All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
It is disliked to blow on food or smell it because it may become polluted with spray from the mouth or nose. Ibn Hajar wrote:
"There are a number of ahaadeeth underlining that it is forbidden to blow into the vessel; it is also forbidden to breathe into the vessel because that may result in some change from the breath, either because the person’s mouth has a bad smell because of eating a certain food, for example, or because he has not used the Siwaak (tooth stick) or rinsed his mouth for a long time, or because vapors from the stomach rise with the breath. In all these cases, blowing is worse than merely breathing into the vessel."
A hadeeth was narrated regarding the forbiddance to smell food like the animals do, and it was cited by Al-Albaani in his compilation of weak ahaadeeth (As-Silsilah Adh-Dha‘eefah). Ibn ‘Aabideen wrote, "Food should not be eaten when it is hot, and it should not be smelled."
That said, you should know that a fundamental principle in Fiqh reads, "Dislike is removed by need." So if the person needs to smell the food for a need such as fearing that it might be rotten or the like, then there is no harm in that, especially if the previously mentioned reason for the dislike does not exist. It should be noted that breathing or blowing into the vessel while drinking is worse than smelling the food, however, scholars held that it is permissible in the absence of the mentioned reason. Ibn Al-Mulaqqin said, "This applies to the case when the person is sharing the food or drink with others. If he is eating or drinking alone, or with his family, or with someone whom he believes would not be disgusted by seeing him blowing into the food or drink as such, then there is no harm in him breathing into the vessel."
Allah knows best.