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Use of a Turbidity Sensor to Characterize Micellar Casein Powder Rehydration: Influence of Some Technological Effects

¹ Laboratoire de Science et Génie Alimentaires, 2 Avenue de la Forêt de Haye, B.P. 172, 54505 Vandoeuvre Les Nancy Cedex, France
² INRA, UMR Sciences et Technologies du lait et de l’uf, 65, Rue de Saint-Brieuc, 35042 Rennes, France

Corresponding author: Sylvie Banon; e-mail: Sylvie.Banon{at}ensaia.inpl-nancy.fr.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
A simplified method to study rehydration of dairy powders was developed for native phosphocaseinate powder. The method involved dispersing powder in a stirred vessel equipped with a turbidity sensor under standardized conditions. The changes of turbidity occurring during powder rehydration highlighted several stages. These stages include particles wetting, and then swelling as the water penetrates into the powder bed, followed by a slow dispersion of the particles. With this tool, some technological effects on powder rehydration were analyzed. Ultrafiltrate incorporation to the casein concentrate before spray drying was found to greatly improve the rehydration, whereas mixing ultrafiltrate powder with casein powder after spray drying did not change the rehydration properties. The effect of granulation on powder rehydration stages was also investigated.

Key Words: turbidity • native phosphocaseinate • rehydration • drying

Abbreviation key: NPC = native phosphocaseinate

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Rehydration is an essential quality attribute of a dairy powder because most powders are dissolved before use (King, 1966). Some common problems are associated with different stages of the rehydration process: wettability, which is the ability to absorb water; sinkability, which is the ability to sink into the water; dispersibility, which is the ability to disperse in single particles throughout the water; and solubility, which is the ability to dissolve in water (Freudig et al., 1999).

The use of a turbidity sensor to continuously follow a physicochemical process is common especially in crystallography (Crawley et al., 1996; Herri et al., 1999; Moscosa-Santillan et al., 2000) and in environmental studies (Huang and Cheng, 1996; Spicer et al., 1998). In dairy research, turbidity sensors are also used for monitoring enzymatic milk clotting and acidification (Hardy and Fanni, 1981; Hardy and Scher, 1986; Banon and Hardy, 1991). Except for the report of de Wit and Klarenbeek (1986), the use of a turbidity sensor to study rehydration stages of a dairy powder has not been reported. However, these authors did not take into account the stirring effect or the different stages occurring during rehydration.

In the present work, we used native phosphocaseinate (NPC), which is obtained by tangential membrane microfiltration of milk followed by spray drying of the diafiltrated retentate (Schuck et al., 1994). This powder is an attractive material due to its high protein content and can be used as a relevant model of milk micelles (Famelart et al., 1999). However, the low water transfer during NPC rehydration makes rehydration a time consuming step. By improving its rehydration properties, NPC would then become an attractive material for the dairy and food industry.

Rehydration properties of dairy powders have been investigated often (King, 1966; Baldwin and Sanderson, 1973; Schubert, 1993) but the rehydration process of NPC is less studied (Davenel et al., 1997; Schuck et al., 2002). The objectives of the present study were (i) to validate a turbidity sensor under standardized conditions to continuously follow NPC rehydration, (ii) to use the turbidity profiles obtained to study its rehydration, and (iii) to point out some technological factors (ultrafiltrate incorporation mode and granulation) improving its rehydration.

	MATERIALS AND METHODS

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NPC and Ultrafiltrate Preparation
The concentrates were supplied by Unité Mixte de Recherche: Sciences et Technologies du lait et de l’uf (INRA Rennes, France). Native phosphocaseinate was separated from skimmed milk by tangential membrane microfiltration followed by purification through water diafiltration according to Pierre et al. (1992) and Schuck et al. (1994). Ultrafiltrate was obtained by membrane tangential UF of microfiltrate collected during NPC production.

Vacuum Evaporation
Ultrafiltrate was concentrated at Bionov (Rennes, France) in a 2-stage pilot plant falling film vacuum evaporator (GEA, Niro Atomizer, St-Quentin en Yvelines, France) at 530 g/kg of total solid. Evaporation capacity was close to 300 kg/h. The temperature of the first effect was 75 ± 1°C and the outlet temperature of the concentrate was 50 ± 1°C.

