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WSU-TFREC/Postharvest Information Network/Determination of Maturity for Long-Term Storage of Apples



Determination of Maturity for Long-Term Storage of Apples


Introduction

In Nova Scotia there has been no application of a standard scientific method to determine the appropriate time to harvest apples assigned to long-term controlled atmosphere (CA) storage. The purpose of this project was to develop a reliable method that could be fast, inexpensive and easily done by any orchard manager without reliance on laboratory facilities.

There are various methods which have been proposed to estimate fruit maturity (Kingston, 1991; Schofield, 1997). During a tour of the Netherlands and Belgium by the Nova Scotia Fruit Growers' Association (NSFGA) Quality and Storage Committee in April, 1995, information on the success of the 'Streif Index' in the European apple industry was obtained. The Streif Index is named after its developer, Dr. Josef Streif, an apple postharvest physiologist working in Southern Germany (Univ. Hohenheim, Versuchsstation Bavendorf, D-88213 Ravensburg, Germany). It is calculated as {firmness/(soluble solids x starch index)}. The appeal of this Index is partly based on its simplicity. All three measurements are accepted already as important individual measurements. These measurements are rapid, inexpensive and easily done by any orchard manager. The application of the Streif Index has been studied on various popular European cultivars grown in different regions of Europe (de Jager et al., 1996). However, most of these studies did not consider popular North American cultivars nor long-term storage in CA (8 months or more). The Quality and Storage committee wanted to evaluate this method under Nova Scotia conditions and a 3-year research project was started in the fall of 1995 with support from participating growers, the NS Tree Fruit Research Foundation, NS Dept. of Agriculture and Marketing and Agriculture (NSDAM) and Agri-Food Canada (AAFC).


Materials and Methods

Three strains of McIntosh (Marshall, Summerland, and Redmax), one strain of Cortland (Redcort) and one strain of Jonagold (Wilmuta) were selected for the study. The experimental design was the same as Streif (1996) with some small modifications. There were three grower sites for each of the McIntosh strains and four grower sites for the Cortland and Jonagold strains. Samples taken every week, five weeks prior to the 'conventional' harvest period, and four weeks during the harvest period were characterized by several standard physical and biochemical measurements, including firmness (N), soluble solids content (%SSC), and starch index (1=starch, 9=no starch). For each apple strain x cultivar combination, the Streif Index {firmness/(soluble solids x starch index)} (y axis) was plotted against the time of sampling ('Day of Year') (x axis) and a regression equation calculated.

Following long-term CA storage (8 months at 3 °C, 2.5% O2, and 4.5% CO2) and a shelf period (7 days at 0 °C plus 5 days at 20 °C), the quality of the samples from the four harvest dates was evaluated by disease & disorder ratings, sensory panel ratings, and the standard measures previously mentioned. These measurements were combined into an overall quality rating. We modified the 10 point rating used by Streif (1996) (Table 1) to a 27 point rating (Table 2), which gave better separation of the four harvest dates. After evaluating the post-storage results, we felt that they could not clearly show the best harvest date for beginning harvest date for long-term storage (BHD). However, the data suggested we could determine the final harvest date for long-term storage (FHD). We defined the FHD for each Orchard x Year sample as the 'Day of the Year' when the quality rating dropped 10% from its maximum score. The FHD value was converted to Streif Index value using the appropriate regression equation mentioned above.


Results and Discussion

The first year's results (Schofield, 1997) and the second year's results have been combined and are included in this report. After the third year's quality data are collected in June of 1998, the FHD and Streif Index values will be re-calculated using all three years. An example of the first 2 years's results for Redmax McIntosh are shown in Figure 1.

As mentioned in the Materials and Methods section, the post-storage evaluations of the four harvest dates did not appear to vary enough to allow estimation of the beginning harvest date (BHD). This would have been possible if the post-storage evaluation score was low for harvest 1, compared with harvest 2 and/or 3. However, we observed that harvest 1 was frequently as good as harvest 2 and/or 3. We speculate that this is a result of our choice of storage duration of 8 months. After such a long period of storage, it becomes more likely that the earliest picked fruit (more pre-climacteric) would be given a higher post-storage rating, compared with the later picked fruit, which would be more senescent and have a lower post-storage rating than earlier in the storage period. This is likely to occur more so in cultivars not known to be good long-term storage cultivars, e.g., McIntosh and Cortland. The combined three years of data may allow estimation of BHD. If not, then future research should use shorter storage periods, e.g., 4 to 6 months, to obtain the necessary differentiation between the first harvest and later harvests.

