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How does 1. cornmeal, 2. but not salt, help whiten clam meat?

Aliza Green's _The Fishmonger's Apprentice : The Expert's Guide to Selecting, Preparing, and Cooking a World of Seafood, Taught by the Masters_. [p 142](https://books.google.com/books?id=hDe3j9hIgw0C&lpg=PA142&ots=9JPvXZJoVR&dq=%22scoop%20the%20clams%20from%20the%20water%2C%20leaving%20the%20sand%22&pg=PA142#v=onepage&q=%22scoop%20the%20clams%20from%20the%20water,%20leaving%20the%20sand%22&f=true).

>To begin, place the clams in a sink or large 
pot of cold saltwater (use 1/2 cup [150 g] 
salt to 1 gallon [3.8 liters] of water). A**dd 
1/2 cup (70 g) cornmeal to help whiten the 
meat if desired.** Leave the clams to purge 
for 30 minutes to 1 hour. Scoop the clams 
from the water, leaving the sand and grit 
behind. Have ready a clam opening knife 
(see "A Few Good Tools"). It is highly rec- 
ommended that you wear a protective glove 
on your nonwriting hand. 

Because you are instructed to purge the clams in salt first, this book implies that salt can't help whiten the clam meat. Why can corn meal, but not salt?