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Impact of edible coatings on nutritional and physiological changes in lightly-processed carrots
Postharvest Biology and Technology 14 (1998) 51 – 60
Impact of edible coatings on nutritional and physiological changes in lightly-processed carrots Peiyin Li a,b, M. Margaret Barth a,b,* a
Department of Nutrition and Food Science, Uni6ersity of Kentucky, Lexington, KY 40506 -0054, USA b Redi-Cut Foods, Inc., 9501 Ne6ada A6enue, Franklin Park, IL 60131, USA Received 13 August 1996; accepted 28 March 1998
Abstract The objective of this study was to evaluate the effects of two edible coatings (EC) of varying pH (2.7 and 4.6) on carotene retention and other physiological changes in lightly-processed (LP) carrots during storage. Carrots were treated with cellulose-based EC, packaged and stored at 1°C for 28 days. Samples were taken at regular intervals for analysis. Our results showed that carotene retention was 15% greater in the EC treatments versus control treatment throughout the 28 days. Samples treated with the lower pH EC had the highest CO2 and lowest O2 concentrations in the headspace. Whiteness index (WI) scores were significantly lower in both coated samples. Ethylene production was greatest in carrots treated with the lower pH coating on removal from sealed bags to air after each storage period. ECs improved carotene retention and retarded surface whitening in LP carrots during postharvest storage. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Carrot; Edible coatings; Lightly processed; b-Carotene
1. Introduction Consumer demand for high quality, fresh, nutritive and conveniently prepared vegetables has increased dramatically in recent years. This has led to the development of lightly processed (LP) * Corresponding author. Present address: Redi-Cut Foods, Inc., 9501 Nevada Avenue, Franklin Park, IL 60131, USA. Tel.: +1 847 2882200; fax: + 1 847 2882205; e-mail: [email protected]
vegetables. LP carrots, prepared by peeling the outer layer of the carrot roots, are one of the most popular vegetable products in the US. Upon surface peeling, LP carrots are susceptible to a variety of physiological changes that limit their shelf-life. The abrasion of the surface may increase the potential for carotene oxidation during storage. Surface abrasion may further increase the respiration of carrot tissue, resulting in increased degradation of proteins, carbohydrates, and
0925-5214/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S0925-5214(98)00020-9
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P. Li, M.M. Barth / Posthar6est Biology and Technology 14 (1998) 51–60
lipids, and the development of off-flavors. This surface wounding may also initiate the production of a new protective layer, known as ‘white blush’, a result of dehydration and lignification, on the carrot surface (Bolin and Huxsoll, 1991). Lignification after wounding is an enzyme-stimulated reaction (Hahlbrock and Grisebach, 1979; Bell, 1981). In addition, wounding also stimulates the production of ethylene. Studies have shown that ethylene production or exposure of carrots to ethylene was highly correlated with the formation of phenolic compounds, such as isocoumarin, that contribute to development of bitter flavor during storage (Phan, 1974; Sarkar and Phan, 1973, 1979; Lafuente et al., 1989). The effects of edible coatings (EC) on shelflife extension of vegetables have been reviewed by many authors (Dhalla and Hanson, 1988; Nisperos-Carriedo and Baldwin, 1988; Bender et al., 1993; Park et al., 1994; Baldwin et al., 1995; Howard and Dewi, 1995). EC may be composed of polysaccharides, proteins, lipids, or a blend of these compounds (Nisperos-Carriedo et al., 1991). The benefits of applying edible coatings to fresh intact or LP vegetables include slowed ripening and browning, and delayed color, flavor, moisture and firmness loss. Edible coatings provide a barrier to moisture, oxygen and solute movement, and consequently, reduce the metabolism and oxidation reaction rates. Although EC have been reported to control surface whitening on LP carrots (Avena-Bustillos et al., 1993), no information is published on carotene retention and other physiological changes of these carrots during postharvest storage. In our study, two cellulose-based EC of varying pH were commercially applied to LP carrots stored in modified atmosphere packaging (MAP) for 28 days. The objectives of our study were to: (1) evaluate the effects of edible coatings on carotene retention; and (2) gain a better understanding of physiological changes, such as respiration, surface whitening, moisture loss, peroxidase (POD) activity, and ethylene production of LP carrots during storage.
2. Materials and methods
2.1. Experimental design Carrot (cv. Daucus caroto6a) samples were prepared by removing the surface layer of the carrot roots (1.0 mm) according to industrial procedures within 2 days after harvest, and shipped overnight on ice in boxed, styrofoam containers from California (Grimmway Farms, Salinas, CA) to the University of Kentucky Postharvest Research Laboratory. Carrot samples were carefully selected for quality and uniformity and placed into the following treatment groups: control, EC1 and EC2. Two types of cellulose-based edible coatings, EC1 (pH= 2.7) and EC2 (pH= 4.6) (EcoScience, Orlando, FL) were applied by dipping the carrots in the coating solution for 10 s followed by draining in a stainless steel colander for 4 h at 1°C, 85% RH. Carrot samples (70 g/bag) from each treatment were placed into polymeric film bags (OTR= 5006 cm3/m2/24 h/atm. at 25°C, 50% RH) (Grimmway Farms, California) and stored in a commercial walk-in cooler maintained at 1°C, 92% RH, for 28 days in the absence of light. Three bags from each treatment were removed at regular intervals (0, 7, 14, 21, and 28 days) for the determination of carotene content, headspace composition, whiteness index score, moisture content, peroxidase activity, and ethylene production. Samples for carotene and enzyme analyses were uniformly ground using a Kitchen Aid food grinder (Model K45SS, KitchenAid, St. Joseph, MI) with 3-mm diameter holes, followed by rapid grinding in liquid nitrogen. The samples were then stored at − 80°C until analysis.
