- Email: [email protected]
Solid-state characterization of spray-dried ice cream mixes
Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
Solid-state characterization of spray-dried ice cream mixes Cesar Vega a , Esther -H.-J. Kim b , Xiao D. Chen b , Yrj¨o H. Roos a,∗ b
a Faculty of Food Science and Technology, University College Cork, Cork, Ireland Department of Chemical & Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand
Received 17 February 2005; accepted 18 July 2005
Dedicated to Mrs. Josefina Vazquez de Morales.
Abstract The main physicochemical properties of spray-dried ice cream mixes (i.e. surface composition, wettability, flowability and microstructure) were analyzed. Emulsions contained 19–44% milk fat on a dry basis and included mixes with no added emulsifier and/or sucrose. The time necessary for complete wetting of the powders correlated with the amount of surface free-fat measured by means of solvent extraction. Non-micellar casein (sodium caseinate) showed to be a better co-encapsulant than micellar casein (skim milk) as demonstrated by surface fat coverage measured by electron spectroscopy for chemical analysis (ESCA). Emulsifiers influenced the fat surface composition of the powders by reducing the amount of surface protein due to their lower interfacial tension. Surface fat caused an initial overestimation of the particle size of the powders due to fat-related caking. Powders showed no flow before and after surface fat extraction which was attributed to fat-related caking and very small particle size (<80 m), respectively. © 2005 Elsevier B.V. All rights reserved. Keywords: Ice cream; Spray-drying; ESCA; Surface fat; Wettability; Emulsion
1. Introduction Transformation of milk into powder by means of spray drying is probably one of the largest food-related operations. When, particularly, whole milk or cream are spray dried, the process allows for microencapsulation of milk fat. In such processes, a core material (in this case, milk fat), is protected from oxidation through the formation of a physical barrier of water soluble substances. The wall material is usually comprised of a mix of carbohydrates and proteins, where the carbohydrates provide structure (through glass formation) and the proteins, emulsification and film forming properties [1]. Nowadays, very complex food and pharmaceutical systems (i.e. multi-component emulsions) are often manufactured by spray-drying [1–6]. In this context, we have previously reported on the spray drying feasibility of ice cream mixes
∗
Corresponding author. Tel.: +353 21 490 2386; fax: +353 21 427 0001. E-mail address: [email protected] (Y.H. Roos).
0927-7765/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2005.07.009
and the overall quality of ice creams made from their corresponding reconstituted powders [6]. The effect of multiple homogenization steps, protein type (micellar versus nonmicellar casein) and fat content (6 and 10%) were included in the design. Product recovery was related to its inherent glass transition temperature, Tg (77 ◦ C), and increased from 40 to 60% as the sucrose content (on a dry basis) decreased from 41.8 to 11.4%, respectively. Free fat content (as g fat/g powder) for all spray-dried powders was below 2%, which indicated relatively high encapsulation efficiency. Reconstitution of spray-dried mixes rendered identical particle size distributions compared to their parent, liquid, emulsions. Effective microencapsulation requires formation of capsules of high physical integrity, i.e. the core material should be completely surrounded and protected by the wall system. An ideal encapsulating material should have bland flavor, high solubility, and possess emulsification and film-forming characteristics. In addition, its concentrated solution should have low viscosity [7]. Flow, reconstitution behavior and morphology of spray-dried emulsions are highly influenced by
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
the composition and physical stability of the encapsulating material. The physicochemical, flow and reconstitution properties of dairy and related powders (i.e. lactose) have been extensively studied [8–15]. These studies have dealt with the properties of mainstream powders available for the food industry as well as model systems composed of lactose and milk proteins aiming at analyzing more fundamental aspects of powder stability. Added value powders, such as dried emulsions have also been developed for other applications, particularly in pharmaceutical areas because of the potential in improving delivery of poorly soluble drugs [5]. A major property of solid-state emulsions manufactured for consumer use is their ease of reconstitution. More important though, is that they reconstitute to the same droplet size distribution of their parent emulsions. The reconstitution process involves wetting, submerging, dispersing and dissolving of the powder. Among these, wetting of particles is often the rate-controlling step [16]. Wettability is understood as the ability of a bulk powder to imbibe a liquid under the influence of capillary forces. Generally, it depends on particle size, density, porosity, surface charge, surface area, presence of amphipathic substances and the surface activity of the particles. Nonetheless, surface composition probably plays the most important role in the wetting process – especially at high surface-fat levels. Whole milk powder (WMP) and cream powder (CP) (1 g/100 mL water) could not be wetted within a reasonable time-frame (>15 min). After a very quick wash with petroleum ether, wetting times improved to 35 s for WMP and 100 s for CP, suggesting that a hydrophobic fat surface coverage was the main reason behind poor wettability [11]. Free fat, defined as the amount of fat that can be extracted from food powders by an organic solvent, has been used to estimate of the amount of fat present on the surfaces of powder particles. It is acknowledged that such extraction (if shaking is applied) also accounts for some fat coming from the interior of the particles since solvents can reach the interior through cracks and pores in the particles—particularly in agglomerated powders [14,17]. Some authors do not seem to take this in consideration and tend to report over-estimated values for surface-free fat. Furthermore, free fat determination techniques are not well standardized across different research groups, which makes comparison of reported values rather difficult. It was until the pioneering work from F¨aldt et al. [18] that a quantitative measurement of different chemical species on the surface of food powders was possible: electron spectroscopy for chemical analysis (ESCA). This technique has been now used by several researchers to study the surface “topography” of a wide range of solid-state emulsions [11,19–23]. Caking is a deleterious phenomenon in which a lowmoisture, free-flowing powder is first transformed into lumps, followed by agglomeration into a solid and ultimately into a sticky material, resulting in loss of functionality and lowered quality. Caking can occur as a result of crystallization,
67
either after melting or solubilization of crystal surfaces; surface wetting followed by water equilibration or cooling; or electrostatic attraction between particles [24]. Free fat on the surface of a powder can also contribute to caking [25]. Methods to assess caking include techniques commonly used to measure powder flowability, angle of repose, inter-particle cohesion [26,27], size distribution and particle morphology [28]. The aim of the present study was to characterize a solidstate dairy emulsion—spray dried ice cream mixes. Surface composition, wettability and flowability properties as well as microstructure were selected for the analysis.
2. Materials and methods 2.1. Liquid emulsion manufacture Spray dried ice cream mixes were elaborated as described elsewhere [6]. Briefly, a total of ten different treatments were studied and their dry basis compositions are summarized in Table 1. The treatments were labeled according to the variable being analyzed: 6-1p (6% fat, 1 homogenization pass [hp], micellar casein); NaCas (6% fat, 1 hp, sodium caseinate—as non-micellar casein); 10-1p (10% fat, 1 hp, micellar casein); and 10-3p (10% fat, 3 hp, micellar casein). For mixes comprising micellar casein, skim milk and cream provided protein, milk-solids-non-fat and fat, whereas the non-micellar casein emulsion consisted of sodium caseinate, whey powder and anhydrous milk fat. Homogenization (22 MPa) was undertaken using a 2-stage (3 MPa in second stage) laboratory homogenizer (Model APV 1000, APV Homogenizers, AS, Albertslund, Denmark). Treatment 10-3p was homogenized three times at increasing pressures of 22, 27 and 32 MPa, respectively. In order to understand the role that emulsifier and sucrose play in the encapsulation and hence, in the powder surface composition and related properties, emulsions were prepared (a) without emulsifier/stabilizers and (b) without emulsifier/stabilizers and without sucrose. The emulsifier Table 1 Spray dried ice cream composition (dry basis) Treatment (% w/w) 10-1pa
Sucrose Fat Lactose Protein (total) Emulsifier Stabilizer
6-1p
NaCas
Fulla
nSucb
Fulla
nSucb
Fulla
nSucb
36.7 28.5 20.9 10.6 0.85 0.37
0 44.9 33 16.7 0 0
41.8 19.6 23.6 12 0.63 0.42
0 33.6 40.6 20.6 0 0
41.2 19.1 23.2 11.8 0.64 0.42
0 32.4 39.5 20.1 0 0
a Composition of formulas with three homogenization passes was the same. b Formulas with no sucrose/emulsifier/stabilizers. The composition of formulas with no stabilizers/emulsifier was considered to be the same as that of the full formulation.
68
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
used (a distilled monoglyceride made from edible, refined rapeseed oil and blended with fully hardened rapeseed oil with iodine value (Iv) of approximately 30) was from Danisco (Brabrand, Denmark).
ing, using a Mastersizer model S equipped with a 300RF (reverse Fourier) lens, a He–Ne (λ = 633 nm) laser and a powder feeder system unit (Malvern Instruments Ltd., Malvern, Worcs., UK).
2.2. Spray drying
2.5. Wettability test
Ice cream mixes were spray dried in a pilot plant scale spray dryer (Niro Atomizer, Denmark). The drier operates co-currently and has a rotary atomizer. The inlet and outlet temperatures were 160 and 70 ◦ C, respectively; the rotation of the rotary cup atomizer was 22,000 rpm. Ice cream mixes were sprayed at their original solids content (i.e. no preconcentration took place).
The wettability of powders was tested by a static wetting test, as described by Kim et al. [11]. A constant amount of powder (0.25 g) was placed on a slide covering a water reservoir (diameter = 50 mm) containing 100 mL of distilled water. By pulling the slide away, the powder layer was brought into contact with water. The wetting time, defined as the time necessary for the submersion of the last powder particle, was measured. Measurements were done by triplicate.
