Eating and Drinking:
A. How much food or drink is it forbidden to consume on Yom Kippur:
It is Biblically forbidden to eat any amount of food or drink on Yom Kippur. All the measurements of food and liquid that are mentioned are only with regard to the penalty of Kareis and the bringing of the sin offering, and not with regards to what is permissible. The necessity to know the measurements is for the need of an ill person which must eat on Yom Kippur. In such a case, we strive to feed him less than the Shiur Kareis, when possible.
B. The amount forbidden to be eaten that is under penalty of Kareis:
The penalty of Kareis only applies to one who eats the measurement of a thick Kuseves [i.e. Kuseves HaGasah, which is a thick date], which is slightly less than a Kebeitza [without its shell], within the amount of time it takes to eat a Pras of bread. [A Kebeitza without its shell regarding Yom Kippur is approximately 43 grams [of foods whose weight and volume are equal, such as water]. Accordingly, the Kuseves is slightly less than 43 grams. Practically, a Kusebes is measured as 30 grams in weight and 30 square centimeters in volume.]
How long does it take to eat a Pras of bread? Some Poskim rule that this is the amount of time it takes to eat four eggs. Other Poskim rule it is the amount of time it takes to eat three eggs. Practically, by a Biblical command, such as eating on Yom Kippur, we are stringent, while by a Rabbinical command we are lenient. [In terms of minutes, this matter is debated amongst the Poskim and the final ruling is to initially consider Kdei Achilas Peras as nine minutes.]
C. The amount of liquid forbidden to be drunk that is under penalty of Kareis:
The penalty of Kareis only applies if one drinks the amount of liquid it takes to fill a single cheek [i.e. Malei Lugmav]. This applies to every individual’s cheek size, and is not a general measurement. Hence, one with a larger mouth will have a larger measurement than one with a smaller mouth. One does not need to fill his entire cheek, but rather an amount that makes the cheek protrude from the side of his mouth is considered full in this regard.
Amount of time of consumption: The amount of time that this measurement needs to be consumed within in order to have a penalty of Kareis is disputed. Some Poskim rule that this is the amount of time it takes to drink a Revius [86 mil] of water. Other Poskim rule it is the amount of time it takes to eat a Pras of bread. Practically, one should be stringent like the second opinion. [In minutes, the measurement of Kdei Achilas Peras is initially measured as nine minutes 9 minutes, as stated above.]
D. Tasting food:
E. Rinsing the mouth:
It is forbidden to rinse one’s mouth on Yom Kippur. This applies even if one will be using less than a Revius of water. [This applies even if it is causing one great discomfort, he may not be lenient to rinse his mouth on Yom Kippur.]
F. Swallowing saliva on Yom Kippur:
It is permitted to swallow saliva on Yom Kippur. It is however forbidden before Yom Kippur to leave a pleasant taste in one’s mouth and then swallow this taste on Yom Kippur.
G. Eating inedible items:
It is forbidden to eat even inedible foods on Yom Kippur. Nevertheless, one who transgresses is not liable for the penalty of Kareis.
H. Inducing vomit:
 Admur 612/16; Michaber 612/5
 All food prohibitions in the Torah apply to even the slightest amount of food which is eaten.
 Admur 612/1-3
 Admur 612/1
 Degul Merivava 612; Tzemach Tzedek Mishnayos Yuma; Mateh Efraim 612/1; Shiurei Torah 3/10; See Nishmas Avraham 612/2
Other opinions: Some Poskim rule the egg is measured with its shell. [Darkei Moshe 456/1; Binyan Tziyon 30]
 Admur 612/3
The reason: The reason behind this measurement is because eating this amount of food is able to calm one down from his hunger [i.e. Misyasheiv Daato], and thus cause him to no longer feel oppressed. Every person, regardless of his weight, receives some nourishing from this amount of food. If this amount of food is not eaten within the amount of time it takes to eat a Pras of bread, then it will not nourish him enough to calm him down from his hunger, and therefore it does not carry with it the penalty of Kareis. [Admur 612/1]
 Shiurei Torah 3/10 that there are various opinions as to the measurement of a Kebeitza without its shell, and regarding Yom Kippur one is to suspect for the smallest measurement.
