Beaumal, Delepine, & Lamballerie-Anton, 2001) was used to
describe flow behavior of emulsion.
where t is the shear stress (Pa), t0 is the yield stress (Pa), K is the
consistency index (Pa sn), g_ is the shear rate (s1
), and n is the flow
index (dimensionless), n < 1 for a shear-thinning fluid, n > 1 for a
shear thickening fluid and n ¼ 1 for a Newtonian fluid.
Creep tests should be performed in the linear viscoelastic region
(LVR). Therefore, the stress sweep test (1e300 Pa for cream, 1 Hz)
was carried out first to determine yield stress (t0) with 20 data
points collected and plotted logarithmically by P35 TiL polished
sensor. In the creep test, samples of foam were subjected to a
constant shear stress of 3 Pa for 300 s. The 4-element Burgers
model (a Kelvin-Voigt and Maxwell model in series) (Xu, Xiong, Li,
& Zhao, 2008) was used in the creep analysis of the samples.
where g is the total strain at time t (s), t (Pa) is the constantly
applied shear stress, l (s) is the mean retardation time, h1 (Pa s) is
the coefficient of viscosity associated with viscosity flow, g_(s1
) is
the shear rate of viscosity flow, E1, E2 (Pa) are the instantaneous
elastic modulus and retarded elastic modulus, respectively. g1, g2
(1et/l) and g_ $t are the instantaneous elastic deformation,
retarded elastic deformation and viscosity flow deformation,
respectively.
Creep data were also analyzed in terms of equilibrium creep
compliance Je (Pa1
), determined by Eq. (4).
where ge is the equilibrium deformation, t (Pa) is the constantly
applied shear stress (Herranz, Tovar, Borderias, & Moreno, 2013).
2.6. Measurement of partial coalescence of fat
The amount of free fat in the emulsion was determined using
the method of Palanuwech, Potineni, Roberts, and Coupland (2003).
Oil Red O (0.15 mg/g) in corn oil was prepared and stirred overnight
using a digital overhead stirrer (RW20, IKA Co. Ltd., Germany). The
absorbance of the solution at 520 nm was measured using a
UVevisible spectrophotometer (PerkineElmer, Lambda 3, Norwalk,
CT). Corn oil was used as the blank. Then dilution technique was
applied: a dye solution in non-polar solvent was poured onto the
surface of the emulsion with gentle mixing, allowing the colored oil
to float at the surface during centrifugation at 10,000 g for 30 min
at 25 C in a Beckman L8-M ultracentrifuge (Beckman, Fullerton,
CA, USA). Any free fat in the emulsion was dissolved in the colored
oil but the emulsified fat remained in the droplets. The diluted-dye
solution fraction was easily transferred from the surface for
absorbance measurements. The change in absorbance indicates
that the mass fraction that is not emulsified in the fat (4d), which is
given by Eq. (5).
where 4d is the mass fraction of fat in the emulsion, mo is the mass
of added Red Oil O, me is the mass of emulsion, a is absorbance ratio
of the Red Oil O solution before and after centrifugation, and f is
the mass fraction of oil in the emulsion. All the measurements were
made in triplicate.
2.7. Measurement of overrun
The overrun of whipped creams was measured according to the
method of Scurlock (1986), i.e. through filling a transparent tub to a
set volume with the testing whipped cream. Complete filling
without including artificial large bubbles was facilitated with
gentle tapping. The overrun was related to the mass of this volume
and the density of the cream before whipping. It was determined by
Eq. (6).
where M1 is the mass of unwhipped cream with the set volume (g),
and M2 is the mass of whipped cream with the same volume (g). All
the measurements were made in triplicate.
2.8. Statistical analysis
Statistical analysis was conducted using SPSS 11.5 (SPSS Inc.,
Chicago, IL, USA) for one-way ANOVA. StudenteNewmaneKeuls
test was used to compare of the mean values among treatments,
and to identify the significance of difference (p < 0.05) among
treatments. Bivariate correlation analysis was used to analyze the
correlationship between the parameters. All the data were
expressed as mean ± standard deviation of triplicate
determinations.
