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Functional and physicochemical properties of non-starch polysaccharides
Functional and physicochemical properties of non-starch polysaccharides
6
Charles Brennan1, Uma Tiwari2 1Department of Food and Tourism, Manchester Metropolitan University, Manchester, UK 2UCD School of Agriculture, Food Science and Veterinary Medicine, University College Dublin, Dublin, Ireland
6.1 Introduction Non-starch polysaccharides (NSPs) or complex carbohydrates are the major part of dietary fiber (DF) and can be measured more precisely than total dietary fiber; they include cellulose, pectins, glucans, gums, mucilages, inulin and chitin (and exclude lignin) (Cummings and Englyst, 1995). NSPs are the principal components of the plant cell wall and constitute a major source of fiber in the diet (Selvendran and Robertson, 1990). Hence, the estimation of NSPs provides Pulse Foods: Processing, Quality and Nutraceutical Applications. DOI: 10.1016/B978-0-1238-2018-1.00007-0 Copyright © 2011 by Elsevier Inc
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a good estimate of fiber from plant foods (Englyst et al., 1994). According to Cummings (1997), NSPs are defined as polysaccharides (DP ≥ 10), which are non-a-glucans that reach the human colon. The key aspect of NSPs is that they are plant materials that are not digested by the enzymes of the human digestive tract but remain fermentable in the large intestine. The non-digestibility of NSP in the small intestine of human subjects has been demonstrated in studies in ileostomy patients (Sandberg et al., 1981; Englyst and Cummings, 1985). However, the criterion of non-digestibility in the small intestine is nowadays the fundamental point of most DF definitions (Champ et al., 2003). Non-starch polysaccharides are traditionally classified into soluble and insoluble fractions (Sasaki et al., 2004), although the exact differences in solubility are not always clear. It must be emphasized that this solubility may be determined under conditions which do not occur in the human small intestine (Topping, 1991). Nevertheless, the terms “soluble fiber” and “insoluble fiber” have entered into common usage and also serve to segregate NSPs on one of their best documented physiological effects – the lowering of plasma cholesterol, an established risk factor for cardiovascular diseases (CVD) (Brennan, 2005; Topping, 2007; Fernando et al., 2008). Physiologically, the NSPs can influence the bacterial flora of the colon (Plaami, 1997). This action is related to the fact that NSPs are potential substrates for colonic fermentation. Similar to bifidogenic compounds, these are food substances not digested by gastrointestinal enzymes that beneficially affect the host by selective stimulation of growth and/or activity of a limited number of colonic bacteria capable of producing short-chain fatty acids (SCFAs), mainly acetate, propionate and butyrate. Small amounts of branched SCFAs may also be formed from indigestible protein (Roberfroid, 2001; Campos-Vega et al., 2009).
6.2 NSP content of pulses In general, pulses contain NSPs in the range of 5.5–19.6% with a higher level of insoluble fraction (Table 6.1). The NSP include celluloses, hemicelluloses, lignin, pectin, gums and mucilages. The insoluble fraction of NSP is normally found on the outer protective layer of plants, whereas soluble NSP is normally found in the inner parts of plants. The major neutral sugars identified in the soluble
Functional and physicochemical properties of non-starch polysaccharides 159
Table 6.1 NSP content (g 100 g−1 DM) of pulses
Pulse
Insoluble NSP
Soluble NSP
Total NSP
White bean
12.73
4.54
17.27
Common bean Pinto bean
9.92 9.64 11.45
7.91 9.34 8.15
17.83 18.98 19.6
9.0 7.6 5.37 8.76 9.29
7.07 2.0 3.41 2.82 1.32
16.07 9.6 8.78 11.58 10.61
8.1 6.74 4.0
2.0 1.92 1.5
10.1 8.66 5.5
Chickpea
Lentils, dried Green lentil Red lentil
Reference Anderson and Bridges (1988) Bravo (1999) Marconi et al. (2000) Anderson and Bridges (1988) Bravo (1999) Periago et al. (1997) Bravo (1999) Marconi et al. (2000) Anderson and Bridges (1988) Stephen et al. (1995) Bravo (1999) Stephen et al. (1995)
and insoluble NSP fractions of horse gram were arabinose, xylose and glucose (Bravo et al., 1999). Variations occur in NSP content of pulses, which are mainly attributed to species, variety and other agronomic factors. Bravo (1999) observed higher levels of insoluble NSP content in beans, lentils and chickpeas compared to soluble NSP. They observed about 80% of total NSP as insoluble fraction in lentils. Gooneratne et al. (1994a) studied the distribution of NSP in mung bean tissue. They observed 0.4% in embryo, 2.5% in hull and 10.4% in cotyledon with a total fiber content of 13.3%, as NSP. Arabinose, galactose and uronic acid (galacturonic) account for 25% of the soluble NSP in mung bean, whereas pectic polysaccharides rich in arabinose were the major non-cellulosic polysaccharides in the insoluble fiber fraction, accounting for 42% of the NSP in mung bean (Gooneratne et al., 1994a).
