034: Effect of Thermal Denaturation of Casein Micelles on Processability Into Natural Cheese

Introduction

Rennet-induced coagulation of the milk and curd syneresis during cooking are important processes in forming a natural cheese structure. It is known when milk is heated, several reactions occur such as the denaturation of casein micelles (CM) via disulfide bonds between κ-casein (κ-CN) on the surface of CM, and β-lactoglobulin (β-Lg), the insolubilization of soluble minerals, and the thermal denaturation of whey proteins themselves. In natural cheese production, milk has undergone these changes by high-temperature sterilization is difficult to use as an ingredient because its rennet-induced coagulation properties are poor. The mechanism of poor coagulation in high-temperature heated milk remains unclear, as several heat-induced changes may each have an effect. Therefore, we aimed to clarify the details of poor rennet-induced coagulation, focusing only on the denaturation of CM via binding between κ-CN and β-Lg. Furthermore, previous studies on the processability of high-temperature heated milk into natural cheese have focused on milk coagulation. Thus, we also investigated curd syneresis during cooking.

Methods

CM was obtained by centrifugation of raw milk. Whey protein isolate was dissolved at 0-0.78% in a solution simulating the mineral composition of milk (SMUF). CM was then dispersed in the solution to prepare reconstituted milk with different whey protein contents. The reconstituted milk was then incubated at 80 °C for 30 min to thermally denature the CM. Then centrifuged to obtain heat-denatured CM. The heat-denatured CM was dispersed in SMUF and used as a sample. Rennet-induced gel strength and weight of water released from the gel when heated at 40°C were measured.

Results

When 8% of β-Lg (the amount of β-Lg in raw milk was 100%) bonded with κ-CN on the surface of CM by heating, rennet-induced coagulation and curd syneresis were decreased than non-denatured CM. Furthermore, coagulation and syneresis were further decreased with increasing binding between κ-CN and β-Lg. And when 27% of β-Lg bonded with κ-CN, coagulation didn’t occur. This result provides direct evidence that denaturation of CM via binding between κ-CN and β-Lg decreases not only rennet-induced coagulation but also curd syneresis.

Significance

This study provides basic knowledge for producing natural cheese from high-temperature sterilized milk.

Authors: Minamo Hayashi, Wataru Ono, Daiki Oka, Tomohiro Noguchi