The book, "Nature's Garden" by Samuel Thayer, has an excellent, extremely thorough explanation of how to find, select, harvest and prepare acorns.
He lists 9 different types of defective acorns to avoid:
- Attached Cup: Ripe acorns loosen from their cups unless they are infected. An acorn with a firmly attached cup indicates that the tree noticed it was infected and prematurely dropped it. Note that a loosely attached cup is fine, as long as it can be easily separated from the nut without using significant force.
- Exit hole: an exit hole about the side of a pencil tip indicates that the acorn had a grub in it.
- Old acorn: Acorns from the previous autumn are dull brown or gray and have lost any attractive color they may have had.
- Dark zone: If part of the shell has a dark area, it usually corresponds with an area of spoiled nut meat inside.
- Shell or disk separation: The disk is the round, pale, nearly-flat area underneath the cap. If this area is partially separated from the shell, it indicates the nutmeat is bad. (Think of it like a popped lid on a jar of home-canned produce.)
- Rippled bottom: Healthy acorns often have subtle ripples in the shell where it attaches to the disk, but extreme or exaggerated ripples indicate a bad nut.
- Dark spot: Tiny dark spots anywhere on the shell but especially on the disk, indicate weevil holes.
- Bulging or sunken disk: Healthy acorn disks can be flat, slightly concave or slightly convex. Severely sunken or bulging disks indicate a bad acorn.
- Dying sprout: Primarily seen on white oaks. A healthy sprout indicates the acorn is fine, but a dying sprout indicates the acorn is bad.
He also notes that while the float test is useful for finding very bad acorns, it will not find acorns that have been recently infested with grubs, because the grub will not have eaten enough of the acorn to make it float. You can freeze or roast your acorns after the float test. This will kill any grubs, but it will also kill the acorns. Dead acorns go bad quickly, so you must immediately shell and dry acorns after freezing or roasting.
Another tip Thayer provides is oaks will drop their nuts in two batches. The first batch is often made up of unhealthy or infected nuts that the tree wants to get rid of. Collect from the second batch, because it will have fewer bad nuts. In red oaks, the two batches will be a month or two apart. In white oaks, the two batches blend together without an obvious gap, but the odds of finding good nuts are better in the latter half of the season. Of course, if you wait too long you might miss the opportunity to harvest at all, because critters will get all the nuts. If you don't know exactly when to expect the start and end of the acorn season, it's probably better to err on the side of too early.
I strongly recommend this book for anyone who wants to harvest acorns. The acorn section is 30 pages long and full of useful tips that will save you a lot of wasted effort.