Comparison of standard 11-mm penetrometer plungers against 8-mm plungers to establish the optimum harvest maturity of late season nectarines using flesh firmness as indices

V. A. DE KOCK

ExperiCo
(Agri Research Solutions), Stellenbosch, South Africa
E-mail: info@experico.co.za

Abstract

The primary aim of this study was to establish if 8-mm diameter penetrometer plungers can provide a versatile alternative to standard 11-mm plungers, to accurately measure the flesh firmness of nectarines intended for sea export, as well as to identify changes to improve existing maturity standards where appropriate. Testing was done over two seasons on Alpine, Summer Fire, August Red and September Bright nectarines, while Regal Pearl, Sunburst, Summer Sweet and Top Pearl were evaluated over one season. The early season cultivars – Alpine and Sunburst – were included as references. Each population of nectarines was harvested at three maturities, approximately one week apart, commencing one week before the start of the commercial optimum maturity window. Harvest maturity was determined with a focus on comparing efficacy of flesh firmness measured, using penetrometers fitted with standard 11-mm diameter plungers against 8-mm plungers. The quality of fruit sampled at each harvest was evaluated after four weeks at -0.5°C, followed by a shelf life of 5 days at 10°C. This study emphasised the importance of flesh firmness as maturity indices for nectarines.
The 11-mm and 8-mm plungers both indicated separation of maturities across harvest dates. The benefit of the 8-mm plunger was that it was suitable for use on firmer nectarines that typically crack with the thicker plunger.
This can be of great value for some new late-season nectarine cultivars, where it seems that harvest may need to commence with the maximum standards higher than the 11.3 kg flesh firmness traditionally measured with the 11-mm plunger.
Use of the 8-mm plunger is also likely to simplify the quality control process since it’s widely used by international clients. A division factor of 1.8 can be used to convert existing standards from 11-mm to 8-mm.
Maturity standard changes are suggested for Summer Fire and August Red, namely: flesh firm-ness between 8.0 and 4.5 kg with an 8-mm plunger (13.6 to 8.0 kg with an 11-mm plunger), irrespective of total soluble solids levels. This implies an earlier optimum harvest window.
Standards for Alpine and September Bright remain unchanged. Additional testing to verify the optimum harvest maturity windows is recommended for Regal Pearl, Sunburst, Summersweet and Top Pearl, as well as all other new cultivars.
Based on this research, Hortgro has recommended to the Department of Forestry and Fisheries, that a Regulation change be made to specify that the 8-mm plunger be used to determine the flesh firmness at harvest for certain nectarine cultivars. Consequently there is an industry wide dispensation to use the 8-mm plunger on the cultivars listed in Table 1. All the existing riders remain applicable.

Table 1. Cultivars for which the 8mm plungers will be used instead of the 11mm plunger.

Introduction

Export volumes of late season nectarines are increasing in South Africa; and rejections at inspection as a consequence of excessive flesh firmness are common. This is despite other harvest maturity indices such as total soluble solids, malic acid, skin colour and fruit size indicating that the nectarines are suitable for picking and sea export. Delayed harvesting is often not a satisfactory solution to achieve the lower levels of flesh firmness required, as a means to comply with the maximum permissible standard to commence harvest. This is because the variability in maturity between individual fruits increases as the nectarines soften. Consequently pack-outs are also likely to be lower due to over-maturity of some of the fruit, which if packed for export, are prone to a myriad other post-storage quality problems. In South Africa, the flesh firmness of nectarines is currently determined by using penetrometers fitted with 11-mm diameter plungers, which may be inaccurate, since the upper readings can be out of range. There is also a tendency for very firm nectarines to split when trying to penetrate the fruit using the 11-mm plunger, hence making it impossible to accurately measure this important maturity indexing parameter. The primary aim of this study was to establish if 8-mm diameter penetrometer plungers can provide a versatile alternative to standard 11-mm plungers, to accurately measure the flesh firmness of nectarines intended for sea export, and where appropriate, identify changes to improve existing maturity standards. Use of 8-mm plungers will make it easier for importers to relate flesh firmness readings at harvest to that on arrival in the market, since most receivers use the thinner plunger. If successful, it is anticipated that more accurate maximum and minimum flesh firmness standards can be set to specify the optimum harvest maturity windows for nectarines in future, in particular late season cultivars.

