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Postharvest moisture loss characteristics of carrot (Daucus carota L.) cultivars during short-term storage
SCIENTIA HORTlCULfllRR ELSEVIER
Scientia Horticulturae 71 (1997) 1-12
Postharvest moisture loss characteristics of carrot (Caucus carota L.) cultivars during short-term storage S.I. Shibairo a, M.K. Upadhyaya
a3*, P.M.A. Toivonen b
aDepartment of Plant Science, University of British Columbia, Vancouuer, BC, Canada V6T 124 b Summerland Research Centre, Agriculture and Agri-Food Canada, Summerland, BC, Canada VOH IZO LArwntd .-““y.-..
17 Anril ._ ‘.y”’
1997 I I,,
Abstract Differences in moisture loss characteristics among carrot cultivars Imperator Special 58, Gold Pak 28, Caro-pride, Paramount, Eagle, Celloking, Top Pak and Caro-choice during short-term storage at 13°C and at either 80% or 35% relative humidity were investigated. Experiments were conducted over two years with an early and late harvest in each year. Moisture loss was significantly greater when carrots were stored at low relative humidity compared to high relative humidity. Consistent cultivar differences in moisture loss characteristics were observed only in the late-harvested carrots at low relative humidity. Cultivars with higher specific surface area and relative electrolyte leakage, and lower water and osmotic potentials exhibited high moisture losses. Regression analysis, however, showed that moisture loss differences among cultivars were mainly associated with the specific surface area of the root. 0 1997 Elsevier Science B.V. Keywords: Osmotic potential; Relative electrolyte leakage; Shelf life; Water potential; Water vapour pressure deficit
1. Introduction Postharvest 1977). .~~~_L______, __ .,, lose __L_ _ ---.-. --. moisture loss causes carrots to become shrivelled (Hurschka. their bright orange appearance, and become susceptible to postharvest decay (van den Berg and Lentz, 1966, 1973). Generally, the rate of moisture loss is proportional to the surface area of the carrot root and the water vapour pressure deficit (WVPD) (Apeland
* Corresponding
author.
0304-4238/97/$17X10 0 1997 Elsevier Science B.V. All rights reserved. PII SO304-4238(97)00077-O
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and Baugerod, 197 l), which is determined by the temperature and relative humidity (RH) of the surrounding air (van den Berg, 1987). Differences in size and shape may affect moisture loss from fruits and vegetables, For example, smaller tangerines have a shorter shelf life due to greater moisture loss compared to larger tangerines (Ketsa, 1990). Lownds et al. (1994) suggested that differences in the rate of moisture loss from the fruits of nine pepper cultivars may be due to differences in their surface area to volume ratios, the presence of pores and/or cracks, cuticle thickness, and epicuticular wax quantity, chemistry and distribution. Apeland and Baugerod (1971) observed that moisture loss per unit surface area from the ‘Nantes’ type carrots increased as their root size decreased. Water potential ($1 and membrane permeability are important factors in determining rates of moisture loss. The levels of sucrose and hexoses, which are major determinants of osmotic potential (I)~> in carrot roots, have been reported to change during storage (Nilsson, 1987). Changes in turgor pressure and I,!+, lead to changes in I/J_When roots have low 4, compared to that of soil solution, they draw moisture from the soil. Whether a low Cc, at harvest facilitates moisture retention by the carrot root during postharvest storage has not been investigated. Plasma membrane semipermeability is essential for maintaining the osmotic balance and hence r+!~of a cell. The plasma membrane permeability increases in carrots in response to temperature and Sclerotiniu sckrotiorum infection (Finlayson et al., 1989), and in potatoes (Spychalla and Desborough, 1990) during storage. A differential increase in plasma membrane permeability during storage may cause differences in moisture loss characteristics among carrot cultivars. Insufficient information is available concerning moisture loss characteristics of carrot cultivars during short-term storage under retail shelf conditions. An understanding of physical and physiological characteristics that influence moisture loss during short-term storage can contribute to the development of ways to enhance shelf life of carrots. The objectives of this study were to determine: (1) if carrot cultivars differ in their postharvest moisture loss characteristics during short-term storage at 13°C and (2) whether these differences can be attributed to differences in physical and/or physiological characteristics of carrot roots.
2. Materials
and methods
Carrot cultivars Imperator Special 58, Gold Pak 28, Caro-pride, Paramount, Eagle, Celloking, Top Pak and Caro-choice, grown in the Fraser Valley of British Columbia, were used in this study. ‘Gold Pak 28’, ‘Eagle’, ‘Imperator Special 58’, ‘Celloking’ and ‘Top Pak’ seeds were supplied by Stokes Seed (St. Catharines, Ontario) and ‘Caro-pride’, ‘Caro-choice’ and ‘Paramount’ seeds by the Asgrow Seed (Newmarket, Ontario). Carrots were grown at the Totem Park Field Station of the University of British Columbia between May and November 1993. The experiment was repeated during the same period in 1994. The soil was a sandy loam with a pH of 6.0, 8.9% organic matter and 9.4, 74.6, 100, 52, 100, and 16 kg ha-’ of nitrate, phosphate, calcium, magnesium, potassium and
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sodium, respectively, determined by the Kelowna extraction method (van Lierop, 1988). Fertilizers 21:0:0, 0:20:0 and 0:0:50 (N:P:K) were broadcast to provide the recommended rates of 70, 8.7 and 62.2 kg of N, P, K ha-‘, respectively, and incorporated by raking before seeding (Anonymous, 1992). N at 40 kg ha-i was top-dressed two months after planting. Carrots were grown in a randomised complete block design replicated four times. Each replication was a 2 X 5 m plot with five rows spaced 0.35 m apart. Carrots were seeded on May 15, 1993 and on May 17, 1994 using a Heistair-Stanhay precision seeder. Seedlings were thinned to 60-80 plants per meter row length three weeks after planting. Overhead irrigation was used to supplement rainfall as needed. The carrots were harvested 87 (early harvest) and 120 days (late harvest) after seeding in 1993, and 98 and 164 days after seeding in 1994. In 1994, the carrots in the second block were not uniform due to transient flooding and were therefore not harvested. Carrots were hand-harvested from the middle three rows, and their shoots were removed. The carrots were washed in cold water, gently blotted with paper towels, and used for the following measurements. 2.1. Carrot size and shape measurements Carrot root length (L>, greatest diameter (01, and weight (W) were measured and the C-value (which indicates carrot shape), and surface area (A) were calculated using the following formulae (Bleadsdale and Thompson, 1963 and H. Baugerod, Dept. of Vegetable Crops, Agric. College of Norway, Vollebekk, Norway, personal communication). C = W/(3.142
x R* x L)
A=(4xCX3.142xRxL)/(l+C) where R = D X 0.5. Since specific gravity of carrots is approximately unity (Bleadsdale and Thompson, 1963), carrot weight in grams (W) gives an accurate estimate of root volume (V). Surface area to weight ratio [specific surface area (SSA)] was, therefore, used as an estimator of the surface area to volume ratio. 2.2. Moisture
loss characteristics
of cultiuars
Six carrots per plot were chosen randomly. Three individually marked carrots were placed in each of two 0.51 X 0.56 m plastic bags perforated with 18, 4 mm diameter holes, to facilitate air and moisture movements. The bags were placed in incubators at 13°C and 35 + 3% or 80? 5% RH to provide low (LRH) and high (HRH) RH conditions, respectively. Carrot weights were recorded periodically over a three-week period. Moisture loss (W p> was expressed as a percentage of the carrot fresh weight as well as per unit surface area [transpiration coefficient (TC)]. Preliminary studies showed that respiration accounted for only a negligible portion of the total weight loss during 21 days of storage under similar conditions. Therefore, weight loss was used as a measure of moisture loss from carrots.
