CONDENSED AND DRIED MILKS

Module 13. Technology of dried milks

Lesson 36
TECHNOLOGY OF DRY MILKS

36.1 Introduction

Milk is unique in its content of valuable nutrients. Both the chemical composition and the physical properties of the milk powder play an important role in its use with quick and complete reconstitution adding convenience to the products. Every day, millions of liters of milk are processed into dry products. By far the largest part ends up as ordinary skim milk and whole milk powders. These products are market commodities with little requirement for product functionality. Consumer demand for specific product properties, particularly in the food industry, has resulted in the development of many different dry dairy products, ranging from instant whole milk powder to speciality food ingredients. The dairy industry invests heavily in the development and production of such products.

During the last few decades there has been a growing market for powders which are instantly soluble in cold water. Ordinary non-agglomerated powders tend to lump when mixed with water, and if strong mechanical stirring is not applied, it may result in an inhomogeneous mixture which is not attractive to the consumer. The quality requirements to instant milk powders are constantly getting stricter as an increasing number of properties must be optimized simultaneously and controlled within still narrower limits in order to obtain a high quality product with the highest degree of uniformity, fulfilling the requirements set by costumers, organizations and industrial and/or legislative standards.

36.2 Instantization

The historic development started with the pioneering research of Mr. David D. Peebles in the beginning of the fifties, and instantized non-fat dry milk was marketed from 1954. Soon it replaced the regular spray dried products on the retail market. Lecithinizing during the drying in the fluid bed is not always applied; it is meant to obtain instant properties.

The principle of instantization is the agglomeration of individual spherical milk particles into clusters and the conversion of lactose from the glass into a microcrystalline form which makes the powder more wettable and less hygroscopic. In the instantization process, the surface of milk powder particles are humidified, or the particles are only partially dried during manufacturing, so that the surface is tacky and partial crystallization of lactose leading to formation of microcrystals occurs before the particles are redried. This produces a clustering of the particles in loose spongy aggregates of low density (known as agglomerates / conglomerates / granulates ) which flow freely and disperse readily in cold water, as water penetrates the spongy structure of these aggregates and allow them to sink and disperse.

The principal purpose of agglomeration, also called instantizing, is to improve the rate and completeness of the reconstitutability of dry milk products. Small single particles dissolve instantly in water. Powder consisting of small particles is, however, difficult to disperse. Big particles are easy to disperse in water, but dissolve only partially.

Agglomeration is a result of wet and/or semi-dry particle collision with size >125 µm. Control is achieved by returning dry fine powder to the wet spray during different stages of spray drying. Mastering the agglomeration techniques is the art of modern spray drying. Plant operation and economics are other important parameters.

36.3 Products

In addition to nonfat dry milk, the instantizing process has been applied to dry whole milk and other milk fat containing dry dairy products with only limited improvement in reconstitutability resulting. The milk fat adversely affects wettability because of its hydrophobic property.

36.4 Major Systems

The process may be carried out using either of these two methods:

• Spray Drying Agglomeration

• Rewet Agglomeration

36.4.1 Spray drying aggomeration

During the spray drying process, the aim is to produce particles with a big surface/mass ratio, i.e. small particles. The reconstitution in water of a powder consisting of small particles is however difficult and requires intensive mixing in order to disperse the powder, before it is totally dissolved. Bigger particles exhibit a better dispersion, but the solubility is negatively affected during the drying operation. By agglomeration both a good dispersion and a complete solution are obtained. In spray drying there are two ways of agglomeration: the spontaneous and the forced, both in a primary and secondary form given in Table 36.1 and in Fig. 36.1 to Fig. 36.4 (Fig. 36.1: Spontaneous primary agglomeration, Fig. 36.2: Forced primary agglomeration, and .

Table 36.1 Types of agglomeration process

In Forced Secondary agglomeration the small dry particles are introduced into the dryer near the atomizing device, where they will meet and collide with atomized wet droplets thus forming agglomerates consisting of many particles stuck together having a size of 100-500 µ, depending on the parameters selected.

