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Effect of hot water treatment on leaf extension growth, fresh weight loss and color of stored minimally processed leeks
Postharvest Biology and Technology 39 (2006) 56–60
Effect of hot water treatment on leaf extension growth, fresh weight loss and color of stored minimally processed leeks Pavlos Tsouvaltzis a , Anastasios S. Siomos a,∗ , Dimitrios Gerasopoulos b b
a Department of Horticulture, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Department of Food Science and Technology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
Received 4 February 2005; accepted 27 June 2005
Abstract Freshly harvested leeks (Allium porrum L.) were heated by immersion in water at 50, 52.5, 55 or 57.5 ◦ C for 0–60, 0–35, 0–20 and 0–15 min, respectively. After hot water treatment, leeks were cooled in water at ambient temperature for 10 min and then cut at 22 cm from the compressed stem of the root base, weighed, had color measured and stored at 4 ◦ C for 9 days. Untreated stalks (without immersion in a hot water bath) were used as controls. Hot water treatments at 50 ◦ C for 40–60 min, 52.5 ◦ C for 25–35 min, 55 ◦ C for 17.5–20 min and 57.5 ◦ C for 10–15 min efficiently controlled postharvest leaf extension growth in stalks stored for 9 days. However, treatments that controlled leaf extension growth showed fresh weight loss significantly higher than the control. There was only a slight effect of heat treatment on color attributes of stored minimally processed leek. © 2005 Elsevier B.V. All rights reserved. Keywords: Allium porrum L.; Telescoping; Quality; Ready to use leek
1. Introduction Leeks may provide a challenge as a minimally processed product since they are a vegetable that requires a minimal processing before consumption, which includes leaf trimming, root cutting, decayed-leaf removal and stalk cutting to a precise length. Attempts of some food chains to offer minimally processed prepackaged leeks has failed due to rapid quality deterioration mainly due to postharvest leaf extension growth, as well as dehydration and discoloration. Hong et al. (2000) has described postharvest leaf extension growth of fresh-cut green onions. This extension is also referred to as ‘telescoping’ (Cantwell et al., 2001). Leaf extension growth causes a rapid deterioration of the overall market quality of the product, reducing its appearance, which is the most significant attribute during the buying stage of minimally processed products (Ragaert et al., 2004). Moreover, during common processing of leek, intensely colored ∗
Corresponding author. Tel.: +30 2310 998646; fax: +30 2310 998609. E-mail address: [email protected] (A.S. Siomos).
0925-5214/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.postharvbio.2005.06.009
pigments are often formed and leek homogenates may turn pink or red within several hours (K¨orner and Berk, 1967; Kubec et al., 2004). During the past few years there has been increasing interest in the use of postharvest heat treatments (Lurie, 1998; Ferguson et al., 2000; Fallik, 2004), since the overall quality of fresh produce that was treated at optimal hot water temperatures and duration was significantly better than that of untreated, as determined by a sharp reduction in decay incidence and maintenance of several quality traits (Fallik, 2004). Postharvest growth-related phenomena of horticultural commodities can also be affected by heat treatment; geotropic curvature of green asparagus spears (Paull and Chen, 1999) and sprouting of potatoes (Ranganna et al., 1998) have been reported to be successfully controlled by short hot water dips. A pre-storage heat treatment of 55 ◦ C for 2 min effectively controlled leaf extension growth of fresh-cut green onions, and in combination with controlled atmosphere, provided a shelf life of more than 2 weeks at 5 ◦ C (Hong et al., 2000; Cantwell et al., 2001).
P. Tsouvaltzis et al. / Postharvest Biology and Technology 39 (2006) 56–60
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2. Materials and methods
in the L* , a* , b* color space. Chroma [C* = (a*2 + b*2 )0.5 ] and hue angle [h◦ = tan−1 (b* /a* ) when a* > 0 and b* > 0 or h◦ = 180◦ + tan−1 (b* /a* ) when a* < 0 and b* > 0) were calculated from a* and b* values (Lancaster et al., 1997). L* refers to the lightness, ranging from 0 = black to 100 = white, chroma represents color saturation which varies from dull (low value) to vivid color (high value) and hue angle is defined as a color wheel, with red–purple at an angle of 0◦ , yellow at 90◦ , bluish-green at 180◦ , and blue at 270◦ (McGuire, 1992).
2.1. Plant material and handling
2.4. Statistical analysis
Leeks (Allium porrum L.) produced in the area of Central Macedonia, Greece under usual production practices, were harvested brought to the laboratory in plastic bags, trimmed (leaf tips, roots and root base part at nearly 1 cm), had the decayed leaves removed and were washed in tap water.
