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Physical and Microscopic Characterization of Dry Whole Milk with Altered Lactose Content. 1. Effect of Lactose Concentration
Physical and Microscopic Characterization of Dry Whole Milk with Altered Lactose Content. 1. Effect of Lactose Concentration CARLOS A. AGUILAR and GREGORY R. ZIEGLER Department of Food Science The Pennsylvania State University University Park 16802 ABSTRACT
The microstructures of spray-dried whole milk powders with 0, 27, 38, 48, 58, and 67% lactose were evaluated. Delactosed milk concentrates were prepared by ultrafiltration and diafiltration, and the lactose concentration in the powders was adjusted by addition of lactose to the spray-dried feed. Milk powder particles with 0% lactose had a more porous matrix with deep dents and wrinkles; lower true and apparent particle densities; and larger median diameter, vacuole volume, surface area, and free fat content. Higher lactose concentrations produced nearly spherical particles with a less porous matrix; higher true and apparent particle densities; and smaner median diameter, vacuole volume, surface area, and free fat content. Whole milk powder with modified lactose content may have potential in the manufacture of milk chocolate. (Key words: lactose, microstructure, spray-dried whole milk powder) Abbreviation key: DWM
= dry
whole milk.
INTRODUCTION
Proposed changes in the standards of identity for milk chocolate will permit the addition of lactose (14) as a substitute for sucrose in the form of high lactose milk powders. The efficient use of these lactose-enriched powders will be directly affected by their physical structure. The microstructure of spray-dried milk is affected by the method of atomization (nozzle
Received August 2. 1993. Accepted November 29, 1993. 1994 J Dairy Sci 77:1189-1197
or centrifugal) and dryer operating conditions: feed temperature, feed concentration, pump pressure, presence of steam, inlet air temperature, outlet air temperature, spray pressure, nozzle size, velocity of the atomizer (8, 11, 13, 23, 24, 26), and composition (10, 18). The main components of dry whole milk (DWM) are carbohydrates (principally lactose), protein, and fat; on a weight basis, lactose is the main component. In spray-dried whole milk powder, amorphous lactose forms a continuous matrix in which proteins, fat globules, and air cells (i.e., vacuoles) are dispersed (16). Micrographs of low heat and high heat skim milk powders show casein micelles, air cells, and fat globules as distinct entities dispersed in an amorphous lactose matrix (3). During spray drying, large concentration gradients are produced in the droplets as a result of fast water evaporation on the surface and slow water diffusion from the inside that lead to the development of a high viscosity layer on the surface and, eventually, a rigid crust (24). Water evaporation causes droplet size reduction, resulting in deep dents and folds if large air bubbles are present (23). During spray drying, the liquid feed is accelerated to high velocities inside the atomizer, dispersing air bubbles into the liquid. Upon leaving the atomizer, larger bubbles are eliminated, but others remain enclosed in the droplets (23, 24). Steam has been introduced into atomizers, replacing the air-liquid interface with a steam-liquid interface, to increase the particle density and to reduce the vacuole volume by reducing the amount of air occluded during atomization (26). In droplets that do not contain air bubbles, vacuoles will not develop, and shrinkage will occur only at the particle surface (24). Nozzle atomization can produce particles with higher density but requires lower feed concentrations than centrifugal atomization (11).
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Skim milk powder particles are round and usually smooth but can also be wrinkled (11, 15). Burna and Henstra (10) observed that spray-dried lactose particles are spherical with practically no surface folds or dents, but particles of spray-dried calcium caseinate and skim milk have dents and surface folds; they (10) hypothesized that dents and surface folds are produced when casein is the predominant component in spray-dried powders. Roetrnan (20) also observed that particles of spray-dried amorphous lactose and spray-dried whey powder have smooth surfaces without dents or folds. However, low fat, high protein milk powder particles with lactose content >38% have a wrinkled surface (18). Mistry and Hassan (17) and Mistry et al. (18) observed particles of delactosed, low fat, high protein milk powder that had smooth surfaces, large dents, and vacuoles that vary in size from 2 to 33 p. in diameter and some large hollow particles having a wall thickness of approximately 2 p.. Wrinkles are not only a result of powder composition but can be formed by high inlet air temperature or large temperature differences between the hot air and the milk droplets (11). The desired final viscosity of chocolate mass is usually attained during conching by the addition of cocoa butter and surface-active agents (e.g., lecithin), both of which are expensive ingredients. Therefore, a chocolate structure is desired in which a minimum of cocoa butter produces the proper flow behavior. Verhey (25) discussed the suitability of rollerand spray-dried milk powders for chocolate making. He demonstrated that consumption of energy and cocoa butter was lower when roller-dried milk powder was used. This difference was attributed to the higher free fat content. Only a portion of the total fat can be extracted by organic fat solvents under standardized conditions (free fat). Some of the fat in DWM is present on the surface of the particles in the form of pools and inside the particles in capillary pores and cracks. Another portion of the fat, which is present in larger amounts in homogenized milk than in unhomogenized milk, is encapsulated in the powder particles, where it is complexed with milk proteins. This encapsulated fat cannot be extracted easily by fat solvents and is resistant to oxidation (6). Journal of Dairy Science Vol. 77, No.5, 1994
In a model proposed by Burna (6) and confirmed by Buchheim (3), the free fat originates from four different sources: 1) surface fat, 2) outer layer fat (i.e., fat from fat globules in the outer layer, which are directly exposed to the solvent), 3) capillary fat (i.e., fat from fat globules in the interior of the powder particles that is extracted through capillary pores or cracks), and 4) dissolution fat (i.e., fat from internal fat globules that can be reached by solvents via the holes left by dissolved fat globules in the outer particle layer close to wide capillaries). The dissolution fat extracted from the interior of milk powder particles contributes a high proportion to the total free fat (3).
