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Soluble solids accumulation and postharvest performance of ‘Hayward’ kiwifruit
Postharvest Biology and Technology 80 (2013) 1–8
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Soluble solids accumulation and postharvest performance of ‘Hayward’ kiwifruit J. Burdon a,∗ , N. Lallu a , P. Pidakala a , A. Barnett b a b
The New Zealand Institute for Plant & Food Research Limited, Private Bag 92169, Auckland Mail Centre, 1142 Auckland, New Zealand The New Zealand Institute for Plant & Food Research Limited, 412 No. 1 Road, RD 2, Te Puke 3182, New Zealand
a r t i c l e
i n f o
Article history: Received 19 April 2012 Accepted 23 January 2013 Keywords: Kiwifruit, Actinidia Soluble solids Storage Softening Chilling injury
a b s t r a c t A soluble solids content (SSC) of 6.2% has been used as a minimum harvest index for ‘Hayward’ kiwifruit for about 30 years. This paper describes a study that examines the pattern of soluble solids accumulation in ‘Hayward’ kiwifruit beyond the simple timing at which fruit reach 6.2% and investigates the relationship between soluble solids accumulation and postharvest performance assessed as softening and expression of chilling injury. This has been done using fruit from 10 orchards harvested at a range of SSC from 5 to 10% during one season. Soluble solids accumulation showed a general trend for a change from slow to more rapid accumulation during the season that could be described by a single logistic curve. The point at which the rate of soluble solids accumulation increased was more or less distinct for fruit from different orchards and occurred when fruit were at SSC between 6.3 and 7.4%. It is also possible that there is not a consistent change in soluble solids accumulation rate, with the rate being dependent on the environmental conditions over several days before measurement. There was a major change in softening pattern and low temperature breakdown susceptibility between fruit harvested at 6.4 and at 8.0% SSC. This change coincided with a change to faster soluble solids accumulation at harvest. It is concluded that the pattern, or rate, of soluble solids accumulation is likely to be a more robust indicator of the physiological state of the fruit, and therefore postharvest performance, than a single SSC value. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The changing soluble solids content (SSC) of kiwifruit during fruit development is a reflection of the carbohydrate supply to the fruit, its partitioning into soluble and insoluble components, and any conversion from starch to sugars. The soluble component is largely soluble sugars and the SSC measurement using a refractometer is a simple way of estimating them, but includes any other soluble material that affects the refractive index of the juice (Harman and Watkins, 1986). The insoluble components include the structural cell wall components and also the starch. The SSC of ‘Hayward’ kiwifruit has been used as a harvest index for about 30 years, based on the initial finding that storage performance was related to SSC (Harman, 1981). A minimum SSC of 6.2% was established in New Zealand for ‘Hayward’ kiwifruit for export. It should be noted that this is a minimum, not optimum, harvest index for storage performance and fruit for long-term storage tend to be harvested at a higher SSC (e.g., 7 – 9%). With a SSC of 6.2% as a threshold for harvesting, there has been much research into being able to predict the time at which fruit
∗ Corresponding author at: The New Zealand Institute for Plant and Food Research Limited, 120 Mt Albert Road, Sandringham, Private Bag 92169, Auckland 1142, New Zealand. Tel.: +64 9 925 7237; fax: +64 9 925 7001. E-mail address: [email protected] (J. Burdon). 0925-5214/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.postharvbio.2013.01.009
would reach 6.2%, and thereby assist harvest scheduling. Much of the research has looked at models that use the temperatures for the whole growing season and sometimes including aspects such as flowering date (Salinger et al., 1993; Snelgar et al., 1993; Hall and McPherson, 1997; Pailly et al., 1999; Zoffoli et al., 1999). Generally, an inverse relationship was found between temperature and the rate of SSC accumulation. However, the change in SSC is not linear and the timing of fruit reaching 6.2% has been determined to be more associated with the temperatures more immediately before measurement (Hopkirk et al., 1989; Seager et al., 1991a, 1991b, 1996, Burdon et al., 2007), rather than a linear accumulation of heat units throughout the growing season. Hence there is a limited capacity to predict the timing of fruit reaching 6.2% more than a couple of weeks ahead, particularly in climates that vary from year to year. The 6.2% harvest index has been adopted elsewhere (OECD, 1992), yet the use of 6.2% as a harvest index in New Zealand is about the physiological state of the fruit at that SSC rather than just the SSC value. In New Zealand, when initially established, the 6.2% SSC index was selected as a time at which the fruit had changed from nett accumulation of starch to nett breakdown, resulting in an increased rate of soluble solids accumulation (Beever and Hopkirk, 1990). This can be seen as a marker for the change from fruit growth to fruit ripening. However, fruit may still reach 6.2% without starch breakdown, through a prolonged period of slow accumulation of soluble carbohydrate.
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J. Burdon et al. / Postharvest Biology and Technology 80 (2013) 1–8
The exposure of fruit to lower temperatures not only increases the rate of soluble solids accumulation, but also acclimates the fruit to withstand low storage temperatures, thereby improving storage performance by reducing the susceptibility to chilling injury (Sfakiotakis et al., 2005; Burdon et al., 2007). How dependent reduced chilling sensitivity is on the increased soluble solids accumulation is not known; it may be coincidental, but with both processes responding to lower environmental temperatures. Thirty years on from the introduction of the 6.2% SSC harvest index, the New Zealand ‘Hayward’ kiwifruit industry is markedly different. Yields have increased from <18 to >35 tonnes per hectare and orchard management practices such as girdling have been adopted to increase carbohydrate flow into the fruit (Patterson and Currie, 2011): these practices also alter the timing of fruit reaching 6.2% SSC (Burdon et al., 2011). The purpose of this study is to examine the soluble solids accumulation in ‘Hayward’ kiwifruit beyond the simple timing at which fruit reach 6.2% SSC and to investigate the relationship between soluble solids accumulation and postharvest performance assessed as the softening and expression of chilling injury (low temperature breakdown; LTB) that occurs during storage. This has been done using fruit from 10 orchards harvested at a range of SSC from 5 to 10% during one season, with soluble solids accumulation monitored during maturation allowing the patterns and rates of soluble solids accumulation to be assessed. 2. Methods 2.1. Fruit ’Hayward’ kiwifruit (Actinidia deliciosa C.F. Liang and A.R. Ferguson) were sourced from ten orchards (designated O1–O10) around Te Puke in the Bay of Plenty Region of New Zealand. On each orchard, fruit came from approximately 12 vines, either as bays or as individual vines, depending on the orchard layout. Fruit monitoring and harvesting commenced on 30 March 2009. Fruit were monitored for SSC using samples of 30 fruit at approximately 4-d intervals and harvested when at on average 5.2, 5.9, 6.4, 8 and 10% SSC, designated H1, H2, H3, H4 and H5, respectively. At each harvest, four crates of fruit (approximately 25 kg of fruit per crate) were taken from each orchard. Fruit were held for two days at ambient conditions under cover to simulate curing before packing into modular bulk (MB; 10 kg loose fill) packs with polybags and storing at 0 ◦ C. Ten MB packs were filled at each harvest, with five packs designated for firmness assessments through storage, and five packs for disorder assessments. 2.2. Fruit assessments For each orchard/harvest, a 30-fruit sample was measured for SSC and firmness. Firmness in storage was measured on 30-fruit samples after 3, 6, 8 and 10 weeks (H1 and H2) or 6, 10, 16 and 20 weeks (H3–H5) at 0 ◦ C. After storage, and when at <9.8 N (1 kgf ) firmness during shelf-life, fruit were assessed for physiological, pathological and physical disorders.
each other. Firmness was measured as kgf and data converted to N, where 1 kgf = 9.8 N. Soluble solids content was determined as the average of the stylar and stem ends of fruit measured separately during maturity monitoring and at harvest using a hand-held refractometer (Master Series, 0–30%, Atago). 2.3.2. Disorders All fruit from 5 MB packs per orchard/harvest were assessed for physiological, pathological and physical disorders. The incidence of low temperature breakdown (LTB) was assessed after cutting each fruit transversely starting from the stylar end and looking for watersoaked and/or mealy appearance in the flesh. With the incidence of disorders other than LTB being low and sporadic across orchards and harvests, only LTB data have been analysed further. 2.4. Data analysis Soluble solids data were investigated for patterns in the rate of accumulation, the rate of accumulation at 6.2%, the timing of reaching 6.2% and any marked increases in the rate of soluble solids accumulation. Graphics and fitted curves were created using Origin v7.5 (OriginLab Corporation, One Roundhouse Plaza, Northampton, MA 01060, USA). A logistic curve, of the form y = ((A1 − A2 )/(1 + (x/x0 )p )) + A2 , where A1 = initial value, A2 = final value, x0 = centre and p = power, or two linear lines, was fitted to the SSC data for each orchard. The SSC data to which linear fits were made were selected by eye. For much of the data, investigation of trends and patterns was emphasised as much as were strict statistical differences among orchards or harvests. The strength of relationships among SSC, the rate of soluble solids accumulation and firmness at harvest with softening in storage and the incidence of LTB after storage were determined as correlation coefficients. Mean LTB values were subjected to analysis of variance (ANOVA) using GenStat Release 8.1 [(PC/Windows XP) Copyright 2003, Lawes Agricultural Trust (Rothamsted Experimental Station)]. The incidences of LTB were angular transformed (arcsin(sqrt(x)) before analysis. LTB data were analysed on the basis of incidence, with no assessment of severity. Data in the table of LTB incidence are untransformed. 3. Results and discussion 3.1. Soluble solids accumulation The SSC of fruit from the end of March to the end of May 2009 was plotted and logistic curves fitted to the data (Fig. 1). For all orchards, the logistic curves fitted with R2 values in excess of 0.95 (Table 1). From the fitted curves, the fruit reached 6.2% SSC at Table 1 Coefficient of regression (R2 ) and the date and rate of soluble solids accumulation when at 6.2% soluble solids content from the ‘Hayward’ kiwifruit soluble solids data in Fig. 1. Orchard
R2
1 2 3 4 5 6 7 8 9 10
0.994 0.992 0.988 0.960 0.972 0.990 0.982 0.967 0.977 0.980
2.3. Fruit assessment methodology 2.3.1. Firmness and soluble solids Fruit firmness was measured using a Fruit Texture Analyser (Güss, model GS14, South Africa) fitted with a 7.9-mm Effegi (Alfonsine, Italy) penetrometer probe after removal of skin and flesh to a depth of approximately 1 mm. The probe was driven into the flesh at 20 mm/s to a depth of 7.9 mm, and the maximum force recorded as the firmness value. Firmness was measured twice at the midpoint of each fruit, with the two measurements taken at 90◦ to
At 6.2% soluble solids content Date
Rate (%/d) × 100
19 April 14 April 14 April 17 April 16 April 18 April 23 April 24 April 19 April 24 April
6.7 5.6 5.8 7.4 4.2 8.8 7.4 7.3 7.2 7.2
J. Burdon et al. / Postharvest Biology and Technology 80 (2013) 1–8
O1
O2
13
O3
13
O4
13 12
11
11
10
10
10
10
10
9 8
9 8
9 8
SSC (%)
12
11
SSC (%)
12
11
SSC (%)
12
11
SSC (%)
12
9 8
8
7
7
7
7
6
6
6
6
6
5
5
5
5
5
13
r r r y y Ma Ap Ap Ma Ma 2 7 1 0 24 8 2 2
O6
13
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O7
13
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O8
13
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O9
13
12
12
11
11
11
11
11
10
10
10
10
10
8
9 8
9 8
SSC (%)
12
SSC (%)
12
SSC (%)
12
9
9 8
8
7
7
7
7
6
6
6
6
6
5
5
5
5
5
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O10
9
7
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O5
9
7
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
SSC (%)
13
SSC (%)
SSC (%)
13
3
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
Fig. 1. Soluble solids accumulation in ‘Hayward’ kiwifruit from 10 orchards (O1–O10) during maturation in 2009. Each data point is the mean of 30 fruit and the fitted curves are logistic functions.
