Introduction

The quality of the top milk foam layer significantly influences key attributes, including appearance and texture, in many cappuccino-style products (Khezri et al., 2017; Walstra, 1989). Foaming properties of milk are mainly determined by its main components such as protein, lactose, and fat. Proteins with amphiphilic nature and the ability to form intermolecular interaction are recognized as excellent foaming agents and able to generate a high volume of stable foam at a very low content (Ho et al., 2022; Sarkar & Singh, 2016; Xiong et al., 2020; Zayas, 1997). Lactose has a high water-holding capacity; thus, its presence in milk typically contributes to the increase of viscosity of milk, hindering molecular diffusion and protein adsorption on interfacial regions, thereby reducing foamability. However, it enhances foam stability by minimizing liquid drainage and air bubble coalescence (Gamboa & Barraquio, 2012). Additionally, lactose was observed to enhance the formation of insoluble aggregates in whey proteins subjected to dry heating in an alkaline environment. This enhancement resulted in improved water-holding capacity for the insoluble fraction and enhanced foam stability for the soluble fraction (Vidotto et al., 2020). Meanwhile, fat has a negative impact on foaming properties of milk; thus, milk with high-fat content has very poorer foamability and less foam stable than milk with a low-fat level (Anderson & Brooker, 1988; Ho et al., 2020, 2021, 2022; Huppertz, 2010; Walstra, 1989). High levels of products resulting from lipase-catalyzed hydrolysis of milk fat, including free fatty acids and monoglycerides, have been reported to negatively impact the foaming properties of milk (Ho et al., 2023a; Kamath et al., 2008a). Fat remaining in skim milk was reported to be the main cause for lower foamability and foam stability of the proteose-peptone fractions extracted from the skim milk than those extracted from whey protein concentrates (Karamoko et al., 2013). All these studies revealed that any alteration of solids concentration of milk affects its foaming properties. However, there have been a few reports dedicating to the effects of solids concentration of milk on its foaming properties; meanwhile, studies in this regard reported contradictory results. Kamath (2007) reported that increasing solids concentration of reconstituted skim milk powder dispersions from 1.5 to 15% (w/w) resulted in decreased foamability and increased foam stability, which was attributed to the increased viscosity at higher solids concentration. However, Martinez-Padilla et al. (2014) found that increasing solids concentration in reconstituted skim milk powder dispersions from 10 to 25% not only increased foam stability but also enhanced foamability by approximately 50%, despite a nearly four-fold increase in viscosity. The variations in reported results between these studies could be attributed to differences in foaming methods employed, with Kamath (2007) utilizing air injection and Martinez-Padilla et al. (2014) employing whipping foaming approaches. The prevalent foaming method in coffee shops involves steam injection, where steam is introduced directly into the milk bulk through a small opening nozzle, creating air bubbles while heating the milk to 65 − 70 °C. Nevertheless, there is a lack of reported information on the foaming properties of milk dispersions at different solids concentration using steam injection.

Concerning the foaming of milk proteins, numerous studies propose that the quantities of milk proteins required to create stable foam are considerably lower than those typically present in typical whole milk commonly used in coffee shops (e.g., 3.2–3.5% protein). Various studies have demonstrated effective foam production with diverse protein concentration, including reconstituted skim milk powder dispersions at 0.5% protein (Kamath, 2007), sodium caseinate and whey protein isolate at 0.25% protein (Britten & Lavoie, 1992), sodium caseinate and whey protein concentrate 0.3 and 1.0% protein, respectively (Marinova et al., 2009), skimmed milk at 1.5% protein (Borcherding et al., 2009), and reconstituted milk protein concentration dispersions with protein content of 1.5–4% (Xiong et al., 2020). However, in these studies, there is a lack of fat in the foaming systems, which leads to the question whether milk proteins can still form excellent foam at such low concentrations in the presence of fat. During foaming, fat can disrupt the formation of intermolecular interactions among proteins, potentially weakening or disrupting the integrity, cohesiveness, and viscoelastic properties of the interfacial films of proteins (Nylander et al., 2019).

