Without digging through my sources, the short answer would be that it's most likely the Austens ate "white" bread.

On of the first marks of a person's advancement up the social ladder would be the ability to say he or she had lost their "rye teeth". That is, they only ate white bread.

Interestingly, white bread was considered healthier. It was also thought to be less suspect to tampering. There were concerns the darker colour could hide all kinds of things (spoiled wheat, sawdust, etc.) OTOH, unscrupulous bakers might improve the look of there white bread by the use of chalk, alum, and other whiteners.

I remain,
Jason E.