The microstructure of skim or low fat milk powders has been studied before as a function of lactose and protein contents. Our objective was to study the effect of lactose content on the microstructure of spray-dried DWM. MATERIALS AND METHODS Statistical Analysis
Data were analyzed in a one-way ANOVA; lactose concentration in the DWM was the independent variable. Six nominal lactose concentrations were considered: 0, 30, 40, 50, 60, and 70%. Means were separated using Tukey' s method of multiple comparisons at P = .05 (19). Calculations used the general linear models procedure of SAS (22). Concentration of Milk
Powders with altered lactose concentration were prepared according to the method of Aguilar and Ziegler (1). Homogenized, pasteurized whole milk (230 kg) was obtained from The Pennsylvania State University Creamery (University Park). The milk was ultrafiltered (55°C) using a sanitary pilot plant unit (model S-I; Abcor, Inc., Wilmington, MA) equipped with 5.6-m2 spiral wound membranes (nominal molecular weight cutoff of 5000 or 10,(00) at a mean transmembrane pressure of 310 kPa (concentration volume ratio = 5.3). The retentate was diafiltered with two volumes (2 x 187 kg) of water that was chlorinated (20 ppm of free chlorine) and treated by reverse osmosis. Approximately 29 kg of the final retentate
LACTOSE AND THE MICROSTRUCTURE OF MILK POWDER
(33.5% total solids, .18% lactose) were collected and stored at 4"C prior to spray drying. Spray Drying
Lactose concentration of the spray-dried feed was adjusted to yield DWM with nominal lactose concentrations of 0, 30, 40, SO, 60, and 70% (wt/wt). Each DWM was prepared in duplicate. Food-grade lactose monohydrate (Land 0' Lakes, Minneapolis, MN) was dissolved in water at SS"C (approximately I h); the concentration varied from 28 to 39%, depending on the desired lactose content of the final DWM. Lactose solution and retentate were mixed (30 min at 5S"C) to achieve the final lactose concentration at a constant feed concentration of 30% total solids (wt/wt). Approximately 1.2 kg of each powder were produced using a Portable Spray Dryer (Niro Atomizer, Hudson, WI): inlet air temperature, 160 to 170'C; outlet air temperature, 70 to 80'C; feed temperature, 5S'C; feed rate, 30 mll min; and centrifugal atomizer, operated at 27,000 rpm. Powders were stored in sealed plastic containers at 20 to 2S'C until analysis. Analyses
Chemical Composition. Lactose concentration was determined by AOAC method 984.15, protein by method 930.29, fat by method 932.06, moisture by method 927.05, and ash by method 930.30 (2). x-Ray Diffraction. The x-ray diffraction pattern of DWM was determined between 10 and 30' using an automated x-ray diffractometer (Rigaku Denki, Co. Ltd., Tokyo, Japan). Particle Size Distribution. Particle size distribution was measured using the MasterSizer® laser light-scattering particle size analyzer equipped with an MS 15 sample presentation unit and lOO-mm lens (Malvern Instruments Ltd., Malvern, England). The DWM was dispersed in isobutanol (Fisher Scientific, Pittsburgh, PA) at room temperature (20 to 22"C) until the obscuration value was .2, corresponding to a volume concentration of .03 to .05%. Ultrasonic dispersion (SO W at 27 kHz for S min) and mechanical stirring were applied to ensure that particles were independently dispersed, and particle size distribution was measured. Scanning Electron Microscopy. The DWM particles were picked up directly onto double
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sticky tape attached to aluminum stubs (lO-mm diameter) and sputter-coated with gold using an argon plasma (International Scientific Instruments, Topcon, Pleasanton, CA) operating at 10 rnA and 1.4 kV for 8 min. A drop of silver paint was applied to the edge of the coated double sticky tape to improve grounding. The samples were viewed with an International Scientific Instruments Model ISI-60 (Topcon) or a JEOL S400 (JEOL U.S.A., Peabody, MA) scanning electron microscope operated at S or to kV. Free Fat. Free fat was determined by suspending 2.S g of DWM in 40 ml of petroleum ether, boiling point 40 to 60"C (Fisher Scientific), for 1 h (7, 12). Every to min, slow mixing was applied for 30 s using an orbital shaker (Sensaur Platform Rotator®; Haydon Mfg. Co., operating at 200 rpm. Fat Inc., Torrington, extracts were filtered under vacuum using glass microfiber filter paper GFfF (Whatrnan International Ltd., Maidstone, England), and the solvent was evaporated. The extracts were dried in a convection oven at ISO"C for 1 h and weighed at room temperature. Apparent and True Particle Densities and Vacuole Volume. Apparent particle density was measured using light mineral oil, 158 maximum Saybolt viscosity, as the displacement liquid (Fisher Scientific). The DWM (I g) and light mineral oil (10 ml) were placed in 25-ml specific gravity bottles (Fisher Scientific), stirred for 10 s using a vortex stirrer, and left undisturbed for 20 min to allow air bubbles to escape before the bottles were filled to the calibration mark. Mass was established to the closest .1 mg using an analytical balance (12). True density was measured using an air comparison pycnometer (model 930; Beckman Instruments, Inc., Fullerton, CA) with helium (20 to 22"C, 34.5 kPa) as the displacement gas (5). The DWM (5 g) was placed in the measuring cup, and the air was evacuated by moving the pistons inward and outward two to three times. The gas was allowed to penetrate the milk sample for 5 min, and then the sample volume was measured. Vacuole volume was calculated from measurements of apparent particle density (PJ and true particle density (Pt) according to the following equation (12):
en
vacuole volume
1 1 = [-] Pa Pt
mlllOO g.
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TABLE 1. Composition of milk powders. Nominal lactose
Lactose
Moisture
.3 1 27.4e 37.7 b 47.7 C 58.1 b 67.2"
2.31" 2.35" 1.75" 1.92" 1.63" 1.6oa .71
Protein
Fat
Ash
41.2" 28.6 b 24.OC 19.5 d 16.8" 12.21 1.30
50.4" 36.5 b 32.6c 28.Qd 20.00 15.6f 1.17
3.61" 2.63 b 2.37c 1.93d 1.57e 1.221 .10
(gIIOO g) 0 30 40 50 60 70 SE
.40
".b.c.d.e.fMeans followed by the same superscript are not significantly different (P > .05).
