After sterilization and cooling, canned food must be inspected after heat preservation or storage to measure whether its indicators meet the standards and commodity requirements, and determine the quality and grade of finished products. There are many inspection items and methods. Here are a few.

1. Appearance inspection: the key point of appearance inspection is to check the double seaming state, observe whether the double seaming is closely combined, whether there are tongue, teeth, crinkling and broken, and check whether the can body seam is normal. The size of double seaming can be measured with special calipers for cans. If there are air leakage micropores in the double seaming, which can not be seen with naked eyes, it can be checked with warm water or with pressure leak detector. The inspection of the can bottom cover mainly depends on whether it is concave inward. The normal can has a certain degree of vacuum, so the can bottom cover should be concave inward.

2. Heat preservation inspection: if there are microorganisms remaining in canned food due to insufficient sterilization or other reasons, it will grow and propagate at a suitable temperature and make canned food deteriorate. Most spoilage bacteria, except some heat-resistant bacteria, produce gas and make cans expand. According to this principle, the method of heat preservation storage is used to create the optimum temperature for the growth and reproduction of microorganisms, to place it in sufficient time for the growth and reproduction of microorganisms, and to observe whether the bottom cover of the can expands, so as to identify whether the quality of the can is reliable and whether the sterilization is sufficient. This method is called the heat preservation inspection of the can. This is a simple and reliable method for inspection of canned products. But some high acid food, if you ensure the correct operation process, ensure the production of good sanitary conditions, ensure adequate sterilization, can not be heat preservation inspection. At present, most factories have adopted the commercial aseptic sampling heat preservation under the condition that the process hygiene is guaranteed and the sterilization conditions are sufficient, but they only use the whole heat preservation inspection for individual products.

3. Inspection: before labeling, use the inspection stick to knock the bottom cover of all cans after heat preservation or storage to identify the quality of cans. Those with clear sound are normal cans, while those with turbid sound are inflated cans. The causes of turbidity can are as follows

① Due to insufficient exhaust and low vacuum degree of can, it can produce turbid sound. The main reasons for the decrease of vacuum in the can are as follows: the exhaust temperature is low or the time is not enough to reach the specified vacuum; too much food in the can will cause the decrease of vacuum, or too little contents will cause too large top gap in the can, so that the air in the can is not easy to exhaust, which will also cause the decrease of true vacuum. The heat transfer of the can is too slow to reach the full exhaust, and the vacuum will be low If the can is not sealed in time, the temperature of the food in the can will drop and the vacuum degree will be low. For the can which is pre-sealed and then vented, because the pre-sealed curl is too tight, the exhaust channel is blocked, so that the air in the can is difficult to be exhausted; it also causes the vacuum degree to drop, or even no vacuum degree at all.

② Turbid sound caused by incomplete sealing. There are micropores in the edge seam and the can body seam, and the air outside the can invades into the can, resulting in the decrease of vacuum. This phenomenon is often found in turbid can.

③ It is a turbid sound caused by gas generated in the can with good sealing performance. There are two reasons: one is the growth and reproduction of bacteria to produce gas. Due to insufficient heating and sterilization, the growth and reproduction of bacteria remaining in the can make the meat decompose and produce gas, resulting in turbid sound; the other is the chemical action to produce gas, resulting in turbid sound. When the raw material is heated and sterilized, it is easy to be decomposed to produce gas, which reduces the vacuum degree of the can. The lower the freshness, the lower the vacuum.

④ The changes of air temperature and air pressure lead to the decrease of vacuum degree in the can and the turbidity of sound. When the air temperature rises, the vacuum of the can will drop, and when the air pressure is low, the vacuum will also drop.

It is very important to inspect the canned products when they are packaged, and the inspection is based on practical experience, so it is necessary to equip specially trained inspectors and make detailed records of the inspection results.

4. Can vacuum is one of the physical indexes of can quality. The vacuum degree of normal cans is generally 0.024 ~ 0.051mpa, and the vacuum degree of large cans can be lower.

The vacuum degree of tinplate is usually measured directly by vacuum meter. A needle tip is arranged at the lower end of the vacuum meter, and a rubber pad is arranged at the back of the needle tip for sealing. When measuring, hold the vacuum meter with the right hand, align the needle tip with the center of the can cover, press down with force, insert the needle tip into the can, and read out the vacuum meter reading, which is the vacuum degree of the can. This method can also be used for tin covered glass cans.

5. Can opening inspection: to understand the change of the state in the can, it is necessary to open the can for inspection. Here we mainly introduce the sensory inspection and weighing inspection.

(1) Sensory examination includes the following three aspects:

① Histological and morphological examination. Canned meat, poultry and aquatic products should be heated in 80-85 ℃ warm water until the soup dissolves. Canned fruits and vegetables in sugar water should be opened at room temperature. The soup should be filtered first, and then the contents should be gently poured into the white porcelain plate. The arrangement of canned fish, individual vegetables (such as cucumbers, beans, etc.) and meat (such as braised pork, etc.) should be checked before they are poured into the porcelain plate. After pouring into the porcelain plate, first observe its shape and structure, and then gently move it with a glass rod to check whether the tissue is complete, the size and number of blocks. For sugar water fruits, it is necessary to observe whether the size is uniform, and whether there are mechanical injury, shrinkage, cracking, boiling and spots. For canned fish, check whether the spine is exposed, whether the bone and meat are continuous, whether the skin is attached to the fish, and whether the can is stuck, and carefully observe whether there are impurities or inclusions.