Materials and methods

Research was conducted over a three year period on eight cultivars of nectarines sourced from commercial farms located in Ceres, Paarl and Franschhoek. While the focus of this research was on late nectarines, the early cultivars (Alpine and Sunburst) were included for reference purposes. For quality assessment after cold storage, the fruit were packed into standard 2.5 kg cartons and enclosed in perforated nectarine wrappers on the day of harvest. In the first year of study, for each maturity sample, half the cartons were subjected to pre-ripening and the remaining fruit not. This was to test the ability of this technique, to control pulpiness and woolliness. The pre-ripening entailed holding the fruit at 20°C and relative humidity of 75%, until ripened to a flesh firmness of 8 kg and 6 kg (11-mm plunger) for white and yellow fleshed nectarines, respectively. Fruit was cold stored for 4 weeks at -0.5°C followed by a shelf life of 5 days at 10°C, at which time quality examinations were conducted using five single carton replicates comprising 10 fruit each, per maturity treatment. The perforated wrappers were opened at the start of shelf life to simulate commercial practice. The fruit quality parameters measured were: soft tips decay, pulpiness, woolliness, overripeness and shrivel.

Standard deviations were conducted on flesh firm-ness readings taken at harvest. While the research reports generated over the three years of research contain statistically evaluated data (Hortgro N01-16), as well as a regression analysis for the two penetrometer plungers tested, it was necessary to limit the amount of information shown in this communication. Consequently, the fruit quality results are presented in a format designed to show tendencies rather than absolute values. The assessments were made on the basis that 0-10% disorder levels were classified as acceptable (signified √ in the tables), while disorders in excess of 10% were considered problematic (signified x in the tables). The exception was decay, where the acceptability threshold was set at 2%. For the purposes of cross referencing, the current harvest maturity standards set by the Department of Forestry and Fisheries (DAFF) for the cultivars tested over two seasons are noted in Table 2.

Table 2: DAFF export standards for Alpine, Summer Fire, August Red and September Bright nectarines, using flesh firmness values specified for penetrometers with 11-mm plungers.

Table 3:

Harvest maturities and post-cold storage quality assessments of Harvest maturities and post-cold storage quality assessments of Alpine Summer Fire nectarines harvested at three nectarines harvested at three stages in each of the 2016 and 2017 seasons with focus on flesh firmness determined using penetrometers with standard 11-mm and alternative 8-mm diameter plungers

¹. Standard deviations are shown in parenthesis.
². Post-storage quality assessment showing trends observed after four weeks’ cold storage at -0.5°C, followed by five days’ shelf life at 10°C. (√) signifies 0-10% disorders recorded, while (x) signifies levels higher than 10%, except for decay where a 2% threshold was applied.

Table 4:

Harvest maturities and post-cold storage quality assessments of Summer Fire nectarines harvested at three stages in each of the 2016 and 2017 seasons with focus on flesh firmness determined using penetrometers with standard 11-mm and alternative 8-mm diameter plungers.

¹. Standard deviations are shown in parenthesis.
². Post-storage quality assessment showing trends observed after four weeks’ cold storage at -0.5°C, followed by five days’ shelf life at 10°C.(√) signifies 0-10% disorders recorded, while (x) signifies levels higher than 10%, except for decay where a 2% threshold was applied.

Results

Results are presented for Alpine, Summer Fire, August Red and September Bright, which were tested over two seasons each. Regal Pearl, Sunburst, Summer Sweet and Top Pearl were only evaluated over one season, so it’s premature to draw definitive conclusions for these cultivars at this stage.

Discussion

The discussion concentrates on two focus areas: the measurement of flesh firmness as maturity indices for nectarines using penetrometers fitted with standard 11-mm diameter plungers, compared to the possible 8-mm plunger alternative; and fruit quality aspects relating to harvest maturity, with suggestions for changes to the current optimum maturity standards, where appropriate.

