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Effect of oligomeric or polymeric additives on glass transition, viscosity and crystallization of amorphous isomalt
Food Research International 33 (2000) 41±51
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Eect of oligomeric or polymeric additives on glass transition, viscosity and crystallization of amorphous isomalt J. Raudonus a, J. Bernard b,*, H. Janûen b, J. Kowalczyk b, R. Carle a a
Institute of Food Technology, Section Plant Foodstu Technology, Hohenheim University, Garbenstraûe 25, D-70599 Stuttgart, Germany b SuÈdzucker AG/ZAFES, Wormser Str. 11, D-67283 Obrigheim/Pfalz, Germany Received 2 December 1999; accepted 5 January 2000
Abstract The main purpose of this study was to further improve the stability of the amorphous state of isomalt by the addition of high molecular weight compounds. Dierential scanning calorimetry (DSC) was used to determine the glass transition temperature Tg of a series of polyol mixtures containing isomalt as a function of water content. As expected, for each mixture Tg decreased with increasing moisture. Only with the addition of more than 75% of higher molecular weight compounds to isomalt an increase of Tg could be achieved. Moisture content and time dependent phase transitions in the metastable amorphous state of the mixtures strongly aect the storage stability of isomalt hard candies. Storage tests indicated a markedly accelerated water absorption and crystallization when oligomeric or polymeric compounds were added to amorphous isomalt. Rheological experiments showed that in contrast to pure isomalt melts, the viscosity of melts containing oligomeric or polymeric additives deviated from the model curve predicted by the Williams±Landel±Ferry-kinetics (WLF) with increasing water content. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Additives; Hard candies; Hygroscopicity; Isomalt; Recrystallization; Viscosity
1. Introduction In hard candies, sugars or sugar alcohols are in an amorphous state. The candy matrix is, in contrast to the crystalline phase, characterized by slightly loose packing of molecules with scattered water molecules. In addition there is no long range order of molecules as there is in the crystal. For the formation of the amorphous state in hard candies, a carbohydrate solution with a very low water content is quickly cooled after boiling. Because of the increasing viscosity with reduced temperature, the mobility of the molecules is severely restricted. Therefore transformation into the crystalline phase is inhibited (Hartel, 1987). The transformation from the glassy to the more liquidlike state, or vice versa, takes place over a range of temperatures. Consequently a glass transition temperature, Tg , cannot be accurately de®ned (Herrington & Bran®eld, 1984). At the glass transition temperature various physical properties such as viscosity drastically change. Above Tg there is an * Corresponding author. Tel.: +49-6359-803483; fax: +49-6359803331. E-mail address: [email protected] (J. Bernard).
increase of molecular mobility and free volume (White & Cakebread, 1966) resulting in an endothermic change in the apparent speci®c heat which can be detected by dierential scanning calorimetry (DSC) (Roos & Karel, 1990). A typical DSC trace over the glass transition range of an amorphous sugar or sugar alcohol as well as the usual de®nitions of Tg are shown in Fig. 1. The most important factors aecting the glass transition temperature are the composition of the material, molecular weight, and plasticizers (Roos & Karel, 1990). The Tg value of anhydrous food homopolymers was reported to decrease linearly with increasing 1/M value according to Eq. (1) (Roos & Karel, 1991c): Tg Tg 1 ÿ
K M
1
where M is molecular weight; K is constant; and Tg 1 is the limiting Tg at high molecular weight. Water is the most prominent plasticizer which decreases the glass transition temperature of amorphous carbohydrates. The eect of water on the Tg of a wide range of carbohydrates can be modeled according to
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Fig. 1. Glass transition temperature (Tg ) range of amorphous sugars or sugar alcohols as determined using dierential scanning calorimetry. The deviation, onset, and at half cp temperatures are abbreviated Tg;d , Tg;o , and Tg;c , respectively. The change of speci®c heat at the Tg region is indicated by cp (Janûen, 1999)..
Eq. (2), originally reported by Gordon and Taylor (1952), and used for the calculation of Tg values of binary polymer mixtures (Karel, Anglea, Buera, Lormasd, Levi & Roos, 1994; Roos, 1993): Tg
w1 Tg1 kw2 Tg2 w1 kw2
2
where w1 and w2 are the weight fractions of the carbohydrate and of water, respectively. Tg1 and Tg2 are the glass transition temperatures of the pure components. k is a constant which describes the sensitivity of Tg to variations of water content. The Tg value of water has been empirically determined by Johari, Hallbrucker and Mayer (1987); the found value of ÿ134 C was applied in this study. In this work Tg values of the dierent carbohydrate solutions were measured at various water contents to obtain the constant k by a least square ®t of a linearized transformation of Eq. (2): ÿ Tg;1 ÿ Tg w ÿ k 2 3 w1 Tg ÿ Tg;2 The viscosity of a glass exceeds 1012 Pas (White & Cakebread, 1966) and decreases dramatically above Tg (Roos, 1995). For a number of carbohydrate systems, the temperature dependence of viscosity above Tg is reported to obey the Williams±Landel±Ferry (WLF) equation (Karel et al., 1994): log
C1 T ÿ Tg g C2 T ÿ Tg
4
where is the viscosity at temperature T and g is the viscosity at Tg . C1 and C2 are constants. In this study C1 (-17.44) and C2 (51.6 K) originally reported by Williams, Landel and Ferry (1955) were
used. The viscosity at glass transition temperature g was adapted for each carbohydrate or carbohydrate mixture examined. The empirically determined values of low molecular sugar or sugar alcohol melts shared good correlation with the calculated curve by using log g 12 (Janûen, 1999). In organic glasses, both increase of moisture content and storage temperature may aect the rate of deteriorative reactions. The most important changes aecting the behaviour of amorphous carbohydrates occur at a quite narrow temperature range above Tg . This is related to the plasticization of amorphous structure which leads to a dramatic decrease in viscosity, and therefore an increase in molecular mobility (Roos & Karel, 1991a). These factors cause dierent time-dependent and viscosity related structural transformations like stickiness, cold ¯ow and crystallization during storage. Because of the high viscosity, crystallization in the glassy state below Tg is kinetically inhibited (Roos, 1995). The rate of crystallization depends on moisture content and temperature. Due to a decreasing viscosity an increase of these parameters allows molecular movement required for crystal growth. At ambient temperature absorption of water is, therefore, necessary for crystallization. In general, additives (``doctoring agents'') are of importance in hard candies to reduce the rate of crystallization of sugar or sugar alcohol glasses once formed (Hartel, 1987). The rate of crystallization depends on type and amount of additive as well as on the water content of the sample (Herrington & Bran®eld, 1984). Iglesias and Chirife (1978) reported delayed crystal growth of amorphous sucrose when polymeric compounds where added (Roos & Karel, 1991a). Several potential mechanisms may be responsible for the inhibitory eect. These include a selective incorporation of part of the additive into the lattice (``tailor