2.3. Surface-free fat 2.6. Flowability test Estimation of surface fat was done according to the method described by Kim et al. [29]. To extract surface-free fat and minimize extraction of free fat from the interior of powders, only a brief wash with an organic solvent was performed. Two grams of the fresh powder were accurately weighed on a filter paper (No. 42, Whatman, Maidstone, Kent, UK), and washed with 10 mL of petroleum ether (b.p. 40–60 ◦ C). This operation was repeated four times as it has been observed that at this stage, surface-free fat extraction reaches a plateau [30]. The filtrate was allowed to evaporate until the extracted fat residue achieved a constant weight. Powders were allowed to dry for a minimum of 4 days in an evacuated desiccator containing phosphorus pentaoxide to subsequently be used for particle size analysis. Extracted fat was reported as grams of surface-free fat per gram of powder. Analysis was performed in quintuplicate for each sample and results were averaged. 2.4. Powder particle size distribution Particle size of the powders before and after surface fat extraction was measured by integrated laser light scatter-
The flowability of powders was determined by measuring the angle of repose (a static measure of relative flowability) as described by Kim et al. [15]. The angle of repose for each of the powders was measured using the simple equipment shown in Fig. 1A. Ten grams of powder were carefully placed on the top box of the equipment with the trap door closed. The trap door was then opened allowing the powder to flow downwards and to form a heap. This method allowed measurement of the drained angle of repose (α) and the poured angle of repose (β), as shown in Fig. 1B. Since the poured angle of repose (β) measured by this method was similar among most of the powders, the drained angle of repose (α) was measured for comparison using horizontal still photographs and a protractor. More freeflowing powders tend to lower drained angles of repose. The flowability of powders could be significantly affected by their water content and water sorption from the surrounding air [13,32]. To minimize this, the powders were dried in a desiccating chamber for 12 h prior to flowability determination. All experimental work was conducted inside the desiccat-
Fig. 1. Devise used for flowability test. (A) Dimension, (B) representation of devise loaded with powder before test (left) and after test showing the drained and poured angles of repose (α) and (β), respectively.
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
ing chamber, which had ≈ 0% relative humidity at room temperature.
69
(VG Microtech, England) and examined with a Philips XL30 S-FEG SEM (Holland) operating at 5 kV accelerating voltage.
2.7. Electron spectroscopy for chemical analysis (ESCA) 2.9. Differential scanning calorimetry (DSC) Ice cream powders have a complex composition. For this reason, aside from full ice cream formulations, systems without either emulsifiers or stabilizers and without sucrose (also without emulsifier and stabilizers) were spray dried to differentiate the effect of these components on surface composition. The components quantified were the three main components of milk—lactose (or total carbohydrate in systems with sucrose), protein and fat. The relative atomic concentrations (%) of carbon, oxygen and nitrogen on the surface layer (∼10 nm) of the powders were analyzed. Elemental composition in the materials was assumed to be a linear combination of the elemental compositions of the pure components making up the sample. By using this relation in a matrix formula, described in detail elsewhere [18], the percentages (relative coverage) of the different components, such as lactose, protein and fat on the powder surface layers were calculated. For the direct comparison with the bulk composition of powders, the relative coverage in the present study was assumed to be mass-based. The elemental compositions of the pure components were estimated by ESCA and were used in calculations (Table 2). The ESCA measurements were made with an XSAM 800 photoelectron spectroscope (Kratos Analytical, UK). The instrument used a non-monochromatic Al K␣ X-ray source. The pressure in the measuring chamber during analysis was less than 1 × 10−7 Torr. The take-off angle of the photoelectrons was perpendicular to the sample. The analyzer operated with a pass energy of 65 eV. The step size was 0.1 eV, and the dwell time was 1000 ms. The powders were loosely packed in stainless-steel sample holders, and the surface was leveled. The analyzed area of the powder was a region of 5 mm × 8 mm.
Differential scanning calorimetry (DSC, Mettler Toledo 821e, Schwerzenback, Switzerland) was used to measure the melting behavior of pure AMF and emulsifier as well as of the fat fractions obtained by solvent-extraction from some of the spray dried emulsions. The instrument was calibrated using gallium (mp 29.8 ◦ C; H = 80 J/g), and indium (mp 156.6 ◦ C; H = 28.45 J/g). Samples were prepared in hermetically sealed aluminum pans (40 L; Mettler ME-2733). The samples (5–10 mg) were firstly melted at 80 ◦ C to erase any previous crystallographic memory. A cooling step to −70 ◦ C was then applied followed by an isotherm at that temperature for 10 min. Samples were then heated at a rate of 2 ◦ C/min to 70 ◦ C. An empty pan was used as a reference. Melting endotherms were analyzed using STARe thermal analysis software, version 6.0 (Mettler Toledo).
3. Results 3.1. Surface-free fat The amount of surface-free fat extracted from the powders is shown in Fig. 2. Regardless of the presence of sucrose and/or emulsifiers and stabilizers, the surface-fee fat content of the powders (by solvent extraction) increased in the following fashion 10-1p > 6-1p > NaCas. In samples with no emulsifiers/stabilizers, the surface fat content decreased sig-
2.8. Scanning electron microscopy (SEM) Powder samples were mounted on aluminum stubs using a double-sided adhesive tape. The samples were then coated with platinum in a Polaron SC7640 sputter coater Table 2 Relative atomic composition for ingredients used in emulsion manufacture that were used in ESCA calculations Ingredient
Lactose Sodium caseinate Anhydrous milk fat Sucrose Locust bean gum -Carrageenan Emulsifier
Relative atomic % C
O
N
S
K
52.9 67.4 88.4 52.9 84.2 56.7 76.8
47.1 19.1 11.6 47.1 15.8 38.2 23.2
– 13.5 – – – – –
– – – – – 2.8 –
– – – – – 2.3 –
Fig. 2. Wetting times and surface-free fat values for samples with different composition(s) before and after fat extraction by a polar solvent. MC, micellar casein 6% fat; MC-Em, no emulsifier; MC-Suc, no emulsifier, no sucrose; NCN, sodium caseinate 6% fat; 10-1p, micellar casein 10% fat. Shaded bars represent wetting time before fat extraction; white bars, after fat extraction. (䊉) represents surface-free fat (g fat/g powder).
70
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
Fig. 3. Particle size distribution for powders before and after surface-free extraction (filled and empty symbols, respectively). Micellar casein (, ); sodium caseinate (, ); 28% fat, one homogenization step (䊉, ); and 28% fat, three homogenization steps (, ♦).
Fig. 4. Particle size distribution for powders made with micellar casein before and after surface-free extraction (filled and empty symbols, respectively): 19% fat, full formulation (, ); 19% fat, nil emulsifier/stabilizer (䊉, ); 33% fat, nil sucrose (, ).
nificantly compared to their full formulation counterparts. Finally, for powders with no sucrose, the surface fat content increased significantly to values even higher than for full formulations, especially for the sample with 45% fat (10-1p). Samples without sucrose showed the greatest variability during extraction which could be associated to the higher amount of fat on the surface.
pared to the full formulation (57 and 67 mm, respectively), but their d(v, 0.9) was significantly higher, 431 m versus 227 m. On the other hand, powders without sucrose had a higher d(v, 0.5) of 111 m and showed a bimodal size distribution. These results suggested that when there was not enough encapsulating material (sucrose) the powder was more prone to fat globule coalescence during drying, which manifested in bigger particle sizes and a higher amount of surface fat.
3.2. Particle size Particle size of the powders was measured as they were obtained from the drier, but this proved to be very problematic as particles were highly agglomerated/caked. There were practically no particles smaller than 20 m and the shape of the distribution was practically the same for all samples as shown in Fig. 3. The volume weighted median diameter d(v, 0.5) was also very similar for all samples (approximately 480 m). For reference, the d(v, 0.5) for skim milk powder (SMP) is 88 m (not shown). Particle size distribution after surface fat extraction showed a dramatic change compared to the original data. The d(v, 0.5) decreased to values around 60–70 m for all samples. As can be deduced from Fig. 3, surface-free fat is an important factor on particle size measurement as there was a seven-fold increment on the apparent sizes of the powder particles before extraction. Fig. 4 shows the comparison of particle size distributions (before and after surface fat extraction) of powders made with 19% fat with and without emulsifiers/stabilizers (6-1p and 6-1p nil emulsifier) and 33% fat (6-1p nil sucrose). Before extraction, powders showed identical behavior to those previously described—they were highly agglomerated and their d(v, 0.5) was between 350 and 400 m. After extraction, all materials had significantly smaller particle sizes. Powders without emulsifiers/stabilizers had similar d(v, 0.5) com-
3.3. ESCA Surface composition showed that powders were mainly covered by fat regardless of their original bulk composition (Fig. 5). For fully formulated powders, the amount of surface fat ranged from 96 to 81%. Sample 10-1p showed the highest surface fat coverage, whereas the sample with non-micellar casein showed the lowest. When no emulsifiers were used, there was a significant difference in the surface composition of all samples. Powders with micellar casein had reduced surface fat (from 85 to 78%) mainly at the expense of higher protein coverage (6–11%). Powders with non-micellar casein had a similar reduction in surface fat (81–73%) as a result of an exchange for surface protein, which increased from 8 to 15%. The same was true for the 28% fat powder (10-1p); its surface protein value went from 0 to 10%. In the absence of sucrose and emulsifier surface fat content increased for all powders with a concomitant decrease in surface carbohydrate (surface protein remained constant). 3.4. Flowability All powders, regardless of their composition, showed very poor flowability as they did not fall into the lower
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
71
3.5. Wettability
Fig. 5. Surface composition of spray-dried ice cream mixes as dependent on composition. Bulk powder composition for full and nil emulsifier formulations: (+), micellar and non-micellar casein; ( ) micellar casein 10-1p. Bulk composition for nil sucrose systems: (×) micellar and non-micellar casein; ( ) micellar casein 10-1p. Composition after spray drying for (a) micellar casein: () full formula; () nil emulsifier/stabilizer; () nil sucrose. (b) Non-micellar casein: ( ) full formula; ( ) nil emulsifier/stabilizer; ( ) nil sucrose. (c) micellar casein 10-1p: () full formula; ( ) full formula 10-3p; (䊉) nil emulsifier/stabilizer; () nil sucrose.
chamber of the flowability test device as shown in Fig. 6. Initially, we attributed this to the high surface-free fat content, so measurements were repeated after its extraction with petroleum ether. Powders showed a similar nonflowing behavior with only one noticeable difference: the volume occupied by the same mass of powder was drastically reduced, which related to a change in packing characteristics. Before surface fat extraction there was a vast amount of inter-particle voids, which disappeared after extraction.