Powder Preparation
Ultrafiltrate was added to NPC according to 2 methods: co-drying and dry-mixing (Figure 1). The co-drying powder was produced by adding the UF solution to NPC concentrate before spray drying. The dry-mixing powder was produced by mixing UF powder with NPC powder after spray drying.

View larger version (16K): [in this window] [in a new window]   Figure 1. Diagrams of the 2 incorporation modes for co-drying (A) and dry-mixing (B) powders. For each method, 2 types of powders were prepared: granulate (G) and nongranulate (NG) powder. CD = Co-drying, DM = dry-mixing, NPC = native phosphocaseinate, UF = ultrafiltrate.

Chemical Analysis
The chemical composition of the powders is reported in Table 1. The amount of total solids was determined by weight loss after drying 1-g samples of powder at 105°C for 5 h. Total protein was determined by Kjeldahl method with a 6.38 conversion factor. Lactose was determined by an enzymatic method using a Enzytec lactose/D-galactose kit (SCIL Diagnostics GmbH, Martinsried, Germany) and fat according to the Röse-Gottlieb method by the FIL 9C:1997 (FIL–IDF, 1997). Ash were measured after incineration at 550°C during 5 h.

Turbidity changes accompanying powder rehydration were followed using a turbidity meter (Analite NEP 160, McVan Instruments, Mulgrave, Australia). This apparatus used light in the near-infrared region (860 nm); the incident beam is reflected back at 180° by any particle in suspension in the fluid to a sensitive electronic receptor. The turbidity meter was connected to a measurement system for continuous monitoring (Almemo 8990-8 V₅, Ahlborn, Holzkirchen, Germany). This data logger was coupled with a PC equipped with software (AMR WinControl for Almemo). Data were collected automatically every second for 1000 s then every 5 s. All runs were carried out at least in triplicate.

Samples and rehydration media.
Rehydration of NPC was studied in 2 media: distilled water and UF solution. Ultrafiltrate solution was prepared from ultra-filtrate powder dissolved in distilled water 24 h before used. Lactose concentration was calculated to be similar in the UF solution and in the NPC+UF solution. In the experiments, the concentrations of powder were chosen such that total protein concentration was constant at 5% (wt/vol) for the 6 powders studied. The powder was poured into the rehydration media in less than 3 s, 30 s after starting the monitoring.

Static Light Scattering
Particle size distributions were measured from a laser light diffusion apparatus (Mastersizer S, Malvern Instruments Ltd., Malvern, UK) with a 5-mW He-Ne laser operating at a wavelength of 632.8 nm with a 300RF lens.

The dry particle size distribution was determined using a dry powder feeder attachment and the standard optical model presentation for particles dispersed in air was used.

From the stirred vessel, 0.5 mL of casein suspension was introduced into 100 mL of prefiltered distilled water (Millipore, membrane diameter 0.22 µm) to reach a correct obscuration. The Malvern small volume sample cell used allowed the maintenance of a stable suspension during the measurement under stirring at 2000 rpm. The refractive indices used were 1.57 for casein and 1.33 for water (Strawbridge et al., 1995). The results obtained are average diameters calculated from Mie theory. The criterion selected was d(50), which means that 50% of the particles have a diameter lower than this criterion. Results are the average of 3 replicate experiments carried out on different days.

Statistical Analyses
Statistical analysis was carried out by using the software KyPlot freeware, version 2.0. For comparisons between NPC powder rehydration in water and other re-hydrations (i.e., co-drying powder in water, dry-mixing powder in water, and NPC powder in ultrafiltrate), a parametric multiple test (Dunnett test with NPC powder rehydration in water as control) was performed. The significance levels were ***P < 0.001, **P < 0.01, *P < 0.05, and ^NSP > 0.05.

	RESULTS

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Typical Turbidity Profile During NPC Rehydration
Typical profile.
Rehydration of 5% granulate NPC powder at 24°C occurred in different stages as shown in Figure 2. Dispersion of powder in the stirred vessel led to a quick increase of turbidity (peak A). Peak A was followed by a decrease of turbidity to a minimum (B) after 250 s. Subsequent to this minimum, an increase in turbidity occurred (phase C) from 250 to 45,000 s. After 45,000 s, the turbidity values stabilized around 13,000 NTU (nephelometric turbidity unit) in phase (D) when a fluid similar to milk is formed.