J. Raudonus et al. / Food Research International 33 (2000) 41±51
made'' impurities), a blockage of growth sites by adsorption of the additive onto the crystal surface (Hartel, 1987), or an increased viscosity and decreased molecular mobility. The aim of the present work was to study the changes of crystallization tendency of isomalt hard candies after addition of viscosity-increasing substances such as hydrogenated starch hydrolysate (HSH) or hydrogenated polydextrose. These sugarfree ingredients are much more hygroscopic than isomalt and consequently less stable during storage. Hard candies made from low to intermediate molecular weight HSH display the phenomenon of ``cold ¯ow''. This describes the tendency for the candy to distort over time, which occurs readily at room temperature (Klacik, 1990). Products made from high molecular weight HSH or polydextrose become sticky due to absorption of water or increase in temperature. The sugar alcohol isomalt is an ideal model material for hard candy production with respect to stability. The low hygroscopicity and the composition of two stereoisomers are responsible for the high stability of amorphous products made from this sugar substitute. Tg of several combinations of isomalt and HSH or polydextrose were determined at various water contents. Rheological experiments should show an increasing viscosity of isomalt melts by the addition of agents with oligomeric or polymeric structure. 2. Materials and methods 2.1. Raw materials The disaccharide alcohol isomalt, a nearly 1:1-mixture of the two stereoisomers 1-O-a-d-glucopyranosyl-dmannitol dihydrate (1,1-GPM dihydrate) and 6-O-a-dglucopyranosyl-d-sorbitol (1,6-GPS) with a purity of 99.5%, was obtained from Palatinit GmbH, Germany. Additives relevant for the production of sugarfree hard candies were used. Apart from a low (Hystar1 3375) and high (Hystar1 6075) molecular hydrogenated starch hydrolysate (HSH) syrup (Lonza, USA), hydrogenated polydextrose Litesse III (Cultor, Denmark) with a mean mass of 5000 g/mol were studied. The compositions of the two HSH syrups used are given in Table 1, where with the exception of the water content (percentage weight of the total syrup), the other contents are expressed as percentage weight of the total solids. 2.1.1. Sample preparation Melts with various water contents were produced by heating solutions that contained approx. 70% of the desired carbohydrate composition at dierent temperatures under atmospheric pressure. Subsequently the
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melts were solidi®ed in a dry-box. The glasses were analyzed for water content by measuring the mass lost after drying in a vacuum oven. Hard candy samples for storage tests were prepared by heating a solution (approx. 70%) of the respective mixture of isomalt and additive to 160 C and evacuating to a pressure less than 100 mbar. The resulting melts contained 1±1.5% water after 75 s. After cooling to 70± 80 C on a cooling table tempered at 60±70 C, the candies were formed with the aid of a motor driven drop roller. The samples were analyzed for water content by using Karl±Fischer titration. 2.2. Methods 2.2.1. Dierential scanning calorimetry (DSC) DSC was used to determine glass transition temperature (Tg ) of amorphous samples at various water contents. A Perkin±Elmer Pyris 1 DSC was used with Pyris software for Windows. The instrument was calibrated using indium (Hm=28.45 J gÿ1, m.p.=156.6 C, Perkin±Elmer standard) and water (m.p.=0 C). The calibration was checked using n-octadecane. Samples were prepared in a dry-box by weighing 10± 15 mg of the amorphous material in 20 ml DSC pans (Perkin±Elmer) which were subsequently hermetically sealed. Desiccated air was blown into the box to keep it dry. An empty aluminium pan was used as reference in all measurements. To avoid condensation of moisture, the sample head was purged with dry nitrogen. Each sample was cooled to about 30 C below Tg , heated at 10 C/min to Tg +30 C, and then cooled at 250 C/min to Tg ÿ30 C. The same cooling and heating procedure was applied to all samples to standardize the thermal history of all glass samples. The samples were then scanned at 10 C/min for determination of Tg .
Table 1 Composition of both HSH syrups used (DP=number-average of polymerisation; molar mass is calculated from composition) Composition
HYSTAR 3375 (HS 3375)% w/w
HYSTAR 6075 (HS 6075)% w/w
D-Sorbitol Maltitol Maltotriositol DP4 DP5 DP6 DP7 DP8 DP9 DP10 DP10+ M (g/mol) Water content
12.9 20.4 11.3 6.2 6.8 8.2 5.3 2.5 1.1 0.7 24.8 504 25%
11.1 7.9 8.1 6.1 6.3 9.2 6.9 2.8 1.5 1.1 39.1 644 25%
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2.2.2. Rheological experiments A Bohlin CSR 10 was used to study the rheological properties and ascertaining viscosity at dierent temperatures of melts varying in composition and water content. The viscosity measurements were carried out with oscillating forces to avoid structural destruction and a changed rheological reaction. The shear stress [ 0 sin !t] to generate a sinusoidal deformation vibration [ 0 sin !t] was registered. Using Eq. (5) the complex viscosity was calculated. !
0 ! sin ÿ i cos
0
5
The complex viscosity could be regarded as a real value if the imaginary part of Eq. (5) is negligible. The measurements carried out in this work shared comparable magnitudes for both complex and real parts of the viscosity. 2.2.3. Rate of crystallization Hard candies made of isomalt with dierent amounts of additives were stored at 25 C and 80% relative humidity in open Petri dishes. During the storage test over 3 weeks, water absorption of the samples was determined gravimetrically. In addition, crystal growth on the sample surface was studied by preparing 10 mm sections of the candy with a microtome. Crystalline areas were detected as yellow or violet glimmering re¯ections under the microscope as observed under polarized light. The thickness of the crystal layers was measured by using an ocular micrometer.
Table 2 Composition of the dierent model glasses and melts examined (differential scanning colorimetry and viscosity measurements) Raw materials
Mixtures with isomalt (dry solids base)
HS 3375 HS 6075 Litesse III
75%, 50%, 25% w/w isomalt 75%, 50%, 25% w/w isomalt 75%, 50%, 25% w/w isomalt
3. Results 3.1. Eect of composition and moisture content on glass transition temperature A selection of glass systems containing dierent amounts of isomalt, HSH, and polydextrose (Table 2) was studied. All materials showed typical thermal behaviour of amorphous materials. The Tg values of the anhydrous additives and mixtures with isomalt examined are shown in Table 3. The plasticizing eect of water is re¯ected by the k values also given in Table 3. The drastic decrease of Tg with increasing water content is in accordance with the ®ndings in previous studies (Roos, 1993; Karel et al., 1994). The eect of the addition of 25% Litesse III, HS 3375 or HS 6075 on Tg of amorphous isomalt is given in Fig. 2. These mixtures are characterized by lower Tg values compared with pure isomalt. This holds for the whole range of water content investigated, that is from 0 to 20% water of total weight. Fig. 3 provides a plot of Tg of the anhydrous combinations of isomalt and additives against isomalt content
Fig. 2. Glass transition temperature of amorphous isomalt containing 25% w/w Litesse III, HS 3375 or HS 6075 compared with pure amorphous isomalt as a function of dry matter concentration.
