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Impact of spray-drying conditions on physicochemical properties and rehydration ability of skim dromedary and cow's milk powders

Abstract The aim of this study was to evaluate the effect of air outlet temperature (75 and 85 °C) and milk type on proximate composition, water activity, particle size distribution, color, and rehydration ability of spray-dried skim dromedary and cow’s milk powders. While the water activity of powders was close to 0.4 when spray-dried at 75 °C air outlet temperature, it ranged from 0.2 to 0.3 for a production at 85 °C air outlet temperature. Skim dromedary milk powder had a lower water activity than skim cow’s milk powder after spray-drying at 85 °C air outlet temperature. Spray-drying yields were greater at the higher air outlet temperature (85 °C) for both skim milk powders. The particle size distributions of spray-dried skim milk powders were centered around 14–20 µm. The particles of skim dromedary milk powders were smaller than those of skim cow’s milk powders as skim dromedary milk was less viscous, leading to smaller sprayed droplets. No significant influence of spray-drying conditions on particle size distribution was observed for dromedary milk powders. Regardless of the spray-drying conditions and the milk type, the produced powders were very bright and had a low color saturation. Scanning electron microscopy images showed that spray-dried powders appeared as agglomerates of small particles with angular shapes rather than individual particles. All investigated spray-dried powders were considered non-wettable, hardly dispersible, and fairly soluble. Skim dromedary milk powder produced at 85 °C air outlet temperature showed a significantly higher solubility index than the same formulation spray-dried at 75 °C outlet air temperature. The opposite was obtained for skim cow’s milk powder. These results demonstrated that the outlet drying air temperature significantly influenced water activity, spray-drying yield and solubility of spray-dried dairy powders.


Introduction
In recent years, a great attention has been paid to the health benefits of dromedary milk, which constitutes an important nutrition source for inhabitants in arid and semi-arid areas. [1] Dromedary milk has been proven to contain great amounts of essential fatty acids, vitamin C as well as natural immuneactive proteins such as lactoferrin, lysozyme, immunoglobulins, and lactoperoxidase. [2] The proximate composition of cow's and dromedary milks is similar. [1] However, dromedary milk is higher in b-casein and lower in OE-casein, making it less suitable for cheesemaking. Moreover, b-lactoglobulin, which is very important for infant nutrition, is absent from dromedary milk. [3] The preservation of dromedary milk involves many processes such as thermal treatment, [4,5] acid coagulation, [6] and microbiological fermentation. [7] Many derivative products of dromedary milk have been developed, such as butter [8] and yogurt. [9] Although these products are stable, they have a short shelf-life due to their sensitivity to physicochemical, biochemical, and microbiological degradations during storage. [10] Therefore, the production of dromedary milk powders could contribute to increase the shelf-life of milk derivatives.
Spray-drying is the most used drying process in all food sectors including dairy products, cereals, vegetables, and eggs. [11][12][13] It is a matured drying technique with a still significant potential for continued development. [14] This process is suitable to dry dairy products due to the short drying time and the ability to obtain a powdered product. [15] In the dairy industry, spraydrying is employed not only to increase the stability of milk products during their storage, but also to facilitate their later use. As observed for bovine milks, the spray-drying process is an excellent option for extending the shelf-life of dromedary milk by converting milk into powder without much changing its nutritional and sensory characteristics. [15,16] However, to the best of our knowledge, only a few studies have investigated the impact of spray-drying on dromedary milk properties. [17,18] Smits et al. [19] have shown that spray-drying of raw whole dromedary milk did not cause any noticeable damage to its protein fraction and dromedary milk powder had a higher solubility than cow's milk powder. Similarly, Zouari et al. [20] have reported that the spray-drying process induces a mild heat treatment, preserving the protein quality of dromedary and cow's milks.
Most of these studies have focused on the evaluation of the physicochemical properties of dromedary milk powders. The number of studies about the influence of spray-drying conditions is limited. In fact, Zouari et al. [21] studied the effect of air outlet temperature and milk fat content on the physicochemical characteristics of spray-dried dromedary milk powder. Recently, Habtegebriel et al. [16] investigated the effect of pretreatments of the feed concentrate prior to spray-drying on cyclone recovery and the physicochemical properties of dromedary milk powders. These authors reported that operating at less severe processing temperature during spray-drying is necessary to retain higher solubility of dromedary milk proteins. They also showed that dromedary milk powders had higher bulk densities than cow's milk powders processed under similar drying conditions. Furthermore, Ogolla et al. [22] studied the influence of inlet drying air temperature and milk flow rate on the physical, optical, and thermal properties of spray-dried dromedary milk powders. They reported that the inlet drying air temperatures significantly influenced moisture content and the colorimetric properties of the powders. Indeed, with the increase in inlet drying air temperatures and a decrease in milk flow rate, the moisture content and the brightness of dromedary milk powders significantly decreased. They also showed that particle morphology was affected by the nature and composition of the feed, degree of heat treatment, and spray-drying parameters (inlet drying air temperature and milk flow-rate). [22] Spray-drying operating conditions play a key role in the functional properties of cow's milk powder.
The properties of cow's milk powder considerably vary depending on the type and composition of the powder, as well as the conditions of concentration and drying processes. [23] The reconstitution properties of cow's milk powder such as solubility, wettability, and dispersibility are mainly affected by the inlet drying air temperature and the dry extract of the feed concentrate. [23] The aim of this study was to evaluate the impact of spray-drying process conditions (75 and 85 C air outlet temperature) and milk type (dromedary or cow's) on physicochemical properties (proximate composition, water activity, particle size distribution, and color) and rehydration ability of skim dromedary and cow's milk powders.

