Heat stability of milk
Mike Lewis
Heat Stability
• Poor heat stability may influence the following
operations:
•
•
•
•
•
Pasteurisation
Sterilisation
UHT
In-container (especially concentrates)
Evaporation
Milk sterilised at 120 C for 15 min
Increasing
additions
Different phosphates, citrate, EDTA and calcium chloride
additions
Effects of poor heat stability
• In UHT treatment, milk having poor heat
stability can give rise to fouling of heat
exchangers and sediment formation. It is best
to avoid UHT treating such milk.
• These problems may be encountered to a
lesser extent in pasteurisation and ESL milks
• Evaporators may also be susceptible to fouling
and hence to longer cleaning regimes
• Fouling of membranes also occurs
Methods for assessing heat stability
•
•
•
•
•
•
•
•
Heat coagulation temperature
Heat coagulation time
Viscosity measurement
Heat exchanger fouling
Sediment formation
Coagulation
Casein micelle size
Two main determinants of heat stability are
pH and ionic calcium
Heat coagulation time-pH profiles
Some observations on this method
• Why does heat stability go through a
maximum and minimum.
• As pH is increased, Ca2+ decreases, micelles
are less susceptible to coagulation.
• At pH maximum, K-casein dissociation occurs,
this has a destabilising effect
• However at minimum, Ca2+ is further educed
and this becomes the dominating influence
• There is an element of subjectivity in
determining the point at which coagulation
first takes place.
• However, it is claimed that it is possible to
obtain reproducible results with practice.
• A variant on this procedure is to determine
the heat coagulation temperature, which is
the temperature required to coagulate milk in
a given time period (Rose, 1962; Horne and
Muir, 1990).
Noteworthy Observations
• However, despite all the research work, problems related
to poor heat stability of milk are still encountered in
commercial processes.
• Singh (2004) stated that the heat coagulation time (heat
stability) often correlates very poorly with the stability of
milk on commercial sterilization.
• He also pointed out that from an industry point of view,
the use of a pilot scale or laboratory scale sterilizer which
simulates sterilization conditions used in practice
provides more reliable results and prediction of
behaviour of milk in commercial plants, although this is
not possible without access to pilot plant facilities.
Some studies on pH and ionic calcium
relationships in cow’s and goat’s milk
Total calcium and ionic calcium in milk of
different species (average values)
Ionic calcium
(mM)
Human
Total Calcium
(mM)
7.5
Cow
30
1.80
Goat
34
2.67
Sheep
55
2.01
Mouse
71
?
Human Blood
2.05-2.5
1.1 – 1.32
3.0
Measuring ionic calcium in milk
• Ion-exchange equilibration, Christianson et al. (1954).
• Murexide complexing agent, Tessier and Rose (1958); White and
Davis (1958).
• Use of an ion selective electrode designed for measuring
calcium in blood (measures activity).
• Development of standards with pH (6.7) and ionic strength
(81mM) similar to milk
• Calibration of the system prior to use daily: (with five standards,
over range 0.25 to 3.00 mM)
Immersion type and flow-though
electrodes
Calibration for ionic calcium meter
log (concentration) mM
0.6
0.5
0.4
0.3
0.2
0.1
0
76
78
80
82
84
mV
86
88
90
92
Guidelines for reliable results
• Immersion probe or flow-through probes
• Divalent ions: ~ 29 mV change for 10 fold
change in concentration
• 8 to 9 mV per doubling of concentration
• Important to check performance and
reproducibility of the electrode
• Calibrate every day prior to use.
