The size of a walnut influences its value, use, and consistency within the commercial market. The widely cultivated English or Persian walnut, Juglans regia, is the subject of nearly all global size standards due to its thin shell and desirable kernel. Categorization is necessary for processors and buyers to ensure quality control, streamline packaging, and accurately determine pricing. The standards established by the United States Department of Agriculture (USDA) define these size parameters, creating a universal language for the trade of this commodity.
Understanding Commercial Walnut Grading Categories
In-shell walnut size classification is determined by passing a sample lot over a series of circular screens to measure the nut’s diameter. The primary categories—Mammoth, Jumbo, Large, and Medium—are defined by the maximum percentage of nuts, by count, that can pass through a specified round opening. This screening process ensures the vast majority of nuts meet the minimum size requirement for that label.
The largest classification, Mammoth, specifies that no more than 12 percent of the nuts pass through a 96/64-inch round opening. The Jumbo category requires that fewer than 12 percent pass through an 80/64-inch screen. The Large classification allows no more than 12 percent of the nuts to fall through a 77/64-inch screen opening.
The Medium size category is defined by both a minimum and a maximum diameter. It requires at least 88 percent of the nuts to pass through a 77/64-inch screen, while restricting the smallest nuts so that no more than 12 percent pass through a 73/64-inch screen. Some standards also include a Baby size, which requires at least 88 percent of the nuts to pass through a 74/64-inch screen.
Measuring In-Shell Diameter vs. Shelled Kernel Pieces
Once the walnut is cracked and shelled, the method of size measurement changes from external diameter to classification based on the size and integrity of the kernel piece. In-shell grading focuses on the whole nut’s physical dimension, while shelled grading focuses on the fragment size of the edible portion. This distinction reflects the different market applications for whole nuts versus processed ingredients.
The most valuable shelled category is Halves, requiring 85 percent or more of the lot, by weight, to consist of half kernels (at least seven-eighths intact). Other standard categories are defined by the size of the screen opening the pieces cannot pass through, often expressed in 64ths of an inch. For example, the standard Pieces category designates fragments that do not pass through a 24/64-inch round opening.
Smaller fragments are classified as Small Pieces, which pass through the 24/64-inch screen but are retained by an 8/64-inch screen. The industry also uses terms like Medium Pieces, which generally must pass through a 32/64-inch screen, though they are not explicitly defined in federal standards. The smallest fragments are often sold as meal or baker’s pieces, used when visual integrity is less important than incorporation into batters or doughs.
Size Comparison of Major Walnut Species
Commercial grading overwhelmingly applies to the English walnut (Juglans regia). Comparing it to the native North American Black Walnut (Juglans nigra) highlights significant biological differences. The English walnut is selectively bred for a thin shell, which facilitates mechanical cracking and yields a high percentage of edible kernel.
Conversely, the Black Walnut is known for its exceptionally hard, thick shell, which has been historically used in industrial applications like sandblasting. Although the total size of the fruit may be comparable to the English variety, the massive shell thickness results in a much lower kernel-to-shell ratio. The intense, earthy flavor of the Black Walnut kernel, combined with the difficulty of extraction, means it is typically used in specialty markets or for extracts, rather than being subjected to common commercial sizing standards.