Gemara

(Beraisa): One may use Tamei Terumah bread or oil for fuel.

(Rava): He puts the bread amidst wood and puts the oil in a disgusting Kli [so no one will eat it].

(Abaye and Rava): The Beraisa does not permit for wheat. Perhaps he will eat it. (It does not become repulsive even if it is amidst wood.)

(R. Yochanan): It is permitted even for wheat.

He does like Rav Ashi answered [elsewhere]. He makes it repulsive through cooking it [and then throws it amidst wood].

Berachos 50b: Shmuel permits using bread for any need. This is like R. Eliezer [who permits washing hands with raw wine. Chachamim forbid.]

(Beraisa): There are four laws of bread... we do not put raw meat over bread... and we do not support a bowl with bread.

Rishonim

Rambam (Hilchos Terumah 12:12): If Terumah bread or oil became Tamei, one casts it amidst wood or puts it in a disgusting Kli until burning it, lest others eat it. If Terumah wheat became Tamei, one cooks it and puts it in a disgusting Kli, so it will be inedible, then uses to heat an oven.

Rav Pe'alim (3 OC 9): Eshel Avraham forbids throwing Orlah fruits in front of a Chasan and Kalah where it is muddy, due to disgrace to food. One may not snub Hash-m's beneficence. It is a Mitzvah to burn them, but not to disgrace them. Pesachim 33a connotes unlike this. One may throw [Tamei Terumah] bread amidst wood, lest one come to eat it. For wheat, he cooks it and then throws it amidst wood to make it repulsive. The Rambam rules like this. Since it is destined to be burned, we are not concerned for disgrace. All the more so we are not concerned for disgrace of throwing [Orlah] fruits in front of a Chasan and Kalah where it is muddy, since they are destined to be burned!

Rambam (Hilchos Berachos 7:9): One may not use bread to support a bowl.

Talmidei R. Yonah (on Rif Berachos 37b, DH Ein): We do not use it to support a bowl, lest soup spill on it. The Rif brought the Beraisa Stam. R. Yonah disagrees, for Shmuel permits using bread for one's needs.

Rosh (Berachos 7:32): R. Chananel rules like Chachamim who argue with R. Eliezer. Also the Beraisa that forbids supporting a bowl with bread is unlike Shmuel, so the Halachah does not follow Shmuel. However, Beitzah 21b is like Shmuel. Bahag rules like Shmuel only regarding uses that do not ruin the food. Maseches Sofrim (3:14) forbids sitting on a box full of olives, dates or dried figs, but one may sit on [a basket full of legumes, or] on a ring of pressed dates, for this is the custom.

Rosh (ibid.): Some eat porridge using bread in place of a spoon, since they eat the bread afterwards. Maseches Sofrim (3:14) permits using a food to eat another food only if they are proper to eat, i.e. together, and he does so. Do not explain that it permits using foods proper to eat. All the more so one may use foods improper to eat, for they are like wood! If so, one may not use bread to eat porridge [if he does not eat them together]. Perhaps Maseches Sofrim is unlike Shmuel. It forbids using food to support food, and Shmuel permits if it will not become despised. We hold like Shmuel, and use bread to eat porridge. Porridge does not ruin the bread. Gedolim would eat a little of the bread each time with the porridge. Shmuel agrees with the Beraisa's four laws of bread, except for the Isur to support a bowl with it.

Poskim

Shulchan Aruch (OC 171:1): One may use bread for any need, as long as it will not become despised.

Mishnah Berurah (2): All the more so, one may use other foods for all his needs.

Mishnah Berurah (4): For the sake of Refu'ah (a cure), we permit even if it will become Ma'us (328:20). We permit any need of man that is normally done with food. We sprinkle wine on the floor and anoint the body with oil, like the Gemara says in many places.

Shulchan Aruch (2): One may not sit on a basket full of figs and dried figs, but one may sit on a ring of pressed dates or on a basket full of legumes.

