Processing of Millet Grains and Effects on Non-Nutrient Antioxidant Compounds

Processing of Millet Grains and Effects on Non-Nutrient Antioxidant Compounds

C H A P T E R 41 Processing of Millet Grains and Effects on Non-Nutrient Antioxidant Compounds Fereidoon Shahidi*, Anoma Chandrasekara† *Department ...

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C H A P T E R

41

Processing of Millet Grains and Effects on Non-Nutrient Antioxidant Compounds Fereidoon Shahidi*, Anoma Chandrasekara† *Department of Biochemistry, Memorial University of Newfoundland, St. John’s, Canada, †Department of Applied Nutrition, Wayamba University of Sri Lanka, Gonawila, Sri Lanka

CHAPTER POINTS • M  illets are rich in bioactive phytochemicals such as phenolics, polyphenolics, and phytic acids which are potent antioxidants in biological systems. • The content of non-nutrient antioxidative compounds of millet grains as well as their activities were affected by different processing conditions experienced during food preparations. • Mechanical operations, hydrothermal processing, soaking, fermentation, and germination/malting affect the content of phenolic compounds and phytates. • Limited data are available on the antioxidant activities of phytochemicals present in millet grains after submitting to different processing operations. • It is necessary to investigate the effect of different processing conditions on the bioactive content of millet grains in order to optimize their antioxidant properties.

INTRODUCTION Millets are the oldest domesticated cereals known to humans from the beginning of civilization. Some archeobotanical evidences have shown that common millet was domesticated as a staple food 10,000 years ago in Northern China. Millets include a group of small seeded cereal grains belonging to the family Poaceae. They serve as a major source of calories and proteins for millions of

Processing and Impact on Active Components in Food http://dx.doi.org/10.1016/B978-0-12-404699-3.00041-X

economically disadvantaged groups of people in Africa and Asia. Millets have additional benefits as subsistence crops due to their unique ability to grow under harsh environmental conditions where other crops yield poorly. Millets are subsistence crops tolerable under drought and low fertile soil conditions. In addition, there are early maturing millet cultivars of 60–90 days and resistance to disease and pest attacks in the field. Furthermore, millet grains can be stored with minimum pest attacks thus constituting a candidate crop for improving food security of vulnerable populations, especially during famines when grain is in short supply. Oxygen, the indispensable element for life, seems to be a paradox due to both its beneficial and toxic effects. A small proportion (1–3%) of oxygen we breathe is freed from the energy production system of oxidative phosphorylation in mitochondria and forms what we collectively refer to as reactive oxygen species (ROS). ROS include free radicals and can be defined as species capable of independent existence that contain one or more unpaired electrons (Halliwell, 2007). Free radicals are generally unstable, but very reactive, thus are capable of initiating a series of damaging chemical changes in biological and food systems. In living systems, non-radical molecules generate radicals when they are brought into contact with radicals and transition metal ions such as those of iron and copper which can move electrons (Halliwell, 2007). At low or moderate levels, ROS exert beneficial effects in cellular signaling systems and the body’s immune function (Halliwell, 2007). At high concentrations, however, ROS generate oxidative stress in biological systems and play a key role in the development of degenerative disorders such as cancer, arthritis, autoimmune disorders, type 2 diabetes, cardiovascular

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41.  PROCESSING EFFECTS ON MILLET ANTIOXIDANTS

and neurodegenerative diseases, and aging. Hence, maintenance of the delicate balance of ROS in vivo is most important for optimum health. The antioxidants function in biological systems by scavenging free radicals and singlet oxygen, or by stabilizing transition metal ions, hence preventing, delaying or inhibiting deteriorative reactions induced by free radicals (Shahidi and Nazck, 2004). Antioxidants may act as chain breaking or oxidation preventing compounds. The body has a number of mechanisms to counteract the oxidative stress caused by internal as well as external factors and when they are overwhelmed under pathophysiological conditions, exogenous antioxidants supplied through foods or supplements are necessary. In foods, processing and storage may lead to the destruction of endogenous antioxidative compounds, thus they may be added to products in order to maintain food quality. Antioxidants include nutritive compounds such as minerals, vitamins, proteins, carbohydrates, and nonnutritive phytochemicals such as phenolics that are capable of donating hydrogen atoms or electrons to a pro-oxidant. This chapter focuses on the effects of different processing and food preparation practices on the content as well as the activities of non-nutritive antioxidants of millet grains.

