J. Chem. SOC., Faraday Trans. I, 1988, 84(8), 2619-2633 Thermomechanical Properties of Small-carbohydrate-Water Glasses and ' Rubbers ' Kinetically Metastable Systems at Sub-zero Temperatures Harry Levine" t and Louise Sladet General Foods Corporation, Technical Center, Tarrytown, New York, U.S.A. Aqueous solutions of 24 small carbohydrates have been analysed by low- temperature differential scanning calorimetry (d.s.c.). The method, based on analogue derivative thermograms, measured two thermomechanical pro- perties characteristic of each non-crystallizing solute : Tg, the sub-zero glass transition temperature of a maximally freeze-concentrated solution and Wg, the amount of unfrozen water (UFW) kinetically immobilized in the glass at q. For 84 low-molecular-weight polyhydroxy compounds (PHCs) analysed to date, Tg ranged from -85 "C for ethylene glycol to - 13.5 "C for maltoheptaose, and increased monotonically with increasing M,. Tg plotted us. Mi1 showed a linear correlation characteristic of an homologous family of glass-forming linear oligomers. Wg ranged from 1.9 g UFW g-' for ethylene glycol to 0.2-0.3 g UFW g-I for several sugars and polyols, including maltoheptaose, and decreased with increasing q, showing fair linear correlations for several series of homologous solutes. We describe here the use of and Wi, as invariant physico-chemical properties of glass-forming solutions at sub-zero temperatures, to interpret thermo- mechanical behaviour of small-carbohydrate-water systems in non- equilibrium glassy and ' rubbery ' states, define structure-activity relation- ships and explain and often predict functional behaviour of such PHCs in various applications. Thermal and mechanical properties of concentrated aqueous solutions of small carbohydrates at sub-zero temperatures have been studied for decades. Nearly 50 years ago, Luyetl reported microscopically observed temperatures of irruptive ice re- crystallization (7J for frozen solutions of sugars and polyhydric alcohols. Today, it is recognized by some2-6that the T, values of such glass-forming aqueous systems'. 7-14 coincide with a characteristic glass-transition temperature, q, measurable calori- metrically. This fact was demonstrated conclusively by our previous study5 of commercial starch hydrolysis products (SHPs), and is illustrated by the results reported here for low-molecular-weight PHCs. It is also recognized that ice re- crystallization is but one of many possible manifestations (referred to as ' collapse ' of the dynamically controlled behaviour of small-carbohydrate-water glasses and ' rubbers ' that exist, at sub-zero temperatures, in kinetically metastable ' states ' rather than equilibrium thermodynamic phases.6* 23-26 [As used here, 'rubbery' refers to the high-viscosity fluid state which exists at < T< crystalline melting temperature (T,) for any glass-forming liquid. In contrast, a true rubber corresponds to a ,viscoelastic network of entangled polymeric chains for a partially crystalline polymer at Tg < T < T,."] Collapse phenomena are diffusion-controlled structural relaxation processes which occur in response to changes in moisture content, temperature and f Present address: Nabisco Brands Inc., Corporate Technology Group, P.O. Box 1943, East Hanover, New Jersey 07936-1943, U.S.A. 2619 Downloaded by Brown University on 07 September 2012 Published on 01 January 1988 on http://pubs.rsc.org | doi:10.1039/F19888402619 View Online / Journal Homepage / Table of Contents for this issue
Transcript
J. Chem. SOC., Faraday Trans. I , 1988, 84(8), 2619-2633
Thermomechanical Properties of Small-carbohydrate-Water Glasses and ' Rubbers '
Kinetically Metastable Systems at Sub-zero Temperatures
Harry Levine" t and Louise Sladet General Foods Corporation, Technical Center, Tarrytown, New York, U.S.A.
Aqueous solutions of 24 small carbohydrates have been analysed by low- temperature differential scanning calorimetry (d.s.c.). The method, based on analogue derivative thermograms, measured two thermomechanical pro- perties characteristic of each non-crystallizing solute : Tg, the sub-zero glass transition temperature of a maximally freeze-concentrated solution and Wg, the amount of unfrozen water (UFW) kinetically immobilized in the glass at q. For 84 low-molecular-weight polyhydroxy compounds (PHCs) analysed to date, Tg ranged from -85 "C for ethylene glycol to - 13.5 "C for maltoheptaose, and increased monotonically with increasing M,. Tg plotted us. Mi1 showed a linear correlation characteristic of an homologous family of glass-forming linear oligomers. Wg ranged from 1.9 g UFW g-' for ethylene glycol to 0.2-0.3 g UFW g-I for several sugars and polyols, including maltoheptaose, and decreased with increasing q, showing fair linear correlations for several series of homologous solutes. We describe here the use of and Wi, as invariant physico-chemical properties of glass-forming solutions at sub-zero temperatures, to interpret thermo- mechanical behaviour of small-carbohydrate-water systems in non- equilibrium glassy and ' rubbery ' states, define structure-activity relation- ships and explain and often predict functional behaviour of such PHCs in various applications.
