Effect of high-pressure homogenization on droplet size distribution and rheological properties of ice cream mixes


Innocente, N.; Biasutti, M.; Venir, E.; Spaziani, M.; Marchesini, G.

Journal of Dairy Science 92(5): 1864-1875

2009


The effect of different homogenization pressures (15/3 MPa and 97/3 MPa) on fat globule size and distribution as well as on structure-property relationships of ice cream mixes was investigated. Dynamic light scattering, steady shear, and dynamic rheological analyses were performed on mixes with different fat contents (5 and 8%) and different aging times (4 and 20 h). The homogenization of ice cream mixes determined a change from bimodal to monomodal particle size distributions and a reduction in the mean particle diameter. Mean fat globule diameters were reduced at higher pressure, but the homogenization effect on size reduction was less marked with the highest fat content. The rheological behavior of mixes was influenced by both the dispersed and the continuous phases. Higher fat contents caused greater viscosity and dynamic moduli. The lower homogenization pressure (15/3 MPa) mainly affected the dispersed phase and resulted in a more pronounced viscosity reduction in the higher fat content mixes. High-pressure homogenization (97/3 MPa) greatly enhanced the viscoelastic properties and the apparent viscosity. Rheological results indicated that unhomogenized and 15/3 MPa homogenized mixes behaved as weak gels. The 97/3 MPa treatment led to stronger gels, perhaps as the overall result of a network rearrangement or interpenetrating network formation, and the fat globules were found to behave as interactive fillers. High-pressure homogenization determined the apparent viscosity of 5% fat to be comparable to that of 8% fat unhomogenized mix.

J.
Dairy
Sci.
92:1864-1875
doi:10.3168/jds.2008-1797
©American
Dairy
Science
Association,
2009.
Effect
of
high-pressure
homogenization
on
droplet
size
distribution
and
rheological
properties
of
ice
cream
mixes
N.
Innocente,
1
M.
Biasutti,
E.
Venir,
M.
Spaziani,
and
G.
Marchesini
Department
of
Food
Science,
Faculty
of
Agriculture,
University
of
Udine, Udine,
Italy
ABSTRACT
The
effect
of
different
homogenization
pressures
(15/3
MPa
and
97/3
MPa)
on
fat
globule
size
and
distribu-
tion
as
well
as
on
structure
-property
relationships
of
ice
cream
mixes
was
investigated.
Dynamic
light
scat-
tering,
steady
shear,
and
dynamic
"theological
analyses
were
performed
on
mixes
with
different
fat
contents
(5
and
8%)
and
different
aging
times
(4
and
20
h).
The
homogenization
of
ice
cream
mixes
determined
a
change
from
bimodal
to
monomodal
particle
size
distri-
butions
and
a
reduction
in
the
mean
particle
diameter.
Mean
fat
globule
diameters
were
reduced
at
higher
pressure,
but
the
homogenization
effect
on
size
reduc-
tion
was
less
marked
with
the
highest
fat
content.
The
rheological
behavior
of
mixes
was
influenced
by
both
the
dispersed
and
the
continuous
phases.
Higher
fat
contents
caused
greater
viscosity
and
dynamic
moduli.
The
lower
homogenization
pressure
(15/3
MPa)
mainly
affected
the
dispersed
phase
and
resulted
in
a
more
pronounced
viscosity
reduction
in
the
higher
fat
con-
tent
mixes.
High-pressure
homogenization
(97/3
MPa)
greatly
enhanced
the
viscoelastic
properties
and
the
apparent
viscosity.
Rheological
results
indicated
that
unhomogenized
and
15/3
MPa
homogenized
mixes
behaved
as
weak
gels.
The
97/3
MPa
treatment
led
to
stronger
gels,
perhaps
as
the
overall
result
of
a
network
rearrangement
or
interpenetrating
network
formation,
and
the
fat
globules
were
found
to
behave
as
interactive
fillers.
High-pressure
homogenization
determined
the
apparent
viscosity
of
5%
fat
to
be
comparable
to
that
of
8%
fat
unhomogenized
mix.
Key
words:
ice
cream
mix,
high-pressure
homogeni-
zation,
particle
size
distribution,
rheology
INTRODUCTION
Homogenization
is
widely
employed
in
the
food
in-
dustry
for
emulsion
stabilization
and
to
improve
the
texture,
taste,
and
fl
avor
of
many
products
including
Received
October
9,
2008.
Accepted
December
16,
2008.
1
Corresponding
author:
nadia.innocenteAuniudit
milk,
milk
cream,
and
ice
cream
mixes.
Homogenization
leads
to
a
reduction
in
size
and
an
increase
in
number
of
solid
or
liquid
particles
of
the
dispersed
phase.
During
this
process,
the
fl
uid
is
subjected
to
several
simultane-
ous
force
-induced
phenomena
such
as
cavitation,
turbu-
lence,
shear,
friction,
heat,
compression,
acceleration,
rapid
pressure
drop,
and
impact
(Phipps,
1975;
Fellows,
1988;
Paquin,
1999;
Floury
et
al.,
2000;
Roach
and
Harte,
2008).
All
these
forces
are
significantly
increased
as
pressure
increases.
For
this
reason,
homogenization
technology
has
evolved
from
systems
operating
at
less
than
50
MPa
(conventional
pressure,
CP)
to
devices
working
at
up
to
200
MPa
[high
pressure
(HP)
and
ultra
-high-pressure
homogenization
(UHP);
Floury
et
al.,
2000,
2002,
2003;
Desrumaux
and
Marcand,
2002;
Sandra
and
Dalgleish,
2005;
Bouaouina
et
al.,
2006;
Roach
and
Harte,
2008].
The
forces
involved
in
HP
homogenization
may
produce
different
effects
on
food
macromolecules
(fat,
proteins,
and
polysaccharides).
With
regard
to
fat,
the
homogenization
process
results
in
smaller
fat
globules
and
a
more
uniform
size
distribution,
thus
limiting
the
rate
of
phase
separation.