TYPES OF MILLET Millet is a generic name used to denote a set of highly variable small seeded cereals. They do not belong to a single species or a single genus. Foremost millet genera are Pennisetum, Elusine, Setaria, Panicum, Paspalum, Eragrostis, Echinochola, and Digitaria. Based on global production, the major millet type is pearl millet (Pennisetum glaucum) which accounts for approximately 46% followed by foxtail (Setaria italica), proso (Panicum milliaceum), and finger (Elusine coracana) millets (FAOSTAT, 2011). Other minor millets include kodo (Paspalum scrobiculatum), little (Panicum sumatrense), Japanese barnyard (Echinochola crus-galli), fonio (Digitaria exilis), and teff (Eragrostis tef ) millets. In general, the importance of individual millet type varies with respect to agro-ecological systems, where they grow, and the food culture of the people. Evidences in Northern China demonstrated that some 4000 years ago noodles were made from proso and foxtail millets. At present, millets occupy the sixth place as the most important cereal in the world. However, to the western world, millets are grains for feed and forage for livestock. In the USA, pearl millet grows as summer grazing and hay crops while proso millet is cultivated for grains in the Great Plains mainly in Dakotas, Colorado, and Nebraska. The significant millet crop in Canada is proso millet cultivated in Canadian prairies (Alberta, Saskatchewan, and Manitoba) and in Southwestern

Ontario. Their popularity as alternating crops in tobacco farms is emerging in Canada. Furthermore, use of millets in multigrain products and in niche markets for gluten-free products and organic cereals is growing in the markets of North America and Europe. Millets have similar nutritional composition to other economically important cereals, namely maize, rice, wheat, barley, sorghum, oat, and rye (FAO, 1995). The protein content of different millet grains ranges from 7–11% which is comparable to that of other cereals such as maize, rice, and wheat (Klopfenstein and Hoseney, 1995). Pearl millets contain the highest fat content of 5%, making them more vulnerable for off-flavour and offodour development due to oxidative deterioration upon grinding into flour when compared to other cereals. Millets are rich in minerals such as iron and phosphorus as well as B complex vitamins, namely niacin, thiamin, and riboflavin. Furthermore, finger millet has a high calcium content of 350 mg/100 g which is the highest amount reported among cereals. The crude fibre content of millets is 2– 7% which is generally higher than that in other cereals (Klopfenstein and Hoseney, 1995). In addition, millets are gluten-free grains which can be tolerated by coeliac patients. Coeliac disease is a condition characterized by damage to the mucosa of the small intestine caused by ingestion of certain proteins in wheat, rye and barley. The gliadins and glutenins of wheat gluten have been shown to cause the problem which can be prevented only by consuming gluten-free cereals. Thus, millets are potential candidate cereals for coeliacs and add variety in their diet.

PRODUCTION AND DISTRIBUTION OF MILLETS According to the 2009 statistics, global millet production is about 27 million tonnes; approximately 90% of which is utilized in developing countries for food (FAOSTAT, 2011). The major millet producing continents in the world are Africa and Asia as they produce 56 and 41% of the total world production, respectively (Figure 41.1). World millet production has been stable during recent decades due to the subsistence nature of the crop in mainly cultivated areas. It is noted that about two thirds of millets produced are consumed as food while the rest is used for planting seeds, animal feed, beer production, and as bird seeds (FAO, 1995).

