…I did not consume it in a state of uncleanliness… (Deuteronomy 26:14)
One may also not eat his second tithe in a state of ritual impurity. This is equally true whether the food is clean and the person unclean, or the person is clean and the food unclean. (If food of the second tithe becomes ritually contaminated, it must be redeemed.)
As with the previous mitzvah, the Torah does not come right out and say “thou shalt not….” Nevertheless, the fact that the Torah requires a person to affirm that he did not do so is tantamount to such an injunction. (The second half of the verse, saying that we have obeyed God’s word in this matter, further underscores that He has, in fact, commanded us to this effect.)
The reason for this is what we have discussed previously regarding sacrificial matters and ritual impurity. Allowing holy things to mix with such uncleanliness is unseemly and shows a lack of proper regard for the Temple service, not to mention the One Whom it serves. Therefore, matters of holiness are to be kept free of the taint of ritual uncleanliness.
This mitzvah applies in Temple times. It is discussed in the Talmud in tractate Yevamos (72b) and in Makkos (19b). It is codified in the Mishneh Torah in the third chapter of Hilchos Maaser Sheini. This mitzvah is #150 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos.