Considerations in Assessing Maturity of Braeburn Apples
The standard Braeburn apple originated as a chance seedling in Nelson, New Zealand in the early 1950s. Although its parentage is unknown, it is thought to be a cross between the late maturing cultivars 'Hamilton's Red' or 'Lady Hamilton' and 'Granny Smith'. Currently, it is the most widely planted cultivar in New Zealand. Although it is relatively new to the Northwest, its growth habits, cultural management, maturity patterns, fruit quality attributes and storability need to be assessed under conditions in Washington state. This is due mainly to the differences in climatic conditions between Hawke's Bay, New Zealand, (approximately 40 degrees south latitude) which occasionally falls below freezing, and north central Washington (approximately 47.5 north latitude) which may easily have 5 winter months of below freezing temperatures (Figures 1 and 2). Whereas the mean difference in high and low temperature for Hawke's Bay (winter low, summer high) may be 40 °F, similar differences for Wenatchee may be 20 °F greater. Figures 1 and 2 show mean monthly (adjusted to Wenatchee Seasons for southern hemisphere) maximum and minimum temperatures for Hawke's bay and Wenatchee for 1992 to 1995.
It should not be surprising that within any given year, the tree itself may undergo a yearly temperature difference of more than 130 °F. These observations concerning climate are merely to illustrate that the Braeburn which was bred originally for the milder New Zealand climate may act quite differently than when grown in a more severe climate.
This report briefly summarizes some of the important observations and data from a 3-year study of Braeburn fruit maturity and the development of BBD in regular and modified atmosphere storage. Included are some preliminary observations concerning possible corrective measures.
In 1994 to 1996, fruit was sampled from eight commercial Braeburn orchards in Washington from as far north as Ellisford, to near Pasco in the south. The trees ranged in age from 3 to 7 years old and were all grafted onto MM.106 rootstock. Twenty trees of similar vigor and crop load within an orchard were selected at blossom time for uniformity of bloom and vigor. Beginning the second week in August 4 similar fruit from each tree were sampled each week until commercial harvest at the end of October.
In the laboratory, the fruit was organized so that there were 4 trays of 20 fruit--one from each tree. One tray was evaluated for color, firmness, soluble solids, acidity, starch and internal ethylene 24 hours after harvest. The other 3 trays were placed in regular storage (RS). Internal ethylene concentration (IEC) was measured in I ml headspace gas in the core of each fruit. Ground color was evaluated with a yellow/green chip set similar to that used in New Zealand. Starch conversion was determined on a scale of 1 to 6 (1 = No conversion; 6 = Complete conversion] using the mid-cross section of an apple. A model EP1 pressure tester with a 1.27-cm head was used to determine firmness. Soluble solids were measured using an Atago Model PR-1 digital refractometer and titratable acidity was measured with an automatic titrator (Radiometer, Copenhagen, Denmark) to pH 8.2 and expressed as a percent malic acid. Braeburn Browning Disorder (BBD) was rated from 1-4 (1= no browning; 4=greater than 50% of a cross sectional cut) and this value was multiplied times 100 for reporting purposes. BBD was assessed each time the fruit were examined; however the worst cases appeared after 6 months in regular atmosphere.
Results and Discussion
Over the 4 to 6 week sampling period, IEC values indicated very slow fruit maturation (data not shown). This physiological feature is apparently a varietal characteristic. At the outset, because of the slow development of internal ethylene, it would appear this would be favorable to the grower because it could give the grower additional time before harvest for development of red color in advance of autocatalytic ethlylene. Maturity of fruit in 1994 was more advanced than in 1996, judging by starch index at 182 days after full bloom.
At some of the harvest sampling dates, starch index was 2 times greater in one year versus the next (Figure 3). Figure 4 shows ground color ratings also for years 1994 to 1996. These values, though quite different at the early harvest periods were similar at the last 3 sampling dates. Therefore, because ground color is the same when the starch rating is so different, starch rating becomes the preferred index upon which to harvest the fruit.
In these particular studies there was very little disorder noted on any of the samples. Bitterpit was present on some of the samples, however, in these orchards and during these years, it was not a problem. Some orchard conditions tend to induce more bitterpit and these factors would be similar to other similarly sensitive cultivars. Regular calcium supplement sprays would help correct this. The other disorder seen in only a few (less than 1%) of the fruit was internal browning otherwise known as Braeburn Browning Disorder (BBD). At harvest, this disorder has the appearance of light to dark brown areas similar in nature to watercore but occurring without pattern, anywhere in the flesh.
It might be as small as the tip of a pencil eraser or may invade the entire fruit. Lastly, the severity of BBD after 6 months storage is shown in Table 1. This indicates a direct correlation between the harvest maturity and development of BBD. The later the harvest, the greater will be the danger of BBD development in storage. Growers should strongly consider harvesting the fruit so that none of the fruit has a starch level above 3.0 before it is stored. Since these initial studies, a great amount of research has been initiated and is underway to examine which parameters are most responsible for the development of this disorder.
This research was supported by the Washington Tree Fruit Research Commission.
Dr. Eric A. Curry, Plant Physiologist
USDA, ARS Tree Fruit Research Laboratory
1104 N. Western Ave., Wenatchee, WA 98801
13th Annual Postharvest Conference