Influence of milk proteins on the development of lactose-induced stickiness in dairy powders

International Dairy Journal 20 (2010) 212–221

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Influence of milk proteins on the development of lactose-induced stickiness in dairy powders S.A. Hogan*, D.J. O’Callaghan Department of Food Processing and Functionality, Teagasc, Moorepark Food Research Centre, Fermoy, Co. Cork, Ireland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 January 2009 Received in revised form 4 November 2009 Accepted 5 November 2009

The stickiness behaviour of a range of spray dried dairy powders differing in protein/lactose ratio was determined using a fluidised bed apparatus. Powders with higher protein/lactose ratios were less susceptible to sticking. Stickiness was related to both the glass transition temperature (Tg) and the temperature increment by which Tg must be exceeded before sticking occurred (TTg). TTg values of approximately 10, 22, 29, 45 and 90  C were found for powders containing 15.5, 26.9, 39.5, 55.7 and 83.4% protein respectively. Composition had different effects on Tg and TTg. The rate at which water was sorbed and desorbed by powders increased with protein content. With increasing protein content, preferential sorption of water by non-amorphous constituents delayed the rate at which lactose underwent the requisite change from the ‘glassy’ to the ‘rubbery’ form in order that powder particles became sticky. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Stickiness during spray drying and caking during transport and storage of dairy powders is an important economic issue resulting in decreased overall production efficiency. It occurs due to the combined effects of temperature and relative humidity (RH), which serve to alter the surface viscosity of powder particles allowing formation of liquid bridges between particles. Powders containing amorphous carbohydrates, such as lactose, are especially susceptible to sticking and caking. Rapid removal of water from super-saturated solutions, during drying, does not allow sufficient time for crystallisation to occur and renders carbohydrates into an amorphous, thermodynamically-unstable state. Such materials subsequently undergo time-dependent physical change ultimately leading to crystallisation (Roos & Karel,1991) and an overall increase in entropy in the material. Stickiness represents an intermediate, viscosityrelated, structural transformation along this thermodynamic path (Roos, 1995). The temperature at which an amorphous material undergoes a transition from a ‘glassy’ to a ‘rubbery’ (fluid) state is known as the glass transition temperature (Tg). Below Tg, the material is ‘kinetically frozen’ (Langrish & Wang, 2006) and although thermodynamically unstable, can be stored for extended periods without significant

* Corresponding author. Tel.: þ353 25 42455; fax: þ353 25 42340. E-mail address: [email protected] (S.A. Hogan). 0958-6946/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2009.11.002

physical change. At and above Tg, an exponential increase in molecular mobility and decrease in viscosity leads to the onset of sticking (Foster, Bronlund, & Paterson, 2006; Roos, 1995). Plasticisation of ‘glassy’ matrices by water accelerates such physical change and decreases Tg. The susceptibility of dairy powders to sticking during drying decreases with increasing Tg and occurs at some temperature above Tg (generally referred to as TTg). Although much research has been carried out on the phenomenon of glass transition in amorphous materials, the relationship between Tg and TTg is less well understood. Most of the available data on TTg, in non-fat systems, established by a variety of methods, has been reported for skim milk powder (SMP), (Hennigs, Kockel, & Langrish, 2001; Hogan, O’Callaghan, & Bloore, 2009; Ozmen & Langrish, 2003; Paterson, Bronlund, Zuo, & Chatterjee, 2007). Only a limited amount of data, however, is available on the influence of milk proteins on TTg and on the physical changes occurring in amorphous lactose, that lead to the development of stickiness in dairy powders. Previous research has shown that crystallisation of lactose may be delayed in the presence of proteins or other higher molecular weight components (Haque & Roos, 2004; Joupilla & Roos, 1994; Thomas, Scher, & Desobry, 2004). Although the precise mechanism(s) have not yet been established, it was suggested by Haque and Roos (2004) that such interactions may also affect other physical changes in amorphous materials. Stickiness represents one such pre-crystallisation phenomenon, where protein/lactose interactions may play an important role.

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In recent years, developments in membrane separation processes have increased the availability of dairy powders such as whey and milk protein concentrates (WPC and MPC) with a large range of protein contents. In general, milk powders with protein contents equal to or greater than skim milk powder (i.e.34%) are less prone to stickiness problems during drying and distribution. In contrast, permeate powders, produced by drying permeate from ultra-filtration of whey or skim milk, are high in lactose, soluble proteins and minerals and are difficult to dry. The pre-crystallisation of lactose in permeate powders prior to spray drying decreases (but does not eliminate) susceptibility to stickiness and caking, and improves stability during storage and transport. Shrestha, Howes, Adhikari, and Bhandari (2008) reported that increasing the level of whey permeate addition to skim milk resulted in greater tendency to stickiness during spray drying (as observed by decreased cyclone recovery). While such observations are useful, data on the relationship between composition and drying behaviour remain limited, particularly with regard to ‘safe’ operating conditions, i.e., the temperature and relative humidity loci within a spray drying system where sticking will not occur. The objectives of this study were to characterise the stickiness behaviour of dairy powders over a wide range of protein/lactose ratios to gain improved insight into the relationships between composition, drying, Tg and TTg. The main approach adopted involved determination of sticking characteristics with respect to composition and relative humidity combined with analysis of moisture sorption behaviour under static and dynamic conditions. Improved understanding of these relationships would contribute to the development of process control systems that maximise production output during spray drying while taking into consideration compositional and ambient factors that contribute to sticking of spray dried powders. 2. Materials and methods 2.1. Milk powders Five powders, ranging in protein/lactose ratio, were produced at Moorepark Technology Ltd., Fermoy, Co. Cork, from skim milk

