Sweet potato dietary fiber Kazunori Takamine1, Meng-Mei Ma2 and Fredrick O. Ogutu2,3 1
Division of Shochu Fermentation Technology, Education and Research Center for Fermentation Studies, Faculty of Agriculture, Kagoshima University, Kagoshima, Japan 2 Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences; Key Laboratory of Agro-Products Processing, Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China 3 Food Technology Division, Kenya Industrial Research and Development Institute, Nairobi, Kenya
Overview of dietary fiber Definition of dietary fiber One definition that has been proposed for dietary fiber is “all of the poorly digestible ingredients in food, that cannot be digested by human digestive enzymes” (Kiriyama, 1980). Dietary fiber includes the structural polysaccharides (cellulose, hemicellulose) and lignin in plant cell walls, the viscous storage polysaccharides such as pectin and guar gum, the indigestible polysaccharides such as chitin and chitosan, as well as others. In addition, it includes starch-like polysaccharides, such as resistant starch and indigestible dextrose, and synthetic polysaccharides, such as polydextrose.
Classification of dietary fiber Dietary fiber is further divided into insoluble and soluble dietary fiber according to the solubility, with the former including cellulose, hemicellulose, lignin, and chitin, and the latter including pectin, plant gums, such as guar gum, and viscous substances, such as glucomannan. Cellulose is the principal component of plant cell walls, which comprises a linear-chain polysaccharide of D-glucose residues linked by β-1,4glycosidic bonds. Cellulose molecules have a degree of polymerization between approximately 3000 and 10,000, and in cell walls they associate with one another through hydrogen bonding to adopt a crystal structure. Cellulose molecules are insensitive to the hydrolytic activities of acid and digestive enzymes. Although cellulose is insoluble in water, it readily absorbs water, which causes it to swell. Hemicellulose is a matrix polysaccharide found in plant cell walls. Its main form is arabinoxylan, which consists of xylan, a polysaccharide with Sweet Potato DOI: https://doi.org/10.1016/B978-0-12-813637-9.00005-3
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β-1,4-bonded-D-xylose residues, and side branches of L-arabinose, D -glucuronic acid, and 4-O-methylglucuronic acid. In cell walls hemicellulose binds to cellulose through hydrogen bonding and this contributes to the structural maintenance of the cell wall. Lignin is not a carbohydrate, but instead is an aromatic polymer that is strongly associated with cellulose through chemical bonds. Wood, for example, contains approximately 20% 30% lignin. Lignin is chemically and enzymatically difficult to break down, and it can be separated as a residue that is insoluble in the presence of 72% (w/w) sulfuric acid. Chitin is a polysaccharide of β-1,4-linked N-acetyl-D-glucosamine residues which is found in a diverse array of organisms, where it plays important structural roles. The shells of crabs and shrimp contain 10% 30% chitin, for example. Chitin is also present in the cell walls of molds, yeast, mushrooms, etc., often in a complex with protein. Chitin is insoluble in water, organic solvents, dilute acids, and dilute alkalis. Pectin is an acidic polysaccharide whose primary constituent is galacturonan, a polysaccharide containing α-1,4-linked residues of the acidic sugar galacturonic acid. Pectin is water soluble, but within cell walls it binds calcium, which renders it insoluble in water. Another form of pectin is rhamnogalacturonan, which has a galacturonan backbone containing L-rhamnose residues interspersed along the chain. The rhamnose residues can also be bound to neutral sugars such as L-arabinose, D-xylose, and D-galactose to create heteropolysaccharides. The carboxyl groups in polygalacturonic acid are also partially methyl-esterified. Guar gum is obtained from the endosperm of the seed of the legume guar. It is a galactomannan composed of galactose and mannose in a 1:2 ratio. Polydextrose is a water-soluble, indigestible synthetic polymer, obtained by mixing glucose, sorbitol, and citric acid, and conducting a condensation polymerization reaction under high temperature and high vacuum conditions.
Uses of dietary fiber In addition to its use as a bulking agent in food, dietary fiber has various physiological effects that allow it play an important role in maintaining health (Ebihara and Kiriyama, 1990). This role is different from that of the essential nutrients such as water, carbohydrates, protein, fats, vitamins, and minerals; thus dietary fiber is often considered to be the seventh
Sweet potato dietary fiber
nutrient. For example, insoluble dietary fiber, when consumed, needs to be chewed more than other types of food. It causes an increase in the secretion of digestive fluids, and suppresses the onset of colorectal cancer by increasing the amount of stool produced, and preventing and reducing the occurrence of constipation (Ebihara and Kiriyama, 1990). When dietary fiber reaches the large intestine, it is fermented by intestinal bacteria in the colon, producing carbon dioxide and organic acids, such as shortchain fatty acids. Among dietary fibers, water-soluble dietary fibers, such as some soluble hemicellulose and pectin, can be easily metabolized to organic acids by intestinal bacteria. It has been suggested that these organic acids are involved in suppressing the onset of colorectal cancer and decreasing plasma cholesterol (Bugaut and Bentéjac, 1993; Ebihara et al., 1993a,b; Salyers et al., 1977). In addition, dietary fiber has a protective effect on the mucosa of the gastrointestinal tract and also suppresses hunger, because it is capable of absorbing water and adopting a gel-like form that contributes to a feeling of fullness.
Dietary fiber from sweet potato Sweet potato pulp Sweet potato pulp is a by-product generated during the production of sweet potato starch (Fig. 5.1). Sweet potato starch is produced by pulverizing the highest quality sweet potatoes in a pulverizer, sequentially wetsieving the material through 80, 150, and 200 mesh sieves, and separating it into the starch fraction and the residue. Foreign substances are removed from the starch fraction with a nozzle separator, and the fraction is refined with a 300-mesh sieve to obtain sweet potato starch. Sweet potato starch is then either dried with warm air or sun-dried, before it is used. Filtrate (300-mesh)
Filtrate (200-mesh) Starch
Filtrate (150-mesh) Sweet potato
Figure 5.1 Process of sweet potato starch and sweet potato pulp.
Sweet potato pulp
Figure 5.2 Microscopic observation of sweet potato pulp.
Meanwhile the residue is dehydrated to approximately 75% water content using a dehydrator to form the final sweet potato pulp. When sweet potato pulp is observed under a biological microscope, it appears as uncrushed, egg-shaped cell walls packed with starch or as crushed cell walls, in which the starch only partially remains (Fig. 5.2). The dry weight of sweet potato pulp consists of 43.5% starch, 2.0% protein, 0.4% lipids, 4.4% ash, and 49.7% dietary fiber. Dietary fiber can be manufactured by efficiently removing only the starch from sweet potato pulp. This chapter describes the efficient manufacturing of dietary fiber from sweet potato pulp (Takamine et al., 2000a), the physical properties of dietary fiber (Mei et al., 2010; Takamine et al., 2000a), a functional evaluation of dietary fiber in rats (Takamine et al., 2005), the extraction of pectin, an ingredient in dietary fiber, and the properties of pectin (Takamine et al., 2000b, 2007; Ogutu and Mu, 2017).
