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Effects of ultrafiltration of whole milk on some properties of spray-dried milk powders
ARTICLE IN PRESS
International Dairy Journal 13 (2003) 995–1002
Effects of ultrafiltration of whole milk on some properties of spray-dried milk powders M. Kieran Keogh*, Cathriona A. Murray, Brendan T. O’Kennedy Teagasc, Dairy Products Research Centre, Moorepark, Fermoy, Co. Cork, Ireland Received 16 September 2002; accepted 30 May 2003
Abstract The objective was to produce spray-dried milk powders for assessment subsequently in chocolate. Milks were ultrafiltered to increase their protein content (3.08–5.33 g 100 g1), concentrated to different solids levels (42.8–52.3 g 100 g1) and spray-dried to produce powders (26–59 g 100 g1 fat). The relationships between the milk protein content, concentrate viscosity and some powder properties were quantified. The free-fat content of the powders increased linearly to 74 g 100 g1 fat with milk protein content for 26 and 40 g 100 g1 fat powders. The particle size and moisture content of the powders increased linearly with concentrate viscosity for 26 g 100 g1 fat powders. Differences between the control and ultrafiltration-treated milk powders were explained. The free-fat content and the particle size increased with the fat content of the control powders. The vacuole volume of the powders was inversely more related to the free-fat content than to the fat content of the control powders. r 2003 Elsevier Ltd. All rights reserved. Keywords: Ultrafiltration; Milk composition; Milk powder; Chocolate
1. Introduction Milk powders with a standard fat content of 26 g 100 g1 fat are usually traded commercially for a variety of dairy and food application end-uses. However, whole milk powders with specific physical properties such as high free-fat content, large particle size and low vacuole volume have been shown to have beneficial effects on the properties of chocolate (Verhey, 1986; Dewettinck, De Moor, & Huyghebaert, 1996; Scheruhn, Franke, & Tscheuschner, 2000; Twomey, Keogh, O’Kennedy, & Mulvihill, 2002). For these reasons, chocolate manufacturers prefer roller-dried to spraydried powders, which typically do not have any of these attributes. However, spray-dried powders with high free-fat contents may be produced in other ways. As the fat content of milk powder increases, the free-fat content is known to increase (Pisecky, 1997; Kelly, Kelly, & Harrington, 2002). High-fat (56 g 100 g1 fat) powders can contain more than 90 g 100 g1 free fat, which can then be blended or co-dried with skim milk powder to obtain standard 26 g 100 g1 fat spray-dried powders for *Corresponding author. Tel.: +353-25-42235; fax: +353-25-42228. E-mail address: [email protected] (M.K. Keogh). 0958-6946/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0958-6946(03)00123-7
chocolate (Twomey, Keogh, O’Kennedy, & Mulvihill, 2000). It would be preferable to avoid the blending step by producing high free-fat powders at the standard powder fat level. This also can be achieved by crystallisation of the amorphous lactose pre-drying by crystal seeding (Bohren, Kuypers, & Meister, 1987) or by exposing the powder to high relative humidity postdrying (Aguilar & Ziegler, 1994b). The presence of crystals caused fine interstices and cracks in the powder particles (Saito, 1985) and increased free fat. A high free-fat spray-dried powder can also be produced by spray drying part of the fat onto a partly skimmed spray powder (Verhey, 1986). It has also been shown (Twomey et al., 2000) that free-fat content of 56 g 100 g1 fat milk powders was linearly related to seasonal changes in the native milk protein content. This suggested the need for a follow-up study to increase the protein level of milk by ultrafiltration prior to evaporation in order to increase free fat in standard 26 g 100 g1 fat powders. Concentration by ultrafiltration as well as evaporation increases protein content, concentrate solids and viscosity. Since these properties, in turn, affect powder particle size and moisture levels (Snoeren, Damman, & de Klok, 1983), it is necessary to quantify the effect of each variable on
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the powder and subsequently on the chocolate properties. The main difficulty is that powder properties are, in many cases, interrelated (Pisecky, 1978) and variables are therefore not always independent. De Vilder, Martens, and Naudts (1976, 1977, 1979) reported the effects of some process variables and total solids of the concentrate after evaporation on some physical properties of standard whole milk powders. The effects of the concentrate total solids level and the viscosity on the moisture content of the powder, free fat, vacuole volume and related factors were outlined but the particle size of the powders was not measured, as laser light-scattering measuring instruments did not become generally available until about 1990. As the total solids and viscosity of non-homogenised concentrates increased, the free fat and vacuole volume of the powders decreased and the moisture contents increased. The decrease in free fat was attributed to the increasing homogenising effect of the atomiser on the milk fat as the level of concentrate solids increased. The higher vacuole volume at lower concentrate total solids was a result of easier air entrapment and more rapid water evaporation. Higher total solids and viscosity of the concentrate slowed down the drying process leading to increased moisture content in the powder. The effects of ultrafiltration (UF) of skim milk on the physical and microstructural properties of skim milk powders have also been studied (Mistry & Hassan, 1991a, b; Mistry, Hassan, & Robison, 1992). The effects of UF on fat and free-fat contents were therefore not included in that work and the particle size was not reported. The first objective of this study was therefore to produce high free fat (>70 g 100 g1 fat) spray-dried powder at the fat content of standard whole milk powder (26 g 100 g1 fat), which generally contains a low level of o10 g 100 g1 free fat. The second objective was to determine the effects of the milk protein content and concentrate viscosity on certain properties of the spraydried powders, especially free fat, particle size and moisture content. A third objective, which will be the subject of another paper, was to determine the effects of the properties of the spray-dried milk powders (made from control and ultrafiltered milks) on the properties of milk chocolate.
2. Materials and methods 2.1. Milks Milks were obtained over 2 years from a spring herd (mean calving date 26/02/2000 and 01/03/2001) and an autumn herd (mean calving date 04/10/2000 and 27/09/ 2001) maintained at this centre. Normally, one-day’s production of milk was pooled, except in early and late lactation when two-day’s production of milk was pooled
so that sufficient milk was available for processing. The milk was stored at 4–6 C. All the milk was separated and standardised with its separated cream to the required composition to give a concentrate and powder of the required fat content. The protein, fat and lactose contents of the milks were measured in duplicate by the Milkoscan (Foss Electric, Hillerod, Denmark).
2.2. Ultrafiltration of milks Skim milk was ultrafiltered at 40 C on a Koch spiralwound membrane UF plant with a surface area of 10 m2 and normal molecular weight cut-off of 10 kDa. In one example, the protein content of the retentate was increased by a factor of 1.25 and 100 L was standardised to give a powder with 26 g 100 g1 fat (UF1). UF was then continued to increase the protein content by a factor of 1.50 to give a second powder (UF2). The protein contents of the control powder, UF1 powder and UF2 powder were 28.6, 32.2 and 34.1 g 100 g1, respectively.
2.3. Preparation of various milk fat powders The standardised and ultrafiltered milks were preheated at 97.5 C for 2 min in an APV heat exchanger (UHT-Pasilac, Silkeborg, Denmark), equivalent to a medium heat treatment (ADMI, 1971). The milks were then concentrated to various levels of total solids ranging from 41.4 to 54.0 g solids 100 g1 (45.3–50.3 g solids for the powders containing 50–59 g fat 100 g1) in a single-effect falling film evaporator (Anhydro F1 Lab, Copenhagen, Denmark). The concentrate (at 50 C) was spray-dried (Anhydro dryer, Model Lab 3, Copenhagen, Denmark; drying capacity of 10 kg h1) using a 2-fluid nozzle atomiser. Drying took place with an air inlet temperature of 190 C and an air outlet temperature of 90 C. The powder was then collected and a sample taken for analysis.