Surface Area and Pore Size Distribution. A mercury porosimeter (model PS-33; Quantachrome Corp., Syosset, NY) with 2-mm (3ml) glass cell and distilled mercury (Fisher Scientific) was used to determine surface area and pore size distribution. The DWM (.15 g) was placed in the glass cell, which was then filled with mercury using a mercury filling apparatus (Quantachrome Corp.) operating at an absolute pressure of 2.1 kPa. The relationship between penetration volume and pressure was measured between 0 and 227.5 MPa. The limiting pore size for mercury penetration was calculated from the penetration pressure assuming circular pore openings, a surface tension for mercury of 4.8 x 10-5 J/cm2 , and a contact angle between mercury and the material of 140· (27).
contents (30%) and other processing conditions were kept constant for all powders to minimize variations. The initial aim in this study was to use higher total solids in the spray-dried feed (60 to 70%); however, lower total solids were required as a result of poor performance of the rotary atomizer at low lactose concentrations. High viscosity at low lactose concentration influenced atomization and was a likely cause of the larger size and greater vacuole volume for DWM containing 0% lactose. The general morphology of the DWM powders was similar to that observed in other studies (11, 15). The absence of lactose crystals confirms the x-ray diffraction patterns in
RESULTS AND DISCUSSION
Table 1 presents the composition of the DWM powders. Lactose concentration had no significant effect on the moisture content of the final powder. The absence of characteristic peaks in the x-ray diffraction pattern indicated that lactose was present in the amorphous state. All DWM powders had unimodal, volumebased particle size distributions (Figure 1). The median volume-based diameter was used to represent the average particle size. The DWM particles containing 0% lactose had a significantly larger median volume-based diameter than that of other lactose concentrations (Table 2). The larger particle size of DWM containing 0% lactose was also confirmed by scanning electron micrographs (Figure 2). Total solids Journal of Dairy Science Vol. 77, No.5, 1994
20
15 ~
e
=
'0
>
10
~ 5
0 .1
1
10
100
1000
Particle size (Ilm) Figure 1. Volume-based particle size distribution of milk powders containing 0% (e) and 67% lactose (0).
LACTOSE AND THE MICROSTRUCTURE OF MILK POWDER
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TABLE 2. Median volume-based diameter. true and apparent densities. and vacuole volume of milk powders. Median volumebased diameter
True density
(%)
(P)
- - - - (glml) - - - -
(ml/100 g)
.3 27.4 37,7 47.7 58.1 67.2 SE
28.514.0b 12.6b 13.lb 14.3 b l4.2 b 2.0
1.1211.190b 1.233 bc 1.278< l.34()d l.346 d
52.638.1 b 40,7 b 38.4b 37.9b 34.5b 2.3
Lactose
.017
Apparent density
.705.819b .82Ib .857 bc .88900 ,919 d .013
Vacuole volume
-,b.c.dMeans followed by the same superscript are not significantly different (P > .05).
which no characteristic peaks were observed. Deep dents and wrinkles in spray-dried milk particles may be a result of high casein concentration (10). This phenomenon was confirmed in milk samples with 0% nominal lactose (i.e., high protein concentration), which had a smooth surface, but many deep dents, indicating the presence of large air bubbles during drying that produced milk particles with large vacuoles. Figure 2, part I, bottom left) shows a broken particle (0% nominal lactose) with a large central vacuole and a thin wall with many small vacuoles. Milk particles became smaller and more spherical with smaller dents and wrinkles as lactose concentration increased (Figure 2, part I, right side; part 2). Differences in dents and wrinkles have been observed (21) to be a function of particle size; small particles have smooth surfaces and no dents, and large particles show wrinkled surfaces and dents. However, the results presented in the present study show that wrinkles and dents were not a function of particle size but were dependent on lactose concentration. For example, dents and wrinkles were large in DWM particles with 0 and 27% lactose even though the DWM samples had significantly different median volume-based diameter. However, milk powder particles with 67% lactose had no large dents or wrinkles, even though the median volume-based diameter was the same as that for powders with 27 and 38% lactose. Vacuoles in milk powders are initially formed during atomization by the entrapment of air bubbles (23); during drying, these bubbles can expand and form large vacuoles or escape, leaving an empty structure that collapses and shrinks under further drying (Figure
2, part 1, top left). Evacuation of air bubbles may occur during droplet formation (by inertia) and drying (by diffusion); vacuoles are formed by air bubbles that do not dissolve before the particle solidifies (23). A high protein content (i.e., low lactose) increases viscosity, may decrease air bubble diffusion to the surface, and promotes rapid formation of a hard casing, which increases the probability of air entrapment and vacuole formation. At high lactose concentrations, small cracks were observed on the particle surface (Figure 2, part 2, top right). Higher lactose concentrations increased the true and apparent densities of milk powders (Table 2), in agreement with previous studies on spray-dried materials (4), which indicated that true densities of whole milk (1.28 glml), milk fat (.94 glml), and calcium caseinate phosphate complex (1.39 glml) are lower than that of amorphous lactose powder (1.52 glml). The vacuole volume of the 0% lactose powder was higher than those of lactose-containing powders and was constant for milk powders with lactose concentrations between 27 and 67% (Table 2). The increase in vacuole volume with higher protein content (i.e., low lactose) can be explained by the entrapment of air bubbles as a result of increased viscosity. The data suggest that a critical lactose concentration may exist (Le., protein concentration) that facilitates the escape of air bubbles during drying and that higher concentrations of lactose have no effect on vacuole volume. Surface area was inversely related to lactose concentration (Table 3). This relationship could not be explained solely on the basis of differences in particle size. For example, milk powders with 0% lactose had the largest median, Journal of Dairy Science Vol. 77. No.5. 1994
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volume-based diameter and the largest surface area, whereas samples with 27. 38.48. 58, and 67% lactose had similar particle sizes. but their surface areas were a function of lactose concentration. Surface area by mercury porosimetry is a measure of internal and external irregularities. For two milk powders with the same particle size. a larger surface area indicates the presence of particles with irregularities such as cracks. holes. dents, wrinkles. and pores. A smaller surface area for the same particle size indicates the presence of smooth particles without such irregularities. A less porous matrix was produced as a result of increased lactose concentration (Figure 3). Milk powders with 0 and 27% lactose (Figure 3. A and B) showed pores of 20 to 30 nm that were not present at higher lactose concentrations (Figure 3C). Also, at high lactose concentra-
tions. small pores (<20 nm) were less numerous (Figure 3C). The presence of holes and cracks in the milk particles may be important during chocolate processing. Holes and cracks may determine the fracture pattern during refining, resulting in changes in particle size distribution as a function of lactose concentration in the powders. More cocoa butter and emulsifiers are required to coat particles with holes and cracks. Breaking of milk particles with high vacuole volume and high porosities exposes internal surfaces that increase the surface area of milk powders even more. Milk powders with high vacuole volume also had high surface areas and porosities. Milk powders with increased lactose concentration had less total fat to be extracted (Table 1). Therefore. the total possible free fat decreased as the lactose concentration in-
Figure 2. Milk powders with lactose concentrations. Part I: 0% (top left), 0% (bottom left), 27% (top right), and 38% (bottom right). Journal of Dairy Science Vol. 77, No.5, 1994
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LACTOSE AND THE MICROSTRUCTURE OF MILK POWDER TABLE 3. Surface area, free fat, and percentage of free fat of milk powders. Lactose (%)
.3 27.4 37.7 47.7 58.1 67.2 SE
Surface area
Free fat
Percentage of free fat
(m2/g) 18.5a 14.O"b 13.9ab 1O.3 bc 8.84bc 7.19c 1.14
(g/loo g of powder)
(gil 00 g of fat)
M.O" 15.2b 4.83c 1.76d 1.34d 1.43d 1.31
87.3" 41.6b 14.8< 6.29< 6.7rf 9.17< 3.71
a.b.c.dMeans followed by the same superscript are not significantly different (P > .05).
creased (Table 3). Furthermore, the ratio of free fat to total fat (percentage of free fat) also decreased as the lactose concentration increased (Table 3). The pores or cracks present in whole milk powder particles are related to the free fat content. Fat globules in milk powder particles can be reached by the penetration of fat solvents into the pores and cracks (9).
