Elsevier

Journal of Food Engineering

Volume 94, Issue 2, September 2009, Pages 163-168
Journal of Food Engineering

Surface composition of industrial spray-dried milk powders. 1. Development of surface composition during manufacture

https://doi.org/10.1016/j.jfoodeng.2008.09.021Get rights and content

Abstract

Development of the surface composition of milk powders during manufacture was investigated in three industrial spray-dried milk powders (skim milk powder, whole milk powder and instant whole milk powder). Samples were obtained from commercial production plants and were collected at different manufacturing stages. As the powder properties of milk powder are defined in spray-drying and the subsequent manufacturing processes, the powder samples were collected at the exit of the spray drying chamber and the exit(s) of the fluidized bed(s), and the surface compositions of the powder samples collected were studied using electron spectroscopy for chemical analysis (ESCA). For all three industrial spray-dried milk powders, no significant differences in surface composition were observed between the samples collected at different manufacturing stages, except for a slight increase (3%) in the surface fat coverage for whole milk powder after the fluidized bed drying process. These results indicate that the surface composition of milk powders is determined to a large extent during the spray drying process and that the subsequent fluidized bed drying and handling processes have no or little effect on the surface composition of milk powders, even though these processes affect the final powder quality (e.g. moisture content, particle size). For whole milk powder and instant whole milk powder, no fat appeared to leak out on to the powder surface during the fluidized bed drying process; however, the fat present on the powder surface after the spray-drying process appeared to flow over the particle surfaces, resulting in a slight increase in the surface fat coverage. After lecithin treatment (instantization), because a mixture of lecithin and anhydrous milk fat was sprayed on the powder, a slight increase in the thickness of the surface fat layer was observed.

Introduction

Spray-dried dairy powders are common ingredients in many food and dairy products. Many properties that are important in the storage, handling and final application (e.g. wettability, dispersibility, flowability and oxidative stability) of dairy powders are influenced by the surface composition of the powder. For example, the presence of fat on the powder surface renders the surface hydrophobic, deteriorating wettability and dispersibility (Faldt and Bergenstahl, 1996, Kim et al., 2002). Fat on the surface acts as a bridge between the particles, reducing flowability (Onwulata et al., 1996, Kim et al., 2005a), and is also readily susceptible to oxidation and the development of rancidity (Granelli et al., 1996, Hardas et al., 2000). Therefore, an understanding of the mechanism behind the formation of the surface composition of the powder and the ability to control this surface composition will be very useful in the improvement of product quality and the development of new products.

In order to understand the mechanism behind the formation of the surface composition, how the individual components of the material being dried govern the development of the surface composition needs to be understood. It has long been assumed that the components will be uniformly distributed in the dried particles, so that the powder surface composition will be the same as the composition of the solution before drying. In our earlier study (Kim et al., 2002), the surface composition of various industrial spray-dried dairy powders was investigated using electron spectroscopy for chemical analysis (ESCA or X-ray photoelectron spectroscopy (XPS)), a method that provides direct chemical analysis of the outermost surface layer (approximately 10 nm). It was found that the surface composition of powders is significantly different from their bulk composition, indicating that there is redistribution of the components during powder production. Particularly pronounced is the accumulation of fat on the powder surface compared with the fat content of the bulk powder. The fat appears to be over-represented on the powder surface even in powders with very low fat contents. As the fat content of the powder is increased, there is a sharp increase in the surface fat coverage and the fat seems to completely cover the whole powder particles. For example, the surfaces of whole milk powder and cream powder, which contain 29% and 75% fat, respectively, are almost totally covered by milk fat (98% and 99% respectively). The next dominating component on the surface of powders is found to be protein, and then lactose.

