Biotechnol. Prog. 1991, 7, 49-53 49

Phase Transitions of Mixtures of Amorphous Polysaccharides and Sugars?

Yrjo ROOS* and Marcus Karel Department of Food Science and Center for Advanced Food Technology, Rutgers University, P.O. Box 231, New Brunswick, New Jersey 08903

Maltodextrins of varying molecular weights, maltose, and sucrose were used to study the effect of molecular weight, water plasticization, and composition on glass transition temperature ( Tg). All maltodextrins were plasticized by water, and the decrease of Tg was linear with water activity over the range of 0.11-0.85. The plasticization effect of water was similar for maltodextrins having various molecular weights. Effects of molecular weight and composition on the Tg of maltodextrins could be correlated by using relationships reported previously for polymers. The quantitative results obtained can be applied to formulate food and related materials having desired processability and storage stability.

Introduction Carbohydrates and proteins may exist in an amorphous

state with thermal and time-dependent physical properties similar to those of polymers (1-3). In foods, amorphous components affect their processability and storage stability (4-9).

An amorphous material undergoes a change from a very viscous “glass” to a “rubber” a t the glass transition temperature (Tg). A t Tg the molecular mobility increases and the viscosity decreases, which may result in structural changes such as collapse and stickiness. In biological materials water plasticizes the amorphous structure, and Tg is decreased with increasing moisture content. Since structural collapse and stickiness occur a t a specific temperature, which depends on moisture content and time, the physical properties have been characterized by collapse temperature (T,) or sticky point (T,) (5, 10-13). Tsour- ouflis e t al. (IO) and To and Flink (11) have reported similarities between the T, and the Tg of polymers. Tsour- ouflis et al. (10) also found collapse to be time-dependent, and To and Flink (11) reported collapse to be phenom- enologically similar to the glass transition in polymers.

Time-dependent physical changes in foods and food components, including crystallization, can be related to temperature above Tg (14). Maltodextrins are widely used as food components to increase viscosity, to retard crystallization, to improve drying characteristics, for encapsulation, and to decrease stickiness and hygroscop- icity of dried materials. They may also be used to improve storage stability of frozen foods (15). The purpose of this study was to determine the effect of water plasticization and molecular weight on Tg of maltodextrins and to quantify the ability of maltodextrins to increase the Tg of sucrose in binary mixtures.

Materials and Methods Sample Preparation. Maltodextrins having dextrose

equivalents of 5, 10, 15, 20, 25, and 36 and respective

+ Paper 7a, AIChE Summer Meeting, San Diego, CA, August 19- 22, 1990.

theoretical molecular weights (M) of 3600,1800,1200,900, 720, and 500 (Maltrin M040, M100, M150, M200, M250, and M365; Grain Processing Corp.), and maltose (Sigma Chemical Co., Grade I) were dissolved in HPLC-grade water (Fisher) as 20% solutions. The solutions were frozen (20 gin a weighing bottle) for 24 h at -30 OC, followed by 3 h over dry ice, and freeze-dried (48 h, Virtis Bench- top, 3 L). The freeze-dried materials were further dehy- drated to a “zero” moisture content in a vacuum desiccator over Pz05 for a t least 1 week. The dried materials were powdered, and 5-10 mg of the material was transferred into aluminum DSC pans (40 pL, Mettler) in a dry box. These pans were weighed rapidly and rehumidified for 2-3 days over saturated salt solutions (Table I) to varying moisture contents in vacuum desiccators. Subsequently the pans were hermetically sealed for differential scanning calorimetry. The moisture content of the samples was obtained as reported by Roos (16). The average value of five replicates was taken as the moisture content (m). Binary mixtures of maltodextrin and sucrose (Sigma, Grade 11) were prepared and analyzed similarly a t the zero moisture level.

Differential Scanning Calorimetry (DSC). The Tg values for the maltodextrins and maltodextrin-sucrose mixtures were determined by using a Mettler T A 4000 system DSC 30s. The instrument was calibrated as reported by Roos (16). The dry and rehumidified samples were scanned over the glass transition region at 5 “C/min, and the Tg was determined as the onset temperature of the glass transition (16). The onset temperature of the Tg was determined by using Mettler GraphWare TA72PS.l. In most cases each sample was analyzed twice from a temperature a t least 20 “C below Tg to a temperature a t least 20 “C above Tg to eliminate the hysteresis effects of thermal relaxations typical of glass transitions (I 7-19), and an average value of five replicate samples was taken as the Tg.