CONCLUSIONS
Lactose concentration of spray-dried milk affected the microstructure of DWM particles. The DWM particles with 0% lactose had a more porous matrix with deep dents, wrinkles, and large vacuoles; lower true and apparent particle densities; and larger median, volumebased diameter, surface area, vacuole volume,
Part 2: 48% (top left), 58% (bottom left), and 67% (top right). Journal of Dairy Science Vol. 77. No.5, 1994
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AGUILAR AND ZIEGLER
6,...-----,---------------,
A
5 4 3
and free fat content. Higher lactose concentrations produced nearly spherical particles with a less porous matrix; higher true and apparent particle densities; and smaller median volumebased diameter, surface area, vacuob volume, and free fat content.
REFERENCES
2
I Aguilar, C. A., and G. R. Ziegler. 1993. Lactose crystallization in spray-dried milk powders exposed to
Oh~....,.---.--::'~.--~e:=~~
1
10
100
1000
10000
Pore radius (om) 6 .-------------::8" 5
4 3 2
....~~.......l
O+---~~~.........;~-
1
10
100
1000
10000
Pore radius (nm)
6.--------------..., C 5 4
3 2
. .~:......;.........l
O+---~~--..:::~~--
I
10
100
1000
10000
Pore radius (om) Figure 3. Pore size distributions of milk powders with lactose concentrations of 0% (Al, 27% (8). and 67% (C). Journal of Dairy Science Vol.
n,
No.5. 1994
isobutanol. Food StruCI. 12:43. 2 Association of Official Analytical Chemists. 1990. Official Methods of Analysis. 15th ed. AOAC. Arlington. VA. 3 Buchheim, W. 1982. Electron microscopic localization of solvent-extractable fat in agglomerated spray-dried whole milk powders. Food Microstruct. 1:233. 4 Burna, T. J. 1%5. The true density of spray milk powders and of certain constituents. Neth. Milk Dairy 1. 19:249. 5 Burna. T. 1. 1966. The physical structure of spray milk powder and the changes which take place during moisture absorption. Netb. Milk Dairy J. 20:91. 6 Burna. T. J. 1971. Free fat in spray-dried whole mille I. General introduction and brief review of literature. Neth. Milk Dairy 1. 25:33. 7 Burna. T. 1. 1971. Free fat in spray-dried whole milk. 2. An evaluation of methods for the determination of free-fat content. Netb. Milk Dairy J. 25:42. 8 Burna. T. 1. 1971. Free fat in spray-dried whole milk. 3. Particle size. Its estimation. influence of processing parameters and its relation to free-fat content. Netb. Milk Dairy J. 25:53. 9 Burna. T. 1. 1971. Free fat in spray-dried whole milk. 8. The relation between free-fat and particle porosity of spray-dried whole milk. Neth. Milk Dairy J. 25: 123. 10 Burna. T. J., and S. Henstra. 1971. Particle structure of spray-dried caseinate and spray-dried lactose as observed by scanning electron microscope. Netb. Milk Dairy J. 25:278. II Cari~. M., and M. Kal6b. 1987. Effects of drying tecbniques on milk powders quality and microstructure: a review. Food Microstruct. 6:171. 12 Daemen. ALH. 1982. The estimation of the mean particle density, the vacuole volume a.I?d the porosity of spray-dried porous powders. Neth. Milk Dairy 1. 36:53. 13 DeViider. J., R. Martens. and M. Naudts. 1976. Influence of process variables on some whole milk powder characteristics. Milchwissenschaft 31:396. 14 Food and Drug Administration. 1992. Cacao products. Amendment of the standards of identity. Fed. Reg. 57: 23989. 15 Kal6b. M. 1979. Scanning electron microscopy of dairy products: an overview. Scanning Electron Microsc. 3:261. 16 King. N. 1%5. The physical structure of dried milk. Dairy Sci. Abstr. 27:91. 17 Mistry, V. V., and H. N. Hassan. 1991. Delactosed. high milk protein powder. 2. Physical and functional properties. 1. Dairy Sci. 74:3716.
LACTOSE AND THE MICROSTRUCTURE OF MILK POWDER 18 Mistry, V. V., H. N. Hassan, and D. I. Robinson. 1992. Effect of lactose and protein on the microstructure of dried milk. Food Struct. II :73. 19 Neter, I., W. Wasserman, and M. H. Kutner. 1990. Applied Linear Statistical Models. 3rd ed. Richard D. Irwin, Inc., Homewood, IL. 20 Roetman, K. 1979. Crystalline lactose and the structure of spray-dried milk products as observed by scanning electron microscopy. Neth. Milk Dairy I. 33: I. 21 Saito, Z. 1985. Particle structure in spray-dried whole milk and instant skim milk powder related to lactose crystallization. Food Microstruct. 4:333. 22 SAS~ User's Guide: Statistics, Version 5 Edition. 1985. SAS Inst.. Inc., Cary, NC. 23 Verhey, I.G.P. 1972. Vacuole fonnation in spray pow-
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der particles 2. Location and prevention of air incorporation. Neth. Milk Dairy J. 26:203. 24 Verhey, J.G.P. 1972. Vacuole fonnation in spray powder particles 3. Atomization and droplet drying. Neth. Milk Dairy J. 27:3. 25 Verhey, J.G.P. 1986. Physical properties of dried milk in relation to chocolate manufacture. Neth. Milk Dairy 1. 40:261. 26 Verhey, I.G.P., and E. A. Vos. 1971. Air-free atomization: a method for producing spray powders without vacuoles. Neth. Milk Dairy 1. 25:73. 27 Wikberg, M., and G. Alderborn. 1992. Compression characteristics of granulated materials: VI. Pore size distributions, assessed by mercury penetration, of compacts of two lactose granulations with different fragmentation properties. Int. 1. Pharmacol. 84: 191.
