"In a pantry" indicates that they could be hold up piles of plates and glasses, or 5-10 pound bags of flour, sugar, potatoes, etc, or just cans of soup or vegetables. That will, pretty quickly, add up to a lot of weight.
The biggest issue I see is that they're floating, which is cool, hip, and modern, but suitable only for light weight knickknacks.
As unappealing as it might be, adding an angled bracket into the studs at the end of each run and a single one in the middle would significantly increase the strength of your shelf without detracting too much from the appearance. These brackets could be metal shelf brackets purchased from the store, they could be more 2x2 with mitered cuts at the ends, they could be 2x2 hardwood to make them more decorative, or even some angle iron for an "industrial chic" look.
One possibility, since you're putting on plywood tops, would be to make plywood gussets that would be screwed into the "stringers" (sticking straight out from the wall) then angled down to rest against the wall. In theory, if you cut them really well, you might get away with just resting them on the wall, but better would be to screw a 2x2 upright to the wall, then screw the gusset to the upright.
Oh, and is pine a bad choice for building something like this? Hint: your whole house is built out of pine just like it. It's the size and constructions details that matter more than the choice of lumber.