Effect of soluble fibre (guar gum and carboxymethylcellulose) addition on technological, sensory and structural properties of durum wheat spaghetti

Food Chemistry 131 (2012) 893–900

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Effect of soluble fibre (guar gum and carboxymethylcellulose) addition on technological, sensory and structural properties of durum wheat spaghetti Nisha Aravind a,b, Mike Sissons a,⇑, Christopher M. Fellows b a b

NSW Department of Primary Industries, Tamworth Agricultural Institute, 4 Marsden Park Road Calala, NSW 2340, Australia School of Science and Technology, The University of New England, Armidale, NSW 2351, Australia

a r t i c l e

i n f o

Article history: Received 25 July 2011 Received in revised form 12 September 2011 Accepted 19 September 2011 Available online 25 September 2011 Keywords: Pasta Guar gum Carboxymethylcellulose Digestibility Soluble fibre Durum wheat

a b s t r a c t Incorporation in pasta of either of two soluble fibres, carboxymethylcellulose sodium salt (CMC) and guar gum (GG), was found to significantly reduce the rate of in vitro starch digestion. The amount of reducing sugars produced over 300 min was reduced by 18% at 1.5% CMC incorporation and 24% at 20% GG incorporation. Negative effects on sensory and technological properties were seen at the high levels of GG needed to reduce the rate of in vitro digestion, and a ‘matty’ layer covering the surfaces of starch granules was observed by scanning electron microscopy and confocal scanning laser microscopy. By contrast, levels of CMC incorporation giving large reductions in in vitro digestion had no negative effects on pasta properties. No significant alteration in pasta structure on CMC incorporation was observed by microscopy. The large difference in the amounts of soluble fibre required to bring about equivalent reductions in digestion rate suggests that different mechanisms may be involved in the two cases. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Dietary fibres are carbohydrates that are resistant to digestion and absorption in the human small intestine and undergo complete or partial fermentation in the large intestine (Trowel & Burkitt, 1986). Diets moderate to high in fibre help reduce the risk of cardiovascular disease, diabetes and cancer (WHO, 2003). Processed cereal products like pasta and bread are not high in dietary fibre, but if fibres are incorporated into these foods the sensory and cooking properties can be changed in an undesirable way. Therefore, careful selection of the amount and type of fibre used is needed to develop foods with satisfactory taste and acceptability while delivering improved nutritional value. Pasta is considered a low glycaemic index (GI) food as it progressively liberates sugars during digestion, reducing the glycaemic and insulinaemic response in humans (Granfeldt & Bjorck, 1991). Addition of dietary fibre can further reduce the GI of pasta and introduce additional health benefits (Gatti et al., 1984; Yokoyama et al., 1997). In producing such foods of acceptable eating quality it is always a challenge to optimise the potential health benefit while retaining the consumer acceptability of the food. Dietary fibres are usually classified as soluble (e.g., pectins, galactomannans, alginate, psyllium fibre, agar and gums) or insoluble (e.g., cellulose, hemicellulose, and lignin), based on their sol⇑ Corresponding author. Tel.: +61 2 67631119; fax: +61 2 67631222. E-mail address: [email protected] (M. Sissons). 0308-8146/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2011.09.073

ubility in water (Brennan, Tudorica, & Kuri, 2002). All fibres are generally odourless and tasteless but can have undesirable effects on food texture. Soluble fibres are frequently useful to increase the fibre content of liquid foods. They can be low in viscosity to ensure good integration into the food, unlike insoluble fibres (Anderson, 1990). Soluble fibres readily absorb water to form viscous polymer networks. For example, galactomannans can affect food microstructure by coating starch granules in a mucilaginous layer, reducing food breakdown rates (Brennan, Blake, Roberts, & Ellis, 1996). Soluble fibres can increase digesta viscosity, control postprandial glucose and insulin response, reduce total cholesterol and LDL, and regulate appetite (Brennan, Blake, Ellis, & Schofield, 1996; Davidson & McDonald, 1988; Ellis, Wang, Rayment, Ren, & RossMurphy, 2001; Peressini & Sensidoni, 2009; Tudorica, Kuri, & Brennan, 2002). Soluble fibre sources have the ability to support selectively the growth of beneficial bacteria in the colon, and can alleviate constipation to some extent (Malkki, 2004; Roberfroid, 1993). Jenkins et al. (1978) compared the effects of various fibres consumed with 50 g of glucose. In general, soluble fibres were most effective in decreasing the glycaemic response and the higher the viscosity of the fibre, the more effective the response. Guar gum is a water-soluble, non-starch polysaccharide galactomannan found in the endosperm of the seed from the Indian cluster bean Cyamopsis tetra-gonoloba. Galactomannan consists of a b-(1– 4)-linked D-mannopyranosyl backbone, partially substituted with a-(1–6)-linked D-galactopyranosyl side chains (Meir & Reid, 1982). Guar gums are often added to food as thickening, binding or

