The quality and safety of canned food has always been the focus of attention of quality inspection departments and consumers. How to prolong the shelf life of canned food without preservatives is the difficult point in the processing of canned food manufacturers. Professor Yang Bangying, a consultant and senior engineer of the China Canning Industry Association, pointed out that the bottleneck of the canning industry in terms of management and sealing technology needs to be broken through. In management, the first is the form of product quality sampling, which makes some unqualified but undetected products drill holes into the market; the second is a large number of outflows. Oral products, due to the attention paid to the hygienic quality of the products abroad, neglect the sensory quality of the products themselves, making a number of poor taste, sensory performance of the products exported abroad. In terms of technology, manufacturers and operators need more attention.

 

 

Canned food preservation is based on the principle of sealed containers, moderate sterilization, do not need or allow to add any preservatives, finished products to achieve commercial aseptic requirements, can be stored at room temperature for a long time.  Therefore, the sealing requirements of canned food for packaging containers are very strict. Packaging container is an indivisible part of canned food. First of all, it should be absolutely sealed. The air and microorganisms outside the cans can not invade the cans to ensure that canned food will not be secondary contaminated and spoiled. Second, the inner wall of the canned container must be able to resist corrosion and ensure that the food is not contaminated. Affected by container corrosion, food safety, hygiene and color and aroma are affected.

 

 

There are many kinds of packaging materials for canned food containers, such as tinplate (commonly known as tinplate), chrome plate, aluminum alloy plate, glass can, aluminum-plastic composite packaging materials, paper-plastic-aluminum composite packaging materials (commonly known as Lile bag), heat-resistant plastic cans sterilization. At present, China produces about 3 million tons of canned food every year, and most of the packaging materials used are tinplate.  Tin-packed containers have many advantages, such as good sealing performance, high temperature and pressure resistance, good heat transfer performance, high-speed mechanical production, good ductility, the current mechanical production of the tank, the speed can reach more than 600 cans per minute; light weight, low price, good corrosion resistance, easy to return Recycle and utilization; 5 years after the abandonment of soil decay, will not cause environmental pollution; can withstand more extensive handling; drastic changes in the external temperature has little impact on the contents of such things.

 

 

In addition to tinplate, canned food packaging used more glass cans, its advantages are very stable chemical properties, and will not have chemical reactions with food, and contaminated content; because of the transparency of glass cans, can clearly see the color and shape of canned content, convenient for consumers to choose and buy; can be reused. However, the mechanical strength of glass and the drastic change of resistance to cold and heat are poor; the weight of glass is 3-4 times heavier than tinplate, the heat transfer rate is about 1/30 of tinplate cans, and the heating time of canned food is usually more than twice longer than tinplate cans. The cooling time is longer and the stage cooling process is adopted. Other kinds of packaging are less used: some canned fish and shellfish use aluminum alloy cans; a small amount of fruit and vegetable juice, milk canned use Lile bag; tomato sauce, concentrated juice use aluminum plastic aseptic large packaging.

 

 

Common corrosion phenomena in the inner wall

 

 

1. Sulfide corrosion: Canned goods with long storage time, the inner wall or bottom cover of the container, often appear cyan-violet, grey-black

The phenomenon of color, even black, when serious, the inner wall of the black substances will be separated, contaminated food causes food discoloration, this phenomenon is called sulfur corrosion.

 

 

In the recent spot check, it was found that many of the inner walls of the milk refiner corroded, polluting the refined milk, causing the refined milk to become "rusty milk", affecting the health of consumers, it belongs to sulfide corrosion. The main reason is that these foods contain a large number of proteins, hydrogen sulfide or other sulfides in the sterilization and storage process and contain - SH clusters, these substances and tin tin tin scratches on the surface of tinplate can process, and eventually produce black sulfide corrosion.

 

 

2. Equilibrium corrosion of the inner wall of the cans: acid food cans often have uniform tin dissolution in the inner wall of the containers, so that the tin layer crystal body of the cans wall is exposed completely, uniform corrosion occurs, tin content in canned food increases a little, the time will not exceed the national hygiene standard 200 mg/kg, food will not have the smell of iron. However, if the storage time is too long and the corrosion continues to develop continuously, the tin layer on the tank wall will peel off and the steel base will be exposed. In addition, the tin content in the food will increase sharply, and the food will have the smell of iron, and a large amount of hydrogen will be produced, resulting in hydrogen expansion of the tank, and serious cracking will be caused. At this point, this canned food has lost its edible value and commodity value.

 

 

3. Local corrosion occurs on the inner wall of the cans: after opening the cans, dark brown corrosion rings (commonly known as oxide rings) are often found at the junction of the liquid level at the top gap. This is due to the corrosion of tinplate caused by the residual oxygen in the top gap, and the contents will be polluted if it is serious.

 

 

4. Centralized corrosion occurs in the inner wall of the tank: a limited area of tin dissolution occurs in the inner wall of the tank, which is the manifestation of concentrated corrosion, such as pores, pits, black spots, serious perforations on the tank wall.

 

 

5. Abnormal Detin Corrosion: Some foods contain special corrosion factors, such as nitrate ion and nitrite ion, which react directly with the inner wall of canned containers, often occurring in a relatively short period of time within two or three months with large area of detin phenomenon.