2.4. Compositional analysis The moisture content of the milk powders was determined by the oven drying method (A/S Niro . Atomizer, 1978a); fat content by the Rose–Gottlieb method (International Dairy Federation, 1987); protein content by Kjeldahl (International Dairy Federation, 1993; lactose by difference and ash by oven-drying at 550 C. The free-fat content was determined by taking 10 g of the powder and shaking gently with CCl4 for 15 min at ambient temperature (A/S Niro Atomizer, 1978b). All tests were carried out in duplicate and the mean calculated.
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2.5. Viscosity of concentrates
2.9. Statistical analysis
A strain-controlled Bohlin VOR rheometer with concentric cylinder geometry was used. Samples of milk concentrate were taken from the evaporator and 13.25 g was weighed into a C25 cup. The samples were subjected to a pre-shear at 461 s1 for 300 s at 45 C, followed by an up and down shear rate sweep from 1.16 to 461 s1 and the mean values of each sweep were taken. The concentrate viscosity profiles obeyed the power law model most closely (r > 0:998), that is
The properties of the standardised milks, concentrates, powders and chocolates were correlated using simple linear regression with intercept using the statistical functions in Microsoft Excel software. The significance levels were based on the analysis of variance using a two-tailed Student’s t-test (Box, Hunter, & Hunter, 1978).
3. Results and discussion s ¼ k’gn ; where s is the shear stress (Pa), g’ is the rate of shear (s1), the exponent n is the power law factor and k is the consistency index (Pa sn1) or the viscosity (Pa sn) at g’ =1 s1. 2.6. Particle size of milk powders The size of the particles in the milk powders was measured using a Malvern Mastersizer X (Malvern Instruments, Malvern, UK) fitted with an MSX15 small volume sample presentation unit. The instrument uses an approximation of the Mie-scattering theory to determine particle size, which utilises the refractive index of the dispersed phase and its absorption. A relative refractive index of 1.095 and an absorption value of 0.1 were used in the calculations. A 2 mW He– Ne laser beam (633 nm) and a 300 RF lens (size range 0.05–879 mm) were used for the measurements. The powders were suspended in propan-2-ol and sonicated (Sonicator, Hielscherr, Germany, model UP 200 H) for 2 min before each determination. The results are expressed as the volume weighted median diameter D(v,0.5). 2.7. Bulk density Bulk density was measured by the standard method (International Dairy Federation, 1995). 2.8. Particle density, vacuole volume and interstitial air The particle density (g mL1) was calculated by measuring the air-free volume of a known weight of powder using a pycnometer (AccuPyc 1330, Micromeritics, Norcross, GA, USA). The pycnometer determined the density and volume of milk powder by measuring the pressure change of helium in a calibrated volume. Using these data and the loose bulk density of the powder, the vacuole volume and interstitial air were calculated (A/S Niro Atomizer, 1978c).
3.1. Relationships between powder properties and other variables In all, 14 26 g 100 g1 fat powders (eight from ultrafiltered milks), one 30 g 100 g1 fat powder, 11 40 g 100 g1 fat powders (five ultrafiltered) and five 50–59 g 100 g1 fat powders (none ultrafiltered) were produced. Compositional details and physical properties of the 26 g 100 g1 fat powders are shown in Table 1(a), in Table 1(b) for 40 g 100 g1 fat powders and in Table 1(c) for 50–59 g100 g1 fat powders. Relationships between powder properties and milk, concentrate or powder variables found to be significant are shown in Table 2a for 26 and 40 g100 g1 fat powders and in Table 2b for control powders (26–59 g 100 g1 fat). The effects of these variables on milk powder properties known to be important in chocolate, namely free-fat content, particle size and vacuole volume are shown in Figs. 1–6. The relationships concentrate viscosity versus free fat and concentrate viscosity versus vacuole volume found previously by other workers (Snoeren et al., 1983; Pisecky, 1997) were not related in these powders. This is because increases in concentrate viscosity due to evaporation lead to a reduction in the free-fat content and an increase in the particle density of the powders (decreasing vacuole volume) (Snoeren et al., 1983). However, increases in concentrate viscosity due to UF resulted in increases in free fat with almost no change in particle density or vacuole volume (Table 1a). However, powder particle size versus particle density (r ¼ 0:576; n ¼ 14; Po0:05) were significantly related and powder moisture versus vacuole volume (r ¼ 0:532) and powder moisture versus interstitial air (r ¼ 0:419) were marginally related (n ¼ 14; Po0:1) in the current work. 3.2. Protein, protein:lactose ratio and free-fat content of milk powders The free-fat content of the milk powders made from spring herd milks increased linearly with the milk protein content in both 26 g 100 g1 fat (Fig. 1) and 40 g100 g1 fat powders (Fig. 2). As the milk protein content increased from 3.08 to 5.33 g 100 g1 due to UF,
(a)
Moisture (g 100 g1)
Particle size D(v.0.5) (mm)
Free fat (g 100 g1 fat)
Particle density (g cm3)
3.08 3.13 3.24 3.43 3.51 3.52 4.16 4.17 4.30 4.31 5.06 5.14 5.33 5.33
49.0 52.3 43.2 48.2 44.2 43.8 43.5 47.3 43.0 43.9 45.2 42.8 43.1 44.6
177 253 — 171 51 43 — 300 49 100 550 227 62 221
3.4 3.7 3.0 4.5 2.9 2.3 3.0 4.2 2.8 2.9 6.7 3.8 3.1 3.9
76.3 59.3 45.0 82.6 54.8 55.7 43.5 75.5 48.8 63.7 121.0 68.3 53.5 68.7
6.0 13.2 28.4 12.6 20.9 25.7 41.0 21.2 44.8 49.3 57.1 74.2 62.7 58.8
