Cereals and Cereal Products ( H.-D Belitzand W.Grosch )

Cereals and Cereal Products Food Chemistry (2nd Ed), 1999 H.-D Belitz and W. Grosch

Major cereals include wheat, rye, rice, barley, millet and oats. „ the most important staple foods of mankind

Production: 60% of cultivated land in the world

Cereals: caryopsis huskless (bared seed)—wheat, rye whusk—oat, barley, rice

Chemical composition of cereals

Celiac Disease (celiac sprue, or gluten-induced enteropathy) „ Wheat, rye and barley (Triticeae sp) „ Prolamin fractions „ Loss of villous structure of the intestinal mucosa Degenerative changes of epithelial cells Severely impaired nutrient absorption function „ Affects both infants and adolescents 0.05% in central Europe 0.33% in Ireland

Individual constituents—Proteins Low in Lysine and methionine

Osbone Fractions of cereals „ 1907 T.B. Osborne separated wheat proteins on the basis of their solubility into four fractions. „ 1.Water extracted: water-soluble albumins 2.Salt extracted: salt-soluble (e.g. 0.4 mol/L NaCl globulins 3.Ethanol extracted: 70% aqueous ethanol-soluble prolamins* *the only aa composition can be correlated to the botanical genealogy 4.Acid extracted: 0.5 mol/L acetic acid soluble glutelins (I) LMW (dissolved in 50% 1-propanol at 60oC with l dithioerythritol) (ii) HMW (ppt. At 60% 1-propanol)

Wheat gluten proteins Monomeric gliadins Polymeric glutenin ω-gliadins α-type γ-type LMW HMW (ω-5, ω-1, ω-2) gliadins gliadins subunits subunits S-poor S-rich HMW Intermediate LMW group group MW group Figure Classification of wheat gluten proteins based on structural homologies and genetical relationships

Protein components of wheat gluten –prolamins:glutelins (2:3) „ In hydrated form, prolamins—dough viscosity glutelins —dough elasticity „ Occur at 9 different complex loci in wheat genome HMW glutenin subunits: Glu-A1, Glu-B1 and Glu-D1 LMW glutenin subunits, ω- and γ-gliadin: Gli-A1, Gli-B1 and Gli-D1 α- and β-gliadin: Gli-A2, Gli-B2 and Gli-D2 „ The relative importance of different alleles for gluten quality: Glu-1>Gli-1>Gli-2

Wheat gluten „ the storage proteins of wheat „ composed of two main groups of proteins—gliadin (a prolamin) and glutenin (a glutelin) „ easy to isolate in relatively pure form (insoluble in water) „ responsible for the cohesive, viscoelastic property of wheat flour dough and the ability of gas retention during dough fermentation, and partly for the setting of the dough during baking „ In hard wheat, 80% of protein is gluten protein; in soft wheat, less than 50% of protein is gluten protein

„ Gliadins added to the glutenins soften the gluten formed „ Gliadins act as plasticizer

Chemical properties of gluten proteins –amino acid composition „ high in glutamic acid (about 35% of the total protein), presented as its amide (glutamine) in protein „ low in the basic amino acids (lysine) „ high in proline (about 14% of the total protein); involved in a ring structure, not form an α-helix „ high levels of amino acids with hydrophobic side chains and relatively low amounts of sulfur- containing amino acids

Chemical properties of gluten proteins „ low level of charges of gluten proteins results in low repulsion forces within the proteins, thus, the protein chains can interact with each other quite readily (by H-bonding, hydrophobic interactions), a condition that appears to be necessary for dough formation, gluten structure stability, the rheological and baking properties of flour

Physical properties of gluten — Gliadins „ a large group of proteins with similar properties, average MW~40,000, monomeric proteins (single chain) associated by H-bonding and hyrophobic interactions, extremely sticky when hydrated, little or no resistance to extension, and appear to be responsible for the dough’s cohesiveness (viscosity)

Physical properties of gluten — Glutenins „ a heterogeneous group of proteins, MW~100,000 to several million, average MW~3 million, polymers by inter-molecular disulfide bonds (multichain), resilient and rubbery but prone to rupture when hydrated, apparently gives dough its property of resistance to extension (elasticity)

The importance of gluten proteins to breadmaking quality „ Breadmaking quality, as indicated by loaf volume, increased linearly with flour protein content for all cultivars „ But, gradients of regression lines (“protein responses”) differed amongst cultivars. „ Breadmaking performance vs the glutenin polymer size distribution: higher proportions of glutenin polymers of greater molecular size have higher breadmaing performance

Structure of wheat gluten „ Protein composition of wheat gluten: know well „ Type and extent of polymerization or aggregation of subunits or on other interactions in native gluten: little information available „ Dough rheology: construction now „ Genetic modification ‘gluten composition, quantity ‘ gluten polymerization or aggregation ‘dough rheology ‘ dough property ‘baking property ‘storage quality Need to be correlated with more information with lots of studies on a molecular level

Enzymes present in cereal kernels –amylases „ α-amylase:minimal activity in mature kernels; increases abruptly during sprouting or germination

