Effect of homogenizing pressure and sterilizing condition on quality of canned high fat coconut milk pps

Journal of Food Engineering 73 (2006) 38–44 www.elsevier.com/locate/ jfoodeng Effect of homogenizing pressure and sterilizing condition on quality of canned high fat coconut milk Naphaporn Chiewchan * , Chanthima Phungamngoen, Suwit Siriwattanayothin Department of Food Engineering, King Mongkut s University of Technology, Thonburi, Tungkru, Bangkok 10140, Thailan d Received 31 August 2004; accepted 6 January 2005 Available online 24 Februar y 2005 Abstract The effect of homogenizing pressure (15–27 MPa) and commercial sterilizing condition (109.3–121.1 C under pressure of 5–15 psi) on the quality of canned high fat (30%) coconut milk was investigated. All heat-treated homogenized samples exhibited pseudoplastic behavior with flow behavior index (n) between 0.719 and 0.971. At similar sterilizing condition, a decrease in n value and an increase in consistency index (K) were observed for samples passing higher homogenizing pressures. A reduction in apparent viscosity was found for the homogenized samples undergoing higher sterilizing temperatures. For color determination, Hunter L/b values of homogenized coconut milk were greater than that for fresh sample and the values increased with increasing pressures. The reduction in L/b values was observed when the homogenized samples were subjected to heat treatment. Sterilizing at 121.1 C for 60 min could provide an acceptable color comparing to fresh coconut milk while heating at lower temperature but for longer time permitted more browning reaction and resulted in an increase of b value. Overall, the results suggested that quality of canned high fat coconut milk in terms of rheological and optical properties was influenced by both homogenizing pressure and sterilizing condition. 2005 Elsevier Ltd. All rights reserved. Keywords: Coconut milk; Color; Homogenizing pressure; Sterilizing temperature; Rheological propertie s 1. Introduct ion Coconut milk is a milky white oil-in-water emuls ion extracted from coconut flesh. It plays an important role in many traditional foods of Asian and Pacific regions . Separation of an emulsion into an aqueous phase and cream phase commonly occurs and leads to an una c- ceptably physical defect of either fresh or pro cessed coconut milk. Canning has been found to be a suit able process for preservation of coconut milk. The process starts from extracting the milk from grated cocon ut meat with or without added water. The pe rcentag e of fat is adjusted before heating at pasteurization tem- perature. The milk is then added with a stabilizer or * Corresponding author. Tel.: +66 2470 9243; fax: +66 2470 9240. E-mail address: naphaporn.rat@kmutt.ac.th (N. Chiewchan ). emulsifier and pass through the homogenizer. Finally, it was filled in can and sterilized in the retor t. Previous research works have demonstrated that fat particle size, dispersion and temperature had significant effects on a stability of foods containing high fat content such as milk, yogurt and cheese (Shaker, Jumah, & Jdayil, 2000; Xu, Nikolov, Wasan, Gonsalves, & Borwankar, 1998). For typical canned coconut milk process, the addition of suitable emulsifiers and homog- enization for redu cing fat globule size are required prior to heat treat ment to retain the emulsion stability. Sringam (1986) reported that type and quality of emulsifier and homogenization affected the stability of coconut milk. Increasing homogenizing pressure from 1000 to 5000 psi resulted in increasing stability of coco- nut milk and two-stage homogenization at 1000 and 2000 psi resulted in greater stability of coconut mil k 0260-8774/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005 .01.003 N. Chiewchan et al. / Journal of Food Engineering 73 (2006) 38–44 39 than single-stage high pressure (5000 psi) homo geniza- tion (Gonzalez, de Leon, & Sanchez, 1990). Adding so- Table 1 Process time