Prediction of dry mass glass transition temperature and the spray drying behaviour of a concentrate using a desorption method

Journal of Food Engineering 105 (2011) 460–467

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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng

Prediction of dry mass glass transition temperature and the spray drying behaviour of a concentrate using a desorption method Peng Zhu a,b,c, Serge Méjean a,b, Eric Blanchard c, Romain Jeantet a,b, Pierre Schuck a,b,⇑ a

INRA, UMR1253, F-35000 Rennes, France Agrocampus Ouest, UMR1253, F-35000 Rennes, France c Laiterie de Montaigu, F-85600 Montaigu, France b

a r t i c l e

i n f o

Article history: Received 6 January 2011 Received in revised form 1 March 2011 Accepted 3 March 2011 Available online 9 March 2011 Keywords: Desorption Water transfer Glass transition Viscosity Spray drying Infant formula

a b s t r a c t A predictive tool (SD2PÒ) based on a drying by desorption method was recently developed in order to determine the key process parameters values prior to spray drying. However, the SD2PÒ software cannot currently take into account the risk of stickiness during the process. In the study reported here, new standardized desorption method was tested and with this new method an equation was proposed to evaluate the dry glass transition temperature (Tg) of a concentrate according to its total solid content, its viscosity and its average evaporation rate. The concentrate’s behaviour during spray drying could then be predicted on the basis of the Tg. Four validation experiments were performed using different infant formula samples; the concentrates prepared from the four infant formula powders were then spray dried using a one-stage pilot dryer. The drying parameters were predicted using the SD2PÒ software. The results showed that the Tg predicted using this method were 18–30 °C higher than Tg measured by DSC (Differential Scanning Calorimetry). The drying temperatures predicted by SD2PÒ corresponded well to the values measured and this new method can correctly predict the behaviour of a concentrate with regard to spray drying. The advantage of the method presented here is that it can easily and quickly evaluate a concentrate’s Tg range and spray drying behaviour by direct analysis of the fresh concentrate sample. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Spray drying is a valuable technique for food preservation, especially in the dairy industry. The principle of this technique is spraying a concentrated liquid in individual small droplets using a device called an atomizer, and then mixed them using a hot air stream (temperature ranging from 100 to 300 °C). Evaporation of water from the droplets is facilitated by heat and vapour transfer through/from the droplets. The wet-bulb temperature of the droplets is believed to be in the range of 30–50 °C and the total duration of drying is only a few seconds (Schuck et al., 2009). Due to the need for a variety of dairy products with more stringent product quality and strict function requirements, dairy fluids with a wide range of components and proportions are currently spray dried. For example, the formulation of an infant/baby formula may involve the mixing of over 40 different constituents before the evaporation and spray drying processing stages. These different products may exhibit different drying behaviours and stickiness-solubility patterns, and the drying of such products may require adjustment of outlet gas temperature and outlet gas ⇑ Corresponding author at: INRA, UMR1253, F-35000 Rennes, France. Tel.: +33 2 23 48 53 32; fax: +33 2 23 48 53 50. E-mail address: [email protected] (P. Schuck). 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.03.003

humidity. The operational parameters for the drying of milk and whey have to date been obtained empirically and/or using plant trials. However, this approach is very expensive, risky and time consuming and thus difficult to apply systematically for each formula (Schuck, 2002). An original drying by desorption method was recently developed at the Institut National de la Recherche Agronomique (INRA) in France (Schuck et al., 2009). This method is based on global mass and energy balance throughout the spray drying system. In this technique, the ratio of bound to unbound water is evaluated using a thermodynamic sensor (Schuck et al., 1998). Taking into account input parameters related to the dryer, the drying conditions and the final product requirements (e.g. evaporation capacity, air flow rates and humidity, total solid contents of the concentrate and temperature, water content of the powder in relation to water activity, energy cost, etc.), the operational parameters such as inlet and outlet air temperatures, concentrate and powder flow rates, specific energy consumption, yield of the dryer and cost (per kg water evaporated or per kg powder produced) can be predicted from mass and energy balance. This approach has been tested for over 30 different dairy concentrates and several spray dryers according to different scales (from 5 kg h1 to 6 t h1). A good match between measured and predicted parameters (±1–5% error) has been reported (Schuck et al., 2009; Zhu et al., 2011). A

