Resistant starch in food: a review.

Resistant Starch in Food: A Review Pinky Raigond, Rajarathnam Ezekiel, Baswaraj Raigond Division of Crop Physiology, Biochemistry & Post Harvest Technology, Central Potato Research Institute, Shimla, India Email: [email protected]

Abstract The nutritional property of starch is related to its rate and extent of digestion and absorption in the small intestine. For nutritional purposes, starch is classified as rapidly available, slowly available and resistant starch (RS). The exact underlying mechanism of relative resistance of starch granules is complicated because those factors are often interconnected. The content of RS in food is highly influenced by food preparation manner and processing techniques. Physical or chemical treatments also alter the level of RS in a food. Commercial preparations of RS are now available and can be added to foods as ingredient for lowering caloric value and improving textural and organoleptic characteristics along with increasing the amount of dietary fiber. RS has assumed great importance due to its unique functional properties and health benefits. The beneficial effects of RS include glycemic control, control of fasting plasma triglyceride and cholesterol levels and absorption of minerals. This review attempts to analyse the information published, especially in the recent past on classification, structure, properties, applications and health benefits of RS. Key words: resistant starch, classification, properties, applications, health benefits

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Introduction Starch is the most abundant storage polysaccharide in plants and is the major component of diet. Digestibility of starch improves during cooking and not all of the starch present in the food is digestible. A part of starch present in the diet escapes digestion and absorption in the small intestine and is fermented in the large intestine of humans, with the production of short chain fatty acids (SCFA). This is termed as ‘Resistant starch’.1 Resistant starch (RS) is a form of dietary fiber and is naturally present in many of the starchy foods. Digestibility of starch is influenced by many factors such as temperature during cooking and storage and interaction of starch with proteins, lipids and other carbohydrates. Most of the starch is consumed in gelatinized form, which can be readily digested. Reduced digestibility of RS is influenced by many internal and external factors such as behaviour and nature of food, botanical origin of starch, food processing and physiology.2 Properties of starch depend on its two major components, amylose and amylopectin and how these molecules are organised within the granule.3 Generally food processing techniques influence the physiological fate and behaviour of dietary carbohydrates. American Association of Cereals Chemists and the Food Nutrition Board of Institute of Medicine of the National Academies have defined RS as a type of dietary fiber. RS is not rapidly digested like ordinary starch and this property is of great biological importance. Based on the origin and physical characteristics of the starch, RS is further categorised into five different types. Reviews on RS have appeared in the past but there is continuous accumulation of newer information as resistant starch continues to attract the attention of researchers. Therefore, it became necessary to update the available information on RS and this review attempts to analyze the information published, especially in the recent past on classification, structure, properties, applications and health benefits of resistant starch. Starch


Starch is a major storage carbohydrate and is second most abundant polysaccharide next to cellulose. Starch is present in the form of granules in cereal or legume seed endosperm, tubers (potato and sweet potato), unripe fruits (banana and mango) and many other plant reserve organs. It is present in diverse shapes like round, oval, lenticular and angular and the shape of starch mainly depends on the botanical source. It can vary among plant species and cultivars of same species.4 The size of starch granules generally range between 1 and 100 µm. Starches are the natural polymer occurring in all plant organs and are one of the main forms of dietary carbohydrates. Starch occurs as granules where carbohydrates are stored in insoluble and tightly packed manner.5 A number of monosaccharide or sugar (glucose) molecules are linked together with α 1-4 and α 1-6 linkages to form starch granules.6 Starch is made up of amylose and amylopectin, where, amylose has a degree of polymerization up to 6000. Amylopectin is highly branched and its degree of polymerization is up to two million. Studies based on X-ray diffraction spectrum revealed that starch is present in two crystalline structures i.e an ‘A’ and ‘B’ type. These two forms vary in proportion of amylopectin. ‘A’ type starches are found in cereals, while ‘B’ type starches are found in tubers and amylose-rich starches. A third type called ‘C’ type appears to be a mixture of both A and B forms and is found in legumes.7 Starch granules are insoluble in cold water. They form starch paste when the granules are gelatinized at higher temperature (over 40°C) in excess of water. The paste obtained after gelatinization is a mixture of dissolved carbohydrate substances and amylose and low polymerized chains of amylopectin. Starch substances bind considerable amount of water on cooling. Changes in starch gel stored at 0°C for long period of time is termed as ageing. Reduction in the viscosity of paste indicates the phenomena of syneresis. The gradual winding up of straight chains in starch paste is the process termed as ‘retrogradation’. This process occurs at a higher rate in starch pastes stored at lower temperature. The retrograded starch is considered as semi crystalline in nature, as both crystalline and amorphous patterns are present. Crystalline structures formed by the double helices of amylopectin have lower thermostability due to its branching and short chains. Heating at temperatures over 120°C and 60°C help in rehydration of product of amylose as well as amylopectin retrogradation, respectively. Amylolytic enzymes can easily hydrolyze the gelatinized starch.8 In general, digestible starches are broken down/ hydrolyzed by the enzymes α amylase, glucoamylase and sucrose-isoamylase in the This article is protected by copyright. All rights reserved