2.2. Carotene content Carotenes were extracted from ground carrot samples (stored at − 80°C) according to the method described in AOAC (1992). The a- and b-carotene were separated and quantitatively measured by reverse-phase high performance liquid chromatography (HPLC) using b-cryptoxanthin (Hoffmann-La Roche, Nutley, NJ) as an internal standard. Analysis was performed using a C18 column (Rainin Instrument, Emeryville, CA)
P. Li, M.M. Barth / Posthar6est Biology and Technology 14 (1998) 51–60
of 4.6× 250 mm length and 5-mm particle size, with the mobile phase of acetonitrile/methylene chloride/octonol (90/25/0.1, v/v/v) running isocratically at a flow rate of 1 ml/min for 30 min. The detection of a- and b-carotene was by a UV/VIS detector (Waters 486, Tunable Absorbance Detector) at 450-nm wavelength.
2.3. Headspace composition The O2 and CO2 concentrations in the headspace of the packaging bags were measured at each interval using an Oxygen and Carbon Dioxide Head Space Analyzer (Model ZR892/HS, Illinois Instrument, McHenry, IL) to estimate the respiration rate of carrots. Two bags were taken from each treatment at each sampling interval and 5 cm3 of headspace gas was injected onto the column of the analyzer. The headspace compositions were expressed as percentages of O2 and CO2.
2.4. Whiteness index (WI)
53
tissue homogenizer (Tissumizer, Model SDT 1810, Tekmar, Cincinnati, OH) at 80 rpm for 1 min. The mixture was held for 1 h at 4°C, followed by centrifugation at 10000 rpm for 15 min (4°C). POD activity of supernatant was determined by spectrophotometer from the rate of H2O2 decomposition with guaiacol serving as hydrogen donor at pH 5.0 (Howard and Griffin, 1993). POD activity was expressed as the increase in the absorbance of 0.001 units at 470 nm/g/min (25°C).
2.7. Ethylene production Carrot samples (80 g) were taken from each treatment and sealed in a glass jar fitted with a rubber septum for 1 h (25°C). The ethylene concentration inside the jar was measured by gas chromatography with a flame ionization detector (SpectraPhysics, SP4270 Integrator) using an external standard.
2.8. Statistical analysis
Six carrots were taken from each treatment per interval and color measurements were made by a Minolta Colorimeter (Model CR-200, Minolta Camera) in contact with the surface of the carrots. The L, a, and b values were obtained and severity of surface whitening was estimated by the whiteness index scores (WI) expressed by the equation 100−[(100 − L)2 +a 2 +b 2]1/2 (Bolin and Huxsoll, 1991).
Data were analyzed by analysis of variance (ANOVA) using the general linear model (GLM) procedure (SAS Institute, Cary, NC). ANOVA was done on data at each sampling interval (7, 14, 21, and 28 days), as well as for all treatment groups. All comparisons were made at the 95% confidence level.
2.5. Moisture content
3. Results
Moisture content of the carrot samples was measured as an indicator of water losses during storage using the oven drying method (Nielsen, 1994). Freshly ground carrot samples (5 g) were weighed onto aluminum weighing dishes. After 24 h in the drying oven (85°C), the samples were reweighed and percent moisture was calculated.
3.1. Carotene retention
2.6. Peroxidase (POD) acti6ity Ground carrot samples (4 g) were homogenized in 8 ml pH 6.5 citrate-phosphate buffer using a
b-Carotene levels decreased steadily in all treatments during 28-day storage (Fig. 1). After 21-day storage, b-carotene retention was significantly higher in the edible coating-treated samples versus control (PB 0.05). However, statistical analysis showed that there was no significant difference between EC1- and EC2-treated samples. By 28 days, b-carotene retention was 50% in the edible coating treatments, compared to 33% in the control samples.
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Fig. 1. Percent of b-carotene retention in lightly-processed carrots treated with edible coatings.
Similar trends could be observed in percent of a-carotene retention (Fig. 2). a-Carotene also decreased in all treatments with time, and samples treated with edible coatings had greater acarotene retention at 14, 21, and 28 days (P B 0.05). In addition, there was no significant difference of a-carotene retention between EC1 and EC2 treatments.
3.2. Headspace analysis The results of the headspace analyses are shown in Fig. 3. Oxygen levels in the bags declined rapidly from 21 to 10% in the first 7 days in all treatments. During the remainder of storage, O2 concentration remained relatively constant, with a slight increase in EC2 and control treatments, and a slight decrease in EC1-treated samples. After 14 days, EC2-treated samples had a significantly higher O2 level than EC1-treated samples (PB0.05). However, no significant differences were detected between EC1 and control samples, or between EC2 and control samples at any intervals.
In contrast to O2 levels, CO2 concentrations in all treatments exhibited an increase from 0 to 2.5% by 7-day storage. EC1-treated samples reached 3.6% by the end of the storage. CO2 levels remained constant for control and EC2 treatments until 21 days followed by a decrease to 1.7 and 1.6% in control and EC2-treated samples by 28 days. EC1 treatment had a significantly higher CO2 level than both EC2 and control treatments (PB 0.05), whereas there was no significant difference in CO2 content between EC2 and control treatments.