The
mean
fat
drop
size
(d)
is
reduced,
with
increasing
pressures
(P),
following
the
exponential
relationship
d
oc
P
-
m,
with
m
related
to
the
applied
pressure
and
the
fat
content
of
the
system
(Phipps,
1975;
Kessler,
1981;
Floury
et
al.,
2000, 2003;
Desrumaux
and
Marcand,
2002).
The
value
of
m
has
been
reported
as
equal
to
0.6
in
emulsions
with
a
low
dispersed
phase
fraction,
whereas
it
decreases
at
higher
fat
contents
(Floury
et
al.,
2000).
With
reference
to
milk
proteins,
there
are
few
works
in
the
literature
on
dynamic
HP
homogenization
ef-
fects.
In
oil
emulsions,
no
change
has
been
observed
in
the
primary
and
secondary
structure
of
13-LG
and
a
-LA
after
HP
treatment
>200
MPa
(Subirade
et
al.,
1998;
Paquin,
1999;
Desrumaux
and
Marcand,
2002).
Nevertheless,
it
has
been
suggested
that
the
protein
architecture
is
stabilized
by
slightly
different
interac-
tions
following
the
treatment
(Subirade
et
al.,
1998).
In
contrast,
Bouaouina
et
al.
(2006)
did
not
observe
significant
changes
in
the
native
structure
of
a
-LA
and
0-LG
when
they
were
in
solution.
In
regard
to
caseins,
their
micelle
sizes
have
been
found
to
be
modified
1864
EFFECT
OF
HIGH-PRESSURE
HOMOGENIZATION
ON
ICE
CREAM
MIXES
1865
through
HP
homogenization
treatment
depending
on
the
pressure
applied
(Hayes
and
Kelly,
2003;
Sandra
and
Dalgleish,
2005;
Roach
and
Harte,
2008).
With
regard
to
polysaccharides,
Paquin
(1999)
found
a
decrease
in
the
average
molecular
weight
of
xanthan
after
homogenization
because
of
an
irrevers-
ible
disruption
of
the
biopolymer.
A
subsequent
change
in
rheological
properties
was
observed.
Similarly,
HP
homogenization
treatment
significantly
reduced
the
average
molecular
weight
of
methylcellulose.
The
emul-
sions
stabilized
by
the
disrupted
molecules
were
found
to
lose
their
shear
thinning
behavior
and
to
undergo
a
large
decrease
in
viscosity
(Floury
et
al.,
2003).
Ice
cream
is
a
multicomponent
system
in
which
air
bubbles,
ice
crystals,
and
fat
globules
are
dispersed
in
a
freeze
-concentrated
continuous
watery
phase
consisting
of
sugars
and
polysaccharides,
milk
proteins,
and
salts
(Goff,
1997;
Muse
and
Hartel,
2004).
Creaminess,
tex-
ture,
and
meltdown
behavior
of
ice
cream
are
strongly
affected
by
partial
coalescence
of
the
destabilized
fat
globules
occurring
during
the
freezing
process
(Mar-
shall
and
Arbuckle,
1996;
Goff,
1997;
Goff
et
al.,
1999).
Fat
globule
destabilization
is
promoted
by
the
modi-
fication
of
the
membrane,
which
is
broken
down
by
homogenization.
A
subsequent
rearrangement
of
the
fat
globule
membrane
occurs
first
with
the
adsorption
of
milk
proteins,
which
are
then
partially
displaced
by
the
emulsifiers.
The
new
recombined
membrane
becomes
weaker
and
thus,
much
less
stable
to
freezing
shear
forces.
This
favors
a
partial
coalescence
of
the
desta-
bilized
fat
globules
leading
to
a
network
that
stabilizes
the
air
bubbles
and
the
foam
structure
(Marshall
and
Arbuckle,
1996;
Goff,
1997).
Homogenization
pressures
employed
in
ice
cream
mixes
generally
range
between
6
and
20
MPa
depending
on
fat
content
(Marshall
and
Arbuckle,
1996);
extensive
literature
is
available
with
regard
to
these
CP
(Schmidt
and
Smith,
1988,
1989;
Koxholt
et
al.,
2001;
Goff,
2002
among
others).
High
pressure
and
UHP
have
been
only
recently
tested
on
ice
cream
mixes
and
very
little
has
been
published
(Hayes
et
al.,
2003).
Ice
cream
mixes
are
hydrocolloidal
dispersions
in
which
proteins
and
polysaccharides
are
present
in
concentrations
above
the
phase
separation
threshold,
determining
a
2
-phase
aqueous
system,
according
to
the
concept
of
thermodynamic
incompatibility
(Tol-
stoguzov,
2003).
Gelation
of
phase
-separated
systems
can
lead
to
gels
filled
with
liquid
or
gel
-like
dispersed
particles.
Gels
are
multicomponent
systems
having
a
substantial
liquid
phase
but
exhibiting
solid
-like
behav-
ior.
Ice
cream
mixes
may
be
considered
emulsion
-filled
gels;
that
is,
macromolecular
gels
containing
dispersed
fat
particles
(filler).
The
latter
may
exhibit
different
filler
—gel
matrix
interactions,
strongly
influencing
structure
and
"theological
properties
(van
Vliet,
1988).
No
literature
is
available
with
regard
to
the
effects
of
different
homogenization
pressures
on
this
complex
structure.
In
regard
to
structure,
some
information
on
the
type
of
interactions
between
the
conformation
and
func-
tional
properties
of
molecules
and
biopolymers
can
be
obtained
by
rheological
analysis
such
as
steady
shear
fl
ow
and
dynamic
tests.
The
aim
of
this
study
was
to
investigate
the
effect
of
different
homogenization
pressures
on
fat
globule
size
and
distribution
as
well
as
on
structure
—property
relationships
of
ice
cream
mixes.
