For what cheeses must one wait six hours before eating meat?
The law: From the letter of the law there is no requirement to wait six hours after eating any form of dairy. Nevertheless, the Ashkenazi custom is to be stringent and wait [six hours] after eating [certain forms of] hard cheese, prior to eating meat. Practically, although one is not to protest those who are lenient, it is proper to be stringent.
The definition of hard cheese: The definition of hard cheese in this regard is disputed as to whether it is dependent only on the cheeses fat quality and content, or is alternatively also dependent on its dryness and toughness of the cheese which causes it to get stuck between the teeth. Some Poskim rule like the former approach and conclude that hard cheese is defined as maggot cheese [i.e. Cazu Mazu] or pungent cheese produced from a very sharp enzyme, such as the stomach, which makes its fat content be extremely potent. It, however, does not apply to cheese that is simply aged. [This fat quality is measured not by the percentage of fat in the cheese but by the quality and sharpness of the fat found in the cheese.] Other Poskim, however, rule like the latter approach and conclude that also cheese which has aged approximately six months is considered hard cheese and requires a six hour wait. [Practically, the final ruling follows the latter opinion that any cheese which is either aged six months, produced with worms or a very strong enzyme which makes it pungent, requires a six hour wait according to the custom. Thus certain Swiss and Holand cheeses require a six hour wait.]
The practical application by today’s cheeses:
Aged or pungent cheese: All cheeses which are aged six months or are pungent, require a six hour wait, in accordance to the ruling and custom stated above. An extensive list of these cheeses can be found on the OU website.
All other hard cheeses: Hard cheeses which have not aged 6 months and are not pungent, such as typical yellow cheese and the majority of forms of hard cheeses sold today on the market, do not fall under the hard cheese definition and thus do not require a 6 hour wait. However, some of today’s Poskim are stringent to require a 6 hour wait by all hard cheeses sold today, including yellow cheese. Practically, the widespread custom is to be lenient, although some follow the stringent opinion.
Must one wait six hours after eating aged cheese which is melted?
Some Poskim rule that aged [non-pungent] cheese [i.e. 6 months] which is melted does not require one to wait six hours.
List of hard 6 month or pungent cheeses:
* Provided on the OU website. See there for extensive listing.
1. Appenzeller Extra (Swiss-made)
2. Appenzeller Surchoix (Swiss-made)
3. Asiago d´Allevo/Mezzano (Aged)
4. Asiago d´Allevo/Stravecchio
5. Asiago d´Allevo/Vecchio
6. Caciocavallo (Aged)
7. Caciocavallo (Semi-aged)
8. Caciotta Alpina
9. Cheddar (Medium, Sharp, Aged)
10. Chevre/ Goat Cheese (Aged)
11. Dry Monterey Jack
12. Emmental/ Swiss Cheese-Switzerland
13. Fiore Sardo
14. Fontina (Aged)
15. Havarti (Aged)
16. Fiore Sardo
17. Fontina (Aged)
18. Kashkaval (Aged)
19. Kashkaval (Young)
20. Marble Cheese (Aged)
21. Mochego Curado
22. Montasio (Aged)
24. Pecorino Romano
25. Pecorino Sardo
26. Pepper Jack (Foreign Market)
27. Piccante Provolone
28. Provola dei Nebrodi
29. Provolone, Piccante
32. Speedy Piccante
 Michaber Y.D. 89:2 “One may eat meat products immediately after eating milk products so long as he verifies that his hands are clean and he does Kinuach and Hadacha to his mouth.”; See Rama ibid in next footnote that it is only a custom to be stringent, and one is not to protest against those who are lenient
 Taz 89:4; P”M 89 S.D. 16 and M”Z 3
Other opinions: Some Poskim rule that according to the original Ashkenazi custom and Zohar there is no need to wait six hours after hard cheese, but rather only one hour. [Sefer Mitzaref Lechochma p. 27a; Implication of Shach 89:16]
 Rama ibid “There are those who are stringent to wait between eating cheese and then eating meat. Practically the custom is to avoid eating any meat, including poultry, after eating hard cheese, for the same amount of time that one waits between eating meat and cheese. However, there are those who are lenient and do not wait prior to eating meat. One should not protest their custom so long as they wash their hands and clean and wash their mouths prior to eating the meat. Nevertheless, it is best for one to be stringent.”
The Sefaradic Custom: Some Sefaradim are accustomed to wait one hour between hard cheese and meat. [Ruling of Rav Aba Shaul, brought in Hakashrus 10:49] Others are accustomed to wait one hour per month of its age. [Ben Ish Chaiy Shlach 15 states this is the custom in Bagdad] Others rule it is not required to wait at all, as is the ruling of the Michaber. [Yabia Omer 6:7]
 Taz 89:4 “Due to the reason of fat”; P”M 89 M”Z 4; Aruch Hashulchan 89:11 that certain hard cheeses have a lot of fat content and this is the reason for the six hour wait; This follows the first reason and opinion in Tur ibid, Taz ibid and Shach ibid, which follows the opinion of Rashi in his reasoning behind waiting six hours, that it is due to the fat
 Peri Chadash 89; P”M 89 S.D. 15 and M”Z 4 that according to Shach 89:15 and Issur Viheter who hold aged cheese is a problem because it gets stuck between the teeth; Aruch Hashulchan 89:11; This follows the reason of Rambam Machalos Assuros 9:28, brought in Tur ibid, Taz 89:1 and Shach 89:2; See however Taz 89:4 who negates this issue according to Rambam and says it only applies regarding meat.
