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J. Dairy Sci. 87:1158-1166
© American Dairy Science Association, 2004.

Lactose/ß-Lactoglobulin Interaction During Storage of Model Whey Powders

M. E. C. Thomas, J. Scher and S. Desobry

Laboratoire de Physico-chimie et Génie Alimentaires, ENSAIA-INPL, 54500 Vandoeuvre-Lès-Nancy, France

Corresponding author: M. E. C. Thomas; e-mail: marie.thomas{at}ensaia.inpl-nancy.fr.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 GENERAL DISCUSSION
 REFERENCES
 
The objective of this study was to evaluate the presence or absence of interaction between lactose and ß-lactoglobulin during storage of model whey powders at different water activities (aw). Model whey powders were prepared by colyophilization of lactose with increasing quantities of ß-lactoglobulin. These colyophilized ß-lactoglobulin:lactose powders, assigned as BL powders, were stored from 0.11 to 0.95 aw. The water sorption behavior of BL powders was studied gravimetrically, and the state of lactose was investigated using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Before storage, BL powders were amorphous. After storage, a loss of water was observed on moisture sorption isotherms of BL powders. It was related to the formation of lactose crystals, detected by DSC and SEM analysis, and to the structural collapse of the powders. Water loss due to lactose crystallization was shifted to higher aw with increasing ß-lactoglobulin content in BL powders. Moreover, kinetics of moisture sorption demonstrated that ß-lactoglobulin was also responsible for a slower crystallization process in BL powders. Then, the water sorption behavior of BL powders was very different from the behavior of the 2 compounds mixed after separate lyophilization. All these results pointed out interaction between lactose and ß-lactoglobulin, which appeared during lyophilization and still occurred during storage. This lactose/ß-lactoglobulin interaction stabilized model whey powders against lactose crystallization.

Key Words: ß-lactoglobulin • storage • lactose crystallization • co-lyophilization

Abbreviation key: aw = water activity, BL powders = colyophilized ß-LG:lactose powders, B + L powders = powders composed of lactose and ß-LG lyophilized separately and mixed, BL(10:90) = colyophilized ß-LG:lactose powder with 10% ß-LG and 90% lactose, BL(40:60) = colyophilized ß-lactoglobulin:lactose powder with 40% ß-lactoglobulin and 60% lactose, DSC = differential scanning calorimetry, SEM = scanning electron microscopy


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 GENERAL DISCUSSION
 REFERENCES
 
The production of whey and whey-derived powders is very important worldwide. In 2002, more than 1.6 million tonne of spray-dried whey were produced. Whey powders have interesting functional properties and are used as ingredients in the food industry (Huffman, 1996). Although their compositions become very specific, lactose and/or whey proteins (mainly ß-lactoglobulin) are the principal compounds (Table 1Go).


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  		Table 1. Composition of powders derived from cheese whey and used as food ingredients.
 
The changes of lactose state particularly influence the shelf life of whey powders (Vuataz, 2002). In whey powders produced by spray drying or freeze-drying, lactose is in a meta-stable glassy state, very sensitive to changes in temperature and water content. During the storage of whey powders in severe conditions, the glass transition zone of lactose is passed. Viscosity strongly decreases, whereas molecular mobility highly increases (Champion et al., 2000; Mazzobre et al., 2001). This change in molecular mobility accelerates physico-chemical alterations. Lactose crystallization occurs and favors other damages, like particles caking, Maillard reaction, lipid oxidation, and so on (Burin et al., 2000; Özkan et al., 2003; Thomas et al., 2003). The preservation of the amorphous phase of lactose or the control of the crystallization process is thus very important for the dairy industry.

Lactose crystallization in milk powders differs from the crystallization of pure lactose, probably because of milk proteins (Jouppila and Roos, 1994; Knudsen et al., 2002). Lactose crystallization can also be inhibited in the presence of other proteins such as catalase (Forbes et al., 1998). According to Costantino et al. (1998), the development of interaction in the solid state between proteins and amorphous sugars could be responsible for the alteration of the crystallization. The fact that lactose is an efficient cryo-protectant for proteins also suggests that direct interaction, especially hydrogen bonds, can form in the solid state between lactose and the polar groups of proteins (Arakawa et al., 2001). Then, solid-state interaction may develop between lactose and milk proteins in whey powders and influence lactose crystallization.

