Ask the Produce Docs (General)
I would like you to help me determine if the system I have adopted for cooling grapes is right or wrong. I came up with an idea where one can both maintain the temperature at -1°C in the precooling chamber by which the core temperatures of the grapes were 0°C, and also keep the Relative Humidity at 97% without any additional humidifier or water spray. The moisture particles were so small that they did not settle on the pallet boxes, floor or the fruits. It is room cooling with a large surface cooling coil and a low Td. The air is of high volume and high velocity. This is circulating and penetrating the boxes. The evaporators are ceiling hanging type. The system is DX system. Thanks for your advice. (S.S.)
Grape storages in California almost exclusively use the method you describe. Grapes are first forced-air cooled in a separate cooling room. Cooling times are 4 to as long as 24 hours depending on packaging method. Fruit packed in bags and placed in a master container with a plastic liner require the longest cooling times. The coolers are not designed for excessively high humidity because temperature difference between the grapes and the cooling air is the main reason for a vapor pressure difference that dries the fruit during initial cooling. Reducing the temperature of the grapes is the first step in preventing moisture loss. After the fruit is cooled to about 0° to 2°C it is moved to a specially designed cold room for final cooling and storage. The storage rooms have large coil surface area and suction pressure control. The suction pressure is increased to the highest possible level which will still provide adequate refrigeration capacity. Humidifiers are often used initially to add water to packaging materials, but after this is accomplished they are turned off. Air speed in the storage is minimized to reduce moisture loss but still maintain uniformly low fruit temperature.
Remember moisture loss is a cumulative process and starts as soon as the fruit is picked. Initial cooling causes about 0.5 to 0.75% moisture loss from the fruit. But it is common for this much or more water to be lost by the fruit during temporary storage in the vineyard after picking and in transport to the cooling facility. Reducing the time between picking and cooling and protecting the fruit from excessive temperatures are key to preventing fruit shrivel. Paper or wood packaging materials also absorb moisture. Use plastic packaging materials if possible or humidifiers to add water to the moisture absorbing materials. –Jim Thompson