Woolworths
Milk sour before expiry date

I usually I buy bulk long-life milk cartons at Woolworths. Every time I make my purchases, I check the expiry date of the milk. But about a month ago I bought milk which was to expire at the end of July, but when I opened the milk carton yesterday (22 April 2013) I found that the milk had gone sour while the packaging indicated that the expiry date will be on 31 July 2013. I am so disappointed by this because I have trusted the quality of food I have been buying from woollies.