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stabilising agents (Ellis, Apling, Leeds, & Bolster, 1981) because GG can be easily dispersed in water and form viscous solutions. Guar gum addition to foods can alter starch digestion, as well as exhibiting other desirable effects of soluble fibre incorporation (see review by Ellis et al., 2001). Jenkins et al. (1978) found GG attenuated the postprandial blood glucose and insulin concentrations in healthy and diabetic subjects. Brennan et al. found the in vitro digestibility and rate of hydrolysis of bread containing 10% guar gum was decreased compared to normal white bread (Brennan, Blake, Ellis, et al., 1996). The combination of guar gum affecting food digesta viscosity (Ellis et al., 2001) and possibly forming a physical barrier to a-amylase– starch interactions was suggested as a mechanism of action in starch-based foods (Brennan & Tudorica, 2008). A broad range of foods besides bread have been developed incorporating guar gum (snack bars, marmalade, biscuits, breakfast cereals), including pasta (Ellis et al., 2001). Addition of 10% guar gum to pasta resulted in a significant reduction in the in vitro starch digestion, compared to regular pasta (Brennan, Mertz, Monro, Woolnough, & Brennan, 2008). Gatti et al. (1984) found that pasta with 20% GG fed to diabetic and normal individuals delayed the increase in plasma glucose significantly more than normal pasta. Carboxymethyl cellulose (CMC) is a cellulose derivative with carboxymethyl groups bound to some of the hydroxyl groups of the glucopyranose monomers which make up the cellulose backbone, with food-grade CMC having a degree of substitution in the range 0.65– 0.95 (Hoefler, 2000). CMC molecules are usually used as the sodium salt and are on average shorter than cellulose, have high cold water solubility and are mainly used to control viscosity without gelling in foods (BeMiller, 2008). The addition of CMC into cereal-based food has shown beneficial affects on blood glucose regulation and fasting plasma cholesterol (Brennan et al., 1996). Komlenic, Ulgarcic-Hardi, and Jukic (2006) added 0.15–0.75% CMC to pasta which not only did not degrade sensory properties, but improved sensory value over regular pasta. This may be too low a concentration to have any desirable physiological effect, however. None of the studies to date on pasta have evaluated the impact of the soluble fibre on the structure, technological, functional and sensory qualities together. This approach is the one required to achieve a true understanding of how the added fibre affects the properties of the pasta and allow intelligent design of functional foods. The objective of this study was therefore to evaluate the effects of substituting semolina at different rates with guar gum and carboxymethylcellulose on the technological, structural, sensory and nutritional properties of pasta. These soluble fibres were chosen based on their solubility, high fibre content and sensory properties, to minimise their probable impact on the properties of the base pasta. 2. Materials and methods 2.1. Materials Commercial semolina was provided by Manildra Mills (Gunnedah, NSW, Australia). Guar gum, G4129 (GG) and carboxymethylcellulose sodium salt, C5678, 50–200 cP as 4% aqueous solution at 25 °C (CMC), were from Sigma–Aldrich, Castle Hill, Australia. GG contained approximately 1% insoluble fibre. At 30 °C in distilled water, the intrinsic viscosity of GG was measured to be 900 mL/g and for CMC, the intrinsic viscosity was measured in 0.1 M NaCl to be 140 mL/g; degree of substitution of CMC was estimated to be 0.9 by 1H-NMR. 2.2. Pasta preparation and analysis Durum semolina was blended with GG or CMC by substituting durum wheat semolina with 0%, 2.5%, 5%, 10%, 15% and 20% w/w