small intestine. It has been shown that B-type starches are resistant to enzymatic digestion, while Atype starches are slowly digestible.9,10 Supramolecular structure i.e packing of crystallites inside starch granules, the ratio of amylose and amylopectin, fine structure of amylose and the surface characteristics of starch granules are the major internal factors that affect the digestibility of raw starch.11 The semi-crystalline structure of starch is formed due to the concentric layers of amorphous and crystalline regions radiating from hilum, and amylopectin is responsible for the semi-crystalline region.12 Classification of Starch The classification of dietary carbohydrates is based on the chemical and physiological properties.13 Based on the action of enzymes and, rate and extent of digestion, starches can be classified into three kinds viz. rapidly digestible starch (RDS),14 slowly digestible starch (SDS)14 and resistant starch15. These three types of starches differ in the time taken for digestion in the small intestine. Rapidly digestible starch Major proportion of dietary starch is rapidly digested. High levels of rapidly digestible starch (RDS) are found in starchy food cooked by moist heat (bread and potatoes). RDS is defined as a ‘type of starch which is rapidly (within 20 minutes) converted to glucose molecules by the enzymatic digestion’.14 If RDS is present in high proportions in food, it will rapidly release glucose to blood and elevate blood glucose and insulin response, which is detrimental to health.16 RDS is significantly correlated with glycemic index, based on in vivo postprandial glycemic response.17,18,19 Slowly digestible starch As the name indicates, slowly digestible starch (SDS) is a proportion of starch that takes long time for digestion and is completely digested in the small intestine. SDS is defined as a ‘type of starch which is converted to glucose after 120 minutes of enzymatic digestion’.14 SDS is basically a physically inaccessible amorphous starch. In vitro Englyst test has revealed that mostly raw cereal starches are rich in SDS, while the slow and prolonged postprandial glucose release profile was confirmed by


human testing of normal corn starch (A type).20 Gelatinization of starches during heat-moisture food processing/cooking decreases the slow digestion property of native cereal starches.21 For the production of SDS in a patented process, partially debranched amylopectin was retrograded after the isoamylase treatment.22,23 According to investigators, it is possible to prepare SDS by modifying the molecular structure of starch molecules including 1-octenyl succinic anhydride (OSA) modified starch, cross-linked starch and enzymatically modified starch with α 1,6 linkages.24,25,26 SDS as well as low glycemic food confers similar health benefits. Foods rich in SDS are beneficial27,28,29 and they delay the occurrence of metabolic syndrome, diabetes and cardiovascular diseases.30,31,32 Resistant Starch The resistance of starch is influenced by the ratio of amylose and amylopectin. Amylose is slowly digested, whereas digestion of amylopectin is fast after retrogradation. Starch is hydrolyzed in the gastrointestinal tract by the activity of amylolytic enzymes and product of hydrolysis i.e glucose is digested and absorbed in the small intestine. This type of hydrolysis and digestion occurs only when starch present in food is gelatinized during cooking and cooked food is consumed immediately after preparation. The term ‘resistant starch’ was coined by Englyst et al to describe a small fraction of starch that resist hydrolysis by α-amylase and pullulanase treatment in vitro.15 When compared to RDS and SDS, RS is not hydrolysed to glucose in the small intestine within 120 minutes of being consumed, but is fermented in the colon. RS is a linear molecule of α 1, 4 D-glucan and is mainly derived from retrograded amylose. The rate and extent of digestion is affected by large number of factors and all of the factors are interlinked, which complicate understanding of the resistant nature of starch granules. Generally, RS content of granular starch is positively correlated with the amylose level of that starch. But there are some exceptions like pea starch, in which amylose content is medium, but RS content is high.33,34,35 A strong correlation between amylose and RS content of maize starch has been reported by Morita et al.36 Because of the unique functional and nutritional properties of amylose, efforts have been made to develop crops containing high amylose starch.37 High amylose potato, barley and wheat have been developed.38,39,40,41 Several in vitro and in vivo methods are available to measure the RS content accurately.


Relationship between RDS, SDS and RS Three starch fractions viz. RDS, SDS and RS are measured during enzymatic hydrolysis. In most processed foods RDS is inversely proportional to SDS and RS i.e if RDS is more, amount of SDS and RS is less. So, while making SDS, first important step is to reduce the RDS amount. Usually SDS and RS coexist in processed foods and are structurally more closer in terms of retrogradation.42 To make SDS from RDS, several physical, chemical and physiological methods are available.43 Table 1[14] Classification and structure of resistant starch Depending on the nature of RS, it has been classified into five sub-types viz. RS1, RS2, RS3, RS4 and RS5. Earlier classification of RS included the three types viz. RS1, RS2 and RS3, and two more types namely RS4 and RS5 were included in successive years.14,44,45,46,47 RS1: RS1 is a type of RS, which is physically inaccessible to digestion, may be due to the entrapment within whole or partly milled grains or seeds and presence of intact cell walls in grains, seeds or tubers. In such cases, amylolytic and digestive enzymes become inaccessible to starch and also degradation of cell wall components is not possible in gastrointestinal tract due to lack of cell wall degrading enzymes.8 This type of starch passes the small intestine as such. It can be completely digested in small intestine only if it is properly milled. Being heat stable it does not break down during normal cooking. RS2: RS2 are the native starch granules that are protected from digestion by the conformation or structure of the starch granules as in raw potato and green bananas. Because of their crystalline nature, native and uncooked starch granules are poorly susceptible to hydrolysis.48 Compact structure of RS2 makes it inaccessible to digestive enzymes and amylases. Digestion of RS1 and RS2 is slow and incomplete in small intestine. The resistant nature of raw potato starch was first observed by Nowotny, in 1937.49 He subjected raw starch of numerous plant species to enzymatic hydrolysis and observed the small extent of enzymatic hydrolysis of raw potato starch. Japanese researchers