for sterilizing high fat canned coconut milk (1985) studied the effect of temperature (15–50 C) and total solids (36.9–51.6%) on the flow properties of coco- nut milk. It was found that coconut milk exhibited midly pseudoplastic behavior. Simuang, Chiewchan, and Tansakul (2004) examined the effects of temperature (70–90 C) and fat content (15–30%) on the rheological properties of coconut milk. Their finding was similar to the work of Vitali et al. (1985) which all samples exhib- ited pseudoplastic behavior . They stated that fat concen- tration resulted in an increase in consistency index ( K ). Furthermore, the previous research work demonstrated that more aggregates of fat globule were clearly ob- served at higher heating temperatures (Simuang et al., 2004). This resulted in the reduction of K values which implied the decrease of the coconut milk stability. From literature described above, homogenizing pres- sure and temperature were significant parame ters affect- ing the stability of the emulsion. This research was aimed to investigate the effect of homogenizing pressur e in the pressure range of 15–27 MPa (11/4– 23/4 MPa ) and commercial sterilizing condition, (109.3–121.1 C under pressure of 5–15 psi) on the stability of canned high fat coconut milk (30%). The information obtaine d from the study could be used as a guideline for de velop- ing of high fat coconut milk canning process . 2. Materials and methods 2.1. Coconut milk prepar ation Fresh coconut milk without added water from a local market was stored at room temperature and pa ssed through the cloth filter before experiments. The initial fat content of coconut milk (35–37%) was determined by Rose–Gottlieb method ( AO AC, 1990 ) and then di- luted to the fat concentration of 30 % w/vby distilled water. 0.6% (w/v) M ontanox 60 (Polyoxyethylene (20) sorbitan monostearate) and 0.6% (w/v) CMC were added while the sample was heating and stirring contin- uously on a hot plate (Framo Geratetechnik Model M21/1, Germany). The sample was held on a hot plate for 1 min once its temperature reached 70 C to inhibit lipase and microbial growth. The prepared sample was passed through a two-stage homogenizer (GEA Model NS200 6L, Italy) at different pressure levels, i.e. 11, 14, 17, 20 and 23 MPa for the first stage and followed by 4 MPa for the second stage. The homogenized sample was then filled in a can (can size 300 · 407, 15 oz.) and a The time in min at 121.1 C that will produce the same degree of sterilization as the given process at its temperature T . Fig. 1. The change in apparent viscosity of high fat coconut milk after sterilization: (a) 30 C (non-heat sample), (b) 109.3 C, (c) 115.6 C and (d) 121.1 C. dium caseinate and stearoyl lactylate (0.5–2.5% of Pressure Temperature Come up Process F 0 a coconut milk) coupled with two-stage homogenization (psi) ( C) time (min) time (min) (min) could enhance the stability. 5 109.3 15 160 5 Processing temperature also has significant effect on 10 115.6 15 110 5 the stability of coconut milk. Vitali, Soler, and Rao 15 121.1 15 60 5 40 N. Chiewchan et al. / Journal of Food Engineering 73 (2006) 38–44 sterilized using a horizontal still retort. The thermal pro- cess conditions are shown in Table 1 . 2.2. Rheological measurement The rheological measurements were carried out using a rotational concentric cylinder viscometer (HA AKE Model VT500, Germany) with NV type measuring sys- tem. Shear rate was increased from 0 to 300 s 1 in 2 min. The temperature of samples was maintained at room temperature (about 30 C) during the measur ement s. 2.3. Microscopic study A few drops of oleoresin dye were added to 10 ml of coconut milk sample and subsequently stirred for at least 1 min to disperse the dye. A few drops of the sam- ple were transferred to the slide and a cover slip was placed over the sample. An optical standard microsco pe (Olympus Model CH30, Japan) was used to determ ine the fat structure at a magnification of 400 · (before ther- mal process) or a magnification of 100 · (after therm al processing) and photographs were taken from typic al fields. 