There appears to be an interesting contradiction in the Bartenura's peirush on the Mishna. In Maaser Sheni 2:1, the mishna notes that if we improve the flavour of maaser sheni wine by adding honey or spices, the extent to which the wine increases in value must be reckoned against a calculation based on the value of the honey and the spices which were chullin. But if we heat maaser sheni dough over a fire, the value of the dough rises as a result and we need subtract nothing for the value of the wood. The Bartenura explains the difference between these two cases as follows (s.v. השבח לשני):
ופודה את הפת בשויו, ואין חולקין השבח לעצים של חולין, אלא כל השבח למעשר שני, לפי שאין שבח עצים ניכר בפת
We can redeem the bread at its full value, and need not apportion out the value of the wood of chullin. Rather, all of the profit is maaser sheni since the improvement conferred by the wood is not recognised in the bread.
Contrast that with what he has to say on the mishna in 'Orlah 3:4! There, the mishna observes that if we cook a stew using husks of 'orlah, the stew needs to be destroyed. The gemara (Pesachim 26b) understands this mishna to be referring to an oven that is lit with husks of 'orlah and that the reason that it is forbidden is because the husks improve the taste. Note the Bartenura (s.v. ידלק):
דיש שבח עצים בתבשיל, וכן בפת
Since wood confers an improvement on stew, and also on bread.
Am I to assume that the Bartenura recognises that the quality of the wood improves the taste of the bread, but in a manner incommensurate with the value of the wood? That makes little sense to me, since the cost of wood is a good indicator of its quality. How else might the contradiction between these two passages be resolved?