Currently, the two varieties Cripps Pink and Rosy Glow that produce Pink Lady® branded apples, are managed the same in storage by industry. However, we have found differences in their storage behaviour that could warrant either separation of the two during storage or an additional 1-MCP treatment being applied to Rosy Glow during storage.
Cripps Pink and Rosy Glow
Rosy Glow is a limb sport of Cripps Pink and is popular because of the greater red colouring of its apples compared with those of Cripps Pink. Apples produced by both varieties can be marketed under the Pink Lady® brand. The major supermarket chains have strict requirements for the colour of Pink Lady apples, which may be difficult to achieve from season to season because of the particular combination of weather conditions, temperature and light before harvest.
For this reason, the redder Rosy Glow has become popular with growers. The earlier red colouration of Rosy Glow apples means they can be picked and marketed sooner than Cripps Pink, potentially gaining market advantage at the beginning of the Pink Lady season. Additionally, consumers are often drawn to a more highly coloured product because they may associate (rightly or wrongly) greater colouration with increased quality and nutritional value. All of the above attributes could account for the popularity of Rosy Glow apples and their increased planting in recent years.
Perhaps because of the genetic similarities between the two varieties, the storage characteristics of Cripps Pink and Rosy Glow had not previously been tested. Yet, it is important to know whether various indicators of storage quality are similar in the two apples. If one does not store as well as the other, any resultant dissatisfaction among consumers could potentially lead to a lack of confidence in the Pink Lady brand overall. We undertook a storage trial of commercially harvested (from a single pick) Cripps Pink and Rosy Glow at a Victorian apple orchard in 2015.
Measuring maturity differences
First we measured maturity at harvest with a non-invasive hand-held device known as a DA meter, which measures the chlorophyll content of the outer part of the fruit flesh. Although the chlorophyll content of the fruit is not visible to the naked eye, the DA meter is sensitive enough to measure the declining amount of chlorophyll in the flesh as fruit matures. The DA meter was developed at the University of Bologna in Italy and was recently introduced to Australia, where it has been used successfully by researchers at Agriculture Victoria on a variety of fruit, including apples, pears, peaches and nectarines. In addition to the DA meter measurements, we also performed the traditional starch plate tests for maturity and compared those scores with the DA meter results. The two methods of measuring maturity aligned well both pre- or post-harvest.
Measuring storage quality indicators
Because some storage quality indicators (such as superficial scald and internal browning) can be related to the stage of maturity at harvest, we divided our commercially harvested fruit into two maturity classes: mature and immature. These maturity classes were based on the DA readings and confirmed by our starch plate and ethylene production results. Half of the fruit from each maturity group was treated commercially with 1-MCP at standard commercial rates within four days of harvest.
The other half of the fruit remained untreated. Then, during our 7.5-month storage trial, we recorded the DA meter readings after each 1.5-month removal, as well as measured ethylene production, carbon dioxide production, starch, firmness and sugars. We also checked for the storage disorders, superficial scald and internal browning at harvest and again at every 1.5-month removal during the 7.5 months of cold (regular air) storage at 0°C. In addition, at each removal ethylene production was tracked during 14 days of simulated shelf life at 20°C. To determine how well the fruit maintained its firmness and sugar content, both immediately after each removal and at the end of each 14-day simulated shelf life, all fruit was subjected to the destructive tests of firmness and sugar content.
We found that, overall, Cripps Pink apples retained the various quality attributes better than Rosy Glow. There was less ethylene and CO2 production over time for each of the storage removals in Cripps Pink than in Rosy Glow.
The ethylene produced during the 14-day simulated shelf life at 20°C could reflect what occurs in a normal supply chain situation where fruit may sit on a retail shelf and then in a consumer’s fruit bowl for a couple of days each before consumption. We measured ethylene production from fruit at zero, seven and 14 days after each removal (see graphs). Treatment with 1-MCP worked well for Cripps Pink apples; they maintained a minimal amount of ethylene production, even up to 7.5 months of storage, with only the fruit from the mature-at-harvest category showing an increase in ethylene production after seven days of shelf life.
