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Role of food emulsifiers in milk coffee beverages
Journal of Colloid and Interface Science 449 (2015) 198–204
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Role of food emulsifiers in milk coffee beverages A. Ogawa ⇑, H. Cho Mitsubishi-Kagaku Foods Corporation, Yokohama, Japan
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“Milk coffee beverage with rich coffee extracts”
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OH
OH
CH2OH
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“electrolytes” δ
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H
-OOCR
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“High hydrophilicity” H
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Article history: Received 23 October 2014 Accepted 23 January 2015 Available online 30 January 2015 Keywords: Beverages Coffee Emulsion Emulsifier Stability Electrolyte
a b s t r a c t To emphasize the coffee flavor, many milk coffee beverages contain coffee extracts; these are the socalled ‘‘rich milk coffee’’ beverages. When the content of the coffee extracts increases, milk coffee beverages become unstable. The milk ring formation, or oiling off, is accelerated in these kinds of drinks. We prepared a ‘‘rich milk coffee’’ beverage and studied the stability of the emulsion. We also investigated the influence of the food emulsifiers on the stability of the emulsion. We tried to test the emulsifier system in order to improve the emulsion stability. When the milk coffee beverage with a low light value for the roasted coffee beans sterilized by UHT was stored at a low temperature, the milk component strongly separated. We found that the sucrose monoester with a high HLB and diglycerol monoester accelerated the milk separation, and the decaglycerol monoester controlled the milk separation. We discussed the milk separation mechanism and showed that maintaining the hydration of the hydrophilic group in the rich milk coffee beverage was related to the combination of emulsifiers that control the milk separation. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction Beverages defined as RTD (‘‘Ready to Drink’’), such as canned or bottled milk coffee, contain milk components that have been subjected to a heat sterilization treatment for subsequent storage. Milk coffee beverages contain about 1% milk fat, forming an oilin-water emulsion. Since the storage term for these milk coffee ⇑ Corresponding author. Fax: +81 45 963 3976. E-mail address: [email protected] (A. Ogawa). http://dx.doi.org/10.1016/j.jcis.2015.01.063 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
beverages is 6–12 months, the milk fat gradually floats towards the air–emulsion interface. Aggregation or flocculation [1–3] follows, and a milk ring is formed on the air–emulsion interface. Aggregation means sticking together of solid particles (like hard spheres) and flocculation means the association of polymer-coated particles (or droplets). Since milk proteins that are biopolymer adsorb on the surface of the oil droplet in milk coffee beverages system, we used the ‘‘flocculation’’ not ‘‘aggregation’’ in this paper. When flocculation happens, even if the milk coffee beverage is repeatedly shaken, the milk ring will not re-disperse and some
A. Ogawa, H. Cho / Journal of Colloid and Interface Science 449 (2015) 198–204
199
lumps of the milk fat will float on the surface. If the degree of flocculation is severe, coagulation occurs in the milk fat and this leads to the separation of the oil and water (oiling off). If this oil sticks to the can or plastic bottle, consumers will get the impression that these beverages are of a low quality and will no longer purchase them. Milk coffee beverages constitute a large percentage of the Japanese soft drink market. The scale of Japanese soft drink market is over 20,000 thousands kl/year. And the scale of Japanese coffee beverage market is 3000 thousands kl/year. To emphasize the coffee flavor, many milk coffee beverages contain coffee extracts; these are the so-called ‘‘rich milk coffee’’ beverages. When the content of the coffee extracts increases, milk coffee beverages become unstable. The milk ring formation, or oiling off, is accelerated in these kinds of drinks. This situation is aggravated by the fact that the plastic bottle is transparent: consumers can see that the milk fat has separated, so the product value depreciates and consumers complain. We prepared a ‘‘rich milk coffee’’ beverage and studied the stability of the emulsion. We also investigated the influence of the food emulsifiers on the stability of the emulsion. We tried to test the emulsifier system in order to improve the emulsion stability. Fig. 1. Emulsifier structures.
2. Materials and methods 2.1. Preparation of milk coffee beverages Roasted coffee beans were purchased from Unicafe Inc. (Japan). UHT sterilized milk was purchased from the Japanese supermarket, it contains 3.3% protein, 3.8% fat, 4.8% lactose, 0.7% minerals (Sodium: 41 mg, Potassium: 150 mg, Calcium: 110 mg per 100 g milk) and 87.4% water. We used the sucrose fatty acid ester sample of Mitsubishi-Kagaku Foods Corp. (Japan). Two milk coffee beverages were prepared. We defined ‘‘Rich1’’ as the milk coffee beverage containing 6.5 wt% coffee beans (as unroasted coffee beans) and 8 wt% milk. ‘‘Rich2’’ is another milk coffee beverage that contains 7.0 wt% coffee beans (as unroasted coffee beans) and 12 wt% milk. The two kinds of roasted coffee beans used were Coffea Arabica (Colombian EX) and Robusta (Indonesian AP-1). As the roasting of coffee beans is proceeding, the color of coffee beans change into dark brown. We generally use the Hunter’s Lab color space as an index of the degree of roasting, because the degree of roasting correlates closely with the light value. We can easily measure the light value with the Hunter color difference meter. The light values (LV) of 20 and 28 were used for ‘‘Rich 1’’, and that of 20, 22, 24, 26 and 28 were used for ‘‘Rich 2’’ milk coffee beverages. Sucrose palmitic acid ester (RYOTO™ Sugar Ester P-1670; HLB 16, with a monoester content of 80%) was used as an emulsifier. Roasted coffee beans were extracted with 10 times de-ionized water as much as roasted coffee beans heated to 95 °C, thereby obtaining a coffee extract solution. The thus obtained coffee extract solution was mixed with milk and granulated sugar and further
Table 1 Composition of milk coffee beverages. Ingredient
Rich1
Rich2
Coffee beans (as unroasted beans) Milk Sugar Emulsifier (Ryoto™ Sugar Ester P-1670) pH (Sodium bicarbonate) Water
6.5% 8.0% 6.0% 0.04% 6.9 79.46%
7.0% 12.0%
Fat Protein
0.30% 0.26%
0.46% 0.40%
6.6 74.96%
with an emulsifier. Then, de-ionized water was added to the mixture to obtain emulsion. After sodium bicarbonate was added to the emulsion to adjusting the pH, the emulsion was intimately mixed and homogenized at a temperature of 60–70 °C under a pressure of 15 MPa/50 MPa using a high-pressure homogenizer. Thereafter, the obtained emulsion was sterilized at 121 °C for 40 min using a retort sterilizer in the case of the canned milk coffee beverages, or was sterilized at 137 °C for 60 s (hold time for sterilization) using a plate-type Ultra High Treated (UHT) pasteurizer in the case of the plastic bottled milk coffee beverages, thereby obtaining a milk coffee beverages [4]. The composition of these milk coffee beverages is depicted in Table 1.
