- Email: [email protected]
Thermal Analysis of Cocoa Butter and Cocoa Butter Alternatives Crystallisation Using DSC Method
Copyright 0 IFAC Control Applications in Post-Harvest and Processing Technology, BUdapest, Hungary, 1998
THERMAL ANALYSIS OF COCOA BUTlER AND COCOA BUTlER ALTERNATIVES CRYSTALLISATION USING DSC MEmOD
Katalin Kerti
Department ofPhysics and Control, University ofHorticulture and Food Somloi ut 14-16, Budapest H-1118. e-mail: [email protected]
Abstract: The use of the Avrami equation for cocoa butter and cocoa butter alternatives are described. The experiments were carried out by using a DSC method for isothermal crystallisation. The crystallisation curves may be linearized according to the empirical Avrami equation. From the resultingAvrami plots the rate constant of crystallisation may be calculated. This method gives valuable results about the specific crystallisation tendency of CB and CBA's. The method seems to be useful for quality control of these fats in the confectionery industry and for process control during the tempering steps of the industrial chocolate production. Copyright © 1998 /FAC Keywords: Avrami equation; classification; chocolate production; heat flows
1. INTRODUCI10N
Obvious advantages of these alternative fats may include some or all of the following (Leissner, et al.,
The quality of food is determined by the quality parameters of raw material. In the confectionery industry fat is the most critical raw material, both in technological and financial terms. The fat in chocolate constitutes the continuous phase which affects a host of parameters during processing, storing and consumption. Cocoa butter itself possesses a good many interesting properties, as well as major limitations.
1991):
-
they do not require tempering, they give longer gloss and shelf life, they enhance ability to withstand tropical climates, they have rheological properties suitable for modem, highly flexible production lines, they provide a totally bland taste, they are highly suitable for use in cost-efficient chocolate production
The primary reason for use of alternative fats in chocolate was, of course, the high and extremely unpredictable price of cocoa butter. Today, however, it is technical considerations that increasingly dictate the use of Cocoa Butter Alternatives (CBA's).
Since cocoa butter is a natural raw material, its quality varies. This fact also explains the industry's interest in alternatives to cocoa butter. CBA's can be divided into two main groups:
The aim of this work was to study the crystallisation behaviour of the cocoa butter and the cocoa butter alternatives using a described DSC method for isothermal crystallisation of these fats.
1. Tempering fats. This group includes Cocoa Butter Equivalents (CBE's) and Cocoa Butter Improvers (CBI's). These fats consist of the same types of triglyceride as cocoa butter and can thus
157 7
be blended with it in any proportion, even up to 100 %. Chocolate based on these fats must be tempered. 2. Non-tempering fats. These fats differ completely in composition from cocoa butter, but give similar properties in chocolate. Tempering is not needed, because these fats crystallise directly as the correct polymorphic form when cooled. They are divided in two sub-groups: 2a. Cocoa Butter Replacers (CBR's) are based on oils rich in C16/C18, ensuring good miscibility with cocoa butter. 2b. Cocoa Butter Substitutes (CBS's). They have a high lauric acid (CI2) content and are known as lauric fats. Because of the great differences in composition between CBS's and cocoa butter, the miscibility of these two groups of fats is poor. These fats are widely used in the confectionery industry as coatings, as a product for enrobing fondants, jellies and marshmallows.
t.
Time, •
Fig. 1. Isothermal crystallisation curve. ts, loa, lmax, loff, le = start, onset, maximum, offset and end of the main crystallisation. MI = crystallisation enthalpy (mW·s).
2.2 Analysis according to the Avrami equation The over the time formed crystal proportion is determined by integration of the DSC measuring curve. This proportion is given by
The crystallisation curves may be linearized according to the empirical Avrami equation. From the resulting Avrami plots the rate constant of crystallisation may be calculated.
a. a..
x=-
(1)
whereby a. is the fat crystallised at present t, and a.s is the maximal crystallizable fat proportion.
2. METIlODS AND MATERIALS
The value x is given by using partial integration of the exothermic DSC peak (Fig. 1); a. means the partial peak area from ts to time t, and a.s the total peak area from 0 to le.
