Milk ultra-fine

Two techniques are used commercially to produce ultra-fine milks of lime, namely slaking at high shear rates and milling conventional milks. 

Ultra-fine milks of lime may be produced by pumping a conventional milk of lime through the mill at a suitable rate. The milk may be recycled if a single pass does not give the required size reduction. 

The chemical reactivities of ultra-fine milks of lime can be measured using a con-ductimetric method [22.1]. A quantity of milk sufficient to give a concentration equivalent to (say) half of the saturation concentration is added, with stirring, to a standard volume of de-ionised water. The rate of increase in the conductivity of the resulting mix is a measure of the rate of solution of the milk of lime. 

Ultra-fine milks of lime can have solution rates, which are measured in seconds, and which are more than 10 times greater than those of standard milks of lime. 

Because the settling rate decreases with increasing fineness, ultra-fine milks can, to a large extent, be treated as conventional liquids rather than as suspensions. This enables them to be used in place of soluble alkalis such as caustic soda. 

The milk of lime should be grit-free and should have as high a surface area as is practicable. Traditionally this has been produced by slaking a low carbonate quicklime under conditions favouring the production of finely divided calcium hydroxide (see chapter 22). More recently, ultra-fine milks of lime (see section 22.8) have proved to be particularly effective lubricant carriers. 

Raw or gently pasteurised milk (e.g. for 10 seconds at 73 °C) has a fine characteristic odour and sweet taste. Typical components present in low concentrations are dimethylsulfide, biacetyl, 2-methylbutan-l-ol, (Z)-hept-4-enal and ( )-non-2-enal. Milk pasteurised at higher temperatures and Ultra High Temperature (UHT) milk present the so-called cooked flavour, the appearance of which is the first measurable manifestation of the chemical changes that occur in heated milk. The substances responsible for the cooked off-flavour are sulfane and other sulfur compounds. Of particular importance are dimethylsulfide, dimethyldisulfide and dimethyltrisullide that are produced from proteins contained in the membranes of fat particles and from thiamine. Also relevant are alkane-2-ones (methylketones) generated by thermal decarboxylation of P-oxocarboxylic acids (mainly hexane-2-one, heptane-2-one and nonane-2-one), y-lactones and 5-lactones produced by dehydration of y- and 5-hydroxycarboxylic acids (mainly 8-decalactone and y- and 8-dodecalactones). Important carbonyl compounds include biacetyl, hexanal, 3-methylbutanal, (Z)-hept-4-enal and ( )-non-2-enal. In the more intensive thermal treatment of milk (sterilisation), products of the Maillard reaction play a role, such as maltol and isomaltol, 5-hydroxymethylfuran-2-carbaldehyde, 4-hydroxy-2,5-dimethyl-2 f-furan-3-one (furaneol) and 2,5-dimethylpyrazine. 

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