Effect of Different Treatment on the Properties of Coconut Milk Emulsions

Transcription

1 Food Science and Technology Research, 22 (1), 83 _ 89, 2016 Copyright 2016, Japanese Society for Food Science and Technology doi: /fstr Original paper Effect of Different Treatment on the Properties of Coconut Milk Emulsions Peipei Jiang, Dong Xiang * and Xibin Wang Food Science Research Group, Food Science College, Hainan University, No. 58 People Road, Haikou, Hainan Province, China Received March 8, 2015 ; Accepted September 9, 2015 Droplet size has an important effect on the stability and sensory quality of emulsions. This study aimed to analyze the effect of different treatments, including treatment under different temperatures and shearing, on the properties of coconut milk emulsions. Various coconut oil droplet sizes of coconut milk emulsion droplets after different treatments were investigated by observing the droplet sizes and micrographs. The stability of these samples was investigated by the creaming index of the emulsions. Result showed that heating increased the average droplet size of coconut milk. Freezing decreased the average droplet size of the emulsion to a minimum of 7.67 µm. Highspeed shearing increased the average droplet size of the emulsion to a maximum of µm. The creaming index of the emulsion squeezed from coconut meat that underwent freezing treatment was 35.10%, which was the minimum value. Thus, freezing treatment on coconut meat could induce the size of the emulsion droplets and improve the stability of the emulsions. Keywords: coconut milk, particle size, pretreatment, stability Introduction An emulsion is a mixture of two immiscible liquids, in which one is the dispersed or internal phase comprising small spherical droplets, and the other is the continuous, external phase. In food systems, the two liquid phases are usually oil and water. Food emulsions can be categorized as oil-in-water or water-in-oil depending upon which phase is continuous (Ariyaprakai and Tananuwong, 2015; Qiao et al., 2015; Schmidt et al., 2015). Coconut milk is a natural oil-in-water emulsion extracted from the endosperm of mature coconut, and it contains about 54% moisture, 35% fat, and 11% solid non-fat (Ng et al., 2014; Saikhwan et al., 2015; Zhu et al., 2014). Coconut milk has been used as an important ingredient for Asian cuisine, as well as in other parts of the world because of its unique flavor and other desirable sensory characteristics (Iguttia et al., 2011). In China, coconut milk is an important ingredient for beverages. According to statistics, about 25% of the outputs of coconuts are consumed as coconut milk (Marina et al., 2009; Tipvarakarnkoon et al., 2010). Coconut milk is stabilized by the coconut proteins and phospholipids (Raghavendra and Raghavarao, 2010). Similar to all emulsion, fresh coconut milk is not physically stable and is prone to phase separation. Within 5 _ 10 h of manufacture, coconut milk will separate into cream and serum layers, known as coconut cream and coconut skim milk, respectively. However, the separated milk can be re-homogenized by shaking. Sometimes it will separate into three or four phases, which are grey precipitate layer, serum layers, cream layer and oil layer, respectively (Chambal et al., 2012; Ng et al., 2014). Many of the properties of an emulsion, i.e., stability, appearance, and texture, depend on the droplet size. The major reason for the instability of coconut milk emulsion is its poor emulsifying properties, low surface activity of coconut protein, and large droplet size (Chambal et al., 2012). Proteins are absorbed at the surface of the droplets and provide repulsive interactions (e.g., electrostatic and steric) that help prevent droplet aggregation (Tangsuphoom and Coupland, 2009a). The emulsifying properties of proteins are influenced by their structures, which are affected by environmental factors (e.g., ph, temperature) *To whom correspondence should be addressed. foods2003@126.com.