2.2. Preparation of milk coffee beverages with various emulsifiers The plastic bottled milk coffee of ‘‘Rich2’’ that contained Colombian EX (LV 26) was prepared by UHT sterilization. Sucrose palmitic acid ester (RYOTO™ Sugar Ester P-1670; HLB 16 (Mitsubishi-Kagaku Foods Corp., Japan)), diglycerol palmitic acid ester (POEM™ DP-95RF; HLB 8 (Riken Vitamin Co., Ltd., Japan)), and decaglycerol stearic acid ester (RYOTO™ polyglycerol Ester S-10D, HLB 14 (Mitsubishi-Kagaku Foods Corp., Japan)) were used as emulsifiers. The structures of these emulsifiers are depicted in Fig. 1.
2.3. Evaluation of milk coffee beverages 2.3.1. Oil drop size and distribution measurements The median oil drop size and drop size distribution were measured using a LA-500 Laser Diffraction Particle Size Analyzer with a flow cell unit (HORIBA, Japan). The measurement of the droplet size analysis uses the Mie light scattering theory. The scattered light strength is decided by the particle diameter parameter a (a = pD/k; p: particles circumference length, D: single particle diameter, k: incoming light’s wavelength) and the particle’s diffractive index m. Particle size distributions are calculated based on the collected scattered light strength’s angle distribution value. Milk coffee samples were poured into the cup of the analyzer and were diluted with de-ionized water. The median oil drop size
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and drop size distribution were calculated with the circulation measurement. 2.3.2. Transmission electron microscopy [5] The samples were rapidly frozen at 210 °C using a JFD-380P slushy nitrogen production device (JEOL Ltd., Japan). Freeze fracture replica preparations were obtained by making reference to the method described by Buchheim [6] using JFD-9010 freeze etching equipment (JEOL Ltd., Japan). The samples were fractured at 165 °C, shadowed with 2 nm of platinum/carbon under an angle of 45°, and coated with 20 nm of carbon. The replicas were cleaned in concentrated sodium hypochlorite and distilled water. All samples were examined with a H-9000NA transmission electron microscope (Hitachi Ltd., Japan) operating at 100 kV accelerating voltage. 2.3.3. Viscosity measurements The viscous behaviors of the milk coffee and milk separation samples were measured using a dynamic shear rheometer with a 4 °C/40 mm cone rotor and the temperature module of the plane plate (Constant Stress Rheometer, CS-10, Bohlin Instruments Ltd., USA). Samples were placed in the temperature-controlled measurement plate and allowed to equilibrate to the required temperature (5 °C or 40 °C) for 3 min prior to making the measurements. The viscosity was acquired as a function of the shear stress (0.06–0.6 Pa). 2.3.4. Preparation of milk solution with coffee components The model milk solution was prepared by dispersing 12 wt% UHT sterilized milk, 6 wt% sugar, and 0.05 wt% emulsifier in water. Sodium citrate (as an organic acid) or caffeine was added to the model milk solution. The concentration of sodium citrate was in the range of 0.1–1.0% and that of caffeine was 1.0–1.5. After stirring, we observed the milk separation. 2.3.5. Electric conductivity measurements Electric conductivity for the extracts of the roasted coffee beans (LV 18, 22, and 26) and for the two milk coffee samples with Indonesian AP-1 (LV 20 and 28) and Colombian EX (LV 20 and 28) was measured using an electric conductivity meter CM-60G with a CT57101C cell (DKK-TOA, Japan). The measurement temperature for the extract solution of the roasted coffee beans was 22 °C and 10 °C for the milk coffee samples. 3. Results and discussion 3.1. Stability of the milk coffee system After the initial preparation, the milk coffee beverages were stored at 5 °C and 40 °C; the emulsion stability was then studied by visual observation. The results are summarized in Table 2.
Canned milk coffee prepared by retort sterilization (used for both Rich1 and Rich2) saw little milk separation, and the milk coffee beverages (which are oil-in-water emulsions) were stable regardless of the kinds of roasted coffee beans or storage temperature. However, the plastic bottled milk coffee beverage prepared by UHT sterilization exhibited strong milk separation with Colombian EX under the conditions of LV 20 at 5 °C; however, the 40 °C storage samples had little milk separation. The milk coffee beverage with Indonesian AP-1 had observable milk separation at 5 °C for both Rich1 and Rich2. The plastic bottled milk coffee beverage with the LV 20 roasted coffee beans exhibited more milk separation than that with the LV 28 roasted coffee beans. It exhibited especially violent separation: there was a complete separation of the milk and coffee that occurred with the LV 20 in Rich2. Milk separation in the samples was observed, even at 40 °C with the LV 20 roasted coffee beans. In order to investigate the influence of the light value, we observed the effects of light on Rich2 with various light values for the Colombian EX beans. Table 3 indicates that milk separation was observed at below LV 24 for these roasted beans. Actually, milk separation occurred through the flocculation of the oil drops (Fig. 2). After the flocculation of the oil drops advanced, these floated either gradually or rapidly to the air–emulsion interface and formed a thick milk layer. When we measured the electrical conductivity of the coffee extracts with the different roasted coffee beans, the lower light value of the coffee beans meant the higher electrical conductivity. We think this means low light value of the coffee extract contains high amount of electrolyte. And we presume that there is the border of milk separation between LV 24 and LV 26 for Columbian EX in the UHT sterilized milk coffee beverages system. It is speculated that an increase of electrolyte caused the milk separation. We found that the milk coffee beverages became unstable under the conditions of UHT sterilization, low storage temperatures, and the low light values of the Robusta beans. These conditions lead to milk separation. The stability of Rich1 was evaluated by measuring the drop size distributions (Table 4). The median size of the drops was 0.44–0.49 lm. After the milk coffee preparation for two different roasted coffee beans, the drop size distribution was almost the same as that of the retort sterilization canned coffee with LV 20 for the Indonesian AP-1 (it had a slightly wide drop size distribution). The milk separation part of Rich1 with a LV 20 for the Indonesian AP-1 sterilized by UHT was evaluated by measuring the droplet size and drop size distributions. The median size of the drops was 0.44 lm, and the drop size distribution was almost same as that of the results shown the above. When the plastic bottled milk coffee beverage was reconstituted by dispersion, this separated milk was evaluated with the same method for the milk separation part; the median size of the drops and the drop size distribution were the same. This result suggests that oil drops interact with
Table 2 Degree of the milk layer separation for various milk coffee (1–2 mm ( ), 5 mm (+), strong separation (++), violent separation (+++)). Sterilization
UHT
Roasted coffee beans
Indonesia AP-1 Columbia EX
Retort
Indonesia AP-1 Columbia EX
Temperature (°C)
5 40 5 40 5 40 5 40
Rich1
Rich2
LV 20
LV 28
LV 20
LV 28
++
+
+++ + ++
+
++
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A. Ogawa, H. Cho / Journal of Colloid and Interface Science 449 (2015) 198–204 Table 3 Influence of the light value of the roasted beans on the milk separation in the Rich2 milk coffee (1–2 mm ( ), strong separation (++)). Sterilization
Temperature (°C)
Light value of the roasted beans (Columbia Ex) 20
22
24
UHT
5 40
++
++
++
Retort
5 40
26
28
Fig. 2. Course of milk separation (UHT sterilization, Indonesian AP-1 (LV 20), 5 °C storage).
