John Lopresti, researcher at Agriculture Victoria, discusses research on the impacts on table grapes from delays between harvest and cooling, part of the Serviced Supply Chain Project.
Video transcript: Impacts on table grapes from delays between harvest and cooling
It's important for table grape growers and exporters to minimize the delay between harvesting their fruit and cooling their fruit, because we've shown that delays beyond three to four hours can significantly impact on grape bunch quality during subsequent cool storage or export. What we looked at, as part of the service supply chains project, was the effective of different delays between harvest and cooling fruit to see what impact they had on subsequent quality during cool storage or export.
So we used 'Luisco' white grapes. We harvested those, as would be done commercially. Packed them commercially, and then delayed cooling for periods beginning with, from 30 minutes all the way through to six hours. This work's been done overseas, but they usually, the delays between harvest and cooling are very unrealistic. We're talking for 24, 48 and up to 72 hours between harvest and cooling, which is not going to occur commercially.
So, we looked at delays of up to six hours, and what we found was delays above four hours, for this particular, this cultivar, significantly increase the rate of quality loss during a storage period of 4 weeks, in terms of rot development, as well as rachis browning.
So, it basically confirmed what growers expect and what we'd expected, that delays, relatively long delays between harvest and cooling your fruit, where the fruit might be sitting in the vineyard for those four to five hours, will significantly impact on fruit, on bunch quality, grape bunch of quality after subsequent cool storage, export, or marketing.
Harvest rot risk and effect of cooling delays on storage quality among ‘Luisco’ grapes
- Determine the effect of delays between harvest and commencement of cooling on subsequent fruit quality during simulated sea freight export and distribution.
- Correlate rot risk potential at harvest via fruit incubation with subsequent rot development during simulated sea freight export and distribution.
- Understand the impact of rot risk potential in the vineyard at harvest on development of storage rots, and impact on the effect of reducing cooling delays.
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Delays between harvest and commencement of cooling have the potential to reduce table grape quality during subsequent low temperature cool storage, particularly the level of bunch rot development, and water loss from berries and rachides (Crisosto et al., 2001). For example Pereira et al. (2018) found that longer cooling delays of up to 48 hours after harvest significantly reduced water loss among grape bunches after long-term cool storage, resulting in significantly more rapid fruit quality loss. Similarly, Fourie (2008) found that delays in cooling after harvest both increased bunch water loss during cool storage and increased the rate of rot development, compared to fruit that was cooled immediately after harvest. Although delays in cooling appear to increase the potential for grape quality loss, the period between harvest and cooling used in studies so far have been unrealistically long (i.e., up to 72 hours) whilst commercially this delay would likely be no more than 6 to 8 hours. Also previous delayed cooling studies have generally not considered the effect of initial rot inoculum load on grapes at harvest, and its interaction with delays in cooling, in increasing the risk of fruit quality loss during storage.
Beginning in mid-April 2021 an experiment utilizing ‘Luisco’ grapes, a late season white cultivar, was conducted to determine the effect on fruit quality of commercially-realistic delays between harvest and commencement of fruit cooling of up to six hours, whilst accounting for differences in rot risk potential among cartons determined at harvest using an incubation test. Incubation results were also utilised to validate a harvest rot risk protocol designed to predict the potential for rot development among grapes during cool storage and marketing.
In collaboration with a commercial table grape grower and exporter, freshly harvested grape bunches were allocated to one of five delayed cooling treatments (0.5, 1, 2, 4, or 6 hours). Fruit for each cooling delay treatment were harvested at an ambient temperature of 24 °C from a plot consisting of 50 m of vine row, and randomly assigned to one of ten cartons (Figure 1), where five cartons (after cooling) were treated with sulphur dioxide, and another five cartons were left untreated (i.e., control fruit), thus delayed cooling and disinfestation treatments were replicated five times.
After the delay treatment, a total of 50 cartons containing approximately 9 kg of grapes each were cooled down to 2°C overnight, and then transported to AgriBio Centre in Melbourne for simulated sea freight and distribution. Prior to cooling after harvest and cool storage at 2 °C, 250 g of grape berries were sampled from each carton, placed in plastic bags, and incubated at 18 °C for 4 days for harvest rot risk assessment (Figure 2). After the incubation period, the incidence of botrytis rot, rots caused by berry damage and total rot incidence was determined for each carton. As of the time of reporting, the first post-storage quality assessment after cool storage at 2 °C for 25 days has been completed.
Assessments to determine the incidence of botrytis rot (as a proportion of bunch weight), rots due to berry damage and cracking, and severity of rachis browning were completed directly out of cool storage, and after a simulated distribution phase of 7 days at 8 °C. Four bunch bags per carton were randomly sampled and assessed out of storage, with berries showing botrytis or damage symptoms removed prior to the subsequent distribution phase. Remaining grapes were placed back in cool storage at 2 °C for quality assessment after a further storage period of 25 days (to be completed). The type and severity of rot symptoms encountered during assessments are shown in Figure 3a and 3b.
