Gemara

(Rav Yitzchak bar Chananya): One may delay letting a waiter taste any food except for meat and wine.

(Rav Chisda): This applies only to fatty meat and old wine.

(Rava): It applies to fatty meat the entire year, and to old wine only in summer.

Rav Anan bar Tachlifa: I was standing in front of Shmuel, and they brought him a mushroom dish. Had I not been given to taste, I would have been in danger.

Rav Ashi: I was standing in front of Rav Kahana, and they brought him sliced turnips in vinegar. Had I not been given to taste, I would have been in danger.

(Rav Papa): Even fat dates can pose a danger.

The rule is, anything with smell or pungency must be tasted.

Avuha bar Ihi or Minyamin bar Ihi allowed his attendant to taste all the cooked foods right away. The other let him eat one now, and taste the rest after the meal. Eliyahu would speak only with the former.

There were two Chasidim (some say that they were Rav Mari and Rav Pinchas, the sons of Rav Chisda). One allowed his attendant to taste before himself, and the other allowed him to taste after serving his master and the guests. Eliyahu would speak only with the former.

Ameimar, Mar Zutra and Rav Ashi were sitting at the doorway of King Azgor. The king's waiters passed, carrying his meal. Rav Ashi saw that Mar Zutra turned pale. Rav Ashi stuck his finger in the food and put it in Mar Zutra's mouth.

The king's officers charged Rav Ashi for ruining the king's meal. He explained that it was not worthy to be served to the king, for it contains leprous pork. This was found to be true. Rav Ashi told Rabanan that he relied on a miracle because he saw a spirit of Tzara'as above Mar Zutra.

A man split open a pomegranate and was eating in front of a woman. She salivated, and swallowed her saliva. He did not give to her. She became very sick and inflated. To cure her, he brought a pomegranate, split it open and ate it in front of her. She salivated. He told her to spit out the saliva, until green came out of her. This cured her.

Rishonim

Rif and Rosh (5:25): One may delay letting a servant taste any food except for meat and wine. This refers to fatty meat the entire year, and old wine in summer. A mushroom dish was brought in front of Rav Anan. Had he not been given to taste, he would have been in danger. Even fat dates can pose a danger. The rule is, anything with smell or pungency must be tasted. Avuha bar Ihi or Minyamin bar Ihi allowed his attendant to taste all the cooked foods right away. The other let him eat one now, and taste the rest after the meal. Eliyahu would speak only with the former.

Ran (DH ha'Kol): We discuss a waiter who does not eat until the diners leave. A person craves fatty meat and old wine. Delaying to taste them causes pain. In summer, the smell of old wine incites a person like poison. Rav Anan was in danger of being seized by Bulmus (a mortally dangerous sickness caused by awesome hunger).

Korban Nesan'el (8): The Rif and Rosh bring the episode with Rav Anan to teach that it is dangerous. The laws of a pregnant woman who smelled food apply. The same applies to a pregnant woman, a waiter or any person.

Chasam Sofer (61b DH Mai): The spirit of Tzara'as showed that Mar Zutra was in great danger. The Ramban says that turning pale showed danger. Perhaps he explains like Mishneh Halachos (14:271), that the Tzara'as came after Mar Zutra ate, showing Rav Ashi that there is Tzara'as in the food.

Rambam (Hilchos Berachos 7:7): The waiter who stands in front of those seated to eat does not eat with them. It is merciful to put into his mouth from every Tavshil (cooked food), to calm his mind.

Rambam (Hilchos Avadim 9:8): One is permitted to make one's Eved Kena'ani work rigorously. It is Midas Chasidus and wise to be merciful on him, and to let him eat from every food and drink. Previous Chachamim used to give to the slave from every Tavshil. They would put food of the animals and slaves before their own food.

Lechem Mishneh: The Kesef Mishneh says that the Rambam learns from Kesuvos 61. There is no proof from there, for that discusses a Yisrael waiter.

Poskim

Shulchan Aruch (OC 169:1): Any food brought in front of a person that has a smell, and people desire it, one must give from it to a waiter immediately. It is Midas Chasidus to give to him immediately from every kind (of food).

Mishnah Berurah (1): The same applies to anything sour.

Magen Avraham (1): One must give to him a small amount. If one stipulated at the time he hired him that he will feed him like other members of the household, he need not give to him from every food, only from those with smell. This is why we do not give to our servants from every food, for it is as if we stipulated. It seems to me that a stipulation does not help, for the law is due to pain. It seems that the law applies only to a waiter who serves, but not to one who sits at the table.

Kaf ha'Chayim (3,4): The SHLaH says that a stipulation exempts from Midas Chasidus for foods without smell. It does not help for foods with smell. The Magen Avraham says that it does not help at all. The SHLaH and Magen Avraham argue like the Rambam and Rashi. We need not give immediately to one who sits at the table, but it is improper not to give to him at all.

Mishnah Berurah (2): We learn from the Gemara that if one smells a food and desires it and cannot obtain it, he should spit out his saliva, for swallowing it can be dangerous.

Mishnah Berurah (4): It is Midas Chasidus to give from each food. Even if it has no smell and there is no danger, it pains him to see others eat and he cannot.

Shulchan Aruch (3): If there is an important person at the meal who knows that one must give to the waiter as much bread as he needs, the waiter need not bless on every piece.

Taz (4): The Tur omits the word 'bread'. The Shulchan Aruch was correct to insert it. The same applies to everything that one is obligated to give (e.g. what has a smell and people desire it). The important person will command that they give to the waiter as much as he needs. The waiter blesses each time he receives other foods, which it is only Midas Chasidus to give, for the important person will not command them about this.

Levushei Serad (3): The Taz connotes that one gives as much as the waiter needs. We learn differently from the Shulchan Aruch, which requires a new Berachah for each food he receives except for bread. The Magen Avraham (1) connotes that a small amount suffices, even for foods that one is obligated to give.

Kaf ha'Chayim (2): It sufficed for Rav Ashi to put his finger in the food and put it to Mar Zutra's mouth (a small amount).

Shulchan Aruch (YD 267:17): It is Midas Chasidus and wise to be merciful on one's Eved Kena'ani, and to let him eat from every food and drink. Previous Chachamim used to give to the slave from every Tavshil. They would put food of the animals and slaves before their own food.