Table 4 The median sizes of Rich1 with Indonesian AP-1 after sterilization. Sterilization
UHT Retort
very weak forces with each other, and can be re-dispersed by slight shaking or stirring.
Light value of roasted coffee beans 20 (lm)
28 (lm)
0.49 0.46
0.44 0.44
3.2. Influence of the emulsifier on milk separation We tested the stability of the plastic bottled milk coffee beverage with a LV 26 for the Colombian EX by adding the three kinds of
Fig. 3. Freeze replica TEM images of the milk separation part: (a) P-1670 and (b) DP-95RF.
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emulsifiers. The plastic bottled milk coffee beverage with P-1670 and DP-95RF exhibited violent milk separation. The milk coffee beverage with S-10D and without the emulsifier did not seem to have any obvious visible milk separation. All four samples exhibited almost the same drop size distribution; the median sizes of the drops were 0.49 lm (for S-10D), 0.52 lm (for P-1670), 0.53 lm (for the sample without an emulsifier), and 0.55 lm (for DP-95RF). According to Table 3, the plastic bottled Rich2 with a LV 26 for the Colombian EX prepared by UHT sterilization was stable. However, because the pH (6.3) was not adjusted in this case, the emulsion seemed to become unstable, and milk separation occurred depending on the kind of emulsifier used. The emulsifier used for Rich2 that resulted in the violent milk separation has a relatively small hydrophilic group compared with that of the hydrophobic group (Fig. 1). Therefore, it is suggested that as the hydrophilicity of the oil droplet decreases, flocculation more easily occurs between the oil droplets. In order to observe the microstructure for the milk separation part, a transmission electron microscope (TEM) was used and the freeze fracture samples for the P-1670 and DP-95RF were observed. These TEM image are depicted in Fig. 3. It was found that each oil drop changed shape for the sample with P-1670. This, however, did not lead to coalescence. Some large oil drops that were coalescent were observed for the sample with DP-95RF. The microstructures of both the P-1670 and DP-95RF sample were characterized by the diamond shaped continuous network structure that was formed by the flocculation of the oil drops. When these milk separation parts were re-dispersed and the milk coffee beverage was reconstituted, the flocculates were not observed in the TEM images. From these results, it is speculated that the oil drops formed the regular flocculates by weak interactions in the separated milk layer. We assume that this weak interactions is originated from the dehydration of the hydrophilic part of emulsifier, as a result the oil droplets flocculated by hydrophobic interaction. The viscous behavior and the formation of the flocculates were investigated by measuring the viscosity of the milk separation part using a rheometer (Fig. 4). The shear stress was increased to 0.6 Pa, and then was decreased to 0.06 Pa. With the increase of the shear stress, the viscosity at 5 °C was gradually decreased for the milk separation part with P-1670. The viscosity recovered with the decrease in the shear stress. This indicates that the destroyed network is easy to recover, and shows that this milk separation part has thixotropy. The milk coffee beverage that had a re-dispersed separation part (viscosity at 5 °C) or the slight milk separation part with P-1670 (viscosity at 40 °C) did not have the property of thixotropy. These flocculates in the milk separation part were destroyed by a high storage temperature or slight shaking. The viscosity of the milk separation part with DP-95RF at 5 °C also showed almost the same behavior as that in the case of P1670 in regards to the increase in the shear stress (Fig. 5). However,
Fig. 4. Rheological properties of the milk coffee beverage with P-1670.
Fig. 5. Rheological properties of the milk coffee beverage with DP-95RF.
in the case of the decrease of the shear stress, the viscosity did not recover to its beginning value. This indicates that the destroyed network is not easy to recover. This result shows the milk separation part with DP-95RF also has thixotropy, but the flocculates are not immediately formed again. The milk coffee beverage that was re-dispersed with this milk separation part (viscosity at 5 °C) showed similar viscous behavior to that of the milk separation part of DP-95RF at 5 °C. It is speculated that the flocculation force is strong between the oil drops: the network of the oil drops is difficult to destroy in the milk coffee beverage with DP-95RF by shaking. From these results, we can suggest that once the network is destroyed, it is easy to recover from the milk separation that occurred with P-1670, but the flocculation force is weak. It also requires a long time to recover from the milk separation with DP-95RF, but in this case, the flocculation force is strong. We speculate that strong force is hydrophobic interactions in the milk coffee beverages with DP-95RF. Since the hydrophilic group size of DP-95RF (diglycerol) is smaller than that of P-1670 (sucrose), steric repulsion is weak and hydrophobic interactions become strong. We investigated the influence of the components in the coffee extract solution on the emulsion stability. In many components, we noticed the organic acid and caffeine were representative of the electrolyte and non-electrolyte, respectively. These components were added to the model milk solution. In the presence of the emulsifier, milk separation was observed above the 0.7 wt% concentration of trisodium citrate as the electrolyte (Table 5). Without an emulsifier and with caffeine as the non-electrolyte, the milk separation was not observed. There are two main effects of citrate addition: pH and ionic strength. When we used the trisodium citrate as an example of electrolyte, ionic strength increased but pH did not lower (Table 5). Therefore, it is speculated that the electrolyte in the coffee extract solution influences the emulsion stability, rendering the emulsion unstable. Also citrate can sometimes act as a sequestrant of unbound calcium ions in milk protein emulsions. We think that it occurred in such a situation. However, there are many electrolytes in the coffee extracts besides citrate, so we speculate that these electrolytes influence the milk separation. Especially electrolytes influence dehydration of emulsifier are not that of milk protein from the result, as seen in the Table 5. Moreover, we measured the electric conductivity of the coffee extract solution with the roasted coffee beans of Colombian EX and the Rich2 milk coffee beverage because the electrolyte content increased with the increase in the electric conductivity value. For the coffee extract solution, the values of the electric conductivity were 3.25 mS/cm (LV 18), 3.16 mS/cm (LV 22), and 3.05 mS/cm
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Table 5 Influence of the roasted coffee beans components on the milk separation of 12% UHT sterilized milk and 6% sugar solution at 5 °C (Emulsifier: Ryoto™ Sugar Ester P-1670, milk separation (+), no milk separation ( ), the table in the parentheses indicate pH). Concentration (%)
With emulsifier (0.05%)
Electrolyte (Trisodium citrate)
0 0.1 0.3 0.5 0.7 1.0
(7.00) (7.46) (7.73) (7.80) +(7.85) +(7.90)
Without emulsifier (6.87) (7.40) (7.72) (7.82) (7.90) (7.93)
Non-electrolyte (Caffeine)
1.0 1.5
(6.98) (6.93)
(6.98) (6.94)
Fig. 6. Influence of the electrolytes in coffee extract solution on the hydration for the hydrophilic group of the emulsifier.
(LV 26). For the Rich2 with Indonesian AP-1, the electric conductivity values were 2.26 mS/cm (LV 20) and 2.14 mS/cm (LV 28); for the Rich2 with Colombian EX, they were 2.29 mS/cm (LV 20) and 2.20 mS/cm (LV 28). These results indicate that the coffee extract solution of the roasted beans with the low light value contains many more electrolytes than the solution with the high light value, and these affect the emulsion stability. The effort to emphasize the coffee flavor in the beverage by increasing the amount of the coffee extract solution causes the milk coffee emulsion to become more unstable. We speculate that the electrolytes affect the hydration of the emulsifier, especially for the hydrophilic groups (Fig. 6). By increasing the electrolytes, the dehydration of the sucrose moiety is proceeding in the milk coffee beverage with P-1670 because the electrolytes replace water molecules. As a result, the hydrophilicity of the oil drops is lower, and flocculation can easily occur (Fig. 6a). The milk coffee with S-10D is different, since the increase in the electrolytes results in the dehydration of the decaglycerol moiety.