Efficient manufacture of dietary fiber from sweet potato pulp The particle size of sweet potato pulp ranges from 45 to 710 μm, with approximately 80% of the particles being between 150 and 350 μm, and most particles are at least 100 μm. Meanwhile, the core particles of sweet potato starch range from 15 to 20 μm, and at most 30 μm. Based on this, a method has been developed for manufacturing dietary fiber that relies on the difference in particle size between sweet potato pulp and starch (Takamine et al., 2000a). Specifically, the method of obtaining sweet potato-derived dietary fiber (SPDF) involves pulverizing sweet potato pulp, and then sorting it using sieves with pore sizes of 45 and 100 μm,
Sweet potato dietary fiber
Table 5.1 Components of sweet potato pulp and sweet potato dietary fiber (%, DW). Components
Sweet potato pulp
Sweet potato dietary fiber
Starch Crude protein Crude lipids Ash Dietary fiber Cellulose Hemicellulose Pectin Lignin
43.5 2.0 0.4 4.4 49.7 32.9 22.8 41.3 3.0
4.7 1.1 0.2 5.9 88.1 33.6 23.4 39.5 3.5
respectively; the material that passes through the 100 μm sieve, but remains on the top of the 45 μm sieve, is the SPDF. The composition of SPDF is shown in Table 5.1. The starch content by dry weight is 4.7%, being approximately one tenth of the quantity of starch found in sweet potato pulp. Proteins and lipids are reduced by approximately a half and the ash content increased slightly to 5.9%. This latter increase arises because of the reduction in protein and lipids which causes a corresponding relative increase in the percentage of ash. The dietary fiber in SPDF is 88.1%, an approximate 1.8-fold improvement over that of sweet potato pulp, a value that compares favorably with that of beet fiber (Nippon Beet Sugar Mfg. Co., Ltd.) and corn fiber (Nihon Shokuhin Kako Co., Ltd.). Also, the cellulose, hemicellulose, pectin, and lignin components of the dietary fiber are almost unchanged in abundance between SPDF and sweet potato pulp dietary fiber, as shown in Table 5.1. The SPDF therefore retains the dietary fiber derived from the sweet potato in an essentially unaltered form.
Physicochemical and functional properties of sweet potato dietary fiber Water and oil retention capacity Both water retention capacity and oil retention capacity are important parameters in the preparation of SPDF. A comparison of the water retention capacity and the oil retention capacity of SPDF obtained by air-dried at 60°C (AD-SPDF), freeze-dried SPDF (FD-SPDF), beet fiber, and corn fiber was shown in Fig. 5.3. The water retention capacity and oil retention capacity are both markedly superior in FD-SPDF, whereas ADSPDF exhibits retention capacities on a par with that of beet fiber. The
Water-holding capacity (g water / g dietary fiber )
Corn fiber Beet fiber
30 20 10 0 64
Centrifugal force (xg)
Oil-holding capacity (g oil / g dietary fiber )
20 15 10 5 0 64
417 1670 Centrifugal force (xg)
Figure 5.3 Water retention capacity (A) and oil retention capacity (B) of dietary fiber.
water retention capacity and oil retention capacity of FD-SPDF decrease as the centrifugal force increases, whereas for AD-SPDF, beet fiber, and corn fiber, the water and oil retention capacities are unaffected by the centrifugal force. The water retained in dietary fiber can be divided into three types: Type 1, water absorbed at the tissue surface; Type 2, water taken up into the tissue pores; and Type 3, free water implanted between the dietary fiber particles (Takeda and Kiriyama, 1995). The majority of water or oil content taken up by FD-SPDF is Type 2, and all the other samples are Type 1. These differences in water-holding capacity and oilholding capacity are thought to be due to the fact that FD-SPDF has a sponge-like, soft structure, as compared with AD-SPDF, beet fiber, and corn fiber, which all have a rigid surface, perhaps as a result of having been air-dried.
Sweet potato dietary fiber
Table 5.2 Color properties.
SPDF Beet fiber Corn fiber a
Index of whiteness
Index of yellowness
9.6 15.8 19.7
81.5 75.0 72.3
16.7 23.5 39.5
Value of a different from a standard white plate. Sweet potato dietary fiber.
Table 5.3 Composition of control and sweet potato dietary fiber diets (g/kg). Constituent
Sweet potato dietary fiber
Casein α-Corn starch Corn oil Sucrose Cellulose powder Sweet potato dietary fiber Mineral mixture Vitamin mixture Choline chloride
200 299 60 300 100 0 30 10 1
200 299 60 300 0 100 30 10 1
Color and sensory evaluation The color difference from a standard white plate (ΔE) and the whiteness value were 9.6 and 81.5, respectively, for AD-SPDF, which was the closest to white, compared with beet fiber and corn fiber. Also AD-SPDF had the lowest yellowness, at 16.7, a value half or less than half that of corn fiber (Table 5.2). In addition, AD-SPDF was virtually odorless, since it contains almost no protein or lipids.