Thermal and mechanical properties of concentrated aqueous solutions of small carbohydrates at sub-zero temperatures have been studied for decades. Nearly 50 years ago, Luyetl reported microscopically observed temperatures of irruptive ice re- crystallization (7J for frozen solutions of sugars and polyhydric alcohols. Today, it is recognized by some2-6 that the T, values of such glass-forming aqueous systems'. 7-14 coincide with a characteristic glass-transition temperature, q, measurable calori- metrically. This fact was demonstrated conclusively by our previous study5 of commercial starch hydrolysis products (SHPs), and is illustrated by the results reported here for low-molecular-weight PHCs. It is also recognized that ice re- crystallization is but one of many possible manifestations (referred to as ' collapse '
of the dynamically controlled behaviour of small-carbohydrate-water glasses and ' rubbers ' that exist, at sub-zero temperatures, in kinetically metastable ' states ' rather than equilibrium thermodynamic phases.6* 23-26 [As used here, 'rubbery' refers to the high-viscosity fluid state which exists at < T < crystalline melting temperature (T,) for any glass-forming liquid. In contrast, a true rubber corresponds to a ,viscoelastic network of entangled polymeric chains for a partially crystalline polymer at Tg < T < T,."] Collapse phenomena are diffusion-controlled structural relaxation processes which occur in response to changes in moisture content, temperature and
f Present address: Nabisco Brands Inc., Corporate Technology Group, P.O. Box 1943, East Hanover, New Jersey 07936-1943, U.S.A.
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time.6 Their kinetics are governed by the free volume, mobility, and viscosity of a water- plasticized glass or ' rubber '.5
In the present study, aqueous solutions of 24 small carbohydrates were analysed by low- temperature d. s .c. When added to 60 other low-molecular- weight compounds analysed previously,6 these PHCs represent a comprehensive series of mono-, di-, and small oligo-saccharides and their derivatives, including many common sugars, polyols arid glycosides, covering an M , range of 62-1 153 dalton. The d.s.c. r n e t h ~ d , ~ based on analogue derivative thermograms, was used to measure two thermomechanical properties characteristic of each non-crystallizing solute : Tg, the particular sub-zero
of the maximally freeze-concentrated solute-UFW amorphous matrix surrounding the ice crystals in a frozen solution;3,12.28 and Wk, the amount of water rendered 'unfreezable' (expressed as g UFW g-l solute) by immobilization with the solute in this kinetically metastable, dynamically constrained solid which forms on slow cooling to T < < . 3 ' 5 We will demonstrate here how and Wk, invariant physico-chemical properties of glass-forming systems at sub-zero temperatures, can be used to describe the thermomechanical behaviour of PHC-water solutions in non-equilibrium glassy and 'rubbery' states. We will also show how these results can be used to define structure-activity relationships, which can be used in turn to explain and often predict functional behaviour of such carbohydrates, many of which are widely used in food applications.
2622 Small-carbohydrate- Water Glasses and ' Rubbers '
Experimental The small PHCs characterized in this study are included in table 1 [updated from ref. (6)]. All the sugars and polyols were reagent-grade chemicals, most of them from Sigma. These materials were analysed as received. Most of the glycosides were synthesized and purified by our colleague, Dr Allan Bradbury.
All solutions for T, and Wi determinations were 20 wt YO solids (i.e. 20.0 g solute per 100.0 g solution), prepared in distilled, deionized water. Samples were produced by mechanical stirring, with gentle heating when necessary, to yield clear solutions.
D.s.c. measurements were performed with a DuPont 990 thermal analyser coupled to a DuPont 910 d.s.c. equipped with a liquid-nitrogen quench-cooling accessory capable of sample cooling at ca. 50 "C min-'. The analogue derivative function on the DuPont 990 allowed precise determination of transition temperatures, with a reproducibility (for duplicate samples) of & 0.5 "C for 7';. In practice, 20-30 mg of solution were hermetically sealed in an aluminium sample pan and scanned (against an empty reference pan) at a heating rate of 5 "C min-l, from a temperature at least 10 "C below 7*, to 25 "C. In all cases, initial cooling to well below ensured maximal freeze concentration and thus maximally frozen samples. Illustrative thermograms were shown, and the method for determining W; was described previously.