1.2085 1.2259 1.1851 1.1120 1.1933 1.1921 1.1779 1.1609 1.1803 1.1935 1.1301 1.2000 1.1824 1.1812
Standardised milk protein (g 100 g1)
Milk concentrate
Moisture (g 100 g1) 3.8 3.0 NA NA NA NA NA NA 1.6 3.5 3.9
Particle size D(v, 0.5) (mm) 100.3 81.9 57.1 57.0 61.6 69.9 57.0 67.9 46.0 90.5 78.8
Free fat (g 100 g1 fat) 39.6 49.4 45.0 45.1 40.0 46.5 61.4 63.1 71.8 67.6 73.9
Vacuole volume (mL 100 g1) 2.3 3.1 3.7 2.7 2.2 3.8 3.6 2.7 5.4 3.5 4.0
2.97 3.13 3.19 3.36 3.37
Total solids (g 100 g1) 54.0 53.6 NAc NA NA NA NA NA 41.4 50.4 48.7
26.7 25.7 27.0 28.6 27.9 29.7 32.2 32.2 32.0 32.9 34.1 35.1 35.9 35.2
Consistency index (mPa sn1) 129 73 NA NA NA NA NA NA 12 222 220
Protein (g 100 g1) 20.4 20.7 22.2 22.2 21.5 21.2 25.5 26.3 25.1 25.9 26.9
Consistency index (mPa sn1) 57 13 25 5 10
Interstitial air (mL 100 g1) 89.6 95.6 92.1 92.3 91.1 101.6 97.7 98.3 119.9 82.4 87.8
Powder
Fat (g 100 g1) 51.7 53.8 51.4 59.2 51.7
Moisture (g100 g1) 2.3 3.2 2.8 2.8 3.0
n, sample size. See text for details of preparation of standardised milks, concentrates and milk powders. c NA, not applicable. b
0.640 0.600 0.560 0.625 0.570 0.590 0.600 0.625 0.560 0.580 0.625 0.500 0.630 0.600
Powder
Milk concentrate
Total solids (g 100 g1) 50.3 47.8 49.4 45.3 46.5
Bulk density (g cm3)
Particle size D(v, 0.5) (mm) 104.1 67.4 93.7 91.6 70.5
Free fat (g 100 g1 fat) 78.4 91.2 74.9 95.5 90.4
Vacuole volume (mL 100 g1) 3.3 2.0 2.5 0.9 2.5
Interstitial air (mL 100 g1) 88.1 52.4 52.7 51.2 53.3
Vacuole volume (mL 100 g1)
Interstitial air (mL 100 g1)
5.5 3.9 6.5 12.4 5.9 7.4 7.3 8.6 7.1 6.8 10.0 6.1 6.7 6.5
73.5 85.1 94.2 73.9 91.6 85.6 81.8 70.1 93.8 88.6 71.5 116.7 74.2 82.0
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Consistency Protein index (mPa sn1) (g 100 g1)
Standardised milk protein (g 100 g1)
a
Powder
Total solids (g 100 g1)
2.95 2.98 3.04 3.05 3.08 3.42 3.50 3.57 3.91 3.93 4.24 (c)
Milk concentrate
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(b)
Standardised milk protein (g 100 g1)
998
Table 1 Composition and physical properties of standardised milks, milk concentrates and powders containing 26 g 100 g1 fat (n ¼ 14)a,b (a); 40 g 100 g1 fat (n ¼ 11)a,b (b); 50–59 g 100 g1 fat (n ¼ 5)a,b (c)
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Table 2 (a) Relationships between properties of powders containing 26 and 40 g 100 g1 fat and other variablesa,b,c Powder property
26 g 100 g1 fat powder
Related variable
Correlation coefficient, r Free fat Free fat Free fat Particle size Particle size
Milk protein Powder protein Milk protein:lactose ratio Concentrate viscosity Powder moisture
0.925 0.899 0.826 0.891 0.920
40 g 100 g1 fat powder
Degrees of freedom
Sig.b
Correlation coefficient, r
Degrees of freedom
Sig.
12 12 12 10 12
0.935 0.919 0.906 0.975c 0.888
9 9 3 3 3
(b) Relationships between properties of control powders containing 26–59 g 100 g1 fata,b,c Powder property
Related property
Correlation coefficient, r
Degrees of freedom
Sig. level
Free fat Particle size Vacuole volume Vacuole volume
Fat Fat Fat Free fat
0.976 0.621c 0.786 0.760
13 12 13 13
a
See text for details of preparation of standardised milks, concentrates and milk powders. Levels of significance: , Po0:001; , Po0:01; , Po0:05: c Relationships were linear in all models, except those marked c were quadratic (curvilinear).
130
80 70 60 50 40 30 20 10 0
Particle size D(v, 0.5) µm
Free fat (g100g-1 fat)
b
110 90 70 50 30
3.0
3.5
4.0
4.5
5.0
0
5.5
200
300
400
500
600
Consistency index k (mPasn-1)
Milk protein (g100g-1 powder) Fig. 1. Effect of protein content of control and ultrafiltered milks on the free-fat content of milk powders containing 26 g 100 g fat1. Regression line () is fitted to experimental values (K). See text for details of preparation of milks and powders.
Fig. 3. Effect of consistency index of control and ultrafiltered concentrated milks on the median particle size of milk powders containing 26 g100 g fat1. Regression line () is fitted to experimental values (K). See text for details of preparation of milks and powders.
130 Particle size D(v, 0.5) µm
80 Free fat (g100g-1 fat)
100
70 60 50 40
110 90 70 50 30
30 2.5
3.0
3.5
4.0
4.5
Milk protein (g100g-1 powder)
Fig. 2. Effect of protein content of control and ultrafiltered milks on the free-fat content of milk powders containing 40 g 100 g fat1. Regression line () is fitted to experimental values (K). See text for details of preparation of milks and powders.
2.0
3.0
4.0
5.0
6.0
7.0
Moisture (g100g-1 powder)
Fig. 4. Effect of moisture content on the median particle size of milk powders containing 26 g 100 g fat1 made from control and ultrafiltered milks. Regression line () is fitted to experimental values (K). See text for details of preparation of milks and powders.
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Free fat (g100g-1 fat)
120
120
100
100
80
80
60
60
40
40
20
20
0
0 20
30
40
50
Particle size D(v, 0.5) µm
M.K. Keogh et al. / International Dairy Journal 13 (2003) 995–1002
1000
60
Fat content (g100g-1 powder)
Vacuole volume (mL100g-1 powder)
Fig. 5. Effect of fat content on the free-fat content (closed symbols) and median particle size (open symbols) of milk powders made from control (not ultrafiltered) milks. The linear and polynomial regression lines () are fitted to the experimental values (K, J). See text for details of preparation of milks and powders.
14 12 10 8 6 4 2 0 20
30
40
50
60
Fat content (g100g-1 powder)
Fig. 6. Effect of fat content on the vacuole volume of milk powders made from control (not ultrafiltered) milks. Regression line () is fitted to experimental values (K). See text for details of preparation of milks and powders.
