August Red nectarines

Packaging and humidity control impact on shrivel

Moisture loss and shrivel can be a problem with certain nectarine cultivars packed and stored for sea export or long-term storage. The objectives of this study were to evaluate plastic packaging liner types and relative humidity (RH) levels in cold stores during forced-air cooling (FAC) and accumulation of fruit prior to export, on moisture loss and shrivel for August Red nectarines.

Fruit was packed on pulp trays in standard single layer cartons with a 300 x 400 mm box footprint. The carton liners tested were high density polyethylene perforated and non-perforated wrappers and perforated bags. Fruit was subjected to FAC and accumulated for seven days in cold rooms with and without humidity control. Thereafter, fruit was transferred to a single cold store for an additional 28 days at -0.5 °C.

The impact of these treatments on fruit quality after application of simulated export shipping regimes was determined. It was clear that carton packaging liners are required to control mass loss and shrivel on August Red. The 36 x 4 mm and 72 x 4 mm perforated bag liners gave best results and warrant further testing. In similar research on plums, the 36 x 4 mm perforated bags also gave good control of mass loss and shrivel (Viljoen, 2019b). Hence, it may be beneficial to test this configuration for use across stone fruit kinds and cultivars that require shrivel control. After 35 days’ cold storage, less shrivel occurred on fruit subjected to higher RH (> 95%) in the cold rooms during FAC and accumulation. This effect was lost over shelf life. Cold store RH levels varying between 77% and 89% during FAC and accumulation had little impact on mass loss and shrivel. It is likely that a higher humidity is required to achieve the desired effects.

The findings of this research supports the best practices recommendation to maintain an RH of 95% during cold storage of fruit. It is anticipated that the risk of decay may increase with use of perforated bags that are less vented than perforated wrappers. Hence, it is cautioned that special care be taken in cases of high decay potential, and/or if sub-standard handling and storage conditions are likely. In such instances it may be safer to use the standard perforated wrappers commonly referred to as shrivel sheets.

Introduction

As with plums (Viljoen 2019b), moisture loss and shrivel can be a problem with certain SA nectarine cultivars that are packed and stored for export by sea or long-term storage. Plastic wrappers – commonly referred to as shrivel sheets – that are used to reduce moisture loss from fruit during cold storage, were commercialised many years ago. Handling systems, cultivars and means to control decay have changed over the years and there are frequent complaints regarding the occurrence of shrivel. Just 5% mass loss from perishable products may cause shrivel (Wills et al., 2007). Produce characteristics influencing moisture loss include aspects such as surface area to volume ratio, with smaller fruit losing water faster.

Plant surfaces also affects water loss dependant on structure and composition of the waxy cuticle, surface cells and underlying tissues, as well as hairs and lenticels on fruit surfaces. Increasing the RH of the air around produce reduces vapour pressure difference between products and their surrounds and hence, less moisture loss occurs. Air movement over pro-duce 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 FAC, the boundary layer is reduced and the rate of moisture loss increases. It remains important to cool and cold store fruit effectively, since moisture loss is higher from fruit held at higher temperatures due to conductance differences from the inside to the surface, as well as higher rates of respiration (Chigwaya, 2016). The extent to which moisture loss is reduced by packaging as a consequence of reduction of air movement or increase in RH, depends on the permeability of the package to water vapour transfer, as well as closeness 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

1. Evaluate different plastic packaging liner types for efficacy to control moisture loss and shrivel on nectarines, without causing other negative fruit quality effects.
2. Determine the effect of RH in cold stores used for FAC and accumulation of pallets prior to loading into shipping containers for export, on moisture loss and shrivel on nectarines

Materials and methods

Materials and methods August Red nectarines sourced from commercial cold stores in the Ceres area of the Western Cape in SA were used for this study conducted in 2017, 2018 and 2019.

The nectarines were harvested according to export standards, using flesh firmness and total soluble solids (TSS) as primary indices. Warm fruit was packed into the various packaging liner treatments on the day of harvest. The five liner treatments tested were: no liner, standard perforated nectarine wrapper (48 x 6 mm), non-perforated wrapper, and respective perforated bags (36 x 4 mm) and (72 x 4 mm). Wrappers and bags were manufactured in high density polyethylene (HDPE) with thickness of 20 and 15 microns, respectively. The fruit was packed on pulp trays in standard single layer 2.5 kg cartons with a 300 x 400 mm box footprint.