between 14 and 24 April (Table 1) and at 6.2% SSC fruit had rates of soluble solids accumulation between 0.04 and 0.09%/d. However, fitting the logistic curve to the range of SSC data collected predetermines that the rate of SSC will increase continuously with time up to the centre of the curve, and decline thereafter. Discussion of soluble solids accumulation often focuses on identifying a point at which there has been a change in rate, from slow to fast, indicative of the change to nett starch breakdown. The logistic fit to the data gives a continually increasing rate over the period of interest, although the full soluble solids accumulation curve would also include a period of slowing accumulation as fruit approached the maximum SSC value depending on the amount of starch available. There was little obvious slowing in soluble solids accumulation within the period sampled. Alternative approaches to describing the soluble solids accumulation data have included fitting a linear component early during fruit development to a later logistic curve (A.J. Hall personal communication) Given that it is the point at which fruit change from a nett starch accumulation to starch breakdown that is important as an indicator of fruit physiology, rather than SSC per se, the data were re-examined by fitting two linear components to data selected by eye. First, an upper SSC component was fitted using the higher SSC values and then a second, lower line fitted for the lower earlier season SSC values (Fig. 2; Table 2). The limitations in connectivity and smoothness of joining two linear fits are acknowledged if they were going to be used to create a single description, but this element of modelling is not relevant to the way the data are being investigated here. The two linear lines create a break-point between the slower and faster periods of soluble solids accumulation. While this is a simple procedure at the end of the season when all the data have been collected, it is much more difficult to identify the break-point in real time.
As with the logistic fits, the two linear fits to the data tended to have high coefficients of regression at >0.9 (Table 2). However, with the reduced sample numbers, the goodness of fit was very dependent on individual samples. Taking the intersection of the two linear lines as the break-point, the timing of the break-points for the orchards was quite consistent at between 27 April and 5 May. These dates are later than the time at which fruit reached 6.2% SSC (Table 1), and occurred when fruit SSC was between 6.3 and 7.4% (Table 2). This variability in SSC at which the change to rapid SSC occurs illustrates the problem of using a single at-harvest SSC value as being predictive of the physiological state of the fruit. The rates of soluble solids accumulation of the lower linear components (0.04–0.07%/d; Fig. 2) were of the same magnitude as for the fruit at 6.2% SSC determined from the logistic fits in Fig. 1. This suggests that the fruit at 6.2% SSC were still in the slower phase of
Table 2 Coefficient of regression (R2 ), rate of soluble solids accumulation and date and soluble solids content (SSC) at the break-point between two (lower and upper) linear fits to the ‘Hayward’ kiwifruit SSC data from Fig. 2. Orchard
1 2 3 4 5 6 7 8 9 10
R2
Rate (%/d) × 100
Break-point
Lower
Upper
Lower
Upper
Date
SSC (%)
0.962 0.972 0.960 0.941 0.968 0.974 0.904 0.889 0.908 0.882
0.978 0.966 0.980 0.723 0.929 0.955 0.904 0.893 0.916 0.885
5.5 6.9 7.4 5.7 4.2 5.8 4.4 3.9 5.5 4.3
16.7 16.9 20.1 12.8 18.5 18.7 15.4 15.2 17.4 21.1
2 May 27 April 29 April 28 April 1 May 30 April 29 April 30 April 3 May 5 May
6.9 7.4 7.4 6.9 7.0 6.8 6.4 6.3 7.1 6.7
4
J. Burdon et al. / Postharvest Biology and Technology 80 (2013) 1–8
O1
O2
13
O3
13
O4
13 12
11
11
10
10
10
10
10
9 8
9 8
9 8
SSC (%)
12
11
SSC (%)
12
11
SSC (%)
12
11
SSC (%)
12
9 8
8
7
7
7
7
6
6
6
6
6
5
5
5
5
5
13
r r y y r Ma Ap Ap Ma Ma 27 10 24 8 22
O6
13
r r y y r Ma Ap Ap Ma Ma 27 10 24 8 22
O7
13
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O8
13
r r y y r Ma Ap Ap Ma Ma 27 10 24 8 22
O9
13
12
12
11
11
11
11
11
10
10
10
10
10
8
9 8
9 8
9 8
SSC (%)
12
SSC (%)
12
SSC (%)
12
9
8
7
7
7
7
6
6
6
6
6
5
5
5
5
5
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O10
9
7
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O5
9
7
r r y y r Ma Ap Ap Ma Ma 27 10 24 8 22
SSC (%)
13
SSC (%)
SSC (%)
13
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
Fig. 2. Soluble solids accumulation in ‘Hayward’ kiwifruit from 10 orchards (O1–O10) during maturation in 2009. Each data point is the mean of 30 fruit and the upper (䊉) and lower () linear fitted lines have been fitted to data selected by eye. Data not used for linear fits ().
soluble solids accumulation, i.e., in a growth phase of development rather than ripening. Under this circumstance, the prediction of the time to reach 6.2% SSC is simple from the linear relationship, but 6.2% SSC would not be indicative of a changed physiological state, with the fruit still being in a growth phase. The rates of soluble solids accumulation for the upper linear components were in the range 0.15–0.21%/d, with no obvious sign of a decline in rate that would be expected as fruit reached their maximal SSC as starch becomes depleted. With dry matter contents in the range 15–18% at harvest (data not presented), ripe fruit SSC (rSSC) would be expected to be approximately 12–15% based on the relationship that exists between dry matter and rSSC (Burdon et al., 2004). This relationship between dry matter and rSSC has also led to suggestions of using the SSC/rSSC ratio as a harvest index (Feng et al., 2003, 2006). However, as with using the SSC value, the ratio is not indicative of the physiological state of the fruit, since the SSC at which the physiological change occurs is not fixed, and therefore the ratio does not improve on the use of SSC alone. For some of the orchards, there was not a clear break-point between the slower and faster phases of soluble solids accumulation, with a gradual change being more apparent (e.g., O6 and O10). This raises the question as to whether a clean break-point should be expected. With the change in soluble solids accumulation rate being a result of the nett effect of starch synthesis and starch degradation, if the soluble solids accumulation rate is the result of the relative rates of each process, and not an on-off switch, then a more or less sharp change could be expected depending on the conditions that modulate the two processes. Starch synthesis and degradation have been shown to occur simultaneously in banana (Hill and Ap Rees, 1994). In tomato, starch turnover has been reported to remain constant during fruit development whilst starch synthesis was dependent on sucrose supply (N’tchobo et al., 1999). While there has been recent research into the control of
starch production (Nardozza et al., 2010, 2011) and general carbohydrate metabolism (Moscatello et al., 2011) in kiwifruit, there has been limited research into starch breakdown and its control (Wegrzyn and MacRae, 1995). This discussion can be extended to consider whether a consistent change in soluble solids accumulation rate should be expected, or whether the rate may fluctuate depending on environmental conditions. If all data points are accepted as real, rather than fitting smoothed logistic or linear lines, then periods of slower or faster soluble solids accumulation may be identified (Burdon et al., 2011). The ability to modulate the rate of soluble solids accumulation by altering temperature has been demonstrated under controlled environmental conditions by Seager et al. (1996). Temperature shifts resulted in predictable changes in soluble solids accumulation, with faster accumulation at lower temperatures. However, soluble solids accumulation can also be affected by the fruit growth rate. Early in the season, SSC may be described as increasing only slowly, if at all, as if to suggest little carbohydrate flow into the fruit. Yet at this time there is a significant flow of carbohydrate into the fruit that allows for SSC to be maintained during the rapid increase in fruit size. Conversely, a slowing of the rate of fruit expansion and a constant inflow of carbohydrate could result in an increase in soluble solids accumulation rate without any need for starch breakdown. In the future, studies of soluble solids accumulation will include aspects of starch metabolism, fruit growth and environmental temperatures. 3.2. Softening in storage There was only limited softening of fruit whilst on the vine, on average from 90 to 70 N (∼9 to 7 kgf ) over the two months between H1 and H5. Fruit at H1 and H2 were harvested at an average SSC of 5.2 and 5.8%, respectively, and only stored for 10 weeks since
J. Burdon et al. / Postharvest Biology and Technology 80 (2013) 1–8
O1
100
Firmness (N)
Firmness (N)
100 80 60 40 20 0
8
12
16
20 0 8
12
16
O5
16
20
4
8
12
16
20
4
8
12
16
20
4
8
12
16
20
4
8
12
16
20
60 40 20
100
60 40 20
O6
80 60 40 20 0
0
4
8
12
16
20
0
O7 Firmness (N)
100
80 60 40 20 0
4
8
12
16
O9
60 40 20 0
60 40 20
100
80
4
8
12
16
0
O10
80 60 40 20 0
20
O8
80
0
20
Firmness (N)
Firmness (N)
12
O4
0
0
Firmness (N)
8
80
20
Firmness (N)
Firmness (N)
4
80
0
4
0 0
100
20
100
40
0
40
0
O3
60
100
60
20
Firmness (N)
Firmness (N)
4
80
100
O2
80
0 0
100
5
0
Storage Period (Weeks) H1;
H2;
Storage Period (Weeks) H3;
H4;
H5
Fig. 3. Firmness of ‘Hayward’ kiwifruit during storage at 0 ◦ C after harvest on five occasions (H1–H5). Fruit were harvested from 10 orchards (O1–O10) between 30 March and 22 May 2009. Values are the mean of 30 fruit.