the free-fat content of the 26 g 100 g1 fat powders increased from 6 to 74 g 100 g1 fat (r ¼ 0:925; df ¼ 12; Po0:001). In the 40 g 100 g1 fat powders, as the milk protein content increased from 2.95 to 4.24 g 100 g1, the free-fat content increased from 40 to 74 g 100 g1 fat (r ¼ 0:935; df ¼ 9; Po0:001). The relationship between the protein content of the powders—which increased from 25.7 to 35.9 g 100 g1 in the 26 g 100 g1 fat powders and from 20.4 to 26.9 g 100 g1 in the 40 g 100 g1 fat powders—and the free-fat content of the powders was also significant (Po0:001; Table 2a). A linear relationship was also found between the milk protein:lactose ratio and the free-fat content of the 26 g 100 g1 (Po0:001) and 40 g 100 g1 fat powders (Po0:05). The free-fat content of six 26 g 100 g1 fat milk powders made from autumn herd milks also increased linearly with the milk protein content, but the correlation coefficient was not significant (not shown). According to Buma’s model (1971b), free fat exists as surface fat, sub-surface fat, pore fat from the pores of the powder particles and dissolution fat that is dissolved
by the solvent. It has already been established that the free-fat content increases either with the protein:lactose ratio or protein content in powders microencapsulated using whey proteins (Young, Sarda, & Rosenberg, 1993; Moreau & Rosenberg, 1993; Keogh & O’Kennedy, 1999), in powders with reduced lactose content (Aguilar & Ziegler, 1994a) and in high-fat (56 g 100 g1) milk powders (Twomey et al., 2000). The porosity of powders to gases also increases with protein content (Moreau & Rosenberg, 1993, 1998). These changes in porosity (Buma, 1971a, b) of microencapsulated milk fat powders (Moreau et al., 1993), powders with altered lactose content (Aguilar et al., 1994b) and skim milk powders (Mistry et al., 1992) with altered protein:lactose ratios have also been studied microscopically. In all these powders, increasing the protein:lactose ratio gave rise to an increasing number of surface cracks and pores in the powder particles, thereby allowing greater access of solvent to the fat, including the newly exposed surface fat. In contrast, skim milk powders with high lactose but also very high protein contents had particle surfaces that appeared smooth (Mistry et al., 1992). Moreover, as the protein level increases, the environment surrounding the fat globules becomes more hydrophobic (Moreau & Rosenberg, 1993), thus increasing the extractability of the fat from the powder particles by organic solvent. Conversely, lactose, being hydrophilic, repels the solvent and reduces the extractability of the fat from the powder (Young et al., 1993; Twomey et al., 2000), even in powders containing fat and lactose only (Buma, 1971b). 3.3. Particle size and moisture content of milk powders The particle size of the powders increased linearly with the viscosity of the concentrates (whether increased by evaporation or by UF and evaporation combined) for powders containing 26 and 40 g 100 g1 fat (Table 2a). The median particle size of powders containing 26 g 100 g1 fat increased from o50 to 121 mm as the concentrate viscosity (at g’ =1 s1) increased from o50 to 550 mPa sn (r ¼ 0:891; df ¼ 10; Po0:001; Fig. 3), and as the powder moisture content increased from 2.3 to 6.7 g 100 g1 (r ¼ 0:920; df ¼ 12; Po0:001; Fig. 4). These results agree with those of De Vilder et al., 1979. As the same two-fluid nozzle was used throughout at relatively low pressure, the increased viscosity of the concentrates was the main cause of the increase in particle size, which in turn lead to increased moisture content of the powders. 3.4. Effect of powder fat and free-fat contents When the control powders were evaluated, the free-fat content increased from a mean of 18 g100 g1 fat for powders containing 26 g 100 g1 fat to 95.5 g 100 g1 fat (Table 1c) for a powder containing 59.2 g 100 g1 fat
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(r ¼ 0:976; df ¼ 14; Po0:001; Table 2b, Fig. 5). This is expected due to the increased ratio of fat to emulsifying protein (Young et al., 1993; F.aldt, Bergensta( hl, & Carlsson, 1993; Keogh & O’Kennedy, 1999). When the control and UF powders were evaluated, the correlation coefficient between free fat and fat contents was lower (r ¼ 0:750; df ¼ 24; Po0:001), for which there may be two reasons. Firstly, the free-fat content increased with the protein content of the UF powders with the same fat content, as outlined earlier. The particle size of the powders increased curvilinearly with the fat content (r ¼ 0:621; df ¼ 12; Po0:05; Fig. 5), but not with the free-fat content (r ¼ 0:524; not significant), because larger particles have a lower surface area resulting in lower levels of surface fat and free fat. The reasons for the wide scatter of data, which indicates that other factors besides fat content affect particle size, will be reported in a future paper. In the control powders, the vacuole volume decreased from 7.0 to o1.0 mL 100 g1 powder, as the powder fat content increased to 59.2 g 100 g1 fat (r ¼ 0:786; df ¼ 13; Po0:001; Fig. 6) and as the free-fat content increased to 95.5 g 100 g1 fat (r ¼ 0:760). The decrease in vacuole volume is usually attributed to the foam depressing effect of free fat (Verhey, 1972).
4. Conclusions It was possible to produce a set of spray-dried milk powders containing 26–59 g 100 g1 fat with a range of values of protein, free fat, particle size, moisture and vacuole volume. This has been achieved mainly by UF and fat-standardisation of milks prior to evaporation and drying. A number of important and highly significant relationships between the properties of the standardised milks or concentrates and the properties of the powders were defined for the first time in these spray-dried products. Differences between the relationships found for the control and UF milk powders were outlined and explained. The results show that it is possible to predict certain properties of powders, such as free-fat content, particle size and vacuole volume from certain properties of the milks or concentrates. All of these relationships were anticipated from previous work with standard whole milk and skim milk powders. Nevertheless, this set of milk powders will allow the production of a series of chocolates at pilot-scale to test and provide greater insight into the effects of certain milk powder properties on the properties of chocolate.
Acknowledgements This project was part-funded by the Irish Government under the National Development Plan 2000–2006. The
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technical assistance of Mr. Robert Kennedy and Mr. John O’Keeffe is greatly appreciated.
References Aguilar, C. A., & Ziegler, G. A. (1994a). Physical and microscopic characterization of dry whole milk with altered lactose content 1. Effect of lactose concentration. Journal of Dairy Science, 77(5), 1189–1197. Aguilar, C. A., & Ziegler, G. A. (1994b). Physical and microscopic characterization of dry whole milk with altered lactose content. 2. Effect of lactose crystallization. Journal of Dairy Science, 77(5), 1198–1204. American Dry Milk Institute (ADMI) (1971). Determination of undenatured whey protein nitrogen in non-fat dry milk (WPNI). In Anon (Ed.), Standards for grades of dry milk including methods of analysis. Bulletin 916, Chicago. A/S Niro Atomizer, Copenhagen, Denmark (1978a). Determination of moisture. In I. Haugaard Sorensen, J. Krag, J. Pisecky, & V. Westergaard (Eds.), Analytical methods for dry milk products (4th ed.) (pp. 10–11). Copenhagen: De Forenede Trykkerier A/S. A/S Niro Atomizer, Copenhagen, Denmark (1978b.) Determination of free-fat on the surface of milk powder particles. In I. Haugaard Sorensen, J. Krag, J. Pisecky, & V. Westergaard (Eds.), Analytical methods for dry milk products (4th ed.) (p. 46). Copenhagen: De Forenede Trykkerier A/S. A/S Niro Atomizer, Copenhagen, Denmark (1978c). Determination of particle density, contents of occluded air and interstitial air. In I. Haugaard Sorensen. J. Krag, J. Pisecky, & V. Westergaard (Eds.), Analytical methods for dry milk products (4th Ed.) (pp. 48–51). Copenhagen: De Forenede Trykkerier A/S. Bohren, H.-U., Kuypers, T. W., & Meister, N. (1987). A process for the production of a milk powder. UK Patent Application 2 183 135 A. Societ!e des Produits Nestl!e S.A., Vevey, Switzerland. Box, G. E. P., Hunter, W. G., & Hunter, J. S. (1978). Significance tests and confidence intervals for means, variances, proportions, and frequencies. In G. E. P. Box (Ed.), Statistics for experimenters: An introduction to design, data analysis, and model building (pp. 107–123). New York: Wiley. Buma, T. J. (1971a). Free fat in spray-dried whole milk. 8. The relation between free-fat content and particle porosity of spray-dried whole milk. Netherlands Milk and Dairy Journal, 25(1), 123–140. Buma, T. J. (1971b). Free fat in spray-dried whole milk. 10. A final report with a physical model for free fat in spray-dried milk. Netherlands Milk and Dairy Journal, 25(1), 159–174. De Vilder, J., Martens, R., & Naudts, N. (1976). Influence of process variables on some whole milk powder characteristics. Milchwissenschaft, 31(7), 396–401. De Vilder, J., Moremans, R., & Martens, R. (1977). The free-fat content and other physical characteristics of whole milk powder. Milchwissenschaft, 32(6), 347–350. De Vilder, J., Martens, R., & Naudts, N. (1979). The influence of the dry matter content, the homogenisation and the heating of concentrate on physical characteristics of whole milk powder. Milchwissenschaft, 34(2), 78–84. Dewettinck, K., De Moor, H., & Huyghebaert, A. (1996). The free-fat content of dried milk products and flow properties of milk chocolate. Milchwissenschaft, 51(1), 25–28. F.aldt, P., Bergenst(ahl, B., & Carlsson, G. (1993). The surface coverage of fat on food powders analysed by ESCA (electron spectroscopy for chemical analysis). Food Structure, 12(2), 225–234. International Dairy Federation (1987). Dried milk, dried whey, dried buttermilk and dried butter serum. Determination of fat content (Rose . Gottlieb reference method). IDF standard 9C.