Measurements of wetting properties showed some variability, but they were sufficient to discriminate among the different treatments. Such variability was a result of (a) how the powder fell into the beaker; and (b) the difficulty of assessing when the last particle was actually sunk. Before surface fat extraction, powders containing 28% fat showed the poorest wettability since they did not wet after 180 min (Fig. 2). For powders with 28% fat (10-1p) but no emulsifier and no emulsifier/sucrose (44% fat), wettability was significantly better as they wetted between 30 and 40 min on average. Powders with 19% fat and micellar casein (61p) needed over 1 h to be reconstituted, while those made with non-micellar casein were wetted in 43 min. Their respective powders without emulsifiers showed significantly shorter average wetting times: 24.5 and 11.5 min for 6-1p and NaCas, respectively. Samples without sucrose/emulsifiers (33% fat) showed average wetting times of 30 and 42 min, respectively. It should be noted that wettability continuously improved for all powders as components were eliminated from the formula, particularly emulsifiers; whereas when sucrose was removed, wettability improved only when compared to the full formulations. Overall, there was a clear correlation between the amount of fat extracted and the wetting time before surface fat extraction. For each of the three powder categories analyzed (i.e. full formulations, nil emulsifier and nil sucrose/emulsifier), the wetting time increased with increasing surface fat, but to different extents. For powders with 19% fat—6-1p and NaCas—wettability was improved by a factor of 10. The powder with 28% fat and one homogenization step was wetted in 29 min, whereas the powder with three homogenization steps wetted in about 15 min. These powders showed the largest improvement since before extraction, they did not wet even after three hours. The only powder without emulsifier analyzed was that containing micellar casein (6-1p). This powder wetted in 5 min which was five times faster than its counterpart with surface-free fat. Powders without sucrose/emulsifiers had the greatest improvement in wettability probably due to the relatively high amount of fat removed; the powders comprised of non-micellar casein showed their wetting time reduced by 50% and finally, those made with micellar casein remained unchanged. 3.6. Microstructure
Fig. 6. Flowability test device showing zero flow for a sample before surfafree fat extraction (left) and after extraction (right).
Micrographs were obtained for the majority of the powders at different magnifications. We centered our attention at the higher magnification pictures so that the focus remained on powder surface characterization. Differences among treatments became obvious at a very high level of magnification (12,800×). Fig. 7 shows that the type of protein rendered a different surface topography: non-micellar casein showed a high level of cracks that covered the particle entirely, in some cases crater-like structures were obvious (Fig. 7B); on
72
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
Fig. 7. Surface microstructure of spray-dried ice cream mixes. Micellar casein: full formulation (A); nil emulsifier/stabilizer (C); nil sucrose (E). Non-micellar casein: full formulation (B); nil emulsifier/stabilizer (D); nil sucrose (F). Micellar casein made with 28% fat: full formulation (G) and nil emulsifier/stabilizer (H). Bar is 5 m.
the other hand, micellar casein showed a smoother surface, but some depressions and cracks were also present (Fig. 7A). Surface imaging for powders without emulsifiers/stabilizers (19% fat) are shown in Fig. 7C and D—micellar and nonmicellar casein, respectively. Cracks were evident (to a lesser degree) for powders with non-micellar casein, but all depressions disappeared; whereas the powder with micellar casein showed a rougher surface and less cracks compared to both, Fig. 7A and D. In the absence of sucrose, the powder with
micellar casein (Fig. 7E) had a higher number of depressions and its sphericity was somewhat distorted by the presence of protrusions. Powders made with non-micellar casein showed no signs of cracks and their surface resembled that of orange peel (Fig. 7F); these powders also showed a high degree of shriveling (not shown). Finally, powders made with 28% fat showed a surface covered by what appeared like sanddunes along with some small depressions. The structure was dramatically different when no emulsifiers were added; the
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
73
surface was very rough and many very deep cracks were observed (Fig. 7G and H).
4. Discussion The lower surface-free fat content in powders containing non-micellar casein suggested that this type of protein has better CO-encapsulation properties than micellar casein. The encapsulating properties of sodium caseinate alone [33], blended with lactose and/or other low and high molecular weight carbohydrates [1,23,34], or with whey proteins [2,3,35] have been extensively studied. The main conclusions have been that (a) sodium caseinate is a better encapsulant than whey proteins since it has a greater emulsifying potential and does not show heat-denaturation during drying; and (b) it is not possible to stabilize a solid-state emulsion having sodium caseinate as both encapsulant and emulsifier since it shrinks during drying and, in excess, it confers instability to the emulsion (even before spraying) due to a mechanism known as depletion flocculation [36,37]. Studies to compare the encapsulating properties of pure micellar casein (i.e. ultrafiltrated skim milk) and those of sodium caseinate are scarce [38]. Our conclusion is that sodium caseinate showed better CO-encapsulation properties than micellar casein because of its looser molecular conformation, higher diffusivity and the strong amphiphilic characteristics of the major individual caseins (␣s1 - and -casein), which might have allowed for a better distribution around the fat globule surface than micellar casein. Sodium caseinate has also been shown to have better resistance against displacement from the oil–water interface by low molecular weight emulsifiers [6,39]; this fact might also have accounted for some of the differences observed. Onwulata et al. [40] showed that anhydrous butter–oil was better encapsulated than milk fat coming from heavy cream, which is consistent with the results shown in this study. In the case of samples with 28% fat (10-3p), the extra homogenization steps made a significant difference by lowering the amount of surface-free fat in comparison to the powder produced with 1 homogenization step (10-1p). For samples with no sucrose added, it was not surprising to find a higher surface-free fat compared to those with added sucrose, since the ratio of non-fat solids to fat was smaller (2.4 instead of 3.9, respectively). The importance of this ratio has been already pointed out by Dollo et al. [41]. Similarly, Buma [13] and Onwulata et al. [42] have reported that the amount of surface fat increased as the total fat content in the original emulsions increased. In the absence of emulsifiers, surface fat coverage decreased by 11, 23 and 31% for samples 6-1p, NaCas and 101p, respectively. We attribute this change to the surface-active nature of emulsifiers. Compared to any of the protein types used in this study, they were the most surface-active components and would be expected to prevail at the air–water interface [43]. They then would be anticipated to displace proteins and reduce their content onto the powder surface;
Fig. 8. DSC melting thermograms for (a) pure anhydrous milk fat; (b) pure emulsifier; (c) surface-free fat extracted from spray-dried ice cream mix; and (d) surface-free fat extracted from pure emulsifier.
