Non-linear properties and yielding of enzymatic milk gels.

One of the first steps of cheese making is to suppress the colloidal stability of casein micelles by enzymatic hydrolysis and initiate milk gelation. Afterwards, the enzymatic milk gel is cut to promote syneresis and expulsion of the soluble phase of milk. Many studies have reported on the rheological properties of enzymatic milk gels at small strain, but they provide limited information on the ability of the gel to be cut and handled. In this study, we aim to characterize the non-linear properties and the yielding behavior of enzymatic milk gels during creep, fatigue and stress sweep tests. We evidence by both continuous and oscillatory shear tests that enzymatic milk gel displays irreversible and brittle-like failure, as reported for acid caseinate gels, but with additional dissipation during fracture opening. Before yielding, acid caseinate gels display strain-hardening only, while enzymatic milk gels also display strain-softening. By varying the gel aging time and the volume fraction of casein micelles, we are able to attribute the hardening to the network structure and the softening to local interactions between casein micelles. Our study highlights the crucial importance of the nanoscale organization of the casein micelles - or more generally of the building block of a gel - to retain the macroscopic nonlinear mechanical properties of the gel.

A sensitive, high-throughput fluorescent method for the determination of lactoperoxidase activities in milk and comparison in human, bovine, goat and camel milk.

Lactoperoxidase (LPO) is one of the major antibacterial ingredients in milk and an extensively employed indicator for milk heat treatment. The traditional method for LPO activity measurement using ABTS (2,2'-azinobis(3-ethylbenzothiazoline-6-sulphonate) cannot achieve high sensitivity and is affected by indigenous milk thiocyanate. A more sensitive microplate fluorescent assay was developed by monitoring generation of red-fluorescent resorufin from LPO catalysed oxidation of Amplex® Red (1-(3,7-dihydroxyphenoxazin-10-yl)ethanone) in this study. The assay is particularly suitable for milk LPO activity measurement as it eliminates the influences of indigenous milk hydrogen peroxide and thiocyanate. The method limit of detection was 7.1x10-6 U/mL of LPO in milk and good intra-run and inter-run precision was obtained. The LPO activities ranked as bovine > goat > camel > human in the four types of milk analysed. The high sensitivity and low cost of this assay makes it suitable for LPO activity analyses in both laboratory and commercial scales.