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according to Fig. 1. Skim milk was supplied by Dairygold, Mitchelstown, Co. Cork, Ireland. Powders (denoted LP15, LP25, SMP, MPC55 and MPC80) had protein contents of 15, 27, 40, 56 and 83% (dry matter) respectively. Skim milk for MPC55 and MPC80 powders was ultra-filtered (Memtech UF, PCI-Memtech, Swansea, Wales) with molecular mass cut-off of 5 kDa (96 m2, operated at 40  C and 5 bar) to 14 and 25% total solids, respectively. The latter was subsequently dia-filtered to give a final total solids content of 28% before low-heat (60  C) treatment and spray drying. LP15 and LP25 powders were produced by mixing milk UF permeate (5% dry matter) with skim milk, to yield mixtures with the required protein/lactose ratios. LP15 was subsequently crystallised prior to spray drying (inlet and outlet temperatures of 160 and 90  C, respectively) to avoid production problems (sticking/blocking) associated with concentrates containing high levels of amorphous lactose. LP15 feed solution was concentrated using a three-stage falling film evaporator (Niro Ltd, Søborg, Denmark) to ca. 60% solids and subsequently crystallised by rapid cooling from ca. 50  C to ca. 32  C, followed by slow cooling, overnight, to 15  C with constant stirring. Concentration of LP25, SMP and MPC55 liquid feeds was carried out by evaporation to ca. 45% solids prior to low-heat treatment (72  C for 14 s) and drying. Concentrates were spray dried in a pilot-scale Niro tall form drier (TFD 25, Niro Ltd, Søborg, Denmark) with pressure nozzle atomisation. All powders, with the exception of LP15 were dried using inlet and outlet temperatures of ca. 185 and 85  C respectively and were agglomerated by fines return. Powders were produced singly and stored (15  C and ambient RH) prior to analyses. All analyses were carried out within three months of manufacture. 2.2. Powder characterisation 2.2.1. Chemical analysis Free moisture content was determined by weight loss following overnight drying of 2 g powder at 102  C. Total moisture content of LP15 powders was determined by Karl-Fischer titration using a 784 KFP Titrino (Metrohm AG, Herisau, Switzerland). Protein (N  6.38) was determined by macro-Kjeldahl (IDF, 2001). Fat was determined by Ro¨se-Gottlieb (IDF, 1987). Ash content was determined after overnight incineration at 550  C. Lactose content was determined

LP15 1

LP25

SMP

MPC552

MPC80

Skim milk 127 kg

Skim milk 900 kg

Skim milk 1500 kg

Skim milk 1600 kg

Skim milk 2518 kg

Ultrafiltration

Ultrafiltration/ Diafiltration

Permeate 930 kg

Permeate 2100 kg

Retentate 663 kg

Retentate 400 kg

+ 1850 kg permeate + 608 kg permeate

3

Recycle to 8% %TS TS

Evaporator 60% TS

Evaporator 48% TS

Evaporator 50% TS

Evaporator 45% TS

28% TS

Crystallisation Slow cooling to 14 °C at 1 °C C/h h-1

Low heat 72 °C, 14 s

Low heat 72 °C, 14 s

Low heat 72 °C, 14 s

Low heat 60 °C

Spray drier

Spray drier

Spray drier

Spray drier

Spray drier

Fig. 1. Production scheme for powders used in this study. Abbreviations used are: LP, low protein; MPC, milk protein concentrate; TS, total solids.

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by difference, and with exception of LP15 powders, was assumed to be in the amorphous state. Titratable acidity was determined according to GEA Niro analytical method A19a (GEA Niro, 1978). The chemical analysis of powders was carried out in duplicate, immediately after manufacture, and the results are shown Table 1. 2.2.2. Physical properties Powder particle size was determined by laser light scattering using a Malvern Mastersizer S with dry powder accessory (Malvern Instruments Ltd., Worcs. England). Powder densities and flowability were determined according to GEA Niro analytical methods A2a and A23a, respectively (GEA Niro, 1978). The physical properties of powders were carried out in duplicate and are shown in Table 1. 2.2.3. Moisture sorption ‘Working’ moisture sorption isotherms were determined by weighing ca. 1 g powder onto aluminium dishes, drying overnight in a vacuum oven (60  C, 20 mbar), and then holding at 19  3  C, in evacuated desiccators containing saturated salt solutions of LiCl, CH3COOK, MgCl2, K2CO3, Mg(NO3)2, NaNO2, giving relative humidities (RH) of 11, 22, 33, 44, 55 and 66% respectively. Powders were equilibrated for a minimum of 11 days, after which no change in weight was observed. Final moisture contents were calculated according to Bell and Labuza (2000) for samples with some initial moisture content. Moisture sorption and desorption rates of powders, were determined using a gravimetric sorption analyser (Sorption Test System SPS 11–10 m, Projekt Messtechnik, Ulm, Germany). This type of instrument has been used for characterisation of the crystallisation kinetics of lactose in pharmaceutical products (Burnett, Thielmann, Sokoloski, & Brum, 2006; Hogan & Buckton, 2001; Lane & Buckton, 2000) and infant food powders (Nasirpour et al., 2007). The test provides information on sorption characteristics under dynamic conditions, over a range of RH similar to the latter stages of spray drying. Samples (ca. 0.1 g) were equilibrated, as defined by the conditions of the test (i.e., weight change less than 0.01% within 40 min), sequentially at 2, 30 and 2% RH at 25  C and changes in mass were recorded. In the initial step, samples stored under ambient conditions lose moisture as they equilibrate to 2% RH. Equilibration required about 5 h. Sorption and desorption rates were calculated using a first order rate curve fitted to the data. 2.2.4. Fourier transform infrared spectroscopy Fourier transform infrared spectroscopy (FT-IR) was used as a complementary technique to provide qualitative information on the physical state of lactose in powders. Attenuated total reflectance (ATR) mid-infrared spectra were acquired using a single reflection diamond ATR instrument (Pike Technologies Inc., Madison, WI, USA) mounted in a nitrogen-purged Bruker Tensor 27 FT-IR spectrometer (Bruker Optik GmbH, Ettlingen, Germany), equipped with a liquid nitrogen cooled mercury cadmium telluride detector.

