MILK WITH ABNORMAL ACIDITY. VI. THE ROLE OF PHOSPHORUS CONTENT AND THE RENNET-COAGULATION PROPERTIES OF ITALIAN FRIESIAN HERD MILKS 1 P. Formaggioni 1 , M. Malacarne 2 , A. Summer 2 , E. Fossa 2 , P. Mariani 2 Introduction Titratable acidity plays a fundamental role in all phases of milk rennet-coagula- tion: reactivity between rennet and casein, aggregation rate of para-casein micelles and syneresis ability of the curd (1). It represents a very important parameter for the technical evaluation of the dairy-technological quality of milk. In the production of valuable cheeses (for example, the Parmigiano-Reggiano cheese), milk with abnor- mal acidity (especially hypoacid milk) is considered, more or less, unsuitable for cheesemaking, because of the remarkably negative consequences on the rheology of the acid-rennet curd and on the textural properties of the cheese paste (2). The pH, strictly correlated with the titratable acidity, markedly affects the rate of hydrolysis of the k-casein by the chymosin. There are many substances which concur in determining the natural acidity of milk. Casein and soluble phosphorus represent more than 4/5 of the titratable acidi- ty of the milk. In normal conditions, the contribution is almost of equal entity, while in abnormal milks, both hypoacid and hyperacid milk, such equilibrium is more or less profoundly altered because of one or both factors. One of the causes of variation in the acidity can be metabolic disorders (3, 4), which can play a very particular role, especially with reference to the acidimetric alterations arising from deficiencies and nutritional and alimentary imbalances (5, 6). The aim of this work was to study, at the level of herd milk samples, the eventu- al relationship between the milk hypoacidity and the contents of casein and phos- phorus, with reference also to the conditions of normal and medium-high acidity level, and evaluate their effects on milk rennet-coagulation. Materials and methods The study was carried out on 51 herd milk samples, equally distributed between low, medium and medium-high acidity, characterised by a normal somatic cell con- 261 1 This work was supported by the experimental programme of the Emilia-Romagna region, Centro Ricerche Produzioni Animali Spa, Corso Garibaldi 42, 42100 Reggio Emilia. 1 Scienze Zootecniche e Qualità delle Produzioni Animali; Dipartimento di Produzioni Animali, Biotecnologie Veterinarie, Qualità e Sicurezza degli Alimenti; Università degli Studi di Parma; Via del Taglio 8, 43100 Parma. 2 Centro Lattiero Caseario, via Torelli 17, 43100 Parma.