PROCESSING OF MILLETS In Africa, East-Asia, and the Indian subcontinent, where millets are traditionally grown, they are prepared for consumption in different ways using the grains as flour/meal and malt. Therefore, millet grains are usually

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Compounds in Millet Grains with Antioxidative Properties

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FIGURE 41.1  Global millet production in million metric tonnes (MMT). Data adopted from FAOSTAT, 2011.

subjected to a wide range of processing conditions such as soaking, decortication, flaking, grinding, malting, fermentation, and heat treatment using traditional or advanced technologies. Thus, millet grains undergo substantial changes in composition during initial postharvest practices such as threshing (separation of grains from stalks), and milling as well as different food manufacturing conditions. In addition, foods prepared from millets vary across continents, countries, and regions within the same country by the nature of preparation as well as other ingredients added. Traditionally, millet is processed by malting or by fermentation and the resultant flour or extracts derived are used in the preparation of weaning foods, infant foods, supplementary food formulations, and beverages such as beer. In general, the different foods prepared from millets include porridges, steam-cooked products, fermented and unfermented breads, boiled rice-like products, alcoholic and non-alcoholic beverages, and snacks (Murty and Kumar, 1995). In African countries and the Indian subcontinent, millets play an integral role as a major component of many meals. Millets are consumed as ‘Couscous’ (steam-cooked product), ‘To’ (stiff porridges) and ‘Ogi’ (thin porridges) which can also be used as a complementary food for infants and young children. In addition, regional specific snack foods such as ‘Halepe’ in Sri Lanka are prepared with finger millet flour.

COMPOUNDS IN MILLET GRAINS WITH ANTIOXIDATIVE PROPERTIES Phenolic Compounds Cereal phenolics are known to possess in vitro antioxidative activities (Shahidi and Naczk, 2004; Chandrasekara and Shahidi, 2010, 2011a). The regular consumption of whole grains is emphasized to contribute to the risk reduction of non-communicable disease load and overall

well-being in populations. Millet grains contain phenolic acids which are located in the pericarp, testa, aleurone layer, and endosperm. Free and conjugated forms of phenolic acids, which include derivatives of hydroxybenzoic and hydroxycinnamic acids, have been identified in millets. Hydroxybenzoic acids reported in different millet grains are gallic, protocatechuic, gentisic, vanillic, syringic, and p-hydroxybenzoic acids (Chandrasekara and Shahidi, 2011b). Hydroxycinnamic acids and their derivatives that are found in millet grains include ferulic, p-coumaric, sinapic, caffeic, and cinnamic acids. In addition, dimers and trimers of ferulic acid that have demonstrated higher antioxidant activities than those of monomers were also identified in several millet grains (Chandrasekara and Shahidi, 2011b). Flavonoids found in millets belong to the antho­ cynidins, flavanols, flavones, flavanones, chalcones, and aminophenolic group of compounds. Phenolic acids and flavonoids are found in different parts of the grain and their content and composition vary depending on the type of millet grain (Chandrasekara and Shahidi, 2011b). It is noted that flavonoids are abundantly present in the free and esterified forms in finger millet and kodo millet grains, among others (Chandrasekara and Shahidi, 2011b). However, composition of flavonoids showed remarkable differences between the two millet types. Catechin, gallocatechin, epicatechin, and epigallocatechin were reported as major flavonoids in finger millets whereas apigenin, vitexin, isovitexin , and luteolin were reported in kodo millet grains (Chandrasekara and Shahidi, 2011b). Condensed tannins, also known as proanthocyanidins are also present in millets. These oligomeric or polymeric flavonoids consist of flavan 3-ol units. Proanthocyanidins are biologically active and are potent antioxidants. Chandrasekara and Shahidi (2010) showed that proanthocyanidin content of millet grains as determined by the vanillin assay ranged from 311– 3 μmol of catechin equivalents (CE)/g of defatted meal. Finger millet had the highest content of proanthocyanidins

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among the examined millet types, namely proso, foxtail, pearl, finger and little millets. Procyanidin B1 and B2 were reported as major dimers in finger millets.