GG, or with 0%, 0.25%, 0.5%, 0.75%, 1.0% and 1.5% CMC. These doses are based on levels used in previously published work (Brennan et al., 2008; Komlenic et al., 2006). Samples were mixed in tumblers for 30 min to ensure uniform blending of the fortified flour. Semolina–fibre blends were mixed with water (ca. 30% by weight) then extruded to prepare spaghetti. The amount of water added in the case of GG pasta was adjusted according to the farinograph water absorption of the GG–semolina dough to account for the higher water absorption capacity of GG (for a 1 kg batch of semolina the amount of water added ranged from 280 mL for 0GG to 448 mL for 20GG). The CMC had negligible effect on the farinograph water absorption. Details of the pasta preparation and drying procedure have been described previously (Aravind, Sissons, Egan, & Fellows, 2012). All pasta samples were cooked to their optimum cooking time (OCT), to assess firmness, stickiness, cooking loss, swelling index and water absorption, as described previously (Aravind et al., 2012). Uncooked spaghetti colour was measured on the Hunter scale for L⁄, a⁄, and b⁄, using a Minolta Chroma metre CR-410 (Biolab Australia, Sydney). Data for colour are the means of three replicate readings along the pasta strand. 2.3. Sensory testing A panel of 12 testers experienced in general food evaluation was trained in pasta assessment by staff of Cereal Partners Worldwide (Rutherglen, Victoria). The sensory testing was determined according to the sensory assessment procedure reported by Aravind et al. (2012). Only statistically significant data are presented for the attributes measured. 2.4. In vitro carbohydrate digestion The procedure for measuring the breakdown of carbohydrates to sugars is that reported by Aravind, Sissons, and Fellows (2011). For relative comparison to the control (unsubstituted durum semolina), the area under the curve (AUC) of the reducing sugar production with time plot was taken as a single number, allowing quantitative comparison of starch digestibility. The AUC of the test samples was divided by the AUC of the control to give a normalised AUC value. This normalised value corrects for variation in starch and moisture content between the samples. Values are the means of triplicate determinations. 2.5. Confocal laser scanning microscopy (CLSM) Cooked pasta samples were prepared and examined by confocal laser scanning microscopy (CLSM) as described previously (Aravind et al., 2012). 2.6. Differential scanning calorimetry (DSC) Differential scanning calorimetry (DSC) characterises the process of starch gelatinisation by measuring the temperature at which different stages of the process occur. Testing was based on the method of Randzio, Flish-Kabulska, and Grolier (2002) using a Mettler Toledo DSC 822e. Prior to testing, the samples were equilibrated overnight to obtain similar moisture content within the samples. Ground sample (4–6 mg) was placed into a hermetic DSC pan and 10 lL of water were added via syringe. The pan was then covered with a lid, and crimped to create a hermetic environment, and allowed to equilibrate overnight at room temperature. A crimped empty pan and lid were used as a reference. The instrument was calibrated daily to an indium standard. During testing, each sample was cooled or heated to 40 °C before slowly ramping up to 140 °C at a rate of 4 °C per minute. A DSC heating or cooling curve was generated in this process. Gelatinisation temperatures of

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Tonset, gelatinisation temperature (peak) and Tendset are obtained in the process. 2.7. Pasta viscosity determined by RVA The pasting properties of the spaghettis were measured with a Rapid Visco Analyzer (RVA4, Perten Instruments, Sydney, Australia) interfaced with a computer equipped with Thermocline software. Spaghettis were dry milled (falling number mill, Perten Instruments, Sydney, Australia) to pass through a 0.8-mm screen. Ground spaghetti (3.5 g) was added to 25 mL of distilled water (adjusted for the moisture content of the samples) and mixed in an RVA canister. The temperature profile used was based on that of Bruneel, Pareyt, Brijs, and Delcour (2010) and included a temperature holding step (1 min at 50 °C), a linear temperature increase to 95 °C at 7.5 °C/min, a holding step (8 min at 95 °C), a linear temperature decrease to 50 °C at 7.5 °C/min and a final isothermal step at 50 °C (10 min). The paddle speed was 960 rpm for the first 7 s and then 160 rpm. The parameters measured were peak viscosity, i.e. the maximum viscosity during the cycle and a measure of shear resistance of particles, and end viscosity (cP), i.e. the viscosity at the end of the run (31 min); trough is the minimum value of viscosity between peak and end; breakdown is the difference between peak and trough; setback is the difference between final and trough. Three replicate runs were run for each sample and the mean values are presented. 2.8. Statistical analysis Data are presented as means and compared by one-way analysis of variance (ANOVA) using Statistical Analysis System (GenStat 11.1, VSN International Ltd.) software. Means were compared to test for significant differences (p < 0.05) using the least significant difference statistic (LSD).