6.3 Cellulose and hemicelluloses Cellulose consists of unbranched polymers of b-linked glucose residues arranged in linear chains which exist as a crystalline array of many parallel, oriented chains – microfibrils – resulting in “amorphous” or paracrystalline regions of the microfibril where the b-1,4-glucan chains are less ordered (Rose and Bennett, 1999). Cellulose content varies depending on the species.
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Hemicelluloses have b-(1-4)-linked backbones of xylose, mannose or glucose residues that can form extensive hydrogen bonds with cellulose. Hemicelluloses are cellulose-binding polysaccharides, which together with cellulose form a network. However, branches and other structural modifications in their structure prevent them from forming microfibrils by themselves. Xyloglucan and arabinoxylans are the most abundant hemicelluloses (Cosgrove, 2005). The hemicelluloses are more abundant in secondary walls than in the primary walls of both dicot and monocot species. Monocot species tend to have significantly more hemicellulose and less pectin than dicots, and also have mixed linkage glucans that make up a major proportion of monocot hemicellulose polysaccharides (Caffall and Mohnen, 2009). Srisuma et al. (1991) observed the cellulose content was the major component of the navy bean seed coat, ranging up to 60%, followed by the hemicellulose, ranging up to 20%, and a small amount of lignin, about 2%. Similarily, Górecka et al. (2000) reported that cellulose was predominant in the hull fraction of lupin whereas in the flour hemicellulose was the predominant fraction. Sosulski and Wu (1988) determined that field pea hulls contained 82.3% total dietary fiber and indicated that cellulose (62.3%) was the predominant form with only 8.2% hemicellulose content, whereas Pérez-Hidalgo et al. (1997) reported that the main component of pulses was hemicellulose with ≈12% for chickpeas and kidney beans and ≈16% for lentils but their cellulose content was low, ranging from 6 to 8%. This clearly illustrates the possible variations that exist in relatively close species.
6.4 Pectin, gums and mucilages The pectins, which are most abundant in the plant primary cell walls and the middle lamellae, represent a class of molecules defined by the presence of galacturonic acid (Caffall and Mohnen, 2009). Pectins include homogalacturonan and rhamnogalacturonan I. Homogalacturonan, which makes up the bulk of pectins, is composed of linear chains of galacturonic acid residues that are methyl-esterified and acetylated to different extents depending on the plant species (Willats et al., 2001). Bailoni et al. (2005) evaluated pectin content of the dietary fiber of legumes, reporting a range from 8 to 13%. Bravo (1999) demonstrated various pulse processing
Functional and physicochemical properties of non-starch polysaccharides 161
techniques and reported the presence of highest pectin content in soluble NSP fractions, except for lentils which appeared to have similar contents of soluble pectins and pectic substances associated with insoluble cell wall polysaccharides. Many processing factors such as boiling and thermal dehydration may degrade the pectin content of legumes (Cheung and Chau, 1998; Brennan and Tudorica, 2008; Aguilera et al., 2009). This may be due to dissolution of the middle lamellae and some breakdown of pectins through b-elimination during heat treatment (McDougall et al., 1996). Gums are used as thickening agent and increase the viscosity of food which has properties that are intermediate: it hydrates rapidly in cold water, but interacts with gel-forming polysaccharides (Harris and Smith, 2006). Mucilages are usually sticky substances and the soluble mucilages amount to over 40% of NSP composition, depending on the variety of legume seed. Mucilages are produced by the outer layer of the seed, i.e epidermis of seed (Harris and Ferguson, 1999). This mucilagenous mixture of compounds has been shown to be useful as a potential food ingredient with guar and locust bean gum (Tudoricã et al., 2002; Brennan et al., 2006).