3. Results and discussion
3.1. Particle characteristics of powder and emulsion
SEM images (Fig. 1) showed that particles of powdered whipped
cream (PWC) were generally in spherical shape with smooth surfaces
and their average diameters were approximate to 10e40 mm.
No discernible pores or cracks on the outer surface but some small
dents, wrinkles or scars. Some PWC particles appeared in clusters
or attached one another indicating the formation of aggregates,
probably due to some mechanical stress induced water removal by
drying and the uneven drying at different locations of the drying
droplets.
As the PWC powders were dispersed in water transforming
emulsion again, a few large irregular fat droplet aggregates
occurred in spray dried emulsion (Fig. 2C). However, micrographs
of both frozen and chilled emulsions appeared to be more evenly
distributed without obvious large droplets (Fig. 2A, B). As evidenced
from the micrographs, the processing conditions had a
distinct influence on particle size distribution of fat droplets in
emulsions. The evenness of particle size distribution of emulsions
decreased in the order of chilled emulsion > frozen
emulsion > spray dried emulsion, while this last one exhibited a
very different pattern to those shown by the frozen and chilled
emulsions. Both the frozen and chilled emulsions exhibited typical
monomodal distributions and similar profiles, ranging from
0.03 mm to 0.8 mm (i.e. average particle size (d4,3) 0.213 ± 0.001 mm
and 0.227 ± 0.000 mm, respectively). No significant difference
(p > 0.05) in mean particle size was shown between them. The
spray dried emulsion (d4,3 ¼ 8.982 ± 0.340 mm) showed a broadening
bimodal distribution with a population of fat droplets occupied
between 0.3 and 3.8 mm, and a large irregular peak shape
between 4.3 and 182.0 mm.
The bimodal distribution of the spray dried emulsion was
probably attributed to the dynamic physical changes caused by the
extra processing steps i.e. spray drying and reconstitution. Thus,
occurrence of free fats, aggregation, formation of large fat droplets
via coalescence and uneven or insufficient emulsification after
reconstitution with normal stirring were all possible (Danviriyakul,
McClements, Decker, Nawar, & Chinachoti, 2002; Hogan, McNamee,
O’Riordan, & O’Sullivan, 2001; Vignolles et al., 2009). In this study,
particle size of spray dried emulsion increased significantly
compared to its parent emulsion (p < 0.05). Although no significant
difference between the average particle size and particle size distribution
of frozen and chilled emulsions was observed, the slight
difference was possibly caused by the fat crystallization and partial
fat droplets coalescence induced by the freeze-thaw process in the
frozen emulsion (Vanapalli, Palanuwech, & Coupland, 2002). Fat
droplets in chilled emulsion partially coalesced to a slightly greater
extent than those of frozen emulsion, which could be evidenced by
the values of partial fat coalescence at 0 min (Fig. 4a).
3.2. Flow behavior of emulsion
Flow and apparent viscosity curves versus shear rate and the
parameters of Herschel-Bulkley model of emulsions under different
processing conditions were shown in Fig. 3 and Table 1, respectively
It appeared that the Herschel-Bulkley model provides an excellent
approximation to the measurements. The flow behaviors exhibited
pronounced shear-thinning tendency, which was characterized by
relatively high viscosity at low shear rate and reached to a plateau
at high shear rate (Fig. 3). Flow index n less than 1 confirmed the
pseudoplasticity of all emulsion samples (Table 1). The values of
consistency index K were in an ascending order as frozen
emulsion < chilled emulsion < spray dried emulsion, which was in
agreement with change tendency of apparent viscosity. The pseudoplastic
behavior with yield stress suggests that weak droplet
network structure was formed at the initial stage. The decrease in
apparent viscosity with increased shear rate might be attributed to
the breakdown or reorganization of droplet network by mechanical
shear (Bellalta, Troncoso, Zúniga, ~ & Aguilera, 2012). At a given shear
rate, both the apparent viscosity and the shear stress of emulsions,
were in the same ascending order. The flow and viscosity curves of
frozen and chilled emulsions were similar and greatly differed from
those of spray dried emulsion, indicating that processing procedures
had distinct influence on flow behavior of emulsion.