In summary, a research gap exists in understanding the foaming properties of milk at various solids concentrations using steam injection and the impact of low milk protein concentration in the presence of fat. In addressing these gaps and enhancing insights into barista-style milk coffee foam, we systematically investigated the foaming properties and structure of (1) reconstituted skim milk powder dispersions (1.5–15% solid content, equivalent to 0.5 − 5% protein), (2) whole milk with added skim milk powder and milk protein concentrate (final protein content of 3.5% and 4%) and butter milk powder (0.5% and 1%), and (3) reconstituted whole milk powder and commercially pasteurized whole milk dispersions (0.5% protein content) using both steam injection and mechanical mixing. Notably, the mechanical mixing system integrates a heating unit, enabling simultaneous heating and foaming. This study provides a systematic understanding of the role of proteins and the impact of incorporating milk powder in milk foaming.

Materials and Methods

Materials

Low-heat skim milk powder and butter milk powder were bought from Tatura Milk Industries Ltd. (Tatura, NSW, Australia). Whole milk powder was obtained from New Zealand Milk Products (NZMP, Richmond, Victoria, Australia). Commercially pasteurized whole milk was obtained from local market (Brisbane, QLD, Australia) and transported cold to the laboratory. Milk protein concentrate was obtained from Maxum Foods (Milton, QLD, Australia). The main components of these milk samples are expressed in Table S1 (Supplementary materials). For the analyses of free fatty acids, the chemicals of analytical grade such as isopropanol (≥ 99.5%), hexane (≥ 95%), and H2SO4 (≥ 99.9%) were purchased from Sigma-Aldrich (Castle Hill, NSW, Australia), while a 0.02 N KOH solution in methanol was obtained from Thermo Fisher Scientific (Scoresby, VIC, Australia).

Preparation of Reconstituted Skim Milk Powder Dispersions at Different Solids Concentrations

Reconstituted dispersions of skim milk powder at solids concentrations of 1.5, 5.0, 8.5, 10, and 15% (w/w) were prepared by gradually introducing skim milk powder into MilliQ water with continuous stirring at a speed of 100 rpm using an IKA® overhead stirrer (LabGear, Milton, QLD, Australia). The mixtures were stirred at 100 rpm for 1 h and left to equilibrate overnight in a cold room at 4 °C for subsequent experiments (Kamath, 2007).

Preparation of Milk Dispersions at Low Protein Content from Different Types of Milk

We chose 0.5% protein of milk dispersions for this study based on the preliminary research results of foaming properties of reconstituted skim milk powder dispersions at varied solids concentrations (“Preparation of Reconstituted Skim Milk Powder Dispersions at Different Solids Concentrations”). It was observed that the foaming properties in terms of foamability, foam stability, and foam structure of milk samples at 1.5% solids concentration, corresponding to about 0.5% (w/w) proteins, were good and were not different from those of milk samples at higher solids concentration (5.5–15%).

Reconstituted whole milk powder dispersions at 0.5% (w/w) protein were prepared dissolving whole milk powder into MilliQ water under mixing at a speed of 100 rpm ((IKA® overhead stirrer, LabGear, Milton, QLD, Australia). The milk dispersions were then mixed at 300 rpm for 1 h. Meanwhile, commercial whole milk dispersion at 0.5% (w/w) protein was prepared by mixing commercial whole milk with MilliQ water at an appropriate ratio under slight stirring at 100 rpm for 15 min. All milk dispersions were equilibrated overnight in the cold room (~ 4 °C) for further experiments.

Preparation of Commercial Whole Milk Dispersions Containing Added Different Types of Milk Powders

To prepare milk dispersions with added milk powders, a certain calculated amount of skim milk powder and milk protein concentrate was dispersed in the commercial whole milk to achieve final total protein contents of 3.5 and 4% (w/w), while butter milk powder was added at 0.5% (w/w) or 1.0% (w/w). To fully disperse milk protein concentrate, whole milk was first heated to 50 °C prior to mixing milk protein concentrate. Skim milk powder was directly dispersed into whole milk at room temperature (~ 25 °C). All the samples were stirred for 4 h to allow rehydration and were then stored at 4 °C overnight to collapse the foam and complete rehydration.

