J . Sci. Food Agric. 1987,39,185-194

Flow Characteristics and the Water Retention Properties of Wheat Bran

Alan Anderson and Martin A. Eastwood

Wolfson Laboratories, Gastrointestinal Unit, Department of Medicine, Western General Hospital, Edinburgh EH4 2XU, UK

(Received 7 April 1986; revised manuscript accepted 24 October 1986)

ABSTRACT

A method is described which enables cereal fibre to be categorised by the rate of flow of water when packed into a column. This flow-rate method enables changes in physical characteristics of fibre, predictive of faecal bulking action, to be readily identified.

Flow rate through coarse wheat bran was 200 ml h-' and 120 ml h-' for fine wheat bran.

Heating wheat bran (4PC, 70°C) has an irreversible enhancing effect on flow rates. The initial ranking of the brans (coarse greatest, fine least) remains consistent despite changes resulting from cooking.

Flow rates are affected by ionic strength of the eluting solutions. A 6 M urea elution reduces the flow rate and this reduction is reversed by eluting with lower ionic strength solutions.

Key words: Water retention properties, wheat bran.

1 INTRODUCTION

The amount and type of fibre in the diet have been considered to be significant factors in altering stool weight and other parameters.' A number of trials have shown that cereal bran is a more effective agent for increasing stool weight than fibre from fruit and vegetable^.^.^

There is a need for reproducible in-vitro methods to classify fibres according to their influence on the human gastrointestinal tract.4 Such predictions can be expressed as physical and chemical characteristics. These are difficult to measure and can vary with the source and mode of preparation of the fibre. There is

185

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186 A . Anderson, M . A . Eastwood

evidence that bacterial activity in the colon5 can modify or totally hydrolyse the structure of dietary fibre, particularly fruit and vegetable fibre. This makes prediction of potential effects on the colon and stool weight a difficult, and as yet an unresolved, task. The effect of wheat bran on the gastrointestinal tract is most conspicuously in the colon, increasing stool weight .6 Wheat bran appears to be much less modified by bacterial action, and the initial water-holding capacity is a reasonable indicator of a potential to increase stool eight.^ Particle size8 and cooking9 wheat bran alters its ability to influence stool weight. Raw wheat bran is not very edible and hence is often made up into more palatable forms. These preparations may well alter the stool bulking ability of such wheat bran. This paper describes flow characteristics and water retention properties of wheat bran, which change with the mode of preparation of the wheat bran.

2 EXPERIMENTAL

2.1 Materials

French soft wheat brans of coarse (FC) and fine (FF) particle size, and coarse Canadian Red Spring wheat (RSW) bran were obtained from Chancelot Mills, Edinburgh. A standardised bran (AACC) from the American Association of

TABLE 1 Analysis of Wheat Brans

Chemical Coarse wheat bran

Starch Pectic substances Hemicellulose Cellulose Neutral detergent

fibre Acid detergent

fibre Lignin Sieving tests AperGre (mm)

4.0 3.55 2.80 1.50 0.75 0.50 0.36 <

Canadian French

(% polymer, DM) RSW

17.5 25.7 0.44 0.35

22.8 16-3 9.7 8.2

4.7 4-0

- - 1 .o 0.5 4.1 3.0

46.1 41.9 31.5 32.9 17.3 21.7 - -

Fine wheat AACC bran bran

French (% polymer, DM)

35.5 15.0 0.34 -

16.9 - 7.9

42.5 -

11.1

4-3

- 0.5 12.4

31.9 57.2 44.0 13.2 16.2 13.2 7.4 3.9

Flow characreristics and water retention of wheat bran 187

Cereal Chemists, St Paul, MN, USA, was judged to be of fine particle size (Table

The chemical composition of all brans was known,6 and is shown in Table 1. The RSW, FC and FF wheat brans were estimated by the Southgate method,1° the AACC by the Van Soest method."

