Difference between lab-grade and food-grade purity?
So-called food-grade stuff is meant to be consumed by humans. It's important that every single compound of a foodstuff is food-grade or at least GRAS. In the European Union the law giver has defined the state when a compound is good for consumption as "save".

Lab-grade chemicals often contain toxic materials, so when something is called lab-grade it is not save per definition. 

For example: $\ce{NaOH}$ is widely used in preparation of foodstuff as well as in the lab. Now it's a fact that lab-grade (standard) $\ce{NaOH}$ (Sigma Aldrich) contains a certain amount of heavy metals. On the other hand, food-grade $\ce{NaOH}$ will not contain any heavy metals (usually food-grade $\ce{NaOH}$ will have a purity of more than $99\%$ while standard $\ce{NaOH}$ pellets are sold with purities less than $97\%$). So using lab-grade $\ce{NaOH}$ in foodstuff is a crime (at least in the EU).

So my advice is: Do not use lab-grade materials for any food-related applications.