For
these
purposes,
dynamic
light
scattering,
steady
shear,
and
dynamic
rheological
analyses
were
performed
on
mixes
with
dif-
ferent
fat
contents
(5
and
8%)
homogenized
at
15/3
MPa
and
97/3
MPa
and
then
aged
for
4
and
20
h
at
4°C.
Unhomogenized
mixes
were
used
as
controls.
MATERIALS
AND
METHODS
Ingredients
and
Ice
Cream
Mix
Formulation
The
following
ingredients
were
used
to
prepare
the
ice
cream
mixes:
commercial
homogenized
and
pasteurized
whole
milk
and
milk
cream
(35%
fat)
and
sucrose
(all
purchased
in
a
local
market),
maltodextrins
(Natural
World
s.r.1.,
Ravenna,
Italy),
30
dextrose
equivalent
glucose
(Cerestar
France,
Haubourdin
Cedex,
France),
skim
milk
powder
(Bayerische
Milchindustrie,
Landshut,
Germany),
whey
protein
concentrate
(Borculo
Domo
Ingredients,
Zwolle,
the
Netherlands),
carboxymethyl-
cellulose
(Comiel
s.r.1.,
Milan,
Italy),
guar
gum
(Indian
Gum
Industries
Ltd.,
Mumbai,
India),
locust
bean
gum
(LBG
Sicilia
s.r.1.,
Ragusa,
Italy),
and
mono-
and
diglycerides
of
fatty
acids
(Natural
World
s.r.1.).
Two
formulations
were
prepared,
containing
5
and
8%
fat,
respectively.
Table
1
reports
their
overall
composition.
Ice
Cream
Mix
Processing
The
dry
ingredients
were
blended
together
and
added
to
the
liquid
ingredients
(milk
and
milk
cream)
at
65°C.
The
mix
was
then
pasteurized
at
82°C
for
8
min
and
divided
into
3
equal
portions.
One
portion
was
not
ho-
mogenized
(CON)
and
the
2
remaining
portions
were
immediately
subjected
to
homogenization
in
a
2
-stage
mode
homogenizer
(Panda
2K,
Niro
Soavi
s.p.a.,
Parma,
Italy).
Conventional
homogenization
(CP)
was
performed
at
a
primary
pressure
of
15
MPa
and
a
sec-
ondary
pressure
of
3
MPa
with
an
inlet
temperature
of
65
to
70°C.
High-pressure
homogenization
(HP)
was
conducted
at
97
MPa
and
3
MPa
with
an
inlet
tem-
perature
of
45
to
48°C
(Hayes
et
al.,
2003).
The
outlet
Journal
of
Dairy
Science
Vol.
92
No.
5,
2009
1866
INNOCENTE
ET
AL.
Table
1.
Composition
of
5
and
8%
fat
ice
cream
mixes
Item
l
5%
fat
8%
fat
Total
sugars
(000
g
of
mix)
from:
19.42
19.42
Sucrose
(%
of
total
sugars)
76.34
76.34
30
DE
glucose
(%
of
total
sugars)
21.37 21.37
Maltodextrins
(%
of
total
sugars)
2.29 2.29
Total
NMS
(000
g
of
mix)
from:
10.83
10.58
Milk
(%
of
total
NMS)
55.40
49.62
SMP
(%
of
total
NMS)
28.99
29.68
WPC
(%
of
total
NMS)
10.99
11.25
Milk
cream
(%
of
total
NMS)
4.62
9.45
Total
fat
(000
g
of
mix)
from:
5.34
7.98
Milk
cream
(%
of
total
fat)
54.68
73.31
Milk
(%
of
total
fat)
43.63
25.56
WPC
(%
of
total
fat)
1.50 1.00
SMP
(%
of
total
fat)
1.09
0.13
Emulsifiers
(000
g
of
mix)
0.5
0.5
Total
stabilizers
(000
g
of
mix)
from:
0.25 0.25
CMC
(%
of
total
stabilizers)
60 60
Guar
gum
(%
of
total
stabilizers)
20 20
Locus
bean
gum
(%
of
total
stabilizers)
20 20
Total
solids
(000
g
of
mix)
36.35
38.73
1
DE
=
dextrose
equivalents;
NMS
=
nonfat
milk
solids;
SMP
=
skim
milk
powder;
WPC
=
whey
protein
concentrate;
CMC
=
carboxymethylcellulose.
temperatures
of
all
samples
did
not
exceed
80°C.
The
CON,
CP,
and
HP
mix
samples
were
cooled
to
4°C
and
aged
for
4
and
20
h
without
stirring.
Particle
Size
Distribution
Analysis
Particle
size
distribution
of
the
mix
samples
was
mea-
sured
by
dynamic
light
scattering,
using
a
Nicomp
380
ZLS
analyzer
(Particle
Sizing
System
Nicomp,
Santa
Barbara,
CA).
All
mix
samples
were
diluted
1:1,000
with
a
dissociation
medium
(aqueous
solution
of
1%
wt/vol
SDS;
Sigma
-Aldrich,
Steinheim,
Germany).
The
measurement
parameters
were
set
at
25°C
constant
tem-
perature,
1.333
refractive
index,
scattering
angle
of
90°
for
CON
and
CP
mix
samples
and
170°
for
HP
samples.
Data
were
integrated
over
3
min.
For
size
analysis,
the
Nicomp
volume
-weighted
distribution
was
used.
Microstructure
Magnifications
(1,000
x
)
of
the
ice
cream
mixes
were
obtained
by
using
an
optical
microscope
(Axiophot,
Carl
Zeiss,
Oberkochen,
Germany)
at
ambient
temperature,
using
differential
interference
contrast
mode.
Samples
were
placed
on
a
glass
microscope
slide,
covered
with
a
glass
coverslip,
and
immediately
observed.
Rheological
Analysis
Rheological
tests
were
carried
out
by
means
of
a
con-
trolled
stress
rheometer
(StressTech
rheometer,
Reo-
logica
Instruments
AB,
Lund,
Sweden)
at
4.0
±
0.2°C,
Journal
of
Dairy
Science
Vol.
