- Email: [email protected]
Effects of baking on polysaccharides in white bread fractions
Journal of Cereal Science 10 (1989) 149-156
Effects of Baking on Polysaccharides in White Bread Fractions E. WESTERLUND, O. THEANDER, R. ANDERSSON and P. AMAN Swedish University of Agricultural Sciences, Department of Chemistry, P.O. Box 7015, S-750 07 Uppsala, Sweden Received 26 July 1988
White bread was baked at 210 °C for 22 or 35 min and divided into outer crust, inner crust and crumb fractions. The content and composition of water-soluble and insoluble dietary fibre polysaccharides were determined in these fractions and the dough by using a gas-liquid chromatographic method. Corresponding values for starch and •resistant' starch were obtained by enzymic methods. Water-insoluble arabinoxylans were rendered partly extractable in Na acetate buffer during baking, whereas the arabinogalactan became less extractable. The total amount of arabinoxylans decreased in the order crumb to outer crust probably due to increasing degradation caused by more severe thermal conditions. The starch values also decreased in the same order due to formation of resistant and probably to chemically modified forms of starch. The former reaction prevailed in crumb, whereas the latter reaction caused a significant increase in non-starch glucans in outer crust. These polysaccharide modifications resulted in increased non-starch glucan content and decreased pentasan content in bread compared with dough.
Introduction
Various reactions of polysaccharides that take place during the baking of bread have significant effects on product structure and quality. Formation of retrograded starch, which is resistant to amylolytic degradation (' resistant starch '), as a result of gelatinisation and cooling, is well-documented 1-3, for example. Other reactions of starch, which can be envisaged as a result of baking, but which have not been studied hitherto, are formation of glucosaccharides by rupture of intramolecular linkages in starch 4 - s. These glucosaccharides possess end units of ~-1,6-anhydro-glucopyranose,and they may in turn react further by transglycosidation with hydroxyl groups in starch or other carbohydrate components present. Branched glucan structures formed in this manner represent an irreversible chemical modification of starch, increasing the non-starch glucan content of dietary fibre 5 • Pentosans, including arabinoxylans and arabinogalactans, are the main components among dietary fibre polysaccharides in wheat flour', and they have several functional properties that are important in baking 8 • For example, water-soluble pentosans may have beneficial effects on bread volume, whereas water-insoluble arabinoxylans have Abbreviation used: OLe
= gas-liquid chromatography.
0733-5210/89/050149+08 $03.00/0
© 1989 Academic Press Limited
150
E. WESTERLUND ET AL.
been reported to decrease the rate of bread staling 9 , Although several structural studies of pentosans in wheat flour have been performed6, 10. 11, little information is available on these polysaccharides in bread 12 , Other polysaccharides present in wheat in significant amounts are glucomannans 13 , cellulose and ~-glucans14, The present paper on breadmaking is focused on modifications that are induced in the starch and non-starch polysaccharide fractions during the baking of white bread. The study was undertaken to evaluate effects on these components in bread fractions as a result of different baking conditions.
Experimental General methods Solutions were concentrated under reduced pressure at bath temperatures not exceeding 40°C, or by freeze-drying. All samples were milled to pass a 0·8 mm screen. Dry matter was determined by oven drying at lOS °C for 18 h. Gas-liquid chromatography (GLC) was conducted on a Packard 427 apparatus equipped with a flame-ionisation detector. Alditol acetates were analysed on a fused-silica CpSil-88 column (9'5 m x 0·25 mm Ld., helium flow approximately 1 m/s) programmed from 170 to 220°C at 4°C/min. All analyses were made at least in duplicate.
Materials The dough and bread samples used were taken from a previous study15, in which white bread had been baked at 210°C for 22 or 35 min (experiments A and B, respectively). The breads had been separated by hand into three fractions on the basis of visible differences in their texture. These fractions, denoted as outer crust, inner crust and crumb, and the corresponding dough, had been extracted successfully by reflux with hexane and 80 % (vIv) aqueous ethanol. The resulting aqueous alcohol extracted residues were analysed in this study to determine the contents and compositions of the polysaccharides present. Bacterial alpha-amylase (Termamyl 120 L, BC 3.2.1.1, Novo A/S, Copenhagen), and amyloglucosidase (14 U/mg, BC 3.2.1. 3, Boehringer, Mannheim) were used for hydrolysis of starch. The glucose-peroxidase reagent used for determination of glucose (Merckotest 3395, Darmstadt) was obtained from Merck.
