Peach Skin Discoloration

Skin discoloration, also known as inking, is a disorder of peaches that has been a problem for 20 years. A symptom of inking is the development of burgundy-colored areas within the red flesh of the peach, that eventually turn purplish black or ink color. Research at Clemson University, University of California, and Rutgers University has demonstrated that any operation causing peaches to rub, roll, or physically abrade against one another can result in discoloration. Discoloration can also be triggered by iron levels in excess of 10 ppm in the hydrocooler and dump tank, leachate from latex-rubber drying rollers, ammonia, and fungicide sprays.

Four major factors influence the development of inking: the cultivar, dump tank water, contaminants, and abrasions to the skin of the fruit.

Cultivar. Early maturing cultivars seem to be more susceptible. Cultivars that have dark red or purplish over color can show the disorder more regularly. Fruit left on the tree too long in an attempt to achieve greater skin color may be more prone to inking. White-fleshed, low-acid cultivars seem to be more prone to discoloration. There are no recommendations as to susceptibility by cultivar, and the susceptibility may vary by the year.

Dump Tank Water. The water used in hydrocoolers and the dump tank should be monitored to keep it at or near a pH of 7.0. Water that is high in iron, copper, or aluminum also tends to increase inking problems. Inspect your hydrocoolers and dump tanks to make sure they are not rusty and allowing additional metals to enter into the water. Change the water regularly to prevent the buildup of metal residues. If your water source is naturally high in iron, consider switching your source or install a filtration system. Inking has also been observed to be more prevalent when fruit are harvested in the rain.

Contaminants. Research in California has shown that contaminants can increase the likelihood of inking. Make sure your harvest crew uses good sanitation when picking. Harvest bags, crates, and bins should be clean. Minimize exposure of the fruit to dust. Ensure that hydrocoolers and grading machines are clean. Avoid the use of foliar applications of materials that have heavy metals during the final swell period of fruit development.

Skin Abrasions. Minimize rough handling of the fruit during harvest. Consider smoothing orchard roads to avoid excessive jostling of the fruit in transit to the packing house. Brushing to remove the "fuzz" can lead to a greater occurrence of inking. Inspect your packing line for places where excessive abrasion can occur.

To help control the problem:

  • Avoid any operation that causes excessive vibration, rubbing, or rolling.
  • Check water used for spraying for heavy metal contamination.
  • Avoid spraying foliar nutrients that contain heavy metals while fruit is on trees.
  • Cool fruit as soon as possible after picking. Hydrocooling is preferred to storing in a cold room. Before hydrocooling, keep field bins in a shaded area out of direct sunlight.
  • Keep hydrocoolers and dump tanks as clean as possible by draining and refilling with fresh water on a regular basis.
  • Monitor iron levels and water pH in the hydrocoolers and dump tanks. Excessive iron in tanks can cause discoloration. Water pH should be maintained between 6.5 and 7.0. Water at a pH of 4.0 to 6.0 has caused problems with discoloration and iron levels.
  • Wash drying rollers, especially new ones, in detergent and rinse well before installing. Polyurethane drying rollers have proven superior to latex rollers for maintaining fruit quality.
  • If using ammonia refrigerant, make sure there are no leaks. Peaches can be adversely affected by ammonia levels under 1 ppm although humans cannot detect concentrations that low.
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