92
No.
5,
2009
using
a
cone
-plate
sensor
geometry
(cone
angle
4°,
40
mm
diameter).
Before
analysis,
the
sample
placed
in
the
rheometer
cell
rested
for
5
min
to
allow
the
stress
induced
during
loading
to
relax.
Steady
Shear
Flow.
Steady
shear
fl
ow
curves
were
determined
at
shear
rates
(y)
ranging
from
0.3160
s
-1
to
152.5
s
-1
.
The
power
law
equation
was
applied
to
samples
exhibiting
shear
thinning
and
the
Herschel
Bulkley
to
samples
with
yield
stress
(Stress
Tech
2.24
software
for
Windows,
Reologica
Instruments
AB).
Ap-
parent
viscosities
(fl
app
)
were
taken
at
a
shear
rate
of
20
s
Dynamic
Oscillatory
Measurements.
Linear
viscoelastic
regions
were
determined
by
stress
sweeps
at
a
fixed
frequency
of
1
Hz.
Mechanical
spectra
were
performed
in
the
frequency
range
0.1
to
10
Hz,
at
shear
stresses
within
the
linear
viscoelastic
region.
Statistical
Analysis
For
each
mix
sample,
at
least
3
experiments
were
in-
dependently
performed
and
all
analyses
were
carried
out
at
least
3
times
on
the
samples.
Thus,
the
data
shown
are
the
averages
of
at
least
9
values.
Student's
t
-test
was
used
to
compare
2
means,
and
one-way
ANOVA
(F
-test)
and
Tukey's
Honestly
Significant
Difference
test
were
used
for
multiple
comparisons.
In
both
cases,
the
differences
between
the
means
were
considered
sta-
tistically
significant
for
P
-values
<0.05.
All
statistical
analyses
were
conducted
using
the
software
Statistical
Discovery
JMP
3.0
for
Windows
(SAS
Institute
Inc.,
Cary,
NC).
EFFECT
OF
HIGH-PRESSURE
HOMOGENIZATION
ON
ICE
CREAM
MIXES
RESULTS
AND
DISCUSSION
Particle
Size
Distribution
As
an
example,
Figure
1
shows
the
particle
size
distributions
of
the
CON,
CP,
and
HP
homogenized
ice
cream
mixes.
The
CON
sample
showed
a
bimodal
distribution
with
a
main
group
of
particles
above
1,000
nm
and
a
second
group
at
about
100
to
150
nm
(Figure
1A).
Both
the
CP
and
HP
samples
were
characterized
by
monomodal
distributions
with
smaller
mean
diam-
eters
with
respect
to
the
main
group
of
CON
(Figure
1B,
C).
In
the
CON
mix,
the
main
group
is
attributed
to
the
fat
globules
derived from
milk
and
milk
cream,
whereas,
presumably,
the
second
group
refers
to
casein
micelles.
Indeed,
commercial
milk
homogenization
at
20
and
10
MPa
followed
by
UHT
treatment
leads
to
a
casein
micelle
diameter
of
110
to
270
nm
(Garcia-Risco
et
al.,
2002).
Moreover,
it
has
been
reported
that
ca-
sein
micelles
are
disaggregated
only
by
homogenization
pressures
of
100
MPa
(Roach
and
Harte,
2008).
The
CP
treatment
of
ice
cream
mixes
could
allow
casein
micelles
to
disaggregate
because
of
the
higher
viscosity
of
the
mixes
with
respect
to
commercial
milk
and
milk
cream.
In
fact,
with
the
increase
in
medium
viscosity,
an
increase
in
the
resulting
pressure
has
been
reported
(Phipps,
1975).
Moreover,
adsorption
of
caseins
to
the
fat
by
the
new
surface
area
created
following
CP
and
HP
may
also
occur.
As
shown
in
Figure
2,
fat
globule
mean
diameter
was
reduced
by
homogenization
even
though
the
milk
and
milk
cream
used
as
ingredients
had
already
been
sub-
jected
to
industrial
homogenization.
As
noted
above
for
casein
micelle
disappearance,
the
conventional
homog-
enization
of
mixes
would
cause
a
further
decrease
in
fat
globule
size
with
respect
to
the
CON
mixes,
because
of
the
higher
viscosity
of
mixes.
Moreover,
although
mean
diameter
was
consistently
decreased
by
CP,
HP
caused
only
a
small
additional
decrease.
Fat
globule
size
is
known
to
vary
as
a
function
of
the
applied
pressure
with
an
exponential
relationship
(Kessler,
1981;
Floury
et
al.,
2000).
In
most
cases,
the
homogenized
mixes
with
5%
fat
showed
smaller
globules
than
the
mixes
with
8%
fat.
Indeed,
fat
content
was
found
to
influence
the
relationship
between
mean
diameter
and
homogeni-
zation
pressure,
with
a
decrease
in
the
absolute
value
of
the
exponent
for
greater
fat
amounts
(Kessler,
1981;
Floury
et
al.,
2000).
Different
aging
times
did
not
affect
globule
size
distribution
as
also
observed
by
Gelin
et
al.
(1994)
and
Alvarez
et
al.
(2005).
Gelin
et
al.
(1994)
attributed
this
lack
of
difference
to
the
fact
that
no
noticeable
destabilization
of
the
fat
globule
membranes
would
occur
during
aging
until
freezing
takes
place
with
the
consequent
physical
changes.
Rd.
100
80
60
40
20
0
50
100
200
500
Rel.
100
80
60
40
20
0
1867
2K
5K
10K
20K
50K
A
-
1
2
5
10
20
50
I00
200
500
1K
B
Rel.
100
80
60
40
20
0
1
10
20
❑iameter
(nm)
C
I
50
100
200
500
1K
Figure
1.
Representative
volume
-weighted
distributions
of
ice
cream
mix
samples:
A)
unhomogenized
(CON);
B)
homogenized
at
15
and
3
MPa
pressure
(CP);
C)
homogenized
at
97
and
3
MPa
pressure
(HP).
