Raw walnut kernel is the shelled seed of Juglans regia, dried to a stable moisture and graded on two axes at once: colour and piece size. Ukraine is one of the largest walnut origins supplying the European Union, and the kernel rather than the inshell nut is the form most European buyers take, because the shelling yield is roughly a third and paying freight on two thirds shell is rarely worth it.
The commodity has an unusual property for a nut: it is bought and sold on appearance first and on chemistry second, but it is rejected on chemistry. A buyer picks a lot because the kernel is light and the halves are intact, and then discovers three months later that the free fatty acid was already climbing when it shipped. Walnut carries the highest polyunsaturated fraction of the common tree nuts, which is exactly why it tastes the way it does and exactly why it goes rancid faster than almond or cashew.
Grades, cuts and calibration
Grading runs on colour, on piece size and on defect count, and the three are priced separately.
Colour. The trade ladder runs from extra light through light, light amber and amber. UNECE Standard DDP-02 defines the classes and the reference is a colour chart, not an instrument. This is where most disputes start, because a supplier grading against their own retained sample and a buyer grading against a chart from a different season will disagree in good faith. Name the standard, name the class, and keep a jointly sealed reference sample from the contracted lot.
Piece size. The ladder is halves, then broken halves or three-quarter pieces, then quarters, then eighths, then small pieces and kernel meal. A halves specification is almost never one hundred per cent halves; the honest way to write it is a minimum percentage of intact halves with a maximum for each smaller fraction, verified on a weighed sieve separation rather than by eye.
Defects. Shell and foreign material, shrivelled kernel, mould, insect damage and rancid kernel each carry their own tolerance. Shell fragments are the commercially important one because they are a physical hazard and because they are what X-ray or laser sorting is bought to remove. A specification that gives a single combined defect percentage is not enforceable against a lot that fails on one defect and passes on the rest.
What the grade does not tell you. Colour class says nothing about oxidative state. Extra light kernel from a poorly managed store is worth less than light amber kernel that was dried gently and held cold. The chemistry lines below are not optional extras on a walnut specification.
Seasonality and availability
Walnut is a single-harvest crop. The nuts fall or are shaken from late September into October, are dried down within a day or two of collection, and are then shelled and graded through the winter and spring. Kernel is available all year from conditioned store, but the crop year on the label is a real quality variable and it belongs in the contract.
New crop kernel from November to about March is the freshest material available and it is where the colour distribution is best, because the sorting lines have not yet been picked over for the extra light fraction. From late summer onward the market is carrying old crop, the light fractions have been sold, and free fatty acid on any given lot has had ten or eleven months to move. Ask for the crop year, the shelling date and the storage temperature history, and treat a supplier who cannot give all three as a supplier who does not have them.
Drying is the step that sets everything afterwards. Nuts dried too hot case-harden and darken; nuts dried too slowly or held wet develop mould in the shell before shelling even starts, which is where an aflatoxin problem is created.
What the buyer checks at intake
- Identity and lot: crop year, shelling date, colour class with the standard named, piece size specification and the establishment approval number, all checked against the packing list before the pallets are broken down.
- Colour against the jointly sealed reference sample under consistent daylight-balanced light, on a spread sample from several cartons rather than from the top layer of one.
- Piece size distribution on a weighed sieve separation, reporting each fraction, because the halves percentage is the line that carries the price difference.
- Kernel moisture and, where the specification carries it, water activity. Moisture above specification on arrival is a store problem waiting to happen, not a rounding difference.
- Free fatty acid and peroxide value from the certificate of analysis, read together. A low peroxide value with a high free fatty acid means an older lot, not a fresher one.
- Aflatoxin result on the certificate, with the sampling plan stated. A result without a sampling plan is a result on an unknown fraction of the lot.
- Shell and foreign matter on a weighed sample, and a sensory check on a cracked-open cross-section for the paint, cardboard or oil-paint note that signals oxidation.
Where it is used
Bakery is the largest outlet by volume and it takes the widest range of grades, because a piece destined for a filling does not have to be an intact half. Colour still matters where the nut is visible on the surface of a product, and the difference between a light and an amber kernel on top of a tart is worth more than the difference in the purchase price.
Confectionery and snack takes the halves and the large pieces, at the light end of the colour ladder, and is the most appearance-sensitive channel. Ingredient and industrial users take pieces and kernel meal into pastes, fillings and bakery mixes, where colour is nearly irrelevant and free fatty acid is nearly everything, because a paste concentrates whatever oxidation the kernel arrived with.
Walnut oil pressing takes the pieces, the meal and the off-colour fractions. It is the outlet that gives a sorting line somewhere to send what the food grades reject, which is why the price of the lowest kernel grade tracks the oil market rather than the food market.
Standards and routes
Kernel is an ambient dry good in transit but it is not an indifferent one. It travels in standard curtainside trailers or dry containers, and the real risks are heat and moisture pick-up rather than freezing. Sustained transit above roughly 25 degrees Celsius measurably advances oxidation on a polyunsaturated kernel, so a summer road leg across southern Europe is worth planning around, and a reefer set to a chilled ambient is a legitimate specification for a high-value light-grade consignment.
The pack does the rest of the work. A polyethylene liner in a carton is the minimum; a vacuum or nitrogen-flushed barrier bag is what a buyer specifies when the lot will be held for months, and it is the single most effective intervention available on shelf life.
Road corridors to Poland and the Netherlands carry most of the volume. The documentary chain is the standard third-country set set out in the export documentation guide, with the aflatoxin certificate added. Supplier assurance normally runs through BRCGS or IFS Food; the contaminant detail sits in the aflatoxin management guide.