Results and Discussion Low-temperature Thermal Properties of Aqueous Solutions of Small Carbohydrates
The theoretical basis for the thermal properties manifested by water-soluble solids in solution at sub-zero temperatures has come to be well unders t~od.~ , 7-12* 16, 28 Assignments of characteristic transitions and temperatures have been reconciled definitively with state diagrams previously reported for various material^.^^ l2 In such diagrams [e.g. fig. 6 of ref. ( 5 ) for poly(viny1 pyrrolidone)] the different cooling-heating paths that can be followed by solutions of monomeric cs. polymeric solutes are revealed. However, in either general case, rewarming forces the system through a glass transition at 7';. In many earlier d.s.c. studies,". 16, 29 performed without benefit of derivative thermograms, a pair of transition temperatures (each independent of initial concentration), called T (antemelting), Tit, and T(incipient melting), Tim, were reported in place of a single 7';. In fact, in many cases, reported values of Tam3' and Ti, bracket that of 7*,. This led us5 to suggest that Tam and Tim actually represent the temperatures of onset and completion of the single thermal event (a glass transition) that must occur at Ti, as defined by the state diagram. Even today, many workers in this field do not recognize the sub-zero T p < T devitrification (7J < 7", sequence of transitions characteristic of frozen solutions of PHCs. Instead, they refer to 'anomalous double glass transitions ' manifested by aqueous solutions of, e.g. propylene glycol and glycerol.31 33 Far from being anomalous, for each solute, the higher Tp of the doublet coincides with Ti.6 Similarly, q, manifested as the onset temperature for opacity during warming of vitrified aqueous solutions and known to be independent of initial concentration, is still a topic of current interest and discussion as to its rigi in,^^-^* but it is not yet universally recognized to coincide with Ti for water-soluble carbohydrates, including several much- studied polyols.
Our previous studies5. 6 , 2 3 9 24 have helped explain why Tg is the most noteworthy feature of low-temperature d.s.c. thermograms. The matrix surrounding the ice crystals in a maximally frozen solution is a supersaturated solution of all the solute in the fraction of water remaining unfrozen. This matrix exists as a kinetically metastable amorphous solid (a glass of constant composition) at any T < TL, but as a viscous liquid (a 'rubbery' fluid) at Tg < T < ice c,. Again with regard to a state diagram for a typical
Fig. 1. Variation of the glass transition temperature, q, for maximally frozen 20 wt % solutions against Mi1 for the sugars, glycosides and polyols in table 1. r = -0.934.
solute that does not readily undergo eutectic crystallization, Tg corresponds to the intersection of an extension of the thermodynamically defined equilibrium liquidus curve and the kinetically determined supersaturated glass curve. As such, Franks12 described T, as representing a quasi-invariant point in the state diagram, invariant in both its characteristic temperature (Tg) and composition (i.e. Ci, expressed as wt YO solute, or Wk, expressed as g UFW g-l solute) for any particular solute. This glass, which forms, e.g. on slow cooling to Tg, serves as a kinetic barrier (of high activation energy) to further ice formation (within the experimental timeframe), despite the continued presence of UFW at all temperatures < T,, as well as to any other diffusion-controlled p r o ~ e s s . ~ Recognizing this, one can begin to appreciate why the temperature of this glass transition is so important to aspects of frozen-food technology involving freezer-storage stability, freeze concentration and free~e-drying,~? all of which are subject to various recrystallization and collapse phenomena.
Measured Tg values for 84 small carbohydrates are listed in table 1. As noted earlier, T, values for various solutes fall between previous literature values for cm and Tim,8 and within a few degrees of values reported for and the collapse temperature, T, (as shown later in table 3). T, values for this non-homologous collection of monodisperse sugars, polyols and glycosides ranged from -85 "C for ethylene glycol ( M , = 62) to - 13.5 "C for maltoheptaose ( M , = 1 153). These results demonstrated a monotonic relationship between increasing T, and M,, that yielded a linear correlation (Y = -0.93) between T, and Mi' , as shown in fig. 1. The major cause of scatter in this plot was the series of chemically different glycosides. In contrast, a plot of T, us. M , for glucose and malto- oligosaccharides of degree of polymerization (D,) 2-7 gave a smooth curve, with Y = -0.99 for the corresponding plot of T, us. The latter results, especially the theoretically predicted linear dependence of Tg on Mil , exemplified the glass-forming behaviour characteristic of a homologous family of n~n-entangl ing ,~~ linear, mono- disperse 01igomers.~~* 35 In contrast, for a polydisperse mixture of solutes, the observed Tg is actually a weight-averaged 23, 26 For example, previous Tg results' for heterogeneous commercial SHPs showed a range from -43 "C for glucose [ M , = 180, dextrose equivalent value (DE) = 1001 to -4 "C for 0.5 DE maltodextrin (number- averaged M,, @, x 3.6 x lo'), with T, = - 13.5 "C for maltodextrins of 15-20 DE (A%, = 900-1200). T, values in table 1 illustrate that glucose is representative of many other monosaccharides, while maltoheptaose is comparable to 15-20 DE maltodextrins.