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Nomenclature AH DE DC m m0 mend Psurface Psat RH RHz

absolute humidity (g kg1 dry air) dextrose equivalent chamber deposition (g deposited 100 g1 total powder) mass (mg) mass of concentrate at the beginning of the desorption (mg) mass of concentrate at the end of the desorption (mg) partial pressure at the surface (Pa) saturated partial pressure (Pa) relative humidity (%) relative humidity of the zeolite (%)

predictive tool based on this drying by desorption method is now marketed under the name ‘Spray drying parameter simulation and determination software’ SD2PÒ in order to determine or optimize the key process parameters. However, the SD2PÒ software cannot currently take into account the risk of stickiness during spray drying. Several authors have found that the stickiness phenomenon is closely related to the glass transition phenomenon: the stickiness that happens during the drying process of dairy products usually occurs at a temperature above the glass transition temperature of the product (Hogan et al., 2009; Ozmen and Langrish, 2002; Truong et al., 2005; Roos and Karel, 1991a). Glass transition is a phase transition of amorphous materials during their gradual transition from a soft state (higher water mobility) to a glassy and relatively hard state (low water mobility) (Roos, 2002; Genin and René, 1995). The temperature during this transition is therefore called glass transition temperature (Tg). This phenomenon has been extensively studied for amorphous polymeric materials, and its influence on the manufacture and storage of foods has been ever more extensively studied over the past twenty years (Biliaderis et al., 1986; Roos, 1987; Zeleznak and Hoseney, 1987; Simatos and Karel, 1988; Roos and Karel, 1990, 1991a). Many dehydrated dairy products contain amorphous components. During the process, if the temperature is higher than the glass transition temperature of the product, the product will be in the visco-elastic state and will thus have a high risk of stickiness. According to Truong et al. (2005) and Roos and Karel, (1991a), this critical temperature is usually from 10 to 23 °C above the glass transition temperature, the risk of stickiness rising with the increase in T–Tg (Lloyd et al., 1996). As the stickiness phenomenon usually occur with low Tg powders, formulations with high molecular weight additives can increase the Tg of the product and thus reduce the risk of stickiness during spray drying, and the Tg can be taken as a parameter to predict a product’s behaviours during spray drying (Tonon et al., 2009; Langrish et al., 2007; Busin et al., 1995). In order to avoid stickiness, it is important to fix the spray drying parameters with regard to the glass transition of the product (Schuck et al., 2007; Boonyai et al., 2004, 2005). The methods of determination of Tg found in the literature fall into two types, i.e. direct measurement by instruments and indirect estimation by mathematical modelling (Couchman and Karaz, 1978). Direct measurement is more precise but this method needs to analyse samples from the dried product whereas it cannot determine the Tg from a liquid sample which has a high water content such as a dairy concentrate. The sugars in an oven-dried sample are usually already crystallized due to the long duration of drying which makes it difficult to use for determination of Tg prior to spray drying. Indirect estimation usually requires the glass transition temperatures and DCp of each component, and is thus difficult to

RHm RHr

ve

Tg TS

g

measured relative humidity (%) real relative humidity (%) average evaporation rate (mg min-1) glass transition temperature (°C) total solids apparent viscosity (mPa s)

Abreviations DSC differential scanning calorimetry INRA Institut National de la Recherche Agronomique SD2PÒ spray drying parameter simulation and determination software

apply to a complex dairy product such as an infant formula. The aim of this study was first to develop a new method to evaluate the glass transition temperature of a concentrate according to its desorption behaviour, and then to predict its drying behaviour in a spray dryer. 2. Materials and methods 2.1. Preparation of the concentrates Concentrates with 40 wt.% and 30 wt.% total solids (TS) were prepared from various powders dissolved in reverse osmosis water. The maltodextrins DE1 (DE, Dextrose Equivalent), DE6, DE19, DE29, DE38, DE47 and dextrose monohydrate were manufactured by Roquette Frères (Lestrem, France); the skim milk powder and lactose-free whey powder were produced at Bionov (Rennes, France). The infant milk powder samples (P1–P4) were provided by the Laiterie de Montaigu (Montaigu, France) (Table 1). 2.2. Standardization of the desorption method A traditional desorption method was developed by Schuck et al. (2009) using an air-tight stainless-steel cylindrical device at 45 °C. The milk concentrate sample (160 ± 1 mg) is contained in a small plastic cup that is carefully placed in a cylindrical device which is filled with strong adsorbent particles (zeolites). The water transfer takes place due to the vapour gradient between the zeolite particles and the milk concentrate. Changes in the relative humidity of the air inside the cylindrical device (due to the vapour transfer from the sample to zeolites) are continuously monitored using a relative humidity (RH) sensor (Rotronic, Bassersdorf, Switwerland) placed close to the surface of the milk sample. The desorption method was standardized in this study in order to eliminate any external influences. The volume of zeolites was fixed at 120 ± 1 ml and the position between the cup and the RH sensor was always kept the same for all the desorption experiments.

