Potato, Onion & Root Crop Storage: Buyer's Guide

Potatoes, onions, carrots, beetroot and other root and bulb crops are often stored in larger volumes than any fruit, yet they are among the most misunderstood cold storage projects. Many growers and processors still think of them as 'simple' stores: a shed, some fans and a refrigeration unit. In practice, a root crop store is a living process. Each tonne of potatoes keeps respiring, releasing heat, moisture and carbon dioxide for months; onions must be dried and cured before they can be held; carrots need near-freezing temperatures and almost saturated air. Get the airflow, humidity or temperature wrong and losses through weight loss, sprouting, rot or sugar build-up can wipe out a season's margin. This guide explains how professional buyers plan root crop storage, what drives scope and how to ask suppliers for proposals that can be compared on equal terms.

Short answer: root crop storage combines refrigeration with high-volume ventilation, humidity control and CO₂ management. Define each crop's end use (fresh market, processing or seed), the tonnage, storage duration and whether you store in bulk or boxes; then specify curing, cooling, holding and pre-delivery phases separately. Compare suppliers on airflow design, insulation, control software and service — not on refrigeration capacity alone.

Why root crops are different from fruit

Fruit stores usually hold palletised product in rooms of a few hundred tonnes. Root crop stores often hold thousands of tonnes in one hall, either as loose bulk piled several metres high or in large wooden or plastic boxes stacked to the ceiling. That changes the engineering. Air must be pushed evenly through a dense mass of product, not just around it. The crop arrives warm and sometimes wet from the field, so the first weeks need drying and wound healing before any cooling starts. And because the product is sold over many months, the store must hold a very stable climate long after harvest.

End use also matters more than with most crops. Potatoes for crisps and fries must be stored warmer than table potatoes, because cold temperatures convert starch to sugar and cause dark frying colour. Seed potatoes can be held colder to limit sprouting. Onions need low humidity to keep their skins dry, while carrots and beetroot need very high humidity to avoid shrivelling. A single building may therefore need several independently controlled compartments.

Typical storage conditions by crop

The figures below are common planning ranges; the right setpoint depends on variety, maturity, field conditions and buyer specifications, so confirm them with your agronomist and the storage temperature chart. - Table (ware) potatoes: roughly 4–6 °C with high humidity, after a curing period of about two weeks at warmer temperatures. - Processing potatoes: usually warmer, around 6–10 °C, to protect frying quality; often combined with sprout control. - Seed potatoes: colder, around 2–4 °C, to keep them dormant until planting. - Onions: after field or forced-air drying, close to 0–2 °C with relatively low humidity to keep skins intact. - Carrots, beetroot and parsnips: close to 0–1 °C with very high humidity, usually stored in boxes or under protective covers.

Bulk storage or box storage?

Bulk storage piles the crop directly on a ventilated floor, with lateral ducts or a slatted floor delivering air from below. It offers the lowest handling cost and the highest volume per square metre, but it requires a strong building (the pile pushes against the walls) and a careful air distribution design. Box storage keeps the crop in large crates of roughly a tonne or more, stacked in rows with air pushed through the boxes from a plenum wall or letterbox system. Boxes make it easier to separate varieties, grade lots and sell in smaller quantities, and they are common for seed and specialty crops. Many modern stores combine both approaches in different compartments.

Airflow, humidity and CO₂: the three control loops

The heart of a root crop store is the ventilation system. Large fans move outside air, recirculated air or refrigerated air through the product; the control system mixes them to reach the target temperature with the least energy. In cold climates, outside air can do much of the cooling for long parts of the season, so refrigeration may only run during warm spells and at the end of storage. In warm climates, mechanical refrigeration carries most of the load.

Humidity control prevents weight loss — and every percentage point of moisture lost is lost revenue when crops are sold by weight. Humidifiers, careful air speeds and well-sized evaporators with small temperature differences keep the air moist without condensation dripping on the crop. CO₂ sensors trigger fresh-air ventilation, because carbon dioxide build-up from respiration can harm skin quality and frying colour. Ask suppliers how they control all three loops, which sensors they place in the pile or boxes, and how alarms reach the operator at night.

Five things that drive project scope

1. Tonnage and number of compartments: separate rooms for different crops, varieties or end uses add doors, controls and refrigeration circuits, but make management far easier.
2. Building envelope: insulation thickness, vapour barrier and airtightness determine energy use and condensation risk; bulk stores also need walls designed for the lateral pressure of the pile.
3. Ventilation design: fan capacity per tonne, duct or plenum layout, and whether outside-air cooling is included.
4. Refrigeration: capacity for pull-down after harvest and for holding in warm weather; refrigerant choice under local rules.
5. Controls and monitoring: product temperature probes, humidity and CO₂ sensors, remote access, data logging for buyer audits.

Early capacity planning helps here. The cold storage capacity calculator and the cooling load calculator give first figures for storage volume and refrigeration duty. Budgets should come from consistent planning ranges in the cold storage cost guide, not from a single quotation for equipment.

Common mistakes to avoid

• Cooling potatoes before curing is finished, which slows wound healing and increases rot.
• Undersizing fans or ducts, so the centre of the pile stays warm while sensors near the walls look fine.
• Storing processing potatoes too cold and losing frying quality.
• Mixing onions and carrots in one compartment despite opposite humidity needs.
• Forgetting reconditioning: processing potatoes are often warmed before delivery, which needs heating capacity.
• Leaving out data logging, then struggling to prove storage conditions to a processor or retailer.

How to write a request suppliers can quote

A strong request for root crop storage lists each crop and end use, tonnage per compartment, bulk or box storage, box dimensions, pile height, harvest dates and storage duration, site climate, available power, preferred refrigerant, whether outside-air cooling is wanted, control and monitoring requirements, and what the supplier should include — building, insulation, ventilation, refrigeration, electrical, commissioning, training and service. Ask each supplier to state fan capacity, airflow per tonne, insulation values and expected energy use so you can compare proposals like-for-like. Our guide on post-harvest cold storage for fruit and vegetables covers the wider packhouse context.

ColdMatch Group works with qualified projects of USD 250K and above. Use the RFQ builder to structure your brief; a specialist reviews it before any supplier is introduced, so you only receive proposals that answer your actual crop and storage plan.

A simple planning illustration

Imagine a grower who wants to hold table potatoes and onions from harvest until late spring. Instead of one large hall, the plan splits the building into two potato compartments and one onion compartment, each with its own fans and controls. Potatoes are cured warm for two weeks, cooled slowly to holding temperature, then held with high humidity; onions are dried with forced air and held colder and drier. Because each compartment can be emptied and cleaned separately, the grower can sell in stages and keep the remaining crop undisturbed. This kind of zoning decision, taken early, usually matters more to long-term returns than small differences in equipment price.