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Quality changes of intact and sliced fennel stored under different atmospheres
Postharvest Biology and Technology 41 (2006) 307–316
Quality changes of intact and sliced fennel stored under different atmospheres V´ıctor H. Escalona, Encarna Aguayo, Francisco Art´es ∗ Postharvest and Refrigeration Group, Department of Food Engineering, Technical University of Cartagena, Paseo Alfonso XIII, 48. 30203 Cartagena, Murcia, Spain Received 28 June 2005; accepted 9 April 2006
Abstract The effect of atmospheres of 5 kPa O2 + 5 kPa CO2 , 5 kPa O2 + 15 kPa CO2 , and 21 kPa O2 + 0 kPa CO2 (as a control) on the metabolic activity and quality of intact and sliced ‘Orion’ fennel was studied. Under these atmospheres, intact bulbs were stored for 28 days at 5 ◦ C followed by 3 days at 15 ◦ C and 60–70% RH in air while sliced fennel was stored under the same atmospheres for 14 days at 5 ◦ C. In both intact bulbs and sliced fennel respiration rates, ethylene production, microbial counts (only for slices), colour, sugar and organic acid contents, and chemical and sensory attributes were evaluated. By using a controlled atmosphere (CA) with low O2 , the respiration rates and ethylene production decreased by more than 50% compared to air. This reduction of the metabolic activity was more substantial for intact bulbs than sliced fennel. CA with 5 kPa O2 delayed the quality loss of slices and fennel bulbs. After 3 weeks of storage, intact bulbs stored in 5 kPa O2 + 15 kPa CO2 suffered physiological damage developing as a brown spot. Using a CA of 5 kPa O2 + 15 kPa CO2 led to the best quality in sliced fennel. However, the recommended atmosphere for intact fennel was 5 kPa O2 + 5 kPa CO2 . © 2006 Elsevier B.V. All rights reserved. Keywords: Respiration rate; Ethylene production; Microbial and sensory qualities; Browning
1. Introduction An increasing demand for fennel from European countries such as Germany, Italy, The Netherlands and United Kingdom has resulted an expansion of the cultivated area into the Spanish Mediterranean coast. In the south-east of Spain the harvest season for fennel extends from December to March, with a yield of about 30–35 t ha−1 (Pico, 2001). Fennel bulbs are harvested when they have an equatorial diameter greater than 6 cm, ensuring that the bulbs are firm, white, sweet, and without injuries. Usually fennel bulbs are not cold-stored for a long time, being harvested according to market demand. However, commercial storage can reach 2 weeks at 0–1 ◦ C and 95% RH. The major postharvest loses of fennel under high humidity and low temperature conditions are due to weight loss and decay associated with bacteria (Erwinia carotovora and ∗
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Pseudomonas spp.) and moulds (Botrytis cinerea and Sclerotinia sclerotiorum) (Snowdon, 1991; Namesny, 1993; Pico, 2001). Browning on the butt end cut zone of fennel bulbs and in the cut surfaces of fresh-cut fennel is the most important cause of deterioration during cold storage and distribution (Albenzio et al., 1998; Escalona et al., 2005a,b). Low O2 and high CO2 levels in controlled atmospheres (CA) or modified atmosphere packaging (MAP) can reduce browning of fennel bulbs after 2 weeks of storage at 0 and 5 ◦ C (Art´es et al., 2002a,b). During minimal fresh processing for preparing ready-toeat fruit and vegetables, tissues suffer damage mainly by cutting, which increases their rate of deterioration. For keeping quality and extending shelf life, these products must be maintained throughout the cold chain in the range of 0–5 ◦ C (Huxsoll and Bolin, 1989; Varoquaux and Wiley, 1994; Ahvenainen, 1996). MAP at 0–5 ◦ C with 1–8 kPa O2 and 5–20 kPa CO2 might be a beneficial tool for maintaining quality of these kinds of vegetables because these conditions
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reduce water loss, respiration rate and ethylene production, inhibit browning and some physiological disorders and retard microbial growth and decay (Varoquaux and Wiley, 1994; Watada et al., 1996; Gorny, 1997; Art´es et al., 1999a). Under these conditions senescent browning in fresh-cut lettuce was delayed due to a reduced production of phenolic compounds (L´opez-G´alvez et al., 1996). For fresh-cut vegetables, most of which fall into the low-acid category (pH 5.8–6.0), the high humidity and the extension of the cut surfaces can provide ideal conditions for the growth of microorganisms (Brackett, 1987). Previously, other authors have found it difficult to create a minimally processed fresh product from fennel largely due to its susceptibility to enzymatic browning (Albenzio et al., 1998). Minimally processed fennel should be considered as a new product and thus has received little attention in the scientific literature (Escalona et al., 2005a,b). The purpose of this work was to determine the effect of low O2 combined with moderate to high CO2 levels on the respiration rate and ethylene production of intact bulbs and sliced fennel at 5 ◦ C (usual commercial temperature). Several chemical and sensory parameters were monitored during chilling storage and retail sale.
2. Materials and methods 2.1. Preparation of fennel bulbs ‘Orion’ fennel (Foeniculum vulgare var. dulce) was fieldgrown in Torre Pacheco (Murcia, Spain) in the Mediterranean coast of Spain. Bulbs were hand-harvested on 8th February, and selected in the field, eliminating the soiled and decayed external leaves. Four to five stalks 10 cm long were maintained per bulb, transported to the laboratory (40 km) and stored at 0 ◦ C. The next morning, bulbs were carefully inspected, selecting only those that were free from defects and with similar appearance. The average weight of bulbs selected was 312 ± 16 g (mean ± S.E.), with equatorial and longitudinal diameters of 8.0 ± 0.2 and 7.6 ± 0.3 cm, respectively. In the clean room at 7 ◦ C, bulbs were washed in a solution of 50 mg L−1 NaOCl (pH 7.5) at 5 ◦ C for 1 min. 2.2. Minimal fresh processing of fennel Minimal processing was conducted in a disinfected cold room at 7 ◦ C. Bulbs selected for processing were cut in slices approximately 0.8 cm thick using a commercial cutting machine (Hallde RG-100, Sweden). Immediately after cutting, the slices were immersed in a 100 mg L−1 NaOCl solution at 5 ◦ C and pH 7.5 for 1 min and then drained. 2.3. Experimental set-up The treatment gas mixtures of 5 kPa O2 + 5 kPa CO2 , 5 kPa O2 + 15 kPa CO2 and 21 kPa O2 + 0 kPa CO2 (as a
control) were selected based on results of our previous experiments (Art´es et al., 2002a,b; Escalona et al., 2005a,b). To simulate commercial practice, the maximum cold storage period requirement for intact bulbs and sliced fennel were stored for 28 and 14 days at 5 ◦ C, respectively. After cold storage, intact bulbs were maintained at 15 ◦ C and 60–70% RH in air for 3 days to simulate a retail sale period. 2.4. Respiration rate and ethylene production Samples of 3–4 intact bulbs and 400 g of sliced fennel were put into 2.6 and 1.5 L glass jars, respectively. For each gas mixture, five jars were connected to a flushing panel with a flow rate of 1–2 L h−1 humidified to 95% RH. The gas mixture was composed of a gas-mixing panel (Flowboard, Davis, CA, USA). The jars were closed and the initial head space composition (O2 , CO2 and N2 ) was monitored using a 0.5 mL gas sample which was injected into a gas chromatograph (GC) (Shimadzu GC-14B, Tokyo, Japan) equipped with a thermal conductivity detector. The C2 H4 production was measured with a GC (Hewlett Packard 5730A, Philadelphia, PA, USA) equipped with a flame ionisation detector on a 1 mL gas sample. The headspaces were analysed again 2–4 h later. The measurements were conducted periodically during the storage period. Between measurements, the jars were continuously flushed with the respective gas mixture. 2.5. Microbiological analysis In order to determine microbial growth on sliced fennel on days 0 (production day) and 14, three random samples were taken. Three jars per treatment were analysed in each evaluation period. A 30 g sample of slices was blended with 270 mL of sterile peptone buffered water (Merck Darmstadt, Germany) for 1 min into a sterile stomacher bag (Model 400 Bags 6141, London, UK) by using a Masticator (Seward Medical, London, UK). Serial dilutions were prepared in 9 mL PPS. From each dilution, 1 mL aliquots were aseptically pipetted for bacteria microflora and 0.1 mL for yeasts and moulds. The following media and incubation conditions were used: plate count agar (MERCK Darmstadt, Germany) incubated for mesophilic and psychrophilic aerobic bacteria, incubated at 32 ◦ C for 48 h and at 7 ◦ C for 7 days, respectively, and Rose Bengal Chloramphenicol agar base (MERCK Darmstadt, Germany) for yeasts and moulds by spread, 5 days at 22 ◦ C. Duplicates were made for each dilution (Pascual and Calder´on, 2000). Microbial counts were reported as log10 colony forming units per g sample (log cfu g−1 ). Microbial quality of the product was evaluated according to the Spanish legislation for minimally fresh processed vegetables (RD 3484/2000, 2001), where maximum microbial counts are 7 log cfu g−1 for aerobic bacteria, and 5 and 3 log cfu g−1 for yeasts and moulds, respectively.