Determination of starch and' resistant' starch Starch was analysed in the aqueous alcohol extracted residues (40'0-50'0 mg) using an enzymic method 16 ,17 involving hydrolysis with thermostable alpha-amylase (Termamyl 120 L) and amyloglucosidase, followed by determination of the glucose released by a glucose-oxidase reagent. Determination of resistant starch was performed on original unextracted dough and bread fractions as follows. Samples (75'0-100'0 mg) were incubated at 96°C for 90 min with Termamyl 120L (100 fll) in 0·1 M Na acetate buffer (pH 5; 8 ml) using glass tubes fitted with Teflon-lined screw caps. After centrifugation (2000 x g, 15 min) and removal of the supernatant with a Pasteur pipette using vacuum suction, the residue was once more treated with Termamyl (50 Ill) and Na acetate buffer as above. The samples were then cooled, incubated with amyloglucosidase (100 fll) at 60°C overnight in a shaking water-bath, centrifuged and washed thoroughly with water (3 x 8 ml), taking care to remove all the supernatant from the last centrifugation. The residues obtained were mixed with 2 M KOH (2 ml) and incubated for 30 min at room temperature with occasional mixing. The suspension was neutralised by adding 0,4 M Na acetate
POLYSACCHARIDES IN WHITE BREAD
151
buffer (pH 4'8, I ml) and 2 M HCI (2 ml), followed by incubation with amyloglucosidase (100 Jll) as above and quantitative transfer to a 25·0 ml volumetric flask. A portion (about 5 ml) of the diluted suspension was centrifuged and a portion (1,0 ml) of the supernatant was treated with the glucose-oxidase reagent. The absorbance was measured at 510 nm and resistant starch was calculated as glucose content x 0·9.
Analysis of non-starch polysaccharides Methods previously developed 18 in this laboratory for analysis of dietary fibre, i.e. non-starch polysaccharides and lignin, were used. In brief, a part of each aqueous alcohol extracted residue (3'00 g) was treated with Termamyl 120L at 96°C for 30 min in 0·1 M Na acetate buffer (pH 5'0) and then with amyloglucosidase at 60°C overnight. The insoluble fibre fraction was isolated by centrifugation and the soluble fibre fraction was recovered by dialysis and freeze-drying of the resulting supernatant. The content and composition of neutral polysaccharide constituents in the respective fractions were determined by GLC of alditol acetates. The amount of non-starch glucans in insoluble fibre fractions was obtained by subtracting values for starch remaining (soluble in 2 M aqueous alkali and analysed enzymically essentially according to Bjorck et al. 19 ) from corresponding GLC-glucan values. Results and Discussion
Baking of white bread was performed at 210 °C for 22 (experiment A) or 35 min (experiment B). The breads were subsequently separated by hand into outer crust, inner crust and crumb fractions. Statistical evaluation of this fractionation procedure showed that it could be reproducibly performed 1.5 • This experimental design also enabled the extent of reactions occurring in the neutral polysaccharides during baking to be determined. Changes in uronic acid residues and lignin were not followed since these components were present in small amounts (less than 0·1 %).
Effects of baking on starch The levels of resistant starch were lower in the outer crust than in the inner crust, which in turn were lower than in the crumb (Table I). This type of modified starch requires dispersion in alkali 2 or dimethylsulphoxide 3 to render it susceptible to hydrolysis by amylolytic enzymes. The levels observed for resistant starch were the same in both baking experiments but slightly lower than the values previously20 observed in crumb (1·1-1·2 %) and crust (0'6-0,8 %) from wheat bread. It is reasonable to assume that this difference is mainly due to the fact that resistant starch in the present study was determined directly on original dough and bread samples and not on isolated fibre fractions. The latter analytical approach is inevitably associated with a risk of genera tion of some resistant starch during the isolation procedure. The decreasing starch contents, which occurred in order crumb to outer crust in each baking experiment, were not completely accounted for by corresponding formation of resistant starch, since the total value of starch and resistant starch also decreased progressively. This suggests that some of the starch may have been transformed chemically to other glucan structures, which are at least partly resistant to amylolytic enzymes, even after solubilisation in alkali. The formation of such chemically modified
