Intermittent Warming during Cold Storage of Peaches Packed in Perforated Polypropylene

Lebensm.-Wiss. u.-Technol., 31, 38–43 (1998)

Intermittent Warming during Cold Storage of Peaches Packed in Perforated Polypropylene J. Pablo Fernandez-Trujillo ´ and Francisco Artes* ´ Postharvest and Refrigeration Laboratory, Food Science and Technology Department, CEBAS – CSIC, PO Box 4195, 30080 Murcia (Spain) (Received February 21, 1997; accepted May 21, 1997)

Firm-breaker peaches (Prunus persica L. Batsch cv. ‘Paraguayo’) were stored for up to 3 weeks at 0.5 °C in perforated polypropylene (PPP) bags and/or subjected to intermittent warming (IW) of three cycles of 1 d at 20 °C every 6 d of storage at 0.5 °C in PPP. Flesh firmness, pH, total soluble solids, titratable acidity, colour, taste and total losses were monitored. The reduction in ethylene emission observed in conventionally stored fruit supports the hypothesis that the ethylene-synthesizing system is impaired during the latency period of chilling injury. The transient peak in respiration activity observed in IW fruit after 12 h warming was probably associated with the restoration to normal metabolism. Both PPP and PPP + IW maintained fruit freshness and prevented chilling injuries and weight losses. PPP fruit required a post-storage ripening period. Although IW slightly increased senescence, the shelf-life was extended and the peaches were preferred for colour and taste. PPP + IW might be a useful commercial method for maintaining quality and extending shelf-life of peaches.

©1998 Academic Press Limited Keywords: chilling injuries; fungal growth; respiration activity; ethylene emission

Introduction Peaches suffer internal browning and other physiological disorders when stored at low temperatures for up to 2 weeks (1). Intermittent warming (IW) has been applied to alleviate such chilling injuries (CI) (1, 2). For example, IW partially alleviated woolliness in peaches and nectarines by promoting normal ripening and maintaining the capacity to produce adequate levels of pectolytic enzymes (mainly endopolygalacturonase) during cold storage (2–5). The use of controlled atmospheres in combination with IW has been shown to limit CI and extend the storage life of fruit to a greater extent than either technique used alone (6). The use of PVC packaging plus IW was also effective in controlling CI (4). Although there is much evidence pointing to the benefits of using IW during the cold storage of peaches, several problems arise when such a technique is applied on a commercial scale due to the development of softening, senescence symptoms and skin injuries (cracking, discoloration and subsequent fungal attacks). The warming parameters must be optimized in order to avoid water condensation (1, 2, 6–8). Modified atmosphere packaging creates a water-saturated atmosphere in the sealed enclosure around the fruit. This reduces water loss and shrinkage, and delays membrane disintegration (9, 10). Alternatively, fruit *To whom correspondence should be addressed.

can be stored in perforated bags without the disadvantages often associated with individual seal-packaging such as the development of off-flavours and the increased likelihood of decay and spoilage due to a phytotoxic atmosphere (low O2 and excessive CO2 and C2H4) which may develop around the enclosed fruit (9–11). In fact, perforated polyethylene bags have been used to store peaches at 2 °C and 85–95% RH for 30 d with good results (12). The present work was conducted to study the effect of IW on the quality parameters of cold stored peaches in PPP, which to the best of our knowledge, has not been previously reported.

Materials and Methods Experimental design Firm-breaker peaches (Prunus persica L. Batsch cv. ‘Paraguayo’) were harvested from an orchard in Cieza (Murcia) and immediately transported by ventilated car 5 km to a packing house, where they were sorted in a commercial line, and selected for uniform size and appearance and freedom from defects. The mean measurements and standard error (SE) of the fruit were: axial diameter 43.2 ± 1.3 mm, longitudinal diameter 74.2 ± 4.3 mm and 137.4 ± 14.4 g weight. The same day, boxes of 5 kg each were transported 35 km by ventilated car to the laboratory at Murcia, where they were forced-air precooled to reach 5 °C internal fruit