2.2 Methods

Before each experiment, each bran was shaken overnight on a 125 p m sieve placed on an orbital shaker to remove loose starch fines. Where the fibres were to be hydrated, they were soaked in a ten-fold excess of liquid (by volume) for a predetermined time of approximately 18 h. Such a soaking has no effect on water- holding capacity (WHC).' Sodium azide (0.1 g litre-l), which has no effect on WHC,* was added to all hydrating solutions as an anti-bacterial agent.

The effect of ionic concentration was studied using NaCl (0.3-1.2%) and an aqueous solution of urea (6 M, which abolishes hydrophobic bondings in a poly- meric system.

1) *

2.2.1 Flow-rate measurements Flow rates were determined at room temperature using Pharmacia K9/30 acrylic plastic chromatography columns of dimension 300x 8 mm (Pharmacia, Uppsala, Sweden) with 10-pm nets at both ends. Hydrated fibre preparations were packed into the columns after degassing for 20 min using suction. A packing funnel at the top of the column ensured that the pre-soaked fibre did not come into contact with air as this was found to introduce gas into the column, which affected the flow rate. A pressure difference of 1 m (lo4 Pa) was used to run solutions through the packed columns by ascending flow, which avoided compaction of the fibre. A constant operating pressure was maintained over the columns using a 51 Mariotte flask which supplied all the columns. Polyethylene tubing (1 mm diameter) was used throughout.

The column effluents were collected in 11 conical flasks over a 1 h period at room temperature (18-20°C). Measurements were made continuously for 7-9 h, and flow rate was expressed as mean hourly flow. After each series of measure- ments, the nets were cleaned by sonication.

After the flow experiments the fibre from the columns was emptied on to a Buchner funnel (Whatman No. 1 filter paper), washed three times with tap water, and dried using suction pressure before being placed in tared beakers to deter- mine the wet weight. The beakers were placed overnight in a force-draught oven at 70°C, and the weight of dry fibre determined.

When ionic solutions were used to elute the columns, they were made up from sodium chloride or urea (Analar, BDH) at room temperature.

2.2.2 Measurement of water-holding capacity WHC was determined by the centrifuging method of Robertson and E a s t w ~ o d . ~ Samples of hydrated fibre estimated to be between 0.3 and 0-5 g dry weight were transferred to tared 25-ml polypropylene centrifuge tubes. After equilibration for 1 h the tubes were centrifuged at 6000Xg for 15 min (MSE Super Minor centri-

188 A . Anderson, M. A . Eastwood

LOO. FLOW RATE

I ml. h-11

300-

2 0 0

100-

fuge, Fisons, Crawley, UK), supernatants and any loosely associated fibre were discarded and the tubes inverted over filter paper to drain for 30 min. The wet weight of the spun fibre was determined before freeze-drying. WHC was expressed as water held g-' dry fibre for a mean of five determinations, unless otherwise stated. This value includes moisture present before hydration, and disregards water loosely associated with the fibre, which is removable by centrifuging. When WHC was determined concomitantly with flow rate, the same batch of hydrated fibre was used.

3 RESULTS

Figure 1 shows the variation in flow rates through four brans over a 7 h period. A trend of decreasing flow rate with time was found in the majority of experiments, and was presumed to result from partial blockage, by fines, of the net at the top of each column. No significant difference in flow rate was found when FC bran was packed into columns of differing size ( 5 0 ~ 0 . 9 cm and 3 0 ~ 1 . 5 cm). Flow was assumed to be a function of fibre type, and not affected significantly by the packing densities used in these experiments. In a previous experiment a mean flow rate of 468 (SEk11) ml h-' was afforded through a column of FC bran with wet and dry weights 4.3 g and 1.3 g, whilst the measured rate through the same type of bran of weights 5.1 g and 1.4 g, respectively, was 463 (SEf6) ml h-l. In all subsequent experiments the dry weight of fibre in the columns was determined at the end of the flow-rate measurement, and no relationship between flow and density could be established.