Figure 1. ‘Luisco’ grapes packed into cartons during rot risk prediction and delayed cooling experiment in Mildura during April 2021.
Figure 2. ‘Luisco’ grapes incubated at 18 °C to determine rot risk potential at harvest.
Figure 3a. Low to moderate severity latent Botrytis rot among ‘Luisco’ grapes during cool storage.
Figure 3b. Rot due to berry damage or cracking among ‘Luisco’ grapes during cool storage.
Harvest rot risk and storage rots
Harvest incubation results are presented in Figure 4. Each plot of ten cartons (i.e., carton 1 to 10, 11 to 20 etc.) represents a continuous 50 m section of vine row, with each plot allocated to one of the five delayed cooling treatments. Cartons with an botrytis rot incidence greater than 20 % after incubation would be considered higher risk fruit, whilst those with less than 20 % botrytis incidence can be considered as lower risk fruit in terms of potential for rot development during sea freight (equivalent to cool storage), and distribution.
Incidence data clearly show that plots 1, 2 and 5 contained 2 to 4 cartons that could be considered high risk, whilst fruit harvested from plots 3 and 4 were of low risk. Significant variation in botrytis incidence was observed between plots, with less variability among cartons within plots. Almost all the botrytis observed within bunches was due to latent infection and not berry cracking or injury, suggesting that disease variation among plots was determined by vineyard agronomic practices such as disease inoculum levels, efficacy of fungicide applications during flowering, crop load, and bunch position along vines. These results further suggest that fruit sampling at the plot level for harvest rot risk assessment may be the best approach when attempting to gauge subsequent postharvest rot risk potential.
Directly out of storage and after the distribution both botrytis incidence and total rot incidence, which includes both incidence of botrytis and rots due to berry damage, were correlated with rot incidence after harvest incubation at 18 °C for 4 days in both sulphur-treated and control fruit. Botrytis incidence after storage for 25 days was well correlated with disease incidence after incubation, among both sulphur-treated fruit (r2 = 0.60) and control fruit (r2 = 0.83). Sulphur treatment reduced average botrytis incidence to below 1 % among all plots, compared to control fruit in which disease incidence ranged between 5 and 12 % (Figure 5). Similarly, total rot incidence out of storage due to latent infection and rots due to berry damage were well correlated with harvest incubation incidence in treated fruit (r2 = 0.72), and untreated fruit (r2 = 0.84) (Figure 6).
Figure 4. Botrytis rot incidence after incubation at harvest for 4 days at 18 °C among plots and cartons of ‘Luisco’ grapes in a commercial vineyard, with each plot approximately 50 m of vine row in length.
Figure 5. Linear relationship between botrytis rot incidence after cool storage and botrytis incidence at harvest after incubation at 18 °C for 4 days for sulphur-treated and control ‘Luisco’ grapes after cool storage at 2 °C for 25 days. Each point is the mean incidence of infected berries by weight among 10 cartons per plot.
Figure 6. Linear relationship between total rot incidence after cool storage and total incidence at harvest after incubation at 18 °C for 4 days in sulphur-treated and control ‘Luisco’ grapes after cool storage at 2 °C for 25 days. Each point is the mean incidence of infected berries by weight among 10 cartons per plot.
After cool storage for 25 days and a simulated distribution period of 7 days at 8°C, an excellent correlation was obtained between botrytis incidence among sulphur-treated fruit and incubation incidence at harvest (r2 = 0.81). This relationship suggests that in this vineyard each 10 % increase in disease incidence after incubation corresponds to an increase of 3 to 4 % incidence among sulphur-treated fruit after distribution (Figure 7). Interestingly in control fruit the correlation was only 0.2 after the distribution phase mainly due to a lower than expected botrytis incidence in fruit from plot 5, likely resulting from the fact that the 0.5 hour delayed cooling treatment was applied to fruit from this plot, which is likely to have suppressed botrytis development during storage and distribution relative to longer delayed cooling treatments.
Figure 7. Linear relationship between botrytis rot incidence after storage and distribution, and botrytis incidence at harvest after incubation at 18 °C for 4 days in sulphur-treated and control ‘Luisco’ grapes after cool storage at 2 °C for 25 days and a distribution phase of 7 days at 8 °C. Each point is the mean incidence of infected berries by weight among 10 cartons per plot.
The above results mostly verify outcomes from previous cool storage experiments conducted on ‘Crimson Seedless’ grapes where very good correlations of between 0.6 to 0.9 were found between botrytis incidence after storage and distribution, and initial disease incidence after incubation at harvest.
