Wow. I find the response to the fillable- refillable squeeze tube idea here to be quite interesting. Dear Mrs. Grumpy and I used those tubes on backpacking trips for years after we discovered them (then marketed by Gerry) in the late 1960s. We also carried fresh eggs in little plastic compartmented containers, with considerable success. Life was good. We did not fall ill.
I would say the only real disadvantage to refillable squeeze tubes is the relatively large volume of gooey stuff they can hold. Good for things like peanut butter and jelly – which themselves constitute the sandwich filling -- but more than a tad large for condiments like mustard, catsup or mayonnaise to serve a party of one or two on a day trip (if completely filled).
Nonetheless, the tubes are closed with reliable screw-on caps at the serving end and seal reliably at the filler end. The screw caps eliminate some of the mess associated with those little restaurant packets so many have advocated here. Also, I kinda like the idea of reusable things as an alternative to throwaways. I only wish the tubes were available in a smaller size.
Maybe
something like this would work as a smaller volume alternative. A little pricey, given the minimum order quantity, but maybe the local pharmacist can make them more affordable. I’d double seal, carrying the thing in a zip-closure plastic bag, though.
By the way, I have to think that if commercially prepped mayo would be considered “safe” on a sandwich in your daypack, then a small refillable container of it likewise should be OK. Personally, I dislike mayo as a rule, except for the homemade stuff on Mrs. G’s potato salad, which is wonderful even on picnics!
Bon appetit! (Did I get that right?)
G.