Liz, Bach generally just wrote the music (and for that matter a lot of the pieces that are normally thought of as his work are actually his arrangements of other folks' tunes). So saying "Bread of Heaven is a hymn by Bach" is both to deny credit to the lyricist (the text you mention is by Josiah Conder) and likely to fail to honor the composer as well. Granted, Bach was a truly great arranger, and granted, when he wrote his own stuff he was a mightly fine composer, but still let's give credit where credit is due. Now as far as the music of "Bread of Heaven" goes, The Cyber Hymnal gives four tunes, with Paul Heinlein's JESU, JESU, DU MEIN HIRT, 1676, as default choice (the one that plays when you load the page), as well as the "tune proper" (i.e. one that has the incipit as its tune name), BREAD OF HEAVEN by WillÂiam D. MacÂLaÂgan, 1875; HOUGHTON-LE-SPRING by SamÂuÂel S. WesÂley, 1860; and NICHT SO TRAURIG by JoÂhann G. EbelÂing, 1666.
The Oremus Hymnal also gives Heinlein's tune (not to be confused with the tune formerly often called HEINLEIN but nowadays usually called AUS DER TIEFEâ€"oddly, this tune, attributed to Martin Herbst, came out in the same year, 1676, as the tune by Heinlein) as the default, but it mentions two others, CHRISTOPHER (about which I have no information except that apparently it is not the Christopher also known as IHR GESTIRN' which is mentioned as an alternate tune for the Christian ROCK OF AGES* usually sung to REDHEAD 76 [also called PETRA and occasionally AJALON] in England and to TOPLADY in the US) and JESU, MEINE ZUVERSICHT by Johann Crüger. This last was arranged by Bach; is it the tune you had in mind?
Hymnologically yours, Haruo
*not to be confused with the Jewish Rock of Ages, tune name MAOZ TSUR, a Hanukkah hymn sometimes used in Christian worship or as a tune for Christian lyrics (e.g. Men and Children Everywhere. The tune is of German folk origin. The English version linked above is a fairly loose adaptation and has only three stanzas unlike the original Hebrew's five.