However, because the decaglycerol monoester has 9 ether groups besides the 11 hydroxyl groups on the molecule, the hydration is kept for the ether groups and the hydrophilicity of the oil drop does not decrease much (Fig. 6b). We suggest that this is the reason for the milk separation in the milk coffee beverage with P-1670 and for the good stability for S-10D.
3.3. Improvement of the emulsion stability by the combination of emulsifiers To control the milk separation in the milk coffee beverage with P-1670, we prepared the Rich2 sample with a combination of P1670 (0.05 wt%) and S-10D (0.1 wt%) for study. A comparison between the milk coffee beverage with P-1670 alone and the milk coffee beverage with the combination of P-1670 and S-10D after the UHT sterilization shows that the former had visible milk separation, but the latter exhibited controlled milk separation.
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Wasan et al. reported that the combination of P-1670 and S-10D in a model milk coffee system made the emulsion stable [7]. They evaluated the effective inter-droplet interactions inside the ring of oil-in-water emulsion using a non-destructive Kossel diffraction technique based on the principle of back light scattering, and the effective energy barrier for emulsifier system was determined from the radial distribution function [8]. The energy barrier for the combined system of P-1670 and S-10D was higher than that for P-1670 system alone, so they concluded the combined system indicated a higher emulsion stability. Moreover, Wasan et al. also reported that the combination of P-1670 and S-570 (RYOTO™ Sugar Ester S-570; HLB 5) in the model milk coffee system [9] made the emulsion stable; they used the Kossel diffraction technique and capillary force balance [10–13] based on the light interference microscopy to study the film-thinning process and the film thickness stability. It was found that a rich milk coffee beverage comprising a sucrose fatty acid ester with an HLB of 10 or higher, a polyglycerol fatty acid ester, and a sucrose fatty acid ester with an HLB less than 10 had a high emulsion stability [14]. The rich milk coffee beverage had a high emulsion stability that was provided by the incorporation of a sucrose fatty acid ester with an HLB of 10 or above, a polyglycerol fatty acid ester with a cloud point of 50 °C or above (as determined in a 20 wt% aqueous solution of sodium chloride at a concentration of 1 wt%), and a sorbitan fatty acid ester having an HLB of 4 or above [15]. It was also effective to use the combined system with an ionic emulsifier. We found that the combination of P-1670 and an ionic emulsifier, such as sodium lactic fatty acid ester or succinyl fatty acid ester, resulted in a more stable emulsion. The electrostatic repulsion may contribute to the emulsion’s stability. A suitable combination of emulsifiers generally produces good results for emulsion stability. This reinforces the interfacial membrane formed on the oil–water interface, because the molecular packing of emulsifiers becomes dense. Arima et al. reported that the combined system of P-1670 and P-170 (RYOTO™ Sugar Ester P-170; HLB 1) in a 20 wt% Palm Mid Fraction-water emulsion controlled the growth of the fat crystallization on the oil–water interface [16]. 4. Conclusions We investigated the emulsion stability of a rich milk coffee beverage with a high content of roasted coffee beans. When the milk coffee beverage with a low light value for the roasted coffee beans sterilized by UHT was stored at a low temperature, the milk component strongly separated. We examined some emulsifiers, since it is well known that emulsifiers improve an emulsion’s stability. We found that the sucrose monoester with a high HLB (16) and diglycerol monoester accelerated the milk separation, and the decaglycerol monoester controlled the milk separation. We also
investigated the microstructure of the separated milk using the oil drop size and distribution measurements, transmission electron microscopy, and viscosity measurement; we found that the network inside the milk separation part is constructed of oil drops with weak interactions. The electrolytes contained in the coffee extract solution were affected by the separation. We discussed the milk separation mechanism and showed that maintaining the hydration of the hydrophilic group in the rich milk coffee beverage was related to the combination of emulsifiers that control the milk separation. Thus, an increase of the roasted coffee beans makes the milk coffee unstable in a certain process or storage conditions. Emulsifiers that possess better steric stability like polyglycerol esters with long hydrophilic part are effective in development of emulsion’s stability. Since rich milk coffee beverages are increasing these days, we hope to apply our results to the related industry. On the other hand, we should investigate the role of the milk proteins, because we speculate that the conformation of proteins, especially whey proteins, is different between UHT sterilization and retort one. This study provides a basis for the solution of some issues with the development of a coffee-rich milk coffee beverage. Acknowledgments We would like to thank the members of our team. We would also like to acknowledge Dr. D. Wasan and Dr. A. Nikolov for their helpful discussions. We congratulate Dr. Wasan on his remarkable achievements in the field of the colloid and surface science. References [1] D.J. McClements, Food Emulsions: Principles, Practices and Techniques, second ed., CRC, Boca Raton, FL, 2005. [2] E. Dickinson, D. Lorient, Food Macromolecules and Colloids, The Royal Society of Chemistry, Cambridge, UK, 1995. [3] E. Dickinson, Introduction to Food Colloids, Oxford University Press, 1992. [4] US Patent 7150893. [5] J.E. Rash, C.S. Hudson, Freeze Fracture: Methods, Artifacts, and Interpretations, Raven Press, New York, 1979. [6] J.-P. Krause, W. Buchheim, Die Nahrung 38 (5) (1994) 455–463. [7] Y. Kong, A. Nikolov, D. Wasan, A. Ogawa, J. Disp. Sci. Technol. 27 (2006) 579– 585. [8] W. Xu, A. Nikolov, D. Wasan, A. Gonsalves, R. Borwankar, J. Food Sci. 63 (2) (1998) 183–188. [9] Y. Kong, A. Nikolov, D. Wasan, A. Ogawa, Ind. Eng. Chem. Res. 47 (23) (2008) 9108–9114. [10] A. Nikolov, D. Wasan, Colloids Surf. A 123 (124) (1997) 375–381. [11] D.T. Wasan, K. Koczo, A. Nikolov, in: A.G. Gaonkar (Ed.), Characterization of Food Emerging Methods, Elsevier Science B.V., 1995, p. 1. [12] A. Nikolov, D. Wasan, S. Friberg, Colloids Surf. A 118 (1996) 221. [13] K. Koczo, A. Nikolov, D. Wasan, R. Borwankar, A. Gonsalves, J. Colloids Int. Sci. 178 (1996) 694. [14] PCT Patent WO 2004049813. [15] PCT Patent WO 2004054382. [16] S. Arima, T. Ueji, S. Ueno, A. Ogawa, K. Sato, Colloids Surf. B 55 (2007) 98.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Role of food emulsifiers in milk coffee beverages A. Ogawa ⇑, H. Cho Mitsubishi-Kagaku Foods Corporation, Yokohama, Japan
g r a p h i c a l a b s t r a c t O CH2O