Effects of sweet potato dietary fiber on cecal fermentation products and intestinal flora in rats Four-week-old male Wister-Hanover rats (BrlHan:WIST@Jcl) were reared in individual cages at room temperatures between 22°C and 24°C, with 12 h alternating light and dark cycles (light from 8:00 to 20:00). Commercially available solid feed (CE-2, Clea Japan, Inc.) was given for 1 week to allow acclimatization to the environment. Following this, the rats were divided into two groups (n 5 7 each) and provided with feed containing 10% cellulose or SPDF, respectively, for 27 days. The detailed composition of the experimental feed is shown in Table 5.3
(Osaki et al., 2001). Rats fed the cellulose-containing feed were used as controls. Free access to feed and drinking water was provided, and body weight was measured every 2 3 days. The quantity of stools produced and the quantity of feed consumed were measured in aggregate in 2- to 3-day increments. The water content of the stools was obtained by heating the stools for 17 h at 105°C. After 27 days of feeding, the rats were anesthetized with diethyl ether, and necropsies were performed. The SPDF group gained less weight than the control group, although this was not a statistically significant difference (Table 5.4). The SPDF group also consumed significantly less feed than the control group, so SPDF has the effect of suppressing appetite. A significant difference was not seen with respect to feed efficiency or cecal content pH. The SPDF group had a higher wet weight of cecal contents and wet weight of cecal tissue, but these differences were not significant. In other studies, rats given feed containing dietary fiber have been shown to generally consume less feed than rats fed standard feed, and large differences in feed efficiency have not been observed (Aritsuka et al., 1992; Hara et al., 1994). In our study the SPDF group also consumed a significantly lower quantity of food than the control group, but the feed efficiency was almost the same in both groups. The stool wet weight was lower in the SPDF group than in the control group, but despite this, the stool water content was significantly higher in the SPDF group than in the control group. The stool dry weight divided by the feed intake quantity was lower in the SPDF group compared with the control group, being 0.21 and 0.26, respectively. When rats are fed a large quantity of dietary fiber, the cecum and colon expand, but because the most marked expansion is observed in rats reared with feed containing an α-glucosidase inhibitor, it is believed that the expansion of the cecum and colon is not a phenomenon specific to dietary fiber, but rather, that expansion depends on an increase in the quantity and water retention properties of undigested material passing through the small intestine into the cecum (Aritsuka et al., 1992; Hara et al., 1994; Oku et al., 1981). Also because both the solid content and water content of the stools increase as dietary fiber intake increases, the stool volume and weight also increase, an effect that is reported to be greater with insoluble dietary fiber than with soluble dietary fiber (Hayakawa et al., 2003; Takahashi et al., 1999). In our study, the wet weight of both the cecal contents and cecal tissue increased with the addition of SPDF, an effect which likely arises because SPDF has a high water
Sweet potato dietary fiber
Table 5.4 Influence of the sweet potato dietary fiber on body weight gain, food intake, cecum, feces, microflora, and organic acid in cecum contents of rats.
Initial body weight (g) Body weight gain (g/27 d) Food intake (g/d) Food efficiencyc Cecum (g/100 g of body weight) Content of cecum (g/100 g of body weight) Tissue of cecum (g/100 g of body weight) pH of cecum Fecal weight (wet, g/d) Fecal weight (dry, g/d) Moisture (%) Clostridium Lactobacillus Bacteroides Peptostreptococcus Enterococcus Bifidobacterium Eubacterium Enterobacteriaceae Total aerobes Total anaerobes Total anaerobes/total aerobes Succinic acid Lactic acid Formic acid Acetic acid Propionic acid Isobutyric acid n-Butyric acid Isovaleric acid Total
129 6 7 150 6 16 24.5 6 2.1 0.22 6 0.01 1.27 6 0.29
130 6 6 136 6 10 21.8 6 1.2b 0.23 6 0.01 1.55 6 0.50
0.87 6 0.25
0.99 6 0.48
0.40 6 0.07
0.56 6 0.12
6.95 6 0.20 7.20 6 1.66 6.38 6 1.52 11.6 6 1.85 5.68 6 0.8d 7.20 6 0.8 8.18 6 0.3 7.59 6 0.7 6.41 6 1.1 n.d.f n.d. 6.23 6 1.4 6.60 6 0.7 8.34 6 0.3 23 1.9 6 1.4 3.0 6 2.7 5.9 6 3.8 28.0 6 7.7 11.5 6 4.0 1.5 6 1.3 5.4 6 3.4 3.1 6 1.6 60.2 6 18.3
6.90 6 0.18 5.48 6 1.36 4.60 6 1.19b 16.0 6 3.42b 7.12 6 1.0 8.00 6 0.8 9.24 6 0.4 8.80 6 0.3 6.70 6 1.2 9.15 n.d. 6.78 6 1.4 7.28 6 0.5 9.45 6 0.3 105 2.6 6 4.0 4.1 6 5.8 5.4 6 6.6 33.3 6 7.8 25.2 6 16.7b 1.8 6 1.2 9.2 6 5.3 6.1 6 5.0 87.6 6 29.8
Data are means 6 SD (n $ 2). a Sweet potato dietary fiber. b Significantly different from the control group (P , .05). c Body weight gain (g)/food intake (g). d Values are expresses as mean 6 SD of bacterial counts. e Frequency of occurrence. f Not detected.
100e 100 86 86 86
100e 100 100 29 57 14 100
retention ability. In fact it has been reported that 9 g, or more, of water can be retained per gram of SPDF (Takamine et al., 2005). In addition, the stool weight in the SPDF group significantly decreased compared with the control group, a result that is likely due to a reduction in feed intake. Similar to the effect on the wet weight of the cecal contents and cecal tissue, the increase in stool water content in the SPDF group compared with the control group is likely to have arisen due to the higher water retention ability of SPDF. Rats in the SPDF group also had a higher bacterial count in their intestinal flora than rats in the control group, and Bifidobacterium, in particular, which was not detected in the control group, was detected in the SPDF group, although only in the intestinal microflora of one rat out of seven (14%). The SPDF group also had a higher percentage of anaerobic bacteria than aerobic bacteria. The concentration of ammonia per gram of cecal contents did not differ much between the control group and the SPDF group; the values being 0.20 6 0.06 and 0.26 6 0.15 mg, respectively. Useful bacteria in the intestinal flora synthesize vitamins and proteins useful to the host, and are intimately involved in the digestion and absorption of food. Useful intestinal bacteria, such as bifidus, also inhibit the proliferation of harmful bacteria and work to purify the intestinal environment. Harmful intestinal bacteria have been shown, for example, to produce decomposition products, such as ammonia, as well as carcinogenic substances (Mitsuoka, 1995). Soluble fiber has been shown to be easily fermented by intestinal bacteria (Hayakawa et al., 2003) and to significantly increase the bacterial bifidus count (Aoe et al., 1988). The total bacterial count was higher in the SPDF group, but because the concentration of ammonia per gram of cecal contents was almost the same between the two groups, it appears that the proliferation of harmful bacteria was not promoted in the SPDF group. In contrast, bifidus was detected in the SPDF group, and it is likely that this is due to the presence of the soluble dietary fiber components hemicellulose and pectin contained in the SPDF. The concentrations of organic acids, including acetic acid, butyric acid, and isovaleric acid, in the intestinal contents were also numerically higher in the SPDF group than in the control group, but these differences in concentrations were not considered to be significant, because of large individual differences. In contrast, the concentration of propionic acid was significantly higher in the SPDF group. Propionic acid is known to inhibit HMG-CoA synthase, HMG-CoA reductase, and acetyl-CoA synthetase,
Sweet potato dietary fiber
and has been reported to be involved in suppressing cholesterol biosynthesis (Ebihara and Kiriyama, 1990; Mitsuoka, 1995). One study has reported that rats that consumed feed containing beet dietary fiber had significantly lower plasma cholesterol than those that consumed feed containing cellulose, but this correlation between short-chain fatty acid content and serum lipid concentration has not been confirmed (Arimandi et al., 1992). Although the propionic acid concentration was significantly higher in the SPDF group than in the control group, an analysis of serum lipids (values are not shown) showed no large differences in total cholesterol, HDL-cholesterol, or triglyceride levels between the two groups. Butyric acid is an energy source used by epithelial cells in the large intestine, and it is also known to inhibit the proliferation of cancer cells (Morishita, 1990). The concentration of butyric acid in the SPDF group was 1.7 times higher than in the control group, although there were large individual differences, and so this difference was not significant. Overall, the SPDF group had demonstrably higher concentrations of propionic acid, and numerically higher levels of butyric acid, and isovaleric acid, so it is possible that SPDF could be expected to have beneficial intestinal regulatory effects through increasing the levels of these organic acids.
