In the Etymologies, the word for "bread" is given as bast , Quenya masta, but it seems that Tolkien later changed his mind and updated the word to bass, as shown in Quenya massánie, Sindarin besain, besoneth "bread-giver", and in the mutated form (i)mbas (apparently prefixed with the article). These latter Sindarin forms are however dubious, as we would rather have expected bessain (as a regular cognate of Quenya massánie) and bassoneth (without i-affection), and possibly a different mutation pattern after the article
Noldorin
bast
noun. bread
bast
noun. bread
This was the word for “bread” in Sindarin and its conceptual precursors for much of Tolkien’s life, derived from the equally long-lived root √MBAS “bake”. The word dates all the way back to the Gnomish Lexicon of the 1910s where G. bast “bread” was derived from the early root ᴱ√M(B)ASA “cook, bake” (GL/22). ᴱN. bast “bread” reappeared in Early Noldorin Word-lists of the 1920s (PE13/138) and appeared again as N. bast “bread” in The Etymologies of the 1930s under the root ᴹ√MBAS “knead” (Ety/MBAS).
It appeared without a final t in the Sindarin version of the Lord’s Prayer from the 1950s: anno ammen sír i mbas ilaurui vín “give us this day our daily bread” (VT44/21). However the t was restored in the phrase penim vast “we have no bread” from around 1959 (PE17/144). The late vacillations on the presence and absence of t are likely connected to Tolkien challenges with the derivation of lembas; see that entry for discussion. Likewise, the mutated forms mbas vs. vast indicate some late uncertainty on whether the primitive form began with mb- or b-.
Neo-Sindarin: For purposes of Neo-Sindarin, I would assume the normal form was bast “bread” from ancient mbasta, so that lenited forms show mb-.