How canned food deteriorates in the packaging process
canned food packaging containers mainly use tin cans, thin-walled glass bottles, cooking bags, empty metal cans and empty glass bottles, which can be directly purchased from designated manufacturers, and cooking bags can be purchased from prefabricated bags or directly produced by canning manufacturers. Empty cans should be checked before use. It is required that the cans are neat, the welds are complete and uniform, the edges of the cans and the cans cover are free of notch or deformation, and the tinned sheet is free of rust and tin removal. In terms of 3D printer, the glass can should be neat in shape, smooth in mouth, without notch, uniform in thickness, and free of bubbles and cracks in the glass. Some empty cans should be cleaned before loading, and should not be placed for a long time after cleaning to avoid re pollution. The production process of canned food changes with the diversification of varieties and raw materials, but the basic packaging process is generally: filling exhaust sealing sterilization cooling. Each process has factors that lead to the deterioration of canned food. The main influencing factors are analyzed as follows:
filling is to put the contents into empty cans. When filling, a certain top gap should be left in the can, which directly affects the capacity of the food in the can, the vacuum degree and the deformation of the can after sterilization. If the top gap is too small, the pressure will increase due to the expansion of food during heating and sterilization, which will often cause the bottom cover of the can to protrude outward, and even cracks will appear; If the top gap is too large, the pressure in the tank will be greatly reduced after sterilization and cooling, and the tank body will often sag by itself. In addition, due to the large top gap, there is more air in the tank, which is easy to cause oxidation and discoloration of food. The size of the top gap is slightly different according to the type of raw materials, tank shape and raw material state. Generally, the top gap mm is reserved during canning. Keep the edge of the tank mouth clean and dry when loading and filling salt liquid. Do not stick raw materials or salt liquid on the tank mouth to affect the sealing.
can filling generally adopts rotary measuring cup canning or drum vibration canning, both of which are volume quantitative. The reasons for the unqualified filling amount are: the capacity of the measuring cup is adjusted incorrectly, the hopper feeding is too slow or unstable, the loading surface of the hopper is too low, the feed pipe is too small, the material flow is not cool, and the feed pipe and the measuring cup are not concentric, which will make the measuring cup dissatisfied. If the running speed of the machine is too fast, and the hopper drops materials too fast, it will cause repeated cyclic loading of materials and excessive filling; Improper adjustment of the telescopic mechanism of the measuring cup will cause excessive reflux; If the container is not concentric with the feed pipe, the beat is not accurate, the container is too small, or the material sticks in the feed pipe so that the feeding lags behind, it will cause material overflow and loss.
exhaust is to remove the air contained in the tissue of canned food as much as possible after the food is filled, so that when the can is heated and sterilized, it will not cause excessive pressure in the can due to the thermal expansion of the air, causing the can body to deform or the can seam to crack; At the same time, a certain vacuum degree is formed in the tank, which can inhibit the activities of aerobic microorganisms remaining in the tank, reduce the oxidation and corrosion of the tank wall, reduce the oxidation loss of nutrients (especially vitamin C), and better preserve the color, aroma and taste of canned food. It is generally required that the central temperature of the exhaust to the sealed tank reach 70 ℃ -80 ℃. Vacuum pumping method is often used for tinplate cans - put the container and its contents in the closed volume, and vacuum the closed volume with a vacuum pump. The air density in the volume is reduced, and vacuum is obtained after sealing, or steam injection method - spray the bottle mouth with steam at a certain temperature and pressure, and the steam condenses after sealing to form a certain vacuum. Thermal exhaust method is commonly used in glass bottles and cans - heat the container and contents before sealing to reduce the density of air in the bottle, seal while hot, and obtain vacuum after cooling. The cooking bag adopts the vacuum pumping method. In order to effectively remove the air in the tank without affecting the quality of the contents, attention should be paid to the exhaust: the varieties with slow heat conduction should be re boiled before canning, and then filled with hot water, and then vented, and the exhaust time should be appropriately extended. During the exhaust process, the vapor condensate must be prevented from dripping into the tank. When using vacuum pumping method, the degree of vacuum pumping should be controlled according to the shape, variety and the temperature of the soup. For varieties with soup, the vacuum degree is too high, and the soup is easy to be extracted. Too low often leads to insufficient vacuum degree in the tank. Generally, the vacuum degree is controlled 7kga is appropriate.