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Table 3 Glass transition temperatures of the anhydrous material and k values of various combinations between isomalt and Litesse III, HS 3375 or HS 6075 Composition
Tg;c ( C)
kc
Composition
Tg;c ( C)
kc
Composition
Tg;c ( C)
kc
isomalt 25% Litesse III 50% Litesse III 75% Litesse III Litesse III
63.6 53 59 69.4 90.8
3.9 3.6 3.9 4.1 4.5
isomalt 25% HS 3375 50% HS 3375 75% HS 3375 HS 3375
63.6 49.6 49.2 49.2 54.6
3.9 3.7 3.9 4.0 5.6
isomalt 25% HS 6075 50% HS 6075 75% HS 6075 HS 6075
63.6 56.2 59.5 64.5 75
3.9 4.3 4.5 4.8 6.7
in the mixture. It is evident that Tg runs through a minimum with decreasing isomalt content. Polymeric Litesse III and HSH type HS 6075 have higher Tg values than pure isomalt. However, the addition of 20± 30% of these substances to isomalt lowers the Tg value by approx. 10 C. Mixtures of polydextrose and isomalt exhibit the most pronounced Tg depression. Only compositions with polydextrose or HS 6075 content higher than 60 or 70% elevates Tg in a temperature range above the Tg of pure isomalt. Mixtures between isomalt and low molecular HSH (HS 3375) are characterized by a broad and shallow range of lowered Tg values along the whole concentration scale. These values are approximately 10±15 C lower than the Tg of pure isomalt. Fig. 4 shows the k values of the mixtures as a function of isomalt content. For Litesse III and HS 3375 a minimal k value for the mixtures with isomalt is, analogous to Tg , located at approximately 75% isomalt content. At this composition a depression of the k value of 0.3 and 0.2 for Litesse III and HS 3375, respectively, was observed. A 1:1 mixture between isomalt and HS 3375 or Litesse III was found to have the same Tg sensitivity as pure isomalt with varying water content. An increase
in the content of additive above 60±70% raises the k values of the mixtures above that of pure isomalt. Mixtures of isomalt and HS 6075 do not show an analogous minimum. In the entire composition range the k values of the mixtures were found to be higher than that of pure isomalt. Pure amorphous HSH, in particular HS 6075, was markedly plasticized by water. In comparison to pure isomalt (3.9) there is an increase in the k values of HS 3375 (5.6) and HS 6075 (6.7) by 1.7 and 2.8, respectively. 3.2. Eect of composition and water content on viscosity The temperature-dependence of a number of melts varying in composition and water content were studied using an oscillation rheometer (Bohlin CSR 10). For each melt the rheological behaviour was investigated at a frequency of 1 Hz. Table 2 shows the composition of the dierent samples examined. The viscosities of various melts with dierent water contents were plotted as a function of the relative variable (T±Tg;d ) and compared with a model curve predicted by the WLF kinetic. The uniform behaviour of low molecular sugars or sugar alcohols (e.g. isomalt) is con®rmed by experimental data
Fig. 3. Dry matter glass transition temperatures of isomalt and mixtures with HS 3375, HS 6075 and Litesse III as a function of isomalt content.
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Fig. 4. The k-value of combinations between isomalt and HS 3375, HS 6075 and Litesse III as a function of isomalt content.
which correlate well with the predictions of the WLF model (Janûen, 1999). The temperature-dependence of the empirical data and calculated viscosity data of an isomalt melt with addition of 25% Litesse III is shown in Fig. 5. For this sample, it was necessary to increase the WLF parameter log Zg from 12, which is valid for pure isomalt, to 12.6. It is obvious that the viscosity of melts containing substances with oligomeric or polymeric structures cannot be treated as a function of (T±Tg ). At lower temperatures above Tg these melts show increasing deviation in viscosity with increasing water content from the expected model behaviour. Consequently a
generalized treatment of viscosity of higher molecular carbohydrate melts with the use of the WLF model seems to be possible only for melts with a low water content. As shown in Fig. 5, viscosity seems to be independent of temperature in the range between 40 and 50 C above Tg;d and below at 1 Hz. In fact, the viscosity becomes dependent on measuring frequency at temperatures slightly above Tg . The rheological properties change from Newtonian to pseudoplastic and plastic behaviour as shown in Fig. 6. Moreover, this change of behaviour occurs at a higher temperature the higher the amount of oligomeric or polymeric material in the mixture.
Fig. 5. Viscosity of an isomalt melt containing 25% w/w Litesse III at various water contents, compared with WLF-model.
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Fig. 6. Viscosity of an isomalt melt containing 25% Litesse III as a function of frequency at dierent temperatures.
Viscosity values of melts showing pseudoplastic properties are independent of the frequency at low frequencies. Therefore the constant viscosity value can be extrapolated to 0 Hz, that is static behaviour. In the temperature range where viscosity displays a plastic behaviour it obeys the exponential law of Ostwald-deWaele (Weipert, Tscheuschner & Windhab, 1993). Therefore, the used method of determining viscosity is no longer applicable. In Fig. 7 the viscosity of a melt containing 75% isomalt and 25% Litesse III measured at 1Hz is compared with its static behaviour. The Newtonian to plastic behaviour transition is indicated by the dierentiation of the static and
dynamic values of viscosity. In this case, the dynamic viscosity is determined and the behaviour transition starts at about 40 C above Tg . It should be kept in mind that this transition temperature depends on the frequency at which the dynamic viscosity is determined as shown in Fig. 6. 3.3. Eect of composition on water absorption and crystallization The progress of the water absorption, expressed as increase in weight, and nucleation front was plotted against storage time for the dierent glass systems.
Fig. 7. Temperature-dependent viscosity values extrapolated to 0 Hz compared with the viscosity measured at 1 Hz of an isomalt melt containing 25% Litesse III (0.4% w/w water content).
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Fig. 8. Water absorption of model glasses made of isomalt with dierent amounts of HS 3375, HS 6075 and Litesse III compared with pure amorphous isomalt (conditions: 25 C, 80% rel. humidity).
Glasses made from isomalt containing 10, 30 or 50% w/w additive (HS 3375, HS 6075, Litesse III) were examined at 25 C and 80% rel. humidity. Each glass sample contained 1±1.5% water at the beginning of the stability study. The water absorption values of these samples are compared with those of pure amorphous isomalt in Fig. 8. The samples containing HS 3375, HS 6075 or Litesse III show a prominent accelerated water absorption with increasing proportions of the agents with oligomeric or polymeric structure. The water absorption approaches a constant value at high water contents. From Fig. 9, it can be seen that crystal growth follows an analogous
course. Upon increasing the amount of additive to 50%, the samples absorb so much water that they ``¯ow'' and crystallize simultaneously. The very damp surface of the candies makes sample preparation with a microtome impossible in order to measure the thickness of crystal layer. The highest acceleration both of water absorption and of crystal growth is initiated in isomalt by adding low molecular HSH syrup HS 3375. The maximum error of the microcrystalline thickness results is estimated as 5%. Representative error bars are included in Fig. 9 for the example of 90% isomalt and 10% HS3375.
