Abstract

This season a range of controlled atmosphere (CA) conditions were applied to ‘Fantasia’ nectarines harvested at local and export market maturity standards. Ethylene (C₂H₄) production at harvest showed that both grades of fruit were starting the climacteric, but the local market fruit were further advanced. These fruit were coolstored under air or CA conditions for 4 or 6 weeks. After 4 weeks, fruit of both maturities stored in air lacked flavour, were softer and developed a mealy texture during shelf life compared to fruit stored under 2% oxygen (O₂) and either 5 or 10% carbon dioxide (CO₂) , or under 10, 15 or 20% CO₂. The CA fruit were juicy, and developed a good flavour. C₂H₄ production of air stored fruit during a shelf life period suggest the fruit were ripening abnormally. Fruit stored under CA retained the capability to produce C₂H₄ after coolstorage, although the more mature local market fruit were showing signs of senescence after 4 and 6 weeks storage. Previously unpublished work is presented that shows these disorders were accompanied by a reduction in activity of the enzymes endo and exo-polygalacturonase. Results suggest that for optimum CA storage nectarines should be placed into CA storage near of onset of the C₂H₄ climacteric.

1 - Introduction

In practice CA has been shown to have variable benefits to stonefruit depending on the variety and cultivars, the atmospheres used and local conditions (Smock, 1979). Research on peaches and nectarines in New Zealand and overseas suggests that elevated CO₂ levels are a critical component for delaying the onset of physiological storage disorders (Lurie, 1992; Streif et al., 1994). However, CO₂ levels above 10% have been shown to have variable effects on quality. Some researchers have found levels up to 20% to be beneficial; others have found such high levels to cause loss of flavour, off flavours and development of storage disorders. The variation in results may be attributed to fruit variety, growing conditions and maturity at harvest.

Controlled atmosphere storage has been shown to be useful in reducing the incidence disorders such as mealiness in New Zealand nectarines. However the quality of nectarines after CA storage can be highly variable, some fruit have been found to ripen abnormally, many lack flavour and develop flesh beakdown during shelf life. These problems may be related to fruit maturity at the onset of CA storage, and the maturity range which is optimum for CA storage may be quite narrow. This season a range of CA conditions were applied to ‘Fantasia’ nectarines of two maturity grades, local and export.

2 - Materials and Methods

Firmness was measured with an Effegi penetrometer using an 8mm head. After removing 2mm of skin from opposite sides of each fruit. Fruit were halved and assessed subjectivly for presence or absence of breakdown and mealiness. For each assessment three replicates of 5 fruit per treatment were examined. Results presented are means for each treatment. Severity of disorders are expressed as percent (%) incidence and quality and flavour were rated by an informal taste panel.

Pectolytic enzyme extraction: 50g of frozen of tissue was macerated in 50ml cold water with 5% PEG6000 and 0.2% potassium metabisulphate pH 5.0 and centrifuged (10,000g X 20 min.). The enzymes were extracted using 50ml 7.5% NaCl:Na₂EDTA (10:1) solution pH 6.5. Polygalacturonase (PG) activities were measured as in Gross, 1982. Endo PG activity was determined by incubating enzyme extract in 0.125% polygalacturonic acid in 50ml 0.2M sodium acetate pH 4.25. Exo PG activity was determined by adding 10mM CaCl₂ to the above PGA solution at pH 5.5.

3 - Results

At harvest, fruit held at 20°C ripened normally , were juicy and had good flavour. Results after 4 weeks cold storage and 3 days at 20°C are outlined in Table 1. Fruit of both maturities stored in air lacked flavour, were softer and developed a mealy texture during shelf life compared to fruit stored under 2% oxygen and either 5 or 10% CO₂ or 10% CO₂. The latter fruit were juicy and had good flavour although fruit in all treatments developed some brown rot. Fruit stored under 15 or 20% CO₂ developed symptoms of tissue breakdown.For all of the CA treatments, the export maturity fruit were of much better quality than the local grade being firmer and with less mealiness and breakdown.

After the 4 week storage plus a 5 day shelf life all air stored fruit were mealy and/or had breakdown (Table 2). The influence of maturity grade on the storage potential of ‘Fantasia’ in CA is evident. The export grade maturity with 2% O₂ plus either 5% CO₂:,or 10% CO₂, and 10% CO₂ in air demonstrating the best potential for maintainence of quality.