depends rigorously on linear, weight-averaged D, (DPw) for such polydisperse solutes, so that linear polymer
Our previous study of polymeric SHPs demonstrated that
chains give rise to higher values than branched chains of equal Mw.6 As illustrated in table 2, from comparisons of the significant T, differences between maltose (1 -+ 4-linked glucose dimer), gentiobiose (1 -+ 6-linked) and isomaltose (1 -+ 6-linked), and between maltotriose (1 -+ 4-linked trimer), panose (1 -+ 4, 1 -+ 6-linked) and isomaltotriose (1 -+
6, 1 -+ 6-linked), current results suggest that 1 -+ 4-linked (linear amylose-like) glucose oligomers manifest greater 'effective ' linear chain lengths in aqueous solution (and, consequently, greater hydrodynamic volumes) than oligomers of the same molecular weight which contain 1 -+ 6 (branched amylopectin-like) links. Table 2 also illustrates the intrinsic sensitivity of the Tg parameter to molecular configuration, in terms of linear chain length, as influenced by the nature of the glycosidic linkages in various non- homologous saccharide oligomers (not limited to glucose units) and the resultant effect on solution conformation. Another interesting comparison is between T, values for the linear and cyclic a-(1 -+ 4)-linked glucose hexamers, maltohexaose (- 14.5 "C) and a- cyclodextrin (- 9 "C). In this case, the higher Tg of the cyclic oligomer leads us to suggest that the ring of a-cyclodextrin apparently has a much larger hydrodynamic volume in solution (owing to its relative rigidity3') than does the linear chain of maltohexaose, which is relatively flexible and can assume a more compact conformation in solution.
the composition of the glass at T, (as a weight of UFW per unit weight of solute), is one of several methods routinely employed in the food industry to determine the so-called 'water-binding capacity' of a solute. Franks39 1 2 , 2 8 9 37 h as reviewed this subject in depth, and pointed out repeatedly that so-called 'bound' water is not truly bound in any energetic sense. It is subject to rapid exchange,38 has thermally labile hydrogen 40 shows cooperative molecular mobility,41 has a heat capacity approximately equal to that of liquid water rather than ice40-42 and has some capability to dissolve Furthermore, it has been firmly e~ tab l i shed ,~~ for water-soluble polymers and monomers alike, that such ' unfreezability ' is not due to tight equilibrium binding by but to purely kinetic retardation of diffusion of water and solute molecules at the low temperatures approaching the vitrification
We showed previ~usly,~ for an homologous series of SHP solutes, that Wi decreases with increasing Tg, with a linear regression coefficient of -0.91. In other words, as
0 10 20 30 40 50 60 70 80 90 100 CL (wt% solute in the glass at T i )
Fig. 2. Variation of the glass transition temperature, Tg, for maximally frozen 20 wt YO solutions against Ck, the composition of the glass at q, in wt YO solute, for homologous series of polyhydric alcohols (r = 0.83) (O), glucose-only solutes (Y = 0.73) (*), fructose-containing (Y = 0.96) ( x ) and galactose-containing saccharides (Y = 0.94) (0) in table 1. [Reproduced, with permission, from
ref. (23).]
solute(s) A?n increased, the fraction of total water unfrozen in the glass at Ti decreased. Again in the context of a typical state diagram, these results showed that as fin of a solute (or mixture of homologous solutes) in an aqueous system increases, the Tg- CI, point generally moves up the temperature axis towards 0 "C and to the right along the composition axis towards 100 wt o/o solute. The critical importance of this fact became clear when we described5 SHP functional behaviour v i s - h i s T,, and the capacity of inhibiting collapse processes in frozen solutions by formulating a system with polymeric SHPs in order to elevate Tg.
Measured WI, values shown in table 1 ranged from 1.90 g UFW g-' for ethylene glycol to 0.20-0.30 g UFW g-l for several sugars and polyols, including maltoheptaose. In apparent contrast to the above-mentioned results for an homologous SHP series of mixed glucose monomer and oligomers, the 7';, us. Wk results for the diverse PHSc in table 1 yielded a regression coefficient of only -0.64.6 Thus, when Franks3 noted that, among ;he (non-homologous) sugars and polyols most widely used as 'water binders' in fabricated foods, 'the amount of unfreezable water does not show a simple dependence on M , of the solute', he was sounding a necessary caution. [In fact, when the Wi data in table 1 were plotted against Mi', Y = 0.47.1 We concluded that such plots could not be used for predictive purposes, so that the prudent approach would be to rely on measured WI, values for each potential ' water-binding ' candidate. However, the situation was not quite as nebulous as it first appeared. When some of the same data were plotted (actually T, us. CL, as shown in fig. 2"), and compounds were grouped by chemical classification into specific homologous series (e.g. polyols, glucose-only solutes, and fructose- or galactose-containing saccharides), better linear correlations became evident. The plots in fig. 2 illustrate the same linear dependence of Tg on the composition of the glass at Tg (i.e. as the amount of unfrozen, plasticizing water in the glass decreases,
2626 Small-carbohydrate- Water Glasses and ' Rubbers '
Tg increases) as did the data for the homologous SHP solutes. Still, Franks' suggestion45 that future investigations of T, and viscosity as functions of solute concentration, and the liquidus curve as a function of solute structure would be particularly worthwhile is a good one.
