[Advances in Food Research] Advances in Food Research Volume 28 Volume 28 || Food Technological Evaluation of Xylitol
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ADVANCES IN I-OOD KEStARCH, VOL.. 28
FOOD TECHNOLOGICAL EVALUATION OF XYLITOL
LEA HYVONEN, PEKKA KOIVISTOINEN,
Department of Food Chemistty and Technology, Uriiversig of Helsinki, Helsinki. Finland
Xyrofn Ltd.. Baar, Switzerland
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. The Occurren
A. Natural Occurrence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Large-Scale Xylitol Production. . . . . . . . . . . . . . . . . . . . . . . . Physicochemical and Food Technological Properties of Xylitol. . A. Physicochemical Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . B. Food Technological Properties . . . . . . . . . . . . . . . . . . . . . . . .
IV. Food Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Confectionery. . . . . . . . . . . . . . . . . . . . . . . . . B. Ice Cream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Jams, Jellies, and Marmalades.. . . . . . . . . . . . . . . . . . . . . . . E. Bakery Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Drinks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V. Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Research Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
of Xylitol . . . . . . . . . . . . . . . .
c. Yogurt . . . . . . . . . . . . . . . . . . . . . . . . . .
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313 314 374 375 378 382 382 389 392 392 396 396 396 398 398 399 399 400
The sensation of sweetness and the concept of a sweetener have undoubtedly been meaningful and important to man throughout his entire existence. During much of the relatively recent culinary history, i.e., the last 15CL200 years, and indeed continuing to the present day sweetness and sweetener have for
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374 LEA HYVONEN ET AL
most people meant the respective taste and functional use of sucrose, which in turn has simply been referred to as sugar.
The world of food science, however, is not so simple. On the one hand, there are numerous substances which have the property of sweetness and hence have the potential to be used as sweeteners.
On the other hand, the various potential sweeteners have many other proper- ties in addition to sweetness which have important and varying functional charac- teristics, both positive and negative in nature.
As knowledge about the various kinds of sweet-tasting substances has in- creased, it has become generally recognized that there are valid roles which each of them can play. Sweetness and the enhancement of food palatability are, perhaps, the common denominators in the use of any sweetener in foods. The choice of sweetener for a particular food system, however, is based on other considerations as well. The food technologist may require bulking, preservative, or humectant functions, or other physical and chemical properties such as sta- bility to heat processing and storage. Most of these requirements are adequately fulfilled by the traditional sucrose or hydrolyzed starch sweeteners.
From the nutritional and health point of view, however, there may also be objectives such as reducing the amount of energy which the sweetening compo- nent brings into the food system, avoidance of too rapidly absorbed carbohy- drates, or reducing the exposure to types of food which are known to cause dental decay, to note only a few of the more obvious considerations.
In recognition of the validity of these other requirements there has been an intensive search in recent years for suitable alternative sweeteners. The search has not been in vain, because there are a number of sweeteners which hold promise in fulfilling some of the divergent special sweetening needs currently being developed and commercialized. One of the most promising of these from the standpoint of special dietary applications, is xylitol, particularly in the areas of noncariogenic confections and disturbances of carbohydrate metabolism, and from the standpoint of fulfilling many of the food technological requirements traditionally expected of the conventional sweeteners.
The metabolic pathways of xylitol and the effects of xylitol on human metabo- lism as well as the tolerability and toxicity of xylitol have been discussed pre- viously in Advances in Food Research by Ylikahri (1979). The dental aspects of xylitol have also been reviewed in this series (Makinen, 1979). The manufacture, properties, and food applications of xylitol are discussed in this article.
II. THE OCCURRENCE AND MANUFACTURE OF XYLITOL
A. NATURAL OCCURRENCE
Xylitol occurs widely in nature. Frerejacque (1943) showed the occurrence of xylitol in lichens, seaweed, and yeast. Kratzl and Silbernagel (1963) found
FOOD TECHNOLOGICAL EVALUATION OF XYLITOL 375
xylitol in mushrooms (Psalliota campestris). Xylitol has been found in small quantities in many fruits, berries, and vegetables (Table I) (Washiittl et al., 1973; Makinen and Soderling, 1980), and is also a normal metabolic intermediate in mammalian carbohydrate metabolism, including that of man (Hollmann and Touster, 1956, 1957; Bassler, 1972). The normal xylitol concentration of blood is 0.03-0.06 mg/100 ml blood.
Commercially produced xylitol is a nature-identical product similar in struc- ture and properties to the natural substance.
Xylitol is by no means a new substance, having been first prepared as a syrup 90 years ago almost simultaneously in the laboratories of Bertrand (1891) and Fischer and Stahel (1891). Wolfrom and Kohn (1942) succeeded in obtaining crystalline xylitol upon hydrogenation of highly purified xylose. Carson et al. (1943) demonstrated the existence of two crystalline forms: the stable rhombic and the unstable monoclinic forms.
Chiang et al. (1958) reduced xylose to xylitol by Penicillium chrysogenum and Onishi and Suzuki (1966) by yeasts. Later Onishi and Suzuki (1969) produced xylitol from glucose via D-arabitol and D-xylulose by certain yeasts.
Since the time when xylitol was found to be a normal intermediate in carbohy- drate metabolism (Touster, 1960) there has been an ever-increasing volume of knowledge about its metabolic behavior in parenteral nutrition (e.g., Horecker et al., 1969; Brian and Miller, 1974; Thomas et al., 1974; Ritzel and Brubacher, 1976; Ylikahri, 1979) as well as its use as a sweetener in diabetic diets, which was first considered by Mellinghoff (1961).
By the end of the 1960s xylitol had drawn the attention of dental scientists as being possibly less cariogenic than other known nutritive sweeteners. Miihlemann and his colleagues (1970) confirmed this in the rat model. Scheinin and Makinen and their colleagues (1 974, 1975a) found in the Turku sugar studies that when xylitol was substituted for sucrose in the human diet the result was a 90% reduction in the incidence of new carious lesions, as well as indications of a remineralizing effect on existing caries. Later Scheinin et al. (1975b) made a 1- year chewing gum study, the findings of which indicated a therapeutic, caries- inhibiting effect of xylitol even for a partial sucrose replacement in the diet.
