In Alma 8:21-22, Alma blesses God after, not before, eating the meal Amulek offered him:
And it came to pass that the man received him into his
house; and the man was called Amulek; and he brought forth bread and meat and
set before Alma. And it came to pass that Alma ate bread and was filled; and he
blessed Amulek and his house, and he gave thanks unto God. (Alma 8:21-22)
This was a common Old World custom that the Nephites
seem to have retained. As Crowell and Tvedtnes noted:
In Judaism, while a brief blessing is recited before
eating, a series of longer blessings, the birkat ha-mazon, follows the
meal. Four blessings come after the consumption of bread, while separate
blessings are offered for other foods depending on their nature and origin. The
basis for this practice is Deuteronomy 8:10:
When thou hast eaten and art full,
then thou shalt bless the Lord thy God
for the good land which he hath given thee.
This observance is also found in a Dead Sea Scrolls
poem, which Gaster calls Invitation to Grace after Meals; this poem
paraphrases the language in the first part of Deuteronomy 8:10. It says,
"whenso they eat and are filled," noting that this is when men should
speak of wisdom and think of the law given by God. (Taken from Theodor H.
Gaster, The Dead Sea Scriptures, 3rd ed. [Garden City, N.Y.: Doubleday,
1976], 219-21)
Blessing after meals is mentioned in Mishnah Berakot
3:3 and Sukkah 2:5. The latter notes that when Rabbi Zadok ate only a
small portion of food, he didn't say the blessing afterward, probably because
Deuteronomy 8:10 calls for a blessing only if one has eaten and is full. (Angela
M. Crowell and John A. Tvedtnes, "The Nephite and
Jewish Practice of Blessing God after Eating One's Fill," Journal
of Book of Mormon Studies 6, no. 2 [1997]: 252)