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International Dairy Federation (1993). Milk. Determination of nitrogen content. IDF standard 20B. International Dairy Federation (1995). Dried milk and dried milk products. Determination of bulk density. IDF standard 134A. Kelly, J., Kelly, P. M., & Harrington, D. (2002). Influence of processing variables on the physicochemical properties of spraydried fat-based milk powders. Le Lait, 82(4), 401–412. Keogh, M. K., & O’Kennedy, B. T. (1999). Milk fat microencapsulation using whey proteins. International Dairy Journal, 9(9), 657–663. Mistry, V. V., & Hassan, H. N. (1991a). Delactosed, high milk protein powder. 1. Manufacture and composition. Journal of Dairy Science, 74(4), 1163–1169. Mistry, V. V., & Hassan, H. N. (1991b). Delactosed, high milk protein powder. 2. Physical and functional properties. Journal of Dairy Science, 74(11), 3716–3723. Mistry, V. V., Hassan, H. N., & Robison, D. J. (1992). Effect of lactose and protein on the microstructure of dried milk. Food Structure, 11(1), 73–82. Moreau, D. L., & Rosenberg, M. (1993). Microstructure and fat extractability in microcapsules based on whey proteins or mixtures of whey proteins and lactose. Food Structure, 12(4), 457–468. Moreau, D. L., & Rosenberg, M. (1998). Porosity of whey proteinbased microcapsules containing anhydrous milkfat measured by gas displacement pycnometry. Journal of Food Science, 63(5), 819–823. Pisecky, J. (1978). Bulk density of milk powders. Dairy Industries International, 43(2), 4–11. Pisecky, J. (1997). Achieving product properties. In Handbook of milk powder manufacture (pp. 149–191). Copenhagen: A/S Niro Atomizer Ltd., Soeborg, Denmark.
Saito, Z. (1985). Particle structure in spray-dried whole milk and in instant skim milk powder as related to lactose crystallization. Food Microstructure, 4(4), 333–340. Scheruhn, E., Franke, K., & Tscheuschner, H.-D. (2000). Einfluss der Milchpulverart auf das Verarbeitungsverhalten und die Fliesseigenschaften von Milchschokoladenmasse, Teil 2 Merkmale des Milchpulvers beeinflussen die Viskosit.at. Zucker und Susswarenwirtschaft, 53(5), 131–136. . Snoeren, T. H. M., Damman, A. J., & de Klok, H. J. (1983). The viscosity of whole milk concentrate and its effect on the properties of whole milk powder. Zuivelzicht, 75(39), 846–849. Twomey, M., Keogh, M. K., O’Kennedy, B. T., & Mulvihill, D. M. (2000). Effect of milk composition on selected properties of spraydried high-fat and skim-milk powders. Irish Journal of Agricultural and Food Research, 39(1), 79–94. Twomey, M., Keogh, M. K., O’Kennedy, B. T., & Mulvihill, D. M. (2002). Seasonal effects of some milk powder characteristics on the rheology of milk chocolate. Irish Journal of Agricultural and Food Research, 41(1), 105–116. Verhey, J. G. (1972). Vacuole formation in spray powder particles— 1. Air incorporation and bubble expansion. Netherlands Milk and Dairy Journal, 26(1), 186–202. Verhey, J. G. (1986). Physical properties of dried milk in relation to chocolate manufacture. Netherlands Milk and Dairy Journal, 40(2), 261–268. Young, S. L., Sarda, X., & Rosenberg, M. (1993). Microencapsulating properties of whey proteins. 1. Microencapsulation of anhydrous milk fat. Journal of Dairy Science, 76(10), 2868–2877.
International Dairy Journal 13 (2003) 995–1002
Effects of ultrafiltration of whole milk on some properties of spray-dried milk powders M. Kieran Keogh*, Cathriona A. Murray, Brendan T. O’Kennedy Teagasc, Dairy Products Research Centre, Moorepark, Fermoy, Co. Cork, Ireland Received 16 September 2002; accepted 30 May 2003
Abstract The objective was to produce spray-dried milk powders for assessment subsequently in chocolate. Milks were ultrafiltered to increase their protein content (3.08–5.33 g 100 g1), concentrated to different solids levels (42.8–52.3 g 100 g1) and spray-dried to produce powders (26–59 g 100 g1 fat). The relationships between the milk protein content, concentrate viscosity and some powder properties were quantified. The free-fat content of the powders increased linearly to 74 g 100 g1 fat with milk protein content for 26 and 40 g 100 g1 fat powders. The particle size and moisture content of the powders increased linearly with concentrate viscosity for 26 g 100 g1 fat powders. Differences between the control and ultrafiltration-treated milk powders were explained. The free-fat content and the particle size increased with the fat content of the control powders. The vacuole volume of the powders was inversely more related to the free-fat content than to the fat content of the control powders. r 2003 Elsevier Ltd. All rights reserved. Keywords: Ultrafiltration; Milk composition; Milk powder; Chocolate
1. Introduction Milk powders with a standard fat content of 26 g 100 g1 fat are usually traded commercially for a variety of dairy and food application end-uses. However, whole milk powders with specific physical properties such as high free-fat content, large particle size and low vacuole volume have been shown to have beneficial effects on the properties of chocolate (Verhey, 1986; Dewettinck, De Moor, & Huyghebaert, 1996; Scheruhn, Franke, & Tscheuschner, 2000; Twomey, Keogh, O’Kennedy, & Mulvihill, 2002). For these reasons, chocolate manufacturers prefer roller-dried to spraydried powders, which typically do not have any of these attributes. However, spray-dried powders with high free-fat contents may be produced in other ways. As the fat content of milk powder increases, the free-fat content is known to increase (Pisecky, 1997; Kelly, Kelly, & Harrington, 2002). High-fat (56 g 100 g1 fat) powders can contain more than 90 g 100 g1 free fat, which can then be blended or co-dried with skim milk powder to obtain standard 26 g 100 g1 fat spray-dried powders for *Corresponding author. Tel.: +353-25-42235; fax: +353-25-42228. E-mail address: [email protected] (M.K. Keogh). 0958-6946/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0958-6946(03)00123-7
chocolate (Twomey, Keogh, O’Kennedy, & Mulvihill, 2000). It would be preferable to avoid the blending step by producing high free-fat powders at the standard powder fat level. This also can be achieved by crystallisation of the amorphous lactose pre-drying by crystal seeding (Bohren, Kuypers, & Meister, 1987) or by exposing the powder to high relative humidity postdrying (Aguilar & Ziegler, 1994b). The presence of crystals caused fine interstices and cracks in the powder particles (Saito, 1985) and increased free fat. A high free-fat spray-dried powder can also be produced by spray drying part of the fat onto a partly skimmed spray powder (Verhey, 1986). It has also been shown (Twomey et al., 2000) that free-fat content of 56 g 100 g1 fat milk powders was linearly related to seasonal changes in the native milk protein content. This suggested the need for a follow-up study to increase the protein level of milk by ultrafiltration prior to evaporation in order to increase free fat in standard 26 g 100 g1 fat powders. Concentration by ultrafiltration as well as evaporation increases protein content, concentrate solids and viscosity. Since these properties, in turn, affect powder particle size and moisture levels (Snoeren, Damman, & de Klok, 1983), it is necessary to quantify the effect of each variable on
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the powder and subsequently on the chocolate properties. The main difficulty is that powder properties are, in many cases, interrelated (Pisecky, 1978) and variables are therefore not always independent. De Vilder, Martens, and Naudts (1976, 1977, 1979) reported the effects of some process variables and total solids of the concentrate after evaporation on some physical properties of standard whole milk powders. The effects of the concentrate total solids level and the viscosity on the moisture content of the powder, free fat, vacuole volume and related factors were outlined but the particle size of the powders was not measured, as laser light-scattering measuring instruments did not become generally available until about 1990. As the total solids and viscosity of non-homogenised concentrates increased, the free fat and vacuole volume of the powders decreased and the moisture contents increased. The decrease in free fat was attributed to the increasing homogenising effect of the atomiser on the milk fat as the level of concentrate solids increased. The higher vacuole volume at lower concentrate total solids was a result of easier air entrapment and more rapid water evaporation. Higher total solids and viscosity of the concentrate slowed down the drying process leading to increased moisture content in the powder. The effects of ultrafiltration (UF) of skim milk on the physical and microstructural properties of skim milk powders have also been studied (Mistry & Hassan, 1991a, b; Mistry, Hassan, & Robison, 1992). The effects of UF on fat and free-fat contents were therefore not included in that work and the particle size was not reported. The first objective of this study was therefore to produce high free fat (>70 g 100 g1 fat) spray-dried powder at the fat content of standard whole milk powder (26 g 100 g1 fat), which generally contains a low level of o10 g 100 g1 free fat. The second objective was to determine the effects of the milk protein content and concentrate viscosity on certain properties of the spraydried powders, especially free fat, particle size and moisture content. A third objective, which will be the subject of another paper, was to determine the effects of the properties of the spray-dried milk powders (made from control and ultrafiltered milks) on the properties of milk chocolate.