Sweet potato pectin Overview of pectin Pectin is found between the cell walls of plant tissues in its calciumbound, water-insoluble state. Pectin is composed of an α-1,4-linked linear chain of the acidic sugar galacturonic acid (GalA), with side chains consisting primarily of the neutral sugars galactose and arabinose (Rolin and Vries, 1990). The rate at which the carboxyl group of GalA in pectin forms an ester bond with methanol differs depending on the plant source or extraction method. Pectin is primarily used as a gelling agent in raw food materials. Whether it forms a gel, and how its gel properties change, generally depends on the pH, the temperature, sugar concentration, calcium concentration, and pectin concentration (EI-Nawai and Heikal, 1995). In addition, the gelling mechanism of pectin is also determined by its degree of esterification. High methoxyl pectin (HM pectin), which is 50% esterified or more, forms a gel in the presence of acid and sugar. Low methoxyl pectin (LM pectin), which is 50% esterified or less, forms a gel in the presence of calcium in particular, as well as in the presence of
alkaline earth metals (Grant et al., 1973; Oakenful, 1991; Walkinshaw and Arnott, 1981). In industry pectin is primarily extracted from the skin of citrus fruits, or the pulp of pressed apple juice (May, 1990). Beet pulp, potato pulp, sunflower, and others have also been reported as being good pectin sources. Pectin can also be used as a food bulking agent in the dieting industry to decrease calories, reduce cholesterol, and improve insulin sensitivity (Hayashi, 1996; Rombouts and Thibault, 1986; Sosulski et al., 1978; Turquois et al., 1999). Dietary fiber manufactured from sweet potato pulp contains approximately 40% pectin. This value is much higher than the 30% (Rombouts and Thibault, 1986) pectin content of the dietary fiber fraction of sugar beets, which are widely considered to be a useful source of pectin. Pectin has been widely used as a stabilizer in the food industry, but in recent years it has become highly prized for its ability to improve bowel regularity, lower blood cholesterol, and lower blood pressure (Yamaguchi et al., 1993).
Extraction of sweet potato pectin To extract pectin from plant cell walls, cool water (Li et al., 1998), warm dilute hydrochloric acid (Ootsuka et al., 1995), or an aqueous solution of ammonium oxalate (Noda et al., 1994) have all been used. However, pectin in sweet potato pulp cannot be efficiently extracted with warm water and warm dilute hydrochloric acid. This is because, when the starch is separated from the sweet potato, a solution of calcium oxide is added to the pulverized sweet potato to facilitate separation, and most of the carboxyl groups in the pectin form calcium salts. Also because sweet potato pulp contains approximately 44% starch (Takamine et al., 2000a), the starch gelatinizes at high temperature, so the starch and pectin must be fractionated in advance. Uronic acid content of sweet potato pulp Sweet potato dietary fiber contains approximately 39.5% pectin (Table 5.1). To extract the pectin, the dietary fiber is mixed with 0.5% of ammonium oxalate solution and stirred for 3 h at 95°C. An ethanol solution containing 1% of hydrochloric acid is added to the extraction solution at twice the volume of the extraction solution and the resulting solution is centrifuged for 20 min at 5400 3 g. The sedimented fraction is then placed in an evaporator to distill away the ethanol, and then the
Sweet potato dietary fiber
fraction is freeze-dried to obtain the final pectin preparation. The quantity of uronic acid in the pectin can be measured as anhydrous GalA using the m-hydroxydiphenyl method (Blumenkrantz and Asbsoe-hensen, 1973). The quantity of uronic acid in pectin extracted from starch has been measured as being 68.5%. This value has been used to calculate the dry weight percentage of uronic acid in sweet potato pulp as being 14.1%. Extraction reagents Pectin can be extracted from sweet potato pulp by adding a reagent to the sweet potato pulp suspension until the concentration reaches 50 mM, and then allowing extraction to proceed at 60°C for 24 h (Table 5.5). When pectin is extracted from citrus fruit using citric acid, hydrochloric acid, or phosphoric acid, the uronic acid extraction rate ranges from approximately 5% to 11%. Meanwhile with ethylenediaminetetraacetic acid (EDTA), the only reagent that can be used for canned or bottled food products, or with ammonium oxalate, which is normally used for pectin analysis, uronic acid can be extracted at rates that range from 82.8% to 94.2%, respectively. In addition, using dipotassium phosphate, tripotassium phosphate, and disodium phosphate, which cause the final pH of the extraction solution to change from neutral to alkaline, uronic acid can be extracted at rates of 80.9%, 76.9%, and 81.8%, respectively. However, hardly any uronic acid can be extracted using monopotassium Table 5.5 Extraction of pectin from sweet potato pulp. Reagent
Potassium dihydrogenphosphate Dipotassium hydrogenphosphate Tripotassium phosphate Sodium dihydrogenphosphate Disodium hydrogenphosphate Trisodium phosphate Trisodium citrate Sodium carbonate Sodium chloride Citric acid Hydrochloric acid Phosphoric acid EDTA Ammonium oxalate
5.7 7.5 10.0 5.9 7.6 10.5 7.6 10.0 6.3 2.5 1.6 1.9 8.4 6.2
2.3 80.9 76.9 2.2 81.8 79.3 81.4 66.4 0.5 5.4 8.1 10.8 94.2 82.8
phosphate, which produces a final extraction solution pH of 6 or below. Thus the extraction rate differs greatly, even among the different forms of phosphate. Extraction temperature and pH Pectin was extracted from 1% of sweet potato pulp suspension by adding 50 mM disodium phosphate. Extraction did not occur at temperatures ranging from 5°C to 30C, but the extraction rate increased significantly at 40°C and above, furthermore at 70°C approximately 82% of the pectin could be extracted. However, the starch in the sweet potato pulp was also extracted at temperatures of 66°C and above (Takamine et al., 2000b). This is because sweet potato starch begins to gelatinize at around 65°C, and is completely gelatinized at 78°C (Takeda and Hizukuri, 1974). Based on these facts, 63°C can be considered as the optimal temperature for extracting pectin without starch gelation occurring. In industry pectin is extracted in a metal acid, such as hydrochloric acid, diluted with warm water at a pH of 1 2 (May, 1990). The pectin solution is stable between pH 3 and 4; when the pH is lower, hydrolysis occurs, and when the pH is higher, the pectin is broken down by deesterification or a β-elimination reaction (Hayashi, 1996). In addition, pectin breaks down more easily as the extraction temperature increases. Interestingly high molecular weight pectin could be extracted, despite the extraction conditions being 63°C and a pH near 7.5. This is believed to be due to the fact that the raw materials for pectin extraction were different, and that calcium was used (Takamine et al., 2000a) during the sweet potato starch manufacturing process. Calcium forms a calcium salt with carboxyl groups and as a result the β-elimination reaction, which would normally occur at this pH and temperature, is inhibited (Hayashi, 1996). Recovering pectin from the extraction solution The amount of ethanol required to recover the pectin fraction from the pectin extract was determined. The data showed that when ethanol was added at 0.3 times the volume of the pectin extract, then complete recovery of the pectin was achieved. However, if higher levels of ethanol were added this caused the extraction reagent (disodium phosphate) to become insoluble and mix with the pectin, so extreme care must be taken to avoid adding too much ethanol in this procedure (Takamine et al., 2007).