Ionic calcium and alcohol stability for 10 different cows
over a complete lactation period
Cow
No
6848
227
6737
6789
6653
6930
36
6790
6747
6596
Ionic Calcium (mM)
Mean
1.53
1.87
1.92
1.97
2.14
2.23
2.30
2.38
2.40
2.67
sd
0.22
0.19
0.29
0.41
0.44
0.65
0.53
0.56
0.38
0.57
Alcohol stability (%)
Mean
84%
87%
79%
79%
75%
78%
79%
72%
76%
68%
sd
5%
7%
7%
7%
10%
12%
6%
11%
5%
7%
Distribution of ionic calcium in cow’s and goat’s milk
20
18
Samples (%)
16
14
12
cow's milk
10
goat's milk
8
6
4
2
0
1.0-
1.2-
1.4-
1.6-
1.8-
2.0-
2.2-
2.4-
2.6-
2.8-
3.0-
3.2-
3.4-
3.6-
3.8-
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
ionic calcium (mM)
Distribution of ethanol stability in cow’s and goat’s
milk
35
samples (%)
30
25
20
cow's milk
15
goat's milk
10
5
0
3539
4044
4549
5054
5559
6064
6569
7074
7579
ethanol stability (%)
8084
8589
9094
9599
Ionic calcium against adjusted pH for several cows
log [Ca2+] (mM)
0.8
0.6
0.4
0.2
0.0
-0.2
5.8
6.0
6.2
6.4
6.6
6.8
7.0
pH
6928, pH 6.71
7191, pH 6.67
6818, pH 6.70
26, pH 6.71
42, pH 6.69
57, pH 6.73
58, pH 6.78
bulk, pH 6.66
3 Ca++ + 2HPO42-  Ca3PO4 + 2H+
46, pH 6.56
Changes in pH and ionic calcium caused by different
events
4.00
ionic calcium (mM)
3.50
RO
evap
3.00
UF
TSC
DSHP
2.50
Series1
Linear (Series1)
CaCl2
2.00
1.50
acid
1.00
R2 = 0.399
sterilise
SHMP
Freeze?
0.50
HTST
UHT
6.45
6.50
6.55
6.60
6.65
pH
6.70
6.75
6.80
6.85
Salts that could be used
•
•
•
•
•
•
•
•
Trisodium citrate (TSC)
Dihydrogen sodium phosphate (DHSP)
Disodium hydrogen phosphate (DHSP)
Sodium hexametaphosphate (SHMP)
EDTA (disodium salt)
EDTA ( tetrasodium salt)
EDTA ( disodium,calcium salt)
Calcium chloride
Some calcium salts
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•
•
•
•
•
•
•
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Calcium carbonates:
Calcium lactate:
Calcium citrates
Calcium phosphates
Calcium chloride
Calcium hydroxide
Calcium gluconate
E170
E327
E333
E341
E509
E526
E578
Calcium acetate (E263); calcium tartrate (E354); calcium
sulphate (E516); calcium oxide (E529), calcium ferrocyanide
(E538), Calcium silicate (E552), calcium diglutamate (E623),
calcium guanylate (E628), Calcium Inosinate (E362); calcium
5’-ribonucleotides (E634)
• Calcium DISODIUM ethylene diamine tetra acetate (E385)
Measuring sediment in milk
• Centrifuge milk at ~ 3000 G for 30 min to
1h
• Useful for assessing heat stability
• Emulsion stability during storage
• All milk contains sediment
• One important question is how much
sediment is required before it can be
detected by taste?
Heat Stability Measurement (our method)
Sediment from whole milk stored for 4
months at different temperatures
In-container and UHT products
• Comparison studies for goat’s milk and cow’s
milk
• Measuring sediment and casein micelle size
• Making goat milk behave like cow milk and
vice versa.
• One conclusion is that on some occasions
there are quite big differences between incontainer and UHT sterilisation.
The effect of added DSHP, TSC and CaCl2 on the sediment
following UHT and in-container sterilisation for three fully
replicated trials for goat milk
6.00
Sediment Percentage (dwb%)
5.00
4.00
In-direct UHT
3.00
In-container
Sterilisation
2.00
1.00
0.00
CaCl2 2.0
(mM)
-1.00
CaCl2 1.0
(mM)
CaCl2 0.5
(mM)
Control
DSHP 6.4
(mM)
TSC 6.4
(mM)
DSHP 12.8
(mM)
TSC 12.8
(mM)
Bovine milk
The effect of added DSHP, TSC and CaCl2 on the sediment
following UHT and in-container sterilisation.