Before 1975 the production of xylitol was centered in Italy, Germany, the Soviet Union, Japan, and China, with the largest quantity being produced in the Soviet Union, where xylitol is the principal nutritive sweetener used in special dietary foods for diabetics. Total world production was estimated to be under 2000 tonslyr. In 1975 the first truly large-scale production of xylitol was begun in Kotka, Finland, at the sucro-chemical plant of the Finnish Sugar Co. Ltd., Helsinki, with a capacity for producing xylitol of over 3000 tons/yr. In 1976 ownership of the Kotka plant was transferred to Xyrofin Ltd., a joint venture
TABLE I OCCURRENCE OF XYLITOL IN FRUITS"
Fruit A B XylitoP
Raspberryd (Rubus idaeus)
Strawberryd (Fragaria vesca)
Red whortleberryd (lingonberry) (Vaccinium vitis idaeu)
Cranberryd (Vaccinium oxycoccus, Oxycoccus quadripetalus)
B il berryd (Vaccinium myrtillus)
Sea buckthornd (Hippophae rhamnoides)
Rowan berryd (Sorbus aucuparia)
1 2 3 1 2 1 2 3 4 5 6 1 2 3 4 1 2 3 1 2 3 4 I 2 3 4
0.030e 0.300 0.420 0.196f 0.740 0.0308 0.040 0.120 0.600 0.740 1.100 0.0128 0.030 0. I28 0.600 0.124e 0.353 2.0 0.310" 0.380 0.400 0.412 0.030h 0.050 0.242 0.410
Unripe, green, hard Half ripe, reddish, hard Ripe, red Half ripe, reddish, hard Ripe, red Unripe, green, hard Unripe, green, hard Unripe, reddish, hard Half ripe, reddish, hard Half ripe, reddish, hard Ripe, red Unripe, reddish, hard Unripe, reddish, hard Half ripe, reddish, hard Ripe Unripe, green, hard Half ripe, reddish, hard Ripe Unripe, slightly orange. hard Unripe, orange, hard Half ripe, orange Ripe, orange Unripe, green, hard Unripe, reddish, hard Half ripe, reddish Ripe, red
26 150 280 58 I 1 36 9
37 18 21 38 28 21 91 15 26 25
160 I30 1 I9 81
1 2 3
0.050e 0.413 1.460 0.250'
1 . o o o e
Unripe, green, hard Half ripe, bluish Ripe, blue Ripe, yellow
Unripe, green, hard Ripe Ripe
Bog whortlebenyd 77 (bog bilbeny) 100 (Vaccinium uliginosum) 34
Black curranv 70 (Rubus chamaemorus)
Red curranv 100
Apple (Malusp 128 Apple, Yellow Cinnamon, 48 Apple, Astrakan' 67
53 20 Pruned
Grapd 105 White wine (Bordeaux
Dubonnet (-77) 135
Plums (a Romanian variety)' 0 Plums (a South African variety)'
UReprinted from Makinen and Soderling (1980). Copyright 0 by the Institute of Food Technologists. bRelative ripeness is given as extinctions (A) determined from sample homogenates, and by estimating the ripeness visually and observing the collection time (B). cThe values are in micrograms per 1 g of edible portion (fresh weight). dCrown in the wild state. eAt 540 nm. fAt 520 nm. gAt 500 nm. hAt 410 nm. 'At 370 nm. Kultured.
378 LEA HYVONEN ET AL.
established between the Finnish Sugar Co. and F. Hoffmann-La Roche & Co. Ltd., Basel, Switzerland.
The annual world production of sugar alcohols was about 345,000 tons in 1978, and of that amount 330,000 tons were sorbitol. The amount of xylitol and mannitol produced was 6000 tons. The production amounts of maltitol, iso- maltitol, galactitol, and lactitol amounted to less than 1000 tons/yr (Albert et al., 1980).
C. LARGE-SCALE XYLITOL PRODUCTION
Production of xylitol by means of extraction from its natural sources is imprac- tical and uneconomical because of the relatively small amounts in which it occurs. Xylose, a pentose which can be hydrogenated to xylitol, is known to be widely distributed in plant material. It does not occur in the free state in plants, but is usually in the form of xylan, a polysaccharide composed of D-xylose units, which occur in association with cellulose. Xylose is also found as part of glyco- sides (Spalt et al . , 1973).
Despite its wide occurrence in nature, xylose is difficult to produce commer- cially because of the problems encountered in separating it, particularly from other carbohydrates such as glucose. However, the fact that xylan is more easily hydrolyzed than cellulose provides the technical possibility for xylose extraction and xylitol production. Accordingly, the recovery of xylose from plant materials and its subsequent hydrogenation is the basic principle of xylitol production (Fig. 1).
Plant materials which contain a suitable amount of xylans to be used in this process include hardwoods such as birch and beech, oat and cottonseed hulls, corn (maize) cobs, sugar cane bagasse, straw, and various nut shells. The xylan or xylose content of such materials is 2&30% of the dry substance.
The choice of raw materials for the manufacture of pure xylitol is important. Most of the alternatives are bulky and of low density. Optimally, therefore, the raw material for large-scale production should be one which is centrally available in large quantities and of relatively high xylan content. In some of the existing processes agricultural by-products are being utilized, e.g., almond shells in Italy and apparently rice and cotton seed hulls, respectively, in China and the Soviet Union. The large Finnish production is based on birchwood chips, whereas other hardwood chips have been utilized in Germany. Xylan-containing sulfite waste from the paper and pulp industries has been proposed as a more economical alternative to hardwoods. Production in the United States will probably be based on corn cobs. All of these raw materials contain relatively small amounts of polymers of other sugars such as glucose, mannose, arabinose, and galactose in their hemicelluloses. The hydrolyzates require extensive purifications and sepa- rations to remove these sugars from xylose and xylitol. Nevertheless, it is possi- ble to recover about 50-60% of the xylans as xylitol.
Hydro l ys i s
H 2 0
D - Xylose
+ c a t a l y s t
FIG. 1. Principle of xylitol production.
Hydro lys is o f pentasan- containinq r a w mate r ia l s
pen tose sugar mater ia l
r - - - - - - - - I
Fina l pu r i f i ca t i on and co lor removal
I I I I
I I puri f ied pentase solution
H - C - O H
Xyl i to l
I polyol so lu t ion
Fractionation and molasses crystallization crystal l izat ion molasses
I 1 _ _ _ _ _ _ _ J +
XYLlTOL FIG. 2. Production of xylitol aild xylose
380 LEA HYVONEN ET AL.
The main steps in the xylitol production process are illustrated in Fig. 2 and described in detail below.