The four brans were removed from the columns and dried overnight in a force-

* 1 2 3 4 5 6 7

01

HOURLY INTERVALS

Fig. 1. Flow rate (ml h-1) through empty column, 0, or packed with bran AACC, 0; French Coarse (FC), 0; Canadian Red Spring Wheat (RSW), U; or French Fine (FF), A , at hourly intervals.

Flow characteristics and water retention of wheat bran

500.

Loo.

300-

FLOW RATE (rnLh-')

189

J AACC

I? FC

I

lLrL

I

RSW d FF

RAW L O Y 70°C RAW 40°C 70°C RAW W C 70°C RAW 40°C 7 0 T

Fig. 2. The effect of drying and cooking on flow rates (ml h-1) through bran packed in columns AACC, French Coarse (FC), Canadian Red Spring Wheat (RSW) and French Fine (FF).

draught oven at 40°C before rehydration with water and repacking. To investigate the effect of dry cooking on flow, the brans were removed and dried at 70°C before rehydration and flow measurement. This gave a set of mean flow rates for four brans after three preparatory regimes; results are shown in Fig. 2. Dry cooking has an irreversible enhancing effect on the flow rates through the brans. The ranking order of flow rates remained consistent throughout, with the excep- tion of RSW after 40°C treatment which increased flow rate in excess of the AACC. In all instances the flow rate through 70°C treated bran was faster (P<O.OOl) than through raw bran, and for 70°C treated AACC, FC and FF faster than 40°C treated bran (P<O.OOl) .

Figure 3 shows how flow rates varied when the ionic strength of the eluting solution was increased. The flow rate of AACC bran was significantly faster with

L r" I r r.

1 AACC FC RSW FF

H20 09% 6M H20 09% 6M H20 0 9 % 6M H20 09% 6M NaCl UREA NaCl UREA NaCl UREA NoCl WEA

Fig. 3. The effect of ionic solutions (water, 0.9% NaCl, 6 M urea) on flow rates through columns packed with bran AACC, French Coarse (FC), Canadian Red Spring Wheat (RSW) and French Fine

(W).

190 A. Anderson, M. A . Eastwood

TABLE 2 Flow Rates Through and Water-Holding Capacities of Four Brans after Sequential

Soaking in Urea and Sodium Chloride Solutions

Urea

Urea/Saline

Saline

Urea/Saline/Water

Saline/Water

Water

Solution

-

A B A B A B A B A B A B

AA CC French coarse

Red Spring French wheat fine

215 kll ' 1.8

412+6" 6.0

381k11d 4.7

387 f 13 7.3

185 t 17d 5.3

4.9 -

312k7f 2.0

389 +23f 9.4

327 227' 5.7

477+6f 12.1

444 +7' 8.4 - 7.8

2 7 2 S b 2.0

36Ok1Sb 6.4

508 k 10 7.7

423 f 17 11.5

451f13 9.6 __ 7.2

243 29"

409 f 9 c 8.0

478 k 10 5.8

399 +26 9.5

466t9 7.5

5.1

1.9

-

A=Flow rate ml h-' (meantSE). B= Water-holding capacity, water held per gram dry fibre (values are means of duplicates). Significance of results with common superscript letters P<0.001.

water and saline than with 6 M urea (P<O.OOl). Saline significantly increased flow rates through the raw brans (P<O.OOl) . The elution with urea reduced flow rate with AACC but not with FF, FC and RSW bran.

