The test for food grade containers or containers that are coming into direct contact with food is indeed end product migration testing.
The migration testing should be conducted by a competent and accredited ISO 17025 lab and should cover the range of uses of the packaging material e.g.
- the samples should be printed if the packaging is printed
- if muliple raw materials are used by the packaging supplier then they all would all need to be tested separately
- should consider the migration laws where the packaging products will be sold and the food consumed
- should cover the range of product and processing conditions of the suppliers food producer customers e.g. hot fill temperature, high acid, high fat etc.
After all that is done then the packaging supplier can specify packaging materials to customers from a position of knowledge based on their distinct use. They need to aks customers about their product and processing etc. during new product discussions.
Then after that it is up to the customer to finally prove it through their process.