Elsevier

LWT - Food Science and Technology

Volume 83, 15 September 2017, Pages 33-41
LWT - Food Science and Technology

Agglomeration during spray drying: Physical and rehydration properties of whole milk/sugar mixture powders

https://doi.org/10.1016/j.lwt.2017.05.002Get rights and content

Highlights

  • Various agglomeration steps to rehydrate sweet milk powders were studied.

  • The rehydration properties depended on the configuration of the dryer.

  • The optimal scheme consisted of steam injection in the internal fluidized bed.

Abstract

In general, milk powders are submitted to agglomeration in order to enhance their rehydration properties. This study evaluated the impact of six different methods of agglomeration of the same sweet whole milk powder on the physical and rehydration properties of the final product: non-agglomerated powder (control); agglomerated powder with fines returned to the top of the dryer, above the internal fluid bed (IFB) or above the cone; and agglomerated powder with fines returned above the IFB with a nozzle that injects steam or sprays water in the middle of the IFB, respectively. As expected, the biochemistry results showed no difference since the same concentrate was used in the whole set of experiments. The physical properties led to higher bulk densities, tapped densities and interstitial air for the control, and higher occluded air and particle size for agglomerated powders, regardless of the agglomeration process. All of these physical properties influenced the rehydratability. In fact, the wetting time and the dispersibility of sweet whole milk powder were significantly improved by the agglomeration method, with the return of fines and steam injection in the IFB. An experimental design of the agglomeration process was devised for this agglomeration approach.

Introduction

The rehydratability of a powder in water is an essential property of food powders for the consumer (King, 1966). Three main stages are usually distinguished in the rehydration process: wetting, dispersion and solubilization (Freudig et al., 1999, Písecký, 1997). These stages are characterized by three indices: the wettability index (WI), the dispersibility index (DI) and the solubility index (SI), respectively. The WI measures the ability of the powder to adsorb water on its surface, to be wetted, and to penetrate the free surface of still water (Sharma, Jana, & Chavan, 2012). The DI represents the ability of a powder to separate into individual particles when dispersed in water with gentle mixing (Sharma et al., 2012), and the SI provides an overall measurement of the ability of a powder to dissolve in water (Schuck, Dolivet, & Jeantet, 2012). In order to realistically consider the rehydratability of a powder, Schuck et al. (2012) proposed considering the ‘instant’ criterion (IC), which applies to powders that are wettable (WI < 20 s), dispersible (DI > 95%) and soluble (SI > 99%) at the same time.

Rehydratability depends on the composition and structure of a powder, especially the affinity between its components and water (Dupas-Langlet et al., 2012, Crowley et al., 2016) and the accessibility of the water to its components, and on the rehydration conditions (Jeantet, Schuck, Six, Andre, & Delaplace, 2010). Whole milk powder cannot be wetted within a reasonable time (Kim, Chen, & Pearce, 2002) and the dispersibility is not satisfactory compared to skim milk powder (Tamime, 2009, Vignolles et al., 2009) due to the hydrophobic nature of the surface of the particle incorporating free fat (Petit et al., 2017). To improve these rehydration properties, manufacturers are required to add natural surfactants such as oil lecithin or a hygroscopic ingredient such as an amorphous carbohydrate.

Apart from this approach based on modifying the composition, powder agglomeration is recognized to be one way to control and improve certain properties of a disperse system, especially with regard to rehydratability (Buffo et al., 2002, Gaiani et al., 2005). Indeed, agglomeration is known to have an impact on certain physical properties of powders such as density, granulometry, stickiness and cakiness, as well as on instant properties. The latter are mainly determined by the agglomeration process and then by the time/temperature to which the product is subjected in the system, including the processing steps prior to spray drying. The agglomeration mechanism consists of joining fines or small primary particles in order to form larger particles. Different agglomeration techniques, either in the dry or the wet state, are currently proposed, but the wet granulation process in a fluid bed is by far the most commonly used in the food industry (Cuq et al., 2013, Gaiani et al., 2007, Turchiuli et al., 2012).

The aim of this study was to determine the impact of six configurations of a semi-industrial drying pilot on the physical and rehydration properties of a given dairy formulation consisting of a sweet whole milk powder whose biochemical composition was kept constant. The efficacy of five different agglomeration processes was compared to a control non-agglomeration process.

Section snippets

Preparation of the concentrate

Concentrates with 40 g.100 g-1 total solids (TS) were recombined at Bionov (Rennes, France) from a non-agglomerated medium-heat Whole Milk Powder (WMP, Lactalis Ingredients, Bourgbarré, France) and a mix of carbohydrates composed of sucrose (Begin-say, Tereos, Lille, France) and lactose (Lactalis Ingredients, Bourgbarré, France) at a 40/60 g/g sucrose/lactose ratio. The final target of the carbohydrate content was 55 g/100 g TS. The carbohydrate solution was dissolved at 70 °C to obtain a

Concentrate

As explained above, the dry matter of the concentrate was good at 40.00 ± 0.01 g/100 g. The D[4.3] of the fat droplet size in the presence of EDTA was 0.76 ± 0.01 μm. The correlation between the free fat content and greater fat droplet diameters has already been established: to achieve low free fat content in powders, the fat droplet size in the concentrate has to be less than 1 μm before spray drying (Vignolles et al., 2009).

Biochemical composition of powders

The average biochemical composition of all the powders was

Conclusion

This study showed that different dryer configurations may have an impact on the physical properties of powders, influencing rehydration properties. In general, powder agglomeration enhanced rehydration properties. For instance, the best technological approach from a rehydratability point of view appeared to be a combination of the reintroduction of fine particles with simultaneous steam injection in the internal fluid bed, but this approach is highly dependent on the specific drying tower

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