Journal of Dairy Science Vol. 77, No.5, 1994
The microstructures of spray-dried whole milk powders with 0, 27, 38, 48, 58, and 67% lactose were evaluated. Delactosed milk concentrates were prepared by ultrafiltration and diafiltration, and the lactose concentration in the powders was adjusted by addition of lactose to the spray-dried feed. Milk powder particles with 0% lactose had a more porous matrix with deep dents and wrinkles; lower true and apparent particle densities; and larger median diameter, vacuole volume, surface area, and free fat content. Higher lactose concentrations produced nearly spherical particles with a less porous matrix; higher true and apparent particle densities; and smaner median diameter, vacuole volume, surface area, and free fat content. Whole milk powder with modified lactose content may have potential in the manufacture of milk chocolate. (Key words: lactose, microstructure, spray-dried whole milk powder) Abbreviation key: DWM
= dry
whole milk.
INTRODUCTION
Proposed changes in the standards of identity for milk chocolate will permit the addition of lactose (14) as a substitute for sucrose in the form of high lactose milk powders. The efficient use of these lactose-enriched powders will be directly affected by their physical structure. The microstructure of spray-dried milk is affected by the method of atomization (nozzle
Received August 2. 1993. Accepted November 29, 1993. 1994 J Dairy Sci 77:1189-1197
or centrifugal) and dryer operating conditions: feed temperature, feed concentration, pump pressure, presence of steam, inlet air temperature, outlet air temperature, spray pressure, nozzle size, velocity of the atomizer (8, 11, 13, 23, 24, 26), and composition (10, 18). The main components of dry whole milk (DWM) are carbohydrates (principally lactose), protein, and fat; on a weight basis, lactose is the main component. In spray-dried whole milk powder, amorphous lactose forms a continuous matrix in which proteins, fat globules, and air cells (i.e., vacuoles) are dispersed (16). Micrographs of low heat and high heat skim milk powders show casein micelles, air cells, and fat globules as distinct entities dispersed in an amorphous lactose matrix (3). During spray drying, large concentration gradients are produced in the droplets as a result of fast water evaporation on the surface and slow water diffusion from the inside that lead to the development of a high viscosity layer on the surface and, eventually, a rigid crust (24). Water evaporation causes droplet size reduction, resulting in deep dents and folds if large air bubbles are present (23). During spray drying, the liquid feed is accelerated to high velocities inside the atomizer, dispersing air bubbles into the liquid. Upon leaving the atomizer, larger bubbles are eliminated, but others remain enclosed in the droplets (23, 24). Steam has been introduced into atomizers, replacing the air-liquid interface with a steam-liquid interface, to increase the particle density and to reduce the vacuole volume by reducing the amount of air occluded during atomization (26). In droplets that do not contain air bubbles, vacuoles will not develop, and shrinkage will occur only at the particle surface (24). Nozzle atomization can produce particles with higher density but requires lower feed concentrations than centrifugal atomization (11).
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Skim milk powder particles are round and usually smooth but can also be wrinkled (11, 15). Burna and Henstra (10) observed that spray-dried lactose particles are spherical with practically no surface folds or dents, but particles of spray-dried calcium caseinate and skim milk have dents and surface folds; they (10) hypothesized that dents and surface folds are produced when casein is the predominant component in spray-dried powders. Roetrnan (20) also observed that particles of spray-dried amorphous lactose and spray-dried whey powder have smooth surfaces without dents or folds. However, low fat, high protein milk powder particles with lactose content >38% have a wrinkled surface (18). Mistry and Hassan (17) and Mistry et al. (18) observed particles of delactosed, low fat, high protein milk powder that had smooth surfaces, large dents, and vacuoles that vary in size from 2 to 33 p. in diameter and some large hollow particles having a wall thickness of approximately 2 p.. Wrinkles are not only a result of powder composition but can be formed by high inlet air temperature or large temperature differences between the hot air and the milk droplets (11). The desired final viscosity of chocolate mass is usually attained during conching by the addition of cocoa butter and surface-active agents (e.g., lecithin), both of which are expensive ingredients. Therefore, a chocolate structure is desired in which a minimum of cocoa butter produces the proper flow behavior. Verhey (25) discussed the suitability of rollerand spray-dried milk powders for chocolate making. He demonstrated that consumption of energy and cocoa butter was lower when roller-dried milk powder was used. This difference was attributed to the higher free fat content. Only a portion of the total fat can be extracted by organic fat solvents under standardized conditions (free fat). Some of the fat in DWM is present on the surface of the particles in the form of pools and inside the particles in capillary pores and cracks. Another portion of the fat, which is present in larger amounts in homogenized milk than in unhomogenized milk, is encapsulated in the powder particles, where it is complexed with milk proteins. This encapsulated fat cannot be extracted easily by fat solvents and is resistant to oxidation (6). Journal of Dairy Science Vol. 77, No.5, 1994
In a model proposed by Burna (6) and confirmed by Buchheim (3), the free fat originates from four different sources: 1) surface fat, 2) outer layer fat (i.e., fat from fat globules in the outer layer, which are directly exposed to the solvent), 3) capillary fat (i.e., fat from fat globules in the interior of the powder particles that is extracted through capillary pores or cracks), and 4) dissolution fat (i.e., fat from internal fat globules that can be reached by solvents via the holes left by dissolved fat globules in the outer particle layer close to wide capillaries). The dissolution fat extracted from the interior of milk powder particles contributes a high proportion to the total free fat (3).
