processing of foxtail millet for improved nutrient availability

PROCESSING OF FOXTAIL MILLET FOR IMPROVEDNUTRIENT AVAILABILITY

VITHAL DEORAO PAWAR1 and GIRISH MAROTIRAO MACHEWAD

Department of Biochemistry and Applied NutritionCollege of Food Technology

Marathwada Agricultural UniversityParbhani (Maharastra), 431 402, India

Accepted for Publication December 16, 2005

ABSTRACT

Foxtail millet grains were soaked in distilled water (1:5, w/v) for 12 h atroom temperature, dehulled; dehulled and soaked; and dehulled, soaked andcooked in distilled water (1:3, w/v), and the effects of removal of polyphenolsand phytate on the in vitro protein digestibility (IVPD) and availability of ironand zinc were measured. The results showed that polyphenols and phytatewere decreased significantly up to 50.92 and 49.89%, respectively. The IVPD,however, increased up to 38.71%. The iron and zinc contents decreased up to18.79 and 18.61%, respectively, but the ionizable iron and zinc were increasedup to 55.45 and 80.18%, respectively. This indicated the suitability of simpleprocessing techniques for improvement of availability of nutrients fromfoxtail millet.

INTRODUCTION

Foxtail millet (Setaria italica) is one of the most important food crops ofthe semiarid tropics, beginning in China, continuing through India and extend-ing over most of Africa and parts of Southern U.S.A. It plays a very importantrole in the agriculture and food of many developing countries because of itssustainability to grow under adverse heat and limited rainfall conditions. Thiscereal is also known as Italian millet. In India, this millet is grown primarily inthe hot drought – prone arid and semiarid zones – and used mostly for foodpurposes especially by people of economically weaker sections. This milletcontains 12.3% crude protein and 3.3% minerals (Gopalan et al. 1987). Itcould be milled to remove the husk, cooked and puffed for use in the devel-

1 Corresponding author. TEL: (02452) 223801; FAX: (02452) 223582; EMAIL: [email protected]

Journal of Food Processing and Preservation 30 (2006) 269–279. All Rights Reserved.© 2006, The Author(s)Journal compilation © 2006, Blackwell Publishing

269

opment of breakfast and specialty foods and malted for use as an adjunct inbrewing and also in the development of high-calorie density of weaning foods(Malleshi and Desikachar 1985). However, because the grains are coarse, theedible portion of this millet is only about 79%, an indication of the presence ofa high amount of fiber. The fiber is associated with polyphenols and phytate,which reduce the availability of proteins and important minerals especiallyiron and zinc. The removal of these by simple processing techniques such asdehulling, soaking and cooking provides an opportunity to test this millet forprotein and phytate and tannin contents.

The current method of polyphenol assay consists of soaking the grain inbleach or alkali to remove the pericarp, so that the testa, if present andcolored, becomes visible. If a testa is seen, the grain is assumed to containhigh amounts of polyphenols. The foxtail millet grain had a testa with aslight yellowish color. The procedure for vanillin (methanol extraction)(Burns 1971) and modified vanillin (1% HCl in methanol extraction)(Maxson and Rooney 1972) is followed for polyphenol estimation aftertaking into account and optimizing several parameters of vanillin assay(Price et al. 1978). Polyphenols and phytate are both considered antinutri-ents because of their complexation with protein, making them insoluble andalso binding with divalent cations such as Fe++, Zn++, Cu++, etc. They alsoinfluence in vitro protein digestibility (IVPD), ionizable iron and solublezinc contents; and the parameters of in vitro bioavailability (Narsinga Raoand Prabhavathi 1978; Paur 1983). A variety of mild alkaline treatments ofsorghum grain that reduce the assayable tannin content and their effect onthe nutritional value of the grain has been reported by Price and Butler(1979). It has been reported on pearl millet that soaking and dehullingresulted in significant improvement in the protein quality because of thereduction of polyphenols and phytates (Pawar and Parlikar 1990). Thepresent study was undertaken to know how much of these antinutrients couldbe removed during simple processing of foxtail millet and what their influ-ence is on IVPD and the availability of ionizable iron and soluble zinc.

MATERIALS AND METHODS

Grain samples of foxtail millet (variety Arjun) were obtained fromMillet Specialist, Millet Improvement Program, Millet Research Station,Aurangabad, India and cleaned to remove surface contamination beforeanalysis. The grains of this millet were dark brown–yellow, and on removalof the pericarp, the testa was seen as light brown–yellow, which is respon-sible for discoloration caused by tannins and other phenolics such asC-glycosylflavones and alkali-labile ferulic acid.

