INSTRUMENTOS Y METODOS
DISCUSIÓN DE RESULTADOS
The physical and functional properties will all contribute to the quality of the powder. The biochemical, microbiological and sensory properties of the powders are just as i mportant when considering the suitability of a powder. Flavour, composition, colour and the presence of scorched particles can all be considered as sensory properties. Scorched particles are overheated or burnt particles that h ave undergone Malliard reactions which result in browning and undesirable flavours.
M i l k powder composition can also affect the physical and functional properties of a powder. The presence of fats on the surface of agglomerates can prevent wettability durin g reconstitution and lecithin is sprayed on the surface of instant whole milk powder to overcome the surface hydrophobici ty. Milk powders with high amorphous l actose contents are also likely to have reduced tlowability due to lumping and caking duri ng storage resulting in handling problems if not dried sufficiently before storage. Storage conditions are also important for powder stability, specifically powder moisture content, water activity, storage temperature and humidity. The term 'water activity' describes the amount of water available for hydration of materials. Roos (2002) states the water activity of dairy powders must be below 0.37 to retain quality. Figure 2. 1 5 demonstrates the effect of water activity on the rate of transformation processes that can occur during storage of dairy powders . Powders can become sticky and lactose may exhibit crystal lisation which may enhance the deterioration of the dairy powders resulting in browning and oxidation. The growth of moulds, yeast and bacteria are processes which specifically need to be avoided during storage to ensure that milk powders are suitable for human consumption.
UJ I-
�
UJ >�
UJ 0::: o Structural transformations\
-Stickiness -Caking -Collapse-Lactose crysta l l isation
Diffusion-limited reactions -Non-enzymatic browning Enzyme activity Loss of lysine Critical a w 0.2 0.4 0.6 WATER ACTIVITY 0.8 1 .0
Figure 2. 1 5 : Stability map for dairy powders containing amorphous l actose (Roos, 2002).
2.7 Chapter Conclusion
In this chapter the phenomenon of agglomeration was introduced and discussed in relation to the existing literature. Agglomeration is still a relatively new area of research, and still needs to be described completely. Agglomeration i s a complex process with many interacting variables that influence product properties. Granulation is a topic which has been widely studied, and spray drying has also been well researched, but little emphasis has been placed on investigating agglomeration during spray drying.
During agglomeration, the many complex processes occurring can be simplified into a few micro-processes; droplet formation ; impact and adherence followed by drying and shrinkage. The literature has defined the conditions for adhesion following col lisions but need to be validated for forced agglomeration. The structure of the agglomerate formed will depend on the above which will infl uence the final properties of the product. Physical and functional properties such as particle size, shape, density, and reconstitution performance are intertwi ned although there is limited knowledge of how they influence each other.
Benchmarking lndustry Performance
CH APTER 3
BENC HMARKING INDUSTRY PERFORM ANCE
To study agglomeration with the ai m of i mproving bulk density control, a clear understanding of the scale of the problem is required. Thi s chapter benchmarks the performance of some Fonterra spray drying plants that produce agglomerated or 'instant' milk powders. It also outlines the currently accepted practise for instant milk powder production and the knowledge that exists in industry regardi ng agglomeration processes. There were five main objectives in this benchmarking investigation:
• Determi ne the level of downgrades where product does not meet target bulk
density and 'instant' properties.
• Identify how operators control the bulk density of instant milk powders • Identify plant configurations for instant milk powders using pressure nozzles
• Investigate the effect of operati ng parameters on agglomerate properties
• Estimate fines recyc le flow rates for one powder on one spray drier
These objectives were c arried out on several plants at one Fonterra site. The fines recycle study was performed on the low capacity drier which has only one fines recycle l ine and therefore required less powder to imitate the fines recycle mass flow rates. The findings of the benchmarking study were later used as a guide for the equipment and experimental design in the experimental part of this project.
3.1 Powder Selection and Level of Downgrades
It was essential to select industry powders to use as a reference for the experimental study. To keep the scope of thi s investigation as narrow as possible, plant operation was investigated for two instant skim milk powder (IS MP) and five i nstant whole milk powder (IWMP) using h igh pressure nozzles. Table 3 . 1 lists the downgrades due to product not meeting the target bulk density for instant whole and skim milk powders at one Fonterra site. Due to the high percentage downgrades for powders that are produced at a high rate of production an ISMP A and an IWMP E produced at one Fonterra site were selected for further study.
Table 3. 1 : Bulk density downgrades for one Fonterra site.
Bulk Density Total
Powder Production % (tonnes) (tonnes) (% ) ISMP A H H 2.9 ISMP B L L 1 .2 IWMP C L H 0. 1 IWMP D L L 0.8 IWMP E H H 1 .5 IWMP F H L 23.7 IWMP G L H 0.2