Sweet potato dietary fiber
Properties of sweet potato pectin The chemical composition of sweet potato pectin Following pectin extraction from sweet potato pulp using disodium phosphate and its recovery by ethanol precipitation, the pectin was dried and pulverized. When this pectin preparation was treated with pectinase and analyzed by HPLC, 3.9% arabinose, 5.1% galactose, 1.7% rhamnose, 0.1% xylose, 0.1% mannose, and 0.3% glucose was found in sweet potato pectin. Furthermore, the water content was 3.0%, the GalA content was 64.8%, the degree of esterification was 1.4%, and the ash content was 21.6%. The vast majority (84%) of the ash content was sodium. The use of a metal ion-containing reagent such as EDTA to extract pectin can cause some carboxyl groups of the GalA in pectin to become neutralized by positive ions, which increases the ash content (Arslan, 1995). For example, pectin extracted from sugar beet residue using EDTA had a higher ash content than pectin extracted using hydrochloric acid or ammonium oxalate (Phatak et al., 1988; Sun and Hughes, 1998). Structure of sweet potato pectin determined by Fourier transform infrared spectroscopy The infrared spectra of pectin which were extracted by sodium polygalacturonate from sweet potato pulp are shown in Fig. 5.4. The peak at 1740 cm21 is due to the CQO stretching vibration of the carboxyl group that had formed a methyl ester, but when the carboxyl group was bonded to a salt, this absorbance was shifted to 1610 cm21 (Chatjigakis et al., 1998; Kamnev et al., 1998). The absorbance at 1200 1000 cm21
Figure 5.4 Infrared spectra of the pectin prepared from sweet potato pulp (1) and sodium polypectate (2).
occurred due to the presence of oxygen in the glycosidic bonds and the stretching vibration of C O in the hydroxyl groups (Usui, 1986). In addition, the absorbance seen in the 833 855 cm21 range can be ascribed to the α-1,4-glycosidic bonds (Usui, 1986). The infrared spectra of pectin and sodium polygalacturonate extracted from sweet potato pulp were found to be almost identical. Molecular weight of sweet potato pectin From both the ash composition and infrared spectra data, most of the carboxyl groups in the pectin extracted from the sweet potato pulp using disodium phosphate were believed to have been substituted with sodium. In addition, the molecular weight of the prepared pectin showed the presence of two forms of pectin with peaks at 785,000 and 242,000 (Fig. 5.5) (Takamine et al., 2007). This pectin was therefore found to be more highly polymerized than commercially available pectin, which has a molecular weight of 50,000 150,000 (Hayashi and Hoshino, 2003). Breaking strength of pectin gel In the presence of divalent ions, such as calcium, it is the accepted belief that a junction zone, which is the fundamental structure of the gel, called an egg box (Oakenful, 1991), is formed between pectin molecules. A three-dimensional network is formed through this junction zone, and water and sugar molecules become trapped in this network completing the gel. In addition, pectin gelling and gel properties are also affected by the degree of methylation, molecular weight, pH, temperature, sugar concentration, pectin concentration, and others (Walkinshaw and Arnott, 1981).
Elution time (min)
Figure 5.5 Molecular weight distribution of sweet potato pectin.
Sweet potato dietary fiber
Breaking pressure of gel (N/m2)
Breaking pressure of gel (N/m2)
12,000 9000 6000 3000 0
20,000 15,000 10,000 5000 0
Concentration of pectin (%)
Breaking pressure of gel (N/m2)
15,000 Breaking pressure of gel (N/m2)
12,000 9000 6000 3000
15,000 12,000 9000 6000 3000 0
Concentration of Ca (mg/g-pectin)
Concentration of sucrose (%)
Figure 5.6 Effects of pH (A), pectin concentrations (B), calcium (C), and sucrose (D) on breaking pressure of pectin gel.
To study the effects of pH, pectin concentration, calcium concentration, and sugar concentration on the gel breaking strength of the pectin extracted from sweet potato pulp, the standard conditions used were a pH of 3.5, a sugar concentration of 30%, a pectin concentration of 1%, and a calcium concentration of 25 mg/g pectin. The effects of pH on the breaking pressure of pectin gel are shown in Fig. 5.6A. At pH 2.5 the gel was soft and not well formed. At pH 3.0 a gel was formed, and the strength and elasticity improved. At pH 4.0 the gel breaking strength was 1.3 3 104 N/m2, which remained almost constant up until pH 5.5. As shown in Fig. 5.6B, at pectin concentrations of 0.1% and 0.25%, a gel could not be formed. At 0.5% the gel form could not be maintained and became jam-like. At a pectin concentration of 2%, the fluid nature of the gel was absent and the gel contained air bubbles. When the pectin concentration increased from 0.5% to 1.5%, the gel breaking strength increased almost linearly. As calcium was changed from a concentration of 0 to 25 mg/g pectin, the breaking strength increased linearly, as shown in Fig. 5.6C. The maximum breaking strength of 1.1 3 104 N/m2 occurred at a calcium
concentration of 25 mg, beyond which the gel became brittle and easily broken; the breaking strength was then found to rapidly decrease at concentrations higher than 25 mg. In order to form a stable gel with LM pectin, a junction zone of an appropriate length is thought to be required (Axelos and Thibault, 1991; Rees, 1982). The gel formation process is thought to be affected by calcium ions (Hayashi and Hoshino, 2003), so that when excess calcium ions are supplied, the junction zone becomes too long, and the quantity of liquid that can be confined in the network tends to decrease. Consequently, when the calcium concentration exceeds 25 mg/g pectin, the gel breaking strength is believed to rapidly decline. As shown in Fig. 5.6D, a gel was also formed without the addition of sugar. The breaking strength of this gel was 8.1 3 103 N/m2, and as the sugar concentration was increased from 0% to 30%, the breaking strength increased with increasing sugar concentration, although only slightly. Compared with the effects of pectin and calcium concentration, the effect of sugar concentration on the breaking strength was minimal.