6.5 Physiological effects of NSP As indicated previously, the NSP component of pulses has been reported for its valuable contribution to a healthy diet in both humans and animals (Goodlad and Mathers, 1991; Stephen et al., 1995; Brennan, 2005). NSPs are the principal components of dietary fiber and the lack of small intestinal digestibility explains the majority of their principal physiological properties. The physicochemical properties of NSPs result in a number of physiological effects that have been related to certain health benefits (Brennan, 2005). NSPs have been claimed to modulate blood glucose and insulin responses to foods (Jenkins et al., 1981), to lower blood cholesterol (Lairon, 1996) and to have beneficial effects on the prevention and treatment of certain diseases like gallstones, diverticular disease, obesity, constipation or colon cancer (Cummings et al., 1992; Cummings and Englyst, 1995). Studies in healthy humans have shown that ingested NSP is essentially unchanged during passage through the stomach and small intestine (Englyst and Cummings, 1985, 1987). The potential function of NSP in appetite control is to modulate the rate of
162 Pulse Foods: Processing, Quality and Nutraceutical Applications
stomach emptying, with a prolonged residence time of the food in the stomach which makes you feel full. The role of NSP in stomach fullness is realized either by increasing viscosity or by forming a gel (Lyly et al., 2004; Lundin et al., 2008). The function of NSP in human health is still not fully understood. In a human nutrition study, Stephen et al. (1995) reported that consuming approximately 12 g of soluble NSP from green lentils effectively increased fecal weight from 131 g day−1 to 189 g day−1. However, some authors argue that not enough evidence has been produced to demonstrate the role of NSP to reduce cholesterol and glucose level (Goodlad and Mathers, 1991). Cobiac et al. (1990) observed that intake of 12 g NSP from canned baked beans did not alter the plasma cholesterol or the glucose concentration in hypercholesterolemic men. Similarly, Key and Mathers (1993) noted that NSP digestibilities were 0.56 and 0.86 for wholemeal bread and beans, respectively, with no evidence that the dietary presence of beans affected digestibility of bread NSP. In an in vitro study Campos-Vega et al. (2009) suggested that the common bean is an excellent source of polysaccharides that can be fermented in the colon and produce short-chain fatty acids which exert health benefits.
6.6 Effect of processing on NSP As mentioned earlier, processing of pulses affects dietary fiber content and functional properties of dietary fiber. Similar to other fiber fractions, NSP content and properties are also influenced by processing. Marconi et al. (2000) studied the effects of both conventional and microwave cooking on the NSP content of chickpeas and common beans. They observed that both of the cooking procedures produced a redistribution of the insoluble to soluble NSP fraction, without affecting the total NSP content. They also observed that the decrease in the insoluble NSP content and increase in the soluble NSP fractions were more pronounced after microwave cooking compared to conventional cooking based on the ratio between the insoluble and soluble fiber fractions. The ratio of insoluble and soluble fiber fractions reduced from 3.11 g 100 g−1 dry matter (raw chickpeas) to 2.80 g 100 g−1 dry matter during traditional cooking and 2.49 g 100 g−1 dry matter after
Functional and physicochemical properties of non-starch polysaccharides 163
microwave cooking. They observed a similar effect in bean samples, where the insoluble:soluble ratio was reduced from 1.03 g 100 g−1 dry matter in the raw beans, to 0.87 g 100 g−1 dry matter after traditional cooking and 0.76 after microwave cooking. This suggests that the depolymerization of cell wall polysaccharides could be more extensive during microwave cooking than during conventional cooking (Marconi et al., 2000). Redistribution and subsequent shift in insoluble:soluble ratio could be caused by a partial solubilization and depolymerization of hemicellulose and insoluble pectic substances (Vidal-Valverde and Frias, 1991; Lintas et al., 1995). Increased cooking causes redistribution of soluble and insoluble NSP of the legume flours which may influence the total NSP content (Gooneratne et al., 1994b; Cheung and Chau, 1998). Periago et al. (1997) reported that both domestic and industrial cooking altered the NSP of chickpea, possibly due to depolymerization of insoluble material and the loss of soluble component as a consequence of processing. The soluble NSP increased in industrially processed pulses at the expense of the insoluble fraction and the reduction of total NSP was probably due to the presence of other food ingredients (Bravo, 1999). Similarly, Marconi et al. (2000) reported that post-cooking