Mıthat Celebı et al., J.Chem.Soc.Pak., Vol. 36, No. 1, 2014 1 Statistically Defining Optimal Conditions of Coagulation Time of Skim Milk 1,2 Mıthat Celebı*, 2,3 Zafer Omer Ozdemır, 4 Emre Eroglu and 5 Ibrahım Guney 1 Yalova University, Faculty of Engineering, Polymer Engineering, Yalova TURKIYE. 2 Yildiz Technical University, Chemical and Metallurgical Faculty, Bioengineering Department, Istanbul, Esenler, TURKIYE. 3 Kirklareli University, Faculty of Arts and Science, Department of Chemistry, Kirklareli, TURKIYE. 4 Kirklareli University, Faculty of Arts and Science, Department of Mathematics, Kirklareli, TURKIYE. 5 Bitlis Eren University, Faculty of Arts and Science, Department of Statistics, Bitlis, TURKIYE. mithat.celebi@yalova.edu.tr* (Received on 7th June 2012, accepted in revised form 8th May 2013) Summary: Milk consist huge amount of largely water and different proteins. Kappa-kazein of these milk proteins can be coagulated by Mucor miehei rennet enzyme, is an aspartic protease which cleavege 105 (phenly alanine)-106 (methionine) peptide bond. It is commonly used clotting milk proteins for cheese production in dairy industry. The aim of this study to measure milk clotting times of skim milk by using Mucor Miehei rennet and determination of optimal conditions of milk clotting time by mathematical modelling. In this research, milk clotting times of skim milk were measured at different pHs (3.0, 4.0, 5.0, 6.0, 7.0, 8.0) and temperatures (20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75 o C). It was used statistical approach for defining best pH and temperature for milk clotting time of skim milk. Milk clotting activity was increase at acidic pHs and high temperatures. Keywords: Milk clotting time, Mucor miehei Rennet, S curve, skim milk, optimization. Introduction Chymosin (calf rennet)– extracted from the fourth stomach (abamosum) of the young calves- has been used worldwide in cheese production for years and it has been preferred for its high quality. However, nowadays there has been a shortage of the chymosin in the world market, so proteolytic enzymes from different sources gained more importance [1]. Mucor miehei rennet is the most thermostable enzyme of Mucor miehei at acidic pH. It is an aspartic protease which cleavege 105 (phenly alanine)-106 (methionine) peptide bond in kappa- kazein (milk protein) [2, 3]. The aspartic proteinases (EC 3.4.23) are characterized by an optimum activity at acidic pH and contain two catalytically essential aspartate residues. A worldwide shortage of the aspartic proteinase chymosin, traditionally used in the production of cheese, has prompted a search for adequate substitues. Two alternatives are aspartic proteinases from fungi Endothia parasitica and Mucor miehei. Although Mucor miehei proteinase have high milk clotting potential relative to its capacity for general proteolysis, high termal stability has precluded its use as a chymosin substitute [4]. The stability of the casein micelle is dependent on the presence of ĸ-casein on the surface of the micelle where it functions as an interface between the hydrophobic caseins of the micelle interior and the aqueous environment. ĸ-casein is also involved in thiol-catalyzed disulfide interchange reactions with the whey proteins during heat treatments and, after rennet cleavage, in the facilitation of micelle coagulation. These functions of ĸ-casein are regulated by three–dimensional structure of the protein on the micelle surface [5, 6]. Rennet coagulation of milk is a two-stage process. In the first stage, casein micelles colloidal suspensions are destabilized by partial hydrolysis of their surface κ- casein, catalyzed by chymosin. Destabilized casein micelles spontaneously aggregate in the second stage as been considered as a diffusion-limited aggregation, according to the von Smoluchowsky model for unstable colloids coagulation [7, 8]. Mathematical models (nonlinear least squares, etc.) are very useful for defining optimal conditions in different study subjects [9, 12]. In curve fitting data, the prevailing assumption regarding model description is that the structure is linear in the model coefficients. In many areas of the physical, chemical, engineering, and biological sciences, knowledge about the experimental situation suggests the use of a less empirical, more theoretically based, * To whom all correspondence should be addressed. GENERAL AND PHYSICAL