 Shiurei Torah 3/10 based on Admur 618/13 that the Shiur of the Kuseves is 2/3 of an egg or a little more; See there that this is the most stringent measurement as there is no exact Shiur recorded, and simply to be on the safe side one is to eat less than this amount. However, in truth it can also be 38 grams, and even 48 grams, and for a sick person, one is to go up until these amounts of Shiurim if 30cc is not enough.
 Admur 612/4; Michaber 612/4
 1st opinion in Admur and Michaber ibid; Rashi Yuma 80b
 2nd opinion in Admur and Michaber ibid; Rambam Yom Kippur 2/4; Rashba Toras Habayis
 See Shiurei Torah 3/15 [p. 303]; Aruch Hashulchan 202/8; Kaf Hachaim 210/5; Piskeiy Teshuvos 210/1; Sefer Haminhagim ibid and footnotes 342-344;
Opinion of 6-7 minutes: The Tzemach Tzedek [Shaar Hamiluim 1/8-10] records 6-7 minutes regarding the Shiur of Achilas Peras. [Ketzos Hashulchan 36 footnote 5; 59 footnote 4; Shiureiy Torah ibid footnote 35; Sefer Haminhaghim [English] p. 93 regarding Tisha B’av]
 Admur 612/12; Michaber 612/9
 Admur 612/12
 Admur 612/15; Michaber 612/10
 1st opinion in Admur and Michaber ibid; Rambam Yom Kippur 2/4
 2nd opinion in Admur and Michaber ibid; Raavad Terumos 10/3
 Admur 612/15; It requires further analysis why in this ruling Admur omitted that one can be lenient in regards to a Rabbinical law. In the previous dispute of the amount of time for food to be eaten this ruling was mentioned.
 Admur 612/7; Rama 612/6
 The reason: As the eating prohibition applies to any pleasure of eating and drinking, even if one spits the food out. [ibid] Vetzaruch Iyun if based on this one may place in his mouth a food that has a bad taste and then spit it out.
 It is evident that the penalty of Kareis was only given when one’s stomach receives pleasure from eating the food. However, the general Biblical prohibition applies even if he only gets pleasure from its taste.
 Admur 613/7; See Piskeiy Teshuvos 613/5; Michaber 567/1 that one may not taste anything on Tisha B’av, even if he will spit out and 567/3 “One who is accustomed to rinse his mouth, it is improper-not Kosher- to do so on a public fast day” based on Terumos Hadeshen 158 regarding the prohibition to taste foods on a fast; M”A 567/6; Drashos Maharil, brought in Darkei Moshe 567/1 and Elya Raba 567/5 and Kaf Hachaim 567/14; Mamar Mordechai 567/2; Levush 567; Nehar Shalom 567/1; Bigdei Yesha 567/6; Kaf Hachaim 567/13; Chayeh Adam 132/20; Ketzos Hashulchan 2 footnote 25 in name of Ateres Zikeinim; Sefer Haminhagim p. 4 [English]
Stringency or prohibition? It is implied from the Michaber ibid that there is no prohibition from the letter of the law involved in rinsing the mouth even on Yom Kippur, being one has no intent to swallow the liquid and it is merely water. [P”M 567 A”A 6]
 The reason: As one may come to swallow the water. [Mamar Mordechai 567/2; Levush 567, brought in P”M ibid]
 Michaber 567/1
 Chayeh Adam 132/20; M”B 567/11; Kaf Hachaim 567/14
 Admur 90/14; M”A 566; Aruch Hashulchan 90/17; See Piskeiy Teshuvos 567/2
Other opinions: Some Poskim rule it is forbidden to swallow saliva on Tisha Beav and Yom Kippur. [Bach, see Aruch Hashulchan ibid]
 Admur 612/8; Michaber 612/6
 Requires further analysis: Why is eating inedible foods forbidden, as seemingly there is no pleasure at all involved in it being eaten, unlike when one eats on a full stomach that although his stomach gets no pleasure from it, nevertheless his taste buds do? Perhaps the mere fact that the person is willingly eaten it shows that there is some pleasure he is getting out of it. Alternatively, perhaps the reason is because nevertheless one does become satiated from the item.
 Admur 608/9; M”A 608/7; Sefer Chassidim 769
 See Admur 328/44
 The reason: As in the process, one may come to eat part of the food which he expels. [Admur ibid]