Estimation of Tg of Maltodextrin-Sucrose Binary Mixtures. The Tg of binary polymer mixtures can be calculated by using eq 1, which has been used to predict glass transition temperatures of polymer mixtures (20):

8756-7938/91/3007-0049$02.50/0 0 1991 American Chemical Society and American Institute of Chemical Engineers

50

Table I.

Biotechnol. Prog., 1991, Vol. 7, No. 1

Glass Transition Temoeratures of Maltodextrins and Their Moisture Content at Various Relative Humiditiesa DE 5d DE 10 DE 15 DE 20 DE 25 DE 36 maltose

salt rH.C r( m T. pzo5 0 0 188 LiCl 11 1.9 135 CHjCOOK 23 4.0 102

KiCOj 43 6.3 87 %'(Nod2 52 8.9 58 NaC1 75 10.6 44 KC1 85 17.7 23

MgC1* 33 4.5 90

m Tg 0 160 2.0 103 4.9 84 5.4 66 6.9 60 8.2 38

10.5 30 18.8 -6

m Tg 0 2.3 99 4.8 83 6.0 65 7.1 57 8.0 40

11.3 8 19.9 -15

m T, 0 141 2.5 86 5 . 5 73 !5.7 42 6.2 40 9.2 37

16.2 -9 26.3 -32

m T, 0 121 2.1 83 4.6 60 5.1 36 6.3 34 8.8 29

17.4 -18 27.4 -39

m T , 0 100 1.7 67 3.9 45 5.1 31 5.7 27

10.9 6 20.5 -35 31.2 -52

m Tg 0 87 1.1 59 2.8 39 4.5 2.9 7.9 11

10.1 -4

Glass transition temperatures, Tg, are given in degrees Celsius; moisture contents ( m ) are given as grams per 100 g of dry matter. * Saturated Dextrose equivalent. salt solution used for rehumidification of dried materials. Relative vapor pressure of the saturated salt solution at 25 "C.

w,T,, + kw,T,, w1 + k w , Tg = (1)

where Tg is the glass transition temperature of the mixture (maltodextrin-sucrose), Tgl and Tg2 are the glass transition temperatures of components 1 (maltodextrin) and 2 (sucrose), w1 and w2 are the weight fractions of components 1 (maltodextrin) and 2 (sucrose), and h is a constant.

This empirical equation was used to predict the Tg values of maltodextrin-sucrose mixtures. The experimental T , values of the mixtures with sucrose concentrations of 20 '( , 4 0 ' ~ , 50'( , 60%, and 80% were used to calculate the constant k. An average value for all maltodextrin-sucrose mixtures (k = 3) was used.

Results Effect of Moisture on T,. Freeze-dried maltodex-

trins showed a glass transition typical of other amorphous carbohydrates and sugars (16). They were significantly plasticized by water, which decreased the Tg of all mal- todextrins even a t extremely low moisture contents. The T, values and respective moisture contents are given in Table I. The Tg values of M 1800, 1200, 900, and 720 maltodextrins were also compared with their corresponding T , values reported by Tsourouflis e t al. (10). The T, values decreased similarly to T , with increasing moisture content, but the T, was 30-70 "C higher than the T, depending on the material and its moisture content (Figure 1).

Similarly to the case of low molecular weight sugars and dried food materials, a linear relationship could be established between the T , values of maltodextrins and their water activity, a,, a t 25 "C (Figure 2). Although the Tg values showed linearity with a, over an extensive range (0.1 1-0.85), the true relationship seemed to be sigmoid. The decrease of T, was about equal for all maltodextrins for the same increase of a,. An average slope of -150 was calculated for the linear decrease of T, with a,.

