"to" in front of a base form of a verb is not considered a preposition. It's sometimes called an infinitival particle. So, of the two, only "in" is a preposition (in that sentence).
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Debpriya De1. This tonic is sweet to the taste.
2. This tonic is sweet in taste.
3. This tonic is sweet to taste.
Sentences 1. and 2. are similar in meaning and structure as "in" and "to" are prepositions in these sentences.
But I don't understand the structure of the third sentence in which "to" is an infinitive marker.
Does it have the same meaning a