ORIGINAL ARTICLE Influence of race and crossbreeding on casein micelles size Denise R. FREITAS, 1 Leorges M. FONSECA, 2 Fernando N. SOUZA, 1 Cristiane V. G. LADEIRA, 1 Soraia A. DINIZ, 1 João Paulo A. HADDAD, 1 Diêgo S. FERREIRA, 3 Marcelo M. SANTORO 4 * and Mônica M. O. P. CERQUEIRA 2 1 Department of Veterinary Preventive Medicine, 2 Technology and Inspection of Animal Products, 3 Pharmaceutical Products and 4 Biochemistry and Immunology, Federal University of Minas Gerais, Brazil, Belo Horizonte, Brazil ABSTRACT Casein (CN) micelles are colloidal aggregates of protein dispersed in milk, the importance of which in the dairy industry is related to functionality and yield in dairy products. The objective of this work was to investigate the correlation of milk CN micelles diameter from Holstein and Zebu crossbreds with milk composition (protein, fat, lactose, total and nonfat solids and milk urea nitrogen), somatic cell count (SCC), age, lactation stage and production. Average casein micelles diameters of milk samples obtained from 200 cows were measured using photon correlation spectroscopy and multiple regression analysis was used to find relationship between variables. CN micelle diameter, SCC and nonfat solids were different between animals with different Holstein crossbreed ratios, which suggests influence of genetic factors, mammary gland health and milk composition. Overall, results indicate the potential use of CN micelle diameter as a tool to select animals to produce milk more suitable to cheese production. Key words: milk urea nitrogen, nonfat solids, protein, somatic cell count. INTRODUCTION The amount of milk processed as derivatives in Brazil has been increasing during the last years (United States Department of Agriculture 2013). For a sustain- able production, milk quality and composition are essential conditions to obtain high-quality and profit- able products (Lucey & Singh 1997). Casein is among the important components of milk, with four major types, αs1, αs2, β and κ, and together with calcium phosphate, 1000s of casein molecules form aggregates of micelles with an average diameter of 150 to 200 nm (de Kruif 1998). The micelle structure influences milk stability to heating, freezing and drying, gelling prop- erties (de Kruif 1998), and functionality of products, such as cheese and yoghurt (Dalgleish & Corredig 2012). Previous reports indicate that genetic, feed manage- ment and milk composition can affect casein micelle size (Devold et al. 2000; de Kruif & Huppertz 2012). Therefore, animal selection toward production of milk with smaller casein micelles might improve milk renneting properties and consequently cheese produc- tion. Smaller micelles are more compact and have higher proportion of κ-casein (Devold et al. 2000) resulting in a firmer gel during initial steps of cheese production (Ford & Grandison 1986). Crossbreeding of Holstein with Zebu races is widely used for dairy cows in Brazil, especially to obtain animals with important economic implications. The Gyr is one of the principal Zebu breeds used for dairy production. Crossbreeding of Holstein and Gyr have a significant heterosis effect for characteristics of eco- nomic impact and adaptability to tropical climate (Madalena 2002). However, to the best of our knowl- edge, no evaluation of crossbreeding influence over casein micelles has been reported. Therefore, the objective of this work was to investigate the average casein micelles size from different Holstein × Zebu crossbreeding ratios, and to correlate it with milk pro- duction and composition, somatic cell count (SCC), animal age, and lactation stage. Correspondence: Denise Ribeiro de Freitas, Department of Veterinary Preventive Medicine, Escola de Veterinária, Universidade Federal de Minas Gerais, Av. Presidente Antônio Carlos, 6627, Belo Horizonte 30123-970, Minas Gerais, Brazil. (Email: deniseribeirof@yahoo.com); Mônica Maria Oliveira Pinho Cerqueira, Department of Technology and Inspection of Animal Products, Federal University of Minas Gerais, Brazil. (Email: monicamopc@ufmg.br) *In Memoriam. Received 2 April 2014; accepted for publication 4 August 2014. Animal Science Journal (2014) ••, ••–•• doi: 10.1111/asj.12323 © 2014 Japanese Society of Animal Science