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2.3. $I and (dr, measurements Three ‘Gold Pak 28’, ‘Paramount’, ‘Eagle’ and ‘Celloking’ carrots harvested from each plot were placed in perforated plastic bags and incubated at LRH as described above. I) and I)~ were measured immediately after harvest and following 7, 14, and 21 days of storage. Carrot discs (5 mm diameter X 20 mm thickness) from the midsection of the carrot root were excised longitudinally from the phloem parenchyma using a cork borer. @ was determined at room temperature (22 f 2°C) by the constant mass method using 0.0, 0.15, 0.2, 0.25, 0.3 and 0.35 molal polyethylene glycol (PEG 4000; Sigma Chemical, St. Louis, MO) solutions. The Cc,of the PEG solution, where no change in the carrot disc weight occurred, was considered equal to the Cc,of the carrot discs. The I) of the PEG solutions was calculated according to Steuter et al. (1981). +!I~ of the carrots, used for I) determination described above, was also measured. Phloem parenchyma was shredded and frozen at - 85°C for two weeks, thawed at room temperature for at least 10 min and crushed with a mortar and pestle. The sap was squeezed from the crushed tissue and sieved through 2 layers of Miracloth (Calbiochem-Novabiochem, La Jolla, CA). I/I,, of the sap was measured using the molecular depression of freezing point method (Salisbury and Ross, 1991), with a thermocouple connected to a micrologger (21-Micrologger, Campbell Scientific, Logan, UT) calibrated with NaCl standards. 2.4. Relative electrolyte
leakage (REL)
REL of three carrots of each cultivar was measured after 0, 7, 14 and 21 days of storage to assess the influence of moisture loss on tissue permeability. Carrot root cores (30 mm long X 4 mm diameter) were excised longitudinally from the phloem parenchyma of the midsection of the carrot root using a No. 2 cork borer. The cores were cut into 1 mm thick discs and rinsed (3 X ) with deionised distilled water. The discs (25) were placed in 25 ml deionised, distilled water in 50 ml glass jars and incubated at 22 f 2°C. After 24 h of incubation, the increase in the conductance of the bathing medium was measured using a conductivity bridge (Model FCM-2A, Weather Measure, Sacramento, CA) to determine electrolyte leakage. At the end of the measurements, the tissue was frozen at - 85°C as described above. After thawing for 24 h, a second conductivity measurement was made. REL was expressed as the ratio of conductivity before and after tissue disintegration by freezing. 2.5. Statistical analysis Analysis of variance among the treatments was carried out using SYSTAT software (Wilkinson et al., 1992). To normalise the distribution of percent moisture loss data, they were transformed prior to analysis using the following formula: W * = As X J(Wp/lOO), where, W * is the arcsin transformed data; As, arcsin; and W p, percent moisture loss.
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Stepwise multiple regression analysis between W * and the attributes measured was performed for late-harvested ‘Gold Pak 28’, ‘Eagle’, ‘Paramount’ and ‘Celloking’ carrots at LRH. Cultivars were coded 1, 2, 3 and 4 for ‘Eagle’, ‘Gold Pak 28’, ‘Paramount’ and ‘Celloking’, respectively, in order of increasing W * . Data were fitted to the model: W*=Constant+(b,XB)+(b,Xcv.)+(b,Xd) +(b,xSSA)+(b,x$)+(b,X&,)+(b,XREL) +(b,xcv.xd)+(b,xcv.XSSA)+(b,,Xcv.xIC,) +(b,,Xcv.X&J+(b,,Xcv.XREL)+(b,,XdXlJ) +(b,4XdX(G;r)+(b,5XdXREL)+(b,6XCv.XdX~) +(b,,~cv.xd~~J+(b~sx~~.xdxREL), where B is the block effect, b, to b,, are partial regression coefficients, cv., cultivar and d, storage duration (days). The best-fit model was chosen from several models on the basis of a high R* value and the optimum Mallow’s coefficient (Cp value) (Neter et al., 1990).
3. Results 3.1. Specific su$ace
area and shape of cultivars
‘Paramount’ had the highest SSA in the late harvest of 1993, but it was statistically different only from ‘Eagle’ (Table 1). ‘Paramount’ and ‘Gold Pak 28’ had significantly (P I 0.05) higher SSA than ‘Eagle’, ‘Imperator Special 58’ and ‘Caro-pride’ in the late harvest of 1994. There was no difference among cultivars in SSA in the early harvests in both years. There was no difference in the C-value (which indicates carrot shape; Bleadsdale and Thompson, 1963) among cultivars in either year (data not shown). 3.2. Effect of cultivar on moisture loss For both early and late harvests in 1993 and 1994, carrots stored at LRH generally lost more moisture than those stored at HRH (Tables 2 and 3). There were no significant differences among cultivars in moisture loss at HRH for the early harvest of 1993, but in the late harvest ‘Gold Pak 28’ and ‘Celloking’ lost more moisture than the other cultivars studied. No differences in moisture losses were observed at HRH for either harvest in 1994. Moisture loss differences among cultivars were found in the early harvest of each year at LRH; however, the ranking of cultivars was not the same in both years. ‘Paramount’ and ‘Eagle’ lost the greatest moisture in the early harvest of 1993, whereas ‘Gold Pak 28’ and ‘Celloking’ lost the most moisture in the early harvest of 1994 under LRH conditions. Differences among cultivars held at LRH were consistent in the late harvests in both years. In 1993, ‘Eagle’ and ‘Imperator Special 58’ lost the least
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Table 1 Root specific surface area (SSA) (cm* gCultivar
‘) of carrot cultivars harvested early and late in 1993 and 1994
1993
Caro-choice Gold Pak 28 Eagle Paramount Imperator Special 58 Caro-pride Celloking Top Pak S.E. cv.
1994
Early harvest
Late harvest
Early harvest
Late harvest
1.79 1.76 1.72 1.92 1.69 1.82 1.82 1.88 0.06 Ns
1.67”b 1.77a.b 1.44s 1.99” 1.64”,b 1.80”,b 1.69a,b 1.90&b 0.06 *
1.92 2.16 1.97 2.17 2.07 2.10 2.00 1.87 0.04 Ns
1.5@ 1.92” 1.41b 1.74” I .49b 1.53b 1.61a,b 1.59a,b 0.04 I
* : Significant at P 20.05. Ns: Not significant at P I 0.05. S.E.: standard error of means. Means in column followed by different ( P I 0.05).
superscripts
are significantly
different
by the Bonferroni
procedure
moisture at LRH and ‘Celloking’, ‘Gold Pak 28’, and ‘Paramount’ the most (Table The same was true in the late harvest carrots held at LRH in 1994 (Table 3). The TC was generally higher in carrots stored at LRH compared to HRH (Table There was no difference in the TC among cultivars under both storage conditions in early harvest of 1993. However, in the late harvest, ‘Eagle’ and ‘Imperator Special
2). 4). the 58’
Table 2 Moisture losses at 7, 14, and 21 days of storage at 13°C and either 80% (HRH) or 35% (LRH) relative humidity for carrots harvested early and late in 1993 Early harvest
Late harvest
HRH d=7 Caro-choice 5.7 Gold Pak 28 4.2 Eagle 3.5 Paramount 3.3 Imperator 3.3 Care-pride 3.5 Celloking 3.3 Top Pak 6.8 S.E. 0.8 cv. d cvxd
LRH
HRH
LRH
d=l4
d=21
d=7
d=l4
d=21
d=7
d=14
d=21
d=l
d=l4
d=21
12.5 11.8 8.8 8.2 10.0 8.7 7.6 12.8 1.2 Ns *
19.0 14.9 11.8 10.6 12.6 11.9 11.0 14.6 1.4
4.8 5.5 6.8 12.6 5.3 6.1 5.1 5.7 1.2
13.3 12.2 16.8 20.9 15.2 17.2 12.3 14.3 1.8 * * *
19.8 15.4 26.3 27.3 21.1 21.2 17.4 17.5 1.8
3.6 6.8 3.5 4.0 3.5 3.4 5.6 3.8 0.7
5.2 10.5 4.9 6.8 5.1 5.6 8.2 5.4 0.9 * * *
7.1 14.1 6.6 8.8 7.0 8.0 10.5 7.1 1.1
13.1 12.9 1.9 10.6 4.5 8.0 17.4 10.4 1.6
18.1 17.7 9.4 16.1 9.0 12.4 19.8 17.8 1.9 I * *
21.5 24.6 13.4 24.9 12.8 17.0 25.6 23.4 1.9
Ns
* : Significant at P I 0.05 level. Ns: Not significant at P 5 0.05 level. ‘Imperator’ is the cultivar Imperator Special 58. cv.: cultivar; d: storage duration (days); S.E.: standard error of means.
