Pre-crystallization of the concentrate shifts the sticking temperature upwards, as the crystallization yields a concentrate with much less amorphous lactose to be dried. It is thus possible to use considerably higher feed concentrations and inlet temperatures.
In order to understand the theoretic background for the crystallization process it is necessary to look into the physico-chemical properties of the lactose which form about 3/4 of the solids of the whey.
Lactose is a disaccharide quite different in its behaviour from other common sugars. A distinctive feature of the lactose is its appearance in different modifications with physico-chemical interrelations determined by the temperature.
In an aqueous solution the lactose molecule is present in an α and a β form as shown in Fig. 126.
Fig. 126 Lactose molecule in ά and β configuration
The α and β forms are in a reversible equilibrium, i.e. there is a continuous transformation of the α form into the β form and vice-versa, called mutarotation, see Fig. 127. The proportion of the α form to the β form is determined by the temperature, see Fig. 128.
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Fig. 127 Mutarotation
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Fig. 128 Transformation of β lactose to ά In 1 hour.
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In whey powder produced according to the process described above with a pneumatic conveying system, the lactose is present in an amorphous or glassy state. This form of lactose is extremely hygroscopic absorbing water from the air forming α-lactose monohydrate.
The α-lactose monohydrate, which is not hygroscopic, is also formed in liquid whey concentrates by crystallization of the lactose from supersaturated solutions. As the α form is less soluble than the β form at a given temperature, the α form reaches the point of supersaturation first and forms crystals of α-lactose monohydrate. In Fig. 129 the dissolving properties of α and β lactose are shown in relation to the temperature.
The removal of v-lactose from the solution due to the crystallization means that the proportion between α- and β-lactose changes, so that the solution contains more β-lactose than corresponding to the equilibrium.
Due to the mutarotation the solution of α-lactose becomes again supersaturated, so that the crystallization continues. This process will continue as long as the solution is supersaturated and will not stop until the saturation point is reached.
Lactose can be supersaturated by:
- Increasing the content of lactose in relation to water content. This is done by evaporation.
- Cooling the solution as lactose becomes less soluble in water at lower temperatures.
A rich crystallization is therefore achieved by concentrating the whey to a solids content as high as possible and by cooling the concentrate.
The rate of mutarotation is directly influenced by the temperature of the solution, and as it appears from Fig. 129 it proceeds relatively fast at high temperatures, whereas it goes very slowly at temperatures near the freezing point. It is thus clear that the conditions promoting mutarotation and crystallization are conflicting.
This means that by cooling the concentrate too fast to a too low temperature the proportion of lactose which will crystallize will be low, even though the crystallization proceeds quickly. This is due to the slow mutarotation resulting in only a small amount of β-lactose being transformed into α- lactose.
Fig. 129 Solubility of ά and β lactose in water at different temperatures
A temperature compromise is therefore necessary to obtain an optimal degree of crystallization.
The actual obtainable solids content depends on the type of whey, degree of denaturation of the whey proteins, pH of the whey, and the ability of the evaporator.
Although the crystallization process is not fully understood, it is a well-known fact that the crystallization takes place on the surface of already existing crystals. In order to promote the crystallization, lactose crystals or well crystallized whey powder is seeded to the supersaturated solution. As the surface area of the seeding material is the important factor, a sufficient amount of small crystals should be added to the concentrate. If lactose crystals (α-lactose monohydrate) are used, approx. 0.1% w/w should be added to the concentrate.
Cooling of the concentrate is done instantaneously to 30ºC in a flash cooler (see page 59) connected to the evaporator after which it is cooled slowly (1-3ºC/h) to about 15ºC in specially designed crystallization tanks, see Fig. 130. This may allow the mutarotation to proceed at a reasonable speed resulting in a rich crystallization of about 80% of the lactose. During the whole crystallization time it is of significant importance that the content of the crystallization tank be vigorously agitated continuously. This is done in order to transport supersaturated solution to the surface of the crystals, simultaneously replacing the saturated solution. The agitation also prevents the viscosity of the thixotropic suspension from getting too high, and furthermore sedimentation of the lactose crystals does not take place.
Fig. 130 Whey crystallization tanks
The viscosity of the crystallized whey concentrate is mainly influenced by:
- Heat treatment before the evaporation
- Solids content of the concentrate
- Size of the lactose crystals
Heat Treatment before the Evaporation
The degree of denaturation of the whey proteins has a great influence on the viscosity of the concentrate as well as on the rate of crystallization, the drying properties, and the final powder properties. The properties of the proteins may also vary in different types of whey, and consequently they may require different heat treatment.
As a guide line, the preheating temperature should be in a narrow range of 80ºC. Higher preheating temperature normally results in higher viscosity, and problems with pumping of the crystallized concentrate may occur. Lower temperature than 80ºC will decrease the viscosity, but at the same time problems with deposits in the drying chamber may occur due to an increased thermo-plasticity calling for higher particle/outlet temperature with an inferior product as a result.
It is thus a general rule that the preheating temperature is kept at such a level that the viscosity is about 2000 cP.
Solids Content of the Concentrate
It goes without saying that the higher solids content the higher viscosity in the concen-trate, but also during the crystallization the viscosity undergoes deep changes, partly due to the decrease in temperature, but also due to the crystallization itself.
Size of the Lactose Crystals
A typical development of the viscosity during the crystallization is shown in Fig. 131. The maximum viscosity at the time t0 max. for a given amount of concentrate is reached after a short time (½-1 hour) and may go as high as several thousand cP. The reason is that the size of the lactose crystals is very small (big specific surface) and their mother liquor solution still has a relatively high solids content, which means that the friction between the crystals and the mother liquor is big. As time passes the lactose crystals grow, and the solids content of the mother liquor solution decreases resulting in a decrease of the viscosity.
Fig. 131 Viscosity development during the crystallization of whey concentrate
The viscosity at t0 max. may be so high that agitation is no longer possible. However, at the time ti non-crystallized concentrate with low viscosity is pumped into the crystallization tank, and thereby the content of the tank is "diluted" in the way of viscosity.
The curve Σto-n is therefore the interesting one, and under normal conditions the viscosity does not present any problems, provided the heat treatment is controlled. The crystal size aimed at in the concentrate is 20-30μ and the biggest crystal should not exceed 50μ. This in order to ensure that all spray particles contain at least one lactose-crystal enabling a further crystallization to proceed during the final drying. This is possible as the mother liquor solution again becomes supersaturated due to the evaporation.
Under normal conditions it is possible to obtain 80-90% crystallization of the lactose. This may be determined in a very simple way by using an ordinary hand refractometer which gives the refractive index or direct % of sugar in solution.
The refractive index or direct % of sugar is measured at short intervals of 15-30 min. direct from the evaporator, and the average arithmetical value is calculated = S1. In an average sample of the crystallized whey concentrate a second refractometric reading is made = S2. The degree of crystallization will with a good approximation be:
%Crystallization = (S1-S2) x 9500 x 100 / L x TS x (95-S2) (27)
where:
S1 = % sugar (ref. index) of the concentrate direct from the evaporator.
S2 = % sugar (ref. index) of the crystallized concentrate
L = % lactose (74% may be used)
TS = total solids content in %