Internal Blackspot (IBS)
Other pages about bruise defects:
In addition to impact-caused, external bruises, there is an impact-caused internal bruise, that is unseen by looking at the tuber surface, called IBS, internal blackspot or just blackspot. It occurs under the skin surface and can only be seen by removing the skin or cutting into the tuber where the discoloration exists. IBS appears as a small, dark, oval area just under the skin (Picture 1). When potatoes are fried, these dark spots appear deep dark near the edge of the tuber surface (Picture 2). For instance in a potato chip, IBS areas appear along the chip's rim.
IBS forms as a result of an impact of the tuber against a hard surface such as the sides of the harvester or piler. The discoloration under the skin appears one to two days after the impact. As with pressure bruising, it results from a phenolic reaction in the wounded cells. Cells collapse; ethylene is released, and further cell wall breakdown occurs resulting in more cell death. The dark discoloration identifying IBS and pressure bruising is from a pigment, melanin, formed in the phenolic reaction. [Note, this is the same pigment that darkens skin in people.] As with pressure bruising, susceptibility to IBS is related to tuber dehydration or firmness. Tuber firmness may be inversely related to specific gravity thereby making tubers with higher solids more susceptible to IBS than those with lower solids (Figure; Sawyer and Collins, 1960). Tubers should be harvested when their internal temperature is above 45o F and less than 65o F. Also, harvesting when the soil is not dry helps minimize the occurrence of damage.
IBS Resistance: A Sad Epilogue on Bruising
Can genetic modification of potato allow tubers to be resistant to blackspot bruising? Several years ago, the biotechnology company NatureMark was disbanded due to pressure against genetically-modified (GMO) potatoes. Prior to its demise, the company had modified some varieties such as Ranger Russet to be bruise resistant and were in the process of field testing these NewLeaf varieties in 1999. The following pictures (3, 4, and 5) compare tubers from standard Ranger Russet (left) to those of NewLeaf anti-blackspot bruise Ranger Russet (right). Bruising is depicted by the increasing darkening of the tubers starting at one end.
Besides anti-bruising, the most highly farmer-requested genetic-modified trait that growers requested were late blight resistance and increased dry matter content. The company was in the process of fulfilling these as well at the time of their forced closure by Greenpeace through pressure on MacDonalds.
Picture 2: Internal black spot on potato chips
Figure 1: Black spot vs. Firmness
Picture 3: Ranger Russet 24 hours after peeling
Picture 4: Ranger russet 48 hours after peeling
Picture 5: Ranger russet after peeling
McGarry, A., C.C. Hole, R.L.K. Drew, and N. Parsons. 1996. Internal damage in potato tubers: a critical review. Postharvest Biology & Technology 8:239-258.
Sawyer, R.L. and G.H. Collins. 1960. Black spot of potatoes. Amer Potato J 37:115-126.