Key Production Stages: Harvest and Storage
After these production stages are done, the tubers need to be removed from the ground; if not marketed directly following harvest, they need to be stored.
Most Critical Actions = bruise avoidance and storing tubers under the right conditions for their intended market.
Key Biological Activities = bruise healing, tuber dormancy, starch breakdown and metabolism.
A key concern in harvesting potatoes is tuber bruising. The types of bruises are cuts made by the harvester, shatter bruise and internal black spot which develop from dropping tubers or impacting them against each other with force, and skinning resulting from rubbing of the tubers (Kleinschmidt and Thornton, 1991). An additional type, pressure bruise, can occur from piling tubers too high in storage. The severity of bruising is related to the maturity, temperature and hydration level of the tubers. Bruises will lead to several problems in storage. Cuts and shatter are good entry points for diseases. All bruises will speed the aging of tubers and are related to the performance of seed. Internal black spot and pressure will discolor during cooking. Skinning will make an unattractive tablestock potato. Current research is uncovering more and more negative effects resulting from bruising.
Storage conditions are critical. Appropriate conditions vary depending on the intended market for the harvested potatoes. Conditions such as temperature, curing and sprout control treatment may differ for potatoes to be marketed as seed or tablestock, and for those to be processed into potato chips or frozen potato products (Pavlista and Ojala, 1996). Storage should be well ventilated to promote wound healing and inhibit condensation. Tubers are alive; they respire, take in oxygen and release carbon dioxide. Relative humidity should be about 95% to reduce tuber weight loss, but condensation needs to be avoided to minimize diseases. Potatoes to be sold for seed and tablestock are usually stored at 36 to 40 degrees F; those to be used for frozen, pre-cooked, products are usually stored at 45 degrees F. Those to be processed into potato chips are stored at 50 degrees F to keep reducing sugars from accumulating in the tubers which lead to darkening (Pavlista and Ojala, 1996). For the chip and fry processing markets, and for some long-term tablestock storage, tubers are quite often treated with sprout inhibitors. Of course, these materials cannot be used on seed potatoes. For early planting of seed, sprout promoters are sometimes used. A fungicide may be applied on tubers going into storage to inhibit dry rot.
The tubers are dormant, that is, they will not sprout for some months under storage. However, they are very much alive! When stored at higher temperatures (above 45 degrees F), the dormancy period is shortened and, for that reason, processing potatoes are treated with sprout inhibitors. At colder temperatures such as for seed, dormancy is extended and sprouting is delayed. Sugars accumulate in potatoes under cold storage since tubers do not burn-off (metabolize, respire) the sugars as rapidly as under warmer conditions. Warm storage temperatures favor disease development in addition to sprouting.
- Kleinschmidt, G. and M. Thornton. 1991. Bruise-free potatoes: Our goal. Univ. Idaho Coop. Extension Bull. #725.
- Pavlista, A.D. and O.C. Ojala. 1995. Potatoes: Chip and french fry processing. In D. Smith (Ed.), Processing fruits and vegetables: Science and technology. Volume IV: Vegetables: Major processed products (in press). Technomics Publ. Co. Inc., Lancaster, PA.