You should be able to find alternative methods in this paper:

Rapid identification of gelatin and casein hydrolysis using TCA

https://pubmed.ncbi.nlm.nih.gov/17289191/

A commonly used method for the hydrolysis of proteins prior to amino acid analysis is to treat the protein with 6M hydrochloric, under vacuum, at 110oC

The protein hydrolysis such as casein can be by acid hydrolysis is carried out above 100°C at pH 1–2 using inorganic (hydrochloric, sulfuric, orthophosphoric) acids. The rate of release and destruction of individual amino acids depends mainly on the nature of the protein.

• REf: Svetlana-Maslennikova , Svetlana-Maslennikova, Acid Hydrolysis of Casein, May 2014, Foods and Raw Materials 2(1):27-30, DOI:10.12737/412