REFRIGERATION supports the table-grape value chain

The need to chill

Grapes remain alive after picking. They generate energy and heat by burning some of their sugars through respiration. Fruit metabolism accelerates as temperatures rise – respiration rates generally double with every 5 °C increase.

Cold-chain breaks of even a few degrees boost fruit metabolism and speed up deterioration. Restoring optimal temperatures slows fruit down, but respiration rates will remain higher than before the break.

The bottom line is that the effects of cold-chain breaks are usually irreversible and additive. The results are stem drying and browning, berry softening, weight loss, and decay – all of which impact value and profitability.

Time is of the essence

Grape quality at harvest is as good as it gets – cold-chain management must start here if fruit quality is to be sustained. Packing and cooling harvested grapes is extremely time-sensitive – a general rule of thumb is that fresh produce loses a day of shelf life for every hour of delay between picking and packing.

Most moisture loss occurs in warm grapes before packing and cooling. Keep in mind that lost moisture cannot be regained.

The preferred method for reaching set-point temperatures in table grapes is forced-air cooling.

Five factors that affect the cooling rate of grapes

1. The centre of a berry will cool faster, the colder its surface is.
2. The surface of a berry will cool faster, the colder the air temperature is.
3. Higher airflows increase the cooling rate by removing heat more quickly.
4. Packaging like liners insulate fruit and reduce cooling rates.
5. Outer packaging like cartons and structures like pallets restrict airflow and reduce cooling rates.

Cooling is most efficient when the chilled air comes into direct contact with the fruit surface – this is called convective cooling. Much less effective conductive cooling occurs when heat must cross a barrier like packaging before removal.

Cooling efficiency also drops, the further the chilled air has to move and the lower the air volumes are.

Temperature reductions during pre-cooling are not linear. As grapes become colder, the temperature gradient between the grapes and the air flattens, so cooling slows. For example, if grapes take two hours to get halfway to the set point, they will take another four hours – twice as long – to get to 87.5% of the set point.

Growers can optimise pre-cooling by good cold-room management.

Nine tips to reduce the heat load in cold rooms

1. Harvest during cooler times of the day and keep grapes shaded, to minimise field heat before putting them in the cold room.
2. Cool grapes quickly to slow down
   their metabolism.
3. Insulate the cold room – including the floor – to keep heat out.
4. Ensure that doors are well-sealed and kept closed. Install strip or air curtains, or double doors to prevent heat from entering.
5. Minimise lighting and use energy-
   efficient bulbs.
6. Install energy-efficient fan systems – about 90% of the electricity used by the fan will end up as heat in the cold room.
7. Minimise human activity in the cold room – people produce heat and introduce warm air when entering and leaving the cold room. Do administrative tasks like stock control outside.
8. Invest in electrical forklifts that generate less heat.
9. Optimise cold-room use, as fuller cold rooms are more efficient to cool than emptier ones.

Packaging pointers

Effective forced-air cooling requires unobstructed airflow, which creates a conundrum for table-grape growers. On the one hand, packaging impedes cooling and can trap too much moisture. On the other, inner packaging is necessary to retain sulphur dioxide and increase relative humidity. Whereas outer packaging protects the fruit during transport and handling.

The compromise is ventilation gaps – inner packaging for table grapes is perforated and outer packaging has slots.

Optimising inner packaging is challenging because cultivars differ in, for example, their sensitivity to sulphur dioxide damage. Applying available research data to best match cultivars and packaging will improve product quality and resilience.

Growers should resist the temptation to save money on inner or outer packaging. Poor packaging elevates the risk of damaged or deteriorated grapes – cheap packaging does not equal cost-effective packaging.

The power of monitoring

Most of the table-grape cold chain is beyond the grower's control. But, thanks to technology, growers can keep a close eye on their grapes during transport and storage. Temperature-recording systems enable growers to respond to logistics-related claims and proactively identify and address shortcomings in the cold chain.

Radio-frequency identification devices can transmit temperature data during export without needing to recover the devices afterwards. Timeous data transmission requires radio- or cellular-network access whenever the product is transferred between storage or transport locations.

The placement of temperature recorders is important – they must provide representative data. Recorders also need to track temperatures during the entire export process. Incomplete temperature data sets are of little use when resolving disputes.

Every grower knows that the fruit always pays. Profitable production requires fruit to pay off. Setting up the cold chain on the farm and monitoring temperatures beyond the farm gate will help make this happen.

References

Dodd, M.C. and J.J. Bouwer. 2014. The supply value chain of fresh produce from field to home: refrigeration and other supporting technologies. In Postharvest Handling: A Systems Approach (pp. 449 – 483). Academic Press.

Nelson, K.E. 1985. Harvesting and Handling California Table Grapes for Market. Bulletin 1913. UCANR Publications.

Additional reading

An on-going SATI project led by Dawie Moelich is further investigating the use of temperature profiling in managing the table-grape cold chain. Read more about some of the findings in the December 2022|January 2023 SA Fruit Journal.