152
E. WESTERLUND ET AL.
TABLE 1. Contents of neutral polysaccharides in dough and bread fractions Neutral non-starch polysaccharide residues
Sample and baking conditions
Resistant Fraction yield Starch starch Glucose
Arabinose Mannose and and xylose galactose
Dough
laO
75-6
0·09
0·60
1·92
0'55
Experiment A, 210 °C for 22 min Crumb Inner crust Outer crust Total (whole bread)
50 38 12 100
75·1 74'6 74'4 74·8
0·95 0·65 0·30 0'76
0·83 0·81 0·95 0'84
1·78 1'86 1·69 1·80
0·56 0·54 0·50 0'55
44
75·8 73'4 72-8 74'4
1·02 0·62 0·30 0·76
0·68 0·75 0·86 0·73
1-67 1-67 1'56 1-66
0·50 0·52 0·49 0'51
Experiment B, 210 °C for 35 min Crumb Inner crust Outer crust Total (whole bread)
44 12
laO
Values are expressed as gj 100 g fraction (dry basis). starch may be of nutritional importance. It is not known, however, to what extent this component is available for digestion. Fragmentation and transglycosidation reactions of starch, giving rise to new glucan structures, are known to be accelerated by heat and dry conditions, as in production of technical dextrins 4 • Thus, it was to be expected that the levels of non-starch glucans observed for the outer crust were significantly higher than in crumb and unheated dough. The previous finding 14 that glucosaccharides with end units of p-I,6-anhydro-Dglucopyranose, and which arise by thermal fragmentation of starch, were present in an 80 % aqueous ethanolic extract of outer crust (experiment B) is also in agreement with the present observation.
Effects of baking on arabinoxylans The amount of arabinoxylans, measured as the sum of arabinose and xylose residues, was slightly lower in bread (0'12-0'26 % units) than in dough (Table I) suggesting the occurrence of degradation reactions during baking. This trend was particularly noticeable in the outer crust fractions. The content of Na acetate buffer-insoluble arabinoxylans decreased significantly during baking (Fig. I). The decrease was evidently related to the extent of heat-treatment as it was largest for fractions of outer crust and also became more apparent with the use of the longer baking time (experiment B). It cannot be taken for granted, however, that the decrease in arabinose residues arises solely from effects on arabinoxylans, since it is known 1o that arabinogalactans also are present in wheat flour. The increasing content of Na acetate buffer extractable arabinoxylans in the order crumb to outer crust in both experiments (Fig. 2) showed that
153
POLYSACCHARIDES IN WHITE BREAD
0·50 0~ 0+-
r::
Ql
0+-
r::
0
U
0·25
Dough
Crumb
Inner crust
Outer crust
Experiment A
Crumb
Inner crus!
Ouier crus!
Experiment B
FIGURE 1. Na acetate buffer-insoluble arabinose and xylose contents of dough and bread fractions. The results are expressed as g anhydrosugar/100 g fraction (dry basis). Experiment A is for bread baked at 210 °C for 22 min and experiment B for bread baked at 210 °C for 35 min. IZl Xylose; 0 arabinose.
0·50
0~ 0+-
r::
Ql
0+-
r::
0
u
0'25
Dough
Crumb
Inner crust Outer crust Experiment A
FIGURE 2. Na acetate buffer-extractable arabinose and xylose contents of dough and bread fractions. The results are expressed as g anhydrosugar/IOO g fraction (dry basis). Experiments A and B as described in Fig. 1. [2J Xylose; 0 arabinose.
E. WESTERLUND ET AL.
154
~ 0·20 +<::
$:! <::
o
U
0'10
o
Dough
Crumb.
Inner crust Outer crust
I
Crumb
Inner crust Outer crust
!
Experiment B
Experiment A
FIGURE 3. Contents of Na acetate buffer-extractable, hexose-containing polysaccharides in dough and bread fractions. The results are expressed as g anhydrohexosejlOO g fraction (dry basis). Experiments A and B as described in Fig. I. I2J Mannose; • glucose; 0 galactose. 0·75,------------------------------,
~
0·50
'1: OJ '1: 0 u 0·25
o
Dough
Crumb
Inner crust Experiment A
Outer crust
Crumb
Inner crust
Outer crust
r
Experiment B
FIGURE 4. Contents of Na acetate buffer-insoluble, hexose-containing polysaccharides in dough and bread fractions. The results are expressed as g anhydrohexose/lOO g fraction (dry basis). Experiments A and B as described in Fig. 1. IZI Mannose; • glucose; 0 galactose.