yellowness (b⁄), a desirable characteristic for consumers, increased at all doses of CMC compared to the control, with the highest b⁄ at 1.5CMC (Table 1). Overall, there were no negative affects on technological properties with a CMC substitution of up to 0.75CMC. At 1CMC, pasta became stickier and absorbed more water but was more yellow than control pasta (Table 1). 1.5CMC absorbed even more water but had an even better yellow colour. The impact of GG addition on farinograph water absorption was large (0GG, 60%; 20GG, 120%) and dough development time increased from 4.5 to 10 min (data not shown). The gluten index could not be determined as a gluten ball failed to form in the glutomatic apparatus. This shows GG has a strong water-absorbing capacity and competes with the starch and gluten for water. Guar gum incorporation had no impact on pasta cooking time (Table 2). Cooked pasta firmness was increased only at 2.5GG compared to control, was no different from control between 5GG and 15GG and was lower at 20GG. Pasta became increasingly stickier with GG substitution compared to control but only significantly at 15GG and 20GG, with 20GG double the stickiness of the control. A similar trend was noted for cooking loss, which increased significantly above control at doses above 10GG. Cooking loss continued to increase with higher doses of GG, an undesirable feature. The pasta absorbed more water than the control at GG incorporation of 5GG and above. A similar trend was obtained with swelling index showing an increase with GG substitution above 5GG. Uncooked pasta colour was also affected by GG with increases in redness at 10GG and above, while decreases in brightness and yellowness occurred above 10GG. The 20GG samples had the lowest L⁄ and b⁄ and the highest a⁄. Overall there were relatively minor impacts of GG with 2.5GG and 5GG substitution on technological properties, however at 10GG and especially above 15GG, pasta became softer, stickier, had higher cooking loss, absorbed more water and was duller and less yellow.

3. Results 3.2. Effect of soluble fibre inclusion on sensory properties 3.1. Effect of soluble fibre inclusion on pasta cooking quality The effect of the addition of CMC on the technological properties of pasta is shown in Table 1. The impact of CMC addition on farinograph water absorption was minimal (FWA 0CMC, 59.2; 1.5CMC, 63.1) with no affect on dough development time (data not shown). Similarly, wet gluten and gluten index were not affected (data not shown). The addition of CMC had no effect on pasta cooking time, swelling index, or cooking loss, and had no significant impact on cooked pasta firmness. For stickiness, only pasta with 0.5CMC and 1CMC were significantly stickier than control. Pasta water absorption showed a steady increase as more CMC was added and became significantly higher than control pasta at 1CMC and 1.5CMC. This data is consistent with the lack of impact of CMC on swelling index. CMC addition had no affect on pasta brightness (L⁄), and while increased redness (a⁄) was seen at 1CMC the value was lower than control at 1.5CMC. Uncooked pasta

Pasta containing CMC had very similar sensory scores to control pasta, differing significantly in only three measures, which are illustrated in the radar plot showing movement relative to control (Fig. 1). CMC reduced the pasta surface roughness and rubberiness scores as more CMC was added, but not to a great extent, and had a more pronounced effect in increasing the floury mouthfeel by up to 100%. These data indicate a minimal impact of CMC on pasta sensorial quality, in line with the low impact on technological properties measured. The general appearance of the dried pasta strands at all CMC levels could not be visually distinguished from the control sample. The stands had a smooth feel, were bright and yellow with no white inclusions or checking. In contrast to CMC, the quantities of GG used had a relatively large impact on the sensory properties of the pasta. Most of the parameters measured were affected and this is indicated by the radar plot showing movement away from the control pasta, espe-

Table 1 Effect of CMC addition to pasta on technological properties. Pasta composition (values% CMC)

OCT (min:s)

Firmness (peak height, g)

Stickiness (peak height, g)

Cooking loss%

Water absorption

Swelling index

L⁄

a⁄

b⁄

0CMC 0.25CMC 0.5CMC 0.75CMC 1.0CMC 1.5CMC LSD p < 0.05

14:00 14:00 14:00 14:00 14:00 14:00

583a 580a 551a 538a 539a 542a 26

14.0a 16.6a,b 22.7b 18.3a,b 22.0b 17.4a,b 3.8

5.2a 5.1a 5.2a 5.1a 5.5a 5.9a 0.4

165a 167a,b 167a,b 170a,b,c 172b,c 175c 3

2.13a 2.21a 2.15a 2.10a 2.13a 2.22a 0.06

69.09a 69.55a 69.32a 69.29a 69.29a 69.35a 0.34

1.95a 1.85a 1.90a 1.88a 2.25b 1.69c 0.11

46.57a 49.16b 49.62b,c 49.62b,c 49.30b 50.26c 0.37

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Table 2 Effect of guar gum (GG) addition to pasta on technological properties. Pasta composition (values% GG)