Experiments were conducted as completely randomized designs with five individual stalks per treatment. Data analysis was done by an analysis of variance, with mean separation by LSD or Duncan’s multiple range test at the 0.05 level.
This study was undertaken to investigate the effects of hot water treatment on minimally processed leeks in order to determine the optimum hot water temperature and duration of exposure that is necessary to control leaf extension growth in addition to minimizing fresh weight loss and color changes during storage.
2.2. Treatments Preliminary experiments were performed under different time–temperature combinations of hot water treatment (HWT) to determine the damage limits. Subsequently, the experiment was performed using time–temperature combinations that caused no damage to the commodity. Five individual leek stalks (22–36 mm in diameter) were immersed in a hot water bath held at temperatures of 50 ◦ C for 10, 20, 30, 40, 50 and 60 min, at 52.5 ◦ C for 5, 10, 15, 20, 25, 30 and 35 min, at 55 ◦ C for 2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20 min and at 57.5 ◦ C for 1, 2, 3, 4, 5, 7.5, 10, 12.5 and 15 min. After hot water treatment, leeks were immediately cooled in water at ambient temperature for 10 min and then placed on absorbent paper to remove the excess surface water, cut at 22 cm from the compressed stem of the root base, weighed, had color measured and were then kept on plastic disks in air at 4 ◦ C for 9 days. Untreated stalks (without immersion in a hot water bath) were used as controls. The trays were covered with wet paper to maintain high humidity conditions. The experiment was conducted twice. The data presented are from the second experiment, but very similar results were obtained from the first one.
3. Results and discussion 3.1. Maximum leaf extension growth (MLEG) Storage of untreated (control) cut stalks at 4 ◦ C for 9 days resulted in MLEG as high as 7.5 mm, following a hyperbolic growth curve (r = 0.788, P < 0.0001) (Fig. 1). Heat treatment of cut stalks to 50 ◦ C for 10 min, to 52.5 ◦ C for 5–10 min or to 57.5 ◦ C for 2–4 min showed appreciably higher MLEG than in the controls by day 9 of storage (Fig. 2). However, MLEG at day 9 of storage was progressively less with increasing treatment duration and it was reduced almost to zero values when treatment duration was more than 30, 20, 15 and 7.5 min at 50, 52.5, 55 and 57.5 ◦ C, respectively (Fig. 2). The pattern of MLEG development during the storage period of 9 days was similar in all temperature treatments and for this reason only results for HWT at 52.5 ◦ C are presented (Fig. 1).
2.3. Evaluations Maximum leaf extension growth (MLEG) of cut leek stalks was measured every three days with a caliper from the cut surface of the leaf base to the end of the most extended portion. The fresh weight of each stalk was monitored every three days and data were expressed as % fresh weight loss (FWL). Color readings were taken every three days in each individual stalk at the base cross section and the mid-surface using a chromameter (Minolta CR-200, Minolta, Osaka, Japan), equipped with an 8 mm measuring head and a C illuminant (6774 K). The meter was calibrated using the manufacturer’s standard white plate. Color changes were quantified
Fig. 1. Maximum leaf extension growth (MLEG) during the storage period of 9 days at 4 ◦ C of cut leek stalks subjected to hot water treatment (HWT) at 52.5 ◦ C for 0–35 min. Each data point is the mean of five replications. The vertical bar represents the 5% LSD.
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P. Tsouvaltzis et al. / Postharvest Biology and Technology 39 (2006) 56–60
Fig. 2. Maximum leaf extension growth (MLEG) of cut leek stalks at the end of the 9-day storage period at 4 ◦ C in relation to duration of hot water treatment (HWT) at 50, 52.5, 55 and 57.5 ◦ C. Each data point is the mean of five replications.
Fig. 3. Fresh weight loss (FWL) during the storage period of 9 days at 4 ◦ C of cut leek stalks subjected to hot water treatment (HWT) at 52.5 ◦ C for 0–35 min. Each data point is the mean of five replications. The vertical bar represents the 5% LSD.
