Not all onions are the same shape — and that changes everything about how they should be sorted. Here's why matching the grader to the onion variety matters more than you think.
The Onion Shape Factor That Most Packers Overlook
Walk into any onion packing house and you'll see one thing in common: onions of all shapes and sizes coming down the line. But here's what many packers don't realize — the shape of the onion determines which grading technology will actually work.
Yellow onions tend to be round or oval. They roll smoothly and predictably. Purple onions (also known as red onions), on the other hand, are flatter and more irregular in shape. They don't roll the same way. They shift, tilt, and can get wedged into gaps that don't truly represent their size.
This might sound like a small detail. But for packers processing high volumes — 6 to 8 tons per hour or more — it's the difference between accurate grades and costly errors.
Two Onion Types, Two Grading Approaches
Round yellow/white onions are best suited for roller graders. The roller principle is simple: as onions move along parallel rollers, the gaps between rollers gradually widen. Round onions roll consistently, falling through at the precise point that matches their diameter. The result? Clean, accurate size separation.
Flat purple onions are a different story. Their irregular shape means they don't roll predictably between rollers. That's why drum graders are the better choice — they use a rotating drum with holes that measure diameter from multiple angles as the onion tumbles, ensuring accurate sizing regardless of shape.
Want to see which grader fits your onion variety and volume? Visit our onion sorting grading machine page for complete specifications and configuration options →
What Makes a High-Capacity Onion Grader Work
For packers processing large volumes — think commercial operations handling 6–8 tons per hour — the equipment needs to do more than just sort. It needs to handle the unique challenges that come with onions:
· Dirt and debris — Onions arrive straight from the field, carrying leaves, dust, and soil that can wear down equipment quickly. Durable materials aren't a luxury; they're a necessity.
· Delicate skins — Onions have multiple layers of thin, papery skin that can tear or bruise during rough handling. Once damaged, they lose shelf life and market value.
· Loose skins — The outermost flaky layers of onion skin can detach during processing, clogging equipment and contaminating the final pack.
· Continuous flow — At 6–8 tons per hour, any jam or slowdown costs real money. The system needs to keep moving.
A well-designed onion grading line addresses all of these challenges in one continuous flow: feeding → debris removal → sizing → packing. The result is consistent grades, minimal waste, and labor costs that are a fraction of what manual sorting would require.
A Smarter Way to Grade Onions at Scale
This is where the right equipment makes all the difference.
Fstsort's onion sorting and grading machines are designed with these real-world challenges in mind. The system typically includes a feeding hopper, elevator, roller inspection conveyor, air blower for loose skin removal, and the size grader itself — all working together in a continuous, high-capacity flow.
What sets it apart:
· No jamming, no downtime — The roller design gradually increases the gap between rollers, so onions never get stuck. Every onion moves through smoothly.
· Gentle on the fruit — The grading process is designed to handle onions with care, minimizing bruising and preserving the outer skin layers that protect the onion during storage and transport.
· Built to last — Onions are tough on equipment. Leaves, dust, and dirt punish every part of the machine, so the materials used are selected for durability in demanding commercial environments.
· Integrated packing — Each grading outlet can be equipped with a bagging device, allowing onions to be packed directly into net bags or cartons as they exit the grader — eliminating an extra handling step.
Fstsort has delivered onion grading solutions to packers across multiple markets, including customized installations for customers in Turkey, Senegal, the Middle East, and the United States.
Want to learn more about our installation stories? Visit our onion sorting machine product page for full details→
Getting It Right from the Start
Onions come in different shapes. The right grader matches the shape. Get it right, and you get consistent grades, higher throughput, and fewer rejects. Get it wrong, and you're leaving money on the table.
The technology exists. The question is whether your line is matched to your onions.
If you are considering upgrading your onion sorter, please don't hesitate to reach out to us.