1.80
percentage of dry sediment %
1.60
UHT
1.40
1.20
1.00
0.80
0.60
In-container
sterilisation
0.40
0.20
0.00
CaCl2 2mM
Control
DSHP 10mM
Milk samples
TSC 10mM
Bovine milk
The effect of added DSHP, TSC and CaCl2 on the casein
micelle size following UHT and in-container sterilisation
1200.00
Casein micelle size d.nm
1000.00
Original
800.00
600.00
UHT
400.00
200.00
.
In-container
sterilisation
0.00
CaCl2 2mM
Control
DSHP 10mM
Milk samples
TSC 10mM
Thee Effect of added DSHP, TSC and CaCl2 on pH
and Ca2+ before and after UHT treatment
Before heat
treatment
Properties
Milk samples
Mean±SD
pH
Control
6.78±0.03b 6.70 - 6.85
6.77±0.03b 6.71 - 6.83
10 mM DSHP
10 mM TSC
6.96±0.03c 6.89 - 7.01
7.05±0.03d 6.99 - 7.11
6.93±0.03c 6.86 - 6.99
7.03±0.03d 6.97 - 7.10
2 mM CaCl2
Control
10 mM DSHP
10 mM TSC
6.71±0.03a
1.91±0.23b
0.87±0.10a
0.93±0.13a
6.70±0.03a
1.81±0.24b
0.84±0.13a
0.92±0.13a
2 mM CaCl2
2.34±0.27c 1.81 - 2.84
Ca2+ (mM)
Range
Indirect UHT
6.65 - 6.78
1.38 - 2.29
0.65 - 1.06
0.68 - 1.14
Mean±SD
Range
6.64 - 6.76
1.43 - 2.44
0.63 - 1.10
0.68 - 1.24
2.14±0.26c 1.75 - 2.63
Table 3. Effect of added DSHP, TSC and CaCl2 on the dry
sediment percentage after UHT treatment
PS: SP=Spring; SM=Summer; A=Autumn; W=Winter;
NS=Non-significant difference
Indirect UHT
Properties
Milk samples
Mean±SD Range
Seasonal
variation
Heat stability
(Dry sediment %)
Control
0.19±0.05a 0.10 - 0.29
SM > A and W
10 mM DSHP
0.24±0.11ab 0.12 - 0.62
SP > A and W SM
>A
10 mM TSC
0.33±0.23b 0.12 - 1.31
SP > A and W
2 mM CaCl2
0.31±0.10b 0.18 - 0.51
SM > SP, A and W
Table 4. Effect of added DSHP, TSC and CaCl2 on
pH and Ca2+ before and after in-container
sterilisation.
In-container
Before heat treatment sterilisation
Properties
Milk samples
Mean±SD
Range
Mean±SD Range
pH
Control
6.78±0.03b
6.70 - 6.85
6.62±0.03c 6.57 - 6.70
10 mM DSHP
6.96±0.03c
6.89 - 7.01
6.59±0.03b 6.52 - 6.65
10 mM TSC
7.05±0.03d
6.99 - 7.11
6.80±0.03d 6.70 - 6.86
2 mM CaCl2
6.71±0.03a
6.65 - 6.78
6.55±0.03a 6.49 - 6.62
Control
1.91±0.23b
1.38 - 2.29
1.85±0.24c 1.35 - 2.20
10 mM DSHP
0.87±0.10a
0.65 - 1.06
0.64±0.15a 0.36 - 1.13
10 mM TSC
0.93±0.13a
0.68 - 1.14
1.01±0.18b 0.72 - 1.44
2 mM CaCl2
2.34±0.27c
1.81 - 2.84
2.23±0.30d 1.60 - 2.64
Ca2+ (mM)
Table 5. Effect of added DSHP, TSC and CaCl2 on the dry
sediment percentage after in-container sterilisation
PS: SP=Spring; SM=Summer; A=Autumn; W=Winter;
NS=Non-significant difference
In-container sterilisation
Seasonal
variation
Properties
Milk samples
Mean±SD Range
Heat stability
(Dry sediment %)
Control
0.24±0.10a 0.02 - 0.56 NS
10 mM DSHP
1.15±0.44c 0.31 - 1.77 W > A
10 mM TSC
W > SM and A;
0.75±0.38b 0.16 - 1.65 SP > A
2 mM CaCl2
0.23±0.07a 0.13 - 0.39 SM > SP
Miniature UHT plant
T
P
P
Hot water in
Main
heater
T
T
Water Product
tank tank
Pump
3-way valve
P
Cold
water in
Out
Cooler
Steam in
Overflow
valve
Backpressure
valve
Filter
Steam
out
Preheater
Flow
meter
T
Out
P
Monitoring Fouling
• Monitoring temperature, pressure and flow
rate and overall heat transfer coefficient.
