This article explores the impact of carton liners and relative humidity in cold stores during accumulation for export on post-storage shrivel levels on Laetitia and African Delight® plums.

Abstract

Moisture loss and shrivel can be a problem with certain plum cultivars packed and stored for sea export, or long-term storage. Objectives of this study were to evaluate plastic packaging liner types and relative humidity levels in cold stores during forced-air cooling and accumulation of fruit prior to export, on moisture loss and shrivel for Laetitia and African Delight® plums. Plums were packed in different polyethylene plastic perforated and non-perforated wrapper and perforated bag treatments, and forced-air cooled and accumulated for seven days in cold rooms with and without humidity control. The impact of these treatments on fruit quality after application of simulated export shipping regimes and subsequent shelf life was determined. The results were quite variable over the three years of study, which is to be expected, since shrivel development is highly complex and seemingly influenced by multiple factors. As far as mass loss and shrivel control were concerned, humidity control in cold stores that delivered approximately 95% relative humidity during forced-air cooling and accumulation, helped maintain plum quality for 35 days’ cold storage. Hence, this is recommended. The impact of liner type on mass loss of plums appeared to be minimised with higher humidity in cold stores. It is suggested to concentrate on further evaluating the 36 x 4 mm perforated bags for plum cultivar prone to shrivel. In the case
of African Delight®, further research is required to compare 54 x 2 mm perforated bags to 36
x 4 mm perforated bags for efficacy to control shrivel. The risk of skin cracking on Laetitia plums stored at high relative humidity must be carefully monitored, as should be the development of internal browning as influenced by liners.

Introduction

Moisture loss and shrivel can be a problem with certain SA plum cultivars that are packed and stored for export by sea, or long-term storage. Plastic wrappers, currently used to reduce moisture loss from fruit, were commercialised many years ago. Handling and shipping systems, as well as cultivars and products to control decay, have changed over the years and there are frequent complaints about the occurrence of shrivel. Just 5% mass loss from perishable products may cause shrivel (Wills et al., 2007). According to them, produce characteristics influencing moisture loss include aspects such as surface area to volume ratio, with smaller fruit losing water faster. Plant surfaces also affect water loss, dependant on structure and composition of the waxy cuticle, surface cells and underlying tissues, as well as presence of hairs and lenticels on fruit surfaces. Increasing the relative humidity (RH) of the air around produce reduces vapour pressure difference between products and their surrounds, and hence, less moisture loss occurs. Air movement over produce has a desiccating effect, because it disturbs the boundary air layer. In this layer the vapour pressure is in equilibrium with that of the produce itself, so movement of water out of the produce is slow. With increased air movement, such as during forced-air cooling (FAC), the boundary layer decreases and the rate of moisture loss increases. The degree to which moisture loss is reduced by packaging, either by reduction of air movement or increase in RH, depends on the permeability of the package to water vapour transfer, as well as close-ness of containment. Polyethylene (PE) films are good vapour barriers. Wills et al., 2007 emphasises that an increase in RH is associated with increased risk of decay. In the context of long-term fruit storage, this risk must also be taken into consideration when seeking moisture loss solutions.

RESEARCH OBJECTIVES

Materials and methods

This research was conducted on Laetitia and African Delight® plums in 2017, 2018 and 2019. The fruit were sourced from commercial cold stores in Franschhoek, located in the Western Cape, SA. The plums intended for export were harvested by growers, accord-ing to the statutory maturity standards, using flesh firmness and total soluble solids (TSS) as primary indices. Fruit was warm-packed into the various packaging liner treatments on the day of harvest and subjected to FAC as per individual cold store practice. The five liner treatments tested on Laetitia were: no-liner, standard perforated plum wrapper (48 x 6 mm), non-perforated wrapper and perforated bags with 36 x 4 mm and 72 x 4 mm hole configurations. In the case of African Delight®, the four treatments applied were: no-liner and perforated bags with 18 x 4 mm, 36 x 4 mm and 72 x 4 mm configurations. The wrappers and bags were manufactured in high density polyethylene (HDPE) format with thickness of 20 and 15 microns, respectively. The plums were packed on pulp trays in standard double layered 5 kg D051 cartons with a 300 x 400 mm box footprint.
Split samples, each comprising five replicate cartons of all the packaging liner treatments were subjected to low and high RH cold stores immediately after packing. The fruit was held under these conditions for the first seven days of cold storage during which FAC was con-ducted, and a simulated accumulation period implemented. In total, the plums were cold stored for 35 days. The accumulation period simulated the “holding” stage of pallets of fruit prior to sea export. For each population of fruit tested, the same low and high humidity cold store was used for both FAC and accumulation. The “low” RH cold rooms had no humidity control equipment and an air temperature set point of -1.0°C.
The “high” RH cold rooms were fitted with humidifiers and also set at -1.0°C. After the seven-day accumulation, fruit was transferred to cold stores without specialised humidity control (± 92.5% RH) for the remainder of the cold storage period. A PD7 dual-temperature regime was used in the case of Laetitia (10 d @ -0.5°C, 7 d @ 7.5°C, 18 d @ -0.5°C) and a PD1 single-temperature regime (35 d @ -0.5°C) was used for African Delight®. A subsequent shelf life comprising five days at 10°C was applied for both cultivars.
Fruit maturity at packing was assessed by measuring flesh firmness (11.0 mm plunger – kg), TSS (%) and titratable malic acid (%). Fruit quality after cold storage, both at the start and end of shelf life, was ascertained by measuring mass loss (fruit mass at harvest com-pared to after storage), shrivel stretching over the shoulders of fruit, flesh firmness, decay and internal quality. The internal disorders gel breakdown [GB], internal browning [IB]) and over-ripeness [OR] were classified as defined by Taylor (1996). Five single carton replicates were used per treatment. The start and end of shelf life data were analysed separately, using Two-way ANOVA’s with Factor (A) liner packaging type and Factor (B) humidity control. Wrappers and/or bags were opened at the start of shelf life to simulate commercial practice. Half of the fruit in each carton were examined before shelf life and the remainder after shelf life.