 Taz 89:4; Issur Viheter brought in Taz 89:4; This follows the first reason and opinion in Tur ibid, Taz ibid and Shach ibid, which follows the opinion of Rashi in his reasoning behind waiting six hours, that it is due to the fat
 See P”M 89 M.Z. 4 that the eating of the worms which is considered like meat gives off a taste of fat for 6 hours just like meat. [See Darkeiy Halacha 89]
 i.e. Casu marzu cheese. This cheese is created by leaving whole Pecorino cheeses outside with part of the rind removed to allow the eggs of the cheese fly Piophila casei to be laid in the cheese. A female Piophila casei can lay more than five hundred eggs at one time. The eggs hatch and the larvae begin to eat through the cheese. The acid from the maggots’ digestive system breaks down the cheese’s fats, making the texture of the cheese very soft. By the time it is ready for consumption, a typical casu marzu will contain thousands of these maggots. There are several other regional varieties of cheese with fly larvae in Europe. For example, goat-milk cheese is left to the open air until Piophila casei larvae are naturally laid in the cheese. Then it is aged in white wine, with grapes and honey, preventing the larvae from emerging, giving the cheese a strong flavor. In addition, other regions in Europe have traditional cheeses that rely on live arthropods for ageing and flavoring, such as the German Milbenkäse and French Mimolette, both of which rely on cheese mites.
 See Taz ibid; P”M ibid; Aruch Hashulchan ibid
 Taz ibid “In my opinion one is to only be stringent by maggot cheese, however cheese which is aged but not wormy is not considered hard cheese and there is thus no need to delay eating meat after eating this cheese, and rather it suffices to wash one’s hands and perform Kinuach/Hadacha, unless one desires to be extra scrupulous and holy.”
The reason: As even cheese which has aged does not have a very sharp fat content and hence there is no need to wait between it and meat, as rules the Rosh and Tur. Now, although the Rambam rules that if the food gets stuck in between the teeth one must wait, and hard cheese does get stuck in between one’s teeth, nevertheless this only applied by meat as there is a verse brought in the Gemara which teaches us that meat in between one’s teeth is still considered meat. However, other foods are not considered food when they are in one’s teeth. [Taz 89:4]
 Pashut! Butter contains 80% fat and all agree it is part of the soft cheese list which does not require a six hour wait. Hence we see it is the fat quality and not quantity that matters.
 Shach 89:15; Toras Chatas p. 12; Issur Viheter, brought in Taz 89:4 [however the Taz himself negates this definition, as stated above]; Peri Chadash 89; P”M 89 S.D. 15 and M”Z 4; This follows the reason of Rambam Machalos Assuros 9:28, brought in Tur ibid, Taz 89:1 and Shach 89:2
 So writes Shach ibid; See Sheivet Haleivi 2:35 that some Achronim rule all hard cheeses require a 6 hour wiat
 The reason: As hard cheese of this nature gets stuck between the teeth. [P”M 89 S.D. 15 and M”Z 4]
 Aruch Hashulchan 89:11
 Aruch Hashulchan ibid; See P”M 89 M”Z 4
 These take approximately 1-2 months to be prepared. [Hakashrus 10 footnote 120]
 Hiskashrus 931 “Majority, if not all, cheeses on the Mehadrin market today are not hard, and one does not need to wait 6 hours after them.” Rav Y. Farkash rules one may be lenient by all hard cheese unless one knows it is aged six months. This is likewise the position of many Kashrus agencies.
The reason: As a) They are not as hard as 6 month aged cheese; b) Their fat content is not pungent; c) The widespread custom is to be lenient.
 See Sheivet Haleivi 2:35 that he is personally stringent; Rav Elyashiv; Rav S.Z. Labkowsky; Rav Eli Landa; See Hakashrus 10 footnote 125]
 The reason: Some are stringent being that although these cheeses are not wormy and have not aged 6 months, nevertheless they maintain a hard cheese quality due their ingredients, and high fat content which is result of the new technology in cheese processing. [See Hakashrus 10 footnote 120, 125 and 126. I heard this also from Rav S.Z. Labkowsky, and so he rules to be stringent by all hard cheeses.] Others are stringent because it is not written on the cheese as to how many months they have been aged and hence, due to doubt, one is to be stringent by all hard cheeses. [See Shevet Halevi ibid]
 Yad Yehuda 89:30