In Forced Secondary agglomeration the small dry particles are introduced into the dryer near the atomizing device, where they will meet and collide with atomized wet droplets thus forming agglomerates consisting of many particles stuck together having a size of 100-500 µ, depending on the parameters selected.

Due to the special air flow pattern in a Multi Stage Dryer, a considerable spontaneous, secondary agglomeration takes place. For production of high quality instant whole or skim milk powder this spontaneous agglomeration suffices, and the fines are just returned to the integrated fluid bed, from where they will get airborne again and reach the atomizing zone again. However, the agglomeration may be further enhanced by forced, primary agglomeration (collision of sprays overlapping each other from different nozzles in a multi-nozzle atomization unit) and/or by returning the fines to the atomization zone (forced, secondary agglomeration). Further flexibility can be gained by designing the atomization unit in a way that allows the distance between the single nozzles or between the nozzles and the fines return tube to be altered. Depending on the atomization device the fines return is designed in different ways as shown in Figures 36.5 to 36.7.

1. Rotary Atomization , Fig. 36.6 &

In modern dryers fines are therefore introduced from above through the air disperser via four fines pipes situated just above the atomizer cloud. Deflector plates at the end of each fines pipe ensure a correct introduction and distribution of the fines. The unit operation of the whole process of agglomeration is illustrated in Table 36.2.

Table 36.2 Unit operations during spray drying / agglomeration

2. Nozzle Atomization

The fines return is an integral part of the nozzle unit with the fines duct in the centre surrounded by nozzles at the periphery. The fines are introduced tangentially into the fines distribution duct or through a center pipe. The nozzles can be welded to the nozzle rod at a certain angle, so that by turning the nozzle rod around its axis the collision point can be altered.

3. Separation

It is the process of separating the part of fines which is entrained in the main drying air leaving the drying chamber. The efficiency of separation is determined by the air flow pattern and air velocities in the drying chamber.

4. Attrition

It is defined as the partial break down of agglomerates in fluid beds or powder conveying systems resulting in creation of either fines, smaller agglomerates (abrasion) or of a number of smaller sized particles (fragmentation). Factors affecting the extent of attrition are the jet velocity, determined by the pressure difference across the perforated plate, the fluidization velocity and the actual design of the perforated plate.

Classification

It is defined as the separation of fines in fluid beds. The efficiency of classification is mainly determined by the fluidization air velocity and also the fluid bed design.

After the final drying, the powder enters the cooling section where the powder is cooled by means of air at ambient temperature followed by cooled, dehumidified air. The powder is finally passed over a sifter where any oversize particles are removed. It is also possible to install a sifter with two nets thus removing any remaining particles/agglomerates of small diameter. Together with the fines, this fraction may be returned to the atomizing device thus producing a powder with a well defined agglomerate size distribution.

The agglomeration is improved by:

¨ High solids content in the concentrate

¨ Bigger quantity of fines returned to the atomizing device

¨ Fines introduction closer to the atomizing device

¨ Shorter distance from nozzle to fluidized layer in a static fluid bed

¨ Higher moisture content from the primary drying stage

¨ Bigger primary particles

¨ Lower pasteurization temperature of the milk prior to the evaporation

When leaving the sifter, the powder should not be exposed to strong mechanical conveying, neither by means of air nor by fast moving mechanical screws. However, today's lenient vacuum-low speed air systems are used without too much damage to the agglomerates. The best thing, however, is to install the plant so high that filling into bags or bins is possible by gravity.

36.4.2 Rewet agglomeration

Since many powders may become instant by the mere agglomeration, many processes have been developed during the past years to agglomerate ordinary powders consisting of single spray particles usually produced in plants with pneumatic conveying system.

The major systems of agglomeration are the Peebles (

Cherry-Burrell, Blaw-Knox and Niro Agglomerator (Fig. 36.9).