In the present work, model whey powders containing lactose and ß-lactoglobulin were prepared by colyophilization. The behavior of moisture sorption of the model powders was studied, and the state of lactose was determined according to the water activity of storage. The objective of our investigation was to determine the influence of ß-LG on lactose crystallization and to evaluate the presence or absence of interaction between lactose and ß-LG during the storage of model whey powders at different water activities.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 GENERAL DISCUSSION
 REFERENCES
 
Preparation of the Model Whey Powders
Amorphous systems were obtained by freeze-drying solutions of lactose, ß-LG, and the 2 compounds at different ratios. Solutions of 10% (wt/vol) alpha-monohydrate lactose (Prolabo, France) and ß-LG (Davisco, Eden Prairie, MN) were prepared in distilled water. The solutions were stirred until complete dissolution. The pH of lactose solution was 4.07, and the pH of all solutions containing ß-LG was between 7.30 and 7.56. The solutions were mixed to obtain ß-lactoglobulin:lactose ratios of (10:90), (20:80), (30:70), and (40:60). Solution samples of 75 mL were immediately dispensed into 500-mL round-bottomed flasks and quickly frozen at –30°C in a refrigerating bath. Frozen samples were stored at –30°C at least 12 h and then freeze-dried 48 h using a Lyovac GT3, Leybold-Heraeus, Orsay, France). The powders were stored in vacuum dessicators over P2O5 at 20°C for 1 wk to complete the dehydration. The powders composed of the 2 compounds were assigned a colyophilized ß-LG:lactose powder with 10% ß-LG and 90% lactose (BL[10:90]), BL(20:80), BL(30:70), and colyophilized ß-LG:lactose powder with 40% ß-LG and 60% lactose (BL[40:60]).

Storage of the Model Whey Powders and Moisture Sorption Isotherms
Samples were stored for 3 mo over the saturated salt solutions described in Table 2Go. The solutions corresponded to water activity (aw) ranging from 0.11 to 0.95.


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  		Table 2. Saturated salts solutions used for moisture sorption isotherms, and corresponding water activity (aw). (Data from Greenspan, 1977)
 
Approximately 0.5 g of powder samples was placed at each aw and stored at 20 ± 1°C. Two replicates were done. The moisture sorption was monitored gravimetrically. Moisture sorption isotherms were drawn up from the percentage of water adsorbed at each water activity (g of water/100 g of dry solid) when equilibrium was reached.

Differential Scanning Calorimetry (DSC)
The state of lactose in model whey powders was determined using a DSC analysis system (Pyris 1 and Pyris manager 2.0 software, Perkin Elmer). The instrument was calibrated using indium. Analysis involved 50-µL open aluminum pans (Perkin Elmer) containing powder samples of 3 to 5 mg. An empty pan was used as a reference. Two replicates of each sample were analyzed. Each sample was heated at a temperature rate of 10°C/min in a temperature range from 25 to 250°C.

Scanning Electron Microscopy (SEM)
Samples of the moisture sorption equilibrium were used for SEM investigation. Thin layers of powder samples were mounted on aluminum stubs, using double-sided adhesive tape. The mounted samples were first sputter-coated with carbon and then with gold/palladium. The coated samples were examined with a Hitachi S2500 scanning electron microscope operating at 17 kV accelerating voltage.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
GENERAL DISCUSSION
 REFERENCES
 
Moisture Sorption Isotherms
The moisture sorption isotherms of lactose, ß-LG, and model whey powders (BL powders) with the 2 compounds are shown in Figure 1Go. The average standard deviation was 0.37% and corresponded to the size of the symbols used in Figure 1Go. The moisture sorption isotherms of all the powders were thus clearly different.