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2.6. Colour measurements Colour changes were measured with a tristimulus colorimeter (Minolta CR-300, Ramsey, NJ, USA) with an 8 mm diameter of viewing aperture, using a white plate as reference (Y = 94.3; x = 0.3142; y = 0.3211, standard CIE illuminant, 2◦ observer). Colour was expressed as L* , a* , b* , hue angle (H◦ = tan−1 b* /a* ) and chroma [(a*2 + b*2 )1/2 ]. Measurement of bulb colour was conducted at six points on the equatorial axis and additionally at three points on the butt end cut zone of the bulbs on days 0 and 28 of storage a 5 ◦ C and after 3 days at 15 ◦ C in air. The butt end cut zone is the most sensitive to enzymatic browning (Art´es et al., 2002a; Escalona et al., 2004). The colour of sliced fennel was measured on each slice taken randomly, 30 per jar (Escalona et al., 2005a). 2.7. Sugar and organic acid contents After each evaluation, juice samples (20 mL) were frozen at −70 ◦ C. At the end of storage, samples were thawed and centrifuged (Sigma 1-13 model, Osterode, Germany) at 10,468 × gn for 10 min. The supernatent was filtered twice, firstly by using a 0.45 m pore size filter (Nylon Filter Media with Polypropylene Housing, Whatman, Clifton, NJ, USA) and secondly with a Sep-Pack Cartridge filter (C-18 Cartridges Waters, Taunton, Ireland). The extracts were analysed on by HPLC (Merck Hitachi, Darmstadt, Germany) as described by Melgarejo et al. (2000). Sugar composition was determined by HPLC equipped with a refractive index detector (Hitachi, L-7490 model, Tokyo, Japan) and a Lichrospher 250-4 NH2 column (Lichrospher, 5 m, 100 NH2 , Merck, Darmstadt, Germany). A 20 L extract sample was injected using a mobile phase of 85:15 acetonitrile: water (Merck, Germany) at a 1.5 mL min−1 flow. Organic acids were analysed in 10 L aliquots with a non-polar derivatives Lichrospher column (RP-Select B, 5 m, Merck, Darmstadt, Germany). The mobile phase (0.3 mL min−1 ) was a combination of water and methanol (99:1, v/v) with a buffer of 50 mM phosphate di-hydrogen potassium with H2 SO4 (pH 3). Samples were read at 210 nm with a UV–vis dectector (Hitachi, L-7400, Tokyo, Japan). Quantification of samples was determined by comparison to authentic standards (Sigma–Aldrich Chemie, Steinheim, Germany). Values were expressed in g per 100 mL of juice. 2.8. Sensory evaluations At the beginning and at the end of cold storage and after the retail sale period (bulbs only), an informal panel of five people (3 men and 2 women, aged 25–55), familiar with the sensory properties of fennel, evaluated appearance, aroma and texture. A nine-point scale to record their perceptions of appearance where 1: inedible, 3: poor, 5: fair, 7: good, and 9: excellent was used. Aroma and texture were scored based on a similar scale, where 1: complete lacking or soft, 5: moderate, and 9: full characteristic or fresh, respectively (adapted from
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Kader et al., 1973). Browning on the butt end cut zone of the bulbs and on cut slices was determined subjectively for each repetition using the scale 1: none; 2: slight, 3: moderate, 4: severe; and 5: very severe. Both 4 and 5 indicate that slices were commercially unacceptable (Escalona et al., 2005a). 2.9. Statistical analysis A completely randomised design with five replicates per treatment, where each glass jar constituted a replicate, was performed. To determine the effect of storage time (at harvest, after cold storage and after retail sale) and gas mixture (5 kPa O2 + 5 kPa CO2 ; 5 kPa O2 + 15 kPa CO2 ; 21 kPa O2 + 0 kPa CO2 ) a two-way ANOVA (P < 0.05, 0.01, 0.001) was carried out. Mean values were compared by the LSD multiple range test to identify significant differences among treatments and significant interactions between factors.