270 V.D. PAWAR and G.M. MACHEWAD

The samples (500 g) were dehulled in a Satake sheller (Satake Corpora-tion, Tokyo, Japan). The hulls were removed by aspiration and weighed, whichaccounted for 13.5% of the weight of the samples. The first lot of 100-gdehulled samples was soaked in distilled water (1:5, w/v) for 12 h at roomtemperature (~28C), and then water was drained out and grains were dried.The second lot of 100-g dehulled samples was cooked in boiling water (1:3,w/v) until the grains become soft, and these grains were then dried. The thirdlot of 100-g samples was soaked and cooked as previously mentioned. Theexcess cook water in both cases was drained and then dried. One un-dehulledwhole sample was taken as control. All the samples were ground in a Udycyclone mill (Tecator, Inc., Boulder, CO) using a 0.4-mm sieve. The nitrogencontent of the ground samples was estimated by micro-Kjeldahl procedure(AOAC 1990) and nitrogen values were converted in protein by multiplying bya factor of 6.25. The total ash content was determined by oxidizing organicmatter in a muffle furnace at 550C for 5–6 h as per AOAC (1990) method. Thepolyphenols were determined after extracting the samples by continuouslyrotating 200 mg of sample and 10 mL of methanol (1% concentrated HCl inmethanol for the modified vanillin assay) as described by Price et al. (1978) inscrew-top test tubes at room temperature for 20 min. The test tubes werecentrifuged and extracts were collected and assayed as previously described(Burns 1971), and extinction coefficients (DA500) were calculated by subtract-ing the blanks (Price and Butler 1977). The catechin was used as a standardand values for these were obtained by the regular method and by correcting forthe blank and were expressed as catechin equivalent. The phytate was esti-mated by a combination of three methods. The extraction and precipitation ofphytate phosphorus were performed (Wheeler and Ferrel 1981) and estimated(Makower 1970). Finally, the iron content was measured (AOAC 1990) usingo-phenanthroline reagent. The iron and phosphorus contents were assumed tobe in a 4:6 ratio to calculate the phytate phosphorus content. The total phos-phorus and iron contents of the samples were determined after dry ashing bythe AOAC (1990) method. The ionizable iron and zinc were determined asfollows: A 2-g sample suspended in 25-mL pepsin–HCl solution (0.5% pepsinin 0.1 M HCl, pH 1.35) was incubated at 37C for 90 min in a metabolic shakerwater bath. The contents were centrifuged and the aliquot of the supernatantwas adjusted to pH 7.5 and then incubated and centrifuged as previouslydescribed. The resultant supernatant was analyzed for a,α′ -dipyridyl-reactiveiron by the procedure of Narasinga Rao and Prabhavathi (1982). This repre-sented the ionizable iron content of the sample. The soluble zinc from thissupernatant and the total zinc from the food sample were estimated by anatomic absorption spectrophotometer (Varian Techtron AAS 1000; VarianTechtron Pty. Ltd., Australia). The ionizable iron and soluble zinc were used topredict their bioavailability (Narasinga Rao and Prabhavathi 1978; Paur 1983).

271PROCESSING OF FOXTAIL MILLET TO ENHANCE MINERAL ABSORPTION

The IVPD of untreated and treated foxtail millet samples was determined bythe method of Fasold and Gundlach (1964) using both pepsin and trypsin.

RESULTS AND DISCUSSION

The data on the contents of protein and ash on a dry weight basis after asimple processing of foxtail millet are shown in Table 1. The foxtail millet wasfound to be a good source of protein as well as ash (12.32% protein and 3.34%ash). On processing of foxtail millet (dehulling, soaking and cooking), therewere marked significant changes in protein and ash contents, being highest inthe dehulled, soaked and cooked samples. Any of the treatments caused asignificant decrease in the protein and ash contents which could be caused bya loss of fiber in dehulling and leaching out of water-soluble solids includingprotein and minerals in dehulling and subsequent soaking and cooking.Panwal and Pawar (1989) also observed similar results in pearl millet (Penn-isetum typhoides) when they soaked and/or cooked it. Pawar and Parlikar(1990) soaked whole and dehulled pearl millet seeds in water for 15 h andobserved a significant decrease in ash and crude fiber contents. Moreover, theyreported 67.6 and 14.8% reduction of polyphenolic pigments and phytatephosphorus, respectively. The maximum decrease of protein in the dehulled,soaked and cooked samples could be partly because of leaching out of proteinsin water during soaking and partly because of hydrolysis of protein duringsoaking and denaturation of the protein, which caused a decrease in the proteincontent. Sankar Rao and Deosthale (1980) also recorded similar observationsin sorghum, and Rao and Deosthale (1983) in pulses.