when heating the exhaust, if the exhaust temperature is too high, it is easy to cause soft and rotten materials in the tank and insufficient net weight; Insufficient exhaust and too low vacuum degree in the can are easy to cause the sudden cover of the can, false expansion of the can and corrosion of the can bone, which can not effectively inhibit the growth of aerobic bacteria remaining in the can, and will affect the pressure difference in the can during sterilization and cooling, and also indirectly affect the leakage during sterilization of the can. The main reason is that the temperature and time of steam or hot water used do not meet the requirements.
when vacuuming and exhausting, the main reasons for insufficient vacuum are: the pumping rate of the vacuum pump decreases; Serious air leakage in the gas circuit or plugging of the extraction pipeline; Vacuum device instrument failure; The sealing rubber ring in the glass cap is damaged.
sealing is to isolate the food from the external environment under the condition of maintaining a vacuum in the packaging container, so that it will no longer be corrupted by the pollution of external air and microorganisms. If the sealing quality does not meet the requirements, the food will be oxidized and deteriorated in contact with the air. The sealing method of tinplate can is double crimping method - two crimping rollers with different groove shapes are used to curl and hook the pre crimped can body and the inner periphery of the flange shaped can cover. The glass can is mainly sealed by screwing - the metal bottle cap with convex claw is rotated relative to the glass bottle with spiral flange at the bottle mouth and moved along the spiral axis by using clamping claw, friction wheel or friction belt, so that the sealant on the top of the cap is tightly pressed on the end of the bottle mouth to form a seal. Soft cans are mainly sealed by heat sealing. The problems of sealing quality are as follows:
(1) the reasons for the lax sealing of double crimping:
false crimping - the crimping body hook and the cover hook are not engaged. The problems are the flanging damage caused during the transportation and transmission of the tank body, the shape of the tank body flanging is unqualified, the round edge of the tank cover is defective or damaged, and the tank cover is not aligned with the tank body during sealing.
large collapse edge - when sealing the can, because the edge of the can body or the can cover is seriously bumped, the can body and the can cover are not hooked together, and there are obvious can body flanging exposed at the lower part of the flanging. The reason is that the flanging damage is caused during the transportation of the can body, the shape of the can body flanging is unqualified, the round edge of the can cover is damaged or the round edge is too large, and the can cover is not aligned with the can body when sealing, etc.
skip sealing - because the curling at the weld is thick, the sealing roller jumps over when passing through the weld of the tank body and fails to compress the curling, because the can sealing machine runs too fast, the buffer spring of the second sealing wheel is too soft, and the joint of the tank body is too thick.
incomplete crimping - partial crimping is not fully compressed due to the slippage of the tank cover on the indenter or the poor rotation of the sealing roller, which is characterized by a normal thickness of part of the crimping and an excessive thickness of part. The reason is that the pressure of the tray is too small, the size or taper of the tank cover and the indenter is not appropriate, the indenter is worn, the height of the indenter shaft is not adjusted properly, the spacing with the tray is too large, the tray spring fails, there is oil on the indenter or tray, the axial movement of the indenter, inaccurate positioning, etc.
crimping "teeth" - the cover hook and the body hook are not overlapped, and the V-shape or protrusion is formed at the lower edge of the crimping, because the sealing curve of one sealing wheel is not ideal, the pre sealing machine or the first sealing roller is adjusted too loosely, and the sealing wheel is worn. Too tight adjustment of the second sealing wheel will aggravate tooth defects, the shape design of the can cover is not ideal, the can cover has wrinkles on the rubber bearing surface, there are sundries in the crimping or too much sealant in the can cover, the tray pressure is too large, the actual can is too much, the bearing of the sealing wheel is not flexible, etc.
