"Pepparn" is some definite pepper, otherwise it would be "peppar". So the translation without "the" is incorrect. The context for this sentence could be "I fed my spider one fly with salt, and one fly with pepper. The spider didn't like the pepper" (on the fly I fed him - a definite instance of pepper.)
In general, In english there are plenty of instances where a substance can be definite - e.g. "pass me the salt, please". In the latter case, the sentence would actually sound wrong without a specifier - it is either "the salt" or "some salt" - "pass me salt please" sounds pretty wrong, at least to my ears.