Table 2 shows that the effect of processing on polyphenols and phytatecontent in the foxtail millet revealed that it contained quite a good amount ofpolyphenols (0.052 mg/100 g) and phytate (1.94 mg/g). Like many othercereal grains, foxtail millet grains have a thick outer layer of pericarp andcolored testa, which contains most of the phenols and tannins. During de-hulling, as the outer layer of the pericarp was removed, the phenols and tanninswere also removed to the extent of 41.49%. However, soaking had a very littleeffect on the reduction of polyphenols with a very marginal 3.75% leachingout in the soaking medium (water). Dehulling and soaking; dehulling andcooking; and dehulling, soaking and cooking, however, caused a significantdecrease in the polyphenol content to the extent of 43.38, 50.92 and 50.92%,respectively. The reduction of polyphenols was observed because of bothremoval of the pericarp where polyphenols are largely concentrated and also oftheir leaching out in the soaking medium and destruction during cooking. Thephenolic substances (tannins) have been reported at significant levels in somecultivars of barley and sorghum (Maxson and Rooney 1972). Normal cooking


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Prot

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12.3

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7362

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procedures do not overcome the harmful effects of tannins in sorghum, whichinclude lower digestibility, reduced mineral bioavailability, possible carcino-genic effects, lower palatability caused by astringency and lower growth ratesin animals. The effect of tannins on animal growth is thought to be caused bytheir ability to bind proteins, thereby making them insoluble and indigestible.Tannins in cereals were generally believed to be limited to high-tanninsorghum grain (0.21–3.11% catechin equivalent).

Ramachandra et al. (1977) studied the relationship between tannin levelsand IVPD in finger millet (Eleusine coracana) and reported the averagecontent of 0.44% tannin as catechin equivalent in 19 Indian varieties of thismillet. They reported that white grain varieties had lower phenolic content thanthe brown grain varieties and dehulling of seeds caused a concomitant increasein IVPD. Lorenz (1983) also studied the tannin and phytate contents in theproso millet and observed that the tannin content (catechin equivalents) is tothe extent of 0.055–0.178%. He further observed that dark-colored seed cul-tivars of these millets had the highest tannin contents, whereas light-coloredcultivars were comparatively low in tannins, and dehulling of the seeds greatlyreduced the levels of tannins and phytate. Pawar and Parlikar (1990) alsoreported that dehulling followed by soaking pearl millet grains in water couldeffectively remove polyphenols and phytate and improve recovery of solubleprotein and its digestibility in vitro.

The phosphorus compounds and phytic acid data (Table 2) revealed thatall the phosphorus compounds were found decreased during the processing ofthis millet. The phytate phosphorus was reduced from 1.94 mg/g in an unproc-essed sample to 0.30 mg/g in a dehulled, soaked and subsequently cookedsample. The reduction of phytate phosphorus was observed maximum in thesetreatments than any of the other treatments. The phytate phosphorus whenexpressed as a percentage of the total phosphorus was found to be 69% in anunprocessed millet sample, which was drastically reduced to 12.74% in adehulled, soaked and cooked sample. The calculated phytic acid was alsofound decreased to a significant extent in all the processing methods, themaximum being in dehulled, soaked and cooked samples. The dehulling,soaking and cooking of foxtail millet resulted in 15.49–49.89% reductions inthe phytic acid content. The value observed after only dehulling was 15.49%,whereas the value observed after dehulling and soaking was 45.13%, which isin good agreement with the values of 27–53% reported on proso millet byLorenz (1983). Because the ferric ion precipitation method is less sensitive ata low concentration of phytate, the phytate content might have underestimatedsomewhat the phytate content in the concentration range reported for thismillet. This could be because of variations in the iron–phosphorus molar ratiosas reported in soybean extract with ferric phytate precipitation methods asreported by Thompson and Erdman (1982).


The untreated foxtail millet contained approximately two times morephytic acid than the treated foxtail millet, which further confirmed that theresults observed in the present investigation are in good agreement to those ofLorenz (1983) on proso millet.

This reduction in phytic acid in foxtail millet during the processing maybe partially because of its removal during dehulling and partially because of itsleaching out in the soaking medium and destruction during cooking. There areseveral reports in the literature to support our observations in the presentinvestigation. Pawar and Parlikar (1990) reported 18.52 and 74.07% reduc-tions in phytate phosphorus on soaking pearl millet in water for 15 h and ondehulling (15.84% degree of dehulling), respectively.