Fig. 9. Crystal growth on the surface of model glasses made of isomalt with dierent amounts of HS 3375, HS 6075 and Litesse III compared with pure amorphous isomalt (conditions: 25 C, 80% rel. humidity).
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Fig. 10. Inhibitive eect of hydrogenated starch hydrolysate on crystal growth on the surface of lactitol hard candies (conditions: 25 C, 80% rel. humidity).
4. Discussion Among the various factors in¯uencing the storage stability of amorphous materials, the glass transition temperature is of outstanding importance. Tg decreases with increasing moisture content. With increasing water content from 0 to 1% a Tg -depression of the carbohydrates and carbohydrate mixtures examined in this work in the magnitude of 7±14 C was found. At high moisture content the glass transition temperature decreases below room temperature leading to the breakdown or stickiness of the product. Some glasses may crystal-
lize at temperatures above Tg because of higher molecular mobility as a result of a decrease in viscosity. Isomalt crystallization during storage is the most important defect of hard candies based on this sugar alcohol. Along with a reduced molecular mobility in the glassy state, one could assume a delayed crystallization of amorphous isomalt by the addition of material which increases viscosity of the melt. As expected, HSH shows an inhibitory eect on super®cial crystal growth of lactitol hard candies (Fig. 10). In the case of isomalt, however, addition of high or low molecular HSH or polydextrose signi®cantly
Fig. 11. Simpli®ed viscosity course of isomalt-HS 3375 systems as a function of (T±Tg ).
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accelerates crystal growth of isomalt glasses (Fig. 9). Because of their higher hygroscopicity (Klacik, 1990), mixtures of isomalt and oligomeric or polymeric substances are less stable during storage. The reduced viscosity caused by increased water absorption nulli®es the viscosity increasing eect of high molecular additives (Fig. 8). In the case of isomalt glasses partial incorporation of the additive into the lattice or an inhibition of growth by adsorption of the additive onto the crystal surface, as proposed by Hartel (1987), seems to be negligible. The occurence of the two isomalt-stereoisomers GPS and GPM is supposed to be most eective in preventing the individual compounds from crystallizing. Samples containing 10% HSH or polydextrose show a dry crystal layer similar to pure isomalt. An increase in the amount of polymeric additives to 30% led to crystallization on the surface which remained damp during storage at 25 C and 80% rel. humidity. Further increase of additives to 50% shows the phenomenon both of cold ¯ow and graining. Minimal stability of isomalt hard candies was found after the addition of the low molecular weight HSH syrup HS 3375, which may be due to the higher hygroscopicity of pure HS 3375 compared with isomalt. The glass transition temperature of amorphous isomalt is dierently aected by adding polydextrose or HSH syrup. In accordance with Eq. (1) (Roos & Karel, 1991c) pure compounds like polydextrose show increasing Tg with increasing molecular weight. Tg values of high molecular weight additives are dicult to analyze because of a broadening of the glass transition region. This behaviour of polymer mixtures is in accordance with the ®ndings of previous studies (Roos & Karel, 1991b). Since isomalt is a mixture of the low molecular stereoisomers GPS and GPM, isomalt signi®cantly decreases the eective dry Tg of the high molecular weight compound polydextrose. Astonishingly, with decreasing isomalt content, Tg of the anhydrous combinations of isomalt and polydextrose runs through a minimum (Fig. 3). Therefore, a high amount (75%) of high molecular additive is needed to increase the eective Tg of pure isomalt. Furthermore, mixtures of isomalt and low (HS 3375) or high (HS 6075) molecular weight HSH syrups show an analogous minimum of Tg mentioned above. Another important parameter for hard candy production is the k value, which re¯ects the sensitivity of Tg to variations of water content. The plasticizing eect of water is much more pronounced on amorphous additives than on isomalt, as expressed by the empirical k values given in Table 3. The found k values can be used for the calculation of Tg as dependent on water content. Although Tg of anhydrous amorphous HS 6075 (Tg;c =75 C) is higher than that of amorphous isomalt (Tg;c =63.6 C), glass transition of glassy HS 6075 and isomalt, both containing the same amount water (2%), occurs approximately at the same
temperature (Tg;c =49 C). The in¯uence of variations in water content on Tg of the hydrogenated carbohydrate melts should be minimized in hard candy production because high variations of Tg implies a loss of control of the production process. The addition of HSH or Litesse III leads to a signi®cant increase in viscosity with increasing amount of these substances. Therefore, all viscosity-dependent procedures such as incorporation of ¯avours or colours and forming of the candies have to be carried out at elevated temperatures, as shown in Fig. 11. Especially the addition of thermally unstable ¯avours may be dicult. Viscosity of low molecular sugar or sugar alcohol melts with dierent water contents was shown to depend uniformly on (T±Tg ), if Tg;d (Fig. 1) is taken as glass transition temperature (Janûen, 1999). (T±Tg ), sticky point and tendency to crystallize should be closely related in these systems. Therefore, Tg , which can be easily determined is useful in the evaluation of viscosity-dependent changes of amorphous state during storage. The treatment of viscosity as function of (T±Tg ) is not possible in the case of melts containing oligomeric or polymeric compounds. Melts with higher water contents show higher deviations in viscosity at constant (T± Tg ). The formation of viscosity increasing structures in the melts through water may be a reason for this phenomenon. Consequently the temperature-dependence of viscosity and viscosity-dependent changes of amorphous materials containing hydrated carbohydrates with higher molecular weight are only predictable by means of WLF at very low water content. 5. Conclusions The stability of the glassy state of isomalt is negatively in¯uenced by the addition of polydextrose or HSH syrup. Water absorption and crystallization of isomalt hard candies increase with increasing percentage of the agents containing oligomeric or polymeric compounds. The addition of high amounts of high molecular weight compounds such as polydextrose or high molecular weight HSH to isomalt is needed to increase Tg above the Tg of pure isomalt. This leads, however, to a new stability issue as a consequence of the marked increase in water absorption. In any case, an increase of Tg cannot be achieved by adding a low molecular weight HSH. Problems in candy production may occur by using high molecular weight additives. Such melts show increasing viscosity with increasing water content at constant (T± Tg ). Therefore, an evaluation of viscosity-dependent processing steps and changes of amorphous state by simply measuring Tg is not practicable. Additional information about the eect of water on the viscosity of melts with oligomeric or polymeric substances is needed.
