Leachables & extractables / odor & taste transfer in packaging have been a perennial source of controversy and product rejections. Nature's penchant for equilibrium results in migration of substances from plastic packaging into the product.
If glass was a new invention, it would be difficult to get FDA approval due to the elements which leach from it. It's cliche but true - anything in quantity is poison.
The ramifications of leaching and taste transfer can range from annoying and inocuous to toxic. The metallic taste in tap water resuls from rusting ductile iron pipe. A faint plastic taste will eventually show up in old bottled water. Harmless but annoying. There is no telling what contaminants can enter the recycling stream in post-consumer reprocessed plastic. For this reason, reprocessed plastic is not an option for food or pharmaceutical packaging. There is one exception - polyethylene terephthalate ( PET #1 ) which is re-polymerized vis-a-vis reprocessed,
Commodity plastics such as polyethylene ( PE ), polypropylene (PP) and polystyrene ( PS ) are popular choices for food grade packaging because they are so inert. For reasons still unknown, some polymers transfer taste to specific products. For example, anyone who has made institutional milk bags knows there is zero tolerance for EVA. The bouquet of EVA ( best described as apple vinegar ) transfers to milk.
The subjective zone often results in vicious disputes. Years ago, a major manufacturer of cereal detected an oxidized odor in the film yet there was no transfer to the product and there were no consumer complaints. The film supplier had to make sandwiches out of the returned film because the cereal manufacturer refused to pay the invoice.
After 40 years and millions of pounds of ice bag film, we no longer make film for ice bags due to subjective, specious odor and taste complaints.
Of course, very real leaching can ruin entire lot of product by the presence of only trace amounts. Many a lot of circuit boards has been ruined by a few hundred parts per million of slip or silicone. Clean room packaging has zero tolerance for slip and silicone.
The threshold of parts per million has very serious legal implications, mainly about compliance with regulations. California prop 65 is not going away. The scope is expanding indefinitely.
Extractables encompass everything from the monomer itself to residual catalysts and additives. So let's break it down:
monomers Plastic is made from simple hydrocarbons. With the exception of PVC, these feedstocks have been shown to be inert and harmless. Old fashioned HDPE is still a popular choice for distilled water.
catalysts are necessary to facilitate a reaction to create the polymer chains. The catalyst is either transformed or used up. Remember the hysteria over BPA ? Turns out it might have been overblown.
Like it or not, the catalyst BPA is here to stay. If BPA is an indisputable carcinogen, we should discontinue the use of ubiquitous 5 gallon polycarbonate carboys today.
Additives is a big subcategory encompassing antioxidants, stabilizers, antiblock, slip and pigments. Additives each have a specific job to do. Without antioxidants, plastics would have an odor like the smoke from a candle which was just extinguished. Consumers would return any food packaged in an "antioxidant free" package. Most resin antioxidants are tocopherols commonly known as vitamin E. Only PVC has phthalates for stabilization.
Antiblock is diatomaceous earth. It is so inert it is used to encapsulate nitroglycerin to make dynamite.
Slip is used to make plastic slippery as the name implies. Most slip additives anymore are pure enough that they can be Kosher certified.
Most US made pigments are HMF, or "heavy metal free" if the end use is food packaging or anything medical. The yellow does not contain lead and the orange no longer contains molybdenum. But most consumers prefer to eagerly expose themselves daily to dermal exposure to levels of lead which are in direct violation of CA prop 65.
Quantifying leachables and extractables is done according to ASTM WK43975. The extraction test is done in two solvents. The extractables must fall below specific threshold amounts to be considered FDA approved.
Of course, what is difficult to capture is goodwil and good faith. If somebody is looking for an excuse to not pay their bill, odor and taste is a handy excuse. As many injection molders found out ( some were put out of business ), the test method of CA prop 65 was modified to obtain the desired lead threshold results. Whether or not the thresholds posed a realistic health hazard was irrelevant.