2.4. Determination of fat globule size distributi on The diffractometer (Malvern Instrument Model Mastersizer-S, UK) equipped with a 3 RF lens an d a n He–N e laser (k = 633 nm) was used to determine size distribution of fat globules of coconut milk. The ster il- ized samples were diluted to approximately 1/100 0 with deionized water before measuring. Size distribution his- tograms are presented in volume of fat particle (%) against droplet diameter (in the range of 0.05–900 l m). The measurements were conducted three times for each sampl es. 2.5. Col or meas urem ent Color of coconut milk was analyzed by measuring the reflectance using a spectrocolorimeter (Juki Model JP7100, Japan). 2 North skylight was used as the light source. The instrument was calibrated against a stan- dard white reference tile ( L = 91.66, a = 0.12, b = 1.37). A glass cell (30 mm diameter) containing the sample was placed above the light source and covered with the lid. Although, three Hunter parameters, namely ‘‘L’’ (lightness), ‘‘a’’ (greenness and redness) and ‘‘ b ’’ (blueness and yellowness) were recorded, only L and b values were required to describe the change in color. 2.6. Experimental design and data analysis The experiments were conducted for five level s of homogenizing pressure (11/4, 14/4, 17/4, 20/4 and 23/4 MPa ) and three levels of sterilizing tempe ratur e (109.3, 115.6 and 121.1 C). A 2-factor factorial design was used in scheduling of the experiments. The results Table 2 Effect of homogenization pressure and sterilizin g temperature on consistency index (K) and flow behavior index ( n ) Table 3 Apparent viscosity at 300 s 1 for high fat coconut milk at different homogenization pressures and sterilizing temperatures Temperatur e Homogen ization K (Pa s n ) n r 2 Temperature Homogenizatio n Apparent viscosity ( C) pressure (MPa) ( C) pressure (MPa) ( g a ; Pa s ) 30 Non homogeni zation 3.62 · 10 2 0.971 0.992 30 Non 1.54 · 10 2 15 (11/4) 5.81 · 10 2 0.858 0.968 15 (11/4) 2.71 · 10 2 18 (14/4) 6.62 · 10 2 0.806 0.979 18 (14/4) 2.80 · 10 2 21 (17/4) 9.34 · 10 2 0.759 0.964 21 (17/4) 3.10 · 10 2 24 (20/4) 10.42 · 10 2 0.740 0.977 24 (20/4) 3.55 · 10 2 27 (23/4) 14.56 · 10 2 0.719 0.954 27 (23/4) 4.55 · 10 2 109.3 15 (11/4) 3.95 · 10 2 0.926 0.981 109.3 15 (11/4) 1.85 · 10 2 18 (14/4) 4.97 · 10 2 0.904 0.981 18 (14/4) 2.27 · 10 2 21 (17/4) 5.64 · 10 2 0.883 0.987 21 (17/4) 2.38 · 10 2 24 (20/4) 7.15 · 10 2 0.852 0.983 24 (20/4) 2.70 · 10 2 27 (23/4) 8.45 · 10 2 0.810 0.982 27 (23/4) 2.94 · 10 2 115.6 15 (11/4) 2.32 · 10 2 0.949 0.984 115.6 15 (11/4) 1.32 · 10 2 18 (14/4) 3.46 · 10 2 0.911 0.982 18 (14/4) 2.05 · 10 2 21 (17/4) 3.61 · 10 2 0.892 0.984 21 (17/4) 2.27 · 10 2 24 (20/4) 4.18 · 10 2 0.863 0.985 24 (20/4) 2.47 · 10 2 27 (23/4) 4.58 · 10 2 0.822 0.958 27 (23/4) 2.66 · 10 2 121.1 15 (11/4) 2.02 · 10 2 0.959 0.988 121.1 15 (11/4) 1.20 · 10 2 18 (14/4) 2.56 · 10 2 0.913 0.984 18 (14/4) 1.43 · 10 2 21 (17/4) 2.82 · 10 2 0.894 0.975 21 (17/4) 1.58 · 10 2 24 (20/4) 2.78 · 10 2 0.869 0.977 24 (20/4) 1.78 · 10 2 27 (23/4) 3.13 · 10 2 0.823 0.981 27 (23/4) 1.96 · 10 2 N. Chiewchan et al. / Journal of Food Engineering 73 (2006) 38–44 41 were reported as an average of three replicates. Anal ysis of variance (ANOVA) of the two factors and inter ac- tions were applied to the different sets of data with a sig- nificant level of 0.05 ( a = 0.05) . 