After 6 weeks storage plus a 3 day shelf life 100% of air stored fruit were of unacceptable quality. The export grade maturity with 2% O₂ : 5% CO₂, or 10% CO₂, and 10% CO₂ CA were still of good eating quality but some fruit did not ripen developing a rubbery texture. The fruit from 15% and 20% CO₂ treatments had skin browning, off flavours and did not ripen during their shelf life (data not shown).

Table 1: Quality of ‘Fantasia’ nectarines stored for 4 weeks at 0°C then 3 days in 20°C

Table 2: Quality of ‘Fantasia’ nectarines after 4 weeks storage in at 0°C in air or CA plus a 5 day shelf life in air at 20°C. Results are the mean of 3 reps of 5 fruit.

3.2 Effects of CA storage on C₂H₄ and CO₂ production

C₂H₄ and CO₂ production at harvest showed that both maturity grades of fruit were climacteric, but the local market fruit were further advanced (Fig. 1 and Fig. 2). After 4 weeks of storage, the fruit stored in air produced much less C₂H₄ during shelf life in comparison to the CA stored fruit (Fig. 3). C₂H₄ production after CA storage of the local grade fruit was much less than the export grade in most cases (Fig. 3). It seems fruit that maintained the ability to produce C₂H₄ ripened normally and were of the best quality.

CO₂ production at harvest was very similar in the export and local grade fruit peaking after 1 day at 20°C then remaining relatively constant (Fig. 2). CO₂ production rates were much lower after storage with the exception of the local grade air stored fruit (Fig. 4) which developed severe breakdown during shelf life (Table 1). Respiration rates were slightly lower for CA than air stored fruit (Fig. 4) and very similar for all the CAs and the different maturity grades ((Fig. 4) 10% 15% and 20% CO₂ treatments not shown)).

3.3 Effects of CA storage on Endo and Exo-polygalacturonase activity

Earlier unpublished work on the of ‘Fantasia’ nectarines showed that fruit harvested prior to the climacteric and coolstored at 0°C for more than four weeks had reduced C₂H₄ production and developed mealiness during shelf life (data not shown). In addition, there was reduction in the activity of the enzymes endo and exo-polygalacturonase by fruit held at 20°C after 5 weeks storage. Fruit held at 20°C after harvest developed relatively high levels of PG activity as fruit softened compared with fruit that had been stored for 5 weeks at 0°C (Fig. 5).

Figure 1: C2H4 production at harvest of ‘Fantasia’ nectarines harvested at local and export maturities. Each data point is the mean of 10 individual fruit.

Figure 2: CO2 production at harvest of ‘Fantasia’ nectarines harvested at local and export maturities. Each data point is the mean of 10 individual fruit.

4 - Discussion

Figure 3: C2H4 production after 4 weeks storage at 0°C ‘Fantasia’ nectarines harvested at local and export maturities. Each data point is the mean of 5 individual fruit.

Mealiness (wooliness) is a common problem in storage of nectarines. The effectiveness of CA to reduce the development of mealiness has had variable success (Lurie, 1992; Streif et al., 1994). Mealiness has been associated with the loss of fruits ability to ripen normally. Our work demonstrates that air storage results in the loss of C₂H₄ biosynthetic capacity and a loss in PG activity. This indicates that mealiness could be a result of ripening disfunction. Similar findings have been reported (Dawson et al., 1992). Preventing ripening disfunction may be the mechanism by which CA ameliorates mealiness in ‘Fantasia’.

Figure 4: CO2 production at 20°C after 4 weeks storage in air or CA at 0°C of ‘Fantasia’ nectarines harvested at local and export maturities. Each data point is the mean of 5 individual fruit.

Figure 5: Endo and exopolygalacturonase activity of ‘Fantasia’at 20°C after harvest and after 5 weeks air storage at 0°C Each data point is the mean of 5 individual fruit.

5 - References

Gross, K.C., A rapid and sensitive method for assaying polygalacturonase using 2-cyanoacetamide. Hortscience, 17:933-934.

Lurie, S. 1992. Controlled Atmosphere Storage to Decrease Physiological Disorders in Nectarines. Intern. J. Food Sci. and Tech. 27: 507-514.

Streif, J., Retamales, J., Cooper, T. 1994. Preventing Cold Storage Disorders in Nectarines. Acta Horticulturae, 368:163-166.

Smock, R.M. 1979. Controlled Atmosphere Storage of Fruits. In: Hort. Rev., 1:301-336.

Tonini, G., Brigatti, S., Caccioni, D. 1989. CA Storage of Nectarines: Influence of Cooling Delay, Ethylene Removal, Low O₂ and Hydorcooling on Rots, Overripening, Internal Breakdown and Taste of Fruits. Acta Horticultrurae, 254: 335-339.