Material and methods
Dromedary and cow's milks Dromedary (Camelus dromedarius) milk was obtained from a local breeding located in the south of Tunisia (Gab es governorate, Tunisia). Dromedary milk constituted of pooled milk samples assembled from twenty different healthy dromedaries, whose lactation time ranged between 2 and 8 months. As for the cow's milk, which was provided by a farm located in the north-east of France (experimental domain of La Bouzule, ENSAIA, Laneuvelotte, France), it was collected from healthy Holstein cows. Both milks were collected in sterile milking cans and 0.02% (w/w) sodium azide was added to ensure their microbiological stability. Milk was skimmed at 3 000 Â g for 20 min at 4 C (Gyrozen 1580MGR, Multi-purpose Centrifuge, Daejeon, Korea). Then, skim milk was stored at À20 C until use for spray-drying.

Spray-drying of skim dromedary and cow's milks
Skim milk was sieved at 50 mm mesh size (stainless steel, 200 mm diameter, 50 mm height, Retsch GmbH, Germany) in order to remove the potential remaining coalesced fat globules. The spray-drying of skim milk was performed in a MicraSpray MS 150 simple effect pilot plant spray-dryer (Anhydro, Soeborg, Danemark) equipped with a bi-fluid nozzle (Fluid Cap 60 100 þ Air Cap 120, Spraying System, Wheaton, Illinois, USA). Aspiration rate was set at 70% for all performed assays, corresponding to 155 kg.h À1 drying air flow rate. The spray-dryer was fed at 27 C with a FAST Load peristaltic pump (VWR, Leuven, Belgium, 48 mm internal pipe diameter) and spraying pressure was fixed at 1 bar. The drying air cocurrently flowed with the spray of milk droplets in the drying chamber ( Figure 1). Two assays differing in spray-drying conditions (30 L milk per assay) were carried out with a view to evaluate the influence of air outlet temperature ( Table 1). The spray-drying conditions were selected based on literature [11] and after preliminary experimentations in order to obtain skim milk powders suitable for storage (i.e. having a moisture content not exceeding 5% (w/w) and water activity values ranging between 0.2 and 0.4). [11] The powders were then collected, immediately packed in polyethylene terephthalate bags and stored at 10 C until analysis. All analyses were carried out within a week after powder production. Spray-drying yield was calculated as the ratio between the dry mass of powder obtained only during the phase where spray-drying conditions remained constant at target values and the dry mass of feed skim milk.
Proximate composition of skim dromedary and cow's milks Dry matter, protein, and ash contents of skim milk were determined according to the AOAC standard methods. [24] Dry matter was determined by drying 10 mL milk at 103 C for 5 h in a capsule containing Fontainebleau sand (1:10 (w/w) milk sample/ Fontainebleau sand). Ash content was determined by weight loss after incinerating 10 mL milk sample in a furnace at 550 C during 6 h. The total nitrogen content was determined according to Kjeldhal method (10 mL of milk sample), and protein content was deduced from total nitrogen content by multiplying by a conversion factor of 6.38. Fat content was determined according to the Gerber acid-butyrometric method (11 mL of milk sample). Carbohydrates content was deduced by difference with other components according to Equation (1) assuming that vitamins were weakly represented: Carbohydrates content ð%Þ ¼ 100 À moisture content þ protein content ð þash content þ fat contentÞ (1)