confirmed these results later.50,51 Resistant nature of raw starch is not fully understood and need further investigations. The resistant nature of fine-grain high amylose maize starch and coarse grain potato starch is similar. High amylose maize starch is a type of RS2, which retains its structure and resistance even during processing and preparation of foods. Potato starch granules are bigger than the cereal starch granules and the size of granules affect the extent of enzyme adsorption on the granule surface. Kimura and Robyt found that there is no relationship between the extent of enzyme adsorption and the degree of hydrolysis.52 Later the degree of starch crystallinity was related to the extent of enzymatic hydrolysis. Amylolytic enzymes first degrade the amorphous region. Therefore, resistance was attributed to the crystalline region. But the degree of crystallinity is not always linked to resistance to amylolytic enzymes. Starch containing more amylose is considered more resistant to enzymatic hydrolysis. Starches with B type crystallinity have been found more resistant to enzymatic hydrolysis.51,53 In B type of starches, enzymatic damage is only on the surface of granules, whereas starches with A type crystallinity undergo deep enzymatic hydrolysis. Gallant et al, Ridout et al and Kossmann and Lloyd observed that B type crystalline structure of potato starch is made of double helices and form large ‘blocklets’ incorporated in ‘hard’ crystalline layers of granules.54,55,56 And these large ‘blocklets’ are responsible for resistant nature of potato starch. In cereal starch granules ‘blocklets’ are smaller than potato starch granule and cereal starch is more susceptible to enzymatic hydrolysis. Therefore, it can be concluded that starch resistance does not depend on any one factor, but a large number of factors including size of granule, shape of granule surface, amylose content, starch crystallinity and size of pores in starch granules are responsible for resistant nature of starch.8 RS3: Physically modified starches come under this category. RS3 are retrograded starches, mainly retrograded/ recrystallized amylose and is formed during cooling of gelatinized starch and in cooked foods that are kept at low or room temperature. RS3 is thermally very stable and is formed in moistheated foods. Being thermally stable, it is an important starch fraction and is also used as an ingredient in wide variety of conventional food.48 RS3 has high water holding capacity than granular starch.57 Cooked and cooled potatoes and cornflakes are some of the good examples of RS3. When the


starch paste/ gel is stored at low or room temperature for some time, amylose double helices aggregate and form a highly thermostable B-type crystalline structure. The aggregates can be rehydrated only above 150°C temperature. Formation of RS is influenced by the storage temperature and duration. More RS is formed when starch paste is stored at low temperature for several hours than at high temperatures.58 B-type crystalline RS is obtained during storage at low temperature, whereas storage at boiling temperature demonstrates A-type of crystallinity.59 Starch gel contains amorphous and crystalline fractions, where amorphous fractions are hydrolyzed by amylolytic enzymes and crystalline fractions remain resistant to the enzymatic activity.60 Interaction of starch with other components also affects the resistance of starch. In starch of many plant species amylose chains are penetrated by lipids and these amylose chains do not participate in retrogradation resulting in less production of RS3.61 Like amylose, amylopectin also form partly crystallized gels. Amylopectin crystalline structures are less stable than amylose crystalline structures and crystallization of amylopectin proceeds very slowly. These can be rehydrated at 55 to 70°C temperature.45 Amylopectin crystalline structures formed during storage of starch paste/gel are also resistant to activity of amylolytic enzymes. Major part of amylose is retrograded and precipitated from solution, but at high temperatures these processes occur only in its fractions.62 Formation of retrograded starch depends on the botanical origin of starch and the procedure applied for its production. The products of amylose retrogradation prepared by both in vitro and in vivo methods are resistant to amylolytic enzymes. Product preparation from amylopectin retrogradation is a lengthy process and may take from a few days to few weeks of paste storage at appropriate temperature. This process becomes efficient with repeated heating and cooling of starch pastes. RS4: RS4 is a group of starches that are chemically modified with similarity to resistant oligosaccharides and polydextrose.63 Starches which have been etherized, esterified or cross-bonded with chemicals to decrease their digestibility come under this category. Depending upon the solubility in water and the method of analysis, RS4 is further subdivided into four subcategories.64 Chemical modification can prevent the digestion of RS by blocking the enzyme access and forming typical linkages such as α (1-4) and α (1-6) linkages and main modifications are conversion, substitution and


cross linking.65 Resistance of acetylated and hydroxypropylated starch increases with increasing degree of substitution.66 Hydroxypropyl distarch phosphate and acetylated distarch phosphate are less susceptible to enzymatic hydrolysis as compared to native starches.67 The degree of chemical modification and the level of resistance are directly proportional. More is the degree of substitution with phosphoric acid, more is the resistance in monostarch phosphate. Heating of product of monostarch phosphate with glycine produced starches with higher resistance to enzymatic hydrolysis than monostarch phosphate itself.68 Also heating of soluble starch saturated with iron (III) and treated with glycine is highly resistant to amylolytic enzyme activity. Chemical modification changes the structure and composition of the starch granules and hence increases their resistance to amylolytic enzymes. Normal chain arrangement of starch is disturbed due to substitution and starch becomes inaccessible to amylolytic enzymes. RS5: RS5 is a type of RS formed due to formation of amylose-lipid complexes. These complexes can be formed during food processing and can also be prepared under controlled conditions. Amyloselipid complexes are generally formed from high amylose starches. Structure and formation of RS5 depends upon the botanical sources. RS5 is a polysaccharide consisting of water insoluble linear polyalpha-1,4-glucan and is resistant to degradation by α-amylases.47 These polysaccharides promote the formation of short chain fatty acids, particularly butyrate which is the most important short chain fatty acid. Table 214,47,64,69,70,71 Properties of RS RS has gained importance because of its functional properties. RS has many desirable physicochemical properties, such as swelling, viscosity increase, gel formation and water binding capacity, which make it useful in a variety of foods.72 RS can be used to replace flour on a 1 for 1 basis without significantly affecting dough handling or rheology. RS is present naturally and is bland in flavour, with white colour and fine particle size. Because of fine particle size RS does not affect the texture of food. RS imparts special characteristics along with dietary fiber fortification in high fiber