Table 1 Main composition of the four infant milk powder samples (P1–P4) (GOS: Galactooligosaccharide.

Protein (g) Carbonhydrate (g) Fat (g) GOS (g) * **

P1

P2

P3

P4

11.0 56.5 25.0 2.0

15.0 53.0 22,5 3.0

15.0 55.1 19.0 3.0

11.6* 57.0** 26.0 0.0

100% Hydrolysed protein. 33 g Lactose and 24 g maltodextrin DE19.

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The duration of desorption was 300 min, the milk samples were weighed using a weight scale (Max 220 g, d = 0.01 mg, Model: wxts205du, Mettler Toledo, Viroflay, France) before and after each desorption process. The desorption experiments were performed in parallel in four desorption devices for each concentrate. 2.3. Chemical and physical analysis 2.3.1. Determination of glass transition temperature The powder samples (10 g) were first dried by 100 g zeolite particles (RH < 0.1% at 45 °C) in a vacuum desiccator for 15 days at 45 °C in order to obtain the dry powders. The dry powders were then used to determine the glass transition temperatures (Tg) using differential scanning calorimetry (DSC, Q-1000, TA Instruments, Saint Quentin en Yvlines, France) calibrated with indium (melting point 156.6 °C). The scanning rate was 3 °C min1 from –10 °C to 200 °C. The Tg range was determined from the reversible heat flow curve. Three Tg values were provided: Tg onset, Tg inflection and Tg endset. Each product was tested twice; the average value of Tg inflection was taken as the Tg of the dry powder. The composition of the P4 showed that the protein in P4 was 100% hydrolyzed, which can significantly reduce the Tg value. Moreover the fat used was vegetable oil (26 g per 100 g powder), which usually has a lower melting point. Thus the glass transition may occur in the same area as the melting of fat, which means that the Tg cannot be determined by the DSC method. The Tg of P4 was therefore measured by the rheological method developed by Hogan et al. (2010). A 2 g sample of powder was compressed between a 40 mm steel parallel plate and a Peltier plate using a laboratory rheometer (AR2000, TA Instruments UK, Ltd., Crawley, England). The sample was heated from 0 to 140 °C at a rate of 2 °C min1 after an initial equilibrium time of 10 s. During heating, the normal force was recorded at a constant gap with an initial compression of 30 N. The Tg was identified by a significant increase in the rate of change in normal force. The sample was tested twice; the average value of Tg was taken as the Tg of the dry powder. Finally, the glass transition temperature of dextrose was obtained from the literature (Katkov and Levine, 2004). 2.3.2. Determination of total solid content Total solids contents were measured according to the weight loss profiles after oven drying of 1 g of the sample mixed with sand at 105 °C for 5 h for the powder and 7 h for the liquid concentrate. 2.3.3. Determination of viscosity The viscosity of the concentrates was determined with an AR1000 rheometer (TA Instruments, Guyancourt, France) in coaxial cylinder geometry (stator inner radius: 25.00 mm; rotor outer radius: 23.05 mm; immersed cylinder height: 30.00 mm; bottom cap: 4000 lm) at 45 °C (Schuck et al., 2005a). Samples were preheated at 45 °C and were equilibrated at the measurement temperature for 5 min, then sheared at increasing rates from 0.1 to 200 s1 followed by decreasing rates from 200 to 0.1 s1. As the concentrate was in a static state during the desorption experiment, a low shear rate (1 s1) was chosen. Apparent viscosity (g) was considered as the viscosity value at the shear rate of 1 s1. 2.4. Modelling of water transfer 2.4.1. Estimation of real relative humidity (RHr) At the beginning of the desorption process, the vapour concentration of the concentrate was near the saturated vapour concentration at the surface of pure water (i.e., RH = 100%). However, the sensor provided a RHm value of around 30%, whatever the product (Schuck et al., 2009). Schuck et al. (1998) assumed that the RHm decreased with the change in thickness of the concentrate during

