Module 1: Grain Anatomy & Biochemistry

Botanically the wheat “grain” is a caryopsis: a dry indehiscent fruit in which the pericarp (fruit wall, maternal ovary tissue) is permanently fused to the seed coat (testa). Inside sit the endosperm and the embryo (germ). A mature kernel is roughly 6–8 mm long, 2.5–4 mm wide, weighs 30–50 mg, and contains approximately 82% endosperm, 15% bran, and 3% germ by mass (Evers & Bechtel 1988).

Longitudinal section reveals a crease on the ventral face (the original ovule funiculus), a brush of epidermal hairs at the distal end (“brush end”), and the basal embryo occupying one corner. The outer layers (pericarp, testa, nucellus, aleurone) together constitute the miller’s “bran”; the bulk starchy endosperm is the target of white-flour milling.

Tissue Inventory (Evers 2002)

• Pericarp — outer fruit wall: epidermis, hypodermis, cross cells, tube cells. Cellulose + arabinoxylan. Pigment strand → outer colour.
• Testa (seed coat) — thin pigmented layer from ovule’s inner integument. Dark-red testa is the origin of “red wheat”.
• Nucellar epidermis (hyaline layer) — thin remnant nucellus; cell-wall polysaccharide only.
• Aleurone layer — a single cuboidal cell layer (two in durum) around the starchy endosperm. Protein-storage vacuoles, phytate globoids, enzyme precursors. Biochemically the most active tissue at germination.
• Starchy endosperm — bulk tissue: thin-walled cells packed with starch granules in a protein matrix (the future gluten network). Sub-aleurone, prismatic, and central endosperm sub-zones.
• Embryo (germ) — embryonic axis (plumule, radicle, coleoptile, coleorhiza) plus the scutellum, a shield-shaped cotyledon that absorbs endosperm sugars during germination.

Although the aleurone represents only ~7% of grain mass, it is biochemically the most active tissue in the kernel. During germination the gibberellin hormone GA&sub3; released by the scutellum diffuses into the aleurone and induces synthesis and secretion of hydrolases that dismantle the endosperm: α-amylase, β-amylase, limit dextrinase, proteinases, and β-glucanases. These enzymes cascade into the starchy endosperm, converting starch to maltose and dextrins that feed the embryo (Fincher 1989).

Protein and Mineral Storage

Aleurone cells contain densely packed aleurone grains: membrane-bounded protein-storage vacuoles in which phytate globoids (inositol hexaphosphate chelated with K, Mg, Ca) sit alongside a matrix of albumins and globulins. The result concentrates minerals at the periphery of the grain, where milling strips them out. Approximately 60% of grain Mg, 80% of P (mostly as phytate), 50% of Fe and Zn, and more than half of the B-vitamins (niacin, thiamine, riboflavin) reside in the aleurone (Brouns 2012).

Arabinoxylans of the Aleurone Cell Wall

The aleurone cell wall is unusually rich in arabinoxylan (~65% dry mass) with characteristic diferulate cross-links. Arabinoxylans have a (1→4)-β-D-xylopyranose backbone decorated with α-L-arabinofuranose side chains. In dough-making water-unextractable arabinoxylans (WU-AX) absorb 4–10× their weight in water, competing with gluten and altering rheology. In nutrition they serve as soluble fermentable fibre supporting short-chain fatty acid production in the colon.

Starch is the dominant biopolymer of the wheat grain, accounting for 65–75% of dry mass. It is synthesised in amyloplasts by the concerted action of ADP-glucose pyrophosphorylase (AGPase), granule-bound starch synthase (GBSS, for amylose), soluble starch synthases (SSS), branching enzymes (BE-I, BE-IIa/b), and debranching enzymes. In wheat the granule population is characteristically bimodal:

• A-type granules: large, lenticular, 15–40 µm, formed during early endosperm development (DPA 4–14). ~75% of starch mass but only ~10% of granule number.
• B-type granules: small, spherical, 2–10 µm, formed later (DPA 10–30). ~25% of mass, vast majority of granule count.
• C-type granules: <2 µm, formed last; usually lumped with B-type.