of chickpea and common beans led to an increase in total NSP content, which may be due to a greater loss of non-fiber components. Rehman and Shah (2004) studied the effect of domestic processing on the cellulose, hemicellulose and lignin contents of black grams, chickpeas, lentils, and red and white kidney beans (Table 6.2). They observed that pressure cooking had a pronounced effect on cellulose, hemicellulose and lignin contents compared to the microwave and ordinary cooking. They also observed that the reduction in hemicellulose was higher compared to cellulose. The reduction in cellulose and hemicellulose is mainly due to breakdown of these products into simple carbohydrates (Rehman and Shah, 2004). Periago et al. (1996) studied the effect of thermal treatment such as freezing, cooking and canning of peas. They observed that frozen cooked peas showed a higher content of total NSP than raw frozen peas whereas canned peas had slightly lower total NSP contents compared with frozen raw peas. The high temperature used in canning may lead to disruption of linkages in cell wall polysaccharides, which could lead to an increase in soluble NSP or to loss of NSP (Periago et al., 1996). In another study, Periago et al. (1996) reported
Table 6.2 Effect of different cooking methods on cellulose, hemicellulose and lignin contents (g 100 g−1 on dry basis) of various food pulses Pulse
Black gram Chickpea Lentil Red kidney bean White kidney bean
Cellulose
Hemicellulose
Lignin
Raw
Ordinary cooking
Pressure cooking
Microwave cooking
Raw
Ordinary cooking
Pressure cooking
Microwave cooking
Raw
Ordinary cooking
Pressure cooking
Microwave cooking
9.70 ± 0.95a 8.45 ± 1.21a 8.10 ± 1.08a 6.22 ± 0.48a
7.65 ± 0.41b 6.70 ± 0.48b 6.80 ± 0.72b 4.98 ± 0.22b
7.75 ± 0.38b 6.00 ± 0.45c 6.60 ± 0.49c 4.44 ± 0.21c
7.55 ± 0.42b 6.90 ± 0.44b 6.75 ± 0.33b 4.88 ± 0.24b
12.0 ± 1.11a 16.00 ± 1.08a 20.3 ± 1.32a 20.2 ± 1.17a
9.24 ± 0.62b 12.2 ± 0.73b 13.5 ± 0.80b 13.78 ± 0.42b
6.89 ± 0.70c 9.23 ± 0.48c 12.6 ± 0.81c 12.1 ± 0.55c
9.10 ± 0.88b 12.1 ± 0.75b 14.0 ± 0.90b 15.1 ± 0.50b
1.70 ± 0.11a 1.08 ± 0.16a 1.42 ± 0.15a 1.21 ± 0.11a
1.90 ± 0.08b 1.18 ± 0.12a 1.58 ± 0.21b 1.30 ± 0.08b
2.07 ± 0.12c 1.25 ± 0.11c 1.19 ± 0.13c 1.40 ± 0.09c
1.80 ± 0.13b 1.15 ± 0.16a 1.19 ± 0.13c 1.39 ± 0.11c
8.00 ± 0.59a
7.00 ± 0.70b
6.64 ± 0.53c
6.85 ± 0.51b
14.9 ± 1.00a
9.41 ± 0.85b
9.10 ± 0.73c
10.79 ± 0.67b
1.32 ± 0.14a
1.50 ± 0.10b
1.40 ± 0.11c
1.50 ± 0.14b
Mean values ± SD (on dry basis). Mean values in a row with different superscripts are significantly different at P < 0.05.
Functional and physicochemical properties of non-starch polysaccharides 165
an increase in NSP content in peas after cooking, which is due to losses of non-fiber material, presumably mainly free sugars. Cheung and Chau (1998) investigated the effect of cooking on the NSPs of Phaseolus angularis, Phaseolus calcaratus and Dolichos lablab flour. They observed an increase in soluble NSP content with cooking time. The increase in soluble fraction could be due to solubilization of cell wall pectic substances as a result of dissolution of the middle lamellae and some breakdown of pectins through b-elimination during boiling (McDougall et al., 1996; Cheung and Chau, 1998). Redistribution of the insoluble fraction and soluble components of NSP has been reported for peas (Periago et al., 1996), black gram and green gram (Gooneratne et al., 1994a) during thermal treatment. The increase in the solubilization of pectic polysaccharides during the cooking of these pulses was important for their therapeutic values as soup ingredients (Cheung and Chau, 1998). Gooneratne et al. (1994a) observed an increase in soluble NSP fraction in extruded mung bean flour. The increased soluble NSP content relative to the raw flour was due to mainly an increased solubility of arabinose-containing pectic polysaccharides resulting from a slight degradation of pectic polysaccharides. Gooneratne et al. (1994a) stated that the notable increase in glucose-containing soluble NSP was probably due to the presence of some “resistant starch” formed during the extrusion process; this starch was resistant to amylolytic digestion.
6.7 Conclusions Pulses are potential sources of non-starch polysaccharides. Variations exist in NSP content of pulses which is mainly due to genotypic variation. There is convincing evidence that NSPs play an important role in the human diet and have several physiological benefits including appetite control, slowing down small intestine transit, lowering blood plasma cholesterol and improving bowel function. However, some of these health benefits are still inconclusive. Compared to pulses there is strong evidence that cereal soluble NSPs have benefits in the control of plasma cholesterol (Topping, 2007). However, more work is needed to investigate the function of pulse NSPs in physiological responses such as regulating obesity and diabetes.