they were known to be of low maturity and prone to disorder if stored long-term. Fruit from H3–H5 were stored for 20 weeks. The softening curves for the fruit from the five harvests from each of the 10 orchards are given in Fig. 3. The generic storage softening curve of ‘Hayward’ kiwifruit is usually described as sigmoidal with three phases: a slow initial phase followed by a fast phase and then a final slow phase (MacRae and Redgwell, 1992). In later harvested fruit, only the second and third phases may be present, giving an exponential decay curve, the fruit having reached a stage of development on the vine such that the first slow phase has already been passed prior to harvest. Differences in firmness during storage may arise through differences in initial softening rate, the timing of the change to rapid softening, the rapid softening rate, the firmness at which the softening rate changes to slow, and the rate during the final slow phase.
Overall, there was little difference in the patterns of softening for fruit from the 10 orchards (Fig. 3). There was a change from a slow linear pattern of softening for fruit from H1 and H2, to an exponential pattern of softening in fruit from H4 and H5 (Fig. 3). The softening curves for fruit from H4 and H5 can be regarded as typical for commercially harvested ‘Hayward.’ Overall, the H3 softening curves tended to be very similar to those for H2, suggesting little change in the capacity to soften between H2 and H3, i.e., for fruit harvested at SSC of 5.8 and 6.4%. Thus, the main change in the capacity to soften occurred between H3 and H4, i.e., for fruit harvested at between 6.4 and 8.0% SSC. Among the softening curves for H4 fruit, there were differences among the 10 orchards. For some orchards, the H4 softening curve tended to be similar to the H5 curve (e.g., O7), whereas for others (e.g., O9) the H4 softening curve was closer to the H3 curve than the H5 curve. This suggests that even at H4
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J. Burdon et al. / Postharvest Biology and Technology 80 (2013) 1–8
Table 3 Rate of softening of ‘Hayward’ kiwifruit over the first 6 weeks of storage at 0 ◦ C for fruit harvested from 10 orchards. Fruit were harvested on five occasions (H1–H5) over the period 30 March–22 May 2009. Values have been calculated from data in Fig. 3. Rate of softening (N/week) for weeks 0–6 in storage Orchard
H1
H2
H3
H4
H5
O1 O2 O3 O4 O5 O6 O7 O8 O9 O10
1.81 2.77 2.85 2.03 2.55 3.24 3.44 3.63 3.83 2.65
3.29 3.57 3.67 2.48 3.70 4.67 2.67 2.61 2.97 3.13
3.32 3.92 4.07 4.07 2.50 5.41 4.82 4.46 4.67 3.83
4.85 5.27 3.69 6.76 4.57 4.92 7.16 6.67 4.57 4.91
6.96 6.06 5.39 7.13 5.83 6.39 8.16 6.71 7.55 6.91
Mean
2.88
3.28
4.11
5.34
6.71
(approximately 8% SSC at harvest), there were differences among the fruit from different orchards in the capacity to soften in storage. The differences in softening are reflected in the rates of softening over the first 6 weeks of storage (Table 3), with rates increasing with maturity, but with clear differences among orchards at each harvest. The data suggest a continuum between H3, H4 and H5, with an increasing rate of softening over the first 6 weeks of storage. However, there is only a limited discrimination possible among orchards since the fruit were first sampled at 6 weeks and any short-term differences earlier in storage will have been missed, i.e., softening may have been slower for 1–2 weeks after harvest, but when not assessed until week 6 this slow period only reduces the overall softening rate. The time in storage taken to reach 9.8 N (1 kgf ) is a commonly used comparison for the storage performance of ‘Hayward’ kiwifruit. However, in the trial, few fruit softened to 9.8 N in storage and the time to 19.6 N (2 kgf ) has been taken for comparisons of softening in storage among fruit from H3–H5, i.e., those fruit harvested at >6.2% SSC. There was a general trend for a shorter time to 19.6 N for fruit from H5 (average 10 weeks) than from H3 and H4 (14–15 weeks), and for some treatments there was little difference between H3 and H4 (range 11–18 weeks; Table 4).
Table 4 Time taken for ‘Hayward’ kiwifruit harvested at >6.2% soluble solids content (SSC) to soften to 19.6 N (2 kgf ) during storage at 0 ◦ C for fruit harvested from 10 orchards. Fruit were harvested on three occasions: H3, H4 and H5, when at 6.4, 8.0 and 10.0% SSC, respectively. Values have been interpolated from the data in Fig. 3. Orchard
Time (weeks) to 19.6 N (2 kgf ) H3
H4
H5
O1 O2 O3 O4 O5 O6 O7 O8 O9 O10
17.2 15.2 15.7 16.0 23.0a 11.0 15.6 14.1 14.4 15.2
18.1 14.9 13.0 15.0 19.4 11.6 13.5 14.2 13.7 15.1
9.1 9.1 10.4 10.4 14.6 8.7 10.6 11.1 9.5 10.9
Mean
15.74 (14.93)b
14.85 (14.34)b
10.44 (9.98)b
a b
Extrapolated value. Value in parenthesis excludes O5 value.
3.3. Correlations between at-harvest fruit characteristics and softening in storage Using data for fruit harvested above 6.2% SSC, i.e., combining H3, H4 and H5 data, there were strong correlations between firmness at harvest, SSC, the rate of soluble solids accumulation at harvest (see Table 5) and the time taken to soften to 19.6 N. These strong correlations are not surprising given the wide range of maturity of the fruit investigated. However, the correlations between harvest firmness, SSC, or rate of soluble solids accumulation at harvest with firmness at the end of storage were not significant (Table 5). The lack of association with firmness at the end of storage is probably because the softening pattern was not linear and if left long enough, firmness values for fruit from all orchards tend to come together during the final slow phase of softening. For fruit from H3, harvested at an average SSC of 6.4%, neither firmness nor SSC at harvest was a good predictor of firmness after 20 weeks of storage, the time taken to soften to 19.6 N, or the rate of softening over the first 6 weeks of storage (Table 5). The lack of associations is perhaps not surprising given the very similar firmness and SSC values for these fruit from the 10 orchards at a single harvest. Commercially, it is important to be able to discriminate among fruit from different orchards for postharvest performance when harvested at about the same time, and when the fruit appear similar by SSC or firmness values. While looking for at-harvest characteristics with which to discriminate among fruit from different orchards, it is important to note that a large component of commercial postharvest performance is dependent on fruit handling and coolstore operation. In contrast to SSC or firmness at harvest, there were relatively strong correlations between the rate of soluble solids accumulation and both the firmness after 20 weeks of storage and the rate of softening over the first 6 weeks of storage for H3 fruit (Table 5). There was a less strong association between soluble solids accumulation rate and time to 19.6 N. These associations were less strong in later harvested fruit (data not shown). The softer fruit after storage and faster initial softening rate of fruit harvested with a more rapid soluble solids accumulation are indicative of the more advanced physiological state and capacity to soften of these fruit, despite there being little difference in firmness at harvest. 3.4. Disorders Disordered fruit mostly showed symptoms of LTB with only sporadic low incidences of other disorders or rots. LTB is a physiological disorder in which the softening of fruit is affected resulting in areas of outer pericarp with a granular appearance or, in more severe instances, with water soaked tissues (Lallu, 1997). Overall, the incidence of LTB decreased in later harvested fruit, which also had higher SSC at harvest. There was a marked decrease in LTB incidence from 35 to 13% for fruit harvested at H1 and H2 (both stored for 10 weeks) and from 30 to 11% for fruit harvested at H3 and H4 or H5 (stored 20 weeks) (Table 6). The nature of LTB, progressive and increasing with longer storage, would mean that the LTB incidence in fruit from H1 and H2 would be higher if stored for longer, giving incidences in excess of those found in H3 fruit by 20 weeks. For the fruit harvested at on average 6.4%, the high incidence of LTB (on average 30%) shows that the fruit were still chilling sensitive, possibly lacking maturity as shown by a low soluble solids accumulation rate suggesting fruit to still be in a growth phase, and also possibly associated with a lack of temperature acclimation (Sfakiotakis et al., 2005, Burdon et al., 2007). The 10% LTB incidence in fruit harvested at H4 and H5 is unusual, as fruit harvested at 8–10% SSC would normally be expected not to be chilling sensitive. This suggests that while the soluble solids
J. Burdon et al. / Postharvest Biology and Technology 80 (2013) 1–8
7
Table 5 ANOVA P-values and correlation coefficients (r) for correlations between at-harvest ‘Hayward’ kiwifruit firmness (FF), soluble solids content (SSC) and rate of soluble solids accumulation (SSC rate) with softening in storage assessed as firmness after 20 weeks, time taken to reach 19.6 N (2 kgf ) and softening rate over the first 6 weeks of storage. Data are for fruit from harvests 3–5 combined (H3–H5) and from harvest 3 (H3) alone. At-harvest characteristic
Anova P-value and correlation coefficient (r) for correlations between at harvest characteristics and softening in storage
H3–H5 FF SSC SSC rate H3 FF SSC SSC rate
Firmness after 20 weeks
Time to 19.6 N (2 kgf )
P
r
P
r
P
r
0.823 0.567 0.871
−0.043 0.109 −0.031
<0.001 <0.001 <0.001
0.632 −0.684 −0.647
0.017 <0.001 <0.001
−0.434 0.749 0.695
0.248 0.640 0.031
−0.403 0.170 −0.679
0.708 0.953 0.104
−0.136 0.022 −0.544
0.357 0.854 0.070
0.327 −0.067 0.594
accumulation rate had increased, the chilling sensitivity had not been completely removed. It is therefore possible that there are different temperature thresholds or different control mechanisms for the two processes (starch breakdown/temperature sensitivity). LTB is not normally a problem commercially in New Zealand, and the incidences found in this trial may reflect the more rapid cooling applied to the trial fruit compared with the slower cooling that would occur commercially in packed and palletised fruit, i.e., overnight compared with a week or more, with the faster cooling exacerbating LTB expression (Lallu, 1997; Lallu and Webb, 1997). The chilling sensitivity and softening patterns are both suggestive of delayed maturation of the fruit, with changes in chilling sensitivity and softening patterns suggesting a change in fruit physiology occurring between H3 and H4, i.e. in fruit harvested at between 6.4 and 8.0% SSC. This change coincides with cooler temperatures from the start of May and the period of increased SSC accumulation. However, any temperature acclimation did not totally remove the susceptibility to chilling. The chilling sensitivity and softening patterns may also be linked by chilling affecting the rate or pattern of softening in storage. With LTB affecting the texture of ripe fruit, it is possible that softening earlier in storage is affected by chilling before LTB symptoms are visible.