We noted with interest the Wb, results for the series of monomeric glycosides listed in table 1, in terms of a possible relationship between glycoside structure (e.g. position and size of the hydrophobic aglycone, which is absent in the parent sugar) and the function reflected by Wb. Wb values for all methyl, ethyl and propyl derivatives were much greater than those for the corresponding parent monosaccharides. However, Wb appeared consistently to be maximized for methyl or ethyl derivatives, but somewhat decreased for the n-propyl derivative. We suggest that these results could indicate that increasing the hydrophobicity (of the aglycone) leads to both decreasing Wi and the demonstrated tendency toward increasing insolubility of propyl and larger glycosides in water.
As described elsewhere, 26 r, of aqueous solutions of low-molecular-weight PHCs does not depend solely on free volume and thus correlates more closely with the weight- averaged M , (M,) of the solute-UFW mixture in the glass of composition defined by Wb than with the corresponding an, or M , of the monodisperse dry solute. For those PHCs in table 1 with measured values of T, and Wi, a plot of T, us. M;' showed a better linear correlation ( r = -0.95) than the plot of Tg us. Mi' in fig. 1 (r = -0.93). In contrast, T, plotted us. Mi' gave an r value of only - 0.71. For the homologous series of malto-oligosaccharides up to DP 7, 7", plotted us. Mi' , Mi l , and a;' gave r values of -0.99, -0.99, and -0.20, respectively. In other words, 7", appears to reflect the hydrodynamic volume, in the glass, of the mobile 'cluster entity'46 represented by a solute molecule and its complement of unfrozen water molecules, rather than a property of the isolated solute.26 One might ask whether it would be preferable to correlate r, with partial molar volume ( Y o ) rather than M , of dry solute, since V", like intrinsic viscosity, gives an indication of the effective solute size in solution. Just as free volume is related to inverse M , of monodisperse solutes (or inverse an for polydisperse solutes) in the limit of zero dilution,2i free volume should be related to inverse V" for comparison of conformational homologues of different M , in the limit of infinite dilution. However, it is for comparison of different conformers of the same M , that we look for an advantage in the use of V" to replace M,. Unfortunately, while M , values are exact, an approach based on V" is not straightforward
Despite a relative wealth of I/" data,4i differences in the values for isomeric sugars are sufficiently small to discourage their use to interpret the influence of conformation on hydration4s and may lie within the range of values reported by different research groups for a single sugar, due in part to differences in anomeric ratios.49 If this complication is removed by comparison of methyl pyranosides, larger values of V o are observed for conformers with equatorial rather than axial OCH, sub~tituents.~' Similarly, for comparison of conformers at C4, contributions to apparent molar volumes are said to be greater for equatorial than for axial hydroxyls. 50 However, the general observation, for PHCs compared to their apolar structural analogues, is that hydroxy groups are effectively invisible to limiting density measurement~.~'. 49 The greater contribution of certain equatorial hydroxyl groups to V" is attributed to greater spatial and orientational compatibility with the pre-existing liquid water i.e. greater effective ' specific hydration '. Data for a few of the same sugars show that intrinsic viscosity increases with both contributions ( M , and ' specific hydration ') to increasing V0.51 We might expect, from this slim evidence, that both contributions to increased V" would also lead to increased Tg. However, there still remain the questions of temperature and concentration dependence of apparent molar volumes of PHCs.