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Acknowledgements The authors wish to acknowledge Dr. M. MensahWilson for the critical review of the English text.
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Eect of oligomeric or polymeric additives on glass transition, viscosity and crystallization of amorphous isomalt J. Raudonus a, J. Bernard b,*, H. Janûen b, J. Kowalczyk b, R. Carle a a
Institute of Food Technology, Section Plant Foodstu Technology, Hohenheim University, Garbenstraûe 25, D-70599 Stuttgart, Germany b SuÈdzucker AG/ZAFES, Wormser Str. 11, D-67283 Obrigheim/Pfalz, Germany Received 2 December 1999; accepted 5 January 2000
Abstract The main purpose of this study was to further improve the stability of the amorphous state of isomalt by the addition of high molecular weight compounds. Dierential scanning calorimetry (DSC) was used to determine the glass transition temperature Tg of a series of polyol mixtures containing isomalt as a function of water content. As expected, for each mixture Tg decreased with increasing moisture. Only with the addition of more than 75% of higher molecular weight compounds to isomalt an increase of Tg could be achieved. Moisture content and time dependent phase transitions in the metastable amorphous state of the mixtures strongly aect the storage stability of isomalt hard candies. Storage tests indicated a markedly accelerated water absorption and crystallization when oligomeric or polymeric compounds were added to amorphous isomalt. Rheological experiments showed that in contrast to pure isomalt melts, the viscosity of melts containing oligomeric or polymeric additives deviated from the model curve predicted by the Williams±Landel±Ferry-kinetics (WLF) with increasing water content. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Additives; Hard candies; Hygroscopicity; Isomalt; Recrystallization; Viscosity
1. Introduction In hard candies, sugars or sugar alcohols are in an amorphous state. The candy matrix is, in contrast to the crystalline phase, characterized by slightly loose packing of molecules with scattered water molecules. In addition there is no long range order of molecules as there is in the crystal. For the formation of the amorphous state in hard candies, a carbohydrate solution with a very low water content is quickly cooled after boiling. Because of the increasing viscosity with reduced temperature, the mobility of the molecules is severely restricted. Therefore transformation into the crystalline phase is inhibited (Hartel, 1987). The transformation from the glassy to the more liquidlike state, or vice versa, takes place over a range of temperatures. Consequently a glass transition temperature, Tg , cannot be accurately de®ned (Herrington & Bran®eld, 1984). At the glass transition temperature various physical properties such as viscosity drastically change. Above Tg there is an * Corresponding author. Tel.: +49-6359-803483; fax: +49-6359803331. E-mail address: [email protected] (J. Bernard).
increase of molecular mobility and free volume (White & Cakebread, 1966) resulting in an endothermic change in the apparent speci®c heat which can be detected by dierential scanning calorimetry (DSC) (Roos & Karel, 1990). A typical DSC trace over the glass transition range of an amorphous sugar or sugar alcohol as well as the usual de®nitions of Tg are shown in Fig. 1. The most important factors aecting the glass transition temperature are the composition of the material, molecular weight, and plasticizers (Roos & Karel, 1990). The Tg value of anhydrous food homopolymers was reported to decrease linearly with increasing 1/M value according to Eq. (1) (Roos & Karel, 1991c): Tg Tg 1 ÿ
K M
1
where M is molecular weight; K is constant; and Tg 1 is the limiting Tg at high molecular weight. Water is the most prominent plasticizer which decreases the glass transition temperature of amorphous carbohydrates. The eect of water on the Tg of a wide range of carbohydrates can be modeled according to
0963-9969/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0963-9969(00)00022-3
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Fig. 1. Glass transition temperature (Tg ) range of amorphous sugars or sugar alcohols as determined using dierential scanning calorimetry. The deviation, onset, and at half cp temperatures are abbreviated Tg;d , Tg;o , and Tg;c , respectively. The change of speci®c heat at the Tg region is indicated by cp (Janûen, 1999)..
Eq. (2), originally reported by Gordon and Taylor (1952), and used for the calculation of Tg values of binary polymer mixtures (Karel, Anglea, Buera, Lormasd, Levi & Roos, 1994; Roos, 1993): Tg
w1 Tg1 kw2 Tg2 w1 kw2
2
where w1 and w2 are the weight fractions of the carbohydrate and of water, respectively. Tg1 and Tg2 are the glass transition temperatures of the pure components. k is a constant which describes the sensitivity of Tg to variations of water content. The Tg value of water has been empirically determined by Johari, Hallbrucker and Mayer (1987); the found value of ÿ134 C was applied in this study. In this work Tg values of the dierent carbohydrate solutions were measured at various water contents to obtain the constant k by a least square ®t of a linearized transformation of Eq. (2): ÿ Tg;1 ÿ Tg w ÿ k 2 3 w1 Tg ÿ Tg;2 The viscosity of a glass exceeds 1012 Pas (White & Cakebread, 1966) and decreases dramatically above Tg (Roos, 1995). For a number of carbohydrate systems, the temperature dependence of viscosity above Tg is reported to obey the Williams±Landel±Ferry (WLF) equation (Karel et al., 1994): log
C1 T ÿ Tg g C2 T ÿ Tg
4
where is the viscosity at temperature T and g is the viscosity at Tg . C1 and C2 are constants. In this study C1 (-17.44) and C2 (51.6 K) originally reported by Williams, Landel and Ferry (1955) were
used. The viscosity at glass transition temperature g was adapted for each carbohydrate or carbohydrate mixture examined. The empirically determined values of low molecular sugar or sugar alcohol melts shared good correlation with the calculated curve by using log g 12 (Janûen, 1999). In organic glasses, both increase of moisture content and storage temperature may aect the rate of deteriorative reactions. The most important changes aecting the behaviour of amorphous carbohydrates occur at a quite narrow temperature range above Tg . This is related to the plasticization of amorphous structure which leads to a dramatic decrease in viscosity, and therefore an increase in molecular mobility (Roos & Karel, 1991a). These factors cause dierent time-dependent and viscosity related structural transformations like stickiness, cold ¯ow and crystallization during storage. Because of the high viscosity, crystallization in the glassy state below Tg is kinetically inhibited (Roos, 1995). The rate of crystallization depends on moisture content and temperature. Due to a decreasing viscosity an increase of these parameters allows molecular movement required for crystal growth. At ambient temperature absorption of water is, therefore, necessary for crystallization. In general, additives (``doctoring agents'') are of importance in hard candies to reduce the rate of crystallization of sugar or sugar alcohol glasses once formed (Hartel, 1987). The rate of crystallization depends on type and amount of additive as well as on the water content of the sample (Herrington & Bran®eld, 1984). Iglesias and Chirife (1978) reported delayed crystal growth of amorphous sucrose when polymeric compounds where added (Roos & Karel, 1991a). Several potential mechanisms may be responsible for the inhibitory eect. These include a selective incorporation of part of the additive into the lattice (``tailor
