Survey of commercial stone fruit cold room relative humidity and temperature to ascertain possible impact on moisture management

Shrivel on stone fruit is a quality concern and is becoming a more difficult challenge to manage. At time of application for this project, there was a good possibility that petroleum-based single-use plastics, especially those under 37 microns in thickness, were likely to be banned soon. This seems to have changed but there is still pressure to reduce plastic and make it more recyclable. An important aspect to reduce moisture loss and shrivel in stone fruit is reducing the vapour pressure deficit (VPD) by increasing relative humidity (RH) in cold rooms. This study was conducted by a survey to establish the range of RH levels in stone fruit cold rooms used for pre-pack bin holding, accumulation and/or forced-air cooling fruit prior to export, and ascertain the types of RH systems employed and the type of monitoring.

Twenty three cold rooms in the three main stone fruit production areas were identified and loggers were used to monitor temperature and RH. Questionnaires were used to get information on the type of RH system used, room function (i.e. pre-pack (bin), forced-air cooling [FAC], FAC & accumulation or just accumulation) and if RH was monitored and how.

Overall the pre-pack (bin) cold rooms exhibited RH results on the higher side of the medium performance range set out in the work plan, with five rooms between 85 – 90%, one room in the high 80’s / low 90’s (Graph 3) and one room in the poor to low medium range <82%. Data from forced-air cooling done in dedicated FAC tunnels (Graphs 8, 10, 11 & 12) showed 3 runs done in the poor RH (< 80%) range and 3 in the good RH (>90%) range, while the rest (17) of the runs were in the medium range (80-90%). The two rooms used for FAC (Graphs 9 & 21) exhibited RH readings of medium (average 84.5% – 83.4%) to poor humidity (average 78.6% – 78.5%). In the accumulation rooms, five of the 11 rooms exhibited RH levels in the high medium to good (>85%) range, while the rest were below 85% RH in the low medium to poor range. None of the cold rooms surveyed monitored RH levels or temperature, and only had readings on return air as per cooling unit and set point.

In previous research it was recommended to aim for a relative humidity of 95%, to limit moisture loss from stone fruit and subsequent shrivel. None of the commercial cold rooms reached the desired RH of 95%. Data will be shared with the respective cold room representatives that participated in this survey, to show what the rooms were running at and to consider if changes are necessary