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The leading authority on skin damage in peaches and nectarines is Carlos Crisosto and his collaborators at UC Davis. They have been working on the problem going back into the early 1990s. Recently they have separated out two distinctive types of skin damages. One they call field inking and the other skin burning. The former occurs in the field and is visible at harvest. Skin burning is damage to the skin observed after packing and handling caused by a combination of pre- and/or post-harvest physical abrasion with exposure to high pH and/or high forced air cooling velocity. The latter disorder is somewhat cultivar dependant depending upon the skin phenolic composition. Unfortunately, since most of the work was done on varieties grown in California we do not know the susceptibility of the more common varieties grown in the mid-Atlantic region.
Inking is caused when metallo-anthocyanin pigments are released from skin damaged cells. The presence of metallic ions such as iron, copper and aluminum have also been implicated as precursors for inking development. The California research team showed that at least 10 ppm Fe iron was enough for the pigments to form resulting in inking. As mentioned previously it can occur in the field but more often in our area it usually shows up after running through the grading process. In their Central Valley Postharvest Newsletter they listed some chemicals that had higher levels of iron, copper or aluminum. The table below lists some of the materials that they tested and believe can affect the incidence of inking. To reduce the incidence of inking they recommended minimizing abrasion of the fruit by handling it gently and transporting it gently. Keep picking baskets clean and free of dirt. If you suspect you might develop inking, delay packing for 48 hours to detect inking damage during grading. You should also check your dump tank water pH. High pH, above 7 can also aggravate the problem.
Skin burning appears as brown and/or black areas on the skin of the fruit. This disorder mainly occurs during the grading procedures at the brushing and washing point. Symptoms appear after packing especially when the fruit is placed in cold storage. The symptoms will increase during storage due to dehydration. Like inking, to reduce the incidence of skin burning, minimize abrasion to the fruit during pre- and post-harvest operations. High velocity forced air cooling can also induce skin burning. Dump tank and hydrocooler water should be maintained at around 6.5 to 7.0 pH.
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