2. Materials and methods 2.1. Milks Milks were obtained over 2 years from a spring herd (mean calving date 26/02/2000 and 01/03/2001) and an autumn herd (mean calving date 04/10/2000 and 27/09/ 2001) maintained at this centre. Normally, one-day’s production of milk was pooled, except in early and late lactation when two-day’s production of milk was pooled
so that sufficient milk was available for processing. The milk was stored at 4–6 C. All the milk was separated and standardised with its separated cream to the required composition to give a concentrate and powder of the required fat content. The protein, fat and lactose contents of the milks were measured in duplicate by the Milkoscan (Foss Electric, Hillerod, Denmark).
2.2. Ultrafiltration of milks Skim milk was ultrafiltered at 40 C on a Koch spiralwound membrane UF plant with a surface area of 10 m2 and normal molecular weight cut-off of 10 kDa. In one example, the protein content of the retentate was increased by a factor of 1.25 and 100 L was standardised to give a powder with 26 g 100 g1 fat (UF1). UF was then continued to increase the protein content by a factor of 1.50 to give a second powder (UF2). The protein contents of the control powder, UF1 powder and UF2 powder were 28.6, 32.2 and 34.1 g 100 g1, respectively.
2.3. Preparation of various milk fat powders The standardised and ultrafiltered milks were preheated at 97.5 C for 2 min in an APV heat exchanger (UHT-Pasilac, Silkeborg, Denmark), equivalent to a medium heat treatment (ADMI, 1971). The milks were then concentrated to various levels of total solids ranging from 41.4 to 54.0 g solids 100 g1 (45.3–50.3 g solids for the powders containing 50–59 g fat 100 g1) in a single-effect falling film evaporator (Anhydro F1 Lab, Copenhagen, Denmark). The concentrate (at 50 C) was spray-dried (Anhydro dryer, Model Lab 3, Copenhagen, Denmark; drying capacity of 10 kg h1) using a 2-fluid nozzle atomiser. Drying took place with an air inlet temperature of 190 C and an air outlet temperature of 90 C. The powder was then collected and a sample taken for analysis.
2.4. Compositional analysis The moisture content of the milk powders was determined by the oven drying method (A/S Niro . Atomizer, 1978a); fat content by the Rose–Gottlieb method (International Dairy Federation, 1987); protein content by Kjeldahl (International Dairy Federation, 1993; lactose by difference and ash by oven-drying at 550 C. The free-fat content was determined by taking 10 g of the powder and shaking gently with CCl4 for 15 min at ambient temperature (A/S Niro Atomizer, 1978b). All tests were carried out in duplicate and the mean calculated.
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2.5. Viscosity of concentrates
2.9. Statistical analysis
A strain-controlled Bohlin VOR rheometer with concentric cylinder geometry was used. Samples of milk concentrate were taken from the evaporator and 13.25 g was weighed into a C25 cup. The samples were subjected to a pre-shear at 461 s1 for 300 s at 45 C, followed by an up and down shear rate sweep from 1.16 to 461 s1 and the mean values of each sweep were taken. The concentrate viscosity profiles obeyed the power law model most closely (r > 0:998), that is
The properties of the standardised milks, concentrates, powders and chocolates were correlated using simple linear regression with intercept using the statistical functions in Microsoft Excel software. The significance levels were based on the analysis of variance using a two-tailed Student’s t-test (Box, Hunter, & Hunter, 1978).
3. Results and discussion s ¼ k’gn ; where s is the shear stress (Pa), g’ is the rate of shear (s1), the exponent n is the power law factor and k is the consistency index (Pa sn1) or the viscosity (Pa sn) at g’ =1 s1. 2.6. Particle size of milk powders The size of the particles in the milk powders was measured using a Malvern Mastersizer X (Malvern Instruments, Malvern, UK) fitted with an MSX15 small volume sample presentation unit. The instrument uses an approximation of the Mie-scattering theory to determine particle size, which utilises the refractive index of the dispersed phase and its absorption. A relative refractive index of 1.095 and an absorption value of 0.1 were used in the calculations. A 2 mW He– Ne laser beam (633 nm) and a 300 RF lens (size range 0.05–879 mm) were used for the measurements. The powders were suspended in propan-2-ol and sonicated (Sonicator, Hielscherr, Germany, model UP 200 H) for 2 min before each determination. The results are expressed as the volume weighted median diameter D(v,0.5). 2.7. Bulk density Bulk density was measured by the standard method (International Dairy Federation, 1995). 2.8. Particle density, vacuole volume and interstitial air The particle density (g mL1) was calculated by measuring the air-free volume of a known weight of powder using a pycnometer (AccuPyc 1330, Micromeritics, Norcross, GA, USA). The pycnometer determined the density and volume of milk powder by measuring the pressure change of helium in a calibrated volume. Using these data and the loose bulk density of the powder, the vacuole volume and interstitial air were calculated (A/S Niro Atomizer, 1978c).