The syrup can is heated in 70-80 ℃ warm water until the juice dissolves, and then it is taken out. After the can is opened, the contents are flat tipped into a wire sieve and left standing for 3 minutes; the juice is allowed to sink to the bottom of the sieve, and the number or pieces of the contents are gently counted with a spoon to observe whether the size is uniform and whether there are fruits such as hardening, cracking, boiling and melting.
After the canned jam is opened at room temperature (15-20 ℃), a piece of jam is extracted with a spoon and placed on a dry white porcelain plate. Within 1 minute, the jam body is inspected for dispersion, juice secretion, impurities and seeds.

② Color check. At the same time of tissue and morphology identification, the color of the content was identified, and whether it met the requirements of the standard was observed.

For canned meat and poultry, the collected soup can be injected into a measuring cylinder, and the color and clarity of the soup can be observed after standing for 3 minutes.

For jam and canned tomato paste, pour all the paste into the white porcelain plate, observe whether the color is uniform and whether it meets the standard, and stir the paste thinly with a spoon, depending on whether there are inclusions.

Syrup can can be collected in the porcelain plate to observe whether it is cloudy, whether there is jelly and whether there are a lot of fruit chips and inclusions.

The juice of canned fruits and vegetables in sugar water can be collected in a small beaker to observe whether the juice is clear and transparent, whether there are inclusions and cloudy pulp debris.

After standing in glass container for half an hour, the degree of precipitation, stratification and oil ring were observed.

③ Taste and fragrance inspection. Participants should have normal sense of taste and smell, and cough before taste evaluation.

Take the solid and soup with spoon, first smell it with nose, then smell it to check whether it has the proper flavor.
The canned meat, poultry and aquatic products shall be inspected for the flavor of cooking (such as eggplant juice, spiced pork, braised pork, fried pork, etc.) and auxiliary materials, oyster flavor and odor, and moderate meat quality.

For fruits and vegetables, check whether they have aroma similar to the original fruits and vegetables. For jam, use a spoon to hold a small amount of jam, and chew it slowly in the mouth to evaluate its flavor. For canned fruit juice, you should smell its aroma first, and then taste it (concentrated fruit juice must be flushed to the specified concentration) to evaluate whether it is sour and sweet.

The whole sensory examination time should not exceed 2 hours.

(2) Weighing inspection

① Net content, solid content, liquid content. Clean the outer wall of the tank and weigh the gross amount of the tank. The canned meat, poultry and aquatic products should be heated in 80-100 ℃ hot water for 5-15min to melt the contents (if the contents are weighed after the sealing inspection, it can not be heated any more, but the contents must be melted). Take out the canned meat, poultry and aquatic products, pour the contents onto a wire sieve of known quality, put the sieve on a funnel with a larger diameter, and connect it with a measuring cylinder to collect the juice. Let it stand for 2-5 minutes to make the liquid run out. Clean the empty can with warm water, wipe it dry and weigh it. Then weigh the sieve and the solid together, and calculate the net content, solid content and liquid content respectively.

② Weighing of meat and oil. For canned meat and poultry, the liquid (including soup and oil) leaked through the sieve hole was collected in a 50ml measuring cylinder, maintained at a suitable temperature, and allowed to stand for 5min to separate the soup and oil into two layers. Pour the obtained liquid into another 50ml measuring cylinder which is dry, clean and of known mass until the full scale. Weigh the liquid and the measuring cylinder together, and then calculate the mass of meat, soup and oil respectively.

For Canned aquatic products. For canned fish with oil, collect the liquid (including oil and water) leaked through the sieve hole into a 100ml measuring cylinder, maintain the appropriate temperature, stand for 5min, make the oil and water into two layers, and read the ml of the water layer. The total ml minus the ml of water layer is the ml of oil layer. The quality of oil is obtained by multiplying ml of oil layer by oil density of 0.9. For canned fish with tomato juice, because all the juice leaked through the sieve hole is banru juice, the net weight minus the weight of the fish on the sieve is the quality of tomato juice.

For canned fruits and vegetables, the quality of the pulp or vegetables on the sieve is the quality of all the pulp or vegetables. However, in canned vegetables with ingredients, the weight of small ingredients must be subtracted from the weight of vegetables.

(3) Inspection of tank inner wall.

After opening the tank, clean the inner wall and observe whether the tin coating on the tank body and bottom cover is eroded, peeled and exposed due to acid or other reasons; observe whether the coating layer is corroded, discolored and peeled; whether there are rust spots and iron sulfide exceeding the requirements; and whether there is rubber flow in the tank.

(4) Chemical inspection shall be carried out according to corresponding standards.