By contrast, Rosy Glow apples began to produce large amounts of ethylene after 4.5 months of storage, and especially after 14 days of simulated shelf life. This occurred even if the fruit was treated
with 1-MCP and was in the ‘immature’ fruit category. By six months of storage, there was a marked increase in ethylene production between when the fruit was removed from storage and the day seven simulated shelf life measurement, regardless of whether Rosy Glow had been treated with 1-MCP or not.
As expected, both Cripps Pink and Rosy Glow fruit that did not receive any 1-MCP treatment had larger amounts of ethylene production, which was evident immediately after harvest and
at each storage removal. This occurred regardless of whether they were classed as ‘mature’ or ‘immature’ at harvest.
Effects on fruit firmness
As for fruit firmness, Cripps Pink apples that had been treated with 1-MCP retained their firmness more than Rosy Glow, particularly during the 14-day simulated shelf life period. This
occurred even after 7.5 months of storage. From three months of storage onwards, Rosy Glow apples exhibited a decrease in firmness over the 14 days of simulated shelf life, even after 1-MCP treatment.
Scald and internal browning
In terms of superficial scald, none was evident for the first six months of storage in Cripps Pink fruit. After 7.5 months of storage, one third of the mature, 1-MCP-treated fruit showed signs of superficial scald, but this was only after 14 days of simulated shelf life, which is likely longer than the fruit would sit in a store prior to purchase.
In Rosy Glow, superficial scald was more evident, particularly in fruit that had not been treated with 1-MCP. These numbers increased to more than 50 per cent of untreated fruit after the 14-day shelf life after six months of storage.
Overall, internal browning was minimal in Cripps Pink fruit, especially when compared with Rosy Glow. The few instances of internal browning in Cripps Pink occurred in the ‘immature’ category of fruit, whether treated with 1-MCP or not, at the 1.5-month removal. It only became evident after 14 days of simulated shelf life and then rarely occurred throughout the rest of the storage trial.
In Rosy Glow, however, internal browning occurred far more frequently than in Cripps Pink. It first became evident after 1.5 months of storage in 7 per cent of mature fruit after the 14-day simulated shelf life period. However, by six months of storage, this amount had increased, with most apples exhibiting some internal browning, regardless of 1-MCP treatment or harvest maturity. The range was quite varied, from 7 per cent in immature fruit to up to 53 per cent in mature fruit.
In conclusion, our research suggests that 1-MCP protects against internal browning and superficial scald up to a point. In contrast, previous studies indicate that 1-MCP protects against scald completely. Further work is therefore needed to determine the factors that have led to these different results, for example, a cultivar-specific response or a physical treatment effect.
Rosy Glow may benefit from extra 1-MCP to lower ethylene
Because Rosy Glow apples produced more ethylene than Cripps Pink, they may need to be stored separately from Cripps Pink. They may also benefit from a further 1-MCP treatment after 4.5–6 months of storage to maintain low ethylene levels. Whether that additional 1-MCP treatment could also help mitigate internal browning and superficial scald in Rosy Glow will require further study. Moreover, a second application of 1-MCP is not the current recommendation and its effects are untested in fruit.
Thanks are extended to Kevin, Peter and Bob Sanders for the use of their orchard, apples and storage and arranging the 1-MCP treatment. Associate Professor Graham Hepworth, the University of Melbourne, provided statistical advice and Christine Frisina did the statistical analyses. Muhammad Naveed Tareen collected most of the data during his Masters project; Fahad Khan and Jane
Rollinson also helped with set-up and data collection, respectively.
We acknowledge funding from the University of Melbourne’s School of Ecosystem and Forest Sciences Masters research project fund and a Frank Keenan Scholarship. This article is based on the recent publication: Williamson, V.G., Frisina, C., Tareen, M.N. and Stefanelli, D. 2018. Storage performance of two Pink Lady® clones diers, but 1-MCP treatment
is beneficial, regardless of maturity at harvest. Scientia
Horticulturae 235: 142-151. The journal article is available from
the authors on request.
About the authors
Dr Virginia Williamson
The University of Melbourne
t: 03 9035 6804 | e: firstname.lastname@example.org
Dr Dario Stefanelli
Department of Economic Development, Jobs, Transport
t: 03 9032 7373 | e: email@example.com