Systematic extrapolation of V" data to sub-zero temperatures of interest for correlation with Tg is hindered by the paucity of data for mean limiting apparent molar expansibilities. Based on two relevant cases for which data are a~ai lable ,~ ' such
a Recrystallization temperatures, from Franks,12 p. 297. Collapse temperatures during freeze-drying, from Franks, l2
p. 313. From Luyet,' p. 564. Antemelting temperatures, from Virtis CO.,~O p. 4. 'Completion of opacity' temperatures, from Forsyth and MacFarlane. l3 f Recrystallization temperature, from Thom and Matthes.14
extrapolations would magnify differences in behaviour predicted from measurements made near room temperature. The concentration dependence of apparent molar volumes is more questionable. One of the most important, but often overlooked, aspects of the glass transition is its cooperative Upon slow cooling, the glass at c, with solute-specific composition WL, represents the greatest dilution that retains this maximally cooperative behaviour. Cooperativity is maximum at the glass transition (where arrestation of large-scale molecular mobility occurs without change in structure), but decreases with increasing temperature or dilution above T p (where retardation of mobility occurs and shows a WLF-type temperature dependence). Of the two extremes, behaviour in the limit of zero dilution is less remote from that of the cooperative system than is behaviour in the limit of infinite dilution. Apparent molar volumes of PHCs in aqueous solution have been shown to be characteristically (in contrast to apolar solutes) independent of c~ncentration,~' yet reported differences between apparent molar volumes for 3 and 10 wt % solutions of a single sugar approach the greatest differences seen between equatorial us. axial conformers at a single con~ent ra t ion .~~ There exists the possibility that a decrease in apparent molar volume upon extrapolation towards T, and an increase upon extrapolation towards CL might counterbalance. Despite these issues the subject is of sufficient interest to merit further exploration elsewhere. 26
Structure-Property Relationships for Small Carbohydrates in Aqueous Solution T, and W,, as physico-chemically invariant but structure-dependent properties of glass- forming solutions at sub-zero temperatures, can be used to interpret thermomechanical behaviour of small-carbohydrate-water systems in non-equilibrium glassy and ' rubbery ' states. For example, as illustrated in table 3, previous established the fundamental identity of T, with transition onset temperatures observed for structural collapse during freeze-drying (T,) and recrystallization during frozen storage (q), for both model solutions and real systems (i.e. foods, pharmaceuticals, biologicals). These collapse phenomena are translational diffusion-controlled processes, for which a
2620 Small-carbohydrate Water Glasses nnd ' Rubbers'
physico-chemical mechanism was described,'. involving viscous flow in the ' rubbery ' state, at T > and viscosity, q < qg = 1011-1014 Pa s."' 2 7 , 4 6 , 5 3 An extensive list of collapse processes, all of which are governed by 7", of frozen systems (or of low- moisture systems), and involve potentially detrimental plasticization by water. has been identified and illustrated.". 'Q z 3 * 24 The non-Arrhenius kinetics of collapse and/or recrystallization in a high-viscosity fluid, which are governed by the free volume and mobility of the water-plasticized matrix, were shown5. 6 . ':' to depend on the magnitude of AT above 7",, as defined by an exponential relationship derived from Williams-- Landel-Ferry (W'LF) free volume theory for glass-forming liquidsz7
Our previous study5 demonstrated the classical behaviour of commercial SHPs as an homologous family of amorphous glucose oligomers and polymers, and revealed a chain ' entanglement coupling 'z79 34, 54 capability for SHPs of DE < 6 and T, 3 - 8 C . For such polymeric SHPs, a lower limit of A?, z 3000 (calculated from DE : MI, = 1801 6DE-l) for entanglement leading to viscoelastic network formation"* 56 was identified. This an value was within the characteristic range of 1250--19 000 for the minimum entanglement M , of many typical synthetic linear high polymers.34 This entanglement capability was shown to correlate well with various functional attributes of low-DE SHPs, including a predictable ability' to form thermo-reversible, partially crystalline gels from aqueous ~ o l u t i o n . ~ ' - ~ ~ Gelation occurred by a mechanism involving crystallization plus entanglement in concentrated solutions undercooled to 12" < 64 In contrast, the PHCs reported here, including malto-oligosaccharides up to DP 7, were found to fall below the Tg limit defined by SHPs for entanglement and the onset of viscoelastic rheological properties, and to be incapable of gelling from solution. It was clear, from the linearity of the 7", zw. M i 1 plot in fig. 1, that these monodisperse sugars, polyols and glycosides did not show evidence of entanglement coupling. For these small carbohydrates, none of which is larger than a heptamer of M , 11 53, the entanglement plateau region (in which remains constant with increasing M,27*35) has not been reached, a finding in accord with the M , range for entanglement limits cited above.
Predicted Functional Behaviour of Small Carbohydrates in Solution
We demonstrated previously5 how insights into structure-function relationships may be gleaned by treating a plot of T, 21s. Mn as a predictive map of regions of functional behaviour for SHP samples. For polymeric SHPs, the entanglement plateau region defines the useful range of gelation, encapsulation, cryostabilization, thermomechanical stabilization and facilitation of drying processes. For high-DE, low- T'L SHPs, low Mn corresponds to the region of sweetness, hygroscopicity, humectancy, browning reactions and cryoprotection. We described' how such a plot can be used predictively to choose individual SHPs or mixtures of SHPs and other small carbohydrates (targetted to a particular T, value) to achieve desired complex functional behaviour for specific product applications. Especially for applications involving such mixtures, similar use can be made of the corresponding results for the PHCs represented in fig. 1. For low-molecular- weight sugars and polyols, as for high-DE SHPs, low Tk values correlate with functional properties of sweetness, hygroscopicity, humectancy, browning and cryoprotection.