OCT (min:s)

Firmness (peak height, g)

Stickiness (peak height, g)

Cooking loss%

Water absorption

Swelling index

L⁄

a⁄

b⁄

0GG 2.5GG 5GG 10GG 15GG 20GG LSD p < 0.05

14:00 14:00 14:00 13:30 14:00 14:00

805a,d 975b 831a,d 822d 742a,c 701c 36.00

21.2a 20.6a 25.1a,b 27.6a,b 31.8b 42.1c 3.70

4.9a 5.0a 5.0a 5.4b 5.6b 6.0c 0.15

163a 165a 171b 178c 186d 189d 2

2.06a 2.04a 2.14a,b 2.17a,b 2.29b,c 2.42c 0.09

69.36a 69.34a 68.47a 67.40b 66.65b,c 65.73c 0.51

1.77a 1.79a 1.89a,b 2.13b,c 1.99a,b,c 2.22c 0.16

47.47a 47.60a 47.05a 44.91b 41.99b 39.94c 0.63

Surface Roughness 60

0CMC 0.25CMC

50

A 1.1

40

0.75CMC 1CMC

30 20 10

Normalised area

0.5CMC

1.0

0.9

0.8

0

0.7 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

% CMC Rubbery

Fig. 1. Influence of CMC on the sensory attribute scores shown in a radar plot.

cially at the higher GG doses (Fig. 2). At 15GG pasta was significantly softer, less rubbery and chewy, had a more floury mouthfeel, higher mouth drying and a more starchy aftertaste. The general appearance of the dried pasta strands at all GG levels could not be visually distinguished from the control sample, consistent with the similar surface roughness reported in the sensory testing.

B

1.1

Normalised area

Floury Mouthfeel

1.0

0.9

0.8

0.7 0

3.3. Effect of soluble fibre inclusion on in vitro starch hydrolysis Pastas enriched with CMC exhibited a significantly lower rate of reducing sugar release than the controls (Fig. 3A). The normalised area under the reducing sugar release versus time curve decreased

5

10

15

50

Flavour Overall

40 30 20 Floury Mouthfeel

Aftertaste Starch

10 0

Chewy

Aftertaste Sour

Rubbery

25

Fig. 3. Normalised area under the reducing sugar production versus time curve for pasta containing various levels of GG (A), CMC and (B). Data are mean ± SD (error bars).

Surface Roughness 60 Mouthdrying

20

Guar Gum %

Firmness

Fig. 2. Influence of GG on the sensory attribute scores shown in a radar plot.

0GG 2.5GG 5GG 10GG 15GG

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Fig. 4. CLSM of cooked spaghetti samples with or without CMC. Magnification 40 and bars = 100 lm. Control (A), 0.5CMC (B), 1CMC (C), 1.5CMC (D).

relative to control pasta (corrected for differences in moisture and starch content). The AUC decreased with as little as 0.25CMC then declined further at 0.75CMC remaining relatively constant thereafter. An overall reduction in AUC of 18% was achieved using 1.5CMC. A similar reduction in AUC occurred with GG substitution where AUC decreased as more GG was added, however much larger quantities of soluble fibre were required. No significant differences in AUC were measured between 2.5 and 10GG, but a further significant decline occurred with 15–20GG (Fig. 3B). An overall reduction in AUC of 24% was achieved with 20GG.

3.4. Effect of soluble fibre inclusion on the internal structure of pasta determined by confocal laser scanning microscopy (CLSM) CLSM is a useful technique for viewing the 3-D structure of thick specimens and with suitable staining can be used to detect the endosperm starch and protein in situ. CLSM images of cooked control and CMC-fortified pasta (Fig. 4A–D) show dual-stained images where many starch granules (in green) are still visible after cooking. The protein stains intensely yellow and shows the typical structure found in pasta; starch granules embedded in a continuous protein matrix (Fig. 4A) (Aravind et al., 2011). This image is not that different with 0.5CMC, except the protein matrix has a different colour, more red-yellow, and this might be caused by the interaction of the CMC with the dyes (Fig. 4B). In the 1CMC sample the intensity of the yellow matrix staining is increased and there are red inclusions in the image (Fig. 4C). The 1.5CMC is similar to 1CMC (Fig. 4D). The green staining of the starch granules has dulled in the 1 and 1.5CMC samples and the granules appear more

