Postharvest Information for Fruits and Nuts
UC Kearney Agricultural Center

PHYSIOLOGICAL DISORDERS

Fruit Freezing Injury

Occurrence
Freezing injury can be encountered in fruit that are purposely stored at near their freezing point or some accidental exposure to subfreezing temperatures because of some malfunction in the refrigeration system.  Injury can occur whenever fruit are exposed to too low temperatures whether during cooling, storage, transport or in distribution centers.

Importance
Occasionally freezing can occur in any type of fruit. Decay development occurs faster on freeze injured fruit.

Symptoms
Freezing injury will appear as glassy, "water soaked" or translucent areas in the flesh.  With time these injured areas will dry leaving open "gas pockets" in the flesh.  The freeze injured tissue of most fruits will begin to brown as a result of enzymatic oxidation of phenols released by the injured tissue.  When freezing occurs at the fruit surface, the glossy or browning symptoms may be visible without cutting.

Often when injury is seen it is necessary to determine whether it is indeed from freezing or whether it is from some other cause.

Similar symptoms can be associated with injury from other causes.  Water core in apples and some senescent breakdown problems can cause flesh translucency.  Many disorders can cause internal tissue browning or even the development of gas pockets.  Surface browning may be confused with scald disorders in apples, Asian and European pears, or even chemical or mechanical injuries on many fruits.

Freezing will occur on the most exposed fruit, i.e. near box openings located on the sides and corners of the pallet. Damage may be worse on the exposed surface of the fruit, and there will be no relationship between freezing injury and the soluble solids content (SSC) patterns within or among fruits.

Freezing injury should thus be seen first in the lowest SSC portions of the fruit and in the lowest SSC fruit within a lot.  Each fruit has a typical SSC pattern.  For pears and apples that we have evaluated, the lowest SSC is in the core area, the highest SSC in the outer flesh near the blossom end of the fruit.  For kiwifruit the lowest SSC is in the flesh nearest the stem end, the highest SSC is in the core and flesh tissue near the blossom end.  While we would expect to see freezing injury appear first in the core area of a pear, for example, we should verify the relationship between SSC and injury with refractometer measurements.

Causes
A fruit freezes because of prolonged exposure to a temperature just below its freezing point, the injury pattern should relate to the pattern of soluble solids content (SSC) of the fruit.  This is because low SSC fruit will freeze at a higher temperature than high SSC fruit.

Control
Maintain temperatures just above freezing.  This requires good equipment and careful management.  Of equal importance is accurate monitoring of soluble solids content (SSC) of fruit as a basis for estimating the freezing point of the tissue.  The relationship between SSC and the freezing point for stone fruit is presented in Table 1.  To safely utilize temperatures near the freezing point of fruits, one must know the SSC variability within fruits.

Table 1.   Relationship between stone fruit soluble solids content (SSC) and the freezing point.

References

Mitchell, F. G.  1987.  Influence of cooling and temperature maintenance on the quality of California grown fruit.  Rev. Int. Froid. 10:77-81

Mitchell, F. G. and Adel A. Kader.  198  .  Storage.  In: J. H. La Rue and R. S. Johnson (eds.).  Peaches, Plums and Nectarines: Growing and Handling for Fresh Market.  University of California Department of Agriculture and Natural Resources Publication No. 3331.  Pp. 158-164.

Skin Discoloration, Inking, Staining, Black Staining

Occurrence
Peach and nectarine fruits

Importance
During the last decade, this has become a frequent problem in California, Washington, Georgia, South Carolina, New Jersey, and Colorado, as well as in other production areas in the world Italy, New Zealand,  Australia, Argentina, and Chile).

Symptoms
Inking symptoms appear as discolored brown and black spots or stripes but are restricted to the skin.

Physiology
Abrasion damage in combination with heavy metal contamination are requirements for inking development.  The damaged skin cells, where the anthocyanin/phenolic pigments are located, collapse and their contents reacts with heavy metals turning their color dark brown/black.   Iron, copper and aluminum (heavy metals) are the most deleterious contaminants.  Only 5-10 ppm iron is enough to induce inking at the physiological fruit pH.  This contamination can occur within 15-20 days before harvest, during harvesting or packing operations.  Foliar nutrient, fungicide and insecticide preharvest sprays which contain heavy metals in combination with abrasion damage have the capacity to induce inking on peach and nectarine fruit when sprayed close to harvest.

