A great number of operational parameters and conditions influence the performance of a spray drying plant. One essential factor is the quality of the processed milk, evaluated not only according to general quality standards, but also composition and physical-chemical properties as influenced by the composition itself.
|Component||% in liquid milk||% in total solids|
|Total protein in non-fat-solids||36.2-48.0 1)|
|Lactose in non-fat-solids||51.7-54.9|
|Protein/lactose ratio||0.63-1.06 1)|
|Whey protein/non-fat solids ratio||7.4-8.3 1)|
|Whey protein/casein ratio||0.19-0.22 1)|
|1) The high range limit applies for Channel Islands breeds and end of season.|
Milk sugar or lactose is a carbohydrate, existing only in milk, in true water solution.
It is a disaccharide C12H22O11 consisting of glucose and galactose and occurs in two isomeric forms α- and β-lactose (see Fig. 9.2). They have different physical properties, especially solubility in water and polarized light rotation. Both forms can crystallize, but for the production of milk and whey powders the most important is α-lactose, which crystallizes as α-lactose-monohydrate from the supersaturated solution of lactose below the temperature of 93.5°C. During the production of normal milk powders, the water evaporation during spray drying is so fast that despite supersaturation, the lactose cannot crystallize but remains in the powder as amorphous lactose, also called lactose glass. Amorphous lactose is very hygroscopic. This can cause caking problems with powders having high content of lactose, as for instance whey powders. To avoid caking the lactose has to be crystallized as α-lactosemonohydrate, which is non-hygroscopic. This is done by pre-crystallization of the concentrates. The rate of crystallization in solutions of lactose is controlled by the rate of mutarotation, i.e. transformation of β-lactose into α-lactose. The rate of mutarotation decreases with falling temperature, being fairly high in the range of 40-20°C, but practically zero at temperatures below 10°C. The solubility of lactose is shown on Fig.9.3.
The specific optical rotation of α-lactose in water is [α]D/20°C = +89.4° and melting point 201.6°C under disintegration. The corresponding values for α-lactose are [α] D/20°C = +33.5° and 252.2°C. The equilibrium specific rotation is [α] D/20°C = +55.3°.
α-lactose-monohydrate crystallizes in prism shapes often called tomahawk. These crystals in milk products are detectable on one’s teeth. With crystals larger than 10 μm there is a “flourish” and over 15 μm a “sandy” feel on the teeth.
The density of pure α-lactose-monohydrate is 1.54 g/ml. The sweetness of lactose is only 30% of that of sucrose. For whey powder manufacturing technology a most important physical property is the heat of crystallization, which is 10.63 kcal/kg. This must be taken into consideration when calculating the consumption of cooling water for the crystallization tanks. The relationships between various forms of lactose acc. to. King  are shown in Fig. 9.4.
where: %F = percent of fat
%NFS = percent of non-fat-solids
%W = percent water
ρfat = specific gravity of fat (acc. Hunziker 0.93)
ρNFS = specific gravity of non-fat-solids (acc. Hunziker 1.608)
ρwater = specific gravity of water (Hunziker used 1 for t=15°C)
Concentrate density at a temperature, t, can be calculated with a reasonable degree of approximation by:
|% lactic acid||111.1||44.44||100||-|
|Strenght NaOH||0.1 N||0.25 N||0.11 N||0.1 N|
|°Th=Thörner, °SH=Soxhlet-Henkel, °D=Dornick, %1.a.=% as lactic acid|
|Maltodextrin DE 36 (MW ~ 550)||100|
|Maltodextrin DE 25 (MW ~ 720)||121|
|Maltodextrin DE 20 (MW ~ 900)||141|
|Maltodextrin DE 10 (MW ~ 1800)||160|
|Maltodextrin DE 5 (MW ~ 3600)||188|
Handbook of Milk Powder Manufacture
The Handbook of Milk Powder Manufacture is a valuable reference book for dairy processing engineers wanting to take a deeper look at the complex world of milk powder processing.