पक्षिजग्धं गवा घ्रातमवधूतमवक्षुतम् ।
दूषितं केशकीटैश्च मृत्प्रक्षेपेण शुध्यति ॥ १२३ ॥
pakṣijagdhaṃ gavā ghrātamavadhūtamavakṣutam |
dūṣitaṃ keśakīṭaiśca mṛtprakṣepeṇa śudhyati || 123 ||
What has been eaten by a bird, what has been smelt by a cow, blown upon, or sneezed at, or defiled by hair and insects, becomes pure by scattering earth.—(123).
The same is repeated in other smṛtis:
Viṣṇu (23.38) — ‘Food nibbled at by a bird, smelt at by a cow, sneezed on, or defiled by hair, or by insects or worms, is purified by earth scattered over it.’
Yājñavalkya (1.189) — ‘Food smelt by the cow, or defiled by hair, flies or insects, should have water and ashes or earth sprinkled over it, for purifying it.’
So, why is food smelt by a cow considered impure? Is there a logical explanation for this because it is rather counterintuitive given that cow dung and cow urine is prescribed as a purification method for other minor offenses. E.g., Manu 11.212 says:
gomūtraṃ gomayaṃ kṣīraṃ dadhi sarpiḥ kuśodakam |
ekarātropavāsaśca kṛcchraṃ sāntapanaṃ smṛtam || 212 ||
Taking cow’s urine, cow-dung, milk, curds, clarified butter, kuśa-water, and fasting for one day,—has been declared to be ‘Sāntapana Kṛcchra’— (212)
Also, Yājñavalkya Smṛti 1.194 says:
CXCIV. — The mouth of a goat and of a horse is pure but not of a cow nor impure excretions of man. The roads are purified by the rays of the moon and sun and by the wind. — 194
Why is horse's and goat's mouth pure but not the cow's?
P.S. I'm aware of a Puranic legend that Surabhi, the mother of all cows was once cursed (for uttering a lie) that whatever it touches with its mouth becomes impure. But I'm not interested in that sort of explanation.