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  		Figure 1. Moisture sorption isotherms of all lyophilized powders after 12-wk storage. The 3 arrows on the graph indicate a sharp water loss, respectively, from lactose above 0.39 water activity (aw); from BL(10:90), BL(20:80), and BL(30:70) above 0.44 aw; and from BL(40:60) above 0.55 aw.

 
ß-Lactoglobulin had a sigmoid moisture sorption isotherm, which is typical of food proteins. ß-Lactoglobulin was very hygroscopic and adsorbed more water than the other powders studied. In BL powders above 0.70 aw, the higher the ß-LG content, the more the powder adsorbed water. For instance, lactose, BL(10:90), BL(20:80), BL(30:70), BL(40:60), and ß-LG stored at 0.85 aw adsorbed 4.2, 6.1, 6.7, 12.0, 13.5, and 26.2% water, respectively.

The isotherms of all powders containing lactose were irregular between 0.39 and 0.55 aw: they released water and their sorption isotherms thus showed a "break." Lactose powder released water at 0.39 aw. The water release occurred at higher aw with increasing ß-LG content in BL powders. The BL powders with 10 to 30% ß-LG released water at 0.43 aw. The sorption isotherms of BL(40:60) with 40% ß-LG only showed a "break" at 0.55 aw.

According to the literature, the release of water by powders containing lactose is due to lactose crystallization (Berlin et al., 1968; Jouppila and Roos, 1994). Water is released when the hygroscopic amorphous lactose crystallizes into its less hygroscopic crystalline forms. The "break" in the sorption curves of BL powders between 0.39 and 0.55 aw could thus correspond to lactose crystallization. However, sorption isotherms alone were not sufficient to assert the amorphous or crystalline state of lactose. ß-Lactoglobulin, which is strongly hygroscopic, could adsorb the water released at the beginning of the crystallization and thus conceal the phenomenon. Then, complementary methods were used to determine the amorphous or crystalline state of lactose at these critical water activities.

Determination of the State of Lactose in Model Whey Powders
Visual observations.
After 3 mo of storage, the particles of lactose powders were agglomerated, and a collapse of their structure was clearly observed at 0.43 aw and above. Similar alterations were observed from 0.55 aw in powders containing 10 to 30% ß-LG and from 0.59 aw in BL(40:60). Since collapse and crystallization of lactose occur simultaneously in milk powders, the visual observations of the samples were consistent with the results from sorption isotherms (Newell et al., 2001).

Differential scanning calorimetry.
The characterization of lactose state in the lyophilized powders before and after storage was undertaken using DSC (Figures 2Go, 3Go, and 4Go).



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  		Figure 2. Differential scanning calorimetry profiles of lyophilized lactose samples before and after a 3-mo storage at water activities between 0.11 and 0.95 at 20°C.

 


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  		Figure 3. Differential scanning calorimetry profiles of BL(10:90) after a 3-mo storage at water activities between 0.11 and 0.95 at 20°C.

 


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  		Figure 4. Differential scanning calorimetry profiles of BL(40:60) after a 3-mo storage at water activities between 0.11 and 0.95 at 20°C.

 
The DSC curves of lyophilized lactose before storage showed an exothermic peak around 175°C and an endothermic peak at 218°C (Figure 2Go). These peaks corresponded to the recrystallization of amorphous lactose during analysis and to melting of crystalline {alpha}-lactose produced, respectively. These DSC profiles were in agreement with those obtained by Corrigan et al. (2002) for spray-dried amorphous lactose. Before storage, lyophilized lactose samples were thus amorphous.

The DSC curves obtained after a 3-mo storage at aw between 0.11 and 0.95 are reported in Figure 2Go. From 0.11 to 0.33 aw, exothermic peaks of lactose crystallization were observed at 175°C, showing that lactose was amorphous.

The DSC trace of lactose sample stored at 0.39 aw presented a {alpha}-lactose melting peak (222°C) and an endothermic one at 84°C, with irregular shape. According to Darcy and Buckton (1997), this peak could correspond to the water released from a collapsed structure of lactose.

