"right kind of powder" is quite hard to get if the murder is not preplanned as in the case.besides he should be quite nervous to do it right in front of a police officer and a noisy kid at such a short notice.
lets be more clear, there can be a variety of other possible alibis ,yet he uses froth which he got unintentionally while pouring his beer before message arrived.
I) either he is unimaginative culprit who could not think a more subtle alibi ,
II) religious culprit who think it was act of god so he must use that for alibi
III) when he entered room with takagi and conan ,he realized that froth could be used as alibi(guilty or innocence does not matter)
IV) subaru is wrong and he is culprit and paranoid enough to have a little bit of "right kind of powder" with him
for such instances
V) he has tablets on him because of medical condition
VI) he has tablets in bag from funeral
case VI in the bag
it is foreign to me as is japan and japanese .
so whether bags from funerals ,in japan, can contain effervescent tablets ? i got no idea but i think it is impossible
Case V medical condition
well it is definitely possible that he took it out when he stumbled across the bag and put a little powder when he bent over. Hats off to chekhov for digging out such possibilities
but it involves coincidence and can be caught by chemical tests or by taste
case IV blasphemy
no subaru can't be wrong
Case III done by god(gosho) / truth
well first guy did put the beer in glass and froth did came out ,so even if he is culprit ,it could be possible by act of god(gosho aoyama) that some froth remained.
if he is innocent ,then no work needded.
case I and II: no discussion
ps : may this end in okiya impressing sato with his skills and body