Is it permissible to dispose of unpalatable or spoiled food by using it as animal feed, and is this considered extravagance and a failure to preserve blessings?

As long as you are keen on managing affairs and what happens contrary to that is unintentional, there is no sin upon you; for Allah Almighty says: "And there is no blame upon you for that in which you have erred." It is natural for a beginner to make mistakes in preparing food and its quantity, and that will disappear with time and experience. Giving surplus food to those who can benefit from it with the intention of charity is a good deed for which you will be rewarded.