desorption. They developed an equation to deduce RHr as a function of RHm and the thickness. However, using this equation, the RHr deduced for pure water increased with time, which could not be the case. A new equation is proposed in this study which considers RHr as a function of RHm. Concentrates tested by the desorption method usually have more than 50% water content. At the very beginning of the desorption, due to the existence of free water in the concentrate, the RH at the concentrate’s surface was around 100% (aw close to 1), and it gradually reduces during drying. However, in the case of desorption of saturated salts, as the solutions were saturated, the RH at the surface was not 100% and it would always retain the same RH during drying by desorption. They could thus be used to determine the correspondence between the RHm and the RHr. Pure water, zeolites and 11 saturated salt concentrates were used to calibrate the real relative humidity (RHr). The water activity (aw) of the 11 saturated salt concentrates (LiCl, CH3COOK, MgCl2, NaI, K2CO3, Mg(NO3)2, NaBr, NaCl, KCl, BaCl2 and K2Cr2O7.) were measured by an aw meter (aw-box, Novasina, Bassersdorf, Switzerland) at 45 °C. The RHr of the concentrates can be calculated from the aw measured at 45 °C. In equilibrium conditions:

RHr ¼

Psurface  100% ¼ aw  100% Psat

ð1Þ

They were then dried by the standardized desorption method. The RHm for each concentrate was taken as the RH value measured after the preheating stage. Thus the relationship between RHr and RHm can be determined from the experimental results. 2.4.2. Calculation of evaporation rate Preliminary experiments (results not presented) showed that the area between the real relative humidity curve and the bulk relative humidity curve can represent the amount of water evaporated during desorption. As the RH value was recorded each minute, this area can be expressed as the cumulated RH (%.min):

RHcumulated ¼

X

ðRHr  RHz Þ

ð2Þ

where RHz is the relative humidity of the zeolite particles at 45 °C. The amount of evaporated water can be calculated from the differences between the weight of concentrate before (m0) and after (mend) desorption. The quantity of evaporated water at time t during desorption (mevaporated water [t]) can thus be calculated from the total amount of evaporated water (m0  mend) and the ratio of cumulated RH at time t (RH cumulated[t]/RHcumulated[total]) (Eq. (3)). The quantity of evaporated water in relation to time is presented in figure 1, the slope of the curve being the instantaneous evaporation rate.

mevaporated water ½t ¼

RHcumulated ½t  ðm0  mend Þ RHcumulated ½total

ð3Þ

As shown in figure 1, drying by desorption can be divided into two periods: the constant evaporation rate stage (from the beginning to about 100 min for a skim milk concentrate at 40% TS, which includes a preheating stage for the first 10 min (not visible in figure 1) (Zhu et al., 2011)) and the decreasing evaporation rate stage (time above 100 min.). The boundary between these two periods can slightly vary for different concentrates. A lower total solid content means a longer constant evaporation rate stage. For example, the constant evaporation rate stage can reach 170 min in the case of skim milk at 20% TS. In order to remove the potential influence of the preheating stage during the first 10 min, the average evaporation rate (ve) of the constant evaporation stage was therefore calculated from the average of instantaneous evaporation rate from 15 to 80 min.

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Quantity of evaporated water (mg)

100 90 80 70

Decreasingevaporation rate stage

Constant evaporation rate stage

60 50 40 30 20 10 0 0

50

100

150

200

250

300

Time (min) Fig. 1. Quantity of evaporated water in 40 wt.% TS skim milk in relation to time.

2.5. Spray drying test Infant formula concentrates P1, P2, and P3 were recombined at 40 wt.% with reverse osmosis water overnight before spray drying and were stored at 4 °C. Infant formula concentrate P4 was prepared in the same way on the morning of the spray drying to avoid significant decantation of the concentrate. The concentrates were spray dried twice (once in the morning, once in the afternoon) in a pilot-scale Mobile Minor™ spray-drier, MM-PSR type (GEA Niro A/S, Søeborg, Denmark), equipped with a two-fluid spray nozzle (orifice diameter: 0.80 mm). The maximum evaporation rate was 5 kg water h1. A Watson-Marlow peristaltic pump, type 520S (Watson-Marlow, Gambais, France), fed the emulsion to the spray-drying chamber, with a feeding rate of around 5 kg h1. Inlet air humidity was controlled and adjusted by a Munters dehumidifier (type M210, Munters Europe AB, Sollentuna, Sweden) to 1 to 1.5 g kg1 of water in dry air (Vignolles et al., 2010; Zhu et al., 2009). Schuck et al. (2008) showed that the moisture content and the aw of the dairy powder would remain constant if the outlet air temperature and humidity do not change. In order to obtain a powder with an aw around 0.2, the outlet air RH was defined as 10% for all the trials. The outlet air temperature was then calculated (78 °C) on the basis of outlet air RH and maximum evaporation capacity of the dryer. The other spray drying parameters were then determined by the SD2PÒ software (Schuck et al., 2009) and adjusted during experiments to obtain the predefined outlet air temperature and RH. The duration of the trial was 1 h with a concentrate flow rate of about 5 kg h1. Manual hammer beating was applied every 10 min to limit the powder sticking to the walls. However, any powder remaining in the drying chamber was collected and weighed after each experiment. The percentage of powder remaining in the drying chamber was called ‘‘deposition in chamber’’ (DC, g deposited.100 g1 total powder), which was calculated on the basis of mass balance in the entire dryer. The calculated outlet air AH was obtained by dividing the total inlet water by total outlet dry air. The DAH was the difference between the calculated outlet air AH and the measured value.

