Re: How is Vitamin C formed from unripe to ripe citrus fruits?

> If a ripe orange is left for many days from the day it was plucked, 
> the Vitamin C declines. Why? 

As you know, Vitamin C can react with oxygen to oxidize.  As that 
oxidation takes place, the Vitamin C gets used up.  Vitamin C is also 
degraded by heat, light and other stresses.  It's one of the most unstable 
vitamins.

> Also when Vitamin C oxidises, what does it form? 

It's breakdown reaction products are very complex (see below).

> In citrus fruits, does Vitamin C turn to glucose? 

No.  Glucose is a sugar, and it's structure is not related to that of 
Vitamin C or ascorbic acid.  The breakdown reactions for Vitamin C are 
very complex and in many cases lead to the formation of complex brown 
pigments.

A detailed diagram can be found in the book "Principles of Food Science - 
Part 1 - Food Chemistry" by Owen Fennema.  The diagram is found on pg 362 
of the 1976 edition.