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flesh temperature within 5 h, and 0.5 °C within 12 h. The following morning, peaches were randomly divided into bags of six fruits each to be sealed in 42.5 ± 3.8-µm thick nonoriented perforated (33 holes of diameter 2 mm per square dm) polypropylene (PPP) film (DERPROSA, Madrid, Spain). Four batches of six bags each were stored continuously at 0.5 °C. Another four batches were periodically exposed to IW (PPP + IW, see below). Fruits from the bags (30 fruits per experimental variant) were inspected and analysed weekly during 3 weeks of storage and after an additional 3 d of ripening at 20 °C and 70–75% RH without opening the bags. In addition, five batches of four replicates of 24 fruit each were placed in a plastic box with the fruit touching. One batch was stored for normal postharvest ripening at 20 °C and 95% RH for 10 d (N20), two in continuous cold storage at 0.5 °C (CS), and two for exposure to IW. One box for each treatment was used for quality parameter analysis and the others for evaluating weight losses, CI and decay (after 10 d at 20 °C in N20 fruit, and after 3 weeks with or without poststorage ripening in CS and IW fruit). Losses were expressed as percent on a fresh weight basis at harvest and CI was divided into four classes (very slight, slight, moderate and severe injury). Only moderate and severe levels were considered as losses according to Artes ´ et al. (2). The fruits were placed inside 360 L gas-tight stainless steel chambers in a cold room at 0.5 ± 0.5 °C and 95% RH. The chambers were equipped with a renewalhumidification system with an air flow of 360 L/h (13). Peaches subjected to continuous cold storage were placed in a cold room at 0.5 ± 0.5 °C and 90–95% RH. In the case of IW storage, two warmings were applied every 6 d by removing the corresponding boxes and PPP bags from the cold room to another room at 20 °C and 95% RH. After 1 d of warming the fruits were retransferred to 0.5 °C (13). All fruits were examined for decay at the beginning of storage and at weekly intervals. In summary, the treatments were: PPP, PPP + IW, N20, CS and IW. The mean half-time to warm and/or cool individual fruit when transferred from the cold (0.5 °C) to warm temperature (20 °C) was determined by periodically recording the temperature indicated by a thermocouple inserted in the pulp touching the pit cavity of the five fruit used for this purpose. For both warming and cooling periods this half-time was 58 min and the standard error (SE) of the temperature measurements was 0.5 °C.

angle colour index [tan–1(b*/a*)] was calculated in the peel (areas free from any blush) and in the flesh (H* ground and H* flesh colour, Minolta CR-300 colorimeter, C standard C.I.E. illumination, 0° viewing). The three fresh firmness values and colour readings were averaged for each peach and the mean of the measurement for five fruits from each replicate was recorded. The overall sensory and visual quality of the peaches which had been ripened for 3 d at 20 °C after 3 weeks of storage were scored using a 5-point hedonic scale (1 = extremely poor; 5 = excellent). The test panel consisted of five men and two women; two bags of six fruits each were used for each treatment.

Carbon dioxide and ethylene emission analysis CO2 and C2H4 measurements were made in five replicates of two fruits each kept inside 650 mL gastight jars for 1 h at 0 °C and 20 min at 20 °C prior to gas sampling. CO2 was determined with a Hewlett Packard 5730A model gas chromatograph with a Porapak column. C2H4 was determined by a Perkin Elmer Autosystem gas chromatograph with FID detector and Porapak column. The measurement error was about 0.1% for CO2 and 1.5% for C2H4, with a detection limit of 0.025 µL C2H4/kg.

Statistical Analysis The experimental design was completely randomized. Each plastic bag containing six fruits constituted a replicate. The percentage of total losses was statistically analysed by converting values to their respective arcsin transformation (15). Given the factorial structure of the two treatments applied, an analysis of variance (ANOVA) and residual analysis (16) was performed according to the following statistical model: Yil = µ + ti + IWj + ti 3 IWj + ε (ij)l

Eqn [1]

where Yij l is the lth replicate of the ith Time (i = 0, 1, 2, 3, in weeks) and jth IW (j = 0, 1). Parameter µ is the mean effect, whereas ε is the error of the model’s estimate. If significant differences were found, Duncan multiple range test or orthogonal polynomial contrasts were performed (15). A one-way analysis of variance and LSD test of quality and sensory parameters after 3 d post-storage ripening (in PPP and PPP + IW) or after 10 d of ripening (in N20) were performed to establish differences between treatments.