Dynamic viscosity of skim dromedary and cow's milks
The dynamic viscosity of skim milk was determined at 4 C using the Malvern Kinexus oscillatory rheometer (Malvern Instruments, Orsay, France) using coneplane geometry at shear rates ranging from 0.1 to 1 000 s À1 during 20 min.
Proximate composition of skim dromedary and cow's milk powders Moisture, protein, and ash contents of skim milk powders were evaluated according to the AOAC  standard methods. [24] Moisture content was determined by weight loss after drying 3 g powder in an air oven at 105 C for 7 h. Ash content was obtained by weight loss after incinerating 3 g powder in a furnace at 550 C during 6 h. The total nitrogen content was determined by the Kjeldahl method (0.1 g of milk powder) (Vapodest, Gerhardt GmbH & Co. KG, K€ onigswinter, Germany) and protein content was deduced from nitrogen content by multiplying by a conversion factor of 6.38. Fat content was determined using the Folch method [25] by performing solvent extraction of 2 g powder using a mix of 2:1 (v/v) chloroform/methanol.
Carbohydrates content was deduced by difference with other components according to Equation (1) assuming that vitamins were weakly represented.

Water activity
Water activity of spray-dried powders (10 g) was measured using a water activity meter (HygroPalm23-AW, Rotronic, France) in quick mode at 25 C.

Particle size distribution
Particle size distributions of skim dromedary and cow's milk powders were determined with a laser granulometer (Mastersizer 3000 Malvern Instruments, UK), equipped with He-Ne laser light (632.8 nm wavelength), using dry dispersion with the Aero S module. Dispersion conditions were as follows: 2 bar, 100% air pressure, 100% feed rate, and 3 mm hopper length. The median diameter in volume was chosen as particle size estimator. Classical granulometric parameters D 10, D 50, and D 90 were recorded (D X corresponds to the size for which X % of the particle population has smaller size). Span was calculated to evaluate the width of the particle size distribution (Equation 2):

Colorimetric analysis
The color of skim dromedary and cow's milk powders was measured using a colorimeter (Konica Minolta, Inc, Japan) in the CIEL Ã a Ã b Ã system. The colorimetric parameters L Ã (lightness), a Ã (redness/greenness), and b Ã (yellowness/blueness) were determined. L Ã , varying from 0 to 100, describes sample lightness (low values for darkness, high values for lightness). a Ã varies from À100 to 100 and represents the green-magenta color scale, with negative values for greenness and positive values for redness. b Ã ranges from À100 to 100 and corresponds to the blue-yellow color scale, with negative values for blueness and positive values for yellowness. The chroma (C Ã ) and hue angle (H ), representing the saturation level and shade of the color, respectively, were calculated as follows (Equations 3 and 4). [26]

Solubility index
Solubility represents the mass percentage of soluble matter in powders, it was determined according to ISO 8156:2005 standard method. [27] First, 2.5 g skim milk powder was vigorously mixed with 17.5 mL distilled water at 22 C in a Falcon tube during 30 s. The suspension was then centrifuged at 1 800 Â g (Heraeus Megafuge 8R Centrifuge, Germany) at room temperature (22 C) and the supernatant was collected. The supernatant dry matter was determined by drying at 103 C in an air oven for 15 h. The solubility index SI was determined using the following Equation (6): where M designates the sediment mass (g).