Phytic Acid Phytic acid, also known as myo-inositol hexaphosphoric acid, is an ubiquitous phytochemical found in foods including cereals as their primary storage form of both phosphate and inositol in seeds. In general, phytates are found in the outer layers of the grain. However, depending on the type of millet, distribution within the grain may vary. Phytate content of pearl millet grain was 752 mg/100 g in germ against 86 and 278 mg/100 g for endosperm and bran, respectively (Simwemba et al., 1984). Phytic acid is generally considered an antinutrient due to its complex formation with essential dietary minerals such as calcium, magnesium, iron, and zinc, to decrease their bioavailability in humans and animals. However, due to its chelating ability of free iron, phytic acid is a potent inhibitor of iron-catalyzed hydroxyl radical formation (Shahidi, 1997). Phytic acid is known to reduce the incidence of colonic cancer by suppressing the oxidative damage caused to the epithelium in the colon (Graf and Eaton, 1990). Thus, phytic acid is a potent antioxidant in certain food sources, including cereals.

EFFECT OF PROCESSING ON PHENOLICS AND THEIR ANTIOXIDANT ACTIVITIES OF MILLET GRAINS Generally, antioxidative non-nutrient compounds and nutrients are not distributed evenly in the grain. Thus, processing of millet grains may exert a noted effect on their bioactivities. The outermost layers of the grains possess a high phenolic content and antioxidant activity (Chandrasekara and Shahidi, 2011c; Chandrasekara et al., 2012). In addition, a variety of processing methods applied during food preparation may exert varying effects on the antioxidant activities of millet grains. Cooking is a common hydrothermal treatment employed during cereal grain preparation as foods. N’Dri et al. (2012) recently demonstrated that cooking of millets may increase the total content of phenolic compounds, whereas some specific phenolic acids may be lost. In pearl millet, the cooking process significantly increased the soluble phenolic acid content as determined by high performance liquid chromatography (HPLC). The soluble phenolic acid contents of raw and cooked pearl millets were 64.8 and 115.2 mg/kg of flour, respectively. However, TPC as determined by Folin–Ciocalteu colorimetric method showed a significant decrease in cooked millets compared to that of their raw counterparts.

TABLE 41.1  Antioxidant Activity of Phenolics from Whole (WG), Dehulled (DG), and Cooked (CG) Millet Grains Finger Millet

Pearl Millet

Antioxidant Activity

WG

DG

CG

WG

DG

CG

DPPH* radical scavenging activitya

35.7

31.6

30.9

13.8

13.8

11.2

Hydroxyl radical scavenging activitya

26.8

9.69

3.0

56.0

2.1

1.5

Hydrogen peroxide scavenging activitya

38.9

37.2

31.1

52.9

57.2

46.2

Oxygen radical absorbance capacityb

101

129

116

60.3

95.9

65.5

Superoxide radical scavenging activityc

1.0

0.7

0.6

1.41

1.42

1.09

*2,2-diphenyl-1-picryllhydrazyl; adetermined as μmol ferulic acid equivalents/g defatted meal; bdetermined as μmol trolox equivalents/g defatted meal; cdetermined as mmol ferulic acid equivalents/g defatted meal. Data adopted from Chandrasekara et al., 2012.

Furthermore, fonio millets showed a slight, but significant decrease in almost all soluble phenolic acids along with an increase in bound phenolic compounds (N’Dri et al., 2012). In addition, Chandrasekara et al. (2012) demonstrated that TPC of cooked millet grains did not differ significantly from their uncooked counterparts except for finger millets. Cooked finger millets showed 11 –36% reduction of TPC and the authors suggested that the reduced TPC in finger millet could be due to the degradation of phenolics upon cooking or leaching of phenolics into the endosperm, thus making them less extractable (Chandrasekara et al., 2012). Furthermore, antioxidant activities presented as scavenging of hydroxyl radical, superoxide radical and hydrogen peroxide, and oxygen radical absorbance capacity of dehulled, cooked grains varied depending on the type of millet (Table 41.1). Pradeep and Guha (2011) showed that germination, steaming and roasting of little millet increased the TPC compared to the unprocessed grains. The TPC of native, germinated, steamed, and roasted were 430, 453, 486, and 521 mg of gallic acid equivalents (GAE)/100 g of sample (dry basis), respectively. Furthermore, there was an increase in the content of total flavonoids as well as tannins. The increase in TPC during roasting of little millet grains may be due to the increase in the release of insoluble bound phenolics by the thermal degradation of cellular constituents. Dehulling, a process for removing the outer layers of the grain, also reduced the total phenolic content by 22% from its original value (Hag et al., 2002). A recent study showed that dehulling and removal of outer layers of the grain decreased the TPC of whole grain millets (Chandrasekara et al., 2012). Hulls had higher TPC than