Control
   1.   Reduce fruit abrasion damage.
          A.   Treat fruit gently.
          B.   Avoid long hauling.
          C.   Keep picking containers dirt free.

  2.   Reduce contamination of fruit.
          A.   Keep harvesting equipment clean.
          B.   Avoid dust contamination on your fruits.
          C.   Check your water quality for heavy metal (Fe, Cu & Al) contamination.
          D.   Do not spray foliar nutrients containing heavy metals during fruit maturation.
          E.   Tentative preharvest application intervals for the following fungicides and foliar
                 nutrients were developed (DBH-days before harvest):
                      Z.I.P.® = 20 DBH
                      Benlate® = 12 DBH
                      Rovral® = 7 DBH
                      Funginex® = 3 DBH
                      Ronilan® = 1 DBH

   3.   In case of a possible inking situation with peach and/or nectarine, delay your packaging
  for 48 hours to detect fruit inking damage during your grading operation.

  4.   As a long term solution, it is suggested that chemical manufacturers attempt to identify
 and remove the possible sources of contamination from their products that may cause
 inking before distributing them.

References

Cheng, G.W. and C.H. Crisosto. 1994. Development of dark discoloration on peach and nectarine (Prunus Persica. L. Batsch) fruit in response to exogenous contamination.  J. Amer. Hort. Sci. 119:529-533.

Crisosto, C.H., R.S. Johnson, J. Luza, and K. Day.  1993. Incidence of physical damage on peach and nectarine skin discoloration development: anatomical studies.  J. Amer. Hort. Sci.  118:796-800.

Internal browning, chilling injury, dry fruit, mealiness, woolliness

Occurrence
Apricot, peach, nectarine, plum and fresh prune

Importance
The number one problem in shipping stone fruit, and the most frequent complaint made by consumers and wholesalers.

Symptoms
Flesh browning, flesh mealiness, black pit cavity, flesh translucency, red pigment accumulation (bleeding), and low flavor.

Causes
These symptoms normally appear after placing fruit at room temperature, while some ripening is occurring, following cold storage.  For this reason, this problem is usually experienced by the consumer, not the grower and/or packer.

Stone fruit cultivars vary greatly in susceptibility to internal breakdown injury (Table 1).  Some of them show no apparent susceptibility when grown under California climatic conditions.  Among peaches and nectarines early season cultivars are least susceptible and late-season cultivars are most susceptible.  Among plum cultivars there is no seasonal pattern of susceptibility.

Even under the best storage and handling conditions, stone fruit have a limited market life.  An estimate of the potential postharvest life of stored stone fruit is presented under market life in Table 1.  Market life means the maximum number of weeks for each cultivar under continuous exposure to 32^oF (0^oC) and 90% RH (where the fruit can be stored until at least 85% remain marketable).  As these ideal conditions do not usually occur during the storage, transportation and handling at the retail end, the maximum potential postharvest life of stone fruit cultivars is never reached and therefore is normally shorter than that given in Table 1.

Control
The ideal way to eliminate this problem would be to produce cultivars resistant to it.  In the meantime, temperature management is the best commercial tool available to delay the onset of internal breakdown.  Storage below 0^oC (32^oF) but above the freezing point is beneficial to delay chilling injury symptoms and extend market life.

Use of controlled atmosphere (CA) conditions in combination with temperatures close to 32^oF has been beneficial in extending the postharvest life of plum, nectarine and peach cultivars.  CA treatment reliability can be attained by understanding the role of fruit size, canopy position, cultivar market life and shipping period.

1. Avoid cultivars susceptible to I.B.

2. Market susceptible cultivars according to their potential postharvest life.

3. Segregate fruit according to their postharvest potential life.

4. Pick fruit "well matured."

5. Enforce proper postharvest handling during transport and at the retailer.
          a.  Keep fruit near 32^oF during storage and transportation.
          b.  Avoid 36-46^oF temperatures during retail handling.

6. Educate warehouse and retail managers on I.B.

References

Crisosto, Carlos H., F. Gordon Mitchell and R. Scott Johnson.  1995.  Factors in fresh market stone fruit quality.  Postharvest News and Information 6(2): 17-21N.

Luza, J. G., R. Van Gorsel, V. S. Polito and A. A. Kader.  1992.  Chilling injury in peaches: A cytochemical and ultra-structural cell wall study.  Amer. Soc. Hort. Sci. 117: 881-886.

Table 1.  Postharvest performance rating (maximum market life) of different stone fruit cultivars grown in California.