Asking someone to pick up a few things from the market can cause one to spend a lot of time explaining what you really want. “We need a vegetable, please grab some green beans to go with dinner tonight.” This allows for several options that the marketplace provides for shoppers today. How much product do you want? What style of green beans do you want: whole, cut, French style? Is convenience a factor, should they be microwaveable? Do you care if the green beans were organically grown? If they are canned, are they also packed in a BPA-free container? There are a lot of decisions to be made for a simple side dish. There’s also many decisions that must be made when manufacturing metal cans that those green beans might be housed in.
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Metal Can Anatomy
The metal container has improved dramatically since it was invented around 1810. By around 1920 it started to look like what is seen today in the marketplace: open top cans with a top lid that is “crimped” (double seamed) to the can body. The size and shape has not changed much over the last nearly 100 years; however, many subtle changes have occurred.
Three-piece cans are named for the number of separated pieces that make up the can: one body and two lids on either side of the can body. The body starts as one flat piece of tinplate that is welded together to form the cylindrical body. The cylindrical body is then manipulated to form flanged ends and ridged body beads (for added container strength) around the circumference of the body. One flanged end is sealed by a lid that is double seamed on the can body. A coating material is then applied to the interior of the can. Cans are then stacked and wrapped to prevent damage and then sold to food producing companies along with the other ends (lids). The food manufacturer fills the can with a food product and then hermetically seals and heat processes it to sterilize the can and the food product.
The welded side seam is a relatively new improvement to the three-piece metal can relative to the overall age of the canning industry. Before welding, the side seams were soldered and the solder contained lead that could leach into the food product and thus be ingested by the consumer. Soldered side seams also presented other problems with sealing containers. The side seam with solder was much thicker than the tinplate, which would then compromise the overlap measurement of the double seam at that point called the side seam juncture. A phase out of lead soldered side seams began in 1979 and by 1991, the FDA issued a final rule to prohibit the sale of lead soldered cans. Other soft metals, like tin and silver, can be used to make solder for can side seams, but that is rarely seen today. Modern can welds greatly reduce the thickness of the side seams at the juncture area compared to the older soldered side seams.
If a company packed and processed a food product that was later found to have a non-hermetic seal and the food product spoiled, how do you know who put on the lid that led to the spoilage? Container manufacturers needed to identify the responsible party and root cause of the issue. Can bodies were soon sold to food processors with one lid on. They also made the other lids that the food processor had to hermetically seal on the open end of the can, after the food was put in and the lid was applied. An embossed code system was then implemented by the food processor to identify its lid.