Generally, as a good indicator of internal network structure
strength, yield stress is calculated by extrapolation of experiment
data in the case of shear rateg_ / 0 (Derkach, Kuhkushkina,
Levachov, & Matveenko, 2011). The frozen emulsion exhibited the
lowest yield stress, while the highest yield stress was observed for
the spray dried emulsion. Factors affecting the yield stress are
particle volume fraction, particle size and strength of interparticle
forces. In general, a decrease of particle size leads to increased yield
stress (Genovese, Lozano, & Rao, 2007). However, yield stress
increased with increasing fat droplets size of emulsions in this
work. It may be interpreted by increasing of interparticle interaction
strength as the dominant factor. The strength of interparticle
interaction is associated with the interconnection of the network
microstructure, which will increase with elimination of weak
linkages as the droplet aggregation increases (Zhou, Solomon,
Scales, & Boger, 1999). Furthermore, apparent viscosity of emulsion
also appeared higher dependence of three-dimensional
network. It meant that the emulsions with firmly network structure
should overcome more interparticle resistance force to flow, in
other words, exhibiting higher viscosity (Boode & Walstra, 1993).
Beaumal, Delepine, & Lamballerie-Anton, 2001) was used todescribe flow behavior of emulsion.where t is the shear stress (Pa), t0 is the yield stress (Pa), K is theconsistency index (Pa sn), g_ is the shear rate (s1), and n is the flowindex (dimensionless), n < 1 for a shear-thinning fluid, n > 1 for ashear thickening fluid and n ¼ 1 for a Newtonian fluid.Creep tests should be performed in the linear viscoelastic region(LVR). Therefore, the stress sweep test (1e300 Pa for cream, 1 Hz)was carried out first to determine yield stress (t0) with 20 datapoints collected and plotted logarithmically by P35 TiL polishedsensor. In the creep test, samples of foam were subjected to aconstant shear stress of 3 Pa for 300 s. The 4-element Burgersmodel (a Kelvin-Voigt and Maxwell model in series) (Xu, Xiong, Li,& Zhao, 2008) was used in the creep analysis of the samples.where g is the total strain at time t (s), t (Pa) is the constantlyapplied shear stress, l (s) is the mean retardation time, h1 (Pa s) isthe coefficient of viscosity associated with viscosity flow, g_(s1) isthe shear rate of viscosity flow, E1, E2 (Pa) are the instantaneouselastic modulus and retarded elastic modulus, respectively. g1, g2(1et/l) and g_ $t are the instantaneous elastic deformation,retarded elastic deformation and viscosity flow deformation,respectively.Creep data were also analyzed in terms of equilibrium creepcompliance Je (Pa1), determined by Eq. (4).where ge is the equilibrium deformation, t (Pa) is the constantlyapplied shear stress (Herranz, Tovar, Borderias, & Moreno, 2013).2.6. Measurement of partial coalescence of fatThe amount of free fat in the emulsion was determined usingthe method of Palanuwech, Potineni, Roberts, and Coupland (2003).Oil Red O (0.15 mg/g) in corn oil was prepared and stirred overnightusing a digital overhead stirrer (RW20, IKA Co. Ltd., Germany). Theabsorbance of the solution at 520 nm was measured using aUVevisible spectrophotometer (PerkineElmer, Lambda 3, Norwalk,CT). Corn oil was used as the blank. Then dilution technique wasapplied: a dye solution in non-polar solvent was poured onto thesurface of the emulsion with gentle mixing, allowing the colored oilto float at the surface during centrifugation