On the same day as packing, split samples comprising 10 cartons each per packaging liner treatment, were placed in low and high RH cold stores and subjected to FAC as per commercial cold store practice. After FAC, the fruit was held under these cold store conditions for seven days to simulate the accumulation period typically involved prior to loading shipping containers for export by sea. In total, the nectarines were cold stored for 35 days at -0.5°C.

Thereafter, a five-day shelf life at 10°C was employed. The “low” RH cold stores had no humidity control equipment and an air temperature set point of  -1.0°C. The “high” RH cold stores were fitted with humidifiers and also set at -1.0°C. Relative humidity was recorded in the standard and the humidity controlled cold stores using LOGTAG. After the seven days’ accumulation, fruit from the low and high humidity accumulation cold stores were transferred to a holding store with 92% RH and air temperature of -0.5°C, for the remainder of the cold storage period.

Fruit maturity at packing was assessed in terms of flesh firmness (8.0 and 11.0 mm plungers – 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 of 20 fruit at harvest compared to after storage), shrivel stretching over the shoulders of fruit, flesh firmness (11.0 mm plunger only) and decay. Internal quality was assessed in terms of overripeness (OR), internal browning (IB), pulpiness, and woolliness. Five single carton replicates were used per treatment at the start and end of shelf life examinations. The start and end of shelf life data were analysed separately, using Two-way ANOVAs with Factor (A) liner packaging type and Factor (B) humidity control. Wrappers and bags were opened at the start of shelf life to simulate commercial practice.

Table 1: 2018 Season – Quality of August Red nectarines packed with different liner treatments that were cooled and held without (N) and with (Y) relative humidity control (82.6% vs 95.6%) in cold rooms during seven days’ accumulation at the start of storage, as determined after 35 days’ cold storage at – 0.5°C

1.  Mass loss calculated for 20 fruit and IB = Internal browning and OR = Overripeness
2. Values in same row followed by different letters indicate significant differences (P<0.05) according to the LSD test
3. Humidity control for 7 days after packing, simulating accumulation period: N = average of 82.6% RH and +0.5°C, Y = average of 95.6% RH and -0.1°C

Table 2: 2018 Season – Quality of August Red nectarines packed with different liner treatments that were cooled and held without (N) and with (Y) relative humidity control (82.6% vs 95.6%) in cold rooms during seven days’ accumulation at the start of storage, as determined after 35 days’ cold storage at -0.5°C, plus shelf life of five days at 10°C

1. Mass loss calculated for 20 fruit and IB = Internal browning and OR = Overripeness
2. Values in same row followed by different letters indicate significant differences (P<0.05) according to the LSD test
3. Humidity control for 7 days after packing, simulating accumulation period: N = average of 82.6% RH and +0.5 °C, Y = average of 95.6% RH and -0.1°C

Results and discussion

Only results for the 2018 and 2019 seasons will be shown because the target humidity differences in cold stores were not achieved in 2017 (Viljoen 2019a).

2018 Season: The flesh firmness of the nectarines at intake was 12.2 kg and 8.8 kg measured with penetrometers fitted with 11.0 and 8.0 mm plungers, respectively (data not shown).

The TSS was 17% and the malic acid concentration was 1.06%. This indicated that the August Red nectarines had been har-vested within the maturity standards for export, which stipulates that if fruit is fully developed and mature with a minimum average TSS of more than 12%, an average maximum pressure of 13.6 kg as determined with an 11 mm plunger is permitted.

Humidity conditions for the seven days simulating the accumulation period were ideal in the cold stores. The cold store without humidity control had an average RH of 82.6% and an average air temperature of 0.5°C, while the cold store with humidity control had an average RH of 95.6% and an air temperature of -0.1°C. After 35 days’ cold storage, at the start of shelf life (Table 1), no significant interactions occurred between the factors, liner packaging type and humidity control. Hence, data were pooled across factors.

High mass loss, indicative of high moisture loss, occurred with nectarines cold stored with no liners (approximately 5.4 g/fruit). All liner types significantly reduced mass loss, but the perforated bags performed significantly better than the perforated and non-perforated wrappers. Comparing the 36 x 4 mm to the 72 x 4 mm perforated bags, there was no significant difference in mass loss. It was clear that liners are imperative to control mass loss and shrivel on August Red nectarines exported by sea.

Despite the large difference in RH between the cold stores with humidity control (95.6% RH) and with-out humidity control (82.6% RH) during FAC and accumulation, no significant difference in mass loss was evident due to this. Shrivel, the primary parameter of interest, was reduced by application of liners, irrespective of type.