ESCA results supported this argument since protein coverage increased at the expense of fat when no emulsifier was used. Nonetheless, the relative atomic composition of the emulsifiers (C 77%, O 23%) did not allow for a clear differentiation between C and O species coming from AMF or the emulsifier per se. This could have caused an over-representation of surface fat. Based on this observation, the melting behavior of surface-free fat extracted from a powder made with non-micellar casein with emulsifier (as it showed the highest difference in surface-free fat content) was measured by DSC and their corresponding thermograms were compared with those of pure AMF, pure emulsifier, and the extracted fraction after subjecting the pure emulsifier to the solvent (following the same protocol as with the powders). The thermograms shown in Fig. 8 illustrate that the surface-free fat extracted from the powder with emulsifier had an identical thermal behavior to pure AMF (traces c and a, respectively). Traces b and d, correspond to the melting behavior of the pure emulsifier and of the fraction obtained after solvent exposure. It can be observed that (i) a negligible amount of fat from the emulsifier was extracted and (ii) that the powder did not show any traces of emulsifier fat (especially those observed in line d between 38 and 44 ◦ C), in other words, surface-free fat came exclusively from milk fat. From this scrutiny we propose that the mechanism by which the addition of emulsifiers increases the surface-free fat in spray-dried emulsions is very similar to what is known as coalescence [33]. The membrane surrounding the fat globule was thinned by the preferential adsorption of emulsifier over proteins, which made the globules more sensitive to shear and/or mechanical stress during drying, manifesting itself as higher surface-free fat contents. Particle size was highly dependent on surface-free fat. A powder maybe considered to have a particle size less than 200 m [44]. Figs. 3 and 4 suggest that before surface fat extraction most of the particles had a size >300 m and that after extraction, approximately 90% of the particles were below 200 m in diameter. This suggested particle caking
74
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
and/or agglomeration because of surface fat. Our findings are in line with those reported by Foster et al. [25]. Fig. 3 also shows that after extraction, all powders had a “monomodal” size distribution with a very small second peak at higher sizes. Interestingly, for all powders analyzed, the lower the surfacefree fat value (both, by solvent extraction and ESCA), the smaller was its size distribution. This also applied for Fig. 3. Lower surface fat values correlated well with progressively smaller “real” particle sizes. This suggested that bridging or agglomerating potential was the highest for the powder made with non-micellar casein—smallest particle size distribution meant largest surface area and as a result, powders were more prone to caking per mass unit of surface fat [45]. Fig. 4 shows that for samples made with micellar casein without sucrose powders did not exhibit monomodal distributions after fat extraction indicating that the amount of encapsulant (sucrose) was limited and hence, collision and fusion of fat globules took place during drying. A similar behavior was found in samples with non-micellar casein and 28% fat (not shown). Flowability was poor for all powders tested before and after surface fat extraction. Before extraction, the interparticle fat-related caking, hindered flow regardless of the surface-free fat content. This suggested that after a certain plateau was reached in terms of surface-free fat per unit area, cohesion forces did not further increase and differences could not be detected [13]. High cohesion forces were evident by the presence of inter-particle voids (Fig. 6). After fat extraction, such void spaces disappeared and powders packed more efficiently. Nonetheless, there was no flow. We attributed this to the “real” particle size of the powders. Changes in particle properties and storage conditions may influence flowability. Particle size has a major influence on powder flowability, and as the size decreases below 200 m, the flowability deteriorates. The volume weighted median diameter d(v, 0.5) after fat extraction for fully-formulated powders was between 55 and 76 m. Fitzpatrick et al. [44] reported that there was a noticeable decrease in flowability when particle size was reduced from 200 to around 80 m. SMP is a free-flowing powder with a d(v, 0.5) of 88 m (data not shown). This reduction in flowability at smaller particle size was due to the increased surface area per unit mass of powder. More surface area was available for cohesive forces, in particular, and frictional forces to resist flow. As mentioned previously, all powders showed spherical geometries but significantly different topographies. Shriveled particles were detected only for powders comprised of sodium caseinate with no sucrose (not shown). This could have been caused by a net decrease in the amount of encapsulant material available and the aforementioned susceptibility of sodium caseinate to shrinkage during drying. This also suggested that micellar casein was not prone to shrinkage. Previous studies have shown that lactose does not contribute to the formation of folds or cracks during spray drying [13]. If this is the case, the abundance of cracks in the sodium caseinate formula relative to that in the micellar casein pow-
der could be attributed solely to the protein type (Fig. 7A and B, respectively). The use of no emulsifier in powders made with non-micellar casein decreased the number of cracks observed (Fig. 7D). On the other hand, the absence of sucrose created a microstructure very similar to that of whole milk powder in powders made with micellar casein (Fig. 7E) [11]. Research is being undertaken in our laboratory on the encapsulating properties of pure micellar and non-micellar caseins in the presence of different disaccharides as main encapsulants in order to elucidate their interactions and the properties they provide to spray-dried model emulsions.
5. Conclusions A detailed analysis of the surfaces of a number of spraydried ice cream mixes was undertaken. It has been shown that their flowability, wettability and microstructure are strongly dependent on surface fat coverage. In this study, since laboratory spray-dried particles are generally smaller than those manufactured on a commercial basis, the sensitivity of flowability test to surface fat coverage was not as dramatic as in the case of commercial particles. Surface-active materials modified the behavior of the air–water interface and this had an influence on the amount of surface fat measured by ESCA and solvent extraction. Agglomerated particles had fat bonds, which were removed during solvent extraction. Powders showed extremely poor flowability due to very small particle size as well as the sticky nature of the fat surfaces. Wettability was drastically improved after surface-free fat extraction.
Acknowledgements Author Cesar Vega is indebted to the Mexican Consejo Nacional de Ciencia y Tecnologia (CONACyT) and to Dippin’ Dots Inc. for providing financial support to undertake this research. Similarly, travel funding provided by Fractec, Ontario, Canada is appreciated. Thanks also to Dr. John J. Fitzpatrick (University College Cork) for his valuable comments.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
P. F¨aldt, B. Bergenst˚ahl, J. Am. Oil Chem. Soc. 72 (1995) 171. P. F¨aldt, B. Bergenst˚ahl, Food Hydrocolloid. 10 (1996) 421. P. F¨aldt, B. Bergenst˚ahl, Food Hydrocolloid. 10 (1996) 431. D.L. Moreau, M. Rosenberg, Food Struct. 12 (1993) 457. G.P. Pedersen, P. F¨aldt, B. Bergenst˚ahl, H.G. Kristensen, Int. J. Pharm. 171 (1998) 257. C. Vega, H.D. Goff, Y.H. Roos, J. Food Sci. 70 (2005) 244. M. Rosenberg, S.L. Young, Food Struct. 12 (1993) 31. S. Miao, Y.H. Roos, J. Agric. Food Chem. 52 (2004) 5250. K. Jouppila, Y.H. Roos, J. Dairy Sci. 77 (1994) 2907. P.R. Rennie, X.D. Chen, C. Hargreaves, A.R. Mackereth, J. Food Eng. 39 (1999) 277.
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75 [11] E.H.J. Kim, X.D. Chen, D. Pearce, Colloid. Surf. B: Biointerfaces 26 (2002) 197. [12] N. Ozcan, N. Walisinghe, X.D. Chen, J. Food Eng. 55 (2002) 293. [13] T.J. Buma, Neth. Milk Dairy J. 25 (1971) 107. [14] T.J. Buma, Neth. Milk Dairy J. 25 (1971) 42. [15] E.H.J. Kim, X.D. Chen, D. Pearce, in preparation. [16] B. Freudig, S. Hogekamp, H. Schubert, Chem. Eng. Process. 38 (1999) 525. [17] W. Buchheim, Food Microstruct. 1 (1982) 233. [18] P. F¨aldt, B. Bergenst˚ahl, G. Carlsson, Food Struct. 12 (1993) 225. [19] K. Landstrom, J. Alsins, B. Bergenst˚ahl, Food Hydrocolloid. 14 (2000) 75. [20] A. Millqvist-Fureby, U. Elofsson, B. Bergenst˚ahl, Colloid. Surf. B: Biointerfaces 21 (2001) 47. [21] K.L. Christensen, G.P. Pedersen, H.G. Kristensen, Int. J. Pharm. 212 (2001) 187. [22] K.L. Christensen, G.P. Pedersen, H.G. Kristensen, Eur. J. Pharm. Biopharm. 53 (2002) 147. [23] A. Millqvist-Fureby, Colloid. Surf. B: Biointerfaces 31 (2003) 65. [24] M. Peleg in, M. Peleg, E. Bagley (Eds.), Physical Properties of Foods, AVI Publishing, 1983, pp. 293–323. [25] K.L. Foster, J. Bronlund, T. Paterson, Int. Dairy J. 15 (2005) 85. [26] M. Peleg in, J.M.V. Blanshard, P.J. Lillford (Eds.), The Glassy State of Foods, Nottingham University Press, 1993, pp. 435–451. [27] M. Peleg, J. Food Process. Preserv. 1 (1977) 303. [28] B. Provent, D. Chulia, J. Cary, Eur. J. Pharm. Biopharm. 39 (1993) 202. [29] E.H.J. Kim, X.D. Chen, D. Pearce, Colloid. Surf. B: Biointerfaces 42 (2005) 1.
75
[30] E.H.J. Kim, Surface composition of spray-dried dairy powders and its formation mechanism(s). PhD Thesis. The University of Auckland, New Zealand, 2005. [32] E. Teonou, J.J. Fitzpatrick, J. Food Eng. 42 (1999) 109. [33] S.A. Hogan, B.F. McNamee, E.D. O’Riordan, M. O’Sullivan, J. Agric. Food Chem. 49 (2001) 1934. [34] S.A. Hogan, B.F. McNamee, E.D. O’Riordan, M. O’Sullivan, Int. Dairy J. 11 (2001) 137. [35] E.L. Sliwinski, B.W.M. Lavrijsen, J.M. Vollenbroek, H.J. van der Stege, M.A.J.S. Van Boekel, J.T.M. Wouters, Colloid. Surf. B: Biointerfaces 31 (2003) 219. [36] E. Dickinson, M. Golding, Food Hydrocolloid. 11 (1997) 13. [37] C. Vega, D.G. Dalgleish, H.D. Goff, Food Hydrocolloid. 19 (2005) 187. [38] M.K. Keogh, B. O’Kennedy, J. Kelly, M. Auty, A. Fureby, A.M. Haahr, J. Food Sci. 66 (2001) 217. [39] E. Davies, E. Dickinson, R.D. Bee, Food Hydrocolloid. 14 (2000) 145. [40] C.I. Onwulata, P.W. Smith, P.H. Cooke, h.G. Holsinger, Lebensm. -Wiss. u. -Technol. 29 (1996) 163. [41] G. Dollo, P. Le Corre, A. Guerin, F. Chevanne, J.L. Burgot, R. Leverge, Eur. J. Pharm. Sci. 19 (2003) 273. [42] C.I. Onwulata, P.W. Smith, J.C. Craig, h.G. Holsinger, J. Food Sci. 59 (1994) 316. [43] A. Millqvist-Fureby, N. Burns, K. Landstrom, B. Bergenst˚ahl, in: E. Dickinson, J. Rodriguez-Patino (Eds.), Food Emulsions and Foams, Royal Society of Chemistry, London, 1999, pp. 236–245. [44] J.J. Fitzpatrick, T. Iqbal, C. Delaney, T. Twomey, M.K. Keogh, J. Food Eng. 64 (2004) 435. [45] T.J. Buma, Neth. Milk Dairy J. 25 (1971) 53.
