When to Make an After-Blessing?

For everything a person eats, they recite a blessing before eating, even if they eat only a small amount of that food, as long as they intend to eat it as food. Therefore, someone who chews gum that has flavor, even though they swallow very little of it, must still make a blessing for the enjoyment of the gum's taste. Similarly, when someone drinks any beverage, they need to recite a blessing even if they only drink a single drop of that drink. However, this applies specifically to the blessing before eating. For the blessing after eating, one only recites it after consuming a significant amount of food, which for eating is the size of a k'zayit (27 grams), and for drinking the measurement is a revi'it (81 grams).

Additionally, the eating or drinking must occur within a time frame where the consumption can be combined together. If someone eats a k'zayit of bread over the course of an entire day, crumb by crumb, they certainly cannot recite the Grace After Meals afterward. Rather, the consumption of a k'zayit must occur within the time period known as "kedei achilat pras" (the time it takes to eat half a loaf). The authorities disagree about the exact length of this time period. Some say it is five minutes, while others say it is about seven and a half minutes (and there are other opinions as well). Therefore, it is proper to be careful to always eat a k'zayit (especially in Shabbat meals) within four and a half minutes to fulfill all opinions.

Regarding the after-blessing for beverages, one must drink a revi'it (81 grams) in one continuous act. If one drank with pauses between sips, no after-blessing is recited at all. Therefore, someone who eats soup with a spoon does not make an after-blessing on drinking that soup, since they did not drink a revi'it in one continuous act. Similarly, someone who drinks coffee or tea when they are hot does not recite an after-blessing for them because they cannot drink them in one continuous act.

Someone who drinks black coffee when it is lukewarm, even if they drank a revi'it in one continuous act, does not recite an after-blessing, because most people do not typically drink black coffee when it is cold, and we follow the practice of most people in this matter. Even if that particular person always drinks black coffee when it is cold, their personal preference is nullified by common practice, and they do not recite an after-blessing for their drinking. Regarding coffee with milk, nowadays many people are accustomed to drinking it cold, so if one drinks the required amount in one continuous act, they should recite an after-blessing.

Rulings of Rabbi Ovadia Yosef courtesy of "Daily Halacha" website