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Effect of Processing on Phenolics and their Antioxidant Activities of Millet Grains

FIGURE 41.2  Effect of sieving and food

4.5

preparation on total phenolic and flavonoid contents of whole finger millet (WFM), sieved finger millet (SFM) and their products. Data adopted from Oghbaei and Prakash, 2012.

4

Total phenolic content (mg tannic acid eq/kg )

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3.5 3 2.5 2 1.5 1 0.5 0

WFM four

WFM wafer

WFM SFM flour SFM vermicelli wafer

WFM four

WFM wafer

WFM SFM flour vermicelli

SFM vermicelli

16

Total flavonoid content (mg quercetin eq /kg)

14 12 10 8 6 4 2 0

those of dehulled grains in kodo, foxtail, proso, pearl, little, and finger millets. Furthermore, TPC of dehulled and cooked grains of millet varieties did not differ significantly except for finger millets (Chandrasekara et al., 2012). Phenolic acids were mainly concentrated in the hulls of millet grains (Chandrasekara and Shahidi, 2011c). Kodo and pearl millet hulls had 34- and 4-fold more ferulic acid, respectively, compared to their dehulled counterparts (Figure 41.2). In addition, p-coumaric acid content in hulls of kodo and pearl millet grains was seven times higher than their corresponding dehulled grains (Chandrasekara and Shahidi, 2011c). Furthermore, dehulling affects the antioxidant activity exerted by millet grains. Dehulled grains showed less inhibition of low density lipoprotein (LDL) cholesterol, DNA strand scission induced by peroxyl radicals and liposome oxidation (Table 41.2).

SFM wafer

SFM vermicelli

Malting changed the content of free and bound phenolic acids in finger millet. The content of bound phenolic acid continuously decreased whereas free phenolic acid content increased upon malting of finger millet grains for 96 h (Rao and Muralikrishna, 2002). Ferulic, caffeic and coumaric acids were found as major bound phenolic acids in native finger millet and upon malting for 96 h, a one- to two-fold decrease in their contents was reported (Rao and Muralikrishna, 2002). Furthermore, malting of finger millets for 96 h decreased the protocatechuic acid content from 45 to 16 mg/100 g. However, the content of other free phenolic acids, namely p-coumaric, gallic and ferulic acids was increased upon 96 h of malting by 2-, 4-, and 10-fold, respectively (Rao and Muralikrishna, 2002). Hag et al. (2002) demonstrated that fermentation and dehulling of pearl millet reduced the TPC as determined by Folin-Denis method. The TPC of standard cultivar of

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TABLE 41.2  Antioxidant Activity of Millet Phenolics as Affected by Dehulling Antioxidant Activity

Kodo Millet

Pearl Millet

WGa

DGb

Hull

WG DG

Hull

Inhibition of LDL cholesterol oxidation (%)

36

28

58

5

1.5

9

Inhibition of DNA strand scission (%)

95

87

85

43

31

75

Inhibition of liposome oxidation (%)

74

63

65

45

44

69

aWG,

whole grain dehulled grain. Data adopted from Chandrasekara and Shahidi, 2011c.

