Having a Bracha Achrona count for only a specific food:

• Example: One at a Kezayis of a piece of chicken or other food and drank a Revius of water and desires to recite a Borei Nefashos on the chicken, but to continue drinking the water. What is one to do?

Some Poskim[1] rule that if one says Borei Nefashos with intent for it to count for only one specific food that he ate, it nevertheless covers all the foods that he ate, and he hence cannot say a second after blessing on the other food. [Accordingly, he would have to say a new before blessing on the drink that he wishes to continue drinking, and only if he drank the Shiur, he would then have to say another after blessing.[2]] Other Poskim[3], however, rule that the after blessing only counts for what one had in mind, and hence in the above scenario, he may continue drinking without a new before blessing and then say an after blessing after he concludes. [Practically, in order to avoid the dispute, it is best to either: A) If possible [will not pass Shiur Ikkul on first food] delay the after blessing until after finishing eating/drinking all foods. B) Exempt all the foods in the after blessing, and then repeat a new before blessing for the food that one desires to continue eating.[4] C) One is to have in mind to exempt only one food, and then change areas from inside to outside or vice versa, and repeat a before blessing on the second food which he plans to continue eating.]

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[1] P”M 207 A”A 1; Pesicha Brachos 200 “Mireish”

[2] See Admur 291:3 that one must recite a new before blessing even if he had in mind to continue eating after saying the after blessing. See Admur 179:5 that if one has Hesech Hadaas he must repeat the blessing, and saying an after blessing is considered Hesech Hadaas.

[3] Rav Poalim 2:32 in length; Shaar Asher 1:4; Har Tzevi 1:96; See Admur 489/12; Rama 489/3; M”A 489/8; Chok Yaakov 489/14 that even according to the opinion who holds that that Mitzvos do not need intent to fulfill ones obligation, if one specifically has in mind to not fulfill his obligation through this counting then everyone agrees he is not Yotzei against his will. See Piskeiy Teshuvos 207:1

[4] This is not considered a Bracha Sheiyno Tzericha, as one is doing so in order to avoid the dispute.