Orlah 2:5

Dostai from Kfar Yisma was one of the students from Beis Shammai. He reported that Shammai himself ruled that even spices and leaven do not transmit ritual impurity in a volume smaller than an egg (counter to the opinion expressed by Beis Shammai in 2:4).

Orlah 2:6

The ruling about things that ferment, flavor and create meduma (a forbidden mixture) in 2:4 is stringent when it comes to a species of produce mixed with its own kind. It is both stringent and lenient when it comes to a species mixed with a different kind. For example, if wheat leaven fell into wheat dough and it is enough to leaven it, then it is prohibited whether it is outnumbered in a ratio of 1-in-100 or not. If it is not outnumbered in the ratio of 1-in-100, then it is prohibited whether it is enough to leaven the dough or not. (In other words, either not being outnumbered 1-in-100 or being enough to leaven the dough is sufficient to render the mixture prohibited. “Both stringent and lenient” is explained in the next mishna.)