Weather conditions or management practices sometimes delay crop maturity. Prompt harvest before the crop reaches chemical maturity may be necessary to avoid autumn frosts. Alternative storage management steps may be required to ensure that chemical immaturity (high sugars) of tubers does not affect processing quality out of long-term storage. Chemical immaturity can cause excessive accumulation of the reducing sugars, which results in darkening of potato products during frying.
Processing tubers are usually held at storage temperatures near 59°F (15°C) with Relative Humidity near 95% for up to two weeks after harvest to promote suberization and wound healing. This curing or preconditioning period may need to be extended if tubers are chemically immature at harvest to prevent excessive reducing sugar accumulation in storage. During this period, sugars are either used up in respiration or are converted into starch. Depending on how much sugar is in tubers at harvest, it may take several weeks of preconditioning before reducing sugars begin to decrease and it may take several more weeks before they have dropped to a level acceptable for processing.
Extended preconditioning would not be recommended for severely diseased tubers or tubers which have been damaged by frost. Prompt marketing of such tubers would be recommended. Tubers can also be reconditioned prior to marketing, which involves gradually raising the storage temperature to lower sugars that accumulated during storage. Reconditioning just before sale may be preferred for diseased or frosted tubers since storage losses may be excessive if tubers are held at high temperatures for extended periods after harvest.
Long exposure of stored tubers to high temperatures will increase the chance of weight loss due to respiration and water loss. The more physically and chemically immature the tubers at harvest, the greater the weight loss. However, the additional weight loss during preconditioning or reconditioning is justified if processing quality is improved.
For further information on storage of immature potatoes refer to the following bulletin on the Department of Plant Science, University of Manitoba Storage of Immature Russet Burbank and Shepody Potatoes
Written by M. Pritchard
Special storage problems
Storage problems most often occur because of conditions in the field and not conditions in storage. Adverse weather, disease or improper harvesting and handling of tubers can cause problems in storage. Tubers that are rotting, frozen, chilled or diseased must be managed differently than mature, sound tubers. Good storage management will help to salvage problem tuber lots, but storage will never improve a poor quality crop.
Summary of the techniques used for storing problem potatoes.
 | | Crop Conditions |
Field or Storage
Period | General
Requirements | Normal | Wet soil | Wet Tubers | Chilled | Frozen | Wet Rot | Dry Rot | Hot Tubers |
| Pre-Harvest | Repair, clean & disinfect the harvest, handling and storage equipment as well as the storage structure. | Under dry soil conditions, apply preharvest irrigation as required to soften clods and prevent black spot bruising. | Segregate affected areas by desiccation or discing. | Do not overirrigate prior to harvest. Keep an eye on low spots to ensure water doesn't pool. | Plan to complete harvest before first week in October to avoid chilling. | Potatoes that are hilled properly are less likely to freeze. Plan to complete harvest before first week in October to avoid frost damage. | Segregate affected areas by desiccation or discing. | Under dry soil conditions, apply preharvest irrigation as required to soften clods and prevent black spot bruising. | |
| Harvest | Follow best management practices for bruise prevention. Record the harvest conditions (tuber temperature, precipitation, etc.) of each load. Record the location of each field in storage. | | Do not harvest segregated areas. | Never harvest when there is precipitation. Tarp loads in case of sudden rain showers and do not windrow too far ahead of the harvester. | Dig when tuber pulp temperature is above 41°F (5°C) to avoid shatter bruising. | Dig after tuber pulp temperature is above 41°F (5°C) to avoid shatter bruising. Let frozen tubers rot before harvest, making it easier to identify and grade out before storing. | Do not harvest segregated areas of field. Grade out affected tubers before storing. | Follow best practices for bruise prevention. for more information. | Do not harvest when tuber pulp temperature exceeds 65°F (18°C). Do not windrow too far ahead in order to keep tubers cool. |
| Post-Harvest (Curing or Suberization | Ventilate to bring storage to suberization temperature, suberize for appropriate period then reduce temperature by 4- 5°F (2-3°C)/week for long term storage. | Ventilate and humidify (R.H. 95%) to bring storage to suberization temperature [50-60°F (10- 15°C)], suberize for 2 weeks then reduce temperature by 4-5°F (2- 3°C)/week for long term storage. When humidifying, ensure there is no free moisture dripping onto the tubers. | Ventilate continuously without humidifying to bring storage to suberization temperature [50-55°F (10-13°C)], suberize for two weeks then reduce temperature by 4-5°F (2-3°C)/week for long term storage. Return to normal humidity (R.H. 95%) after soil on tubers is dry. | Market immediately or segregate affected lots in storage. Ventilate continuously without humidifying to bring storage to suberization temperature [50-55°F (10- 13°C)], suberize for two weeks then reduce temperature by 4-5°F (2-3°C)/week for long term storage. Return to normal humidity (R.H. 95%) after tuber surface is dry. | Ventilate continuously and humidify (95% R.H.) to warm storage to suberization temperature [59°F (15°C)]. Carry out normal suberization process for table and seed potatoes, and for processing potatoes suberize until fry colour improves. Then reduce temperature by 4-5°F (2- 3°C)/week for long term storage. When humidifying, ensure there is no free moisture
dripping onto the tubers. | Market immediately or segregate affected lots in storage. Ventilate continuously without humidifying to warm storage to 47-50°F (8- 10°C). Consider adding supplemental heat to remove excess moisture. Reduce temperature by 4- 5°F (2-3°C)/week for long term storage and return to normal humidity (R.H. 95%) after frozen tubers are mummified. | Market immediately or segregate affected lots in storage. Ventilate continuously without humidifying to bring storage to 47-50°F (8- 10°C). Consider adding supplemental heat to remove excess moisture. Reduce temperature by 4- 5°F (2-3°C)/week for long term storage and return to normal humidity (R.H. 95%) after affected tubers are mummified. | Apply appropriate fungicides to tubers going into storage. Ventilate and humidify (R.H. 95%) to cool storage to suberization temperature [50-60°F (10-15°C)],suberize for 2 weeks then reduce temperature by 4- 5°F (2-3°C)/week for long term storage. When humidifying, ensure there is no free moisture dripping on to the tubers | Ventilate and humidify (R.H. 95%) to bring storage to suberization temperature [50-60°F (10- 15°C)], suberize for 2 weeks then reduce temperature by 4-5°F (2- 3°C)/week for long term storage. |
| Long-Term Storage | Long Term Storage Temperatures: Table 41- 43°F (5-6°C), Processing 47-50°F (8-10°C) Shepody 50-54°F (10- 12°C), Seed 37-39°F ( 3- 4°C) | Fan operation - continuous or intermittent or reduce fan speed with VFDs (Variable Fan Drives). Inspect storage frequently. Monitor oxygen levels, and add fresh air as necessary. Fresh air may require warming to prevent shocking the tubers. Heaters should be electrical or exhausted to the outside of the storage. |
| Marketing | a. Warm to reduce handling damage.
b. Recondition.
c. Pre-warm to break dormancy. | | Market as soon as possible. | Market as soon as possible | | | | | |