Foaming Methods

Mechanical mixing (Breville cafe milk frother, BMF600, Breville Group Ltd., Alexandria, NSW, Australia) and steam injection (Café Series EM6910, Sunbeam, NSW, Corrimal, Australia) foaming methods were employed to evaluate foaming properties of milk dispersions. Before foaming, milk dispersions and their containers were cooled to 5 °C in an ice bath, and the foaming process was stopped as the milk temperature reached 65 °C. Due to technical requirements, 250 mL of milk dispersions was used to produce foam in the mechanical mixing, while only 100 mL of milk dispersions was used in the steam injection. Details of these foaming methods have been described in our previous study (Ho et al., 2019). Briefly, in mechanical mixing, a heating unit is integrated into the mixer, allowing the heating of the milk to 65 °C during foaming. In steam injection, the milk container was gradually lowered to keep the steam wand about 2 mm below the milk surface, enabling air incorporation. When the milk reached 45 °C, the container was raised, lowering the steam wand tip into the milk to texture and heat it to 65 °C.

Determination of Foam Properties

Foamability

Foamability of milk dispersions was evaluated as volume of foam (mL) generated from 100 mL of milk samples (mL/100 mL). The foam volume was determined by measurement directly from the graduated plastic jug (steam injection) or pouring the foam into a 500 mL plastic cylinder. In both cases, the foam volume was measured immediately as the interfacial layer between liquid and foam was observed.

Foam Stability

Foam stability was evaluated from the foam volume remaining after 10 min of collapsing, which is based on the fact that most cappuccino-style drinks are typically consumed to half their volume within 10 min. Foam stability was expressed as the percentage reduction in foam volume after 10 min (%), as per Eq. 1 (VF0 and VF10 are volume of foam at t = 0 and t = 10 min, respectively).

$$\%\;\mathrm{reduction\;in\;}{V}_{F}\;{\text{after}}\;10\;{\text{min}}= \frac{{V}_{F0}-{V}_{F10}}{{V}_{F0}}*100$$
(1)

Foam Structure

Images of the foam surface at t = 0 and t = 10 min of destabilization process were taken using a light microscope (Prism Optical, Eagle Farm, QLD, Australia) fitted with a 5.0 MP camera system using TSView7 software (Tucsen Image Technology Co., Ltd., Fuzhou, China). During imaging, an Olympus LG-PS2 lamp (Eagle Farm, Queensland, Australia) was used to illuminate the foam. To determine the size and size distribution of air bubbles in foam, we followed the method reported by Kamath (2007) and Kamath et al. (2008b). For each sample, the diameter of at least 1000 air bubbles from three foam images was determined using Image-Pro Plus 6.0 software (Media Cybernetics Inc, Bethesda, USA). It is noted that determining the diameter for individual air bubbles in the sub-surface layers was challenging and unable to be automatically performed by the Image-Pro Plus 6.0 software due to their visibility through the air space within the air bubbles on the surface layer. Hence, manual measurements were conducted with the help of the Image-Pro Plus 6.0 software. Considering the non-spherical shape of most air bubbles, the longest diameter of air bubbles was determined. Graphical representations (histograms) of the log diameter of air bubbles were created from the measured diameters. A smooth curve based on the probability density function was fitted to generate continuous visualization of the underlying data distribution using Minitab 17® software (Minitab Inc., USA).