a redistribution from insoluble to extractable arabinoxylans had occurred as a result of increased heat-treatment. Similar thermal effects have been observed previously21 for industrially heat-processed products of wheat flour. It has also been found 22 that the content of soluble fibre increased during baking of white bread. The lower content of Na acetate buffer-extractable arabinose and xylose residues found in some bread fractions compared with the dough might be due to thermal depolymerisation of arabinoxylans, followed by leakage of polysaccharide fragments during the dialysis used in isolation of the water-soluble fibre fraction. Another possible explanation is that partial hydrolysis of acid-labile arabinofuranosidic linkages, leading
POLYSACCHARIDES IN WHITE BREAD
155
to loss of arabinose residues, had occurred. Detailed structural studies of pentosans extractable with water from the various bread fractions are in progress. Effects of baking on hexose-containing polysaccharides
Despite the fact that several polysaccharides were present in the fractions studied, some conclusion could be drawn based on the amounts of individual sugar residues observed. Thus, the content of water-soluble glucose residues (Fig. 3) was significantly higher in outer crust fractions compared with dough. This suggested that formation ofNa acetate buffer-extractable, non-starch glucans had occurred through fragmentation/transglycosidation reactions of starch. The content of extractable mannose residues was slightly decreased by baking, whereas the amount of extractable galactose residues decreased substantially in the order crumb to crust and was also lower than in dough. The latter finding suggested that a Na acetate buffer-extractable water-soluble polymer containing galactose residues was rendered partly insoluble as a result of more severe thermal treatment. This was confirmed by the observation that there was a higher level of insoluble galactose residues in bread fractions compared with dough (Fig. 4). A waterextractable arabinogalactan linked to protein is known to be present in wheat f1our 10 , and it may be that denaturation of the protein part of this glycoprotein is responsible for the effects observed. The values observed for the contents of insoluble mannose residues in dough and bread fractions (Fig. 4) were more than twice as high as that in the original flour (0'09 %), most likely due to the presence of yeast mannans. The content ofNa acetate buffer-insoluble glucose residues, corrected for the presence of starch soluble in 2 M aqueous alkali, was significantly higher in baked fractions than in the dough. This suggested that the formation of non-starch glucans by thermal reactions of starch had taken place during baking as already discussed. Conclusions
The present study indicates that arabinoxylans were rendered partly extractable in Na acetate buffer during baking and were increasingly lost at higher baking temperature. The non-starch glucan content, on the other hand, increased slightly in bread compared with dough presumably due to chemical modification of starch to new glucan structures that, at least in part, resist amylolytic digestion. These polysaccharide modifications may have nutritional implications and significant effects on bread properties. References 1. Berry, C. S. J. Cereal Sci. 4 (1986) 301-314. 2. 3. 4. 5. 6.
Englyst, H., Wiggins, H. S. and Cummings, 1. H. Analyst 107 (1982) 307-318. Englyst, H. and Cummings, J. H. Analyst 109 (1984) 937-942. Greenwood, C. T. Adv. Carbohydr. Chern. 22 (1967) 483-515. Theander, O. and Westerlund, E. Sttirke 39 (1987) 88-93. Siljestrom, M., Bj6rck, 1. and Westerlund, E. Starke 41 (1989) 95-100.
156
E. WESTERLUND ET AL.
7. Amado, R. and Neukom, H. in 'New Approaches to Research on Cereal Carbohydrates' (R. D. Hill and L. Munck, eds.), Elsevier Science Publishers, Amsterdam (1985) pp 241-251. 8. Hoseney, R. C. Food Technol. (London) 38 No.1 (1984) 114-117. 9. Kim, S. K. and D'Applonia, B. L. Cereal Chern. 54 (1977) 225-229. 10. Neukom, H. Lebensrn. Wiss. Technol. 9 (1976) 143-148. 11. Meuser, F. and Suckow, P. in 'Chemistry and Physics of Baking' (J. M. V. Blanshard, ed.), Royal Society, London (1986) pp 43-61. 12. D'Applonia, B. L. Cereal Chern. 50 (1973) 27-36. 13. Mares, D. J. and Stone, B. A. Aust. J. Bioi. Sci. 26 (1973) 793-812. 14. Henry, R. J. J. Sci. Food Agric. 36 (1985) 1243-1253. 15. Westerlund, E., Theander, O. and Aman, P. J. Cereal Sci. 10 (1989) 139-147. 16. Salomonsson, A.-C., Theander, O. and Westerlund, E. Swed. J. Agric. Res. 14, Ill-ll7. 17. Aman, P., Westerlund, E. and Theander, O. Methods Carbohydr. Chern. X, 1989, in press. 18. Theander, O. and Westerlund, E. J. Agric. Food Chern. 34 (1986) 330-336. 19. Bjorck, I., Nyman, M., Pedersen, B., Siljestrom, M., Asp, N.-G. and Eggum, B. O. J. Cereal Sci. 4 (1986) 1-1 I. 20. Johansson, C.-G., Siljestrom, M. and Asp, N.-G. Z. Lebensrn. Unters. Forsch. 179 (1984) 24-28. 21. Siljestrom, M., Westerlund, E., Bjorck, 1., Holm, J., Asp, N.-G. and Theander, O. J. Cereal Sci. 4 (1986) 315-323. 22. Ranhotra, G. and Gelroth, J. Cereal Chern. 65 (1988) 155-156.
