Does the Borer restrictions apply when separating edible foods from amongst edible foods?
A. Edible food from edible food of same species: 
There is only a prohibition of separating when one separates waste [I.e. inedible parts] from the food or vice versa. However if one separates food from food [of the same species], even if he utterly only wishes to eat one of them while the second one he wants to take and throw out, [nevertheless this unwanted food] is not considered waste due to this since it is [nevertheless] fit to be eaten. Therefore it is allowed to separate it from the food which one wishes to eat and doing so does not contain [the prohibition] of separating waste from food. [See Q&A]
Separating large pieces from small pieces: As long as [the mixture] is of a single species, then even though one separates large pieces from small pieces [nevertheless] this is not [Halachicly] considered separating at all being that the entire [mixture] is fit to be eaten, and there is thus no waste [in the mixture]. [It is therefore] allowed to separate from it in any way that he wishes and even in order to eat it later on that day [meaning while it is still Shabbos]. [See Q&A]
Edible food from food of same species that is only edible in pressing times: If [the unwanted food] is only fit to be eaten in a pressing time then even though it is not considered actual waste and there is [thus] no Biblical prohibition in separating it, nevertheless it is Rabbinically forbidden [to separate it] just like is the law by actual waste. [See Q&A]
Removing the rotten lettuce leaves: Therefore it is forbidden to remove the rotten leaves from the vegetable (called lettuce) even if they are fit to be eaten in times of need. Rather one is to separate the vegetable from the leaves, which is [separating] the food from the waste. [See Q&A regarding if this restriction applies even when the leaves are still attached to the stem!]
If a same species mixture contains parts which are Halachicly forbidden to eat may one nevertheless remove the forbidden part?
It is permitted to remove the forbidden parts of the meat of an animal on Yom Tov even if one was able to do so before Yom Tov [and the same applies for any other case see Q&A]
The reason for this is because:  removing the forbidden parts of the meat is not a complete Melacha as although one is separating food from food, this poses no Halachic meaning being that it is all one species and it is only that the Torah has forbidden it from being eaten. Now, the prohibition of Borer only applies when one is separating two species of foods from each other or is separating waste from food which is considered like two species.
If the species contains two different parts such as an egg white and yolk : [To separate the yolk of an egg from its white for the purpose of eating it contains all the separating restrictions.] Nevertheless, this ruling would only apply in this case [which involves an egg] being that although the white is considered a food nevertheless it is not edible in its current state, as well as that it is common to throw it out together with the waste and thus appears like waste in comparison to the yolk which is eaten the way it is together with the mustard [or other food], and would [thus be] Rabbinically forbidden [to separate in order to eat]. [See Q&A] However in other scenarios [involving different parts of the same food] there would be no prohibition at all, even Rabbinically, in separating [one part of the food from another part] of the same food. Meaning that one may even separate small pieces from slightly large pieces, even if he throws the small pieces to the ground, as they nevertheless do not appear like waste due to this since they are edible and are commonly eaten.
If the two different parts of the same species are liquid substances and combine well: If one places the curd of milk into an elastic vessel and the whey drips from inside it then this is an offshoot of separating food from waste through using a sieve and sifter and one is thus liable.
The reason: Now, although the curd and the milk are both a single species of food and there is [thus] no waste [here] at all, nevertheless since they are liquid substances which mix well which he is [now] separating, therefore this contains the separating [prohibition] just like one who is separating food from waste. [Regarding turning milk to cheese on Shabbos- See Halacha 2 above. Regarding removing the fat off the surface of milk-See Halacha 8]
B. Edible food from edible food of different species:
However the above allowance to separate food from food only refers to when both foods are all the same species. However if one had in front of him two different types of foods which are mixed together and he wishes to separate [in order] to eat one of them right away and to place the second [food] aside to be eaten later on, then that species [of food] which one wishes to eat right away is considered the food while the other [species of] food is considered like the waste. Therefore one must separate with his hands that [food] which he wishes to eat right away from amongst the other [food], thus [separating] the food from the waste and not vice versa. If one separates it and sets it aside to be eaten later on, even [later on] that day, he is liable. [See Q&A]
Two species of fish: Two species of fish are considered two species of foods and it is [thus] forbidden to separate [even] by hand unless [one separates] that species which he wants to eat right away and [he is] not [separating it] from [prior to the coming] meal for [the need of the next] meal.
