Pe'ah, Chapter 5, Mishnah 4. This mishnah too deals with the question of which kinds of items we may believe a poor person about when he claims that what he is holding came to him from the gifts to the poor.
"Ne'emanim al hayarak chai":
A poor person is believed when he says about raw vegetables in his possession that they came to him from the gifts to the poor. It is worth noting that the law of pe'ah does not apply to vegetables, as was mentioned at the beginning of the tractate, that leafy vegetables are not subject to the laws of pe'ah; the tithe for the poor, however, does apply to them, and therefore the poor person is believed when he says that these raw vegetables came to him as ma'aser ani, the tithe for the poor.
"Ve'ein ne'emanim al hamevushal":
About cooked vegetables the poor person is not believed to say that they came to him from the tithe for the poor, "ela im ken hayah lo davar mu'at" - unless it is only a small amount. And the reason is: "shekein derech ba'al habayis lihyos motzi me'ilpaso" - this is the ordinary practice of the householder, the farmer, to take out the tithe for the poor from the cooked vegetables in his pot.
Here is the explanation: sometimes the householder forgets to separate the tithe from the vegetables while they are still raw, and remembers only after they have been cooked. At that point he separates whatever amount he has on hand, and that is a small amount. Therefore, if the poor person claims that the householder gave him cooked vegetables, he is believed only when a small quantity is involved.
But a large quantity of cooked vegetables is not plausible, since it is not the way of a farmer to take his tithe for the poor, cook it, and only afterward give it out as gifts to the poor. With such a quantity the poor person is believed only when the vegetables are raw and not cooked.
In summary: In this mishnah we learned that a poor person is believed about raw vegetables that he received them as the tithe for the poor, whereas about cooked vegetables he is believed only when the quantity is small - because the householder's way is to separate the tithe for the poor from the pot only when he remembers after the cooking, and such a separation is by its nature a small one.