foods.73 Calorific content of RS is low (1.6 to 2.8calories per g) and can be used to compliment reduced fat and reduced sugar formulations. Its physical properties, particularly its low water-holding capacity, provide good handling in processing, as well as crispiness, expansion and improved texture in the end-product. RS has high gelatinization temperature, good extrusion and film forming qualities and lower water holding properties than traditional fiber products. It increases the coating crispness of products and the bowl life of breakfast cereals. Because of the above properties, RS has been used successfully in a range of baked and extruded products. RS is especially suitable for grain-based, lowmoisture and moderate moisture food systems. Applications of Resistant Starch RS has attracted the attention of nutritionists and food processors because of its potential physiological benefits and unique functional properties. Due to increasing awareness about healthy and nutritious foods, consumers are now concerned with supplementary health merits derived from its regular ingestion along with traditional nutritional aspects of the food.74 Looking into the consumer’s awareness, food manufacturers, researchers and producers are aiming at production of improved food with better digestion and health benefits.75 RS is present naturally in broad range of starchy products, so it can be added as a functional ingredient. RS fortified foods are becoming popular among people. Consumers are accepting food products enriched with RS in order to increase their dietary fiber intake.76 RS containing starch ingredients are commonly sold as ‘resistant starch’ at commercial level. A large fraction of these products is fully digestible and is considered as RS-suppliers.77 First commercially available product of RS was reported in the mid-1990s. Nowadays, RS rich-powders are prepared by a number of companies employing technologies such as that developed at Kansas State University using an amylose-rich starch from maize hybrids.78 RS products are of high quality and cannot be replaced by traditional insoluble fibers.79 RS is used in the production of moisture free food products. Cross-linked starches prepared from maize, tapioca and potato are used for formulations that need pulpy texture, smoothness, flowability, low pH storage and high temperature storage.80 To imrove the textural properties and health benefits, baked products, pasta products and beverages are fortified with RS. Arimi et al have successfully replaced most or all of the fat in


imitation cheese with RS, without adversely affecting meltability or hardness and conferring the wellestablished benefits of RS as a functional fiber.81 Large number of products rich in fiber are available in the market, for example, high-fiber bread and breakfast cereals.82 But others, like white bread, biscuits and cakes are not fortified with fiber. Availability of process tolerant RS has now made it possible to prepare food rich in dietary fiber. RS has been added during preparation of pasta and beverages, where dried pasta products containing up to 15% resistant starch can be made with little or no effect on dough rheology during extrusion. Pasta prepared with addition of RS was lighter in colour, but a firm texture was obtained in the same cooking time as a control that contained no added fiber.69 Resistant starches generally require suspension and add opacity to beverages and may be used in thickened, opaque health drinks where insoluble fiber is desired. Fibers other than RS generally have strong flavour, possesses coarse texture and poor as well as dry mouth feel. But RS imparts a less gritty mouth feel and masks flavours to a lesser extent. Table 364,70,80,83 The first commercial RS was introduced as ‘Hi-Maize’, by the Starch Australia Ltd (Australia). Later another commercial sources of RS such as CrystaLean® (RS3), Novelose®240 (RS2), Novelose®260 (RS2), Novelose®330 (RS3), Eurylon® (RS2), Amylomaize VII (RS2) and Neo-amylose (RS3) were introduced to increase the dietary fiber content in foods. CrystaLean® is a RS3 preparation, produced by starch retrogradation of high amylose maize starch ae-VII hybrid. ‘Hylon VII’ a natural high amylose maize starch, was introduced by the National Starch & Company (USA). The above mentioned RS3 products have been prepared by heating and cooling of high amylose corn starch under controlled moisture and temperature conditions and these processes help in manufacture of granular forms of concentrated RS containing 47% to 60% RS. Using maltodextrins as starting material, a natural, highly crystalline RS3 {(Actistar®), (Act*-RS3)} has also been developed. The taste of Act*-RS3 is very natural due to the raw material and production process used. FibersymTM HA is a product of high-amylose corn and is suitable for use in wide array of lower net carbohydrate food products. This product provides more than 70% dietary fiber in food and is used in a wide variety of products namely pizza crust, breads, tortillas, cookies, muffins, breakfast cereals, snack


products and nutritional bars. Fibersym 80ST is a product derived from potatoes and it has slightly higher water-holding properties, which can impact finished food products, such as cookie spread or volume of a muffin. RS content is also high in products such as Roquette’s Nutriose FB06 or ADM/Matsutani’s Fibersol-2 and these products deliver 85% and 90% fiber content, respectively. Using maltodextrins as starting material, a natural, highly crystalline RS3 has been developed. Commercial preparations of RS4 such as Fibersym 80ST, Fibersym® RW, Fibersym HA and Fibersol-2 etc. are available in the market. It is less susceptible to enzymatic hydrolysis and confers health benefits by reducing the cholesterol, triglyceride and glucose levels in blood.[8] RS preparations reduce the availability of some saccharides in food, without affecting the organoleptic properties of food products. The quality of product remains same after addition of components with artificially increased RS and the quality does not deteriorate during baking and hence does not affect the quality of bakery products. It also does not affect the organoleptic properties of extruded products and confectionery.84 Table 442,70,85 Health Benefits of RS The international food industry is investigating ways to produce innovative food products with health benefits to fulfill the growing demands of consumers for functional foods due to increasing health awareness. Development of carbohydrate products with low glycemic index is gaining attention due to its health benefits. Products with low glycemic index can improve control of obesity and diabetes and, subsequently, reduce the risk of cardiovascular disease.86 All types of resistant starch are not beneficial to the cholesterol level in blood. The production of short chain fatty acids by bacterial fermentation of resistant starch in the large intestine is determined by composition and properties of RS.87 Slow digestibility of RS leads to the slow release of glucose. RS has physiological benefits of soluble fibers and has a positive impact on colonic health by increasing the crypt cell production rate, or decreasing the colonic epithelial atrophy in comparison with no fiber diets. Although RS would not normally be used in un-heated foods, for physiological studies of the health benefits of RS, raw RS