Effect of Molecular Weight on Tg. T, of homopoly- mers increases with increasing molecular weight. Ac- cording to Fox and Flory (1 7). the effect of molecular weight can be calculated by using

(2) where M is the molecular weight, K , is a constant, Tg(m) is the limiting T, at a high molecular weight, and Tg is the glass transition temperature. This equation was used to study the effect of molecular weight of maltodextrins on their glass transition temperature. The T, of maltodex- trins was plotted against M-l, which was found to give a linear relationship at all a, values (Figure 3). An average slope was calculated to give K , = -25 000. Tg of Maltodextrin-Sucrose Mixtures. The T, of

dry sucrose is 57 "C (14). This value was found to be increased in two-component single-phase maltodextrin-

T , = Tg(m) - K K '

250

+ Tg M1800 - TC M 1800 Tc M 1200 + Tg M 900 - ' - . - TC M 900 * To M 720 - - - TC M 720

200 + Tg M ,200 .......... G e

a 150

K

c

W 100

n

c 50 B

0

- 5 0 ' ' ' ' ' " ' " " ' ' " " ' ' 0 5 1 0 1 5 2 0

MOISTURE CONTENT (g/lOOg dry matter)

Figure 1. Glass transition temperature of maltodextrins having various molecular weights and their collapse temperature (10).

- 200

E 1801) M 3600

E M 1800

A M 1200 0 M 900 0 M 720 A M 500 x Maltase

4 -40 ' -60 0.0 0.1 0.2 0.3 0.4 0 . 5 0 . 6 0 . 7 0.8 0.9 1.0

WATER ACTIVITY

- 2 0 L

Figure 2. Glass transition temperature of maltodextrins having various molecular weights. The glass transition temperature decreased linearly as a function of water activity (relative vapor pressure used to rehumidify the samples) at 25 O C . At extremely low and high water activities the shape of the line was presumed to be sigmoid, probably because of the very high viscosity of the nonplasticized material and low glass transition temperature of water, respectively.

sucrose glasses. The Tg values of maltodextrin-sucrose mixtures are shown in Figure 4. The predicted values of Tg of the mixtures obtained by using eq 1 followed the experimental points, giving a correlation coefficient of 0.991. The effect of maltodextrins and their molecular weight a t concentrations below 50% was small in com- parison to the increase a t concentrations above 50%.

Discussion The physical properties of amorphous food materials

depend on temperature and moisture content, but they are also time-dependent. Maltodextrins have thermal behavior typical of amorphous polymers, and they can be

Biotechnol. Prog., 1991, Vol. 7, No. 1 51

"" . 0 . 0 0 0 0 . 0 0 1 0 . 0 0 2 0 . 0 0 3

1 i M

Figure 3. Effect of molecular weight on the Tg of maltodextrins a t various relative humidities. The equation Tg = T&m) - 25000M-1 gives the T,for materials having the same wateractivity.

- 2001 I -0- M 3600 * M 1800

M 900 M 3600 Predicted M 1800 Predicted M 900 Predicted

0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . o WEIGHT FRACTION OF MALTODEXTRIN

Figure 4. Glass transition temperatures obtained for malto- dextrin/sucrose mixtures. Mixtures containing small amounts of maltodextrins showed only a slight increase of TE, and the increase was almost independent of molecular weight. At high maltodextrin concentrations the Tg of mixtures was increased by both concentration and molecular weight, and the Tg of malto- dextrins was substantially decreased with increasing sucrose concentration.

classified as thermoplastics with T, ranging from 100 t o 200 "C. Similar behavior is also typical of amorphous sugars, which have lower T, values (14).

The magnitude of physical changes above T, can be estimated by applying the Williams-Landel-Ferry equa- tion (21), which relates the T, to the temperature dependence of the relaxation time of mechanical properties a t a temperature range approximately from Tg to T, + 100 "C. The water plasticization that decreases T, has therefore a similar effect on physical properties, such as increased temperature. This effect is usually observable above the T, of maltodextrins by a structural change from powder to a clear paste, which is achievable by increasing either moisture content or temperature. In this study it was shown that maltodextrins having various molecular weights show similar effects of water on Tg (Table I, Figure 2). The decrease of Tg with increasing moisture content agreed well with the results for maltose, maltotriose, and maltohexaose reported by Orford et al. (22). In practice, temperatures above the T, can be used in food processing or relatively short-term storage, because the relaxation times of changes in the vicinityof the Tg are long. However, small differences in the relative vapor pressure change the moisture content of the amorphous materials and may significantly change the physical properties. In powders this is observed by agglomeration and in some cases by crystallization.