2 84 (Ariyaprakai et al., 2013; Ariyaprakai and Tananuwong, 2015; Neta et al., 2012; Tangsuphoom and Coupland, 2009b). In China, coconut milk squeezed from coconut meat as raw material is usually used as a beverage. In factories, coconut meat is processed within 8 h without cooling. Therefore studies on the effect of low temperature on the properties of coconut milk are needed. Coconut meat is usually treated at 80 prior to squeezing, which can inactivate the coconut protein enzyme in the material. During coconut milk production, coconut milk needs to be subjected to high-speed shear homogeneous pretreatment. Generally, this type of pretreatment may enhance the stability of the product. In this study, the authors investigated the effects of different pretreatment temperatures on coconut meat by measuring the fat content, droplet size, and creaming index. Moreover, microstructures were observed using optical microscopy. The authors also investigated the influence of high-speed shearing process on the droplet size and stability of coconut milk. Materials and Methods Materials Whole mature coconuts (aged 8 _ 10 months) were purchased from a local retailer. Sample preparation Mature coconuts were shelled and peeled using a traditional coconut cutter, and the fresh white coconut meat was left in the shells. Under normal temperature treatment at 30, coconut meat was used to squeeze coconut milk directly without any other pretreatment. In heating process, coconut meat was treated in a water bath (HH4 digital constant temperature water bath pot, Changzhou Aohua instrument Co., Ltd., Changzhou, China) at 80 for 15 min. During low temperature treatment (4 and _ 18 ), coconut meat was placed in the refrigerator for 24 h. The different pretreatments are presented in Table 1. Coconut milk was produced by squeezing the shredded coconut meat and filtering it through a piece of double-layered cheesecloth. Under machine treatment, coconut milk was sheared by a highspeed shearing machine (FJ-200, Shanghai Specimen model factory, Shanghai, China) at a rotation of 11,000 r/min for 1 min. Determination of fat content of coconut milk Aliquots of coconut milk (10 ml) were transferred into a liposuction bottle. Approximately 1.25 ml of aqueous ammonia was added into the bottle, which was fully mixed. The bottle was treated in a water bath at 60 for 5 min, shaken manually for 2 min, added with 10 ml of ethanol, and mixed thoroughly. Subsequently, 25 ml of diethyl ether were added into the bottle, shaken for 0.5 min, 25 ml of light petroleum were then added followed by shaking for 0.5 min, and let stand for 30 min. The volume of the top layer was measured. Some liquid in the ether layer was transferred to an empty flask by sucking the liquid gently. The flask was distilled to recycle ether and petroleum ether. After being placed in a loft drier for 1.5 h, the sample was weighed. This process was repeated until the flask reached a constant weight. The fat content was calculated using Eq. (1). P. Jiang et al. Fat content = (m 1 _ m 2 ) / (m*v 1 /v)*100% Eq. 1 Where m 1 is the mass of the empty flask (g), m 2 is the mass of the flask and fat, m is the mass of sample, v is the volume of the measured ether layer, and v 1 is the volume of the liquid transferred to the flask. Table 1. Coconut meat with different thermal treatments Code Determination of the particle size of coconut milk emulsion droplets The mean size of coconut milk emulsion droplets was measured using a laser particle analyzer (WJL-602, Shanghai instrument physical optics instrument Co., LTD., Shanghai, China). Set the continuous phase as water, its relative index is 1.33, and the dispersed phase as coconut oil. Then samples of coconut milk (~0.5 ml) were placed into the sample cell with 400 ml distilled water in it. The droplet size was given by D 10, D 90 and Dav, which are the representative diameters of 10%, 90% and the mean diameter of coconut milk emulsion droplets, respectively. The droplet size distribution determination was based on the Mie scattering theory. Microscopy Samples of coconut milk (~1 μl) were placed on a microscope slide, gently covered with a cover slip, and observed at 100 magnification using an optical microscope (OPTEC BDM500, Chongqing Optec instrument Co.,Ltd., Chongqing, China) equipped with a color video camera. The optical micrographs were analyzed using image analysis software (OPTPro2012, Chongqing Optec instrument Co.,Ltd., Chongqing, China). Pictures were obtained from different fields on each slide, and representative images are presented. Evaluation of physical stability About 10 ml of coconut milk, including non-sheared and sheared emulsion, was transferred to flat-bottomed test tubes. These tubes were then covered and incubated at 30 for 12 h. All samples resulted in the formation of two phases, namely, the opaque layer at the top and transparent aqueous phase at the bottom during storage. Upon measuring the height of the bottom layer, the creaming index was calculated using equation Eq. (2): Creaming index = h / H *100% Eq. 2 Where h means the height of the bottom layer, and H means the total height of coconut milk in the tube. The smaller the creaming index, the more stable the coconut milk. Thermal treatment A Normal temperature at 30 B Chilling at 4 C Freezing at _ 18 D Heating at 80 E Heating at 80 followed by chilling at 4 F Heating at 80 followed by freezing at _ 18 Statistical analysis All the experiments were conducted in triplicate, and freshly prepared coconut milk was used in all