3.1. Relationships between powder properties and other variables In all, 14 26 g 100 g1 fat powders (eight from ultrafiltered milks), one 30 g 100 g1 fat powder, 11 40 g 100 g1 fat powders (five ultrafiltered) and five 50–59 g 100 g1 fat powders (none ultrafiltered) were produced. Compositional details and physical properties of the 26 g 100 g1 fat powders are shown in Table 1(a), in Table 1(b) for 40 g 100 g1 fat powders and in Table 1(c) for 50–59 g100 g1 fat powders. Relationships between powder properties and milk, concentrate or powder variables found to be significant are shown in Table 2a for 26 and 40 g100 g1 fat powders and in Table 2b for control powders (26–59 g 100 g1 fat). The effects of these variables on milk powder properties known to be important in chocolate, namely free-fat content, particle size and vacuole volume are shown in Figs. 1–6. The relationships concentrate viscosity versus free fat and concentrate viscosity versus vacuole volume found previously by other workers (Snoeren et al., 1983; Pisecky, 1997) were not related in these powders. This is because increases in concentrate viscosity due to evaporation lead to a reduction in the free-fat content and an increase in the particle density of the powders (decreasing vacuole volume) (Snoeren et al., 1983). However, increases in concentrate viscosity due to UF resulted in increases in free fat with almost no change in particle density or vacuole volume (Table 1a). However, powder particle size versus particle density (r ¼ 0:576; n ¼ 14; Po0:05) were significantly related and powder moisture versus vacuole volume (r ¼ 0:532) and powder moisture versus interstitial air (r ¼ 0:419) were marginally related (n ¼ 14; Po0:1) in the current work. 3.2. Protein, protein:lactose ratio and free-fat content of milk powders The free-fat content of the milk powders made from spring herd milks increased linearly with the milk protein content in both 26 g 100 g1 fat (Fig. 1) and 40 g100 g1 fat powders (Fig. 2). As the milk protein content increased from 3.08 to 5.33 g 100 g1 due to UF,
(a)
Moisture (g 100 g1)
Particle size D(v.0.5) (mm)
Free fat (g 100 g1 fat)
Particle density (g cm3)
3.08 3.13 3.24 3.43 3.51 3.52 4.16 4.17 4.30 4.31 5.06 5.14 5.33 5.33
49.0 52.3 43.2 48.2 44.2 43.8 43.5 47.3 43.0 43.9 45.2 42.8 43.1 44.6
177 253 — 171 51 43 — 300 49 100 550 227 62 221
3.4 3.7 3.0 4.5 2.9 2.3 3.0 4.2 2.8 2.9 6.7 3.8 3.1 3.9
76.3 59.3 45.0 82.6 54.8 55.7 43.5 75.5 48.8 63.7 121.0 68.3 53.5 68.7
6.0 13.2 28.4 12.6 20.9 25.7 41.0 21.2 44.8 49.3 57.1 74.2 62.7 58.8
1.2085 1.2259 1.1851 1.1120 1.1933 1.1921 1.1779 1.1609 1.1803 1.1935 1.1301 1.2000 1.1824 1.1812
Standardised milk protein (g 100 g1)
Milk concentrate
Moisture (g 100 g1) 3.8 3.0 NA NA NA NA NA NA 1.6 3.5 3.9
Particle size D(v, 0.5) (mm) 100.3 81.9 57.1 57.0 61.6 69.9 57.0 67.9 46.0 90.5 78.8
Free fat (g 100 g1 fat) 39.6 49.4 45.0 45.1 40.0 46.5 61.4 63.1 71.8 67.6 73.9
Vacuole volume (mL 100 g1) 2.3 3.1 3.7 2.7 2.2 3.8 3.6 2.7 5.4 3.5 4.0
2.97 3.13 3.19 3.36 3.37
Total solids (g 100 g1) 54.0 53.6 NAc NA NA NA NA NA 41.4 50.4 48.7
26.7 25.7 27.0 28.6 27.9 29.7 32.2 32.2 32.0 32.9 34.1 35.1 35.9 35.2
Consistency index (mPa sn1) 129 73 NA NA NA NA NA NA 12 222 220
Protein (g 100 g1) 20.4 20.7 22.2 22.2 21.5 21.2 25.5 26.3 25.1 25.9 26.9
Consistency index (mPa sn1) 57 13 25 5 10
Interstitial air (mL 100 g1) 89.6 95.6 92.1 92.3 91.1 101.6 97.7 98.3 119.9 82.4 87.8
Powder
Fat (g 100 g1) 51.7 53.8 51.4 59.2 51.7
Moisture (g100 g1) 2.3 3.2 2.8 2.8 3.0
n, sample size. See text for details of preparation of standardised milks, concentrates and milk powders. c NA, not applicable. b
0.640 0.600 0.560 0.625 0.570 0.590 0.600 0.625 0.560 0.580 0.625 0.500 0.630 0.600
Powder
Milk concentrate
Total solids (g 100 g1) 50.3 47.8 49.4 45.3 46.5
Bulk density (g cm3)
Particle size D(v, 0.5) (mm) 104.1 67.4 93.7 91.6 70.5
Free fat (g 100 g1 fat) 78.4 91.2 74.9 95.5 90.4
Vacuole volume (mL 100 g1) 3.3 2.0 2.5 0.9 2.5
Interstitial air (mL 100 g1) 88.1 52.4 52.7 51.2 53.3
Vacuole volume (mL 100 g1)
Interstitial air (mL 100 g1)
5.5 3.9 6.5 12.4 5.9 7.4 7.3 8.6 7.1 6.8 10.0 6.1 6.7 6.5
73.5 85.1 94.2 73.9 91.6 85.6 81.8 70.1 93.8 88.6 71.5 116.7 74.2 82.0
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Consistency Protein index (mPa sn1) (g 100 g1)
Standardised milk protein (g 100 g1)
a
Powder
Total solids (g 100 g1)
2.95 2.98 3.04 3.05 3.08 3.42 3.50 3.57 3.91 3.93 4.24 (c)
Milk concentrate
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(b)
Standardised milk protein (g 100 g1)
998
Table 1 Composition and physical properties of standardised milks, milk concentrates and powders containing 26 g 100 g1 fat (n ¼ 14)a,b (a); 40 g 100 g1 fat (n ¼ 11)a,b (b); 50–59 g 100 g1 fat (n ¼ 5)a,b (c)
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Table 2 (a) Relationships between properties of powders containing 26 and 40 g 100 g1 fat and other variablesa,b,c Powder property
26 g 100 g1 fat powder
Related variable
Correlation coefficient, r Free fat Free fat Free fat Particle size Particle size
Milk protein Powder protein Milk protein:lactose ratio Concentrate viscosity Powder moisture
0.925 0.899 0.826 0.891 0.920
40 g 100 g1 fat powder
Degrees of freedom
Sig.b
Correlation coefficient, r
Degrees of freedom
Sig.
12 12 12 10 12
0.935 0.919 0.906 0.975c 0.888
9 9 3 3 3
(b) Relationships between properties of control powders containing 26–59 g 100 g1 fata,b,c Powder property
Related property
Correlation coefficient, r
Degrees of freedom
Sig. level
Free fat Particle size Vacuole volume Vacuole volume
Fat Fat Fat Free fat
0.976 0.621c 0.786 0.760
13 12 13 13
a
See text for details of preparation of standardised milks, concentrates and milk powders. Levels of significance: , Po0:001; , Po0:01; , Po0:05: c Relationships were linear in all models, except those marked c were quadratic (curvilinear).
130
80 70 60 50 40 30 20 10 0
Particle size D(v, 0.5) µm
Free fat (g100g-1 fat)
b
110 90 70 50 30
3.0
3.5
4.0
4.5
5.0
0
5.5
200
300
400
500
600
Consistency index k (mPasn-1)
Milk protein (g100g-1 powder) Fig. 1. Effect of protein content of control and ultrafiltered milks on the free-fat content of milk powders containing 26 g 100 g fat1. Regression line () is fitted to experimental values (K). See text for details of preparation of milks and powders.
Fig. 3. Effect of consistency index of control and ultrafiltered concentrated milks on the median particle size of milk powders containing 26 g100 g fat1. Regression line () is fitted to experimental values (K). See text for details of preparation of milks and powders.