I . Hydrolysis
In mass production plant material is treated with a dilute acidic solution under heat and pressure to hydrolyze the hemicelluloses and to precipitate the lignins. The monomeric sugars dissolve in the reaction media together with other soluble products. Fortunately, the cellulose is not attacked, otherwise the xylose would be contaminated with large amounts of glucose which would be troublesome and costly to separate. The simultaneous occurrence of undesired side reactions and the considerable nonspecificity are the restrictions of acid hydrolysis.
Von Puls et al. (1978) have described the use of immobilized xylanolytic enzymes in the total hydrolysis of xylans. An enzymatic hydrolysis would be a more subtile method without chemicals, high temperatures, and high pressures, but the specificity of xylanases may disturb hydrolysis and therefore a number of different xylanases are required to complete hydrolysis. However, enzyme hy- drolysis has not yet been used in mass production.
2. Xylose Purification
In the next phase of the process the hydrolysate is processed via a series of complicated purification steps to remove the undesirable by-products. These substances originally comprised part of the hemicelluloses and were solubilized during the hydrolysis. Two basic routes have been reported for the desired purification. These differ in whether or not xylose is isolated as such.
a. Isolation ofXylose. A patented process obtaining xylose from vegetable matter uses oxalic acid treatment (Steinert and Lindlar, 1970). Relatively pure crystalline xylose is produced from the hydrolysate by successive operations of ion exchange, decolorization, and crystallization from methanol (Jaffe et al., 1974). In an alternate process xylose is isolated from impurities with alcohol precipitation and crystallized from an aqueous concentrate diluted with acetic acid (Spalt et al., 1973).
The pentose-rich solution obtained by acid hydrolysis is purified by mechan- ical filtration and ion-exclusion techniques for color removal and desalting. This solution is then subjected to chromatographic fractionation to obtain a highly purified solution of xylose (Melaja and Hamalainen, 1977).
In this approach the hydrolysate is treated in a series of ion-exchange exclusion and decolorization processes to remove all by- products except the carbohydrates from the main xylose stream. The mixed xylose and other carbohydrates contained in the solution are in a high state of chemical purity (Melaja and Hamalainen, 1977).
b. Nonisolation of Xylose.
FOOD TECHNOLOGICAL EVALUATION OF XYLITOL 38 1
1 L - -
3 . Hydrogenation
Xyl i to l - r ich f r a c t i o n
For the conversion to xylitol the isolated xylose dissolved in water or the mixed xylose-carbohydrate solution is hydrogenated at temperatures ranging from 80 to 140C and hydrogen pressures up to 50 atm, in the presence of a metal catalyst. With the nonisolated xylose stream, all other sugars present are also hydrogenated to their respective polyols (Wisniak et al., 1974). Some 80% of the world production of sugar alcohols is manufactured in batch suspension pro- cesses using Raney nickel catalysts (Albert et al., 1980).
4. Xylitol Purification
After removal of the catalyst by filtration and ion exchange the hydrogenated solutions are further processed to obtain xylitol by purification, concentration, and crystallization. In the isolated xylose route, decolorization and crystalliza- tion from either alcoholic solvents or aqueous solutions have been used for the isolation of pure crystalline xylitol (Jaffe et al., 1974; Melaja and Hamalainen, 1977).
Pur i t i ed pentose so lut ion
From i so la ted
Xyl l tol Crysta l l izat ion
Recycle of xyl i tol-r ich f rac t i on
fract ionation X Y l l to1 solution
FIG. 3. Chromatographic fractionation and crystallization of xylitol.
382 LEA HYVONEN ET AL
2 6 10 1L lk 2 2 26 3L 38 FRACTIONS 18-30 T lMEx 10 min
FIG. 4. Distribution of xylitol and other polyols in ion-exchange chromatography. From Melaja and Hamalainen (1977). (1) Arabinitol, (2) xylitol, (3) rnannitol, (4) galactitol, ( 5 ) sorbitol, (6) unhydrogenated sugars and unknown impurities. Cationic resin: Ca2+ form; bed: 350 cm, 4 22.5 cm; temperature, 49C; feed, 17 litersihr.
With the nonisolated xylose, the separation of nonxylitol polyols must be made before xylitol crystallization (Fig. 3 ) . This purification has been effectively carried out by ion-exchange chromatographic fractionation with cationic ex- change resins (Fig. 4). Pure xylitol is then crystallized from aqueous solutions separated in the fractionation (Melaja and Hamalainen, 1977).
Ill. PHYSICOCHEMICAL AND FOOD TECHNOLOGICAL PROPERTIES OF XYLITOL
A. PHYSICOCHEMICAL PROPERTIES
I . Structure of Xylitol
Xylitol is a pentahydric sugar alcohol, or pentitol with the empirical formula C,H,,O, and MW of 152.15. Xylitol is a meso compound completely lacking in optical activity in solution. Its structure is indicated in Fig. 5 .
2 . Crystdlization
a. Bimorphism and Melting Point. Wdfrom and Kohn (1942) reported the first successful attempt at crystallization. They obtained hygroscopic crystals,
FOOD TECHNOLOGICAL EVALUATION OF XYLITOL 383
H-C-OH I CHZOH
FIG. 5 . The structure of xylitol.
melting at 61C. Carson et al. (1943), when repeating the former experiment, produced a new form of crystals melting at 93-94.S"C. By seeding with the low- or high-melting material they were able to grow either form. No melting point depression was shown in a mixture of both, but the low-melting form changed into the high-melting form after a few days. The stable form melting at 94C was found to be orthorhombic, whereas the metastable form melting at 61C was monoclinic. Apparently the monoclinic form is very elusive, since Kim and Jeffrey (1969), among others, were unable to crystallize the monoclinic form. Instead Kim and Jeffrey (1969) obtained two different morphologies of the orthorhombic form.
This behavior is reminiscent of that observed with D-mannitol, in which poly- morphism has been reported, but it is difficult to reproduce the crystals (Berman et al., 1968; Kim et al., 1968). All xylitol produced by industrial processes, microbiologically or chemically, is in the orthorhombic form with a melting point of 94C.
One of the outstanding properties of xylitol is its capability to form metastable melts under certain conditions. The phenomenon is known for a number of organic and inorganic substances. When completely melted and subsequently cooled to ambient temperature in a closed container, xylitol will remain in the molten state. The melt is colorless, clear, and of a honey-like viscosity. In addition to seeding with xylitol microcrystals, crystal- lization can be triggered by ultrasonic cavitation or by scratching the container's inner surface (Voirol, 1979).
Contaminants, such as dust, soil, iron powder, or sodium chloride, added to the crystals before melting did not influence the metastability of the melt. How- ever, 10% sorbitol or 5% mannitol will cause crystallization of the mixture after 1 hr or S min, respectively (Voirol, 1979).