View larger version (15K): [in this window] [in a new window]   Figure 2. Turbidity typical profile as a function of time during rehydration of 5% granulate native phosphocaseinate at 24°C for 50,000 s. Inset shows the first 2500 s. A = Peak of turbidity corresponding to powder wetting; B = minimum of turbidity corresponding to powder swelling; C = turbidity increased corresponding to powder disintegration and dispersion stage; and D = stabilization of the turbidity corresponding to the end of rehydration.

View larger version (14K): [in this window] [in a new window]   Figure 3. Turbidity profile as a function of time during rehydration of granulate native phosphocaseinate () and nongranulate native phosphocaseinate () powder at 24°C for 300 s. The wetting time (Tw) is defined as the time to reach the turbidity peak and/or shoulder.

As a consequence of the swelling, a disintegration of the wet particles and their progressive dispersion could explain the d(50) decrease and consequently the turbidity increase. Phase C was related then to the powder dispersion.

The time for the powder to fully rehydrate was determined by the time needed to obtain turbidity stabilization (phase D). Indeed, the turbidity stabilization time around 45,000 s obtained with the turbidity profile is related to the particle size stabilization time obtained by static light scattering.

Effect of Added Ultrafiltrate on the Rehydration Process
No ultrafiltrate added: NPC powder.
The rehydration times of granulate and nongranulate NPC powders are summarized and given as a reference in Tables 3 and 4. The wetting and swelling times were both lengthened for nongranulate NPC powder, whereas the time to rehydrate was shortened. The time to rehydrate was around 45,051 s for granulate and 36,138 s for nongranulate powder.

For nongranulate powder (Table 4), no differences (P > 0.05) were noticed for the wetting time; the time to rehydrate the powder was reduced (P < 0.001) from 36,138 to 1710 s. No swelling time (minimum of turbidity) was observed. The minimum of turbidity may be masked by the quick rehydration of the powder, the swelling stage being mixed up with the wetting stage. To slow down reactions, the rehydration stages of the powder were repeated at lower temperature (10°C; results not shown). All stages were lengthened and the minimum of turbidity was shown to correspond to particle swelling, confirming our hypothesis.

Ultrafiltrate incorporation by dry-mixing: NPC powder + UF powder.
Ultrafiltrate powder incorporation after spray-drying did not improve the rehydration times. For granulate powder (Table 3), no differences (P > 0.05) were noticed for the wetting, swelling, and time to rehydrate. For nongranulate powders (Table 4), the swelling time was lengthened (P < 0.01) from 1045 to 1702 s. The wetting time and the time to rehydrate were not modified (P > 0.05).

Rehydration of NPC powder in ultrafiltrate media.
Rehydration of NPC in ultrafiltrate media instead of water did not change the rehydration times. For granulate powder (Table 3), the times obtained were not different (P > 0.05) from those in water. For nongranulate powders (Table 4), the swelling time was lengthened from 1045 to 1838 s. The wetting time and the time to rehydrate were not modified (P > 0.05).

Rehydration Times Obtained by Comparison with a Standard
Standards for the determination of powder wettability and dispersibility are commonly used, but these are often empirical and difficult to perform (American Dry Product Institute, 2002, and Fédération Internationale Laitière, 1985). The FIL determination of wettability consists of measuring the time necessary for all particles to absorb water in an unstirred vessel. As shown in Table 5, the wetting times obtained in our experiment correlated well with the standard wetting times. The experimental values found were always shorter than the standard values due to the stirring effect. Moreover, it is often impossible with the standard method to determine the wettability for poor-wettable powder such as nongranulate powder (time > 10,000 s) in this study.

View larger version (15K): [in this window] [in a new window]   Figure 4. Relation between the dispersibility (ADPI standard) and the time to rehydrate the powder obtained in our experiment (R2 > 0.97). G = Granulate, NG = nongranulate, NPC = native phosphocaseinate, UF = ultrafiltrate.