Solid-state characterization of spray-dried ice cream mixes Cesar Vega a , Esther -H.-J. Kim b , Xiao D. Chen b , Yrj¨o H. Roos a,∗ b
a Faculty of Food Science and Technology, University College Cork, Cork, Ireland Department of Chemical & Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand
Received 17 February 2005; accepted 18 July 2005
Dedicated to Mrs. Josefina Vazquez de Morales.
Abstract The main physicochemical properties of spray-dried ice cream mixes (i.e. surface composition, wettability, flowability and microstructure) were analyzed. Emulsions contained 19–44% milk fat on a dry basis and included mixes with no added emulsifier and/or sucrose. The time necessary for complete wetting of the powders correlated with the amount of surface free-fat measured by means of solvent extraction. Non-micellar casein (sodium caseinate) showed to be a better co-encapsulant than micellar casein (skim milk) as demonstrated by surface fat coverage measured by electron spectroscopy for chemical analysis (ESCA). Emulsifiers influenced the fat surface composition of the powders by reducing the amount of surface protein due to their lower interfacial tension. Surface fat caused an initial overestimation of the particle size of the powders due to fat-related caking. Powders showed no flow before and after surface fat extraction which was attributed to fat-related caking and very small particle size (<80 m), respectively. © 2005 Elsevier B.V. All rights reserved. Keywords: Ice cream; Spray-drying; ESCA; Surface fat; Wettability; Emulsion
1. Introduction Transformation of milk into powder by means of spray drying is probably one of the largest food-related operations. When, particularly, whole milk or cream are spray dried, the process allows for microencapsulation of milk fat. In such processes, a core material (in this case, milk fat), is protected from oxidation through the formation of a physical barrier of water soluble substances. The wall material is usually comprised of a mix of carbohydrates and proteins, where the carbohydrates provide structure (through glass formation) and the proteins, emulsification and film forming properties [1]. Nowadays, very complex food and pharmaceutical systems (i.e. multi-component emulsions) are often manufactured by spray-drying [1–6]. In this context, we have previously reported on the spray drying feasibility of ice cream mixes
∗
Corresponding author. Tel.: +353 21 490 2386; fax: +353 21 427 0001. E-mail address: [email protected] (Y.H. Roos).
0927-7765/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2005.07.009
and the overall quality of ice creams made from their corresponding reconstituted powders [6]. The effect of multiple homogenization steps, protein type (micellar versus nonmicellar casein) and fat content (6 and 10%) were included in the design. Product recovery was related to its inherent glass transition temperature, Tg (77 ◦ C), and increased from 40 to 60% as the sucrose content (on a dry basis) decreased from 41.8 to 11.4%, respectively. Free fat content (as g fat/g powder) for all spray-dried powders was below 2%, which indicated relatively high encapsulation efficiency. Reconstitution of spray-dried mixes rendered identical particle size distributions compared to their parent, liquid, emulsions. Effective microencapsulation requires formation of capsules of high physical integrity, i.e. the core material should be completely surrounded and protected by the wall system. An ideal encapsulating material should have bland flavor, high solubility, and possess emulsification and film-forming characteristics. In addition, its concentrated solution should have low viscosity [7]. Flow, reconstitution behavior and morphology of spray-dried emulsions are highly influenced by
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
the composition and physical stability of the encapsulating material. The physicochemical, flow and reconstitution properties of dairy and related powders (i.e. lactose) have been extensively studied [8–15]. These studies have dealt with the properties of mainstream powders available for the food industry as well as model systems composed of lactose and milk proteins aiming at analyzing more fundamental aspects of powder stability. Added value powders, such as dried emulsions have also been developed for other applications, particularly in pharmaceutical areas because of the potential in improving delivery of poorly soluble drugs [5]. A major property of solid-state emulsions manufactured for consumer use is their ease of reconstitution. More important though, is that they reconstitute to the same droplet size distribution of their parent emulsions. The reconstitution process involves wetting, submerging, dispersing and dissolving of the powder. Among these, wetting of particles is often the rate-controlling step [16]. Wettability is understood as the ability of a bulk powder to imbibe a liquid under the influence of capillary forces. Generally, it depends on particle size, density, porosity, surface charge, surface area, presence of amphipathic substances and the surface activity of the particles. Nonetheless, surface composition probably plays the most important role in the wetting process – especially at high surface-fat levels. Whole milk powder (WMP) and cream powder (CP) (1 g/100 mL water) could not be wetted within a reasonable time-frame (>15 min). After a very quick wash with petroleum ether, wetting times improved to 35 s for WMP and 100 s for CP, suggesting that a hydrophobic fat surface coverage was the main reason behind poor wettability [11]. Free fat, defined as the amount of fat that can be extracted from food powders by an organic solvent, has been used to estimate of the amount of fat present on the surfaces of powder particles. It is acknowledged that such extraction (if shaking is applied) also accounts for some fat coming from the interior of the particles since solvents can reach the interior through cracks and pores in the particles—particularly in agglomerated powders [14,17]. Some authors do not seem to take this in consideration and tend to report over-estimated values for surface-free fat. Furthermore, free fat determination techniques are not well standardized across different research groups, which makes comparison of reported values rather difficult. It was until the pioneering work from F¨aldt et al. [18] that a quantitative measurement of different chemical species on the surface of food powders was possible: electron spectroscopy for chemical analysis (ESCA). This technique has been now used by several researchers to study the surface “topography” of a wide range of solid-state emulsions [11,19–23]. Caking is a deleterious phenomenon in which a lowmoisture, free-flowing powder is first transformed into lumps, followed by agglomeration into a solid and ultimately into a sticky material, resulting in loss of functionality and lowered quality. Caking can occur as a result of crystallization,
67
either after melting or solubilization of crystal surfaces; surface wetting followed by water equilibration or cooling; or electrostatic attraction between particles [24]. Free fat on the surface of a powder can also contribute to caking [25]. Methods to assess caking include techniques commonly used to measure powder flowability, angle of repose, inter-particle cohesion [26,27], size distribution and particle morphology [28]. The aim of the present study was to characterize a solidstate dairy emulsion—spray dried ice cream mixes. Surface composition, wettability and flowability properties as well as microstructure were selected for the analysis.
2. Materials and methods 2.1. Liquid emulsion manufacture Spray dried ice cream mixes were elaborated as described elsewhere [6]. Briefly, a total of ten different treatments were studied and their dry basis compositions are summarized in Table 1. The treatments were labeled according to the variable being analyzed: 6-1p (6% fat, 1 homogenization pass [hp], micellar casein); NaCas (6% fat, 1 hp, sodium caseinate—as non-micellar casein); 10-1p (10% fat, 1 hp, micellar casein); and 10-3p (10% fat, 3 hp, micellar casein). For mixes comprising micellar casein, skim milk and cream provided protein, milk-solids-non-fat and fat, whereas the non-micellar casein emulsion consisted of sodium caseinate, whey powder and anhydrous milk fat. Homogenization (22 MPa) was undertaken using a 2-stage (3 MPa in second stage) laboratory homogenizer (Model APV 1000, APV Homogenizers, AS, Albertslund, Denmark). Treatment 10-3p was homogenized three times at increasing pressures of 22, 27 and 32 MPa, respectively. In order to understand the role that emulsifier and sucrose play in the encapsulation and hence, in the powder surface composition and related properties, emulsions were prepared (a) without emulsifier/stabilizers and (b) without emulsifier/stabilizers and without sucrose. The emulsifier Table 1 Spray dried ice cream composition (dry basis) Treatment (% w/w) 10-1pa
Sucrose Fat Lactose Protein (total) Emulsifier Stabilizer
6-1p
NaCas
Fulla
nSucb
Fulla
nSucb
Fulla
nSucb
36.7 28.5 20.9 10.6 0.85 0.37
0 44.9 33 16.7 0 0
41.8 19.6 23.6 12 0.63 0.42
0 33.6 40.6 20.6 0 0
41.2 19.1 23.2 11.8 0.64 0.42
0 32.4 39.5 20.1 0 0
a Composition of formulas with three homogenization passes was the same. b Formulas with no sucrose/emulsifier/stabilizers. The composition of formulas with no stabilizers/emulsifier was considered to be the same as that of the full formulation.
68
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
used (a distilled monoglyceride made from edible, refined rapeseed oil and blended with fully hardened rapeseed oil with iodine value (Iv) of approximately 30) was from Danisco (Brabrand, Denmark).
ing, using a Mastersizer model S equipped with a 300RF (reverse Fourier) lens, a He–Ne (λ = 633 nm) laser and a powder feeder system unit (Malvern Instruments Ltd., Malvern, Worcs., UK).
2.2. Spray drying
2.5. Wettability test
Ice cream mixes were spray dried in a pilot plant scale spray dryer (Niro Atomizer, Denmark). The drier operates co-currently and has a rotary atomizer. The inlet and outlet temperatures were 160 and 70 ◦ C, respectively; the rotation of the rotary cup atomizer was 22,000 rpm. Ice cream mixes were sprayed at their original solids content (i.e. no preconcentration took place).
The wettability of powders was tested by a static wetting test, as described by Kim et al. [11]. A constant amount of powder (0.25 g) was placed on a slide covering a water reservoir (diameter = 50 mm) containing 100 mL of distilled water. By pulling the slide away, the powder layer was brought into contact with water. The wetting time, defined as the time necessary for the submersion of the last powder particle, was measured. Measurements were done by triplicate.