Xylitol melt at 20C in an open container will crystallize within a few hours wherever dust particles have fallen. A sample of open melt kept for 6 weeks in a low-dust atmosphere did not crystallize, confirming the role of dust in initiating crystallization at the surface. Supercooled melts can be kept stable in closed aluminum tubes, sealed plastic bags, and rubber-stoppered glass flasks (Voirol, 1979).
b. Supercooled Melts.
384 LEA HYVONEN ET AL
3 . Boiling Point
In contrast to sugars (sucrose, glucose, and fructose), xylitol has a distinct boiling point below decomposition. It will show only slight discoloration when boiled at a constant temperature of 216C under atmospheric pressure (Kracher, 1975a).
4. Specific Heat
The specific heat of liquid xylitol between the melting point and 25C is 167.9 J/g (40.1 cal/g) as determined by differential thermoanalysis (Schildknecht, per- sonal communication). The heat required to bring crystalline xylitol from room temperature to the melting point (AHs) , the heat required for melting (AH,), the heat liberated by supercooling back to room temperature (AH,), and the subse- quent heat of crystallization (AH,) represent a cyclic process (Fig. 6) in which the energy balance is zero
AH, + AHm + AH,+ AHc = 0 I I I
AH, (-189.2 J l g )
AH, . 257.7 J l g
A HS 99 L Jlgl
25 50 75 93 T('C)
Room Temperature Melting Temperature
FIG. 6 . Heat capacity of xylitol ( A H = 0 at 25C). H,, heat capacity of the solid phase; H,, heat of melting; H,, specific heat of the liquid phase; H,, heat of crystallization. From J. Schildknecht (personal communication).
FOOD TECHNOLOGICAL EVALUATION OF XYLITOL 385
Calorimetric measurements of crystallization heat have shown 189.2 J/g (45.3 cal/g) to be available in supercooled melts. It is difficult to find a substance capable of forming metastable melts with a higher heat of crystallization (Voirol, 1979).
The solubility of xylitol is the same as that of sucrose (68 g/lOO g solution) at 30C. Below that temperature it is less, above it is more soluble than sucrose (Ape1 and Rossler, 1959; Manz et al., 1973; Virtanen, 1973). The increase of xylitol solubility with increasing temperature is significantly greater than that of sucrose solubility (Fig. 7). Xylitol is only slightly soluble in alcohol: 1.2 g/100 g solution of 96% ethanol, and 6.0 g/lOO g of 96% methanol (Kracher, 1975a).
6. Heat of Solution
Another remarkable characteristic of xylitol is its endothermic dissolution. The heat required to dissolve 1 g of this pentitol is the highest of known sugars or sugar alcohols (Mangold, personal communication). The heats of solution of the common alternative sweeteners are as follows:
sucrose: 18.1 Jig (4.34 calig), dextrose: 59.4 Jig (14.2 calig), sorbitol: 97.0 J/g (23.2 calig), xylitol: 153.0 Jig (36.6 calig).
In food use this means that the consumption of xylitol in crystalline form results in an actual cooling of the saliva. This property lends a true cooling effect to
- I I ,
10 20 30 LO 50 60 70
FIG. 7. (1968).
Solubility in water of xylitol and sucrose. Data from Virtanen (1973) and Schneider et al.
386 LEA HYVONEN ET AL.
foods containing solid xylitol. The cooling effect is desirable in some foods, often proclaimed and even patented (Hammond and Streckfus, 1975).
Ten percent xylitol reduces the temperature of an aqueous solution by 3"C, whereas the preparation of a 50% xylitol syrup reduces the temperature by 12C (Voirol, 1980).
The viscosity of sugars and sugar alcohols depends on many factors: solids concentration in solution, molecular weight, temperature, and composition of solids (von Graefe, 1975). Consequently, the viscosity of a xylitol solution is, for instance, significantly lower than that of the sugar alcohol or sugar of a higher molecular weight (Fig. 8). The viscosity of a saturated xylitol solution is signifi- cantly lower than that of sucrose, for instance.
The viscosities of sugar solutions as well as that of the xylitol solution de- creases with increasing temperature (Fig. 9). The temperature dependence of viscosity for a saturated aqueous xylitol solution shown by H. E. Keller (un- published) is presented in Table 11.
10 20 30 LO 50 60 % SOLIDS(WIWI
FIG. 8. fructose, (4) xylitol. From Nicol (1980).
Viscosity of sweetener solutions at 20C. ( I ) DE 42 glucose syrup, (2) sucrose, ( 3 )
FOOD TECHNOLOGICAL EVALUATION OF XYLITOL 387
- I000 a k
0 - > v) 0 0
0 LO 80 ' c TEMPERATURE r C )
FIG. 9. Viscosity of some carbohydrate sweeteners at different temperatures. Glucose syrup DE 40, 78 wt. %; glucose syrup DE 60, 77 wt. %; isoglucose, 70 wt. %; fructose solution, 70 wt. %; xylitol solution, 70 wt. %. From von Hertzen and Lindqvist (1980).
Figure 10 shows the lower density of aqueous xylitol solutions as a function of concentration in comparison with solutions of sorbitol and sucrose.
The density of a supercooled melt at 20C was determined to be 1.42 and that of xylitol crystals 1.49, indicating an approximate contraction of 4.7% at the point of crystallization (Voirol, 1980).
Sorption isotherms show that an equilibrium moisture content of xylitol is low at relative air humidities lower than 80%, after which the moisture adsorption
TABLE I1 TEMPERATURE DEPENDENCE OF VISCOSITY FOR A SATURATED AQUEOUS
Temperature Viscosity ("C) (CP)
20 37 40 15 60 7 70 5 80 4
aFrom H . E. Keller (unpublished).
388 LEA HYVONEN ET AL
xyl i to l sorbitol
1,000 1.100 1.200 1.300
FIG. 10. from Hirschmuller (1953) and G. Pongracz (personal communication).
Densities of xylitol, sorbitol, and sucrose solutions as a function of concentration. Data
increases sharply (von Schiweck, 1971; Kammerer, 1972). Fructose, sorbitol, and corn starch are distinctly more hygroscopic than xylitol at relative air humid- ities between 60 and 80% (Fig. 11).
There is hardly any difference between the behavior of crystalline and powdered xylitol during storage. Both show an increasing tendency to pick up moisture above 70% relative air humidity. Below 60% relative air humidity they behave similarly to sucrose and powdered sugar (W. J. Mergens, personal communication).