Powder samples were kept in optical contact with the diamond surface with consistent application of low pressure. Spectra, at 4 cm1 resolution, were vector normalised over the wavelength ranges: 1720–1480 and 1200–800 cm1, which are associated with protein and lactose, respectively, and corrected using the instrument ‘Atmospheric Compensation’ function for water vapour in the spectroscopy software (Bruker OPUS Version 5.5). Powder samples were analysed following equilibration over saturated salt solutions as described above. 2.2.5. Scanning electron microscopy Scanning electron microscopy (SEM) was carried out using a field emission scanning electron microscope (FE-SEM, Zeiss Supra Gemini, Darmstadt, Germany). Powders were lightly coated with a chromium target at 100 mA for 30 s and imaged at 1 kV. 2.2.6. Stickiness A powder fluidisation technique, previously described by Hogan et al. (2009), was used to determine the sticking behaviour of spray dried powders in a miniature fluidised bed system. Powders (ca. 0.4 g) were suspended at a series of fixed temperatures (dry bulb temperatures, equivalent to spray drier outlet temperature) in a stream of air saturated by bubbling through water at controlled temperatures. The miniature fluid beds (6) were held in an insulated, aluminium heat exchange column heated with a central element. The temperature of the water bath was gradually increased (0.8  C per min), thus raising the relative humidity of fluidising air in contact with suspended powder particles, until the powder became sticky. The sticking points were recorded by noting the water bath temperature (saturated air temperature or dew point) at which powders, in each fluidised bed, ceased to fluidise and/or by the appearance of ‘rat-holes’ (channels formed by air flowing through a solid (sticky) powder mass). RH is determined from the dry bulb temperature and the dew point. Stickiness curves were generated by plotting the air temperature versus RH, at which fluidisation in each bed ceased. 3. Results and discussion 3.1. Powder characterisation Protein and lactose contents ranged from 15.5 to 83.4% (wet basis) and 74.0–2.57% (wet basis) for LP15 and MPC80 powders, respectively (Table 1). Bulk density and tapped bulk densities increased with lactose content. Such findings are consistent with previous findings in milk powders (Aguilar & Ziegler, 1994; Shrestha et al., 2008). The very low bulk density of MPC80 powders was probably exacerbated by a larger amount of occluded air resulting from spray drying at low solids concentration (28%, w/w) (Mistry, 2002). With the exception of SMP, powder particle size was in the region of 80 mm and decreased slightly with decreasing lactose

Table 1 Compositions and physical characteristics of the powders evaluated in the study.a Powders

Protein (g 100 g1)

Lactose (g 100 g1)

Fat (g 100 g1)

Ash (g 100 g1)

Free moisture (g 100 g1)

Particle sizeb (mm)

Bulk density (g mL1)

TBDc (g mL1)

Flowability (s)

TAd (%)

LP15 LP25 SMP MPC55 MPC80

15.5 26.9 39.5 55.7 83.4

74.0 59.4 48.0 32.0 2.57

0.85 0.88 0.70 1.20 1.77

7.49 8.08 8.22 8.00 8.26

2.16 4.74 3.58 3.10 4.00

90.6 87.1 130.0 80.2 77.5

0.44 0.41 0.31 0.27 0.15

0.51 0.48 0.36 0.35 0.18

12.7 16.3 14.7 23.0 39.3

0.13 0.14 0.16 0.16 0.10

a b c d

Each result is a mean of duplicate analyses. Particle size is a volume average diameter (D4,3). TBD: tapped bulk density (g mL1 after 100 taps). TA: titratable acidity.

S.A. Hogan, D.J. O’Callaghan / International Dairy Journal 20 (2010) 212–221

content. Shrestha et al. (2008) concluded that skim milk powder particle size was not strongly influenced by increasing whey permeate (high-lactose) or protein levels. Powder particle sizes observed probably also reflect the strength of agglomerates and the relative friability of powders during drying, handling and particle size measurement. In the case of LP15, crystallisation of lactose prior to spray drying meant that the lactose present in spray dried powders existed in a partially crystalline state, i.e., a mixture of both crystalline and amorphous lactose. Scanning electron micrographs show the presence of lactose crystals in LP15 powders (Fig. 2a) but no visible evidence of crystallisation in the other powders (e.g., Fig. 2b), which is consistent with the view that lactose in non-pre-crystallised spray dried powders exists in the amorphous state (Roos, 1995). There remains a possibility, however, that small crystals may have been present in the interior of powder particles and hence were not visible by SEM (Warburton & Pixton, 1978). In such case, it is likely, given their sub-surface location, that any influence on moisture sorption and stickiness would have been negligible. The content of a-lactose monohydrate in LP15 powders was estimated at w68% of total lactose (i.e., w32% of total lactose was present in the amorphous form) according to the formula:

WC  19  100 L

(1)