at 10,000 g for 30 minat 25 C in a Beckman L8-M ultracentrifuge (Beckman, Fullerton,CA, USA). Any free fat in the emulsion was dissolved in the coloredoil but the emulsified fat remained in the droplets. The diluted-dyesolution fraction was easily transferred from the surface forabsorbance measurements. The change in absorbance indicatesthat the mass fraction that is not emulsified in the fat (4d), which isgiven by Eq. (5).where 4d is the mass fraction of fat in the emulsion, mo is the massof added Red Oil O, me is the mass of emulsion, a is absorbance ratioO น้ำมันแดงแก้ปัญหาก่อน และ หลัง centrifugation และ f เป็นเศษส่วนโดยรวมของน้ำมันในอิมัลชัน มีวัดทั้งหมดทำใน triplicate2.7 การวัดมากเกินไปมากเกินไปของครีมที่ถูกตีถูกวัดตามวิธีของ Scurlock (1986), เช่นผ่านอ่างโปร่งใสเพื่อกรอกข้อมูลการตั้งเสียง ด้วยครีม whipped ทดสอบ กรอกสมบูรณ์โดยไม่รวมฟองอากาศใหญ่ประดิษฐ์ถูกกลืนแตะเบา ๆ มากเกินไปไม่เกี่ยวข้องกับมวลของไดรฟ์ข้อมูลนี้และความหนาแน่นของครีมก่อน whipping มันถูกกำหนดโดยEq. (6)โดยที่ M1 คือ มวลของครีม unwhipped กับไดรฟ์ข้อมูลการตั้งค่า (g),และ M2 คือ มวลของครีมที่ถูกตีด้วยปริมาณเดียวกัน (g) ทั้งหมดการประเมินการทำ triplicate2.8. สถิติวิเคราะห์วิธีวิเคราะห์ทางสถิติโดยใช้โปรแกรม 11.5 (โปรแกรม Inc.ชิคาโก IL สหรัฐอเมริกา) สำหรับการวิเคราะห์ความแปรปรวนแบบทางเดียว StudenteNewmaneKeulsใช้ทดสอบเพื่อเปรียบเทียบค่าเฉลี่ยระหว่างรักษาและระบุความสำคัญของความแตกต่าง (p < 0.05) ระหว่างรักษา วิเคราะห์ความสัมพันธ์ bivariate ถูกใช้ในการวิเคราะห์การcorrelationship ระหว่างพารามิเตอร์ ข้อมูลทั้งหมดได้แสดงเป็นเฉลี่ย±ส่วนเบี่ยงเบนมาตรฐานของของ triplicatedeterminations3. ผลลัพธ์ และสนทนา3.1 ลักษณะอนุภาคของผงและอิมัลชันภาพ SEM (Fig. 1) แสดงให้เห็นว่า อนุภาคของผงถูกตีครีม (PWC) มีรูปร่างเป็นทรงกลมมีผิวเรียบโดยทั่วไปand their average diameters were approximate to 10e40 mm.No discernible pores or cracks on the outer surface but some smalldents, wrinkles or scars. Some PWC particles appeared in clustersor attached one another indicating the formation of aggregates,probably due to some mechanical stress induced water removal bydrying and the uneven drying at different locations of the dryingdroplets.As the PWC powders were dispersed in water transformingemulsion again, a few large irregular fat droplet aggregatesoccurred in spray dried emulsion (Fig. 2C). However, micrographsof both frozen and chilled emulsions appeared to be more evenlydistributed without obvious large droplets (Fig. 2A, B). As evidencedfrom the micrographs, the processing conditions had adistinct influence on particle size distribution of fat droplets inemulsions. The evenness of particle size distribution of emulsionsdecreased in the order of chilled emulsion > frozenemulsion > spray dried emulsion, while this last one exhibited avery different pattern to those shown by the frozen and chilledemulsions. Both the frozen and chilled emulsions exhibited typicalmonomodal distributions and similar profiles, ranging from0.03 mm to 0.8 mm (i.e. average particle size (d4,3) 0.213 ± 0.001 mmand 0.227 ± 0.000 mm, respectively). No significant difference(p > 0.05) in mean particle size was shown between them. Thespray dried emulsion (d4,3 ¼ 8.982 ± 0.340 mm) showed a broadeningกระจาย bimodal มีประชากรหยดไขมันที่ครอบครองระหว่าง 0.3 และ 3.8 มม. และรูปร่างสูงใหญ่ผิดปกติระหว่าง 182.0 และ 4.3 มม.