powders were added to beverages and jellies at levels of 24-60 g RS/d.88 Now-a-days viewpoints on nutrition have changed. Until recently, consumption of carbohydrate-rich foods was highly recommended. Nutritional value of food is expressed by a tendency to reduce the caloric value of meals, observed especially in developed countries. Civilizational changes have also contributed to an increased intake of dietary fiber, indispensable for proper functioning of the organism. RS gained greater interest because it is a natural food component, which is neutral to the organism and add little caloric value to the food. Dietary fibers, including RS, promote beneficial physiological effects including laxation, blood cholesterol attenuation and blood glucose attenuation. RS: A Type of Dietary Fiber Dietary fiber is usually defined as a food component that resists digestion by human enzymes in the small intestine and that pass into the large intestine where they may or may not be fermented by gut bacteria, for example oligosaccharides and resistant starch. Another definition of dietary fiber is that it is a part of a plant, as chemical substance, according to indigestibility in the small intestine and/or by its beneficial digestive and physiological effects and metabolic fate.89 The recent definition of dietary fiber is ‘the edible part of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine’.90 The main components of dietary fiber are non-starch polysaccharides (NSP), lignin, resistant starch and non-digestible oligosaccharides. NSP include cellulose and hemicellulose, (glucans, gums and pectin). The principal components of dietary fiber are NSP and lignin, where lignin is a noncarbohydrate component of plant cell wall. More recently, it has been suggested that non digestible oligosaccharides such as raffinose, stachyose, oligofructose and inulin also function as dietary fiber. NSP (β-linked polymers) are completely resistant to amylolytic enzymes.91 Consumption of soluble dietary fiber, such as β-glucan and arabinoxylan leads to the formation of viscous solutions and increase the viscosity in the intestine slowing intestinal transit, which in turn delays gastric emptying and slows glucose and sterol absorption by the intestine. It can also lower serum cholesterol, postprandial blood glucose, and insulin levels.92 Insoluble dietary fiber such as lignin, cellulose and hemicellulose usually has high water-holding capacity which contributes to increased faecal bulk.


Role of dietary fiber in body is influenced by degree of fermentation and dietary fiber that is not fermented in body is excreted in the faeces. Dietary fiber confers health benefits and physiological effects including laxation and/or blood cholesterol attenuation, and/or blood glucose attenuation.93

RS Promotes Probiotic Bacteria RS has been found to function as a prebiotic and interest is increasing in its prebiotic potential. Prebiotics are defined as ‘non-digestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacterial species already resident in the colon and thus attempt to improve host health. There is a symbiotic relationship between prebiotics and probiotics.94 One of the best examples of prebiotic is fructo-oligosaccharide but certain other carbohydrates also have potential as prebiotics. RS functions as a prebiotic as well as symbiotic.95 Some examples of typical prebiotics includes inulin and oligofructose and inulin type fructans.76 RS promotes the growth and activity of probiotic bacteria and can interact with other prebiotic dietary fibers such as β-glucans and hence acts as a prebiotic component.96,97 Probiotics improves the human health by acting on the specific colonization of human (or animal) gastrointestinal tract by one or more bacterial species. The ingestion of RS helps in extending the viability of some probiotic organisms in the colon. As a prebiotic, RS protects some of the ingested organisms on their path to the colon and effectively increases the initial levels of the desirable species, once they reach the colon. In the colon, RS may initiate its role as susbtrate for a portion of the probiotic organisms.94 High amylose starches (HAS) act as a prebiotic and are also a source of RS2. The health benefits of HAS is through the promotion of faecal excretion of probiotic organisms. HAS also prevents the development of non-reversible insulin resistance and lowers plasma cholesterol and triglyceride concentrations compared with a diet rich in amylopectin starch in humans.98 RS has close relationship with colonic health as it affects faecal bulk and short chain fatty acid metabolism.69 The role of RS has been investigated in relation to hypocholesterolemic and protective effects against colorectal cancer.99 The high fermentation rate of retrograded RS3 and production of short chain fatty acids (SCFA), with a high proportion of butyrate by action of the intestinal microflora are mainly responsible for better colonic health.100 In case of RS4, enzymes are inaccessible due to chemical