	DISCUSSION

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NPC Rehydration
From comparison of static light scattering and turbidity measurements, it appears that NPC rehydration occurred in different stages: wetting and swelling followed by very slow dispersion of the particles to finally reach a fluid similar to milk. By viscosimetry measurements, Gaiani et al. (C. Gaiani, S. Banon, J. Scher, P. Schuck, and J. Hardy, unpublished data, 2005) noted also the wetting, swelling, and dispersion stages during NPC rehydration. By a nuclear magnetic resonance method, Davenel et al. (1997) observed 2 stages: water absorption by powder, and solubilization of particles (i.e., swelling and dispersion stages). They found an instantaneous increase of the particle hydration up to 4 g of water/g of powder in less than 3 min after powder pouring; our results were similar as the maximum of swelling was observed after 4 min. The full rehydration process clearly takes time to achieve a stable dispersion. Depending on granulation, the time to rehydrate NPC powder was between 36,000 and 45,000 s. In the study by Povey et al. (1999), casein dissolution kinetics was monitored for hours and days before complete rehydration using an ultrasound apparatus.

Effect of Ultrafiltrate Addition on Powder Rehydration
For co-drying powders of NPC and ultrafiltrate, the time to rehydrate was the most influenced parameter during rehydration, 14 and 20 times less for granulate and nongranulate powders, respectively. Turbidity stabilization was rapidly reached, demonstrating that the presence of ultrafiltrate favored a short rehydration. By the hygroscopic nature of UF, addition of ultrafiltrate concentrate before spray drying greatly improved water transfer in dried particles as observed by Schuck et al. (2002) when adding NaCl to NPC by co-drying.

Comparison of dry-mixing powders with NPC showed that the wetting and swelling time as well as the time to rehydrate were not improved. For NPC rehydration in ultrafiltrate media, similar times were obtained. As the global composition of dry-mixing NPC + UF powders in water and NPC powders in UF was the same, we hypothesized that during rehydration of dry-mixing powders, UF powder quickly rehydrated first followed by the slow rehydration of NPC powder. Rehydration of NPC in ultrafiltrate media is similar to rehydration of (NPC + UF) dry-mixing powder in water. In previous studies, Davenel et al. (1997) did not find any improvement of NPC rehydration if the incorporation of additives occurred after spray drying. On the other hand, Schuck et al. (2002) established a minor improvement of the rehydration time by dry-mixing mineral salts and NPC.

Granulation Effect
The wettability time was systematically better for granulate particles. Indeed, fast wetting is favored with large particles forming large pores, high porosity, and small contact angle between powder surface and the penetrating water. Freudig et al. (1999) found an optimal size for wettability of around 400 µm, close to the size of the granulate powders used in this study. The wetting stage is often described as a rate-controlling step (Schubert, 1993; Freudig et al., 1999). Baldwin and Sanderson (1973) demonstrated, for whole milk powders, that the most significant improvement in rehydration properties was obtained by improving the wetting properties. On the contrary, in our powders, the dispersion stage seemed to be the rate-controlling step. Indeed, even with a shorter wetting time, a granulate powder was slower to rehydrate than a nongranulate powder.

The time for the powder to fully rehydrate was largely influenced by the granulation, as granulation was shown to systematically slow down the rehydration process. Granulate particles penetrated the water quickly but then dispersed slowly, whereas nongranulate particles presented poor wettability (powder floated on the water surface) but then dispersed more quickly.

	CONCLUSIONS

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The use of a stir vessel equipped with a turbidity sensor to continuously follow NPC rehydration was validated in this study. The interpretation of the turbidity profiles coupled with static light scattering seems to be a promising tool to appreciate casein powder rehydration and parameters like wetting time, swelling time, and rehydration time.

The experiments reported showed clearly that ultra-filtrate incorporation to NPC concentrate before spray drying improved the rehydration time and that the incorporation mode and the granulation were of importance. There is an ongoing need to develop innovative dairy-based ingredients including native phosphocaseinate. By improving its rehydration properties, NPC could become an attractive material for the food industry. Co-drying of ultrafiltrate solution and NPC concentrate could be one way to extend its application. Further studies applying this approach to other dairy powders are in progress.

	ACKNOWLEDGEMENTS

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The authors are indebted to ARILAIT RECHERCHES for stimulating discussions and financial support.

Received for publication February 21, 2005. Accepted for publication April 4, 2005.

	REFERENCES

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