The microstructure of skim or low fat milk powders has been studied before as a function of lactose and protein contents. Our objective was to study the effect of lactose content on the microstructure of spray-dried DWM. MATERIALS AND METHODS Statistical Analysis
Data were analyzed in a one-way ANOVA; lactose concentration in the DWM was the independent variable. Six nominal lactose concentrations were considered: 0, 30, 40, 50, 60, and 70%. Means were separated using Tukey' s method of multiple comparisons at P = .05 (19). Calculations used the general linear models procedure of SAS (22). Concentration of Milk
Powders with altered lactose concentration were prepared according to the method of Aguilar and Ziegler (1). Homogenized, pasteurized whole milk (230 kg) was obtained from The Pennsylvania State University Creamery (University Park). The milk was ultrafiltered (55°C) using a sanitary pilot plant unit (model S-I; Abcor, Inc., Wilmington, MA) equipped with 5.6-m2 spiral wound membranes (nominal molecular weight cutoff of 5000 or 10,(00) at a mean transmembrane pressure of 310 kPa (concentration volume ratio = 5.3). The retentate was diafiltered with two volumes (2 x 187 kg) of water that was chlorinated (20 ppm of free chlorine) and treated by reverse osmosis. Approximately 29 kg of the final retentate
LACTOSE AND THE MICROSTRUCTURE OF MILK POWDER
(33.5% total solids, .18% lactose) were collected and stored at 4"C prior to spray drying. Spray Drying
Lactose concentration of the spray-dried feed was adjusted to yield DWM with nominal lactose concentrations of 0, 30, 40, SO, 60, and 70% (wt/wt). Each DWM was prepared in duplicate. Food-grade lactose monohydrate (Land 0' Lakes, Minneapolis, MN) was dissolved in water at SS"C (approximately I h); the concentration varied from 28 to 39%, depending on the desired lactose content of the final DWM. Lactose solution and retentate were mixed (30 min at 5S"C) to achieve the final lactose concentration at a constant feed concentration of 30% total solids (wt/wt). Approximately 1.2 kg of each powder were produced using a Portable Spray Dryer (Niro Atomizer, Hudson, WI): inlet air temperature, 160 to 170'C; outlet air temperature, 70 to 80'C; feed temperature, 5S'C; feed rate, 30 mll min; and centrifugal atomizer, operated at 27,000 rpm. Powders were stored in sealed plastic containers at 20 to 2S'C until analysis. Analyses
Chemical Composition. Lactose concentration was determined by AOAC method 984.15, protein by method 930.29, fat by method 932.06, moisture by method 927.05, and ash by method 930.30 (2). x-Ray Diffraction. The x-ray diffraction pattern of DWM was determined between 10 and 30' using an automated x-ray diffractometer (Rigaku Denki, Co. Ltd., Tokyo, Japan). Particle Size Distribution. Particle size distribution was measured using the MasterSizer® laser light-scattering particle size analyzer equipped with an MS 15 sample presentation unit and lOO-mm lens (Malvern Instruments Ltd., Malvern, England). The DWM was dispersed in isobutanol (Fisher Scientific, Pittsburgh, PA) at room temperature (20 to 22"C) until the obscuration value was .2, corresponding to a volume concentration of .03 to .05%. Ultrasonic dispersion (SO W at 27 kHz for S min) and mechanical stirring were applied to ensure that particles were independently dispersed, and particle size distribution was measured. Scanning Electron Microscopy. The DWM particles were picked up directly onto double
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sticky tape attached to aluminum stubs (lO-mm diameter) and sputter-coated with gold using an argon plasma (International Scientific Instruments, Topcon, Pleasanton, CA) operating at 10 rnA and 1.4 kV for 8 min. A drop of silver paint was applied to the edge of the coated double sticky tape to improve grounding. The samples were viewed with an International Scientific Instruments Model ISI-60 (Topcon) or a JEOL S400 (JEOL U.S.A., Peabody, MA) scanning electron microscope operated at S or to kV. Free Fat. Free fat was determined by suspending 2.S g of DWM in 40 ml of petroleum ether, boiling point 40 to 60"C (Fisher Scientific), for 1 h (7, 12). Every to min, slow mixing was applied for 30 s using an orbital shaker (Sensaur Platform Rotator®; Haydon Mfg. Co., operating at 200 rpm. Fat Inc., Torrington, extracts were filtered under vacuum using glass microfiber filter paper GFfF (Whatrnan International Ltd., Maidstone, England), and the solvent was evaporated. The extracts were dried in a convection oven at ISO"C for 1 h and weighed at room temperature. Apparent and True Particle Densities and Vacuole Volume. Apparent particle density was measured using light mineral oil, 158 maximum Saybolt viscosity, as the displacement liquid (Fisher Scientific). The DWM (I g) and light mineral oil (10 ml) were placed in 25-ml specific gravity bottles (Fisher Scientific), stirred for 10 s using a vortex stirrer, and left undisturbed for 20 min to allow air bubbles to escape before the bottles were filled to the calibration mark. Mass was established to the closest .1 mg using an analytical balance (12). True density was measured using an air comparison pycnometer (model 930; Beckman Instruments, Inc., Fullerton, CA) with helium (20 to 22"C, 34.5 kPa) as the displacement gas (5). The DWM (5 g) was placed in the measuring cup, and the air was evacuated by moving the pistons inward and outward two to three times. The gas was allowed to penetrate the milk sample for 5 min, and then the sample volume was measured. Vacuole volume was calculated from measurements of apparent particle density (PJ and true particle density (Pt) according to the following equation (12):
en
vacuole volume
1 1 = [-] Pa Pt
mlllOO g.
Journal of Dairy Science Vol. 77, No.5. 1994
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AGUILAR AND ZIEGLER
TABLE 1. Composition of milk powders. Nominal lactose
Lactose
Moisture
.3 1 27.4e 37.7 b 47.7 C 58.1 b 67.2"
2.31" 2.35" 1.75" 1.92" 1.63" 1.6oa .71
Protein
Fat
Ash
41.2" 28.6 b 24.OC 19.5 d 16.8" 12.21 1.30
50.4" 36.5 b 32.6c 28.Qd 20.00 15.6f 1.17
3.61" 2.63 b 2.37c 1.93d 1.57e 1.221 .10
(gIIOO g) 0 30 40 50 60 70 SE
.40
".b.c.d.e.fMeans followed by the same superscript are not significantly different (P > .05).
Surface Area and Pore Size Distribution. A mercury porosimeter (model PS-33; Quantachrome Corp., Syosset, NY) with 2-mm (3ml) glass cell and distilled mercury (Fisher Scientific) was used to determine surface area and pore size distribution. The DWM (.15 g) was placed in the glass cell, which was then filled with mercury using a mercury filling apparatus (Quantachrome Corp.) operating at an absolute pressure of 2.1 kPa. The relationship between penetration volume and pressure was measured between 0 and 227.5 MPa. The limiting pore size for mercury penetration was calculated from the penetration pressure assuming circular pore openings, a surface tension for mercury of 4.8 x 10-5 J/cm2 , and a contact angle between mercury and the material of 140· (27).