bDG,

pearl millet decreased from 304 to 122 mg/100  g upon fermentation for 14  h. The combination of different processing methods used in the preparation for consumption of millet grains may change their phenolic contents. Gupta and Nagar (2010) showed that cooking, fermentation, dehulling and type of utensils used in the preparation of pearl millet flour rabadi had an effect on flavonoid (quercitin and pelargonidin) content. Quercitin content increased when rabadi was prepared using two stages of fermentation (4  h before cooking and 16 h after cooking) and fermented in clay pots. Progressive decortication from 0–50% and cooking reduced the concentration of C-glycosylflavones (vitexin and orientin) of pearl millets (Akingbala, 1991). In addition, conversion of pearl millet (Pennisetum americanum) into Ogi, a thin porridge which is a traditional Nigerian fermented food, reduced the flavanol content of the native grain by 38% (Akingbala et al., 2002); its flavanol content was the least and was 643 mg glycosylvitexin equivalents/kg dry matter. The bran fraction contains a higher amount (37%) of C-glycosylflavanol (2985 mg glycosylvitexin equivalents/kg dry matter) than the endosperm. Thus, the removal of pericarp and the germ as the bran which contained the highest concentration of flavanol was responsible for this observed reduction (Akingbala et al., 2002). According to Matuschek et al. (2001), cooking, soaking and germination reduced the total phenolic content (TPC) of sorghum udo and finger millet. Fermentation and germination of finger millets decreased their 2,2-diphenyl-1-pcryllhydrazyl (DPPH) radical scavenging ability compared to their raw counterparts (Siripriya et al., 1996). According to Shobana and Malleshi (2004) hydrothermal treatment and decortication of finger millets reduced polyphenolic content by 14 and 74%, respectively. In addition, Towo et al. (2003) showed that hydrothermal treatment of finger millet reduced the TPC by 1.7 times compared to that of the raw grain. Processing may increase the bioavailability of bioactive compounds in the grains; but may decrease their levels.

Several studies have shown that the outermost layers of grains possess a high phenolic content and antioxidant activity. In a recent study, Oghbaei and Prakash (2012) demonstrated that sieving decreased the content of polyphenols and flavonoids in whole finger millet flour. However, sieved finger millet flour showed increased in vitro bioaccessibility. Sieving of whole finger millet flour separates the outer coating of the grain which is resistant to fine grinding. However, the outer coating is the fraction of grain rich in bioactive phytochemicals including phenolic compounds and tannin. Wafers and vermicelli are basically prepared by mixing the flour with water, heating the slurry (for gelatinization), extruding or pressing the hot paste, and finally sun drying (Oghbaei and Prakash, 2012). The sieving of finger millet flour and preparation of different food products significantly reduced the TPC (Figure 41.3). Opoku et al. (1981) showed that germination of pearl millet (Pennisetum typhoides) decreased the level of tannins from 1.6 to 0.83%. Conversely, in a recent study Pradeep and Guha (2011) showed that germination of little millet (Pannicum sumatrense) grains increased the tannin content by 17% compared to the native grain, the reason for which remains unclear. Tannins are capable of binding to proteins, carbohydrates, and minerals, thereby reducing digestibility of these nutrients. Pushparaj and Urooj (2011) showed that different food processing methods such as boiling, pressure cooking, roasting, and germination could affect the available tannin content of foods prepared with pearl millets. Semirefining of pearl millet flour lowered the tannin levels. In addition, bran-rich millet flour had high tannin content attributed to the high concentration of tannins in the seed coat of the grain. Tannin contents were reduced upon wet and dry heat treatments due to their possible loss upon leaching into cooking water, as tannins readily dissolve in water. In addition, during heat processing, tannins may bind to proteins, carbohydrates, or minerals, thus making their extraction difficult (Pushparaj and Urooj, 2011).

EFFECT OF PROCESSING ON PHYTIC ACID CONTENT The distribution of phytates in the millet grain affects their reduction after submitting to different processing conditions. According to Lestienne et al. (2007), 4 and 8% of the phytates of pearl millet grains were removed from Gampela and IKMP-5 cultivars, respectively, at 12% decortication. The initial phytate content was 0.72 and 0.80 g/100 g of dry matter for Gampela and IKMP-5 cultivars, respectively. Thus, findings of this study further revealed that phytates are mainly distributed in the starchy endosperm and the germ of pearl millet grains (Lestienne et al., 2007).