Flesh firmness as maturity indicator and comparison of plunger diameter

This study revealed that for the eight populations (4 cultivars over two years each) of nectarines studied (Tables 3, 4, 5 and 6), the flesh firmness values measured using both the 11-mm and 8-mm plungers, showed declining trends over harvest time in seven populations (87%). The only exception was Summer Fire sampled in 2016 (Table 4). These trends in flesh firmness were expected, since fruit will mature on the tree with delayed harvest. In the case of TSS, only four of the eight populations of fruit represented in Tables 3 to 6 showed the expected consistent increasing trend over harvest time (50%). With malic acid, the trends were even less consistent, with only one in eight populations showing the expected decrease over harvest time (12%). This information re-emphasised the importance of flesh firmness as a maturity indexing parameter for nectarines. Flesh firmness measured using penetrometers, whether hand held or with automated fruit texture analysers, also complies with most of the pre-requisites specified for maturity indices by Kader (2002). These include factors such as: the equipment being simple and easy to use; equipment being relatively inexpensive; the provision of objective readings; and the imperative that clear changes over harvest days are shown distinctly and that accuracy has been adequately proven. Having reconfirmed the importance of flesh firm-ness as maturity indices for late season nectarines, the question was: what is the most suitable plunger diameter? The results in this research clearly showed that both the 11-mm and the 8-mm plunger gave the same trends showing separation of maturities across harvest dates. It is clear from the data contained in Tables 3, 4, 5 and 6 that the 8-mm plunger consistently gave readings with lower standard deviations than the 11-mm plunger, thereby indicating less variation in the absolute values. However, when the standard deviations were interpreted as percentages of the average flesh firmness values, the variations were similar. These findings suggest that the efficacy of penetrometers with 8-mm plungers is similar to the 11-mm plungers that have been used with great success for “traditional” nectarine cultivars in the past. The major benefit of the 8-mm plunger is that it is suitable for use on firmer nectarines that typically crack when using the 11-mm plunger.
It is felt that this can be of great value for use on certain new late season nectarine cultivars, where it seems that harvest may need to commence with the maximum standards considerably higher than the 11.3 kg traditionally measured with the 11-mm plunger. Use of the 8-mm plunger is also likely to simplify the quality control process, since it’s widely used by international clients. If a decision is made to change to the 8-mm plunger for nectarine maturity indexing, a division factor of 1.8 can be used to convert maturity flesh firmness standards from 11-mm to 8-mm (vice versa also applies). When rounded off, it is recommended that the current 11.3 kg maximum and 4.5 kg minimum flesh firmness with the 11-mm plunger be converted to 6.5 kg and 3.0 kg with the 8-mm plunger (the latter slightly higher than a 1.8 conversion to lower quality risks). In cases where a flesh firmness of
13.6 kg with the 11-mm plunger is used for cultivars with total TSS higher than 12%, the value equates to 8.0 kg with the 8-mm plunger. Irrespective of plunger diameter or maturity indices employed, it remains vitally important to methodically set the appropriate optimum maturity windows of new cultivars to achieve good post-storage fruit quality. Lastly, as is the standard commercial procedure, all changes relating to maturity standards must be done via DAFF as the statutory authority.

Table 5:

Harvest maturities and post-cold storage quality assessments of August Red nectarines harvested at three stages in each of the 2016 and 2017 seasons with focus on flesh firmness, determined using penetrometers with standard 11-mm and alternative 8-mm diameter plungers.

¹. Standard deviations are shown in parenthesis.
². Post-storage quality assessment showing trends observed after four weeks’ cold storage at -0.5°C, followed by five days’ shelf life at 10°C.(√) signifies 0-10% disorders recorded, while (x) signifies levels higher than 10%, except for decay where a 2% threshold was applied.

Table 6:

Harvest maturities and post-cold storage quality assessments of September Bright nectarines harvested at different maturities in the 2016 and 2017 seasons with focus on flesh firmness determined using penetrometers with standard 11-mm and alternative 8-mm diameter plungers.

¹. Standard deviations are shown in parenthesis.
². Post-storage quality assessment showing trends observed after four weeks’ cold storage at -0.5°C, followed by five days’ shelf life at 10°C.(√) signifies 0-10% disorders recorded, while (x) signifies levels higher than 10%, except for decay where a 2% threshold was applied.

ALPINE (Table 3):

Overall, the quality assessment after cold storage and subsequent shelf life was good for fruit tested in 2016 and 2017. There was a risk for decay in the more mature fruit sampled at Harvest 3 (H3) in both years. It is common knowledge that more mature fruit presents a higher decay risk, due to multiple physiological and pathological factors.
In 2017, pulpiness at a level of 22% (data not shown) occurred in the Harvest 1 (H1) fruit harvest-ed at 12.0 kg (11-mm plunger), which is higher than the maximum standard set for the optimum window. This suggested that the current optimum harvest maturity standard is correct for Alpine and that it is best to harvest using the maximum of 11.3 kg measured with the 11-mm plunger (6.5 kg with 8-mm plunger). Pre-ripening was conducted for Alpine in 2016, but caused soft tips and over-ripeness. Hence, pre-ripening is not recommended.

A condensed summary of the optimum flesh firmness harvest maturity suggestions is presented below: ALPINE

(Unchanged maturity standard)

Note: It is less risky to harvest at the less mature side of the optimum window.