3. Results and discussion 3.1. Estimation of real RH (RHr) of concentrate surface During the desorption of pure water, a gradient of vapour concentration was formed between the water surface (RH = 100%) and the zeolite surface (RH  0%). As the RH sensor was sited around

15 mm above the concentrate surface, it measured the RH of air passing through the sensor which had a RH value of around 30%. As the concentrate thickness was only 1–2 mm, the change in the concentrate thickness did not have a significant influence on the RHm. Using the saturated salt concentrate RHr/RHm correspondence (Table 2), the relationship between RHr and RHm can be deduced as (Figure 2):

RHr ¼ 3:38  RHm - 0:01

ð4Þ

2

R = 0.98 3.2. Estimation of Tg of maltodextrins The Tg values measured by DSC are presented in Table 3. These values showed that the variation in Tg in relation to DE can be considered as linear (Figure 3):

T g ¼ 1:17  ðDEÞ þ 153:91 R2 ¼ 1

ð5Þ

Busin et al. (1996) also reported a linear relationship between Tg and DE:

T g ¼ 1:4  ðDEÞ þ 176:4 R2 ¼ 0:98

ð6Þ

Measured Tg values are generally smaller than those estimated by Eq. (6), the difference varying from 0.4 to 22 °C with decreasing DE (Figure 3). As the scanning rate used in this study was 3 °C/min, whereas Busin et al. (1996) used 10 °C/min, this might be the

Table 2 Real relative humidity (RHr) measured by aw meter and measured relative humidity (RHm) measured by desorption experiment.

Zeolites LiCl CH3COOK MgCl2 NaI K2CO3 Mg(NO3)2 NaBr NaCl KCl BaCl2 K2Cr2O7 Water

RHr (%)

RHm (%)

0.1 11.7 25.8 34.0 43.7 47.9 54.2 59.2 74.5 85.0 89.9 97.7 100.0

1.0 ± 0.2 5.3 ± 0.3 8.4 ± 0.5 10.7 ± 0.9 13.0 ± 1.0 13.9 ± 0.9 15.7 ± 0.2 17.0 ± 0.8 22.5 ± 2.4 25.5 ± 1.3 27.3 ± 2.1 29.4 ± 1.8 29.9 ± 2.2

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Real relative humidity (RHr ; %)

1.2

y = 3.38x - 0.01 R2 = 0.98

1

0.8

0.6

0.4

0.2

0 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Measured relative humidity (RHm ; % ) Fig. 2. RHr as a function of RHm (experimental results of water, zeolite and saturated salt concentrates).

Table 3 Glass transition maltodextrins.

3.3. Relationship between Total solids, Tg, viscosity and temperature

(Tg)

values

DE

Tg (°C)

6* 29* 38* 47* 100 (dextrose)**

145 ± 0 120 ± 6 113 ± 0 98 ± 3 36

* Measured by DSC with a scanning rate of 3 °C min1 from 10 to 200 °C. ** Cited from Roos, (1993).

origin of the difference in Tg values. The scanning rate of DSC can significantly influence the determination of Tg (Rahman et al., 2007). Nevertheless, despite these differences, it can be considered that the Tg values obtained in this study are in the same order of magnitude as those of Busin et al. (1996). As the Tg of some high molecular weight maltodextrins are not visible on DSC thermodiagrams (Roos and Karel, 1991b; Busin et al., 1996), it is possible to estimate the Tg values of maltodextrins at different DE from the Eq. (5). In this study, the Tg values of maltodextrins DE1 and DE19 were estimated from Eq. (5).

