John observed that left over pineapple kept at 15 oC in the refrigerator tastes sweater than pineapple consumed just after being peeled (at room temperature). He thinks that this occurs because of an enzyme which acts at the lower temperature to hydrolyze the stored sucrose to glucose and sucrose

No, John is wrong.

Explanation:

Pineapples are one of the popular fruits among people and people eat pineapple when they are ripen. It tastes sweet and juicy. But unlike most of the other fruits, pineapple does not ripe once it is plucked. It tastes very sweet when it is properly ripen.

In the context, John kept the left over pineapple in the fridge at about 15 degree Celsius and he found that the pineapple tastes sweeter the, This is because the sugary juices from the parts that is ripen trickles and spreads to all the parts of the fruit. So it tastes sweeter. It is not because of any enzymes.

google my dude. if it doesn't show up ask me.

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.