Annual Transactions of the Nordic Rheology

Caseins were crosslinked by Trichoderma reesei tyrosinase in both raw and heat-treated skim milk. Tyrosinaseinduced crosslinking was beneficial in the preparation of acidified milk gels from raw milk, but it did not have an effect on acid gels made from heat-treated milk INTRODUCTION Enzymes that form covalent bonds between or within proteins are potential tools in improving the texture of dairy products such as yogurt. The beneficial effect of transglutaminase (EC 2.3.2.13) in acidified milk gels is well known. Transglutaminase (TG) catalyses the formation of covalent crosslinks between glutamine and lysine residues in protein molecules. Tyrosinase (EC 1.14.18.1) is an interesting alternative to TG, as its mode of action is very different from the action of TG. In the presence of oxygen tyrosinase catalyses the hydroxylation of a monophenol to an o-diphenol, which is further oxidised to the corresponding oquinone. Quinones are highly reactive compounds which can further react nonenzymatically with each other or with thiol and/or primary amino groups. In addition to its natural substrates tyrosine and dihydroxyphenylalanine (DOPA), tyrosinase is capable of oxidizing tyrosine residues (Fig. 1) in protein molecules, resulting in the formation of covalent tyrosine-tyrosine, tyrosine-cysteine or tyrosine-lysine crosslinks. The substrate specifity of tyrosinase is highly dependent on its origin. Tyrosinase extracted from mushroom (Agaricus bisporus) is known to crosslink whey proteins in model systems. The tyrosinase from the fungus Pycnoporus sanguineus has proven to be an efficient crosslinker of casein. In this work the application of a novel tyrosinase from the fungus Trichoderma reesei in acidified milk gels is reported. The action of T. reesei tyrosinase is compared with the action of the some commercially available crosslinking enzymes, i.e. mushroom tyrosinase and TG. MATERIALS AND METHODS Enzymes Two different tyrosinases were used in the work: 1) a commercial mushroom (Agaricus bisporus) tyrosinase (AgaTYR) (Fluka, Switzerland) and 2) a tyrosinase from the filamentous fungus Trichoderma reesei (TrTYR). Tyrosinase activity was assayed using 15 mM L-DOPA (Sigma, USA) as substrate at pH 7 and room temperature according to the method of Robb. Activa MP transglutaminase (TG) preparation was obtained from Ajinomoto Co., Inc. (Japan) and further purified at VTT. The effect of the crosslinking enzyme tyrosinase on gel formation and texture of acid-induced milk gels Dilek Ercili-Cura, Martina Lille, Johanna Buchert & Raija Lantto VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Finland