3. Results and discussion 3.1. Respiration rates After 4 days at 5 ◦ C, the respiration rates of bulbs stored in air (18 mg CO2 kg−1 h−1 ) was higher than those in 5 kPa O2 + 5 or 15 kPa CO2 (7–11 mg CO2 kg−1 h−1 ). During cold storage, control bulbs showed a larger reduction in their respiration rate (Fig. 1A). CA-stored bulbs from either treatment showed a relatively constant respiration rate throughout 28 days at 5 ◦ C. The respiration rate at 5 ◦ C was significantly reduced when O2 levels decreased from 21 to 5 kPa. The CO2 level of 15 kPa stimulated the respiration rate as compared to a CA of 5 kPa CO2 . This could be due to ‘Orion’ fennel bulbs stored under high CO2 level showing a physiological disorder (‘brown spot’) on the external leaves starting at the third week of storage. However, the respiration rate of ‘Clio’ fennel bulbs at 0 ◦ C was reduced when in a CA of 5 kPa O2 , the CO2 level increased from 5 to 20 kPa (Art´es et al., 2002a). In fennel bulbs stored at 0 ◦ C in air, a low to moderate nonclimacteric respiratory behaviour with a respiration rate of 8–9 mg CO2 kg−1 h−1 was found (Escalona et al., 2004). Respiration rates of control fennel slices were about 25% higher than under 5 kPa O2 + 15 kPa CO2 , and 100% compared to 5 kPa O2 + 5 kPa CO2 . After 3 days of cold storage, the rates were 25.3 mg CO2 kg−1 h−1 for control, 12.6 and 20 mg CO2 kg−1 h−1 for 5 kPa O2 + 5 or 15 kPa CO2 , respectively (Fig. 1B). Therefore, the slices in a CA of 15 kPa CO2 had higher respiration rates than in a CA of 5 kPa CO2 , although they maintained lower rates than in air. Furthermore, in contrast to results in intact bulbs, under a CA of 15 kPa CO2 , no physiological disorders on fennel slices were found. This was probably due to lesser storage time. After minimal fresh processing the respiration rate increased around 25% compared to intact bulbs. This rate was slightly higher than the lower limit of the range between
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Fig. 1. Respiration rates of intact bulbs (A) and slices of fennel (B) under different atmospheres for 27 and 13 days at 5 ◦ C, respectively. The symbols are the average value for 5 replicates and bars ± S.E. () 21 kPa O2 + 0 kPa CO2 ; () 5 kPa O2 + 5 kPa CO2 ; () 5 kPa O2 + 15 kPa CO2 ).
Fig. 2. Ethylene production of intact bulbs (A) and slices of fennel (B) in different atmospheres for 27 and 13 days at 5 ◦ C, respectively. The symbols are the average value for 5 replicates and bars ± S.E. () 21 kPa O2 + 0 kPa CO2 ; () 5 kPa O2 + 5 kPa CO2 ; () 5 kPa O2 + 15 kPa CO2 ).
3.2. Ethylene production 1.2- and seven-fold cited for this kind of produce (Cantwell and Suslow, 2002; Aguayo et al., 2004), depending on the cultivars, maturity stage, O2 and CO2 levels, water vapour pressure, presence of inhibitors, severity of wounding, and temperature. Therefore, the sliced cut did not seriously harm the fennel tissues as recommended for processing of bulbs. According to previous reports, the respiration rates of diced fennel and intact bulbs for 13 days at 0 ◦ C in air were 11–17 mg CO2 kg−1 h−1 and 9–14 mg CO2 kg−1 h−1 , respectively. For this reason, the effect of cutting resulted in CO2 production 50% higher than that of the intact bulbs (Escalona et al., 2005a). Fennel slices showed a slight decrease in respiration when in air storage with rates of 24–20 mg kg−1 h−1 at 5 ◦ C and 16–14 mg kg−1 h−1 at 0 ◦ C (Escalona et al., 2005b). No other studies on minimal fresh processing of fennel have been published so far. Nevertheless, an increased respiration rate of diced onion in air compared to 5 kPa O2 + 0 kPa CO2 after 4 days at 4 ◦ C was found (Blanchard et al., 1996). On the other hand, freshcut carrot showed a decreased respiration rate under 0.5 kPa O2 + 10 kPa CO2 of about 55% at 0 ◦ C, 65% at 5 ◦ C and 75% at 10 ◦ C compared to air (Izumi et al., 1996).
Intact bulbs under air and CA showed a decrease in ethylene production until day 17 of storage (Fig. 2A). At the end of the experiment, the ethylene production slightly rose in all treatments due to the start of decay on the bulb stalks. After 4 days at 5 ◦ C, the bulbs reached a production rate of 0.9 L C2 H4 kg−1 h−1 in air and 0.7 and 0.5 L C2 H4 kg−1 h−1 in a CA of 5 kPa O2 combined with 5 and 15 kPa CO2 , respectively. Therefore, under both CA conditions, the metabolic activity of bulbs was reduced 50% compared to that in air. In ‘Orion’ fennel bulbs an ethylene production of 0.2–0.5 mL C2 H4 kg−1 h−1 at 0 ◦ C was found (Escalona et al., 2004). Low O2 levels reduced ethylene synthesis of vegetables and high CO2 levels delayed sensitivity of bulbs to ethylene (Saltveit, 2003). Consequently, low O2 and high CO2 levels are interesting for storage of intact fennel because very high ethylene could negatively affect their quality. Therefore, a knowledge of respiration rates and ethylene production might be a useful tool to predict rate of decay and development of physiological disorders. The ethylene production of the sliced fennel decreased throughout storage, particularly during the first 6 days, showing a similar behaviour to that of the respiration rate (Fig. 2B).
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Table 1 Mesophilic, psychrophilic, yeast, and mould counts in fennel slices at day 0 and after 14 days at 5 ◦ C under different atmospheres Treatments
Mesophilic
Psychrophilic
Yeast
Initial
5.2 ± 0.3
5.2 ± 0.4
≤2.0
After 14 days at 5 ◦ C 21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
8.9 ± 0.1 8.8 ± 0.2 9.3 ± 0.2
8.9 ± 0.2 8.6 ± 0.2 9.2 ± 0.3
≤2.0 ≤2.0 ≤2.0
Mould 2.2 ± 0.2 ≤2.0 ≤2.0 ≤2.0
Data are mean values of three replicates and are expressed as log cfu g−1 ± S.E.
