For any food one eats, one must recite a blessing before eating the food, such as the “Shehakol” or “Ha’etz” blessings. Even if one only plans on eating a small amount of that food, one must nevertheless recite a blessing before eating it, as long as one is consuming it as a form of “eating” (as opposed to merely “tasting” a small amount of a given food in order to see if requires more spices and the like, in which case one would not recite a blessing at all). Thus, one who chews flavored chewing gum must recite a blessing before beginning the enjoyment of the flavor of the gum although one is only swallowing a minute amount. Similarly, before drinking any beverage, one must recite a blessing before drinking even a drop of that beverage (besides for water which only requires a blessing when one is drinking it to quench one’s thirst).
Nevertheless, the above only applies to blessings recited before eating. However, after-blessings (blessings recited after eating or drinking a given food or beverage) are only recited when one has eaten or drunk a “significant quantity.” A “significant quantity” is considered the amount of a “Kezayit” (twenty-seven grams) of food and a “Revi’it” (81 cc or 2.7 fluid ounces) of beverage.
Additionally, in order to qualify for reciting an after-blessing, the eating or drinking must be performed in a specific amount of time such that the various stages of eating or drinking can be combined and considered one big eating or drinking. This means that of one takes an entire day to consume a “Kezayit” of bread by eating one crumb after another, such an individual can certainly not recite Birkat Hamazon. Rather, one must consume a “Kezayit” of food within the amount of time it takes to finish half a loaf of bread. The Poskim disagree regarding exactly how much time this comes out to: Some say this equals five minutes while others say it equals approximately seven-and-a-half minutes. It is therefore correct to always make sure to eat a “Kezayit” (especially during the Shabbat meals) within four-and-a-half minutes in order to fulfill one’s obligation according to all opinions. (This is especially true on the Seder night when eating the correct amount of Matzah is a Torah obligation, as we have explained elsewhere. Nevertheless, if this proves too difficult for one to accomplish, one may eat the prescribed amount of Matzah within seven-and-a-half minutes as well.)
Regarding reciting an after-blessing on beverages, one must drink a “Revi’it” (2.7 fluid ounces) of the beverage in one shot (not one gulp; rather, drinking this amount continuously, without interruption), for if one interrupts between sips before reaching this prescribed amount, one should not recite an after-blessing at all. Thus, if one sips soup with a spoon, one will not recite an after-blessing on the soup, for one has not drunk a “Revi’it” of the beverage continuously. Similarly, if one sips hot tea or coffee, one should not recite an after-blessing on them, for one cannot drink a “Revi’it” of such hot beverages continuously.