modifications such as conversion, substitution, or cross-linking and also due to formation of a typical linkages, for example 1 → 2, 1 → 3, 1 → 4, and 1 → 6. Most of the biopolymers are digested and metabolized in the colon by a large number of microorganisms and enzymes present. Bacterial amylases hydrolyze RS and leads to production of glucose which is further metabolized into organic acids such as lactic acid and gases (CO2, H2, CH4).101,102 Among all the dietary fibers, only RS during fermentation produces higher butyrate, which is known as the principal nutrient of the colonocytes and its lack would increase the risk of some colonic diseases such as colon cancer. Among SCFA, butyrate, have been implicated in improving colonic health and preventing chances of colorectal cancer. Butyrate provides energy to the epithelial cells and inhibits the malignant transformation of such cells in vitro, this makes easily fermentable RS fractions especially useful in preventing colonic cancer. The site of production of SCFA is dependent on the rate of fermentation in the colon and explains the low risk of flatus after ingestion of significant levels of RS.89 Liu and Xu showed that suppression of the formation of colonic aberrant crypt loci is dependent on RS dose, only when it was present during the phase of genotoxic carcinogen in middle and distal colon.103 This shows that RS may retard the development of neoplastic lesions in the colon. RS can be beneficiary to adults with colonic lesions. It has the potential to improve human health and lower the risk of many diet-related diseases.75,104 Hypoglycemic effect of RS Hypoglycemia mainly occurs as side effect in the patients with diabetes mellitus, but can also occur in other diseases. The rate of digestion is slow in foods rich in RS. Inaccessibility of starch to digestive enzymes (α-amylase, isoamylase, pullulanase) in starchy products is responsible for nutraceutical potential of starchy products as well as reduction in postprandial blood glucose and insulin response. Glycemic index is used to rank the products with respect to their influence on postprandial glycemia.3 RS has been reported to play a role in the reduction of glycemic and insulinemic responses to a food. In 1998, FAO recommended increased intake of low glycemic index (GI) foods, with emphasis on diabetics and subjects with impaired glucose tolerance.13 In case of type II diabetes, RS has assumed great importance due to its slow rate of glucose release as well as the time it takes to metabolize.


Higher SDS content in diet can also help to maintain good health in both diabetic as well as nondiabetic persons.14,105 The nutritional quality of cereal grains can be improved by reducing the starch digestibility during processing and increasing the SDS and RS content of product. Whole grains which contain higher concentration of dietary fibers may lower the sugar release to blood and are a good example of low GI foods.106 The outer layer and germ of wheat, barley, rye and oat grains contain many bioactive components such as dietary fibers, antioxidants, phenolic compounds, lignin, vitamins and minerals.107 These bioactive compounds have been linked to reduced risk of cardiovascular disease, cancer, diabetes, obesity and other chronic diseases and coronary heart disease.108 Due to increasing awareness of consumers about the relationship between diet and disease, the food industry is now focusing on producing functional foods based on various cereal wholegrain flours and other low GI products. Hypocholesterolemic effects RS particularly affect lipid metabolism. Total lipids, total cholesterol, low density lipoproteins (LDL), high density lipoproteins (HDL), very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), triglycerides and triglyceride-rich lipoproteins have been affected by RS.64 RS diets, containing 25% raw potato, markedly raised the cecal size and the cecal pool of short-chain fatty acids (SCFA), as well as SCFA absorption in colon and lowered plasma cholesterol and triglyceride levels in rats. Also low concentration of cholesterol in all lipoprotein fractions, especially the HDL1 and reduction in triglycerides concentration in the triglyceride-rich lipoprotein fraction has been observed in rats.69 Cassava starch extrudated with 9.9% oat fiber or cassava starch extruded with 9.7% RS confers hypocholesterolemic properties and these starches can be used in foods to improve cardiovascular health.109 According to Hashimoto et al RS ingestion may decrease the serum cholesterol level in rats fed a cholesterol-free diet.110 There are still contradictions regarding the role of RS in altering triglyceride and cholesterol levels. Hence more research is needed to help us better understand the effects of RS on lipid metabolism in humans. Energy and weight management


RS provides balanced energy for hours after consumption, because the energy of RS is partially released in the small intestine as glucose and partially released in the large intestine as fermentation by-products (such as acetate). The energy value of RS is very low and has been calculated as approximately 8 kJ/g (2kcal/g), whereas, the energy value for completely digestible starch is 15 kJ/g (4.2 kcal/g).111 Role of RS in modifying fat oxidation, as a satiety agent and in weight management has been studied by many authors64,70,112. RS rich diets may potentially enhance the mobilisation and use of fat stores as an indirect result of any reduction in insulin secretion.113 RS rich foods provide fewer calories along with lower glucose response as compared to digestible glucose-based carbohydrates. By RS consumption, total and regional body fat accumulation is reduced with lower adipocyte volume. RS is associated with increased lipid oxidation at the expense of carbohydrate oxidation, and decreased lipid production. The physiological properties mentioned above are important to control weight and reduce obesity.2 Keenan et al reported that use of RS in a diet as a bioactive functional food component is a natural, endogenous way to increase gut hormones that are effective in reducing energy intake.114 This may be an effective natural approach to the treatment of obesity. Reduction of gall stone formation Consumption of digestible starch causes greater secretion of insulin leading to gall stone formation. Insulin is also reported to stimulate cholesterol synthesis.69 Therefore, RS consumption helps in reducing the incidence of gallstones. Incidences of gallstones are less frequent in southern India where whole grains are consumed rather than flour, as in northern India.82 The dietary intake of RS is 2 to 4 fold lower in the United States, Europe, and Australia, compared to populations consuming highstarch diets, such as in India and China, which may be reflected in the difference in the number of gallstone cases in the latter countries.69,115 Absorption of minerals RS increases the ileal absorption of a number of minerals in rats and humans. Enhanced absorption of only calcium has been observed in humans, whereas enhanced absorption of calcium, magnesium, zinc, iron and copper has been reported in rats fed with RS-rich diets.97 A great increase in calcium


