Most grains/extract typically contain around 10-12% protein, while flaked wheat and barley have around 14-16%. Maize (corn) has a lower protein content, less than 5%.
Even though there are different amounts of protein in different grists, there's nearly always enough protein in the wort to make the boiled wort look like egg drop soup if you can get the protein to precipitate during boil. Here are some factors that help get the protein to precipitate out of the wort:
- hops: polyphenols from the hops attract the proteins to form a complex with later flocculates out.
- a vigorous rolling boil. This is necessary to move about the polyphenols and the protein about vigorously so there is more chance of contact. This will produce a good hot break - more coagulated proteins. These proteins that coagulate in the hot break would cause off flavours and instability in the beer if left in there.
- calcium - at least 50ppm of calcium in the wort is recommended to help proteins precipitate out.
- pH - a pH of 5.0-5.5 creates an efficient hot break.
- kettle finings (irish moss/whirfloc etc..) these comprise large particles that attract proteins (and beta glucans), causing them to fall out of solution, clearing the wort further.
With all these factors working to precipitate out the protein, much of the unwanted protein is removed from the beer. Additionally, a fast chill after the boil will form a cold break, comprising yet more proteins, which reduces chill haze.
As to why you got more protein this time round - it could be any of the factors above. It's probably not the flaked corn that's giving you more protein this batch. Rather, your new mill may have created a finer crush with more flour, bringing more protein into the wort, so there was more protein to precipitate.