The data on IVPD and ionizable iron and soluble zinc (Table 3) revealedthat IVPD of untreated foxtail millet was found to be 62.30%. It was observedthat with the decrease in polyphenol and phytate contents on the processing offoxtail millet, there was a significant increase in IVPD from 62.30 to 82.68%.The increase in IVPD was observed to be 30.17, 22.35, 29.35, 32.24 and32.17% in dehulled; soaked; dehulled and soaked; dehulled and cooked; anddehulled, soaked and cooked foxtail millet samples, respectively.

The polyphenols and phytate have been reported to decrease the nutritionalvalue of food grains. Jambunathan and Mertz (1973) have shown that tannins aremostly located in the pericarp layers of grains and dehulling of such grainssignificantly improved their food value. Polyphenols complex with protein andthereby make it insoluble and unavailable to the body. The relationship betweentannins and IVPD in finger millet was studied by Ramachandra et al. (1977);they reported an increase in IVPD on dehulling. Arora and Luthra (1974)observed a similar increase in digestibility by a decrease in the tannin content ingrain sorghum. Pawar and Parlikar (1990) also reported a significant improve-ment in the IVPD of pearl millet on soaking the grains in water for 15 h and ondehulling (8.10–15.84% degrees of dehulling). The harmful effect of polyphe-nols on protein digestibility can be minimized by the normal cooking processbut do not overcome the harmful effects of tannin on sorghum (Price et al. 1980),which include lower protein digestibility and mineral bioavailability.

The data presented in Table 3 further show that foxtail millet contained afair amount of iron and zinc. Ionizable iron expressed as the percentage of ironis considered as a parameter of bioavailability of food iron (Narasinga Rao andPrabhavati 1982), and soluble zinc expressed as the percentage of total zinc isconsidered as a parameter of in vitro zinc availability (Paur 1983). Ionizableiron as the percentage of total iron and soluble zinc as the percentage of totalzinc were found to be 12.48 and 35.20%, respectively, in the untreatedsamples. These observations are broadly consistent with those of Sankar Raoand Deosthale (1980) who reported 22% of total iron in the grains as ionizableand 38% of the total zinc in the grain as soluble zinc in the white ragi varieties.


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(%)

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(mg/

100

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ble

iron

(mg/

100

g)To

tal

iron

(%)

Tota

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nc(m

g/10

0g)

Solu

ble

zinc

(mg/

100

g)R

educ

tion

(%)

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trol

62.3

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1.66

0.33

12.4

82.

310.

8135

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30.1

72.

230.

7835

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2.01

0.93

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5So

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76.5

522

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1.40

0.25

18.1

51.

510.

5335

.00

Deh

ulle

dan

dso

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80.5

929

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1.01

0.42

41.7

51.

110.

6255

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Deh

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dan

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82.3

932

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0.83

0.39

48.1

80.

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.


Polyphenols are known to affect the bioavailability of zinc. The absence ofphytate and relatively low polyphenol content of grain may explain the highlevel of available iron of white foxtail millet varieties. These observationssuggest that the polyphenol content of the grains was the factor determiningthe iron and zinc availabilities in the millet. The percentage increase of theionizable iron and soluble zinc in dehulled, soaked and cooked samples offoxtail millet was caused by the removal of polyphenols and also because ofthe breaking down of polyphenols–protein–minerals complexes during theseprocesses. The foxtail millet sample soaked overnight resulted in the loss ofiron and zinc caused by leaching out, and therefore, the values for iron and zincwere decreased in the soaked grain samples than in the control one. During thewater-soaking treatment, there was an increase in the ionizable iron andsoluble zinc contents. With the combined processing methods, there was aprogressive increase in ionizable iron and soluble zinc contents. The availableiron as judged by the ionizable iron rose progressively from 12.48 to 55.45%,whereas the available zinc as judged by the soluble zinc rose progressivelyfrom 35.20 to 80.18%. This increase in both divalent cations was found to besignificant, which could possibly be partially because of a decrease in thepolyphenol content and partially because of a decrease in the phytate content.

CONCLUSIONS

The simple processing of foxtail millet like dehulling, soaking andcooking resulted in a significant decrease in antinutrients such as polyphenolsand phytate, and improved the bioavailability of minerals such as iron and zincand also protein digestibility in vitro.