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Table 3 Moisture losses at 7, 14, and 21 days of storage at 13°C and either 80% (HRH) or 35% (LRH) relative humidity for carrots harvested early and late in 1994 Early harvest
Late harvest
HRH
Caro-choice Gold Pak 28 Eagle Paramount Imperator Caro-pride Celloking Top Pak SE. cv. d cv.xd
LRH
LRH
HRH
d=7
d=14
d=21
d=l
d=14
d=21
d=7
d=14
d=21
d=7
d=14
d=21
6.5 4.3 4.1 6.7 5.1 7.5 6.1 5.8 0.7
9.2 6.7 6.4 10.9 8.0 11.1 9.1 8.6 1.0 Ns *
12.1 9.6 8.5 14.3 9.8 14.0 11.1 11.9 1.1
8.9 13.3 6.8 8.9 7.3 10.3 12.5 9.6 1.4
14.3 19.3 12.7 13.1 11.7 16.3 19.2 14.5 I .8 * * *
18.5 23.3 15.0 15.9 14.2 21.2 23.1 17.9 2.0
4.3 4.6 3.3 4.6 2.8 3.8 3.9 3.2 0.6
5.5 6.7 5.3 7.0 5.1 6.0 8.9 5.4 0.9 Ns *
9.2 9.6 7.3 9.5 7.4 8.5 13.1 7.1 0.9
9.5 12.0 7.1 12.9 9.0 9.5 9.1 8.5
16.1 19.5 12.1 21.6 12.6 15.3 16.9 14.4 1.2 * * *
22.5 27.2 16.6 30.2 18.0 21.6 23.6 19.3 1.3
Ns
1.2
Ns
* : Significant at P 5 0.05 level. Ns: Not significant at P 5 0.05 level. ‘Imperator’ is the cultivar Imperator Special 58. cv.: cultivar; d: storage duration (days); S.E.: standard error of means.
Table 4 Transpiration coefficient (mg mm -*) of carrot cultivars at 13°C and either 80% (HRH) or 35% (LRH) relative humidity in early and late harvests of 1993 and 1994 1993
1994
Early harvest
Caro-choice Gold Pak 28 Eagle Imperator Caro-pride Celloking Top Pak S.E. cv. H cv.x H
Late harvest
Early harvest
Late harvest
HRH
LRH
HRH
LRH
HRH
LRH
HRH
LRH
1.06 0.87 0.70 0.56 0.68
1.12 0.85 1.53 1.41 1.33 1.21 0.96 0.96 0.10 Ns *
0.46 0.79 0.44 0.47 0.41 0.46 0.59 0.37 0.05
1.38 1.52 0.81 1.30 0.77 1.02
0.62 0.46 0.44 0.66 0.48 0.69 0.57 0.65 0.05
0.98 1.06 0.77 0.76 0.69 0.97 1.13 0.97 0.10 * * *
0.48 0.42 0.43 0.47 0.41 0.46 0.71 0.39 0.06
1.21 1.11 0.97 1.55 0.98 1.09 1.26 0.99 1.10 I *
0.61 0.76 0.07
Ns
* : Significant at P 5 0.05 level. Ns: Not significant at P 5 0.05 level. ‘Imperator’ is the cultivar Imperator Special 58. cv.: cultivar; H: storage condition (relative humidity).
1.93 1.24 0.10 * * *
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Table 5 Water potential ($), osmotic potential (ICI,) and relative electrolyte 1993 and 1994 $
1993 Gold Pak 28 Eagle Paramount Celloking SE.
leakage @EL) for late harvested carrots in
eV (MPa)
@IPa)
REL (%)
d=l
d=14
d=21
d=l
d= 14
d=21
d=7
d= 14
d=21
-0.8 -0.8 -0.9 -0.7
-0.8 - 0.7 - 1.0 -0.8 0.02
- 1.0 -0.9 - 1.0 -0.9
-
1.3 1.2 1.2 1.1
-
1.2 1.1 1.7 1.2 0.05
-1.7 -1.5 -2.1 - 1.6
43 41 55 32
46 45 6.5 38 3.1
55 51 63 76
-0.9 -0.8 -1.0 -0.9
-
1.1 1.2 1.1 1.0
-
1.3 1.2 1.4 1.3 0.02
- 1.6 -1.3 -1.8 - 1.5
30 31 31 25
29 33 31 39 0.90 *
34 33 44 44
CV.
d cv.x d
*
1994
Gold Pak 28 Eagle Celloking SE.
-0.8 -0.7 -0.8 -0.7
CV.
d cv.xd
-0.7 -0.7 -0.8 - 0.7 0.01
* *
* *
* : Significant at P IO.05 level. Ns: Not significant at P I 0.05 level. ‘Imperator’ is the cultivar Imperator Special 58. cv.: cultivar; d: storage duration in days; SE.: standard error of means.
had low TC values at LRH. In the early harvest ‘Imperator Special 58’ had lower TC values than ‘Imperator Special 58’, and ‘Top Pak’ had low ‘Celloking’ had high TC values at LRH in the late
in 1994, ‘Eagle’, ‘Paramount’, and the other cultivars at LRH. ‘Eagle’, TC values, while ‘Paramount’ and harvest in 1994.
3.3. 4, CCI,and REL Except for ‘Paramount’ in 1993, I) decreased after day 14 in all cultivars in both years (Table 5). At the end of storage, I) of ‘Paramount’ was lower than that of ‘Eagle’ in both years. In ‘Eagle’, ‘Gold Pak 28’ and ‘Celloking’ in 1993, h did not change up to day 14 and decreased sharply thereafter. &,, declined continuously in ‘Paramount’ over the storage period. In 1994, I)~ of ‘Eagle’ did not change up to day 14, but declined thereafter. It declined continuously in ‘Gold Pak 28’, ‘Paramount’ and ‘Celloking’ over the storage period. At the end of storage, qn was relatively low for ‘Paramount’ and high for ‘Eagle’ in both years. REL increased in ‘Paramount’ up to day 14 and did not change thereafter in 1993. In ‘Eagle’ and ‘Gold Pak 28’, it increased steadily over the storage period. ‘Celloking’ had the lowest REL up to day 14 but increased sharply at day 21 in 1993. In 1994, REL did
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Table 6 Parameters and statistics for best subset multiple regression models of the relationship between moisture loss (W * > and various attributes for the late-harvested 1993 and 1994 carrots stored at iow relative humidity Potential independent
variable
C”.
SSA * REL dxJ, dxREL cv.x d cv.xSSA cv.xREL cv.xdxREL
r2
b’
0.52 0.02 -0.11 - 0.42 - 1.70 0.12 -0.01 _ -
Constant B
1994
1993 b
0.10 _
_ 0.04 - 0.20 -0.87 -0.29 0.22 - 0.01 _ 0.16 -
b
0.14 0.14 0.25 0.03 0.02 0.22
0.05 - 0.08 0.21 0.01
_ -
_
-0.01 0.29 0.01 -
0.13 0.42 0.01 _ _
0.02 0.01
0.02 0.01 _
0.10 0.20
0.24 - 0.02
0.34 - 0.02
-
0.24 _
r2
b
-
-
0.14 0.18
- : Parameter not selected in the moisture loss model. Cp: Mallow’s coefficient; R2: model coefficient of determination; b: partial regression coefficient; b’: standard partial regression coefficient; r2: partial coefficient of determination. B: the block effect; cv.: cultivar; d: duration in storage; SSA: specific surface area (cm’ 8-l); I/J: water potential (MPa); &: osmotic potential (MPa); REL: relative electrolyte leakage. C, = 9.2 and R2 = 0.89 for 1993; C, = 4.15 and R2 = 0.88 for 1994.
not change significantly in ‘Eagle’, did not change up to day 14 but increased thereafter in ‘Gold Pak 28’, and steadily increased up to 21 days in ‘Paramount’ and ‘Celloking’. While only results for $, I& and REL at days 7, 14 and 21 are presented (Table 5), results similar to those at day 7 were obtained at day 0 in both years. 3.4. Multiple attributes
regressions
between
W * and the various physical
and physiological
SSA was the attribute that had the highest partial coefficient of determination (r*>, and was thus accountable for most of the variation in W * in both years (Table 6). SSA, followed by J/J, due to their high standard partial regression coefficients (b’), were more important than other attributes in estimating W * in 1993 (Table 6). However, in 1994, the interaction cv. x REL, followed by SSA, had the highest standard partial regression coefficient.