Determination of Milk Properties

Properties of milk dispersions in terms of viscosity, surface tension, zeta potential, free fatty acids, and particle size were determined. Details of these analytical methods have been well-reported in our previous studies (Ho et al., 2019, 2023a, b). Viscosity of milk dispersions at 25 °C was measured with an AR 1500 Rheometer (TA Instruments, Cheshire, UK) using a cone and plate geometry (diameter 40 mm and gap 0.2 mm), and the viscosity values at a shear rate of 50 1/s were reported. Surface tension of milk samples was measured by tensiometer (ST9000, Nima Technology, Coventry, UK) using a platinum Nima Wilhelmy plate (10.25 × 0.16 mm). Zeta potential of milk dispersions was determined using a Zetasizer Nano (Malvern Panalytical Ltd., Great Malvern, UK) and a disposable polycarbonate cuvette (ATA Scientific, DTS1061). To avoid multi-scattering, the samples were diluted with milliQ water at a ratio of 1:100. Free fatty acid content (µ.equiv/mL) in milk samples was determined using a mixture of isopropanol, hexane, and 4 N H2SO4, (40:10:1, by volume) to extract free fatty acids from 3 mL of milk, followed by titration of the extracts with 0.02 N methanolic KOH with few drops of 1% methanolic phenolphthalein as an indicator. The particle size of milk samples in terms of volume-based diameter (D[4,3]) and surface area-based diameter (D[3,2]) and particle size distribution curves were determined using a Malvern Mastersizer 2000 (Malvern Panalytical Ltd., Great Malvern, UK) with refractive indices of milk fat and water being 1.462 and 1.330, respectively.

The Design of Experiment and Statistical Analysis

The experiments were performed with three replications and two measurements were performed for each replicate, and the results are expressed as mean values (± standard errors). The experimental data were subjected to one-way ANOVA followed by post hoc Tukey’s test to differentiate foamability, foam stability, and milk properties among the samples. Statistically significant differences were assessed for p < 0.05 at a confidence level of 95% using the Minitab 17® software (Minitab Inc., USA).

Results and Discussion

Properties of Reconstituted Skim Milk Powder Dispersions at Different Solids Concentrations and Their Foaming Properties

Properties of Reconstituted Skim Milk Powder Dispersions

The calculated main components, viscosity, surface tension, and zeta potential of reconstituted skim milk powder dispersions at various solids concentrations (1.5–15%, w/w) are shown in Table 1. Apparently, milk dispersions containing the higher solids concentration had a higher amount of proteins (~ 0.49 − 4.88%), fat (~ 0.01 − 0.12%), and lactose (~ 0.83 − 8.25%). Viscosity values of milk dispersions at 1.5–15% solids concentration were 1.93–3.36 mPa.s, which are in agreement with results reported by Kamath (2007) in which viscosity of milk increased significantly with increasing solid content (p < 0.05). Lactose with a high water holding capacity could be a reason for increasing viscosity of milk dispersions at high solid content (Gamboa & Barraquio, 2012). Increasing solids concentration did not affect zeta potential of reconstituted skim milk powder dispersions (p > 0.05), with absolute values ranging from 25.82 to 27.22 mV. This indicates that the zeta potential value of reconstituted skim milk powder dispersions is independent of solids concentration. Similar results were reported for the zeta potential of skim milk at various solids concentrations of 9, 17, and 25% (Markoska, 2018). Regarding surface tension, it is well-known that surface tension of milk is reduced with the presence of the surface-active components of milk such as proteins and fat (Kamath, 2007). It was reported that increasing the concentration of whey protein isolates resulted in a reduction in surface tension. However, this effect was marginal because at higher concentration, the proteins occupied the entire interface, thus, no longer affecting surface tension further (Nastaj & Solowiej, 2020). In contrast, this study observed a significant increase (p < 0.05) in surface tension from 56.0 to 59.74 mN/m with an increase in the solids concentration of reconstituted skim milk powder dispersions from 1.5 to 15% (i.e., an increase in protein and fat content). Williams et al. (2005) stated that the determination of milk surface tension using the Wilhelmy plate method, as employed in this study, is primarily influenced by the viscosity of the sample. The viscosity affects the drainage rate of milk from the plate, and thus, the recorded force includes liquid that has not yet drained away. Consequently, milk samples with higher viscosity, as observed in the case of higher concentrations of reconstituted skim milk powder dispersions, exhibited higher surface tension. Similarly, Borcherding et al. (2009) reported an increase in the surface tension of skimmed milk from 45.4 to 47.1 mN/m, along with an increase in viscosity from 0.7 to 1.5 mPa.s, corresponding to an increase in protein content from 1.0 to 6.0%. Additionally, variations in surface tension were noted in homogenized milk samples with different components including protein, fat, and lactose (Mukherjee et al., 2005), as well as in milk dispersions containing different types of milk powders such as whey protein concentrate 65, whey protein concentrate 80, and whey protein isolate (Nastaj, Solowiej et al., 2020).