where: WC is the content of water of crystallisation of lactose and L is the total lactose content. WC was calculated as the difference between total moisture (5.15%, w/w, as determined by Karl-Fischer titration) and free moisture (2.18%, w/w, as determined by oven drying) according to the method described by Schuck and Dolivet (2002), which also takes into account the contribution of protein and mineral fractions to free moisture. The method provides an estimate only and overlooks the polymorphic nature of lactose and the possible presence of other crystalline forms in the powder (Listiohadi, Hourigan, Sleigh, & Steele, 2005). 3.2. Sorption experiments Inspection of experimental moisture sorption isotherms for the range of powders throws some light on the effects of protein and lactose during hydration (Fig. 3). Moisture contents for powders equilibrated at 66% RH ranged from 6.7 to 14.0 g 100 g1 for LP15 and MPC55, respectively. Overall, the extent of hydration was the lowest in LP15. MPC80 powders had the highest moisture contents up to 33% RH and showed an almost linear increase in moisture content up to 66% RH. The sorption behaviour of MPC55 was similar to LP25 and SMP up to 33% RH but sorbed more than those at RH > 44%. Crystallisation of lactose, as evidenced by a flattening or decrease in the sorption curves due to release of moisture (Berlin, Anderson, & Pallansch, 1968), was observed in LP15, LP25 and SMP powders and occurred at around 33% RH for LP15 and around 44% RH for the latter powders. This is consistent with reports that increasing the lactose content in milk powders decreases the RH at which crystallisation begins (Haque & Roos, 2004; Shrestha, Howes, Adhikari, Wood, & Bhandari, 2007), and that the presence of high molecular weight compounds such as proteins delays the onset of crystallisation in milk powders (Joupilla & Roos, 1994). In the case of LP15, any evidence of crystallisation was subtle and identified as a flattening of the curve at RH > 33%. The extent of moisture loss due to crystallisation provides a semi-quantitative indication of amorphous lactose content and suggests a decrease in amorphous lactose content in the order SMP > LP25 > LP15. This observation was supported by compositional measurements and estimation of the amorphous lactose content of LP15, as stated above. The final moisture contents of LP15 powders were lower than other powders because crystalline lactose sorbs very little moisture (Bronlund & Paterson, 2004) and protein was not present in sufficient quantity to offer significant potential for hydration. No evidence of lactose crystallisation was evident from MPC55 or MPC80 isotherms although some caking occurred in the former Moisture content (g H2O 100 g-1 powder)

% Crystallisation ¼

215

14 12 10 8 6 4 2 0 10

20

30

40

50

60

70

Relative humidity (%) Fig. 2. Representative scanning electron micrographs of (A) LP15 powder showing presence of lactose crystals and (B) LP25 showing no evidence of lactose crystallisation (i.e., consistent with lactose in amorphous state).

Fig. 3. Moisture sorption isotherms for powders stored for 11 days at 19  3  C: LP15 (>), LP25 (,), SMP (), MPC55 (þ) and MPC80 (B). Moisture contents of powders expressed on wet weight basis. SD ¼ 0.07 g 100 g1.

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at 66% RH. The amount of lactose present in MPC80 powders may have been insufficient for caking to have occurred. Any loss of moisture due to crystallisation of lactose in MPC55 powders may also have been masked by protein re-absorption. It is likely that crystallisation of MPC55 or MPC80 is accelerated under equilibrium conditions > 66% RH. In SMP, following crystallisation, the moisture content was higher compared with LP15 and LP25 powders. This was probably due to sorption of moisture by the protein components of the powder, an effect previously suggested by Shrestha et al. (2007) and Haque and Roos (2004). The rate and extent to which powders sorb or desorb moisture, under RH conditions similar to those in the final stages of spray drying (i.e., 2–30% RH), was dependent on powder composition (Fig. 4) and increased as the protein/lactose ratio increased. This supports the hypothesis that competitive or preferential sorption of moisture by proteins delays the uptake of moisture by amorphous lactose. Such an effect would result in delayed plasticisation of lactose by water and a decrease in the rate at which Tg decreases as a function of RH. 3.3. Analysis of powders by Fourier transform infrared spectroscopy Systematic differences in FT-IR spectra have been demonstrated for glass–rubber transition, crystallisation and melting of amorphous sugars acquired during temperature-induced perturbation of freeze-dried powders (Ottenhof, MacNaughtan, & Farhat, 2003). In this study, FT-IR spectra, derived from samples stored at a range of equilibrium relative humidities, allowed differentiation between lactose in its ‘glassy’ and crystalline forms (Fig. 5). Curve shapes in the spectral region between 1200 and 850 cm1 progress from a series of smooth, less defined peaks to a more jagged appearance following crystallisation of lactose. The latter curve shape is reflective of the three-dimensional repeating structure of the crystalline form (Listiohadi, Hourigan, Sleigh, & Steele, 2009; Ottenhof et al., 2003). Such spectra, in complement with moisture sorption isotherm data, provide further information on the effects of relative humidity on the physical state of lactose-containing milk powders. In the case of LP15 powders, spectra from powders stored under low RH conditions (Fig. 6) were more jagged than other powders held under equivalent conditions but were not as jagged as powders in which the lactose was fully crystallised. The shape of these spectra reflected the partially crystalline nature of LP15 powders following

20

-1 -2

10

44 % 33 % 22 %

1200

1150

1100

1050

1000

950

900

850

-1

Wavenumber (cm ) Fig. 5. Representative FT-IR spectra of the lactose region of spray dried SMP, showing lactose in amorphous (RH 11, 22 and 33%) and crystalline forms (RH 44, 55, 66%).

drying. The development of more jagged peaks in powders stored under RH conditions > 33% is indicative of crystallisation of the remaining amorphous lactose fraction. Crystallisation of lactose in LP25 and SMP powders was observed at 44% RH as evidenced by moisture sorption isotherm data. Crystallisation of MPC55 was observed in FT-IR spectra at 66% RH despite no evidence to this effect in moisture sorption isotherms. Moisture loss due to crystallisation was probably masked in sorption isotherms by re-absorption by the protein fraction. It would appear that the lactose was not present in sufficient quantity for the physical properties of the powder to have been adversely affected by the state transitions leading to crystallisation. Thus, it can be inferred that the absence of a ‘gap’ in a sorption isotherm does not exclude the possibility of crystallisation of lactose. No evidence of crystallisation was evident from FT-IR spectra of MPC80 powders, which complements the data from sorption isotherms and confirms that the lactose remained amorphous even at 66% RH. Kher, Udabage, McKinnon, McNaughton, and Augustin (2007) used FT-IR to establish a correlation between changes in b-sheet structure (type of protein) and powder solubility following storage. No evidence of changes to protein secondary structure, within the spectral region from 1700 to 1450 cm1, was observed in any of the powders examined in the present study (data not shown).