3.3. Whiteness index (WI) Surface whitening was significantly retarded by applying edible coatings. By the end of 3-week storage, both EC1- and EC2-treated carrots maintained fresh appearance whereas an obvious whiteness developed in the non-treated samples. The severity of surface whitening in LP carrots during storage was quantitatively illustrated by whiteness index (WI) scores (Fig. 4). Higher WI scores indicate greater development of surface
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Fig. 2. Percent of a-carotene retention in lightly-processed carrots treated with edible coatings.
whiteness. The initial WI score for all treatments was 30.1. Control samples exhibited the highest rate of increase in WI scores while EC2-treated samples showed the lowest WI scores. At 28 days, WI scores increased 30% in the control samples (39.3) whereas only 8.6 and 0.7% increases were observed in the EC1- (32.7) and EC2- (30.2) treated samples, suggesting the most severe development of surface whitening in the untreated samples compared to the treated samples (P B 0.05). EC1-treated samples showed significantly higher WI scores than EC2-treated samples at 21 and 28 days.
3.4. Moisture content The percent moisture in carrot tissues decreased in the untreated samples, but the changes were unremarkable, with moisture losses less than 1% during the entire storage period. In the edible coating-treated samples, water losses were not significant. A significantly higher moisture content was observed in the edible coating treated samples versus control samples after 14 days (P B 0.05).
3.5. Peroxidase acti6ity The initial level of POD activity in the LP carrots was 0.473 (absorbance increase/g/min). A constant increase in POD activity was shown in control samples during storage (Fig. 5). A slight decrease, followed by an increase of POD activity occurred in both EC1- and EC2-treated samples. The trends in POD activity in the carrot samples were similar to those of WI scores, with highest POD levels in control samples and lowest activity in EC2-treated samples. A significant difference among treatments was shown at 7 days and 21 days (PB 0.05). By 21 days storage, POD activity increased by 89.1% in control samples, 77.8% in EC1-treated samples, and 58.1% in EC2-treated samples.
3.6. Ethylene production Ethylene production increased slightly in EC2 and control samples, but remarkably in EC1 samples (Fig. 6). EC1-treated samples started to show higher ethylene production than EC2 and control
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treatment after 14 days (P B0.05). By the end of storage, ethylene production was 0.773 ml/g/h (fresh weigh basis, gas volume at 20°C, 101 kPa) in EC1-treated samples, whereas levels were 0.123 and 0.084 ml/g/h in the control and EC2 treatments, respectively. These results indicate a sevento ninefold higher ethylene production in EC1 treatment than control or EC2 treatments. Control samples showed a significantly higher ethylene production than EC2-treated samples only at 28 days.
4. Discussion
4.1. Carotene losses Possible reasons for carotene losses in LP carrots during storage are autoxidation, which occurs
spontaneously when the carotenes combine with oxygen in the air, and enzymatic oxidation, which is catalyzed by oxidative enzymes (Gross, 1991). The abrasion of the carrot surface exposes the phloem, where carotenes are most concentrated, to the air and light. Edible coatings serve as a protective layer and control the permeability of O2 and CO2, thus decreasing the autoxidation potential of carotenes. Headspace analysis showed the highest O2 concentration occurred inside the EC2 packages, followed by control and EC1 bags. O2 levels did not appear to correlate with the trends for carotene losses, suggesting that either the carotene losses in LP carrots are not sensitive to oxygen levels, or the permeability of edible coatings has a greater effect on the gas exchanges of the plant tissue as compared to the use of packaging films, and may result in reduced internal oxygen levels in the coating-treated samples. Edible coating treatments of vegetables have been reported to retard respiration and metabolism (Nisperos-Carriedo and Baldwin, 1988; Bender et al., 1993; Park et al., 1994), indicating that the activities of enzymes involved in the oxidation of carotenes may be suppressed or the substrate (O2) limited by the application of edible coatings. This may lead to decreased carotene losses in LP carrots during storage. No significant difference of carotene retention was detected between EC1- and EC2-treated samples, suggesting that the pH of the edible coatings did not affect carotene oxidation in our study.
4.2. Respiration le6els
Fig. 3. Headspace analysis of O2 and CO2 in lightly-processed carrots treated with edible coatings.
The patterns of the headspace composition of CO2 and O2 inside the packages in our study were in agreement with those previously described for many packaged fruits and vegetables (Henig and Gilbert, 1975; Rizvi, 1981; Carlin et al., 1990), showing a rapid change of CO2 and O2 concentration in the first few days, followed by equilibration of gas composition during the market life of the product. In our modified atmosphere system for untreated samples, the equilibrium concentrations of O2 and CO2 inside bags were approximately 10 and 2.5%, respectively, which never reached critical values (1% O2 and 30% CO2;
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Fig. 4. Whiteness index scores in lightly-processed carrots treated with edible coatings.
Carlin et al., 1990), indicating that the amount of samples in the bags was small enough and the permeability of the film was high enough to avoid anaerobic metabolism of the untreated carrots. Permeability in the edible coatings should be considered in EC1- and EC2-treated samples when evaluating their impact on atmospheric modification in carrot tissue.