2.1 isothermal crystallisation by DSC instrument A DSC instrument type microDSC rn, SETARAM was used for the measurements. 30 mg cocoa butter and CBA's were measured into the cup of the instrument. The reference was an empty container. The samples were warmed up to 60 QC, kept there for 30 minutes for melting all crystal modifications then they were cooled to the measuring temperatures with a rate of 1.2 QC/min. They were kept isotherm at the measuring temperatures until the measurements ended.
Plotted x against t yields an S formed curve, which achieves at le the value x = 1. The absolute value a.s of the crystallizable fat proportion doesn't have to be known. It varied naturally with the temperature and in case of cocoa butter with the origin. By Avrami means empirically:
A typical measuring curve (Fig. 1) was analysed after ts, loa, tm.x, loff, le, so start, onset, maximum, offset and end of the main crystallisation. MI is the crystallisation enthalpy (mW·s).
1-x=e
_k·tn
(2)
k is a rate constant and n is a dimensionless exponent.
Like earlier investigations showed cocoa butter always crystallises between the used measuring temperatures (19-23 QC) with the mentioned conditions in modification 6' with a fusion zone of 26-30 QC. That is the same in the case of CBE. CBR and CBR fats also crystallise in that modification without tempering (Ziegleder, 1990).
With the submitted test conditions the germ formation takes place thermally, i.e. under sporadic developing of germs with constant rate of forming, and the shperulites grown three dimensionally (Ziegleder, 1990).
158
Figure 2 clarifies the differences among cocoa butter, CBR., CBS, CBE fats in each case measured at 20°C. As shown Akomax (CBE) needs more time for total crystallisation, while Akopol (CBR) and Nobletan (CBS) crystallise more rapidly in a half an hour.
In that case are these Avrami constants valid: n=4
(3a)
k = ~·G3.1 3
(3b)
G = linear growth velocity (min- I ) 1 = rate of core formation (min- I )
Heat Flow! mW 10.0
Akopol (CBR)
,f.
After transformation ofEq. (2):
IExo
19 ( -In (I-x» = 19 k + n . 19 t
Nobletan (CBS)
7.'
(4)
,., Plotted 19 ( -In (I-x» against 19 t a straight line is given which gradient is n, and from its axis intercept 19 ( -In (I-X» = 0 19 k can be calculated.
4.'
" Because of the uncertainty in the definition of ts and ~ were only measured values 0.1< x< 0.9 used. Additionally the DSC curves had to be corrected in order to linearise them later.
Cocoa Butter (CB) 1\ Akomax (CBE)
I.'
'-\. /
".::: 4'"
one s
1000
Fig. 2. DSC isotherm curves measured at 20°C As earlier investigations showed (Ziegleder, 1990) that at the beginning of the exothermic peak Is x = 0 is by definition, but in reality a small crystal proportion approximately xI=0.03 is already present and all further x - values are to be corrected as follows:
xI
(x +0.03)
=
1.03
Deviations in the fatty acid composition are responsible for the different ways of crystallisation. The crystallisation isotherms in Fig. 2 and 3 could be transformed into Avrami straight lines (Figs. 3,4,5,6) under the mentioned boundary conditions.
(5) 06,.-----------------,
At the end of the crystallisation (~) XI= x= 1. Because of the uncertainty in the determination of the starting point (Is) an other time is used for the definition of the crystallisation start (ton) which is given by extrapolation. This value is independent of the weight of the sample and it can be determined as "the time to the crystallisation".
20~~~~21~
04
......
'?
02
0
// .... ••
-
..••..'
.Q6 .QS
_I
.:
....
.-: ..-
•
•
•
•
•
•
•
••
+-.c...--+------1r----+----+--+----1 3.4
3. RESULTS
•
..
••
.~22~
35
3.6
3.7
3.8
3.9
4
Igt
Cocoa butter, Akopol (CBR), Nobletan (CBS) and Akomax (CBE) were measured in the DSC. The measurements resulted crystallisation's isotherms similar to Fig. 1, however with a very broad span of crystallisation time and with different ways.
