Why is there a Minhag not to eat sesame in Erev Yom Kippur?

Question

I found sources that say that there is a Minhag not to eat sesame in Erev Yom Kippur, since it is considered as a heavy food and might disturb the dawening in Yom Kippur. I would like to understand why the sesame has such an impact. Can it be s small garlic that might sound same?

Here is one of the sources I found for the Minhag of not eating sesame in Erev Yom Kippur:

ובסעודה המפסקת לא יאכל בשמים וכורכום ושומשומין רק מאכלים קלים כדי שלא יהא שבע יותר מדי ויפריע לו להתפלל [...] one should not eat spices, turmeric, sesame. Only light foods, so that he will not be too full and that it will not interfere with praying.

Answer 1

The reason behind this is that sesame seeds are one of the foods that can cause a person to regurgitate and causes one to feel full (of eating the meal). The Alter Rebbe in his Shulchan Aruch HaRav (608:8) writes:

On the day preceding Yom Kippur, even during the morning meal, one should eat only foods that are readily digestible (so that he will not be [overly] satiated),39 e.g., fowl and fish, so that he will not be satisfied and proud while praying on the day preceding Yom Kippur.

Sesame should be avoided, since it can cause reflux. It can also cause nausea, vomiting, coughing etc... See also here:

Sesame is the ninth most common food allergy among children and adults in the U.S. The edible seeds of the sesame plant are a common ingredient in cuisines around the world, from baked goods to sushi. Several reports suggest this allergy has increased significantly worldwide over the past two decades.1

When a person with an allergy to sesame is exposed to sesame, proteins in the sesame bind to specific IgE antibodies made by the person’s immune system. This triggers the person’s immune defenses, leading to reaction symptoms that can be mild or very severe.

Starting January 1, 2023, sesame will become the ninth major allergen that must be labeled in plain language on packaged foods in the U.S. While some manufacturers may begin labeling for sesame sooner, they are not required to do so.

Thus, sesame should be avoided, since it can cause one to regurgitate.

Shulchan Aruch HaRav 608:10

One should not eat sesame seeds on the day preceding Yom Kippur, [lest] they cause regurgitation on Yom Kippur.

Also, garlic and eggs should also be avoided, since these are products that causes the human body to produce sperm (see: footnote 9).

Answer 2

It goes back to R. Yehudah ha-Hasid's Sefer Hasidim (רס"ה). The original context is where he says that if you want someone to regurgitate like an animal that raises its cud, you should have the person eat all that he needs and then take a fistful of sesame and have him swallow it, and then he'll be regurgitating all day long... and for that reason one should not consume sesame (presumably in this aforementioned way) on שבת lest they raise up their food whilst traversing the public domain and thereby violate the prohibition on carrying.

אם תחפוץ מאדם שיהא מעלה גרה כבהמה לא תודיעהו שאתה מתכוין לכך לאחר שיאכל וישתה כל צרכיו תן לו מלא אגרופיך שומשמין ויכוס אותן ויבלע כל היום יהא מעלה גרה כבהמה לפיכך לא יאכל אדם שומשומין בשבת וכיוצא בו לאחר אכילתו שמא יצא לרשות הרבים ומאכל בפיו

Later codifiers (Magen Abhraham, Be'er Hetebh, et al), taking R. Yehudah ha-Hasid's folk-medicine (and attendant halakhic advice) as factual, extrapolated from it and suggested that one ought not eat sesame on the eve of Yom Kippur, lest it cause one to regurgitate on YK itself and thereby either violate the prohibition of eating or cause an interruption to prayer.

It does not seem to be the case that contemporary medicine finds sesame to possess an emetic property, however it would be interesting to see if there are any 12th c. gentile sources from Germany or thereabouts that make similar claims regarding sesame.

Given that the minhag appears to be based on antiquated medical knowledge, it presents an interesting question concerning whether it ought be deemed authoritative/binding upon those that had previously accepted it.