Table 6 The incidence of low temperature breakdown (LTB) in ‘Hayward’ kiwifruit from 10 orchards after harvest on five occasions (H1–H5) and storage at 0 ◦ C for 10 (H1 and H2) or 20 (H3–H5) weeks before assessment during shelf-life at 20 ◦ C when firmness was <9.8 N (1 kgf ). Values are the means of five packs with approximately 90 fruit per pack. Orchard
LTB incidence (%) H1
O1 50 31 O2 43 O3 47 O4 O5 9a O6 38 26 O7 40 O8 39 O9 O10 33 Mean 36 Statistical analysis: ANOVA <0.001 P values 7.2 L.S.D. a
H2
H3
H4
H5
12 23 16 7 17 15 10 8 10 15 13
44 30 39 33 29 24 12 26 26 38 30
7 12 8 11 9 9 10 22 10 11 11
10 3 9 6 6 14 22 16 20 8 11
<0.001 5.1
<0.001 6.8
<0.001 3.7
<0.001 3.8
Fruit did not soften to <9.8 N (1 kgf ) during 2 weeks at 20 ◦ C.
Softening weeks 0–6
3.5. Correlations between at-harvest measurements and disorders Correlations between firmness at harvest, the SSC at harvest, or rate of soluble solids accumulation at harvest and LTB incidence were investigated (Table 7). When the data for H3–H5 were combined, there were significant relationships between the incidence of LTB and firmness at harvest, SSC at harvest, and the rate of soluble solids accumulation at harvest. This is not surprising given the wide range of fruit development investigated and the general association that exists in kiwifruit between reduced chilling susceptibility and advanced fruit development (Burdon et al., 2007). However, when only H3 data were used, there were no significant associations between firmness, SSC, or the rate of soluble solids accumulation at harvest with LTB incidence (Table 7). 4. Summary and conclusion Soluble solids accumulation in ‘Hayward’ kiwifruit showed a general trend for a change from slow (0.04–0.07%/d) to more rapid (0.1–0.2%/d) accumulation during the season. The point at which the rate increased was more or less distinct for fruit from different orchards and occurred when fruit were at between 6.3 and 7.4% SSC. Overall, the pattern or rate of soluble solids accumulation appears to be more indicative of the physiological state of the fruit than the SSC value alone. There was a major change in softening pattern and LTB susceptibility between fruit harvested at H3 and H4, i.e., fruit harvested at 6.4 and 8.0% SSC. This change between fruit at H3 and H4 coincided with a change to faster soluble solids Table 7 ANOVA P-value and correlation coefficient (r) for correlations between at-harvest ‘Hayward’ kiwifruit firmness (FF), soluble solids content (SSC) and rate of soluble solids accumulation (SSC rate) with the incidence of low temperature breakdown (LTB) after storage. Data are for fruit from harvests 3, 4 and 5 (H3–H5) combined and for harvest 3 (H3) alone. At-harvest characteristic
H3–H5 FF SSC SSC rate H3 FF SSC SSC rate
ANOVA P-value and correlation coefficient (r) for correlations between at-harvest characteristics and LTB incidence after storage P
r
0.001 <0.001 <0.001
0.568 −0.635 −0.663
0.128 0.521 0.205
−0.514 0.231 −0.439
8
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accumulation and a change to cooler weather after the start of May. The association between softening in storage and soluble solids accumulation rate was stronger than with either SSC or firmness values at harvest. Susceptibility to LTB was reduced in later harvested fruit, but with little association with SSC, soluble solids accumulation rate or firmness within an individual harvest. The 10% incidence of LTB in fruit at H4 and H5 is unusual. Both the LTB incidence and the softening curves are indicative of the fruit having delayed maturation. It is concluded that 6.2% SSC is still a useful marker as a minimum threshold for harvesting, but may not be completely reliable as an indicator of postharvest performance in all seasons. The pattern, or rate, of soluble solids accumulation is likely to be a more robust indicator of the physiological state of the fruit, and therefore postharvest performance, than a single SSC value. Acknowledgements The authors gratefully acknowledge the assistance of the orchard owners and managers for facilitating the sourcing of fruit and the orchard staff at Plant & Food Research, Te Puke Research Centre for assistance with harvesting. This research was funded by ZESPRI Group Ltd., the New Zealand Foundation for Research, Science and Technology Programme C06X0706 and the Plant & Food Research KRIP Project 2010-05. References Beever, D.J., Hopkirk, G., 1990. Fruit development and fruit physiology. In: Warrington, I.J., Weston, G.C. (Eds.), Kiwifruit Science and Management. Ray Richards Publisher/New Zealand Society for Horticultural Science Inc., Auckland/Wellington, pp. 97–126. Burdon, J., McLeod, D., Lallu, N., Gamble, J., Petley, M., Gunson, A., 2004. Consumer evaluation of ‘Hayward’ kiwifruit of different at-harvest dry matter contents. Postharvest Biol. Technol. 34, 245–255. Burdon, J., Lallu, N., Francis, K., Boldingh, H., 2007. The susceptibility of kiwifruit to low temperature breakdown is associated with pre-harvest temperatures and at-harvest soluble solids content. Postharvest Biol. Technol. 43, 283–290. Burdon, J., Lallu, N., Pidakala, P., Barnett, A., 2011. Is the 6.2◦ Brix soluble solids harvest index suitable for ‘Hayward’ kiwifruit from high productivity orchard management systems? Acta Hort. 913, 539–546. Feng, J., MacKay, B.R., Maguire, K.M., 2003. Variation in firmness of packed Hayward kiwifruit. Acta Hort. 610, 211–217. Feng, J., Maguire, K.M., MacKay, B.R., 2006. Discriminating batches of ‘Hayward’ kiwifruit for storage potential. Postharvest Biol. Technol. 41, 128–134.
Hall, A.J., McPherson, H.G., 1997. Predicting fruit maturation in kiwifruit (Actinidia deliciosa). J. Hort. Sci. 72, 949–960. Harman, J.E., Watkins, C.B., 1986. Fruit maturity testing: refractometer use. New Zealand Ministry of Agriculture and Fisheries, Horticultural produce and practice information sheet AgLink HPP 212. Harman, J.E., 1981. Kiwifruit maturity. Orchardist NZ 54, 130, 126-127. Hill, S.A., Ap Rees, T., 1994. Fluxes of carbohydrate metabolism in ripening bananas. Planta 192, 52–60. Hopkirk, G., Snelgar, W.P., Horne, S.F., Manson, P.J., 1989. Effect of increased preharvest temperature on fruit quality of kiwifruit (Actinidia deliciosa). J. Hort. Sci. 64, 227–237. Lallu, N., 1997. Low temperature breakdown in kiwifruit. Acta Hort. 444, 579–585. Lallu, N., Webb, D.J., 1997. Physiological and economic analysis of precooling kiwifruit. Acta Hort. 444, 691–697. MacRae, E., Redgwell, R., 1992. Softening in kiwifruit. Postharvest News and Information 3, 49N–52N. Moscatello, S., Famiani, F., Proietti, S., Farinelli, D., Battistelli, A., 2011. Sucrose synthase dominates carbohydrate metabolism and relative growth rate in growing kiwifruit (Actinidia deliciosa, cv Hayward). Sci. Hort. 128, 197–205. Nardozza, S., Boldingh, H.L., Richardson, A.C., Costa, G., Marsh, H., MacRae, E.A., Clearwater, M.J., 2010. Variation in carbon content and size in developing fruit of Actinidia deliciosa genotypes. Funct. Plant Biol. 37, 545–554. Nardozza, S., Hallett, I.C., McCartney, R., Richardson, A.C., MacRae, E.A., Costa, G., Clearwater, M.J., 2011. Is fruit anatomy involved in variation in fruit starch concentration between Actinidia deliciosa genotypes? Funct. Plant Biol. 38, 63–74. N’tchobo, H., Dali, N., Nguyen-Quoc, B., Foyer, C.H., Yelle, S., 1999. Starch synthesis in tomato remains constant throughout fruit development and is dependent on sucrose supply and sucrose synthase activity. J. Exp. Bot. 50, 1457–1463. OECD, 1992. International standardisation of fruit and vegetables. Kiwifruit, ISBN: 92-64-03697-0. Pailly, O., Battini, M., Polidori, I.J., 1999. Predicting kiwifruit maturity in orchard by the use of daily mean air temperature accumulation. Acta Hort. 498, 239–246. Patterson, K., Currie, M.B., 2011. Optimising kiwifruit vine performance for high productivity and superior fruit taste. Acta Hort. 913, 257–268. Salinger, M.J., Kenny, G.J., Morley-Bunker, M.J., 1993. Climate and kiwifruit cv ‘Hayward’. 1. Influences on development and growth. New Zealand J. Crop Hort. Sci. 21, 235–245. Seager, N.G., Hewett, E.W., Warrington, I.J., MacRae, E.A., 1991a. The effect of temperature on the rate of kiwifruit maturation using controlled environments. Acta Hort. 297, 247–253. Seager, N.G., Hewett, E.W., Warrington, I.J., 1991b. How temperature affects Brix increase. New Zealand Kiwifruit J. 75 (February), 17–18. Seager, N.G., Warrington, I.J., Hewett, E.W., 1996. Maturation of kiwifruit grown at different temperatures in controlled environments. J. Hort. Sci. 71, 639–652. Sfakiotakis, E., Chlioumis, G., Gerasopoulos, D., 2005. Preharvest chilling reduces low temperature breakdown incidence of kiwifruit. Postharvest Biol. Technol. 38, 169–174. Snelgar, W.P., Hopkirk, G., McPherson, H.G., 1993. Predicting harvest date for kiwifruit: variation of soluble solids concentration with mean temperature. New Zealand J. Crop Hort. Sci. 21, 317–324. Wegrzyn, T., MacRae, E., 1995. Alpha-amylase and starch degradation in kiwifruit. J. Plant Physiol. 147, 19–28. Zoffoli, J.P., Gil, G.F., Crisosto, C.H., 1999. Determination of harvest period of Chilean kiwifruit in relation to fruit quality and temperature during maturation. Acta Hort. 498, 247–254.
