Kernel Hardness

In wheat, hardness is controlled to a large degree by the puroindoline genes, which influence how firmly starch granules bind to the surrounding protein matrix in the endosperm. Variants of these genes associated with softness leave a friable endosperm that fractures easily along cell walls; variants associated with hardness produce an endosperm where fracture cuts through starch granules themselves. Because this is a genetic rather than an environmental effect, a given wheat variety is reliably hard or soft across seasons and locations in a way that attributes like protein content are not — though environment can still shift hardness somewhat within a variety's characteristic range.

This stability is exactly why hardness, rather than a more environmentally variable attribute, is used to define wheat market classes such as hard red winter, hard red spring, soft red winter, hard white, and soft white. A market class is a description of what the wheat is suited to be milled and used for; it is set primarily by hardness together with kernel colour and, for some classes, growth habit, and it is a distinct concept from a commercial grade, which describes the condition of a lot within its class.

• Milling energy: harder kernels require more mechanical energy to break down and reduce to flour.
• Particle size distribution: hard wheats mill to a coarser, more granular flour; soft wheats mill to a finer, more powdery flour.
• Starch damage: fracturing through starch granules (characteristic of hard wheat milling) damages more starch than fracturing along cell boundaries (characteristic of soft wheat milling).
• Water absorption: damaged starch absorbs more water, so hard-wheat flours generally absorb more water in dough than soft-wheat flours, a property bakers rely on when formulating recipes.

These downstream effects are why millers select wheat by hardness class for a specific end product before they ever look at protein content: hardness sets the physical milling behaviour and the baseline water absorption, while protein and gluten quality separately determine dough strength and baking performance within that class.

In durum wheat particularly, vitreousness — whether a kernel appears translucent and glassy or opaque and starchy-white in cross-section — is assessed as its own visual and optical trait, separately from hardness measured by mechanical or spectroscopic methods. The two properties are correlated, since a densely packed, vitreous endosperm structure tends to also be harder, but they are not the same measurement, are not always concordant in an individual kernel, and are recorded as distinct attributes in grading and quality assessment rather than treated as interchangeable.