Results and discussion

This research was conducted in commercial cold stores. In retrospect, it may have been better to do preliminary testing under laboratory conditions, to guarantee the desired experimental humidity and temperature conditions in cold stores during FAC and accumulation. In some instances, the target high RH conditions in cold stores fitted with humidifiers were compromised because warm fruit was frequently transferred into the room for purposes of FAC. This necessitated frequent opening of the cold store doors rendering it impossible to achieve high RH levels and compromising temperature control. In one instance the temperature was approximately 3°C higher than the -1°C set point.

While this research was conducted over three years, only the 2018 seasons data for Laetitia (Tables 1 and 2) and African Delight® plums (Tables 3 and 4) will be shown to illustrate the observed cold store RH and packaging liner impact on post-storage plum quality. The full research report is available from Hortgo Science (Viljoen, 2019).
At intake, the Laetitia plums exhibited a flesh firmness of 6.3 kg, TSS of 10.4% and a malic acid of 1.49% (data not shown). While the flesh firmness complied with the export standard, the TSS was below the 11% minimum. In the case of African Delight®, the plum maturity was within the optimum range, with a flesh firmness of
9.6 kg, TSS of 19.2% and malic acid content of 0.91% (data not shown).

After 35 days’ cold storage, at the start of shelf life, clear trends relating to mass loss which are indicative of moisture loss, were evident with Laetitia plums (Table 1). At 67.6 g, the mass loss from plums in the no-liner treatment was significantly higher where no humidity control was applied in the cold store (74.2% RH), com-pared to 39.5 g mass loss in the cold store with humidity control (94.9% RH). The liner packaging type had a more profound effect on mass loss when the plums were cooled and accumulated at the lower RH, in the cold store without humidity control, compared to the higher RH. At low humidity, the perforated bags limited moisture loss significantly better than wrappers, and both these liner formats gave better control of moisture loss than the no-liner treatment. In this comparison across treatments accumulated in the cold store without humidity control, differences in bag perforation configuration had no significant effect on mass loss. The liner format, namely: whether wrapper
or bag, was a more dominant influence. With humidity control in the cold store that result-ed in higher RH, the liner packaging type had less impact on moisture loss. This suggested that under conditions of high humidity in cold rooms (and probably shipping containers), the impact of liner type on moisture loss may be reduced. The non-pooled mass loss results showed a somewhat clearer picture of the influence of humidity control and liner packaging type, than the shrivel data that were pooled due to a non-significant interaction. With shrivel, the liner type had no significant impact on incidence. However, shrivel levels were significantly lower (13.6%) under conditions of high RH in the cold store during FAC and accumulation, compared to where the RH was lower (19.1% shrivel). Flesh firmness was significantly higher, albeit a negligible 0.3 kg, for fruit accumulated under higher humidity conditions. Small differences were also record-ed across liner treatments. IB was significantly higher, albeit at low levels, in the Laetitia plums packed without a liner. It is postulated that this may have been a consequence of fast-er cooling without the air barrier effect created by liners. It is well known that fast cooling of Laetitia may exacerbate internal fruit quality disorders (de Kock et al., 2006). This effect was lost over shelf life (Table 2). Decay and GB were recorded at relatively low levels, with no significant differences across treatments, while no OR was detected (Table 1).
After 35 days’ cold storage, plus five days’ shelf life, the mass loss from Laetitia plums packed in perforated bags was lower compared to the no-liner and wrapper treatments, irrespective of whether humidity control was applied or
not (Table 2). In most cases this difference was significant. Similar to the start of shelf life, best overall control of mass loss occurred for fruit packed in the 36 x 4 mm perforated bags. The shrivel levels across treatments were lower in the plums stored with humidity control (94.9% RH), compared to those accumulated without humid-ity control (74.2% RH). Once again, this showed the importance of maintaining a high RH during FAC and accumulation to limit shrivel in Laeti-tia plums after shipping to overseas markets. Overall, the best shrivel control was achieved for plums accumulated at high RH using 36
x 4 mm perforated bags. Flesh firmness was significantly higher for the plums stored under high RH conditions, compared to low RH, with no differences across liner types. Decay, GB and OR levels remained low across treatments at the end of shelf life. For some inexplicable reason, IB was higher in plums stored in the standard perforated wrapper (12.0%) and the 36 x 4 mm perforated bags (13.5%) compared to the other treatments. This aspect should be carefully monitored in semi-commercial trials.