In an attempt to establish whether differences in flow rates using solutions other than water were irreversible, the four types of bran were soaked in 6 M urea overnight and washed three times on a Buchner funnel with sodium chloride solution (9 g litre-'). After packing, columns were eluted with the saline. When flow measurements were completed, this procedure was repeated, washing this

TABLE 3 Variations in Water-Holding Capacity and Flow Rate Under Different Experimental

Conditions

Solution Flow rate Water-holding capacity (ml h-l) (g g-' dry fibre)

French French French French coarse fine coarse fine bran bran bran bran

Water 446t3" 403 f 9 c 8.2+0.3g 5.1 f0.09h (3 g litre-') NaCl 444k2" 460k3d 7.2f0.3 5.2k0.08 (6 g litre-l) NaCl 447 k 7" 455 f 2 ' 7.1 f0.4h 5-6 k0.09" (12 g litre-') NaCl 381 f 12h 398 f 1Y 6.3 k0.4k 5.5 k0.2"

Significance: a-b, c-d, c-e, e-5 g-h, g-k, h-m, h-n, P<O.OOl; d-f, P<O.O1. Similar letters, e.g. a-a, not significant. Values represent meankSE.

Flow characteristics and water retention of wheat bran 191

time with water (urea/saline and urea/saline/water brans). In a separate experi- ment, brans soaked only in saline were washed and then eluted with water (saline/water brans). WHC were determined from the same batches, and results are shown in Table 2.

Initial flow rates were reduced (P<O-O01) when bran was eluted with 6 M urea compared to similar initial elutions with saline. Washing this hydrated fibre with saline and water restored the flow rate to the initial value. Oddly, the AACC bran had a decreased flow rate when saline was replaced by water.

To investigate specifically the effect on flow of saline solutions of differing strengths through brans of differing particle ranges, five samples of FC and FF brans were taken in 5-g batches and soaked in water, 3 , 6 and 12 g litre-l sodium chloride solutions. Five columns were packed with the same type of bran and run for 8 h. WHC determinations were made at the same time and results are shown in Table 3. It became apparent that, although the flow rates for each column were consistent, there was considerable variation between the columns despite packing density being constant to within 0.1 g.

4 DISCUSSION

A simple reproducible in-vitro test to predict how dietary fibre from a particular source affects human gastrointestinal function would be useful. It is widely felt that the ability of fibre to take up and retain water is an important factor in the manner in which an increased faecal output is elicited. There is as yet no single method by which in-vivo properties can be linked conclusively to the physical characteristics of a fibre source.

The mode in which water is held by fibre may affect the ability of that fibre to influence stool weight. This is of particular importance for wheat bran which has a predictable effect on stool weight.' In contrast the more readily hydrated fruit, vegetable, mucilage and gum fibres provide a substrate for proliferation of bacteria; faecal water may then be bacterially intracellular and not available for reabsorption by the colon.

If WHC and faecal bulking ability of wheat bran are interlinked then a method to separate the phases by which water is bound would be useful. The centrifuga- tion method1* does not distinguish the phases of water and also presents problems because of the elasticity of the fibre, and there is a degree of 'rebound' after the compression of centrifugation. The more recent method of Robertson and East- woods does make some distinction between water phases but does not readily give values which can describe a bran. The dialysis bag method gives interesting and useful results and insights into the physical properties of water soluble fibres and has recently been shown to be of predictive value for fibre and faecal bulking ability, after bacterial fermentati~n.'~ The method described in this paper attempts to give an easy method for WHC, reflecting phases of held water.

Coarse French Wheat and Canadian RSW brans have been shown to have a greater effect on stool weight than fine bran from the same milling.6 We have no results for the effect of AACC wheat bran on stool weight in our own human

192 A . Anderson, M. A . Eastwood

experiments. French soft wheat flour is used for cake-making whilst Canadian RSW produces bread flour; AACC wheat bran is a reference standardised bran that has undergone steam heat-treatment. In the drying experiments, AACC bran showed anomalous behaviour which may be a consequence of its preparation.

Dietary fibre may be classified in a number of ways. A simple value for total fibre is of limited value in predicting the effect on stool weight; 1 g of pectin fibre does not have any effect on stool weight, whereas bran certainly does. This difference is because pectin is totally fermented in the colon. Some fermented fibres result in an increase in faecal weight by virtue of bacterial proliferation, but this is an unpredictable relationship. Wheat bran is fermented to a limited extent, but the relationship between stool weight and WHC persists.”