SUMMER FIRE (Table 4):

Overall, the quality assessment after cold storage and shelf life was good for fruit sampled in 2016 and 2017. In 2016, there was a tendency for shrivel-ling, but this was not a problem for fruit sampled in 2017. Of concern was the relatively high level of soft tips (16% data not shown) in the more advanced maturity H3 fruit sampled in 2017. Scrutiny of the full data set suggests that it may be prudent to set the maximum flesh firmness at 13.6 kg using the 11-mm plunger (8.0 kg with 8-mm plunger), with a minimum of 8.0 kg with the 11-mm plunger (4.5 kg with 8-mm plunger). As is the case with most nectarine cultivars it’s less risky to harvest at the less mature side of the optimum window to reduce levels of post-cold storage disorders. Since Summer Fire is a semi-clingstone type, flesh firmness may not always be adequate as the sole maturity indexing parameter. Hence, it is recommended to also utilise skin colour and TSS to determine optimum maturity. Pre-ripening as a means to limit pulpiness and woolliness in Summer Fire was tested in 2016, but because none of these disorders developed during storage it was not possible to draw conclusions. Consequently, pre-ripening cannot be recommended on the basis of information currently available.

SUMMER FIRE

(Revised maturity standard suggested)

Note: Above recommendation is irrespective of TSS level.

AUGUST RED (Table 5):

While pulpiness was recorded in the H3 fruit sampled in 2016, the quality was good overall. In 2017, August Red showed problems with soft tips irrespective of harvest maturity. Advanced maturity (H3) fruit sampled with a flesh firm-ness of 9.8 kg (11-mm plunger) or 5.8 kg (8-mm plunger) and a high TSS of 16% developed
a multitude of problems. This caused great concern. Soft tips (36%), decay (3.5%), pulpiness (26%), woolliness (26%) and overripeness (27%) were recorded. Hence, it is less risky – from a quality point of view – to harvest August Red at the less mature side of the optimum maturity window. It is postulated that the quality prob-lems experienced on August Red harvested with a low flesh firmness, were due to the fruit ripening from the inside of the mesocarp, around the stone, to the outside. This probably leads to inaccuracy when measuring flesh firmness on the cheek of the fruit; and it’s likely that the level of inaccuracy increases alongside the ripeness of the fruit. An optimum maturity window between 13.6 kg maximum with the 11-mm plunger (8.0 kg with 8-mm plunger), and 8.0 kg minimum with the 11-mm plunger (4.5 kg with the 8-mm plunger) is suggested, irrespective of TSS level. Subjecting August Red to the pre-ripening exacerbated pulpiness in some fruit, therefore this technique is not recommended.

AUGUST RED

(Revised maturity standard suggested)

Note: Above recommendation is irrespective of TSS level and it is lower risk to harvest at the less mature side of the optimum window.

SEPTEMBER BRIGHT (Table 6):

The quality of the H3 fruit sampled in 2016 was not as good as the high quality observed for earlier harvested H1 and H2 fruit, due to development of decay and pulpiness at approximately 2% and 30%, respectively. While pre-ripening helped control pulpiness (data not shown), it caused unacceptable overripeness and consequently cannot be recommended. Unfortunately, in 2017 the H3 fruit was harvested commercially before the trial could be completed, so it was not possible to verify the previous year’s results. Based on findings to date, it is suggested to leave the flesh firmness standards unchanged. This is a maximum flesh firmness standard of 11.3 kg measured using a 11-mm plunger (6.5 kg with the 8-mm plunger), and a minimum of 4.5 kg with the 11-mm plunger (3.0 kg with the 8-mm plunger). The current provision to harvest firmer fruit if the TSS is higher than 12% remains.

SEPTEMBER BRIGHT

(Unchanged maturity standard)

Note: The current provision to harvest firmer fruit if TSS is higher than 12% remains (see table 2), and it is lower risk to harvest at the less mature side of the optimum window.

REGAL PEARL, SUNBURST, SUMMERSWEET AND TOP PEARL

Regal Pearl, Sunburst, Summersweet and Top Pearl were only tested for one year. Hence, final conclu-sions cannot be made from the data generated to date. Generally, soft tips were a cause for concern across harvest maturities and Summersweet also seemed prone to shrivel. The results emphasised the importance of setting accurate optimum harvest maturity standards to achieve the opti-mum post-storage fruit quality. Additional testing of these cultivars, to set the optimum harvest maturity window is recommended.

Acknowledgements

Thanks to the South African Stone Fruit Producers Association
(SASPA) for financing this research and to HORTGO for placing the research at ExperiCo (Agri-Research Solutions).

References

MATURATION AND MATURITY INDICES, MICHAEL S REID. In: Postharvest Technology of Horticultural crops – Third edition; Kader, A.A. 2002 pp 55 – 62.