Results and Discussion Quality parameter analysis and sensory test The quality parameters measured in triplicate on five fruits each were firmness (Fruit Pressure Tester Effegi 327 penetrometer, 7.9-mm probe tip, readings at 20 °C), total soluble solids (Atago N1 hand refractometer, readings at 20 °C), titratable acidity (AOAC (14), in meq malic acid/L) and pH (Crison pH meter). The Hue

At the end of 3 weeks of cold storage, total losses remained less than 10% w/w for all treatments and woolliness was not detected in any treatment (Table 1). Transfer from 0 to 20 °C resulted in slight water condensation within the bags, particularly during the first hours of the warming period. However, after poststorage ripening of PPP + IW fruit only slight

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symptoms of fungal growth (3.3% by Alternaria sp. and 3.2% by Penicillium sp.) occurred. These fungal attacks could be due to slight condensation occurring during the rewarming periods or to secondary contamination associated to senescent fruit in the high RH within the bags. Therefore, it may be advisable to warm the fruit gradually over a 12-h period to reduce condensation on the fruit and minimize possible skin injuries and discolorations (8). No evidence of fungal attacks appeared in IW fruit in air, but N20 fruit were strongly affected (30% w/w Rhizopus nigricans, 2% of Monilinia spp. and 1% of Cladosporium spp.) (Table 1). CI (mainly woolliness associated with vitrescence and scald in the outer part of the mesocarp under the skin) appeared after post-storage ripening (particularly in CS fruit), with no significant difference between PPP and PPP + IW treatments (Table 1). Some slight vitrescence and woolliness occurred in PPP and PPP + IW fruit that could be related to the slight difficulty some fruit had in ripening. At firm-breaker stage of maturity, the latency period to CI could be less than 6 d at 0.5 °C. A more advanced ripening stage at harvest may therefore be desirable for this peach cultivar (to avoid the development of woolliness in IW storage). Afterwards, the reduced CI in PPP fruit could be related to storage temperature. In a previous experiment with firm-breaker and firm-mature fruit stored at 2 °C in PPP bags for 3 weeks plus post-storage ripening, weight losses were 3.6% and 3.9%, respectively, and 51% and 55% (%w/w) PPP fruit was lost due to CI (data not shown). Alleviation of water stress in PPP during post-storage ripening could be related with the low incidence of woolliness that was not detected in control fruit in air. In fact, the atmosphere surrounding PPP fruit (near to water vapour saturation) presumably delayed disintegration of the cell membranes (17), which is a primary effect of chilling-induced stress (18). Weight losses in PPP + IW were between 31 to 36% higher during cold storage than in PPP fruit (0.6% weight loss per week), while 3 d of post-storage ripening reduced this difference to 7%, in agreement with a previous report (2). The slight differences in weight losses among treat-

ments subjected or not to IW did not sufficiently explained the effect of IW in the alleviation of CI (Table 1). Fruit subjected to IW did not need a post-storage ripening period because this exacerbated overripeness and subsequent fungal attacks, in agreement with previous results (14). However, PPP fruit needed this period to reach commercial acceptability. The best sensory quality after 3 weeks of cold storage was shown by PPP + IW fruit (4 points in a 0–5 hedonic scale; LSD (5%) = 0.4) and by the same fruit after post-storage ripening (4.7 points, LSD (5%) = 0.3). PPP fruit also showed a good external appearance at the end of cold storage (3.5 points), but remained at the firm-breaker stage after post-storage ripening, when it appeared less attractive (3 points, LSD (5%) = 0.4), due to the different maturity stage and slight symptoms of a dry and mealy texture. There was slight browning but no off-flavours were detected. For visual quality, PPP + IW fruits were preferred to PPP fruit by all panellists (4 points to 3 points, respectively, LSD (5%) = 0.6), but differences were reduced after the post-storage ripening period (4.8 points to 4 points, LSD (5%) = 0.5) due to the yellow colour which developed in PPP fruit. PPP + IW fruit remained similar to N20 fruit. IW induced controlled ripening in peaches (Fig. 1; Table 2), which was mainly manifested by a two-phase reduction in flesh firmness (a linear reduction in the first week with a deceleration or deviation from linear thereafter), and an enhancement of H* ground and flesh colour, in agreement with results obtained in nectarines (5). Flesh firmness of PPP fruit did not attain the levels of N20 or PPP + IW fruit, probably due to abnormal ripening. No differences in pH values were found, but slight changes in total soluble solids and titratable acidity between PPP and PPP + IW fruit were found. Results in PPP fruit agree with a previous report (12). The slight differences in colour parameters between PPP and PPP + IW after post-storage ripening indicated that ethylene production in PPP fruit was great enough to induce a fast degradation of chlorophyll (mainly in the flesh). Changes in the H* value of

