Walnuts

A walnut fruit is a drupe with a green husk over a hard shell containing the kernel. As the nut matures the husk splits and begins to separate, which is the harvest signal — the crop is shaken to the ground and gathered, then hulled to strip the husk away and dried before storage. The window between the nut hitting the ground and being hulled and dried is where kernel colour is won or lost, because a nut lying in a wet husk stains rapidly.

In-shell walnuts move as whole nuts and are a major trade form in their own right, especially into markets with a tradition of buying nuts to crack at home and for seasonal demand. Shelled walnuts are traded as halves — the highest value, because an intact half demonstrates careful cracking — and as progressively smaller pieces, with the size and intactness of the kernel forming a large part of the grade.

Walnut grading scales run from the palest kernels down through progressively darker categories, and buyers pay accordingly. Light kernels signal a nut that was gathered promptly, hulled without delay, and dried under controlled conditions; darker kernels indicate the opposite, and the darkening is irreversible. Rain on nuts lying on the orchard floor, delay before hulling, and drying at excessive temperature all push a lot down the scale.

Drying itself is a balance. Under-drying leaves a lot vulnerable to mould and to mycotoxin risk in storage; over-drying and excessive drying temperature darken the kernel, make the shell brittle, and increase breakage during cracking — costing halves, which is where the value sits. The commodity therefore rewards a narrow, well-controlled drying operation more than most.

Walnut kernels are among the most oil-rich of the traded nuts, and their oil is unusually unsaturated. That is why the commodity's practical life is ended by oxidation rather than by rot: a properly dried walnut will not spoil microbially, but it will go rancid, developing the paint-like off flavour that makes it unsaleable. Warmth, light, and oxygen all accelerate the process.

The trade responds by holding kernels cold and, for long storage, under reduced oxygen or in modified-atmosphere packaging — treatments a dry commodity would not otherwise need. The shell affords real protection, which is one reason in-shell walnuts hold longer under ordinary conditions than shelled kernels do. Storage insects are the other standing threat to stored lots, and hygiene and monitoring are routine.

Shelled walnuts are graded on colour, size and the proportion of intact halves, and the incidence of defects. In-shell walnuts are assessed on shell cleanliness and integrity, the proportion of nuts that are well filled, and the ease with which they crack. Shrivelled or poorly filled kernels — nuts whose kernel never developed to fill the shell — are a persistent downgrade that cannot be seen without cracking, and this is why in-shell lots are sampled and cracked to assess kernel condition.

• Dark kernel colour from delayed hulling, rain, or excessive drying heat
• Rancidity in kernels held warm, bright, or long
• Shrivelled and poorly filled kernels
• Broken halves and excessive small pieces from cracking
• Mould growth and mycotoxin risk in under-dried lots
• Insect damage and shell staining

Walnut kernels are eaten as a snack and used widely in bakery, confectionery, and cooking, with halves preferred where the nut is visible and pieces used where it is not. Pressing kernels yields walnut oil, a small, high-value culinary product. The shells left by cracking are collected and used as an abrasive blast medium, a filler, and a fuel — an established industrial by-product outlet rather than waste — while the husks removed at hulling are a bulk residue handled at the huller.

• Food — kernels as halves and pieces for snacking, bakery, and confectionery
• Oil — walnut oil pressed from kernels for culinary use
• Industrial — shells used as abrasive media, filler, and fuel