broken curling - the outer iron sheet of the curling at the joint of the tank body is broken, because the second sealing wheel is adjusted too tightly, the tank cover material is defective, there are too many seals in the cover, and sundries are sandwiched in the curling. Because the first sealing wheel is adjusted too tightly, the folding of the tank cover is too long. Compactness is widely used and does not meet the requirements in various fields, because the curve of a sealing wheel is unreasonable, the adjustment of the sealing wheel is inappropriate, the curve of a sealing wheel is seriously worn, the shape of the rubber bearing surface of the tank cover is unreasonable or the rubber bearing surface of the tank cover is wrinkled, and the bearing of the sealing wheel is not flexible.
crimping damage - the damage of the appearance of the crimping part or the falling off of the coating, because the curve of the first sealing wheel is unreasonable, the smoothness of the sealing surface of the first or second sealing wheel is low, or the wheel is adjusted tightly, etc.
(2) the reason for the lax sealing of glass bottles: the gap between the bottle feeding turntable and the bottle guard frame is uneven or the whole machine moves uncoordinated, causing bottle breakage. If the belt speed is too fast, the bottle mouth will be crushed or the bottle cap will not be tightly sealed due to too low belt speed.
(3) the reason why the opening of the heat sealing bag is not sealed firmly or the seal is burned through or damaged: the former is mainly due to the low sealing temperature or insufficient sealing pressure and heat sealing time; There are sundries at the mouth of the bag, which makes the seal not firm; The heat sealer is bonded with melted plastic; When making bags, the former is not aligned with the material, so that the seam edge of the bag is not aligned. The latter is mainly caused by too long heat sealing time or too high temperature or pressure.
the purpose of sterilization of canned food is to kill microorganisms that are destructive and toxic to food. During sterilization, try to preserve the original color, flavor, texture and nutritional value of food. If the sterilization is not complete, the microorganisms in the can will multiply in large numbers when the conditions are suitable for growth, causing food spoilage.
canned food is often sterilized by thermal sterilization - heating the product to a certain temperature and maintaining it for a certain time to kill the bacteria in it
steam or hot water is often used as the heat source. Each can must be in direct contact with the heat source during sterilization. There are many factors that affect the sterilization effect, mainly including the following points: the sterilization formula of canned food is improper (the temperature or time cannot meet the requirements); Insufficient steam sterilization exhaust or poor hot water sterilization convection effect cause cold spots in the sterilization pot; For food containing soup, too high solid content will affect the convective heat transfer effect in the tank or the mixing function of the top gap gas will be affected if the tank is filled too full. Too low initial temperature will prolong the time required for cans to achieve sterilization effect, making the sterilization formula inappropriate. The high temperature of can sterilization will temporarily soften the rubber ring on the bottom cover. When the vacuum is formed when the can is cooled after sterilization, a small amount of cooling water may be sucked in, especially when the crimping quality is poor, micro leakage will occur. The nature and chemical composition of food are the environmental conditions in which microorganisms exist during sterilization. The acid, sugar, protein, oil, salt, etc. of food can affect the heat resistance of microorganisms. Other factors such as low pH value of food, high salt concentration, nitrite and organic acid can enhance the thermal effect of sterilization. The high concentration of sugar solution has a protective effect on bacterial spores. The size of food lumps, the content of solids and the way of canning will affect the heat transfer in the tank. During sterilization, the microbial contamination in the tank is high, and because the microbial residue in the tank after sterilization is directly proportional to the initial bacterial count, after the same sterilization conditions are adopted, the bacterial content in the tank is high, and the residual count is also high, which will make the can fail to meet the commercial sterility requirements
after sterilization, the cans must be cooled quickly, otherwise the food in the cans will remain quite high
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