Should the energy evolved per gram of a snack food be positive or negative after undergoing combustion

I get negative values, but the energy content of the snack food should not be negative though

Exothermic reactions have a negative sign by definition (ΔHprod - ΔHreact).
Since food releases energy it will have a negative sign.

If you're just putting a value on it, use the negative sign.
If you're answer is part of a sentence, the sign can be implied by the wording.
for example "contains", "will release", or "requires" show that you know the direction the energy is going.

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Dietary labels tend to have positive values. You could think of that as the energy that will be absorbed by the body (endothermic, +ΔH) rather the energy released by the food (exothermic, -ΔH).

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That really depends how you set up your sign convention. Usually, things you get out of the system are positive and things you put in are negative.