One major area of practical significance of the current results on small carbohydrates can be explained in the context of 'cryostabilization', a concept we introduced" to describe a new industrial technology for the storage stabilization of frozen, freezer- stored and freeze-dried foods. This technology emerged from a basic research programme in food polymer sciencez3 and developed from a fundamental understanding of the critical physico-chemical and thermomechanical structure--property principles which underlie the behaviour of water in non-equilibrium food systems at sub-zero temperature^.^, l2 Cryostabilization is a means of protecting products, stored for long periods at typical freezer temperature ( q = - 18 "C), from the deleterious changes in
Fig. 3. Idealized state diagram of temperature us. wt 94 solute for an aqueous solution of a hypothetical small carbohydrate (representing a model frozen-food system), illustrating the critical relationship between Tg and freezer temperature (q), and the resulting impact on the
physical state of the freeze-concentrated amorphous matrix.
texture (e.g. ' grain growth ' of ice, solute crystallization), structure (e.g. shrinkage, collapse) and chemical composition (e.g. enzymatic activity, oxidative reactions, flavour or colour degradation) usually encountered. Such collapse-related changes are exacerbated in typical fabricated foods whose formulae are dominated by small carbohydrates with low T, and high Wi values. The key to protection and resulting improvement in storage stability lies in controlling the physico-chemical and thermomechanical properties, by controlling the structural state, of the freeze- concentrated amorphous matrix surrounding the ice crystals in a frozen system. This insight is illustrated by the idealized state diagram (modelled after one for fructose- water23) shown in fig. 3. If the matrix is maintained as a kinetically metastable mechanical solid (i.e. if TI < Q, then diffusion-controlled processes that typically result in reduced quality and stability can be virtually prevented or greatly inhibited. If, on the other hand, a natural material is improperly stored at too high a q, or a fabricated product is improperly formulated, so that the matrix is allowed to exist at T2 (> T,) as a 'rubbery' fluid in which translational diffusion is free to occur, then storage stability is reduced. The optimum Tr value for a natural material or optimum formula for a fabricated product is dictated by the characteristic Ti of the specific amorphous solute(s)-UFW matrix, which is governed in turn by Hw of the particular combination of water-soluble solids in a complex food system. Moreover, the dynamic behaviour of 'rubbery' frozen- food products during storage is kinetically controlled, and rates of diffusion-controlled deterioration processes are quantitatively determined by AT between and Tg. These rates increase exponentially with increasing AT above 7",, as defined by WLF, rather than Arrhenius, kinetics.
For the cryostabilization of products such as ice cream (with smooth, creamy texture) against ice crystal growth during storage, inclusion of low-DE SHPs elevates the composite T, of a mix of soluble solids, which is usually dominated by low-molecular- weight sugars and other PHCS.~ In practice, a retarded rate of migratory ice recrystallization ('grain growth'7) at results and an increase in observed T,. In such
2630 Small-carbohydrate- Water Glasses and ' Rubbers '
0 CRY0 (STABILIZER +
PROTECTANT 1 S i~ PPG
STABILIZERS - 10
- 20 w hydroxypropylinulln * #hydroxypropylinulin stachyose
-30
- 40
-50
- 60
- 70
-801 - 90' I I I I I I I I I
I ene 01 - -
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 W i / g UFW g-'
Fig. 4. Variation of the glass transition temperature, Tg, for maximally frozen 20 wt % solutions against Wi, the composition of the glass at q, in g UFW g-' solute, for a series of water- compatible compounds, including many small carbohydrates listed in table 1, illustrating the cryostabilization technology 'spectrum ' from monomeric cryoprotectants to polymeric
cryostabilizers.
products, ice recrystallization is known to involve a diffusion-controlled maturation process with a mechanism analogous to ' Ostwald ripening ', whereby larger crystals grow with time at the expense of smaller ones which eventually d i~appear . " .~ ' -~~ The rate at q, and thus also AT (T- Q, is reduced by formulating with low-DE SHPs of high 7';. We showed23 that ice recrystallization exemplifies a collapse process whose kinetics can be described quantitatively by the WLF equation." Because typical ice- cream products have 7'; values in the range - 28 to -43 OC, they exist as 'rubbery' fluids (with embedded ice and fat crystals) under typical freezer storage at - 18 OC, and WLF (rather than Arrhenius) kinetics describe the rate of ice crystal growth. In practice, the technological utility of Th and Wk results for sugars, polyols (table 1) and SHPs,' in combination with corresponding relative sweetness data, was demonstrated by the successful formulation of ice-cream products68' 69 with an optimum combination of stability and softness in a - 18 "C home freezer.