In the DSC profile at 0.43 aw, a small shoulder at 150°C indicated the presence of {alpha}-lactose monohydrate, and a melting peak of anhydrous ß-lactose was shown at 242°C (Figure 2Go). Lactose samples stored at 0.43 aw had thus crystallized to form mainly anhydrous ß-lactose and {alpha}-lactose monohydrate. This is in agreement with Buckton et al. (1998) who demonstrated that samples that began to crystallize at 44% relative humidity very clearly exhibited the characteristic peak of ß-lactose (DSC and infrared spectral shape). From 0.55 to 0.95 aw, lactose samples contained {alpha}-lactose monohydrate, since all DSC curves showed an endothermic peak at 150°C. Moreover, the melting peaks of crystalline lactose were irregular with temperatures between 220 and 240°C. This could indicate that different {alpha}-lactose and ß-lactose crystalline forms were produced in addition to {alpha}-lactose monohydrate. Although mutarotation could occur during DSC analysis and modify the final proportion of lactose crystalline forms (Angberg, 1995), these results were consistent with those of Darcy and Buckton (1997), Buckton et al. (1998), and Corrigan et al. (2002). The authors demonstrated that at aw above 0.55, lactose crystallized in mostly crystalline anhydrous {alpha}- and ß-lactose, and subsequently mutarotated in the solid state to crystalline {alpha}-lactose monohydrate during storage.

Then, lactose samples stored above 0.43 aw were crystallized. This was consistent with the moisture sorption isotherm of lactose (Figure 1Go). The loss of water at aw above 0.39 corresponded to lactose crystallization. The lactose samples stored at 0.39 aw were thought to be amorphous. The DSC analysis showed that a very early step of alteration already occurred, since lactose started to collapse. According to state diagrams of lactose and results of the literature, the collapse temperature of lactose is known to be 10 to 15°C above glass transition temperature (Tg), whereas the crystallization temperature is sometimes 50°C higher than Tg (Jouppila and Roos, 1994; Champion et al., 2000). As a consequence, the water activity of collapse is lower than the aw of lactose crystallization. Collapse occurs prior to crystallization (Newell et al., 2001). Corrigan et al. (2002) noticed that a lactose-collapsed structure could be stable more than 1 yr, since collapsed amorphous lactose desorbed water very slowly (Darcy and Buckton, 1997). This explains why the moisture sorption isotherm of lactose was stable.

The DSC curves for BL(10:90) and BL(20:80) powders stored from 0.11 to 0.43 aw presented a small exothermic peak at 180°C, which meant that all samples contained amorphous lactose. From 0.55 to 0.95 aw, DSC traces showed a clear endothermic peak at 140 to 150°C, corresponding to the release of water by {alpha}-lactose monohydrate (Figure 3Go). Thus, BL(10:90) and BL(20:80) samples were crystallized at water activities above 0.55 after 3 mo of storage.

The higher content of ß-LG in BL(30:70) and BL(40:60) partly concealed the DSC analysis of the state of lactose. The DSC traces of BL(40:60) are shown Figure 4Go. The DSC curves were relatively flat for samples stored below and at 0.55 aw. However, above 0.55 aw, DSC traces exhibited melting peaks of crystalline lactose around 200°C. This melting temperature, which did not correspond to pure {alpha}- or ß-lactose crystals, may be due to the specific crystallization products formed in the presence of proteins. Then, the lactose contained in BL(40:60) seemed to be amorphous until 0.55 aw and to crystallize above 0.59 aw.

The break observed in the moisture sorption isotherms of the BL powders (Figure 1Go) may correspond to the water release upon lactose crystallization. The more the model whey powders contained ß-LG, the higher the aw of lactose crystallization.

