Vol 60, No 1 February 2007 International Journal of Dairy Technology 37 ORIGINAL RESEARCH *Author for correspondence. E-mail: ulrich.kulozik@ wzw.tum.de © 2007 Society of Dairy Technology Blackwell Publishing Ltd Oxford, UK IDT International Journal of Dairy Technology 1364-727X Society of Dairy Technology 2004 54 ORIGINAL RESEARCH ORIGINAL RESEARCH Water mobility during renneting and acid coagulation of casein solutions: a differentiated low-resolution nuclear magnetic resonance analysis RUTH HINRICHS, SELDA BULCA and ULRICH KULOZIK* Chair for Food Process Engineering and Dairy Technology, Technische Universität München, Weihenstephaner Berg 1, D-85350 Freising-Weihenstephan, Germany ABSTRACT Changes in water mobility during renneting and fermentation (acidification) of casein micelle solutions measured by nuclear magnetic resonance (NMR) are presented. Casein solutions (native and ultra-high temperature or UHT-treated) were renneted and fermented directly in the NMR instrument to detect changes in water mobility due to the induced structural changes online. The data were analysed using a newly developed method capable of differentiating several fractions of water in terms of their mobility according to the T 2 relaxation times in four distinct ranges, that is, immobile, weakly mobile, mobile and very mobile. Whereas obvious changes in the water mobility take place during acidification, no changes in the water mobility during renneting were observed. This is explained by proposing a model building on different mechanisms of coagulation between acidification and renneting. *Author for correspondence. E-mail: dvanhekken@errc.ars.usda.gov Keywords Casein solution, Fermentation, NMR, Rennet, UHT-treatment, Water mobility. INTRODUCTION Curd formation by aggregation and coagulation of proteins is a process with both micro- and macro- structural changes. During renneting of milk, casein micelles coagulate and form a gel due to the enzymatic separation of caseinomacropeptide (CMP) and precipitation of para-casein micelles. Presumably, these structural changes should include a change in water mobility. Many techniques, like vibrational systems, electrical conductivity, optical systems and dynamic light-scattering techniques have been used to monitor the process of curd setting as reviewed by O’Callaghan et al. (2002). None of the methods mentioned by O’Callaghan et al. (2002), however, deals with water mobility. Therefore, nuclear magnetic resonance (NMR) is of interest because it has the potential to provide additional information beyond routine tests applied so far. NMR uses the fact that protons ( 1 H), when exposed to a magnetic field, can absorb and emit energy through electromagnetic radiation depend- ing on their own mobility and their immediate environment. Various authors have published results offering first insights regarding the change of water mobility upon gelification. The gelling of renneted milk has already been described by means of 1 H low-resolution NMR (20 MHz) by Lelievre and Creamer (1978) and high-resolution NMR (250 MHz) by Tellier et al. (1993). However, no significant change in the molecular mobility of the water protons could be detected during renneting in these studies despite the fact that gelification obviously takes place. Mariette (2003) also mentioned, without providing evidence, that it is not possible to study the renneting with NMR. After gelling, that is during the con- traction of the gel, which is linked with the phe- nomenon of syneresis, an increase in the mobility of the mobile phase was observed (Lelievre and Creamer 1978; Tellier et al. 1993). 1 H NMR diffu- sion measurements also did not show any differences in the mobility of water in casein solutions and ren- neted casein gels (Mariette et al. 2002). In contrast to renneted gels, using single-phase T 2 relaxation time analysis, HCl-acidified casein gels (Roefs et al. 1989) and glucono-δ-lactone-acidified milk and caseinate solutions (Mariette et al. 1993; Famelart et al. 1997) show obvious changes in the overall water mobility depending on the pH value. Changes in the T 2 relaxation time during fermenta- tion by lactic acid bacteria can also be observed (Laligant et al. 2003). Making a distinction between various phases of immobilized and mobile water, definite changes in the water mobility and the structure during yogurt fermentation (R Hinrichs, personal communication) could be observed by a differentiated analysis using modern NMR instru- ments. Lelievre and Creamer (1978) dealt only with a single-phase T 2 relaxation time analysis