Effects of Baking on Polysaccharides in White Bread Fractions E. WESTERLUND, O. THEANDER, R. ANDERSSON and P. AMAN Swedish University of Agricultural Sciences, Department of Chemistry, P.O. Box 7015, S-750 07 Uppsala, Sweden Received 26 July 1988
White bread was baked at 210 °C for 22 or 35 min and divided into outer crust, inner crust and crumb fractions. The content and composition of water-soluble and insoluble dietary fibre polysaccharides were determined in these fractions and the dough by using a gas-liquid chromatographic method. Corresponding values for starch and •resistant' starch were obtained by enzymic methods. Water-insoluble arabinoxylans were rendered partly extractable in Na acetate buffer during baking, whereas the arabinogalactan became less extractable. The total amount of arabinoxylans decreased in the order crumb to outer crust probably due to increasing degradation caused by more severe thermal conditions. The starch values also decreased in the same order due to formation of resistant and probably to chemically modified forms of starch. The former reaction prevailed in crumb, whereas the latter reaction caused a significant increase in non-starch glucans in outer crust. These polysaccharide modifications resulted in increased non-starch glucan content and decreased pentasan content in bread compared with dough.
Introduction
Various reactions of polysaccharides that take place during the baking of bread have significant effects on product structure and quality. Formation of retrograded starch, which is resistant to amylolytic degradation (' resistant starch '), as a result of gelatinisation and cooling, is well-documented 1-3, for example. Other reactions of starch, which can be envisaged as a result of baking, but which have not been studied hitherto, are formation of glucosaccharides by rupture of intramolecular linkages in starch 4 - s. These glucosaccharides possess end units of ~-1,6-anhydro-glucopyranose,and they may in turn react further by transglycosidation with hydroxyl groups in starch or other carbohydrate components present. Branched glucan structures formed in this manner represent an irreversible chemical modification of starch, increasing the non-starch glucan content of dietary fibre 5 • Pentosans, including arabinoxylans and arabinogalactans, are the main components among dietary fibre polysaccharides in wheat flour', and they have several functional properties that are important in baking 8 • For example, water-soluble pentosans may have beneficial effects on bread volume, whereas water-insoluble arabinoxylans have Abbreviation used: OLe
= gas-liquid chromatography.
0733-5210/89/050149+08 $03.00/0
© 1989 Academic Press Limited
150
E. WESTERLUND ET AL.
been reported to decrease the rate of bread staling 9 , Although several structural studies of pentosans in wheat flour have been performed6, 10. 11, little information is available on these polysaccharides in bread 12 , Other polysaccharides present in wheat in significant amounts are glucomannans 13 , cellulose and ~-glucans14, The present paper on breadmaking is focused on modifications that are induced in the starch and non-starch polysaccharide fractions during the baking of white bread. The study was undertaken to evaluate effects on these components in bread fractions as a result of different baking conditions.
Experimental General methods Solutions were concentrated under reduced pressure at bath temperatures not exceeding 40°C, or by freeze-drying. All samples were milled to pass a 0·8 mm screen. Dry matter was determined by oven drying at lOS °C for 18 h. Gas-liquid chromatography (GLC) was conducted on a Packard 427 apparatus equipped with a flame-ionisation detector. Alditol acetates were analysed on a fused-silica CpSil-88 column (9'5 m x 0·25 mm Ld., helium flow approximately 1 m/s) programmed from 170 to 220°C at 4°C/min. All analyses were made at least in duplicate.
Materials The dough and bread samples used were taken from a previous study15, in which white bread had been baked at 210°C for 22 or 35 min (experiments A and B, respectively). The breads had been separated by hand into three fractions on the basis of visible differences in their texture. These fractions, denoted as outer crust, inner crust and crumb, and the corresponding dough, had been extracted successfully by reflux with hexane and 80 % (vIv) aqueous ethanol. The resulting aqueous alcohol extracted residues were analysed in this study to determine the contents and compositions of the polysaccharides present. Bacterial alpha-amylase (Termamyl 120 L, BC 3.2.1.1, Novo A/S, Copenhagen), and amyloglucosidase (14 U/mg, BC 3.2.1. 3, Boehringer, Mannheim) were used for hydrolysis of starch. The glucose-peroxidase reagent used for determination of glucose (Merckotest 3395, Darmstadt) was obtained from Merck.