-3

Relative absorbance

0

55 %

0.25

Relative humidity (%)

1

66 %

11 %

30

2

Change in mass (%)

RH

Relative absorbance

216

0.2

0.15

0.1

0.05

-4 0

200

400 600 Time (mins)

800

1000

Fig. 4. Change of mass against time during sorption (2–30% RH) and desorption (30–2% RH) cycles for powders held at 25  C. Equilibrium conditions, defined by the conditions of the test (change in mass of less than 0.01% over 40 min), were established for all powders prior to each RH step change: LP15 (>), LP25 (,), SMP (), MPC55 (þ) and MPC80 (B); RH (%) (- -).

0 850

900

950

1000

1050

1100

1150

1200

Wavenumber (cm-1) Fig. 6. Representative FT-IR spectra of the lactose region of spray dried LP15 powder stored at 11% RH (–) and 66% RH (––) showing lactose in partially crystalline and crystalline forms respectively. Representative FT-IR spectrum of LP25 powder stored at 11% RH () with lactose in amorphous state is included for comparative purposes.

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Zhou et al. (2006) used FT-IR to examine the thermal stability of milk powders and reported that the sequence of temperatureinduced changes to the main powder ingredients (protein, fat, lactose) was dependent on overall milk powder composition. In the case of SMP, lactose was observed to undergo physical change at a lower temperature than protein components. As expected, it would appear from this study that the stability of non-fat powders (where storage under different RH conditions was used as perturbation) is determined largely by lactose. 3.4. Stickiness 3.4.1. Stickiness of powders examined The fluidised bed apparatus used to generate stickiness curves provides information on the temperature and RH conditions required for powders to cease fluidising in a stream of air. The sticking point, thus generated, reflects powder behaviour under conditions similar to those pertaining at the spray drier outlet or in fluidised beds i.e. where particles are moving in a dynamic environment. Areas above and below each curve, (Fig. 7), represent ‘sticky’ and ‘non-sticky’ states, respectively, and provide information on safe operating conditions for drying powders differing in composition. For the powders produced in this study, higher sample temperatures (equivalent to spray drier outlet temperatures) decreased the RH at which sticking occurred (Fig. 7). The susceptibility of powders to stickiness decreased in the order LP15 > LP25 > SMP > MPC55 > MPC80. It is established that the stickiness of powders containing amorphous materials is closely associated with Tg (Boonyai, Bhandari, & Howes, 2004; Paterson, Brooks, Bronlund, & Foster, 2005). When the surface temperature of the powder particle exceeds Tg, a decrease in viscosity of the amorphous material from a ‘glassy’ to a ‘rubbery’ state allows formation of liquid bridges between particles in contact with each other or with equipment surfaces. Given the short contact times between particles moving rapidly during drying or pneumatic transport, decreases in surface viscosity must be sufficient to allow almost instantaneous formation of such bridges. Traditionally Tg curves have been determined by techniques such as differential scanning calorimetry (DSC) but can also be extrapolated from equilibrium powder moisture contents, generated from moisture sorption isotherms, using established mathematical relationships. These include the Gordon–Taylor equation for binary polymer mixtures (Gordon & Taylor, 1952) and the Couchmann– Karasz equation for multi-component, polymer mixtures

217

(Couchmann & Karasz, 1978). In the present study, Tg curves were derived using the latter equation:

Pn Tg ¼

i¼1 P n

Wi $DCpi $Tgi

i¼1

(2)

Wi $DCpi

where: Tgi is the glass transition temperature of each component (i.e., water, lactose, casein and whey protein), DCpi is the change in heat capacity at Tg (J kg1  C1) and Wi is the weight fraction of each component. Wi values for lactose were arrived at by difference and those for casein and whey protein by assuming that the N  6.38 value could be split in the ratio 4:1. Values used for Tgi and DCpi were taken from Schuck et al. (2005). The influence of powder composition on theoretical Tg curves, derived using the equation above, (Fig. 8), is relatively minor compared with its effect on stickiness. The Tg of lactose-containing milk powders, being a function of composition, is determined primarily by sorbed moisture acting as a plasticizer of amorphous lactose. In a multi-component system, the contribution of proteins to overall Tg is less significant than amorphous carbohydrates (Bhandari & Howes 1999). Shrestha et al. (2007) reported that increasing or decreasing the protein in SMP/ lactose mixtures did not affect the Tg of powders and that Tg was only significantly affected by the lactose component. In contrast, Shrestha et al. (2008) reported that increasing the level of whey permeate addition to skim milk lowered the Tg of powders and resulted in greater tendency to stickiness during spray drying (as observed by decreased cyclone recovery). The stickiness behaviour of SMP/ lactose mixtures was not reported. The differences in effects of whey permeate or lactose addition on the Tg of resultant powders was attributed to the presence of hygroscopic milk salts in the former system. Such findings suggest that the interactions between milk powder components during drying and storage are complex and remain poorly understood. In the powders examined, it is thought that stickiness was associated, predominantly, with lactose in its amorphous state. The relative positioning of the stickiness curves, LP15 apart, is reflective of the amorphous lactose content of the powders, i.e., powders became less susceptible to sticking (higher RH) with decreasing lactose content, assumed to be in the amorphous state. High protein powders tend not to cause sticking during drying. Shrestha et al. (2008) observed that increasing the whey permeate concentration or decreasing the protein content of SMP/whey permeate powders

Glass transition temperature (Tg)

140

110

Temperature (°C)

100 90 80 70 60

120 100 80 60 40 20 0 0

50 0

20

40 60 Relative humidity (%)

80

100

Fig. 7. Stickiness curves for powders examined in the study: LP15 (>), LP25 (,), SMP (), MPC55 (þ) and MPC80 (B). Individual observations (a minimum of five per powder type) are shown at each temperature.

