Conveying Roasted Nuts Off the Roaster: 145 °C, Salt Dust and the Oil That Stays Tacky

The drum door swings open at 07:20 and 25 kg of almonds come out at 148 °C, hissing faintly, smelling of hot oil and something close to burnt sugar. The operator tips the batch into the discharge chute and the nuts hit the cooling belt in a moving heap, 400 mm deep at the crown, spreading as they go. Steam rises off the first two metres. Salt goes on at the drum, so the belt is receiving hot nuts, hot salt and a fine dust of both at the same time — and the belt under the heap is the part of the line nobody thinks about until the splice cracks nine months later.

1. A Batch at the Drum Door

2. What Roasted Nuts Are, Physically

3. Where Conveying Sits on This Line

4. Three Places It Goes Wrong

5. Why This Surface and Not Another

6. The Numbers That Set the Design

7. Changeover and Cleaning

8. The Sentence That Decides It

The chute drop is 300–600 mm: nothing for a dense object, a great deal for a hot kernel with an open seam. At 148 °C the kernel is plastic: it deforms at the point of impact and keeps the deformation. Two metres later the surface is at 90 °C and the kernel has started to stiffen. By the end of the cooling run it is brittle, and the same drop that dented it an hour earlier now takes a corner off.

The cooling belt is 6–10 m long and the batch takes four to eight minutes to travel it, depending on depth and air. Everything below happens in that space.

Roasted tree nuts and peanuts — the category that runs through a drum or a continuous roaster and out onto a cooling belt — share a set of properties that no other food product quite combines:

· Hard and abrasive. A roasted almond is harder than most conveyor surfaces it meets, and its edges are the abrasive part. Wear appears on chute liners, side guides and belt edges long before it appears anywhere structural.

· Hot, 120–160 °C at discharge, cooling to a packing target usually in the 30–40 °C range.

· High oil, 45–60%. Oil migrates to the surface during roasting and stays there. It is the flavour, and it is also the film that collects every particle of dust on the belt.

· Salt and seasoning dust, 1–2% by weight, applied hot so that it adheres. Salt is hygroscopic; seasoning blends are worse.

· Bulk density around 0.5–0.65 t/m³, which means a 400 mm bed on a 600 mm wide belt is a significant mass per metre.

· Two-stage texture. Plastic when hot, brittle when cold. Almost every breakage argument in a nut plant is an argument about which stage a particular drop happens in.

The two cooling stages exist for a reason, and not for symmetry. The first belt has to remove most of the sensible heat without letting the surface oil polymerise onto a hot surface. The second has to bring the product to a temperature at which the pack does not fog, and it is where a nut that is still warm in the centre will steam the inside of a bag three days later.

The belt becomes the heat sink. A 145 °C discharge landing on any belt creates a hot zone directly under the product flow, and the belt surface under a 400 mm bed runs 20–30 °C hotter than the air above it. For a PU belt rated to 80–90 °C continuous, the first two metres are outside the rating for as long as the batch is passing. What happens is not a dramatic failure: the belt hardens, loses elasticity at the splice, and starts to show fine cracks across the join. The splice is the first thing to go, and it goes at a moment when the line is running a full batch.

Salt becomes brine. Above roughly 55% relative humidity, salt on a belt picks up moisture from the air, and once it is wet it is aggressive in a way dry salt is not. It collects at the frame joints, under the belt edge and around fixings, and it sits there between washes. A cooling room that is comfortable for operators, with washdown water on the floor, is exactly the wrong room for a salt-covered conveyor to live in without a cleaning routine built around it.

Fines, oil and a night of warm air. Every batch sheds fines — broken tips, skins, salt crystals. On an open surface they fall through or collect at the edges; on a solid surface they stay in the oil film. A fine particle has a very large surface per gram, it is already coated in oil, and it is sitting in a warm room. Overnight it oxidises, and the smell that greets the morning shift is the smell of a belt that was not scraped at the end of the previous one. The flavour consequence is worse than the smell: oxidised oil on the return air is the beginning of a rancidity complaint six weeks downstream.

An open mesh — the classic choice for a roasting line — lets air pass through the bed rather than over it, so the bottom of the heap cools at something like the rate of the top. It also lets fines fall through to a catch tray, which keeps the carrying surface cleaner but moves the cleaning problem to a tray that has to be emptied. On the hot end, a stainless mesh has no temperature argument at all.

A solid belt is simpler, quieter and easier to wipe, and it cools from one side only. The bottom of a 400 mm bed stays hot for minutes while the top is already at 60 °C, which is why solid-belt cooling runs are usually longer than mesh runs for the same duty. It also holds every fine that lands on it, in the oil.

The practical split is by product size: large whole kernels tolerate mesh, small pieces and splits fall through it, and a solid surface with a scraper and a catch tray wins. Where the section is short and the room is dry, a solid belt does the job with less cleaning. Where the bed is deep and the room is humid, the mesh is doing thermodynamic work that no amount of belt length replaces.

· Discharge temperature and batch rhythm — how hot, how often, and how long the belt runs empty between batches. An empty belt heats faster than a loaded one.

· Batch weight and bed depth, which together set the mass per metre and the air path through the product.

· Target temperature at the packing infeed, and how it is measured — surface or centre.

· Residence time available, which is whatever the room allows, and is usually shorter than the cooling curve wants.

· Salt or seasoning rate, and whether it is applied before or after the cooling belt. Applying hot salt on a hot belt is a different cleaning regime from applying it cold.

· Room temperature and relative humidity in the cooling area, and the washdown frequency. These two decide whether the salt problem is theoretical or daily.

Dry first, and dry properly. Scrape or vacuum the belt, the catch tray and the chute before any water touches the line, because water turns salt and fines into a brine that migrates into places a brush cannot reach. Then a low-pressure wash from the discharge end, working back against product travel, followed by a rinse in the same direction.

The catch tray under a mesh belt is the part that gets forgotten. It holds fines, oil and salt together, and it is the single most reliable source of the smell in a nut plant.

Drying matters as much as washing. A damp mesh restarted with a batch on it steams the product from below for the first metre.

The cooling belt on a nut line is judged by the temperature of the product when it reaches the bag, and it is the belt surface under the heap — not the air above it — that decides how long that takes.

Orange Zhou, Technical Sales Manager · orange@xstlconveyor.com