Wetting time
The wetting time is defined as the time expressed in seconds required for a powder to become completely wet. [28] It was determined by gently pouring 26 g powder at the surface of 250 g distilled water at 20 C in a 250-mL beaker.

Dispersibility index
The dispersibility index (DI) is the amount of dry matter that can be dispersed in water, expressed as a mass percentage. [28] To evaluate the dispersibility index, 26 g powder was added to 250 g distilled water at 20 C and the mixture was stirred with a spatula for 20 s. A filtration of the mixture was carried out using a metallic sieve. The dispersibility index DI was determined using the following Equation (7): With T: dry matter content of the resulting liquid (% (w/w)); W: moisture content of the powder sample (% (w/w)).

Statistical analysis
All analyses of this study were carried out in triplicate and reported values were means ± standard deviations. Statistical analysis was performed using the DSAASTAT add-on for Excel 2010 (Microsoft, Redmond, USA). The presence of significant differences between sample results was investigated by oneway ANOVA and the means were separated by Tukey's HSD test at p 0.05.

Results and discussion
Proximate composition of skim dromedary and cow's milks The proximate composition of skim dromedary and cow's milks is summarized in Table 2. The proximate composition was significantly different between milk samples. Total solids content was significantly lower for skim dromedary's milk (9.52% (w/w)) than for skim cow's milk (12.05% (w/w)). Protein and ash contents were significantly higher for skim dromedary's milk. Carbohydrates content was significantly lower in the case of skim dromedary's milk. These differences could be attributed to the milk type, animal feed, geographic location, and veterinary practices. [1] Similar results were obtained by Al Haj and Al Kanhal [1] who reported that ash content of dromedary milk ranged between 0.60 and 0.90% (w/w) on dry basis. Zouari et al. [21] obtained similar results for proteins and ash but lower carbohydrates (lactose) content for skim dromedary and cow's milks. The proximate composition of skim cow's milk obtained in the current study was in agreement with the ranges of proximate composition of skim cow's milk reported by other researchers. [16] Although both milks were skimmed in the current study, the fat content of both samples was still higher than that usually observed for skim dromedary and cow's milks. These results could be justified by the fact that the conditions used for milk skimming by centrifugation in this study were soft compared to what is commonly realized in the literature. Therefore, both milks were considered to be partially skimmed milks.
Proximate composition of skim dromedary and cow's milk powders Table 3 displays proximate composition, water activity, and spray-drying yield for skim dromedary and cow's milk powders. Total solids content, ranging from 92.96 to 97.01% (w/w), significantly differed between all spray-dried skim milk powders. Skim dromedary and cow's milk powders resulting from the second spray-drying assay led to the highest proportions of total solids (97.01 and 96.63% (w/w), respectively). This was consistent with the fact that a higher air outlet temperature generally induces the formation of powder with a higher total solids content. [11] According to Chever et al., [29] the total solids content of milk powder is affected by milk proximate composition, the air outlet temperature during spray-drying, and the residence time in the spray-dryer. It has also been reported that a small increase in the air outlet temperature during spray-drying can have a substantial effect on powder moisture content. In fact, an increase in outlet drying air temperature of 1 C is reported to cause a decrease in powder moisture content of 0.2% (w/w) on wet basis. [11,30] Similar results have been reported by Habtegebriel et al. [16] for skim dromedary milk powder, Langrish et al. [31] for skim cow's milk powder, and de Oliveira et al. [32] for whole goat milk powder. Spray-drying conditions did not have a significant impact on the proximate composition of studied milk powders. As it can be seen from Tables 2 and 3, the proximate compositions of skim milk powders are comparable to those of their corresponding milks. Besides, protein contents of skim milk powders were neither significantly affected by the milk type nor the spray-drying conditions. The protein contents reported for skim dromedary milk powders spray-dried at 75 and 85 C were equal to 22 and 23% (w/w on dry basis), respectively. The lower protein contents obtained for skim dromedary milk powders could be attributed to the lower protein content of dromedary milk used to feed the spray-dryer.