contents (30%) and other processing conditions were kept constant for all powders to minimize variations. The initial aim in this study was to use higher total solids in the spray-dried feed (60 to 70%); however, lower total solids were required as a result of poor performance of the rotary atomizer at low lactose concentrations. High viscosity at low lactose concentration influenced atomization and was a likely cause of the larger size and greater vacuole volume for DWM containing 0% lactose. The general morphology of the DWM powders was similar to that observed in other studies (11, 15). The absence of lactose crystals confirms the x-ray diffraction patterns in
RESULTS AND DISCUSSION
Table 1 presents the composition of the DWM powders. Lactose concentration had no significant effect on the moisture content of the final powder. The absence of characteristic peaks in the x-ray diffraction pattern indicated that lactose was present in the amorphous state. All DWM powders had unimodal, volumebased particle size distributions (Figure 1). The median volume-based diameter was used to represent the average particle size. The DWM particles containing 0% lactose had a significantly larger median volume-based diameter than that of other lactose concentrations (Table 2). The larger particle size of DWM containing 0% lactose was also confirmed by scanning electron micrographs (Figure 2). Total solids Journal of Dairy Science Vol. 77, No.5, 1994
20
15 ~
e
=
'0
>
10
~ 5
0 .1
1
10
100
1000
Particle size (Ilm) Figure 1. Volume-based particle size distribution of milk powders containing 0% (e) and 67% lactose (0).
LACTOSE AND THE MICROSTRUCTURE OF MILK POWDER
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TABLE 2. Median volume-based diameter. true and apparent densities. and vacuole volume of milk powders. Median volumebased diameter
True density
(%)
(P)
- - - - (glml) - - - -
(ml/100 g)
.3 27.4 37,7 47.7 58.1 67.2 SE
28.514.0b 12.6b 13.lb 14.3 b l4.2 b 2.0
1.1211.190b 1.233 bc 1.278< l.34()d l.346 d
52.638.1 b 40,7 b 38.4b 37.9b 34.5b 2.3
Lactose
.017
Apparent density
.705.819b .82Ib .857 bc .88900 ,919 d .013
Vacuole volume
-,b.c.dMeans followed by the same superscript are not significantly different (P > .05).
which no characteristic peaks were observed. Deep dents and wrinkles in spray-dried milk particles may be a result of high casein concentration (10). This phenomenon was confirmed in milk samples with 0% nominal lactose (i.e., high protein concentration), which had a smooth surface, but many deep dents, indicating the presence of large air bubbles during drying that produced milk particles with large vacuoles. Figure 2, part I, bottom left) shows a broken particle (0% nominal lactose) with a large central vacuole and a thin wall with many small vacuoles. Milk particles became smaller and more spherical with smaller dents and wrinkles as lactose concentration increased (Figure 2, part I, right side; part 2). Differences in dents and wrinkles have been observed (21) to be a function of particle size; small particles have smooth surfaces and no dents, and large particles show wrinkled surfaces and dents. However, the results presented in the present study show that wrinkles and dents were not a function of particle size but were dependent on lactose concentration. For example, dents and wrinkles were large in DWM particles with 0 and 27% lactose even though the DWM samples had significantly different median volume-based diameter. However, milk powder particles with 67% lactose had no large dents or wrinkles, even though the median volume-based diameter was the same as that for powders with 27 and 38% lactose. Vacuoles in milk powders are initially formed during atomization by the entrapment of air bubbles (23); during drying, these bubbles can expand and form large vacuoles or escape, leaving an empty structure that collapses and shrinks under further drying (Figure
2, part 1, top left). Evacuation of air bubbles may occur during droplet formation (by inertia) and drying (by diffusion); vacuoles are formed by air bubbles that do not dissolve before the particle solidifies (23). A high protein content (i.e., low lactose) increases viscosity, may decrease air bubble diffusion to the surface, and promotes rapid formation of a hard casing, which increases the probability of air entrapment and vacuole formation. At high lactose concentrations, small cracks were observed on the particle surface (Figure 2, part 2, top right). Higher lactose concentrations increased the true and apparent densities of milk powders (Table 2), in agreement with previous studies on spray-dried materials (4), which indicated that true densities of whole milk (1.28 glml), milk fat (.94 glml), and calcium caseinate phosphate complex (1.39 glml) are lower than that of amorphous lactose powder (1.52 glml). The vacuole volume of the 0% lactose powder was higher than those of lactose-containing powders and was constant for milk powders with lactose concentrations between 27 and 67% (Table 2). The increase in vacuole volume with higher protein content (i.e., low lactose) can be explained by the entrapment of air bubbles as a result of increased viscosity. The data suggest that a critical lactose concentration may exist (Le., protein concentration) that facilitates the escape of air bubbles during drying and that higher concentrations of lactose have no effect on vacuole volume. Surface area was inversely related to lactose concentration (Table 3). This relationship could not be explained solely on the basis of differences in particle size. For example, milk powders with 0% lactose had the largest median, Journal of Dairy Science Vol. 77. No.5. 1994
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AGUILAR AND ZIEGLER
volume-based diameter and the largest surface area, whereas samples with 27. 38.48. 58, and 67% lactose had similar particle sizes. but their surface areas were a function of lactose concentration. Surface area by mercury porosimetry is a measure of internal and external irregularities. For two milk powders with the same particle size. a larger surface area indicates the presence of particles with irregularities such as cracks. holes. dents, wrinkles. and pores. A smaller surface area for the same particle size indicates the presence of smooth particles without such irregularities. A less porous matrix was produced as a result of increased lactose concentration (Figure 3). Milk powders with 0 and 27% lactose (Figure 3. A and B) showed pores of 20 to 30 nm that were not present at higher lactose concentrations (Figure 3C). Also, at high lactose concentra-
tions. small pores (<20 nm) were less numerous (Figure 3C). The presence of holes and cracks in the milk particles may be important during chocolate processing. Holes and cracks may determine the fracture pattern during refining, resulting in changes in particle size distribution as a function of lactose concentration in the powders. More cocoa butter and emulsifiers are required to coat particles with holes and cracks. Breaking of milk particles with high vacuole volume and high porosities exposes internal surfaces that increase the surface area of milk powders even more. Milk powders with high vacuole volume also had high surface areas and porosities. Milk powders with increased lactose concentration had less total fat to be extracted (Table 1). Therefore. the total possible free fat decreased as the lactose concentration in-
Figure 2. Milk powders with lactose concentrations. Part I: 0% (top left), 0% (bottom left), 27% (top right), and 38% (bottom right). Journal of Dairy Science Vol. 77, No.5, 1994
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LACTOSE AND THE MICROSTRUCTURE OF MILK POWDER TABLE 3. Surface area, free fat, and percentage of free fat of milk powders. Lactose (%)
.3 27.4 37.7 47.7 58.1 67.2 SE
Surface area
Free fat
Percentage of free fat
(m2/g) 18.5a 14.O"b 13.9ab 1O.3 bc 8.84bc 7.19c 1.14
(g/loo g of powder)
(gil 00 g of fat)
M.O" 15.2b 4.83c 1.76d 1.34d 1.43d 1.31
87.3" 41.6b 14.8< 6.29< 6.7rf 9.17< 3.71
a.b.c.dMeans followed by the same superscript are not significantly different (P > .05).
creased (Table 3). Furthermore, the ratio of free fat to total fat (percentage of free fat) also decreased as the lactose concentration increased (Table 3). The pores or cracks present in whole milk powder particles are related to the free fat content. Fat globules in milk powder particles can be reached by the penetration of fat solvents into the pores and cracks (9).