Ultrasonic modification of sweet potato pectin Introduction of ultrasonic technology Ultrasonics is the science and technology of applying sound waves with frequencies above human hearing ability, or energy generated by sound waves of frequencies in the range of 18 kHz to 1 GHz. Sweet potato pectin exhibits a low degree of methyl esterification, which is rich in galactose and arabinose (Takamine et al., 2007). Moreover, application of sonication in pectin offers a novel, green, cost-effective, and easy to upscale method to modify pectin, which could yield better products for wider applications (Rastogi, 2011). Indeed, several low molecular weight polymers have shown higher activity than their large molecular weight counterparts.
Effect of pectin concentration and sonication time on sweet potato pectin sonolysis Concentration is one of the key factors that affect sonication efficiency. The pectin at concentration 2.5 mg/mL showed higher sonolysis, followed by 5 and 10 mg/mL. A 2.5 mg/mL pectin dispersion treated for 20 min at 200 W had a molecular weight reduced about twofold compared to 5 and 10 mg/mL pectin solutions, which are shown in Fig. 5.7A.
Sweet potato dietary fiber
(B) 2.5 mg/mL pectin 5 mg/mL pectin 10 mg/mL pectin
1.5 Polydispersity %
600 400 200
0 min 5 min 10 min 20 min
0 min 5 min 10 min 20 min
0 0 min 5 min 10 min 20 min
Molecular weight (kDa)
Figure 5.7 Effects of pectin concentration and sonication time on molecular weight (A) and polydispersity (B) of sweet potato pectin at constant power 200 W and duty cycle 60%.
Pectin polydispersity did not show consistent change with increasing sonication time, which characterizes random scission of pectin chains (Fig. 5.7B), such a phenomenon was similar to that previously reported by Huang et al. (2015). In addition, increased sonication time resulted in clearer liquid compared to initial pectin dispersion (data not shown). Both pectin concentration and sonication time had significant (P , .0001) influences on pectin molecular weight change, with concentration contributing 77.50% influence, whereas sonication time had 18.69% influence, and interaction of the factors had insignificant (P 5 .1667) influence on molecular weight change according to two-way analysis of variance. Sonolysis had a direct correlation with time as shown in Fig. 5.7A. However, pectin concentration had a higher effect than that of sonication time. Concentration is known to affect cavitation, which in turn influences sonochemical impact.
Effect of sonication power on sweet potato pectin sonolysis Pectin molecular weight reduction had a positive correlation with sonication power, that is, increasing the power led to a decrease in molecular weight, which is shown in Fig. 5.8A. Polydispersity did not show a consistent trend with the increasing power, which corresponded to a random scission of pectin (Fig. 5.8B). Pectin molecular weight decreased rapidly between 100 and 400 W. In addition, the polydispersity of pectin was increased when the power increased to 100 W, and then decreased when the power further increased to 400 W.
Molecular weight (kDa)
Figure 5.8 Effects of ultrasonic power on molecular weight (A) and polydispersity (B) of sweet potato pectin.
Sonication reduces the molecular weight of a polymer to a definite point and further increases in the sonication power or time will not lead to an additional reduction in molecular weight. High intensity produced by high sonic power induces higher cavitation in the medium, hence a higher cavitation yield, leading to stretching and uncoiling in molecules, and hence the structure and flexibility of molecules determine their susceptibility to sonication. Cavitation growth depends on ultrasonic intensity, as high intensity ultrasound causes bubbles to have cavities that result in faster and more violent bubble growth and bursting. High-intensity ultrasound expands the cavity so rapidly during the negative-pressure cycle that the cavity does not shrink during the positive-pressure cycle (Kasaai, 2013). In this process therefore cavities grow faster in a single cycle, thus increasing sonochemical activity. On the other hand, low intensity ultrasound leads to the size of the cavities oscillating in phase with the expansion and compression cycles, hence less impact (Suslick, 1989). Overall the study results lend credence to the growing body of evidence showing that acoustic power is a vital parameter for the sonochemical effect, with increasing power resulting in an increased sonochemical effect.
Effect of sonication duty cycle on sweet potato pectin sonolysis Duty cycle is related to sonication time, the higher the duty cycle, the longer the sonication time. There was a direct relationship between duty cycle and molecular weight reduction. As expected, molecular weight decreased with increasing duty cycle, and the highest ultrasonic activity
Sweet potato dietary fiber
Molecular weight (kDa)
(A) 400 300 200
Duty cycle (%)
Duty cycle (%)
Figure 5.9 The effects of ultrasonic duty cycle on molecular weight (A) and polydispersity (B) of sweet potato pectin.
was obtained at 80% duty cycle. The average molecular weight of pectin sonolyzed at 80% duty cycle was 140.41 6 11.49 kDa, while that at 20% duty cycle was 391.15 6 63.14 kDa, that is, a fourfold increase in duty cycle resulted in an approximately 2.8-fold decrease in molecular weight (Fig. 5.9A). Increasing the duty cycle led to an increased sonolysis effect, causing a significant decrease in pectin molecular weight with P 5 .0475 and the correlation coefficient (R2 5 0.9073) at α 5 0.05. When the duty cycle was lower than 60%, such as 20%, and 40%, the total sonication time could have been less to degrade substantial amount of pectin. However, above 60% duty cycle a significant molecular weight reduction occurred, which could be due to sufficient time being allowed for cavitation formation. Based upon our results on duty cycle, we selected 60% duty cycle for the experiments on the effect of ultrasonic power, time, and concentration on pectin sonolysis. It was reported that in pulse ultrasound treatment, the duty cycle length and interval had a significant effect on sonochemical activity, and from 50% duty cycle significant sonolysis was realized (Sun and Ye, 2013). The pulse duration is related to cavitation, hence the longer the duty cycle, the higher the cavitation and consequently, higher sonolysis. The polydispersity did not show a clear trend in the tested duty cycle range (Fig. 5.9B), indicating that sonolysis was most probably random.