and iron absorption has been observed when a meal containing 16.4% RS was consumed.116 According to Schulz et al, some RS (RS2 but not RS3) could improve calcium and magnesium absorption by enhancing the mineral solubility in the cecum and/or large intestine.117 However, no effect of RS on the absorption of glucose produced by the digestion of the digestible starch fraction has been demonstrated and also, no evidence of the influence of RS on amino acids and vitamins has been reported. Raw potato starch is considered to be RS2. Feeding raw potato starch enhances the fermentation in the distal parts of the digestive tract. As a result, absorption of Ca, Mg, Fe, Zn and Cu has been increased due to hypertrophy of the cecal wall and cecal acidification.118 Other Health Benefits of RS RS has been found to reduce the inflammatory bowel diseases like ulcerative colitis and other large bowel problems such as diverticulitis and constipation.64 As RS increases SCFA and butyrate production in vivo, it may prove as a useful adjunct to the traditional treatments of ulcerative colitis (SCFA enemas in human patients). RS2 and RS3 rich diets fed to rats with chemically induced colitis have resulted in normalization of cell functions such as activation of colonic cell proliferation, restoration of apoptotic responses and uptake of SCFA, increased cecal level of butyrate, and improved cecal and distal macroscopic and histological observations.119 However, due to the beneficial effects of RS on supplementation of fecal bulk, fecal consistency and transit time for example, consumption of RS may be helpful in improving these conditions. RS has also been reported to affect the immune functions through production of pro-inflammatory cytokines and the expression of number of receptors on T and B-lymphocytes that are required for the initiation of the immune responses.120,121 Future Trends Resistant starch in food is an active topic for research and the increasing number of research papers that appeared on this topic in recent years is testimony to the importance of RS. It will continue to interest researchers in the future as well. Several commercial preparations of RS are now available and they can be used to increase the fiber content of the food. More research will be needed to produce RS with improved functional properties in terms of texture, solubility and resistance to


thermal processes. Health benefits of RS are well recognised. However, further studies will be needed for developing RS preparations with optional characteristics to claim specific health benefit such as improved bowel health and lowering the glycemic response. With the available information on RS, it is very difficult to suggest a figure for RS consumption for general health benefit, although some figures have been proposed for specific health benefits.122 More research will be needed before recommendations on minimum RS consumption can be made for general health benefit. Modern food processing practices have led to lower RS consumption. Because of the desirable functional and physiological properties of RS, there is an increasing trend to incorporate commercial preparations of RS in processed foods. In developed countries where processed foods daily intake is considerable, RS may constitute a substantial part of dietary fiber intake. RS can be used in a diet as a bioactive functional food component to increase gut hormones that are effective in reducing energy intake for the treatment of obesity. RS is considered as promising and innovative food ingredient due to its physiological benefits, and may form an essential ingredient of innovative foods to be developed in the future as the demands for healthier foods increase. More of tailor-made starch derivatives with multiple modifications are likely to be developed in the future.82 Development of insoluble, resistant maltodextrins with a functionality similar to RS has been reported by Buttriss and Stokes.76 More innovations of this type can be expected in the future. Chemically modified starch derivatives, which are non-digestible and can be categorized as RS are likely to find increasing applications in food formulations.123 Although there are some reservations regarding chemically modified starches, use of nutritionally harmless chemicals such as citric acid for chemical modification of starches can be helpful in addressing such reservations. Extrusion has been reported to be useful in increasing RS content of native starches.124 and more extruded products with increased RS content can be expected to be produced in the future. Production of RS with resistance to thermal processing will be needed to increase its use in processed food formulations. Summary RS has assumed great importance due to its potential health benefits and functional properties in foods. A reasonably good understanding of RS with respect to its structure and mechanism has been


achieved through research on its physical, chemical and physiological aspects. Food consumption studies have shown that RS consumption has declined over the last decades. This decline could be due to reduced intake of foods containing natural fibers and increased intake of fast foods. To overcome this problem, food scientists have developed a number of RS-enriched products. RS is ideal for use in ready-to-eat cereals, snacks, pasta, noodles, baked goods and fried foods and permits for easy labelling as simply starch, conferring additional nutraceutical benefits. Its content can be increased by modifying processing conditions such as, number of heating and cooling cycles, pH, temperature and time, freezing and drying. RS shows improved crispness and expansion in certain products, which have better mouthfeel, colour and flavour than products produced with traditional fibers. RS is considered as a type of dietary fiber and shows physiological properties that can reduce the risk of several diseases including colon cancer and diabetes, and can be very useful in controlling diabetes and obesity. RS fortified products have found better consumer acceptability because of the unique physicochemical properties of RS and its bland taste.

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Table 1: Classification of starch14 Starch fraction

RDS

Digestion timeline (invitro)/place Examples

Within 20 minute, mouth 20-120 minute, and small intestine intestine Freshly cooked food

Amount (g/100g Boiled hot potato: 65 dry matter) Main Rapid source of energy physiological property Structure

Mainly amorphous

SDS

RS (Type 1-4)

small >120 minute, not in the small intestine, main action in colon Native waxy maize Raw potato, staled bread starch, millet, legumes Boiled millet: 28 Raw potato starch: 75 Slow and sustained Effects on gut health (e.g. source of energy and prebiotic, fermentation to sustained blood glucose butyrate with hypothesised anticarcinogenic effects ) Amorphous/ Crystalline Depending on the type, mainly crystalline


Table 2 Classification of types of resistant starch (RS), food sources, and factors affecting their resistance to digestion in the colon14,47,64,69,70,71

RS Type RS1 RS2

RS3

RS4

RS5

Description

Food sources

Physically protected

Whole- or partly milled grains and seeds, legumes Raw potatoes, green bananas, some legumes, high amylose corn

Ungelatinized resistant granules with type B crystallinity, slowly hydrolysed by áamylase Retrograded starch

