I have purchased today at the McDonald's closest to my home (so as to control time exposed to the elements) 3 plain hamburgers. I've taken photos of the original receipt with time-stamp on it. I've also verified the metadata on my camera is correct and I won't modify anything from the original shots.
My idea is to have one burger representative of what was claimed. And I will keep it sealed in a Ziploc brand plastic bag. I expect this will provide a better seal than her Welch's box, but I want to know she wasn't cheated. Thing is she said she uses it as a prop, but doesn't say how often nor how long it's out of its container. So I plan to break the seal on the Ziploc bag once a week for 30 minutes for air. And I'll weigh it at that time.
Now the other two. One I've sealed, untouched since put into the paper it's wrapped in, sealed in a Ziploc freezer bag, and left in the refrigerator. I plan to leave it this way until I see significant change in our exposed weekly burger. The other I have taken out of the wrapper placed in the same type of Ziploc freezer bag, and will leave sealed next to the exposed one.
I've only weighed the one burger that will be regularly exposed, and hope we'll all be satisfied that all 3 will be within +/- 5g of each other at creation. I expect McDonald's has pretty tight controls on product manufacture.
Here are the first of what I expect will be many pictures and updates.