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Postharvest Biology and Technology journal homepage: www.elsevier.com/locate/postharvbio
Soluble solids accumulation and postharvest performance of ‘Hayward’ kiwifruit J. Burdon a,∗ , N. Lallu a , P. Pidakala a , A. Barnett b a b
The New Zealand Institute for Plant & Food Research Limited, Private Bag 92169, Auckland Mail Centre, 1142 Auckland, New Zealand The New Zealand Institute for Plant & Food Research Limited, 412 No. 1 Road, RD 2, Te Puke 3182, New Zealand
a r t i c l e
i n f o
Article history: Received 19 April 2012 Accepted 23 January 2013 Keywords: Kiwifruit, Actinidia Soluble solids Storage Softening Chilling injury
a b s t r a c t A soluble solids content (SSC) of 6.2% has been used as a minimum harvest index for ‘Hayward’ kiwifruit for about 30 years. This paper describes a study that examines the pattern of soluble solids accumulation in ‘Hayward’ kiwifruit beyond the simple timing at which fruit reach 6.2% and investigates the relationship between soluble solids accumulation and postharvest performance assessed as softening and expression of chilling injury. This has been done using fruit from 10 orchards harvested at a range of SSC from 5 to 10% during one season. Soluble solids accumulation showed a general trend for a change from slow to more rapid accumulation during the season that could be described by a single logistic curve. The point at which the rate of soluble solids accumulation increased was more or less distinct for fruit from different orchards and occurred when fruit were at SSC between 6.3 and 7.4%. It is also possible that there is not a consistent change in soluble solids accumulation rate, with the rate being dependent on the environmental conditions over several days before measurement. There was a major change in softening pattern and low temperature breakdown susceptibility between fruit harvested at 6.4 and at 8.0% SSC. This change coincided with a change to faster soluble solids accumulation at harvest. It is concluded that the pattern, or rate, of soluble solids accumulation is likely to be a more robust indicator of the physiological state of the fruit, and therefore postharvest performance, than a single SSC value. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The changing soluble solids content (SSC) of kiwifruit during fruit development is a reflection of the carbohydrate supply to the fruit, its partitioning into soluble and insoluble components, and any conversion from starch to sugars. The soluble component is largely soluble sugars and the SSC measurement using a refractometer is a simple way of estimating them, but includes any other soluble material that affects the refractive index of the juice (Harman and Watkins, 1986). The insoluble components include the structural cell wall components and also the starch. The SSC of ‘Hayward’ kiwifruit has been used as a harvest index for about 30 years, based on the initial finding that storage performance was related to SSC (Harman, 1981). A minimum SSC of 6.2% was established in New Zealand for ‘Hayward’ kiwifruit for export. It should be noted that this is a minimum, not optimum, harvest index for storage performance and fruit for long-term storage tend to be harvested at a higher SSC (e.g., 7 – 9%). With a SSC of 6.2% as a threshold for harvesting, there has been much research into being able to predict the time at which fruit
∗ Corresponding author at: The New Zealand Institute for Plant and Food Research Limited, 120 Mt Albert Road, Sandringham, Private Bag 92169, Auckland 1142, New Zealand. Tel.: +64 9 925 7237; fax: +64 9 925 7001. E-mail address: [email protected] (J. Burdon). 0925-5214/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.postharvbio.2013.01.009
would reach 6.2%, and thereby assist harvest scheduling. Much of the research has looked at models that use the temperatures for the whole growing season and sometimes including aspects such as flowering date (Salinger et al., 1993; Snelgar et al., 1993; Hall and McPherson, 1997; Pailly et al., 1999; Zoffoli et al., 1999). Generally, an inverse relationship was found between temperature and the rate of SSC accumulation. However, the change in SSC is not linear and the timing of fruit reaching 6.2% has been determined to be more associated with the temperatures more immediately before measurement (Hopkirk et al., 1989; Seager et al., 1991a, 1991b, 1996, Burdon et al., 2007), rather than a linear accumulation of heat units throughout the growing season. Hence there is a limited capacity to predict the timing of fruit reaching 6.2% more than a couple of weeks ahead, particularly in climates that vary from year to year. The 6.2% harvest index has been adopted elsewhere (OECD, 1992), yet the use of 6.2% as a harvest index in New Zealand is about the physiological state of the fruit at that SSC rather than just the SSC value. In New Zealand, when initially established, the 6.2% SSC index was selected as a time at which the fruit had changed from nett accumulation of starch to nett breakdown, resulting in an increased rate of soluble solids accumulation (Beever and Hopkirk, 1990). This can be seen as a marker for the change from fruit growth to fruit ripening. However, fruit may still reach 6.2% without starch breakdown, through a prolonged period of slow accumulation of soluble carbohydrate.
2
J. Burdon et al. / Postharvest Biology and Technology 80 (2013) 1–8
The exposure of fruit to lower temperatures not only increases the rate of soluble solids accumulation, but also acclimates the fruit to withstand low storage temperatures, thereby improving storage performance by reducing the susceptibility to chilling injury (Sfakiotakis et al., 2005; Burdon et al., 2007). How dependent reduced chilling sensitivity is on the increased soluble solids accumulation is not known; it may be coincidental, but with both processes responding to lower environmental temperatures. Thirty years on from the introduction of the 6.2% SSC harvest index, the New Zealand ‘Hayward’ kiwifruit industry is markedly different. Yields have increased from <18 to >35 tonnes per hectare and orchard management practices such as girdling have been adopted to increase carbohydrate flow into the fruit (Patterson and Currie, 2011): these practices also alter the timing of fruit reaching 6.2% SSC (Burdon et al., 2011). The purpose of this study is to examine the soluble solids accumulation in ‘Hayward’ kiwifruit beyond the simple timing at which fruit reach 6.2% SSC and to investigate the relationship between soluble solids accumulation and postharvest performance assessed as the softening and expression of chilling injury (low temperature breakdown; LTB) that occurs during storage. This has been done using fruit from 10 orchards harvested at a range of SSC from 5 to 10% during one season, with soluble solids accumulation monitored during maturation allowing the patterns and rates of soluble solids accumulation to be assessed. 2. Methods 2.1. Fruit ’Hayward’ kiwifruit (Actinidia deliciosa C.F. Liang and A.R. Ferguson) were sourced from ten orchards (designated O1–O10) around Te Puke in the Bay of Plenty Region of New Zealand. On each orchard, fruit came from approximately 12 vines, either as bays or as individual vines, depending on the orchard layout. Fruit monitoring and harvesting commenced on 30 March 2009. Fruit were monitored for SSC using samples of 30 fruit at approximately 4-d intervals and harvested when at on average 5.2, 5.9, 6.4, 8 and 10% SSC, designated H1, H2, H3, H4 and H5, respectively. At each harvest, four crates of fruit (approximately 25 kg of fruit per crate) were taken from each orchard. Fruit were held for two days at ambient conditions under cover to simulate curing before packing into modular bulk (MB; 10 kg loose fill) packs with polybags and storing at 0 ◦ C. Ten MB packs were filled at each harvest, with five packs designated for firmness assessments through storage, and five packs for disorder assessments. 2.2. Fruit assessments For each orchard/harvest, a 30-fruit sample was measured for SSC and firmness. Firmness in storage was measured on 30-fruit samples after 3, 6, 8 and 10 weeks (H1 and H2) or 6, 10, 16 and 20 weeks (H3–H5) at 0 ◦ C. After storage, and when at <9.8 N (1 kgf ) firmness during shelf-life, fruit were assessed for physiological, pathological and physical disorders.
