The Clarifying Reagent, or how to make the analysis of milk and dairy products easier
Difficulties with the analysis of opaque milk and dairy products can be resolved by rendering the sample transparent. This allows direct photometric measurements of the reaction mixtures, and all sample pre-treatment steps become unnecessary. For this, several reagents have been reported in literature. Among them, the ‘Clarifying Reagent’ is mainly described here: its composition is given because it is now released from patenting restrictions. Its main property is a low absorbance between 340 and 800 nm. So it can be used in many chemical and enzymatic colorimetric measurements for estimating the quality of milk and dairy products.
Screening procedure for evaluating heat load in commercial milks
The evaluation of heat damages in commercial milk samples has been studied by an ana" lytical procedure which uses two different and complementary methods. The first is the rapid total sul- phydryl (SH) group measurement which evaluates whey protein denaturation. The second involves the measurement of absorbance at 340 nm (A340) of milks modified with the Clarifying Reagent'", At .this wavelength sorne coloured compounds resulting from the non-enzymic browning still absorb. There is a satisfying correlation between A340 and lactulose content (r =0.98) measured in commercial milk samples by using capillary electrophoresis or high performance liquid chromatography (HPLC). Our analytical procedure using SH and A340 measurements has been applied to 130 raw and commercial milk samples. Analysis by Student's r-test shows that the values of the two parameters obtained for raw and heat-treated milk samples were significantly different (P < 0.01). This study demonstrates that this simple protocol makes possible rapid evaluation of heat damages in commercial milk sam- pies as il could be done with official but more sophisticated methods. © Inra/Elsevier, Paris. heat damage / commercial milk / sulphydryl group / absorbance A340 / transparent-modified miIk / analytical procedure
Reciprocal effect of apolipoprotein C-II on the lipoprotein lipase-catalyzed hydrolysis of p-nitrophenyl butyrate and trioleoylglycerol.
Interaction of purified bovine milk lipoprotein lipase (LpL) with sonicated vesicles of dipalmitoyl phosphatidylcholine in the gel phase is associated with an increase in the rate of the LpL-catalyzed hydrolysis of p-nitrophenyl butyrate. There is a 6-fold increase in Vmax. Apolipoprotein C-II, the activator protein for LpL, inhibits the LpL-catalyzed hydrolysis of p-nitrophenyl butyrate. With 0.5 mol % tri[14C]oleoylglycerol present in the dipalmitoyl phosphatidylcholine vesicles and in the presence of 20 mM Ca2+, the rate of p-nitrophenyl butyrate hydrolysis is decreased reciprocally compared to trioleoylglycerol hydrolysis and is dependent on apolipoprotein C-II. These results suggest that apolipoprotein C-II enhances the activity of LpL by increasing the affinity of the active site of LpL for triacylglycerol.