2.3. Surface-free fat 2.6. Flowability test Estimation of surface fat was done according to the method described by Kim et al. [29]. To extract surface-free fat and minimize extraction of free fat from the interior of powders, only a brief wash with an organic solvent was performed. Two grams of the fresh powder were accurately weighed on a filter paper (No. 42, Whatman, Maidstone, Kent, UK), and washed with 10 mL of petroleum ether (b.p. 40–60 ◦ C). This operation was repeated four times as it has been observed that at this stage, surface-free fat extraction reaches a plateau [30]. The filtrate was allowed to evaporate until the extracted fat residue achieved a constant weight. Powders were allowed to dry for a minimum of 4 days in an evacuated desiccator containing phosphorus pentaoxide to subsequently be used for particle size analysis. Extracted fat was reported as grams of surface-free fat per gram of powder. Analysis was performed in quintuplicate for each sample and results were averaged. 2.4. Powder particle size distribution Particle size of the powders before and after surface fat extraction was measured by integrated laser light scatter-
The flowability of powders was determined by measuring the angle of repose (a static measure of relative flowability) as described by Kim et al. [15]. The angle of repose for each of the powders was measured using the simple equipment shown in Fig. 1A. Ten grams of powder were carefully placed on the top box of the equipment with the trap door closed. The trap door was then opened allowing the powder to flow downwards and to form a heap. This method allowed measurement of the drained angle of repose (α) and the poured angle of repose (β), as shown in Fig. 1B. Since the poured angle of repose (β) measured by this method was similar among most of the powders, the drained angle of repose (α) was measured for comparison using horizontal still photographs and a protractor. More freeflowing powders tend to lower drained angles of repose. The flowability of powders could be significantly affected by their water content and water sorption from the surrounding air [13,32]. To minimize this, the powders were dried in a desiccating chamber for 12 h prior to flowability determination. All experimental work was conducted inside the desiccat-
Fig. 1. Devise used for flowability test. (A) Dimension, (B) representation of devise loaded with powder before test (left) and after test showing the drained and poured angles of repose (α) and (β), respectively.
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
ing chamber, which had ≈ 0% relative humidity at room temperature.
69
(VG Microtech, England) and examined with a Philips XL30 S-FEG SEM (Holland) operating at 5 kV accelerating voltage.
2.7. Electron spectroscopy for chemical analysis (ESCA) 2.9. Differential scanning calorimetry (DSC) Ice cream powders have a complex composition. For this reason, aside from full ice cream formulations, systems without either emulsifiers or stabilizers and without sucrose (also without emulsifier and stabilizers) were spray dried to differentiate the effect of these components on surface composition. The components quantified were the three main components of milk—lactose (or total carbohydrate in systems with sucrose), protein and fat. The relative atomic concentrations (%) of carbon, oxygen and nitrogen on the surface layer (∼10 nm) of the powders were analyzed. Elemental composition in the materials was assumed to be a linear combination of the elemental compositions of the pure components making up the sample. By using this relation in a matrix formula, described in detail elsewhere [18], the percentages (relative coverage) of the different components, such as lactose, protein and fat on the powder surface layers were calculated. For the direct comparison with the bulk composition of powders, the relative coverage in the present study was assumed to be mass-based. The elemental compositions of the pure components were estimated by ESCA and were used in calculations (Table 2). The ESCA measurements were made with an XSAM 800 photoelectron spectroscope (Kratos Analytical, UK). The instrument used a non-monochromatic Al K␣ X-ray source. The pressure in the measuring chamber during analysis was less than 1 × 10−7 Torr. The take-off angle of the photoelectrons was perpendicular to the sample. The analyzer operated with a pass energy of 65 eV. The step size was 0.1 eV, and the dwell time was 1000 ms. The powders were loosely packed in stainless-steel sample holders, and the surface was leveled. The analyzed area of the powder was a region of 5 mm × 8 mm.
Differential scanning calorimetry (DSC, Mettler Toledo 821e, Schwerzenback, Switzerland) was used to measure the melting behavior of pure AMF and emulsifier as well as of the fat fractions obtained by solvent-extraction from some of the spray dried emulsions. The instrument was calibrated using gallium (mp 29.8 ◦ C; H = 80 J/g), and indium (mp 156.6 ◦ C; H = 28.45 J/g). Samples were prepared in hermetically sealed aluminum pans (40 L; Mettler ME-2733). The samples (5–10 mg) were firstly melted at 80 ◦ C to erase any previous crystallographic memory. A cooling step to −70 ◦ C was then applied followed by an isotherm at that temperature for 10 min. Samples were then heated at a rate of 2 ◦ C/min to 70 ◦ C. An empty pan was used as a reference. Melting endotherms were analyzed using STARe thermal analysis software, version 6.0 (Mettler Toledo).
3. Results 3.1. Surface-free fat The amount of surface-free fat extracted from the powders is shown in Fig. 2. Regardless of the presence of sucrose and/or emulsifiers and stabilizers, the surface-fee fat content of the powders (by solvent extraction) increased in the following fashion 10-1p > 6-1p > NaCas. In samples with no emulsifiers/stabilizers, the surface fat content decreased sig-
2.8. Scanning electron microscopy (SEM) Powder samples were mounted on aluminum stubs using a double-sided adhesive tape. The samples were then coated with platinum in a Polaron SC7640 sputter coater Table 2 Relative atomic composition for ingredients used in emulsion manufacture that were used in ESCA calculations Ingredient
Lactose Sodium caseinate Anhydrous milk fat Sucrose Locust bean gum -Carrageenan Emulsifier
Relative atomic % C
O
N
S
K
52.9 67.4 88.4 52.9 84.2 56.7 76.8
47.1 19.1 11.6 47.1 15.8 38.2 23.2
– 13.5 – – – – –
– – – – – 2.8 –
– – – – – 2.3 –
Fig. 2. Wetting times and surface-free fat values for samples with different composition(s) before and after fat extraction by a polar solvent. MC, micellar casein 6% fat; MC-Em, no emulsifier; MC-Suc, no emulsifier, no sucrose; NCN, sodium caseinate 6% fat; 10-1p, micellar casein 10% fat. Shaded bars represent wetting time before fat extraction; white bars, after fat extraction. (䊉) represents surface-free fat (g fat/g powder).
70
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
Fig. 3. Particle size distribution for powders before and after surface-free extraction (filled and empty symbols, respectively). Micellar casein (, ); sodium caseinate (, ); 28% fat, one homogenization step (䊉, ); and 28% fat, three homogenization steps (, ♦).
Fig. 4. Particle size distribution for powders made with micellar casein before and after surface-free extraction (filled and empty symbols, respectively): 19% fat, full formulation (, ); 19% fat, nil emulsifier/stabilizer (䊉, ); 33% fat, nil sucrose (, ).
nificantly compared to their full formulation counterparts. Finally, for powders with no sucrose, the surface fat content increased significantly to values even higher than for full formulations, especially for the sample with 45% fat (10-1p). Samples without sucrose showed the greatest variability during extraction which could be associated to the higher amount of fat on the surface.
pared to the full formulation (57 and 67 mm, respectively), but their d(v, 0.9) was significantly higher, 431 m versus 227 m. On the other hand, powders without sucrose had a higher d(v, 0.5) of 111 m and showed a bimodal size distribution. These results suggested that when there was not enough encapsulating material (sucrose) the powder was more prone to fat globule coalescence during drying, which manifested in bigger particle sizes and a higher amount of surface fat.
3.2. Particle size Particle size of the powders was measured as they were obtained from the drier, but this proved to be very problematic as particles were highly agglomerated/caked. There were practically no particles smaller than 20 m and the shape of the distribution was practically the same for all samples as shown in Fig. 3. The volume weighted median diameter d(v, 0.5) was also very similar for all samples (approximately 480 m). For reference, the d(v, 0.5) for skim milk powder (SMP) is 88 m (not shown). Particle size distribution after surface fat extraction showed a dramatic change compared to the original data. The d(v, 0.5) decreased to values around 60–70 m for all samples. As can be deduced from Fig. 3, surface-free fat is an important factor on particle size measurement as there was a seven-fold increment on the apparent sizes of the powder particles before extraction. Fig. 4 shows the comparison of particle size distributions (before and after surface fat extraction) of powders made with 19% fat with and without emulsifiers/stabilizers (6-1p and 6-1p nil emulsifier) and 33% fat (6-1p nil sucrose). Before extraction, powders showed identical behavior to those previously described—they were highly agglomerated and their d(v, 0.5) was between 350 and 400 m. After extraction, all materials had significantly smaller particle sizes. Powders without emulsifiers/stabilizers had similar d(v, 0.5) com-
3.3. ESCA Surface composition showed that powders were mainly covered by fat regardless of their original bulk composition (Fig. 5). For fully formulated powders, the amount of surface fat ranged from 96 to 81%. Sample 10-1p showed the highest surface fat coverage, whereas the sample with non-micellar casein showed the lowest. When no emulsifiers were used, there was a significant difference in the surface composition of all samples. Powders with micellar casein had reduced surface fat (from 85 to 78%) mainly at the expense of higher protein coverage (6–11%). Powders with non-micellar casein had a similar reduction in surface fat (81–73%) as a result of an exchange for surface protein, which increased from 8 to 15%. The same was true for the 28% fat powder (10-1p); its surface protein value went from 0 to 10%. In the absence of sucrose and emulsifier surface fat content increased for all powders with a concomitant decrease in surface carbohydrate (surface protein remained constant). 3.4. Flowability All powders, regardless of their composition, showed very poor flowability as they did not fall into the lower
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
71
3.5. Wettability
Fig. 5. Surface composition of spray-dried ice cream mixes as dependent on composition. Bulk powder composition for full and nil emulsifier formulations: (+), micellar and non-micellar casein; ( ) micellar casein 10-1p. Bulk composition for nil sucrose systems: (×) micellar and non-micellar casein; ( ) micellar casein 10-1p. Composition after spray drying for (a) micellar casein: () full formula; () nil emulsifier/stabilizer; () nil sucrose. (b) Non-micellar casein: ( ) full formula; ( ) nil emulsifier/stabilizer; ( ) nil sucrose. (c) micellar casein 10-1p: () full formula; ( ) full formula 10-3p; (䊉) nil emulsifier/stabilizer; () nil sucrose.
chamber of the flowability test device as shown in Fig. 6. Initially, we attributed this to the high surface-free fat content, so measurements were repeated after its extraction with petroleum ether. Powders showed a similar nonflowing behavior with only one noticeable difference: the volume occupied by the same mass of powder was drastically reduced, which related to a change in packing characteristics. Before surface fat extraction there was a vast amount of inter-particle voids, which disappeared after extraction.