Microstructure
Figure
3
shows
the
microstructures
of
CON,
CP,
and
HP
mixes
at
5
and
8%
fat
content.
Spheroidal
elements
are
fat
globules
that
were
smaller
in
the
CP
and
HP
samples
compared
with
the
CON
mix.
A
further
reduc-
tion
in
size
was
not
as
evident
as
it
was
when
observed
with
dynamic
light
scattering
measurements.
The
8%
Journal
of
Dairy
Science
Vol.
92
No.
5,
2009
1868
Mean
Diameter
(nm)
Mean
Diameter
(nm)
1000
-
900
-
800
-
700
-
600
-
500
-
400
-
300
-
200
-
100
-
0
a
844
1000
909
-
809
-
700
-
609
-
500
-
409
-
300
-
209
-
100
-
0
A
T
818
5%
CON
8%
CON
a
A
874
5%
CON
8%
CON
INNOCENTE
ET
AL.
1)
I
289
13
36
5%_CP
8%
CP
b
I
329
I
376
5')/o
CP
89/0
CP
C
[
219
1
B
5%
HP
8°/0
HP
C
216
2
277
5'%L
HP
8%
HP
Figure
2.
Mean
fat
globule
volume
-weighted
diameters
of
ice
cream
mixes
with
5%
and
8%
fat
content
and
aged
for
4
h
(panel
1)
or
20
h
(panel
2).
CON
=
unhomogenized;
CP
=
homogenized
at
15
and
3
MPa
pressure;
HP
=
homogenized
at
97
and
3
MPa
pressure.
Statistical
analysis
(Tukey's
HSD
test,
P
<
0.05)
was
carried
out within
groups
at
different
fat
content;
lowercase
letters
refer
to
5%
fat
content
mixes,
and
uppercase
letters
refer
to
8%
fat
content
mixes.
Asterisks
mark
statistically
different
pairs
(Student's
t
-test,
P
<
0.05).
fat
HP
sample
showed
a
more
pronounced
3
-dimen-
sional
structure.
Steady
Shear
Flow
Behavior
Steady
shear
fl
ow
determinations
allow
the
shear
stress
(6
-
)
curves
as
a
function
of
shear
rate
(y)
to
be
determined.
Figure
4
shows
representative
steady
shear
fl
ow
curves
of
5
and
8%
fat
ice
cream
mixes
aged
for
4
and
20
h.
All
samples
were
non
-Newtonian.
The
CON
and
CP
samples
were
shear
-thinning
and
hence,
were
described
by
the
power
law
equation
(R
2
=
1).
The
HP
sample
showed
a
yield
stress,
which
is
the
characteristic
behavior
fitted
by
the
Herschel-Bulkley
equation
(R
2
=
0.999).
In
shear
-thinning
systems
(n
<
1),
dispersed
particles
may
disaggregate
or
orient
in
the
direction
of
fl
ow,
opposing
less
resistance
(Lapasin
and
Pricl,
1995;
Aguilera
and
Stanley,
1999).
Both
fat
globules
and
sta-
bilizers
(locust
bean
gum
and
guar
gum)
may
be
re-
sponsible
for
the
shear
-thinning
behavior
of
the
mixes
(Cottrell
et
al.,
1980;
Goff
et
al.,
1994).
With
regard
to
HP
mixes,
the
logarithmic
plot
of
shear
stress
versus
shear
rate
reported
in
Figure
5
enables
the
plastic
be-
havior
of
these
samples
to
be
further
highlighted
(Lapa-
sin
and
Pricl,
1995).
In
systems
with
a
yield
point,
a
network
structure
has
to
be
broken
prior
to
fl
ow.
High-
er
yield
stress
values
marked
the
8%
fat
mixes
(Figure
4),
indicating
more
firmly
structured
systems
and,
ac-
cording
to
Rao
(2007),
more
stable
emulsions.
In
par-
ticular,
mean
yield
stress
values
increased
from
about
20
Pa
for
5%
fat
HP
mixes
to
about
80
Pa
for
8%
fat
HP
mixes.
In
general,
the
magnitude
of
the
yield
stress
Journal
of
Dairy
Science
Vol.
92
No.
5,
2009
EFFECT
OF
HIGH-PRESSURE
HOMOGENIZATION
ON
ICE
CREAM
MIXES
1869
25
}gym
.4
"
A
'
14
4
"
411
25
rem
.
25
}ari
$•
.0 2.
. ,
4
:
:•
-•
1
,
7
f;.-:/:
r
•.=
-
CON
CP
HP
25
rim
25
8
,4
6
'
,
;
-
./04
t
••
f
ox
,
..
•-•.••
-
si"
I
fl
If
a
CP
HP
Figure
3.
Optical
light
microscope
images
of
mixes
at
5%
(left
column)
and
8%
(right
column)
fat
content.
CON
=
unhomogenized;
CP
=
homogenized
at
15
and
3
MPa
pressure;
HP
=
homogenized
at
97
and
3
MPa
pressure.
Magnification
1,000x;
scale
bar
=
25
µm.
increases
with
increasing
particle
volume
fraction,
de-
creasing
particle
size,
and
increasing
magnitude
of
in-
terparticle
forces
(Poslinski
et
al.,
1988;
Genovese
et
al.,
2007).
Flow
curves
of
the
5%
fat
samples
(Figure
4)
changed
following
aging,
and
a
shift
to
higher
stress
values
was
observed.
Analogously,
apparent
viscosity
at
20
s
-1
of
5%
fat
CON
and
CP
mixes
significantly
in-
creased
after
20
h
of
aging
(Table
2).
This
was
expected
because
the
hydration
of
proteins
and
stabilizers
during
aging
causes
a
viscosity
increase
(Marshall
and
Ar-
buckle,
1996;
Goff,
1997)
possibly
because
of
protein
displacement
from
the
surface
back
to
the
serum
phase.