The bimodality is wheat-specific. Rice has a single polyhedral granule class; barley and rye are similarly bimodal. The size distribution influences water binding, pasting behaviour, and freeze–thaw stability of dough.

Amylose and Amylopectin

Wheat starch composition: ~25% amylose, ~75% amylopectin. Amylose is essentially a linear α(1→4)-glucan of 500–6000 glucose residues, forming double helices with ~6 glucoses per turn. Amylopectin is a giant (~10⁶ residues) branched α(1→4) glucan with α(1→6) branches every ~20 residues, organised into alternating amorphous and crystalline lamellae of ~9 nm period (Jenkins 1994, Waigh 2000).

Gelatinisation Thermodynamics

In excess water, heating collapses the semi-crystalline lamellar structure in a first-order-like melting transition. Differential scanning calorimetry (DSC) of bread-wheat starch typically shows onset (Tₒ) ~55 °C, peak (Tᵗ) ~62 °C, and conclusion (Tₐ) ~70 °C, with melting enthalpy 8–12 J/g. A second endotherm near 95–100 °C is the amylose–lipid complex (V-type helices). These parameters directly govern baking: crumb temperature must reach ≥Tₒ for the starch network to set.

Wheat grain contains 8–16% protein, partitioned classically (Osborne 1907) by solvent solubility into four fractions:

• Albumins (water-soluble, ~10% of grain protein): enzymes, protease inhibitors including the amylase–trypsin inhibitors (ATIs) implicated in non-celiac wheat sensitivity.
• Globulins (salt-soluble, ~5%): triticins and other storage globulins; chiefly in embryo and aleurone.
• Gliadins (70% ethanol-soluble, 30–50%): monomeric prolamins, 30–40 kDa, contribute dough viscosity and extensibility. Detailed in Module 2.
• Glutenins (dilute-acid-soluble, 30–50%): polymeric prolamins cross-linked by inter-chain disulfide bonds; HMW-GS and LMW-GS subunits provide dough elasticity. Module 2.

Gliadins + glutenins together are the “gluten complex”. They accumulate in the starchy endosperm during grain filling and are deposited in protein bodies derived from the endoplasmic reticulum and the vacuolar system. At maturity the protein bodies coalesce into an amorphous matrix embedding the starch granules, and rehydration of this matrix generates the viscoelastic gluten network on kneading (Module 2).

Non-storage Proteins

Albumins and globulins contain the grain’s enzymes, ribosomal proteins, and a diverse set of protein-inhibitor defenders (Bowman–Birk trypsin inhibitors, α-amylase inhibitors). Amylase–trypsin inhibitors represent 2–4% of wheat grain protein and have attracted recent attention as potential triggers of innate immunity (Junker 2012): they stimulate TLR4 on intestinal monocytes and may underlie a component of non-celiac wheat sensitivity distinct from the FODMAP-driven IBS component.

Total lipid content of wheat grain is a modest ~2% of dry mass, the bulk confined to the germ (~8% lipid) and bran (~5%); endosperm is only 1–1.5% lipid. Most germ lipids are triacylglycerols rich in linoleic (C18:2) and oleic (C18:1) fatty acids, accompanied by tocopherols (vitamin E) and plant sterols. Endosperm lipids are predominantly polar: lysophosphatidylcholines, phosphatidylethanolamines, and digalactosyldiacylglycerols associated with starch granule surfaces and the amylose–lipid complex.

Dietary Fibre

Total dietary fibre in bread wheat is ~12% of dry mass; in wholemeal ~15% on a wet basis. The principal components are arabinoxylan (~7%), β-glucan (~1%), cellulose (~2%), lignin (trace), and fructans (~1.5%). Arabinoxylans and β-glucans are concentrated in aleurone and sub-aleurone cell walls; fructans (inulin-type kestose, nystose) are a soluble FODMAP prominent in wheat and the primary dietary driver of IBS-type symptoms in non-celiac wheat sensitivity (Biesiekierski 2013).