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made'' impurities), a blockage of growth sites by adsorption of the additive onto the crystal surface (Hartel, 1987), or an increased viscosity and decreased molecular mobility. The aim of the present work was to study the changes of crystallization tendency of isomalt hard candies after addition of viscosity-increasing substances such as hydrogenated starch hydrolysate (HSH) or hydrogenated polydextrose. These sugarfree ingredients are much more hygroscopic than isomalt and consequently less stable during storage. Hard candies made from low to intermediate molecular weight HSH display the phenomenon of ``cold ¯ow''. This describes the tendency for the candy to distort over time, which occurs readily at room temperature (Klacik, 1990). Products made from high molecular weight HSH or polydextrose become sticky due to absorption of water or increase in temperature. The sugar alcohol isomalt is an ideal model material for hard candy production with respect to stability. The low hygroscopicity and the composition of two stereoisomers are responsible for the high stability of amorphous products made from this sugar substitute. Tg of several combinations of isomalt and HSH or polydextrose were determined at various water contents. Rheological experiments should show an increasing viscosity of isomalt melts by the addition of agents with oligomeric or polymeric structure. 2. Materials and methods 2.1. Raw materials The disaccharide alcohol isomalt, a nearly 1:1-mixture of the two stereoisomers 1-O-a-d-glucopyranosyl-dmannitol dihydrate (1,1-GPM dihydrate) and 6-O-a-dglucopyranosyl-d-sorbitol (1,6-GPS) with a purity of 99.5%, was obtained from Palatinit GmbH, Germany. Additives relevant for the production of sugarfree hard candies were used. Apart from a low (Hystar1 3375) and high (Hystar1 6075) molecular hydrogenated starch hydrolysate (HSH) syrup (Lonza, USA), hydrogenated polydextrose Litesse III (Cultor, Denmark) with a mean mass of 5000 g/mol were studied. The compositions of the two HSH syrups used are given in Table 1, where with the exception of the water content (percentage weight of the total syrup), the other contents are expressed as percentage weight of the total solids. 2.1.1. Sample preparation Melts with various water contents were produced by heating solutions that contained approx. 70% of the desired carbohydrate composition at dierent temperatures under atmospheric pressure. Subsequently the
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melts were solidi®ed in a dry-box. The glasses were analyzed for water content by measuring the mass lost after drying in a vacuum oven. Hard candy samples for storage tests were prepared by heating a solution (approx. 70%) of the respective mixture of isomalt and additive to 160 C and evacuating to a pressure less than 100 mbar. The resulting melts contained 1±1.5% water after 75 s. After cooling to 70± 80 C on a cooling table tempered at 60±70 C, the candies were formed with the aid of a motor driven drop roller. The samples were analyzed for water content by using Karl±Fischer titration. 2.2. Methods 2.2.1. Dierential scanning calorimetry (DSC) DSC was used to determine glass transition temperature (Tg ) of amorphous samples at various water contents. A Perkin±Elmer Pyris 1 DSC was used with Pyris software for Windows. The instrument was calibrated using indium (Hm=28.45 J gÿ1, m.p.=156.6 C, Perkin±Elmer standard) and water (m.p.=0 C). The calibration was checked using n-octadecane. Samples were prepared in a dry-box by weighing 10± 15 mg of the amorphous material in 20 ml DSC pans (Perkin±Elmer) which were subsequently hermetically sealed. Desiccated air was blown into the box to keep it dry. An empty aluminium pan was used as reference in all measurements. To avoid condensation of moisture, the sample head was purged with dry nitrogen. Each sample was cooled to about 30 C below Tg , heated at 10 C/min to Tg +30 C, and then cooled at 250 C/min to Tg ÿ30 C. The same cooling and heating procedure was applied to all samples to standardize the thermal history of all glass samples. The samples were then scanned at 10 C/min for determination of Tg .
Table 1 Composition of both HSH syrups used (DP=number-average of polymerisation; molar mass is calculated from composition) Composition
HYSTAR 3375 (HS 3375)% w/w
HYSTAR 6075 (HS 6075)% w/w
D-Sorbitol Maltitol Maltotriositol DP4 DP5 DP6 DP7 DP8 DP9 DP10 DP10+ M (g/mol) Water content
12.9 20.4 11.3 6.2 6.8 8.2 5.3 2.5 1.1 0.7 24.8 504 25%
11.1 7.9 8.1 6.1 6.3 9.2 6.9 2.8 1.5 1.1 39.1 644 25%
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2.2.2. Rheological experiments A Bohlin CSR 10 was used to study the rheological properties and ascertaining viscosity at dierent temperatures of melts varying in composition and water content. The viscosity measurements were carried out with oscillating forces to avoid structural destruction and a changed rheological reaction. The shear stress [ 0 sin !t] to generate a sinusoidal deformation vibration [ 0 sin !t] was registered. Using Eq. (5) the complex viscosity was calculated. !
0 ! sin ÿ i cos
0
5
The complex viscosity could be regarded as a real value if the imaginary part of Eq. (5) is negligible. The measurements carried out in this work shared comparable magnitudes for both complex and real parts of the viscosity. 2.2.3. Rate of crystallization Hard candies made of isomalt with dierent amounts of additives were stored at 25 C and 80% relative humidity in open Petri dishes. During the storage test over 3 weeks, water absorption of the samples was determined gravimetrically. In addition, crystal growth on the sample surface was studied by preparing 10 mm sections of the candy with a microtome. Crystalline areas were detected as yellow or violet glimmering re¯ections under the microscope as observed under polarized light. The thickness of the crystal layers was measured by using an ocular micrometer.
Table 2 Composition of the dierent model glasses and melts examined (differential scanning colorimetry and viscosity measurements) Raw materials
Mixtures with isomalt (dry solids base)
HS 3375 HS 6075 Litesse III
75%, 50%, 25% w/w isomalt 75%, 50%, 25% w/w isomalt 75%, 50%, 25% w/w isomalt
3. Results 3.1. Eect of composition and moisture content on glass transition temperature A selection of glass systems containing dierent amounts of isomalt, HSH, and polydextrose (Table 2) was studied. All materials showed typical thermal behaviour of amorphous materials. The Tg values of the anhydrous additives and mixtures with isomalt examined are shown in Table 3. The plasticizing eect of water is re¯ected by the k values also given in Table 3. The drastic decrease of Tg with increasing water content is in accordance with the ®ndings in previous studies (Roos, 1993; Karel et al., 1994). The eect of the addition of 25% Litesse III, HS 3375 or HS 6075 on Tg of amorphous isomalt is given in Fig. 2. These mixtures are characterized by lower Tg values compared with pure isomalt. This holds for the whole range of water content investigated, that is from 0 to 20% water of total weight. Fig. 3 provides a plot of Tg of the anhydrous combinations of isomalt and additives against isomalt content
Fig. 2. Glass transition temperature of amorphous isomalt containing 25% w/w Litesse III, HS 3375 or HS 6075 compared with pure amorphous isomalt as a function of dry matter concentration.