2

4

6

8

10

Moisture content (%) Fig. 8. Theoretical glass transition curves (Tg), derived using Couchmann–Karasz equation for multi-component polymer systems, showing the effects of moisture content (0–10%) and powder type: LP15 (>), LP25 (,), SMP (), MPC55 (þ) and MPC80 (B). The LP15 curve assumes all lactose in amorphous state.

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lowered cyclone recovery and increased the incidence of powders sticking to the drier walls. The findings of the present study provide empirical information on ambient conditions leading to such effects. The relative susceptibility to stickiness of LP15 powders, compared with LP25 and SMP, was somewhat unexpected. Precrystallisation of lactose is carried out during production of nonhygroscopic whey, permeate and lactose powders to prevent stickiness and caking during drying and subsequent storage. In the case of LP15 powders, however, partial crystallisation did not appear to confer any immunity to sticking. Titratable acidity of LP15 powders was not significantly higher than other powders, suggesting that lactic acid, which can be a cause of sticking, was not the reason for the stickiness behaviour observed. Assuming that about one third of the total lactose in LP15 powders was in the amorphous state, it is possible that such levels are excessive in terms of conferring stability against sticking. Highlactose powders, in which the lactose has been crystallised prior to spray drying, typically have crystalline lactose contents of 75–80%. At such levels, surface coverage of amorphous lactose may be insufficient to cause rapid sticking even under conditions where the viscosity of amorphous lactose is conducive to so doing. Schuck et al. (2005) reported lower Tg in crystallised whey powders compared with non-crystallised powders (at equivalent moisture contents) and observed sticking in crystallised whey powders during drying. As stickiness is a particle surface phenomenon, it is possible that where surface coverage of amorphous lactose is sufficient to cover any lactose crystals, powder particles behave more like those with high bulk amorphous lactose content. The evidence here suggests that any stabilising effects conferred by pre-crystallisation of high-lactose powders may be lost if the process does not yield a sufficiently high degree of crystallinity. The observed behaviour of LP15 powders, in this study, does not fit the generally accepted understanding of the stickiness phenomenon, i.e., that stickiness in lactose-containing powders is directly related to the proportion of amorphous lactose. The effect of percentage amorphicity on stickiness in partially crystalline powders has not been the subject of previous investigation and may represent an avenue for future research. 3.4.2. Relationship between stickiness and glass transition Although an association between Tg and stickiness has been established, the extent to which particle temperature must exceed Tg (i.e., TTg) before sticking occurs appears to be both system (i.e., dependent on measurement technique) and composition dependent (Hogan et al., 2009). TTg represents a temperature increment, at a given RH, at which surface viscosity decreases to an extent sufficient to induce stickiness. It has been proposed, based on a variety of stickiness measurement techniques that the rate of stickiness development is related to the magnitude of TTg, as opposed to the specific temperature and RH used to generate the sticking point (Foster et al., 2006; Murti, Paterson, Pearce, & Bronlund, 2009; Paterson et al., 2005, 2007). It is also apparent that during the production process the temperature excess over Tg, at which sticking will occur, is likely to be dependent on the velocity, trajectory and volume fraction of particles. Decreasing contact time and/or area allow less opportunity for particles to stick compared with particles in static or compressed powder beds. Values for TTg can be established by superimposing a stickiness curve onto a Tg curve plotted as a function of RH, i.e., Tg values were plotted as a function of RH using measured equilibrium moisture contents (Fig. 9). It was found that TTg increases as the proportion of protein increases. TTg for LP15, LP25, SMP, MPC55 and MPC80 powders were established at approximately 10, 22, 29, 45 and 90  C respectively. In the case of SMP, TTg values ranging from 14 to 22  C

(Ozmen & Langrish, 2003), 23.3  C (Hennigs et al., 2001), ca. 38  C (Paterson et al., 2007) and 33.6  C (Murti et al., 2009), established using a variety of techniques, have been reported. The TTg value of ca. 29  C presented here for agglomerated SMP represents an intermediate figure compared with those outlined above. Paterson et al. (2007) also reported a TTg value of 48  C for an MPC powder (containing lactose and protein at 31.3 and 59.4% respectively). Such findings are very close to those reported in this study for powders of similar composition (MPC55). In contrast, Schuck et al. (2005) reported that TTg values of 18  C resulted in sticking of sodium caseinate during drying. No other data on the relationship between sticking and Tg in high protein powders are available for comparison. TTg values appear to result from a balance between the proportions of amorphous lactose (which tends to increase stickiness) and milk protein (which tends to decrease stickiness). Increasing the proportion of milk proteins affects powder Tg in a relatively minor way (Fig. 8) but has a more pronounced effect on the temperature or RH increment by which the Tg curve must be exceeded before initiation of sticking occurred. Such findings suggest that Tg and TTg are not affected by composition in the same manner. This can be understood as being related to contrasting methods for determination of stickiness and Tg, namely that Tg is measured at constant moisture, whereas stickiness as it arises in spray drying, and as determined in this study, is associated with variation in powder moisture, where the different sorption characteristics of protein and lactose come into play. The effect of protein/lactose ratio on stickiness in powders may be understood in terms of thermodynamic instability. Amorphous lactose exists in a thermodynamically-unstable state (i.e., it exists in a higher energy state than the crystalline form) and, as such, is subject to time-dependent crystallisation (Joupilla & Roos, 1994; Roos & Karel, 1991) and an associated increase in entropy. The time required for crystallisation to occur is reduced by increasing temperature and moisture (Roos & Karel, 1991; 1992). Such conditions induce thermal and water-plasticised molecular motion leading to re-arrangement of molecules and eventual crystallisation. As such, stickiness, along with other physical changes in amorphous materials (such as caking and collapse) represents a possible intermediate event occurring along a thermodynamic path leading from the ‘glassy’ to the crystalline state. Previous studies have shown that inclusion of proteins can delay the onset of glass transition and crystallisation of lactose (Haque & Roos, 2004; Joupilla & Roos, 1994). Haque and Roos (2004) showed that TcrTg (where Tcr is the crystallisation temperature) increases in the presence of proteins and suggested that crystallisation was affected by lactose–protein interactions at a molecular level and that such interactions may also affect the rates of other changes above and below an observed Tg. Ibach and Kind (2007) and Nijdam, Ibach, and Kind (2008), proposed that crystallisation of amorphous lactose, in partially crystalline whey powders, is delayed due to the concentration of proteins and salts in the non-crystalline powder matrix. Ibach and Kind (2007) suggested that the presence of proteins and salts effectively dilutes the concentration of amorphous lactose and lengthens the mean path length required for lactose molecules to migrate to bond with other lactose molecules. Thomas et al. (2004) reported that b-lactoglobulin was responsible for slower crystallisation rates in co-lyophilised lactose/blactoglobulin powders and that such an effect may have been due to preferential sorption of water by proteins. It was also observed that b-lactoglobulin sorbed water more rapidly than lactose, an effect consistent with the sorption/desorption rates of powders in this study. Constantino, Curley, Wu, and Hsu (1998) suggested that solid– sate interactions between sugars and proteins reduce the availability of water-binding sites and resulted in crystallisation occurring at