Determination of starch and' resistant' starch Starch was analysed in the aqueous alcohol extracted residues (40'0-50'0 mg) using an enzymic method 16 ,17 involving hydrolysis with thermostable alpha-amylase (Termamyl 120 L) and amyloglucosidase, followed by determination of the glucose released by a glucose-oxidase reagent. Determination of resistant starch was performed on original unextracted dough and bread fractions as follows. Samples (75'0-100'0 mg) were incubated at 96°C for 90 min with Termamyl 120L (100 fll) in 0·1 M Na acetate buffer (pH 5; 8 ml) using glass tubes fitted with Teflon-lined screw caps. After centrifugation (2000 x g, 15 min) and removal of the supernatant with a Pasteur pipette using vacuum suction, the residue was once more treated with Termamyl (50 Ill) and Na acetate buffer as above. The samples were then cooled, incubated with amyloglucosidase (100 fll) at 60°C overnight in a shaking water-bath, centrifuged and washed thoroughly with water (3 x 8 ml), taking care to remove all the supernatant from the last centrifugation. The residues obtained were mixed with 2 M KOH (2 ml) and incubated for 30 min at room temperature with occasional mixing. The suspension was neutralised by adding 0,4 M Na acetate
POLYSACCHARIDES IN WHITE BREAD
151
buffer (pH 4'8, I ml) and 2 M HCI (2 ml), followed by incubation with amyloglucosidase (100 Jll) as above and quantitative transfer to a 25·0 ml volumetric flask. A portion (about 5 ml) of the diluted suspension was centrifuged and a portion (1,0 ml) of the supernatant was treated with the glucose-oxidase reagent. The absorbance was measured at 510 nm and resistant starch was calculated as glucose content x 0·9.
Analysis of non-starch polysaccharides Methods previously developed 18 in this laboratory for analysis of dietary fibre, i.e. non-starch polysaccharides and lignin, were used. In brief, a part of each aqueous alcohol extracted residue (3'00 g) was treated with Termamyl 120L at 96°C for 30 min in 0·1 M Na acetate buffer (pH 5'0) and then with amyloglucosidase at 60°C overnight. The insoluble fibre fraction was isolated by centrifugation and the soluble fibre fraction was recovered by dialysis and freeze-drying of the resulting supernatant. The content and composition of neutral polysaccharide constituents in the respective fractions were determined by GLC of alditol acetates. The amount of non-starch glucans in insoluble fibre fractions was obtained by subtracting values for starch remaining (soluble in 2 M aqueous alkali and analysed enzymically essentially according to Bjorck et al. 19 ) from corresponding GLC-glucan values. Results and Discussion
Baking of white bread was performed at 210 °C for 22 (experiment A) or 35 min (experiment B). The breads were subsequently separated by hand into outer crust, inner crust and crumb fractions. Statistical evaluation of this fractionation procedure showed that it could be reproducibly performed 1.5 • This experimental design also enabled the extent of reactions occurring in the neutral polysaccharides during baking to be determined. Changes in uronic acid residues and lignin were not followed since these components were present in small amounts (less than 0·1 %).
Effects of baking on starch The levels of resistant starch were lower in the outer crust than in the inner crust, which in turn were lower than in the crumb (Table I). This type of modified starch requires dispersion in alkali 2 or dimethylsulphoxide 3 to render it susceptible to hydrolysis by amylolytic enzymes. The levels observed for resistant starch were the same in both baking experiments but slightly lower than the values previously20 observed in crumb (1·1-1·2 %) and crust (0'6-0,8 %) from wheat bread. It is reasonable to assume that this difference is mainly due to the fact that resistant starch in the present study was determined directly on original dough and bread samples and not on isolated fibre fractions. The latter analytical approach is inevitably associated with a risk of genera tion of some resistant starch during the isolation procedure. The decreasing starch contents, which occurred in order crumb to outer crust in each baking experiment, were not completely accounted for by corresponding formation of resistant starch, since the total value of starch and resistant starch also decreased progressively. This suggests that some of the starch may have been transformed chemically to other glucan structures, which are at least partly resistant to amylolytic enzymes, even after solubilisation in alkali. The formation of such chemically modified