Effect of delayed cooling on grape quality after storage and marketing
Due to high variation in disease inoculum levels between plots in the vineyard, the effect of delayed cooling on disease development during storage and distribution was confounded with harvest rot risk as measured after fruit incubation. An Analysis of Covariance (CANOVA) indicated that incubation disease incidence had a partial influence on subsequent disease incidence after storage and distribution, but generally this covariate was not significant. Analysis of disease data after harvest incubation and after storage and distribution did demonstrate that that delays of 4 and 6 hours between harvest and commencement of cooling significantly increased total rot incidence compared to delays of 2 hours or less, particularly among control fruit (Figure 8, top).
Figure 8. Effect of delayed cooling after harvest for 0.5, 1, 2, 4 and 6 hour on total rot incidence in untreated (top) and sulphur-treated (bottom) ‘Luisco’ grapes. Rot incidence was measured after incubation of fruit prior to cooling ("Incubation at harvest") and after cool storage at 2 °C for 25 days and a distribution phase of 7 days at 8 °C ("After storage & distribution"). Different letters between delayed cooling treatments after incubation, and after storage and distribution, indicate a significant difference at P < 0.05.
These results suggest that reducing delays between harvest and cooling can suppress postharvest disease development even under conditions where fruit are determined to be at high risk of rot development during storage and distribution using the harvest incubation method. A similar pattern of harvest incubation rot incidence was observed among cooling delay treatments in fruit assigned to sulphur treatment (Figure 8, bottom). Although no significant difference in rot incidence was found between delayed cooling treatments after storage and distribution, excessive cooling delays of 4 or 6 hours did appear to marginally increase rot incidence, and again a shorter delay between harvest and commencement of cooling appeared to suppress disease incidence among high risk fruit. Furthermore a comparison of the change in total rot incidence between harvest incubation, and the end distribution after cool storage, among control grapes exposed to different cooling delays, indicates that disease incidence was reduced only among the 0.5 hour delay treatment (Table 1). This result is further evidence that minimizing the time delay between harvest and commencement of cooling can reduce the potential for rot development during cool storage, particularly among higher rot risk fruit.
Table 1. Effect of delayed cooling on the change in total rot incidence between harvest, and the end of distribution after cool storage and, among untreated ‘Luisco’ grapes.
|Delay||Increase or decrease in total rot incidence |
- Harvest rot risk (via fruit incubation) was well correlated to both latent Botrytis, and total disease incidence observed directly after cool storage, and at the end of a distribution phase after storage, suggesting that this protocol can be used to estimate likelihood of rot development during cool storage.
- Fruit sampling at the plot level for harvest rot risk assessment is likely to be the best approach when attempting to gauge subsequent postharvest rot risk potential.
- Reducing delays between harvest and commencement of cooling is likely to suppress postharvest disease development, particularly under conditions where fruit are determined to be at high risk of rot development using the harvest incubation method.
- In this trial the potential benefits of reducing delays between harvest and cooling were highly confounded with the rot risk potential of plots to which cooling delay treatments were applied.
- Vineyard factors including bunch disease latent infection and inoculum load at harvest greatly influence postharvest rot development even when using recommended and optimum cooling and sulphur treatment practices.
Crisosto C.H. J.L. Smilanick and N.K. Dokoozlian (2001). Table grapes suffer water loss, stem browning during cooling delays. California Agriculture, 55(1):39-42.
Fourie J.F. (2008). Harvesting, handling and storage of table grapes (with focus on pre- and post-harvest pathological aspects). Acta Horticulturae, 785:421-424. http://dx.doi.org/10.17660/ActaHortic.2008.785.54
Pereira E., R.G.B. de Silva, W.A. Spagnol and V.S. Junior (2018). Water loss in table grapes: model development and validation under dynamic storage conditions. Food Sci. Technol (Campinas), 38(3):473-479. https://doi.org/10.1590/1678-457X.08817
John Lopresti, Janine Jeager, Kristen Pitt & Glenn Hale, Agriculture Victoria
Department of Jobs, Precincts and Regions
1 Spring Street Melbourne Victoria 3000
Telephone (03) 9651 9999
© Copyright State of Victoria,
Department of Jobs, Precincts and Regions 2021
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This document is also available in an accessible format at economicdevelopment.vic.gov.au
The Serviced Supply Chains project is funded by the Hort Frontiers Asian Markets Fund, part of the Hort Frontiers strategic partnership initiative developed by Hort Innovation with co-investment from the Department of Agriculture and Fisheries, Queensland; Department of Jobs, Precincts and Regions (Victoria); Manbullo (mangoes); Montague Fresh (summerfruit); Glen Grove (citrus); and the Australian Government plus in-kind support from The University of Queensland and the Chinese Academy of Sciences.