Ethylene production decreased when CO2 increased from 0 to 15 kPa, reaching on day 3 a rate of 0.9, 0.63 and 0.47 L C2 H4 kg−1 h−1 under air, 5 kPa O2 + 5 kPa CO2 and 5 kPa O2 + 15 kPa CO2 , respectively. Fennel slices had a similar ethylene production to that of intact bulbs until day 5 of cold storage. Subsequently the ethylene production of the slices continued decreasing less than for the intact bulbs. This behaviour was unexpected because after cutting, ethylene production commonly increases (Varoquaux and Wiley, 1994; Ahvenainen, 1996; Cantwell and Suslow, 2002). However, these results confirm our previous report on diced fennel at 0 ◦ C (Escalona et al., 2005a) and also those with sliced tomato at 2 ◦ C (Art´es et al., 1999b) and pears (Rosen and Kader, 1989). Izumi et al. (1996), found that the ethylene production of fresh-cut carrot was lower than 0.1 L kg−1 h−1 in air and in a CA of 0.5 kPa O2 + 10 kPa CO2 . 3.3. Microbiological quality Results for aerobic mesophilic and psychrophilic bacteria counts were similar to each other and increased during the cold storage period of 14 days from 5.2 log cfu g−1 at day 0 to 8.6–9.3 log cfu g−1 in all gas mixtures (Table 1). The bacterial counts were higher than the mesophilic maximum limit (7 log cfu g−1 ) prescribed by the Spanish standard (RD 3484/2000, 2001) and than the psychrophilic limit of 8 log cfu g−1 (Debevere, 1996). After 14 days at 0 ◦ C, fennel dices in an MAP of 11–13 kPa O2 + 9–12 kPa CO2 had mesophilic counts of 5.3–5.4 log cfu g−1 (Escalona et al., 2005a). Similar to this work, mesophilic and psychrophilic counts in diced onion increased from 4 log cfu g−1 at day 0 to 8 log cfu g−1 after 14 days at 4 ◦ C under 2 kPa O2 + 0 or 5 kPa CO2 and air (Blanchard et al., 1996). However, for diced onion, the psychrophilic count decreased slightly in 2 kPa O2 + 10 kPa CO2 due to the antimicrobial activity of high CO2 levels (Kader et al., 1989). Barriga et al. (1991), found a significant increase of mesophilic and psychrophilic growth on fresh-cut lettuce from 4 to 7 log cfu g−1 after 12 days at 4 ◦ C in 3 kPa O2 + 3 or 5 kPa CO2 and air. These bacteria counts slightly decreased in 3 kPa O2 + 10 kPa CO2 at 4 ◦ C. Nguyen-The and Carlin (1994) reported mesophilic counts in fresh-cut vegetables from 3 to 6 log cfu g−1 after processing and from 3 to 9 log cfu g−1 after cold storage. Additionally, in spinach leaves stored under a CA of 0.8 kPa O2 combined with 0 or 10 kPa CO2 at 5 ◦ C, the
mesophilic and psychrophilic growth decreased in comparison with air atmosphere. In low O2 alone or combined with high CO2 levels at 5 ◦ C, the bacterial counts decreased between 10- and 100-fold compared to air. However, these atmospheres did not have any effect at 10 ◦ C (Babic and Watada, 1996). Nguyen-The and Carlin (1994), also reported that an increased CO2 level delayed the microbial growth on chicory leaves at 2 and 6 ◦ C, but not at 10 ◦ C. The aerobic bacterial counts on fresh-cut celery were 7–7.7 log cfu g−1 after 14 days in MAP at 2 ◦ C (Robbs et al., 1996). After 15 days at 4 ◦ C, celery sticks stored under MAP of 6 kPa O2 + 7 kPa CO2 showed a fresh quality better than air reaching mesophilic and psychrotrophic counts of 2.9 and 1.6 log cfu g−1 , respectively compared to 3.3 and 4.6 log cfu g−1 in air (G´omez and Art´es, 2005). In all gas mixtures, fennel slices registered a yeast and mould growth (2.0–2.2 log cfu g−1 ) substantially lower than the Spanish legal limit (5 log cfu g−1 for yeasts and 3 log cfu g−1 for moulds). Similar mould and yeast counts have been reported in sliced fennel (Escalona et al., 2005a), diced onion (Blanchard et al., 1996) and diced kohlrabi (Escalona et al., 2003). In contrast, Barriga et al. (1991) found that 3 kPa O2 + 10 kPa CO2 did not depress the yeast growth on cut lettuce. 3.4. Colour change In all intact bulb treatments a decrease in lightness (L* ) of the external leaves from 80.4 (at harvest) to 77.7–78.9 after cold storage was found (data not shown). A significant interaction between time and atmosphere factors (P < 0.05) was observed. An unexpected increase in L* values was measured after the retail sale period in comparison with 28 days of storage in all gas mixtures. Significant changes in b* , chroma and hue angle values were found at harvest, after cold storage and after retail sale. These colour differences were affected by the increase in temperature during the retail sale period. Therefore, the colour values on external leaves obtained by using a colorimeter were not a good indicator of fennel bulbs as browning does not affect the leaves. A significant interaction between time and atmosphere factors (P < 0.001) was found for all colour parameters on the butt end cut zone of the bulbs (Table 2). Browning was demonstrated by darkening (decrease in L* ), an increase in chroma values and a hue angle change toward red colours.
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Table 2 Colour changes of the butt end cut zone of ‘Orion’ fennel bulbs at harvest and after 28 days at 5 ◦ C under different atmospheres followed by 3 days at 15 ◦ C in air Time (day)
Atmosphere
0
L*
a*
b*
Chroma
Hue angle
81.5
−3.5
12.3
12.8
106.0
28
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
68.1 71.7 73.6
0.9 −1.3 −1.2
26.3 21.6 22.1
26.4 21.7 22.1
88.7 93.6 93.5
28 + 3
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
64.0 73.7 68.8
2.7 −1.8 0.2
29.9 21.6 21.4
30.1 21.7 21.4
85.3 94.9 89.9
Time Atmosphere Time × atmosphere
(2.79)*** (2.79)*** (4.83)***
(2.21)*** (2.21)*** (3.83)***
(2.21)*** (2.21)*** (3.82)***
(3.42)*** (3.42)*** (4.58)*
b*
Chroma
(1.29)*** (1.29)*** (2.24)***
LSD values are in parentheses, probability: ns, not significant. * Significant at P < 0.01. *** Significant at P < 0.001. Table 3 Colour changes on ‘Orion’ fennel slices at day 0 and after 14 days at 5 ◦ C under different atmospheres Time (day)
L*
a*
75.4
−3.1
7.9
8.5
111.5
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
68.1 69.5 70.7
−3.1 −3.9 −3.4
16.9 12.8 11.5
17.2 13.4 12.0
101.2 106.6 106.9
Time Atmosphere Time × atmosphere
(3.33)*** ns ns
(0.27)** ns ns
(1.52)*** (1.87)*** (2.64)***
(1.51)*** (1.85)*** (2.62)***
Atmosphere
0 14
Hue angle
(1.82)*** (2.23)*** (3.15)***
LSD values are in parentheses, Probability: ns, not significant. ** Significant at P < 0.05. *** Significant at P < 0.001.
The colour changes were larger for control bulbs than for both CA treatments. However, the CA-stored bulbs showed no colour changes compared to colour at harvest. Under 5 kPa O2 + 5 kPa CO2 , the bulbs showed less browning than that at harvest. After the retail sale period, both CA treatments delayed colour changes on the butt end cut zone of the bulbs, with the treatment of 5 kPa O2 + 5 kPa CO2 performing the best. This result agrees with that reported previously by Art´es
et al. (2002a,b). In chicory, where a red colouration is the most important disorder on the butt end cut zone, a CA of 10 kPa O2 + 10 kPa CO2 prevents the development of the red colour during 21 days at 5 ◦ C (Vanstreels et al., 2002). The lightness (L* ) of fennel slices diminished during storage at 5 ◦ C for 14 days irrespective of atmosphere treatment (Table 3). A significant interaction between time and atmosphere factors (P < 0.001) was detected increasing the chroma
Table 4 Changes in fructose, mannose, glucose, sucrose levels and in glucose:fructose ratio of ‘Orion’ fennel bulbs throughout 28 days at 5 ◦ C under different atmospheres followed by 3 days at 15 ◦ C in air Time (days)
Atmosphere
0
Fructose
Mannose
Glucose
Sucrose
Glucose/fructose
1.91
0.16
1.46
0.30
0.76
28
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
1.44 1.79 1.44
0.14 0.15 0.16
1.06 1.03 0.90
0.26 0.36 0.20
0.74 0.58 0.63
28 + 3
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
1.90 1.78 1.99
0.26 0.19 0.20
1.05 1.28 1.12
0.18 0.13 0.10
0.56 0.72 0.56
Time Atmosphere Time × atmosphere
ns ns ns
ns (0.08)** ns
ns ns ns
(0.10)** ns ns
ns ns ns
LSD values are in parentheses. Probability: ns. not significant. The mean values (n = 5) are g per 100 mL juice. ** Significant at P < 0.05.