Three of the brans discussed in this paper, RSW, French soft wheat coarse and fine, have been studied for their faecal bulking properties and the order of flow rate is in accord with faecal bulking ability.6

The flow rate through each column declines very modestly with time. Differ- ences in flow rates through the bran could not be attributed to the amounts of fibre contained within the columns. It is therefore assumed that the differences in flow are due to differences in the water retention properties.

Heating the bran affects its flow characteristics. All the brans have increased their flow rates when modestly heated at 40 and 70°C. It would appear that the effect of heating on these fibre measurements is to increase flow rate towards the values observed for AACC bran.

The effect of cooking on bran reduces its enhancing action on stool bulking.14 The effect of cooking bran in the presence of water, as in bread-making, may not be the same as that of drying or of steam heat-treatment. Wholemeal bread (small particles, cooked bran) has a very modest effect on stool weight (1620% increase). l5

However, in these experiments the effect of cooking the bran is an increase in the flow rate, suggesting a loss of the avidity of water holding, which will persist into the colon.

When 0.9% sodium chloride was used as the eluting solution, flow rates were significantly higher than those with water for all brans except AACC. Urea (6 M) changed the physical appearance of the hydrated brans, making them softer and more pliable than when hydrated in water or saline. This change may explain the decrease in flow rate through the urea-soaked bran. When brans were soaked and eluted with urea, then washed, soaked and eluted in 9 g litre-’ saline, flow rates were increased to values close to those for brans soaked and eluted in saline alone. The flow rate values were inconclusive but WHC values were significantly increased each time a solution of lower osmolality was used to hydrate the brans. This suggests that soaking in very strong solutions irreversibly alters those parts of the physical structure that confer water-retaining properties to the bran. This effect was less marked when 9 g litre-l sodium chloride was used.

With French bran of both coarse and fine particle ranges there was no signifi- cant changes in flow rates between water, 3 and 6 g litre-’ saline solutions; when 12 g litre-l was used, flow rate was reduced with coarse bran, but not with the

Flow characteristics and water retention of wheat bran 193

fine. WHC was decreased in FC bran when 12 g litre-' was used, but increased when the bran was FF. Strength of solution had no effect on WHC except where the physical or chemical structure may have been altered.

Values for WHC vary according to the method of determination, the method of fibre preparation and its s o ~ r c e . ~ , ~ It has been suggested that the way in which water is bound may be more important than the total a m ~ u n t . ~ - ~ Variability within values for WHC are consistent with variability between flow rates. Flow-rate measurements are highly reproducible for a single column but substantial varia- tion exists between columns. Measurement of column flow provides no better a predictor of change in stool weight than absolute WHC. The coarser bran fibres allow more water through per hour than do the fine brans. However, the AACC bran allows most water through. There have been no comparisons of AACC bran with other raw brans in faecal bulking experiments so the functional implication of this finding is not clear. The WHC of AACC bran, 4 g, is not high and is suggestive of a modest influence on stool weight.

The method described in this paper may be of value in readily resolving how a dietary fibre will affect stool weight. If an insoluble fibre is intended for consump- tion as a faecal bulking agent it is possible that it will be extracted, coated or heated. If so it is important to follow the manufacturing process and to identify stages in preparation where modifying changes in WHC may take place. It would appear that ionic strength has effects only at high concentrations whereas heat treatment permanently changes WHC. Clearly, if flow rate remains unaltered with a particular mode of preparation, e.g. heating or extraction, then the fibre may be reasonably regarded as being of similar characteristics to the original material. When the flow-rate characteristics are altered then such an assumption may not be justified.

ACKNOWLEDGEMENTS

We thank the Incorporated National Association of British and Irish Millers Ltd for their financial assistance.

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194 A . Anderson, M . A . Eastwood

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1986, 3, 343-349.