Fig. 3. Avrami straight lines of Cocoa Butter at 20, 21 and 22°C As seen on Fig. 3 the measured isotherm DSC curve of cocoa butter at 20°C couldn't be transformed into a straight line. The reason of this can be found in the uncertainty of the determination of~. Therefore in further transformation an other time (loff) was used, which can be given also by extrapolation (Fig. 1).
The exothermic crystallisation process begins according to Fig. 1 with ts, attains a maximum and ends apparently with ~. In the time after ~ may follow a slow secondary crystallisation, which cannot be measured because of the small heat flow in the DSC. The onset can be better determined then t. and therefore it is determined than crystallisation start.
159
Therefore other measuring temperatures were chosen for further investigations (Fig. 2).
06r---------------., 18"C 04 II II
Q2 II
06
I I
~
I
-0.6
-1
0
i-O.
I
+---+--+---+---t----+----j 14
16
35
17
18
39
•• • •
4
4
••
-0.8 -1
Fig. 4. Avrami straight lines of Akomax (CBE) by 18, 19, 20°C
3
I
I
I
•
12
.&
••
• ••
I I
•
• ••
I
11
./ /
&
I
I
27 "C
/
I
••
-0.6
Igt
••• • •
•
j.Q2
I
-0.8
.'
Q2
I I
26 "C
25 "C
04
I I
14
13
15
3.6
3.7
Igt Fig.
The shape of the lines in Fig. 4 proves the asymmetric distribution of the heat flow curves near the observed peak. The measuring temperatures were chosen according to the mean of the DSC measurements, because in these temperature ranges the curves show significant maximums.
6.
Avrami plots by 25, 26, 27 QC
of
Nobletan
(CBS)
The mentioned grounds give reasons for choosing higher temperatures in case of Nobletan (CBS), too. The cxystallisation process of cocoa butter substitute fat is closer to Akopol than cocoa butter.
Comparing the results between cocoa butter and cocoa butter equivalent fat (Akomax) there is no significant difference between the slopes of the fitted lines (see Fig. 3 and Fig. 4).
4. CONCLUSION It was concluded that the Avrami equation can describe the cxystallisation of cocoa butter and CBA's.
The collected data at 20°C in case of cocoa butter and Akomax has the same trend after Avrami transformation. Therefore both confectionexy fats have the same or similar chemical and physical features as it was expected in the introduction.
This method gives valuable results about the specific cxystallisation tendency of cocoa butter and cocoa butter alternatives and about temperature influence on cxystallisation times.
Fig. 5 presents the straight lines produced by Avrami transformation from Akopol data sets measured at 24°C, 25 °c and 26°C. Higher temperatures were selected because of the same reasons mentioned before.
The method seems to be useful for quality control of cocoa butters and other vegetable fats and for process control during the tempering in the industrial chocolate production.
06
24 "C,
04
~
~
•
-0.8
&
• I
•
-0.6
•
•
•
I
-1
28
3
Ali, A.RM and P.S. Dimick (1997). Melting and solidification characteristics of confectionexy fats: anhydrous milk fat, cocoa butter and palm kernel stearin blends. J. Am. Oil Chem. Soc., 71, 803806. Leissner, R et al. (1991). Cocoa Butter Alternatives. Karlshamns Oil & Fats AB, Karlshamns. Vieweg, R and D. Braun (1975). KunststojJHandbuch. Band I: Grundlagen, pp. 252-256. Carl Hanser Verlag, Mfinchen-Wien. Ziegleder, G. (1990). DSC - Thermoanalyse und Kinetik der Kristallisation von Kakaobutter. Fat Sei. Technol., 92, 481-485.
•
I
•
REFERENCES
•• ••• •
•
•
~ -0.4
26 "C,
I
• •
0
:5.Q2 ~
25 "C •
•
Q2
12
14
16
18
Igt Fig. 5. Avrami transformation of the DSC curves by Akopol (CBR) on 24, 25, 26°C
In general Akopol starts its cxystallisation already during the cooling to the measuring temperature at lower temperatures. That means the DSC curve does not begin at 0 mW, but at higher heat flow values.