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under air and 2 kPa O2 alone during 12 days at 4 ◦ C could be due to hydrolysis of sucrose toward glucose and fructose. Under 2 kPa O2 + 10 kPa CO2 , the hydrolysis of sucrose was delayed because the respiration rate of diced onion was reduced.
and decreasing the hue angle values on control slices. A CA of 5 kPa O2 + 15 kPa CO2 was found to be the best to delay browning on fennel slices. Escalona et al. (2005a) also reported that MAP of 11–13 kPa O2 + 9–12 kPa CO2 reduced browning on the cut zones of fennel dices for while at 0 ◦ C for 14 days.
3.6. Organic acid contents 3.5. Sugar contents Oxalic acid was the main organic acid of fennel bulbs with an amount of 30.4 mg per 100 mL−1 juice at harvest (Table 6). This value was higher than that reported by Souci et al. (1986) of 5 mg per 100 g fresh weight although our results were expressed per fresh juice. After cold storage and the retail sale period, the oxalic acid decreased to 11.56–14.81 mg per 100 mL−1 juice, independent of the gas mixtures. These results agree with the reduction of malic acid content in green tomatoes after 4 weeks at 12 ◦ C. The citric acid changes were different depending on cultivar. However, storage in air and 5 kPa O2 + 5 kPa CO2 had no effect on acid content (Goodenough et al., 1982). Fennel slices showed a decrease in oxalic acid contents after cold storage (Table 6). A CA of 5 kPa O2 + 5 to 15 kPa CO2 did not delay the oxalic acid loss following the same pattern as for sugar contents (Table 5). After cold storage, the oxalic acid content of slices was similar to that of intact bulbs, possibly because the respiration rate in slices was two-fold higher than in bulbs although their storage time was half. Therefore, no clear influence of gas mixtures on the oxalic acid consumption of fennel stored at 5 ◦ C was found.
Fructose and glucose were the predominant sugars in fennel bulbs with an initial level of 1.91 and 1.46 g 100 mL−1 , respectively (Table 4). These sugars did not register significant differences after cold storage and the retail sale period. However, sucrose content significantly decreased after retail sale in all gas mixtures. Probably the reduction of sucrose was related to the increased respiration rate caused by the higher temperature during this period. The sucrose consumption could maintain the fructose and glucose levels in the glycolysis pathway. The initial mannose content (0.16 g 100 mL−1 ) was maintained for 28 days at 5 ◦ C and slightly increased after the retail sale due to the weight loss (Table 4). The glucose:fructose ratio was lower than one, indicating a higher synthesis in opposition to glucose. Fennel slices had no change in fructose and mannose contents for 14 days at 5 ◦ C (Table 5). Fructose is the most important sugar and respiratory metabolic substrate of fennel. However, glucose and mainly sucrose could be the sources to generate new fructose for maintaining the energy requirements of the cells. For that reason, glucose and sucrose contents decreased during storage in all gas mixtures. The slices also showed a glucose:fructose ratio lower than one, as in the intact bulbs. Similar results were previously reported in diced fennel stored 14 days at 5 ◦ C (Escalona et al., 2005a). Heimdal et al. (1995), found that the sugar content in shredded iceberg lettuce slightly declined after 10 days at 5 ◦ C under MAP. The glucose and fructose contents decreased about 10–17% and 6–17%, respectively. L´opez-G´alvez et al. (1997), reported similar results in fresh-cut romaine lettuce stored in MAP. The reduction of total sugar content was from 2.1 to 1.7 g/100 g after 15 days at 5 ◦ C. According to Blanchard et al. (1996), the sucrose loss found in diced onion
3.7. Decay and physiological disorders At the end of cold storage, control bulbs developed bacterial soft-rot (Erwinia and Pseudomonas), gray mould (Botrytis cinerea), and watery rot (Sclerotinia spp.) on the end of bulb stalks. According to Snowdon (1991), these kinds of decay are normal in fennel bulbs stored at chilling temperatures. During the retail sale period, the dehydration of the stalks reduced decay growth. The decay of control bulbs was helped by the prolonged storage of 28 days at 5 ◦ C.
Table 5 Changes in fructose, mannose, glucose, sucrose levels and glucose:fructose ratio of ‘Orion’ fennel slices throughout 14 days at 5 ◦ C under different atmospheres Time (days)
Atmosphere
Fructose 1.91
0.16
1.46
0.30
0.76
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
1.46 1.06 1.12
0.13 0.16 0.10
0.82 0.72 0.72
0.06 0.03 0.04
0.56 0.68 0.64
Time Atmosphere Time × atmosphere
ns ns ns
ns ns ns
(0.71)* ns ns
(0.20)*** ns ns
ns ns ns
0 14
Mannose
Glucose
LSD values are in parentheses. Probability: ns, not significant. The mean values (n = 5) are g 100 mL−1 juice. * Significant at P < 0.01. *** Significant at P < 0.001.
Sucrose
Glucose/fructose
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Table 6 Changes in oxalic acid content of whole bulbs and slices of fennel throughout 28 days at 5 ◦ C under different atmospheres followed by 3 days at 15 ◦ C in air and troughout 14 days at 5 ◦ C under different atmospheres Time (days)
Atmosphere
Intact bulbs
Slices
0
30.43
30.43
After cold storage 28
28 d 5 ◦ C
14 d 5 ◦ C 14.81 11.56 14.11
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
24.63 9.97 12.27
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
28 d 5 ◦ C + 3 d 15 ◦ C 13.50 11.97 12.43
After retail sale period
(12.20)*** ns ns
Time Atmosphere Time × atmosphere
(12.61)*** ns ns
LSD values are in parentheses, probability: ns. not significant, the mean values (n = 5) are mg 100 mL−1 juice. *** Significant at P < 0.001.
Browning on the butt end cut zone of the fennel bulbs increased in all treatments. In air, browning was scored as severe after cold storage and retail sale periods (Table 7). However, browning was delayed under CA conditions, this being more effective at 15 than 5 kPa CO2 . A significant interaction was found between time and atmosphere factors (P < 0.001), showing differences between evaluation periods and air compared to CA conditions. After 3 weeks of storage, fennel bulbs stored under 15 kPa CO2 showed little brown spots on the external leaves that affected around 10% of the surface. This disorder did not increase subsequently after the retail sale period. An MAP of 16–17 kPa O2 + 6–7 kPa CO2 within baskets and 6–7 kPa O2 + 10 to 12 kPa CO2 within bags inhibited browning of the butt end cut of fennel for 14 days at 0 ◦ C. However, only the bag treatments showed a residual effect for delaying browning at the end of the retail sale period (Escalona et al., 2004).
