TEMPERATURE PROFILING for improved grape quality

Identifying and implementing cold-chain handling improvements

Monitoring table-grape pulp temperature

Utilising "type T" thermocouples (TC) has been proven to be a cost-effective mechanism for measuring the status of pulp temperature in pallet stacks. In addition, modern technologies make it possible to monitor pulp temperatures in real time, providing the opportunity to have access to temperature deviations without having to wait for quality claims and retrospective evidence.

There are several products available (Figure 1). Modern devices can record temperatures at specific intervals and upload data to online platforms. Specific sections of the postharvest chain can be accessed with a zoom function, using online dashboards. These products use mostly radio frequency (RF) or subscriber identity model (SIM) card technology to communicate with the customised online platforms.

Figures 2A to 2D provide an example of a pulp temperature profile, which has been installed with an RF device in the centre of a grape-pallet stack destined for Europe. The temperature was monitored from the packhouse to the market, and demonstrates the value of having visibility of all the steps involved in the value chain.

Figure 2A shows pulp temperature in a table-grape pallet stack during export to Europe – the full journey from the packhouse to the market. The data in Figure 2B shows the dwell time in the packhouse (5 hours) and duration of the forced-air cooling cycle (12 hours) to reach the target pulp temperature (0 ˚C).

Figure 2C shows the pulp temperatures (-0.2 ˚C to 0.2 ˚C) during the sea voyage in the container and in Figure 2D the pulp temperature can be seen after arrival in the destination market.

The consignment in this example was monitored during January and February 2021. The upward creep of the pulp temperature measured towards the end of the export chain occurred after the off-loading of the consignment at a distribution centre, where the capacity for optimal cold storage was limited and the pallets remained under suboptimal cooling conditions for a period of 18 days.

The value of pulp temperature profiling

This situation was ascribed to the accumulation of stock when the regular supply and demand, and related supply chain operations, were disrupted by the COVID-19 pandemic. To maintain the desired quality of exported table grapes, producers are encouraged to profile both the air and pulp temperature of commercial consignments on a regular basis, using real-time devices.

Real-time monitoring of pulp temperature provides an excellent opportunity to identify shortcomings in the cold chain, and to regularly engage with service providers whilst the outcome can still be controlled.

Handling parameters specified in service level agreements can then be revisited and updated as required, to align with the perishability of the product. The insights obtained from using these real-time, online technologies and dashboards can be used to ensure accountability along the full export chain. They also present opportunities to optimise all steps of the export value chain, as part of a "systems thinking" approach.

This situation demonstrated the danger of using a single probe reading on the periphery of a pallet stack, as an accurate indicator of the pulp temperature status for regulatory purposes.

At the time when the carton stacked in the "commercial TC" position (Figure 4, position B) had reached the target, the pulp temperature in carton position E was still at 1.4 ˚C and in position C, at 3.5 ˚C.

This result indicates that in this specific carton design, the TC would not give a reliable indication of the overall pulp temperature situation in the pallet stack during all circumstances of the forced-air cooling process, due to the unbalanced air flow through the pallet. Therefore, this carton design requires adaptation of the ventilation openings to improve the heterogeneity of the pallet stack and concurrently, ensure the accuracy of temperature checks before load-out.

Featured: Grape harvesting underway