3.3. Color Horticultural commodities that include meristems are subject to postharvest growth phenomena (Gerasopoulos and Chebli, 1998; Siomos, 2003). Removal of the base of the leek stalk that contains the meristem, considerably reduces extension growth in both the removed or the used, remaining stalk part (Tsouvaltzis et al., unpublished data). Moreover, a pre-storage heat treatment of 55 ◦ C for 2 min effectively controlled extension growth of fresh-cut green onions, and in combination with CA, provided a shelf life of more than 2 weeks at 5 ◦ C (Hong et al., 2000). It seems that produce of the Alliaceae family require relatively high heat treatment temperatures. However, other produce, such as green asparagus spears, require 47.5 ◦ C for 2–5 min to control postharvest geotropic curvature (Paull and Chen, 1999). 3.2. Fresh weight loss (FWL) Untreated cut leek stalks (control) lost fresh weight linearly with storage time at 4 ◦ C (r = 0.940, P < 0.0001) to as much as 2.64% of initial fresh weight, on day 9 of storage (Fig. 3). All cut leek stalks subjected to HWT had a similar pattern of FWL during the 9-day storage period and for this reason only results for HWT at 52.5 ◦ C are presented (Fig. 3). However, on day 9 of storage, stalks treated at 50 and 52.5 ◦ C for more than 10 min, as well as at 55 and 57.5 ◦ C for more than 1 min lost more fresh weight than the control (Fig. 4). Treatments that controlled MLEG showed FWL that ranged from 3.60 to 5.78%, which was significantly higher than the control. It is likely that the more extreme heat treatments removed surface waxes, resulting in greater water loss. A slight increase in FWL of the hot water treated broccoli was also observed by Forney (1995). In contrast, Paull and Chen (1999) and Siomos et al. (unpublished data) observed a lower FWL in some HWT, than the control, in green and white asparagus, respectively.
3.3.1. Color of mid-surface Analysis of variance for color attributes of stalk midsurface showed that HWT at all temperatures had no significant effect on chroma and hue angle, while only HWT at 57.5 ◦ C significantly affected lightness. On day 9 of storage, stalks treated at 57.5 ◦ C for more than 7.5 min (treatments that controlled MLEG) showed significantly lower lightness than the control (Table 1), indicating that outer leaves became darker. Changes of color attributes of the mid-surface indicate that outer leek leaves did not turn yellow within the 9 days of storage at 4 ◦ C, unlike fresh cut green onions (Hong et al., 2000). Higher lightness of heat-treated than untreated commodities has been found in strawberry fruit (Civello et al., 1997; Vicente et al., 2002) and broccoli heads (Forney, 1995; Tian et al., 1996). The response of a particular commodity will result from a combination of factors, such as preharvest environmental conditions, the physiological age of the commodity and many others (Lurie, 1998).
Fig. 4. Fresh weight loss (FWL) of cut leek stalks at the end of the 9-day storage period at 4 ◦ C in relation to duration of hot water treatment (HWT) at 50, 52.5, 55 and 57.5 ◦ C. Each data point is the mean of five replications.
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Table 1 Lightness values of the mid-surface of cut leek stalks at the end of the 9-day storage period at 4 ◦ C in relation to duration of hot water treatment (HWT) at 50, 52.5, 55 and 57.5 ◦ C Hot water treatment (HWT) at 50 ◦ C
52.5 ◦ C
Time
a b
Time 78.61a 78.03 76.66 76.53 76.06 76.40 74.43
0 10 20 30 40 50 60
55 ◦ C
0 5 10 15 20 25 30 35
57.5 ◦ C
Time 78.61 77.78 76.51 76.88 76.03 75.56 75.58 73.38
0 2.5 5 7.5 10 12.5 15 17.5 20
Time 78.61 76.90 76.39 77.98 77.76 77.88 76.64 75.14 74.37
0 1 2 3 4 5 7.5 10 13 15
78.61 abb 79.72 a 77.67 ab 77.38 ab 77.96 ab 77.59 ab 76.54 bc 74.55 bc 72.80 de 71.75 e
Each data point is the mean of five replications. Means within columns followed by different letters are significantly different according to Duncan’s multiple range test at the 0.05 level.
3.3.2. Color of base cross section Analysis of variance for color attributes of the base cross section showed that HWT at all temperatures had no significant effect on hue angle, while HWT at 52.5 ◦ C significantly affected chroma and lightness (Tables 2 and 3), and HWT at 57.5 ◦ C significantly affected only lightness (Table 3). On day 9 of storage, stalks treated at 52.5 ◦ C for more than 25 min (treatments that controlled MLEG) showed significantly higher chroma (Table 2) and lower lightness (Table 3) than the control, indicating that the base cross section had a vivid and darker color than the control. Stalks treated at 57.5 ◦ C for more than 10 min (treatments that controlled MLEG) also showed significantly lower lightness than the control (Table 3). Trimming at the leek root base resulted in a slight dull color. It is likely that wounding-induced PAL activity would increase in this part, leading to tissue browning (Ke and
Salveit, 1986; L´opez-G´alvez et al., 1996). Perhaps the base cross section should be done on the leaf part of the leek base rather that on the root area. In this case there might have been additional benefit due to removal of the meristem in controlling extension growth, as well as FWL (Tsouvaltzis et al., unpublished data). In conclusion, HWT at 50 ◦ C for 40–60 min, 52.5 ◦ C for 25–35 min, 55 ◦ C for 17.5–20 min and 57.5 ◦ C for 10–15 min efficiently controlled postharvest leaf extension growth in minimally processed leek stalks stored at 4 ◦ C for 9 days. However, these treatments showed a significantly higher FWL than the control, indicating that additional measures to reduce water loss are necessary for handling hot water treated minimally processed leeks, such as modified atmosphere packaging. There were no appreciable color changes of stored minimally processed leeks following HWT.
