Brown Core, a Disorder of CA Stored Pears
Brown core, also known as pithy brown core, is a physiological disorder affecting d'Anjou, Bosc and Bartlett pears stored in boxes in sealed polyethylene liners in controlled atmosphere (CA) storage. There is evidence that several factors promote this disorder. Among them are preharvest weather, fruit maturity and the balance between oxygen and carbon dioxide in storage.
This disorder is characterized by a pithy brown discolored area in the core region of pear fruits. It may be restricted to brown flecks between the carpers, or it may encompass the entire core and extend into the surrounding flesh. In some instances the tissues collapse to produce cavities. The affected tissues are dry and pithy in contrast to the soft, watery texture found in core breakdown.
Recently, several scientists have noticed an association between dark brown flecks on the skin of the fruit and pithy brown core. These flecks appear in varying amounts and especially on the calyx end of the fruit. Not all pithy brown core fruit has external flecks, but the appearance of the characteristic flecks indicates the presence of pithy brown core in the fruit. The flecks have a different appearance than scald or belt burn.
Unfortunately, there are no methods yet available to determine fruit susceptibility to brown core.
In The Orchard
A number of orchard factors seem to be associated with increased fruit susceptibility to brown core. Specifically, a cool season will produce more brown core susceptible fruit than a warm season. Also, fruit from orchards located at cooler sites should be watched for symptoms of brown core in storage.
Low tree vigor or increased stress on trees will result in weak fruit.
Pears harvested at advanced stages of maturity and placed in long-term CA storage will be more susceptible to brown core than will fruit harvested at the proper stage of maturity. Pressure is probably the best indication of pear fruit maturity. Check it regularly during the growing season.
In The Warehouse
Pears for long-term storage must be harvested and handled rapidly to slow respiration and storage life. Weak fruit is a special problem and should only go into shorter term storage. Both delaying storage and slow cooling in storage shorten potential storage life. Place fruit in storage within 24 hours of harvest and cool to a core temperature of 30 to 31 °F in 6 to 8 days. If the respiration rate is not slowed rapidly and sufficiently, the fruit will become more susceptible to brown core.
Experiments done several years ago show that the method of stacking packed boxes of pears on a pallet will determine the rate at which the fruit is cooled. Closely stacked boxes will take 2½ times as long to reach a core temperature of 32°F on the top layer of the pallet as it will if boxes are stacked with spaces around each box.
Polyethylene box liners have been used to prevent moisture loss in packed pears in storage. Pears have been stored for several years using perforated polyethylene box liners with 12 to 16 holes per liner to promote gas exchange. Recent experience has shown that where brown core has been a problem in a grower lot, fruit at the bottom of a tightly packed box will have more damage than fruit at the top or in the middle of the box. In addition, fruit in boxes in the middle of a closely stacked pallet will have more damage than fruit in boxes at the top of the pallet.
There may be several reasons for this. Fruit at the bottom of a box does not cool as quickly as fruit at the top of the box, and therefore, respires more rapidly than cooler fruit at the top. Consequently, the level of oxygen will be lower and the level of carbon dioxide higher in fruit at the bottom of the box. Conditions are similar when comparing fruit stored in boxes at the top of the pallet and in the center of the pallet. It would be worthwhile to consider a wider spread stacking (or "chimney") pattern to achieve better cooling and reduce the amount of brown core.
Brown core has been considered CO2 damage; however, it is important to recognize that the ratio of carbon dioxide and oxygen is the critical factor. Pears. in regular CA at 2 to 2½% oxygen can be stored without damage even if the CO2 level rises to 1%. However, recent research with low oxygen storage (oxygen is maintained at about 1%) has shown that the CO2 level must be kept at 0.1% (1/10%) or less, or brown core will result. Close control of CO2 is critical if a warehouse is to obtain good' results with pears in CA storage.
SummaryIn summary, a number of factors are responsible for brown core of pears stored in CA. Cool growing conditions, advanced maturity at harvest, and delayed reduction in temperatures increase fruit susceptibility. Of critical importance is the balance between the carbon dioxide and oxygen levels in the box, which may be moderated by the poly liner and stacking pattern on the pallet. An examination of the practices at the warehouse will help to reduce cullage from brown core.
Blanpied, G. D., 1975. Pithy Brown Core occurrence in 'Bosc' pears during controlled atmosphere. J. Amer. Soc. Hort. Sci. 100(1):81-84.
Hansen, E. 1957. Reactions of Anjou pears to carbon dioxide and oxygen content of the storage atmosphere. Proc. Amer. Soc. Hort. Sci. 69:110-115.
Hansen, E., and W. M. Mellenthin. 1962. Factors influencing susceptibility of pears to carbon dioxide injury. Proc. Amer. Soc. Hort. Sci. 80:146-153.
Hansen, E., and W. M. Mellenthin. 1979. Commercial handling and storage practices for winter pears. Oregon State University Agric. Expt. Sta. Special Report No. 550.
Meheriuk, M. 1980. Post harvest physiology of pears. Proc. Tri State Tree Fruit Shortcourse Post Harvest Physiology 1-28.
Pierson, C. F., M. J. Ceponis, and L. P. McCulloch. 1971. Market diseases of apples, pears and quince. USDA-ARS Agric. Handbook No. 376:88.
Sainsbury, G. F., and H. A. Schomer. 1957. Influence of carton stacking patterns on pear cooling rates. USDA Marketing Research Report No. 171.
Dr. Eugene Kupferman, Postharvest Specialist
WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Post Harvest Pomology Newsletter, 5(2): 15-17