each other. Firmness was measured as kgf and data converted to N, where 1 kgf = 9.8 N. Soluble solids content was determined as the average of the stylar and stem ends of fruit measured separately during maturity monitoring and at harvest using a hand-held refractometer (Master Series, 0–30%, Atago). 2.3.2. Disorders All fruit from 5 MB packs per orchard/harvest were assessed for physiological, pathological and physical disorders. The incidence of low temperature breakdown (LTB) was assessed after cutting each fruit transversely starting from the stylar end and looking for watersoaked and/or mealy appearance in the flesh. With the incidence of disorders other than LTB being low and sporadic across orchards and harvests, only LTB data have been analysed further. 2.4. Data analysis Soluble solids data were investigated for patterns in the rate of accumulation, the rate of accumulation at 6.2%, the timing of reaching 6.2% and any marked increases in the rate of soluble solids accumulation. Graphics and fitted curves were created using Origin v7.5 (OriginLab Corporation, One Roundhouse Plaza, Northampton, MA 01060, USA). A logistic curve, of the form y = ((A1 − A2 )/(1 + (x/x0 )p )) + A2 , where A1 = initial value, A2 = final value, x0 = centre and p = power, or two linear lines, was fitted to the SSC data for each orchard. The SSC data to which linear fits were made were selected by eye. For much of the data, investigation of trends and patterns was emphasised as much as were strict statistical differences among orchards or harvests. The strength of relationships among SSC, the rate of soluble solids accumulation and firmness at harvest with softening in storage and the incidence of LTB after storage were determined as correlation coefficients. Mean LTB values were subjected to analysis of variance (ANOVA) using GenStat Release 8.1 [(PC/Windows XP) Copyright 2003, Lawes Agricultural Trust (Rothamsted Experimental Station)]. The incidences of LTB were angular transformed (arcsin(sqrt(x)) before analysis. LTB data were analysed on the basis of incidence, with no assessment of severity. Data in the table of LTB incidence are untransformed. 3. Results and discussion 3.1. Soluble solids accumulation The SSC of fruit from the end of March to the end of May 2009 was plotted and logistic curves fitted to the data (Fig. 1). For all orchards, the logistic curves fitted with R2 values in excess of 0.95 (Table 1). From the fitted curves, the fruit reached 6.2% SSC at Table 1 Coefficient of regression (R2 ) and the date and rate of soluble solids accumulation when at 6.2% soluble solids content from the ‘Hayward’ kiwifruit soluble solids data in Fig. 1. Orchard
R2
1 2 3 4 5 6 7 8 9 10
0.994 0.992 0.988 0.960 0.972 0.990 0.982 0.967 0.977 0.980
2.3. Fruit assessment methodology 2.3.1. Firmness and soluble solids Fruit firmness was measured using a Fruit Texture Analyser (Güss, model GS14, South Africa) fitted with a 7.9-mm Effegi (Alfonsine, Italy) penetrometer probe after removal of skin and flesh to a depth of approximately 1 mm. The probe was driven into the flesh at 20 mm/s to a depth of 7.9 mm, and the maximum force recorded as the firmness value. Firmness was measured twice at the midpoint of each fruit, with the two measurements taken at 90◦ to
At 6.2% soluble solids content Date
Rate (%/d) × 100
19 April 14 April 14 April 17 April 16 April 18 April 23 April 24 April 19 April 24 April
6.7 5.6 5.8 7.4 4.2 8.8 7.4 7.3 7.2 7.2
J. Burdon et al. / Postharvest Biology and Technology 80 (2013) 1–8
O1
O2
13
O3
13
O4
13 12
11
11
10
10
10
10
10
9 8
9 8
9 8
SSC (%)
12
11
SSC (%)
12
11
SSC (%)
12
11
SSC (%)
12
9 8
8
7
7
7
7
6
6
6
6
6
5
5
5
5
5
13
r r r y y Ma Ap Ap Ma Ma 2 7 1 0 24 8 2 2
O6
13
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O7
13
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O8
13
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O9
13
12
12
11
11
11
11
11
10
10
10
10
10
8
9 8
9 8
SSC (%)
12
SSC (%)
12
SSC (%)
12
9
9 8
8
7
7
7
7
6
6
6
6
6
5
5
5
5
5
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O10
9
7
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O5
9
7
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
SSC (%)
13
SSC (%)
SSC (%)
13
3
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
Fig. 1. Soluble solids accumulation in ‘Hayward’ kiwifruit from 10 orchards (O1–O10) during maturation in 2009. Each data point is the mean of 30 fruit and the fitted curves are logistic functions.
between 14 and 24 April (Table 1) and at 6.2% SSC fruit had rates of soluble solids accumulation between 0.04 and 0.09%/d. However, fitting the logistic curve to the range of SSC data collected predetermines that the rate of SSC will increase continuously with time up to the centre of the curve, and decline thereafter. Discussion of soluble solids accumulation often focuses on identifying a point at which there has been a change in rate, from slow to fast, indicative of the change to nett starch breakdown. The logistic fit to the data gives a continually increasing rate over the period of interest, although the full soluble solids accumulation curve would also include a period of slowing accumulation as fruit approached the maximum SSC value depending on the amount of starch available. There was little obvious slowing in soluble solids accumulation within the period sampled. Alternative approaches to describing the soluble solids accumulation data have included fitting a linear component early during fruit development to a later logistic curve (A.J. Hall personal communication) Given that it is the point at which fruit change from a nett starch accumulation to starch breakdown that is important as an indicator of fruit physiology, rather than SSC per se, the data were re-examined by fitting two linear components to data selected by eye. First, an upper SSC component was fitted using the higher SSC values and then a second, lower line fitted for the lower earlier season SSC values (Fig. 2; Table 2). The limitations in connectivity and smoothness of joining two linear fits are acknowledged if they were going to be used to create a single description, but this element of modelling is not relevant to the way the data are being investigated here. The two linear lines create a break-point between the slower and faster periods of soluble solids accumulation. While this is a simple procedure at the end of the season when all the data have been collected, it is much more difficult to identify the break-point in real time.
As with the logistic fits, the two linear fits to the data tended to have high coefficients of regression at >0.9 (Table 2). However, with the reduced sample numbers, the goodness of fit was very dependent on individual samples. Taking the intersection of the two linear lines as the break-point, the timing of the break-points for the orchards was quite consistent at between 27 April and 5 May. These dates are later than the time at which fruit reached 6.2% SSC (Table 1), and occurred when fruit SSC was between 6.3 and 7.4% (Table 2). This variability in SSC at which the change to rapid SSC occurs illustrates the problem of using a single at-harvest SSC value as being predictive of the physiological state of the fruit. The rates of soluble solids accumulation of the lower linear components (0.04–0.07%/d; Fig. 2) were of the same magnitude as for the fruit at 6.2% SSC determined from the logistic fits in Fig. 1. This suggests that the fruit at 6.2% SSC were still in the slower phase of
Table 2 Coefficient of regression (R2 ), rate of soluble solids accumulation and date and soluble solids content (SSC) at the break-point between two (lower and upper) linear fits to the ‘Hayward’ kiwifruit SSC data from Fig. 2. Orchard
1 2 3 4 5 6 7 8 9 10
R2
Rate (%/d) × 100
Break-point
Lower
Upper
Lower
Upper
Date
SSC (%)
0.962 0.972 0.960 0.941 0.968 0.974 0.904 0.889 0.908 0.882
0.978 0.966 0.980 0.723 0.929 0.955 0.904 0.893 0.916 0.885
5.5 6.9 7.4 5.7 4.2 5.8 4.4 3.9 5.5 4.3
16.7 16.9 20.1 12.8 18.5 18.7 15.4 15.2 17.4 21.1
2 May 27 April 29 April 28 April 1 May 30 April 29 April 30 April 3 May 5 May
6.9 7.4 7.4 6.9 7.0 6.8 6.4 6.3 7.1 6.7
4
J. Burdon et al. / Postharvest Biology and Technology 80 (2013) 1–8
O1
O2
13
O3
13
O4
13 12
11
11
10
10
10
10
10
9 8
9 8
9 8
SSC (%)
12
11
SSC (%)
12
11
SSC (%)
12
11
SSC (%)
12
9 8
8
7
7
7
7
6
6
6
6
6
5
5
5
5
5
13
r r y y r Ma Ap Ap Ma Ma 27 10 24 8 22
O6
13
r r y y r Ma Ap Ap Ma Ma 27 10 24 8 22
O7
13
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O8
13
r r y y r Ma Ap Ap Ma Ma 27 10 24 8 22
O9
13
12
12
11
11
11
11
11
10
10
10
10
10
8
9 8
9 8
9 8
SSC (%)
12
SSC (%)
12
SSC (%)
12
9
8
7
7
7
7
6
6
6
6
6
5
5
5
5
5
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O10
9
7
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
O5
9
7
r r y y r Ma Ap Ap Ma Ma 27 10 24 8 22
SSC (%)
13
SSC (%)
SSC (%)
13
r r r y y Ma Ap Ap Ma Ma 27 10 24 8 22
Fig. 2. Soluble solids accumulation in ‘Hayward’ kiwifruit from 10 orchards (O1–O10) during maturation in 2009. Each data point is the mean of 30 fruit and the upper (䊉) and lower () linear fitted lines have been fitted to data selected by eye. Data not used for linear fits ().