Separating a large species from amongst a small species:  [Furthermore,] even if one of the species are large and thus each species is [easily] recognizable and it is hence possible that there is no separating [prohibition] involved here at all, nevertheless one may not be lenient being that [doing so] is in question of being liable for a sin offering
[meaning that it is questionable if this is a Biblical prohibition, and thus one must be stringent].
Separating large pieces of both species from amongst small pieces:  If the [mixture] contains two different species and one [wishes to] separate from amongst both of them together the large pieces from the small pieces, then it is allowed since one is not separating one species from another. [See Q&A]
Summary-Separating from mixtures which contain edible foods:
The same species of foods, and the entire mixture is equally edible/useable:
There are no separating restrictions involved in separating food from food of the same species and level of edibility. Thus one may separate big pieces from small pieces or vice versa.
Regarding a food which contains two different parts, such as an egg which has a white and yolk, then if one wishes to eat one of them and throw out the other then if the food which one wishes to throw out is commonly thrown out despite it being edible, all the Borer restrictions Rabbinically apply. However if both parts of the food are commonly eaten then no Borer restrictions apply, unless it is a food which in essence contains two united parts, such as curd and whey of milk which are joined together, then separating one from the other does contain a separating prohibition.
A mixture which contain the same species of foods but some of the mixture is only edible in pressing times:
Such as the withered leaves of lettuce.
It Rabbinically contains all the separating restrictions mentioned above.
A mixture containing two different species of food:
Q&A regarding what is the definition of a same species mixture and a different species mixture?
The Rule: Any two foods which have the same name, taste and use are defined as being the same species. If either the name, taste or use is different than it is considered a different species.
Examples of foods which are considered two different species and retain all separating restrictions:
- Two different types of fish either in name or taste
- Stuffed fish with plain fish: Two fish from the same species but one is stuffed while the other is not.
- Candies which have different tastes or has the same taste but different color.
- Cooked with roasted: A cooked piece of fish/meat with a fried piece of fish/meat, even if they are from the same piece of fish/meat.
- Cooked with raw: A cooked/fried piece of fish/meat with a raw piece of fish/meat.
- Meat with liver or other parts of the cow.
- Meat with bones even if the bone has marrow.
- Meat with chicken.
- Different types of meats or poultry.
- Hot with cold: A hot piece of meat with a cold piece of meat of the same species is considered like two different species.
- Cookies: Cookies of different types which have different tastes, such as honey cookies with butterscotch cookies.
- Bread: breads which have different appearance or taste such as white and whole wheat.
- Fruits: Red grapes/plums/apples with yellow grapes/plums/apples. As well sweet fruits with sour fruits of the same species, or even if both fruits are sweet and same color but have a different taste of sweetness, as is common with different types of grapes, they are considered two different species.
- Nuts: A mixture of different types of almonds, or of whole nuts with crushed nuts [designated for baking etc] even if they are of the same species.
- Sugar: Ground sugar with sugar cubicles. As well regular sugar with powdered sugar [used to place on cakes].
- Cutlery: Bowls of different size or depth are considered two different species.
- Keys: Of different doors are considered two different species.
- Clothing: Shabbos with weekday clothing, or clothing of two different people [which are particular not to use each other’s clothes] are considered two different species even if they are the same article of clothing.
- Books: See Halacha 12 below
- Egg white and yolk: See above Halacha.
May one separate a piece of fish with many bones from other pieces of that same fish?
May one separate the different parts of the chicken from each other without restriction, such as to separate a thigh from a breast?
There are opinions which hold that the upper and lower parts of the chicken are considered two different species and therefore retain all the separating restrictions. According to others however it has the status of one species.
May one separate different sized diapers without restriction? 
Depends. If all the diapers in the mixture are useable for the child then yes. If only a certain size is useable then it has the same status as separating good from bad, and has all the separating restrictions apply.
Regarding separating diapers of different thickness, such as is common in a mixture of different brands of diapers: SSH”K rules that it contains all the separating restrictions. However Rav Farkash rules that if the difference in thickness was not manufactured intentionally to be used for different purposes, then one may separate without restriction. However if it was done intentionally, so the thicker ones can be used at night to intake more liquid, then it has all the separating restrictions.
May one separate large and small bread/Matzah crumbs from each other without restriction?
No. All the separating restrictions apply, as anything which is as small as flour which is together with a slightly large crumb receives the same status as a mixture of two different species.