Minerals and Vitamins

• Macrominerals: P (mostly phytate, 0.3–0.4%), K (0.4–0.5%), Mg (~0.15%), Ca (~0.05%).
• Trace minerals: Fe 30–60 mg/kg, Zn 20–40 mg/kg, Se 0.1–0.8 mg/kg (geographically variable).
• B-vitamins: niacin (B3) ~50 mg/kg, thiamine (B1) ~4 mg/kg, riboflavin (B2) ~1 mg/kg, pyridoxine (B6) ~3 mg/kg, folate (B9) ~0.5 mg/kg. Concentrated in aleurone and germ.
• Tocopherols: α- and γ-tocopherol predominantly in the germ.

Phytate and Mineral Bioavailability

Phytic acid (inositol hexaphosphate, InsP₆) is the main phosphorus-storage molecule in the aleurone. Its six phosphate groups avidly chelate Fe²⁺, Zn²⁺, Ca²⁺, and Mg²⁺, reducing bioavailability in monogastric humans who lack endogenous phytase. Leavened bread (active yeast plus endogenous cereal phytase) degrades ~60% of phytate during fermentation; unleavened flatbreads retain nearly all of it (Sandberg 2002). This is a major nutritional driver of the “high extraction” vs. “white” flour debate: bran-rich flours deliver more minerals but also more phytate.

Modern roller milling subjects the kernel to a sequence of break (corrugated) and reduction (smooth) roll passes interleaved with sifting and purifying stages. The goal is to separate endosperm from bran and germ with minimum cross-contamination, yielding flour streams that can be blended to specification.

1. Tempering: wheat is conditioned to 15–17% moisture for 12–24 h. Moisture toughens the bran so it breaks in sheets rather than pulverising, and mellows the endosperm.
2. First break roll (B1): corrugated rollers (~10 corrugations/cm) fracture the kernel longitudinally along the crease, releasing large endosperm chunks.
3. Plansifter: nested sieves classify stock into semolina (coarse endosperm), middlings, and “throughs” (near-flour).
4. Purifier: air-aspirated sifter that separates endosperm particles from attached bran flecks by density difference.
5. Reduction rolls (C1–C12): smooth rollers progressively reduce semolina to flour particle size (80–120 µm median).
6. Flour blending: individual streams are combined to hit protein, ash, and colour targets for a market class.

Extraction Rate

The extraction rate (ER) is the mass fraction of grain recovered as flour. Typical targets:

• Patent flour: ER ~60–65%, purest endosperm, lowest ash, brightest white.
• Standard white flour: ER ~72–78%, the commodity baseline.
• Light brown / wheatmeal: ER ~82–85%, some aleurone retained.
• Wholemeal: ER = 100% (no separation).

Nutrient Losses

White flour at ER = 72% loses ~80% of fibre, ~70% of niacin, ~60% of folate, ~80% of Fe, and ~75% of Zn relative to the whole grain. Statutory fortification (iron, niacin, thiamine, riboflavin, folic acid) restores a subset of these in many jurisdictions (US since 1941, UK since 1984). High-extraction and wholemeal flours retain nutrients but darken and shorten shelf life because of endogenous germ lipases and lipoxygenases.

Three cultivated wheats dominate the modern grain trade: tetraploid durum, hexaploid bread wheat, and hexaploid spelt. All share the basic caryopsis architecture but differ in endosperm texture, aleurone thickness, and hull retention.

Durum (T. turgidum ssp. durum, AABB)

Durum has an extraordinarily vitreous (translucent) endosperm due to tight packing of protein and starch, and a double aleurone layer (most hexaploid bread wheats have a single layer). Grains are amber from carotenoids (lutein, zeaxanthin) that carry through to semolina. Milling yields semolina (coarse endosperm particles ~250–350 µm) preferred for pasta: the tight protein matrix resists cooking losses and retains al dente texture. Durum lacks D-genome HMW-GS alleles, giving a distinctive rheology (strong but less extensible).