Table 1 Calculated percentage (w/w) of main components (protein, fat, and lactose) and physical properties (viscosity, surface tension, and zeta potential) of reconstituted skim milk powder dispersions at various solids concentration
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Foaming Properties and Foam Structure

Foamability and foam stability of reconstituted skim milk powder dispersions at different solids concentrations in two foaming methods (mechanical mixing and steam injection) are shown in Fig. 1. It is noticed that the higher percentage of foam volume reduction after 10 min indicates less foam stability. Both foaming methods exhibited a similar trend, wherein the foamability and foam stability of reconstituted skim milk powder dispersions at low solids concentrations (e.g., 1.5%) were not significantly different from those of the milk samples at high solids concentrations (e.g., 15%) (p > 0.05). These results differed from those reported by Kamath (2007). The authors observed a decrease in foamability and an increase in foam stability in reconstituted skim milk powder dispersions as solids concentration increased from 1.5 to 15%, which was attributed to the increased viscosity at high solids concentration. Conversely, Martinez-Padilla et al. (2014) observed improved foamability and foam stability with increased solids concentration from 10 to 25%, despite a nearly fourfold increase in viscosity. Meanwhile, in this study, an increase in viscosity due to rising solids concentration did not impact the foaming properties of reconstituted skim milk powder dispersions. The findings in these reports indicated that the effects of solids concentration of reconstituted skim milk powder dispersions on foaming properties are highly dependent on foaming approaches including distinct mechanisms and temperatures (e.g., air injection, whipping, steam injection, and mechanical mixing). The high shearing force in the whipping method reduces viscosity resistance, facilitating protein diffusion for foam production, while elevated temperatures in steam injection and mechanical mixing (~ 65 °C) decrease milk viscosity during foaming. Dependence of foaming properties of milk samples on foaming techniques was well reported (Goh et al., 2009; Ho et al., 2019). Kamath (2007) also noted that variations in foamability and stability of reconstituted skim milk powder dispersions at different solids concentration depended on foaming temperature.

Fig. 1
figure 1

Foamability (a) and foam stability (b) of reconstituted skim milk powder dispersions at different solids concentration in mechanical mixing and steam injection foaming methods. Within foam property and foaming method, different letters (a, b, c… or A, B, C…) indicated significant differences among samples at a significance level of 0.05. The p values for foamability and foam stability in steam injection were 0.2049 and 0.0606, respectively, and the corresponding values for the mechanical mixing foaming method were 0.0648 and 0.2227

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While fat hinders milk foaming, protein, and lactose enhance it (Huppertz, 2010). Despite varying solids concentration, the protein/fat ratio remained approximately 40:1 (Table 1). The low-fat content in reconstituted skim milk powder dispersions may not significantly impact foaming, especially as the process occurs at a higher temperature (e.g., 65 °C) than the fat’s melting point (e.g., 40 °C). The foaming results indicate that milk proteins generate good foam at 0.5% protein, rendering increased protein concentration unnecessary. Similar findings were reported for reconstituted skim milk powder dispersions at 0.5% protein (Kamath, 2007), sodium caseinate and whey protein isolate at 0.25% protein (Britten & Lavoie, 1992), and sodium caseinate at 0.3 (Marinova et al., 2009). In this study, a 10-min timeframe for observing foam collapsing may be insufficient to detect stability differences at various solids concentrations, which requires further investigation.