152
E. WESTERLUND ET AL.
TABLE 1. Contents of neutral polysaccharides in dough and bread fractions Neutral non-starch polysaccharide residues
Sample and baking conditions
Resistant Fraction yield Starch starch Glucose
Arabinose Mannose and and xylose galactose
Dough
laO
75-6
0·09
0·60
1·92
0'55
Experiment A, 210 °C for 22 min Crumb Inner crust Outer crust Total (whole bread)
50 38 12 100
75·1 74'6 74'4 74·8
0·95 0·65 0·30 0'76
0·83 0·81 0·95 0'84
1·78 1'86 1·69 1·80
0·56 0·54 0·50 0'55
44
75·8 73'4 72-8 74'4
1·02 0·62 0·30 0·76
0·68 0·75 0·86 0·73
1-67 1-67 1'56 1-66
0·50 0·52 0·49 0'51
Experiment B, 210 °C for 35 min Crumb Inner crust Outer crust Total (whole bread)
44 12
laO
Values are expressed as gj 100 g fraction (dry basis). starch may be of nutritional importance. It is not known, however, to what extent this component is available for digestion. Fragmentation and transglycosidation reactions of starch, giving rise to new glucan structures, are known to be accelerated by heat and dry conditions, as in production of technical dextrins 4 • Thus, it was to be expected that the levels of non-starch glucans observed for the outer crust were significantly higher than in crumb and unheated dough. The previous finding 14 that glucosaccharides with end units of p-I,6-anhydro-Dglucopyranose, and which arise by thermal fragmentation of starch, were present in an 80 % aqueous ethanolic extract of outer crust (experiment B) is also in agreement with the present observation.
Effects of baking on arabinoxylans The amount of arabinoxylans, measured as the sum of arabinose and xylose residues, was slightly lower in bread (0'12-0'26 % units) than in dough (Table I) suggesting the occurrence of degradation reactions during baking. This trend was particularly noticeable in the outer crust fractions. The content of Na acetate buffer-insoluble arabinoxylans decreased significantly during baking (Fig. I). The decrease was evidently related to the extent of heat-treatment as it was largest for fractions of outer crust and also became more apparent with the use of the longer baking time (experiment B). It cannot be taken for granted, however, that the decrease in arabinose residues arises solely from effects on arabinoxylans, since it is known 1o that arabinogalactans also are present in wheat flour. The increasing content of Na acetate buffer extractable arabinoxylans in the order crumb to outer crust in both experiments (Fig. 2) showed that
153
POLYSACCHARIDES IN WHITE BREAD
0·50 0~ 0+-
r::
Ql
0+-
r::
0
U
0·25
Dough
Crumb
Inner crust
Outer crust
Experiment A
Crumb
Inner crus!
Ouier crus!
Experiment B
FIGURE 1. Na acetate buffer-insoluble arabinose and xylose contents of dough and bread fractions. The results are expressed as g anhydrosugar/100 g fraction (dry basis). Experiment A is for bread baked at 210 °C for 22 min and experiment B for bread baked at 210 °C for 35 min. IZl Xylose; 0 arabinose.
0·50
0~ 0+-
r::
Ql
0+-
r::
0
u
0'25
Dough
Crumb
Inner crust Outer crust Experiment A
FIGURE 2. Na acetate buffer-extractable arabinose and xylose contents of dough and bread fractions. The results are expressed as g anhydrosugar/IOO g fraction (dry basis). Experiments A and B as described in Fig. 1. [2J Xylose; 0 arabinose.
E. WESTERLUND ET AL.
154
~ 0·20 +<::
$:! <::
o
U
0'10
o
Dough
Crumb.
Inner crust Outer crust
I
Crumb
Inner crust Outer crust
!
Experiment B
Experiment A
FIGURE 3. Contents of Na acetate buffer-extractable, hexose-containing polysaccharides in dough and bread fractions. The results are expressed as g anhydrohexosejlOO g fraction (dry basis). Experiments A and B as described in Fig. I. I2J Mannose; • glucose; 0 galactose. 0·75,------------------------------,
~
0·50
'1: OJ '1: 0 u 0·25
o
Dough
Crumb
Inner crust Experiment A
Outer crust
Crumb
Inner crust
Outer crust
r
Experiment B
FIGURE 4. Contents of Na acetate buffer-insoluble, hexose-containing polysaccharides in dough and bread fractions. The results are expressed as g anhydrohexose/lOO g fraction (dry basis). Experiments A and B as described in Fig. 1. IZI Mannose; • glucose; 0 galactose.