May one separate sesame seeds from on top of Chalah?
One may do so for right away use.
May one separate the cream off ones piece of cake?
If he does not want to eat the cream: Only if a) one leaves a recognizable sliver of cream on the cakes surface and thus does not totally remove all the cream. Or b) One slices off a recognizable portion of the surface of the cake together with the cream that is on it.
If he wants to eat the cream or to give it to someone else to eat: Then so long as one plans to eat it right away the cream may be removed without restriction.
May one scrape off a dip from ones bread?
Has the same laws as removing cream from on top of cake, see above.
May one separate burnt pieces of fish and the like from the other fully edible pieces of fish?
May one separate fruits which have rotten parts on them from fully edible fruits of the same species?
If the fruit is rotten and spoiled to the point that they are only considered edible in pressing times, then it is forbidden. If however only a minute amount of the fruit is spoiled to the point that people will regularly just cut off the spoiled part and eat the rest, it is allowed.
May one cut off the burnt part of a pastry?
Although from the letter of the law it is permitted to do so prior to eating, nevertheless Lechatchilah one should cut together with it part of the edible side of the pastry.
May one remove rotten grapes from a vine?
Q&A on separating from same species foods which contain a forbidden part
May one remove the Kefula area of a Matzah on Pesach or does this pose a problem of Borer?
It may be removed, as explained with regards to removing the forbidden parts off meat.
Yes as explained above that forbidden parts of the same food do not contain the Borer restrictions.
May one separate whole Matzahs from broken Matzahs?
May one separate without restriction the yolk from the white or vice versa if he also plans to eat the white?
Q&A that relate directly to the Halachas inside
May one separate the large pieces of all the species in a mixture and in the process place the large pieces of each species separately?
For example, one has a vegetable salad and desires to remove all the large pieces of vegetables from the small pieces.
May one remove all the large tomatoes and put them on one side and then all the large cucumbers and place them on another side?
This may not be done as in the end of the day one has still separated one species from another which is forbidden. Thus when doing so the large pieces of all species must remain mixed.
May one separate from within the same species even with a vessel?
May one separate from within the same species the big pieces from the small in order to give them to two different people, the big pieces to one person and the small to another?
This is allowed.
Does the restriction of separating bad lettuce leaves apply even when the rotten leaves are still attached to the lettuce head?
The above law that one may only remove the good leaves from the bad only refers to when the leaves have already been cut up. However when the leaves are still attached to the lettuce head one may remove the bad leaves from the lettuce head immediately prior to the meal, as is the law with regards to peeling any peel off an item.
General Q&A on Separating from amongst different species
If one does not care to eat either food until later on, at which time he will eat them both at that time, may he separate them now?
This matter is disputed in the Poskim. Practically it seems that one is to be stringent.
If one desires to eat both foods during the meal, but desires to eat one prior to the other, do the separating restrictions apply?
- For example: If one desires to serve for the current course soup without the vegetables, and for the next course plans to serve the vegetables, may one separate the two without the separating restrictions, or may he only separate the soup [what he currently wants] from the vegetables?
There are some codifiers which rule as follows: If one also desires to serve the vegetables without any soup, then the separating restrictions do not apply and one may even remove the vegetables from the soup even though he is currently only interested in the soup for the coming course and the vegetables will only be served in a later course.
However if one does not care to separate the vegetables from the soup and only cares to separate the soup from the vegetables then the separating restrictions apply and one may only separate that food which he now wants.
Other Codifiers however rule that since both foods are to be eaten in the same meal, it therefore poses no separating restrictions.
If one is interested in both foods right away may he separate them with a designated vessel? 
- For example if one has soup and would like to serve the liquid separately from the vegetables which he will also serve, may he use a strainer to remove the vegetables?
This matter is disputed amongst Poskim. Practically one is to be stringent.
Does the full regulation of “Right away” apply also when separating food that one desires to eat from food which he does not desire to eat?
Although there are Poskim which are lenient in this matter to say that that one may separate even 4-5 hours prior, nevertheless one is to be stringent and follow all the regulations explained in the previous Halacha with regards to separating the food for right away use. Although in a case of great need one may be lenient to separate the wanted food within an hour before the meal.
 Meaning even to separate the unwanted food from the wanted food.
 As otherwise this is considered preparing on Shabbos for after Shabbos which is forbidden.
