The effect of intrinsic fluoride in cows' milk on in vitro enamel demineralization.

The fluoride concentration in cows' milk has been reported to vary with the fluoride levels in drinking water but it seldom exceeds 0.5 microg/ml. This raised a question as to whether any caries-protective effect could be attributed to the intrinsic fluoride of milk. Two samples of cows' milk with intrinsic fluoride concentrations of 0.03 and 0.3 microg/ml, respectively, were assessed for their protective effect on enamel in an in vitro demineralization model at relatively severe and mild acidic challenges (pH 4.6 and 5.0, respectively). Polished enamel discs were incubated individually in 5.0 ml of demineralization solution for 20 h per day alternated with 1-hour incubations in 1.0 ml of milk or control buffers: group 1, demineralization solution only (negative control); group 2, milk with 0.03 microg/ml fluoride; group 3, milk with 0.03 microg/ml fluoride; supplemented with NaF to 0.3 microg/ml fluoride; group 4, milk with 0.3 microg/ml fluoride; group 5, 0.3 microg/ml fluoride in 20 mM HEPES, pH 6.7; group 6, milk with 0.03 microg/ml fluoride supplemented with NaF to 5.0 microg/ml fluoride (positive control). The solutions were renewed each day and the calcium concentration in the demineralization solutions was followed during 4 days. The results showed that the protective effect of intrinsic milk fluoride on enamel is limited by the severity of the acidic challenge: There was a significant inhibition of the demineralization in groups 3-6 compared to groups 1 and 2, but only at pH 5.0 (p<0.0001) and not at pH 4.6 (p = 0.2). The organic components of milk had limited protection against demineralization because milk and HEPES with the same fluoride concentration gave similar results. The 36% reduction in calcium loss at pH 5.0 by treatment with milk with only 0.3 microg/ml fluoride is an indication that intrinsic milk fluoride has some caries-protective properties.

Enzymatic release of sequestered cows' milk fluoride for analysis by the hexamethyldisiloxane microdiffusion method.

The concentrations of diffusible and total fluoride in cows' milk samples from areas with widely different fluoride levels in drinking water were determined using a fluoride electrode. The diffusible fluoride was determined by direct hexamethyldisiloxane microdiffusion while for total fluoride, samples were subjected to either open ashing or digestion with proteolytic enzymes before microdiffusion. Magnesium nitrate was studied as a new fixative for milk during open ashing and compared with magnesium acetate. Diffusible fluoride ranged from 0.024 to 0.28 microgram ml-1 while total fluoride ranged from 0.05 to 0.31 microgram ml-1. The use of proteolytic enzymes before microdiffusion resulted in total fluoride measurement. It was concluded that all fluoride in milk is inorganic in nature with the bound fluoride being physically or chemically sequestered in the milk proteins. The proposed method is convenient for total fluoride analysis in milk.

Effect of interfering ions on hexamethyldisiloxane microdiffusion method.

Acid diffusion in the presence of hexamethyldisiloxane (HMDS) enables complete recovery of ionic fluoride from standards containing varying concentrations of aluminium as one of the main interfering ions. Acid diffusion without HMDS shows a decrease in fluoride recovery as aluminium ion concentration increases. The fluoride concentration in the trapping solution is determined directly on the diffusion cover with a combination fluoride electrode after neutralising and buffering. The same procedure was used for the analysis of fluoride in soil and plant materials containing high concentrations of aluminium ions. For the same samples, the concentrations of aluminium, iron and silicon were determined using atomic absorption spectrophotometer (AAS).