The impact of relative humidity in commercial cold rooms during accumulation of pallets for shipping on shrivel with plums

The objective of this project was to generate evidence that good relative humidity (RH) management in cold rooms prior to shipment of plums to overseas markets, during the accumulation period, impacts on fruit quality, in particular shrivel. A one-year survey (ST 01-20) in 2020 was conducted to establish RH levels in stone fruit cold rooms used for pre-pack bin holding, accumulation and forced-air cooling of fruit prior to export, and to ascertain the types of RH and monitoring systems. From this survey it was established that most of the commercial cold rooms monitored were operating at below 90% RH, which leads to high vapor pressure deficits (VPD) between the fruit and the surrounding atmosphere.

Fruit was subjected to 42 days cold storage which included 7 and 12 days accumulation periods in commercial cold rooms. Both accumulation periods were done at high and low RH levels. After accumulation, fruit was transported to Stellenbosch to undergo a simulated container shipment overseas. Cold stored as per export shipping protocols, Laetitia (PD7 regime) and African Delight (PD1 regime) plums were packed with shrivel sheets and perforated bags, respectively. The fruit was weighed at intake and after cold storage, with full quality evaluations done after cold storage and subsequent shelf-life.

Data generated showed that more attention must be given to cold room monitoring and control of RH and temperature. It was clear that if cold rooms are not operated at optimum RH and temperature, with good pallet handling procedures, there could be a negative impact on fruit quality in terms of mass loss, shrivel and flesh firmness.

It is important to monitor RH and temperature constantly in plum accumulation cold rooms and limit hot spots. Reduce the time of accumulation if possible. Forced-air cooling is another critical area where RH must be monitored and improved, and fans are to be turned off as soon as the fruit reach the target temperature.

Temperature and RH monitoring is of critical importance, in forced-air and accumulation cold rooms. Make sure targets for these factors are met for best maintenance of fruit quality.