Uneven grading

Potatoes are graded on size (by roller, screen or mesh graders) and on quality (by visual or optical inspection against defect tolerances). The exact size cut-offs differ between sources and markets, so check the buyer's specification before setting equipment.

Size grading

• Cultivation and post-harvest guidance (FAO/CIP/university extension) gives these grades: under 45 mm (smalls/seed), 45-65 mm (standard table), 65-80 mm (large) and over 80 mm (baking).
• USDA-based mechanization guidance uses different bands: seed under 55 mm, ware 55-85 mm, oversize over 85 mm.
• Roller or screen graders handle 5-50 tonnes per hour depending on equipment size (Springer), and their gap settings can be adjusted for different market requirements (USDA).
• Electronic weighing cells can also sort individual tubers by weight.
• For fresh-market regulation, EU Regulation 543/2011 and UNECE FFV-52 set a minimum size of 28 mm for early potatoes and 35 mm for ware potatoes. Within a lot, the difference between the smallest and largest tuber must not exceed 20 mm for tubers under 40 mm, or 30 mm for tubers of 40 mm and above (UNECE FFV-52).

Quality grading

• US grades (USDA AMS, 7 CFR 51.1540-51.1566) are US Extra No. 1, US No. 1, US No. 2, US Commercial and Unclassified. They assess external appearance, internal quality (hollow heart, internal necrosis, vascular discoloration), size and defect tolerances.
• US Grade A defect tolerances: soft rot 0%; severe damage ≤2%; sprouting usually ≤2%, depending on the market; greening usually under 1% because retailers tolerate very little.
• EU requirements: tubers must be intact, clean, firm and pest-free, with sprouts no longer than 5 mm and no more than 1/5 of the surface green. Up to 10% by number or weight may fall short. Labels must show variety, country of origin, net weight and class.
• UNECE FFV-52 Class I allows slight shape and skin defects, with superficial scab covering no more than 1/4 of the surface and a 10% tolerance. Class II allows scab up to 1/3 of the surface, also with a 10% tolerance.

Equipment by end use

My sources don't give a formal equipment-to-use chart, so the following matches the published size bands and technologies to each use:

• Seed: Roller or screen graders with adjustable gaps, set to the seed band (under 45 mm or under 55 mm, depending on the standard). Seed is packed in mesh bags or bulk bins.
• Ware (fresh market): Roller or screen graders for size, plus optical sorting. Camera systems detect green, cuts, bruises, scab and cracks at 5-50 tubers per second (Noordam et al., 2000), and pneumatic air jets or mechanical fingers reject defective tubers. Packaging is typically mesh bags of 2-10 kg, film bags, clam shells or bulk bags of 25-50 kg. Washing is mainly for the fresh market, and potatoes must be fully dried before bagging. Washed potatoes store poorly because of skin damage, so washing is not always done for storage crops.
• Processing: Processing is generally shipped in bulk. Near-infrared (NIR) sorting can non-destructively detect internal defects such as hollow heart and internal browning (Walsh et al., 2020). Hyperspectral imaging is emerging for comprehensive quality assessment (Gowen et al., 2007). I don't have published processor-specific size specifications, which usually come from the buyer or processor.

Key takeaway: Size is set by roller, screen or weight graders. Defects are caught by optical sorting for external flaws and NIR for internal ones. Size bands and tolerances vary by standard (USDA, EU 543/2011, UNECE FFV-52), so match the equipment settings to the target market.