160
THERMAL ANALYSIS OF COCOA BUTlER AND COCOA BUTlER ALTERNATIVES CRYSTALLISATION USING DSC MEmOD
Katalin Kerti
Department ofPhysics and Control, University ofHorticulture and Food Somloi ut 14-16, Budapest H-1118. e-mail: [email protected]
Abstract: The use of the Avrami equation for cocoa butter and cocoa butter alternatives are described. The experiments were carried out by using a DSC method for isothermal crystallisation. The crystallisation curves may be linearized according to the empirical Avrami equation. From the resultingAvrami plots the rate constant of crystallisation may be calculated. This method gives valuable results about the specific crystallisation tendency of CB and CBA's. The method seems to be useful for quality control of these fats in the confectionery industry and for process control during the tempering steps of the industrial chocolate production. Copyright © 1998 /FAC Keywords: Avrami equation; classification; chocolate production; heat flows
1. INTRODUCI10N
Obvious advantages of these alternative fats may include some or all of the following (Leissner, et al.,
The quality of food is determined by the quality parameters of raw material. In the confectionery industry fat is the most critical raw material, both in technological and financial terms. The fat in chocolate constitutes the continuous phase which affects a host of parameters during processing, storing and consumption. Cocoa butter itself possesses a good many interesting properties, as well as major limitations.
1991):
-
they do not require tempering, they give longer gloss and shelf life, they enhance ability to withstand tropical climates, they have rheological properties suitable for modem, highly flexible production lines, they provide a totally bland taste, they are highly suitable for use in cost-efficient chocolate production
The primary reason for use of alternative fats in chocolate was, of course, the high and extremely unpredictable price of cocoa butter. Today, however, it is technical considerations that increasingly dictate the use of Cocoa Butter Alternatives (CBA's).
Since cocoa butter is a natural raw material, its quality varies. This fact also explains the industry's interest in alternatives to cocoa butter. CBA's can be divided into two main groups:
The aim of this work was to study the crystallisation behaviour of the cocoa butter and the cocoa butter alternatives using a described DSC method for isothermal crystallisation of these fats.
1. Tempering fats. This group includes Cocoa Butter Equivalents (CBE's) and Cocoa Butter Improvers (CBI's). These fats consist of the same types of triglyceride as cocoa butter and can thus
157 7
be blended with it in any proportion, even up to 100 %. Chocolate based on these fats must be tempered. 2. Non-tempering fats. These fats differ completely in composition from cocoa butter, but give similar properties in chocolate. Tempering is not needed, because these fats crystallise directly as the correct polymorphic form when cooled. They are divided in two sub-groups: 2a. Cocoa Butter Replacers (CBR's) are based on oils rich in C16/C18, ensuring good miscibility with cocoa butter. 2b. Cocoa Butter Substitutes (CBS's). They have a high lauric acid (CI2) content and are known as lauric fats. Because of the great differences in composition between CBS's and cocoa butter, the miscibility of these two groups of fats is poor. These fats are widely used in the confectionery industry as coatings, as a product for enrobing fondants, jellies and marshmallows.
t.
Time, •
Fig. 1. Isothermal crystallisation curve. ts, loa, lmax, loff, le = start, onset, maximum, offset and end of the main crystallisation. MI = crystallisation enthalpy (mW·s).
2.2 Analysis according to the Avrami equation The over the time formed crystal proportion is determined by integration of the DSC measuring curve. This proportion is given by
The crystallisation curves may be linearized according to the empirical Avrami equation. From the resulting Avrami plots the rate constant of crystallisation may be calculated.
a. a..
x=-
(1)
whereby a. is the fat crystallised at present t, and a.s is the maximal crystallizable fat proportion.
2. METIlODS AND MATERIALS
The value x is given by using partial integration of the exothermic DSC peak (Fig. 1); a. means the partial peak area from ts to time t, and a.s the total peak area from 0 to le.
