I am installing new shelves in my pantry, 12" x 55" with walls on three sides. The shelving material is laminate over particleboard. Considering how heavy a shelf full of canned goods is, can I get away with just using 1" nailers on all three walls to rest the shelves on, or do I need L brackets in the middle as additional support?
55 inches is a rather long span for a 12 inch wide shelf that is supported on the ends and back side by the wall. One way that you could provide some more support for the shelves would be to add some center front supports for the shelves working up from the floor to the top shelf.
Other important factors are how thick the shelving is, and what the shelf to shelf vertical spacing is, and how the back edge is attached, if at all, to the support. Assuming typical home improvement store shelving and common 12" to 16" vertical spacing, and simple nail attachment, the shelving is structurally adequate (meaning it will not actually collapse), but over time, a heavily loaded shelf will develop a very noticeable, unsightly bow without added support.(Which in itself could be called structurally inadequate by some) Particle board will creep, or slowly deflect, over time even under modest loads.
An alternative to L brackets might be a deep banding strip attached to the front of the shelving. The attachment would need to be carefully detailed, as you can't simply nail the banding into the edge of particle board and expect satisfactory results.
Another way to add some support to the shelves, expanding from my first answer, is to add angle supports as shown. This keeps the front edge of the shelves open and takes advantage of the fact that the back side support strip under the next shelf down is fastened securely to the wall. This approach works well in that the angle pieces do not need to be aligned or anchored into a stud.
I agree with the answer by @bcworkz and would add an alternative to L brackets. Wooden brackets would be more attractive and offer comparable support