4.3. Surface whitening WI scores indicated that surface whitening in LP carrots was largely inhibited by applying the edible coatings, and samples treated with high pH edible coatings showed a better result than the samples treated with low pH edible coatings. The possible reasons for the whiteness development on the carrot surface are dehydration and lignification. Although moisture content was significantly lower in the control carrot samples after 14-day storage (B 0.05%), it cannot be concluded that moisture losses contributed to increased surface whitening in these samples. It is possible that ECs reduced whitening by physically filling the air-filled surface tissue of the abraded carrots.
We measured the POD activity in the carrot tissues because it is a key enzyme involved in lignification on the carrot surface (Bell, 1981). Trends of POD activity were similar to those of WI scores. These results are in agreement with the report by Howard and Griffin (1993) who suggested POD activity was highly correlated with the development of lignification in LP carrots during storage. The elevated POD activity in control samples may be related to the stimulation of lignin formation and, overall, the application of edible coatings on the surface of LP carrots may retard surface whitening by suppressing peroxidase activity. EC1-treated samples showed higher WI scores and POD activity than EC2-treated samples, indicating that the pH of EC1 may be too low to have an optimal effect on retarding surface whitening. The optimal pH for peroxidase activity in carrot tissue is 5.0 and the pH for EC1 and EC2 coatings is 2.7 and 4.6, respectively. Therefore, the suppression of peroxidase activity is likely not due to the pH. It is possible that the EC1 may not have been as effective as a filler or was somewhat phytotoxic to the surface cells of the carrot.
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Fig. 5. Peroxidase activity in lightly-processed carrots treated with edible coatings.
Moisture contents and POD activity may also have an impact on the losses of carotenes, given that carotenes are more rapidly oxidized in dehydrated products and peroxidase is involved in the enzymatic oxidation of carotenes (Gross, 1991). The untreated samples showed the highest carotene losses along with the greatest moisture losses and POD activity compared to the ECtreated samples.
4.4. Ethylene production Increase in ethylene production may be attributed to the biosynthesis of wound ethylene (Abeles et al., 1992). Edible coatings may serve as a protective layer for the wounded surface of the carrots and reduce wounding responses. This was demonstrated in the EC2 treatment where significantly lower ethylene levels than in the untreated samples were observed at 28 days. The reason why EC1-treated samples showed such a remarkably high ethylene production in LP carrots is unclear. It is possible that the lower pH coating was somewhat phytotoxic to the carrot tissue.
Ethylene is also considered to be a causative agent in the biosynthesis of isocoumarin, a metabolite in phenolic metabolism and the bitter compound produced in the carrot tissues during storage (Sarkar and Phan, 1973, 1979). However, this only occurs when the ethylene inside the packages reaches a sufficiently high level. Ethylene levels inside the packages depend on the ethylene production of the carrots as well as the permeability of the packaging materials. The ethylene produced in EC1-treated samples in our study may be high enough to cause the alteration of phenolic metabolism and is under investigation in our laboratory.
5. Conclusion In summary, our results suggest that the cellulose-based edible coatings have an overall beneficial effect on retention of carotenes, retardation of surface whitening, and extension of shelf-life of the LP carrots. The pH in edible coatings did not affect the carotene losses in the LP carrots. It appeared to affect, however, the respiration, sur-
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Fig. 6. Ethylene production in lightly-processed carrots treated with edible coatings.
face whitening, and ethylene production in these carrots.
References Abeles, F.B., Morgan, P.W., Saltveit, M.E., 1992. Ethylene in Plant Biology, 2nd ed. Academic Press, San Diego. AOAC, 1992. Official Methods of Analysis, 13th ed. Association of Official Analytical Chemists, Washington, DC. Avena-Bustillos, R.J., Cisneros-Zevallos, L.A., Krochta, J.M., Saltveit, M.E., 1993. Optimization of edible coatings on minimally processed carrots using response surface methodology. Am. Soc. Agric. Engineers 36 (3), 801. Baldwin, E.A., Nisperos-Carriedo, M.O., Shaw, P.E., Burns, J.K., 1995. Effect of coatings and prolonged storage conditions on fresh orange flavor volatiles, degree brix, and ascorbic acid levels. J. Agric. Food Chem. 43, 1321. Bell, A.A., 1981. Biochemistry of disease resistance. Ann. Rev. Plant Physiol. 32, 40. Bender, R.J., Brecht, J.K., Sargent, S.A., Navarro, J.C., Campbell, C.A., 1993. Ripening initiation and storage performance of avocados treated with an edible-film coating. Acta Hort. 343, 184. Bolin, H.R., Huxsoll, C.C., 1991. Control of minimally processed carrot (Daucus caroto6a) surface discoloration caused by abrasion peeling. J. Food Sci. 56 (2), 416.