4. Discussion 4.1. Cultivar d@erences Except for the more mature carrots (late harvest), the cultivar differences in moisture loss characteristics at LRH were not consistent in the two years of this study. In late harvested carrots, ‘Eagle’ and ‘Imperator Special 58’ lost the least moisture, and
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‘Paramount’, ‘Gold Pak 28’, ‘Celloking’ and ‘Caro-choice’ the most. A relationship between cultivar maturity and postharvest moisture loss during the short-term storage has not been shown previously. Hole et al. (1987) showed that carrot cultivars differ in size during growth, reporting that relative growth rates of seedling of nine carrot cultivars were similar during the first 41 days after sowing, but changed thereafter. They suggested that genetic differences affecting partitioning between the shoot and root in the latter stages of growth were responsible for the differences in root size at the time of harvest. Our results agree with this in that cultivar differences in SSA, which partly depend on carrot size, were more pronounced in late-harvested carrots and may explain their differences in moisture loss at LRH. This study, therefore, suggests that the time of harvest is important when evaluating cultivars for shelf life improvement. Carrot shelf life has been defined as the number of days carrots remain at specified storage conditions before attaining the maximum permissible moisture loss of 8% of the initial root weight (Robinson et al., 1975). Carrots that exceed 8% moisture loss are less acceptable to consumers. When days to 8% moisture loss were calculated from the results of our study, the cultivars differed by up to 4 to 6 days in their shelf life. ‘Eagle’ and ‘Imperator Special 58’ carrots from the late harvest lost 8% moisture after about 13 and 9 days of storage at LRH in 1993 and 1994, respectively, whereas ‘Paramount’, ‘Gold Pak 28’ and ‘Celloking’ lost 8% moisture in 7th and 5th days of storage at LRH in 1993 and 1994, respectively. Carrots stored at LRH lost more moisture than those at HRH, as found by van den Berg and Lentz (1966, 1973) and van den Berg (1981). The driving force for transpiration is the WVPD, which depends on temperature and RH (Ben-Yehoshua, 1987). The rate of moisture loss is lower at HRH because of reduced WVPD between the carrot surface and the surrounding air. Inconsistent (in 1993) or lack of (in 1994) differences among cultivars in moisture loss at HRH suggests that RH during storage is more important than cultivar differences in determining the rate of moisture loss in carrots. 4.2. Multiple regressions between W * and various physical and physiological
attributes
Physical attributes of carrots can influence moisture loss. The carrot cultivars in this study had similar C-values (data not shown), indicating that shape was not responsible for the differences in their moisture loss characteristics. The average C-value was 0.5, indicating that all carrots were generally conical (Apeland and Baugerod, 1971). The SSA significantly correlated with moisture loss in both years (Table 5). ‘Eagle’ and ‘Imperator Special 58’, showed the lowest moisture loss at LRH in late harvests, and also had the smallest SSA in both years. Conversely, ‘Paramount’, ‘Gold Pak 28’ and ‘Celloking’, which showed high storage moisture loss, had high SSA in both years. These results suggest that SSA should be taken into consideration while selecting cultivars for longer shelf life. The significance of SSA in accounting for the variation in moisture loss was more important in 1994 than in 1993 (Table 6) and this could be due to later harvesting in 1994, when cultivar size differences were more pronounced. The TC, an index of the ease with which the surface of a product allows transpirational moisture loss (van den Berg, 1987), is influenced by the temperature and relative humidity (i.e. WVPD) of the storage environment, periderm thickness, and interstitial,
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cell wall and plasma membrane resistances (Kays, 1991). REL has been used as an indicator of plasma membrane permeability and cellular integrity (Finlayson et al., 1989). Cells with higher REL, therefore, are expected to exhibit higher TCs. In the late harvest at LRH, ‘Eagle’, the cultivar with the lowest moisture loss, had low REL, while ‘Paramount’ and ‘Celloking’, which exhibited the highest moisture losses, had high REL. In addition, the interaction cv X REL was selected in the best model for the 1994 data. This suggests an association of membrane permeability with high moisture loss in the late harvested carrots. Water movement in plants is governed by the I/I gradient and the conductance (permeability) of the flow path (Salisbury and Ross, 1991). Plant structures with low $ lose less moisture through transpiration (Kramer, 1983). However, we have found (Shibairo (1996) and our unpublished results) high W * in carrots with low I), suggesting greater involvement of other factors in the regulation of moisture loss. Low II, may be counteracted by a high membrane permeability, indicated by an increase in REL. An increase in cell membrane permeability may therefore be important in determining the TC; hence, the moisture loss during storage. In summary, the differences in moisture loss characteristics of cultivars became apparent only when the carrots were stored at low relative humidity. At high relative humidity, the cultivars did not differ significantly. Consistent cultivar differences are seen only in more mature (late-harvested) carrots. The differences in moisture loss characteristics appear mainly to be due to differences in the surface area to volume ratio.
Acknowledgements The authors thank the Natural Sciences and Engineering Research Council of Canada, Science Council of BC, BC Agricultural Research Council, and BC Coast Vegetable Coop. for financial support. Mr. S. Shibairo thanks the Canadian International Development Agency for a postgraduate scholarship.
References Anonymous, 1992. Vegetable production guide for commercial growers. Province of British Columbia. Ministry of Agriculture, Fisheries and Foods. Apeland, J., Baugerod, H., 1971. Factors affecting moisture loss in carrots. Acta Hart 20, 92-97. Ben-Yehoshua, S., 1987. Transpiration, water stress, and gas exchange. In: Weichmann, J. (Ed.), Postharvest Physiology of Vegetables. Marcel Dekker, New York, pp. 113-170. Bleadsdale, J.K.A., Thompson, R., 1963. An objective method of recording and comparing the shapes of carrot roots. J. Hort. Sci. 38, 232-241. Finlayson, J.E., Pritchard, M.K.. Rimmer, S.R., 1989. Electrolyte leakage and storage decay of five carrot cultivars in response to infection by Sclerotinia sclerotiorum. Can. J. Plant Pathol. 11, 313-316. Hole, CC., Morris, G.E.L., Cowper, A-S., 1987. Distribution of dry matter between shoot and storage root of field-grown carrots: I. Onset of differences between cultivars. J. Hort. Sci. 62, 335-341. Hurschka, H.W., 1977. Post-harvest moisture loss and shrivel in five fruits and vegetables. US Dept. Agric., Agric. Res. Ser., Marketing Res. Rep. No. 1059. Kays, S.J., 1991. Movement of gases, solvents, and solutes within harvested products and their exchange between the product and its external environment. In: Postharvest Physiology of Perishable Plant Products. Van Nostrand-Reinhold, New York. pp. 409-456.
