<p>What materials which are <a href="http://www.shapeways.com/materials/">commonly used in 3D printing</a>, are food-safe?</p>

<p>Are there any certifications/grading process for such materials, which can help me with my cross-checking and selection?</p>

<hr>

<p>I have been using an <a href="https://en.wikipedia.org/wiki/Fused_deposition_modeling">FDM printer</a>.</p>

Food safety is a property of both the process and the material. You can't stick food-safe material in a printer that has previously been used to print something food-dangerous and expect the result to be food safe.
The only way to know if a given material is food-safe is to ask your supplier, but a lot depends on how you then process it. For instance, FDM printers often have brass nozzles, which contain lead. To print food-safe materials, you need to use a stainless steel nozzle.
Food safe materials can be identified by mean of
an universal symbol
.
Moreover, to ensure food-safety of a 3D printed model you may need to further process it (for instance, by vapor smoothing or coating with a food-safe lacquer). Some claims circulate on the internet that 3D printed models may have surface porosity in which bacteria can grow, but I've not been able to find a reliable source for this claim. Still, you need to be cautious.