In some cases this reduction in shrivel was significant relative to the no liner treatment. Shrivel levels at 6.1% were significantly lower for fruit accumulated in the cold store with high RH, compared to 10.4% shrivel for fruit held in a cold store with the lower RH. No decay or internal disorders occurred and flesh firmness differences across treatments were not significant. After a five-day shelf life (Table 2), results again revealed that all the liners tested gave significant control of mass loss, compared to the no liner treatment. Again, the perforated bags gave better control of mass loss than the wrappers. The 36 x 4 mm perforation configuration proved most effective.

Despite the 12% difference in RH between the cold stores with and without humidity control, no difference in mass loss was evident as a consequence of humidity levels in the cold stores during FAC and accumulation. The shrivel levels were generally low and no differences occurred due to application of liners and/or humidity control in cold stores. No significant differences in decay, internal disorders and flesh firmness occurred across treatments. In summary, it was clear that liners are required to control mass loss and shrivel on August Red nectarines. All fruit quality aspects considered, but in particular the control of mass loss and shrivel, the 36 x 4 mm and 72 x 4 mm perforated bag liners gave best results after 35 days’ cold storage, at the start and end of shelf life. At the start of shelf life, less shrivel occurred in fruit subjected to higher RH in the cold rooms during FAC and ac-cumulation, but this effect was lost over shelf life. It should also be noted that the higher RH room operated at a lower temperature than the lower RH room, and this also could have had an effect on shrivel incidence.

2019 Season: At intake, the August Red nectarines exhibited flesh firmness of 10.2 kg (11.0 mm plunger), 6.2 kg (8.0 mm plunger), TSS 12.2% and malic acid of 0.82% (data not shown), thus complying with export standards.

During FAC and the seven days’ accumulation, the cold store without humidity control had an average RH of 77.0% and an average air temper-ature of -0.3 °C, while the cold store with humid-ity control had 12% higher RH (88.9%) and an average air temperature of -0.5°C. The high RH was lower than the desired target of 95%, but the absolute difference relative to the lower RH was quite substantial.

As was the case in 2018, the 2019 mass loss re-sults for August Red nectarines at the start of shelf life (Table 3) showed that liners were essential to limit moisture loss. Once again, the perforated bags proved more effective than wrappers, with no significant difference between the two bag treatments. Mass loss was not influenced by the different humidity levels in the cold stores with and without humidity control, with RH of 88.9% and 77.0%, respectively. Similarly, at these humid-ity levels, shrivel was not affected by the RH in the cold stores. However, liners irrespective of type, were essential to control shrivel on August Red nectarines. No significant differences in decay, internal disorders or flesh firmness occurred across treatments.

Similar to previous results, liners per se gave significant control of mass loss and shrivel, with the best results achieved using the 36 x 4 mm and 72 x 4 mm perforated bags (Table 4). With accumulation of nectarines in the cold store with no humidity control and consequently with a lower RH, the perforated bags gave significantly better control of mass loss and shrivel than the standard perforated wrapper. However, at high RH, all liner types exhibited a more consistent performance across treatments.

This suggested that the type of liner is more critical if the RH in cold stores is too low. It also makes a case to seriously test the perforated bag options more comprehensively in the commercial environment. This is critical be-cause there may be a risk of increased decay, and possibly of more rapid fruit ripening (2017 results – data not shown).

Non-pooled data showed somewhat inconsistent trends regarding the impact of humidity control and resultant RH in cold stores during accumulation on mass loss and shrivel (Table 4). In addition, with nectarines packed without liners, no differences in mass loss or shrivel occurred between fruit stored at low (77.0% RH) versus high humidity (88.9% RH) levels.

As previously stated, based on the start of shelf life results (Table 3), it can be concluded that humidity levels in the range of 77 to 89% had little impact on mass loss and shrivel of August Red nectarines. However, at the end of shelf life (Table 4), IB and pulpiness were significantly less in fruit accumulated at the higher RH. This indicated that for optimisation of fruit quality, it is best to adhere to the best practices recommendation to maintain an RH of 95 % (Wills et al., 2007).

No significant differences across treatments occurred for decay, flesh firmness, IB pulpiness or OR. In summary, liners irrespective of type, were essential to control shrivel on August Red nectarines. As was the case in 2018, the 36 x 4 mm and 72 x 4 mm perforated bag liners gave best results after 35 days’ cold storage at the start and end of shelf life. Cold store RH levels during FAC and accumulation, in the range of 77% to 89%, had no impact on mass loss and shrivel at the start of shelf life, and an inconsistent effect at the end of shelf life.