Table 1 Mean lossesa after ripening in air at 20°C (N20), or cold storage. Measurements were made after 3 weeks of cold storage at 0.5°C (ECS) and after 3 d of post-storage ripening at 20°C (EPR) Treatments Losses (% w/w)

N20 (20°C) 10 d

Dehydration Fungal attacks Senescence Chilling injuries (CI) Total

CS Continuous 0.5°C

IWb

PPPc

PPP+IW

ECS

EPR

ECS

EPR

ECS

EPR

ECS

EPR

3.7 f 33 b 10 b 0a

1.2 a 1.4 a 0a 0a

11.9 g 1.2 a 1.2 ab 23.4 c

2.2 cd 0a 0a 0a

12.8 g 0a 8.5 b 3.6 b

1.6 b 0a 0a 0a

2.9 de 0a 0a 3.3 ab

2.1 cd 0a 3.3 ab 0a

3.1 ef 6.4d a 9.7 b 3.3 ab

46.7 d

2.6 a

37.7 cd

2.2 a

24.9 bc

1.6 a

6.2 a

5.6 a

22.5 b

Means in the same rows followed by the same letter are not significantly different at the P <0.05 level, according to Duncan’s multiple range test. aPercent (w/w) on a fresh weight basis at harvest. bCycle of warming: 6 d at 0.5°C+1 d at 20°C. cPPP: Non-oriented perforated (33 holes of diam. 2 mm per square dm) polypropylene film. dAppearance of fungus in wounds resulting from senescence.

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the PPP + IW fruit occurred faster in the flesh than in the skin with a linear decrease in H* flesh of 4.89 units per week during cold storage (Fig. 1; Table 2). The gaseous composition within packages in PPP film were 215.5 g/L O2 and 1.4 g/L CO2 (practically that of air). C2H4 concentrations within the bags during storage and during post-storage ripening were less than 0.3 p.p.m. in both PPP and PPP + IW treatments. Peaches exhibited a climacteric behaviour (Fig. 2). The reduced respiration activity and ethylene emission in woolly fruit compared to that in healthy fruit during post-storage ripening may be due to abnormal ripening or to CI, in agreement with previous results (19, 20). These results confirm in peaches the hypothesis of Brecht and Kader for nectarines (21) about the role for CI in the loss of the C2H4 biosynthesis capacity.

It appears that an increase in C2H4 plays an important role in preventing woolliness in peaches, confirming a previous hypothesis (20). In comparison with CS fruit, ethylene emission was 10 to 20 times higher in IW fruit when retransferred to 0.5 °C after the first warming period and even higher after the second warming period (Fig. 2). The pathway was concomitant with decreased firmness and colour enhancement. Nevertheless, chilling inhibited ethylene emission in CS fruits during the first week until a minimum was reached. This difference between CS and IW fruit may be due to the effect of IW has on the recovery of ethylene biosynthesis before it reaches this minimum. This lag may be termed the latency period, confirming previous results (2, 20). The data support the hypothesis that the impairment of the ethylene-

Fig. 1 Flesh firmness, titratable acidity (TA), total soluble solids, pH and H* ground and H* flesh colour parameters in fruit stored in polypropylene bags (PPP, X) for 3 weeks at 0.5 °C or subjected to intermittent warming storage of 1 d at 20 °C every 6 d (PPP + IW, e). Solid arrows indicate intermittent warming. Bars are SE. Dashed line represents normal or post-storage ripening at 20 °C. Letters indicate differences between PPP, PPP + IW and 20 °C ripening at the end of post-storage ripening at 20 °C (LSD test at P = 0.01)