No
aging
effect
was
observed
in
5%
fat
HP
mixes
or
in
any
of
the
8%
fat
mixes.
The
latter
had
more
surface
area
and
hence,
more
adsorbed
proteins,
resulting
in
a
faster
stabilizing
effect.
Apparent
viscosities
were
lower
for
CP
and
higher
for
HP
compared
with
CON
for
both
5%
fat
and
8%
fat
samples
(Table
2).
Schmidt
and
Smith
(1989)
ob-
served
a
viscosity
decrease
in
mixes
homogenized
at
CP
close
to
the
values
used
in
the
present
study.
Those
authors
attributed
the
lowering
of
viscosity
to
changes
in
the
size
of
the
fat
globules.
With
smaller
fat
glob-
ules,
internal
resistance
is
less
than
with
larger
ones
or
linear
chains
of
globules.
High-pressure
homogenization
caused
a
marked
increase
in
the
T
app
of
mixes
(Table
2),
as
was
reported
by
Hayes
et
al.
(2003),
who
at-
tributed
the
viscosity
increase
to
greater
amounts
of
Journal
of
Dairy
Science
Vol.
92
No.
5,
2009
1870
12.
de
r#)
120
00
80
60
A•
INNOCENTE
ET
AL.
5
%
A'
CON4h
—0—
CON20h
CP4h
AA
.46A
40
-
—0—
CP20h
20
n-
0
-
-
HP4h
0
Shear
stress
(Pa)
0
30
60
90
120
Shear
rate
(s
A
)
150
180
A
HP2Oh
200
8%
-6-
CON4h
180
.J
160
CON20h
140
120
.
100
-6-
CP4h
80
60
CP20h
40
20
A
-
HP4h
0
0
30
60
90
120 150
180
Shear
rate
(s
-1
)
A
HP2Oh
Figure
4.
Representative
steady
shear
flow
curves
of
5%
fat
and
8%
fat
ice
cream
mixes
aged
for
4
and
20
h.
CON
=
unhomogenized;
CP
=
homogenized
at
15
and
3
MPa
pressure;
HP
=
homogenized
at
97
and
3
MPa
pressure.
absorbed
proteins
or
more
tightly
packed
proteins
at
the
oil
—water
interface.
Viscosity
changes
after
process-
ing
were
more
pronounced
in
high
fat
content
samples,
which
also
had
a
higher
initial
rl
app
.
Similarly,
Hayes
et
al.
(2003)
reported
higher
viscosities
for
the
8%
fat
HP
and
CP
mixes
compared
with
the
5%
fat
mixes.
They
also
reported
comparable
viscosities
for
5%
fat
HP
and
8%
fat
CP
ice
cream
mixes.
In
the
present
case,
the
viscosity
of
the
5%
fat
HP
mix
was
comparable
to
that
of
the
8%
fat
CON
mix.
This
difference
can
be
attributed
to
the
mix
formulations
used
in
the
present
study,
which
differed
from
those
of
Hayes
et
al.
(2003)
mainly
because
of
a
greater
amount
of
stabilizers
and
emulsifiers.
Dynamic
Properties
Mechanical
spectra
were
performed
in
the
frequency
range
0.1
to
10
Hz,
at
shear
stresses
within
the
linear
viscoelastic
region
(previously
determined
by
frequency
Journal
of
Dairy
Science
Vol.
92
No.
5,
2009
Log
shear
stress
(Paxs)
1.5
1
-
EFFECT
OF
HIGH-PRESSURE
HOMOGENIZATION
ON
ICE
CREAM
MIXES
-
CON
5%
m
CON
8%
0—
CP
5%
—0—
CP
8%
HP
5%
A
HP
8%
I--
N-L.
I
\
.
6
_
.i
.
..
.1
.1
Log
Shear
stress
a
0
1
-1
Log
shear
rate
(s
)
2
3
Log
Shear
rate
1871
Figure
5.
Logarithmic
plot
of
shear
stress
versus
shear
rate
of
ice
cream
mixes.
CON
=
unhomogenized;
CP
=
homogenized
at
15
and
3
MPa
pressure;
HP
=
homogenized
at
97
and
3
MPa
pressure.
Inner
rectangle:
Theoretical
shear
thinning
(a)
and
plastic
(b)
behaviors
reported
in
Lapasin
and
Pricl
(1995).
stress
sweeps).
Linear
viscoelastic
ranges
were
seen
to
be
wider
and
at
higher
stress
values
for
HP
-treated
samples
(data
not
shown).
Critical
deformations;
that
is,
the
maximum
strain
characterizing
the
limit
of
the
linear
viscoelastic
regimen,
were
always
less
than
ap-
proximately
6%.
In
the
literature,
linear
viscoelastic
strain
is
reported
to
extend
to
approximately
20%
for
strong
gels,
whereas
for
weak
gels
it
is
usually
<5%,
but
can
be
very
much
smaller
(up
to
1,000
times
smaller;
Ross
-Murphy,
1995).
An
investigation
of
gel
properties
was
performed
by
means
of
oscillatory
measurements
at
a
constant
stress
in
the
frequency
range
0.1
to
10
Hz.
Figure
6
shows
a
representative
mechanical
spectrum
of
an
HP
mix
in
which
the
elastic
modulus
(G')
is
always
higher
than
the
viscous
modulus
(G")
and
both
are
slightly
frequency
dependent;
the
phase
angle
is
within
20
to
30°
and
also
slightly
frequency
dependent;
complex
viscosity
decreases
with
increasing
frequency.
Systems
with
these
characteristics
are
referred
to
as
weak
gels
(Lapasin
and
Pricl,
1995;
Ross
-Murphy,
1995).
Similar
trends,
which
can
be
mainly
attributed
to
the
colloidal
phase,
were
obtained
for
all
samples,
including
the
CON
mixes.
Viscoelastic
properties
may
arise
from
pasteurization,
which
causes
some
protein
denaturation
and
contributes
to
gel
formation.