2.1 isothermal crystallisation by DSC instrument A DSC instrument type microDSC rn, SETARAM was used for the measurements. 30 mg cocoa butter and CBA's were measured into the cup of the instrument. The reference was an empty container. The samples were warmed up to 60 QC, kept there for 30 minutes for melting all crystal modifications then they were cooled to the measuring temperatures with a rate of 1.2 QC/min. They were kept isotherm at the measuring temperatures until the measurements ended.
Plotted x against t yields an S formed curve, which achieves at le the value x = 1. The absolute value a.s of the crystallizable fat proportion doesn't have to be known. It varied naturally with the temperature and in case of cocoa butter with the origin. By Avrami means empirically:
A typical measuring curve (Fig. 1) was analysed after ts, loa, tm.x, loff, le, so start, onset, maximum, offset and end of the main crystallisation. MI is the crystallisation enthalpy (mW·s).
1-x=e
_k·tn
(2)
k is a rate constant and n is a dimensionless exponent.
Like earlier investigations showed cocoa butter always crystallises between the used measuring temperatures (19-23 QC) with the mentioned conditions in modification 6' with a fusion zone of 26-30 QC. That is the same in the case of CBE. CBR and CBR fats also crystallise in that modification without tempering (Ziegleder, 1990).
With the submitted test conditions the germ formation takes place thermally, i.e. under sporadic developing of germs with constant rate of forming, and the shperulites grown three dimensionally (Ziegleder, 1990).
158
Figure 2 clarifies the differences among cocoa butter, CBR., CBS, CBE fats in each case measured at 20°C. As shown Akomax (CBE) needs more time for total crystallisation, while Akopol (CBR) and Nobletan (CBS) crystallise more rapidly in a half an hour.
In that case are these Avrami constants valid: n=4
(3a)
k = ~·G3.1 3
(3b)
G = linear growth velocity (min- I ) 1 = rate of core formation (min- I )
Heat Flow! mW 10.0
Akopol (CBR)
,f.
After transformation ofEq. (2):
IExo
19 ( -In (I-x» = 19 k + n . 19 t
Nobletan (CBS)
7.'
(4)
,., Plotted 19 ( -In (I-x» against 19 t a straight line is given which gradient is n, and from its axis intercept 19 ( -In (I-X» = 0 19 k can be calculated.
4.'
" Because of the uncertainty in the definition of ts and ~ were only measured values 0.1< x< 0.9 used. Additionally the DSC curves had to be corrected in order to linearise them later.
Cocoa Butter (CB) 1\ Akomax (CBE)
I.'
'-\. /
".::: 4'"
one s
1000
Fig. 2. DSC isotherm curves measured at 20°C As earlier investigations showed (Ziegleder, 1990) that at the beginning of the exothermic peak Is x = 0 is by definition, but in reality a small crystal proportion approximately xI=0.03 is already present and all further x - values are to be corrected as follows:
xI
(x +0.03)
=
1.03
Deviations in the fatty acid composition are responsible for the different ways of crystallisation. The crystallisation isotherms in Fig. 2 and 3 could be transformed into Avrami straight lines (Figs. 3,4,5,6) under the mentioned boundary conditions.
(5) 06,.-----------------,
At the end of the crystallisation (~) XI= x= 1. Because of the uncertainty in the determination of the starting point (Is) an other time is used for the definition of the crystallisation start (ton) which is given by extrapolation. This value is independent of the weight of the sample and it can be determined as "the time to the crystallisation".
20~~~~21~
04
......
'?
02
0
// .... ••
-
..••..'
.Q6 .QS
_I
.:
....
.-: ..-
•
•
•
•
•
•
•
••
+-.c...--+------1r----+----+--+----1 3.4
3. RESULTS
•
..
••
.~22~
35
3.6
3.7
3.8
3.9
4
Igt
Cocoa butter, Akopol (CBR), Nobletan (CBS) and Akomax (CBE) were measured in the DSC. The measurements resulted crystallisation's isotherms similar to Fig. 1, however with a very broad span of crystallisation time and with different ways.