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Table 3 Glass transition temperatures of the anhydrous material and k values of various combinations between isomalt and Litesse III, HS 3375 or HS 6075 Composition
Tg;c ( C)
kc
Composition
Tg;c ( C)
kc
Composition
Tg;c ( C)
kc
isomalt 25% Litesse III 50% Litesse III 75% Litesse III Litesse III
63.6 53 59 69.4 90.8
3.9 3.6 3.9 4.1 4.5
isomalt 25% HS 3375 50% HS 3375 75% HS 3375 HS 3375
63.6 49.6 49.2 49.2 54.6
3.9 3.7 3.9 4.0 5.6
isomalt 25% HS 6075 50% HS 6075 75% HS 6075 HS 6075
63.6 56.2 59.5 64.5 75
3.9 4.3 4.5 4.8 6.7
in the mixture. It is evident that Tg runs through a minimum with decreasing isomalt content. Polymeric Litesse III and HSH type HS 6075 have higher Tg values than pure isomalt. However, the addition of 20± 30% of these substances to isomalt lowers the Tg value by approx. 10 C. Mixtures of polydextrose and isomalt exhibit the most pronounced Tg depression. Only compositions with polydextrose or HS 6075 content higher than 60 or 70% elevates Tg in a temperature range above the Tg of pure isomalt. Mixtures between isomalt and low molecular HSH (HS 3375) are characterized by a broad and shallow range of lowered Tg values along the whole concentration scale. These values are approximately 10±15 C lower than the Tg of pure isomalt. Fig. 4 shows the k values of the mixtures as a function of isomalt content. For Litesse III and HS 3375 a minimal k value for the mixtures with isomalt is, analogous to Tg , located at approximately 75% isomalt content. At this composition a depression of the k value of 0.3 and 0.2 for Litesse III and HS 3375, respectively, was observed. A 1:1 mixture between isomalt and HS 3375 or Litesse III was found to have the same Tg sensitivity as pure isomalt with varying water content. An increase
in the content of additive above 60±70% raises the k values of the mixtures above that of pure isomalt. Mixtures of isomalt and HS 6075 do not show an analogous minimum. In the entire composition range the k values of the mixtures were found to be higher than that of pure isomalt. Pure amorphous HSH, in particular HS 6075, was markedly plasticized by water. In comparison to pure isomalt (3.9) there is an increase in the k values of HS 3375 (5.6) and HS 6075 (6.7) by 1.7 and 2.8, respectively. 3.2. Eect of composition and water content on viscosity The temperature-dependence of a number of melts varying in composition and water content were studied using an oscillation rheometer (Bohlin CSR 10). For each melt the rheological behaviour was investigated at a frequency of 1 Hz. Table 2 shows the composition of the dierent samples examined. The viscosities of various melts with dierent water contents were plotted as a function of the relative variable (T±Tg;d ) and compared with a model curve predicted by the WLF kinetic. The uniform behaviour of low molecular sugars or sugar alcohols (e.g. isomalt) is con®rmed by experimental data
Fig. 3. Dry matter glass transition temperatures of isomalt and mixtures with HS 3375, HS 6075 and Litesse III as a function of isomalt content.
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Fig. 4. The k-value of combinations between isomalt and HS 3375, HS 6075 and Litesse III as a function of isomalt content.
which correlate well with the predictions of the WLF model (Janûen, 1999). The temperature-dependence of the empirical data and calculated viscosity data of an isomalt melt with addition of 25% Litesse III is shown in Fig. 5. For this sample, it was necessary to increase the WLF parameter log Zg from 12, which is valid for pure isomalt, to 12.6. It is obvious that the viscosity of melts containing substances with oligomeric or polymeric structures cannot be treated as a function of (T±Tg ). At lower temperatures above Tg these melts show increasing deviation in viscosity with increasing water content from the expected model behaviour. Consequently a
generalized treatment of viscosity of higher molecular carbohydrate melts with the use of the WLF model seems to be possible only for melts with a low water content. As shown in Fig. 5, viscosity seems to be independent of temperature in the range between 40 and 50 C above Tg;d and below at 1 Hz. In fact, the viscosity becomes dependent on measuring frequency at temperatures slightly above Tg . The rheological properties change from Newtonian to pseudoplastic and plastic behaviour as shown in Fig. 6. Moreover, this change of behaviour occurs at a higher temperature the higher the amount of oligomeric or polymeric material in the mixture.
Fig. 5. Viscosity of an isomalt melt containing 25% w/w Litesse III at various water contents, compared with WLF-model.
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Fig. 6. Viscosity of an isomalt melt containing 25% Litesse III as a function of frequency at dierent temperatures.
Viscosity values of melts showing pseudoplastic properties are independent of the frequency at low frequencies. Therefore the constant viscosity value can be extrapolated to 0 Hz, that is static behaviour. In the temperature range where viscosity displays a plastic behaviour it obeys the exponential law of Ostwald-deWaele (Weipert, Tscheuschner & Windhab, 1993). Therefore, the used method of determining viscosity is no longer applicable. In Fig. 7 the viscosity of a melt containing 75% isomalt and 25% Litesse III measured at 1Hz is compared with its static behaviour. The Newtonian to plastic behaviour transition is indicated by the dierentiation of the static and
dynamic values of viscosity. In this case, the dynamic viscosity is determined and the behaviour transition starts at about 40 C above Tg . It should be kept in mind that this transition temperature depends on the frequency at which the dynamic viscosity is determined as shown in Fig. 6. 3.3. Eect of composition on water absorption and crystallization The progress of the water absorption, expressed as increase in weight, and nucleation front was plotted against storage time for the dierent glass systems.
Fig. 7. Temperature-dependent viscosity values extrapolated to 0 Hz compared with the viscosity measured at 1 Hz of an isomalt melt containing 25% Litesse III (0.4% w/w water content).
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Fig. 8. Water absorption of model glasses made of isomalt with dierent amounts of HS 3375, HS 6075 and Litesse III compared with pure amorphous isomalt (conditions: 25 C, 80% rel. humidity).
Glasses made from isomalt containing 10, 30 or 50% w/w additive (HS 3375, HS 6075, Litesse III) were examined at 25 C and 80% rel. humidity. Each glass sample contained 1±1.5% water at the beginning of the stability study. The water absorption values of these samples are compared with those of pure amorphous isomalt in Fig. 8. The samples containing HS 3375, HS 6075 or Litesse III show a prominent accelerated water absorption with increasing proportions of the agents with oligomeric or polymeric structure. The water absorption approaches a constant value at high water contents. From Fig. 9, it can be seen that crystal growth follows an analogous
course. Upon increasing the amount of additive to 50%, the samples absorb so much water that they ``¯ow'' and crystallize simultaneously. The very damp surface of the candies makes sample preparation with a microtome impossible in order to measure the thickness of crystal layer. The highest acceleration both of water absorption and of crystal growth is initiated in isomalt by adding low molecular HSH syrup HS 3375. The maximum error of the microcrystalline thickness results is estimated as 5%. Representative error bars are included in Fig. 9 for the example of 90% isomalt and 10% HS3375.