(CH2) 14CH3
C O
H
CH2OH
H
H HO
O
HO H
“Milk coffee beverage with rich coffee extracts”
O
H
H
OH
OH
OH
CH2OH
H
“electrolytes” δ
᧧
H
-OOCR
OH
O
H
K+ H
H
O H
O H H
O O
“High hydrophilicity” H
a r t i c l e
O H
O H
O
OH
CHCH2 CH2CHCH2 OH
O
H
H
K+ H
OH
O
OOCR
O
H
HOH2C
H -
OH
O
O
H
CH2 CH
CH2O
C(CH2)16CH3
8
i n f o
Article history: Received 23 October 2014 Accepted 23 January 2015 Available online 30 January 2015 Keywords: Beverages Coffee Emulsion Emulsifier Stability Electrolyte
a b s t r a c t To emphasize the coffee flavor, many milk coffee beverages contain coffee extracts; these are the socalled ‘‘rich milk coffee’’ beverages. When the content of the coffee extracts increases, milk coffee beverages become unstable. The milk ring formation, or oiling off, is accelerated in these kinds of drinks. We prepared a ‘‘rich milk coffee’’ beverage and studied the stability of the emulsion. We also investigated the influence of the food emulsifiers on the stability of the emulsion. We tried to test the emulsifier system in order to improve the emulsion stability. When the milk coffee beverage with a low light value for the roasted coffee beans sterilized by UHT was stored at a low temperature, the milk component strongly separated. We found that the sucrose monoester with a high HLB and diglycerol monoester accelerated the milk separation, and the decaglycerol monoester controlled the milk separation. We discussed the milk separation mechanism and showed that maintaining the hydration of the hydrophilic group in the rich milk coffee beverage was related to the combination of emulsifiers that control the milk separation. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction Beverages defined as RTD (‘‘Ready to Drink’’), such as canned or bottled milk coffee, contain milk components that have been subjected to a heat sterilization treatment for subsequent storage. Milk coffee beverages contain about 1% milk fat, forming an oilin-water emulsion. Since the storage term for these milk coffee ⇑ Corresponding author. Fax: +81 45 963 3976. E-mail address: [email protected] (A. Ogawa). http://dx.doi.org/10.1016/j.jcis.2015.01.063 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
beverages is 6–12 months, the milk fat gradually floats towards the air–emulsion interface. Aggregation or flocculation [1–3] follows, and a milk ring is formed on the air–emulsion interface. Aggregation means sticking together of solid particles (like hard spheres) and flocculation means the association of polymer-coated particles (or droplets). Since milk proteins that are biopolymer adsorb on the surface of the oil droplet in milk coffee beverages system, we used the ‘‘flocculation’’ not ‘‘aggregation’’ in this paper. When flocculation happens, even if the milk coffee beverage is repeatedly shaken, the milk ring will not re-disperse and some
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lumps of the milk fat will float on the surface. If the degree of flocculation is severe, coagulation occurs in the milk fat and this leads to the separation of the oil and water (oiling off). If this oil sticks to the can or plastic bottle, consumers will get the impression that these beverages are of a low quality and will no longer purchase them. Milk coffee beverages constitute a large percentage of the Japanese soft drink market. The scale of Japanese soft drink market is over 20,000 thousands kl/year. And the scale of Japanese coffee beverage market is 3000 thousands kl/year. To emphasize the coffee flavor, many milk coffee beverages contain coffee extracts; these are the so-called ‘‘rich milk coffee’’ beverages. When the content of the coffee extracts increases, milk coffee beverages become unstable. The milk ring formation, or oiling off, is accelerated in these kinds of drinks. This situation is aggravated by the fact that the plastic bottle is transparent: consumers can see that the milk fat has separated, so the product value depreciates and consumers complain. We prepared a ‘‘rich milk coffee’’ beverage and studied the stability of the emulsion. We also investigated the influence of the food emulsifiers on the stability of the emulsion. We tried to test the emulsifier system in order to improve the emulsion stability. Fig. 1. Emulsifier structures.
2. Materials and methods 2.1. Preparation of milk coffee beverages Roasted coffee beans were purchased from Unicafe Inc. (Japan). UHT sterilized milk was purchased from the Japanese supermarket, it contains 3.3% protein, 3.8% fat, 4.8% lactose, 0.7% minerals (Sodium: 41 mg, Potassium: 150 mg, Calcium: 110 mg per 100 g milk) and 87.4% water. We used the sucrose fatty acid ester sample of Mitsubishi-Kagaku Foods Corp. (Japan). Two milk coffee beverages were prepared. We defined ‘‘Rich1’’ as the milk coffee beverage containing 6.5 wt% coffee beans (as unroasted coffee beans) and 8 wt% milk. ‘‘Rich2’’ is another milk coffee beverage that contains 7.0 wt% coffee beans (as unroasted coffee beans) and 12 wt% milk. The two kinds of roasted coffee beans used were Coffea Arabica (Colombian EX) and Robusta (Indonesian AP-1). As the roasting of coffee beans is proceeding, the color of coffee beans change into dark brown. We generally use the Hunter’s Lab color space as an index of the degree of roasting, because the degree of roasting correlates closely with the light value. We can easily measure the light value with the Hunter color difference meter. The light values (LV) of 20 and 28 were used for ‘‘Rich 1’’, and that of 20, 22, 24, 26 and 28 were used for ‘‘Rich 2’’ milk coffee beverages. Sucrose palmitic acid ester (RYOTO™ Sugar Ester P-1670; HLB 16, with a monoester content of 80%) was used as an emulsifier. Roasted coffee beans were extracted with 10 times de-ionized water as much as roasted coffee beans heated to 95 °C, thereby obtaining a coffee extract solution. The thus obtained coffee extract solution was mixed with milk and granulated sugar and further
Table 1 Composition of milk coffee beverages. Ingredient
Rich1
Rich2
Coffee beans (as unroasted beans) Milk Sugar Emulsifier (Ryoto™ Sugar Ester P-1670) pH (Sodium bicarbonate) Water
6.5% 8.0% 6.0% 0.04% 6.9 79.46%
7.0% 12.0%
Fat Protein
0.30% 0.26%
0.46% 0.40%
6.6 74.96%
with an emulsifier. Then, de-ionized water was added to the mixture to obtain emulsion. After sodium bicarbonate was added to the emulsion to adjusting the pH, the emulsion was intimately mixed and homogenized at a temperature of 60–70 °C under a pressure of 15 MPa/50 MPa using a high-pressure homogenizer. Thereafter, the obtained emulsion was sterilized at 121 °C for 40 min using a retort sterilizer in the case of the canned milk coffee beverages, or was sterilized at 137 °C for 60 s (hold time for sterilization) using a plate-type Ultra High Treated (UHT) pasteurizer in the case of the plastic bottled milk coffee beverages, thereby obtaining a milk coffee beverages [4]. The composition of these milk coffee beverages is depicted in Table 1.