3. Results and discussion 3.1. Rheological properties The plot of apparent viscosity against shear rate of coconut milk homogenized at five pressure levels before and after sterilizing are shown in Fig. 1. The rheogram s obtained were similar for all conditions. Power law model was applied to describe the rheological beh avior of the sampl es. s ¼ K c _ n ð 1 Þ where s is the shear stress, c _ is the shear rate, K is the consistency index (Pa s n ) and n is the flow be havior index. The excellent fits were obtained with high co rrelatio n coefficients (r 2 = 0.954–0.992). The values of K and n are shown in Table 2. It was revealed that all sampl es exhib- ited pseudoplastic behavior with the flow behavior index (n) between 0.719 and 0.971. It was found that the apparent viscosity decreased with increasing shear rate during the early period of measurement. After a sharp reduction, the apparent viscosity changed slightly and became steady at higher shear rates. A s coconut milk is a colloidal system containing fat globules dispersed in water phase, the fat particles may rearrange them- selve s into parallel direction with shear force and fat globule aggregates may break into smaller ones by shear force. These particles could flow easily as a result of resistance arising from particle–particle inter action Fig. 2. Micrograph s ( · 400 magnification) of high fat coconut milk samples passing different homogenization pressures: (a) non-homogenization, (b) 11/4 MPa, (c) 14/4 MPa, (d) 17/4 MPa, (e) 20/4 MPa and (f) 23/4 MPa. 42 N. Chiewchan et al. / Journal of Food Engineering 73 (2006) 38–44 which decreased viscosity (Charm, 1962 ). Whe n the aggregates were completely disrupt ed, further in- crease in shear rate did not affect the apparent viscosity (Campanella, Dorward, & Singh, 1995 ). At the same temperature, a decrease in n value and a n increase in K value were obtained for the samples pass- ing higher homogenizing pressures. The increase in pres- sure level permitted the size reduction. Thi s meant that higher numbers of droplet were presented in the co lloi- dal system and obstructed the flow. Therefore, an increase in pressure caused an increase in apparent vis- cosity and the more pseudoplasticity. Thermal process- ing also had significant effect on the viscosity of coconut milk. A reduction in apparent viscosity of coco- nut milk was observed with increasing sterilizing tempe ratur e. Tabl e 3 shows the values of apparent viscosity (g) at maximum shear rate (300 s 1 ). It was found that the emulsions were more viscous after passing higher pres- sures. From the results, coconut milk exhibited a power-law pseudoplastic behavior, characterized by n values less than 1 at all homogenizing pressures and ster- ilizing temperatures. Experimental results have shown that passing the coconut milk through a hom ogenizer 100 90 80 70 60 50 40 30 20 10 0 0.1 (a) 1 10 100 1000 Particle diameter (um) 100 90 80 70 60 50 40 30 20 10 0 0.1 100 80 60 40 20 0 0.1 (b) 1 10 100 1000 Particle diameter (um) (c) 1 10 100 1000 Particle diameter (um) Fig. 3. Micrograph s ( · 100 magnification) of coconut milk samples at homogenization pressure 23/4 MPa with different sterilizing tempera- tures: (a) 109.3 C, (b) 115.6 C and (c) 121.1 C. Fig. 4. Effect of sterilizing temperatures: (a) 109.3 C, (b) 115.6 C, (c) 121.1 C on droplet size distribution at different homogenizing Vo lu me of par ticl e (% ) Vo lu me of par ticl e (% ) Vo lu me of par ticl e (% ) pressures: 11/4 MPa ( s ), 14/4 MPa ( ), 17/4 MPa ( n ), 20/4 MPa ( j ) and 23/4 MPa ( · ). N. Chiewchan et al. / Journal of Food Engineering 73 (2006) 38–44 43 was accompanied with an increase in pseud oplastici ty and was shown by a decrease in values of flow be havior index (n). This observation was consistent with the work of Floury, Desrumaux, and Legrand (2002). They re- ported that the emulsion obtained at low homo genizin g pressure show Newtonian flow behavior with quite low viscosity because there was no interaction between par- ticles. As homogenizing pressure increa sed, appa rent viscosity of the emulsion increased, with a strong shift of the fluid from a Newtonian to pseudoplastic behav- ior, indicative of resistance arising from particle–particle interaction in the emulsions (Charm, 1962 ). The consistency index (K) is an indicator of the vis- cous nature of the system and was observed to be in- creased with the