Bread wheat (T. aestivum ssp. aestivum, AABBDD)

Bread wheat is free-threshing, with mealy-to-semi-hard endosperm depending on grain-hardness alleles at the Ha locus on 5DS (puroindoline-a and -b, Morris 2002). Puroindoline mutants give “hard” wheats (North American hard red spring/winter, suited to bread), wild-type gives “soft” wheats with friable endosperm (pastry, biscuits). Grain hardness controls milling energy input and starch-damage levels in the flour.

Spelt (T. aestivum ssp. spelta, AABBDD)

Spelt is hulled: glumes remain firmly attached after threshing, requiring a separate dehulling step that consumes ~20% of grain weight as chaff. Endosperm is very similar to bread wheat, but protein content is somewhat higher (12–16%); dehulling limits extraction, and the grain retains a characteristic nutty flavour prized by artisan bakers.

Grain Hardness and the Ha Locus

Puroindolines PINA and PINB are small cysteine- and tryptophan-rich lipid-binding proteins of the starch-granule surface. In soft wheats wild-type PINA/PINB lubricate the interface between starch granules and the protein matrix; milling cleaves cleanly along this interface with little starch damage. Null or missense alleles (Pina-D1b, Pinb-D1b, Pinb-D1d) disrupt the lubricating layer, producing hard wheats in which milling shears through starch granules, damaging 6–14% of granules and elevating flour water-absorption (crucial for bread dough hydration).

The wheat grain develops over 35–50 days after anthesis (DPA, days post-anthesis), beginning with a cell-division phase (DPA 0–10) that sets the number of endosperm cells, followed by a cell-enlargement and reserve-deposition phase (DPA 10–40) during which starch and protein accumulate, and ending with a desiccation phase (DPA 40–50) during which grain moisture falls from ~45% to the mature ~12%.

Temporal Sequence of Starch and Protein Deposition

• DPA 0–4: coenocytic nuclear divisions in the primary endosperm; cellularisation begins at DPA 3–4.
• DPA 4–14: A-type starch granule initiation in amyloplasts; first detectable HMW-GS mRNA.
• DPA 10–30: rapid grain filling; B-type granules initiate around DPA 10–15; HMW-GS and gliadins accumulate in parallel.
• DPA 25–35: maximum dry-matter accumulation rate (30–50 mg/grain/day); nitrogen remobilisation from senescing flag leaf supplies grain amino acids.
• DPA 35–50: desiccation; disulfide bonding consolidates; phytate globoids crystallise in aleurone protein bodies.

The rate of grain filling is temperature-sensitive: a 1 °C rise above the optimum (~20 °C mean temperature) shortens the filling duration by ~3 days and compresses reserve deposition, reducing individual grain weight. This is a dominant yield sensitivity to heat stress (Asseng 2015, Module 8).

Nitrogen Remobilisation

Approximately 70% of grain nitrogen derives from the remobilisation of amino acids from vegetative tissues (chiefly the senescing flag leaf), with the remaining 30% from post-anthesis root uptake. The enzymatic machinery includes chloroplast-localised proteases (SAG12 papain-family, DEG plastid proteases) that dismantle the photosynthetic apparatus during monocarpic senescence, releasing amino acids loaded into the phloem and delivered to the developing grain via the crease-vein pathway.

Starchy endosperm cell walls are thin (1–2 µm) and compositionally unusual for plant tissues: they contain essentially no cellulose (<2%). Instead they are dominated by arabinoxylan (~70%) and mixed-linkage (1,3;1,4)-β-D-glucan (~20%). These walls are digested by aleurone-secreted hydrolases during germination but persist as fibre in the milled flour, where they exert outsized influence on dough rheology and baking.