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Summary

FIGURE 41.3  Effect of dehulling on the content

700

Ferulic acid content (µg/g defatted meal)

600

Kodo millet

of ferulic acid of millet grains. Data adopted from Chandrasekara and Shahidi, 2011c.

Pearl millet

500 400 300 200 100 0

Whole grain

Dehulled grain

Eyzaguirre et al. (2006) demonstrated that cooking alone did not reduce phytate content of wholemeal flour of pearl millet (variety IKMP-5). Cooking in kanwa slightly reduced the level of phytates, possibly due to the formation of complexes with the minerals during cooking. Kanwa is a naturally occurring alkaline rock salt, mainly composed of sesquicarbonates (Na2CO3, NaHCO3.xH2O) containing various elements such as Ca, Fe, S, Cl, Si, P, K, and Al (Eyzaguirre et al., 2006). In West and Central Africa, kanwa is used as a tenderizer and for reducing the cooking time of beans and other tough foods. Soaking of pearl millet (Pennisetum glaucum) grains followed by wet grinding and cooking reduced phytic acid to IP5 (inositol pentaphosphate) and IP4 (inositol tetraphosphate) configurations (Eyzaguirre et al., 2006). Soaking grains in water can result in passive diffusion of water-soluble phytate, which will be removed by decanting the water. Germination and lactic acid fermentation can partially decompose phytic acid. However, phytate reduction by the action of endogenous phytases is inhibited at low pH during fermentation due to the production of lowmolecular-weight organic acids such as citric, and malic, lactic acids. The phytate content of whole grain of pearl millet was reduced by soaking for 24 h in water from 419 to 250 mg/100 g on a dry weight basis (Eyzaguirre et al., 2006). Furthermore, soaking for 24 h and germination for 4 days decreased the phytate content by 39% of the original value of the whole grains of pearl millets (Eyzaguirre et al., 2006). In the same study authors showed that phytase treatment of whole meal flour of pearl millet for 4 h reduced phytate content by 18% (Figure 41.4). Germination/malting processes increase the activity of endogenous phytase through de novo synthesis, activation of intrinsic phytase, or by both actions.

Hulls

Cooking in kanawa Cooking in water Acidification Phytase treatment for 4 h Fermentation for 24 h Whole meal 0

100

200

300

400

500

Phytate content (mg / 100g ( dry matter)

FIGURE 41.4  Effect of different processing conditions on phytate content of pearl millet grains. Data adopted from Eyzaguirre et al., 2006.

SUMMARY Millets are the first cereals domesticated by the human civilization, yet they remain underutilized even in countries where they are produced. In developed countries, millets are utilized for feeding animals whereas they are the main staple for millions of economically disadvantaged people in parts of Africa and Asia. Millets are rich in bioactive phytochemicals such as phenolics, polyphenolics, and phytic acids which are potent antioxidants in biological systems. Furthermore, millets may serve as staple food substitutes for coeliac patients as they are gluten free. A wide variety of traditional foods may be prepared using millet grains and this is dictated by the culture of the people in the region. This contribution showed that the content of nonnutrient antioxidative compounds of millet grains as well as their activities were affected by different processing conditions experienced during food preparations. These include mechanical operations (dehulling, decortication, grinding, sieving), hydrothermal processing

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41.  PROCESSING EFFECTS ON MILLET ANTIOXIDANTS

(boiling, pressure cooking, roasting, steaming), soaking, fermentation, and germination/malting. Depending on the type of millet, processing operations affect the content of phenolic compounds and phytates. Limited data are available on the antioxidant activities of phytochemicals present in millet grains after submitting to different processing operations to evaluate their significance as functional food ingredients in the reduction of ailments such as hypertension, cardiovascular disease, and type 2 diabetes. Thus, it would be interesting to investigate the effect of different processing conditions on the bioactive content of millet grains in order to optimize their antioxidant properties.

