Handbook of Food Science and Technology 3

Particle size, determined by particle size analysis, determines several 
physical and functional properties (flow, density, solubility, wettability, etc.). 
Dry particle size is mainly influenced by the size of droplets during spraying.
Density 
High-density powders can reduce transport costs. The density of milk 
powders is a complex property that depends on primary factors such as the 
true or absolute density of the product, the amount of occluded air in each 
particle and the amount of interstitial air between each particle. Bulk density is 
mainly influenced by the properties of the concentrate (dry matter, 
temperature, intensity of the heat treatment, foaming capacity), the drying air 
(thermodynamic properties at the entry and exit of the facility) and the powder 
(particle size, residual moisture content). 
Hygroscopicity 
The hygroscopicity of a powder is characterized by its final moisture 
content after equilibration with air of controlled relative humidity under
- Acidity 
- Microbial properties 
- Fat content state 
•
physical 
•
chemical 
- Organopleptic properties 
- Fat content 
- Dry matter 
- Additives 
- Low solubility 
- Burned particles 
- Free fat 
- Foam formation 
- White flake 
- Particle size 
- Density
•
apparent 
•
true 
- Water content 
-
a
w
- Rehydration
properties
•
wettability 
•
dispersibility 
•
solubility 
- Flowability 
- Floodability 
- WPNI 
- Hygroscopicity 
- Cacking 
- Thermostability 

---

36 Handbook of Food Science and Technology 3 
defined temperature conditions. A powder is assumed to be non-hygroscopic if 
its hygroscopicity percentage is less than 10%. The hygroscopicity of a milk 
powder is determined by the hydrophilic nature of the components (mainly 
lactose and amorphous minerals). Reducing particle size, by increasing the 
powder surface in contact with air of controlled relative humidity, promotes 
water adsorption and the hygroscopicity of the powder.
Flowability and floodability 
The ability of a powder to flow has a significant impact on storage, 
discharge, weighing, mixing, compression, transfer and so on. Carr’s method 
[CAR 65] is used to determine two types of behavior: flowability and 
floodability. Flowability involves measuring the angle of repose, the angle of 
spatula, cohesion and compressibility. Floodability involves measuring the 
angle of fall, the angle of difference, dispersibility in the air and the value of 
the flowability index. The two main factors affecting the flowability of 
powders are particle size distribution and the state of the particle surface. 
Powders produced using spray nozzles have a higher level of flowability 
compared to powders produced using heated rollers. Two-stage drying also 
yields better flow results compared to single-stage drying. Other factors that 
improve flowability are powder agglomeration, a low level of fines, the 
addition of a flow agent (silica), the addition of hygroscopic compounds 
(carbohydrates, whey) and low levels of free fats.
1.3.3.3.
 Technological properties of milk powders 
Rehydration properties 
The ability of a milk powder to rehydrate in water is an essential property 
for industrial users of dried ingredients and can be characterized by three 
properties: wettability, dispersibility and solubility. They depend on powder 
composition and the affinity between these components and water, and the 
accessibility of water in terms of structure (porosity and capillarity) to the 
powder components.
Wettability, the ability of a powder to immerse itself once placed on the 
surface of water, reflects the capacity of powder to absorb the water on its 
surface. The swelling ability (swellability) of a powder is also linked to 
wettability. The structure of a powder disappears when the various

---

From Milk to Dairy Products 37 
components (in particular proteins) are dissolved or dispersed. Factors 
influencing wettability include: 
– the presence of large primary particles, such as agglomerated particles: 
this is a desired effect with the granulation (with or without recycling fines) of 
milk powders; 
– powder density; 
– the presence of fat on the surface of powder particles (free fats); 
– porosity and capillarity of powder particles as well as the presence of 
interstitial air. 
Dispersibility is probably the best individual criterion for assessing the 
rehydration ability of a milk powder, since to a certain extent, it is influenced 
by wettability and solubility. Dispersibility is improved by: 
– a decrease in protein content; 
– an optimal particle size of 200 µm; 
– drying at low temperatures (low heat powder). 
The insoluble materials formed during the production of milk powder are 
usually due to the denaturation of soluble proteins and the precipitation of 
calcium phosphate. Thus, solubility is particularly influenced by heat 
treatment before drying, the viscosity and biochemical composition of the 
concentrate, the drying air temperature and the particle size of the powder.
Use of recombined milk in cheese processing 
The use of recombined milk from powder is justified for several reasons: 
for economic, nutritional, dietary and geographical purposes, and also for 
sensory and technical purposes. It allows the transfer of cheese production to 
countries where milk production is insufficient, and where milk production has 
a high seasonality (in the case of goat’s or sheep’s milk). 
Milk powders used in cheese production must have a level of 
microbiological quality that is in compliance with regulations and acceptable 
for cheese making. These factors depend on the initial quality of the milk used 
and the intensity of the heat treatments during processing into powder, which 
are the source of physicochemical changes resulting in reduced coagulation 
properties when milk powder is reconstituted with water. In order to meet 

---

38 Handbook of Food Science and Technology 3 
these microbiological and technological requirements, HTST (pasteurization at 
75°C for 20 s) is recommended prior to drying in order to ensure hygienic 
quality while maintaining a high level of coagulation. These recommendations 
only apply if the milk is of good microbiological quality. Otherwise, the 
intensity of the heat treatment must be higher, therefore compromising 
coagulation properties: in this case, the milk powders obtained cannot be used 
in the production of cheese.
Cross-flow microfiltration (1.4 µm) followed by vacuum evaporation at 
low temperature and spray drying is well suited for the production of an “ultra-
low-heat” powder (low level of denaturation of soluble proteins); milk 
reconstituted from this powder, according to regulatory microbiological 
requirements, has the same level of rennet coagulation as the original raw milk 
([SCH 94], Figure
 
1.16).