12
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Ketsa, S., 1990. Effect of fruit size on moisture loss and shelf life of tangerines. J. Hort. Sci. 65, 485-488. Kramer, P.J., 1983. Water relations of plants. Academic Press, New York. Lownds, N.K., Banaras, M., Bosland, P.W., 1994. Postharvest water loss and storage quality of nine pepper (Cap&m) cultivars. HortScience 29, 191-193. Neter, J., Wasserman, W., Kutner, H.M., 1990. Regression, analysis of variance, and experimental designs. In: Applied Linear Statistical Models, 3rd edn. Irwin, Burr Ridge, IL, pp. 286-287 and 447-450. Nilsson, T., 1987. Carbohydrate composition during long-term storage as influenced by the time of harvest. J. Hort. Sci. 62, 191-203. Robinson, J.E., Browne, K.M., Burton, W.G., 1975. Storage characteristics of some vegetables and soft fruits. Ann. Appl. Biol. 81, 399-408. Salisbury, F.B., Ross, C.W., 1991. Plant Physiology, 4th edn. Wadsworth, Belmont, CA, p. 49. Shibairo, S.I., 1996. A study of postharvest moisture loss in carrots (Daucus carora L.) during short-term storage. PhD Thesis, Univ. of British Columbia, Vancouver, BC, Canada, p. 125. Steuter, A.A., Mozafar, A., Goodin, J.R., 1981. Water potential of aqueous polyethylene glycol. Plant Physiol. 67, 64-67. Spychalla, J.P., Desborough, S.L., 1990. Fatty acids, membrane permeability, and sugars of stored potato tubers. Plant Physiol. 94, 1207-1213. van den Berg, L., 1981. The role of humidity, temperature, and atmospheric composition in maintaining vegetable quality during storage. In: Teranishi, R., Barrera-Benitez, H., (Eds.), Quality of Selected Fruits and Vegetables of North America. Am. Chem. Sot., Washington, DC, pp. 95-107. van den Berg, L., 1987. Water vapour pressure. In: Weichmann, J. (Ed.), Postharvest Physiology of Vegetables. Marcel Dekker, New York, pp. 203-230. van den Berg, L., Lentz, C.P., 1966. Effect of temperature, relative humidity and atmospheric composition on changes in quality of carrots during storage. Food Technol. 20, 104-107. van den Berg, L., Lentz, C.P., 1973. High humidity storage of carrots, parsnips, rutabagas and cabbage. J. Am. Sot. Hort. Sci. 98, 129-132. van Lierop, W., 1988. Determination of available phosphorus in acid and calcareous soils with the Kelowna multi-element extractant. Soil Sci. 146, 284-291. Wilkinson, L., Hill, M.A., Welma, P.J., Birkenbeuel, K.G., 1992. SYSTAT for Windows: Statistics, Version 5 edn. SYSTAT, Evanston, IL.
Scientia Horticulturae 71 (1997) 1-12
Postharvest moisture loss characteristics of carrot (Caucus carota L.) cultivars during short-term storage S.I. Shibairo a, M.K. Upadhyaya
a3*, P.M.A. Toivonen b
aDepartment of Plant Science, University of British Columbia, Vancouuer, BC, Canada V6T 124 b Summerland Research Centre, Agriculture and Agri-Food Canada, Summerland, BC, Canada VOH IZO LArwntd .-““y.-..
17 Anril ._ ‘.y”’
1997 I I,,
Abstract Differences in moisture loss characteristics among carrot cultivars Imperator Special 58, Gold Pak 28, Caro-pride, Paramount, Eagle, Celloking, Top Pak and Caro-choice during short-term storage at 13°C and at either 80% or 35% relative humidity were investigated. Experiments were conducted over two years with an early and late harvest in each year. Moisture loss was significantly greater when carrots were stored at low relative humidity compared to high relative humidity. Consistent cultivar differences in moisture loss characteristics were observed only in the late-harvested carrots at low relative humidity. Cultivars with higher specific surface area and relative electrolyte leakage, and lower water and osmotic potentials exhibited high moisture losses. Regression analysis, however, showed that moisture loss differences among cultivars were mainly associated with the specific surface area of the root. 0 1997 Elsevier Science B.V. Keywords: Osmotic potential; Relative electrolyte leakage; Shelf life; Water potential; Water vapour pressure deficit
1. Introduction Postharvest 1977). .~~~_L______, __ .,, lose __L_ _ ---.-. --. moisture loss causes carrots to become shrivelled (Hurschka. their bright orange appearance, and become susceptible to postharvest decay (van den Berg and Lentz, 1966, 1973). Generally, the rate of moisture loss is proportional to the surface area of the carrot root and the water vapour pressure deficit (WVPD) (Apeland
* Corresponding
author.
0304-4238/97/$17X10 0 1997 Elsevier Science B.V. All rights reserved. PII SO304-4238(97)00077-O
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and Baugerod, 197 l), which is determined by the temperature and relative humidity (RH) of the surrounding air (van den Berg, 1987). Differences in size and shape may affect moisture loss from fruits and vegetables, For example, smaller tangerines have a shorter shelf life due to greater moisture loss compared to larger tangerines (Ketsa, 1990). Lownds et al. (1994) suggested that differences in the rate of moisture loss from the fruits of nine pepper cultivars may be due to differences in their surface area to volume ratios, the presence of pores and/or cracks, cuticle thickness, and epicuticular wax quantity, chemistry and distribution. Apeland and Baugerod (1971) observed that moisture loss per unit surface area from the ‘Nantes’ type carrots increased as their root size decreased. Water potential ($1 and membrane permeability are important factors in determining rates of moisture loss. The levels of sucrose and hexoses, which are major determinants of osmotic potential (I)~> in carrot roots, have been reported to change during storage (Nilsson, 1987). Changes in turgor pressure and I,!+, lead to changes in I/J_When roots have low 4, compared to that of soil solution, they draw moisture from the soil. Whether a low Cc, at harvest facilitates moisture retention by the carrot root during postharvest storage has not been investigated. Plasma membrane semipermeability is essential for maintaining the osmotic balance and hence r+!~of a cell. The plasma membrane permeability increases in carrots in response to temperature and Sclerotiniu sckrotiorum infection (Finlayson et al., 1989), and in potatoes (Spychalla and Desborough, 1990) during storage. A differential increase in plasma membrane permeability during storage may cause differences in moisture loss characteristics among carrot cultivars. Insufficient information is available concerning moisture loss characteristics of carrot cultivars during short-term storage under retail shelf conditions. An understanding of physical and physiological characteristics that influence moisture loss during short-term storage can contribute to the development of ways to enhance shelf life of carrots. The objectives of this study were to determine: (1) if carrot cultivars differ in their postharvest moisture loss characteristics during short-term storage at 13°C and (2) whether these differences can be attributed to differences in physical and/or physiological characteristics of carrot roots.
2. Materials
and methods
Carrot cultivars Imperator Special 58, Gold Pak 28, Caro-pride, Paramount, Eagle, Celloking, Top Pak and Caro-choice, grown in the Fraser Valley of British Columbia, were used in this study. ‘Gold Pak 28’, ‘Eagle’, ‘Imperator Special 58’, ‘Celloking’ and ‘Top Pak’ seeds were supplied by Stokes Seed (St. Catharines, Ontario) and ‘Caro-pride’, ‘Caro-choice’ and ‘Paramount’ seeds by the Asgrow Seed (Newmarket, Ontario). Carrots were grown at the Totem Park Field Station of the University of British Columbia between May and November 1993. The experiment was repeated during the same period in 1994. The soil was a sandy loam with a pH of 6.0, 8.9% organic matter and 9.4, 74.6, 100, 52, 100, and 16 kg ha-’ of nitrate, phosphate, calcium, magnesium, potassium and
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3
sodium, respectively, determined by the Kelowna extraction method (van Lierop, 1988). Fertilizers 21:0:0, 0:20:0 and 0:0:50 (N:P:K) were broadcast to provide the recommended rates of 70, 8.7 and 62.2 kg of N, P, K ha-‘, respectively, and incorporated by raking before seeding (Anonymous, 1992). N at 40 kg ha-i was top-dressed two months after planting. Carrots were grown in a randomised complete block design replicated four times. Each replication was a 2 X 5 m plot with five rows spaced 0.35 m apart. Carrots were seeded on May 15, 1993 and on May 17, 1994 using a Heistair-Stanhay precision seeder. Seedlings were thinned to 60-80 plants per meter row length three weeks after planting. Overhead irrigation was used to supplement rainfall as needed. The carrots were harvested 87 (early harvest) and 120 days (late harvest) after seeding in 1993, and 98 and 164 days after seeding in 1994. In 1994, the carrots in the second block were not uniform due to transient flooding and were therefore not harvested. Carrots were hand-harvested from the middle three rows, and their shoots were removed. The carrots were washed in cold water, gently blotted with paper towels, and used for the following measurements. 2.1. Carrot size and shape measurements Carrot root length (L>, greatest diameter (01, and weight (W) were measured and the C-value (which indicates carrot shape), and surface area (A) were calculated using the following formulae (Bleadsdale and Thompson, 1963 and H. Baugerod, Dept. of Vegetable Crops, Agric. College of Norway, Vollebekk, Norway, personal communication). C = W/(3.142
x R* x L)
A=(4xCX3.142xRxL)/(l+C) where R = D X 0.5. Since specific gravity of carrots is approximately unity (Bleadsdale and Thompson, 1963), carrot weight in grams (W) gives an accurate estimate of root volume (V). Surface area to weight ratio [specific surface area (SSA)] was, therefore, used as an estimator of the surface area to volume ratio. 2.2. Moisture
loss characteristics
of cultiuars
Six carrots per plot were chosen randomly. Three individually marked carrots were placed in each of two 0.51 X 0.56 m plastic bags perforated with 18, 4 mm diameter holes, to facilitate air and moisture movements. The bags were placed in incubators at 13°C and 35 + 3% or 80? 5% RH to provide low (LRH) and high (HRH) RH conditions, respectively. Carrot weights were recorded periodically over a three-week period. Moisture loss (W p> was expressed as a percentage of the carrot fresh weight as well as per unit surface area [transpiration coefficient (TC)]. Preliminary studies showed that respiration accounted for only a negligible portion of the total weight loss during 21 days of storage under similar conditions. Therefore, weight loss was used as a measure of moisture loss from carrots.