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B 110

120

100

100

90

Temperature (°C)

Temperature (°C)

A

T-T g ~ 10 °C

80

T

70 60

T-T g ~ 22 °C

80

T 60 40

Tg

50

Tg

20

40

0 0

10

20

30

40

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Relative humidity (%)

Relative humidity (%)

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D 140

120

120

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T-T g ~ 29 °C

80

T

60

100

Temperature (°C)

Temperature (°C)

219

40

T

60 40

Tg

Tg

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T-Tg ~ 45 °C

80

20

0

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10

Relative humidity (%)

20

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Relative humidity (%)

E 140 120

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100

Tg

80

T

60

T-Tg ~ 90 °C

40 20 0 0

20

40

60

80

100

-20

Relative humidity (%)

Fig. 9. Relationship between sticking temperature (T, ,) and glass transition temperature (Tg, -) for (A) LP15, (B) LP25, (C) SMP, (D) MPC55 and (E) MPC80 powders. Tg values were calculated by Couchman–Karasz equation using measured values for equilibrium moisture contents at the corresponding RH values as described in Materials & Methods. The fitted curves for Tg and T illustrate the respective trends and the approximate magnitude of TTg.

higher RH. Mazzobre, Soto, Aguilera, and Buera (2001) reported that although inclusion of trehalose to lactose did not affect the Tg value of co-lyophilised powders, the presence of the former species delayed crystallisation. It was proposed that the presence of a second component in the system may modify the molecular environment by means of possible thermodynamic, geometric or kinetic factors. While the mechanisms, proposed above, relate to the onset of crystallisation in amorphous materials, they are also likely to play a role in the physical changes leading to sticking in spray dried powders. In the powders studied here, TTg appeared to be related to the rate at which moisture (vapour) was preferentially sorbed or desorbed by non-amorphous species, i.e., by proteins and hygroscopic milk salts (Fig. 10).

The development of stickiness in dairy powders can come about through either sorption (hydration of dry powders) or desorption (de-hydration of wet particles during drying). In the former, preferential sorption of water vapour by proteins reduces the rate at which ‘glassy’, amorphous lactose is plasticised and is converted to the ‘rubbery’ form. Higher ambient RH is therefore required to do so. In the case of desorption-induced stickiness, the presence of proteins increases the rate of mass transfer of moisture from the particles, thereby minimising the time at which lactose exists in the rubbery state. Such findings are supported by the suggestion of Shrestha et al. (2007) that the stability of spray dried SMP/lactose mixtures is enhanced by proteins, which lower the availability of water for plasticisation and crystallisation of lactose. The kinetics of

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administered by the Irish Department of Agriculture & Food. The authors would like to acknowledge Vivian Gee of the National Food Imaging Centre, Teagasc Moorepark, Co. Cork for assistance in preparing and imaging samples for this research.

Weight change (µg min-1)