The T, results reported here for PHCs and previously' for SHPs led to an interesting suggestion, which, in the present context, emphasizes the distinction in functional behaviour between non-entangling small carbohydrates and polymeric saccharides capable of entanglement. A freeze-concentrated glass at Tk of an SHP cryostabilizer (of DE < 6) would contain entangled polymer chains, while in a glass at Tg of a low- molecular-weight carbohydrate cryoprotectant, solute molecules could not be entangled. By analogy, various high-molecular-weight polysaccharide gums are believed to be capable of improving freezer-storage stability of ice-containing fabricated foods, in some poorly understood way. In the absence of direct evidence of any effect on ice crystal size, and despite recent work which showed definitively that such 'stabilizers' have no significant effect on either ice nucleation or propagation rates,'"," the effect has been attributed to increased viscosity.66* 7 2 Such gums may owe their limited success not only
to their ability to increase microscopic viscosity (a property shared by all glass-formers, regardless of M,, in the sense that ylg at Tg is independent of MW2'), but to their capability to undergo entanglement and increase macroscopic viscosity in the freeze-concentrated, amorphous matrix of a frozen food. Entanglement might provide an enhanced mouthfeel which masks the limited ability of gums to inhibit diffusion-controlled processes.
Tg and WL results for a broad spectrum of water-soluble solids (fig. 4) led to the identification of a class of common food ingredients (including low-DE SHPs) called 'polymeric cryostabilizers '.' Our investigations of these solutes led to a theoretically based understanding of their stabilizing functionality (via their influence on the structural state of a complex amorphous matrix). This functionality derives from their high molecular weight, and the resulting elevating effect of such materials on a complex food system's characteristic T,. Increased TL leads to decreased AT, which in turn results in decreased rates of change during storage, and so increased stability of hard-frozen products. In contrast, the present results on small, carbohydrates (for which low molecular weight translates to low 7", and high W;) illustrate the predicted utility of sugars, glycosides and polyols as 'monomeric cryoprotectants ' (see fig. 4) in freezer- stored foods with desirable soft-frozen texture (but undesirably poor stability).
For many of the 10w-T~ reducing sugars in table 1, sweetness, hygroscopicity, humectancy and browning reactions are salient functional properties. 5, 7 3 A less familiar one involves the potential for cryoprotection of biological materials, for which the utility of various other low-molecular-weight, glass-forming sugars and polyols is well known.12-14,289 31 By analogy with earlier results for SHPs, the r", results in fig. 1 led us to predict, and d.s.c. experiments confirmed, that such small PHCs, in sufficiently concentrated solution, can be quench-cooled to a completely vitrified state, so that all the water is immobilized in the solute-UFW glass.' Such vitrification has also been suggested as a natural intracellular cryoprotective mechanism in winter-hardened poplar tree^,^' and demonstrated as a means of cryoprotecting whole body organs and ernbryo~. '~ The essence of this cryoprotective activity, indefinite avoidance of ice formation and solute crystallization in concentrated, undercooled solutions of small carbohydrates which have high Wk values, also has an apparent relationship to food applications involving soft, spoonable, or pourable-from-the-freezer products. One example is Rich Corporation's patented ' Freeze-Flo ' beverage concentrate formulated with high fructose corn syrup.76 Part of the underlying physico-chemical basis of such products involves colIigative freezing-point depression. This is illustrated by the idealized state diagram in fig. 3, which reveals a hatched liquid solution zone, bordered by the Tm and T p curves and extending below q2, for wt O h solute concentrations between x and y , which bracket CL. However, contrary to first appearances, functionality of specific PHC solutes in practical products depends more on the kinetically controlled properties of T, and WL than on simple colligative depression of equilibrium freezing point. This fact was demonstrated by model-system experiments with 60 wt % solutions of fructose and mannose (3.3 mol dmP3), methyl fructoside (3.1 mol dm-3), ethyl fructoside, ethyl mannoside and ethyl glucoside (2.9 mol dm-3), in which these samples have remained pourable fluids (completely ice- and solute-crystal free) under kinetic control during - 18 "C freezer storage (i.e. at T < the theoretical freezing point due to colligative depression) for over 4 years to date.23
We thank our former colleagues at General Foods: Drs Timothy Schenz and Allan Bradbury for their contributions to the work reported here; Dr William Eisenhardt for his encouragement of our research programme and Dr William Hall for his support of our research partnership. We especially thank our consultant and mentor, Prof. Felix Franks (University of Cambridge), for invaluable suggestions, discussions, encouragement and support over the years.
2632 Small-carbohydrate- Water Glasses and ' Rubbers '
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