‘Leave Aside Babylonian Kutach’
The Sages (Pesachim 43b) assert that a person does not receive kareis or lashes for eating kutach habavli (a sour milk dip containing bread crumbs) on Pesach. The Gemara (Nazir 36b) explains that one who uses kutach as a dip normally does not consume a kezayis of breadcrumbs within the required span of time it takes to eat a peras (equivalent to the volume of three or four eggs). Therefore, even if one consumes more than a kezayis of bread crumbs throughout the meal, the crumbs do not combine to form the required minimum shiur of a kezayis since they were consumed over an overly lengthy period of time.
This rule is also relevant for the laws regarding berachah acharonah. One is not obligated (or even permitted) to recite a berachah acharonah unless one eats or drinks the proper shiur (i.e., a kezayis of food or a revi’is of beverage) within the time it takes to eat a peras. (Note: Some [see Rambam, Hilchos Ma’achalos Assuros 14:9] maintain that the time span allotted for drinking is the time it takes to drink a revi’is, which is a much shorter period of time than the time it takes to eat a peras.)
A Measure That Varies
The Kenesseth HaGedolah (cited by Ba’er Heitev, O.C. 204:12) asserts that the time it takes to eat a peras varies according to the food (or beverage) consumed. He rules that since it is common to sip hot coffee or tea at a slow pace, one should recite a berachah acharonah after drinking coffee or tea even if one did not drink the minimum shiur within the span it takes to eat a peras (and certainly not within the smaller span of time it takes to drink a revi’is).
The Minchas Chinuch (Mitzvah 313:5) rules in accordance with the Kenesseth HaGedolah, explaining that the time it takes to eat a peras does not depend on how long it takes to eat a peras of bread, but rather on how much time it takes to consume a three-egg volume of whatever food one is eating. If one eats a kezayis of meat, then it is essential that one eat a kezayis within the time it takes to eat a peras of meat. If one drinks coffee, then one’s obligation to recite a berachah acharonah depends on whether one drank the crucial amount within the time it takes to drink a peras of coffee.
A Measure That Is Constant
The Kappos Temarim (cited by Tosefos Yom HaKippurim to Yoma 80b) disputes the ruling of the Kenesseth HaGedolah and Minchas Chinuch and adduces proof from our Gemara that the minimal time span is constant. He argues that if the critical time span expands for foods that are commonly eaten at a slow pace, a person who eats kutach on Pesach should be subject to lashes even if it takes him longer than the time to eat a peras of bread to finish eating a kezayis of crumbs. After all, he consumed the kezayis of kutach within the normal time span it takes to eat a peras of kutach.
The Minchas Chinuch, however, explains that even though the individual may have consumed the kutach at a normal pace, our focus is not on the kutach but on the bread crumbs. Bread crumbs are bread, and so we focus not on the time span of eating kutach but on the time of eating bread. Since he didn’t eat a kezayis within the required span of time it takes to eat a peras of bread, he doesn’t receive kareis or lashes.