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Honeycrisp, one of the most popular cultivars of apple Malus Ã- domestica Borkh. was developed in Minnesota and released in 1991 (Bedford, 2001). The fruit has become an important variety in the fresh fruit markets of the northern regions of the United States and Canada. Approximate 950,000 'Honeycrisp' trees had been planted in the U.S. with the largest number in Michigan and New York as of January 2001. The cultivar was also being grown in Europe, New Zealand, Australia and South Africa (Tong et al., 2003). Its unique texture, outstanding flavor, unusual crispness, and long storage-life (nine months) had granted favorable prices for growers and help the apple industry renovated (DeEll, 2005; Delong et al., 2006).
However, the ardor of industries for 'Honeycrisp' has been restrained by considering on the high frequent occurrence of quality-related storage disorders in fruit, like bitter pit, soft scald, soggy breakdown, water core, skin punctures, decay and internal browning, which vary in severity with the growing season and region (Evans, 2001; Greene and Weis, 2001; Schwallier 2001; Prange et. al., 2002). The first three disorders are the key factors to affect the fruit's market, thus the report will focus on the methods to reduce the incidence of bitter pit, soft scald and soggy breakdown. 'Honeycrisp' was extremely susceptible to bitter pit (Figure 1.) and usually can cause more than 50% of the fruit unmarketable (Dell, 2005). It could happen on the tree or develop during storage, especially with early harvest, delayed cooling, or warmer storage temperatures (Rosenberger et al., 2004). However, bitter pit could be controlled by cultural practices, like calcium spray applications in the field because bitter pit is majorly caused by calcium-deficiency (Rosenberger et al., 2004). While the random incidence of soft scald and soggy breakdown in the fruit are the greatest concern for apple growers, which have resulted in great negative effects for the marketing of this cultivar.
Both soft scald and soggy breakdown were the low temperature injuries and should be worse at temperatures below 35 to 38 0F (Watkins et al., 2005). Soft scald (Figure 2.) was characterized by the development of sharply defined brown lesions on the apple peel, which could extend into the flesh, and the lesions were often invaded by the secondary infections (Watkins and Rosenberger, 2002; Moran et al., 2010). In some regions, soft scald has greatly affected the commercial value of 'Honeycrisp'. Nova Scotia was one of the typical examples, where has the highest incidence of soft scaldwhen compared to other storage disorders, sometimes reaching 100% incidence after harvest (Prange et al., 2002; Delong et al., 2006). Susceptible cultivars include 'Fuji', 'Jonathan', 'McIntosh', 'Delicious', 'Golden Delicious', and 'Honeycrisp'. Another common disorder is soggy breakdown (Figure 2.), which is an internal disorder. The symptom of soggy breakdown was that the flesh of apple would become moist and brown, moreover, the whole tissues looked like sponge (Watkins and Rosenberger, 2002; Watkins et al., 2004). Soggy breakdown developed majorly by inadequate ventilation of containers and storage rooms, a prolonged storage period, temperatures lower than 32 0F, variations in fertilizer treatments, and climatic conditions (Plagge and Maney, 1937). When some apples become immune to soggy breakdown, they should be more susceptible to soft scald. Sometimes both disorders could happen concurrently on the same fruit (Plagge, 1929; Watkins et al., 2003).
Current Technologies to Control Soft Scald and Soggy Breakdown
In common storage (40-600F) or when fruit was stored at temperatures higher than 320F, soft scald and soggy breakdown did not develop. Since the industry had started to store apples at low temperatures of 30-320F, these two disorders began to be observed (Harley and Fisher, 1930). At that time, they are common disorders in apple industries. Thus the low temperature plays a vital role in inducing soft scald and soggy breakdown. Considering the efficiency and economic issues for storing apples, low temperature storage will still be used commonly by producers. Under this condition, special postharvest treatments have been studied to prevent the storage disorders, which could reduce the losses for apple growers.