2.2. Preparation of milk coffee beverages with various emulsifiers The plastic bottled milk coffee of ‘‘Rich2’’ that contained Colombian EX (LV 26) was prepared by UHT sterilization. Sucrose palmitic acid ester (RYOTO™ Sugar Ester P-1670; HLB 16 (Mitsubishi-Kagaku Foods Corp., Japan)), diglycerol palmitic acid ester (POEM™ DP-95RF; HLB 8 (Riken Vitamin Co., Ltd., Japan)), and decaglycerol stearic acid ester (RYOTO™ polyglycerol Ester S-10D, HLB 14 (Mitsubishi-Kagaku Foods Corp., Japan)) were used as emulsifiers. The structures of these emulsifiers are depicted in Fig. 1.
2.3. Evaluation of milk coffee beverages 2.3.1. Oil drop size and distribution measurements The median oil drop size and drop size distribution were measured using a LA-500 Laser Diffraction Particle Size Analyzer with a flow cell unit (HORIBA, Japan). The measurement of the droplet size analysis uses the Mie light scattering theory. The scattered light strength is decided by the particle diameter parameter a (a = pD/k; p: particles circumference length, D: single particle diameter, k: incoming light’s wavelength) and the particle’s diffractive index m. Particle size distributions are calculated based on the collected scattered light strength’s angle distribution value. Milk coffee samples were poured into the cup of the analyzer and were diluted with de-ionized water. The median oil drop size
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and drop size distribution were calculated with the circulation measurement. 2.3.2. Transmission electron microscopy [5] The samples were rapidly frozen at 210 °C using a JFD-380P slushy nitrogen production device (JEOL Ltd., Japan). Freeze fracture replica preparations were obtained by making reference to the method described by Buchheim [6] using JFD-9010 freeze etching equipment (JEOL Ltd., Japan). The samples were fractured at 165 °C, shadowed with 2 nm of platinum/carbon under an angle of 45°, and coated with 20 nm of carbon. The replicas were cleaned in concentrated sodium hypochlorite and distilled water. All samples were examined with a H-9000NA transmission electron microscope (Hitachi Ltd., Japan) operating at 100 kV accelerating voltage. 2.3.3. Viscosity measurements The viscous behaviors of the milk coffee and milk separation samples were measured using a dynamic shear rheometer with a 4 °C/40 mm cone rotor and the temperature module of the plane plate (Constant Stress Rheometer, CS-10, Bohlin Instruments Ltd., USA). Samples were placed in the temperature-controlled measurement plate and allowed to equilibrate to the required temperature (5 °C or 40 °C) for 3 min prior to making the measurements. The viscosity was acquired as a function of the shear stress (0.06–0.6 Pa). 2.3.4. Preparation of milk solution with coffee components The model milk solution was prepared by dispersing 12 wt% UHT sterilized milk, 6 wt% sugar, and 0.05 wt% emulsifier in water. Sodium citrate (as an organic acid) or caffeine was added to the model milk solution. The concentration of sodium citrate was in the range of 0.1–1.0% and that of caffeine was 1.0–1.5. After stirring, we observed the milk separation. 2.3.5. Electric conductivity measurements Electric conductivity for the extracts of the roasted coffee beans (LV 18, 22, and 26) and for the two milk coffee samples with Indonesian AP-1 (LV 20 and 28) and Colombian EX (LV 20 and 28) was measured using an electric conductivity meter CM-60G with a CT57101C cell (DKK-TOA, Japan). The measurement temperature for the extract solution of the roasted coffee beans was 22 °C and 10 °C for the milk coffee samples. 3. Results and discussion 3.1. Stability of the milk coffee system After the initial preparation, the milk coffee beverages were stored at 5 °C and 40 °C; the emulsion stability was then studied by visual observation. The results are summarized in Table 2.
Canned milk coffee prepared by retort sterilization (used for both Rich1 and Rich2) saw little milk separation, and the milk coffee beverages (which are oil-in-water emulsions) were stable regardless of the kinds of roasted coffee beans or storage temperature. However, the plastic bottled milk coffee beverage prepared by UHT sterilization exhibited strong milk separation with Colombian EX under the conditions of LV 20 at 5 °C; however, the 40 °C storage samples had little milk separation. The milk coffee beverage with Indonesian AP-1 had observable milk separation at 5 °C for both Rich1 and Rich2. The plastic bottled milk coffee beverage with the LV 20 roasted coffee beans exhibited more milk separation than that with the LV 28 roasted coffee beans. It exhibited especially violent separation: there was a complete separation of the milk and coffee that occurred with the LV 20 in Rich2. Milk separation in the samples was observed, even at 40 °C with the LV 20 roasted coffee beans. In order to investigate the influence of the light value, we observed the effects of light on Rich2 with various light values for the Colombian EX beans. Table 3 indicates that milk separation was observed at below LV 24 for these roasted beans. Actually, milk separation occurred through the flocculation of the oil drops (Fig. 2). After the flocculation of the oil drops advanced, these floated either gradually or rapidly to the air–emulsion interface and formed a thick milk layer. When we measured the electrical conductivity of the coffee extracts with the different roasted coffee beans, the lower light value of the coffee beans meant the higher electrical conductivity. We think this means low light value of the coffee extract contains high amount of electrolyte. And we presume that there is the border of milk separation between LV 24 and LV 26 for Columbian EX in the UHT sterilized milk coffee beverages system. It is speculated that an increase of electrolyte caused the milk separation. We found that the milk coffee beverages became unstable under the conditions of UHT sterilization, low storage temperatures, and the low light values of the Robusta beans. These conditions lead to milk separation. The stability of Rich1 was evaluated by measuring the drop size distributions (Table 4). The median size of the drops was 0.44–0.49 lm. After the milk coffee preparation for two different roasted coffee beans, the drop size distribution was almost the same as that of the retort sterilization canned coffee with LV 20 for the Indonesian AP-1 (it had a slightly wide drop size distribution). The milk separation part of Rich1 with a LV 20 for the Indonesian AP-1 sterilized by UHT was evaluated by measuring the droplet size and drop size distributions. The median size of the drops was 0.44 lm, and the drop size distribution was almost same as that of the results shown the above. When the plastic bottled milk coffee beverage was reconstituted by dispersion, this separated milk was evaluated with the same method for the milk separation part; the median size of the drops and the drop size distribution were the same. This result suggests that oil drops interact with
Table 2 Degree of the milk layer separation for various milk coffee (1–2 mm ( ), 5 mm (+), strong separation (++), violent separation (+++)). Sterilization
UHT
Roasted coffee beans
Indonesia AP-1 Columbia EX
Retort
Indonesia AP-1 Columbia EX
Temperature (°C)
5 40 5 40 5 40 5 40
Rich1
Rich2
LV 20
LV 28
LV 20
LV 28
++
+
+++ + ++
+
++
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A. Ogawa, H. Cho / Journal of Colloid and Interface Science 449 (2015) 198–204 Table 3 Influence of the light value of the roasted beans on the milk separation in the Rich2 milk coffee (1–2 mm ( ), strong separation (++)). Sterilization
Temperature (°C)
Light value of the roasted beans (Columbia Ex) 20
22
24
UHT
5 40
++
++
++
Retort
5 40
26
28
Fig. 2. Course of milk separation (UHT sterilization, Indonesian AP-1 (LV 20), 5 °C storage).