Collapse in drying and during storage of amorphous foods leads to stickiness and rapid loss of flavor compounds. The T, of maltodextrins has been shown to be a similar function of moisture content and molecular weight as the T,of polymers (10, 111, whichsupports our previous results on the relationship between Tg, collapse temperature, and sticky point (14). The results of this study showed that T, is a consequence of the glass transition of the amorphous material. However, Tg, T,, and T, have different tem- perature values because stickiness and collapse are ob- served at a critical viscosity allowing flow at the specific time used for the experiment. Therefore, both the extent of stickiness and collapse a t a constant temperature depend on mobility. This has also been shown to apply for the crystallization of amorphous sugars, which is governed by diffusivity above Tg (14).

The Tg of food materials is decreased linearly with increasing a, (14, 16). The results of this study showed a linear relationship also for maltodextrins. The slope of the regression line was constant for all molecular weights. The linearity seems to exist in a wide a, range and probably results from the increased free volume due to water plasticization. This range covers the whole practical range applicable to food processing and storage. At extremely low moisture contents Tg increases exponentially, probably because of a slower increase of the molecular mobility in the nonplasticized material. This is also noticed as retarded crystallization (23). At a, values close to 1 a similar exponential lowering of Tg can be expected because of the low theoretical T, of water, -135 "C (24, 25). However, a t high water contents T, is below the freezing point, maltodextrin solutions are freeze-concentrated, and a higher glass transition temperature for the freeze- concentrated solution is obtained (15). The observed linear relationship between a, and T, is useful since it can be applied to predict proper processing and storage conditions for food materials. Use of the constant slope allows calculation of T, a t any practical a, independently of the molecular weight. Tg can also be predicted by using eq 1, as proposed by Orford et al. (22).

The effect of molecular weight on T, was studied at relative humidities ranging from 0 to 85%. The Tg of maltodextrins as a function of M-l showed linearity, and the slope above zero moisture content was independent of the relative vapor pressure (Figure 3). This linear rela- tionship between T, and M-l is typical of polymers (1 7) and seems to apply also to maltodextrins. This has been shown to apply also to the collapse temperature of mal- todextrins (1 l ) , indicating the similarity with T,. However, T, values of maltotriose and maltohexaose reported by Orford et al. (22) are higher than those of maltodextrins with the same respectively theoretical molecular weight (Figure 5), probably because of low molecular weight substances present in starch hydrolysis products.

In previous studies starch has been shown to be a partially amorphous polymer (26-28). On the basis of the linear relationship between Tg and relative humidity, and the linear relationship between Tg and M-l, the Tg of starches can be calculated by extrapolation with eq 2. Maul rice et al. (26) reported a Tg of rice starch to be between 55 and 70 " C a t intermediate moisture levels, which is higher than would be expected by the extrapolation of the curve obtained for maltodextrins a t 85% relative vapor pressure. Also, the value of dry rice starch reported by Biliaderis et al. (27) is about 50 "C higher than the value based on the extrapolation. Zeleznak and Hoseney (28) reported T, values for wheat starch with increasing moisture content above 13% moisture. They reported

Biotechnol. Prog., 1991, Vol. 7, No. 1 52

!? - W K 3 c 4 K W n 9 z P t a v) z K

v) v)

L? 9

’ O 0 i This Study Orford et ai (1989) Maltose 180 - MaltOSe .......... Maltotriose (M 500) Maltohexaose (M 900)

Native Wheal Starch (Zeleznak and Hoseney 1987)

0 5 1 0 1 5 2 0 2 5 3 0 MOISTURE CONTENT (g/lOOg dry matter)

Figure 5. Glass transition temperatures of maltose, maltotri- ose, and maltohexaose (22) compared with maltodextrins having about the same Tg values and with native wheat starch (28). Maltodextrins have higher molecular weight than oligosaccha- rides with corresponding Tg values because of low molecular weight substances, which lower the Tg as shown in Figure 4.

t h e Tg of low mois ture content s ta rches t o be broad and ha rd t o determine. The Tg of d r y high molecular weight mal todext r ins also became broad and difficult to analyze. T h e Tg values of wheat starch agree well wi th t h e mal - todext r in data (Figure 51, and i t is likely that t h e T, values of pregelatinized s ta rches (28) can be closely related to t h e mal todext r in data. However, mos t s tud ies of t h e T, in s ta rch have no t specified relative vapor pressures used.