Table I11 shows the relative hygroscopicity of sucrose and three sugar alcohols at a high relative air humidity and room temperature. Sorbitol is the most hygro- scopic and sucrose the least hygroscopic in these conditions. The moisture pick- up of mannitol increases only slightly, whereas that of xylitol clearly increases with time (W. J. Mergens, personal communication).
corn s ta rch !Nl&&zzdY 20 20 L O 60 80 100 RELATIVE HUMIDITY (%)
FIG. 11. with permission from Kakao and Zucker.
Adsorption isotherms for crystalline carbohydrates. From Kammerer (1972). Reproduced
FOOD TECHNOLOGICAL EVALUATION OF XYLITOL 389
TABLE 111 MOISTURE PICKUP OF GRANULAR MATERIAL DURING STORAGE AT 84% RELATIVE
HUMIDITY AND 20"Ca
Days Sucrose Mannitol Sorbitol Xylitol
1 0.03 0.51 1.89 0.05 2 0.05 0.60 3.20 0.08 3 0.05 0.59 4.45 0.14 4 0.04 0.58 5.53 0.13 9 0.07 0.66 10.53 0.33
0.68 11 0.08 0.65 - 18 0.08 0.67 15.90 1.10 65 0.04 0.67 29.00 13.89
aFrom W. J. Mergens (personal communication).
B. FOOD TECHNOLOGICAL PROPERTIES
1 . Caloric Value
Calorimetric determinations have shown xylitol to be isocaloric with most carbohydrates. Its combustion value is 16.7 kJ/g (4.06 kcal/g).
2. Browning Reactions
Due to the absence of aldo or keto groups, xylitol does not take part in browning reactions of the Maillard type. This may be regarded as an advantage or a disadvantage when contemplating its use as a food ingredient.
No color formation or sweetness reduction was noted in an aqueous xy- litol-aspartame solution (27.17 g xylitol + 0.6467 g aspartame/l000 ml) after sterilization (20 min at 12 1"C), whereas the sweetness of fructose-aspartame solution (34.3 g fructose + 0.5571 g aspartame/l000 ml) was noticeably reduced after sterilization. The color of the solution was yellow and had a honey-like flavor, probably due to the Maillard reaction (Hyvonen, 1981).
Xylitol even does not caramelize at elevated temperatures (Kammerer, 197 1 ) . A slight yellow color formed when heated over 150C is thought to be due to small amounts of aldose impurities in xylitol (Kracher, 1975a).
Most microorganisms are incapable of utilizing xylitol. It has been shown that xylitol is not fermentable by cariogenic oral microorganisms (Gehring et a l . , 1974; Lamas et a l . , 1974). The common baking yeast Saccharomyces cere-
390 LEA HYVONEN ET AL.
visiae cannot ferment xylitol either. The buns sweetened with xylitol did not rise and even the fermentation of sucrose in the buns, where xylitol was also used, was retarded (Varo et al., 1979; Hyvonen and Espo, 1981b).
Salminen and Branen (1978) noted a prolonged fermentation time in pre- sweetened xylitol yogurt. A lower acid production was also noted in xylitol- sweetened yogurt than in sucrose-sweetened yogurt by Hyvonen and Slotte (1981).
4 . Sweetness
a. Chemical Basis. Xylitol, a meso-pentitol, has little structural similarity to sucrose, but they have been reported to taste almost equally swket on a weight basis (Gutschmidt and Ordynsky, 1961; Yamaguchi et al., 1970a; Hyvonen et al., 1977).
Qualitatively the sweetness of xylitol tended to fall near that of fructose and glucose in a three-dimensional space by a multidimensional scaling procedure (Schiffman et al., 1979).
In assessing the sweetness of several pentitols, Lindley et al. (1976) found that xylitol was much sweeter than the stereoisomers, L-( -)-arabitol and ribitol. On the basis of molecular models the oxygen-oxygen distances between all four pairs of oxygen atoms of xylitol in a planar zigzag conformation is 2.9-3.0 A, which is ideal for eliciting sweetness according to the AH, B theory (Shallen- berger and Acree, 1967). A strong IR absorption peak at 3440 cm- suggests that the nonbonded hydroxyl groups must cause the intense sweetness of xylitol, whereas the intramolecular hydrogen bonding reduces the sweetness of ribitol and arabitol (Lindley et al., 1976).
b. Relative Sweetness. Relative sweetness of a sweet-tasting compound is determined as the relation of the concentrations needed to evoke the same sweet- ness perception. Sucrose has mainly been used as the reference. The relative sweetness is dependent on concentration. The relative sweetness of xylitol was found to increase from 86 to 115 as concentration increased from 1 to 20% (Gutschmidt and Ordynsky, 1961). According to Yamaguchi et al. (1970a), the change was from 96 to 118, when concentration increased from 2.5 to 30%. According to Hyvonen et al. (1977), the relative sweetness values of xylitol solutions tasted at room temperature varied from 103 to 115 as compared to 5-20% sucrose references.
The sweetness of xylitol was thought to be largely invariant with temperature, since as a sugar alcohol it does not undergo mutarotation in solution (Fratzke and Reilly, 1977). However, this proved incorrect. The relative sweetness of xylitol decreased significantly, for instance, from 103 to 78, when a 5% sucrose refer- ence was used and when the temperature changed from 5 to 50C (Hyvonen et al., 1977).
FOOD TECHNOLOGICAL EVALUATION OF XYLITOL 39 1
In general the relative sweetness of xylitol was noted to be slightly reduced in acid solutions (Hyvonen et al., 1978a). In 0.0175% o-phosphoric acid solution the relative sweetness of xylitol was exceptional. This acid caused a significant reduction (from 103 to 97) in the sweetness of xylitol at refrigerator temperature, and at hot drink temperature the sweetness was significantly higher in the phos- phoric acid solution (87) than in the corresponding water solution (80) (Table IV) .
c . Synergistic Effects in Xylitol-Containing Mixtures. Synergism is in- ferred when the sweetness of a mixture of sweeteners is greater than the sum of the sweetnesses of its components. Synergistic effects have been noted especially in the mixtures of sweeteners with greatly diverging chemical structures and dissimilar relative sweetnesses.
Weickmann et al. (1969) suggested that synergism is at its maximum when the components of a mixture contribute about the same amount to the sweetness of a mixture, which applies to xylitol-saccharin mixtures also. Yamaguchi et al. (1970b) also reported synergistic interrelationships in xylitol-saccharin and xy- litol-cyclamate mixtures.