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2.3. $I and (dr, measurements Three ‘Gold Pak 28’, ‘Paramount’, ‘Eagle’ and ‘Celloking’ carrots harvested from each plot were placed in perforated plastic bags and incubated at LRH as described above. I) and I)~ were measured immediately after harvest and following 7, 14, and 21 days of storage. Carrot discs (5 mm diameter X 20 mm thickness) from the midsection of the carrot root were excised longitudinally from the phloem parenchyma using a cork borer. @ was determined at room temperature (22 f 2°C) by the constant mass method using 0.0, 0.15, 0.2, 0.25, 0.3 and 0.35 molal polyethylene glycol (PEG 4000; Sigma Chemical, St. Louis, MO) solutions. The Cc,of the PEG solution, where no change in the carrot disc weight occurred, was considered equal to the Cc,of the carrot discs. The I) of the PEG solutions was calculated according to Steuter et al. (1981). +!I~ of the carrots, used for I) determination described above, was also measured. Phloem parenchyma was shredded and frozen at - 85°C for two weeks, thawed at room temperature for at least 10 min and crushed with a mortar and pestle. The sap was squeezed from the crushed tissue and sieved through 2 layers of Miracloth (Calbiochem-Novabiochem, La Jolla, CA). I/I,, of the sap was measured using the molecular depression of freezing point method (Salisbury and Ross, 1991), with a thermocouple connected to a micrologger (21-Micrologger, Campbell Scientific, Logan, UT) calibrated with NaCl standards. 2.4. Relative electrolyte
leakage (REL)
REL of three carrots of each cultivar was measured after 0, 7, 14 and 21 days of storage to assess the influence of moisture loss on tissue permeability. Carrot root cores (30 mm long X 4 mm diameter) were excised longitudinally from the phloem parenchyma of the midsection of the carrot root using a No. 2 cork borer. The cores were cut into 1 mm thick discs and rinsed (3 X ) with deionised distilled water. The discs (25) were placed in 25 ml deionised, distilled water in 50 ml glass jars and incubated at 22 f 2°C. After 24 h of incubation, the increase in the conductance of the bathing medium was measured using a conductivity bridge (Model FCM-2A, Weather Measure, Sacramento, CA) to determine electrolyte leakage. At the end of the measurements, the tissue was frozen at - 85°C as described above. After thawing for 24 h, a second conductivity measurement was made. REL was expressed as the ratio of conductivity before and after tissue disintegration by freezing. 2.5. Statistical analysis Analysis of variance among the treatments was carried out using SYSTAT software (Wilkinson et al., 1992). To normalise the distribution of percent moisture loss data, they were transformed prior to analysis using the following formula: W * = As X J(Wp/lOO), where, W * is the arcsin transformed data; As, arcsin; and W p, percent moisture loss.
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Stepwise multiple regression analysis between W * and the attributes measured was performed for late-harvested ‘Gold Pak 28’, ‘Eagle’, ‘Paramount’ and ‘Celloking’ carrots at LRH. Cultivars were coded 1, 2, 3 and 4 for ‘Eagle’, ‘Gold Pak 28’, ‘Paramount’ and ‘Celloking’, respectively, in order of increasing W * . Data were fitted to the model: W*=Constant+(b,XB)+(b,Xcv.)+(b,Xd) +(b,xSSA)+(b,x$)+(b,X&,)+(b,XREL) +(b,xcv.xd)+(b,xcv.XSSA)+(b,,Xcv.xIC,) +(b,,Xcv.X&J+(b,,Xcv.XREL)+(b,,XdXlJ) +(b,4XdX(G;r)+(b,5XdXREL)+(b,6XCv.XdX~) +(b,,~cv.xd~~J+(b~sx~~.xdxREL), where B is the block effect, b, to b,, are partial regression coefficients, cv., cultivar and d, storage duration (days). The best-fit model was chosen from several models on the basis of a high R* value and the optimum Mallow’s coefficient (Cp value) (Neter et al., 1990).
3. Results 3.1. Specific su$ace
area and shape of cultivars
‘Paramount’ had the highest SSA in the late harvest of 1993, but it was statistically different only from ‘Eagle’ (Table 1). ‘Paramount’ and ‘Gold Pak 28’ had significantly (P I 0.05) higher SSA than ‘Eagle’, ‘Imperator Special 58’ and ‘Caro-pride’ in the late harvest of 1994. There was no difference among cultivars in SSA in the early harvests in both years. There was no difference in the C-value (which indicates carrot shape; Bleadsdale and Thompson, 1963) among cultivars in either year (data not shown). 3.2. Effect of cultivar on moisture loss For both early and late harvests in 1993 and 1994, carrots stored at LRH generally lost more moisture than those stored at HRH (Tables 2 and 3). There were no significant differences among cultivars in moisture loss at HRH for the early harvest of 1993, but in the late harvest ‘Gold Pak 28’ and ‘Celloking’ lost more moisture than the other cultivars studied. No differences in moisture losses were observed at HRH for either harvest in 1994. Moisture loss differences among cultivars were found in the early harvest of each year at LRH; however, the ranking of cultivars was not the same in both years. ‘Paramount’ and ‘Eagle’ lost the greatest moisture in the early harvest of 1993, whereas ‘Gold Pak 28’ and ‘Celloking’ lost the most moisture in the early harvest of 1994 under LRH conditions. Differences among cultivars held at LRH were consistent in the late harvests in both years. In 1993, ‘Eagle’ and ‘Imperator Special 58’ lost the least
XI. Shibairo et al. /Scientia Horticulturae 71 (1997) l-12
6
Table 1 Root specific surface area (SSA) (cm* gCultivar
‘) of carrot cultivars harvested early and late in 1993 and 1994
1993
Caro-choice Gold Pak 28 Eagle Paramount Imperator Special 58 Caro-pride Celloking Top Pak S.E. cv.
