The temperature obtained from the last preheater is in multi-effect evaporators lower than the boiling temperature in the first effect. Additional preheating is therefore necessary to obtain the required 2-3ºC above the boiling temperature of the first effect. A separate preheater heated by live steam, usually via a thermo-compressor, is then used. However, some products may require higher temperatures, but the primary purpose of heat treatment in an evaporator, apart from bacteriological requirements, is not "pasteurization", but obtaining a tool to get functional properties in the final powder. The reasons for the heat treatment are:

• Bacteriological requirements
• Heat classified skim milk powder
• Keeping quality of whole milk
• High-heat heat-stable milk powder
• Instant whole milk powder
  

Bacteriological Requirements

A pasteurization directly before the evaporation will naturally influence the bacteria count in the final powder, and the higher the temperature and the longer the holding the more efficient the killing.

Heat Classified Skim Milk Powder

Skim milk powder is often produced according to a fixed degree of denaturation of the whey proteins and is classified according to the whey protein nitrogen index (mg WPNI/g powder) which expresses the content of undenaturated whey proteins. Different temperature and time combinations have an influence on the index as shown in Fig. 22, as well as % denaturation of β-lactoglobulin in milk in Fig. 22a.

Fig. 22 Mg WPNI/g powder as a function of the pasteurization intensity, a relation between temeprature and time

Fig. 22a  % denaturation off ß-lactoglobulin

Keeping Quality of Whole Milk

When producing whole milk powder one problem is the shelf-life, as the fat easily be-comes oxidized, if the powder is not packed using an inert gas. As a lot of powder is shipped in bags, it is not possible to protect the powder effectively, and antioxidants are in most cases not permitted.

By pasteurizing (direct) the milk prior to the evaporation to 90-95ºC and keeping the temperature for ½-1 min., some natural antioxidants will be formed, as -SH groups, originating from the aminoacids cystine, cysteine and methionine, are liberated and will act as antioxidants. Higher pasteurization temperatures will form more -SH groups, but they will react with casein and not be found in free form. See Fig. 23. The free -SH groups will at the same time give the milk a cooked flavour, which, however, is liked by many consumers.

Fig. 23 Development of free -SH groups as a function of pasteurization temperature

High-Heat Heat-Stable Milk Powder

This type of powder is used for reconstitution for making evaporated, sterilized milk, especially in the Far East. After reconstitution to 25-27% TS the product has to be sterilized using temperatures of 120ºC or higher during 20 min. The heat stability of the recombined product is controlled by the pasteurization temperature/time combination prior to the evaporation and drying. A direct contact heating system gives a better result.

Pasteurization	Temperature Interval ºC
indirect ºC	from 60 to 80
direct ºC	from 80 to 110 *)
direct ºC	from 110 to 125
holding min.	2-4

 *) In the heating interval from 80 to 110C a very fast heating is important to avoid interaction between the whey proteins, in order to produce low viscous products with good heat stability.

Instant Whole Milk Powder

To produce instant whole milk powder with good reconstitution properties in cold water and at the same time with a good "coffee stability" - that is no coagulation should take place when the powder is added to hot coffee as a "whitener" - it is recommended to use a temperature/time combination to achieve a WPNI of  ≥ 3.5 mg/g, which corresponds to approx. 45% denaturation of β-lactoglobulin, see further Fig. 22a.

The pasteurization can be carried out in different ways, either:

• Indirect in plate-, spiral- or straight-tube heat exchangers
• Direct steam injections into the milk or milk into a steam atmosphere

Indirect Pasteurization

The indirect heaters are working as ordinary heat exchangers, either the plate-tube or spiral-tube type. If temperatures up to 110ºC are wanted, it is recommended to have two heaters, where one is in operation while the other one is being cleaned, see Fig. 24.

Fig. 24  Switchable indirect heat exchangers 

The advantage of the indirect heating is that the product will not be mixed with the condensating steam and neither will the product be diluted. The disadvantage is that it takes long time for the product to be heated in the interval from 80ºC to 110ºC resulting in a concentrate with high viscosity, because the whey proteins, when unrolled, will react/denature with each other before the Kappacasein is formed. For improved efficiencies one or more regeneration systems can be incorporated, see Fig. 25.

Fig. 25  Indirect spiral-tube heat exchager working in series, heated by steam and flash vapour

Direct Pasteurization

The direct pasteurization is done in two different ways, either by direct steam injection, where the live steam is mixed into the milk using a Tangential Swirl Heater (TSH), see Fig. 26. It offers a controlled and short residence time with no mechanical impact, even at temperatures >120ºC. It can operate 20 hours or more without inter-mediate cleaning. Alternatively, milk is sprayed into a steam atmosphere (infusion) at a sufficient pressure. The steam must be of good quality, i.e. for use in products for human consumption. Culinary steam boilers, where milk condensate is heated up in an indirect coil-type heater by means of live steam, can be used. The advantage of direct pasteurization is the short time it takes to reach the desired temperature. This means that both unrolled whey proteins and Kappacasein are available, and they can react/denature with each other resulting in a low viscosity.

Fig. 26  Direct pasteurizing system with direct contact flash regenerative system

The direct heating will further have a less pronounced effect on the denaturation of the whey proteins at the same pasteurization temperature/time.

	Whey protein denaturation	Thiamin loss
Direct system	35%	0.5 - 0.8%
Indirect system	65%	1.4 - 4.4%

As for the indirect pasteurization, regenerative flash chambers are used, if high pasteurizing temperatures are needed. The temperature of the milk will drop due to the evaporation, and the vapours are used for preheating prior to the pasteurizer. The regenerative flash chamber can be either indirect as shown in Fig. 25, or direct contact as shown in Fig. 26. The direct contact regenerative system is preferable, as there is no heat contact surface, where deposits (bio-film) can develop.

Tangential Swirl Heater

The holding is practically always done in holding tubes, with specific length and diameter to give the desired holding time. Holding vats, the so-called "hot well", have been used, if holding times as long as 30 min. - especially for whey or permeate - are required. It is, however, difficult to control the exact time and some of the product might pass through in 5 min., whereas some would take very long.

The pasteurization temperature will of course have a direct influence on the total steam consumption which will increase by increasing the temperature. For the same pasteurization temperature the direct pasteurization will result in higher steam consumption compared with indirect pasteurizing due to the need of evaporation of the extra water formed by the condensation. However, the additional steam used is - after flashing off - used as heating medium in the subsequent calandrias and some of the applied energy is reused.