A mother should not compete with her daughter

 

by putting her down every chance she gets.  It's very unhealthy.  Make sure you are not doing that.

You may have disrespected each other long before this bread-vs-bun argument.

She may just disagree to provoke a response from you in order to feel her new-found power.  Don't take the bait.

You may say: "I see you have contempt for people," with an even tone, "contempt on the surface may be a compensation for your insecurity underneath," with a smile, "a teenager is allowd to feel insecure, but it's better for you to look inside yourself, rather than distract yourself by focusing on others."

Respect her as an individual.

It's OK to have feelings.  But dwell on feelings does not solve problems, only makes them worse.  Be a model of rational thinking to your daughter.  She needs you now more than ever.