1994
Early harvest
Late harvest
Early harvest
Late harvest
1.79 1.76 1.72 1.92 1.69 1.82 1.82 1.88 0.06 Ns
1.67”b 1.77a.b 1.44s 1.99” 1.64”,b 1.80”,b 1.69a,b 1.90&b 0.06 *
1.92 2.16 1.97 2.17 2.07 2.10 2.00 1.87 0.04 Ns
1.5@ 1.92” 1.41b 1.74” I .49b 1.53b 1.61a,b 1.59a,b 0.04 I
* : Significant at P 20.05. Ns: Not significant at P I 0.05. S.E.: standard error of means. Means in column followed by different ( P I 0.05).
superscripts
are significantly
different
by the Bonferroni
procedure
moisture at LRH and ‘Celloking’, ‘Gold Pak 28’, and ‘Paramount’ the most (Table The same was true in the late harvest carrots held at LRH in 1994 (Table 3). The TC was generally higher in carrots stored at LRH compared to HRH (Table There was no difference in the TC among cultivars under both storage conditions in early harvest of 1993. However, in the late harvest, ‘Eagle’ and ‘Imperator Special
2). 4). the 58’
Table 2 Moisture losses at 7, 14, and 21 days of storage at 13°C and either 80% (HRH) or 35% (LRH) relative humidity for carrots harvested early and late in 1993 Early harvest
Late harvest
HRH d=7 Caro-choice 5.7 Gold Pak 28 4.2 Eagle 3.5 Paramount 3.3 Imperator 3.3 Care-pride 3.5 Celloking 3.3 Top Pak 6.8 S.E. 0.8 cv. d cvxd
LRH
HRH
LRH
d=l4
d=21
d=7
d=l4
d=21
d=7
d=14
d=21
d=l
d=l4
d=21
12.5 11.8 8.8 8.2 10.0 8.7 7.6 12.8 1.2 Ns *
19.0 14.9 11.8 10.6 12.6 11.9 11.0 14.6 1.4
4.8 5.5 6.8 12.6 5.3 6.1 5.1 5.7 1.2
13.3 12.2 16.8 20.9 15.2 17.2 12.3 14.3 1.8 * * *
19.8 15.4 26.3 27.3 21.1 21.2 17.4 17.5 1.8
3.6 6.8 3.5 4.0 3.5 3.4 5.6 3.8 0.7
5.2 10.5 4.9 6.8 5.1 5.6 8.2 5.4 0.9 * * *
7.1 14.1 6.6 8.8 7.0 8.0 10.5 7.1 1.1
13.1 12.9 1.9 10.6 4.5 8.0 17.4 10.4 1.6
18.1 17.7 9.4 16.1 9.0 12.4 19.8 17.8 1.9 I * *
21.5 24.6 13.4 24.9 12.8 17.0 25.6 23.4 1.9
Ns
* : Significant at P I 0.05 level. Ns: Not significant at P 5 0.05 level. ‘Imperator’ is the cultivar Imperator Special 58. cv.: cultivar; d: storage duration (days); S.E.: standard error of means.
XI. Shibairo et al. /Scientia Horticulturae 71 (1997) 1-12
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Table 3 Moisture losses at 7, 14, and 21 days of storage at 13°C and either 80% (HRH) or 35% (LRH) relative humidity for carrots harvested early and late in 1994 Early harvest
Late harvest
HRH
Caro-choice Gold Pak 28 Eagle Paramount Imperator Caro-pride Celloking Top Pak SE. cv. d cv.xd
LRH
LRH
HRH
d=7
d=14
d=21
d=l
d=14
d=21
d=7
d=14
d=21
d=7
d=14
d=21
6.5 4.3 4.1 6.7 5.1 7.5 6.1 5.8 0.7
9.2 6.7 6.4 10.9 8.0 11.1 9.1 8.6 1.0 Ns *
12.1 9.6 8.5 14.3 9.8 14.0 11.1 11.9 1.1
8.9 13.3 6.8 8.9 7.3 10.3 12.5 9.6 1.4
14.3 19.3 12.7 13.1 11.7 16.3 19.2 14.5 I .8 * * *
18.5 23.3 15.0 15.9 14.2 21.2 23.1 17.9 2.0
4.3 4.6 3.3 4.6 2.8 3.8 3.9 3.2 0.6
5.5 6.7 5.3 7.0 5.1 6.0 8.9 5.4 0.9 Ns *
9.2 9.6 7.3 9.5 7.4 8.5 13.1 7.1 0.9
9.5 12.0 7.1 12.9 9.0 9.5 9.1 8.5
16.1 19.5 12.1 21.6 12.6 15.3 16.9 14.4 1.2 * * *
22.5 27.2 16.6 30.2 18.0 21.6 23.6 19.3 1.3
Ns
1.2
Ns
* : Significant at P 5 0.05 level. Ns: Not significant at P 5 0.05 level. ‘Imperator’ is the cultivar Imperator Special 58. cv.: cultivar; d: storage duration (days); S.E.: standard error of means.
Table 4 Transpiration coefficient (mg mm -*) of carrot cultivars at 13°C and either 80% (HRH) or 35% (LRH) relative humidity in early and late harvests of 1993 and 1994 1993
1994
Early harvest
Caro-choice Gold Pak 28 Eagle Imperator Caro-pride Celloking Top Pak S.E. cv. H cv.x H
Late harvest
Early harvest
Late harvest
HRH
LRH
HRH
LRH
HRH
LRH
HRH
LRH
1.06 0.87 0.70 0.56 0.68
1.12 0.85 1.53 1.41 1.33 1.21 0.96 0.96 0.10 Ns *
0.46 0.79 0.44 0.47 0.41 0.46 0.59 0.37 0.05
1.38 1.52 0.81 1.30 0.77 1.02
0.62 0.46 0.44 0.66 0.48 0.69 0.57 0.65 0.05
0.98 1.06 0.77 0.76 0.69 0.97 1.13 0.97 0.10 * * *
0.48 0.42 0.43 0.47 0.41 0.46 0.71 0.39 0.06
1.21 1.11 0.97 1.55 0.98 1.09 1.26 0.99 1.10 I *
0.61 0.76 0.07
Ns
* : Significant at P 5 0.05 level. Ns: Not significant at P 5 0.05 level. ‘Imperator’ is the cultivar Imperator Special 58. cv.: cultivar; H: storage condition (relative humidity).
1.93 1.24 0.10 * * *
8
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Table 5 Water potential ($), osmotic potential (ICI,) and relative electrolyte 1993 and 1994 $
1993 Gold Pak 28 Eagle Paramount Celloking SE.
leakage @EL) for late harvested carrots in
eV (MPa)
@IPa)
REL (%)
d=l
d=14
d=21
d=l
d= 14
d=21
d=7
d= 14
d=21
-0.8 -0.8 -0.9 -0.7
-0.8 - 0.7 - 1.0 -0.8 0.02
- 1.0 -0.9 - 1.0 -0.9
-
1.3 1.2 1.2 1.1
-
1.2 1.1 1.7 1.2 0.05
-1.7 -1.5 -2.1 - 1.6
43 41 55 32
46 45 6.5 38 3.1
55 51 63 76
-0.9 -0.8 -1.0 -0.9
-
1.1 1.2 1.1 1.0
-
1.3 1.2 1.4 1.3 0.02
- 1.6 -1.3 -1.8 - 1.5
30 31 31 25
29 33 31 39 0.90 *
34 33 44 44
CV.
d cv.x d
*
1994
Gold Pak 28 Eagle Celloking SE.
-0.8 -0.7 -0.8 -0.7
CV.
d cv.xd
-0.7 -0.7 -0.8 - 0.7 0.01
* *
* *
* : Significant at P IO.05 level. Ns: Not significant at P I 0.05 level. ‘Imperator’ is the cultivar Imperator Special 58. cv.: cultivar; d: storage duration in days; SE.: standard error of means.
had low TC values at LRH. In the early harvest ‘Imperator Special 58’ had lower TC values than ‘Imperator Special 58’, and ‘Top Pak’ had low ‘Celloking’ had high TC values at LRH in the late
in 1994, ‘Eagle’, ‘Paramount’, and the other cultivars at LRH. ‘Eagle’, TC values, while ‘Paramount’ and harvest in 1994.