irregular in shape, which is also seen in 0.5CMC sample, but the spacing between granules is otherwise similar to the control. There is no indication of any covering over the granules. Compared to the control 0GG sample, the appearance of 2.5GG (Fig. 5B) is similar, but there are numerous voids, the granules are more intensely stained and, in addition to the yellow-stained protein matrix, there are darker, red stained regions not present in the control. With 10GG the image changes again, the starch granules are harder to detect, and the appearance is much darker red (Fig. 5C). In the 20GG sample the fluorescence intensity increases greatly and it appears as if the granules are coated in a ‘‘matty’’ covering. Although the granules are visible, they are clearly surrounded by a more extensive matrix than observed in the control (Fig. 5D). 3.5. Effect of soluble fibre inclusion on gelatinisation and viscosity of the pasta There were no changes in any of the DSC parameters caused by CMC inclusion in the pasta (Table 3). Similarly GG inclusion in pasta had no affect on DSC measures except for one sample (5GG, endset). The CMC inclusion also had no impact on pasta viscosity profile obtained by the RVA program chosen (Table 4). However, GG inclusion had a marked affect on viscosity, reducing viscosity for all the parameters measured (Table 4). 4. Discussion Komlenic et al. (2006) investigated the affect of adding 0.15– 0.75% CMC sodium salt (degree of substitution 0.5–0.7, viscosity

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Fig. 5. CLSM of cooked spaghetti samples with or without GG. Magnification 40 and bars = 100 lm. Control (A), 2.5GG (B), 10GG (C), 20GG (D).

Table 3 Thermal properties (DSC measurements) for uncooked pasta with and without GG or CMC. Numbers with alike letters in the same column are not significantly different, p < 0.05. Uncooked pasta sample

Tonset (°C)

Gelatinisation temp. (°C)

TEndset (°C)

0CMC 0.25CMC 0.5CMC 0.75CMC 1.0CMC 1.5CMC 0GG 2.5GG 5GG 10GG 15GG 20GG LSD

56.27a 56.02a 57.17a 56.88a 56.48a 56.72a 56.29a 56.65a 56.94a 58.29a 58.3a 57.79a 2.59

61.79a 61.72a 62.11a 62.23a 62.18a 62.4a 61.82a 61.78a 62.94a 62.57a 63.75a 63.13a 1.32

67.41a 67.61a 67.22a 67.67a 67.49a 68.2a 67.67a 66.53a 72.09b 67.39a 70.33a,b 69.36a,b 1.55

of 1% aqueous solution at 25 °C > 3000 cP (Gimeno, Moraru, & Kokini, 2004) to pasta. They found increased cooking loss and decreased water absorption with CMC addition, which they suggested was because CMC absorbs water readily to inhibit starch swelling and absorption of water by the pasta. The data reported here, by contrast, show an increase in water absorption and no change in swelling index or cooking loss. Komlenic et al. (2006) obtained improved sensory scores for CMC pasta due to lower stickiness and consistency, otherwise minimal changes were observed, in line with our data on sensory and instrumental stickiness. Olfat,

Table 4 The effect of CMC or GG incorporation in pasta on RVA pasting properties of dry milled spaghettis. Numbers with alike letters in the same column and group are not significantly different, p < 0.05. Pasta sample

Peak viscosity (cP)

Trough (cP)

Breakdown (cP)

Final viscosity (cP)

Setback (cP)

0CMC 0.25CMC 0.5CMC 0.75CMC LSD 0GG 2.5GG 5GG 10GG 15GG LSD

1747a 1721a 1728a 1713a 54.9 1812a 1709a,b 1622a,b,c 1576b,c 1482c 97.6

1180a 1157a 1179a 1185a 35.9 1216a 1183a,b 1136b,c 1037c 1080c 38.2

567a 564a 549a 528a 31.6 596a 526a,b 486a,b 539a,b 401b 73.4

2547a 2529a 2541a 2548a 37.6 2667a 2411b 2213c 1801d 1836d 97.6

1367a 1372a 1362a 1363a 29.7 1451a 1228b 1076b 764c 756c 77.4

Yaseen, and Aziza (2006) added a CMC–whey protein complex to macaroni and found improved cooking quality (increased cooked volume) and improved sensory scores for colour, flavour and appearance using 3% CMC–whey protein compared to 100% semolina pasta. This suggests that much higher levels of CMC could be used in pasta than used in our work, to give further improvements in the in vitro starch digestion. CMC has been used in non-durum wheat pasta (oat, amaranthus and quinoa pasta) to overcome in part the problems in these products caused by the absence of gluten (Chillo, Laverse, Falcone, & Del Nobile, 2007; Chillo, Suriano,

