Skin layer formation in drying droplet of dairy protein mixes

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Dairy products represent one of the most common sources of high-quality proteins, and from a nutritional point of view, with egg white, are considered as products containing reference proteins. The nutritional benefits that result from their consumption and the ongoing globalization push forward the market demand growth, in particular in emerging dairy markets such as milk deficient regions in Asia.
Thereafter, the global milk production reached 881 million tonnes (mlt) all milk species included in 2019, on a steady growth trend of 2.2% close to the average over the last decade (2.4%; (IDF, 2020)). Global milk production is driven by three main world regions, i.e. Asia with 42% total milk followed by Europe and North & Central America with 20% and 15% total milk, respectively. At the same time, the average per capita milk consumption reached 114.7 kg at the global scale in 2019 (+1.1%;Figure 1). This parameter does not reflect the strong differences in consumption habit and level existing between the different regions of the world. As an example, milk and fermented products are significantly consumed in India compared to some European countries, contrary to the case of cheese. Moreover, the average milk equivalent per capita annual consumption shows a large dispersion, ranging from 42 kg in Africa to approx. 275 kg in Europe and North America.
Cow milk represents the main portion of milk production, with 714 million tonnes. The growth is mainly driven by the dynamic trend in both cow and buffalo’s milk production in Asia, in particular in India (188 mlt; +6.5%), China (32 mlt; +4,1%) and Pakistan (48 mlt; +3.3%), whereas traditional excess production regions with regard to local consumption (EU, Oceania, USA) added only marginal volume (Figure 2). However, the Asian dairy market is growing faster than its growing milk production, making it challenging for the local production to cover the consumer demand: indeed, the self-sufficiency rate in Asia decreased from 93% in 2010 to 90% in 2019, despite a growth production rate of +4.0% per year on this period.
This milk deficit is the starting point of the development of the dairy world trade to supply the market of regions with high demand, especially developing / emerging countries, from those that have a surplus in milk production in relation to their domestic demand, namely western countries. As such, the world trade is dominated by Oceania (New Zealand and Australia), EU and USA, representing all together 73% of the export shares (Figure 3).
World dairy trade in 2019 grew modestly by just over 1% (compared to the +9% growth since 2015), reaching a volume of about 82.1 million tonnes in milk equivalent and therefore representing close to 9% of global milk production. Thus, exports generally constitute a small proportion of production and most of the global milk is consumed locally without crossing any borders. In order to be exported in safe conditions, milk has first to be processed in a stable form, mostly dairy powders (skim milk powder, SMP; whole milk powder, WMP), cheese and butter. Figure 4 shows the exported volume of each of these categories: given the conversion factor in milk equivalent, the powder form (including whey powders, not represented on Figure 4 but accounting approx. 14 mlt milk equivalent) represents by far the first share of world trade, close to 69% (FAO, 2021). This can be explained by the fact that powder processing is the major way to stabilize dairy products over long periods of time (up to 2 years) and to reduce their volume (by a factor ranging from 2.5 to 8.0, in the case of milk and whey, respectively) and by consequence the cost of storage and shipping.
As shown in Table 1, the global production of skim and whole milk dairy powders remained stable between 2014 and 2018, while it showed a sharp increase of 29% and 20% in the previous 4 years (2010-2014; (CNIEL, 2016)). In contrast, whey powder production, mainly originating from cheese production regions (EU and North and Central America), continued to grow at a level close to that of the previous period: +8.3% between 2014 and 2018, versus +10% between 2010 and 2014. The growing trend of the whey powders exports is the signature of the demand for nutritional products on the dairy market, especially in Asia. Indeed, the label “whey powders” may cover whey powder for feed or food application, this latter being connected to infant milk formulae (IMF) formulation.
As an example, the main dry products exported by France in 1988 were whey powders (42% in volume) and WMP (33% in volume); at this time, the share of IMF powders was only 5% in volume (FranceAgriMer, 2019). In 2018, the breakdown still places whey powders as leading product (36% in volume), in front of SMP (26%) and mainly IMF powders (21%). France has therefore gradually turned to the export of products with higher added value, as IMF powders represented in 2018 the main share of income of powder products (43%), 36% of the IMF exports in volume concerning China (CNIEL, 2020). At the global scale, Baker et al. (P. Baker et al., 2021) reported that China accounted for 32.5% of total IMF sales volume at a global scale, close to 0.7 mlt.
Indeed, China is biggest importer since 2008, from which its production level has been kept stable (Figure 5), approx. 32 mlt. The imported dairy products can be divided into five categories: whole milk powder (WMP), butter, cheese, skim milk powder (SMP) and other products. In Figure 5, their share in total import is estimated. Before 2008, the total dairy imports lied between 3 and 4 mlt, and other products (namely whey powders, IMF, fresh products, etc.) accounted for about half of imports. Since 2008, the total volume of imported dairy products has been multiplied by a factor of 4, WMP and other products accounting for 40% each of the whole. China represents the world’s second largest infant and young child population (P. Baker et al., 2021). Moreover, a further increase of total dairy imports is predictable due to the second-child policy encouraged recently by the Chinese government.
China’s population and economic growth, increasing wealth of the middle classes and women’s employment (representing 45% of the total labor share) are factors pushing forward the IMF market, as it makes it necessary to access a substitute replacing breast milk for providing all the premium nutrition for baby growth. Recently, Baker (P. Baker et al., 2021) represented the relation between the IMF powder consumption per child in 77 countries from 2005 to 2019 as a function of IMF powder compounding annual growth rate (CAGR) for the different formula categories (standard, follow-up, toddler and special) (Figure 6). Although a wide variation in category sales, growth rates between regions and country income categories, and between countries at the same income level can be observed, it is clear that much of the IMF sale growth can be attributed to upper-middle income countries (UMIC) with an average CAGR of +8.6% in 2005–19. This growth was led by China with a CAGR around 10% and per child consumption around 20 kg and 25 kg for standard (0-6 month) and follow-up (7-12 month) categories, respectively, and experienced the highest growth rate in special formulas. In contrast, India has remarkably low per child volumes across all categories and negative growth in the standard category, although being the world’s largest infant and young child population.
During the first half of 2020, dairy world trade showed almost the same level as in the same period of 2019, coming with apparently few COVID-19 impact except for milk powders that suffered a significant declining demand (IDF, 2020). Moreover, Van Tulleken et al. (van Tulleken et al., 2020) reported that IMF industry has been actively exploiting concerns about COVID-19 to increase sales, in violation of the WHO International Code of Marketing of Breast-milk Substitutes and national law in many countries. Large manufacturers have spread messages about unnecessary hygiene measures and the separation of mothers from their babies, whereas the growing food insecurity coming with the upcoming economic and social crisis makes breastfeeding even more important. On a longer term, and although IMF Chinese market forecast appears to be positively oriented, the main development should concern the refocusing of Chinese domestic demand on IMF products made in China, rather than from abroad as has been the case since the melamine crisis. In 2021 already, major IMF players such as Reckitt (Mead Johnson) and Danone have sold their positions in China, arguing from “sluggish sales caused by intense competition from local Chinese baby formula brands” (Reuters).

