In the evaporator the whey is preheated to 60-70ºC (depending on the running conditions) after which it is pasteurized. By means of the temperature and holding time combination it is possible to control the degree of denaturation of the proteins, and thus it is possible to produce powders meeting specific requirements.
Generally speaking the viscosity of the whey concentrate increases and the non-caking properties become better by increasing the pasteurizing temperature/time. The whey is evaporated to a solids content suitable for the subsequent spray drying process and may vary from 40% to 60% solids. As the raw whey contains only 6% solids, it means that e = 10 (see Formula (2) on page 17). The evaporator must therefore be equipped with feed flow control and density control, since even small variations will result in tremendous fluctuations in the solids content of the concentrate. See diagram Fig. 123.
Fig. 123 Evaporation ratio: C as a function of mass-flow and evaporation capacity
Special attention should also be paid to Ca++ content in the whey. Ca-phosphate is a salt, which will precipitate, if the concentration and temperature increase. Especially, it may present a problem in the first effect and form deposits on the tubes, if not removed in a decalcification process, see page 273. Alternatively, an intermediate acid wash after 6-8 hours is required.
Since raw whey contains less solids than skim milk, it will in an evaporator designed for skim milk result in less concentrate, as the evaporation capacity is practically the same. This may have the effect that a good coverage of the tubes in the last stage can no longer be achieved, and recycling or splitting is necessary.
Removing part of the water from the whey in a hyperfiltration plant to 10-15% solids may also be done to increase the output from the evaporator.
A pre-evaporation may also be done, if it is impossible to obtain sufficient whey (>400,000 l/day) to make the whey powder production profitable. Small dairies may then pre-evaporate their whey in an evaporator to approx. 30% solids, cool it and transport the concentrate to a factory centrally located. The transport costs are thereby reduced. The pre-condensed whey may then be mixed with raw whey from the factory's own production, after which the final evaporation takes place followed by the spray drying, see Fig. 124. It is well known that it is cheaper to remove water in an evaporator than in a spray dryer (about 1:20), and it is thus not astonishing that the efforts in the development work have been concentrated on increasing the solids content from the evaporator. The outlet temperature of the spray dryer must be increased (about 1ºC/1% TS), which means that the powder temperature will get into the sticking point temperature zone resulting in severe problems in the dryer such as wall deposits and blocked cyclones.
Fig. 124 Preconcentration of whey prior to transport to centrally placed drying installation
The sticking point temperature depends on the composition of the whey powder and mainly on the following components:
- Lactic acid content and salts
- Amorphous lactose content
- Moisture content
Lactic Acid Content
It has been found that the sticking point temperature of one particular sweet whey powder sample was 98ºC. By gradual addition of lactic acid up to 16% the sticking point temperature decreased linearly down to 57ºC. (The samples had a moisture content of 1.5-3.5% which also influences the sticking point temperature, however, not to such a great extent). Partial or complete neutralization shifts the sticking point temperature up again, however, not up to the same level as that of sweet whey due to the increased content of salts.
Amorphous Lactose Content
The conversion of amorphous lactose into α-lactose monohydrate by crystallization (see page 251) results in a rise in the sticking point temperature. It has been found that a whey powder with 80% crystallized lactose had a sticking point of 78ºC, while the same product, but precrystallized to only 45%, had a sticking point temperature of 60ºC. This allows for an increase in the solids content of approx. 18%, if the concentrate is properly pre-crystallized leaving only a small part of amorphous lactose in the final powder.
Moisture Content
The moisture content of whey powder - also the intermediate moisture content of the semi-dried product as found in the spray dryer between atomization and powder outlet has a considerable influence on the sticking point. The graph in Fig. 125 shows the relation between the outlet temperature (to) and the sticking temperature (ts). When keeping other parameters constant it is obvious that the powder moisture depends on the outlet temperature. The temperature of the product particles in the spray dryer is lower than the outlet temperature, and this difference in temperature increases by increasing moisture content. The line (ts) shows the sticking point temperature of a particular whey powder as influenced by the moisture content. This line crosses the product temperature line at a point (Tpc) which represents the maximum (critical) moisture content - and at the same time the minimum (critical) outlet temperature (Toc) that may be used for drying this particular whey concentrate without having trouble with sticking.
Fig. 125 Relationship between the moisture content, air temperature, particle temperature and the sticking point temperature
It may be seen that an outlet temperature of 90ºC, giving a product temperature of about 82.5ºC and a moisture content of 3%, corresponds to a sticking point of 84.5C. So under these conditions the dryer is running with a safety margin of 2ºC below the sticking point, corresponding to about 0.2% moisture.
However, the conditions may vary somewhat during the run causing the product tem-perature to rise above the sticking point. This would for example happen under the following conditions:
- The outlet temperature drops by 2ºC
- The feed concentration increases by 1%
- The absolute humidity of the inlet drying air increases by about 5 g/m3
- The acidity of the processed whey increases by about 0.02% lactic acid
The above example is valid for one type of sweet whey when running at an inlet tem-perature of 180ºC and with a feed concentration of 44% straight from the evaporator with no pre-crystallization. These conditions are quite common for sweet whey with-out pre-crystallization.