. Sterility inspection

The cans to be inspected must be cooled to room temperature, and the cans that have been swollen after expansion test should be put in the refrigerator to cool them first.

(1) can opening and sampling

Before opening cans, the cans to be inspected should be numbered for record. It is carried out in a sterile environment.

A, for fat listening

It can be disinfected with 70% alcohol solution containing 4% iodine, and dried with a sterile towel. It can't be cauterized with a lighted alcohol cotton ball, in case the internal gas is heated and the expansion of the can is intensified, so that cracks appear and the contents are ejected.

Puncturing the top of the 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 from the central part of the can should be taken aseptically, and the sampling amount should be sufficient for re-inspection.

B, for cans with normal appearance

You can use alcohol cotton balls to wipe off the dirt and oil stains that may exist at the open end, then dry them with a clean towel, and then burn the open end with flame until all the attached water evaporates.

Puncture the top of the can with a sterilized can opener, and aseptically take samples from the center of the can, and the sampling amount should be sufficient for re-inspection.

② Inspection

Take two tubes of broth (or bromocresol purple glucose broth) and two tubes of liver slice broth (or just boiled to quickly cool the blister meat culture medium), and inoculate samples at the same time. The inoculation amount is 1 ~ 2ml for liquid samples and1? 2g, both of them are sometimes, so we should take half each. After inoculation, aerobic bacteria culture and anaerobic bacteria culture were examined at 37 ~ C. At the same time, the samples were smeared, stained with Gram (or other dyes) and examined under microscope.

③ Result analysis

If there is no bacterial growth in all aerobic medium tubes and anaerobic medium tubes, the sterility test is qualified, and further pathogen inspection is not needed.

If bacteria grow in 2 tubes of aerobic culture medium and are also found in the smear, it is necessary to test the samples for pathogenic cocci and intestinal pathogens.

If bacteria grow in two tubes of anaerobic culture medium and bacteria are also found in the smear, the samples should be tested for Clostridium botulinum and Clostridium welchii. If the expansion test is positive, the outgassing test is hydrogen, but the culture does not grow. This expansion is mostly caused by the hydrogen generated by the chemical action of the contents of the can on the wall of the can, that is, hydrogen expansion.

If the outgassing gas is not hydrogen or carbon dioxide, and the culture test is positive, the aerobic Bacillus will decompose the additives in some canned meat. Nitrate is caused by carbon monoxide and nitrogen.

Three, canned food food poisoning bacteria inspection

1, in the aseptic test of canned food, if cocci are found, pathogenic staphylococci and pathogenic streptococci must be tested.

2. If Gram-negative bacilli are found, the intestinal pathogenic bacteria such as Salmonella and Escherichia coli shall be tested.

3. If Gram-positive bacilli are found, Clostridium botulinum, Clostridium perfringens and botulinum toxin shall be tested.

4. If the sterility test of canned food is negative or its pH is below 4.6, there is no need to test the food poisoning bacteria.

Four, canned food acid bacteria inspection

The canned food suspected of acid-fast corruption should be tested for acid-fast bacteria.

Specific methods:

Randomly take a certain number of samples, keep them in a 55℃ incubator for 3 days, take them out, perform aseptic operation, suck the contents of cans 1g(mL), inoculate them in bromocresol purple glucose broth, and cultivate them at 55℃ for 5 days.

The main characteristics of typical acidophilic bacteria are 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, deep in the center, flat and slightly protruding, with neat or irregular edges.

In addition, if there is no obvious acid reaction in bromocresol purple glucose broth culture medium at 55 degrees, or there is an acid reaction, but there is an alkaline reverse reaction and a 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 liver slice broth culture medium. After 5 days of anaerobic culture at 55℃, the culture solution was crossed and inoculated into egg yolk agar plate containing 0.1%thioglycolate, and then anaerobically cultured at 50℃ for 24 hours. After 48 hours, Gram-positive colored colonies were selected for pure culture, and the results were analyzed according to table 1 1? 3 for further identification.

Clostridium thermophilus, Clostridium nigricans.

2. Test of mesophilic anaerobic bacteria

Section III 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 spoiling the most canned food. Therefore, effective control of microbial contamination of canned food is an important method to prevent canned food from deteriorating and reduce the occurrence of toxic phenomenon of Eucommia ulmoides. The control of microbial contamination of canned food is a complex systematic project, and a set of feasible comprehensive measures must be taken in production to minimize pollution.

First, the control of microbial contamination in the pre-processing of canned food

The raw materials used for canned food must be fresh, clean and hygienic. For meat food, raw materials must come from healthy animals to 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 removed.

Cleaning is an important process in the pre-processing of canned food. Cleaning not only removes the dirt and dirt on the surface of raw materials, but also reduces the microorganisms on the surface, so the cleaning water must be clean and hygienic, otherwise, if it is contaminated by microorganisms, it will aggravate the food pollution. 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 pollution in this book.

Second, the control of microbial contamination in canned food processing

As mentioned earlier, the main sources of micro-pollution of canned food are incomplete sterilization and leakage of canned food. Therefore, the most effective way to control canned food pollution is to cut off these two pollution sources, which involves the manufacturing process and sterilization procedures of canned food. Under the premise of maintaining the nutritional value and sensory characteristics of canned food, the microorganisms left in the can should be killed as much as possible.

Minimize the residual amount of oxygen in the tank. The heat-treated 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 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 stored in a freezer. In the process of storage and sale, it is found that the cans are rusted, deformed and cracked in the tank wall, etc., and it is not allowed to be sold and forbidden to eat.