Habtegebriel et al. [16] showed that spray-dried skim dromedary milk powders contained 44-49% (w/w on dry basis) proteins. The higher total lipids content of produced powders might explain the lower content in proteins found in the present study. In addition to the significant difference observed in ash content between skim dromedary and cow's milks (Table 2), spray-drying process conditions had a significant impact on ash content ( Table 3). The ash contents of skim dromedary milk powders of the current study were similar to the 10% (w/w) ash content reported by Habtegebriel et al. [16] for skim dromedary milk powder. For skim cow's milk powders, the obtained ash contents, ranging from 6.41 to 6.63% (w/w), were a little lower than those reported by Deeth and Hartanto, [33] ranging from 7.5 to 8.0% (w/w). The water activity of spray-dried powders was mainly affected by spray-drying conditions. In fact, it was close to 0.4 at 75 C air outlet temperature, whereas it ranged between 0.2 and 0.3 at 85 C air outlet temperature. This was consistent with the fact that powder water activity is negatively correlated with total solids content, [11] which was also observed in this study. Skim dromedary and cow's milk powders had close water activity after spray-drying at 75 C air outlet temperature, but skim dromedary milk powder had a lower water activity than skim cow's milk powder after spray-drying at 85 C air outlet temperature. Water activity is an important parameter for food preservation. At water activities around 0.3, microbiological growth, lipid oxidation, and Maillard reaction have very low reaction rates. [34] Hence, it has been shown that a water activity between 0.2 and 0.3 at 25 C is suitable to achieve optimal stability of milk powders. [34] Therefore, in our study, it was noticed that the conditions of the second spray-drying assay (85 C air outlet temperature) were more favorable to the preservation of skim dromedary and cow's milk powders.
The spray-drying yield is an important parameter for the cost-effectiveness of food preservation. For all spray-drying conditions, the process yields reported in this study (Table 3) seemed to be low for several reasons. Indeed, the pilot spray-dryer used in this study has a large wall area relative to the drying room volume compared to industrial equipment, which promotes material loss by wall deposition. Besides, the low total solids content of the feeding concentrate (skim milk here) contributes to the formation of small-sized particles. [35] A non-negligible proportion of the powder was therefore not retained by the cyclone and escaped with the outlet air. Finally, the presence of residual fat could also favor the stickiness of produced powders. [31] Powder stickiness is the primary factor affecting the spray-drying yield. A sticky powder is able to adhere to the walls of the spray-drying chamber, enhancing material losses. Stickiness occurs when using drying temperatures slightly over the glass transition temperature of lactose in milks, i.e. in the temperature range comprised between lactose glass transition temperature (55.6 C) and about 30 C over. [36] Recently, Zouari et al. [21] reported that using an air outlet temperature of 76.5 C, the glass transition temperatures were 40 and 37.8 C for skim dromedary and cow's milk powders, respectively. With the increase in air outlet temperature up to 86.8 C, the same authors obtained glass transition temperatures of 44 and 41.5 C for skim dromedary and cow's milk powders, respectively. Based on these results, it can be concluded that the two air outlet temperatures used in our study were over the sticky points of skim dromedary and cow's milk powders, thus leading to a great proportion of powder loss by sticking to the spray-dryer walls. The residual fat and the fineness of produced powders could probably explain the low spray-drying yields.
In the present study, spray-drying yields were greater at the higher air outlet temperature (85 C), especially for skim dromedary milk powders (37.0% at 85 C outlet air temperature vs 19.7% at 75 C outlet air Table 3. Proximate composition, water activity, and spray-drying yield of skim dromedary and cow's milk powders produced in the two spray-drying assays. temperature). Spray-drying yield was inversely correlated to water activity (Pearson's correlation coefficient r of À0.99). Therefore, it was noted that spray-drying conditions had a greater effect on dromedary milk powders. In accordance with these results, De Oliveira et al. [32] reported that higher yields are obtained at lower relative humidity of the outlet air, i.e. when the outlet temperature is high and/or the feed flow-rate is low. Habtegebriel et al. [16] also reported that the increase in the air outlet temperature is likely to enhance cyclone recovery. Actually, it can be predicted that a greater air outlet temperature increases heat and mass transfer in the drying air, thus improving the drying rate and limiting powder stickiness. [16] Gallo et al. [37] also evidenced that increasing the drying air temperature improves the spray-drying yield.

