Which sorting machine suits which product?

A sorting machine is the heart of almost every processing line in the potato, vegetable, fruit, and flower bulb industries. However, the right machine for one product is not necessarily the right machine for another. An onion sorter works fundamentally differently from a cucumber sorter, and an asparagus sorter requires a completely different approach than a roller sorter for carrots. In this blog, we outline per product group which sorting techniques are available, what the differences are, and what to consider when making your choice.

Broadly speaking, we distinguish between grading machines (sorting by size, weight, or length) and optical sorters (sorting by quality, color, and defects). These two are used for slightly different purposes within a line: size and quality. Want to jump directly to a specific category? In our full range of used sorting machines, you will find all machines per crop and machine type.

Sorting machines for open field vegetables

For open field products such as potatoes, onions, flower bulbs, (baby) carrots, parsnips, red beets, chicory roots, and Brussels sprouts, grading machines are almost always used. The most commonly used techniques are the shaker grader, the (radial) roller grader, the length grader, and the web grader. Your choice depends on the product, the desired capacity, and the level of accuracy you want to achieve. Below, we explain how the different sorters for open field vegetables work.

Mechanical shaker graders

A shaker grader works with vibrating or shaking sieve decks with progressively larger perforations. The product is moved forward by the shaking motion and falls through the sieve openings into the correct size class. This type of sorter is widely used for onions, potatoes, flower bulbs, and Brussels sprouts. Its advantages are simplicity, robustness, and relatively low maintenance costs. Special variants with multiple grading levels stacked on top of each other are available for potatoes.

Mechanical roller graders (radial roller graders)

A roller grader, also known as a radial roller grader, sorts by size by allowing products to fall between two diverging rollers. The distance between the rollers gradually increases, so the product drops through the gap at exactly the right point. This type is highly suitable for flower bulbs, (baby) carrots, parsnips, red beets, chicory roots, (set) onions, and potatoes.

For Brussels sprouts, specific roller graders are available with smaller rollers and more precise drop-through, as sprouts are smaller than onions or potatoes and more sensitive to damage.

Length graders (break graders)

For carrots and parsnips, length is at least as important as diameter. A length grader, also known as a break grader, typically uses a system of cords, rollers, or belts with increasing gaps. Short pieces fall through earlier than long ones, allowing you to sort the product neatly by length. This removes broken pieces and leaves you with uniform, high-quality bunches. This type of machine is often an indispensable part of a complete carrot or parsnip processing line.

Web graders (string-/belt graders)

A web grader works with continuous sorting webs or belts in which the openings gradually increase. This type is widely used for potatoes and, in adapted form (with different belts and a softer contact surface), also for fruit. The major advantage is modularity: each size grading is often a separate unit, making it easy to place multiple units in sequence to sort into different sizes. Rule of thumb: width = capacity

Additional note

For all the above grading machines, one important rule of thumb applies: the wider the grading section, the higher the capacity. A 60 cm wide shaker grader is suitable for a smaller operation, but for industrial capacities you quickly move towards widths of 1.0 to 1.2 meters or more. Always choose the width in proportion to your average and peak input, so the machine never becomes the bottleneck in your line.

Optical sorting: based on quality rather than size

In addition to size grading, open field crops are nowadays almost always also sorted optically, either manually or by machine. An optical sorter evaluates each individual product using cameras to detect defects, damage, green spots, rot, stones, and foreign material. The primary goal is to remove damaged or spoiled products, not to sort by size. These two steps are therefore often separate within the line.

A lot has changed in this category in recent years. Whereas cameras previously could only recognize color and contours, modern optical sorters are now self-learning with the help of AI: the software continuously learns to recognize more deviations and can filter out specific defects (such as scab, soil residues, or internal discoloration) that are barely visible to the human eye. Sorting out is usually done by short air blasts that eject defective products from the flow, or by mechanical grippers that remove them. However, even in the most modern lines, part of the inspection is still done manually. On many operations, human inspection on the sorting belt remains, although this is not technically a sorting machine.

In our range, you will find a selection of optical sorters for quality grading. View the current stock in our category of optical sorting machines for quality.

Sorting machines for fruit and greenhouse vegetables

Where open field vegetables are typically sorted by allowing the product to “fall through” the machine, the opposite applies to fruit and greenhouse vegetables: the product is individually carried to prevent damage. For this purpose, cup sorters are used: each fruit sits in its own cup, moves along weighing and camera stations, and is dropped at the correct point.

Cup types per product

A cup sorter is almost always equipped with cups suited to the specific product. Broadly, there are three categories:

• Standard cups: for apples, pears, kiwis, citrus fruits, stone fruits, and other round fruits of limited diameter.
• Larger cups: for peppers, (beef) tomatoes, mangoes, avocados, and other larger round fruits. The Aweta KG3-16+1 tomato sorting machine is a representative example.
• Elongated cups: for cucumbers, zucchinis, eggplants, and sometimes also sweet potatoes. For eggplants, the machine must be extra gentle, with special exits and accumulating belts; eggplants are very delicate and bruise easily.

Load cells, chain conveyors, and cameras

Modern cup sorters are usually much more than simple grading machines. They can be equipped with load cells to sort each item precisely by weight, which is more accurate than estimating volume optically. For the highest capacities, the cup itself is gradually disappearing: the latest generation of sorting machines uses chain conveyors instead of individual cups for certain products. This significantly increases capacity, although it comes with specific requirements for product shape and handling.

In addition, virtually every cup sorter is now camera-controlled. This opens the door to a much richer set of sorting criteria:

• Size / grade
• Color
• External quality (shape, damage, bruises)
• Internal quality (density, ripeness)
• Presence or absence of crown (in tomatoes)

Here too, the rule applies: the more lines of cups or chain conveyors, the higher the capacity. A 2-lane machine is excellent for smaller companies, while specialized packers often use 6 to 10 parallel lines to handle incoming volumes.

Asparagus sorting machines: a separate category

Asparagus sorting machines deserve their own chapter within this category. Asparagus—both white and green—requires a sorting approach unlike any other product. Most asparagus sorters are cup sorters, but with specific cups, special infeed systems, and often an integrated cutting section.

Depending on the configuration, an asparagus sorter can be used to sort by:

• Length
• Head size
• Head closure
• Diameter
• Curvature
• Shape
• Color (especially relevant for green asparagus)

Many machines are also equipped with a cutting section that immediately trims the asparagus stalk to the correct length, so the products come out of the machine ready for sale.

How do you choose the right sorting machine?

There is, of course, no universal answer to the question “which sorting machine is the best?”. The right choice always depends on at least four factors:

• Product: a roller grader for carrots is not a sorter for apples. Always start with the product and choose the machine family that matches it.
• Capacity: don’t calculate based on the average, but on peak input. The width or number of lines determines whether the machine can handle that peak without bottlenecks.
• Accuracy: for export quality, you are more likely to choose a cup sorter with load cells and cameras; for bulk products, a simple shaker or roller grader may suffice.
• Integration in the line: how does the machine connect to your infeed, buffer, packaging, and WMS? A good sorting machine stands or falls with how well it integrates with the rest of your line.

At Duijndam Machines, we check every used sorting machine before it appears in our inventory. This way, you know exactly which model you are buying and in what condition. Not sure which machine best suits your crop or do you have other questions? Get in touch, send a photo of your current line, or browse our blogs for more background articles on sorting, packaging, and reuse of horticultural machinery.