Table 2 Chroma values of the base cut section of leek stalks at the end of the 9-day storage period at 4 ◦ C in relation to duration of hot water treatment (HWT) at 50, 52.5, 55 and 57.5 ◦ C Hot water treatment (HWT) at 50 ◦ C
52.5 ◦ C
Time 0 10 20 30 40 50 60
a b
Time 23.83a 24.23 23.70 26.48 26.39 26.34 26.45
0 5 10 15 20 25 30 35
bcb
23.83 25.98 b 21.77 c 23.28 bc 23.41 bc 23.67 bc 30.31 a 31.02 a
55 ◦ C
57.5 ◦ C
Time
Time
0 2.5 5 7.5 10 12.5 15 17.5 20
23.83 25.20 23.86 23.21 23.30 22.48 23.92 25.95 24.53
0 1 2 3 4 5 7.5 10 13 15
Each data point is the mean of five replications. Means within columns followed by different letters are significantly different according to Duncan’s multiple range test at the 0.05 level.
23.83 24.70 25.81 25.51 24.15 22.53 25.44 26.60 27.86 26.37
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Table 3 Lightness values of the base cut section of leek stalks at the end of the 9-day storage period at 4 ◦ C in relation to duration of hot water treatment (HWT) at 50, 52.5, 55 and 57.5 ◦ C Hot water treatment (HWT) at 50 ◦ C
52.5 ◦ C
Time
a b
Time 79.38a 75.87 79.60 78.33 78.40 78.72 78.87
0 10 20 30 40 50 60
55 ◦ C
0 5 10 15 20 25 30 35
57.5 ◦ C
Time 79.38 abb 77.21 bc 79.58 a 79.07 ab 79.38 ab 79.11 ab 76.42 cd 74.32 d
0 2.5 5 7.5 10 12.5 15 17.5 20
Time 79.38 79.22 78.76 78.70 79.72 79.68 78.53 78.53 77.81
0 1 2 3 4 5 7.5 10 12.5 15
79.38 a 79.34 a 76.06 bc 76.61 bc 78.23 abc 80.32 a 78.51 ab 78.27 abc 75.27 c 75.76 c
Each data point is the mean of five replications. Means within columns followed by different letters are significantly different according to Duncan’s multiple range test at the 0.05 level.
References Cantwell, M.I., Hong, G., Suslow, T.V., 2001. Heat treatments control extension growth and enhance microbial disinfection of minimally processed green onions. HortScience 36, 732–737. Civello, P.M., Mart´ınez, G.A., Chaves, A.R., Aˇno´ n, M.C., 1997. Heat treatments delay ripening and postharvest decay of strawberry fruit. J. Agric. Food Chem. 45, 4589–4594. Fallik, E., 2004. Prestorage hot water treatments (immersion, rinsing and brushing). Postharvest Biol. Technol. 32, 125–134. Ferguson, I.B., Ben-Yehoshua, S., Mitcham, E.J., McDonald, R.E., Lurie, S., 2000. Postharvest heat treatments: introduction and workshop summary. Postharvest Biol. Technol. 21, 1–6. Forney, C.F., 1995. Hot-water dips extend the shelf life of fresh broccoli. HortScience 30, 1054–1057. Gerasopoulos, D., Chebli, B., 1998. Effects of scape-injected 1aminocyclopropane-1-carboxylic acid (ACC) on vase life of ’Testarossa’ cut gerberas. J. Am. Soc. Hort. Sci. 123, 921– 924. Hong, G., Peiser, G., Cantwell, M.I., 2000. Use of controlled atmospheres and heat treatment to maintain quality of intact and minimally processed green onions. Postharvest Biol. Technol. 20, 53–61. Ke, D., Salveit, M.E., 1986. Effects of calsium and auxin on russet spotting and phenylanine ammonia-lyase activity in iceberg lettuce. HortScience 21, 1169–1171. K¨orner, B., Berk, Z., 1967. The mechanism of pink-red pigment formation in leeks. Adv. Front. Plant Sci. 18, 39–52. Kubec, R., Harb´acˇ ov´a, M., Musah, R.A., Veliˇsek, J., 2004. Allium discoloration: precursors involved in onion pinking and garlic greening. J. Agric. Food Chem. 52, 5089–5094.