Particle size distributions of skim dromedary and cow's milk powders
Particle size distribution is a very important physical property of milk powders, known for its significant impact on the reconstitution properties of milk powders. Indeed, large particles within a narrow particle size distribution could enhance the rehydration properties of milk samples. [38] Particle size distributions and granulometric parameters of spray-dried powders are respectively displayed in Figure 2 and Table 4, respectively.
The particle size distributions of spray-dried skim milk powders were monomodal and centered around 14-20 mm, consistently with previous results obtained with the same combination of pilot spray-dryer and bi-fluid nozzle. [39,40] The dispersion of particle sizes around the median value was relatively low, as indicated by the moderate values of span ranging from 1.75 to 1.93. Although the spray-drying conditions seemed not to influence the particle size distribution, it was noted that the particles of skim dromedary milk powder were smaller than those of skim cow's milk powders.
Furthermore, the higher total solids content of feed skim cow's milk may justify this result. Indeed, its viscosity was higher (3.89 ± 0.12 mPa.s) than that of feed skim dromedary milk (2.96 ± 0.13 mPa.s) ( Table 2), leading to larger sprayed droplets, [35] resulting in larger spray-dried particles. Another cause may be attributed to the higher fat content of skim cow's milk powder, which makes it stickier. In fact, fat, composed of sticky components (with a fusion temperature inferior to 80 C), is known to concentrate at particle surface during spray-drying due to buoyancy in sprayed droplets, [41][42][43][44] making them more prone to agglomeration, thus leading to particle size increase. According to Fournaise et al., [45] median particle size of whole cow's milk powder produced in similar spray-drying conditions was 27.7 lm with a span of 3.0, which is consistent with the present study. At last, the higher total solids of feed cow's milk may have  limited particle shrinkage during drying owing to the faster solidification of surface-active molecules located at the droplet/particle surface (caused by the higher molecular concentration at the droplet/particle surface and the lower mass transfer inside the droplet/particle due to its higher viscosity), thus also acting in favor of higher particle size. Significant differences were obtained for the granulometric parameters of spraydried powders, both owing to milk type and spraydrying conditions. No significant influence of spray-drying conditions was observed for dromedary skim milk powders. However, when increasing the air outlet temperature, cow's skim milk powders presented slightly higher median particle sizes. The difference was statistically significant, with median particle size of 17.1 lm at 75 C air outlet temperature and 19.2 lm at 85 C air outlet temperature for skim cow's milk powders. This difference could be attributed to the fact that a higher drying temperature accelerates the drying rate of droplets, promoting the fast formation of a crust at their surface, and therefore limit shrinkage, which leads to larger particles. [46] The span showed small significant differences between all spray-dried milk powders. There is no marked effect of either the milk type or the spray-drying conditions on the span. Indeed, for skim dromedary milk powders, the increase in the air outlet temperature resulted in a slight decrease in the span values from 1.93 to 1.76. For skim cow's milk powders, span values were about 1.75 and 1.81 at 75 and 85 C air outlet temperature, respectively. Table 4 displays the color parameters of skim dromedary and cow's milk powders. Whatever the spraydrying conditions and the milk type, the produced powders were very bright (L Ã was elevated), had a low color saturation (C Ã was very low), and their main color was green-yellow (owing to H values). Hence, spray-dried powders appeared relatively white, as such a lowly-saturated color cannot be detected by the human eye at this high level of brightness. Skim cow's milk powders were significantly yellower than skim dromedary milk powders whatever the spray-drying conditions. This could probably be attributed to the lower content in b-carotene of skim dromedary milk powders compared to skim cow's milk powders. [47,48] Although spray-drying at higher air temperature seemed not to affect brightness and main color, it slightly increased color saturation, which could contribute to the promotion of consumer acceptability of these powders. This increase in powder color saturation may be related to the Maillard reaction susceptible to occur between milk proteins and lactose, known to be enhanced at higher temperature. [34,49,50] These findings could also be attributed to the lower moisture content of powders spray-dried at higher temperatures. [22] Similar results were previously obtained by Ogolla et al. [22] and Sulieman et al. [18] who investigated the effect of the inlet air temperatures on the color of spray-dried dromedary milk powders. These authors reported that higher inlet drying air temperatures resulted in decreased L Ã and hue angles while increasing the a Ã , b Ã and chroma values of spray-dried dromedary milk powders.

