Protein

• Polymers of aa:20 different aa

• Primary structure – aa sequence

• Secondary structure- chains take up conformations which may crosslink to form helices ie α helix and βpleated sheet

• Tertiary structure- helices fold in various ways to form bulky irregular shapes. Diff non-covalent bonds are responsible for tertiary structure

• Quaternary structure- three dimensional structure that determines its properties

Functionality of Protein

• aa content controls functionality of proteins

• Proteins are denatured by heating, resulting in viscosity increases functional structure is lost, increased digestibility

• Primary structure is not disrupted

• Change in pH, salts may also cause denaturation

• During processing such as breadmaking proteins are exposed to these effects

• Denaturation is non reversible event, but can be reversed in some cases- when denaturation agent has been removed

Protein as enzymes

• Act as enzymes in enzymes ie proteinases , lipases

• Hydrolytic enzymes make a critical contribution in the malting of barley

• Provides nitrogenous nutrients for germinating and in growing plants

• Provides nutrients for fermentation processes

• Provides rheological properties especially wheat and rye to form extensible, elastic, gas retaining doughs in breadmaking

• Need right type of glutens to make extensible doughs

Protein dough forming prperties

• Among all cereals wheat has the ability to form a strong cohesive dough due to presence of gluten

• In milling wheat to white to white flour the total protein content decreases

• Protein of other grains do not have dough forming properties.

• Protein content important in the determination of price in animal feeding

Protein composition

• Composition provides valuable indication of genotype, for distinguishing between varieties within a species,

• Serves in analyses of taxonomic relationships . • Critical at several levels, namely, as amino acids

(composition being relevant to feedvalue),as the amino acid sequence(relating to characterization),as polypeptides (valuable to indicate genotype), and as native proteins (as enzymes or with other functional properties).

• All these aspects of protein composition relate to grain quality for wheat.

• Other proteins of wheat endosperm are identified as markers of important quality attributes:

• Granule-bound starch synthase (for starch properties),

• The purindolines (in relation to grain hardness), and various hydrolytic enzymes(produced in relation to grain defects)

• Protein content is a critical aspect of grain quality for all grain whether protein relates to feed quality or to specific functional properties, such as for barley and wheat.

Protein composition

Protein content

• Varies from between 6-27% due to genetic and environmental effects

• Other effects of protein content : availability of nitrogen ie drought

• Protein content is important for nutritional as well as functional reasons

• As plant produces more protein less is required for physiological functions and more is available for storage

Protein Content

• Corn, sorghum and pearl millet are used to make dough type products eg tortilla,roti or chapati of India

• In maize, protein occurs in the endosperm as discrete protein bodies and as matrix and has high levels of glutamic acid and leucine

• Endosperm contains 5% albumins and globulins, 44% zein, 28%glutelins. The other fraction 17% found in wheat by Osborne –Mendel classification

Protein content

• A higher protein content generally attracts better market value,

• A lower protein content (not too low) is desirable for malting barley as a high potential for starch modification is needed during malting and brewing

Protein content

Protein Classification

Osborne Classification

• Wheat protein divided into 4 classes based on solubility

1. Albumins- soluble in water :metabolically active

2. Globulins- soluble in dilute salt solutions (10% NaCl) but insoluble in water: metabolically active

Metabolically active proteins contain lower glutamic acid and prolinewith higher lysine and arginine ( higher biological value)

3. Gliadins- soluble in 70-90% alcohol (storage protein)

4. Glutenins- insoluble in neutral aqueous solutions, saline solutions, alcohol but soluble in acids and bases (storage proteins)

*** Gliadin and Glutenin reserved for wheat proteins

Prolamins and Glutelin equivalent for protein fractions of other cereals

• Storage proteins have large proportions of glutamic acid and proline and small amounts of lysine , arginine, threonine and tryptophan

Protein Classification- Osbourneclassifcation

Advantages:

• Fractionation of protein gives reproducible results

• Gives some information about proteins

Disadvantages:

• Fractions are mixtures of different proteins

Each protein group has subgroups and none of the subgroups consists of a pure protein

Some proteins do not fall into any of the four groups

• There is residue protein even after extraction procedure of Osborne (some proteins are just insoluble)

Protein Classification

Importance of protein classification

• 1.Aquire nutritive as well as biological value knowledge

• 2.Location of such protein in grain to allow manipulation of milling process so as to get required protein content for processed foods

• 3.Grading of a particular flour/grit depending on protein content and aa availability

• 4.Protein or aa supplementation to compliment different grains thus maintaining product consistency

Possible Protein classification

On basis ofmorphology

On basis of biological function

According to solubility (Osborne)

On basis of chemical composition

Endosperm proteinsProteins of the aleurone layerProteins of the embryo

Metabolically active cytoplasmic proteinsEnzymes

Membarane ptnsRibosome ptnsRegulatory ptnsOther proteins

Storage proteinsLow mwt ptnsHigh mwt ptns

AlbuminsGlobulinsProlaminsGlutelinsResidue proteins

Simple proteinsComplex proteins

LipoproteinsGlycoproteinsNucleoproteinsMetalloproteinsChromoproteimsPhosphoproteins

Protein –Starch Complex

• Starch surrounded by protein in a matrix-endosperm

• Explains kernel hardness

• In soft grains protein starch bond ruptures easily and kernel crashes with minimal force

• In harder grains protein starch bond is stronger

• Matrix affects hardness as well as appearance ie vitreous /hornlike/transluscent and opaque/mealy/floury

• Opacity- air spaces in kernel diffract and diffuse light and make the kernel appear opaque and floury

• Air spaces are formed during grain drying. As grain loses water protein shrinks, ruptures and leaves air spaces

• Translucent -tightly packed kernels with no air spaces light is diffracted at air grain interface but travels through grain without being diffracted again and again

Protein –Starch Complex

• Vitreous - endosperm shrinks but remain intact giving a denser kernel

• In general high protein hard wheat tend to be vitreous and low protein soft wheat tend to be opaque

• Hardness also caused by genetically controlled strength of the bond between protein and starch in the endosperm

Protein –Starch Complex

Lipids

• The relatively small amount of lipid of cereal grains is mainly associated with the germ (embryo) and scutellum tissues.

• lipids in cereals contain a high concentration of unsaturated fatty acids,

• UFA are protected in the intact grain from oxidation by the presence of tocopherols.

• The oil from wheat germ was the starting material for the isolation of α-tocopherol (vitamin E).

•Cereal grains stored as whole kernel

• Milling destroys integrity of the grain, bringing lipases in contact with their substrates.

•Oat grain has a relatively high lipid content, and milled oats are steamed to inactivate lipase activity immediately after removal of the husks by abrasive milling.

Lipids

• Hydrolytic and oxidative rancidity are estimated by determining the free fatty acid content:

• The amount (in mg) of potassium hydroxide required to neutralize the free fatty acids from 100 g of moisture-free grain= fat acidity value

• Milling removes the germ to reduce lipid concentration and the risk of oxidative rancidity to extend shelf life.

Lipids

Average gross composition % Dry weight basis Cereal Grains Protein Fat Starch Fiber Ash

Wheat 12.2 1.9 71.9 1.9 1.7

Rye 11.6 1.7 71.9 1.9 2.0

Barley 10.9 2.3 73.5 4.3 2.4

Oats 11.3 5.8 55.5 10.9 3.2

Maize 10.2 4.6 79.5 2.3 1.3

Millet 10.3 4.5 58.9 8.7 4.7

Sorghum 11.0 3.5 65.0 4.9 2.6

Rice 8.1 1.2 75.8 0.5 1.4