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Lamacchia, & Del Nobile, 2009). To our knowledge, this is the only report showing the effect of CMC on the in vitro starch hydrolysis in pasta. The CLSM images do not show a ‘matty’ covering of CMC on the starch granules and no effect of CMC on swelling behaviour was observed by differential scanning calorimetry (DSC), so it is unlikely that the very marked effect of CMC on reducing digestion rates is due to such a localised inhibition of starch granule swelling/hydrolysis. Rather, the overall effect of CMC on increasing the viscosity of the digesta may retard a-amylase diffusion and be responsible for the effect. However, analysis of the RVA of the CMC pasta shows no change in viscosities. It is probable therefore that the inhibition of a-amylase by CMC does not arise from the effect of CMC on material properties, but from a more direct physical mechanism. To our knowledge there have been no reports of direct competitive inhibition of a-amylase by CMC. Electrostatic effects may play a role as at pH = 6.9, a-amylase should bear an overall negative charge (pI = 5.7 and 6.5; Furuichi & Takahashi, 1989), so repulsion of the enzyme by negativelycharged carboxymethylcellulose salt adsorbed on starch granules, even if it does not form a cohesive layer, is possible. Guar gum inclusion into foods can reduce the rate and extent of starch digestion (Brennan, 2005; Brennan, Blake, Roberts, et al., 1996a; Brennan et al., 2008; Brennan & Tudorica, 2008; Jenkins et al., 1984). This polysaccharide appears to have a regulatory role on the rate of starch hydrolysis, presumably by affecting the movement of water in the food matrix and thus affecting enzyme access to starch during the hydrolysis process. We observed a swelling of the starch in the pasta and increased pasta water absorption (Table 2) with a reduction in starch hydrolysis in pasta, consistent with these reports. Brennan, Blake, Roberts, et al. (1996) suggested the guar galactomannan acts as a physical ‘barrier’ to a-amylase– starch interactions, due to formation of a coating layer over the granules. A similar effect was observed in pasta containing guar gum (Brennan et al., 2008). The CSLM data reported here suggest the presence of a coating in the starch–gluten matrix due to the GG inclusion. Brennan and Tudorica (2008) stated that GG acting as a barrier, combined with its affects on digesta viscosity, is sufficient to explain why a reduction in postprandial glycaemic and insulinaemic responses is found when starchy cereal foods containing guar gum are ingested (Ellis, Apling, Leeds, & Bolster, 1981; Slaughter, Ellis, Jackson, & Butterworth, 2002). Another possible mechanism is that a-amylase could bind directly to the galactomannan, inactivating the enzyme (Slaughter et al., 2002). Guar gum has also been used in combination with gluten and other ingredients to manufacture satisfactory pasta from non-wheat sources such as rice (Raina, Singh, Bawa, & Saxena, 2005), although it cannot by itself form a gluten replacement network. In RVA experiments a significant reduction in pasta viscosity was observed with increasing GG content, which is consistent with a competition between GG and starch for available water, inhibiting starch pasting and retrogradation, and/or disruption of the starch/protein matrix by incorporation of the soluble fibre.

5. Conclusion The superficially similar soluble fibres carboxymethylcellulose and guar gum can both impart a significant improvement in the in vitro starch digestion behaviour of pasta, reducing the amount of reducing sugars produced over 300 min by up to 18% (1.5% CMC incorporation) and 24% (20% GG incorporation). While significant negative effects on sensory and technological properties were seen on incorporation of enough GG to reduce the rate of in vitro digestion, equivalent reductions with CMC incorporation were accompanied only by an increase in the yellowness parameter, b⁄, which is a desirable consumer attribute. The dramatic differ-