Images of the foam surface and size distribution of air bubbles in foam produced from reconstituted skim milk powder dispersions at different concentrations are shown in Fig. 2. Foam produced from all reconstituted skim milk powder dispersions had good structure and appearance with small size of air bubbles. At t = 0, foam from milk samples with 1.5% solids concentration exhibited significantly larger air bubbles than that from samples with 5.0% solids concentration. Further increases up to 15% solids concentration only marginally reduced air bubble size. These results imply that increased solids concentration in reconstituted skim milk powder dispersions leads to smaller air bubbles, possibly due to increased viscosity hindering the coalescence and growth of air bubbles. This aligns with Martinez-Padilla et al.’s (2014) findings, indicating a 50% reduction in air bubble size (from 161 to 87 μm) as solids concentration increased from 10 to 25%. Similarly, studies by Borcherding et al. (2009) and Marinova et al. (2009) noted a decrease in air bubble size for skimmed milk and milk protein foam with increased protein content. After 10 min of the destabilization process, it became evident that the air bubble size increased over time in reconstituted skim milk powder dispersions at the same solids concentration. The air bubbles became larger, and differences in size among milk samples with varying solids concentration diminished. This growth in air bubble size over time could be attributed to the drainage of the liquid phase in the foam, reducing the stabilizing effect of the liquid, facilitating coarsening, and consequently leading to an increase in air bubble size.

Fig. 2
figure 2

Images of the foam surface and size distribution of air bubbles in foam produced from reconstituted skim milk powder dispersions at different solids concentration, and taken at t = 0 and t = 10 min of destabilization process. Scale bar = 1000 µm. For the foam images (on the left-hand side), the numbers of 1.5, 5.0, 8.5, 10, and 15 represent solids concentration of skim milk dispersions; and t0 and t10 represent time at which the images were taken at t = 0 and t = 10 min of destabilization process, respectively. As an example, 1.5-t = 0 means skim milk dispersion of 1.5% total solids concentration at time 0. For the size distribution of air bubbles (on the right-hand side), the numbers 1.5, 5.0, 8.5, 10, and 15 represent solids concentration of skim milk dispersions, while the values such as “2.108 ± 0.3335” denote average and standard deviation values, respectively, of air bubbles. As an example, “1.5: 2.108 ± 0.3335” means that the average size and standard deviation of air bubbles in foam produced from skim milk dispersion of 1.5% total solids concentration were 2.108 and 0.3335, respectively

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Properties of Milk Dispersions at Low Protein Concentration Prepared from Skim Milk Powder, Whole Milk Powder, and Commercial Whole Milk and Their Foaming Properties

Properties of Milk Dispersions

As shown in Table 2, these milk samples had low protein content (0.5%) and quite similar in lactose content (0.7–0.8%), which could be the reason for their similarities in viscosity (2.0–2.5 mPa.s). Variations in fat and free fatty acid content between skim milk powder and whole milk powder dispersions (or whole milk dispersions) resulted in differences in surface tension and zeta potential values. Whole milk dispersions, characterized by the highest fat content (0.5%) and free fatty acid concentration (0.95 µ.equiv/mL), displayed the lowest surface tension (50.82 mN/m) and the highest absolute zeta potential (33.46 mV). Conversely, skim milk powder dispersions exhibited the opposite trend. Previous studies have reported that surface-active components, such as fat and free fatty acids, reduced surface tension in whole milk compared to skimmed milk, and homogenization further lowers surface tension by releasing the surface active components from the milk fat globule membrane (Kamath, 2007; Michalski & Briard-Bion, 2003).

Table 2 Main components (%) and physicochemical properties of milk dispersions at 0.5% protein prepared from skim milk powder (SMP), whole milk powder (WMP), and commercial whole milk (WM)
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The results in Table 2 and Figure S1 (supplementary materials) demonstrate significant differences in particle size and distribution curves among the milk samples. Skim milk powder dispersions had the smallest particle size (D[4,3] ≈ 0.162 µm and D[3,2] ≈ 0.133 µm), aligning with the size of caseins (Crofcheck et al., 2002) and displayed a single peak in size distribution within the range of 0.08–1.0 µm. In the case of whole milk dispersions, two peaks were observed with a volume ratio of 1:1, corresponding to the size of casein micelles and homogenized fat globules (1–10 µm). This resulted in an average particle size of 0.616 µm for D[4,3] and 0.241 µm for D[3,2]. Meanwhile, whole milk powder dispersions had the largest particle size (D[4,3] ≈ 3.388 µm and D[3,2] ≈ 0.364 µm) with three peaks observed on the distribution curves. The particle size results suggest that the reconstitution of whole milk powder was unable to perfectly replicate the original liquid whole milk. Variations in particle size between reconstituted whole milk powder and whole milk dispersions can be attributed to the processing steps involved in powder production and the subsequent rehydration process when dissolved in water.

