Answer: Elmer's Lim
There is just a little bit unpleasant about the milk in cereal products. It is so brilliantly white, opaque, and the grain is so accurate in it. You might be tempted to think that it's a bit of Photoshop wizardry when it's actually a trick that is far older than Photoshop.
It's glue. For years, food stylists have used Elmer glue, the same milky white things that schoolchildren like to mess with, instead of milk. It's richer, it's thicker, and the moments get better, and best of all, with its thickness, you can place elements (like individual pieces of grain and fruit refills) with the kind of precision a successful photography requires.
But in a strange way, it's kind of full circle. If you remember Elmer's glue from your childhood, you can remember that the company logo is a bull. The logo is a bull (which seems odd when you think about it) because the original formulation of Elmer's glue used casein, a by-product of milk. Today, the glue is completely synthetic without any milk by-product (or any other animal product in it), but for a short period in the advertising history cereals were photographed in milk-based glue.