To a n d Fl ink ( I I ) showed that t h e collapse t empera tu re of b inary maltodextrin-sugar mixtures increases wi th increasing maltodextrin concentration. The increase could be predicted by applying a n equation for polymer mixtures. The Gordon a n d Taylor (20) equat ion used in this s tudy is of a similar t ype as t h e equat ion used by T o and Fl ink (11) and gives about t h e same predicted Tg values. However, o ther equat ions used to predic t compositional effects on t h e Tg of polymers (29, 30) m a y be applicable t o amorphous food materials. The prediction of Tg by using e q 1 was compared wi th the exper imenta l results, and a close correlation between t h e measured and predicted T, values was obta ined (Figure 4). Both the exper imenta l and predicted Tg values show that t h e Tg values of b inary mixtures increase with increasing mal todext r in concen- t r a t ion but t h e concent ra t ion needed for a significant change is more than 207c . Also, at low mal todext r in concentrations the molecular weight has only a small effect on increasing Tg of t h e mixture . However, small a m o u n t s of sucrose subs tan t ia l ly decreased the Tg of t h e b inary mixture. T h i s quant i ta t ive information can be used to formula te b inary mixtures having desired physical prop- erties required in processing and storage of food and related materials. It was also noticed that mal todext r ins effec- tively re ta rd crystallization of amorphous sucrose, and the crystallization becomes totally inhibited at high mal - todext r in concentrations.

Notation a, water activity k constant K , constant m moisture content M molecular weight Tc collapse temperature TP glass transition temperature TP1

Tg2 glass transition temperature of component 1 glass transition temperature of component 2

Ts sticky point w1

w2 weight fraction of component 1 weight fraction of component 2

Acknowledgment T h i s is Publication No. D-10535-7-90 of the New Jersey

Agricultural Exper iment S ta t ion suppor ted by t h e Acad- e m y of F in land , by the State of New Jersey Funds, and the Center for Advanced Food Technology. The center for Advanced Food Technology is a New Jersey Com- mission on Science and Technology Center .

Literature Cited (1) Kauzmann, W. The nature of the glassy state and the behavior

of liquids a t low temperatures. Chem. Reu. 1948,43,219-256. (2) Kakivaya, S. R.; Hoeve, C. A. J. The glass point of elastin.

Proc. Natl. Acad. Sei. U.S.A. 1975, 72, 3505-3507. (3) Struik, L. C. 8: Physical Aging in Amorphous Polymers and

Other Materials; Elsevier Scientific Publishing Co.: Amster- dam, 1978.

(4) White, G. W.; Cakebread, S. H. The glassy state in certain sugar-containing food products. J . Food Technol. 1966, I ,

(5) Bownton, G. E.; Flores-Luna, J. L.; King, C. J. Mechanism of stickiness in hygroscopic, amorphous powders. Znd. Eng. Chem. Fundam. 1982,21, 447-451.

(6) Flink, J. M. Structure and structure transitions in dried carbohydrate materials. In Physical Properties of Foods; Pe- leg, M., Bagley, E. B., Eds.; AVI: Westport, CT, 1983; pp 473- 521.

(7) Hoseney, R. C.; Zeleznak, K.; Lai, C. S. Wheat gluten. A glassy polymer. Cereal Chem. 1986, 63, 285-286.

(8) Slade, L.; Levine, H.; Finlay, J. W. Protein-water interac- tions: Water as a plasticizer of gluten and other protein polymers. In Protein Quality and the Effects o f Processing; Phillips, R. D., Finlay, L. W., Eds.; Marcel Dekker, Inc.: New York, 1989; pp 9-124.

(9) Levine, H.; Slade, L. Influences of the glassy and rubbery states on the thermal, mechanical and structural properties of doughs and bakery products. In Dough Rheology and Baked Product Texture; Faridi, H., Faubion, J. M., Eds.; AVI: New York, 1990; pp 157-330.

