How to choose a potato size grader

Potatoes are graded by size (using calibrated roller or mesh graders) and by quality (visual or optical inspection against defect tolerances). Size bands differ slightly between sources, so check the specification of your buyer or market.

Size grades

• General post-harvest practice: under 45 mm (smalls/seed), 45-65 mm (standard table), 65-80 mm (large), over 80 mm (baking).
• USDA mechanization guidance: seed under 55 mm, ware 55-85 mm, oversize over 85 mm.
• EU marketing standards (Regulation 543/2011) and UNECE FFV-52: minimum 28 mm for early potatoes and 35 mm for ware potatoes. The size range within a lot is limited to 20 mm for tubers under 40 mm and 30 mm for tubers of 40 mm or more (UNECE).

Quality grades

• USDA AMS: US Extra No. 1, US No. 1, US No. 2, US Commercial and Unclassified. These cover external appearance, internal quality (hollow heart, internal necrosis, vascular discoloration), size and defect tolerances.
• Typical US Grade A tolerances: soft rot 0%, severe damage ≤2%, sprouting usually ≤2%, greening usually under 1%.
• UNECE FFV-52: Class I allows slight defects, with scab on ≤1/4 of the surface and 10% tolerance. Class II allows scab on ≤1/3 of the surface and 10% tolerance.
• EU rules require tubers to be intact, clean, firm and free from pests, with sprouts no longer than 5 mm and green area no more than 1/5 of the surface. Labels must show variety, origin, net weight and class.

Equipment

• Roller or screen graders: separate tubers by size with adjustable gaps and handle 5-50 tonnes per hour depending on size (USDA, Springer). They suit seed, ware and oversize sorting.
• Optical (camera) sorters: detect green tubers, cuts, bruises, scab and cracks at 5-50 tubers per second, with air jets or mechanical fingers rejecting defects (Noordam et al., 2000). These suit fresh-market ware, where appearance matters most.
• NIR sorting: detects internal defects such as hollow heart and internal browning non-destructively (Walsh et al., 2020). It is useful for premium ware and baking potatoes.
• Weight grading: electronic weighing cells sort individual tubers. My sources cut off before giving details.

By end use

• Seed: size grading is the main step. Packaging is in mesh bags or bulk bins.
• Ware (fresh): size grading, then optical inspection, light brushing and complete drying. Bag in mesh, film bags or bulk bags of 25-50 kg.
• Processing: shipped in bulk. My sources don't list processing-specific size or defect specifications, so check processor contracts.

Washed potatoes store poorly because of skin damage, so wash only those going straight to market.

Key takeaway: Roller graders handle size for all three uses, while optical and NIR sorters add defect detection mainly for fresh ware and premium lines.