Carlin, F., Nguyen-The, C., Chambroy, Y., Reich, M., 1990. Effects of controlled atmospheres on microbial spoilage, electrolyte leakage and sugar content of fresh, ‘ready-touse’ grated carrots. Int. J. Food Sci. Technol. 25, 100. Dhalla, R., Hanson, S.W., 1988. Effect of permeable coatings on the storage life of fruits. II. Pro-long treatment of mangoes (Mangifera Indica L. cv. ‘Julie’). J. Food Sci. Technol. 23, 99. Gross, J., 1991. Pigments in Vegetables. AVI, Van Nostrand Reinhold, New York. Hahlbrock, K., Grisebach, H., 1979. Enzymic control in the biosynthesis of lignins and flavonoids. Ann. Rev. Plant Physiol. 30, 105. Henig, Y.S., Gilbert, S.G., 1975. Computer analysis of the variables affecting respiration and quality of produced packaged in polymeric films. J. Food Sci. 40, 1033. Howard, L.R., Dewi, T., 1995. Sensory, microbiological and chemical quality of mini-peeled carrots as affected by edible coating treatment. J. Food Sci. 60, 142. Howard, L.R., Griffin, L.E., 1993. Lignin formation and surface discoloration of minimally processed carrot sticks. J. Food Sci. 58 (5), 1065. Lafuente, M.T., Cantwell, M., Yang, S.F., Rubatzky, V., 1989. Isocoumarin content of carrots as influenced by ethylene concentration, storage temperature and stress conditions. Acta Hort. 258, 532. Nielsen, S.S., 1994. Introduction to the Chemical Analysis of Foods. Jones and Bartlett, Boston, p. 93.
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Nisperos-Carriedo, M.O., Baldwin, E.A., 1988. Effect of two types of edible films on tomato fruit ripening. Proc. Fla. State Hortic. Soc. 101, 217. Nisperos-Carriedo, M.O., Baldwin, E.A., Shaw, P.E., 1991. Development of an edible coating for extending postharvest life of fruits and vegetables. Proc. Fla. State Hortic. Soc. 104, 122. Park, H.J., Chinnan, M.S., Shewfelt, R.L., 1994. Edible coating effects on storage life and quality of tomatoes. J. Food Sci. 59 (3), 568.
.
Phan, C.T., 1974. Biochemical studies on the development of bitterness in stored carrots. Acta Hort. 38, 309. Rizvi, S.S.H., 1981. Requirements for foods packaged in polymeric films. Crit. Rev. Food Sci. Nutr. 14, 111. Sarkar, S.K., Phan, C.T., 1973. Effect of ethylene on the qualitative and quantitative composition of the phenol content of carrot roots. Physiol. Plant 30, 72. Sarkar, S.K., Phan, C.T., 1979. Naturally-occurring and ethylene-induced phenolic compounds in the carrot root. J. Food Prot. 42 (6), 526.
Impact of edible coatings on nutritional and physiological changes in lightly-processed carrots Peiyin Li a,b, M. Margaret Barth a,b,* a
Department of Nutrition and Food Science, Uni6ersity of Kentucky, Lexington, KY 40506 -0054, USA b Redi-Cut Foods, Inc., 9501 Ne6ada A6enue, Franklin Park, IL 60131, USA Received 13 August 1996; accepted 28 March 1998
Abstract The objective of this study was to evaluate the effects of two edible coatings (EC) of varying pH (2.7 and 4.6) on carotene retention and other physiological changes in lightly-processed (LP) carrots during storage. Carrots were treated with cellulose-based EC, packaged and stored at 1°C for 28 days. Samples were taken at regular intervals for analysis. Our results showed that carotene retention was 15% greater in the EC treatments versus control treatment throughout the 28 days. Samples treated with the lower pH EC had the highest CO2 and lowest O2 concentrations in the headspace. Whiteness index (WI) scores were significantly lower in both coated samples. Ethylene production was greatest in carrots treated with the lower pH coating on removal from sealed bags to air after each storage period. ECs improved carotene retention and retarded surface whitening in LP carrots during postharvest storage. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Carrot; Edible coatings; Lightly processed; b-Carotene
1. Introduction Consumer demand for high quality, fresh, nutritive and conveniently prepared vegetables has increased dramatically in recent years. This has led to the development of lightly processed (LP) * Corresponding author. Present address: Redi-Cut Foods, Inc., 9501 Nevada Avenue, Franklin Park, IL 60131, USA. Tel.: +1 847 2882200; fax: + 1 847 2882205; e-mail: [email protected]
vegetables. LP carrots, prepared by peeling the outer layer of the carrot roots, are one of the most popular vegetable products in the US. Upon surface peeling, LP carrots are susceptible to a variety of physiological changes that limit their shelf-life. The abrasion of the surface may increase the potential for carotene oxidation during storage. Surface abrasion may further increase the respiration of carrot tissue, resulting in increased degradation of proteins, carbohydrates, and
0925-5214/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S0925-5214(98)00020-9
52
P. Li, M.M. Barth / Posthar6est Biology and Technology 14 (1998) 51–60
lipids, and the development of off-flavors. This surface wounding may also initiate the production of a new protective layer, known as ‘white blush’, a result of dehydration and lignification, on the carrot surface (Bolin and Huxsoll, 1991). Lignification after wounding is an enzyme-stimulated reaction (Hahlbrock and Grisebach, 1979; Bell, 1981). In addition, wounding also stimulates the production of ethylene. Studies have shown that ethylene production or exposure of carrots to ethylene was highly correlated with the formation of phenolic compounds, such as isocoumarin, that contribute to development of bitter flavor during storage (Phan, 1974; Sarkar and Phan, 1973, 1979; Lafuente et al., 1989). The effects of edible coatings (EC) on shelflife extension of vegetables have been reviewed by many authors (Dhalla and Hanson, 1988; Nisperos-Carriedo and Baldwin, 1988; Bender et al., 1993; Park et al., 1994; Baldwin et al., 1995; Howard and Dewi, 1995). EC may be composed of polysaccharides, proteins, lipids, or a blend of these compounds (Nisperos-Carriedo et al., 1991). The benefits of applying edible coatings to fresh intact or LP vegetables include slowed ripening and browning, and delayed color, flavor, moisture and firmness loss. Edible coatings provide a barrier to moisture, oxygen and solute movement, and consequently, reduce the metabolism and oxidation reaction rates. Although EC have been reported to control surface whitening on LP carrots (Avena-Bustillos et al., 1993), no information is published on carotene retention and other physiological changes of these carrots during postharvest storage. In our study, two cellulose-based EC of varying pH were commercially applied to LP carrots stored in modified atmosphere packaging (MAP) for 28 days. The objectives of our study were to: (1) evaluate the effects of edible coatings on carotene retention; and (2) gain a better understanding of physiological changes, such as respiration, surface whitening, moisture loss, peroxidase (POD) activity, and ethylene production of LP carrots during storage.