3.3. 4, CCI,and REL Except for ‘Paramount’ in 1993, I) decreased after day 14 in all cultivars in both years (Table 5). At the end of storage, I) of ‘Paramount’ was lower than that of ‘Eagle’ in both years. In ‘Eagle’, ‘Gold Pak 28’ and ‘Celloking’ in 1993, h did not change up to day 14 and decreased sharply thereafter. &,, declined continuously in ‘Paramount’ over the storage period. In 1994, I)~ of ‘Eagle’ did not change up to day 14, but declined thereafter. It declined continuously in ‘Gold Pak 28’, ‘Paramount’ and ‘Celloking’ over the storage period. At the end of storage, qn was relatively low for ‘Paramount’ and high for ‘Eagle’ in both years. REL increased in ‘Paramount’ up to day 14 and did not change thereafter in 1993. In ‘Eagle’ and ‘Gold Pak 28’, it increased steadily over the storage period. ‘Celloking’ had the lowest REL up to day 14 but increased sharply at day 21 in 1993. In 1994, REL did
XI. Shibairo et al./Scientia
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Table 6 Parameters and statistics for best subset multiple regression models of the relationship between moisture loss (W * > and various attributes for the late-harvested 1993 and 1994 carrots stored at iow relative humidity Potential independent
variable
C”.
SSA * REL dxJ, dxREL cv.x d cv.xSSA cv.xREL cv.xdxREL
r2
b’
0.52 0.02 -0.11 - 0.42 - 1.70 0.12 -0.01 _ -
Constant B
1994
1993 b
0.10 _
_ 0.04 - 0.20 -0.87 -0.29 0.22 - 0.01 _ 0.16 -
b
0.14 0.14 0.25 0.03 0.02 0.22
0.05 - 0.08 0.21 0.01
_ -
_
-0.01 0.29 0.01 -
0.13 0.42 0.01 _ _
0.02 0.01
0.02 0.01 _
0.10 0.20
0.24 - 0.02
0.34 - 0.02
-
0.24 _
r2
b
-
-
0.14 0.18
- : Parameter not selected in the moisture loss model. Cp: Mallow’s coefficient; R2: model coefficient of determination; b: partial regression coefficient; b’: standard partial regression coefficient; r2: partial coefficient of determination. B: the block effect; cv.: cultivar; d: duration in storage; SSA: specific surface area (cm’ 8-l); I/J: water potential (MPa); &: osmotic potential (MPa); REL: relative electrolyte leakage. C, = 9.2 and R2 = 0.89 for 1993; C, = 4.15 and R2 = 0.88 for 1994.
not change significantly in ‘Eagle’, did not change up to day 14 but increased thereafter in ‘Gold Pak 28’, and steadily increased up to 21 days in ‘Paramount’ and ‘Celloking’. While only results for $, I& and REL at days 7, 14 and 21 are presented (Table 5), results similar to those at day 7 were obtained at day 0 in both years. 3.4. Multiple attributes
regressions
between
W * and the various physical
and physiological
SSA was the attribute that had the highest partial coefficient of determination (r*>, and was thus accountable for most of the variation in W * in both years (Table 6). SSA, followed by J/J, due to their high standard partial regression coefficients (b’), were more important than other attributes in estimating W * in 1993 (Table 6). However, in 1994, the interaction cv. x REL, followed by SSA, had the highest standard partial regression coefficient.
4. Discussion 4.1. Cultivar d@erences Except for the more mature carrots (late harvest), the cultivar differences in moisture loss characteristics at LRH were not consistent in the two years of this study. In late harvested carrots, ‘Eagle’ and ‘Imperator Special 58’ lost the least moisture, and
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‘Paramount’, ‘Gold Pak 28’, ‘Celloking’ and ‘Caro-choice’ the most. A relationship between cultivar maturity and postharvest moisture loss during the short-term storage has not been shown previously. Hole et al. (1987) showed that carrot cultivars differ in size during growth, reporting that relative growth rates of seedling of nine carrot cultivars were similar during the first 41 days after sowing, but changed thereafter. They suggested that genetic differences affecting partitioning between the shoot and root in the latter stages of growth were responsible for the differences in root size at the time of harvest. Our results agree with this in that cultivar differences in SSA, which partly depend on carrot size, were more pronounced in late-harvested carrots and may explain their differences in moisture loss at LRH. This study, therefore, suggests that the time of harvest is important when evaluating cultivars for shelf life improvement. Carrot shelf life has been defined as the number of days carrots remain at specified storage conditions before attaining the maximum permissible moisture loss of 8% of the initial root weight (Robinson et al., 1975). Carrots that exceed 8% moisture loss are less acceptable to consumers. When days to 8% moisture loss were calculated from the results of our study, the cultivars differed by up to 4 to 6 days in their shelf life. ‘Eagle’ and ‘Imperator Special 58’ carrots from the late harvest lost 8% moisture after about 13 and 9 days of storage at LRH in 1993 and 1994, respectively, whereas ‘Paramount’, ‘Gold Pak 28’ and ‘Celloking’ lost 8% moisture in 7th and 5th days of storage at LRH in 1993 and 1994, respectively. Carrots stored at LRH lost more moisture than those at HRH, as found by van den Berg and Lentz (1966, 1973) and van den Berg (1981). The driving force for transpiration is the WVPD, which depends on temperature and RH (Ben-Yehoshua, 1987). The rate of moisture loss is lower at HRH because of reduced WVPD between the carrot surface and the surrounding air. Inconsistent (in 1993) or lack of (in 1994) differences among cultivars in moisture loss at HRH suggests that RH during storage is more important than cultivar differences in determining the rate of moisture loss in carrots. 4.2. Multiple regressions between W * and various physical and physiological
attributes
Physical attributes of carrots can influence moisture loss. The carrot cultivars in this study had similar C-values (data not shown), indicating that shape was not responsible for the differences in their moisture loss characteristics. The average C-value was 0.5, indicating that all carrots were generally conical (Apeland and Baugerod, 1971). The SSA significantly correlated with moisture loss in both years (Table 5). ‘Eagle’ and ‘Imperator Special 58’, showed the lowest moisture loss at LRH in late harvests, and also had the smallest SSA in both years. Conversely, ‘Paramount’, ‘Gold Pak 28’ and ‘Celloking’, which showed high storage moisture loss, had high SSA in both years. These results suggest that SSA should be taken into consideration while selecting cultivars for longer shelf life. The significance of SSA in accounting for the variation in moisture loss was more important in 1994 than in 1993 (Table 6) and this could be due to later harvesting in 1994, when cultivar size differences were more pronounced. The TC, an index of the ease with which the surface of a product allows transpirational moisture loss (van den Berg, 1987), is influenced by the temperature and relative humidity (i.e. WVPD) of the storage environment, periderm thickness, and interstitial,
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cell wall and plasma membrane resistances (Kays, 1991). REL has been used as an indicator of plasma membrane permeability and cellular integrity (Finlayson et al., 1989). Cells with higher REL, therefore, are expected to exhibit higher TCs. In the late harvest at LRH, ‘Eagle’, the cultivar with the lowest moisture loss, had low REL, while ‘Paramount’ and ‘Celloking’, which exhibited the highest moisture losses, had high REL. In addition, the interaction cv X REL was selected in the best model for the 1994 data. This suggests an association of membrane permeability with high moisture loss in the late harvested carrots. Water movement in plants is governed by the I/I gradient and the conductance (permeability) of the flow path (Salisbury and Ross, 1991). Plant structures with low $ lose less moisture through transpiration (Kramer, 1983). However, we have found (Shibairo (1996) and our unpublished results) high W * in carrots with low I), suggesting greater involvement of other factors in the regulation of moisture loss. Low II, may be counteracted by a high membrane permeability, indicated by an increase in REL. An increase in cell membrane permeability may therefore be important in determining the TC; hence, the moisture loss during storage. In summary, the differences in moisture loss characteristics of cultivars became apparent only when the carrots were stored at low relative humidity. At high relative humidity, the cultivars did not differ significantly. Consistent cultivar differences are seen only in more mature (late-harvested) carrots. The differences in moisture loss characteristics appear mainly to be due to differences in the surface area to volume ratio.
Acknowledgements The authors thank the Natural Sciences and Engineering Research Council of Canada, Science Council of BC, BC Agricultural Research Council, and BC Coast Vegetable Coop. for financial support. Mr. S. Shibairo thanks the Canadian International Development Agency for a postgraduate scholarship.
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