Storage Temperature Storage temperature is a critical factor in the development of storage disorders. Normally, apple was stored at temperatures above 32 0F. Fruit stored at 32 0F or lower develops more soft scald than fruit stored at 36 0F. The appropriate storage temperature for 'Honeycrisp' to control soggy breakdown was 38 to 40 0F preferably at lower temperatures (Schwallier, 2001; Nichols et al., 2004; Watkins et al., 2004). Soft scald and soggy breakdown development could be reduced when temperatures higher than 330F (Watkins et al., 2003).
Timing of Harvest The optimum harvest period for 'Honeycrisp' varied with the growing season and growing place (Wargo and Watkins, 2003). Fruits that harvested too early do not develop varietal flavor and are almost tasteless. If harvested too late, 'Honeycrisp' could develop fermentation products, such as ethanol and acetaldehyde, which caused undesirable flavors. A combination of harvest index testing and visual observations was used frequently to recognize the exact harvest time rule for every orchard. Prange et al. (2002) noticed that soft scald happened in 'Honeycrisp' harvested early and late-harvested with a 4-week storage period at temperature higher than 380F before CA storage at 380F had very low incidence of disorders. Later harvest 'Honeycrisp' would be easier to develop soft scald and soggy breakdown in fruit (Watkins et al., 2003), while in Maine, harvested early could be prone to develop soft scald and soggy breakdown (Moran unpublished data). In the Champlain region, the perfect harvest time for 'Honeycrisp' would be about the second or third week of September (Watkins et al., 2003).
Preconditioning Delaying the cooling of fruit prior to storage is a beneficial step in minimizing disorder incidence (Delong et al., 2004). Research at Cornell University and The University of Minnesota showed that a warm treatment (500F for 1 week) before cold storage resulted in the greatest reduction of soft scald (Bedford, 2001). Several recent reports suggested that a quick, delayed cooling period of 50 to 680F for one week before cold storage decreased the occurrence of soft scald and soggy breakdown (Watkins and Nock, 2003; Delong et al., 2004; Watkins et al., 2004). 'Honeycrisp' apple treated with a delayed cooling regime consisting of 770F for 1-2 days, or 860F for 1 day, strongly suppresses or entirely eliminates the occurrence of both soft scald and soggy breakdown (Delong et al., 2009).
Controlled Atmosphere Storage (CA) Controlled atmosphere were usually defined as the content of 2.5 kPa O2 and 1.0-1.5 kPa CO2. For many apple cultivars, an atmosphere of 2 to 3% CO2 with 2 to 3% O2 was thought to be suitable at cool storage temperatures. Controlled atmosphere storage could completely prevent Jonathan spot when only 2% CO2 was present and could reduce the incidence of core flush and various forms of flesh breakdown (Wills et al., 1998). CA storage was not recommended, because the great susceptibility of 'Honeycrisp' to CA injury had been detected by growers in different growing regions (Beaudry, 2009).
Diphenylamine (DPA) Watkins et al. (2004) had indicated that although DPA could decrease the incidence of soft scald in some cases, it could not effectively control this disorder happened when 'Honeycrisp' were stored at 33 0F. DPA did not have big effect on prevention of the soft scald and the impact on internal CO2 injury is in process (Beaudry, 2009). 'Honeycrisp' is not susceptible to superficial scald, so DPA has not been used by the industry to prevent soft scald (Moran, personal communication).
The above methods used to prevent or reduce soft scald and soggy breakdown were not always effective and could lead to other disorders and quality loss (Watkins et al., 2005), such as the effect of preconditioning on soft scald and soggy breakdown of 'Honeycrisp' was very inconsistent (Moran et al., 2010). Thus some new methods are in need to be investigated to better satisfy the customer demand.