130 Particle size D(v, 0.5) µm
80 Free fat (g100g-1 fat)
100
70 60 50 40
110 90 70 50 30
30 2.5
3.0
3.5
4.0
4.5
Milk protein (g100g-1 powder)
Fig. 2. Effect of protein content of control and ultrafiltered milks on the free-fat content of milk powders containing 40 g 100 g fat1. Regression line () is fitted to experimental values (K). See text for details of preparation of milks and powders.
2.0
3.0
4.0
5.0
6.0
7.0
Moisture (g100g-1 powder)
Fig. 4. Effect of moisture content on the median particle size of milk powders containing 26 g 100 g fat1 made from control and ultrafiltered milks. Regression line () is fitted to experimental values (K). See text for details of preparation of milks and powders.
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Free fat (g100g-1 fat)
120
120
100
100
80
80
60
60
40
40
20
20
0
0 20
30
40
50
Particle size D(v, 0.5) µm
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1000
60
Fat content (g100g-1 powder)
Vacuole volume (mL100g-1 powder)
Fig. 5. Effect of fat content on the free-fat content (closed symbols) and median particle size (open symbols) of milk powders made from control (not ultrafiltered) milks. The linear and polynomial regression lines () are fitted to the experimental values (K, J). See text for details of preparation of milks and powders.
14 12 10 8 6 4 2 0 20
30
40
50
60
Fat content (g100g-1 powder)
Fig. 6. Effect of fat content on the vacuole volume of milk powders made from control (not ultrafiltered) milks. Regression line () is fitted to experimental values (K). See text for details of preparation of milks and powders.
the free-fat content of the 26 g 100 g1 fat powders increased from 6 to 74 g 100 g1 fat (r ¼ 0:925; df ¼ 12; Po0:001). In the 40 g 100 g1 fat powders, as the milk protein content increased from 2.95 to 4.24 g 100 g1, the free-fat content increased from 40 to 74 g 100 g1 fat (r ¼ 0:935; df ¼ 9; Po0:001). The relationship between the protein content of the powders—which increased from 25.7 to 35.9 g 100 g1 in the 26 g 100 g1 fat powders and from 20.4 to 26.9 g 100 g1 in the 40 g 100 g1 fat powders—and the free-fat content of the powders was also significant (Po0:001; Table 2a). A linear relationship was also found between the milk protein:lactose ratio and the free-fat content of the 26 g 100 g1 (Po0:001) and 40 g 100 g1 fat powders (Po0:05). The free-fat content of six 26 g 100 g1 fat milk powders made from autumn herd milks also increased linearly with the milk protein content, but the correlation coefficient was not significant (not shown). According to Buma’s model (1971b), free fat exists as surface fat, sub-surface fat, pore fat from the pores of the powder particles and dissolution fat that is dissolved
by the solvent. It has already been established that the free-fat content increases either with the protein:lactose ratio or protein content in powders microencapsulated using whey proteins (Young, Sarda, & Rosenberg, 1993; Moreau & Rosenberg, 1993; Keogh & O’Kennedy, 1999), in powders with reduced lactose content (Aguilar & Ziegler, 1994a) and in high-fat (56 g 100 g1) milk powders (Twomey et al., 2000). The porosity of powders to gases also increases with protein content (Moreau & Rosenberg, 1993, 1998). These changes in porosity (Buma, 1971a, b) of microencapsulated milk fat powders (Moreau et al., 1993), powders with altered lactose content (Aguilar et al., 1994b) and skim milk powders (Mistry et al., 1992) with altered protein:lactose ratios have also been studied microscopically. In all these powders, increasing the protein:lactose ratio gave rise to an increasing number of surface cracks and pores in the powder particles, thereby allowing greater access of solvent to the fat, including the newly exposed surface fat. In contrast, skim milk powders with high lactose but also very high protein contents had particle surfaces that appeared smooth (Mistry et al., 1992). Moreover, as the protein level increases, the environment surrounding the fat globules becomes more hydrophobic (Moreau & Rosenberg, 1993), thus increasing the extractability of the fat from the powder particles by organic solvent. Conversely, lactose, being hydrophilic, repels the solvent and reduces the extractability of the fat from the powder (Young et al., 1993; Twomey et al., 2000), even in powders containing fat and lactose only (Buma, 1971b). 3.3. Particle size and moisture content of milk powders The particle size of the powders increased linearly with the viscosity of the concentrates (whether increased by evaporation or by UF and evaporation combined) for powders containing 26 and 40 g 100 g1 fat (Table 2a). The median particle size of powders containing 26 g 100 g1 fat increased from o50 to 121 mm as the concentrate viscosity (at g’ =1 s1) increased from o50 to 550 mPa sn (r ¼ 0:891; df ¼ 10; Po0:001; Fig. 3), and as the powder moisture content increased from 2.3 to 6.7 g 100 g1 (r ¼ 0:920; df ¼ 12; Po0:001; Fig. 4). These results agree with those of De Vilder et al., 1979. As the same two-fluid nozzle was used throughout at relatively low pressure, the increased viscosity of the concentrates was the main cause of the increase in particle size, which in turn lead to increased moisture content of the powders. 3.4. Effect of powder fat and free-fat contents When the control powders were evaluated, the free-fat content increased from a mean of 18 g100 g1 fat for powders containing 26 g 100 g1 fat to 95.5 g 100 g1 fat (Table 1c) for a powder containing 59.2 g 100 g1 fat
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(r ¼ 0:976; df ¼ 14; Po0:001; Table 2b, Fig. 5). This is expected due to the increased ratio of fat to emulsifying protein (Young et al., 1993; F.aldt, Bergensta( hl, & Carlsson, 1993; Keogh & O’Kennedy, 1999). When the control and UF powders were evaluated, the correlation coefficient between free fat and fat contents was lower (r ¼ 0:750; df ¼ 24; Po0:001), for which there may be two reasons. Firstly, the free-fat content increased with the protein content of the UF powders with the same fat content, as outlined earlier. The particle size of the powders increased curvilinearly with the fat content (r ¼ 0:621; df ¼ 12; Po0:05; Fig. 5), but not with the free-fat content (r ¼ 0:524; not significant), because larger particles have a lower surface area resulting in lower levels of surface fat and free fat. The reasons for the wide scatter of data, which indicates that other factors besides fat content affect particle size, will be reported in a future paper. In the control powders, the vacuole volume decreased from 7.0 to o1.0 mL 100 g1 powder, as the powder fat content increased to 59.2 g 100 g1 fat (r ¼ 0:786; df ¼ 13; Po0:001; Fig. 6) and as the free-fat content increased to 95.5 g 100 g1 fat (r ¼ 0:760). The decrease in vacuole volume is usually attributed to the foam depressing effect of free fat (Verhey, 1972).
4. Conclusions It was possible to produce a set of spray-dried milk powders containing 26–59 g 100 g1 fat with a range of values of protein, free fat, particle size, moisture and vacuole volume. This has been achieved mainly by UF and fat-standardisation of milks prior to evaporation and drying. A number of important and highly significant relationships between the properties of the standardised milks or concentrates and the properties of the powders were defined for the first time in these spray-dried products. Differences between the relationships found for the control and UF milk powders were outlined and explained. The results show that it is possible to predict certain properties of powders, such as free-fat content, particle size and vacuole volume from certain properties of the milks or concentrates. All of these relationships were anticipated from previous work with standard whole milk and skim milk powders. Nevertheless, this set of milk powders will allow the production of a series of chocolates at pilot-scale to test and provide greater insight into the effects of certain milk powder properties on the properties of chocolate.
Acknowledgements This project was part-funded by the Irish Government under the National Development Plan 2000–2006. The
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technical assistance of Mr. Robert Kennedy and Mr. John O’Keeffe is greatly appreciated.
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