Measurements of wetting properties showed some variability, but they were sufficient to discriminate among the different treatments. Such variability was a result of (a) how the powder fell into the beaker; and (b) the difficulty of assessing when the last particle was actually sunk. Before surface fat extraction, powders containing 28% fat showed the poorest wettability since they did not wet after 180 min (Fig. 2). For powders with 28% fat (10-1p) but no emulsifier and no emulsifier/sucrose (44% fat), wettability was significantly better as they wetted between 30 and 40 min on average. Powders with 19% fat and micellar casein (61p) needed over 1 h to be reconstituted, while those made with non-micellar casein were wetted in 43 min. Their respective powders without emulsifiers showed significantly shorter average wetting times: 24.5 and 11.5 min for 6-1p and NaCas, respectively. Samples without sucrose/emulsifiers (33% fat) showed average wetting times of 30 and 42 min, respectively. It should be noted that wettability continuously improved for all powders as components were eliminated from the formula, particularly emulsifiers; whereas when sucrose was removed, wettability improved only when compared to the full formulations. Overall, there was a clear correlation between the amount of fat extracted and the wetting time before surface fat extraction. For each of the three powder categories analyzed (i.e. full formulations, nil emulsifier and nil sucrose/emulsifier), the wetting time increased with increasing surface fat, but to different extents. For powders with 19% fat—6-1p and NaCas—wettability was improved by a factor of 10. The powder with 28% fat and one homogenization step was wetted in 29 min, whereas the powder with three homogenization steps wetted in about 15 min. These powders showed the largest improvement since before extraction, they did not wet even after three hours. The only powder without emulsifier analyzed was that containing micellar casein (6-1p). This powder wetted in 5 min which was five times faster than its counterpart with surface-free fat. Powders without sucrose/emulsifiers had the greatest improvement in wettability probably due to the relatively high amount of fat removed; the powders comprised of non-micellar casein showed their wetting time reduced by 50% and finally, those made with micellar casein remained unchanged. 3.6. Microstructure
Fig. 6. Flowability test device showing zero flow for a sample before surfafree fat extraction (left) and after extraction (right).
Micrographs were obtained for the majority of the powders at different magnifications. We centered our attention at the higher magnification pictures so that the focus remained on powder surface characterization. Differences among treatments became obvious at a very high level of magnification (12,800×). Fig. 7 shows that the type of protein rendered a different surface topography: non-micellar casein showed a high level of cracks that covered the particle entirely, in some cases crater-like structures were obvious (Fig. 7B); on
72
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
Fig. 7. Surface microstructure of spray-dried ice cream mixes. Micellar casein: full formulation (A); nil emulsifier/stabilizer (C); nil sucrose (E). Non-micellar casein: full formulation (B); nil emulsifier/stabilizer (D); nil sucrose (F). Micellar casein made with 28% fat: full formulation (G) and nil emulsifier/stabilizer (H). Bar is 5 m.
the other hand, micellar casein showed a smoother surface, but some depressions and cracks were also present (Fig. 7A). Surface imaging for powders without emulsifiers/stabilizers (19% fat) are shown in Fig. 7C and D—micellar and nonmicellar casein, respectively. Cracks were evident (to a lesser degree) for powders with non-micellar casein, but all depressions disappeared; whereas the powder with micellar casein showed a rougher surface and less cracks compared to both, Fig. 7A and D. In the absence of sucrose, the powder with
micellar casein (Fig. 7E) had a higher number of depressions and its sphericity was somewhat distorted by the presence of protrusions. Powders made with non-micellar casein showed no signs of cracks and their surface resembled that of orange peel (Fig. 7F); these powders also showed a high degree of shriveling (not shown). Finally, powders made with 28% fat showed a surface covered by what appeared like sanddunes along with some small depressions. The structure was dramatically different when no emulsifiers were added; the
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
73
surface was very rough and many very deep cracks were observed (Fig. 7G and H).
4. Discussion The lower surface-free fat content in powders containing non-micellar casein suggested that this type of protein has better CO-encapsulation properties than micellar casein. The encapsulating properties of sodium caseinate alone [33], blended with lactose and/or other low and high molecular weight carbohydrates [1,23,34], or with whey proteins [2,3,35] have been extensively studied. The main conclusions have been that (a) sodium caseinate is a better encapsulant than whey proteins since it has a greater emulsifying potential and does not show heat-denaturation during drying; and (b) it is not possible to stabilize a solid-state emulsion having sodium caseinate as both encapsulant and emulsifier since it shrinks during drying and, in excess, it confers instability to the emulsion (even before spraying) due to a mechanism known as depletion flocculation [36,37]. Studies to compare the encapsulating properties of pure micellar casein (i.e. ultrafiltrated skim milk) and those of sodium caseinate are scarce [38]. Our conclusion is that sodium caseinate showed better CO-encapsulation properties than micellar casein because of its looser molecular conformation, higher diffusivity and the strong amphiphilic characteristics of the major individual caseins (␣s1 - and -casein), which might have allowed for a better distribution around the fat globule surface than micellar casein. Sodium caseinate has also been shown to have better resistance against displacement from the oil–water interface by low molecular weight emulsifiers [6,39]; this fact might also have accounted for some of the differences observed. Onwulata et al. [40] showed that anhydrous butter–oil was better encapsulated than milk fat coming from heavy cream, which is consistent with the results shown in this study. In the case of samples with 28% fat (10-3p), the extra homogenization steps made a significant difference by lowering the amount of surface-free fat in comparison to the powder produced with 1 homogenization step (10-1p). For samples with no sucrose added, it was not surprising to find a higher surface-free fat compared to those with added sucrose, since the ratio of non-fat solids to fat was smaller (2.4 instead of 3.9, respectively). The importance of this ratio has been already pointed out by Dollo et al. [41]. Similarly, Buma [13] and Onwulata et al. [42] have reported that the amount of surface fat increased as the total fat content in the original emulsions increased. In the absence of emulsifiers, surface fat coverage decreased by 11, 23 and 31% for samples 6-1p, NaCas and 101p, respectively. We attribute this change to the surface-active nature of emulsifiers. Compared to any of the protein types used in this study, they were the most surface-active components and would be expected to prevail at the air–water interface [43]. They then would be anticipated to displace proteins and reduce their content onto the powder surface;
Fig. 8. DSC melting thermograms for (a) pure anhydrous milk fat; (b) pure emulsifier; (c) surface-free fat extracted from spray-dried ice cream mix; and (d) surface-free fat extracted from pure emulsifier.