Recently, several researchers had reported that 1-Methycyclopropene (Smartfresh, 1-MCP) could better prevent some storage disorders and maintain the fruit quality compared to previous methods. 'Red Delicious' treated with 1-MCP could get much higher levels of firmness and acidity when compared with control (Bates, 2001). After treated with 1-MCP, the quality of 'Honeycrisp' and other cultivars, such as 'Granny Smith', 'Golden Delicious', 'Fuji', and 'Braeburn' had been maintained well and prolonged the storage time, this is because 1-MCP could reduce ethylene production, respiration rate, slower loss of firmness, reduce the occurrence of soft scald, delay production of ripe aroma, and other quality-related index (Argenta et al., 2001; Matthesis et al., 2002). Repeated weekly applications of 1-MCP (Mir et al., 2001; Mir and Beaudry 2001; Jayanty et al., 2004) helped prevent apple softening at 680F more than at 320F. 1-MCP was reported to have a great impact on delaying or retarding the softening and upgrade the fruit quality by enhancing crispiness (Watkins et al., 2000; Mir et al., 2001; Mattheis et al., 2005; Moran and McManus 2005). Apple treated with 1-MCP, then storaged at atmosphere temperature could get better firmness than that storage at cold temperature, however, the results varied with cultivars (Fan et al., 1999b; Mir et al., 2001). It is recommended that 'Honeycrisp' be stored at temperatures above 36 0F, but this can lead to other problems, like higher decay and less acceptable flavor (Watkins et al., 2003). 1-MCP could be used on 'Honeycrisp' apples to prevent quality loss at warmer than optimum storage temperatures (Mir and Beaudry, 2001)
Green color loss in the ripening process could be prevented by 1-MCP treatment (Saftner et al., 2003; Zanella, 2003). Moreover, soft scald, brown core and senescent breakdown could also be reduced by 1-MCP (Fan et al., 1999a; Watkins et al., 2000; Zanella, 2003; Delong et al., 2004; Moran and McManus, 2005). The development of greasiness in some apple cultivars, such as 'Honecrisp', was also a process that develops in cold storage, and this was inhibited by 1-MCP (Watkins and Nock, 2005). A number of apple cultivars including 'Delicious, 'Granny Smith', 'Fuji' and 'Gala' have been reported that when 1-MCP used at harvest not only could improve their storage life and delay ripening, but also reduce superficial scald and other physiological disorders (Fan et al., 1999a; Fan et al., 1999b).
1-MCP as a new technology, was reported to inhibit the ethylene action by preventing apple tissues from responding to ethylene that through the combination with ethylene receptor. This chemical compound had been thought as a promising breakthrough in apple storage technology that preserves the fresh-picked apple crunchiness, taste and juice content. Moreover, it could also help the apple industry satisfy the consumer demand for higher quality apples, additionally retain steady quality to consumers in any period of one year (Argenta eral., 2001; and Mattheisi et al., 2002).
'Honeycrip' treated with 1-MCP has great potential to improve fruit quality when less than ideal storage practices are used. This is because it has successfully improved other apple cultivars' quality, which as mentioned above. Another reason is that it has special characteristics which will beneficial fruit storage. Compared to other chemical methods, it is nontoxic to human and has activity at very low concentration (100ppb to 1ppm). When it is ready for a full commercial use, it will be packaged in a self-contained, one-step, single use delivery system. 1-MCP powder is very easy to be dissolved in water and release its active ingredients to storage room in a short time (Ian, 2003a; Bruce Bates, 2001). Ian (2003b) indicated that SmartFreshâ„¢ could easily complements the current systems of apple storage (CA and RA storage) and had benefited the South African apple industry. For majority of cultivars, when apple were harvested at control atmosphere and then treated with 1-MCP immediately, the storage disorders could be reduced to the lowest level (Argenta et al., 2001; Matthesis et al., 2001). Using 1-MCP in storage 'Honeycrisp' apples could also reduce ethylene production, respiration, and greasiness (DeEll and Murr, unpublished data).
Apple treated with 1-MCP can improve many quality characteristics during storage, but few reports on the application of 1-MCP on 'Honeycrisp' have been published. Therefore the potential effect of 1-MCP on preventing storage disorders and improving fruit quality of 'Honeycrisp' is necessary to investigate.
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Figure 1: Bitter pit in apple (Photo by Patricia McManus)
Figure 2: Soft scald (left) in 'Suncrisp' and soggy breakdown (right) in 'Honeycrisp' apples (Photo by Dr. R.E. Moran)