Although
all
samples
exhibited
gel
-like
properties,
differences
in
the
absolute
value
(compared
at
1
Hz)
of
dynamic
moduli
were
observed.
Figure
7
shows
the
Table
2.
Mean
values
and
standard
deviations
of
apparent
viscosity
(Pa
-s;
shear
rate:
20
s
-1
)
of
5%
fat
and
8%
fat
ice
cream
mixes
subjected
or
not
to
homogenization
and
aged
for
4
and
20
h
5%
fat
8%
fat
Ice
cream
mix
1
4h
20h
4h
20h
CON
0.85
a6
±
0.67*
1.44
6
±
0.05*
2.07
6
±
0.06
2.06
6
±
0.32
CP
0.31
6
±
0.07*
0.40
c
±
0.03*
0.53'
±
0.05
0.52'
±
0.01
HP
1.85'
±
0.79
2.28
a
±
0.55
4.70
a
±
0.72
4.57
a
±
0.18
a
-
`Different
letters
within
the
same
column
refer
to
statistical
differences
(Tukey's
HSD
test,
P
<
0.05).
I
CON
(control)
=
unhomogenized
mix;
CP
(conventional
pressure)
=
mix
homogenized
at
15
and
3
MPa;
HP
(high
pressure)
=
mix
homogenized
at
97
and
3
MPa.
*Within
the
same
row,
asterisks
indicate
statistical
differences
between
4
and
20
h
aging
time
(Student's
t
-test,
P
<
0.05).
Journal
of
Dairy
Science
Vol.
92
No.
5,
2009
1872
log
G*,
G',
G"
(Pa),
11*
(Pa•s)
INNOCENTE
ET
AL.
4
3,5
3
2,5
2
1,5
ea
0.)
to
A
G'
1
1
*
--+---
Phase
0,5
0,5
0
-1
-0,5
0
log
t
(Hz)
0,5
Figure
6.
Representative
mechanical
spectrum
of
an
HP
(high-pressure)
mix
(homogenized
at
97
and
3
MPa).
mean
values
and
the
standard
deviations
of
complex
moduli
(G*)
of
CON,
CP,
and
HP
ice
cream
mixes
at
5
and
8%
fat
content
aged
for
4
and
20
h.
The
complex
modulus
G*
was
chosen
as
an
indication
of
the
system
strength;
G*
of
5%
fat
CP
mixes
was
the
lowest
and
G*
of
HP
8%
fat
the
highest.
All
5%
fat
mixes
showed
a
G*
increase
at
20
h
compared
with
4
h.
Conversely,
in
the
8%
fat
samples,
G*
was
almost
constant
suggesting
that
higher
fat
contents
had
a
stabilizing
effect
on
the
viscoelastic
properties
of
mixes,
as
was
also
observed
for
apparent
viscosity
(Table
2).
The
HP
samples,
both
ez-
5
and
8%
fat,
showed
higher
G*.
As
described
above,
ice
cream
mixes
may
be
considered
as
emulsion
-filled
gels.
The
elastic
modulus
G'
in
HP
mixes
markedly
increased
from
258
Pa
in
5%
fat
mixes
to
1,804
Pa
in
8%
fat
mixes;
therefore
it
can
be
inferred
that,
in
these
systems,
fat
behaves
as
an
interactive
filler.
This
may
arise
from
HP
treatment,
because
CP
and
CON
did
not
show
an
increase
in
G'.
Another
useful
parameter
to
characterize
a
viscoelas-
tic
system
is
the
ratio
G":G',
named
loss
tangent
(tan
8),
which
is
a
measure
of
the
viscous/elastic
ratio
for
a
10000-
CI
CON
5%
El
CP
5%
1000-
HP
5%
100-
El
CON
8%
OCP
8%
10-
In
HP
8%
1
4h
20
h
Figure
7.
Mean
values
and
standard
deviations
of
complex
modulus
of
ice
cream
mixes
at
5%
and
8%
fat
content
aged
for
4
and
20
h.
CON
=
unhomogenized;
CP
=
homogenized
at
15
and
3
MPa
pressure;
HP
=
homogenized
at
97
and
3
MPa
pressure.
Journal
of
Dairy
Science
Vol.
92
No.
5,
2009
EFFECT
OF
HIGH-PRESSURE
HOMOGENIZATION
ON
ICE
CREAM
MIXES
1873
Table
3.
Mean
values
and
standard
deviations
of
calculated
tan
6
of
ice
cream
mixes
at
5%
and
8%
fat
content
aged
for
4
and
20
h
Ice
cream
mix
l
5%
fat
8%
fat
4h
20h
4h
20h
CON
0.70
±
0.08
0.57
±
0.02
0.66
±
0.09
0.54
±
0.04
CP
0.72
±
0.02
0.53
±
0.03
0.55
±
0.02
0.53
±
0.02
HP
0.38
±
0.03
0.36
±
0.04
0.33
±
0.01
0.35
±
0.02
I
CON
(control)
=
unhomogenized
mix;
CP
(conventional
pressure)
=
mix
homogenized
at
15
and
3
MPa;
HP
(high
pressure)
=
mix
homogenized
at
97
and
3
MPa.
material
at
frequency
LA.;
(Ross
-Murphy,
1995).
The
loss
tangent
curve
did
not
show
variations
over
the
frequency
range
0.1
to
10
Hz
(data
not
shown).
Values
of
tan
8
at
1
Hz
frequency
are
compared
in
Table
3.
Both
5
and
8%
fat
HP
mixes
showed
tan
8
lower
than
those
of
CP
and
CON,
indicating
a
more
structured
gel
-like
system.
The
HP
homogenization
would
give
structure
to
the
system
and
strengthen
the
elastic
component,
possibly
as
a
consequence
of
pressure
-induced
colloidal
interactions
and
network
formation.