After cold storage, fennel slices showed neither physiological disorders nor decay in any treatment. However, browning on the cut was scored as severe (higher than the limit of marketability) in control slices, but it was efficiently delayed when the CO2 level increased from 5 to 15 kPa (Table 8). Hamza et al. (1996), found that after 16 days at 4 ◦ C under 1 kPa O2 + 10 kPa CO2 fresh-cut romaine lettuce showed lower browning compared to air. 3.8. Sensory attributes After cold storage and retail sale periods, the appearance at harvest decreased in all treatments mainly in control bulbs showing decay development (Table 7). At the end of the retail sale, the bulbs stored under 5 kPa O2 + 15 kPa CO2 reached a score slightly lower than 5 due to little brown spots developing on the external leaves. However, under 5 kPa O2 + 5 kPa
Table 7 Changes in appearance, aroma texture, and browning on the butt end cut zone of ‘Orion’ fennel bulbs troughout 28 days at 5 ◦ C under different atmospheres followed by 3 days at 15 ◦ C in air Time (Days) 0 28
28 + 3
Atmosphere
Appearance
Aroma
Texture
Browning
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
8.0 5.0 6.3 5.5
6.8 5.5 5.8 5.8
8.5 5.8 6.9 7.1
1.0 4.3 3.4 2.4
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
2.5 6.0 4.5
5.8 6.0 5.8
5.7 6.8 6.7
4.1 3.0 2.3
Time Atmosphere Time × atmosphere
(0.94)*** (0.94)*** (1.63)***
ns ns ns
(0.74)*** (0.41)** ns
(0.55)*** (0.55)*** (0.95)***
Appearance scored on 1: inedible, 5: fair, and 9: excellent. Aroma and texture scored on 1: complete lacking or soft, 5: moderate, and 9: full characteristic or fresh, respectively. Browning scored on 1: none; 3: moderate, and 5: very severe. LSD values are in parentheses, probability: ns, not significant. ** Significant at P < 0.05. *** Significant at P < 0.001.
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315
Table 8 Changes in appearance, aroma, texture, and browning of ‘Orion’ fennel slices troughout 14 days at 5 ◦ C under different atmospheres Time (days) 0 14
Atmosphere
Appearance
Aroma
Texture
Browning
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
8.0 2.8 5.4 5.9
6.8 3.3 2.4 2.3
8.5 7.0 7.0 6.9
1.0 4.0 2.1 1.6
Time Atmosphere Time × atmosphere
(0.61)*** (0.75)*** (1.06)***
(1.66)*** ns ns
(1.14)*** ns ns
(0.23)*** (0.28)*** (0.40)***
Appearance scored on 1: inedible, 5: fair, and 9: excellent. Aroma and texture scored on 1: omplete lacking or soft, 5: moderate, and 9: full characteristic or fresh, respectively. Browning scored on 1: none; 3: moderate, and 5: very severe. LSD values are in parentheses, probability: ns, not significant, significant at P < 0.01. *** Significant at P < 0.001.
CO2 a better appearance with a score higher than acceptable for consumption was found. The texture of the bulbs diminished during cold storage although all treatments had a high humidity. The aroma did not show any significant change during the experiment. After the retail sale, the bulbs kept in 5 kPa O2 + 5 kPa CO2 showed the best sensory quality in texture and appearance (Table 7). In agreement with our previous results in bulbs stored at 0 ◦ C under CA after the retail sale period, browning of the butt end cut and external leaves was almost inhibited (Art´es et al., 2002a). The sensory quality of the fennel slices decreased after 14 days at 5 ◦ C, appearance being the attribute most affected by cold storage (Table 8). The appearance scores reached 5.4 and 5.9 in slices from 5 kPa O2 combined with 5 or 15 kPa CO2 , respectively, but control slices had a low score. The aroma decreased from 6.8 to 2–3 in all gas mixtures. Rating of the initial texture also was significantly reduced after cold storage, although it was considered as good (about 7) without significant differences between treatments. The texture changes could be more related to the high relative humidity than to O2 and CO2 levels. It was previously reported that 11–13 kPa O2 + 9–12 kPa CO2 resulted in slight browning on the surface of diced fennel, although chemical, sensory, and microbial quality of the dices were suitable for commercial purposes (Escalona et al., 2005a). Additionally, an MAP with 2–6 kPa O2 + 10–20 kPa CO2 at 0 and 5 ◦ C maintained good quality of sliced fennel (Escalona et al., 2005b). However, our results for browning development contrast with those reported by Albenzio et al. (1998), who considered enzymatic browning as the main cause of postharvest deterioration of minimally processed fennel, probably due to different fennel cultivar sensitivities to browning. Shredded iceberg lettuce had a sensory quality that decreased after 10 days at 5 ◦ C maintaining a better appearance under 2 kPa O2 and 7 kPa CO2 . Low O2 levels delayed enzymatic browning because PPO showed a lower affinity for O2 (Heimdal et al., 1994). The visual quality of diced onion was higher under 2 kPa O2 and 10–15 kPa CO2 than air after 12 days at 4 ◦ C (Blanchard et al., 1996). In other reports,
the visual quality of fresh-cut lettuce significantly decreased when the storage conditions were air or 3 kPa O2 + 0–5 kPa CO2 , but the appearance improved in 10 kPa CO2 (Barriga et al., 1991). According to Mateos et al. (1993), the phenol content in cut lettuce decreased under air enriched with 20 kPa CO2 after 10 and 20 days at 2.5 ◦ C, probably due to a reduction of cytoplasmic pH and PAL activity. However, when the cut lettuces were placed in air at 20 ◦ C, the cytoplasmic pH recovered the normal value and PAL activity. 4. Conclusions 4.1. Fennel bulbs The respiration rate of bulbs decreased under CA with 5 kPa O2 compared to air. The increase of the CO2 level from 5 to 15 kPa raised the respiration of the bulbs. A gas mixture with low O2 and moderate CO2 levels was more recommended for storage of bulbs up to 28 days at 5 ◦ C. Browning on the butt end cut of the bulbs was delayed under 5 kPa O2 combined with 5 or 15 kPa CO2 . Fennel bulbs are sensitive to 15 kPa CO2 level from the 3rd week at 5 ◦ C. 4.2. Fennel slices Under CA conditions, the respiration rate of slices was 50% lower than in air. CA with 5 kPa O2 could be recommended for keeping quality of sliced fennel up to 14 days at 5 ◦ C combined with 5 or 15 kPa CO2 . Under CA, chemical and sensory attributes of slices suffered minimal changes in initial values. Browning of the cutting zones was particularly delayed under 5 kPa O2 + 15 kPa CO2 . Acknowledgements The authors are grateful to financial support of the Spanish Ministry for Education and Science, project AGL200408004/ALI, and to CEBAS-CSIC (Murcia) and FRUVEG Soc. Coop. (Torre Pacheco, Murcia) for providing facilities and fennel, respectively.
