Baba Basra 95b - Bracha on Spoiled Food

The gemara discusses the proper bracha to make on wine that begins to spoil, whether you make hagafen or shehakol. The rashbam and tosafos conclude that so long as it still has the taste of wine, even if it smells like vinegar, we make hagafen - as the shulchan aruch paskens 204:3. The same is true with other foods as well as the braisa says on 95b - on spoiled bread you make shehakol rather than hamotzi. In both cases, you only make shehakol as long as it is still eatable, but if it is so spoiled that it cannot be eaten then you wouldn't make hagafen as shulchan aruch writes (204:2) by wine, and the m.b. (204:1) writes by bread.

The Biur Halacha raises a very interesting question. The Rambam writes that all these spoiled items where the bracha rishona is shehakol, the bracha achrona is borei nefashos. The Biur Halacha asks that although bracha rishona is within the power of the rabbonon to determine that the bracha on partially spoiled food should be reduced to shehakol, but birchas hamazon is d'oraysa! Since the bread is still eatable and for yom kippur we consider achila al yidei ha'dchak to be an achila, so on a d'oraysa level we consider it to be an achila (furthermore, the sha'ar hatziyun 18 says a sevara that on yom kippur we are stricter about what we consider to be an achila since we require yi'suvei da'atei, whereas by brachos all we require is ha'na'ah). How then can one just make a borei nefashos, since m'doraysa he should be required to say birchas hamazon? The Biur Halacha suggests that perhaps we are speaking only when one ate a shiur kezayis so that his chiyuv to bentch is only d'rabonon, but if one would eat a shi'ur se'viah, he would actually be obligated to say a full birchas hamazon. However, it is not mashma from the rambam that you only make a borei nefashos when you eat less that a k'dei se'viah. Perhaps the rambam goes li'shitaso who holds that the bracha me'ein shalosh on the 7 minim, which includes non-bread items made from grains, is only d'rabonon. This is also the implication of the shulchan aruch (209:3) that only on bread should one say birchas hamazon mi'safeik d'oraysa, but not on a tavshil of the 7 minim. It could be that spoiled bread, although it is considered food, it looses its chashivus as "lechem" and is demoted to be just like a tavshil of the 7 minim where the bracha is only d'rabonon, therefore the rabbonon were able to say that borei nefashos is sufficient.