increase in homogenizing pressur e, Furthermore, a decrease in consistency index (K) was observed with the increasing temperature, indicating a decrease in apparent viscosity at higher tempe ratur es. 3.2. Effect on fat structure of coconut milk The effect of homogenizing pressure on fat struc ture of coconut milk were conducted using optical standar d microscope ( Fig. 2). It was found that the non hom oge- nized sample had larger fat globule sizes than homoge- nized ones. During the homogenization, the high shear forces acted on dispersed phase to redu ce droplet size perature, some heat labile proteins were destroy ed (Seow & Gwee, 1997) and fat globules tended to form aggregates. Therefore, the emulsion syst em co ntaine d less suspended single fat globules to resist the flow. The micrographs supported the results from the rheo- logical studies that decreasing in viscosity of heated trea- ted homogenized coconut milk was caused from the change in microstructure. The droplet size distribution and mean droplet diam- eter were also determined as shown in Fig. 4 and Tabl e 4 . The patterns of the size distribution data were changed noticeably at higher heating temperature. The effect of homogenizing pressure on the droplet size was clear ly seen as the data from different pressures were discrete from each other. Furthermore, new large droplets in the range of 10–100 l m were detected which resulted in the increase of the mean droplet diameter obtained for all samples passing higher heating level. The results suggested that the stabili ty of canned coconut milk was influenced by both homogenizing pressure and sterilizing condition. Table 4 Effect of homogenization pressure on fat particle diameter (D m ) of canned high fat coconut milk ( Floury et al., 2002). Small fat globule sizes were ob- tained at higher homogenizing pressures. Reduction in Temperature ( C) Homogen ization pressure (MPa) Fat particle diamet er ( D m ) ± SD ( l m) the fat particle diameters resulted in an increase in K value and thus improved the product stability ( Go nzalez et al., 1990; Srithunma, 2002 ). When the homogenized coconut milk sampl es were subjected to heat treatments, small fat globules formed irregular rearrangement of aggrega tes. Naturally, coco- nut milk composes of fat globules surrounded by the aqueous protein solution ( Gonzal ez et al., 1990). Addi- tion of emulsifier and stabili zer helped in the stability of coconut milk by lowering the interfacial tension be- tween two phases, therefore fat globules could disperse throughout the water pha se. Fig. 3 exemplifies the effect of sterilizing tempe ratur e o n the structure of fat globule. When the samples were heated at high sterilizing tem- 109.3 15 (11/4) 3.57 ± 0.25 18 (14/4) 3.43 ± 0.24 21 (17/4) 3.26 ± 0.23 24 (20/4) 3.06 ± 0.21 27 (23/4) 2.81 ± 0.19 115.6 15 (11/4) 4.40 ± 0.35 18 (14/4) 4.31 ± 0.21 21 (17/4) 4.29 ± 0.31 24 (20/4) 4.12 ± 0.28 27 (23/4) 3.81 ± 0.26 121.1 15(11/4) 5.94 ± 0.34 18 (14/4) 5.49 ± 0.27 21 (17/4) 5.44 ± 0.38 24 (20/4) 5.42 ± 0.41 27 (23/4) 5.01 ± 0.24 Table 5 Effect of homogenizing pressure and sterilizing temperature on L/b values of high fat coconut milk Homogenizing pressure (MPa) Temperatur e 30 C 109.3 C 115.6 C 121.1 C L b L/b L b L/b L b L/b L b L/b Non homogenization 77.92 4.85 16.07 – – – – – – – – – 15 (11/4) 79.35 4.30 18.44 74.58 8.54 8.73 73.54 6.42 11.49 77.90 4.83 16.13 18 (14/4) 79.52 4.29 18.51 72.44 8.14 8.90 71.55 6.18 11.58 77.89 4.79 16.24 21 (17/4) 79.96 4.25 18.78 73.26 8.09 9.06 70.87 6.07 11.68 78.55 4.75 16.53 24 (20/4) 80.47 4.26 18.88 73.69 7.90 9.32 72.02 6.05 11.90 78.49 4.72 16.61 27 (23/4) 80.49 4.26 18.96 72.91 7.80 9.34 71.88 6.01 11.95 78.79 4.69 16.77 44 N. Chiewchan et al. / Journal of Food Engineering 73 (2006) 38–44 3.3. Effect on color of coconut milk The color changes of coconut milk as affected by homogenizing pressure and thermal processing were investigated and the color values were presented in