The inside of the onion is turning brown... Could it be "scale rot"? There's also a risk of food poisoning.

Table of Contents

1. Introduction

2. What is "Scale Rot Disease"?

3. Main Causative Bacteria and Mechanism of Onset

4. Reasons for Browning: Physiological and Environmental Factors

5. Relation to Food Poisoning: Domestic and International Cases

6. How to Identify at Home and Safety Standards

7. How to Cook Safely

8. Proper Storage, Purchasing, and Disposal

9. Control and Distribution Management at Production Sites

10. Conclusion: Actions Consumers Should Take
    
    

1. Introduction

Onions are an essential versatile vegetable in Japanese cuisine, but recently there have been increasing reports of "the center was brown when cut" or "it had a sweet pungent smell." Most of these phenomena are due to "scale rot disease" caused by bacteria. Its occurrence has been confirmed in domestic production areas, and farmers, distributors, and consumers are all required to take appropriate measures.Ishikawa Prefectural Office

2. What is "Scale Rot Disease"?

Scale rot disease is a bacterial disease that invades from the leaf sheath or neck of onions, causing the internal scales to rot in a water-soaked manner. Even if the exterior appears healthy, one or several internal scales may turn yellow-brown to dark brown, emitting a sweet and sour distinctive odor.iplant-j.jp

Conditions Prone to Onset

• After rainfall during high temperatures (25-30°C)

• Tissue damage from wind or insect damage

• Rough drying and high humidity storage after harvest

3. Main Causative Bacteria and Mechanism of Onset

According to diagnostic cases from the Ishikawa Prefectural Agricultural Experiment Station, Burkholderia gladioli and Pantoea ananatis are frequently detected in Japan. Overseas, Enterobacter cloacae and Pantoea stewartii have also been reported. These bacteria invade from the plant surface and secrete pectin-degrading enzymes, breaking down the cell walls.PubMedIshikawa Prefectural Office

Latency and Onset

There are cases where the rot is limited to the internal scales and cases where it spreads to the entire bulb during storage, with the latter posing an "invisible risk" during distribution.Penn State Extension

4. Reasons for Browning: Physiological and Environmental Factors

The brown color of the rot is due to pigments (melanoidins) generated by the oxidative polymerization of phenolic compounds after tissue destruction. The reaction progresses easily when high humidity and oxygen supply are present, and the cut surface turns dark brown the moment it is exposed to air. The stagnation of moisture transfer from wilted leaves also promotes internal discoloration.Kumiai Chemical

5. Relation to Food Poisoning: Domestic and International Cases

5-1 U.S. Gills Onions Incident (2023)

• Number of Patients: 80 people

• Hospitalizations: 18 people, 1 death

• Cause: Diced onions contaminated with Salmonella Thompson

• Route: Cross-contamination in the processing plant suspected
  U.S. Food and Drug AdministrationCDC

5-2 Scale Rot Disease Onions and Opportunistic Infections

Burkholderia gladioli is also an opportunistic pathogen that causes lung infections and bacteremia in humans, with a reported case of febrile gastroenteritis in an immunocompromised patient who consumed rotten onions (University of Utah Hospital case report, 2024).scholarspace.manoa.hawaii.edu

5-3 Current Situation in Japan

In Japan, "scale rot disease-derived" is not recorded in food poisoning statistics, but there is a confirmed case where onion residues with detected Enterobacter species were disposed of at a school lunch center, as confirmed by a municipal survey (Hyogo Prefectural Agricultural Research Institute, 2024).Ishikawa Prefectural Office

6. How to Identify at Home and Safety Standards

*If even one brown scale is found, cut away a large portion leaving at least 2 cm of edible part. If it spreads throughout, disposal is recommended (Consumer Affairs Agency guidelines).Ishikawa Prefectural Office

7. How to Cook Safely

• Heating: Core temperature at 75°C for at least 1 minute. Salmonella and Listeria are almost completely killed.

• Prevent Cross-Contamination: Immediately wash knives and cutting boards.

• Avoid Raw Consumption: Do not use suspicious onions for salads or other non-heated purposes.

• Acid Washing: Pickling in vinegar or pickles can suppress growth if pH is below 4.2, but it is not complete sterilization.U.S. Food and Drug Administration

8. Proper Storage, Purchasing, and Disposal

1. When Purchasing: Choose bulbs with tight, firm necks. Be cautious of moisture on the outer skin.

2. Room Temperature Storage: In a well-ventilated dark place (10-15°C, humidity below 70%).

3. Refrigerated Storage: After cutting, seal with wrap and refrigerate (4°C), and cook within 48 hours.

4. Disposal: Double-bag rotten onions in garbage bags and isolate from other foods. Composting is not recommended.

9. Control and Distribution Management at Production Sites

• Field Management: Avoid excessive nitrogen fertilizers and consider introducing disease-resistant varieties.

• Timely Harvesting and Quick Drying: Reducing neck moisture content to below 5% suppresses bacterial growth.

• Copper Spraying: Control within 48 hours after rainfall (overseas tests reduced disease rate from 45% to 12%).agsci.oregonstate.edu

• Cold Chain: Maintaining below 6°C during distribution delays rot progression.

10. Conclusion: Actions Consumers Should Take

Scale rot disease is a serious cause of reduced yield for agricultural producers and poses an invisible risk of food poisoning for us consumers."Even if the appearance is clean, cut to check","If you sense an unusual smell or discoloration, dispose of it without hesitation"—by adhering to these basics, we can greatly reduce health hazards at home. Pay particular attention during the hot and humid summer months and enjoy safe and delicious onions.##HTML_TAG_