Lancaster, J.E., Lister, C.E., Reay, P.F., Triggs, C.M., 1997. Influence of pigment composition on skin color in a wide range of fruits and vegetables. J. Am. Soc. Hort. Sci. 122, 594–598. L´opez-G´alvez, G., Salveit, M., Cantwell, M., 1996. Wound-induced phenylalanine ammonia lyase activity: factors affecting its induction and correlation with the quality of minimally processed lettuces. Postharvest Biol. Technol. 9, 223–233. Lurie, S., 1998. Postharvest heat treatments. Postharvest Biol. Technol. 14, 257–269. McGuire, R.G., 1992. Reporting of objective color measurements. HortScience 27, 1254–1255. Paull, R.E., Chen, N.J., 1999. Heat treatment prevents postharvest geotropic curvature of asparagus spears (Asparagus officinalis L.). Postharvest Biol. Technol. 16, 37–41. Ragaert, P., Verbeke, W., Devlieghere, F., Debevere, J., 2004. Consumer perception and choice of minimally processed vegetables and packaged fruits. Food Qual. Pref. 15, 259–270. Ranganna, B., Raghavan, G.S.V., Kushalappa, A.C., 1998. Hot water dipping to enhance storability of potatoes. Postharvest Biol. Technol. 13, 215–223. Siomos, A.S., 2003. Quality, handling and storage of white asparagus. In: Dris, R., Niskanen, R., Jain, S.M. (Eds.), Crop Management and Postharvest Handling of Horticultural Products. Science Publishers Inc., Enfield (NH), USA, pp. 65–88. Tian, M.S., Woolf, A.B., Bowen, J.H., Ferguson, I.B., 1996. Changes in color and chlorophyll fluorescence of broccoli florets following hot water treatment. J. Am. Soc. Hort. Sci. 121, 310–313. Vicente, A.R., Mart´ınez, G.A., Civello, R.M., Chaves, A.R., 2002. Quality of heat-treated strawberry fruit during refrigerated storage. Postharvest Biol. Technol. 25, 59–71.
Effect of hot water treatment on leaf extension growth, fresh weight loss and color of stored minimally processed leeks Pavlos Tsouvaltzis a , Anastasios S. Siomos a,∗ , Dimitrios Gerasopoulos b b
a Department of Horticulture, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Department of Food Science and Technology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
Received 4 February 2005; accepted 27 June 2005
Abstract Freshly harvested leeks (Allium porrum L.) were heated by immersion in water at 50, 52.5, 55 or 57.5 ◦ C for 0–60, 0–35, 0–20 and 0–15 min, respectively. After hot water treatment, leeks were cooled in water at ambient temperature for 10 min and then cut at 22 cm from the compressed stem of the root base, weighed, had color measured and stored at 4 ◦ C for 9 days. Untreated stalks (without immersion in a hot water bath) were used as controls. Hot water treatments at 50 ◦ C for 40–60 min, 52.5 ◦ C for 25–35 min, 55 ◦ C for 17.5–20 min and 57.5 ◦ C for 10–15 min efficiently controlled postharvest leaf extension growth in stalks stored for 9 days. However, treatments that controlled leaf extension growth showed fresh weight loss significantly higher than the control. There was only a slight effect of heat treatment on color attributes of stored minimally processed leek. © 2005 Elsevier B.V. All rights reserved. Keywords: Allium porrum L.; Telescoping; Quality; Ready to use leek
1. Introduction Leeks may provide a challenge as a minimally processed product since they are a vegetable that requires a minimal processing before consumption, which includes leaf trimming, root cutting, decayed-leaf removal and stalk cutting to a precise length. Attempts of some food chains to offer minimally processed prepackaged leeks has failed due to rapid quality deterioration mainly due to postharvest leaf extension growth, as well as dehydration and discoloration. Hong et al. (2000) has described postharvest leaf extension growth of fresh-cut green onions. This extension is also referred to as ‘telescoping’ (Cantwell et al., 2001). Leaf extension growth causes a rapid deterioration of the overall market quality of the product, reducing its appearance, which is the most significant attribute during the buying stage of minimally processed products (Ragaert et al., 2004). Moreover, during common processing of leek, intensely colored ∗
Corresponding author. Tel.: +30 2310 998646; fax: +30 2310 998609. E-mail address: [email protected] (A.S. Siomos).
