Before I go on to talk about the question, I just need to chat very briefly about “Oxidation of Tea“. We all know that the cut side of an apple is left alone, it will turn brown. This is the result of oxidation.
When tea leaves are plucked and rolled prior to the application of heat (known as the process of fixation), the enzymes (such as Polyphenols Oxidase and Peroxidase) in the leaves accelerate the speed in which the polyphenols molecules react with oxygen in the air thus breaking it down and creating theaflavins (yellowish component of tea) which contributes to the brisk and bright taste of teas.
Now through the application of heat, the oxidation process is controlled carefully and expertly such that one could create the unoxidised green, the very slightly oxidised white, the slightly oxidised yellow, the semi-oxidised blue/oolong teas, the fully-oxidised black or the post-oxidised red teas. At high heat, these enzymes (essentially made of proteins) would be killed or stunned so that they are unable to continue to hasten the oxidation of polyphenols in tea leaves.
When these enzymes are allowed to do its work, the theaflavins then continue to react with polyphenols to form thearubins (the reddish component of tea) thus turning the tea leaf from green to copper in colour and setting the strength of tea. These thearubins then react with the sugars and amino acids in the leaf to form highly polymerized substances which give rise to the distinctive flavour, colour and aroma profiles of the respective teas.
Now back to the question: Why is it that green tea continues to oxidise even after all these enzymes are killed or stunned during the fixation process?
Actually the process of oxidation happens all around (and inside) us at a molecular level. Iron can react with oxygen to form rust. Oxidation of fats and oils leads to rancidity and, in fruits and vegetables, it can result in the formation of compounds which discolour the fruit.
So even after killing the oxidation causing enzymes in tea leaves through the application of heat, the tea leaves would still continue to oxidise if it is allowed to be in contact with oxygen. The oxidation process is, however, a lot slower than when the enzymes in the tea leaves are still alive. This is why one should be careful in storing unoxidised or lightly oxidised teas in air-tight, light-proof containers (and preferably in a fridge) to stop or halt further oxidation of tea. (See Caring for your teas.)