 This follows the ruling of the Magen Avraham [319/16] as well as Taz [319/12] and Olas Shabbos that the white and yolk are considered one food, and it is only Rabbinically that one may not separate without restriction. However the Mishneh Berurah [319/58] rules that it is considered two different foods and thus it Biblically retains all the separating restrictions. The difference between these two opinions would be that if one desires to use both the white and yolk according to the opinion of Admur and others one would be allowed to use a designated vessel to separate, as when one desires also the white the Rabbinical decree no longer applies. However according to the M”B all the separating regulations would still apply and thus a designated vessel may not be used to separate. [Shabbos Kihalacha Vol. 2 p. 187]
 This refers to a vessel made of material that can be stretched out do to having tiny holes in it, and thus allows the milk to drip out. Similar to a plastic bag.
 319/ 6 However if one of the species is large while the other is small then it is only questionable as to whether the separating prohibitions apply, although nevertheless one must be stringent.
 Shabbos Kehalacha Vol. 2 p. 176
 Shabbos Kehalacha Vol. 2 p. 184
 So rules Rav SZ”A and Sefer Binyan Shalom
 Sefer Ayel Meshulash
 Shabbos Kehalacha Vol. 2 p. 180
 Shabbos Kehalacha Vol. 2 p. 181
 So rules Admur in 504/6
 Shabbos Kehalacha Vol. 2 p. 235
 Shabbos Kehalacha Vol. 2 p. 178
 Shabbos Kehalacha Vol. 2 p. 178-179
 Shabbos Kehalacha Vol. 2 p. 235
 Shabbos Kehalacha Vol. 2 p. 316
 Shabbos Kehalacha Vol. 2 p. 104
 The foam of a drink is forbidden to be drank as stated in Hilchos Shemiras Haguf Vehanefesh Halacha 9
“One who drinks the foam that come on the surface of a beverage can lead to Ririn which comes from the nose, and one who blows it with his mouth can lead to a difficulty in the head, and one who pushes it to the side can lead to poverty.”
 Shabbos Kehalacha Vol. 2 p. 102
 Shabbos Kehalacha Vol. 2 p. 104-105
 To note however that the SSH”K rules that this is forbidden being that according to them if one item is forbidden and one is permitted they are considered like two different species despite the fact that they are all from the same species. This however does not follow the ruling of Admur and others as explained above in Halacha 6 that a prohibited part is not considered another species in the case of removal of forbidden fats. This is also the opinion of the Magen Avraham and Mishneh Berurah. Vetzaruch Iyun on the Ketzos Hashulchan which seems to imply that two Lulavim of two different people on Sukkos would be considered Borer on the first day of Sukkos when one cannot fulfill his obligation with another person’s Lulav. See Shabbos Kehalacha Vol. 2 p. 102.
 Shabbos Kehalacha Vol. 2 p. 187
 Shabbos Kehalacha Vol. 2 p. 176
Shabbos Kehalacha Vol. 2 p. 174
 So is evident from the wording of Admur in the case of straining eggs.
 Shabbos Kehalacha Vol. 2 p. 102
 So rules Rav S”Z Aurbauch in Shulchan Shlomo, thus coming to a conclusion of the doubt he raised in SSH”K [3 footnote 62].
 Shabbos Kehalacha Vol. 2 p. 233
 So rules Peri Megadim, Kaf Hachayim, Ben Ish Chaiy, unlike the Mamar Murdechaiy which rules that one must remove the good leaves from the bad.
 Shabbos Kehalacha Vol. 2 p. 188
 Shabbos Kehalacha Vol. 2 p. 192
 Brought in name of Rav Moshe Feinstein in an English book on Hilchos Shabbos. In Az Nidbaru 1/79/42 this question is left in doubt, and so is brought in one Sefer in the name of Rav SZ”A that he too was in doubt regarding this question. However see next footnote that others bring that Rav SZ”A ruled leniently in this
 Sheivet Halevy, Rav SZ”A as brought in Migeleas Sefer, and so leans Rav Farkash.
 Shabbos Kehalacha Vol. 2 p. 191
 Shabbos Kehalacha Vol. 2 p. 172
 Beis Yosef, brought in Magen Avraham. So rules also Aruch Hashulchan.
 So rules M”B , Machatzis Hashekel in the opinion of Rama and so seems to be the opinion of Admur.