a redistribution from insoluble to extractable arabinoxylans had occurred as a result of increased heat-treatment. Similar thermal effects have been observed previously21 for industrially heat-processed products of wheat flour. It has also been found 22 that the content of soluble fibre increased during baking of white bread. The lower content of Na acetate buffer-extractable arabinose and xylose residues found in some bread fractions compared with the dough might be due to thermal depolymerisation of arabinoxylans, followed by leakage of polysaccharide fragments during the dialysis used in isolation of the water-soluble fibre fraction. Another possible explanation is that partial hydrolysis of acid-labile arabinofuranosidic linkages, leading
POLYSACCHARIDES IN WHITE BREAD
155
to loss of arabinose residues, had occurred. Detailed structural studies of pentosans extractable with water from the various bread fractions are in progress. Effects of baking on hexose-containing polysaccharides
Despite the fact that several polysaccharides were present in the fractions studied, some conclusion could be drawn based on the amounts of individual sugar residues observed. Thus, the content of water-soluble glucose residues (Fig. 3) was significantly higher in outer crust fractions compared with dough. This suggested that formation ofNa acetate buffer-extractable, non-starch glucans had occurred through fragmentation/transglycosidation reactions of starch. The content of extractable mannose residues was slightly decreased by baking, whereas the amount of extractable galactose residues decreased substantially in the order crumb to crust and was also lower than in dough. The latter finding suggested that a Na acetate buffer-extractable water-soluble polymer containing galactose residues was rendered partly insoluble as a result of more severe thermal treatment. This was confirmed by the observation that there was a higher level of insoluble galactose residues in bread fractions compared with dough (Fig. 4). A waterextractable arabinogalactan linked to protein is known to be present in wheat f1our 10 , and it may be that denaturation of the protein part of this glycoprotein is responsible for the effects observed. The values observed for the contents of insoluble mannose residues in dough and bread fractions (Fig. 4) were more than twice as high as that in the original flour (0'09 %), most likely due to the presence of yeast mannans. The content ofNa acetate buffer-insoluble glucose residues, corrected for the presence of starch soluble in 2 M aqueous alkali, was significantly higher in baked fractions than in the dough. This suggested that the formation of non-starch glucans by thermal reactions of starch had taken place during baking as already discussed. Conclusions
The present study indicates that arabinoxylans were rendered partly extractable in Na acetate buffer during baking and were increasingly lost at higher baking temperature. The non-starch glucan content, on the other hand, increased slightly in bread compared with dough presumably due to chemical modification of starch to new glucan structures that, at least in part, resist amylolytic digestion. These polysaccharide modifications may have nutritional implications and significant effects on bread properties. References 1. Berry, C. S. J. Cereal Sci. 4 (1986) 301-314. 2. 3. 4. 5. 6.
Englyst, H., Wiggins, H. S. and Cummings, 1. H. Analyst 107 (1982) 307-318. Englyst, H. and Cummings, J. H. Analyst 109 (1984) 937-942. Greenwood, C. T. Adv. Carbohydr. Chern. 22 (1967) 483-515. Theander, O. and Westerlund, E. Sttirke 39 (1987) 88-93. Siljestrom, M., Bj6rck, 1. and Westerlund, E. Starke 41 (1989) 95-100.
156
E. WESTERLUND ET AL.
7. Amado, R. and Neukom, H. in 'New Approaches to Research on Cereal Carbohydrates' (R. D. Hill and L. Munck, eds.), Elsevier Science Publishers, Amsterdam (1985) pp 241-251. 8. Hoseney, R. C. Food Technol. (London) 38 No.1 (1984) 114-117. 9. Kim, S. K. and D'Applonia, B. L. Cereal Chern. 54 (1977) 225-229. 10. Neukom, H. Lebensrn. Wiss. Technol. 9 (1976) 143-148. 11. Meuser, F. and Suckow, P. in 'Chemistry and Physics of Baking' (J. M. V. Blanshard, ed.), Royal Society, London (1986) pp 43-61. 12. D'Applonia, B. L. Cereal Chern. 50 (1973) 27-36. 13. Mares, D. J. and Stone, B. A. Aust. J. Bioi. Sci. 26 (1973) 793-812. 14. Henry, R. J. J. Sci. Food Agric. 36 (1985) 1243-1253. 15. Westerlund, E., Theander, O. and Aman, P. J. Cereal Sci. 10 (1989) 139-147. 16. Salomonsson, A.-C., Theander, O. and Westerlund, E. Swed. J. Agric. Res. 14, Ill-ll7. 17. Aman, P., Westerlund, E. and Theander, O. Methods Carbohydr. Chern. X, 1989, in press. 18. Theander, O. and Westerlund, E. J. Agric. Food Chern. 34 (1986) 330-336. 19. Bjorck, I., Nyman, M., Pedersen, B., Siljestrom, M., Asp, N.-G. and Eggum, B. O. J. Cereal Sci. 4 (1986) 1-1 I. 20. Johansson, C.-G., Siljestrom, M. and Asp, N.-G. Z. Lebensrn. Unters. Forsch. 179 (1984) 24-28. 21. Siljestrom, M., Westerlund, E., Bjorck, 1., Holm, J., Asp, N.-G. and Theander, O. J. Cereal Sci. 4 (1986) 315-323. 22. Ranhotra, G. and Gelroth, J. Cereal Chern. 65 (1988) 155-156.