ESCA results supported this argument since protein coverage increased at the expense of fat when no emulsifier was used. Nonetheless, the relative atomic composition of the emulsifiers (C 77%, O 23%) did not allow for a clear differentiation between C and O species coming from AMF or the emulsifier per se. This could have caused an over-representation of surface fat. Based on this observation, the melting behavior of surface-free fat extracted from a powder made with non-micellar casein with emulsifier (as it showed the highest difference in surface-free fat content) was measured by DSC and their corresponding thermograms were compared with those of pure AMF, pure emulsifier, and the extracted fraction after subjecting the pure emulsifier to the solvent (following the same protocol as with the powders). The thermograms shown in Fig. 8 illustrate that the surface-free fat extracted from the powder with emulsifier had an identical thermal behavior to pure AMF (traces c and a, respectively). Traces b and d, correspond to the melting behavior of the pure emulsifier and of the fraction obtained after solvent exposure. It can be observed that (i) a negligible amount of fat from the emulsifier was extracted and (ii) that the powder did not show any traces of emulsifier fat (especially those observed in line d between 38 and 44 ◦ C), in other words, surface-free fat came exclusively from milk fat. From this scrutiny we propose that the mechanism by which the addition of emulsifiers increases the surface-free fat in spray-dried emulsions is very similar to what is known as coalescence [33]. The membrane surrounding the fat globule was thinned by the preferential adsorption of emulsifier over proteins, which made the globules more sensitive to shear and/or mechanical stress during drying, manifesting itself as higher surface-free fat contents. Particle size was highly dependent on surface-free fat. A powder maybe considered to have a particle size less than 200 m [44]. Figs. 3 and 4 suggest that before surface fat extraction most of the particles had a size >300 m and that after extraction, approximately 90% of the particles were below 200 m in diameter. This suggested particle caking
74
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75
and/or agglomeration because of surface fat. Our findings are in line with those reported by Foster et al. [25]. Fig. 3 also shows that after extraction, all powders had a “monomodal” size distribution with a very small second peak at higher sizes. Interestingly, for all powders analyzed, the lower the surfacefree fat value (both, by solvent extraction and ESCA), the smaller was its size distribution. This also applied for Fig. 3. Lower surface fat values correlated well with progressively smaller “real” particle sizes. This suggested that bridging or agglomerating potential was the highest for the powder made with non-micellar casein—smallest particle size distribution meant largest surface area and as a result, powders were more prone to caking per mass unit of surface fat [45]. Fig. 4 shows that for samples made with micellar casein without sucrose powders did not exhibit monomodal distributions after fat extraction indicating that the amount of encapsulant (sucrose) was limited and hence, collision and fusion of fat globules took place during drying. A similar behavior was found in samples with non-micellar casein and 28% fat (not shown). Flowability was poor for all powders tested before and after surface fat extraction. Before extraction, the interparticle fat-related caking, hindered flow regardless of the surface-free fat content. This suggested that after a certain plateau was reached in terms of surface-free fat per unit area, cohesion forces did not further increase and differences could not be detected [13]. High cohesion forces were evident by the presence of inter-particle voids (Fig. 6). After fat extraction, such void spaces disappeared and powders packed more efficiently. Nonetheless, there was no flow. We attributed this to the “real” particle size of the powders. Changes in particle properties and storage conditions may influence flowability. Particle size has a major influence on powder flowability, and as the size decreases below 200 m, the flowability deteriorates. The volume weighted median diameter d(v, 0.5) after fat extraction for fully-formulated powders was between 55 and 76 m. Fitzpatrick et al. [44] reported that there was a noticeable decrease in flowability when particle size was reduced from 200 to around 80 m. SMP is a free-flowing powder with a d(v, 0.5) of 88 m (data not shown). This reduction in flowability at smaller particle size was due to the increased surface area per unit mass of powder. More surface area was available for cohesive forces, in particular, and frictional forces to resist flow. As mentioned previously, all powders showed spherical geometries but significantly different topographies. Shriveled particles were detected only for powders comprised of sodium caseinate with no sucrose (not shown). This could have been caused by a net decrease in the amount of encapsulant material available and the aforementioned susceptibility of sodium caseinate to shrinkage during drying. This also suggested that micellar casein was not prone to shrinkage. Previous studies have shown that lactose does not contribute to the formation of folds or cracks during spray drying [13]. If this is the case, the abundance of cracks in the sodium caseinate formula relative to that in the micellar casein pow-
der could be attributed solely to the protein type (Fig. 7A and B, respectively). The use of no emulsifier in powders made with non-micellar casein decreased the number of cracks observed (Fig. 7D). On the other hand, the absence of sucrose created a microstructure very similar to that of whole milk powder in powders made with micellar casein (Fig. 7E) [11]. Research is being undertaken in our laboratory on the encapsulating properties of pure micellar and non-micellar caseins in the presence of different disaccharides as main encapsulants in order to elucidate their interactions and the properties they provide to spray-dried model emulsions.
5. Conclusions A detailed analysis of the surfaces of a number of spraydried ice cream mixes was undertaken. It has been shown that their flowability, wettability and microstructure are strongly dependent on surface fat coverage. In this study, since laboratory spray-dried particles are generally smaller than those manufactured on a commercial basis, the sensitivity of flowability test to surface fat coverage was not as dramatic as in the case of commercial particles. Surface-active materials modified the behavior of the air–water interface and this had an influence on the amount of surface fat measured by ESCA and solvent extraction. Agglomerated particles had fat bonds, which were removed during solvent extraction. Powders showed extremely poor flowability due to very small particle size as well as the sticky nature of the fat surfaces. Wettability was drastically improved after surface-free fat extraction.
Acknowledgements Author Cesar Vega is indebted to the Mexican Consejo Nacional de Ciencia y Tecnologia (CONACyT) and to Dippin’ Dots Inc. for providing financial support to undertake this research. Similarly, travel funding provided by Fractec, Ontario, Canada is appreciated. Thanks also to Dr. John J. Fitzpatrick (University College Cork) for his valuable comments.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
P. F¨aldt, B. Bergenst˚ahl, J. Am. Oil Chem. Soc. 72 (1995) 171. P. F¨aldt, B. Bergenst˚ahl, Food Hydrocolloid. 10 (1996) 421. P. F¨aldt, B. Bergenst˚ahl, Food Hydrocolloid. 10 (1996) 431. D.L. Moreau, M. Rosenberg, Food Struct. 12 (1993) 457. G.P. Pedersen, P. F¨aldt, B. Bergenst˚ahl, H.G. Kristensen, Int. J. Pharm. 171 (1998) 257. C. Vega, H.D. Goff, Y.H. Roos, J. Food Sci. 70 (2005) 244. M. Rosenberg, S.L. Young, Food Struct. 12 (1993) 31. S. Miao, Y.H. Roos, J. Agric. Food Chem. 52 (2004) 5250. K. Jouppila, Y.H. Roos, J. Dairy Sci. 77 (1994) 2907. P.R. Rennie, X.D. Chen, C. Hargreaves, A.R. Mackereth, J. Food Eng. 39 (1999) 277.
C. Vega et al. / Colloids and Surfaces B: Biointerfaces 45 (2005) 66–75 [11] E.H.J. Kim, X.D. Chen, D. Pearce, Colloid. Surf. B: Biointerfaces 26 (2002) 197. [12] N. Ozcan, N. Walisinghe, X.D. Chen, J. Food Eng. 55 (2002) 293. [13] T.J. Buma, Neth. Milk Dairy J. 25 (1971) 107. [14] T.J. Buma, Neth. Milk Dairy J. 25 (1971) 42. [15] E.H.J. Kim, X.D. Chen, D. Pearce, in preparation. [16] B. Freudig, S. Hogekamp, H. Schubert, Chem. Eng. Process. 38 (1999) 525. [17] W. Buchheim, Food Microstruct. 1 (1982) 233. [18] P. F¨aldt, B. Bergenst˚ahl, G. Carlsson, Food Struct. 12 (1993) 225. [19] K. Landstrom, J. Alsins, B. Bergenst˚ahl, Food Hydrocolloid. 14 (2000) 75. [20] A. Millqvist-Fureby, U. Elofsson, B. Bergenst˚ahl, Colloid. Surf. B: Biointerfaces 21 (2001) 47. [21] K.L. Christensen, G.P. Pedersen, H.G. Kristensen, Int. J. Pharm. 212 (2001) 187. [22] K.L. Christensen, G.P. Pedersen, H.G. Kristensen, Eur. J. Pharm. Biopharm. 53 (2002) 147. [23] A. Millqvist-Fureby, Colloid. Surf. B: Biointerfaces 31 (2003) 65. [24] M. Peleg in, M. Peleg, E. Bagley (Eds.), Physical Properties of Foods, AVI Publishing, 1983, pp. 293–323. [25] K.L. Foster, J. Bronlund, T. Paterson, Int. Dairy J. 15 (2005) 85. [26] M. Peleg in, J.M.V. Blanshard, P.J. Lillford (Eds.), The Glassy State of Foods, Nottingham University Press, 1993, pp. 435–451. [27] M. Peleg, J. Food Process. Preserv. 1 (1977) 303. [28] B. Provent, D. Chulia, J. Cary, Eur. J. Pharm. Biopharm. 39 (1993) 202. [29] E.H.J. Kim, X.D. Chen, D. Pearce, Colloid. Surf. B: Biointerfaces 42 (2005) 1.
75
[30] E.H.J. Kim, Surface composition of spray-dried dairy powders and its formation mechanism(s). PhD Thesis. The University of Auckland, New Zealand, 2005. [32] E. Teonou, J.J. Fitzpatrick, J. Food Eng. 42 (1999) 109. [33] S.A. Hogan, B.F. McNamee, E.D. O’Riordan, M. O’Sullivan, J. Agric. Food Chem. 49 (2001) 1934. [34] S.A. Hogan, B.F. McNamee, E.D. O’Riordan, M. O’Sullivan, Int. Dairy J. 11 (2001) 137. [35] E.L. Sliwinski, B.W.M. Lavrijsen, J.M. Vollenbroek, H.J. van der Stege, M.A.J.S. Van Boekel, J.T.M. Wouters, Colloid. Surf. B: Biointerfaces 31 (2003) 219. [36] E. Dickinson, M. Golding, Food Hydrocolloid. 11 (1997) 13. [37] C. Vega, D.G. Dalgleish, H.D. Goff, Food Hydrocolloid. 19 (2005) 187. [38] M.K. Keogh, B. O’Kennedy, J. Kelly, M. Auty, A. Fureby, A.M. Haahr, J. Food Sci. 66 (2001) 217. [39] E. Davies, E. Dickinson, R.D. Bee, Food Hydrocolloid. 14 (2000) 145. [40] C.I. Onwulata, P.W. Smith, P.H. Cooke, h.G. Holsinger, Lebensm. -Wiss. u. -Technol. 29 (1996) 163. [41] G. Dollo, P. Le Corre, A. Guerin, F. Chevanne, J.L. Burgot, R. Leverge, Eur. J. Pharm. Sci. 19 (2003) 273. [42] C.I. Onwulata, P.W. Smith, J.C. Craig, h.G. Holsinger, J. Food Sci. 59 (1994) 316. [43] A. Millqvist-Fureby, N. Burns, K. Landstrom, B. Bergenst˚ahl, in: E. Dickinson, J. Rodriguez-Patino (Eds.), Food Emulsions and Foams, Royal Society of Chemistry, London, 1999, pp. 236–245. [44] J.J. Fitzpatrick, T. Iqbal, C. Delaney, T. Twomey, M.K. Keogh, J. Food Eng. 64 (2004) 435. [45] T.J. Buma, Neth. Milk Dairy J. 25 (1971) 53.