CONCLUSIONS
Lactose concentration of spray-dried milk affected the microstructure of DWM particles. The DWM particles with 0% lactose had a more porous matrix with deep dents, wrinkles, and large vacuoles; lower true and apparent particle densities; and larger median, volumebased diameter, surface area, vacuole volume,
Part 2: 48% (top left), 58% (bottom left), and 67% (top right). Journal of Dairy Science Vol. 77. No.5, 1994
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AGUILAR AND ZIEGLER
6,...-----,---------------,
A
5 4 3
and free fat content. Higher lactose concentrations produced nearly spherical particles with a less porous matrix; higher true and apparent particle densities; and smaller median volumebased diameter, surface area, vacuob volume, and free fat content.
REFERENCES
2
I Aguilar, C. A., and G. R. Ziegler. 1993. Lactose crystallization in spray-dried milk powders exposed to
Oh~....,.---.--::'~.--~e:=~~
1
10
100
1000
10000
Pore radius (om) 6 .-------------::8" 5
4 3 2
....~~.......l
O+---~~~.........;~-
1
10
100
1000
10000
Pore radius (nm)
6.--------------..., C 5 4
3 2
. .~:......;.........l
O+---~~--..:::~~--
I
10
100
1000
10000
Pore radius (om) Figure 3. Pore size distributions of milk powders with lactose concentrations of 0% (Al, 27% (8). and 67% (C). Journal of Dairy Science Vol.
n,
No.5. 1994
isobutanol. Food StruCI. 12:43. 2 Association of Official Analytical Chemists. 1990. Official Methods of Analysis. 15th ed. AOAC. Arlington. VA. 3 Buchheim, W. 1982. Electron microscopic localization of solvent-extractable fat in agglomerated spray-dried whole milk powders. Food Microstruct. 1:233. 4 Burna, T. J. 1%5. The true density of spray milk powders and of certain constituents. Neth. Milk Dairy 1. 19:249. 5 Burna. T. 1. 1966. The physical structure of spray milk powder and the changes which take place during moisture absorption. Netb. Milk Dairy J. 20:91. 6 Burna. T. J. 1971. Free fat in spray-dried whole mille I. General introduction and brief review of literature. Neth. Milk Dairy 1. 25:33. 7 Burna. T. 1. 1971. Free fat in spray-dried whole milk. 2. An evaluation of methods for the determination of free-fat content. Netb. Milk Dairy J. 25:42. 8 Burna. T. 1. 1971. Free fat in spray-dried whole milk. 3. Particle size. Its estimation. influence of processing parameters and its relation to free-fat content. Netb. Milk Dairy J. 25:53. 9 Burna. T. 1. 1971. Free fat in spray-dried whole milk. 8. The relation between free-fat and particle porosity of spray-dried whole milk. Neth. Milk Dairy J. 25: 123. 10 Burna. T. J., and S. Henstra. 1971. Particle structure of spray-dried caseinate and spray-dried lactose as observed by scanning electron microscope. Netb. Milk Dairy J. 25:278. II Cari~. M., and M. Kal6b. 1987. Effects of drying tecbniques on milk powders quality and microstructure: a review. Food Microstruct. 6:171. 12 Daemen. ALH. 1982. The estimation of the mean particle density, the vacuole volume a.I?d the porosity of spray-dried porous powders. Neth. Milk Dairy 1. 36:53. 13 DeViider. J., R. Martens. and M. Naudts. 1976. Influence of process variables on some whole milk powder characteristics. Milchwissenschaft 31:396. 14 Food and Drug Administration. 1992. Cacao products. Amendment of the standards of identity. Fed. Reg. 57: 23989. 15 Kal6b. M. 1979. Scanning electron microscopy of dairy products: an overview. Scanning Electron Microsc. 3:261. 16 King. N. 1%5. The physical structure of dried milk. Dairy Sci. Abstr. 27:91. 17 Mistry, V. V., and H. N. Hassan. 1991. Delactosed. high milk protein powder. 2. Physical and functional properties. 1. Dairy Sci. 74:3716.
LACTOSE AND THE MICROSTRUCTURE OF MILK POWDER 18 Mistry, V. V., H. N. Hassan, and D. I. Robinson. 1992. Effect of lactose and protein on the microstructure of dried milk. Food Struct. II :73. 19 Neter, I., W. Wasserman, and M. H. Kutner. 1990. Applied Linear Statistical Models. 3rd ed. Richard D. Irwin, Inc., Homewood, IL. 20 Roetman, K. 1979. Crystalline lactose and the structure of spray-dried milk products as observed by scanning electron microscopy. Neth. Milk Dairy I. 33: I. 21 Saito, Z. 1985. Particle structure in spray-dried whole milk and instant skim milk powder related to lactose crystallization. Food Microstruct. 4:333. 22 SAS~ User's Guide: Statistics, Version 5 Edition. 1985. SAS Inst.. Inc., Cary, NC. 23 Verhey, I.G.P. 1972. Vacuole fonnation in spray pow-
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der particles 2. Location and prevention of air incorporation. Neth. Milk Dairy J. 26:203. 24 Verhey, J.G.P. 1972. Vacuole fonnation in spray powder particles 3. Atomization and droplet drying. Neth. Milk Dairy J. 27:3. 25 Verhey, J.G.P. 1986. Physical properties of dried milk in relation to chocolate manufacture. Neth. Milk Dairy 1. 40:261. 26 Verhey, I.G.P., and E. A. Vos. 1971. Air-free atomization: a method for producing spray powders without vacuoles. Neth. Milk Dairy 1. 25:73. 27 Wikberg, M., and G. Alderborn. 1992. Compression characteristics of granulated materials: VI. Pore size distributions, assessed by mercury penetration, of compacts of two lactose granulations with different fragmentation properties. Int. 1. Pharmacol. 84: 191.
Journal of Dairy Science Vol. 77, No.5, 1994