We assumed with our desorption device that the average evaporation rate (ve) was mainly influenced by the total solid content of the concentrate(TS), the apparent viscosity of the concentrate (g) and the dry Tg of the concentrate. The Tg of a two component mixture can be calculated from the Tg and weight fraction of each component (Young and Lovell, 1991). The Tg of water was reported to be 130 to 135 °C (Genin and René (1995), De Graaf et al. (1993), Johari et al. (1987), Roos (2002)), which can be considered as constant. Thus the wet Tg of an aqueous solution can be expressed as a function of TS and TS  Tg dry mass. Based on the experimental findings (Table 4) for maltodextrins (DE1, 6, 19, 29, 38, and 47), dextrose, skim milk and lactose-free whey, ve can be empirically expressed as a function of g, TS  Tg dry mass and TS (Eq. (7)):

v e ¼ 4:49  105 ðgÞ þ 1:13  105



 TS  T g dry mass  2:97

 103 ðTSÞ þ 0:75

ð7Þ

2

R = 0.96 Eq. (7) shows that the ve decreased with increasing g and TS, whereas it increased with increasing TS  Tg dry mass. Higher viscosity and higher TS promote crust formation, thus reducing the water evaporation rate. A component with a higher Tg usually means a

200 180 160 140

Tg (°C)

Tg measured by DSC (°C)

120

Busin et al. (1996)

100 80 60 40 20 0 0

20

ve

of

40

60

80

100

Dextrose equivalent (DE) Fig. 3. Tg as a function of dextrose equivalent value (DE) (: Tg measured by DSC in this study; j: Tg reported by Busin et al., 1996).

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P. Zhu et al. / Journal of Food Engineering 105 (2011) 460–467 Table 4 Experimental results for different concentrates (TS: total solids; Tg: glass transition temperature; g: apparent viscosity; ve: average evaporation rate). Concentrates

* **

Tg

g

Table 5 Characteristics of infant formula concentrates and predicted Tg (ve: average evaporation rate; TS: total solids; g: apparent viscosity; Tg: glass transition temperature).

ve

Target TS wt.%

TS

Concentrates

wt.%

°C

mPa s

mg min

DE 1* DE 6 DE 19* DE 29 DE 38 DE 47 Dextrose** Skim milk

40

38.3 ± 0.1 38.3 ± 0.5 38.4 ± 0.2 37.5 ± 0.0 37.5 ± 0.0 38.5 ± 2.0 36.6 ± 0.1 38.0 ± 0.1

153 145 132 120 113 98 36 95

603.5 ± 54.4 79.2 ± 0.0 9.4 ± 1.1 5.6 ± 1.1 6.4 ± 0.0 7.6 ± 0.0 9.4 ± 2.8 16.1 ± 5.2

0.67 ± 0.01 0.70 ± 0.01 0.69 ± 0.01 0.69 ± 0.01 0.69 ± 0.01 0.68 ± 0.01 0.66 ± 0.01 0.67 ± 0.01

Lactose-free whey DE 1* DE 19* DE 29 DE 38 DE 47 Dextrose** Skim milk

30

38.5 ± 0.2

66

848.9 ± 103.0

0.63 ± 0.01

28.5 ± 0.3 27.8 ± 1.7 27.9 ± 0.0 28.3 ± 0.1 28.8 ± 0.0 30.4 ± 0.0 28.7 ± 0.0

153 132 120 113 98 36 95

40.0 ± 1.7 13.7 ± 0.0 5.4 ± 0.0 1.5 ± 0.0 17.8 ± 0.9 4.7 ± 0.5 7.6 ± 4.4

0.72 ± 0.01 0.71 ± 0.01 0.71 ± 0.01 0.70 ± 0.01 0.69 ± 0.01 0.67 ± 0.01 0.69 ± 0.01

1

Calculated from Eq. (5). Cited from Roos (1993).