Each varies in equipment and operation details. The general features in common are:

• Wetting of surface of the particles with steam, atomized water, or a mixture of both.

• Agglomeration, whereby the particles collide due to the turbulence and adhere to each other forming clusters.

• Redrying with hot air.

• Cooling and sizing to eliminate the very large agglomerates and the very small particles.

1. Wetting

Wetting of the surface of the particles is done with humid air, steam, atomized water either pure or containing milk solids, sugar or other soluble components. The water may further contain additives such as vitamins (water soluble), minerals, colour and surface-active agents. The atomization of the moistening agent can be carried out by means of nozzles or a rotary atomizer. If a rotary atomizer with two feed pipes is used, it is possible to use a combination of steam and water or use two moistening agents, which cannot be mixed for various reasons. If the product is insoluble, an adhesive can be dissolved in the moistening agent. When doing so products otherwise impossible to agglomerate can be agglomerated with a good result.

2. Agglomeration

Agglomeration, whereby the moist sticky particles collide due to the turbulence and adhere to each other forming agglomerates, is essential for the rewet process. As powders with different compositions do not behave in the same way during the rewetting and agglomeration process, different equipment is needed to obtain an optimal agglomeration. In principle there are two ways of performing the agglomeration:

¨ Droplet agglomeration

¨ Surface agglomeration

Droplet Agglomeration

In the droplet agglomeration process, the powder particles are wetted with droplets of liquid atomized by means of a nozzle or a rotary atomizer while suspended in air. The powder may either be introduced around the rotary atomizer or the nozzle by means of gravity or pressure air conveying, or from below by means of pressure conveying.

The droplet agglomeration process is especially used for powders containing fat such as whole milk powder and powders with a high content of sugar such as cocoa-milk-sugar mixtures. If cold-water instant whole milk powder is produced, a lecithin dosing equipment is installed between two Vibro-Fluidizers. However, the final product quality will never be as good as that produced on a drying plant equipped with fines return, fluid beds and lecithin dosing equipment.

Surface Agglomeration

In the surface agglomeration, either steam or warm moist air with a high relative humidity is used as the moistening agent. The surface of the individual dry particles is wetted due to condensation of the water vapour on the colder particles, whereby the stickiness required for the agglomeration is created. The subsequent agglomeration will take place, if the particles are exposed to sufficient mechanical impact. The warm humid air is usually made by spraying steam into warm air at a given temperature to obtain a relative humidity of 100%.

The surface agglomeration is mainly used for skim milk powder when large agglomerates are aimed at. The final product properties depend to a great extent on the raw material used for the rewetting, and Table-36.3. below shows specification for a recommendable basis powder:

Table 36.3 Specifications of basis powder for agglomeration

The surface agglomeration can also be used for whole milk powder, but the agglomerates get too compact for obtaining a powder with good rehydrating properties.

1. Redrying

As the basis powder used for agglomeration is remoistened to obtain the desired surface characteristic for an optimal stickiness, this additional moisture has to be evaporated again in order to reach the specified moisture content. The agglomerates may break down again, if they are exposed to extensive mechanical handling, such as in a pneumatic conveying system. It is therefore necessary to perform the drying in a Vibro-Fluidizer.

2. Cooling

Like the redrying, the cooling is performed best in a Vibro-Fluidizer.

3. Sizing

Usually, there is a well defined requirement to the agglomerate size distribution of the final powder. It is therefore necessary to sift the powder. This is done in a sieve with two different net sizes placed above each other. Thus it is possible to remove any agglomerates/lumps considered to be too big in the final product. This oversize fraction may be milled and returned to the process. Powder passing through the upper net may be further fractionated on the lower net into a main fraction and a fines fraction consisting of single particles and agglomerates being too small. This fines fraction is together with the cyclone fraction from the Vibro-Fluidizer recycled back to the process.