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Klopfenstein, C.F., Hoseney, R.C., 1995. Nutritional properties of sorghum and the millets. In: Dandy, D.A.V. (Ed.), Sorghum and Millets: Chemistry and Technology. American Association of Cereal Chemists, Inc., St. Paul, MN, USA, pp. 125–166. Lestienne, I., Buisson, M., Lullien-Pellerin, V., Picq, C., Tre`che, S., 2007. Losses of nutrients and anti-nutritional factors during abrasive decortication of two pearl millet cultivars (Pennisetum glaucum). Food Chem. 100, 1316–1323. Matuschek, E., Towo, E., Savanberg, U., 2001. Oxidation of polyphenols in phytate-reduced high-tannin cereals: Effect on different phenolic groups and on in vitro accessible iron. J. Agric. Food Chem. 49, 5630–5638. Murty, D.S., Kumar, K.A., 1995. Traditional uses of sorghum and millets. In: Dandy, D.A.V. (Ed.), Sorghum and Millets: Chemistry and Technology. American Association of Cereal Chemists, Inc., St. Paul, MN, USA, pp. 185–221. N’Dri, D., Mazzeo, T., Zaupa, M., Ferracane, R., Fogliano, V., Pellegrini, N., 2012. Effect of cooking on the total antioxidant capacity and phenolic profile of some whole-meal African cereals. J. Sci. Food Agric. DOI:10.1002/jsfa.5837. Oghbaei, M., Jamuna Prakash, J., 2012. Bioaccessible nutrients and bioactive components from fortified products prepared using finger millet (Eleusine coracana). J. Sci. Food Agric. 92, 2281–2290. Opoku, A.R., Ohenhen, S.O., Ejiofor, N., 1981. Nutrient composition of millet (Pennisetum typhoides) grains and malts. J. Agric. Food Chem. 29, 1247–1248. Pradeep, S.R., Guha, M., 2011. Effect of processing methods on the nutraceutical and antioxidant properties of little millet (Panicum sumatrense) extracts. Food Chem. 126, 1643–1647. Pushparaj, F.S., Urooj, A., 2011. Influence of processing on dietary fibre, tannin and in vitro protein digestibility of pearl millet. Food Nutr. Sci. 2, 895–900. Rao, M.V.S.S.T.S., Muralikrishna, G., 2002. Evaluation of the antioxidant properties of free and bound phenolic acids from native and malted finger millet (Ragi, Elucine coracana Indaf-15). J. Sci. Food Agric. 50, 889–892. Shahidi, F., 1997. Natural antioxidants: An overview. In: Shahidi, F. (Ed.), Natural Antioxidants: Chemistry, Health Effects, and Applications. American Oil Chemists Society Press, Washington, DC, USA, pp. 1–11. Shahidi, F., Nazck, M., 2004. Phenolics in Food and Nutraceuticals. pp 1–82. CRC press, Boca Ratón, FL, USA. Shobana, S., Malleshi, N.G., 2004. Preparation and functional properties of decorticated finger millet (Eleusine coracana). J. Food Eng. 79, 529–538. Simwemba, C.G., Hoseney, R.C., Varriano-Marston, E., Zeleznak, K., 1984. Certain B vitamin and phytic acid contents of pearl millet [Pennisetum americanum (L.) Leeke]. J. Agric. Food Chem. 32, 31–34. Sripriya, G., Chandrasekaran, K., Murty, V.S., Chandra, T.S., 1996. ESR spectroscopic studies on free radical quenching action of finger millet (Elusine coracana). Food Chem. 57, 537–540. Towo, E.E., Svanberg, U., Ndossi, G.D., 2003. Effect of grain pretreatment on different extractable phenolic groups in cereals and legumes commonly consumed in Tanzania. J. Sci. Food Agric. 83, 980–986.

6.  GRAIN, BEANS, PULSES, NUTS AND SEEDS

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