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Ruminant milk (cow, goat, sheep) represents a complete natural food ideally suited to the needs of the young. Their composition depends on the species and breed, stage of lactation, herd management, the age of the animal and its health status. Milk provides water (87-88% w / w on average for cow’s milk), energy (fat and lactose) and essential nutrients (amino acids, fatty acids, vitamins, etc.) allowing to cover the main physiological functions. First in this section, the different categories of milk components will be reminded (content, structure, physicochemical properties), bovine milk serving as an example and reference. The comparison of human and bovine milk will be then provided.


Apart from traces of glucose, galactose and a minor fraction of oligosaccharides, lactose is the main carbohydrate and major dry matter constituent of cow’s milk with a concentration in between 4.5 and 5.0 w/w % (Jeantet & Croguennec, 2018). In combination with the mineral elements of milk (K+, Na+, Cl- for the most part), it helps controlling the osmotic pressure in the mammary gland. From a structural point of view, lactose is a reducing D-galactose / D-glucose disaccharide, present in two configurations ( and ) in thermodynamic equilibrium in solution (mutarotation). Lactose (especially the form) is a poorly soluble carbohydrate: at 20°C, its solubility limit (saturation) is approximately of 19 g / 100 g of water. Milk is therefore an aqueous solution that is unsaturated with lactose, unlike whey and permeates concentrates obtained by evaporation which are supersaturated. In these concentrates, lactose crystallizes when conditions are favorable according to a mechanism involving two kinetics, that of the formation of stable seed crystals (nucleation) and that of crystal growth (growth). Crystallization takes place under normal conditions in the form, and greatly reduces the hygroscopicity of the powders finally obtained. Indeed, lactose in the amorphous state is highly hygroscopic, and thermodynamically unstable when it is brought to supersaturation in the dry state, coming with a risk evolution towards a crystalline state in case of humidity uptake or temperature increase, releasing water and inducing caking and non-enzymatic browning of the powders.