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Charles Brennan1, Uma Tiwari2 1Department of Food and Tourism, Manchester Metropolitan University, Manchester, UK 2UCD School of Agriculture, Food Science and Veterinary Medicine, University College Dublin, Dublin, Ireland
6.1 Introduction Non-starch polysaccharides (NSPs) or complex carbohydrates are the major part of dietary fiber (DF) and can be measured more precisely than total dietary fiber; they include cellulose, pectins, glucans, gums, mucilages, inulin and chitin (and exclude lignin) (Cummings and Englyst, 1995). NSPs are the principal components of the plant cell wall and constitute a major source of fiber in the diet (Selvendran and Robertson, 1990). Hence, the estimation of NSPs provides Pulse Foods: Processing, Quality and Nutraceutical Applications. DOI: 10.1016/B978-0-1238-2018-1.00007-0 Copyright © 2011 by Elsevier Inc
158 Pulse Foods: Processing, Quality and Nutraceutical Applications
a good estimate of fiber from plant foods (Englyst et al., 1994). According to Cummings (1997), NSPs are defined as polysaccharides (DP ≥ 10), which are non-a-glucans that reach the human colon. The key aspect of NSPs is that they are plant materials that are not digested by the enzymes of the human digestive tract but remain fermentable in the large intestine. The non-digestibility of NSP in the small intestine of human subjects has been demonstrated in studies in ileostomy patients (Sandberg et al., 1981; Englyst and Cummings, 1985). However, the criterion of non-digestibility in the small intestine is nowadays the fundamental point of most DF definitions (Champ et al., 2003). Non-starch polysaccharides are traditionally classified into soluble and insoluble fractions (Sasaki et al., 2004), although the exact differences in solubility are not always clear. It must be emphasized that this solubility may be determined under conditions which do not occur in the human small intestine (Topping, 1991). Nevertheless, the terms “soluble fiber” and “insoluble fiber” have entered into common usage and also serve to segregate NSPs on one of their best documented physiological effects – the lowering of plasma cholesterol, an established risk factor for cardiovascular diseases (CVD) (Brennan, 2005; Topping, 2007; Fernando et al., 2008). Physiologically, the NSPs can influence the bacterial flora of the colon (Plaami, 1997). This action is related to the fact that NSPs are potential substrates for colonic fermentation. Similar to bifidogenic compounds, these are food substances not digested by gastrointestinal enzymes that beneficially affect the host by selective stimulation of growth and/or activity of a limited number of colonic bacteria capable of producing short-chain fatty acids (SCFAs), mainly acetate, propionate and butyrate. Small amounts of branched SCFAs may also be formed from indigestible protein (Roberfroid, 2001; Campos-Vega et al., 2009).
6.2 NSP content of pulses In general, pulses contain NSPs in the range of 5.5–19.6% with a higher level of insoluble fraction (Table 6.1). The NSP include celluloses, hemicelluloses, lignin, pectin, gums and mucilages. The insoluble fraction of NSP is normally found on the outer protective layer of plants, whereas soluble NSP is normally found in the inner parts of plants. The major neutral sugars identified in the soluble
Functional and physicochemical properties of non-starch polysaccharides 159
Table 6.1 NSP content (g 100 g−1 DM) of pulses
Pulse
Insoluble NSP
Soluble NSP
Total NSP
White bean
12.73
4.54
17.27
Common bean Pinto bean
9.92 9.64 11.45
7.91 9.34 8.15
17.83 18.98 19.6
9.0 7.6 5.37 8.76 9.29
7.07 2.0 3.41 2.82 1.32
16.07 9.6 8.78 11.58 10.61
8.1 6.74 4.0
2.0 1.92 1.5
10.1 8.66 5.5
Chickpea
Lentils, dried Green lentil Red lentil
Reference Anderson and Bridges (1988) Bravo (1999) Marconi et al. (2000) Anderson and Bridges (1988) Bravo (1999) Periago et al. (1997) Bravo (1999) Marconi et al. (2000) Anderson and Bridges (1988) Stephen et al. (1995) Bravo (1999) Stephen et al. (1995)
and insoluble NSP fractions of horse gram were arabinose, xylose and glucose (Bravo et al., 1999). Variations occur in NSP content of pulses, which are mainly attributed to species, variety and other agronomic factors. Bravo (1999) observed higher levels of insoluble NSP content in beans, lentils and chickpeas compared to soluble NSP. They observed about 80% of total NSP as insoluble fraction in lentils. Gooneratne et al. (1994a) studied the distribution of NSP in mung bean tissue. They observed 0.4% in embryo, 2.5% in hull and 10.4% in cotyledon with a total fiber content of 13.3%, as NSP. Arabinose, galactose and uronic acid (galacturonic) account for 25% of the soluble NSP in mung bean, whereas pectic polysaccharides rich in arabinose were the major non-cellulosic polysaccharides in the insoluble fiber fraction, accounting for 42% of the NSP in mung bean (Gooneratne et al., 1994a).