Chemically modified starches due to crosslinking with chemical reagents Amylose-lipid complexes

Cooked and cooled potatoes, bread, cornflakes, food products with repeated moist heat treatment Foods in which modified starches have been used (e.g breads, cakes) Foods with high amylose content


Resistance minimized by Milling, chewing Food processing and cooking

Digestion in small intestine Slow rate, Partial degree Totally digested if properly milled Very slow rate, Little degree Totally digested when freshly cooked

Processing conditions

Slow rate, Partial degree Reversible digestion;digestibility improved by reheating

Less susceptible to digestibility in vitro Not susceptible to hydrolysis by α-amylase

A result of chemical modification, can resist hydrolysis Can resist digestion

Table 3 Functional properties and advantages of commercial sources of RS2 and RS364,70,80,83 Natural sources

Increase coating crispness of products

Bland in flavour

Increase bowl life of breakfast cereals

White in color

Functional food ingredients

High gelatinization temperature

Lowering the calorific value of foods

Fine particle size (which causes less interference with texture)

Lower water properties than traditional fiber products

Useful in products for coeliacs as bulk laxatives and in products for oral rehydration therapy

Good extrusion and film-forming qualities

Allow the formation of low-bulk high-fiber products with improved texture, appearance, and mouth feel (such as better organoleptic qualities) compared with traditional high fiber products


Table 4 Commercially manufactured resistant starches commonly used in various foods42,70,85 Brand name of commercial RS

Hi maize

CrystaLean

Novelose 240

Novelose 260

Novelose 300

C*Actistar

TM

Fibersym HA

Type

RS/TDF% content

47% RS

60% RS

Lowers glycemic response when used as a substitute for flour and other rapidly digested carbohydrates

National Starch and Chemicals Co., USA.

< 30% TDF

Lowers glycemic response when used as a substitute for flour and other rapidly digested carbohydrates

National Starch and Chemicals Co., USA

30–60% TDF

RS3

19.2–41% RS

RS2

RS3

RS3

RS4

53% RS

> 70% TDF

FibersymTM 80ST

RS4

80% TDF

Nutriose FB

-

85% TDF

Fibersol 2

-

90% TDF

HylonR VII

RS2

Manufacturer

Prebiotic properties. Lowers faecal pH. Increases the level of SCFA (in particular butyrate which may reduce cancer risk). Increases bowel action with its mild laxative effect. Increases the bowels’ beneficial microflora. Prebiotic effect. Increases proportion of butyrate. Increases cell proliferation in proximal colon (in rats). Provides soluble dietary fiber and prebiotic effects. Low glycemic index. Lowers glycemic response when used as a substitute for flour and other rapidly digested carbohydrates

RS2

RS2

Physiological and/or health benefits

23% TDF

Health benefit potential. Prebiotic effect. Source of butyrate. Supports the immune system. Reduced glycemic response. Low calorific value. Easily fermentable RS. Very well tolerated. Acts a sprebiotic, Reduces the glycemic and insulin response of healthy individuals as well as type 2 diabetics

National Starch and Chemicals Co., USA. Opta Food Ingredients Inc., USA. National Starch and Chemicals Co., USA.

Cerestar ( a Cargill company) MGP Ingredients, Inc. (Atchison, Kans.) and Cargill

Acts a sprebiotic, Reduces the glycemic and insulin response of healthy individuals as well as type 2 diabetics

MGP Ingredients, Inc. (Atchison, Kans.) and Cargill

Low calorific value

Roquette, Freres, France

Probiotics effect, Intestinal regularity and blood sugar regulation Increase level of SCFA

ADM/Matsutani National Starch and Chemicals Co., USA.

Neo -amylose RS3

87 or 95% RS

Prebiotic. Protects against inflammatory intestinal disease. May protect against colorectal cancer. May help control blood glucose levels in diabetics.

Protos-Biotech. (Celanese Ventures GmbH)

Sensory characteristics of high-amylose maize-resistant starch in three food products.

Non-starch polysaccharides (dietary fiber) and resistant starch.

Slowly digestible starch--a review.

Physical modification of food starch functionalities.

Resistant starch: promise for improving human health.

Acute Consumption of Resistant Starch Reduces Food Intake but Has No Effect on Appetite Ratings in Healthy Subjects.

Measurement of resistant starch: factors affecting the amount of starch escaping digestion in vitro.

The effect of resistant starch on colon function in humans.

Production of resistant starch by enzymatic debranching in legume flours.

Increasing resistant starch content in rice for better consumer health.

Structural characteristics of slowly digestible starch and resistant starch isolated from heat-moisture treated waxy potato starch.

Structure characterization and hypoglycemic effects of dual modified resistant starch from indica rice starch.

Preparation and characterization of resistant starch III from elephant foot yam (Amorphophallus paeonifolius) starch.

A resistant-starch enriched yogurt: fermentability, sensory characteristics, and a pilot study in children.

Granulomatous peritonitis and appendicitis of food starch origin.

High pressure intensification of cassava resistant starch (RS3) yields.

Starch utilization by yeasts: mutants resistant of carbon catabolite repression.

Induced mutations in the starch branching enzyme II (SBEII) genes increase amylose and resistant starch content in durum wheat.

Oxygen absorbers in food preservation: a review.

Resistant starch may reduce colon cancer risk from red meat.

Banana resistant starch and its effects on constipation model mice.

Enzymatic modification of corn starch with 4-α-glucanotransferase results in increasing slow digestible and resistant starch.

A single amino acid mutation of OsSBEIIb contributes to resistant starch accumulation in rice.

Resistant starch alters colonic contractility and expression of related genes in rats fed a Western diet.