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ence in the amounts of soluble fibre required to bring about equivalent reductions in digestion rate points to the possibility that different mechanisms may be involved in the two cases. While confocal laser scanning microscopy showed the apparent existence of a ‘‘matty’’ coating of the starch granules at high levels of GG incorporation, consistent with previous speculation by Brennan, et al. (2008) and Brennan and Tudorica (2008), no such coating could be observed in CSLM of the CMC-containing samples. A specific competitive inhibitory or electrostatic effect of the interaction of the negatively-charged CMC with a-amylase may have been responsible for slower starch digestion rate seen with CMC-containing pasta. Acknowledgements Nisha Aravind thanks the (1) Primary Industries Innovation Centre for the PhD award, (2) the Australian Microscopy and Microanalysis Research Facility (AMMRF) for Travel and Access Program, (3) the Electron Microscopy Unit of University of Sydney for CSLM analysis, and (4) Uncle Toby’s for sensory analysis. References Anderson, J. W. (1990). Dietary fibre and human health. Horticultural Science, 25, 1488–1495. Aravind, N., Sissons, M. J., Egan, N., & Fellows, C. (2012). Effect of insoluble dietary fibre addition on technological, sensory, and structural properties of durum wheat spaghetti. Food Chemistry, 130, 299–309. Aravind, N., Sissons, M. J., & Fellows, C. (2011). Can variation in durum wheat pasta protein and starch composition affect in vitro starch hydrolysis? Food Chemistry, 124, 816–821. BeMiller, J. N. (2008). Hydrocolloids. In E. K. Arendt, & F. Dal Bello (Eds.), Gluten-Free Cereal Products and Beverages (pp. 203–215). Brennan, C. S. (2005). Dietary fibre, glycaemic response, and diabetes. Molecular Nutrition and Food Research, 49, 560–570. Brennan, C. S., Blake, D. E., Ellis, P. R., & Schofield, J. D. (1996). Effects of guar galactomannan on wheat bread microstructure and on the in vitro and in vivo digestibility of starch in bread. Journal of Cereal Science, 24, 151–160. Brennan, C. S., Blake, D. E., Roberts, F. G., & Ellis, P. R. (1996). A microscopical evaluation of the location and effects of galactomannan deposits in guar and wheat bread products: Implications for wheat starch digestion and absorption. Journal of Cereal Science, 24, 151–160. Brennan, M. A., Merts, I., Monro, J., Woolnough, J., & Brennan, C. S. (2008). Impact of guar gum and wheat bran on the physical and nutritional quality of extruded breakfast cereals. Starch, 60, 248–256. Brennan, C. S., & Tudorica, C. M. (2008). Evaluation of potential mechanisms by which dietary fibre additions reduce the predicted glycaemic index of fresh pastas. International Journal of Food Science and Technology, 43, 2151–2162. Brennan, C. S., Tudorica, C. M., & Kuri, V. (2002). Soluble and insoluble dietary fibres (non-starch polysaccharides) and their effects on food structure and nutrition. Food Industry Journal, 5, 261–272. Bruneel, C., Pareyt, B., Brijs, K., & Delcour, J. A. (2010). The impact of the protein network on the pasting and cooking properties of dry products. Food Chemistry, 120, 371–378. Chillo, S., Laverse, J., Falcone, P. M., & Del Nobile, M. A. (2007). Effect of carboxymethylcellulose and pregelatinized corn starch on the quality of amaranthus spaghetti. Journal of Food Engineering, 83, 492–500. Chillo, S., Suriano, N., Lamacchia, C., & Del Nobile, M. A. (2009). Effects of additives on the rheological and mechanical properties of non-conventional fresh handmade tagliatelle. Journal of Cereal Science, 49, 163–170. Davidson, M. H., & McDonald, A. (1988). Fibre: Forms and functions. Nutrition Research, 18, 617–624. Ellis, P. R., Apling, E. C., Leeds, A. R., & Bolster, N. R. (1981). Guar bread: Acceptability and efficacy combined Studies on blood glucose, serum insulin and satiety in normal subjects. British Journal of Nutrition, 46, 267–276. Ellis, P. R., Wang, Q., Rayment, P., Ren, Y., & Ross-Murphy, S. B. (2001). Guar gum: Agricultural and botanical aspects, physiochemical and nutritional properties and its use in the development of functional foods. In S. S. Cho & M. L. Dreher (Eds.), Handbook of dietary fiber (pp. 613–657). New York: Marcel Dekker. Furuichi, Y., & Takahashi, T. (1989). Purification and characterization of porcine salivary amylase. Agricultural and Biological Chemistry, 53, 292–294. Gatti, E., Catenazzo, G., Camisasca, E., Torri, A., Denegri, E., & Sirtori, C. R. (1984). Effects of guar-enriched pasta in the treatment of diabetes and hyperlipidemia. Annals of Nutrition and Metabolism, 28, 1–10. Gimeno, E., Moraru, C. I., & Kokini, J. L. (2004). Effect of xanthan gum and CMC on the structure and texture of corn flour pellets expanded by microwave heating. Cereal Chemistry, 81, 100–107. Granfeldt, Y., & Bjorck, I. (1991). Glycemic response to starch in pasta-a case study of mechanisms of limited enzyme availability. Journal of Cereal Science, 14, 47–61.

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