The in vitro digestion of lysozyme and casein roasted at 180-250°C was investigated by use of pancreatin, trypsin, papain and pronase. Lysozyme roasted at 180°C for 20 min was digested completely by proteases within 24 hr. On roasting lysozyme over 210°C for 20 min, however, the residue became water-insoluble in a ratio over 70% and the digestibility extremely decreased. Similarly, on roasting casein over 210°C its digestibility decreased to almost the same degree as in the case of lysozyme. It is considered that such decreases in the digestibility are partially based on the decomposition and racemization of amino acid residues on lysozyme and casein.
It has so far been believed that firming (the process by which the tissues of vegetables become to be unable to be macerated by heating) is promoted by calcium and other metal ions. In the present study, however, the firming of Japanese radishes and beets was found to be retarded by calcium ion. It is considered that firming prevents the excessive disintegration, together with the maceration, of vegetables.