Fresh Sweetcorn

Sweetcorn is harvested at what growers call the milk stage: the kernels have filled out and are plump and tender, and a punctured kernel releases a milky liquid rather than a clear one or a doughy paste. Before this the ear is watery and thin; after it, the kernels toughen, their skins get chewy, and the milky interior turns doughy as starch accumulates. The saleable window is a matter of days in the field, and it moves with the weather.

What sweetcorn cultivars have in common is a genetic block on the normal maize programme of converting sugar to starch in the developing kernel — that block is what makes the kernel sweet at this stage, and cultivars differ in how strongly it holds and therefore how quickly the ear loses sweetness after picking. But no cultivar stops the process. The plant is making a cereal grain; sweetcorn intercepts it partway, and the kernel goes on trying to finish the job after it has been picked.

A snapped ear of sweetcorn is a living tissue cut off from the plant that was feeding it, and it keeps doing what a developing kernel does: converting its sugars into starch, and respiring away sugar for energy. Both processes are running from the moment of harvest, and both are spending the exact thing the ear is sold for. Sweetness is not a stable attribute of this commodity — it is a depleting stock, and the ear starts drawing it down immediately.

Both processes are also strongly temperature-driven, and slow markedly as the tissue is chilled. This is why sweetcorn is precooled as fast and as hard as any commodity in the trade — commonly by hydrocooling or with package ice, and often within hours of picking, with fields cut to match cooling capacity rather than the other way round. Sweetcorn is not chilling-sensitive, so it can be taken close to freezing without injury, and the practice is to do exactly that. The old advice about putting the pot on before picking is folk wisdom about a real biochemical race; the trade wins it with refrigeration instead.

Sweetcorn is graded largely on things the buyer can see without opening the ear — husk colour and freshness, silk condition, ear size and shape — plus what is found on inspection: kernel tenderness, fill, and tip fill, the condition of the tip being a standing grievance because a poorly filled tip is what a shopper notices first. The husk is central to grading twice over: it is the freshness signal a buyer reads, and it is genuine protection, keeping the kernels from drying and shielding them from damage, which is why sweetcorn is traded in husk despite the weight and volume it adds.

• Loss of sweetness — the defining quality loss, invisible until the ear is eaten
• Toughened, starchy kernels from over-mature harvest or warm holding
• Husk yellowing, drying, and wilting
• Dry, discoloured silks
• Poor tip fill and missing or shrivelled kernels
• Earworm and insect damage at the ear tip
• Kernel denting and drying from water loss

Note the gap this list exposes. The most important attribute of the commodity cannot be graded by looking at it: an ear that has been left warm loses its sweetness while its husk still looks perfectly fresh. Visual grade and eating quality can therefore come apart completely, and the only real safeguard is the cooling record rather than the appearance of the lot.

Fresh sweetcorn moves as whole ears in husk, iced or refrigerated, in crates and cartons; trimmed and husked ears packed on trays are a convenience product with a shorter life, since husking removes the ear’s protection. As with several vegetable commodities, a large share of the crop never touches the fresh chain: ears delivered for canning and freezing are cut from the cob and processed close to the field, which sidesteps the sugar-loss problem by stopping the clock within hours of harvest.