6.3 Cellulose and hemicelluloses Cellulose consists of unbranched polymers of b-linked glucose residues arranged in linear chains which exist as a crystalline array of many parallel, oriented chains – microfibrils – resulting in “amorphous” or paracrystalline regions of the microfibril where the b-1,4-glucan chains are less ordered (Rose and Bennett, 1999). Cellulose content varies depending on the species.
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Hemicelluloses have b-(1-4)-linked backbones of xylose, mannose or glucose residues that can form extensive hydrogen bonds with cellulose. Hemicelluloses are cellulose-binding polysaccharides, which together with cellulose form a network. However, branches and other structural modifications in their structure prevent them from forming microfibrils by themselves. Xyloglucan and arabinoxylans are the most abundant hemicelluloses (Cosgrove, 2005). The hemicelluloses are more abundant in secondary walls than in the primary walls of both dicot and monocot species. Monocot species tend to have significantly more hemicellulose and less pectin than dicots, and also have mixed linkage glucans that make up a major proportion of monocot hemicellulose polysaccharides (Caffall and Mohnen, 2009). Srisuma et al. (1991) observed the cellulose content was the major component of the navy bean seed coat, ranging up to 60%, followed by the hemicellulose, ranging up to 20%, and a small amount of lignin, about 2%. Similarily, Górecka et al. (2000) reported that cellulose was predominant in the hull fraction of lupin whereas in the flour hemicellulose was the predominant fraction. Sosulski and Wu (1988) determined that field pea hulls contained 82.3% total dietary fiber and indicated that cellulose (62.3%) was the predominant form with only 8.2% hemicellulose content, whereas Pérez-Hidalgo et al. (1997) reported that the main component of pulses was hemicellulose with ≈12% for chickpeas and kidney beans and ≈16% for lentils but their cellulose content was low, ranging from 6 to 8%. This clearly illustrates the possible variations that exist in relatively close species.
6.4 Pectin, gums and mucilages The pectins, which are most abundant in the plant primary cell walls and the middle lamellae, represent a class of molecules defined by the presence of galacturonic acid (Caffall and Mohnen, 2009). Pectins include homogalacturonan and rhamnogalacturonan I. Homogalacturonan, which makes up the bulk of pectins, is composed of linear chains of galacturonic acid residues that are methyl-esterified and acetylated to different extents depending on the plant species (Willats et al., 2001). Bailoni et al. (2005) evaluated pectin content of the dietary fiber of legumes, reporting a range from 8 to 13%. Bravo (1999) demonstrated various pulse processing
Functional and physicochemical properties of non-starch polysaccharides 161
techniques and reported the presence of highest pectin content in soluble NSP fractions, except for lentils which appeared to have similar contents of soluble pectins and pectic substances associated with insoluble cell wall polysaccharides. Many processing factors such as boiling and thermal dehydration may degrade the pectin content of legumes (Cheung and Chau, 1998; Brennan and Tudorica, 2008; Aguilera et al., 2009). This may be due to dissolution of the middle lamellae and some breakdown of pectins through b-elimination during heat treatment (McDougall et al., 1996). Gums are used as thickening agent and increase the viscosity of food which has properties that are intermediate: it hydrates rapidly in cold water, but interacts with gel-forming polysaccharides (Harris and Smith, 2006). Mucilages are usually sticky substances and the soluble mucilages amount to over 40% of NSP composition, depending on the variety of legume seed. Mucilages are produced by the outer layer of the seed, i.e epidermis of seed (Harris and Ferguson, 1999). This mucilagenous mixture of compounds has been shown to be useful as a potential food ingredient with guar and locust bean gum (Tudoricã et al., 2002; Brennan et al., 2006).
