SATI Beyond The Bunch 3rd Quarter 2025

Ventilation

ADOBE STOCK

Where does SO 2 fit in? SO 2 is highly effective at preventing postharvest decay when it’s present at the correct concentrations. Too little SO 2 won’t control decay organisms, and too much can damage fruit. Although other SO 2 -release systems exist, this discussion will focus on SO 2 sheets designed to be placed inside liners. The sheets generate SO 2 from the reaction of sodium metabisulfite with water. Different sheets have different SO 2 -release profiles, but all are subject to similar consider ations regarding temperature, humidity, and ventilation. The direct effect of ventilation is obvious – increased air circulation reduces SO 2 concentrations, sometimes to levels below the effective threshold. The indirect effect of more ventilation is to lower humidity. Lower humidity re Ventilation – air movement – is central to packaging performance because it’s pivotal to temperature and humidity levels. The relationship between these factors explains a familiar packaging conundrum: trying to balance rapid cooling with optimal humidity. Let’s start with temperature. Fruit can be cooled by conduction or convection. Conduction involves the direct transfer of heat from a warmer object to a cool er object or environment. For example, grapes inside a non-perforated liner cool by conduction. With convection, a cold gas or liquid flows around a warm object, carrying the heat away. For example, perforated packaging can allow cold-room air to cool grapes through convection. Convection generally cools fruit more efficiently than conduction, so venti lation is a crucial consideration in packaging design. Unfortunately, moving air doesn’t only carry heat away. It also removes the layer of moisture around the berries and stems. In still air, the moisture lost by the fruit will be concentrated in the boundary layer – the air immediately adjacent to the bunch. The boundary layer reduces the vapour pressure deficit and slows moisture loss. Convection strips away the boundary layer and maintains a high vapour pressure deficit, which keeps pulling water from the grapes. On the other hand, less air movement increases the relative humidity around bunches. This increases the risk of condensation asso ciated with cold-chain breaks. Cold-chain breaks cause the grapes to lose more water because warmer air has a higher water-holding capacity than cooler air. Once cold temperatures are re-established, the cooler air can no longer hold the extra water, which condenses onto the packaging and the bunches.

Poor ventilation increases the risk of condensation on the grapes.

duces SO 2 release by sheets but also inhibits decay-caus ing organisms. In contrast, less ventilation allows SO 2 accumulation. Less ventilation also promotes higher humidity and increases SO 2 release, further raising SO 2 levels inside the liner. Ex cessive humidity or, even worse, free water can trigger SO 2 spikes, leading to phytotoxicity. SO 2 is attracted to water and can dissolve in condensation on bunches, causing unsightly bleaching of berries. Less ventilation can also result in higher temperatures. Higher temperatures accelerate SO 2 release by sheets, as well as potentially increasing fruit sensitivity to SO 2 dam age. Faster SO 2 release can also prematurely deplete the sheets, leaving the fruit unprotected.

BEYOND THE BUNCH • 6 • QUARTER 3 • 2025