CONDENSED AND DRIED MILKS

Module 14. Physico-chemical changes taking place during manufacture of dried milks

Lesson 38
THE MILK POWDER SYSTEM

38.1 Introduction

The milk powder manufactured undergo several changes while conversion from liquid milk to dry product. They are:

• The manufacturing process employed on milk (~87% water, aw=0.99) eliminates majority of water till it reaches the final stage of drying, the powder (~4% moisture, aw<0.6). The process not only eliminates the water from milk but also changes the physical structure i.e. the arrangement of the components of milk in space. In the process, the sol changes to a dust.

• The dried milk exhibits a well pronounced dual physical structure – the primary structure, which is the internal build-up of the powder particles from the milk solids and small amounts of mois¬ture and air and – the secondary structure which represents a typical powder, a system of closely packed solid particles in a gas.

• The structural elements of dried milk particles are – lactose, either amorphous or partly crystalline, casein micelles and whey protein particles, fat in globular or non-globular (free fat) forms and air as spherical cells. The fat, protein and air are presumably dispersed in continuous phase of amorphous lactose.

• The whole milk and infant milk powders are rich in fat (20-37%) and hence need protection against oxygen and moisture, therefore, generally tinplate cans are used and the packaging is done under nitrogen cover to replace air with nitrogen in the tins. Skim milk powders have very low fat (1.5%) and as such a good moisture barrier material like multi wall paper sacks with one polythene liner is used. Recently, some skim milk powder has also been sold on polythene bags and even high density polythene bottles are being tried for retail packs.

• The milk powders are concentrated mass of milk components in the form of particles with small amount of moisture and air. If sufficient care is not taken to protect them from high humidity, high temperature of storage and the entry of air, the deterioration processes are accelerated.

These changes while manufacture occurs in a sequence. They are discussed here:

38.2 Changes of State of the Drying Droplets

• Atomizing pure water in a drying chamber in the usual way causes the water droplets to reach the wet-bulb temperature and to vaporize within 0.1s at this temperature.

• The presence of dry matter in the droplets, however, makes an enormous difference. T he diffusion coefficient of water decrease substantially with increasing dry-matter content. Accordingly, the vaporization is significantly slowed down.

• In the drying droplet, temperature equalization occurs in less than about 10 ms. i.e. the temperature is virtually constant throughout a droplet during drying.

38.3 Drying Stages

• Initially, the droplet has a very high velocity relative to the drying air.

• Therefore, there is a first stage during which circulation of liquid in the droplet occurs; this circulation greatly enhances transfer of heat and mass.

• For a droplet of 50 µ m diameter, this stage lasts for ~2 ms. In this time, the droplet covers a distance of about 10 cm and loses a small percentage of its water.

• Its velocity compared to the air decreases to the extent that the formed surface tension gradient of the drop surface arrests internal circulation of liquid.

• But in the second drying stage, the difference in velocity between drop and air is still great enough to accelerate water transport.

• The transport in the droplet occurs by diffusion but in the air by convection.

• After about 25 ms the relative velocity of the droplet has decreased so far that the water transport has become essentially equal to that from a stationary droplet.

• Relative to the air, the droplet then has covered a distance of a few decimeters and has lost about 30% of its original water.

• In the third stage, lasting at least a few seconds, the droplet loses the rest of the water by diffusion.

38.4 Drying Process

• During drying the air temperature decreases and the humidity of the drying air increases. Moreover, the droplets vary in size and the smallest ones will dry fastest.

• In practice, the mixing is always between the two extremes. For driers with a spinning disk, the situation tends to be fairly close to perfect mixing. In most driers with nozzles, it tends to be closer to concurrent flows. In all situations, the drying time may very by a factor of, say, 100 between the smallest and the largest drops. This is of great importance for fouling of the drying chamber; the largest drops have the greatest chance of hitting the wall and of being insufficiently dry to prevent sticking to the wall.

• Another factor that affects drying rate is the presence of vacuoles in the drying drops. It leads to significantly faster drying.

38.4.1 Concentration gradients

A rapid decrease in drying rate of the droplets occurs after the water content is reduced to, say, 15%. A strong concentration gradient forms rapidly during drying. The higher the drying temperature, the stronger the effect. Therefore, stronger gradients occur for cocurrent drying. A dry outer layer, i.e., a kind of rind, is formed and because of this, the water transport is slowed down considerably. The temperature can rise significantly in the dry outer layer because a dried material assumes the air temperature, not the wet-bulb temperature. In other words, the decrease in temperature near the surface of the droplet caused by consumption of the heat of vaporization becomes far smaller because the vaporization of water is slower. Because temperature equalization happens very quickly, the whole drying droplet increases in temperature.

The concentration gradients are not of a lasting nature. The relatively dry outer layer of the droplet soon becomes firm and eventually glassy. This causes the droplet to resist further shrinkage. The droplet can react by forming vacuoles or by becoming dimpled. Especially at a low water content of the particles, so-called hair cracks may be formed.

38.4.2 Aroma retention

Besides water, the drying droplets lose other volatile components, including flavor compounds (aroma). In many cases, however, the loss of flavor compounds is far less than expected, in spite of their volatility because of the following conditions

• The effective diffusion coefficient of most flavor components in the relatively dry outer layer of the droplet decreases far stronger with decreasing water content than the diffusion coefficient of water does due to the greater molar mass.

• Thus, the aroma retention - retaining flavor compounds during drying - increases with

* Droplet size because in larger droplets, the outer layer from which the flavor components get lost has a relatively smaller volume and

* Drying temperature because, at a higher temperature a solid rind forms more rapidly.

• Formation of vacuoles diminishes aroma retention, especially if hair cracks develop in the particles and the vacuoles come into contact with the surrounding air.

• Dissolved milk powder often has a cooked flavor, which results from the flavor compounds formed during preheating and possibly during evaporation.

• During drying, conditions are mostly not such that off-flavors are induced. On the contrary, a considerable part of the volatile sulfhydryl compounds is removed. A cooked flavor mainly results from methyl ketones and lactones formed by heating of the fat and from Maillard products which are almost absent in skim milk powder.

38.4.3 Water Activity

If the water content of a product decreases, its water activity ( a _w ) also decreases. Water activity is expressed as a fraction. In pure water a _w = 1; in a system without water, a _w = 0.

Different dairy products have following values of a_w as indicated in Table 38.1

Table 38.1 Water activity values of different dairy products

Due to the very low water activity in milk powders, it exhibits following changes

• The hygroscopicity increases: Usually, a (dry) product is called hygroscopic if a small increase of a _w causes a considerable increase in water content. Obviously, this mainly concerns milk powder with very low water content.

Diffusion coefficients decrease: At low water content the effect is very strong. The diffusion coefficient of water decreases from approximately 10^-9m² s^-1 in milk to 10^-16m²s^-1 in skim milk powder with a small percentage of water.