soluble solids accumulation, i.e., in a growth phase of development rather than ripening. Under this circumstance, the prediction of the time to reach 6.2% SSC is simple from the linear relationship, but 6.2% SSC would not be indicative of a changed physiological state, with the fruit still being in a growth phase. The rates of soluble solids accumulation for the upper linear components were in the range 0.15–0.21%/d, with no obvious sign of a decline in rate that would be expected as fruit reached their maximal SSC as starch becomes depleted. With dry matter contents in the range 15–18% at harvest (data not presented), ripe fruit SSC (rSSC) would be expected to be approximately 12–15% based on the relationship that exists between dry matter and rSSC (Burdon et al., 2004). This relationship between dry matter and rSSC has also led to suggestions of using the SSC/rSSC ratio as a harvest index (Feng et al., 2003, 2006). However, as with using the SSC value, the ratio is not indicative of the physiological state of the fruit, since the SSC at which the physiological change occurs is not fixed, and therefore the ratio does not improve on the use of SSC alone. For some of the orchards, there was not a clear break-point between the slower and faster phases of soluble solids accumulation, with a gradual change being more apparent (e.g., O6 and O10). This raises the question as to whether a clean break-point should be expected. With the change in soluble solids accumulation rate being a result of the nett effect of starch synthesis and starch degradation, if the soluble solids accumulation rate is the result of the relative rates of each process, and not an on-off switch, then a more or less sharp change could be expected depending on the conditions that modulate the two processes. Starch synthesis and degradation have been shown to occur simultaneously in banana (Hill and Ap Rees, 1994). In tomato, starch turnover has been reported to remain constant during fruit development whilst starch synthesis was dependent on sucrose supply (N’tchobo et al., 1999). While there has been recent research into the control of
starch production (Nardozza et al., 2010, 2011) and general carbohydrate metabolism (Moscatello et al., 2011) in kiwifruit, there has been limited research into starch breakdown and its control (Wegrzyn and MacRae, 1995). This discussion can be extended to consider whether a consistent change in soluble solids accumulation rate should be expected, or whether the rate may fluctuate depending on environmental conditions. If all data points are accepted as real, rather than fitting smoothed logistic or linear lines, then periods of slower or faster soluble solids accumulation may be identified (Burdon et al., 2011). The ability to modulate the rate of soluble solids accumulation by altering temperature has been demonstrated under controlled environmental conditions by Seager et al. (1996). Temperature shifts resulted in predictable changes in soluble solids accumulation, with faster accumulation at lower temperatures. However, soluble solids accumulation can also be affected by the fruit growth rate. Early in the season, SSC may be described as increasing only slowly, if at all, as if to suggest little carbohydrate flow into the fruit. Yet at this time there is a significant flow of carbohydrate into the fruit that allows for SSC to be maintained during the rapid increase in fruit size. Conversely, a slowing of the rate of fruit expansion and a constant inflow of carbohydrate could result in an increase in soluble solids accumulation rate without any need for starch breakdown. In the future, studies of soluble solids accumulation will include aspects of starch metabolism, fruit growth and environmental temperatures. 3.2. Softening in storage There was only limited softening of fruit whilst on the vine, on average from 90 to 70 N (∼9 to 7 kgf ) over the two months between H1 and H5. Fruit at H1 and H2 were harvested at an average SSC of 5.2 and 5.8%, respectively, and only stored for 10 weeks since
J. Burdon et al. / Postharvest Biology and Technology 80 (2013) 1–8
O1
100
Firmness (N)
Firmness (N)
100 80 60 40 20 0
8
12
16
20 0 8
12
16
O5
16
20
4
8
12
16
20
4
8
12
16
20
4
8
12
16
20
4
8
12
16
20
60 40 20
100
60 40 20
O6
80 60 40 20 0
0
4
8
12
16
20
0
O7 Firmness (N)
100
80 60 40 20 0
4
8
12
16
O9
60 40 20 0
60 40 20
100
80
4
8
12
16
0
O10
80 60 40 20 0
20
O8
80
0
20
Firmness (N)
Firmness (N)
12
O4
0
0
Firmness (N)
8
80
20
Firmness (N)
Firmness (N)
4
80
0
4
0 0
100
20
100
40
0
40
0
O3
60
100
60
20
Firmness (N)
Firmness (N)
4
80
100
O2
80
0 0
100
5
0
Storage Period (Weeks) H1;
H2;
Storage Period (Weeks) H3;
H4;
H5
Fig. 3. Firmness of ‘Hayward’ kiwifruit during storage at 0 ◦ C after harvest on five occasions (H1–H5). Fruit were harvested from 10 orchards (O1–O10) between 30 March and 22 May 2009. Values are the mean of 30 fruit.
they were known to be of low maturity and prone to disorder if stored long-term. Fruit from H3–H5 were stored for 20 weeks. The softening curves for the fruit from the five harvests from each of the 10 orchards are given in Fig. 3. The generic storage softening curve of ‘Hayward’ kiwifruit is usually described as sigmoidal with three phases: a slow initial phase followed by a fast phase and then a final slow phase (MacRae and Redgwell, 1992). In later harvested fruit, only the second and third phases may be present, giving an exponential decay curve, the fruit having reached a stage of development on the vine such that the first slow phase has already been passed prior to harvest. Differences in firmness during storage may arise through differences in initial softening rate, the timing of the change to rapid softening, the rapid softening rate, the firmness at which the softening rate changes to slow, and the rate during the final slow phase.
Overall, there was little difference in the patterns of softening for fruit from the 10 orchards (Fig. 3). There was a change from a slow linear pattern of softening for fruit from H1 and H2, to an exponential pattern of softening in fruit from H4 and H5 (Fig. 3). The softening curves for fruit from H4 and H5 can be regarded as typical for commercially harvested ‘Hayward.’ Overall, the H3 softening curves tended to be very similar to those for H2, suggesting little change in the capacity to soften between H2 and H3, i.e., for fruit harvested at SSC of 5.8 and 6.4%. Thus, the main change in the capacity to soften occurred between H3 and H4, i.e., for fruit harvested at between 6.4 and 8.0% SSC. Among the softening curves for H4 fruit, there were differences among the 10 orchards. For some orchards, the H4 softening curve tended to be similar to the H5 curve (e.g., O7), whereas for others (e.g., O9) the H4 softening curve was closer to the H3 curve than the H5 curve. This suggests that even at H4
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J. Burdon et al. / Postharvest Biology and Technology 80 (2013) 1–8
Table 3 Rate of softening of ‘Hayward’ kiwifruit over the first 6 weeks of storage at 0 ◦ C for fruit harvested from 10 orchards. Fruit were harvested on five occasions (H1–H5) over the period 30 March–22 May 2009. Values have been calculated from data in Fig. 3. Rate of softening (N/week) for weeks 0–6 in storage Orchard
H1
H2
H3
H4
H5
O1 O2 O3 O4 O5 O6 O7 O8 O9 O10
1.81 2.77 2.85 2.03 2.55 3.24 3.44 3.63 3.83 2.65
3.29 3.57 3.67 2.48 3.70 4.67 2.67 2.61 2.97 3.13
3.32 3.92 4.07 4.07 2.50 5.41 4.82 4.46 4.67 3.83
4.85 5.27 3.69 6.76 4.57 4.92 7.16 6.67 4.57 4.91
6.96 6.06 5.39 7.13 5.83 6.39 8.16 6.71 7.55 6.91
Mean
2.88
3.28
4.11
5.34
6.71
(approximately 8% SSC at harvest), there were differences among the fruit from different orchards in the capacity to soften in storage. The differences in softening are reflected in the rates of softening over the first 6 weeks of storage (Table 3), with rates increasing with maturity, but with clear differences among orchards at each harvest. The data suggest a continuum between H3, H4 and H5, with an increasing rate of softening over the first 6 weeks of storage. However, there is only a limited discrimination possible among orchards since the fruit were first sampled at 6 weeks and any short-term differences earlier in storage will have been missed, i.e., softening may have been slower for 1–2 weeks after harvest, but when not assessed until week 6 this slow period only reduces the overall softening rate. The time in storage taken to reach 9.8 N (1 kgf ) is a commonly used comparison for the storage performance of ‘Hayward’ kiwifruit. However, in the trial, few fruit softened to 9.8 N in storage and the time to 19.6 N (2 kgf ) has been taken for comparisons of softening in storage among fruit from H3–H5, i.e., those fruit harvested at >6.2% SSC. There was a general trend for a shorter time to 19.6 N for fruit from H5 (average 10 weeks) than from H3 and H4 (14–15 weeks), and for some treatments there was little difference between H3 and H4 (range 11–18 weeks; Table 4).
Table 4 Time taken for ‘Hayward’ kiwifruit harvested at >6.2% soluble solids content (SSC) to soften to 19.6 N (2 kgf ) during storage at 0 ◦ C for fruit harvested from 10 orchards. Fruit were harvested on three occasions: H3, H4 and H5, when at 6.4, 8.0 and 10.0% SSC, respectively. Values have been interpolated from the data in Fig. 3. Orchard
Time (weeks) to 19.6 N (2 kgf ) H3
H4
H5
O1 O2 O3 O4 O5 O6 O7 O8 O9 O10
17.2 15.2 15.7 16.0 23.0a 11.0 15.6 14.1 14.4 15.2
18.1 14.9 13.0 15.0 19.4 11.6 13.5 14.2 13.7 15.1
9.1 9.1 10.4 10.4 14.6 8.7 10.6 11.1 9.5 10.9
Mean
15.74 (14.93)b
14.85 (14.34)b
10.44 (9.98)b
a b
Extrapolated value. Value in parenthesis excludes O5 value.
