Heat-induced association of /Mactoglobulin (/?-lg) and casein micelles, mainly at 85-90 °C, was studied by means of preparative ultracentrifugation, using [ 3 H]-labelled /?-lg. Qualitative aspects were studied by gel electrophoresis and electron microscopy. In this association, the formation of intermolecular S-S bonds between /?-lg and /c-casein plays a role, but hydrophobic bonds are also involved.After heating mixtures containing 25 g/kg casein and 4 g/kg /?-lg at 90 °C for 20 min, the amount of /?-lg sedimented with the casein micelles by ultracentrifugation decreased by approximately 30 % when the milk salt buffer system was reduced to 0-25 of its normal concentration; it decreased by about 20% when the pH was increased from 6 -8 to 7 -3 and increased by 15% when the pH was reduced from 6 -8 to 5-8. When the /?-lg concentration was decreased from 4 to 2 g/kg, the amount of sedimented /?-lg decreased by 25 % after heating at 90 °C for 6 min. After heating in milk salt buffer, the amount of sedimented /?-lg was 15% higher than after heating in a salt solution which contained neither >Ca nor citrate ions.Although a-lactalbumin (a-la) is involved in the heat-association of /?-lg and casein micelles, no influence of a-la on the amount of associated /?-lg was found. In addition to the heat-association product of /?-lg and whole casein micelles, an association product consisting mainly of /?-lg and K-casein was also formed. This product was observed by electron microscopy as noodle-like particles. The size of these particles depended on the /?-lg//e-casein ratio.

The primary structure of the sweet-tasting protein thaumatin has been elucidated. The protein consists of a single polypeptide chain of 207 residues. The sequence of the N-terminal part of the chain was determined by sequenator analysis. As the protein contains only one methionine residue, it was possible to deduce the N-terminal sequence of the C-terminal cyanogen bromide fragment by automatic sequencing of the cyanogen-bromide-cleaved, succinylated protein. To arrive at the sequence of the whole protein tryptic and Staphylococcus protease peptides, together with chymotryptic peptides and a 2-(2-nitrophenylsulfenyl)-3-methyl-3'-bromoindolenine (BNPS-skatole) fragment were also sequenced.Comparing the amino acid sequence of thaumatin with that of the other sweet-tasting protein, monellin, we have located five sets of identical tripeptides. Since immunological cross-reactivity of thaumatin antibodies with monellin has recently been described, one or more of these tripeptides might be part of a common antibody recombination site and possibly be involved in the interaction with the sweet-taste receptor.

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