Foaming Properties and Foam Structure

For foamability (Fig. 3a), in mechanical mixing, skim milk powder samples yielded approximately 16 times more foam volume than whole milk powder and whole milk dispersions. The foaming trend was similar for steam injection, but differences in foamability between skim milk powder and whole milk powder or whole milk dispersions were less pronounced than those seen in mechanical mixing. For foam stability (Fig. 3b), assessing foam stability under mechanical mixing is challenging due to the minimal foam volume generated from whole milk powder and whole milk dispersions (approximately 10 mL per 100 mL of milk), resulting in a small percentage reduction in foam volume after a 10-min destabilization process. This may explain the lower stability of skim milk powder foam compared to that of whole milk powder and whole milk foam. In steam injection foam, all milk samples, particularly whole milk powder and whole milk foams, collapsed rapidly within the initial 10 min, with approximately 60% of the initial foam volume collapsing.

Fig. 3
figure 3

Foamability (a) and foam stability (b) of milk dispersions prepared from skim milk powders (SMP), whole milk powders (WMP), and commercial whole milk (WM) by mechanical mixing and steam injection. Within foam property and foaming method, different letters (a, b, c… or A, B, C…) indicated significant difference among samples at a significance level of 0.05. The p values for foamability and foam stability in steam injection were 0.0000 and 0.0021, respectively, and the corresponding values for the mechanical mixing foaming method were 0.0000 and 0.0000

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Differences in foamability and foam stability among these samples may be attributed to particle size. While previous studies have suggested that reducing particle size through homogenization enhances foaming properties in milk (Deeth & Smith, 1983; Huppertz, 2010), this study found almost identical foaming properties in whole milk powder and whole milk dispersions, despite their significant differences in particle size, free fatty acids, surface tension, and zeta potential (Table 2). Notably, the protein-to-fat ratio in skim milk powder dispersions was 49:1, whereas in whole milk powder and whole milk dispersions, it was similar at 1:1. In the foaming process of a milk system containing both proteins and fat, there is a competition between them to adsorb on the interfacial region for foam production and stabilization (Nylander et al., 2019). Consequently, the excellent foaming properties of skim milk powder solution may be attributed to the small amount of fat in the skim milk powder dispersion, reducing interference with proteins in the formation and stabilization of foam.

In terms of foam structure, as shown in Fig. 4, all milk samples produced good-looking foam with a significant proportion of small air bubbles, even after a 10-min destabilization process. At t = 0, the average size of air bubbles was smallest in whole milk powder foam, followed closely by whole milk foam. However, due to liquid phase drainage in the foam, promoting air bubble coalescence, the foam of all milk samples increased in size after 10 min of destabilization.

Fig. 4
figure 4

Images of the foam surface and distribution curves of air bubble size in foam from skim milk powders (SMP), whole milk powders (WMP), and commercial whole milk (WM) at time 0 (t0) and 10 min (t10) of destabilization process. Scale bar = 1000 µm. For the size distribution of air bubbles, the values such as 2.038 ± 0.3503 denote average and standard deviation values, respectively, of air bubbles. As an example, “WM_t0: 2.038 ± 0.3503” means that the average size and standard deviation of air bubbles in foam produced from whole milk and taken at time 0 were 2.038 and 0.3503, respectively

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Properties of Whole Milk Dispersions Containing Added Milk Powders and Their Foaming Properties

Properties of Milk Samples

The particle size distribution of whole milk, and whole milk containing added milk powders is presented in Figure S2. It can be found that whole milk and whole milk containing added milk powders showed a similar bimodal particle size distribution. The peak around 0.1 μm represents the casein micelles, while the peak around 1.0 μm represents the homogenized fat globules (Ho et al., 2021). The average particle size of whole milk and whole milk containing added milk powders, including D[4,3] and D[3,2], are shown in Table 3. All the samples showed similar D[4,3] and D[3,2] values. These results indicate that the addition of the three types of milk powders at different concentration levels did not affect the particle size of whole milk, suggesting their complete rehydration in whole milk.