3.3. Correlations between at-harvest fruit characteristics and softening in storage Using data for fruit harvested above 6.2% SSC, i.e., combining H3, H4 and H5 data, there were strong correlations between firmness at harvest, SSC, the rate of soluble solids accumulation at harvest (see Table 5) and the time taken to soften to 19.6 N. These strong correlations are not surprising given the wide range of maturity of the fruit investigated. However, the correlations between harvest firmness, SSC, or rate of soluble solids accumulation at harvest with firmness at the end of storage were not significant (Table 5). The lack of association with firmness at the end of storage is probably because the softening pattern was not linear and if left long enough, firmness values for fruit from all orchards tend to come together during the final slow phase of softening. For fruit from H3, harvested at an average SSC of 6.4%, neither firmness nor SSC at harvest was a good predictor of firmness after 20 weeks of storage, the time taken to soften to 19.6 N, or the rate of softening over the first 6 weeks of storage (Table 5). The lack of associations is perhaps not surprising given the very similar firmness and SSC values for these fruit from the 10 orchards at a single harvest. Commercially, it is important to be able to discriminate among fruit from different orchards for postharvest performance when harvested at about the same time, and when the fruit appear similar by SSC or firmness values. While looking for at-harvest characteristics with which to discriminate among fruit from different orchards, it is important to note that a large component of commercial postharvest performance is dependent on fruit handling and coolstore operation. In contrast to SSC or firmness at harvest, there were relatively strong correlations between the rate of soluble solids accumulation and both the firmness after 20 weeks of storage and the rate of softening over the first 6 weeks of storage for H3 fruit (Table 5). There was a less strong association between soluble solids accumulation rate and time to 19.6 N. These associations were less strong in later harvested fruit (data not shown). The softer fruit after storage and faster initial softening rate of fruit harvested with a more rapid soluble solids accumulation are indicative of the more advanced physiological state and capacity to soften of these fruit, despite there being little difference in firmness at harvest. 3.4. Disorders Disordered fruit mostly showed symptoms of LTB with only sporadic low incidences of other disorders or rots. LTB is a physiological disorder in which the softening of fruit is affected resulting in areas of outer pericarp with a granular appearance or, in more severe instances, with water soaked tissues (Lallu, 1997). Overall, the incidence of LTB decreased in later harvested fruit, which also had higher SSC at harvest. There was a marked decrease in LTB incidence from 35 to 13% for fruit harvested at H1 and H2 (both stored for 10 weeks) and from 30 to 11% for fruit harvested at H3 and H4 or H5 (stored 20 weeks) (Table 6). The nature of LTB, progressive and increasing with longer storage, would mean that the LTB incidence in fruit from H1 and H2 would be higher if stored for longer, giving incidences in excess of those found in H3 fruit by 20 weeks. For the fruit harvested at on average 6.4%, the high incidence of LTB (on average 30%) shows that the fruit were still chilling sensitive, possibly lacking maturity as shown by a low soluble solids accumulation rate suggesting fruit to still be in a growth phase, and also possibly associated with a lack of temperature acclimation (Sfakiotakis et al., 2005, Burdon et al., 2007). The 10% LTB incidence in fruit harvested at H4 and H5 is unusual, as fruit harvested at 8–10% SSC would normally be expected not to be chilling sensitive. This suggests that while the soluble solids
J. Burdon et al. / Postharvest Biology and Technology 80 (2013) 1–8
7
Table 5 ANOVA P-values and correlation coefficients (r) for correlations between at-harvest ‘Hayward’ kiwifruit firmness (FF), soluble solids content (SSC) and rate of soluble solids accumulation (SSC rate) with softening in storage assessed as firmness after 20 weeks, time taken to reach 19.6 N (2 kgf ) and softening rate over the first 6 weeks of storage. Data are for fruit from harvests 3–5 combined (H3–H5) and from harvest 3 (H3) alone. At-harvest characteristic
Anova P-value and correlation coefficient (r) for correlations between at harvest characteristics and softening in storage
H3–H5 FF SSC SSC rate H3 FF SSC SSC rate
Firmness after 20 weeks
Time to 19.6 N (2 kgf )
P
r
P
r
P
r
0.823 0.567 0.871
−0.043 0.109 −0.031
<0.001 <0.001 <0.001
0.632 −0.684 −0.647
0.017 <0.001 <0.001
−0.434 0.749 0.695
0.248 0.640 0.031
−0.403 0.170 −0.679
0.708 0.953 0.104
−0.136 0.022 −0.544
0.357 0.854 0.070
0.327 −0.067 0.594
accumulation rate had increased, the chilling sensitivity had not been completely removed. It is therefore possible that there are different temperature thresholds or different control mechanisms for the two processes (starch breakdown/temperature sensitivity). LTB is not normally a problem commercially in New Zealand, and the incidences found in this trial may reflect the more rapid cooling applied to the trial fruit compared with the slower cooling that would occur commercially in packed and palletised fruit, i.e., overnight compared with a week or more, with the faster cooling exacerbating LTB expression (Lallu, 1997; Lallu and Webb, 1997). The chilling sensitivity and softening patterns are both suggestive of delayed maturation of the fruit, with changes in chilling sensitivity and softening patterns suggesting a change in fruit physiology occurring between H3 and H4, i.e. in fruit harvested at between 6.4 and 8.0% SSC. This change coincides with cooler temperatures from the start of May and the period of increased SSC accumulation. However, any temperature acclimation did not totally remove the susceptibility to chilling. The chilling sensitivity and softening patterns may also be linked by chilling affecting the rate or pattern of softening in storage. With LTB affecting the texture of ripe fruit, it is possible that softening earlier in storage is affected by chilling before LTB symptoms are visible.
Table 6 The incidence of low temperature breakdown (LTB) in ‘Hayward’ kiwifruit from 10 orchards after harvest on five occasions (H1–H5) and storage at 0 ◦ C for 10 (H1 and H2) or 20 (H3–H5) weeks before assessment during shelf-life at 20 ◦ C when firmness was <9.8 N (1 kgf ). Values are the means of five packs with approximately 90 fruit per pack. Orchard
LTB incidence (%) H1
O1 50 31 O2 43 O3 47 O4 O5 9a O6 38 26 O7 40 O8 39 O9 O10 33 Mean 36 Statistical analysis: ANOVA <0.001 P values 7.2 L.S.D. a
H2
H3
H4
H5
12 23 16 7 17 15 10 8 10 15 13
44 30 39 33 29 24 12 26 26 38 30
7 12 8 11 9 9 10 22 10 11 11
10 3 9 6 6 14 22 16 20 8 11
<0.001 5.1
<0.001 6.8
<0.001 3.7
<0.001 3.8
Fruit did not soften to <9.8 N (1 kgf ) during 2 weeks at 20 ◦ C.
Softening weeks 0–6
3.5. Correlations between at-harvest measurements and disorders Correlations between firmness at harvest, the SSC at harvest, or rate of soluble solids accumulation at harvest and LTB incidence were investigated (Table 7). When the data for H3–H5 were combined, there were significant relationships between the incidence of LTB and firmness at harvest, SSC at harvest, and the rate of soluble solids accumulation at harvest. This is not surprising given the wide range of fruit development investigated and the general association that exists in kiwifruit between reduced chilling susceptibility and advanced fruit development (Burdon et al., 2007). However, when only H3 data were used, there were no significant associations between firmness, SSC, or the rate of soluble solids accumulation at harvest with LTB incidence (Table 7). 4. Summary and conclusion Soluble solids accumulation in ‘Hayward’ kiwifruit showed a general trend for a change from slow (0.04–0.07%/d) to more rapid (0.1–0.2%/d) accumulation during the season. The point at which the rate increased was more or less distinct for fruit from different orchards and occurred when fruit were at between 6.3 and 7.4% SSC. Overall, the pattern or rate of soluble solids accumulation appears to be more indicative of the physiological state of the fruit than the SSC value alone. There was a major change in softening pattern and LTB susceptibility between fruit harvested at H3 and H4, i.e., fruit harvested at 6.4 and 8.0% SSC. This change between fruit at H3 and H4 coincided with a change to faster soluble solids Table 7 ANOVA P-value and correlation coefficient (r) for correlations between at-harvest ‘Hayward’ kiwifruit firmness (FF), soluble solids content (SSC) and rate of soluble solids accumulation (SSC rate) with the incidence of low temperature breakdown (LTB) after storage. Data are for fruit from harvests 3, 4 and 5 (H3–H5) combined and for harvest 3 (H3) alone. At-harvest characteristic
H3–H5 FF SSC SSC rate H3 FF SSC SSC rate
ANOVA P-value and correlation coefficient (r) for correlations between at-harvest characteristics and LTB incidence after storage P
r
0.001 <0.001 <0.001
0.568 −0.635 −0.663
0.128 0.521 0.205
−0.514 0.231 −0.439
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accumulation and a change to cooler weather after the start of May. The association between softening in storage and soluble solids accumulation rate was stronger than with either SSC or firmness values at harvest. Susceptibility to LTB was reduced in later harvested fruit, but with little association with SSC, soluble solids accumulation rate or firmness within an individual harvest. The 10% incidence of LTB in fruit at H4 and H5 is unusual. Both the LTB incidence and the softening curves are indicative of the fruit having delayed maturation. It is concluded that 6.2% SSC is still a useful marker as a minimum threshold for harvesting, but may not be completely reliable as an indicator of postharvest performance in all seasons. The pattern, or rate, of soluble solids accumulation is likely to be a more robust indicator of the physiological state of the fruit, and therefore postharvest performance, than a single SSC value. Acknowledgements The authors gratefully acknowledge the assistance of the orchard owners and managers for facilitating the sourcing of fruit and the orchard staff at Plant & Food Research, Te Puke Research Centre for assistance with harvesting. This research was funded by ZESPRI Group Ltd., the New Zealand Foundation for Research, Science and Technology Programme C06X0706 and the Plant & Food Research KRIP Project 2010-05. References Beever, D.J., Hopkirk, G., 1990. Fruit development and fruit physiology. In: Warrington, I.J., Weston, G.C. (Eds.), Kiwifruit Science and Management. Ray Richards Publisher/New Zealand Society for Horticultural Science Inc., Auckland/Wellington, pp. 97–126. Burdon, J., McLeod, D., Lallu, N., Gamble, J., Petley, M., Gunson, A., 2004. Consumer evaluation of ‘Hayward’ kiwifruit of different at-harvest dry matter contents. Postharvest Biol. Technol. 34, 245–255. Burdon, J., Lallu, N., Francis, K., Boldingh, H., 2007. The susceptibility of kiwifruit to low temperature breakdown is associated with pre-harvest temperatures and at-harvest soluble solids content. Postharvest Biol. Technol. 43, 283–290. Burdon, J., Lallu, N., Pidakala, P., Barnett, A., 2011. Is the 6.2◦ Brix soluble solids harvest index suitable for ‘Hayward’ kiwifruit from high productivity orchard management systems? Acta Hort. 913, 539–546. Feng, J., MacKay, B.R., Maguire, K.M., 2003. Variation in firmness of packed Hayward kiwifruit. Acta Hort. 610, 211–217. Feng, J., Maguire, K.M., MacKay, B.R., 2006. Discriminating batches of ‘Hayward’ kiwifruit for storage potential. Postharvest Biol. Technol. 41, 128–134.
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