Commentary Par XVII 58

Strangely enough, the meaning of this verse is much debated.  From the beginning, all have agreed that it refers to the bitter taste of bread (or anything else) eaten in bitter conditions.  The 'unofficial commentary tradition,' that is, ordinary readers, however, senses a reference to the way bread is prepared in Florence (to this day): It is baked without salt.  Pietrobono (DDP Pietrobono.Par.XVII.58-60) is the first commentator even to refer to that fact and simply denies its relevance (thus revealing that some discussants had raised this issue), insisting on the larger and obvious meaning.  (He cites the often-cited passage in Convivio [Conv.I.iii.4] in which Dante laments his exilic experience.)  Fallani (DDP Fallani.Par.XVII.58-60) explains that the salty taste is supplied by the exile's tears.

Longfellow (DDP Longfellow.Par.XVII.58) cites several pertinent passages, including Ecclesiasticus (29:24 [29:31-32 in the Vulgate] and 40:28-29 [29-30]): IIt is a miserable thing to go from house to house; for where thou art a stranger, thou darest not open thy mouth.  Thou shalt entertain and feast, and have no thanks: moreover, thou shalt hear bitter words....'; 'My son, lead not a beggar's life, for better it is to die than to beg. The life of him that dependeth on another man's table is not to be counted for a life.'  He also cites Shakespeare's Richard II (III.1):

Myself

Have stooped my neck under your injuries,

And sighed my English breath in foreign clouds,

Eating the bitter bread of banishment.