TABLE IV RELATIVE SWEETNESS OF XYLITOL IN WATER AND ACID SOLUTIONS
Acid ( W ) Relative sweetness
6 t 2C
23 t 2C
50 2 3C
No acid Citric acid (0.01) Citric acid (0.05) Malic acid (0.007) Malic acid (0.035) Phosphoric acid (0.0035) Phosphoric acid (0.0175) No acid Citric acid (0.01) Citric acid (0.05) Malic acid (0.007) Malic acid (0.035) Phosphoric acid (0.0035) Phosphoric acid (0.0175) No acid Citric acid (0.01) Citric acid (0.05) Malic acid (0.007) Malic acid (0.035) Phosphoric acid (0.0035) Phosphoric acid (0.0175)
103 t 3 100 2 1 102 t 5 105 t 2 102 t 2 104 t 1
103 t 2 102 2 1 101 2 3 102 t 1 102 t 1 103 t 1 99 2 2 80 2 2 79 ? 4 77 2 2 79 2 3 78 2 2 77 2 4 87 2 1c
97 t I
aFrom Hyvonen e t a / . (1978a). Copyright 0 by Forster Publishing, Inc. bDifference significant by t-test at 10% risk level.
392 LEA HYVONEN ET AL
About 5&60% of extra sweetness was noted in aqueous xylitol-saccharin solutions at the predicted isosweetness with a 5% sucrose solution (Hyvonen et al., 1978b). In the corresponding xylitol-cyclamate mixture the degree of syner- gism was 6&66% at the maximum. At the higher sweetness level, at the pre- dicted isosweetness with a 10% sucrose solution, the degree of synergism in the xylitol-cyclamate mixture was still greater, as high as 7 6 9 8 % in the most ideal combination at each of the temperatures 8 , 25, and 50C (Sipila, 1977; Hyvonen and Sipila, 1977). In xylitol-aspartame mixtures the synergism noted was 77% at maximum (Hyvonen, 1981).
Enhanceh sweetness of the sweetener mixtures could be used advantageously in reducing the energy content of sweetened drinks such as coffee, tea, juice, and soft drinks. Conventional sweetness levels with 50-70% less calories could be achieved without the deterioration of other taste qualities (Hyvonen and Sipila, 1977; Hyvonen, et al., 1978b).
IV. FOOD APPLICATIONS
Not all confections can be made using xylitol as the only sweetener. There are problems in all those sweet preparations which require crystallization inhibitor. Many standard formulations call for both sucrose and glucose syrup in specified proportions. Due to crystallization properties, it is not possible to make fondant creams, chews, toffees, and transparent hard candies with xylitol alone.
Glucose should not be used as a crystallization inhibitor in xylitol confection- ery because its cariogenicity would preclude the products primary intended use.
1. Chewing Gum
Chewing gum is a cariogenic product, since through constant release of sugar during chewing the time of contact with the teeth is quite long and intensive.
In normal chewing gum glucose syrup acts as a softener. It has been shown by Kracher (1975b) that a pure xylitol gum can be made by replacing glucose with gum arabic solution. The use of gum arabic has been found necessary because xylitol solutions have insufficient viscosity, which would otherwise appreciably lengthen the mixing times. The kneading operation also can be controlled with an aqueous xylitol solution or with glycerol (Voirol, 1978).
Gums made of pure xylitol and others made of polyol mixtures are on the market in a number of countries. The forms manufactured include laminated sticks, extruded bubble gum-type blocks, and coated gums.
One of the first xylitol-containing chewing gums was given to students four to
FOOD TECHNOLOGICAL EVALUATION OF XYLITOL 393
five times a day during 1 year in a caries study carried out by Scheinin et al. (197%). Under a moderately cariogenic diet the regular chewing of xylitol chewing gum was shown to be an effective means of caries prevention.
There are only a few processing steps by which xylitol gum manufacture differs slightly from that of the normal sucrose/glucose type (Kracher, 1975b). The xylitol used must be in the form of a powder. Particle size of the powdered xylitol should not exceed 50 p,m. Kneading of the gum base should take place at a temperature on average 10C below that normally used in sucrose/glucose gum. The unusually low melting point of xylitol entails the danger of caking. The addition of water should be kept at a minimum to avoid hardening. Furthermore, the shorter structure may require an adjustment of extruder or roller param- eters (Voirol, 1978).
2. Hard Candy
Hard caramels (hard candy, high-boiled sweets) represent another class of high-caries-risk confectionery. Since they are mainly consumed by children, the substitution of sucrose by xylitol, yielding a product both noncariogenic and acceptable in taste, would be meaningful indeed.
However, the production of hard candy using xylitol is problematic. Trials carried out with high-boiled hard candy made exclusively with xylitol produced a product in which crystallization had already begun during the cooling phase. The drops became brittle (Manz et al., 1973).
The industrial manufacture of normal hard candy takes place in either of two ways: pulling and die-cutting of a formable mass or depositing (casting into molds). The plastic method requires the use of glucose syrup, which must be excluded if the product is to be noncariogenic. Because no plastic phase exists in pure xylitol, this leaves the depositing method, for which a procedure has been developed, resulting in a pure xylitol candy. The product is hard as glass and suckable but not transparent.
The process involves melting the xylitol, adding natural coloring agents, heat- ing to 120C to evaporate all water introduced with the color, cooling to slightly below the melting point (92C), seeding to 25% of the total weight with powdered xylitol, adding crystalline acid and flavor, and mixing. If the mixture is stirred constantly and thermostatically kept between 88 and 92C in a hopper, melt and microcrystals will coexist as do ice and water at 0C. The viscosity of the mass at these temperatures is sufficient to use customary nozzles and molds. Teflon-coated aluminum molds with expeller pins give the best results. In con- trast to sucrose/glucose deposited sweets, xylitol candies do not need long con- veyors or cooling tunnels. Although heat is generated during crystallization, the setting time is ;bout 1 min for a 1.3-g, lenticular-shaped deposit and 3 min for a 4-g, oval deposit (Voirol and Brugger, 1976).
394 LEA HYVONEN ET AL.
Soft caramels containing milk solids can be made with xylitol. The charac- teristic flavor forms when heating the mass to obtain Maillard-type browning reactions between lactose and milk proteins. Xylitol does not participate in these reactions; thus in caramels it is merely a sweetener, not a flavor precursor.