Whole milk is a natural emulsion of fat in an aqueous phase (skimmed milk). The fat represents around 4 w / w % of the overall composition of the milk, and is dispersed in the form of fat globules with an average diameter of around 3 µm and distributed from 1 to 10 µm (Briard et al., 2003). The triglycerides, which represent about 97 w/w % of the total milk fat, are thus protected by a complex biological membrane.
The fatty acid composition of milk fat varies depending on diet, stage of lactation, season and geographic factors (Bornaz et al., 1992; Chilliard et al., 2001). There are over 400 fatty acids, although only 13 of them are present in amounts greater than 1% in mol/kg (JENSEN & NEWBURG, 1995). These are mainly saturated fatty acids (65-75% of total fatty acids). The unsaturated fraction (25-35% of total fatty acids) includes in particular trans fatty acids.
Representing about 2 w/w % of the total milk fat, the fat globule membrane is structured in a triple layer and contains minor proteins (1 w/w % of the fat globule: butyrophilin, xanthine oxidase, etc .; (Patton & Huston, 1986) and mainly polar milk lipids (0.2 to 1 w/w % of total lipids: phosphatidylcholine, phosphatidylethanolamine, sphingomyelin, etc.). It plays an essential role in maintaining the integrity and preventing the coalescence of fatty globules, as well as acting as a natural physical barrier against oxidation and lipolysis (Jeantet & Croguennec, 2018). However, its fragility allows deep compositional and structural changes during the implementation of technological treatments (e.g., homogenization, heat treatment). During homogenization, the interface of the fatty globules (initially with a surface area of 50 to 110 m2 per liter of milk) is considerably increased with simultaneous enrichment in milk proteins (casein micelles and whey proteins).


The protein concentration of bovine milk varies between 3.1 and 3.7 w/w %; even more than the breed, it varies especially from one cow to another depending on genetic, health and food factors (Marshall, 1995). About 80% of the nitrogen fraction in cow’s milk corresponds to the casein fraction and 20% to soluble proteins and the non-protein nitrogen fraction (NPN: urea, creatine, uric acids, vitamins, peptides, ammonia, etc.) (Walstra et al., 1999). The concentration of major milk proteins (concentration ≥ 0.01 w/w %) is shown in Table 2.

Table of contents :

1.1. Worldwide dairy market
1.2. Composition, structure and physicochemical properties of cow milk
1.2.1. Lactose
1.2.2. FAT
1.2.3. Proteins
1.2.4. Minerals
1.2.5. Key differences between Human milk and cow milk
1.3. Infant formula processing and manufacture
1.3.1. Raw materials and preparation of the mix
1.3.2. Heat treatment
1.3.3. Homogeneisation
1.3.4. Concentration by evaporation
1.3.5. Spray-drying
1.4. Drying mechanisms in droplets of dairy fluids
1.4.1. Pilot spray dryers
1.4.2. Single droplet approach
1.4.3. The physics of the drying process in colloidal droplets
1.4.4. Skin formation and characterization in dairy protein drying
2.1. Investigation on skin formation
2.1.1. Skin formation at the early stage of the drying
2.1.2. Later stage of the drying
2.1.3. Time evolution of the rheological properties of the skin
2.2. Modelling the drying kinetics
3.1. Sample preparation
3.1.1. Preparation of Whey protein isolates (WPI), Native phosphocaseinate (NPC) and Sodium caseinate (SC) solutions
3.1.2. Mixture sample preparation
3.2. Single droplet drying experiment
3.2.1. Hydrophobic substrate
3.2.2. Drying conditions
3.2.3. Droplet profile observation
3.2.4. Droplet top view observation
3.2.5. Droplet evaporation rate measurement
3.2.6. Particle section structure and shell thickness
3.3. Droplet interfacial properties
3.3.1. Surface tension measurement
3.3.2. Optimization of experimental parameters for droplet dilation tests
3.4. Monodisperse spray drying process
3.4.1. Analyzing powder’s shape factors
3.5. Glass filament drying experiments
3.5.1. Diameter measurement
3.5.2. Mass measurement
3.5.3. The temperature measurement of droplet
4.1. Skin layer formation in drying droplet of dairy protein mixes
4.1.1. Introduction
4.1.2. Experiment strategies
4.1.3. Results and discussion
4.1.4. Complementary results
4.1.5. Conclusions
4.2. Phase diagram of dairy protein mixes obtained by single droplet drying experiments
4.2.1. Introduction
4.2.2. Experiment strategy
4.2.3. Results and discussion
4.2.4. Conclusions
4.3. Exploring the properties of particles obtained from flying droplets
4.3.1. Introduction
4.3.2. Experiment strategies
4.3.3. Results and Discussion
4.3.4. Conclusions
4.4. Coupling Reaction Engineering and Energy Map approaches
4.4.1. Introduction
4.4.2. Modelling strategies
4.4.3. Results and discussion
4.4.4. Conclusions


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