6.5 Physiological effects of NSP As indicated previously, the NSP component of pulses has been reported for its valuable contribution to a healthy diet in both humans and animals (Goodlad and Mathers, 1991; Stephen et al., 1995; Brennan, 2005). NSPs are the principal components of dietary fiber and the lack of small intestinal digestibility explains the majority of their principal physiological properties. The physicochemical properties of NSPs result in a number of physiological effects that have been related to certain health benefits (Brennan, 2005). NSPs have been claimed to modulate blood glucose and insulin responses to foods (Jenkins et al., 1981), to lower blood cholesterol (Lairon, 1996) and to have beneficial effects on the prevention and treatment of certain diseases like gallstones, diverticular disease, obesity, constipation or colon cancer (Cummings et al., 1992; Cummings and Englyst, 1995). Studies in healthy humans have shown that ingested NSP is essentially unchanged during passage through the stomach and small intestine (Englyst and Cummings, 1985, 1987). The potential function of NSP in appetite control is to modulate the rate of
162 Pulse Foods: Processing, Quality and Nutraceutical Applications
stomach emptying, with a prolonged residence time of the food in the stomach which makes you feel full. The role of NSP in stomach fullness is realized either by increasing viscosity or by forming a gel (Lyly et al., 2004; Lundin et al., 2008). The function of NSP in human health is still not fully understood. In a human nutrition study, Stephen et al. (1995) reported that consuming approximately 12 g of soluble NSP from green lentils effectively increased fecal weight from 131 g day−1 to 189 g day−1. However, some authors argue that not enough evidence has been produced to demonstrate the role of NSP to reduce cholesterol and glucose level (Goodlad and Mathers, 1991). Cobiac et al. (1990) observed that intake of 12 g NSP from canned baked beans did not alter the plasma cholesterol or the glucose concentration in hypercholesterolemic men. Similarly, Key and Mathers (1993) noted that NSP digestibilities were 0.56 and 0.86 for wholemeal bread and beans, respectively, with no evidence that the dietary presence of beans affected digestibility of bread NSP. In an in vitro study Campos-Vega et al. (2009) suggested that the common bean is an excellent source of polysaccharides that can be fermented in the colon and produce short-chain fatty acids which exert health benefits.
6.6 Effect of processing on NSP As mentioned earlier, processing of pulses affects dietary fiber content and functional properties of dietary fiber. Similar to other fiber fractions, NSP content and properties are also influenced by processing. Marconi et al. (2000) studied the effects of both conventional and microwave cooking on the NSP content of chickpeas and common beans. They observed that both of the cooking procedures produced a redistribution of the insoluble to soluble NSP fraction, without affecting the total NSP content. They also observed that the decrease in the insoluble NSP content and increase in the soluble NSP fractions were more pronounced after microwave cooking compared to conventional cooking based on the ratio between the insoluble and soluble fiber fractions. The ratio of insoluble and soluble fiber fractions reduced from 3.11 g 100 g−1 dry matter (raw chickpeas) to 2.80 g 100 g−1 dry matter during traditional cooking and 2.49 g 100 g−1 dry matter after
Functional and physicochemical properties of non-starch polysaccharides 163
microwave cooking. They observed a similar effect in bean samples, where the insoluble:soluble ratio was reduced from 1.03 g 100 g−1 dry matter in the raw beans, to 0.87 g 100 g−1 dry matter after traditional cooking and 0.76 after microwave cooking. This suggests that the depolymerization of cell wall polysaccharides could be more extensive during microwave cooking than during conventional cooking (Marconi et al., 2000). Redistribution and subsequent shift in insoluble:soluble ratio could be caused by a partial solubilization and depolymerization of hemicellulose and insoluble pectic substances (Vidal-Valverde and Frias, 1991; Lintas et al., 1995). Increased cooking causes redistribution of soluble and insoluble NSP of the legume flours which may influence the total NSP content (Gooneratne et al., 1994b; Cheung and Chau, 1998). Periago et al. (1997) reported that both domestic and industrial cooking altered the NSP of chickpea, possibly due to depolymerization of insoluble material and the loss of soluble component as a consequence of processing. The soluble NSP increased in industrially processed pulses at the expense of the insoluble fraction and the reduction of total NSP was probably due to the presence of other food ingredients (Bravo, 1999). Similarly, Marconi et al. (2000) reported that post-cooking of chickpea and common beans