Enzymes present in cereal kernels –lipases, lipoxygenases „ Oxidize linoleci acid to 9-hydroperoxy acids and cooxidizes carotenoids at a slow rate

„ Addition of lipoxygenase (type II)-active soy flour (1%) –increases mixing tolerance, improve dough rheology, increases bread volume

Enzymes present in cereal kernels –glutathione dehydrogenase „ Catalyzes the oxidation of glutathione (GSH) in the presence of dehydroascorbic acid (DHAA) as H-acceptor „ Relatively high in activity in wheat flour

„ Small amount (2-6 g AA/100 kg flour) caused a pronounced increase in both dough strength and bread volume (Jorgensen, 1935) „ Inhibit the SH/SS interchange, which starts immediately on dough making and leads to depolymerization of gluten proteins weakening of gluten is limited <> X X <> X X

Other nitrogen compounds –glutathione, cysteine „ Free state as thiol compd (GSH, CSH) „ Oxidized forms (GSSG, CSSC) „ Protein-bound forms (GSSProt, CSSProt) „ During dough making GSH+ProtSSProt’ProtSSG+ProtSH If HM gluten proteins are cleaved, the viscosity of the dough drops. „ GSH: predominantly in germ and aleurone layer „ GSSG and CSSC are rheologically inactive, however, they are converted to rheologically active thiols by reduction or SS/SH interchange.

Enzymes –microbial phytase „ 70% phosphorus in wheat is bound to phytin (1% of the kernels), formation of water-insoluble salts in the intestinal absorption of calcium and iron ions „ Can be hydrolyzed during dough making by microbial phytases— nutritionally and physiologically desirable

Distribution of Carbohydrate in wheat

Carbohydrate—Wheat starch „ 65~70% (14% moisture) of flour of 80% extraction, 75~80% (db) of endosperm (endosperm is about 83% of wheat kernel) „ one starch granule per amyloplast „ often divided into two types A-type: large lecticular B-type: small shperical „ About 25% amylose, 75% amylopectin „ Lipid (predominantly lysolecithin in wheat) complexed within the starch granules retard swelling and increase starch gelatinization temperature; thus influence the baking behavior of cereals and the properties of the baked products

Functionality of starch as related to bread products „ dilute gluten to desirable consistency „ furnishes sugar through amylase action „ furnishes surface suitable for strong union with gluten „ becomes flexible but does not disintegrate during partial gelatinization „ sets structure to the final loaf of bread

Stages of starch granular dispersion in baked goods „ Swollen Scottish short-bread „ Gelatinized Biscuits Cakes „ Disrupted Bread „ Dispersed Wafers „ Enzymatic degraded

Polysaccharides other than starch –Pentosans „ water soluble ( absorb 15~20 times more water) and water insoluble pentosans „ Rye flour: 6~8% (15~25% water soluble); wheat flour: 2~3% (1~1.5% water soluble) „ water soluble pentosan: absorb 15~20X water, form highly viscous solutions consists mainly of a linear arabinoxylan and highly branched arabinogalactan „ water insoluble pentosan: swell extensively in water, responsible for the rheological properties of dough and the baking behavior of rye, and increases the crumb juiciness and chewability of baked products (optimal starch:pentosan=16:1,w/w) „ in wheat bread

„ Polymerization caused by enzymic phenol oxidation lack of solubility of most pentosans

Polysaccharides other than starch — β-glucan (lichenins) „ Barley: 3~7%, Oats: 2.2~4.2%; wheat and rye: 0.5~2% „ Linear D-glucopyranose joined by β-1,3 and β-1,4 linkages „ Proivde a high viscosity to water solutions „ Barley β-glucan can interfere in wort filtration in beer production

Polysaccharides other than starch — Glucofructans „ wheat flour contains 1% water-soluble, non-reducing oligosaccharides, predominates in durum wheats


Lipids „ Cereal kernels contain relatively low levels of lipids oat: 6~8%, wheat: 1.6% „ Stored mainly in the germ, smaller extent in the aleurone layer „ Not differ significantly in their fatty acid composition among cereal lipids

Wheat total lipid (3~4%) Germ (30%) Aleurone (25%) Endosperm (45%) (Rich in TG, GI, PhL) Non-starch lipid (29%) Starch lipid (16%) (TG, Digalactosyl diacylglycerides) (FFA, Lysophatidylcholine) Figure Lipid distribution in various fractions of wheat kernels

Lipids „ Rheological dough properties are affected by nonstarch- bound lipids „ Free lipid: 90% of total nonpolar lipids and 20% of total polar lipids „ By kneading, glycolipids become completely bound to gluten extent of binding of TG depends on dough handling (intensive oxygen aeration, addition of lipoxygenase increase free lipids)

Gas-holding capacity and baking volume are positively influenced by polar lipids, while negatively influenced by nonpolar lipids *Polar lipids get concentrated in the boundary layer gas/liquid and stabilized the gas bubbles against coalescence *Lipid vesicles seal the pores which are formed in the protein films on kneading

Lipids—carotenoids and tocopherols „ Carotenoids: Wheat flour 5.7 mg/kg, durum wheat 7.3 mg/kg corn 0.6~57.9 mg/kg

Facebook Yorumları

Bir Cevap Yazın