(10) Tsourouflis, S.; Flink, J. M.; Karel, M. Loss of structure in freeze-dried carbohydrates solutions. Effect of temperature, moisture content and composition. J . Sei. Food Agric. 1976,

(11) To, E.; Flink, M. “Collapse”, a structural transition in freeze dried carbohydrates. 11. Effect of solute composition. J. Food Technol. 1978, 13, 567-581.

(12) Thijssen, H. A. C. Optimization of process conditions during drying with regard to quality factors. Lebensm.- Wiss. Tech- nol. 1979, 12, 308-317.

(13) Alexander, K.; King, C. J. Factors governing surface morphology of spray-dried amorphous substances. Drying Technol. 1985, 3, 321-348.

(14) Roos, Y.; Karel, M. Plasticizing effect of water on thermal behavior and crystallization of amorphous food models. J . Food Sei., in press.

(15) Levine, H.; Slade, L. A polymer physico-chemical approach to the study of commercial starch hydrolysis products (SHPs)- . Carbohydr. Polym. 1986, 6, 213-244.

(16) Roos, Y. H. Effect of moisture on the thermal behavior of strawberries studied using differential scanning calorimetry.

(17) Fox, T. G.; Flory,P. J. Second-order transition temperatures and related properties of polystyrene. I. Influence of molecular weight. J . Appl . Phys. 1950, 21, 581-591.

(18) Tant, M. R.; Wilkes, G. L. An overview of the nonequi- librium behavior of polymer glasses. Polym. Eng. Sei. 1981,

(19) Wunderlich, B. The basis of thermal analysis. In Thermal Characterization of Polymeric Materials; Turi, E., Ed.; Academic Press: London, 1981; pp 91-234.

73-82.

27, 509-519.

J . Food Sei. 1987, 52, 146-149.

21, 874-895.

Biotechnol. Prog., 1991, Vol. 7, No. 1

(20) Gordon, M.; Taylor, J. S. Ideal copolymers and the second- order transitions of synthetic rubbers. I. Non-crystalline copolymers. J . Appl. Chem. 1952,2, 493-500.

(21) Williams, M. L.; Landel, R. F.; Ferry, J. D. The temperature dependence of relaxation mechanisms in amorphous polymers and other glass-forming liquids. J . Am. Chem. SOC. 1955, 77,

( 2 2 ) Orford, P. D.; Parker, R.; Ring, S. G.; Smith, A. C. Effect of water as a diluent on the glass transition behavior of malto- oligosaccharides, amylose and amylopectin. Znt. J . Biol. Mac- romol. 1989, 11, 91-96.

(23) Roos, Y.; Karel, M. Differential scanning calorimetry study of phase transitions affecting the quality of dehydrated materials. Biotechnol. Prog. 1990, 6, 159-163.

(24) Angell, C. A. Supercooled water. Annu. Reo. Phys. Chem.

( 2 5 ) Johari, G. P.; Hallbrucker, A.; Mayer, E. The glass-liquid transition of hyperquenched water. Nature 1987, 330, 552- 553.

(26) Maurice, T. J.; Slade, L.; Sirett, R. R.; Page, C. M. Polysac- charide-water interactions-Thermal behavior of rice starch.

3701-3707.

1983, 34, 593-630.

53

In Properties of Water in Foods; Simatos, D., Multon, J. L., Eds.; Martinus Nijhoff Publishers: Dordrecht, The NethBr- lands, 1985; pp 211-227.

(27) Biliaderis, C. G.; Page, C. M.; Maurice, T. J.; Juliano, B. 0. Thermal characterization of rice starches: A polymeric ap- proach to phase transitions of granular starch. J . Agric. Food Chem. 1986,34,6-14.

(28) Zeleznak, K. J.; Hoseney, R. C. The glass transition in starch. Cereal Chem. 1987, 64, 121-124.

(29) Gordon, J. M.; Rouse, G. B.; Gibbs, J. H.; Risen, W. M., Jr. The composition dependence of glass transition properties. J . Chem. Phys. 1977, 66,4971-4976.

(30) Couchman, P. R. Compositional variation of glass transition temperatures. 2. Application of the thermodynamic theory to compatible polymer blends. Macromolecules 1978,11,1156- 1161.

Accepted November 14, 1990.

Registry No. Maltodextrin, 9050-36-6; maltose, 69-79-4; sucrose, 57-50-1.