0925-5214/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.postharvbio.2005.06.009
pigments are often formed and leek homogenates may turn pink or red within several hours (K¨orner and Berk, 1967; Kubec et al., 2004). During the past few years there has been increasing interest in the use of postharvest heat treatments (Lurie, 1998; Ferguson et al., 2000; Fallik, 2004), since the overall quality of fresh produce that was treated at optimal hot water temperatures and duration was significantly better than that of untreated, as determined by a sharp reduction in decay incidence and maintenance of several quality traits (Fallik, 2004). Postharvest growth-related phenomena of horticultural commodities can also be affected by heat treatment; geotropic curvature of green asparagus spears (Paull and Chen, 1999) and sprouting of potatoes (Ranganna et al., 1998) have been reported to be successfully controlled by short hot water dips. A pre-storage heat treatment of 55 ◦ C for 2 min effectively controlled leaf extension growth of fresh-cut green onions, and in combination with controlled atmosphere, provided a shelf life of more than 2 weeks at 5 ◦ C (Hong et al., 2000; Cantwell et al., 2001).
P. Tsouvaltzis et al. / Postharvest Biology and Technology 39 (2006) 56–60
57
2. Materials and methods
in the L* , a* , b* color space. Chroma [C* = (a*2 + b*2 )0.5 ] and hue angle [h◦ = tan−1 (b* /a* ) when a* > 0 and b* > 0 or h◦ = 180◦ + tan−1 (b* /a* ) when a* < 0 and b* > 0) were calculated from a* and b* values (Lancaster et al., 1997). L* refers to the lightness, ranging from 0 = black to 100 = white, chroma represents color saturation which varies from dull (low value) to vivid color (high value) and hue angle is defined as a color wheel, with red–purple at an angle of 0◦ , yellow at 90◦ , bluish-green at 180◦ , and blue at 270◦ (McGuire, 1992).
2.1. Plant material and handling
2.4. Statistical analysis
Leeks (Allium porrum L.) produced in the area of Central Macedonia, Greece under usual production practices, were harvested brought to the laboratory in plastic bags, trimmed (leaf tips, roots and root base part at nearly 1 cm), had the decayed leaves removed and were washed in tap water.
Experiments were conducted as completely randomized designs with five individual stalks per treatment. Data analysis was done by an analysis of variance, with mean separation by LSD or Duncan’s multiple range test at the 0.05 level.
This study was undertaken to investigate the effects of hot water treatment on minimally processed leeks in order to determine the optimum hot water temperature and duration of exposure that is necessary to control leaf extension growth in addition to minimizing fresh weight loss and color changes during storage.
2.2. Treatments Preliminary experiments were performed under different time–temperature combinations of hot water treatment (HWT) to determine the damage limits. Subsequently, the experiment was performed using time–temperature combinations that caused no damage to the commodity. Five individual leek stalks (22–36 mm in diameter) were immersed in a hot water bath held at temperatures of 50 ◦ C for 10, 20, 30, 40, 50 and 60 min, at 52.5 ◦ C for 5, 10, 15, 20, 25, 30 and 35 min, at 55 ◦ C for 2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20 min and at 57.5 ◦ C for 1, 2, 3, 4, 5, 7.5, 10, 12.5 and 15 min. After hot water treatment, leeks were immediately cooled in water at ambient temperature for 10 min and then placed on absorbent paper to remove the excess surface water, cut at 22 cm from the compressed stem of the root base, weighed, had color measured and were then kept on plastic disks in air at 4 ◦ C for 9 days. Untreated stalks (without immersion in a hot water bath) were used as controls. The trays were covered with wet paper to maintain high humidity conditions. The experiment was conducted twice. The data presented are from the second experiment, but very similar results were obtained from the first one.
3. Results and discussion 3.1. Maximum leaf extension growth (MLEG) Storage of untreated (control) cut stalks at 4 ◦ C for 9 days resulted in MLEG as high as 7.5 mm, following a hyperbolic growth curve (r = 0.788, P < 0.0001) (Fig. 1). Heat treatment of cut stalks to 50 ◦ C for 10 min, to 52.5 ◦ C for 5–10 min or to 57.5 ◦ C for 2–4 min showed appreciably higher MLEG than in the controls by day 9 of storage (Fig. 2). However, MLEG at day 9 of storage was progressively less with increasing treatment duration and it was reduced almost to zero values when treatment duration was more than 30, 20, 15 and 7.5 min at 50, 52.5, 55 and 57.5 ◦ C, respectively (Fig. 2). The pattern of MLEG development during the storage period of 9 days was similar in all temperature treatments and for this reason only results for HWT at 52.5 ◦ C are presented (Fig. 1).
