I'm not trying to start a flame war, but I do believe there needs to be some clarification regarding propylene glycol because I'm seeing some information disseminated here that even if it is well-intentioned is a bit misleading.
Propylene glycol is GRAS-certified (generally-regarded as safe) by the FDA. Ethylene glycol is toxic, pure propylene is not. The only reason PG would ever be toxic would be other additives and there's the rub when it is suggested to use inhibited versions of this.
Far as I know, USP would indicate something has gone to the lengths of certification to be used as a direct food additive. It can have the identical make up as a non-USP grade but the non-USP grade was simply not manufactured or inspected under conditions which would be used to manufacture your Dr. Pepper or KY-Jelly. It may not be appropriate for recirculating systems. If you look at the GP at Tractor Supply it is for helping to cure ketosis in cattle. It may well lack properties suitable for recirculating systems.
In a previous lifetime, I was a regional manager for a chemical company which provided chemical and analytical services for HVAC chemicals for large commercial and industrial cooling and heating systems. That was 15 years back so forgive me if I've forgotten a few things. No I am not a chemist and I don't have a CE degree, I was on the marketing end of the business. I had to know enough about what the chemicals do, how to dose them, and what precautions there were with any given blend but I could not tell you which way the electrons are spinning that is the job of the CE's.
Propolene glycol for closed loop HVAC systems may use sodium nitrite or yellow metal inhibitors like tolytriazole because heating and cooling systems do have a combination of ferrous and non-ferrous metals. Sodium nitrite is commonly used as a pacifier for ferrous metals and to my knowledge on these beer line powerpacks we are using there is nothing ferrous in the pumps. Your pump housing is most likely brass (mine is on my Perlick) and your pump vanes are likely stainless.
Phosphate is used as a corrosion inhibitor as well and would be safer in my opinion than nitrite and TTA.
Glycol generally will not break down into glycolic acid when it is used for chilled water applications (it may well with age but it is a much slower process than when it is used at hot temps). It will eventually break down when heated which is why it was recommended to get your radiator flushed every few years. I don't seem to see that pushed as much by the auto repair shops as it used to be so I'm assuming auto anti-freeze must be using some buffers to help with that these days.
In other words, you really should have no need for corrosion inhibitors in the glycol blend for our purposes of keeping a sub-freezing reservoir of liquid. Even when used as beer tower coolers, there simply are no ferrous parts designed into those systems I can think of and brass and copper have naturally very good corrosion resistance with no additives.
The reason PG packaged for RV use is supposedly not suitable for a glycol system is it is designed to basically be poured in and sit stagnant so your lines don't freeze. It is not designed for recirculating systems, at least this is what power pack manufacturers with their own branded PG say. By nature, RV grade needs to be non-toxic because it is used to winterize potable as well as gray or black water systems.
As a bit of disclosure, I did use RV grade in my chiller which passed my check out of the MSDS sheet and had no issues for 1.5 years. Eventually my pump did take a dump but it was due to a ball bearing race which eventually failed not from the coolant side but on the exterior of the pump which means it was corrosion from condensate. Perlick's chillers come with a styrofoam casing on the pump head for insulation so I suspect it was retained condensate from intermittant use which led to the pump failure. Poking a small drain hole in the bottom of that I hope will solve the problem for the future.
The main issue I was concerned about the toxicity aside from a possible excursion into a fermenter was what happened if there was a leak and could it be disposed of through the sanitary sewer in small amounts.
However, the conversation of PG suitable for small recirculating systems has intrigued me and I wonder if a small amount of mineral oil might be added for lubrication though PG is fairly slick and may have some good lubricity properties.
If you really want to cut through the BS on what is safe, pull up the material safety data sheet (MSDS) and read it to see what is in it. One I did pull up said the corrosion inhibitor was proprietary and non hazardous so it was a protected trade secret, even at 12% utilization. (I'd bet anyone a tasty home brew this is a phosphate compound or straight phosphate).
Personally, if I realized I was missing several ounces of coolant while chilling a 10 gallon or 15 gallon batch and it was not on the floor, I would dump the batch of beer, food grade or not. It may not kill you but it doesn't belong in beer. Treat it as a flaw as you would a high temp excursion or an infection.
This would be my starting point to find suitable PG for your chiller- check the MSDS sheets from someone like Rapids Whoesale or another retailer who sells glycol for these systems. If they can't or won't provide an MSDS, keep looking. That is the best proof if the PG you are buying does or does not have any hazardous additives in it. The other rule is, if the glycol went missing, "food grade" or not I would not serve that beer.
Basically, your evaluation needs to make sure it's not hazardous with incidental contact and it should be considered appropriate for our small recirculating systems.