higher molecular weight and fewer hydrophilic groups compared with lower molecular weight components; thus it is less hygroscopic and therefore permits an easier water evaporation (Bhandari et al., 1993; Tonon et al., 2009). It can be seen from Eq. (7) that the influence of g can be negligible for small g values, whereas it would be non-negligible for high g values such as maltodextrin DE1 and lactose-free whey. For most monosaccharide and disaccharide solutions at the same TS, the relationship between ve and Tg can be considered as linear. Eq. (7) makes it possible to predict dry Tg values while measuring ve, g and TS. 3.4. Prediction of glass transition temperature Four infant formula powders (P1, P2, P3 and P4) were used to test the dry Tg prediction obtained from Eq. (7). The characteristics of the concentrates prepared with the four powders and the predicted Tg obtained are presented in Table 5. The measured and predicted Tg followed the same trend for all 4 powders: P1 > P2 > P3 > P4. The predicted Tg for P1, P2 and P3 were higher than the measured Tg, the difference varying from 18 to 30 °C, while for P4, the predicted Tg was 10 °C lower than the measured Tg. These differences between P1–3 and P4 might result from the different measurement methods used (DSC vs. Rheometer). Indeed, the Tg measured by DSC in this study were the Tg inflection. According to Hogan et al. (2010), the Tg measured by rheometer was closer to the Tg endset, which was higher than the Tg inflection value. The differences between measured and predicted Tg may be due to the high sensitivity to the RH measurement: indeed, a change of 0.01 mg min1 in ve can cause a change in predicted Tg value of around 20 °C. Using a weight scale instead of a RH sensor to measure the weight change during desorption directly may improve the precision of prediction. 3.5. Validation experiments using a pilot dryer The spray drying parameters are presented in Table 6. It can be seen that the outlet air temperature was 78.3 ± 1.6 °C and the outlet air RH was 10.6 ± 1.0, whatever the experiment. The measured inlet air temperatures for the four concentrates were close to the predicted values. The difference between measured and predicted inlet air temperatures was generally lower than 5 °C, which

ve mg.min

P1 P2 P3 P4 * **

1

0.69 ± 0.01 0.69 ± 0.01 0.68 ± 0.00 0.65 ± 0.01

TS

g

wt.% 40.1 ± 0.0 39.9 ± 0.1 39.5 ± 0.0 39.4 ± 0.0

mPa.s

Tg measured °C

Tg predicted °C

6.5 ± 1.7 14.1 ± 0.9 6.6 ± 0.5 16.8 ± 0.3

100 ± 4* 92 ± 2* 86 ± 4* 48 ± 3**

126 122 104 38

Tg measured by DSC. Tg measured by the rheological method.

represents less than 2% difference, as presented by Schuck et al. (2009). The variation in the inlet air temperature for the four infant formula samples (from 256 to 282 °C) were due to the different desorption behaviours of the four concentrates: the more the water was bound to the constituents, the greater the extra energy needed to unbind water. Since the outlet air temperatures were the same, a concentrate which was more difficult to dry would have a higher inlet air temperature. The concentrate flow rate measured was 22–34% greater than that predicted by SD2PÒ. This difference may due to the low accuracy of the RH sensor within the experimental conditions, the deposition of powder on the filter surface of the RH sensor significantly slowing down the sensor’s reaction time. As a result, the RH values displayed might be smaller than the real RH values. A higher concentrate flow rate was therefore used in order to obtain the predefined outlet air RH values. As expected, this resulted in higher outlet air absolute humidity (AH) than predicted: the measured outlet air AH was in fact from 1.3 to 2.8 g kg1 dry air higher than the predicted value. As presented in Section 3.4, the glass transition temperatures of the four infant formula samples were in order P1 > P2 > P3 > P4. The DC for P1, P2 and P3 were the reverse of the order of Tg 5.6, 9.4, and 26.9 g deposited.100 g1 total powders, respectively. On the other hand, the DC for P4 was less than that of P3, whereas P4 is known to be much more difficult to dry than P3 according to the industry and according to their Tg. P1, P2, and P4 were at the same level with regard to the moisture content of the powders, while the moisture content of P3 was about twice as high. Higher humidity in the powder corresponds to a higher level of DC. For P4, the particles tended to adhere to each other easily even at low moisture content. They were easy to get off from the dryer walls due to their increase in weight. This may be one reason why its DC value was low during experiments, especially when there was a hammer beating every 10 min. Nevertheless, P4 may still be more likely to cake and even stick to the walls of the drying chamber or the cyclone. This requires confirmation with further experiments. The difference between calculated and measured outlet air AH showed that P4 had the highest DAH values, followed by P3, P2, and P1, respectively. As the calculation of AH was based on the hypothesis that all the water was removed by the hot air, any large difference between calculated and measured AH means that part of the water remained in the powder, corresponding to lower water removal and favouring sticking (Schuck et al., 2005b). Schuck et al. (2005b) reported that sticking happened in a three-stage pilot dryer (Evaporation capacity 70–120 kg h1) when DAH was above 2 g kg1. A high DAH value was observed in all cases in this study, explained by the lack of precision of the RH sensor and the fact that a small one-stage pilot dryer with high mass and energy loss (up to 7% and 25%, respectively, according to Zhu et al. (2009)) encourages stickiness whatever the concentrate. In view of the industrial and DAH values, it can be concluded that the dry Tg of a concentrate predicted by this method