Table 4 The median sizes of Rich1 with Indonesian AP-1 after sterilization. Sterilization
UHT Retort
very weak forces with each other, and can be re-dispersed by slight shaking or stirring.
Light value of roasted coffee beans 20 (lm)
28 (lm)
0.49 0.46
0.44 0.44
3.2. Influence of the emulsifier on milk separation We tested the stability of the plastic bottled milk coffee beverage with a LV 26 for the Colombian EX by adding the three kinds of
Fig. 3. Freeze replica TEM images of the milk separation part: (a) P-1670 and (b) DP-95RF.
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emulsifiers. The plastic bottled milk coffee beverage with P-1670 and DP-95RF exhibited violent milk separation. The milk coffee beverage with S-10D and without the emulsifier did not seem to have any obvious visible milk separation. All four samples exhibited almost the same drop size distribution; the median sizes of the drops were 0.49 lm (for S-10D), 0.52 lm (for P-1670), 0.53 lm (for the sample without an emulsifier), and 0.55 lm (for DP-95RF). According to Table 3, the plastic bottled Rich2 with a LV 26 for the Colombian EX prepared by UHT sterilization was stable. However, because the pH (6.3) was not adjusted in this case, the emulsion seemed to become unstable, and milk separation occurred depending on the kind of emulsifier used. The emulsifier used for Rich2 that resulted in the violent milk separation has a relatively small hydrophilic group compared with that of the hydrophobic group (Fig. 1). Therefore, it is suggested that as the hydrophilicity of the oil droplet decreases, flocculation more easily occurs between the oil droplets. In order to observe the microstructure for the milk separation part, a transmission electron microscope (TEM) was used and the freeze fracture samples for the P-1670 and DP-95RF were observed. These TEM image are depicted in Fig. 3. It was found that each oil drop changed shape for the sample with P-1670. This, however, did not lead to coalescence. Some large oil drops that were coalescent were observed for the sample with DP-95RF. The microstructures of both the P-1670 and DP-95RF sample were characterized by the diamond shaped continuous network structure that was formed by the flocculation of the oil drops. When these milk separation parts were re-dispersed and the milk coffee beverage was reconstituted, the flocculates were not observed in the TEM images. From these results, it is speculated that the oil drops formed the regular flocculates by weak interactions in the separated milk layer. We assume that this weak interactions is originated from the dehydration of the hydrophilic part of emulsifier, as a result the oil droplets flocculated by hydrophobic interaction. The viscous behavior and the formation of the flocculates were investigated by measuring the viscosity of the milk separation part using a rheometer (Fig. 4). The shear stress was increased to 0.6 Pa, and then was decreased to 0.06 Pa. With the increase of the shear stress, the viscosity at 5 °C was gradually decreased for the milk separation part with P-1670. The viscosity recovered with the decrease in the shear stress. This indicates that the destroyed network is easy to recover, and shows that this milk separation part has thixotropy. The milk coffee beverage that had a re-dispersed separation part (viscosity at 5 °C) or the slight milk separation part with P-1670 (viscosity at 40 °C) did not have the property of thixotropy. These flocculates in the milk separation part were destroyed by a high storage temperature or slight shaking. The viscosity of the milk separation part with DP-95RF at 5 °C also showed almost the same behavior as that in the case of P1670 in regards to the increase in the shear stress (Fig. 5). However,
Fig. 4. Rheological properties of the milk coffee beverage with P-1670.
Fig. 5. Rheological properties of the milk coffee beverage with DP-95RF.
in the case of the decrease of the shear stress, the viscosity did not recover to its beginning value. This indicates that the destroyed network is not easy to recover. This result shows the milk separation part with DP-95RF also has thixotropy, but the flocculates are not immediately formed again. The milk coffee beverage that was re-dispersed with this milk separation part (viscosity at 5 °C) showed similar viscous behavior to that of the milk separation part of DP-95RF at 5 °C. It is speculated that the flocculation force is strong between the oil drops: the network of the oil drops is difficult to destroy in the milk coffee beverage with DP-95RF by shaking. From these results, we can suggest that once the network is destroyed, it is easy to recover from the milk separation that occurred with P-1670, but the flocculation force is weak. It also requires a long time to recover from the milk separation with DP-95RF, but in this case, the flocculation force is strong. We speculate that strong force is hydrophobic interactions in the milk coffee beverages with DP-95RF. Since the hydrophilic group size of DP-95RF (diglycerol) is smaller than that of P-1670 (sucrose), steric repulsion is weak and hydrophobic interactions become strong. We investigated the influence of the components in the coffee extract solution on the emulsion stability. In many components, we noticed the organic acid and caffeine were representative of the electrolyte and non-electrolyte, respectively. These components were added to the model milk solution. In the presence of the emulsifier, milk separation was observed above the 0.7 wt% concentration of trisodium citrate as the electrolyte (Table 5). Without an emulsifier and with caffeine as the non-electrolyte, the milk separation was not observed. There are two main effects of citrate addition: pH and ionic strength. When we used the trisodium citrate as an example of electrolyte, ionic strength increased but pH did not lower (Table 5). Therefore, it is speculated that the electrolyte in the coffee extract solution influences the emulsion stability, rendering the emulsion unstable. Also citrate can sometimes act as a sequestrant of unbound calcium ions in milk protein emulsions. We think that it occurred in such a situation. However, there are many electrolytes in the coffee extracts besides citrate, so we speculate that these electrolytes influence the milk separation. Especially electrolytes influence dehydration of emulsifier are not that of milk protein from the result, as seen in the Table 5. Moreover, we measured the electric conductivity of the coffee extract solution with the roasted coffee beans of Colombian EX and the Rich2 milk coffee beverage because the electrolyte content increased with the increase in the electric conductivity value. For the coffee extract solution, the values of the electric conductivity were 3.25 mS/cm (LV 18), 3.16 mS/cm (LV 22), and 3.05 mS/cm
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Table 5 Influence of the roasted coffee beans components on the milk separation of 12% UHT sterilized milk and 6% sugar solution at 5 °C (Emulsifier: Ryoto™ Sugar Ester P-1670, milk separation (+), no milk separation ( ), the table in the parentheses indicate pH). Concentration (%)
With emulsifier (0.05%)
Electrolyte (Trisodium citrate)
0 0.1 0.3 0.5 0.7 1.0
(7.00) (7.46) (7.73) (7.80) +(7.85) +(7.90)
Without emulsifier (6.87) (7.40) (7.72) (7.82) (7.90) (7.93)
Non-electrolyte (Caffeine)
1.0 1.5
(6.98) (6.93)
(6.98) (6.94)
Fig. 6. Influence of the electrolytes in coffee extract solution on the hydration for the hydrophilic group of the emulsifier.