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Quality changes of intact and sliced fennel stored under different atmospheres V´ıctor H. Escalona, Encarna Aguayo, Francisco Art´es ∗ Postharvest and Refrigeration Group, Department of Food Engineering, Technical University of Cartagena, Paseo Alfonso XIII, 48. 30203 Cartagena, Murcia, Spain Received 28 June 2005; accepted 9 April 2006
Abstract The effect of atmospheres of 5 kPa O2 + 5 kPa CO2 , 5 kPa O2 + 15 kPa CO2 , and 21 kPa O2 + 0 kPa CO2 (as a control) on the metabolic activity and quality of intact and sliced ‘Orion’ fennel was studied. Under these atmospheres, intact bulbs were stored for 28 days at 5 ◦ C followed by 3 days at 15 ◦ C and 60–70% RH in air while sliced fennel was stored under the same atmospheres for 14 days at 5 ◦ C. In both intact bulbs and sliced fennel respiration rates, ethylene production, microbial counts (only for slices), colour, sugar and organic acid contents, and chemical and sensory attributes were evaluated. By using a controlled atmosphere (CA) with low O2 , the respiration rates and ethylene production decreased by more than 50% compared to air. This reduction of the metabolic activity was more substantial for intact bulbs than sliced fennel. CA with 5 kPa O2 delayed the quality loss of slices and fennel bulbs. After 3 weeks of storage, intact bulbs stored in 5 kPa O2 + 15 kPa CO2 suffered physiological damage developing as a brown spot. Using a CA of 5 kPa O2 + 15 kPa CO2 led to the best quality in sliced fennel. However, the recommended atmosphere for intact fennel was 5 kPa O2 + 5 kPa CO2 . © 2006 Elsevier B.V. All rights reserved. Keywords: Respiration rate; Ethylene production; Microbial and sensory qualities; Browning
1. Introduction An increasing demand for fennel from European countries such as Germany, Italy, The Netherlands and United Kingdom has resulted an expansion of the cultivated area into the Spanish Mediterranean coast. In the south-east of Spain the harvest season for fennel extends from December to March, with a yield of about 30–35 t ha−1 (Pico, 2001). Fennel bulbs are harvested when they have an equatorial diameter greater than 6 cm, ensuring that the bulbs are firm, white, sweet, and without injuries. Usually fennel bulbs are not cold-stored for a long time, being harvested according to market demand. However, commercial storage can reach 2 weeks at 0–1 ◦ C and 95% RH. The major postharvest loses of fennel under high humidity and low temperature conditions are due to weight loss and decay associated with bacteria (Erwinia carotovora and ∗
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Pseudomonas spp.) and moulds (Botrytis cinerea and Sclerotinia sclerotiorum) (Snowdon, 1991; Namesny, 1993; Pico, 2001). Browning on the butt end cut zone of fennel bulbs and in the cut surfaces of fresh-cut fennel is the most important cause of deterioration during cold storage and distribution (Albenzio et al., 1998; Escalona et al., 2005a,b). Low O2 and high CO2 levels in controlled atmospheres (CA) or modified atmosphere packaging (MAP) can reduce browning of fennel bulbs after 2 weeks of storage at 0 and 5 ◦ C (Art´es et al., 2002a,b). During minimal fresh processing for preparing ready-toeat fruit and vegetables, tissues suffer damage mainly by cutting, which increases their rate of deterioration. For keeping quality and extending shelf life, these products must be maintained throughout the cold chain in the range of 0–5 ◦ C (Huxsoll and Bolin, 1989; Varoquaux and Wiley, 1994; Ahvenainen, 1996). MAP at 0–5 ◦ C with 1–8 kPa O2 and 5–20 kPa CO2 might be a beneficial tool for maintaining quality of these kinds of vegetables because these conditions
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reduce water loss, respiration rate and ethylene production, inhibit browning and some physiological disorders and retard microbial growth and decay (Varoquaux and Wiley, 1994; Watada et al., 1996; Gorny, 1997; Art´es et al., 1999a). Under these conditions senescent browning in fresh-cut lettuce was delayed due to a reduced production of phenolic compounds (L´opez-G´alvez et al., 1996). For fresh-cut vegetables, most of which fall into the low-acid category (pH 5.8–6.0), the high humidity and the extension of the cut surfaces can provide ideal conditions for the growth of microorganisms (Brackett, 1987). Previously, other authors have found it difficult to create a minimally processed fresh product from fennel largely due to its susceptibility to enzymatic browning (Albenzio et al., 1998). Minimally processed fennel should be considered as a new product and thus has received little attention in the scientific literature (Escalona et al., 2005a,b). The purpose of this work was to determine the effect of low O2 combined with moderate to high CO2 levels on the respiration rate and ethylene production of intact bulbs and sliced fennel at 5 ◦ C (usual commercial temperature). Several chemical and sensory parameters were monitored during chilling storage and retail sale.
2. Materials and methods 2.1. Preparation of fennel bulbs ‘Orion’ fennel (Foeniculum vulgare var. dulce) was fieldgrown in Torre Pacheco (Murcia, Spain) in the Mediterranean coast of Spain. Bulbs were hand-harvested on 8th February, and selected in the field, eliminating the soiled and decayed external leaves. Four to five stalks 10 cm long were maintained per bulb, transported to the laboratory (40 km) and stored at 0 ◦ C. The next morning, bulbs were carefully inspected, selecting only those that were free from defects and with similar appearance. The average weight of bulbs selected was 312 ± 16 g (mean ± S.E.), with equatorial and longitudinal diameters of 8.0 ± 0.2 and 7.6 ± 0.3 cm, respectively. In the clean room at 7 ◦ C, bulbs were washed in a solution of 50 mg L−1 NaOCl (pH 7.5) at 5 ◦ C for 1 min. 2.2. Minimal fresh processing of fennel Minimal processing was conducted in a disinfected cold room at 7 ◦ C. Bulbs selected for processing were cut in slices approximately 0.8 cm thick using a commercial cutting machine (Hallde RG-100, Sweden). Immediately after cutting, the slices were immersed in a 100 mg L−1 NaOCl solution at 5 ◦ C and pH 7.5 for 1 min and then drained. 2.3. Experimental set-up The treatment gas mixtures of 5 kPa O2 + 5 kPa CO2 , 5 kPa O2 + 15 kPa CO2 and 21 kPa O2 + 0 kPa CO2 (as a
control) were selected based on results of our previous experiments (Art´es et al., 2002a,b; Escalona et al., 2005a,b). To simulate commercial practice, the maximum cold storage period requirement for intact bulbs and sliced fennel were stored for 28 and 14 days at 5 ◦ C, respectively. After cold storage, intact bulbs were maintained at 15 ◦ C and 60–70% RH in air for 3 days to simulate a retail sale period. 2.4. Respiration rate and ethylene production Samples of 3–4 intact bulbs and 400 g of sliced fennel were put into 2.6 and 1.5 L glass jars, respectively. For each gas mixture, five jars were connected to a flushing panel with a flow rate of 1–2 L h−1 humidified to 95% RH. The gas mixture was composed of a gas-mixing panel (Flowboard, Davis, CA, USA). The jars were closed and the initial head space composition (O2 , CO2 and N2 ) was monitored using a 0.5 mL gas sample which was injected into a gas chromatograph (GC) (Shimadzu GC-14B, Tokyo, Japan) equipped with a thermal conductivity detector. The C2 H4 production was measured with a GC (Hewlett Packard 5730A, Philadelphia, PA, USA) equipped with a flame ionisation detector on a 1 mL gas sample. The headspaces were analysed again 2–4 h later. The measurements were conducted periodically during the storage period. Between measurements, the jars were continuously flushed with the respective gas mixture. 2.5. Microbiological analysis In order to determine microbial growth on sliced fennel on days 0 (production day) and 14, three random samples were taken. Three jars per treatment were analysed in each evaluation period. A 30 g sample of slices was blended with 270 mL of sterile peptone buffered water (Merck Darmstadt, Germany) for 1 min into a sterile stomacher bag (Model 400 Bags 6141, London, UK) by using a Masticator (Seward Medical, London, UK). Serial dilutions were prepared in 9 mL PPS. From each dilution, 1 mL aliquots were aseptically pipetted for bacteria microflora and 0.1 mL for yeasts and moulds. The following media and incubation conditions were used: plate count agar (MERCK Darmstadt, Germany) incubated for mesophilic and psychrophilic aerobic bacteria, incubated at 32 ◦ C for 48 h and at 7 ◦ C for 7 days, respectively, and Rose Bengal Chloramphenicol agar base (MERCK Darmstadt, Germany) for yeasts and moulds by spread, 5 days at 22 ◦ C. Duplicates were made for each dilution (Pascual and Calder´on, 2000). Microbial counts were reported as log10 colony forming units per g sample (log cfu g−1 ). Microbial quality of the product was evaluated according to the Spanish legislation for minimally fresh processed vegetables (RD 3484/2000, 2001), where maximum microbial counts are 7 log cfu g−1 for aerobic bacteria, and 5 and 3 log cfu g−1 for yeasts and moulds, respectively.
