Physical Damage To Potatoes During Harvest Leads To Lost Profits
It seems as though there are some issues that are so important that no matter how much we’ve discussed them over the years, seasonal reminders are still necessary. As we all plunge headlong into the whirlwind of harvest activities, this message bears repeating yet another time: Damage to potato tubers including wounds, bruises, abrasions, and other types of mechanical mayhem are to be avoided at all costs.
All of these types of damage have the potential to cause grading defects that can lead to reduced value or even rejection of your crop by the purchaser, whether the potatoes are destined for fresh market, processing, or seed.
Abrasion or “skinning” injury leaves unsightly, discolored areas on the tuber skin because the areas that were skinned must heal. The healing process creates a new type of skin called “wound periderm.” Skinning is unsightly enough on russet-skinned varieties, but at least with russet varieties the areas of the tuber covered with wound periderm are brown, though they are not the same shade of brown as the normal periderm and they lack the characteristic russet texture. Skinned areas on white, red, or tubers with other skin colors are more of a problem because the damaged areas end up being brown as well. Wound periderm provides excellent protection for the damaged areas of the tuber but it contains no pigments.
Bruises are another type of damage that should be avoided. There are three types of bruises in potato tubers, and all of them are bad. Two bruise types, shatter bruise and black spot bruise, occur during harvest and handling.
Shatter bruises occur when the damage leads to a bruise that breaks the tuber skin. These complex wounds often go deep into the tuber and can be difficult to heal properly. Shatter bruise is more prevalent in young, turgid tubers because the cells within the tuber are not completely adhered to one another and the energy from a physical blow tends to cause cracks and fissures that ultimately break the skin.
Black spot bruise is much more insidious in that there is no external evidence that damage has occurred and tubers must be cut open before the damage becomes evident. Black spot occurs more frequently in flaccid tubers where the tougher outer layers of the tuber deform from a physical blow but the skin remains intact. It is also more prevalent on the stem end of the tuber. While the skin is not broken, some of the cells as much as a centimeter or more into the tuber flesh become damaged. The resultant internal leakage from the damaged cells allows enzymes to mix with substrates, and an internal blackened zone or “black spot bruise” is the result.
Incidentally, pressure bruise is very similar to black spot bruising in that cells deep within the tuber become damaged, begin to leak fluids, and the same reactions that lead to black spot occur. The difference between the two types of bruising lies in how the damage is inflicted. Pressure bruise occurs in the lower portions of the potato pile due to the sheer weight of the tubers above. This type of damage is best avoided by maintaining high relative humidity in the storage facility.
Cuts, nicks, and other types of outright physical damage can also result in tubers with a less than stellar appearance but also have the added disadvantage of providing an entry point for storage diseases. Pythium leak, late blight, early blight, and fusarium dry rot are all examples of storage diseases that are greatly favored by wounding.
The other major factor to take into account when harvesting is temperature. Every one of the defects and the diseases listed can be magnified by elevated temperatures. It is best not to attempt to harvest when temperatures are above 65°F and even this upper limit can be very risky if cool night time air or refrigeration is not available. Lower temperatures, below 45°F, should be avoided as well. In general, tuber damage also increases as tubers get colder.