3 Effect of Different Treatment on the Properties of Coconut Milk Emulsions 85 experiments. Data of fat content, average droplets size and creaming index were analyzed using statistical software (Minitab 15, Minitab Inc, America). The standard deviation (SD) and Tukey s multiple range tests were used to evaluate significant differences (p < 0.05) between the samples. Data are presented as the mean values ± SD. Results and Discussion Effect of different thermal treatment on the fat content of coconut milk The fat content of coconut milk from coconut meat exposed to different pretreatments is described in Table 2. The fat content of coconut milk after freezing process at _ 18 was the lowest (15.94% fat), possibly because of the crystallization of coconut oil (Ariyaprakai and Tananuwong, 2015), which indicated that coconut oil could easily crystallize below 25, and frozen coconut meat was difficult to thaw in a short time period, so parts of coconut oil were not extracted from coconut meat, and decreased the content of coconut oil in coconut milk. The fat content of coconut milk after heating at 80 followed by freezing at _ 18 C was the highest (28.17% fat), which was slightly higher than the fat content of normal temperature at 30. Maybe because that cells, cellulose, and other structures of the coconut were damaged under the two kinds of extreme temperature conditions which made more oil extracted and more coconut protein denatured. Moreover, the oil combined to the denatured protein, thereby leading to more fat being squeezed out. Chilling, heating, and heating followed by chilling for coconut meat had no significant effects on the extraction of coconut milk fat, and the fat contents after these three treatments were all lower than that after normal temperature at 30, but comparing with others, there are significant differences. And freezing for coconut meat had significant effects on the extraction of coconut milk fat. Effect of thermal pre-treatments on the droplet size of coconut milk Effect of different pre-treatments on the coconut oil droplet size of non-sheared process. As shown in Table 3, the coconut oil droplet sizes of coconut milk changed with different thermal process of fresh coconut meat. In terms of mass, the average droplet sizes of coconut milk after heating process at 80 were larger than that of coconut milk without heating pretreatment. After heating process, the droplet size at 10% cumulative volume (D 10 ), the droplet size at 90% cumulative volume (D 90 ) and the mean droplet size (Dav) increased. The Dav increased to and 9.60 µm after heating at 80 for 15 min and heating at 80 followed by freezing at _ 18, respectively. Chilling at 4 and freezing at _ 18 decrease the D 10, D 90 and Dav. Freezing at _ 18 resulted in the smallest droplet size Dav and D 90, 7.67 and µm. Chilling process results in the smallest D 10, 3.15 µm. The Dav was approximately between 8.12 and 9.33 µm after normal temperature at 30, chilling at 4, and heating at 80 followed by chilling at 4. Fig. 1 shows the microstructure of coconut milk after different Table 2. Fat content of coconut milk squeezed from coconut meat with different treatments prior to extraction thermal treatment. Upon comparing the left and right images, the average droplet sizes with heating process were bigger compared with those without heating process. The difference in average droplet size among the normal temperature at 30, chilling at 4, and heating at 80 C followed by chilling at 4 was small. Freezing process resulted in the smallest average droplet size, but heating process followed by freezing process remarkably increased the particle sizes. Code Fat content (%) A B C D E F Effect of different pretreatments on the coconut oil droplets size of sheared process ± 1.42 a ± 1.21 b ± 0.50 c ± 1.94 b ± 0.84 b ± 0.62 a Mean values ± standard deviation. Values with different superscript are significantly different (p < 0.05) (Tukey test). A represents normal temperature at 30, B represents chilling at 4, C represents freezing at _ 18, D represents heating at 80, E represents heating at 80 followed by chilling at 4, and F represents heating at 80 followed by freezing at _ 18. Table 4 shows the results of coconut oil droplets size of coconut milk with high-speed shearing for 1 min followed by undergoing different thermal process. The coconut oil average droplets sizes (Dav) of sheared coconut milk after normal temperature at 30, chilling at 4 and freezing at _ 18 were 9.43, 8.84 and 8.48 µm, the Dav of sheared coconut milk after undergoing heating, heating followed by chilling, and heating followed by freezing were nearly 9.71, 9.18 and µm, which demonstrate that heating could increase the droplet size. Among all the treatment groups, heating increased the D 10, D 90, and Dav, But there was no significant difference of D 10 between normal temperature at 30 and heating at 80. Freezing decreased the coconut oil droplet size, D 10, D 90 and Dav to 3.74, 14.54, 8.48 µm. heating followed by freezing increased D 10, D 90, and Dav to a maximum value of 4.46, and µm. Fig. 2 shows the droplet size and distribution of coconut oil of sheared coconut milk after different thermal treatment. The coconut oil droplets size after chilling at 4, normal temperature at 30, heating at 80, and heating at 80 followed by chilling at 4 were larger compared with the droplets size after freezing. These findings were consistent with the results in Table 4. Based on the measured results and microscope observation, the coconut oil droplets size of coconut milk with heating pretreatment were larger than those of coconut milk without heating pre-treatment. The reason may be that coconut milk is a natural

4 P. Jiang et al. 86 Table 3. Coconut oil droplet sizes of non-sheared coconut milk from coconut milk exposed to different thermal treatments prior to coconut milk extraction Code A B C D E F Coconut oil droplet size (µm) D10 D90 Dav ± 0. 00a ± 0. 04c ± 0. 05c ± 0. 06a ± 0. 01b ± 0. 01a ± 0. 21b ± 0. 47c ± 0. 08d ± 0. 05a ± 0. 02c ± 0. 05b ± 0. 10c ± 0. 22d ± 0. 03e ± 0. 01a ± 0. 00c ± 0. 03b Mean values ± standard deviation. a-e.values with different superscript letters in the same column are significantly different (p < 0.05) (Tukey test). A represents normal temperature at 30, B represents chilling at 4, C represents freezing at _18, D represents heating at 80 C, E represents heating at 80 C followed by chilling at 4, and F represents heating at 80 C followed by freezing at _18. Fig. 1. Micrographs of oil in non-sheared coconut milk. A represents normal temperature at 30, B represents chilling at 4, C represents freezing at _18, D represents heating at 80, E represents heating at 80 followed by chilling at 4, and F represents heating at 80 followed by freezing at _18.