Effect of sonication power on neutral sugar composition of sweet potato pectin The neutral sugar composition of sonicated pectin showed that there was a slight change in pectin structure at different sonication power levels at 60% duty cycle for 20 min (Table 5.6). The most noticeable change
Table 5.6 Effect of ultrasound power on neutral sugar content of sweet potato pectin at 60% duty cycle for 20 min. Treatment
Native 100 W 200 W 400 W
7.23 6 1.34 4.92 6 1.96c 6.64 6 0.08b 11.10 6 0.12a b
17.73 6 0.62 16.57 6 1.01c 18.37 6 0.53bc 30.49 6 1.85a bc
32.73 6 0.15 33.01 6 0.73c 39.72 6 1.41b 47.91 6 0.32a
32.77 6 1.09 34.62 6 6.51a 28.31 6 2.10b 1.80 6 0.03c a
Ara 1 Gal/Rha
9.54 6 0.78 10.86 6 0.92a 6.96 6 1.13c 8.69 6 0.69b a
6.81 9.92 8.74 7.06
Data are means 6 SD (n $ 2). Values within columns with different letters are significantly different (P , .05). Rha, Rhamnose; Ara, arabinose; Gal, galactose; Glc, glucose; Xyl, xylose.
Sweet potato dietary fiber
occurred in 400 W-treated pectin, which showed a general increase in galactose content and a decrease in glucose content with increasing sonication intensity, as well as a decrease in arabinose and rhamnose content, while xylose content had less change. The glucose content was lowest (1.80%). The most dominant neutral sugars in pectin were arabinose and galactose, with glucose possibly coming from starch and cellulose hydrolysis during extraction. Based upon the neutral sugar composition, pectin is a copolymer of homogalacturonan and rhamnogalacturonan, with arabinose and galactose originating from a rhamnogalacturonan side chain, while rhamnose was from the parental chain joined to GalA and forming the branching point, and xylose could have come from xylogalactan (Yapo and Koffi, 2013). The degree of branching of rhamnogalacturonan I, represented as (Ara 1 Gal)/Rha ratio, is indicative of the number of Rha residues branched with Ara and Gal residues, irrespective of the length of Ara and/or Gal residues-containing side chains. Hence the greater the quantity of (Ara 1 Gal) than Rha the lower the amount of branching in the pectin polymer (Sila et al., 2009). According to Table 5.5, sonication at 200 and 400 W led to reduced branching compared to native pectin and pectin sonolyzed at 100 W. In a previous study it was observed that there was no change in the main chain while the decrease of pectin side chains was noted (Liu et al., 2013). The main chain is mostly composed of GalA which is resistant to sonication compared to the neutral sugars.
Effect of sonication power on the degree of methoxylation and galacturonic acid content of sweet potato pectin Sonication led to increased GalA content and decreased degree of methoxylation (DM), as shown in Table 5.7. GalA content increased significantly (P , .0001) with the increase in sonication power applied. Table 5.7 Effect of ultrasound power on GalA and DM of sweet potato pectin at 20 min and 60% duty. Sample treatment
Native pectin 100 W 200 W 400 W
72.0 6 2.1 85.6 6 1.6b 89.29 6 3.2ab 92.00 6 2.7a
12 6 3.0a 5.46 6 1.1b 6.28 6 0.92b 5.25 6 1.3b
Data are means 6 SD (n $ 2). Values within columns with different letters are significantly different (P , .05).
The increase in GalA content could be attributed to the scission of the pectin side chain, which resulted in more GalA-rich homogalacturonan chains, hence GalA increased from 72.0% 6 2.1% in native pectin up to 92.00% 6 2.7% in 400 W-sonicated pectin. The DM of the sonicated pectin decreased significantly (P , .0001) compared to that of native pectin, but there was no significant difference between three different sonication treatments (100, 200, and 400 W). Liu et al. (2013) noted that GalA was more resistant to sonication compared to neutral sugars, which could explain the increasing GalA content with increased sonication intensity. Reduced DM could have been due to hydrolysis of ester groups by cavitation, or ester groups reacting with ionized groups generated during sonolysis. For instance, it was noted that chitosan’s degree of esterification reduced during sonolysis possibly due to the sonic hydrolysis of ester bonds (Baxter et al., 2005). In the same manner ester bonds in pectin could have been hydrolyzed leading to the formation of more free carboxyl groups, and hence increasing GalA content. The effect of sonication on ester groups was reported, and the sonication effect was found to be more pronounced in more hydrophobic ester groups (Piiskop et al., 2007). The team noted that ester groups are vulnerable to sonolysis, notwithstanding the solvent, and that long carbon chain length increases led to increased sonolysis in alcohol (ethyl, propyl, and butyl) esters.
Effect of sonication power on the structure of sweet potato pectin Fourier transform infrared spectroscopy (FTIR) was used to assess the changes in pectin structure due to sonication. Assignment of peaks was based upon the work of Filippov (1974), the peaks representing various groups within pectin chain were noted. The pectin peaks were similar to those reported by Sato et al. (2011) on sweet potato pectin. The pectin FTIR profile was shown in Fig. 5.10. The band at 3400 cm21 represented O H stretching, and the band at 2940 cm21 represented C H stretching of the CH2 group. A similar band profile was reported in sweet potato pulp pectin by Takamine et al. (2007). The FTIR spectra of all pectin showed a lower absorbance at 1750 cm21 (COOR) than at 1650 cm21 (COO ), indicating a LM pectin which corroborated the DM measured via the titration method. The bands at 1100 and 1070 cm21 represented rhamnogalacturonan, and there were bold bands at 1070 and 1043 cm21. The band at 1063 cm21 represented monopyranose component, and a higher peak at between 1250 to 1486 cm21 showed CH deformation
Sweet potato dietary fiber
(A) 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 4000
200 W 100 W 400 W Native CH2
Wavenumber (cm–1) (B)
20% 40% 60% 80% Native
0.7 0.6 0.5 0.4 0.3 0.2 0.1 4000
Wavenumber (cm–1) Native 5 min 10 min 20 min
0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 4000
Figure 5.10 Effects of sonication power (100, 200 and 400 W) (A), duty cycle (20, 40, 60 and 80%) (B), and sonication time (5, 10 and 20 min) (C) on pectin structure of sweet potato pectin determined by Fourier transform infrared spectroscopy (FTIR).
vibration, with three peaks associated with the carboxylic acid (COO ), the methyl ester (COOCH3), and the primary amide (CO NH2) groups. The fingerprint region below 1500 cm21 corresponded principally to coupled C C, C O C, and C OH vibration modes of the carbohydrate ring and to the glycosidic linkage vibrations. Ultrasound degradation had no recognizable effect on the absorption at around 3400 cm21 (OH), 2938 cm21 (CH), 1148.7 cm21 (COC), and 1101.3 cm21 (C C), which indicated that sonic power, time, and duty cycle had no noticeable effect on the main chain and the pectin side chain could have been sonolyzed by scission (Chen et al. 2012). The peak at 1750 cm 1 for native pectin was relatively smaller than that of other pectin, which could explain the reduced DM and increased GalA content during sonication.