Scanning electron microscopy.
Scanning electron microscopy was used as a complementary technique to confirm the DSC analysis of model whey powders, especially with high ß-LG content. Micrographs in Figure 5Go show particles of (a) lactose, (b) ß-LG, and (c) BL(10:90) stored 3 mo at 0.23 aw. The particles of all powders studied had miscellaneous irregular size and shape. However, the surfaces of all particles were very smooth, which is characteristic of amorphous systems (Lai and Schmidt, 1990; Aguilera et al., 1994). The critical water activity of crystallization was determined from the micrographs. Below this aw, particle surfaces were smooth, since lactose remained amorphous. Above the critical aw, lactose crystals clearly appeared on particle surfaces (Figure 6Go) (Lai and Schmidt, 1990). The critical aw of crystallization was 0.43 in lactose powders and 0.55 in BL powders containing 10 to 30% ß-LG. The critical aw of BL(40:60) with 40% ß-LG was 0.59, since lactose crystals were observed on particle surfaces only at this water activity and above (Figure 7Go).



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  		Figure 5. Scanning electron micrographs of (a) ß-LG, (b) lactose, and (c) BL(10:90) stored 3 mo at 20°C and 0.23 water activity (x1000 magnification).

 


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  		Figure 6. Scanning electron micrograph of BL(10:90) stored 3 mo at 20°C and 0.55 water activity. Lactose crystals are clearly observed on particle surfaces.

 


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  		Figure 7. Scanning electron micrographs of BL(40:60) after 3-mo storage at 20°C and at (a) 0.55 water activity (aw) and (b) 0.59 aw. Particle surfaces were smooth at 0.55 aw, whereas lactose crystals were clearly observed at 0.59 aw.

 
The SEM analysis was consistent with the results obtained from moisture sorption isotherms, visual observations, and DSC analysis. Model whey powders were stabilized against lactose crystallization. In BL powders containing from 10 to 30% ß-LG, lactose crystallization was delayed to 0.55 aw, and BL powders with 40% ß-LG were stabilized until 0.59 aw.

Kinetics of Moisture Sorption
The kinetics of moisture sorption were considered in order to better understand the delayed crystallization of lactose in BL powders. The moisture sorption isotherms of (a) BL(10:90) and (b) BL(40:60), determined after 1, 2, 4, and 9 wk of storage, are shown in Figure 8Go.



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  		Figure 8. Kinetics of moisture sorption of (a) BL(10:90) and (b) BL(40:60). The graphs collect the moisture sorption isotherms of the powders after 1, 2, 4, and 9 wk of storage at 20°C.

 
The BL(10:90) and BL(20:80) reached the equilibrium of moisture sorption from the first week of storage (Figure 8aGo). On the contrary, the percentage of water adsorbed by BL(30:70) and BL(40:60) gradually decreased until the ninth week of storage at 0.55 and 0.59 aw, respectively, (Figure 8bGo). There was a time-delayed crystallization in model whey powders containing 30 and 40% ß-LG, whereas lactose crystallization occurred faster in BL powders with less ß-LG.

Knudsen et al. (2002) previously noticed that a more gradual (i.e., slower) lactose crystallization occurred in whole milk powder with 26 to 29% milk protein than in infant formula with 12% milk protein. However, this phenomenon is not fully explained yet. Forbes et al. (1998) reported that the protein hygroscopicity could be involved in the inhibition of lactose crystallization. In our study, ß-LG adsorbed water more rapidly than lactose did, and the equilibrium of moisture content was almost reached after 24 h exposure of the ß-LG samples to moisture. The amplification of crystallization might thus be limited because the water normally required for the growth of lactose crystals could be preferentially adsorbed by ß-LG.

Finally, ß-LG could physically limit the access of water vapor to the molecules of amorphous lactose, generate a delay in saturating the powder bed with moisture, and thus cause a gradual crystallization of lactose (Stubberud et al., 1996). The steric hindrance of ß-LG could also limit the propagation of lactose crystals.