Bach (2): One may sit on a hard wooden basket, like ours. It does not bend when one sits on it. The food does not become despised. There is no disgrace, even if it is full of Seforim. The Tur forbids sitting on a basket of leather or sackcloth, for it bends when one sits on it. Even so, he permits if it is full of legumes. They do not become despised at all. If there is a ring of pressed dates inside, sometimes they get mashed. Even so, the custom is to be lenient, since usually they do not, since they stick to each other well. Only this depends on custom, but letter of the law we permit legumes. They do not become crushed at all, and there is no disgrace to food at all.

Taz (3): This is even if it is a wooden basket, like Stam baskets in the Gemara. In any case the figs are mashed if the basket is full and uncovered, and one sits on top. If it is covered, it is permitted. One may not sit on a box of Seforim, even if it is covered (YD 282:7), unlike the Bach.

Magen Avraham (2): This is only for a wicker basket. The reeds bend when one sits in them, and the food becomes despised. One may sit on a wooden basket, even if it is full of Seforim, and all the more so if there is food inside (Bach). The Rema (Teshuvah 34) forbids sitting on a box of Seforim. The Taz (YD 282:4) permits if it is fixed in the wall. This is the custom in the Beis ha'Keneses.

Kaf ha'Chayim (15): Also Eliyahu Rabah forbids sitting on a box of Seforim. The Shach (Nekudas ha'Kesef YD 282:7) permits. Bechor Shor is lenient if the box is not special for Seforim, and especially if there are also other things inside. If the custom is to sit on benches in the Beis ha'Keneses with Seforim and Tefilin inside, and we do not protest. Ohel Yakov 1,2 is lenient if there is a gap of a Tefach above the Seforim. Sha'arei Teshuvah (40:5) says that one should be stringent.

Mishnah Berurah (12): It is forbidden only regarding a wicker basket. One may sit on a wooden box.

Mishnah Berurah (13): Pressed dates ate hard. They do not get mashed.

Mishnah Berurah (14): We permit legumes even in a wicker basket. They themselves are hard.

Shulchan Aruch (3): One may eat porridge (cooked peeled wheat) with bread in place of a spoon, if he eats the bread afterwards.

Beis Yosef (DH Kasav): The Rosh, Tur, Tosfos and Hagahos Maimoniyos permit this.

Beis Yosef (DH u'Mah): The Tur explains that the Rosh permits only if they are proper to eat together. He cannot exclude what is not proper to eat, for all the more so one may use it like wood!

Magen Avraham (3): It is disgraceful to use bread like wood. Since they eat some of the bread each time, it keeps the status of food. It becomes despised [only] to others.

Gra (DH Mutar): It is permitted because we hold like Shmuel. This is why he must eat the bread at the end.

Mishnah Berurah (15): One may do his needs with bread when it does not become despised. Here, it is not despised to the one who eats with it.

Kaf ha'Chayim (18): If it is not proper to eat, one may use it like wood.

Kaf ha'Chayim (19): One may use bread to eat porridge in his bowl. After eating from a slice, one may not put it a serving bowl from which all eat.

Shulchan Aruch (ibid.): Meticulous people eat some of the bread each time they put the bread in their mouth with the porridge.

Beis Yosef (DH u'Mashma): Those who eat porridge with bread in place of a spoon, they hold like Shmuel. They eat the bread afterwards, for one who eats porridge with bread does not despise the bread. Therefore, he does not waste food. If he would not eat it, no one else would eat it after he put it in his mouth, so he overtly ruined the food. It seems that Maseches Sofrim forbids even if he eats it afterwards. It disgraces bread to use it like wood. Therefore, one may not use a food (perhaps this should say "bread" - PF) to eat another food unless he eats them together. Then, the bread is not secondary to the porridge. The porridge is secondary to the bread! It accompanies the bread! Great people eat some of the bread each time they put it in their mouth with the porridge, to fulfill Maseches Sofrim. Then, it looks like the porridge accompanies the bread! Eating a little of the bread each time suffices to show that the bread does not serve the porridge. These great people must eat the entire slice at the end, for no one else would eat it.

Gra (DH veha'Medakdekim): The first opinion holds that Maseches Sofrim argues with Shmuel.

Rema: They eat afterwards what remains from the bread.