Morphology and surface appearance of skim dromedary and cow's milk powders
The scanning electron micrographs of skim dromedary and cow's milk powders are presented in Figure 3. Whatever the spray-drying conditions and the milk type, spray-dried powders appeared as agglomerates of small particles rather than single particles. Small agglomerates with angular and homogeneous shape are visible in Figure 3. Furthermore, the particles had a shriveled appearance as well as an irregular and rough surface, which is the classical shape of skim milk powders. Habtegebriel et al. [16] reported that, independently of spray-drying conditions, whole dromedary milk powders had a smooth surface, covered by a fat layer, whereas skim dromedary milk powders presented a rough surface, in accordance with the results of the present study. Similar results were reported by other researchers [23,31] which showed that skim cow's milk powders are composed of flat, wrinkled, and collapsed particles. Habtegebriel et al. [16] added that the presence of pores at the particle surface might facilitate water diffusion into the powder, thus enhancing its rehydration ability. Few pores could be detected at the surface of milk powders investigated in the current study, as pointed out by arrows in Figure 3 D2 f and C2 f. This could be due to the presence of surface fat, which may be related to the hypothesized stickiness of particles.

Rehydration ability of skim dromedary and cow's milk powders
The reconstitution properties of skim dromedary and cow's milk powders are presented in Table 4. All spray-dried powders could be considered as non-wettable, as their wettability time was higher than 120 s. [51] This poor wettability can mainly be attributed to the fineness of powders produced with the combination of pilot spray-dryer and bi-fluid nozzle employed in this study. [39] In fact, the larger the particle, the higher its weight, and thus the more easily the particle can overcome water surface tension and immerse in water. It has been reported in the literature that a median particle size over 200 lm promotes powder wetting. [52] Moreover, it has previously been reported that large particles are more prone to have large surface pores, which results in fast wetting. [38] According to Gaiani et al., [41] the wetting step mainly depends on particle surface charge, surface activity, size, porosity, surface composition (especially, proportion of hydrophilic compounds), contact angle between the powder bed and water, and powder density. Besides, the poor wettability of dairy powders has also often been attributed to the presence of various hydrophobic compounds (lipids, caseins, etc.) at the particle surface. [38] Kim et al. [38] reported a wetting time over 15 min for whole milk powder. Spray-dried powders produced in the current study had a nonnegligible fat content, which can lead to a large fraction of the surface covered by lipids and explain their poor wettability.
Moreover, all investigated spray-dried powders could be considered as hardly dispersible. Indeed, their dispersibility indices were well lower than 95%. [51] Spray-drying conditions had an opposite effect on the rehydration behavior of the two types of milk powders. It is evident from Table 4 that dispersibility was positively correlated with the air outlet temperature for skim dromedary milk powder. Actually, the dispersibility index increased from 34.8% at 75 C air outlet temperature to 41.8% at 85 C air outlet temperature. Concerning the air outlet temperature, it had no significant difference on dispersibility indices of skim cow's milk powders. These results could also be attributed to granulometric characteristics of skim dromedary and cow's milk powders. In fact, and generally, the larger the mean particle size, the higher the dispersibility index. [53] According to Chever et al., [29] a rapid dispersion requires particle sizes in the range from 150 to 200 lm, which is not the case in our study. However, for powders produced in the present study, dispersibility and median particle size were highly positively correlated (Pearson's correlation coefficient of 0.89), in agreement with the literature. [53] The higher fat content of cow's milk probably led to the formation of stickier particles, thus more difficult to disperse due to the presence of more interparticular interactions.
In general, milk powders are considered to be soluble if their solubility index is higher than 99%. [51] Solubility is known to be mainly affected by powder composition, especially by the nature and structure of proteins. [54] Skim dromedary and cow's milk powders produced in our study could be considered as fairly soluble, as their solubility indices ranged between 78.3 and 95.3%. Besides, the spray-drying conditions did not have the same effect on the rehydration properties depending on the milk type. On one hand, skim dromedary milk powder produced at 85 C air outlet temperature showed a significantly higher solubility index. On the other hand, skim cow's milk powder revealed significantly higher solubility when spray-dried at 75 C outlet air temperature. These results could be related to the significant lower fat content of skim dromedary milk powders and the difference in particle size distributions of studied powders. Indeed, Pearson's correlation coefficient between solubility and median particle size was 0.66, indicating a medium positive correlation. Besides, whatever the spray-drying conditions, dromedary skim milk powders had slightly better rehydration properties, which could also be related to the sensitivity of cow's proteins to thermal denaturation. In fact, it has been reported that spray-drying of whole dromedary milk did not cause any noticeable damage to its protein fraction, resulting in a higher solubility of dromedary milk powder than cow's milk powder. [19] Additionally, Anandharamakrishnan et al. [55] reported that increasing the outlet air temperatures causes partial protein denaturation and subsequent protein aggregation, inducing solubility loss, as observed for cow's milk powders produced in the present work. From these results, it can be concluded that no significant change of powder properties could be attributed to the small modification of air outlet temperature. In fact, increasing air outlet temperature did not improve the rehydration characteristics of dromedary and cow's skim milk powders, as rehydration mainly depends on particle size, which was very little affected by changing the outlet air temperature.