5 Effect of Different Treatment on the Properties of Coconut Milk Emulsions 87 Table 4. Coconut oil droplet sizes of sheared coconut milk from coconut meat treated with different thermal treatment prior to coconut milk extraction Code A B C D E F Coconut oil droplet size (µm) D10 D90 Dav ± 0. 04b ± 0. 04d ± 0. 02d ± 0. 02b ± 0. 00c ± 0. 01a ± 0. 39b ± 0. 03b ± 0. 01c ± 0. 07a ± 0. 46b ± 0. 11a ± 0. 24c ± 0. 01e ± 0. 00e ± 0. 03b ± 0. 22d ± 0. 03a Mean values ± standard deviation. a-e. Values with different superscript letters in the same column are significantly different (p < 0.05) (Tukey test). A represents normal temperature at 30, B represents chilling at 4, C represents freezing at _18, D represents heating at 80, E represents heating at 80 followed by chilling at 4, and F represents heating at 80 C followed by freezing at _18. Fig. 2. Micrographs of oil in sheared coconut milk. A1 represents normal temperature at 30, B1 represents chilling at 4 C1 represents freezing at _18, D1 represents heating at 80, E1 represents heating at 80 followed by chilling at 4, and F1 represents heating at 80 followed by freezing at _18.

6 88 emulsion, with stability that is supported by its own coconut proteins (globulin and albumin) and phospholipid. The quality and content of coconut protein determine the stability of coconut milk, and coconut protein is highly sensitive to temperature, and it will result in coagulation when heated to 80 (Saikhwan et al., 2015; Zhu et al., 2014). Thus, proteins on the coconut meat surface were destroyed by heating, which resulted in the protein content in coconut milk and the protein adhering to the oil drop surface decreased, subsequent caused the oil droplets inter-attraction, flocculation, and even coalescence, hence, increased coconut oil droplet size of coconut milk, and instability. The main reasons for the biggest difference in the droplet size between freezing process and heating process followed by freezing process are discussed as follows. Coconut fiber may have been destroyed by freezing and coconut oil crystallized, so more coconut proteins and little coconut oil (Table 2) were extracted through squeezing coconut milk. More proteins adhered to the oil drop surface, which inhibited the interattraction of coconut oil, thereby decreasing the particle size. However, heating followed by freezing markedly increased the particle size. The fiber and cells of coconut meat were damaged and became loose, and heating thawed coconut oil, most of the coconut proteins were denatured when heated, but more oil were extracted (Table 2), only a small amount of proteins adhered to the oil drop surface, which resulted in the inter-attraction of coconut oil, flocculation, and coalescence; hence, the average droplet size increased. However, the mechanism underlying this phenomenon needs to be further researched in the future. The difference of coconut oil droplet size between non-sheared and sheared coconut milk By comparing Tables 3 and 4 and Figs.1 and 2, we can conclude that high-speed shearing process could increase the coconut oil droplet size. Among all the treatment groups, the coconut oil droplets size D 10, D 90 and Dav of sheared coconut milk after heating followed by freezing were the biggest at nearly 4.46, and µm, which is larger than that of nonsheared coconut milk. The Dav of sheared coconut milk after freezing, heating followed by chilling, and heating followed by freezing were 8.48, 9.18, µm respectively, which is larger than that of non-sheared coconut milk significantly. However, the coconut oil droplets size D 90 of sheared coconut milk after normal temperature at 30 and heating process were smaller than that of non-sheared coconut milk. The Dav of sheared coconut milk after heating was 9.71 µm, which is also smaller than that of non-sheared coconut milk. Theoretically, high-speed shearing to emulsion will cause centrifugal shearing force, extrusion and collision of coconut milk to broke and scatter oil droplets; thus, oil droplets become smaller (Wu, 2014). However, the data in Tables 3 and 4 show that, highspeed shearing resulted in larger average droplet sizes. The possible reasons for this finding are followed. First, coconut milk has foaming properties, so high-speed shearing process can produce a large number of bubbles with large droplet sizes that may interfere P. Jiang et al. with the measurement results, which could be observed during the shearing process of this experiment. Second, in high-speed shearing process, the strong shear force will result in the droplets of coconut milk to randomly move at a high speed, in addition to the poor quality and quantity of coconut protein, oil droplet, as a kind of liquid, maybe easily cause extrusion