เป็นอิมัลชันพ่นกระจาย bimodalอาจเกิดจากการเปลี่ยนแปลงทางกายภาพแบบไดนามิกที่เกิดจากการการประมวลผลเพิ่มเติมขั้นตอนเช่นสเปรย์แห้งและ reconstitution ดังนั้นเกิดไขมันฟรี รวม การก่อตัวของหยดไขมันขนาดใหญ่coalescence และ emulsification ปริมาณไม่เพียงพอ หรือไม่สม่ำเสมอหลังจากreconstitution กับกวนปกติได้ทั้งหมดไป (DanviriyakulMcClements เหล็กสองชั้น Nawar, & Chinachoti, 2002 Hogan, McNameeO'Riordan และโรงง 2001 Vignolles et al., 2009) ในการศึกษานี้ขนาดอนุภาคของสเปรย์แห้งอิมัลชันที่เพิ่มขึ้นอย่างมีนัยสำคัญเมื่อเทียบกับของอิมัลชันหลัก (p < 0.05) แม้ว่าไม่สำคัญขนาดอนุภาคเฉลี่ยและการกระจายขนาดอนุภาคแตกต่างกันของแข็ง และเย็น emulsions ถูกสังเกต เล็กน้อยความแตกต่างอาจจะเกิดจากการตกผลึกไขมันและบางส่วนเกิดจากกระบวนการแช่แข็ง-thaw ใน coalescence หยดไขมันอิมัลชันน้ำแข็ง (Vanapalli, Palanuwech, & Coupland, 2002) ไขมันหยดในอิมัลชันเย็นบางส่วน coalesced จะมากกว่าเล็กน้อยขอบเขตกว่าของอิมัลชันแช่แข็ง ซึ่งอาจเป็นหลักฐานด้วยค่าของ coalescence ไขมันบางส่วนที่ 0 นาที (Fig. 4a)3.2 การลักษณะการทำงานไหลของอิมัลชันไหลและเส้นโค้งความหนืดที่ชัดเจนเมื่อเทียบกับอัตราเฉือนและparameters of Herschel-Bulkley model of emulsions under different
processing conditions were shown in Fig. 3 and Table 1, respectively
It appeared that the Herschel-Bulkley model provides an excellent
approximation to the measurements. The flow behaviors exhibited
pronounced shear-thinning tendency, which was characterized by
relatively high viscosity at low shear rate and reached to a plateau
at high shear rate (Fig. 3). Flow index n less than 1 confirmed the
pseudoplasticity of all emulsion samples (Table 1). The values of
consistency index K were in an ascending order as frozen
emulsion < chilled emulsion < spray dried emulsion, which was in
agreement with change tendency of apparent viscosity. The pseudoplastic
behavior with yield stress suggests that weak droplet
network structure was formed at the initial stage. The decrease in
apparent viscosity with increased shear rate might be attributed to
the breakdown or reorganization of droplet network by mechanical
shear (Bellalta, Troncoso, Zúniga, ~ & Aguilera, 2012). At a given shear
rate, both the apparent viscosity and the shear stress of emulsions,
were in the same ascending order. The flow and viscosity curves of
frozen and chilled emulsions were similar and greatly differed from
those of spray dried emulsion, indicating that processing procedures
had distinct influence on flow behavior of emulsion.
Generally, as a good indicator of internal network structure
strength, yield stress is calculated by extrapolation of experiment
data in the case of shear rateg_ / 0 (Derkach, Kuhkushkina,
Levachov, & Matveenko, 2011). The frozen emulsion exhibited the
lowest yield stress, while the highest yield stress was observed for
the spray dried emulsion. Factors affecting the yield stress are
particle volume fraction, particle size and strength of interparticle
forces. In general, a decrease of particle size leads to increased yield
stress (Genovese, Lozano, & Rao, 2007). However, yield stress
increased with increasing fat droplets size of emulsions in this
work. It may be interpreted by increasing of interparticle interaction
strength as the dominant factor. The strength of interparticle
interaction is associated with the interconnection of the network
microstructure, which will increase with elimination of weak
linkages as the droplet aggregation increases (Zhou, Solomon,
Scales, & Boger, 1999). Furthermore, apparent viscosity of emulsion
also appeared higher dependence of three-dimensional
network. It meant that the emulsions with firmly network structure
should overcome more interparticle resistance force to flow, in
other words, exhibiting higher viscosity (Boode & Walstra, 1993).
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