Lactose/ß-LG Interaction
The moisture sorption isotherms of 2 powders mechanically mixed after separate lyophilization (B + L [10 + 90] and B + L [40 + 60]) were drawn up and compared to the corresponding theoretical curves calculated from the sorption isotherms of lyophilized lactose and ß-LG, and to the sorption curves of BL(10:90) and BL(40:60) (Figure 9aGo, b). The water sorption curves of powders composed of lactose and ß-LG lyophilized separately and mixed (B + L powders) with the 2 components lyophilized separately were similar to the theoretical curves. The behavior of water sorption of these mixed powders corresponded to the addition of the behavior of each compound taken separately, neglecting ß-LG/lactose interaction. Lactose crystallization (confirmed by DSC and SEM analysis) occurred at the water activity predicted from individual lactose and ß-LG powders. Contrary to the mixed powders, the model whey powders BL(10:90) and BL(40:60) had specific sorption curves that differed from the theoretical curves, especially between 0.39 and 0.60 aw. The BL powders adsorbed more water than predicted at the aw before lactose crystallized—i.e., at 0.43 aw for BL(10:90) and at 0.43 and 0.55 aw for BL(40:60). Moreover, in BL powders, water release during lactose crystallization occurred clearly at higher water activity than predicted. The obvious differences between theoretical and experimental moisture sorption isotherms of BL powders showed evidence of solid-state interaction between lactose and ß-LG. This molecular interaction stabilized model whey powders against lactose crystallization. Jouppila and Roos (1994) noticed a delayed crystallization of lactose in milk powders compared with pure lactose, and they reported that milk proteins could be responsible for the phenomenon. Although suspected, the result we obtained for model whey powders was, to our knowledge, not clearly demonstrated between 2 milk components.



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  		Figure 9. Theoretical and experimental moisture sorption isotherms of (a) BL(10:90) and (b) BL(40:60) after a 9-wk storage. Theoretical curves were calculated from the moisture sorption isotherms of each component considered separately. B + L powders were composed of lactose and ß-LG mixed after separate lyophilization. BL = ß-LG.

 

   GENERAL DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 GENERAL DISCUSSION
REFERENCES
 
The delay of lactose crystallization to higher water activity was previously observed in the presence of high molecular weight compounds (Berlin et al., 1968; Karmas et al., 1992). This effect was attributed to the higher glass transition temperature (Tg) of the mixture with respect to pure lactose. However, some molecules like poly(vinyl pyrolidone), trehalose, or human growth hormone delay sugar crystallization without affecting the Tg value of the systems (Costantino et al., 1998; Mazzobre et al., 2001; Stubberud and Forbes, 1998). In these combined systems, other factors, like the modification of molecular environment, might be involved.

First, noncovalent interaction between lactose and ß-LG, especially hydrogen bonds, could develop from freeze drying and direct the repartition of the 2 compounds within the system. Kim et al. (2002) and Millqvist-Fureby et al. (1999) demonstrated that proteins were largely located on the particle surfaces of industrial spray-dried dairy powders and lyophilized lactose/bovine serum albumin systems. In BL powders, ß-LG could thus largely cover the surface of powder particles and limit the access of water vapor to amorphous lactose. This spatial organization may be responsible for the shift of lactose crystallization to higher water activity.

Covalent interactions may also develop between lactose and ß-LG. Maillard reaction probably occurred during storage of BL powders, since the white BL powders became more yellow, especially between 0.59 and 0.85 aw. Lactosylation may have occurred in the powder state, since a restricted water environment increases the rate of Maillard reaction (Morgan et al., 1999; Schebor et al. 1999; Burin et al., 2000). Moreover, the lactosylation of ß-LG by 10 lactose molecules, which corresponds to a solid-state glycation (Morgan et al., 1999) increases the surface hydrophilicity of the protein, and this could modify the hydrophilic interaction within the BL powders (French and Harper, 2001).

Other investigations will be carried out to pinpoint the nature of the lactose/ß-LG interaction. First, the development of browning in BL powders will be studied. The repartition of ß-LG and lactose on the particle surfaces of the BL powders could also be evaluated by electron spectroscopy for chemical analysis. Moreover, 1H-RMN or infrared spectroscopy techniques could be undertaken to characterize the nature of the molecular bonding between the 2 compounds.

Then, the stability of lactose-rich powders used as food ingredients, in which a small part of lactose often remains amorphous, could be improved by complementation in ß-LG before spray drying.

Received for publication June 12, 2003. Accepted for publication October 6, 2003.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 GENERAL DISCUSSION
 REFERENCES
 


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