REFERENCES

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ARORA, S.K. and LUTHRA, Y.P. 1974. The in vitro protein digestibilityof promising Indian varieties of sorghum and its relation with tannincontents. Ind. J. Nutr. Dietet. 11, 233–235.

BURNS, R.E. 1971. Method for estimation of tannin in grain sorghum.Agron. J. 63, 511–512.

FASOLD, H. and GUNDLACH, G. 1964. Characterization of peptidesand proteins with enzymes. In Methods of Enzymatic Analysis (H.U.Bergmeyer, ed.) p. 350, Academic Press, New York.


GOPALAN, C., RAMASASHTRI, B.V. and BALASUBRAMANIAN, S.C.1987. Nutritive Value of Indian Foods, p. 60, National Institute ofNutrition, ICMR, Hyderabad, India.

JAMBUNATHAN, R. and MERTZ, E.T. 1973. Relationship between tanninslevels, rat growth and distribution of protein in sorghum. J. Agric. FoodChem. 21, 692–696.

LORENZ, K. 1983. Tannins and phytate contents in proso millet (Panicummiliaceum). Cereal Chem. 60, 424–426.

MAKOWER, R.U. 1970. Extraction and determination of phytic acid inbeans. Cereal Chem. 47, 288–295.

MALLESHI, N.G. and DESIKACHAR, H.S.R. 1985. Milling, popping andmalting characteristics of some minor millets. J. Food Sci. Technol. 22,400–403.

MAXSON, E.D. and ROONEY, L.W. 1972. Two methods of tanninanalysis for Sorghum bicolor (L.) Moench grain. Crop Sci. 12, 253–255.

NARASINGA RAO, B.S. and PRABHAVATHI, T. 1978. An in vitro methodfor predicting the bioavailability of iron from foods. Am. J. Clin. Nutr. 31,169–175.

NARASINGA RAO, B.S. and PRABHAVATHI, T. 1982. Tannin content offood commonly consumed in India and its influence on ionizable iron. J.Sci. Food Agric. 33, 89–96.

PANWAL, J.H. and PAWAR, V.D. 1989. Effect of soaking pearl millet in acidon the bleaching and functional properties of flours. J. Food Sci. Technol.26, 79–82.

PAWAR, V.D. and PARLIKAR, G.S. 1990. Reducing the polyphenols andphytate and improving the protein quality of pearl millet by dehulling andsoaking. J. Food Sci. Technol. 27, 140–143.

PRICE, M.L. and BUTLER, L.G. 1977. Rapid visual estimation and spectro-photometric determination of tannin content of sorghum grain. J. Agric.Food Chem. 25, 1268–1273.

PRICE, M.L. and BUTLER, L.G. 1979. Treatments of sorghum grains thatreduce the assayable tannin content and their effects on the nutritionalvalue of grain. Proceedings of a Symposium on Polyphenols in Cerealand Legumes Held During 36th IFT Annual Meeting, Institute of FoodTechnologist, St. Louis, MO, p. 39.

PRICE, M.L., VAN SCOYOC, S. and BUTLER, L.G. 1978. A critical evalu-ation of the vanillin reaction as an assay for tannin in sorghum grain. J.Agric. Food Chem. 26, 1214–1218.

PRICE, M.L., HAGERMAN, A.E. and BUTLER, L.G. 1980. Tannin insorghum grain. Effect of cooking on chemical assays and nutritionalproperties in rats. Nutr. Rep. Internat. 21, 761–763.


PUAR, M.D. 1983. Essential trace element profile and zinc availability inIndian foods and diets. PhD Thesis, Maharaja Sayajirao University,Baroda, India.

RAMACHANDRA, G., VIRUPAKSHA, T.K. and SHADAKSHA-RASWAMY, M. 1977. Relationship between tannin level and in vitroprotein digestibility in finger millet (Eleusine coracana Gaertn). J. Agric.Food Chem. 25, 1101–1104.

RAO, U.P. and DEOSTHALE, Y.G. 1983. Effect of germination and cookingon mineral composition of pulses. J. Food Sci. Technol. 20, 125–128.

SANKAR RAO, D.S. and DEOSTHALE, Y.G. 1980. Effect of pearling onmineral and trace elements. Compositional and ionizable iron content ofsorghum. Nutr. Rep. Internat. 22, 723–726.

THOMPSON, D.B. and ERDMAN, J.W. 1982. Structural model for ferricphytate. Implication for phytic acid analysis. Cereal Chem. 59, 525–528.

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processing of foxtail millet for improved nutrient availability

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