Internal Browning of Fuji: Central California Experiences
Western apple growers and packers are learning quickly to grow, handle, and market high quality Fuji apples. Over the fifteen years Fuji has been commercially grown in California, a number of pre- and postharvest problems have been encountered. Many have been successfully resolved; others remain as challenges for the future. Included among the latter is the problem of internal browning (IB) during controlled atmosphere (CA) storage.
Our experience with internal browning began in January 1992, when a packer observed that some lots of Fuji apples held in controlled atmosphere storage from the 1991 harvest had moderate to severe browning upon removal from storage. The disorder was greatest in large fruit from late-harvested lots. Past research linked IB to high storage carbon dioxide (CO2) levels in other varieties, so there was some question whether a failure to adequately monitor or control storage CO2 levels may have contributed to the observed problem.
Apples with IB have patchy areas of dark brown flesh discoloration, often beginning near the core of the apple. IB-affected tissues are typically less firm than unaffected tissues but are otherwise similar in texture and consistency to normal tissues. Margins of IB-affected tissues are smooth and distinct. Small air cavities are sometimes found associated with the brown tissue. When IB-affected fruit are cut lengthwise, the patchy brown areas are often found more frequently near the stem than the calyx end. Except in the most severely affected fruit, browning rarely extends into core area or to the apple surface.
Since 1992, the reported incidence of IB has varied from year to year but has been generally low. In 1994, there were many more reports of IB in CA-stored Fujis than in previous years. At least one packer observed initial signs of IB, at harvest in 1994. Another reported observing a high incidence in packed air-stored fruit.
Laboratory IB Studies
Our experiments are based on the hypothesis that IB is caused by accumulation of injurious levels of respiratory CO2 in fruit tissues. Factors which may contribute to increased internal CO2 concentrations include high fruit respiration rates occasioned by high temperatures or other factors; reduced rates Of CO2 diffusion through the cortex, skin, and/or wax coating applied to apples; and high CO2 in air surrounding fruit.
We initiated IB studies in 1992 and expanded those studies in 1993 and 1994. The objective of our studies was to explore the influences of harvest timing and CA CO2 levels on IB incidence and severity.
In the three years' tests, we used apples from a well-managed commercial Fuji orchard near Modesto, California, with a history of IB. While our sampling procedures have varied slightly from year to year, we have harvested apples four times each year over a range of dates corresponding roughly to the grower's commercial harvest schedule. We made measurements of flesh firmness, soluble solids, titratable acidity, and starch-iodine maturity rating at harvest. We then stored apples from each harvest for four months at 32°F in air or one of three controlled atmospheres: 2% O2 + 0.5% CO2; 2% O2 + 1.5% CO2; or 2% O2 + 3.0% CO2. After storage, and 5 days at room temperature to simulate market handling conditions, apples were evaluated and rated as having no, slight, moderate, or severe IB.
In 1992, there was little or no IB in fruit from all but the last harvest. There was little IB in air-stored fruit from the last harvest, but IB increased with CA CO2 level in fruit from the last harvest (see figure at left). IB incidence was lower in 1993 than 1992. It was low to nil in early-harvested fruit stored in air or low CO2, but high in late-harvested fruit held at 3% CO2. In 1994, IB incidence and severity were much higher than in previous years. As in 1992, IB was low in early harvested fruit stored in air or low CO2, but increased with harvest date and CA CO2 level.
Managing IB: Suggestions for 1995
There are many aspects of this problem which remain unclear and warrant further study. Year-to-year and orchard-to-orchard variability in IB incidence and severity suggest that orchard and/or environmental factors may play a role in predisposing fruit to browning. Preharvest factors are under preliminary investigation, but little is known about this aspect of IB. Alternative storage strategies also need to be investigated, including effects of storage O2 storage levels other than 2% and temperatures other than 32°F.
From results of our studies to date, we suggest the following provisional guidelines for managing IB:
Begin and end harvest as early as possible. Results of our studies suggest that IB incidence and severity are low in fruit harvested before 180 to 190 days after bloom.
Sample, cut and inspect fruit during harvest, especially in known IB orchards.
CA store only early-harvested fruit free of IB.
Expedite handling of late-harvested lots: Do not CA store.
Keep CA CO2 levels as far below 0.5% as possible.
Joseph A. Grant, Pomology Farm Advisor and Elizabeth J. Mitcham, Postharvest Pomologist
UC Cooperative Extension, Stockton CA
Department of Pomology, University of California, Davis
11th Annual Postharvest Conference,
March 15-16, 1995