The milk powder manufacturing process involves a series of continuous or semi-continuous steps, as presented schematically in Fig. 1. The drying process used in the dairy industry, in which the concentrated milk is converted to a powder form and the powder properties are defined, is usually in two or three stages, comprising spray drying and fluidized bed(s) drying. In two-stage drying, the milk concentrate is pumped through the spray nozzle at a very high pressure or through a rotary atomizer, which spins at a very high speed, to form fine droplets in the hot air stream at 180–220 °C in the drying chamber. Much of the remaining water is evaporated in the drying chamber, leaving a powder of around 6% moisture content. Following the spray drying, the powder particles fall into an external vibrating fluidized bed, which consists of one or two vibrating sections. In the vibrating fluidized bed, hot air at 80–120 °C is blown through a layer of fluidized powder and the powder is further dried to give a product of 2–4% moisture content. At the end of this section of the system, air at ambient conditions is supplied for faster cooling of the particles. This two-stage drying method may also improve the quality of the powder by powder agglomeration. In three-stage drying, a static fluidized bed, in addition to the vibrating fluidized bed, is introduced into the conical base of the spray drier to better control particle agglomeration and drying. During these drying processes, small (fine) powder particles leaving the drier are recovered in cyclones and are returned to the spray drying chamber in close proximity to the atomizer. The wet concentrate droplets collide with the fines and stick together, forming larger, irregular shaped “agglomerates”. After drying and cooling in the vibrating fluidized bed(s), the powder is then conveyed to packaging operations and is stored until its end use (Pearce, Singh and Newstead, 1992, Cruz et al., 2005).

The surface composition of milk powders not only is determined by the gross composition of the milk concentrate being dried, as discussed earlier, but also is expected to be strongly influenced by the manufacturing operations and the specific processing conditions employed, as are other milk powder properties (e.g. powder moisture content, particle size distribution, particle morphology) (Fig. 2). Powder storage may also affect the surface composition of the powder. To obtain a comprehensive understanding of the mechanism behind the formation of the surface composition, how the surface composition of milk powder develops during the manufacturing process and how the processing conditions and storage affect the surface composition need to be understood. A number of studies to explore these effects were undertaken and will be reported in a series of papers. In this first paper, the effects of the manufacturing processes on the development of the surface composition of milk powders are investigated. The second paper (Kim et al., 2008a) will deal with the evaluation of the effects of various spray-drying conditions on the surface composition of the powders. The third paper (Kim et al., 2008b) will report how the surface composition of the powders changes during “practical” long-term storage.

The objective of the present work was to investigate how the manufacturing processes affect the development of the surface composition of milk powders. Samples of three industrial spray-dried milk powders (skim milk powder (SMP), whole milk powder (WMP) and instant whole milk powder (IWMP)) were collected at various points in the manufacturing process and were analyzed for surface composition. SMP was chosen to represent non-fat milk powders, whereas WMP was chosen to represent fat-containing milk powders. IWMP was chosen to investigate the effect of the instantization process (lecithin treatment) on the surface composition of powders.

Section snippets

Materials

Three industrial spray-dried milk powders (SMP, WMP and IWMP) were sampled at various production stages from commercial dairy plants in New Zealand. As the powder properties are defined in spray drying and the subsequent manufacturing processes, the powder samples were collected at the exit of the spray drying chamber and the exit(s) of the fluidized bed(s). The details of the production plants and the positions from which the powder samples were collected are as follows:

  • Spray-dried SMP was

Skim milk powder

In order to study the effects of the manufacturing processes on the development of the surface composition of SMP, powder samples were collected after different drying stages in the production process, and the surface compositions of the powder samples were analyzed using ESCA (Fig. 3). The results showed that the surface compositions of the SMP samples collected after the first and second vibrating fluidized beds were very similar to the surface composition of the powder sample collected

Conclusions

In this work, the development of the surface composition of milk powders during manufacture was investigated in three industrial spray-dried milk powders (SMP, WMP and IWMP). It was found that the surface composition of milk powders is fixed to a large extent during the spray drying process and that the subsequent fluidized bed drying and handling processes have no or little effect on the surface composition of milk powders, even though these processes affect the final powder quality (e.g.

Acknowledgement

The first two authors thank the Fonterra Research Centre for a Ph.D scholarship, which facilitated a research programme in the area.

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