2. Materials and methods
2.1. Experimental design Carrot (cv. Daucus caroto6a) samples were prepared by removing the surface layer of the carrot roots (1.0 mm) according to industrial procedures within 2 days after harvest, and shipped overnight on ice in boxed, styrofoam containers from California (Grimmway Farms, Salinas, CA) to the University of Kentucky Postharvest Research Laboratory. Carrot samples were carefully selected for quality and uniformity and placed into the following treatment groups: control, EC1 and EC2. Two types of cellulose-based edible coatings, EC1 (pH= 2.7) and EC2 (pH= 4.6) (EcoScience, Orlando, FL) were applied by dipping the carrots in the coating solution for 10 s followed by draining in a stainless steel colander for 4 h at 1°C, 85% RH. Carrot samples (70 g/bag) from each treatment were placed into polymeric film bags (OTR= 5006 cm3/m2/24 h/atm. at 25°C, 50% RH) (Grimmway Farms, California) and stored in a commercial walk-in cooler maintained at 1°C, 92% RH, for 28 days in the absence of light. Three bags from each treatment were removed at regular intervals (0, 7, 14, 21, and 28 days) for the determination of carotene content, headspace composition, whiteness index score, moisture content, peroxidase activity, and ethylene production. Samples for carotene and enzyme analyses were uniformly ground using a Kitchen Aid food grinder (Model K45SS, KitchenAid, St. Joseph, MI) with 3-mm diameter holes, followed by rapid grinding in liquid nitrogen. The samples were then stored at − 80°C until analysis.
2.2. Carotene content Carotenes were extracted from ground carrot samples (stored at − 80°C) according to the method described in AOAC (1992). The a- and b-carotene were separated and quantitatively measured by reverse-phase high performance liquid chromatography (HPLC) using b-cryptoxanthin (Hoffmann-La Roche, Nutley, NJ) as an internal standard. Analysis was performed using a C18 column (Rainin Instrument, Emeryville, CA)
P. Li, M.M. Barth / Posthar6est Biology and Technology 14 (1998) 51–60
of 4.6× 250 mm length and 5-mm particle size, with the mobile phase of acetonitrile/methylene chloride/octonol (90/25/0.1, v/v/v) running isocratically at a flow rate of 1 ml/min for 30 min. The detection of a- and b-carotene was by a UV/VIS detector (Waters 486, Tunable Absorbance Detector) at 450-nm wavelength.
2.3. Headspace composition The O2 and CO2 concentrations in the headspace of the packaging bags were measured at each interval using an Oxygen and Carbon Dioxide Head Space Analyzer (Model ZR892/HS, Illinois Instrument, McHenry, IL) to estimate the respiration rate of carrots. Two bags were taken from each treatment at each sampling interval and 5 cm3 of headspace gas was injected onto the column of the analyzer. The headspace compositions were expressed as percentages of O2 and CO2.
2.4. Whiteness index (WI)
53
tissue homogenizer (Tissumizer, Model SDT 1810, Tekmar, Cincinnati, OH) at 80 rpm for 1 min. The mixture was held for 1 h at 4°C, followed by centrifugation at 10000 rpm for 15 min (4°C). POD activity of supernatant was determined by spectrophotometer from the rate of H2O2 decomposition with guaiacol serving as hydrogen donor at pH 5.0 (Howard and Griffin, 1993). POD activity was expressed as the increase in the absorbance of 0.001 units at 470 nm/g/min (25°C).
2.7. Ethylene production Carrot samples (80 g) were taken from each treatment and sealed in a glass jar fitted with a rubber septum for 1 h (25°C). The ethylene concentration inside the jar was measured by gas chromatography with a flame ionization detector (SpectraPhysics, SP4270 Integrator) using an external standard.
2.8. Statistical analysis
Six carrots were taken from each treatment per interval and color measurements were made by a Minolta Colorimeter (Model CR-200, Minolta Camera) in contact with the surface of the carrots. The L, a, and b values were obtained and severity of surface whitening was estimated by the whiteness index scores (WI) expressed by the equation 100−[(100 − L)2 +a 2 +b 2]1/2 (Bolin and Huxsoll, 1991).
Data were analyzed by analysis of variance (ANOVA) using the general linear model (GLM) procedure (SAS Institute, Cary, NC). ANOVA was done on data at each sampling interval (7, 14, 21, and 28 days), as well as for all treatment groups. All comparisons were made at the 95% confidence level.
