Orwell once observed that he thought the disappearance of the milk was the turning-point of the novel in Animal Farm. In what sense is this true?

The pigs taking the milk and apples is the turning point of the novel because it demonstrates that the pigs were taking charge and reserving luxuries for themselves.

After the animals revolt and the humans are forced off the farm, the cows need to be milked.  They are very uncomfortable it they do not get milked.  The pigs are able to milk them successfully, and the other animals begin eying the milk.  Milk is a...

The pigs taking the milk and apples is the turning point of the novel because it demonstrates that the pigs were taking charge and reserving luxuries for themselves.

After the animals revolt and the humans are forced off the farm, the cows need to be milked.  They are very uncomfortable it they do not get milked.  The pigs are able to milk them successfully, and the other animals begin eying the milk.  Milk is a hot commodity on the farm.  All of the animals like it.

"What is going to happen to all that milk?" said someone.

"Jones used sometimes to mix some of it in our mash," said one of the hens.

"Never mind the milk, comrades!" cried Napoleon, placing himself in front of the buckets. "That will be attended to. The harvest is more important. …” (Ch. 2)

What does happen to the milk, and the apples, is actually very significant.  These two luxuries seem to disappear and the animals do not know where they are going.  Soon they find out what happened.

It was mixed every day into the pigs' mash. The early apples were now ripening ... The animals had assumed as a matter of course that these would be shared out equally; one day, however, the order went forth that all the windfalls were to be collected and brought to the harness−room for the use of the pigs. (Ch. 3)

Squealer explains to the animals that the milk has to go to the pigs because they are the farm’s brain trust.  Since they work harder than everyone, and they are in charge, they deserve the milk and apples.

This is a turning point for the animals and for the reader. It is the first time we realize that Animal Farm is not going to be the egalitarian paradise that Old Major foresaw.  Corruption is already setting in.  Taking the milk and apples for themselves is only the first abuse the pigs engage in.  Soon they are worse masters than the people they replaced.