This is said by Squealer, the full passage is:

"Comrades'" he cried. "You do not imagine, I hope, that we pigs are doing this in a spirit of selfishness and privilege. Many of us actually dislike milk and apples. Our sole object in taking these things is to preserve our health. Milk and apples (this has been proven by Science, comrades) contain substances absolutely necessary to the well-being of a pig ... We pigs are brainworkers ... Day and night we are watching over your welfare. It is for your sake that we drink that milk and eat those apples. Do you know what would happen if we pigs failed in our duty? Jones would come back."

In Chapter 3 Squealer is sent to calm the upset animals of the farm down because the animals finally realized that the milk had not gone missing, but it was being mixed into the pigs' mash every day. They were also upset to learn that while they all loved apples, only the pigs would be getting the apples from now on. Squealer, a very convincing spokespig, told the animals that the pigs didn't even like milk and apples, but in order to continue looking out for the animals welfare they had to eat them. He makes it sounds as if the pigs are doing something they hate in order to take care of all the animals on the farm. The animals don't argue because they do not want Mr. Jones and his men to come back and so the pigs continue to get all the milk and apples their bellies can hold.