The
Cox-Merz
superposition
rule
constitutes
a
further
criterion
for
the
distinction
between
gel
systems
and
biopolymer
entangled
solu-
tions,
by
combining
oscillatory
and
steady
shear
mea-
surements.
In
the
Cox-Merz
superposition
the
curves
of
ft
versus
shear
rate
and
ft*
versus
LA.;
are
plotted
on
the
same
diagram.
The
results,
reported
in
Figure
8,
show
that
in
all
cases
the
curves
are
not
superposed,
which
is
typical
of
colloidal
and
particulate
gel
networks
(Ross
-
Murphy,
1995).
In
CON
samples,
the
curves
tend
to
cross
with
increasing
shear
rate
and
frequency,
whereas
in
CP
mixes,
this
tendency
was
less
clear.
In
HP
mixes
the
2
curves
were
in
parallel.
The
convergence
of
the
2
curves
refers
to
weak
gels,
whereas
the
parallelism
is
typical
of
strong
gels
(Ross
-Murphy,
1995).
According
to
these
data,
CON
and
CP
mixes
behaved
as
weak
gels
and
HP
samples
as
stronger
gels.
High-pressure
homog-
enization
on
a
model
oil
-in
-water
emulsion
was
used
by
Floury
et
al.
(2000)
and
they
related
the
effect
of
HP
homogenization
not
only
to
the
change
in
emulsion
droplet
size
but
also
to
the
properties
of
the
stabilizer
molecules.
The
authors
hypothesized
an
unfolding
or
partial
denaturation
of
the
globular
proteins
caused
by
treatments
at
high
pressure
and
simultaneously
high
temperature.
A
similar
mechanism
may
be
suggested
to
explain
the
HP
structuring
effect
on
the
ice
cream
mixes;
that
is,
the
HP
treatment
could
cause
the
unfolding
or
partial
denaturation
of
both
whey
protein
and
caseins
leading
to
a
network
formation
or
rearrangement
or
to
interpenetrating
networks
within
the
colloidal
phase.
In
contrast,
CP
pressures
would
not
enable
changes
at
such
extents.
Similarly,
Floury
et
al.
(2002)
reported
a
strong
gel
-type
behavior
for
soy
protein
-stabilized
emulsions
when
homogenized
at
UHP
(250
and
350
MPa).
The
authors
suggested
that
disulfide
bonding
and
noncovalent
bonds
such
as
hydrophobic
interaction
and
ionic
and
hydrogen
bonding
may
be
involved
in
the
gel
-like
formation
when
produced
at
homogenizing
pressures
>200
MPa.
CONCLUSIONS
Homogenization
of
ice
cream
mixes
resulted
in
a
change
from
bimodal
to
monomodal
particle
size
dis-
tributions
with
a
progressive
reduction
in
the
mean
particle
diameter
as
pressure
increased
from
conven-
tional
to
high
values.
Because
the
milk
and
milk
cream
used
as
ingredients
in
mix
formulation
were
already
conventionally
homogenized,
the
further
modification
in
the
particle
sizes
and
distributions
following
the
CP
process
of
the
mixes
could
be
attributed
to
the
effects
of
the
increased
viscosity
of
ice
cream
mixes.
Fat
globule
mean
diameters
were
reduced
at
higher
pressure,
but
the
homogenization
effect
on
size
reduction
was
limited
with
the
highest
fat
content.
The
rheological
behavior
of
ice
cream
mixes
was
determined
by
both
the
dispersed
and
the
continuous
phase.
In
most
cases,
the
dispersed
phase
accounted
mainly
for
viscosity,
whereas
the
colloidal
phase
was
responsible
for
the
viscoelastic
properties
and
gel
behavior.
Higher
fat
contents
resulted
in
both
higher
viscosity
and
dynamic
moduli.
Homogenization
at
CP
mainly
affected
the
dispersed
phase
and
determined
a
more
pronounced
viscosity
reduction
in
the
higher
fat
content
mixes.
This
effect
was
related
to
fat
globule
size
reduction,
whereas
the
viscoelastic
properties
were
only
slightly
influenced
by
the
process.
Homogenization
at
HP
caused
a
further
slight
decrease
in
particle
size
with
respect
to
CP,
but
a
strong
enhancement
of
the
viscoelastic
properties
and
apparent
viscosity.
Hence,
the
HP
treatment
acted
mainly
on
the
colloidal
phase.
Rheological
results
indicated
that
unhomogenized
and
CP
mixes
behaved
as
weak
gels,
whereas
HP
treat-
ment
led
to
stronger
gels
possibly
as
the
overall
result
of
a
network
rearrangement
or
interpenetrating
network
Journal
of
Dairy
Science
Vol.
92
No.
5,
2009
1874
1000
100
0.
a.
a.
I0
0,1
100
0,1
'7
INNOCENTE
ET
AL.
CON
5%
0I
1'0
uo
tttritE
144,
44,0041,0404.00
:
44•4#
100
CP
5%
0I
1000
-
100
-
10
-
0,1
10
100
HP
5%
s
ae
2222
0,1
1'0
!Oa
(s
-
');
a
(rad-s
-1
)
Figure
8.
Cox-Merz
superposition
of
steady
shear
flow
viscosity
(II)
El)
for
samples
at
5%
fat
(left
column)
and
8%
fat
(right
column)
aged
formation
and
the
fat
globules
were
found
to
behave
as
active
filler.
It
should
be
noted
that
the
HP
pro-
cess
produced
an
apparent
viscosity
in
the
5%
fat
mix
comparable
to
that
of
the
8%
fat
CON
mix.
Further
research
needs
to
be
undertaken
to
determine
whether
HP
homogenization
of
ice
cream
mixes
could
enable
the
production
of
lower
fat
ice
creams
with
similar
charac-
teristics
to
those
of
higher
fat
ice
creams.
ACKNOWLEDGMENTS
The
authors
are
grateful
to
Enrico
Maltini
(Depart-
ment
of
Food
Science,
University
of
Udine)
for
his
help-
ful
advice.
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