Table 3 Particle size in terms of volume-based D[4,3] and surface area-based D[3,2], viscosity, zeta potential, and surface tension of milk samples containing added milk powders
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Table 3 summarizes the viscosity, surface tension, and zeta potential of whole milk and whole milk containing added milk powders. There was a significant increase in viscosity with the addition of skim milk powder or milk protein concentrate at higher protein concentration level (4.0%, w/w). However, increasing the protein content to 3.5% (w/w) by adding skim milk powder or milk protein concentrate or increasing the total solid content by 0.5 or 1% (w/w) by adding butter milk powder did not significantly affect the viscosity of whole milk. Additionally, no significant differences in surface tension and zeta potential were found after the addition of milk powder. These results echoed the findings of a previous study (Xiong et al., 2020) for reconstituted protein dispersions, where there was no significant effect of protein content on surface tension or zeta potential.

Foaming Properties and Foam Structure

The foamability and foam stability of whole milk and whole milk containing added milk powders using steam injection and mechanical mixing methods are shown in Fig. 5. No significant difference in foamability and foam stability was found in this study regardless of the foaming method used (p > 0.05). Although there is evidence that the foam stability increased with increasing the total solid or protein content (Martinez-Padilla et al., 2014), in this case, it is possible that the increased protein contents or total solids are limited compared to previous studies, resulting in no significant change in foam stability.

Fig. 5
figure 5

Foamability and foam stability of whole milk (WM), and that of WM containing added skim milk powder (SMP) and milk protein concentrate (MPC) at final protein content of 3.5 and 4%, and butter milk powder (BMP) at total solids content of 0.5 and 1%. Within foam property and foaming method, different letters (a, b, c… or A, B, C…) indicated significant difference among samples at a significance level of 0.05. The p values for foamability and foam stability in steam injection were 0.7610 and 0.8110, respectively, and the corresponding values for the mechanical mixing foaming method were 0.9910 and 0.8790

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Figure 6 gives representative light microscopic images of foam surface at t = 0 min (immediately after foaming) and t = 10 min of whole milk and whole milk containing added milk powders. There were no significant differences in the average air bubble size of all samples. These results, coupled with foamability and foam stability results, suggest that the addition of milk powders including skim milk powder, milk protein concentrate, or butter milk powder at the concentration levels used in this study did not affect the foaming properties of whole milk.

Fig. 6
figure 6

Images of the foam surface and size distribution of air bubbles of foam produced by steam injection of whole milk (WM), and those of WM containing added skim milk powder (SMP) and milk protein concentrate (MPC) at final protein content of 4%, and butter milk powder (BMP) at total solids content of 1%. Scale bar = 1000 µm. In size distribution figures, values such as “2.321 ± 0.252” represent the average size of air bubbles (2.321) and standard deviation (0.252)

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Conclusion

The study demonstrated that increasing solids concentration of reconstituted skim milk powder dispersions from 1.5 to 15% (corresponding to 0.5–5% protein) under steam injection and mechanical foaming methods did not affect foaming properties, despite increased lactose, viscosity, and surface tension. Similar results were observed for the foaming properties of whole milk with added milk powders (skim milk powder, milk protein concentrate, and butter milk powder). Milk proteins exhibited excellent foaming at 0.5%, yet their performance depended on the protein-fat ratio, highlighting the significance of protein dominance for achieving good foam in the foaming system. For optimal foam quality and stability, selecting milk types with higher protein content, such as skim milk powder dispersions, is recommended.

The findings in this study can guide food manufacturers to optimize foaming processes for milk-based products, ensuring stable foam production across different solids concentrations and milk-based product formulations. It emphasizes the importance of maintaining a favorable protein-fat ratio for superior foam quality. Further investigation is needed to elucidate the specific mechanisms underlying the dependence of foaming properties of milk proteins on the protein-fat ratio and to explore the impact of different protein-fat ratios on the intermolecular interactions during the foaming process. This will determine the critical ratio at which foaming properties of milk proteins undergo a significant shift.