Fig. 9. Crystal growth on the surface of model glasses made of isomalt with dierent amounts of HS 3375, HS 6075 and Litesse III compared with pure amorphous isomalt (conditions: 25 C, 80% rel. humidity).
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Fig. 10. Inhibitive eect of hydrogenated starch hydrolysate on crystal growth on the surface of lactitol hard candies (conditions: 25 C, 80% rel. humidity).
4. Discussion Among the various factors in¯uencing the storage stability of amorphous materials, the glass transition temperature is of outstanding importance. Tg decreases with increasing moisture content. With increasing water content from 0 to 1% a Tg -depression of the carbohydrates and carbohydrate mixtures examined in this work in the magnitude of 7±14 C was found. At high moisture content the glass transition temperature decreases below room temperature leading to the breakdown or stickiness of the product. Some glasses may crystal-
lize at temperatures above Tg because of higher molecular mobility as a result of a decrease in viscosity. Isomalt crystallization during storage is the most important defect of hard candies based on this sugar alcohol. Along with a reduced molecular mobility in the glassy state, one could assume a delayed crystallization of amorphous isomalt by the addition of material which increases viscosity of the melt. As expected, HSH shows an inhibitory eect on super®cial crystal growth of lactitol hard candies (Fig. 10). In the case of isomalt, however, addition of high or low molecular HSH or polydextrose signi®cantly
Fig. 11. Simpli®ed viscosity course of isomalt-HS 3375 systems as a function of (T±Tg ).
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accelerates crystal growth of isomalt glasses (Fig. 9). Because of their higher hygroscopicity (Klacik, 1990), mixtures of isomalt and oligomeric or polymeric substances are less stable during storage. The reduced viscosity caused by increased water absorption nulli®es the viscosity increasing eect of high molecular additives (Fig. 8). In the case of isomalt glasses partial incorporation of the additive into the lattice or an inhibition of growth by adsorption of the additive onto the crystal surface, as proposed by Hartel (1987), seems to be negligible. The occurence of the two isomalt-stereoisomers GPS and GPM is supposed to be most eective in preventing the individual compounds from crystallizing. Samples containing 10% HSH or polydextrose show a dry crystal layer similar to pure isomalt. An increase in the amount of polymeric additives to 30% led to crystallization on the surface which remained damp during storage at 25 C and 80% rel. humidity. Further increase of additives to 50% shows the phenomenon both of cold ¯ow and graining. Minimal stability of isomalt hard candies was found after the addition of the low molecular weight HSH syrup HS 3375, which may be due to the higher hygroscopicity of pure HS 3375 compared with isomalt. The glass transition temperature of amorphous isomalt is dierently aected by adding polydextrose or HSH syrup. In accordance with Eq. (1) (Roos & Karel, 1991c) pure compounds like polydextrose show increasing Tg with increasing molecular weight. Tg values of high molecular weight additives are dicult to analyze because of a broadening of the glass transition region. This behaviour of polymer mixtures is in accordance with the ®ndings of previous studies (Roos & Karel, 1991b). Since isomalt is a mixture of the low molecular stereoisomers GPS and GPM, isomalt signi®cantly decreases the eective dry Tg of the high molecular weight compound polydextrose. Astonishingly, with decreasing isomalt content, Tg of the anhydrous combinations of isomalt and polydextrose runs through a minimum (Fig. 3). Therefore, a high amount (75%) of high molecular additive is needed to increase the eective Tg of pure isomalt. Furthermore, mixtures of isomalt and low (HS 3375) or high (HS 6075) molecular weight HSH syrups show an analogous minimum of Tg mentioned above. Another important parameter for hard candy production is the k value, which re¯ects the sensitivity of Tg to variations of water content. The plasticizing eect of water is much more pronounced on amorphous additives than on isomalt, as expressed by the empirical k values given in Table 3. The found k values can be used for the calculation of Tg as dependent on water content. Although Tg of anhydrous amorphous HS 6075 (Tg;c =75 C) is higher than that of amorphous isomalt (Tg;c =63.6 C), glass transition of glassy HS 6075 and isomalt, both containing the same amount water (2%), occurs approximately at the same
temperature (Tg;c =49 C). The in¯uence of variations in water content on Tg of the hydrogenated carbohydrate melts should be minimized in hard candy production because high variations of Tg implies a loss of control of the production process. The addition of HSH or Litesse III leads to a signi®cant increase in viscosity with increasing amount of these substances. Therefore, all viscosity-dependent procedures such as incorporation of ¯avours or colours and forming of the candies have to be carried out at elevated temperatures, as shown in Fig. 11. Especially the addition of thermally unstable ¯avours may be dicult. Viscosity of low molecular sugar or sugar alcohol melts with dierent water contents was shown to depend uniformly on (T±Tg ), if Tg;d (Fig. 1) is taken as glass transition temperature (Janûen, 1999). (T±Tg ), sticky point and tendency to crystallize should be closely related in these systems. Therefore, Tg , which can be easily determined is useful in the evaluation of viscosity-dependent changes of amorphous state during storage. The treatment of viscosity as function of (T±Tg ) is not possible in the case of melts containing oligomeric or polymeric compounds. Melts with higher water contents show higher deviations in viscosity at constant (T± Tg ). The formation of viscosity increasing structures in the melts through water may be a reason for this phenomenon. Consequently the temperature-dependence of viscosity and viscosity-dependent changes of amorphous materials containing hydrated carbohydrates with higher molecular weight are only predictable by means of WLF at very low water content. 5. Conclusions The stability of the glassy state of isomalt is negatively in¯uenced by the addition of polydextrose or HSH syrup. Water absorption and crystallization of isomalt hard candies increase with increasing percentage of the agents containing oligomeric or polymeric compounds. The addition of high amounts of high molecular weight compounds such as polydextrose or high molecular weight HSH to isomalt is needed to increase Tg above the Tg of pure isomalt. This leads, however, to a new stability issue as a consequence of the marked increase in water absorption. In any case, an increase of Tg cannot be achieved by adding a low molecular weight HSH. Problems in candy production may occur by using high molecular weight additives. Such melts show increasing viscosity with increasing water content at constant (T± Tg ). Therefore, an evaluation of viscosity-dependent processing steps and changes of amorphous state by simply measuring Tg is not practicable. Additional information about the eect of water on the viscosity of melts with oligomeric or polymeric substances is needed.
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Acknowledgements The authors wish to acknowledge Dr. M. MensahWilson for the critical review of the English text.
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