It is likely that a higher RH is required to achieve the desired effects. The type of liner employed had a greater impact on shrivel control when fruit was cooled and accumulated in cold stores at low RH.

Table 3: 2019 Season – Quality of August Red nectarines packed with different liner treatments that were cooled and held without (N) and with (Y) relative humidity control (77.0% vs 88.9%) in cold rooms during seven days’ accumulation at the start of storage, as determined after 35 days’ cold storage at -0.5°C

1.  Mass loss calculated for 20 fruit and IB = Internal browning and OR = Overripeness
2. Values in same row followed by different letters indicate significant differences (P<0.05 indicated in red) according to the LSD test
3. Humidity control for 7 days after packing, simulating accumulation period: N = average of 77.0% RH and -0.3°C, Y = average of 88.9% RH and -0.5°C

Table 4: 2019 Season – Quality of August Red nectarines packed with different liner treatments that were cooled and held without (N) and with (Y) relative humidity control (77.0% vs 88.9%) in cold rooms during seven days’ accumulation at the start of storage, as determined after 35 days’ cold storage at -0.5°C, plus shelf life of 5 days at 10°C

1.  Mass loss calculated for 20 fruit and IB = Internal browning and OR = Overripeness
2. Values in same row followed by different letters indicate significant differences (P<0.05 indicated in red) according to the LSD test
3.  Humidity control for 7 days after packing, simulating accumulation period: N = average of 77.0% RH and -0.3°C, Y = average of 88.9% RH and -0.5°C

Conclusions

It was clear that carton packaging liners are required to control mass loss and shrivel on August Red nectarines packed on pulp trays in standard single layer cartons, with a 300 x 400 mm box footprint and subjected to 35 days’ cold storage. All fruit quality aspects considered, but in particular the control of mass loss and shrivel, 36 x 4 mm and 72 x 4 mm perforated bag liners gave the best results and warrant further testing. In similar research on plums, the 36 x 4 mm perforated bags also gave good control of mass loss and shrivel on plums (Viljoen, 2019b). Hence, it may be beneficial to test this configuration for use across stone fruit kinds and cultivars that require shrivel control. After 35 days’ cold storage at the start of shelf life, less shrivel occurred in fruit subjected to higher RH (> 95%) in the cold rooms during FAC and accumulation. This effect was lost over shelf life. Cold store RH levels in the range of 77% to 89% during FAC and accumulation had little impact on mass loss and shrivel at the start of shelf life and inconsistent effects at the end of shelf life.
It is likely that a higher RH than 89% is required to achieve the desired effects. The type of liner employed had a greater impact on shrivel control when fruit was cooled and accumulated in cold stores at low RH. The findings of this research sup-port the best practices recommendation to maintain an RH of 95% during cold storage of fruit. In general, when using perforated bags to con-trol shrivel on fruit, it is important to close bags properly, to ensure that the correct perforation configuration is used and that all the holes are open. The 2017 to 2019 research on plums and nectarines has shown that the 36 x 4 mm and 72 x 4 mm HDPE perforated bags controlled mass loss and shrivel more effectively than perforated wrappers (shrivel sheets) or non-perforated wrappers. It is anticipated that the risk of decay may increase with use of the less vented perforated bags due to the higher humidity which will develop in the air space surrounding the fruit. While not evident from this research, it is cautioned that special care be taken in cases of high decay potential and/or if sub-standard handling and storage conditions are likely. In such instances it may be safer to use the standard shrivel sheets.

References

CHIGWAYA, K. 2016. Moisture loss studies in nectarines. Thesis – Master of Agriculture (Horticultural Science) at Stellen-bosch University.

VILJOEN, H. 2019A. Final Report – Identify and test improved packaging systems, as well as humidity control in cold stores to reduce moisture loss and shrivel in nectarines. Hortgro N 01-17.

VILJOEN, H.W. 2019B. – Impact of carton liners and relative humidity in cold stores on post-storage shrivel levels on Laetitia and African Delight® plums. Submitted to SA Fruit Journal in November 2019.

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

Acknowledgements

Thanks to Hortgro Stone for financing this research, HORTGRO for placing the research at ExperiCo and Dr Martin Taylor for assistance with the preparation of this publication.