and collision with each other, subsequent cause flocculation, and even coalescence, thereby increasing the particle size of oil droplets. The phenomenon of flocculation and coalescence was observed under the microscope. Effect of different treatments on the stability of coconut milk Effect of different thermal pretreatments on the stability of coconut milk Creaming index is an important indicator to evaluate the stability of coconut milk emulsions. The smaller the creaming index, the more stable the coconut milk emulsion. As shown in Fig.3, the creaming index of coconut milk squeezed from coconut meat with chilling was not significantly different from that of coconut milk from meat with normal temperature at 30. By contrast, freezing process decreased the creaming index of both sheared and non-sheared coconut milk to below 40%. The creaming index of samples with heating and heating followed by chilling increased, and both treatments exhibited a creaming index approaching 70%. The creaming index of samples with heating followed by freezing was the largest. In terms of mass, the creaming index of samples with heating pretreatment significantly increased, especially between freezing and heating followed by freezing. The creaming index of samples with freezing pretreatment was the smallest, and the stability was the best. These data also illustrated that smaller coconut oil droplet sizes of coconut milk emulsion resulted in highly stable coconut milk. Coconut milk is a natural emulsion stabled by coconut protein, which is sensitive to temperature, thus heating denatured the coconut protein, which is the main factor to make coconut milk unstable. Freezing process made coconut oil crystallize, fiber and cell break, so less oil and more coconut protein extracted from coconut meat, than more protein adhered to the coconut oil droplet surface, which inhibited the attraction of coconut oil, thus freezing improve the stability of coconut milk. However, heating followed by freezing markedly decreased the stability of coconut milk, maybe because that the fiber and cells of coconut meat were damaged and became loose, and heating thawed coconut oil, most of the coconut proteins were denatured when heated, but more oil were extracted, only a small amount of proteins adhered to the oil droplet surface, which resulted in the attraction of coconut oil, flocculation, and coalescence, hence, the stability was the worst. Effect of high-speed shearing on the stability of coconut milk Fig. 3 shows the results of the creaming index with machine treatment. Compared with non-sheared coconut milk, the creaming index of coconut milk squeezed from coconut meat with chilling, heating, and heating followed by freezing decreased and their stability improved, but that with normal temperature at 30 and

7 Effect of Different Treatment on the Properties of Coconut Milk Emulsions 89 Fig. 3. Effect of different treatment on the creaming index of coconut milk. A represents normal temperature at 30, B represents chilling at 4, C represents freezing at _ 18, D represents heating at 80, E represent heating at 80 followed by chilling at 4, and F represents heating at 80 followed by freezing at _ 18. Open bars represent the creaming index of non-sheared coconut milk, and filled bars represent the creaming index of sheared coconut milk. freezing increased insignificantly, that with heating followed by chilling has no significantly change. These data illustrated that the major reason resulted in the larger droplet size after shearing process, may be lots of bubbles produced during the shearing process. Conclusions Similar to oil-in-water emulsions, coconut milk emulsions are unstable. The oil droplets in coconut milk are attracted to each other, which results in flocculation or coalescence. The droplets size of coconut milk emulsion is an important indicator to assessing the stability of coconut milk. Different pretreatments of coconut meat clearly changed the sizes of droplets. In this study, heating treatment of coconut meat could increase the size of droplets, freezing decreased the content and the droplet size of coconut oil, high-speed shearing caused larger droplet sizes. In china, in the process of producing coconut milk, coconut meat will be heating at 80 before squeezed, and the fresh coconut milk will be sheared before adding surfactants, which were considered to inactivate the enzymes on the surface of coconut meat and improve the stability of coconut milk. But the result of this experiment shows that heating and high-speed shearing is not the ideal method to improve the stability of coconut milk. This study provided a theoretical basis for the factory producing. 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