Hovering over a food item, starting from the left and moving 1px right each screenshot:
On the third screenshot you can see some pink lines to the left of each saturation icon.

This is something that popped up in 1.7.2 (the same code in 1.6.4 didn't have this issue) and I've never been able to figure out why it happens, although I haven't looked too deeply.

It seems to be related to drawing scaled icons. Relevant code:

https://github.com/squeek502/AppleSkin/blob/1.10.2/java/squeek/appleskin/client/TooltipOverlayHandler.java#L136-L162

I had a similar issue with scaled icons on the Forestry Beealyzer. I solved it by... not scaling icons. :(
My best guess is it's a UV issue, where the scaled icons UVs do not exactly hit the right icon in the texture map due to extremely tiny float rounding errors.
Maybe you could make a mini version of the saturation icon to use?

Took your suggestion and added smaller versions of the icons. Fixed in v1.0.1: https://github.com/squeek502/AppleSkin/releases/tag/v1.0.1