Water-Extractable vs Unextractable Arabinoxylans

• Water-extractable arabinoxylans (WE-AX): ~25% of total AX; low-MW, less cross-linked; solubilise during mixing; increase dough viscosity, improve loaf volume by stabilising gas-cell walls.
• Water-unextractable arabinoxylans (WU-AX): ~75% of total AX; heavily cross-linked by diferulate bridges; absorb 4–10× their mass in water; compete with gluten for hydration and reduce loaf volume if excessive.

Added xylanase enzymes (from Aspergillus niger or Bacillus subtilis) are standard industrial improvers: they selectively hydrolyse WU-AX into soluble fragments, freeing bound water for gluten hydration and releasing the oligosaccharides that further stabilise gas cells. Over-dosing xylanase, however, collapses the loaf by destroying the AX network completely.

Beta-Glucan

Wheat endosperm contains only ~1% mixed-linkage β-glucan (vs. oats at 3–7%, barley at 3–5%). β-glucan is a linear (1→3;1→4)-β-D-glucan whose solubility increases with the ratio of cellotriosyl to cellotetraosyl units. High-MW soluble β-glucan forms viscous solutions credited with the cholesterol-lowering effects of oat bran (FDA health claim 1997). Wheat’s low β-glucan content means this health benefit is minimal in wheat products.

The anatomy and biochemistry of the wheat kernel set the stage for every downstream process. Starch granule bimodality controls water uptake and pasting. Protein bodies in the endosperm coalesce on hydration into the gluten network (Module 2). Aleurone enzymes fire the germination programme. Bran and germ carry the bulk of vitamins and minerals that milling must choose to retain or reject. Lipids and phenolics shape flavour and shelf life.

In the next module we zoom in on the gluten proteins. We will examine how the gliadin monomers and glutenin polymers form a unique viscoelastic network on hydration, how this network is characterised by small-amplitude oscillatory shear (SAOS) rheology, and how it comes to misbehave in the context of celiac disease.

During reduction milling of hard wheats, a fraction of starch granules are mechanically sheared, puncturing the amylose-crystalline shell and exposing internal branched amylopectin. This “damaged starch” has dramatic functional consequences:

• Water absorption: damaged granules absorb ~200% water at room temperature vs ~45% for intact granules. Flour water absorption rises approximately 2% per 1% damaged starch.
• Amylase susceptibility: damaged granules are readily hydrolysed by endogenous α- and β-amylase to maltose, feeding yeast during proofing.
• Falling Number: the Hagberg falling number measures flour alpha-amylase activity; high damaged starch combined with pre-harvest sprouting gives low Falling Number and baking failures.
• Viscosity: intact granules gelatinise at Tₒ ~55 °C; damaged granules swell at room temperature, elevating cold paste viscosity (Brabender amylograph).

Optimal bread-wheat flour has 6–10% damaged starch. Below 5% the flour absorbs too little water and gives a dry dough; above 12% the dough becomes sticky and collapses. Mill operators control damaged starch by roll gap, pressure, and the hardness of the incoming wheat (which is itself a function of the Ha-locus puroindoline alleles).

Pasting Profile (RVA)

The Rapid Visco Analyser subjects a flour-water slurry to a standard thermal cycle (heat to 95 °C, hold, cool to 50 °C) and records viscosity. Characteristic landmarks:

• Pasting temperature: T at which viscosity first rises (~65 °C).
• Peak viscosity: max viscosity during heating (granule swelling).
• Hot-paste viscosity / trough: viscosity after shear-thinning at 95 °C.
• Breakdown: peak minus trough; indicates granule fragility.
• Setback: final viscosity minus trough; indicates amylose retrogradation on cooling.
• Final viscosity: the cold-paste viscosity at 50 °C.

RVA profiles differentiate wheat classes: durum semolina gives low breakdown and high setback (pasta al dente); soft-wheat flour gives high breakdown; pre-gelatinised flours show no peak at all. The RVA has largely replaced the Brabender amylograph as the standard pasting instrument in cereal labs.