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Table 6 Spray drying parameters and experiment results for the four infant formulas. (AH: absolute humidity; TS: total solids; RH: relative humidity; DC: deposition in chamber (g deposited.100 g1 total powder); N/A: not applicable). P1

Inlet air temperature Inlet air AH Total inlet dry air flow rate Concentrate TS Concentrate flow rate Outlet air temperature Outlet air RH Outlet air AH Powder moisture content DC*

* ** ***

P3

P4

SD2P

Measured values SD2P

Measured values SD2P

Measured values SD2P

Measured values

°C g kg1 dry air kg dry air.h1 wt.% kg h1 °C % g kg1 dry air wt.% g 100 g1 powders

256 1.0 89.0 40.0 4.0 78 10.0 28.0 2.0 N/A

255.7 ± 0.6 1.8 ± 0.2 88.8 ± 0.2 39.7 ± 0.0 5.0 ± 0.0 78.2 ± 0.1 10.9 ± 0.4 30.8 ± 1.3 1.3 ± 0.4 5.6 ± 2.8

259.9 ± 2.4 1.5 ± 0.0 89.2 ± 1.1 39.8 ± 0.0 5.0 ± 0.0 78.3 ± 1.6 10.3 ± 0.7 29.3 ± 0.4 1.6 ± 0.2 9.4 ± 4.5

260.7 ± 6.6 1.5 ± 0.1 89.6 ± 2.3 40.1 ± 0.0 5.2 ± 0.1 78.2 ± 0.8 10.5 ± 1.0 29.7 ± 2.0 3.4 ± 0.6 26.9 ± 1.6

282.1 ± 0.0 1.0 ± 0.1 85.6 ± 1.8 39.9 ± 0.0 5.5 ± 0.0 78.4 ± 1.4 10.7 ± 0.2 30.7 ± 1.1 1.5 ± 0.4 12.5 ± 3.9

Spray drying difficulty**

Calculated outlet air AH DAH of outlet air***

P2

Unit

1

g kg dry air g kg1 dry air

264 1.0 89.0 40.0 4.1 78 10.0 28.0 2.0 N/A

265 1.0 89.0 40.0 4.1 78 10.0 28.0 2.0 N/A

282 1.0 89.0 40.0 4.0 78 10.0 28.0 2.0 N/A

Easy

Easy

Difficult

Very difficult

Calculated values

Calculated values

Calculated values

Calculated values

35.5 ± 0.2 4.7 ± 1.1

34.9 ± 0.4 5.7 ± 0.1

36.2 ± 1.4 6.5 ± 0.6

39.2 ± 0.8 8.5 ± 0.4

Based on mass balance entire dryer. Observation from industrial spray dryer. DAH = calculated outlet air AH – Measured outlet air AH.

demonstrates the difficulty and the risk of stickiness of the concentrate during spray drying. For a new concentrate, it is possible to predict dry Tg using this method in order to evaluate its drying behaviour. If the predicted dry Tg of concentrate is low, the concentrate should be considered as a delicate concentrate. The dryer’s evaporation capacity should therefore be reduced during the spray drying process for this concentrate by reducing the inlet air temperature and the concentrate flow rate in order to avoid stickiness and to maintain the quality of the powder. 4. Conclusions This study reports a standardized desorption method which can be used to evaluate the dry glass transition temperature of a concentrate and then to predict its behaviour during spray drying directly using a concentrate ready for processing to avoid undesirable phenomena such as stickiness or caking. First, a standardized desorption method was proposed. The relationship between the average evaporation rate, the total solid content, the apparent viscosity and the glass transition temperature was then studied and formalised empirically through an equation which makes it possible to evaluate the dry glass transition temperature of a concentrate. For the dairy products tested in this study, the predicted Tg were 18–30 °C higher than Tg measured by DSC. However, this standardized desorption method respects the ranking of concentrates according to their spray drying behaviour according to the order of increase in Tg. Finaly, validation experiments were performed with four infant formula samples using a one-stage pilot dryer. The drying parameters were determined using SD2PÒ software. The drying temperatures predicted by SD2PÒ corresponded well to the values measured, except for concentrate flow rates, which were higher than the predicted values due to the lack of precision of the RH sensor in experimental conditions. The results showed that this new method can predict the spray drying behaviour of a concentrate correctly. The only parameters needed are the total solid content, the apparent viscosity (viscometer) and the average evaporation rate (measured by the standardized desorption method). This method is easy and fast for the industry to apply to a new concentrate. Despite these preliminary positive results, improvements are needed to enhance the predictions of Tg, such as using a weight balance to determine the mass change instead of the current RH sensor.

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