The differing properties of xylitol in relation to those of sucrose only have a slight effect on the properties of toffee-like products. Since the proportions of the individual raw materials (proteins, fat, polysaccharides) are not critical, appeal- ing products are obtained by the use of xylitol. Xylitol toffees tend to have a shorter structure, a structure similar to that which would be obtained using a low glucose content (Kracher, 1975b).
4 . Gum Drops
Chewable confectionery using gum arabic, pectin, or gelatin with xylitol as the sweetener tends to harden during storage. Formulations have been developed using a minimum amount of sorbitol to prevent crystallization. Storage tests have shown that the shelf life of combination products exceeds 12 months.
Telemetric tests on humans using gum drops of this type have shown that the plaque pH remains above the critical value during consumption, which justifies the claim tooth saving according to Swiss regulations (Imfeld, 1977).
5 . Confectionery Jellies
Pectin jellies with conventional soluble solids cannot be made with xylitol. Kracher (1975b) states that crystallization will occur if 75% xylitol is used. If the xylitol proportion is lowered, the pectin will no longer gel. A favorable effect has been obtained by slightly increasing the proportion of gelatin or agar-agar.
6. Compressed Tablets
Tablets can be compressed either from crystalline material or granulated xy- litol. The Finnish market offers peppermint-flavored tablets consisting of 99.25% xylitol. The direct compression technique uses crystalline xylitol (0.6-0.4 mm), 1.6% stearic acid, and flavor (R. Etter, personal communication).
The main problem in the process appears to be the friability of the tablets. On a laboratory scale good results have been obtained by sintering the tablet surface in a hot air stream so that only the surface is melted and the core is protected.
This process approaches a coating procedure in which no coating pan is
FOOD TECHNOLOGICAL EVALUATION OF XYLITOL 395
required. It is another example of new technology made possible by the proper- ties of xylitol (low melting point). If a regular coating layer by melting and subsequently crystallizing the surface is desired, the airflow must be laminar and of homogenous temperature over the surface of the tablets. It will be necessary to reverse the flow or turn the tablets over on the sieves for processing the other side. If the initial friability of the tablets is low enough, better results are obtained in a fluidized bed dehydrator (R. Etter, personal communication).
Xylitol-only coatings have been successfully applied to centers of compressed tablets, hard candies, and chocolate. Certain deviations from the customary technique using sucrose/glucose must be observed.
The best results were obtained with a supersaturated (85%) aqueous xylitol solution at 40C panned in layers in a hot air stream (60C). Calcium carbonate can be used as an isolation powder, if the centers must not be visible through the coating. Dusting is preceded and followed by application of the warm syrup (W. Thurkauf, M. Grossmann, and K. Munzel, personal communication).
Contrary to sucrose, xylitol will not cover irregular surfaces of the centers. Twenty percent of a 1:2 gum arabic-water solution in the coating syrup helps in obtaining a smooth surface. The final coatings may contain pigments if desir- able, and wax can be applied for surface polishing. The surface of the finished dragCes is milky-opaque, and natural colors, applied with the final coating syrup, tend to be less stable on it than on sucrose surfaces. Cross sections examined under the microscope seem to indicate a rougher surface. Flavors applied in the form of natural oils improve the surface structure. Peppermint oil, added at 0.5% in the syrup during final coatings, results in a pearl-glossy surface (Voirol, 1979, 1980).
When sucrose is replaced by xylitol in chocolate on a weight basis, some slight changes must be made in the production process, mainly because of the lower viscosity of the xylitol product. The viscosity may be adjusted by the use of additives (Kracher, 1975b).
A coarse sandy texture was noted in the xylitol chocolate after storage, when the relative humidity of the atmosphere in the manufacturing locality had exceed- ed 85%. This may have been caused by a kind of hydrate film around the xylitol particles (Voirol, 1978).
The concentration of xylitol in the chocolate bars sold in Finland, the Federal Republic of Germany, and the Soviet Union ranges from 17 to 42% (Voirol, 1979).
396 LEA HYVONEN ET AL
B. ICE CREAM
In principle, sucrose can be replaced by xylitol on a weight basis in ice cream, however, the melting properties of the product are appreciably altered. Xylitol ice cream has a considerably softer consistency than sucrose ice cream at the same temperatures.
Kracher (1975b) reported that no change occurred in xylitol ice cream during storage for 6 months at -24"C, in particular, there was no recrystallization.
The melting of xylitol ice cream as well as its overall acceptance were judged to be better than those of the sucrose reference after 3-month storage at -25C (Hyvonen and Torma, unpublished).
Both Salminen and Branen (1978) and Hyvonen and Slotte (1981) found 8% xylitol to be the most preferred concentration in yogurt. Xylitol had no effect on the pH of yogurt when added after incubation. If xylitol was added to the milk before incubation, the pH of the xylitol-sweetened yogurt was distinctly higher (4.4) than that of the corresponding sucrose-sweetened yogurt (4.0) (Hyvonen and Slotte, 1981).
The post-incubation-sweetened xylitol yogurt was judged to be as good as the sucrose reference by sensory evaluation. The flavor of the presweetened xylitol yogurt was regarded as poorer than that of the corresponding sucrose reference, mainly due to the lower acidity of the xylitol yogurt according to Hyvonen and Slotte (1981), whereas Salminen and Branen (1978) reported that the lower acidity of the xylitol yogurt was the reason for its more preferred flavor. This could reflect differences in national taste habits.
The viscosity of the presweetened xylitol yogurt was lower than that of the sucrose-sweetened yogurt. However, the texture of the xylitol yogurt was not scored lower in the sensory evaluation (Hyvonen and Slotte, 1981).
D. JAMS, JELLIES, AND MARMALADES
In jams, jellies, and marmalades sugar acts as a preserving agent by its osmo- tic pressure. Xylitol, in addition to its nonfermentability by most yeasts, molds, and bacteria, is an effective preserving agent due to its higher osmotic pressure even at low concentrations. For instance, a 30% xylitol solution and a 70% sucrose solution have about the same osmotic pressure.
Sucrose also has an important role in the gelatinization of gel products. Ka- wabata et al. (1976) studied the effect of sugars and sugar alcohols on the texture of pectin jellies and found that in HM-pectin jellies the jelly strength of the xylitol jelly had a pattern as a function of concentration similar to that of the sucrose jelly (Fig. 12). In LM-pectin jellies the increase of xylitol or sorbitol
FOOD TECHNOLOGICAL EVALUATION OF XYLITOL 397
55 60 65 70 75 5 5 60 65 70 75
Concentration of sugars (%I Concentration of sugars (%I FIG. 12. Effect of concentration of sucrose, sorbitol, and xylitol on the hardness and adhesiveness of the HM-pectin jelly: (0) sucrose, (0) sorbitol, (0) xylitol. From Kawabata et al. (1976). Reproduced with permission from the Japanese Journal of Nutrition.
concentration did not change the jelly strength; when using sucrose the jelly strength increased with increasing sucrose concentration (Fig. 13).
