Transubstantiation and John 6:27 Do not labor for the food which perishes, but for the food which endures to everlasting life, which the Son of Man will give you, because God the Father has set His seal on Him."
  NKJV John 6:27


I am an Evangelical but have been worshiping in a high church for many years mainly because my friends worship there too. I know the doctrine of transubstantiation which teaches that the bread and wine literally change into Jesus' flesh and blood in the Eucharist. Most defenders of this teaching use John 6 as the main evidence. But in v27 Jesus not to work for FOOD WHICH PERISHES: i.e food that will rot and mould, but work for the food that will endure to eternal life. This is the "food" which Jesus says he'll give.

My question is: how does one who believes in transubstantiation go about understanding v27, considering that the consecrated bread in the Eucharist WILL eventually "perish"/mould/rot over time if left unconsumed. How can Jesus be referring to the Eucharist as his "food", when he says that HIS food will NOT perish (unlike the consecrated bread)?

I am not looking for an argument, I am just interested in how Catholics interpret this verse.
St. Thomas Aquinas explains that verse in his commentary on St. John's Gospel ch. 6 (my emphasis):


  
  The power of this food is seen in the fact that it does not perish. In this respect we should point out that material things are likenesses of spiritual things, since they are caused and produced by them; and consequently they resemble spiritual things in some way. Now just as the body is sustained by food, so that which sustains the spirit is called its food, whatever it might be. The food that sustains the body is perishable, since it is converted into the nature of the body; but the food that sustains the spirit is not perishable, because it is not converted into the spirit; rather, the spirit is converted into its food. Hence Augustine says in his Confessions: “I am the food of the great; grow and you will eat me. But you will not change me into yourself, as you do bodily food, but you will be changed into me.” 
  


Related:


Summa Theologica III q. 77 a. 4: "Whether the sacramental species can be corrupted [i.e., decompose]?"


cf. this answer to this question regarding the nutritional aspect of Holy Eucharist