2.5. Moisture content
3. Results
Moisture content of the carrot samples was measured as an indicator of water losses during storage using the oven drying method (Nielsen, 1994). Freshly ground carrot samples (5 g) were weighed onto aluminum weighing dishes. After 24 h in the drying oven (85°C), the samples were reweighed and percent moisture was calculated.
3.1. Carotene retention
2.6. Peroxidase (POD) acti6ity Ground carrot samples (4 g) were homogenized in 8 ml pH 6.5 citrate-phosphate buffer using a
b-Carotene levels decreased steadily in all treatments during 28-day storage (Fig. 1). After 21-day storage, b-carotene retention was significantly higher in the edible coating-treated samples versus control (PB 0.05). However, statistical analysis showed that there was no significant difference between EC1- and EC2-treated samples. By 28 days, b-carotene retention was 50% in the edible coating treatments, compared to 33% in the control samples.
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Fig. 1. Percent of b-carotene retention in lightly-processed carrots treated with edible coatings.
Similar trends could be observed in percent of a-carotene retention (Fig. 2). a-Carotene also decreased in all treatments with time, and samples treated with edible coatings had greater acarotene retention at 14, 21, and 28 days (P B 0.05). In addition, there was no significant difference of a-carotene retention between EC1 and EC2 treatments.
3.2. Headspace analysis The results of the headspace analyses are shown in Fig. 3. Oxygen levels in the bags declined rapidly from 21 to 10% in the first 7 days in all treatments. During the remainder of storage, O2 concentration remained relatively constant, with a slight increase in EC2 and control treatments, and a slight decrease in EC1-treated samples. After 14 days, EC2-treated samples had a significantly higher O2 level than EC1-treated samples (PB0.05). However, no significant differences were detected between EC1 and control samples, or between EC2 and control samples at any intervals.
In contrast to O2 levels, CO2 concentrations in all treatments exhibited an increase from 0 to 2.5% by 7-day storage. EC1-treated samples reached 3.6% by the end of the storage. CO2 levels remained constant for control and EC2 treatments until 21 days followed by a decrease to 1.7 and 1.6% in control and EC2-treated samples by 28 days. EC1 treatment had a significantly higher CO2 level than both EC2 and control treatments (PB 0.05), whereas there was no significant difference in CO2 content between EC2 and control treatments.
3.3. Whiteness index (WI) Surface whitening was significantly retarded by applying edible coatings. By the end of 3-week storage, both EC1- and EC2-treated carrots maintained fresh appearance whereas an obvious whiteness developed in the non-treated samples. The severity of surface whitening in LP carrots during storage was quantitatively illustrated by whiteness index (WI) scores (Fig. 4). Higher WI scores indicate greater development of surface
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Fig. 2. Percent of a-carotene retention in lightly-processed carrots treated with edible coatings.
whiteness. The initial WI score for all treatments was 30.1. Control samples exhibited the highest rate of increase in WI scores while EC2-treated samples showed the lowest WI scores. At 28 days, WI scores increased 30% in the control samples (39.3) whereas only 8.6 and 0.7% increases were observed in the EC1- (32.7) and EC2- (30.2) treated samples, suggesting the most severe development of surface whitening in the untreated samples compared to the treated samples (P B 0.05). EC1-treated samples showed significantly higher WI scores than EC2-treated samples at 21 and 28 days.
3.4. Moisture content The percent moisture in carrot tissues decreased in the untreated samples, but the changes were unremarkable, with moisture losses less than 1% during the entire storage period. In the edible coating-treated samples, water losses were not significant. A significantly higher moisture content was observed in the edible coating treated samples versus control samples after 14 days (P B 0.05).
3.5. Peroxidase acti6ity The initial level of POD activity in the LP carrots was 0.473 (absorbance increase/g/min). A constant increase in POD activity was shown in control samples during storage (Fig. 5). A slight decrease, followed by an increase of POD activity occurred in both EC1- and EC2-treated samples. The trends in POD activity in the carrot samples were similar to those of WI scores, with highest POD levels in control samples and lowest activity in EC2-treated samples. A significant difference among treatments was shown at 7 days and 21 days (PB 0.05). By 21 days storage, POD activity increased by 89.1% in control samples, 77.8% in EC1-treated samples, and 58.1% in EC2-treated samples.
3.6. Ethylene production Ethylene production increased slightly in EC2 and control samples, but remarkably in EC1 samples (Fig. 6). EC1-treated samples started to show higher ethylene production than EC2 and control
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treatment after 14 days (P B0.05). By the end of storage, ethylene production was 0.773 ml/g/h (fresh weigh basis, gas volume at 20°C, 101 kPa) in EC1-treated samples, whereas levels were 0.123 and 0.084 ml/g/h in the control and EC2 treatments, respectively. These results indicate a sevento ninefold higher ethylene production in EC1 treatment than control or EC2 treatments. Control samples showed a significantly higher ethylene production than EC2-treated samples only at 28 days.