Conclusion
This study pointed out that spray-drying is a good technique to stabilize skim dromedary milk. In this study, skim dromedary and cow's milks were spraydried without prior concentration to avoid introducing another source of process variability. It is worth noting that these findings should be read with care, as they were obtained via only two unrepeated trials. Overall, applying an air outlet temperature of 85 C led to better chemical and physical properties for both milk powders. Indeed, a higher air outlet temperature (85 C) resulted in higher spray-drying yields and skim dromedary and cow's milk powders with higher total solid contents and lower water activity, which is favorable for preservation purposes. When increasing the air outlet temperature, skim cow's milk powder presented higher median particle size but no influence was denoted for skim dromedary milk powder. Spraydrying at 85 C outlet temperature seemed not to affect powder brightness and main color, but it slightly increased color saturation. The results of the present study revealed that regardless of process conditions and milk type, skim milk powders spray-dried at pilot scale could be considered as non-wettable, hardly dispersible, and fairly soluble. However, slightly better rehydration properties were obtained for dromedary skim milk powders spray-dried at 85 C outlet air temperature. The SEM analysis showed that spray-dried skim milk powders were composed of agglomerates of small particles with angular and homogeneous shapes. Therefore, it will be interesting in future research works to investigate the influence of larger changes of outlet drying air temperature on physical properties (particle size and shape distributions) of skim dromedary and cow's milk powders spray-dried at pilot scale. Higher outlet drying air temperatures could be investigated, with a view to improve spray-drying yields and enhance powder production rate while maintaining powder quality. A focus on powder flowability should also be performed, as this functional property is as crucial as rehydration properties for industrial applications. from the experimental domain of La Bouzule for cow's milk supply. Pr. Leila MAHFOUDHI, Emeritus Teacher of English at the Faculty of Science of Sfax, should also be thanked for proofreading and refining the manuscript language.

Disclosure statement
On behalf of the authors and as the corresponding author of this publication, I confirm that there are no known conflicts of interest associated with this publication and that there has been no financial support for this work that could have influenced its outcome.

Funding
This project was financially supported by the Ministry of Higher Education and Scientific Research of Tunisia through the scholarship program under the project called Mobility to Encourage Young Tunisian Researchers (N 18PJEC12-20, 2018). The authors acknowledge also support of the LIBio by the "Impact Biomolecules" project of the "Lorraine Universit e d'Excellence" (Investissements d'avenir -ANR).