Experimental cookery

Some changes in the proteins during denaturation. All investi- gators agree that denaturation is brought about in two steps. The first step is a preliminary alteration of the protein or denaturation. The second is a physical change which leads to coagulation or aggregation. Clayton in discussing "Foods as Colloid Systems" reviews some of the theories of protein denaturation. ''Hydrolysis has been frequently reported as the cause of denaturation, but present views incline to the idea of some struc- tural rearrangement within the molecule. Thus, the refractive index in- creases during heat denaturation, whilst X-ray diffraction patterns lead to the view that coagulation is accompanied by the elimination of water between NHo and COOH groups. . . . Cubin holds that denaturation is the distortion or opening up of the protein unit, whilst flocculation is the HEAT COAGULATION 19 process following this and rendered possible by it. Interaction of NH2 and COOH groups situated on contiguous colloid units leads to aggregation and, hence, coagulation." No matter how denaturation is brought about, the denatured product has sulfur atoms, the combination of which differs from those in the native protein. Mirsky and Anson have shown that in native egg albumin no sulfhydryl (SH) and disulfide (S-S) groups are detectable by certain methods. But in completely coagulated protein the number of SH and S-S groups detectable is the same as in hydrolyzed protein. These workers have also shown that in partially coagulated protein when the soluble and insoluble fractions are separated the soluble portion contains no detectable SH or S-S groups, but the insoluble fraction has the number of reactive SH and S-S groups characteristic of the completely denatured protein. In the interfacial coagulation of a protein, i.e., when a film of insoluble protein forms at the surface of a protein solution, SH and S-S groups appear, the number being the same as that found in the hydrolyzed protein. Also when the proteins are denatured by ultra-violet light, by acids, or by other means the SH and S-S groups appear. From these results they con- clude that the formation of insoluble proteins and increase in detectable SH and S-S groups are closely linked phenomena; that denaturation is a definite chemical reaction; and that a given protein molecule is either completely native or completely denaturated. In a later paper Mirsky and Anson report that the number of detectable SH and S-S groups in different proteins varies with the pH and the tem- perature. To illustrate, native hemoglobin had no detectable SH groups at pH 6.8. But with increase of pH the SH groups become detectable in increasing numbers up to pH 9.6. But native egg albumin showed no detectable group at pH 6.8 or pH 9.6. However, denatured hemoglobin had detectable groups at pH 6.8 and still more at pH 9.6. They found that intact, unhydrolyzed proteins possess in addition to SH groups other reducing groups which can be oxidized by ferricyanide. The number and activity of these groups vary from protein to protein. They are probably contained in the tyrosine and tryptophane component of proteins. "It can now be seen that the activation of SH and S-S groups in protein denatura- tion is part of a more general process."