According to Hyvonen and Torma (1981), as well, the gelling properties of xylitol with LM-pectin differed from those of sucrose. A calcium salt addition was needed for gelatinization and yet the texture of the strawberry jam prepared with xylitol was softer than the sucrose-sweetened one.
Xylitol jams and marmalades prepared for the Turku sugar studies had good keeping qualities. The tastes and flavors of xylitol products were judged to be better than those of the corresponding sucrose references. Better color stability was noted in xylitol cranberry and strawberry jams than in the sucrose-sweetened jams (Manz et al . , 1973). Good color stability of the xylitol-sweetened strawber- ry jam was noted also in the studies of Hyvonen and Torma (1981).
10 20 30 LO 50 10 20 30 LO 50
Concentration of sugars 1%) Concentration of sugars 1%) FIG. 13. Effect of concentration of sucrose, sorbitol, and xylitol on the hardness and adhesiveness of the LM-pectin jelly: (0) sucrose, (0) sorbitol, (0) xylitol. From Kawabata er al. (1976). Reproduced with permission from the Japanese Journal of Nutrition.
398 LEA HYVONEN ET AL
E. BAKERY PRODUCTS
For the sake of complete replacement of sucrose by xylitol, the test subjects in the Turku sugar studies were regularly provided with fresh bakery products. If such products ever become of commercial interest, they would have to be weakly cariogenic or more suitable for diabetics.
The characteristic baking flavor is the result of a series of nonenzymatic browning reactions dependent on the presence of keto or aldo groups as in added inverted sucrose, fructose, or glucose. With only xylitol added, this flavor pre- cursor is missing. Nevertheless, some browning can be expected from the reduc- ing sugars present in the flour.
In fact, xylitol proved to be a good substitute for sucrose in sugar cake. The color and texture of the xylitol cake closely resembled those of sucrose cake. Xylitol was also a good sweetener in this type of product (Hyvonen and Espo, 1 98 1 a).
Xylitol cookies were brown-spotted, probably because of the poor solubility of xylitol in the cookie dough, which contained fat. Xylitol cookies were found to be more friable than the sucrose reference and the mouthfeel of xylitol cookies was more finely divided than that of the sucrose cookies (Hyvonen and Espo, 1 98 1 a).
The fact that xylitol is not fermented by Saccharomyces cerevisiae was dis- tinctly seen when buns were sweetened with xylitol. The xylitol buns did not rise, having small volumes, moist interiors, and dense textures (Hyvonen and Espo, 1981b). In addition, the presence of xylitol seemed to retard the inversion of sucrose added for the nourishment of yeast into the bun dough (Varo et al., 1979). Consequently, xylitol is not a suitable sweetener in yeast-leavened doughs, in addition to its unsuitability for the nourishment of yeast.
Due to its laxative effect xylitol alone is not recommended in beverages such as soft drinks, where consumption may easily exceed the recommended single dose intake.
Hyvonen and Sipila (1 977) used a mixture of xylitol and cyclamate to sweeten citrus-base and cola-type soft drinks. The drinks contained 3.9% xylitol and 0.133% cyclamate. The energy content of the mixture-sweetened drinks was 60% lower than that of the isosweet sucrose reference. Using mixtures of suitable sweeteners is one means to reduce the carbohydrate and energy content of a drink and to produce a dietetic drink also suitable for diabetics.
Four percent xylitol proved to be a suitable amount of the sweetener in an UHT-sterilized milk-base chocolate drink. The physical properties (viscosity, color) of xylitol product did not significantly differ from the sucrose reference.
FOOD TECHNOLOGICAL EVALUATION OF XYLITOL 399
The sensory properties of the xylitol drink were judged to be as good as those of the sucrose drink, both when fresh and after 1-month storage at room tempera- ture (L. Hyvonen and A. Espo, unpublished).
It is realistic to expect only a relatively small future replacement of sugar by xylitol. Health consciousness in the industrialized countries will, however, in- crease the demand for suitable sugar substitutes. The new alternative sweeteners meet many requirements. They are expected to be physiological substances (well-tolerated, natural, or nature-identical). Their sweetnesses and tastes should be as similar to those of sucrose as possible. Diabetics should be able to consume them when advised of their energy content, and they should not be cariogenic. In food processing they should not pose unusual technological problems.
Xylitol fulfills most of these requirements satisfactorily, which makes it not only a valuable alternative to sucrose and sucrose substitutes but also one of the few new discoveries in the field of foods today.
However, the production and technological realities dictate that the price of xylitol will never fall to the level of common sugars, which will limit its use. Xylitols special characteristics, particularly its dental and metabolic aspects, justify its higher price. Indeed, the Turku sugar studies have shown that 5-10 g xylitol/day as between-meal sweets is as effective in bringing dental caries under control as the complete dietary substitution of xylitol for sucrose. Consequently, a realistic and useful application of xylitol in foods will be in confectionery and snack products, where the unique properties of xylitol can be best utilized.
VI. RESEARCH NEEDS
1. The published data on the physicochemical properties of xylitol are insuffi- cient. A filling of this gap of knowledge is needed.
2. The threshold level of xylitol to stop microbial growth in substrates con- taining available carbohydrates is of interest.
3. The gelling properties of different types of xylitol-containing gels, and the inhibition of xylitol crystallization in jellies should be studied.
4. The color stability-improving effect of xylitol should be studied thoroughly.
5 . Crystallization inhibitors for xylitol melts of supersaturated xylitol solu- tions are needed. Since the primary xylitol application in foods is in non- cariogenic sweets, such additives should also fulfill the prerequisite of being noncariogenic. Until a permissible noncariogenic crystallization inhibitor can be found, xylitol chews will be an unsolved problem.
400 LEA HYVONEN ET AL.
6. The possibility of preparing high-acid fruit-flavored xylitol candies which do not become sticky should be clarified. A promising fact is that when xylitol is used, no inversion takes place and no fructose is formed.
7. The combinations of xylitol and artificial sweeteners seem to have many potential applications. A nonsweet and possibly noncaloric bulking agent for these combinations to make the sweetening mixture isosweet with sucrose on a volume basis is worth studying.
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