SATI Beyond The Bunch 3rd Quarter 2025

Maintaining table-grape quality after harvest rests on three practices: lowering temperature to slow respiration and senescence, controlling humidity to reduce dehydration, and applying SO 2 to prevent decay. Packaging can help or hinder these processes – figuring out how starts with a grasp of the relationship between temperature and humidity.

Temperature and humidity

Vapour pressure deficit Water vapour pressure is the pressure exerted by the water molecules in water vapour. Vapour pressure in air reaches its maximum when the air is saturated with water vapour. The difference between the maximum possible vapour pressure in saturated air and the actual vapour pressure is the vapour pressure deficit. Water vapour tends to move from areas of higher vapour pres sure to areas of lower vapour pressure. The intercellular spaces in berries and stems have a high vapour pressure because they are saturated with water vapour, while the air outside the fruit is almost always unsaturated. This difference drives water loss from bunches. Warm bunches lose water faster than cold bunches because the vapour pressure of water increases with temperature. For example, the vapour pressure of water at 35 °C is more than nine times greater than that of water at 0 °C. When hot fruit is first put into a cold room, the bunches lose water rapidly because a) the fruit has a higher temperature than the room and b) the air inside the fruit is saturated, but the cold-room air isn’t. Reducing the temperature difference as fast as possible helps limit water loss. However, bunches will continue slowly losing water even after reaching cold-room temperatures because the plant tissues are saturated, and the cold-room air isn’t. The lower the rel ative humidity in the cold room, the more water the bunches will lose. Continuous cooling during transport is necessary not only be cause shipping containers are imperfectly insulated but also to remove the heat generated by the living and respiring fruit.

Absolute humidity is the amount of water vapour in grams of water per mass or volume of air. Warm air holds more water vapour than cold air, so absolute humidity is usually interpret ed with the aid of charts or calcula tors that account for temperature. Relative humidity is more useful be cause it expresses the amount of water vapour as a percentage of the amount in saturated air at a given temperature. Air is saturated when it contains the maximum possible water vapour. For example, at sea level, a cubic metre of cold-room air at 0 °C can hold about 5 grams of water vapour, whereas a cube of pack-house air at 25 °C can hold about 24 grams of water vapour. The practical implication is that cool ing reduces the water-holding ca pacity of air. Excess water vapour will condense into droplets on surfaces or suspended in air. This is why con densation forms on cold grapes when they’re brought into a warm room on a humid day.

Forced-air cooling Table grapes experience the largest vapour pressure deficits during forced-air cooling because a) they are at their warmest relative to the cooling air and b) the high air velocity strips the boundary layer from the bunches. To minimise moisture loss, the vapour pressure deficit must be reduced as fast as possible by rapid cooling and humidifying the cooling air.

BEYOND THE BUNCH • 5 • QUARTER 3 • 2025