4. Discussion
4.1. Carotene losses Possible reasons for carotene losses in LP carrots during storage are autoxidation, which occurs
spontaneously when the carotenes combine with oxygen in the air, and enzymatic oxidation, which is catalyzed by oxidative enzymes (Gross, 1991). The abrasion of the carrot surface exposes the phloem, where carotenes are most concentrated, to the air and light. Edible coatings serve as a protective layer and control the permeability of O2 and CO2, thus decreasing the autoxidation potential of carotenes. Headspace analysis showed the highest O2 concentration occurred inside the EC2 packages, followed by control and EC1 bags. O2 levels did not appear to correlate with the trends for carotene losses, suggesting that either the carotene losses in LP carrots are not sensitive to oxygen levels, or the permeability of edible coatings has a greater effect on the gas exchanges of the plant tissue as compared to the use of packaging films, and may result in reduced internal oxygen levels in the coating-treated samples. Edible coating treatments of vegetables have been reported to retard respiration and metabolism (Nisperos-Carriedo and Baldwin, 1988; Bender et al., 1993; Park et al., 1994), indicating that the activities of enzymes involved in the oxidation of carotenes may be suppressed or the substrate (O2) limited by the application of edible coatings. This may lead to decreased carotene losses in LP carrots during storage. No significant difference of carotene retention was detected between EC1- and EC2-treated samples, suggesting that the pH of the edible coatings did not affect carotene oxidation in our study.
4.2. Respiration le6els
Fig. 3. Headspace analysis of O2 and CO2 in lightly-processed carrots treated with edible coatings.
The patterns of the headspace composition of CO2 and O2 inside the packages in our study were in agreement with those previously described for many packaged fruits and vegetables (Henig and Gilbert, 1975; Rizvi, 1981; Carlin et al., 1990), showing a rapid change of CO2 and O2 concentration in the first few days, followed by equilibration of gas composition during the market life of the product. In our modified atmosphere system for untreated samples, the equilibrium concentrations of O2 and CO2 inside bags were approximately 10 and 2.5%, respectively, which never reached critical values (1% O2 and 30% CO2;
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Fig. 4. Whiteness index scores in lightly-processed carrots treated with edible coatings.
Carlin et al., 1990), indicating that the amount of samples in the bags was small enough and the permeability of the film was high enough to avoid anaerobic metabolism of the untreated carrots. Permeability in the edible coatings should be considered in EC1- and EC2-treated samples when evaluating their impact on atmospheric modification in carrot tissue.
4.3. Surface whitening WI scores indicated that surface whitening in LP carrots was largely inhibited by applying the edible coatings, and samples treated with high pH edible coatings showed a better result than the samples treated with low pH edible coatings. The possible reasons for the whiteness development on the carrot surface are dehydration and lignification. Although moisture content was significantly lower in the control carrot samples after 14-day storage (B 0.05%), it cannot be concluded that moisture losses contributed to increased surface whitening in these samples. It is possible that ECs reduced whitening by physically filling the air-filled surface tissue of the abraded carrots.
We measured the POD activity in the carrot tissues because it is a key enzyme involved in lignification on the carrot surface (Bell, 1981). Trends of POD activity were similar to those of WI scores. These results are in agreement with the report by Howard and Griffin (1993) who suggested POD activity was highly correlated with the development of lignification in LP carrots during storage. The elevated POD activity in control samples may be related to the stimulation of lignin formation and, overall, the application of edible coatings on the surface of LP carrots may retard surface whitening by suppressing peroxidase activity. EC1-treated samples showed higher WI scores and POD activity than EC2-treated samples, indicating that the pH of EC1 may be too low to have an optimal effect on retarding surface whitening. The optimal pH for peroxidase activity in carrot tissue is 5.0 and the pH for EC1 and EC2 coatings is 2.7 and 4.6, respectively. Therefore, the suppression of peroxidase activity is likely not due to the pH. It is possible that the EC1 may not have been as effective as a filler or was somewhat phytotoxic to the surface cells of the carrot.
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Fig. 5. Peroxidase activity in lightly-processed carrots treated with edible coatings.
Moisture contents and POD activity may also have an impact on the losses of carotenes, given that carotenes are more rapidly oxidized in dehydrated products and peroxidase is involved in the enzymatic oxidation of carotenes (Gross, 1991). The untreated samples showed the highest carotene losses along with the greatest moisture losses and POD activity compared to the ECtreated samples.
4.4. Ethylene production Increase in ethylene production may be attributed to the biosynthesis of wound ethylene (Abeles et al., 1992). Edible coatings may serve as a protective layer for the wounded surface of the carrots and reduce wounding responses. This was demonstrated in the EC2 treatment where significantly lower ethylene levels than in the untreated samples were observed at 28 days. The reason why EC1-treated samples showed such a remarkably high ethylene production in LP carrots is unclear. It is possible that the lower pH coating was somewhat phytotoxic to the carrot tissue.
Ethylene is also considered to be a causative agent in the biosynthesis of isocoumarin, a metabolite in phenolic metabolism and the bitter compound produced in the carrot tissues during storage (Sarkar and Phan, 1973, 1979). However, this only occurs when the ethylene inside the packages reaches a sufficiently high level. Ethylene levels inside the packages depend on the ethylene production of the carrots as well as the permeability of the packaging materials. The ethylene produced in EC1-treated samples in our study may be high enough to cause the alteration of phenolic metabolism and is under investigation in our laboratory.
5. Conclusion In summary, our results suggest that the cellulose-based edible coatings have an overall beneficial effect on retention of carotenes, retardation of surface whitening, and extension of shelf-life of the LP carrots. The pH in edible coatings did not affect the carotene losses in the LP carrots. It appeared to affect, however, the respiration, sur-
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Fig. 6. Ethylene production in lightly-processed carrots treated with edible coatings.
face whitening, and ethylene production in these carrots.
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