(LV 26). For the Rich2 with Indonesian AP-1, the electric conductivity values were 2.26 mS/cm (LV 20) and 2.14 mS/cm (LV 28); for the Rich2 with Colombian EX, they were 2.29 mS/cm (LV 20) and 2.20 mS/cm (LV 28). These results indicate that the coffee extract solution of the roasted beans with the low light value contains many more electrolytes than the solution with the high light value, and these affect the emulsion stability. The effort to emphasize the coffee flavor in the beverage by increasing the amount of the coffee extract solution causes the milk coffee emulsion to become more unstable. We speculate that the electrolytes affect the hydration of the emulsifier, especially for the hydrophilic groups (Fig. 6). By increasing the electrolytes, the dehydration of the sucrose moiety is proceeding in the milk coffee beverage with P-1670 because the electrolytes replace water molecules. As a result, the hydrophilicity of the oil drops is lower, and flocculation can easily occur (Fig. 6a). The milk coffee with S-10D is different, since the increase in the electrolytes results in the dehydration of the decaglycerol moiety.
However, because the decaglycerol monoester has 9 ether groups besides the 11 hydroxyl groups on the molecule, the hydration is kept for the ether groups and the hydrophilicity of the oil drop does not decrease much (Fig. 6b). We suggest that this is the reason for the milk separation in the milk coffee beverage with P-1670 and for the good stability for S-10D.
3.3. Improvement of the emulsion stability by the combination of emulsifiers To control the milk separation in the milk coffee beverage with P-1670, we prepared the Rich2 sample with a combination of P1670 (0.05 wt%) and S-10D (0.1 wt%) for study. A comparison between the milk coffee beverage with P-1670 alone and the milk coffee beverage with the combination of P-1670 and S-10D after the UHT sterilization shows that the former had visible milk separation, but the latter exhibited controlled milk separation.
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Wasan et al. reported that the combination of P-1670 and S-10D in a model milk coffee system made the emulsion stable [7]. They evaluated the effective inter-droplet interactions inside the ring of oil-in-water emulsion using a non-destructive Kossel diffraction technique based on the principle of back light scattering, and the effective energy barrier for emulsifier system was determined from the radial distribution function [8]. The energy barrier for the combined system of P-1670 and S-10D was higher than that for P-1670 system alone, so they concluded the combined system indicated a higher emulsion stability. Moreover, Wasan et al. also reported that the combination of P-1670 and S-570 (RYOTO™ Sugar Ester S-570; HLB 5) in the model milk coffee system [9] made the emulsion stable; they used the Kossel diffraction technique and capillary force balance [10–13] based on the light interference microscopy to study the film-thinning process and the film thickness stability. It was found that a rich milk coffee beverage comprising a sucrose fatty acid ester with an HLB of 10 or higher, a polyglycerol fatty acid ester, and a sucrose fatty acid ester with an HLB less than 10 had a high emulsion stability [14]. The rich milk coffee beverage had a high emulsion stability that was provided by the incorporation of a sucrose fatty acid ester with an HLB of 10 or above, a polyglycerol fatty acid ester with a cloud point of 50 °C or above (as determined in a 20 wt% aqueous solution of sodium chloride at a concentration of 1 wt%), and a sorbitan fatty acid ester having an HLB of 4 or above [15]. It was also effective to use the combined system with an ionic emulsifier. We found that the combination of P-1670 and an ionic emulsifier, such as sodium lactic fatty acid ester or succinyl fatty acid ester, resulted in a more stable emulsion. The electrostatic repulsion may contribute to the emulsion’s stability. A suitable combination of emulsifiers generally produces good results for emulsion stability. This reinforces the interfacial membrane formed on the oil–water interface, because the molecular packing of emulsifiers becomes dense. Arima et al. reported that the combined system of P-1670 and P-170 (RYOTO™ Sugar Ester P-170; HLB 1) in a 20 wt% Palm Mid Fraction-water emulsion controlled the growth of the fat crystallization on the oil–water interface [16]. 4. Conclusions We investigated the emulsion stability of a rich milk coffee beverage with a high content of roasted coffee beans. When the milk coffee beverage with a low light value for the roasted coffee beans sterilized by UHT was stored at a low temperature, the milk component strongly separated. We examined some emulsifiers, since it is well known that emulsifiers improve an emulsion’s stability. We found that the sucrose monoester with a high HLB (16) and diglycerol monoester accelerated the milk separation, and the decaglycerol monoester controlled the milk separation. We also
investigated the microstructure of the separated milk using the oil drop size and distribution measurements, transmission electron microscopy, and viscosity measurement; we found that the network inside the milk separation part is constructed of oil drops with weak interactions. The electrolytes contained in the coffee extract solution were affected by the separation. We discussed the milk separation mechanism and showed that maintaining the hydration of the hydrophilic group in the rich milk coffee beverage was related to the combination of emulsifiers that control the milk separation. Thus, an increase of the roasted coffee beans makes the milk coffee unstable in a certain process or storage conditions. Emulsifiers that possess better steric stability like polyglycerol esters with long hydrophilic part are effective in development of emulsion’s stability. Since rich milk coffee beverages are increasing these days, we hope to apply our results to the related industry. On the other hand, we should investigate the role of the milk proteins, because we speculate that the conformation of proteins, especially whey proteins, is different between UHT sterilization and retort one. This study provides a basis for the solution of some issues with the development of a coffee-rich milk coffee beverage. Acknowledgments We would like to thank the members of our team. We would also like to acknowledge Dr. D. Wasan and Dr. A. Nikolov for their helpful discussions. We congratulate Dr. Wasan on his remarkable achievements in the field of the colloid and surface science. References [1] D.J. McClements, Food Emulsions: Principles, Practices and Techniques, second ed., CRC, Boca Raton, FL, 2005. [2] E. Dickinson, D. Lorient, Food Macromolecules and Colloids, The Royal Society of Chemistry, Cambridge, UK, 1995. [3] E. Dickinson, Introduction to Food Colloids, Oxford University Press, 1992. [4] US Patent 7150893. [5] J.E. Rash, C.S. Hudson, Freeze Fracture: Methods, Artifacts, and Interpretations, Raven Press, New York, 1979. [6] J.-P. Krause, W. Buchheim, Die Nahrung 38 (5) (1994) 455–463. [7] Y. Kong, A. Nikolov, D. Wasan, A. Ogawa, J. Disp. Sci. Technol. 27 (2006) 579– 585. [8] W. Xu, A. Nikolov, D. Wasan, A. Gonsalves, R. Borwankar, J. Food Sci. 63 (2) (1998) 183–188. [9] Y. Kong, A. Nikolov, D. Wasan, A. Ogawa, Ind. Eng. Chem. Res. 47 (23) (2008) 9108–9114. [10] A. Nikolov, D. Wasan, Colloids Surf. A 123 (124) (1997) 375–381. [11] D.T. Wasan, K. Koczo, A. Nikolov, in: A.G. Gaonkar (Ed.), Characterization of Food Emerging Methods, Elsevier Science B.V., 1995, p. 1. [12] A. Nikolov, D. Wasan, S. Friberg, Colloids Surf. A 118 (1996) 221. [13] K. Koczo, A. Nikolov, D. Wasan, R. Borwankar, A. Gonsalves, J. Colloids Int. Sci. 178 (1996) 694. [14] PCT Patent WO 2004049813. [15] PCT Patent WO 2004054382. [16] S. Arima, T. Ueji, S. Ueno, A. Ogawa, K. Sato, Colloids Surf. B 55 (2007) 98.