2.3. Evaluations Maximum leaf extension growth (MLEG) of cut leek stalks was measured every three days with a caliper from the cut surface of the leaf base to the end of the most extended portion. The fresh weight of each stalk was monitored every three days and data were expressed as % fresh weight loss (FWL). Color readings were taken every three days in each individual stalk at the base cross section and the mid-surface using a chromameter (Minolta CR-200, Minolta, Osaka, Japan), equipped with an 8 mm measuring head and a C illuminant (6774 K). The meter was calibrated using the manufacturer’s standard white plate. Color changes were quantified
Fig. 1. Maximum leaf extension growth (MLEG) during the storage period of 9 days at 4 ◦ C of cut leek stalks subjected to hot water treatment (HWT) at 52.5 ◦ C for 0–35 min. Each data point is the mean of five replications. The vertical bar represents the 5% LSD.
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P. Tsouvaltzis et al. / Postharvest Biology and Technology 39 (2006) 56–60
Fig. 2. Maximum leaf extension growth (MLEG) of cut leek stalks at the end of the 9-day storage period at 4 ◦ C in relation to duration of hot water treatment (HWT) at 50, 52.5, 55 and 57.5 ◦ C. Each data point is the mean of five replications.
Fig. 3. Fresh weight loss (FWL) during the storage period of 9 days at 4 ◦ C of cut leek stalks subjected to hot water treatment (HWT) at 52.5 ◦ C for 0–35 min. Each data point is the mean of five replications. The vertical bar represents the 5% LSD.
3.3. Color Horticultural commodities that include meristems are subject to postharvest growth phenomena (Gerasopoulos and Chebli, 1998; Siomos, 2003). Removal of the base of the leek stalk that contains the meristem, considerably reduces extension growth in both the removed or the used, remaining stalk part (Tsouvaltzis et al., unpublished data). Moreover, a pre-storage heat treatment of 55 ◦ C for 2 min effectively controlled extension growth of fresh-cut green onions, and in combination with CA, provided a shelf life of more than 2 weeks at 5 ◦ C (Hong et al., 2000). It seems that produce of the Alliaceae family require relatively high heat treatment temperatures. However, other produce, such as green asparagus spears, require 47.5 ◦ C for 2–5 min to control postharvest geotropic curvature (Paull and Chen, 1999). 3.2. Fresh weight loss (FWL) Untreated cut leek stalks (control) lost fresh weight linearly with storage time at 4 ◦ C (r = 0.940, P < 0.0001) to as much as 2.64% of initial fresh weight, on day 9 of storage (Fig. 3). All cut leek stalks subjected to HWT had a similar pattern of FWL during the 9-day storage period and for this reason only results for HWT at 52.5 ◦ C are presented (Fig. 3). However, on day 9 of storage, stalks treated at 50 and 52.5 ◦ C for more than 10 min, as well as at 55 and 57.5 ◦ C for more than 1 min lost more fresh weight than the control (Fig. 4). Treatments that controlled MLEG showed FWL that ranged from 3.60 to 5.78%, which was significantly higher than the control. It is likely that the more extreme heat treatments removed surface waxes, resulting in greater water loss. A slight increase in FWL of the hot water treated broccoli was also observed by Forney (1995). In contrast, Paull and Chen (1999) and Siomos et al. (unpublished data) observed a lower FWL in some HWT, than the control, in green and white asparagus, respectively.
3.3.1. Color of mid-surface Analysis of variance for color attributes of stalk midsurface showed that HWT at all temperatures had no significant effect on chroma and hue angle, while only HWT at 57.5 ◦ C significantly affected lightness. On day 9 of storage, stalks treated at 57.5 ◦ C for more than 7.5 min (treatments that controlled MLEG) showed significantly lower lightness than the control (Table 1), indicating that outer leaves became darker. Changes of color attributes of the mid-surface indicate that outer leek leaves did not turn yellow within the 9 days of storage at 4 ◦ C, unlike fresh cut green onions (Hong et al., 2000). Higher lightness of heat-treated than untreated commodities has been found in strawberry fruit (Civello et al., 1997; Vicente et al., 2002) and broccoli heads (Forney, 1995; Tian et al., 1996). The response of a particular commodity will result from a combination of factors, such as preharvest environmental conditions, the physiological age of the commodity and many others (Lurie, 1998).
Fig. 4. Fresh weight loss (FWL) of cut leek stalks at the end of the 9-day storage period at 4 ◦ C in relation to duration of hot water treatment (HWT) at 50, 52.5, 55 and 57.5 ◦ C. Each data point is the mean of five replications.