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the tissue to metabolize the excess of intermediates (phenols) or toxic substances (free radicals) accummulated during chilling. Additionally, IW might help to replenish any substances which were depleted or which could not to be synthesized during chilling (15, 18). In summary, storage of fruit in PPP prevented water stress, delayed senescence and kept CI at acceptable levels. The application of PPP + IW was a practical method to prevent peaches from mechanical damages due to transfer from cold to warm room and vice versa. When storing fruit in perforated bags, a more advanced maturity stage may be desirable in order to acquire their best sensory attributes after post-storage ripening. IW allows normal ripening and prevents woolliness. However, despite these benefits of PPP + IW, the

synthesizing system during storage could be responsible for the failure of peaches to ripen normally. This pattern has been described as the first step of the effect of chilling on cell membrane integrity (18). In IW fruit, respiration activity after 12 h of warming sharply increased but only transiently, while the normal metabolism was re-established after 24 h of warming. This phenomenon was related to the mitochondrial oxidation of different substrates that might arise from an uncoupling of oxidative phosphorylation due to chilling (16). This damage would reduce ATP levels which, in turn, could result in tissue injury (18). When fruit are still at a reversible stage of CI (during the latency period), higher metabolic activities can be induced by raising the temperature by IW. This allows

Table 2 Analysis of variance for the significance of the overall effect of treatments in the quality parameters analysed during cold storage of ‘Paraguayo’ peaches in polypropylene bags (PPP) at 0.5°C or subjected to intermittent warming of 1 d at 20°C every 6 d at 0.5°C (PPP+IW) Flesh firmness

Titratable acidity

TSS

pH

Hue ground

Hue flesh SS P

Source of variation

df.

SS Pa

SS P

SS P

SS P

SS P

Storage time (ST) linear ST quadratic ST cubic

1 1 1

20.3*** 1.6*** 0.1*

54.5*** 4.5 n.s. 0.0 n.s.

0.0 n.s. 0.0 n.s. 16.9*

78.0*** 6.0 ** 4.4*

0.4 n.s. 0.5 n.s. 2.0 n.s.

16.5** 0.3 n.s. 1.5 n.s.

Intermittent warming ST linear × IWb ST qc IW ST cubic × IW

1 1 1 1

83.3*** 7.7*** 2.3*** 0.5***

0.5 n.s. 0.0 n.s. 4.5 n.s. 9.1 n.s.

22.3* 10.1 n.s. 0.0 n.s. 2.1 n.s.

0.2 n.s. 0.9 n.s. 0.1 n.s. 0.9 n.s.

42.7*** 34.6*** 0.1 n.s. 5.0*

42.0*** 20.4*** 0.1 n.s. 0.1 n.s.

14.7 85.3

19.1 80.9

Residual % variance explained

16 7

0.5 99.5

27.3 72.7

48.3 51.7

11.1 88.9

TSS=total soluble solids. SS =Sum of squares in percentage of the total. aProbability: n.s. = not significant. *significant at P <0.05; **P < 0.01; ***P< 0.001. bST×IW interaction is shown directly decomposed in linear to cubic trends.

70

Conventional storage 0.5 °C

Intermittent warming Post-storage ripening at 20°C

60

Post-storage ripening at 20 °C

200

40

C2H4 (µL/kg/h)

CO2 (mg/kg/h)

50 20 °C storage

30

150

100

10 50 20

0

0 0

5

10

15 Time (d)

20

25

0

5

10

15 Time (d)

20

25

Fig. 2 Respiration activity (G) and ethylene emission (d) in fruit stored in air during normal ripening at 20 °C, and during conventional storage at 0.5 °C plus post-storage ripening at 20 °C, or subjected to intermittent warming storage of 1 d at 20 °C every 6 d at 0.5 °C and after transfer to post-storage ripening at 20 °C. Arrows indicate intermittent warming. Bars are SE

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warming cycles must be optimized or used with other coadjutants to reduce senescence and their associated risk of fungal attacks. The application of this technique on a commercial scale (pallets or boxes of 2 to 5 kg) deserves further research with different combinations of IW and perforated films with or without fog-avoiding coating to extend storage life by up to 3 weeks.

Acknowledgements The authors are grateful to CICYT (ALI-95/0530 Project) and Iltmo. Ayto. de Cieza (Murcia) for financial support. J.P. Fernandez-Trujillo ´ is indebted to CSIC for his Ph.D. grant; this study is part of the Ph.D. of the first author. We thank Mr A. Hernandez, ´ Mr J.A. Mart´ınez and Mr A. Cano for technical help, COFRUCIEZA Soc. Coop for providing peaches and packing house facilities and DERPROSA for providing films.

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