60 50 40 30

References

20

Aguilar, C. A., & Ziegler, G. R. (1994). Physical and microscopic characterisation of dry whole milk and altered lactose content. 2. Effect of lactose crystallization. Journal of Dairy Science, 77, 1198–1204. Bell, L. N., & Labuza, T. P. (2000). Moisture sorption. Practical aspects of isotherm measurement and use (2nd ed.). St Paul, MN, USA: American Association of Cereal Chemists, Inc. Berlin, E., Anderson, B. A., & Pallansch, M. J. (1968). Water vapour sorption properties of various dried milks and wheys. Journal of Dairy Science, 51, 1339–1344. Bhandari, B. R., & Howes, T. (1999). Implication of glass transition for the drying and stability of dried foods. Journal of Food Engineering, 40, 71–79. Bloore, C. (2000). Developments in food drying technology – overview. In Proceedings of the International Food Drying Conference – 2000 and beyond, Melbourne, Australia. Boonyai, P., Bhandari, B., & Howes, T. (2004). Stickiness measurement techniques for food powders: a review. Powder Technology, 145, 34–46. Bronlund, J., & Paterson, T. (2004). Moisture sorption isotherms for crystalline, amorphous and predominantly crystalline lactose powders. International Dairy Journal, 14, 247–254. Burnett, D. J., Thielmann, F., Sokoloski, T., & Brum, J. (2006). Investigating the moisture-induced crystallization kinetics of spray-dried lactose. International Journal of Pharmaceutics, 313, 23–28. Constantino, H. R., Curley, J. C., Wu, S., & Hsu, C. C. (1998). Water sorption of lyophilised protein–sugar systems and implications for solid-state interactions. International Journal of Pharmaceutics, 166, 211–221. Couchmann, P. R., & Karasz, F. E. (1978). A classical thermodynamic discussion of the effect of composition on glass transition temperatures. Macromolecules,11,117–119. Fa¨ldt, P., & Bergenståhl, B. (1994). The surface composition of spray-dried proteinlactose powders. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 90, 183–190. Foster, K. D., Bronlund, J. E., & Paterson, A. H. J. (2006). Glass transition related cohesion of amorphous sugar powders. Journal of Food Engineering, 77, 997–1006. GEA Niro. (1978). Analytical methods for dry milk products (4th ed.). Søborg, Denmark. Gordon, M., & Taylor, J. S. (1952). I Non-crystalline copolymers. Journal of Applied Chemistry, 2, 493–500. Haque, M. K., & Roos, Y. H. (2004). Water plasticization and crystallization of lactose in spray-dried lactose/protein mixtures. Journal of Food Science, 69, 23–29. Hennigs, C., Kockel, T. K., & Langrish, T. A. G. (2001). New measurements of the sticky behavior of skim milk powder. Drying Technology, 19, 471–484. Hogan, S. E., & Buckton, G. (2001). The application of near infrared spectroscopy and dynamic vapour sorption to quantify low amorphous contents of crystalline lactose. Pharmaceutical Research, 18, 112–116. Hogan, S. A., O’Callaghan, D. J., & Bloore, C. G. (2009). Application of fluidised bed stickiness apparatus to dairy powder production. Milchwissenschaft, 64, 308–312. Ibach, A., & Kind, M. (2007). Crystallisation kinetics of amorphous lactose, wheypermeate and whey powders. Carbohydrate Research, 342, 1357–1365. ¨ se Gottlieb reference method. IDF StanIDF. (1987). Determination of fat content – Ro dard 9C. Brussels, Belgium: International Dairy Federation. IDF. (2001). Determination of nitrogen content. IDF Standard 20-2. Brussels, Belgium: International Dairy Federation. Joupilla, K., & Roos, Y. H. (1994). Glass transition and crystallisation in milk powders. Journal of Dairy Science, 77, 2907–2915. Kher, A., Udabage, P., McKinnon, I., McNaughton, D., & Augustin, M. A. (2007). FTIR investigation of spray-dried milk protein powders. Vibrational Spectroscopy, 44, 375–381. Kim, E. H. J., Chen, X. D., & Pearce, D. (2003). On the mechanisms of surface formation and the surface compositions of industrial milk powders. Drying Technology, 21, 265–278. Lane, R. A., & Buckton, G. (2000). The novel combination of dynamic vapour sorption gravimetric analysis and near infra-red spectroscopy as a hyphenated technique. International Journal of Pharmaceutics, 207, 49–56. Langrish, T., & Wang, E. (2006). Crystallisation of powders of spray-dried lactose, skim milk and lactose–salt mixtures. International Journal of Food Engineering, 2. Article 8. Listiohadi, Y. D., Hourigan, J. A., Sleigh, R. W., & Steele, R. J. (2005). Properties of lactose and its caking behaviour. Australian Journal of Dairy Technology, 60, 33–52. Listiohadi, Y., Hourigan, J. A., Sleigh, R. W., & Steele, R. J. (2009). Thermal analysis of amorphous lactose and a-lactose monohydrate. Dairy Science and Technology, 89(1), 43–67. Mazzobre, M. F., Soto, G., Aguilera, J. M., & Buera, M. P. (2001). Crystallization kinetics of lactose in systems co-lyophilized with trehalose. Analysis by differential scanning calorimetry. Food Research International, 34, 903–911. Mistry, V. V. (2002). Manufacture and application of high milk protein powder. Lait, 82, 515–522.

10 0 LP15

LP25

SMP

MPC55

MPC80

Fig. 10. Effect of composition on sorption ( ) and desorption (,) rates of spray dried powders. Sorption rates were calculated from changes in mass during humidification of powders (2–30% RH). Desorption rates were calculated from changes in mass during dehumidification of powders (30–2% RH). Sorption and desorption rates were calculated from data shown in Fig. 4. Equilibrium conditions were defined by the conditions of the test, i.e., change in mass of less than 0.01% over 40 min. Error bars represent SD of duplicates.

sorption and desorption by non-amorphous constituents appear, therefore, to be an important determinant of sticking. It has also been established that surface composition of spray dried powders is significantly different from bulk composition (Kim, Chen, & Pearce, 2003). Over-representation of protein at the particle surface in spray dried, non-fat systems (Fa¨ldt & Bergenståhl, 1994; Shrestha et al., 2007), such as those examined here, may have contributed to delayed moisture sorption by lactose or as a steric barrier to lactose–lactose contact between particles. Surface protein may also affect surface viscosity in a way that lowers susceptibility to sticking, as particle sticking can only occur where surface viscosity is such that particles do not bounce off each other (viscosity too high) or slip apart following contact (viscosity too low) (Bloore, 2000). 4. Conclusions The relative stickiness behaviour of a range of powders varying in lactose and protein content was established, as a function of temperature and relative humidity, using a fluidised bed apparatus. Increasing the proportion of protein decreased the susceptibility of powders to sticking due to the combined influence of both Tg and TTg. Inclusion of protein in powders has a relatively minor effect on Tg but significantly increases TTg. Such findings showed that proteins delay the rate at which amorphous lactose undergoes physical change from the free-flowing, though thermodynamically unstable, ‘glassy’ form to the unstable, ‘rubbery’ (sticky) condition and subsequently to the stable crystalline state. The rates at which powders sorbed or desorbed moisture increased with protein content, suggesting that a competitive sorption mechanism may delay the rate at which amorphous lactose undergoes such thermodynamic change. The findings of this study provide useful additional information on the effects of protein on the physical behaviour of lactose in spray dried powders. Improved understanding of the relationships and physical states of major ingredients during processing and storage is critical to the development of functional food systems. Such knowledge is also essential for the development of effective process control systems for spray drying of food powders. Acknowledgements Funding was provided under the National Development Plan, through the Food Institutional Research Measures (FIRM),

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