Effect of sonication power on the antioxidant capacity of sweet potato pectin Oxidative radical absorbance capacity The antioxidant capacity of sweet potato pectin determined by oxidative radical absorbance capacity (ORAC) is shown in Fig. 5.10. The Trolox standard curve was plotted between 0 and 60 μg/concentration, and its NetAUC standard equation was Y 5 0.899x 1 2.581 (R2 5 0.993). Trolox equivalent (TE) is expressed in micromoles of TE/100 g of sample (Progress et al., 2011). ORAC values of 200 and 400 W-sonicated pectin were higher than those of native and 100 W-sonicated pectin. While 400 W-sonicated pectin at 4 mg/mL had 104,000 6 220 μM TE/100 g, which was fivefold higher than native pectin, which was higher than that of honey and red wine, but in the same range as acacia fruit/pulp/skin. On the other hand, the 200 W-sonicated pectin had ORAC value 34,420 6 180 μM TE/100 g as shown in Fig. 5.11, which was equally higher than that of many foodstuffs and spices according to the United States Department of Agriculture (USDA) ORAC value of foods (Haytowitz and Bhagwat, 2010). The 100 W-treated and native pectin showed almost equal ORAC values; this could have been due to less sonication at 100 W compared to 200 and 400 W, and hence pectin had insignificant structural change to improve its antioxidant activity relative to native pectin. Sonication increased the ORAC value of pectin and its effect was more pronounced at 200 and 400 W sonication power. Previous studies had shown that low molecular weight polysaccharide derivatives had higher bioactivity than high molecular weight ones
Sweet potato dietary fiber
Trolox equivalent µM/100g of pectin
ORAC activity of sonicated pectin 150,000
Native pectin 100 W 200 W 400 W
FRAP activity of pectin 50
Native pectin 100 W 200 W 400 W
40 30 20 10
%Activity compared to 1mM FeSo4 set at 100%
Pectin Concentration (mg/mL)
Pectin Concentration (mg/mL)
Figure 5.11 Effects of sonication power on the antioxidant capacity of sweet potato pectin determined by oxidative radical absorbance capacity (ORAC) (A) and ferric reducing antioxidant power (FRAP) (B).
(Xia et al., 2011), which could explain the increased ORAC value of sonicated sweet potato pectin in the present study. Ferric reducing antioxidant power The ferric reducing antioxidant power (FRAP) values of pectin are shown in Fig. 5.11. The standard curve was plotted for FeSO4 with a concentration range of 0.2 1 mM. There was a strong correlation between FeSO4 concentration and antioxidant capacity (R2 5 0.992) and the equation was Y 5 0.1958x 2 0.2452. The standard curve displayed a linear trend
between 0.2 and 1 mM FeSO4. There was a strong correlation between pectin concentration and antioxidant activity for all the pectin samples tested (R2 5 0.9778, 0.9885, 0.9742, and 0.9954), for the native, 100, 200, and 400 W-degraded pectin, respectively. Sonicated pectin had increased antioxidant activity with 400 W-treated pectin having 43% relative FRAP activity at 4 mg/mL; the same concentration of native pectin had a lower relative FRAP activity 16.4% compared to FeSO4. There was generally increased antioxidant activity with increasing sonication power applied. The results are consistent with the observations of Pokora et al. (2013), who reported that enzymatic hydrolysis of egg yolk protein and egg white protein improved their radical scavenging (DPPH) capacity, ferric reducing power, and chelating of iron activity. Native pectin is a complex molecule with a complex side group structure, and during sonolysis the large molecule is depolymerized yielding a low degree of polymerization of pectin, thus exposing previously hidden functional groups and creating functional groups at the scission sites, for example, carbonyl groups. The reducing agents mostly act as hydrogen/electron atom donors, thus reducing the radical species. Hydrolysis of protein into peptides is also known to increase its antioxidant capacity, because peptides have more functional groups per surface area than native protein, due to exposure of hidden functional groups. Increased FRAP activity correlated with GalA content, and reduced molecular weight due to more COO-groups. Moreover, sonication caused depolymerization which possibly created new functional groups within the low molecular weight pectin formed. Another possibility is that the hydrogen ion (H1) formed during water sonolysis is very reactive and a strong reducing agent. In line with this argument, it was reported that sonication of Ƙ-carrageenan yielded more oligo-carrageenan with a stronger reducing power. On the contrary, Zhou et al. (2014) found that the antioxidant capacity of proanthocyanidins increased with the increasing degree of polymerization, which was partly attributed to its numerous functional groups which increase proportionally with molecular size, hence large phenolic compounds quench more radicals than smaller molecules. Even though the two antioxidant assays work differently, the results show that sonicated pectin is an effective radical scavenger and oxidative species reducing agent. Taken together, the FRAP and ORAC results demonstrate that sonication serves as an innovative and green method of not only pectin molecular size reduction but also of enhancing the antioxidant activity of pectin.
Sweet potato dietary fiber
Application prospect of sweet potato dietary fiber Sweet potato starch residues contain large amounts of dietary fiber, which is rich in cellulose, hemicellulose, lignin, and pectin, and exhibits good physicochemical and functional properties. In particular, sweet potato pectin obtained by sonication modification showed high antioxidant capacity. These results showed that sweet potato dietary fiber could be potentially used in normal foods, functional foods, health care products, and pharmaceuticals in the near future.
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Further reading Aoe, S., 1995. Dietary fiber materials used for food. In: Inami, S., Kiriyama, S. (Eds.), Dietary Fiber. Dai-ichi Press, Tokyo, p. 341. Renard, C.M.G.C., Thibault, J.F., 1993. Structure and properties of apple and sugar-beet pectins extracted by chelating agents. Carbohyd. Res. 244, 99 114.