Fig. 3. Avrami straight lines of Cocoa Butter at 20, 21 and 22°C As seen on Fig. 3 the measured isotherm DSC curve of cocoa butter at 20°C couldn't be transformed into a straight line. The reason of this can be found in the uncertainty of the determination of~. Therefore in further transformation an other time (loff) was used, which can be given also by extrapolation (Fig. 1).
The exothermic crystallisation process begins according to Fig. 1 with ts, attains a maximum and ends apparently with ~. In the time after ~ may follow a slow secondary crystallisation, which cannot be measured because of the small heat flow in the DSC. The onset can be better determined then t. and therefore it is determined than crystallisation start.
159
Therefore other measuring temperatures were chosen for further investigations (Fig. 2).
06r---------------., 18"C 04 II II
Q2 II
06
I I
~
I
-0.6
-1
0
i-O.
I
+---+--+---+---t----+----j 14
16
35
17
18
39
•• • •
4
4
••
-0.8 -1
Fig. 4. Avrami straight lines of Akomax (CBE) by 18, 19, 20°C
3
I
I
I
•
12
.&
••
• ••
I I
•
• ••
I
11
./ /
&
I
I
27 "C
/
I
••
-0.6
Igt
••• • •
•
j.Q2
I
-0.8
.'
Q2
I I
26 "C
25 "C
04
I I
14
13
15
3.6
3.7
Igt Fig.
The shape of the lines in Fig. 4 proves the asymmetric distribution of the heat flow curves near the observed peak. The measuring temperatures were chosen according to the mean of the DSC measurements, because in these temperature ranges the curves show significant maximums.
6.
Avrami plots by 25, 26, 27 QC
of
Nobletan
(CBS)
The mentioned grounds give reasons for choosing higher temperatures in case of Nobletan (CBS), too. The cxystallisation process of cocoa butter substitute fat is closer to Akopol than cocoa butter.
Comparing the results between cocoa butter and cocoa butter equivalent fat (Akomax) there is no significant difference between the slopes of the fitted lines (see Fig. 3 and Fig. 4).
4. CONCLUSION It was concluded that the Avrami equation can describe the cxystallisation of cocoa butter and CBA's.
The collected data at 20°C in case of cocoa butter and Akomax has the same trend after Avrami transformation. Therefore both confectionexy fats have the same or similar chemical and physical features as it was expected in the introduction.
This method gives valuable results about the specific cxystallisation tendency of cocoa butter and cocoa butter alternatives and about temperature influence on cxystallisation times.
Fig. 5 presents the straight lines produced by Avrami transformation from Akopol data sets measured at 24°C, 25 °c and 26°C. Higher temperatures were selected because of the same reasons mentioned before.
The method seems to be useful for quality control of cocoa butters and other vegetable fats and for process control during the tempering in the industrial chocolate production.
06
24 "C,
04
~
~
•
-0.8
&
• I
•
-0.6
•
•
•
I
-1
28
3
Ali, A.RM and P.S. Dimick (1997). Melting and solidification characteristics of confectionexy fats: anhydrous milk fat, cocoa butter and palm kernel stearin blends. J. Am. Oil Chem. Soc., 71, 803806. Leissner, R et al. (1991). Cocoa Butter Alternatives. Karlshamns Oil & Fats AB, Karlshamns. Vieweg, R and D. Braun (1975). KunststojJHandbuch. Band I: Grundlagen, pp. 252-256. Carl Hanser Verlag, Mfinchen-Wien. Ziegleder, G. (1990). DSC - Thermoanalyse und Kinetik der Kristallisation von Kakaobutter. Fat Sei. Technol., 92, 481-485.
•
I
•
REFERENCES
•• ••• •
•
•
~ -0.4
26 "C,
I
• •
0
:5.Q2 ~
25 "C •
•
Q2
12
14
16
18
Igt Fig. 5. Avrami transformation of the DSC curves by Akopol (CBR) on 24, 25, 26°C
In general Akopol starts its cxystallisation already during the cooling to the measuring temperature at lower temperatures. That means the DSC curve does not begin at 0 mW, but at higher heat flow values.
160