Of the three measurements used in this study (firmness, SSC and starch index), the starch index values were found to most strongly influence the relationship between Streif Index (y) and Day of Year (x). It affected both the magnitude of Streif Index values at the FHD and the Streif Index trends during maturation. This agrees with many of the reports in de Jager et al. (1996). For 3 of the 4 cultivar strains, the decline in Streif Index was linear as shown for Redmax McIntosh (Figure 1). However, for Wilmuta Jonagold the decline was curvi-linear (data not shown), similar to reports on Jonagold in de Jager et al. (1996). During the harvest period the Streif Index value was very low and changed less, compared with the other three cultivars. The explanation is the rather high starch index (high amount of starch degradation) that occurs in Jonagold before it is acceptable for harvest. The high starch index values result in low Streif Index values for Jonagold during the harvest period. This can make it more difficult to estimate the beginning and the end of the optimum harvest period for Jonagold, compared with cultivars that have a higher amount of starch (low starch index) during harvest.

In general, the FHD for long-term storage of apples was characterized by a 1 day (Summerland McIntosh) to 7 day (Redcort Cortland) range. The maturity sequence was as follows (earliest to latest) with the FHD range expressed as Day of Year in parentheses: Marshall McIntosh (256.5 - 261.1), Summerland McIntosh (263.5), Redmax McIntosh (263.5 - 268.1), Redcort Cortland (270.5 - 277.5) and Wilmuta Jonagold (274.6 - 280.4). The above maturity sequence is the same as previously reported, using the starch index and internal ethylene (DeEll and Prange, 1993). The corresponding Streif Index values, determined from regression of FHD against Streif Index, such as shown in Figure 1, were: 4.82 - 3.64 (Marshall McIntosh), 3.51 (Summerland McIntosh), 2.99 - 1.83 (Redmax McIntosh), 5.29 - 4.29 (Redcort Cortland) and 2.10 - 1.24 (Wilmuta Jonagold).


Summary

The results from the first 2 seasons are clearly demonstrating that the Streif Index is a practical and useful end-of-harvest guide. The Streif Index appears to be insensitive to year and location variation, giving it good potential for regional utilization. Other advantages are its ease of measurement and its independence from skin color. In the last harvest season, Fall 1997, the Streif Index was used for the first time to recommend the end of harvesting of McIntosh, Cortland and Jonagold for long-term storage in Nova Scotia. After the end of the third storage season and before the beginning of the 1998-1999 storage season, we intend to summarize the results in a manner useful to orchard managers and storage operators.


Literature Cited

de Jager, A., D. Johnson and E. Hohn (Editors). 1996. European Commission COST 94: The postharvest treatment of fruit and vegetables - Determination and prediction of optimum harvest date of apples and pears. Proc. of June, 1994 workshop, Loftus, Norway.

DeEll, J. and R.K. Prange. 1993. Update on apple storage research at Kentville. Nova Scotia Fruit Growers' Annual Report 130:75-79.

Kingston, C.M. 1991. Maturity indices for apple and pear. Horticultural Reviews 13:407-432.

Schofield, R.A. 1997. Determination of the final harvest window for long-term controlled atmosphere storage of apples. B.Sc. (Hons.) Thesis, Biology Dept., Acadia Univ., Wolfville, Nova Scotia.

Streif, J. 1996. Optimum harvest date for different apple cultivars in the 'Bodensee' area, p. 15-21. In: A. de Jager, D. Johnson and E. Hohn (Editors). 1996. European Commission COST 94: The postharvest treatment of fruit and vegetables - Determination and prediction of optimum harvest date of apples and pears. Proc. of June, 1994 workshop, Loftus, Norway.

Dr. Robert K. Prange and Dr. John M. DeLong, Postharvest physiologists

Agriculture and Agri-Food Canada, Atlantic Food and Horticulture Research Centre

14th Annual Postharvest Conference, Yakima, Washington
March 10-11,  1998

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