led to an increase in total NSP content, which may be due to a greater loss of non-fiber components. Rehman and Shah (2004) studied the effect of domestic processing on the cellulose, hemicellulose and lignin contents of black grams, chickpeas, lentils, and red and white kidney beans (Table 6.2). They observed that pressure cooking had a pronounced effect on cellulose, hemicellulose and lignin contents compared to the microwave and ordinary cooking. They also observed that the reduction in hemicellulose was higher compared to cellulose. The reduction in cellulose and hemicellulose is mainly due to breakdown of these products into simple carbohydrates (Rehman and Shah, 2004). Periago et al. (1996) studied the effect of thermal treatment such as freezing, cooking and canning of peas. They observed that frozen cooked peas showed a higher content of total NSP than raw frozen peas whereas canned peas had slightly lower total NSP contents compared with frozen raw peas. The high temperature used in canning may lead to disruption of linkages in cell wall polysaccharides, which could lead to an increase in soluble NSP or to loss of NSP (Periago et al., 1996). In another study, Periago et al. (1996) reported
Table 6.2 Effect of different cooking methods on cellulose, hemicellulose and lignin contents (g 100 g−1 on dry basis) of various food pulses Pulse
Black gram Chickpea Lentil Red kidney bean White kidney bean
Cellulose
Hemicellulose
Lignin
Raw
Ordinary cooking
Pressure cooking
Microwave cooking
Raw
Ordinary cooking
Pressure cooking
Microwave cooking
Raw
Ordinary cooking
Pressure cooking
Microwave cooking
9.70 ± 0.95a 8.45 ± 1.21a 8.10 ± 1.08a 6.22 ± 0.48a
7.65 ± 0.41b 6.70 ± 0.48b 6.80 ± 0.72b 4.98 ± 0.22b
7.75 ± 0.38b 6.00 ± 0.45c 6.60 ± 0.49c 4.44 ± 0.21c
7.55 ± 0.42b 6.90 ± 0.44b 6.75 ± 0.33b 4.88 ± 0.24b
12.0 ± 1.11a 16.00 ± 1.08a 20.3 ± 1.32a 20.2 ± 1.17a
9.24 ± 0.62b 12.2 ± 0.73b 13.5 ± 0.80b 13.78 ± 0.42b
6.89 ± 0.70c 9.23 ± 0.48c 12.6 ± 0.81c 12.1 ± 0.55c
9.10 ± 0.88b 12.1 ± 0.75b 14.0 ± 0.90b 15.1 ± 0.50b
1.70 ± 0.11a 1.08 ± 0.16a 1.42 ± 0.15a 1.21 ± 0.11a
1.90 ± 0.08b 1.18 ± 0.12a 1.58 ± 0.21b 1.30 ± 0.08b
2.07 ± 0.12c 1.25 ± 0.11c 1.19 ± 0.13c 1.40 ± 0.09c
1.80 ± 0.13b 1.15 ± 0.16a 1.19 ± 0.13c 1.39 ± 0.11c
8.00 ± 0.59a
7.00 ± 0.70b
6.64 ± 0.53c
6.85 ± 0.51b
14.9 ± 1.00a
9.41 ± 0.85b
9.10 ± 0.73c
10.79 ± 0.67b
1.32 ± 0.14a
1.50 ± 0.10b
1.40 ± 0.11c
1.50 ± 0.14b
Mean values ± SD (on dry basis). Mean values in a row with different superscripts are significantly different at P < 0.05.
Functional and physicochemical properties of non-starch polysaccharides 165
an increase in NSP content in peas after cooking, which is due to losses of non-fiber material, presumably mainly free sugars. Cheung and Chau (1998) investigated the effect of cooking on the NSPs of Phaseolus angularis, Phaseolus calcaratus and Dolichos lablab flour. They observed an increase in soluble NSP content with cooking time. The increase in soluble fraction could be due to solubilization of cell wall pectic substances as a result of dissolution of the middle lamellae and some breakdown of pectins through b-elimination during boiling (McDougall et al., 1996; Cheung and Chau, 1998). Redistribution of the insoluble fraction and soluble components of NSP has been reported for peas (Periago et al., 1996), black gram and green gram (Gooneratne et al., 1994a) during thermal treatment. The increase in the solubilization of pectic polysaccharides during the cooking of these pulses was important for their therapeutic values as soup ingredients (Cheung and Chau, 1998). Gooneratne et al. (1994a) observed an increase in soluble NSP fraction in extruded mung bean flour. The increased soluble NSP content relative to the raw flour was due to mainly an increased solubility of arabinose-containing pectic polysaccharides resulting from a slight degradation of pectic polysaccharides. Gooneratne et al. (1994a) stated that the notable increase in glucose-containing soluble NSP was probably due to the presence of some “resistant starch” formed during the extrusion process; this starch was resistant to amylolytic digestion.
6.7 Conclusions Pulses are potential sources of non-starch polysaccharides. Variations exist in NSP content of pulses which is mainly due to genotypic variation. There is convincing evidence that NSPs play an important role in the human diet and have several physiological benefits including appetite control, slowing down small intestine transit, lowering blood plasma cholesterol and improving bowel function. However, some of these health benefits are still inconclusive. Compared to pulses there is strong evidence that cereal soluble NSPs have benefits in the control of plasma cholesterol (Topping, 2007). However, more work is needed to investigate the function of pulse NSPs in physiological responses such as regulating obesity and diabetes.
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