Seperating challah from dough which is tameh

The gemara says in Pesachim (32a) that if you seperate terumah from chametz on Pesach then it does not become terumah;

...מפריש תרומת חמץ דברי הכל אינה קדושה

“Everyone agrees that if you separate terumah from chametz on Pesach it does not become terumah.”

The gemara in daf 33 explains that this is becuase the passuk says that you have to give terumah to a cohen for him to eat, here the cohen can only use the chametz for making a fire (that is, even according to the ma'an de'amar that chametz on Pesach is mutar behana'ah the maximum benefit the cohen could have is to use the chametz as fire-wood);

...מנא הני מילי? אמר רב נחמן בר יצחק אמר קרא (דברים יח, ד) תתן לו ולא לאורו

“How do we know this? Rav Nachman bar Yitzchak said, because the passuk says “תתן לו ולא לאורו” – “You should give to the cohen (to eat) - and not to his fire.”  

The asks from a beraisah that says that chametz can become terumah; the gemara explains -

"…התם היתה לו שעת הכושר הכא לא היתה לו שעת הכושר. ודלא היתה לו שעת הכושר היכי דמי? כגון דאחמיץ במחובר”

“Whether or not the grain that is seperated becomes terumah depends on when the grain became chametz;

• In a case where the terumah could have been taken before the grain became chametz (היתה לו שעת הכושר) then the terumah is terumah.
• In a case where the terumah could never have been seperated without being chametz (לא היתה לו שעת הכושר) then it does not become terumah at all.

How can you find a case where it could never have been seperated? In a case where the grain became chametz while it was still growing.”

If the grain became chametz before the being harvested then, because it was never possible to give the separated terumah to the cohen to eat it does not become terumah. If the grain was harvested and then became chametz then if terumah is seperated, it does become terumah.

The Shaagas Aryeh asks as follows;

The mishna in challah (2:3) discusses the halacha concerning taking challah from dough that is tameh.

מי שאינו יכול לעשות עסתו בטהרה, יעשנה קבין, ואל יעשנה בטמאה. ורבי עקיבא אומר, יעשנה בטמאה ואל יעשנה קבים…

“If someone is tameh and is unable to tovel in a mikvah before he makes his dough, he should split the dough into small amounts of one kav each (this is less than the shiur which is chayav in challah) and he should not make a large dough (so he will not have to destroy the challah which is tameh ). Rebbi Akiva says, he should rather make a large dough and take challah betumah …”

According to Rebbi Akiva it is preferable to make a large dough and take challah, according to the tanna kamma it is preferable to avoid having to take the challah and then burn it. According to everyone however, the dough that is seperated does become challah.  This mishna seems to contradict the gemara in Pesachim becuase dough only becomes chayav to have challah seperated from it once it is kneaded, and when this dough was kneaded it was already tameh (as the person who kneaded the dough was tameh).  If so, there was never a time when challah could have been taken and given  to the cohen to eat and according to the gemara in Pesachim it should not be possible in this case to seperate challah at all. Just as when grain became chametz when it is growing it is not possible to seperate terumah on Pesach becuase there was never a shaas ha'kosher (a time when the terumah could have been seperated and given to the cohen to eat), so too if dough became tameh before it was kneaded it should not be possible to take challah from it at all?

The Kehillos Yaakov (Zeraim 24) answers as follows;

The gemara in Pesachim does not mean that it has to be possible to take terumah after the obligation to take terumah has begun. Rather, even if the produce was not chametz before the pile of grain was smoothed down in the granary (miruach ha'kri) grain that was seperated would still become terumah. The reason that there is a problem in the gemara in Pesachim where the grain became chametz while it was still growing is becuase when a plant is still growing it is not considered to be food at all. If so, as soon as the grain became food (when it was harvested) it was chametz and that is why it never had a shaas ha'kosher (a time when terumah could theoretically have been seperated.)

However, in the case of challah, although the dough was tameh as soon as it became dough (when the obligation to take challah starts), it was still potentially possibly to make dough from the flour before it became tameh and that is enough of a connection to the halachah of challah to say that if challah was seperated from dough which is tameh it will still become challah.

The Steipler proves that plants which are still growing do not have a halacha of food at all from the gemara in Bava Basra (80a). The gemara discusses the halachos of honey which is still in the beehive;

...דבש בכוורת אינו לא אוכל ולא משקה ...רבא אמר רבי אליעזר היא דתנן 'כוורת דבורים ר"א אומר הרי היא כקרקע וכותבין עליה פרוזבול ואינה מקבלת טומאה במקומה והרודה ממנה בשבת חייב חטאת'

“The beraisoh says that honey in a beehive cannot become tameh because it is not considered to be either a food or a drink. Rava explained, this beraisoh goes according to Rebbi Eliezer as we have learnt, ‘A bee hive; Rebbi Eliezer says - it is like ground (so is acquired in the same way as land), you can write a pruzbul on your debts if you own a beehive, it is not mekabel tumah and if someone takes honey from it on Shabbos he is chayav a chatas.’”

The gemara profves from the fact that the hive is considered like the ground that the honey in the hive is not considered to be food at all. Thus you see from the gemara that if something is connected to the ground then it is neither a food or a drink. Therefore we can explain that the problem with grain that became chametz while it was still growing is that  when it became food (when reaped) terumah taken could not be eaten by the cohen. However regarding challah that is taken from a dough that was tameh as soon as it was kneaded, becuase it was possible previously to knead the dough be'taharah, the challah that is seperated does become challah.