• OHTC = U  GC p
A
lm
Where U = overall heat transfer coefficient in kcal/hr-m2
G = mass flow rate in kg/h
Cp = specific heat of milk in kcal/kg -C
Δθ = Temperature difference in °C
A = surface area of the tubings in m2
ΔTlm = log mean temperature difference in °C =
[(steam in – milk out)-(steam out – milk in)]/ log [(steam in –milk
out)/(steam out – milk in)]
Comparison of cows’ and
goats’ milk
Cow VS Goat
3500
2500
2000
1500
Cow
1000
Goat
500
Time, S
3180
2940
2700
2460
2220
1980
1740
1500
1260
1020
780
540
300
0
60
OHTC, W/m
2
K
3000
Table showing the effect of additives on pH, ethanol
stability and ionic calcium for goat’s milk
pH
Ionic
Calcium
(mMol)
Ethanol
stability
(%)
Fresh goat milk
6.63
2.55
54
Fresh goat milk with
0.2% tri-sodium citrate
6.76
1.24
76
Fresh goat milk with
0.2% sodium hexa
metaphosphate
6.73
0.83
98
Fresh goat milk with
resins (0.3%)
6.62
1.72
68
Comparison of ionic calcium
reduced milk with raw goats’ milk
3500
2500
Phosphate
2000
Citrate
1500
Resins
Raw
1000
500
Time, S
2040
1860
1680
1500
1320
1140
960
780
600
420
240
0
60
2
OHTC, W/m K
3000
Table showing the effect of CaCl2 on pH,
ethanol stability and ionic calcium in cows’ milk
pH
Ionic Calcium
(mMol)
Ethanol
stability (%)
Fresh cow milk
6.76
1.75
86
Fresh cow milk with
0.01% CaCl2
6.73
1.99
80
Fresh cow milk with
0.03% CaCl2
6.70
2.36
78
Fresh cow milk with
0.05% CaCl2
6.64
2.97
58
Effect of increased ionic calcium level in
Cows’ milk
3500
2500
2000
1500
0.01%
1000
0.03%
500
0.05%
0
60
30
0
54
0
78
0
10
20
12
60
15
00
17
40
19
80
22
20
24
60
27
00
29
40
31
80
2
OHTC, W/m K
3000
Time, S
Effects of heat on whey
• whey easily coagulates on heat treatment
• eg production of ricotta (whey cheese)
• coagulation can be prevented by heating and
controlled shear
• controlled aggregation/ microparticulation
• effects of pH on whey protein denaturation!!
Upbeat(whey protein (8%) drink)
Some challenges
• Fouling of evaporators has been little
researched.
• Microparticulation improves the hea stability
of whey protein. However, there is scope for
further improvements
• Heat stability of milk will be better understood
when pH and ionic calcium can be measured
at sterilisation temperatures
Sterilised evaporated milk
samples with different
stabilisers
pH and ionic calcium in UF permeates at
different temperatures
Samples
pH
Ca2+ (mM)
Milk
6.70±0.04
1.76±0.55
Permeate at 80°C
6.41±0.04
0.57±0.19
Permeate at
5.91±0.03
0.29±0.03
5.65±0.04
0.19±0.04
120°C
Permeate at
140°C
Changes in pH and ionic calcium during UHT
and in-container sterilisation
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