The citric acid in lemon juice and other citrus juices will denature proteins in meat/fish which is why it is often used as a tenderizer; however it won't significantly change its nutritional efficacy.
Once you ingest proteins from any source the much stronger hydrochloric acid and pepsin in your stomach will denature the proteins into amino acids for use in your body, citric acid just gives this process a head start.
Jacqueline Sikoski, MS,RD,LN
Manager, Nutrition Services & Diabetes Center
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