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Table 1 Lightness values of the mid-surface of cut leek stalks at the end of the 9-day storage period at 4 ◦ C in relation to duration of hot water treatment (HWT) at 50, 52.5, 55 and 57.5 ◦ C Hot water treatment (HWT) at 50 ◦ C
52.5 ◦ C
Time
a b
Time 78.61a 78.03 76.66 76.53 76.06 76.40 74.43
0 10 20 30 40 50 60
55 ◦ C
0 5 10 15 20 25 30 35
57.5 ◦ C
Time 78.61 77.78 76.51 76.88 76.03 75.56 75.58 73.38
0 2.5 5 7.5 10 12.5 15 17.5 20
Time 78.61 76.90 76.39 77.98 77.76 77.88 76.64 75.14 74.37
0 1 2 3 4 5 7.5 10 13 15
78.61 abb 79.72 a 77.67 ab 77.38 ab 77.96 ab 77.59 ab 76.54 bc 74.55 bc 72.80 de 71.75 e
Each data point is the mean of five replications. Means within columns followed by different letters are significantly different according to Duncan’s multiple range test at the 0.05 level.
3.3.2. Color of base cross section Analysis of variance for color attributes of the base cross section showed that HWT at all temperatures had no significant effect on hue angle, while HWT at 52.5 ◦ C significantly affected chroma and lightness (Tables 2 and 3), and HWT at 57.5 ◦ C significantly affected only lightness (Table 3). On day 9 of storage, stalks treated at 52.5 ◦ C for more than 25 min (treatments that controlled MLEG) showed significantly higher chroma (Table 2) and lower lightness (Table 3) than the control, indicating that the base cross section had a vivid and darker color than the control. Stalks treated at 57.5 ◦ C for more than 10 min (treatments that controlled MLEG) also showed significantly lower lightness than the control (Table 3). Trimming at the leek root base resulted in a slight dull color. It is likely that wounding-induced PAL activity would increase in this part, leading to tissue browning (Ke and
Salveit, 1986; L´opez-G´alvez et al., 1996). Perhaps the base cross section should be done on the leaf part of the leek base rather that on the root area. In this case there might have been additional benefit due to removal of the meristem in controlling extension growth, as well as FWL (Tsouvaltzis et al., unpublished data). In conclusion, HWT at 50 ◦ C for 40–60 min, 52.5 ◦ C for 25–35 min, 55 ◦ C for 17.5–20 min and 57.5 ◦ C for 10–15 min efficiently controlled postharvest leaf extension growth in minimally processed leek stalks stored at 4 ◦ C for 9 days. However, these treatments showed a significantly higher FWL than the control, indicating that additional measures to reduce water loss are necessary for handling hot water treated minimally processed leeks, such as modified atmosphere packaging. There were no appreciable color changes of stored minimally processed leeks following HWT.
Table 2 Chroma values of the base cut section of leek stalks at the end of the 9-day storage period at 4 ◦ C in relation to duration of hot water treatment (HWT) at 50, 52.5, 55 and 57.5 ◦ C Hot water treatment (HWT) at 50 ◦ C
52.5 ◦ C
Time 0 10 20 30 40 50 60
a b
Time 23.83a 24.23 23.70 26.48 26.39 26.34 26.45
0 5 10 15 20 25 30 35
bcb
23.83 25.98 b 21.77 c 23.28 bc 23.41 bc 23.67 bc 30.31 a 31.02 a
55 ◦ C
57.5 ◦ C
Time
Time
0 2.5 5 7.5 10 12.5 15 17.5 20
23.83 25.20 23.86 23.21 23.30 22.48 23.92 25.95 24.53
0 1 2 3 4 5 7.5 10 13 15
Each data point is the mean of five replications. Means within columns followed by different letters are significantly different according to Duncan’s multiple range test at the 0.05 level.
23.83 24.70 25.81 25.51 24.15 22.53 25.44 26.60 27.86 26.37
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Table 3 Lightness values of the base cut section of leek stalks at the end of the 9-day storage period at 4 ◦ C in relation to duration of hot water treatment (HWT) at 50, 52.5, 55 and 57.5 ◦ C Hot water treatment (HWT) at 50 ◦ C
52.5 ◦ C
Time
a b
Time 79.38a 75.87 79.60 78.33 78.40 78.72 78.87
0 10 20 30 40 50 60
55 ◦ C
0 5 10 15 20 25 30 35
57.5 ◦ C
Time 79.38 abb 77.21 bc 79.58 a 79.07 ab 79.38 ab 79.11 ab 76.42 cd 74.32 d
0 2.5 5 7.5 10 12.5 15 17.5 20
Time 79.38 79.22 78.76 78.70 79.72 79.68 78.53 78.53 77.81
0 1 2 3 4 5 7.5 10 12.5 15
79.38 a 79.34 a 76.06 bc 76.61 bc 78.23 abc 80.32 a 78.51 ab 78.27 abc 75.27 c 75.76 c
Each data point is the mean of five replications. Means within columns followed by different letters are significantly different according to Duncan’s multiple range test at the 0.05 level.
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