Understanding the cold sterilisation of dual-temperature plums. By Esté Beerwinkel.
Yin and yang describe how seemingly opposite forces may actually be complementary. Dual-temperature plums need both moderate and cold temperatures to be successful on the international markets. In this Q&A, ExperiCo’s Handré Viljoen sheds light on the yin and yang of dual-temperature plums and cold sterilisation in conjunction with the use of SmartFresh.
Why was this research done?
We started research on this topic in November 2013 when Bactrocera dorsalis became a problem in the North.
Some countries have strict phytosanitary regulations on fruit exporters, forbidding the importation of certain plants, insects, or their larvae — we needed a mitigation treatment if Bactrocera became a problem.
One of these phytosanitary regulations is cold sterilisation. This means uninterrupted storage at -0,55 °C for 22 consecutive days.
While cold sterilisation suffices for single temperature regimes (PD1), it isn’t a solution for dual-temperature regimes. Varying temperatures are needed to prevent chilling injuries such as internal flesh browning or gel breakdown around the stone — without this, chilling injuries can cause shorter shelf life, shipment rejection and financial losses.
Having this tool means that if this problem ever occurs, we’d still be able to reach our target markets.
What are the particular maturity parameters for this?
Optimum harvest maturity — optimum mature fruit from pack houses was used for this research.
How long is the cold-steri period, and at which temperatures?
A temperature of -0.55 °C for 22 consecutive days.
How long can one store the produce?
Store standard dual-temperature regime plum cultivars (PD7) for up to 43 days. This includes the five-day waiting period before distribution.
Keep fruit at 20°C for three days before cold sterilisation—this is especially important for colour development. Thereafter, keep fruit at -0.55°C for 22 days during shipment.
Using SmartFresh is key during this process. It releases 1-methylcyclopropene (1-MCP) in the storage room and interacts with ethylene receptors on the fruit, blocking them temporarily until the fruit leaves the storage facility. At ambient temperature, the fruit develop new ethylene receptors and continues to ripen normally.
Will cold-steri period commence on-shore or at time of shipping?
At the time of shipping. This is if SmartFresh was applied during the accumulation period. In the 22 days of cold sterilisation, the temperature should not rise above 1.11°C or it would negate the entire process.
Will one be able to access all existing markets?
Yes. Commercial markets will still have to be investigated, as research was done solely on a lab basis. Protocols still need to be set up for containers.
USDA Cold Treatment Manual
T107-e Apricot, Citrus, Grape, Nectarine, Peach, Plum
Thaumatotibia leucotreta (false codling moth), Ceratitis capitata (Mediterranean fruit fly), C. quinaria (five-spotted Zimbabwean fruit fly), C. rosa (Natal fruit fly), and Bactrocera dorsalis.
T107.E Cold treatment
Temperature: 31 °F (-0.55 °C) or below 1°C
Exposure period: 22 days
Temperature exposure period
The treatment shall not commence until all sensors are reading 31 °F (-0.55°C) or below. If the temperature exceeds 31.5°F (-0.27°C), the treatment shall be extended one-third of a day for each day or part of a day the temperature is above 31.5°F (-0.27°C). If the exposure period is extended, the temperature during the extension period must be 34°F (1.11°C) or below. If the temperature exceeds 34°F (1.11°C) at any time, the treatment is nullified. Also, some freeze damage to the fruit may occur if the pulp temperature is allowed to drop below approximately 29.5°F (-1.38°C) (this varies with the commodity).