terms of Hunter L/b (Table 5). It was found that L/b values of homogenized coconut milk were great er than that for fresh coconut milk and increa sed with increasing homogenizing pressure ( P < 0.05). Smaller droplets were produced when the higher homogenizing pressures were applied. The reflectance increased with increasing drop- let concentration and decreasing droplet size (Chantrap- ornc hai, Clyd esdale , & McClements, 1999). This occurrence resulted in the higher lightness values ( L ). For the effect of thermal processing, lightness ( L ) of product at any sterilizing temperatures were not signi fi- cantly different while b values decreased with ster ilizing temperature. Therefore, L/b value increased with increasing sterilizing tempe ratur e. In low acid food such coconut milk (pH about 6), non-enzymatic browning reaction occurred when high heating temperatures (>100 C) were applied (Ames & Hofmann, 2001). In this research, three levels of steriliz- ing temperature, i.e. 109.3, 115.6 and 121.1 C were cho- sen and the process time to approach F 0 = 5 min were different (Table 1). The higher b values were found for the sample passing the thermal process at 115.6 C a n d 109.3 C, respectively. The reason was that heating at lower temperature had taken longer time to achieve F 0 = 5 min. Therefore, there was longer period of tim e to permit the browning reaction to occur. This resul ted in the significant reduction in L/b value. 4. Conclusions Following the power law model, coc onut milk sampl es passing through a 2-stage homogenizer and heating pro- cess (sterilization) in the range of experimental conditio ns exhibited pseudoplastic behavior with the flow be havior index (n) between 0.719 and 0.971. Increasing hom oge- nizing pressure caused a decrease o f fat droplet size whi ch resulted in an increase of apparent viscosity. How ever, heat treatment at higher temperature led to the aggrega t- ing of fat particle and this phenomenon caused the reduc- tion of apparent viscosity. For optical property determination, Hunter L/b values of homogenized coco- nut milk were greater than that for fresh sample and the values increased with increasing homogenizing pressures. Comparing among commercial ster ilizing conditions of study, heating at 121.1 C for 60 min provided an accept- able color comparing to fresh coconut milk. Acknowledgments Thi s work was supported by the National Center for Genetic Engineering and Biotechnology, Thailand (BIOTEC). The authors wish to thank Adinop co mpany for kindly providing the emulsifying agents (Mon tanox 60 and Montane 80). And the National Metal and M ate- rials Technology Center (MTEC) for allowing the use of the Mastersizer-S. Ref erences Ames, J. M., & Hofmann, T. F. (2001). Chemistry and physiology of selected food colorants. Washington, DC: ACS, p. 227. Association of Official Analytical Chemistry (AOAC). (1990). Official method of analysis (15th ed.). The Association of Official Agricul- tural Chemists, Virginia. Campanella, O. K., Dorward & Singh, H. (1995). A study of the rhelogical properties of concentrated food emulsions. Journal of Food Engineering, 25 , 427–440 . Chantrapornchai, W., Clydesdale, F., & M cClemen ts, D. J. (1999). Influence of droplet characteristics on the optical properties of colored oil-in-water emulsion. 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Journal of Food Science, 63(2), 183–188 . . effect of homogenizing pressure (15–27 MPa) and commercial sterilizing condition (109.3–121.1 C under pressure of 5–15 psi) on the quality of canned high fat (30%) coconut milk was. Journal of Food Engineering 73 (2006) 38–44 www.elsevier.com/locate/ jfoodeng Effect of homogenizing pressure and sterilizing condition on quality of canned high fat coconut milk Naphaporn. the pressure range of 15–27 MPa (11/4– 23/4 MPa ) and commercial sterilizing condition, (109.3–121.1 C under pressure of 5–15 psi) on the stability of canned high fat coconut milk