At the end of 35 days’ cold storage, the mass loss from African Delight® plums packed in perforated bags was significantly less than with the no-liner treatment (Table 3). No significant differences in mass loss occurred across the different bag perforation treatments. However, there was an increase in moisture loss with an increase in the number of perforations per bag. Pooled data revealed that mass loss from plums held in the accumulation cold store without humidity control (89.9% RH), was significantly higher than where humidifiers were used (95.6% RH). Despite differences in mass loss across liner packaging type and humidity control treatments, shrivel levels across treatments were low (<9%) with no significant differences between treat-ments. Flesh firmness did not differ between treatments, and no decay, GB and IB were recorded at the start of shelf life.
After 35 days’ cold storage plus five days’ shelf life, the mass loss from African Delight® plums followed the same profile as at the start of shelf life. The mass loss from plums in the perforated bag treatments was significantly lower than with fruit packed with no liner (Table 4). Fruit stored under humidity control (95.6% RH) lost signif-icantly less moisture than those accumulated in a store without humidity control (89.5% RH). Shrivel was significantly lower for fruit packed in the 36 x 4 mm and 72 x 4 mm perforated bags and accumulated in the cold room with higher humidity, compared to all the other treatments, except fruit packed in 18 x 4 mm perforated bags and accumulated at the lower RH. One would expect that the plums packed in the 18 x 4 mm perforated bags and accumulated in the cold store with humidity control would exhibit the lowest shrivel due to less perforations and higher RH, but this was not the case. The reason for this is unclear. Commercially, 54 x 2 mm perforated 20 micron LDPE bags with a total vent area of 170 mm2 per bag (5 kg carton) are sometimes used for African Delight®. Further research is required to compare efficacy of this bag type to the 36 x 4 mm bags tested in this study, which have a vent area of 452 mm2. Flesh firmness did not differ significantly between treatments, and no decay, GB, IB and OR were detected.
While decay development is known to be a higher risk on stored fruit at high humidity
(>95%), decay levels on plums in this study were low. It is presumed that this is due to good post-harvest decay control that is achieved through use of effective products and applications. Consequently, decay control efficacy must be taken into consideration when any changes are made to fruit packaging liners or RH levels in the air, surrounding the produce.

Conclusions

For Laetitia plums, as far as mass loss and shrivel control were concerned, 95% RH during FAC and accumulation at the start of storage helped optimise plum quality after 35 days’ cold storage. Regarding optimal liner packaging to control shrivel specifically, the variable results recorded over the three years of study, made it difficult to identify the single best option. The impact of liner type packaging on mass loss appeared to be minimised at higher humidity in cold stores. It is suggested to expand commercial testing of 36 x 4 mm perforated bags, used in combination with good humidity control during FAC and accumulation. The risk of skin cracking on Laetitia plums stored at high RH must be carefully monitored, and so should development of internal browning as influenced by liners.
In the case of African Delight® plums, the 36 x 4 mm perforated bags showed promise in combination with good humidity control during FAC and accumulation. Further research is required to compare the efficacy of 36 x 4 mm perforated bags to 54 x 2 mm perforated bags.
Overall, it is likely that different plum cultivars react differently to moisture loss prevention technologies such as humidity control in cold stores and application of liners. Hence, cultivar-specific testing is recommended. This is especially important from an environmental point of view, where plastic – even if it is recycled – should be avoided, unless absolutely necessary for fruit quality maintenance.

Acknowledgements

Thanks to the SA Stone Fruit Producers Association (SASPA) for financing this research, HORTGO for placing the research at ExperiCo, and Dr Martin Taylor for assistance with the preparation of this publication.

References

DE KOCK, V.A., KHUMALO, P., DAVIDS, J. AND JOOSTE, M. 2006. Forced-air cooling recommendations for plums. SA Fruit Journal 5(3): 17.

TAYLOR, M.A. 1996. Internal disorders in South African plums: A research review, revision of internal disorder definitions and advice on how to prevent internal disorders in export plums. Deciduous Fruit Grower 46(9): 328-335.

VILJOEN, H. 2019. Final Report – Identify and test improved packaging systems as well as humidity control in cold stores to reduce moisture loss and shrivel in plums. Hort-gro PL 03-17.

WILLS, R.B.H., MCGLASSON, W.B., GRAHAM, D. AND JOYCE, D.C. 2007. Water Loss and Humidity. In: Postharvest – An introduction to the physiology and handling of fruit, vegetables and ornamentals (5th Edition), CABI, UK, pp.67 – 82.