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2.6. Colour measurements Colour changes were measured with a tristimulus colorimeter (Minolta CR-300, Ramsey, NJ, USA) with an 8 mm diameter of viewing aperture, using a white plate as reference (Y = 94.3; x = 0.3142; y = 0.3211, standard CIE illuminant, 2◦ observer). Colour was expressed as L* , a* , b* , hue angle (H◦ = tan−1 b* /a* ) and chroma [(a*2 + b*2 )1/2 ]. Measurement of bulb colour was conducted at six points on the equatorial axis and additionally at three points on the butt end cut zone of the bulbs on days 0 and 28 of storage a 5 ◦ C and after 3 days at 15 ◦ C in air. The butt end cut zone is the most sensitive to enzymatic browning (Art´es et al., 2002a; Escalona et al., 2004). The colour of sliced fennel was measured on each slice taken randomly, 30 per jar (Escalona et al., 2005a). 2.7. Sugar and organic acid contents After each evaluation, juice samples (20 mL) were frozen at −70 ◦ C. At the end of storage, samples were thawed and centrifuged (Sigma 1-13 model, Osterode, Germany) at 10,468 × gn for 10 min. The supernatent was filtered twice, firstly by using a 0.45 m pore size filter (Nylon Filter Media with Polypropylene Housing, Whatman, Clifton, NJ, USA) and secondly with a Sep-Pack Cartridge filter (C-18 Cartridges Waters, Taunton, Ireland). The extracts were analysed on by HPLC (Merck Hitachi, Darmstadt, Germany) as described by Melgarejo et al. (2000). Sugar composition was determined by HPLC equipped with a refractive index detector (Hitachi, L-7490 model, Tokyo, Japan) and a Lichrospher 250-4 NH2 column (Lichrospher, 5 m, 100 NH2 , Merck, Darmstadt, Germany). A 20 L extract sample was injected using a mobile phase of 85:15 acetonitrile: water (Merck, Germany) at a 1.5 mL min−1 flow. Organic acids were analysed in 10 L aliquots with a non-polar derivatives Lichrospher column (RP-Select B, 5 m, Merck, Darmstadt, Germany). The mobile phase (0.3 mL min−1 ) was a combination of water and methanol (99:1, v/v) with a buffer of 50 mM phosphate di-hydrogen potassium with H2 SO4 (pH 3). Samples were read at 210 nm with a UV–vis dectector (Hitachi, L-7400, Tokyo, Japan). Quantification of samples was determined by comparison to authentic standards (Sigma–Aldrich Chemie, Steinheim, Germany). Values were expressed in g per 100 mL of juice. 2.8. Sensory evaluations At the beginning and at the end of cold storage and after the retail sale period (bulbs only), an informal panel of five people (3 men and 2 women, aged 25–55), familiar with the sensory properties of fennel, evaluated appearance, aroma and texture. A nine-point scale to record their perceptions of appearance where 1: inedible, 3: poor, 5: fair, 7: good, and 9: excellent was used. Aroma and texture were scored based on a similar scale, where 1: complete lacking or soft, 5: moderate, and 9: full characteristic or fresh, respectively (adapted from
309
Kader et al., 1973). Browning on the butt end cut zone of the bulbs and on cut slices was determined subjectively for each repetition using the scale 1: none; 2: slight, 3: moderate, 4: severe; and 5: very severe. Both 4 and 5 indicate that slices were commercially unacceptable (Escalona et al., 2005a). 2.9. Statistical analysis A completely randomised design with five replicates per treatment, where each glass jar constituted a replicate, was performed. To determine the effect of storage time (at harvest, after cold storage and after retail sale) and gas mixture (5 kPa O2 + 5 kPa CO2 ; 5 kPa O2 + 15 kPa CO2 ; 21 kPa O2 + 0 kPa CO2 ) a two-way ANOVA (P < 0.05, 0.01, 0.001) was carried out. Mean values were compared by the LSD multiple range test to identify significant differences among treatments and significant interactions between factors.
3. Results and discussion 3.1. Respiration rates After 4 days at 5 ◦ C, the respiration rates of bulbs stored in air (18 mg CO2 kg−1 h−1 ) was higher than those in 5 kPa O2 + 5 or 15 kPa CO2 (7–11 mg CO2 kg−1 h−1 ). During cold storage, control bulbs showed a larger reduction in their respiration rate (Fig. 1A). CA-stored bulbs from either treatment showed a relatively constant respiration rate throughout 28 days at 5 ◦ C. The respiration rate at 5 ◦ C was significantly reduced when O2 levels decreased from 21 to 5 kPa. The CO2 level of 15 kPa stimulated the respiration rate as compared to a CA of 5 kPa CO2 . This could be due to ‘Orion’ fennel bulbs stored under high CO2 level showing a physiological disorder (‘brown spot’) on the external leaves starting at the third week of storage. However, the respiration rate of ‘Clio’ fennel bulbs at 0 ◦ C was reduced when in a CA of 5 kPa O2 , the CO2 level increased from 5 to 20 kPa (Art´es et al., 2002a). In fennel bulbs stored at 0 ◦ C in air, a low to moderate nonclimacteric respiratory behaviour with a respiration rate of 8–9 mg CO2 kg−1 h−1 was found (Escalona et al., 2004). Respiration rates of control fennel slices were about 25% higher than under 5 kPa O2 + 15 kPa CO2 , and 100% compared to 5 kPa O2 + 5 kPa CO2 . After 3 days of cold storage, the rates were 25.3 mg CO2 kg−1 h−1 for control, 12.6 and 20 mg CO2 kg−1 h−1 for 5 kPa O2 + 5 or 15 kPa CO2 , respectively (Fig. 1B). Therefore, the slices in a CA of 15 kPa CO2 had higher respiration rates than in a CA of 5 kPa CO2 , although they maintained lower rates than in air. Furthermore, in contrast to results in intact bulbs, under a CA of 15 kPa CO2 , no physiological disorders on fennel slices were found. This was probably due to lesser storage time. After minimal fresh processing the respiration rate increased around 25% compared to intact bulbs. This rate was slightly higher than the lower limit of the range between
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Fig. 1. Respiration rates of intact bulbs (A) and slices of fennel (B) under different atmospheres for 27 and 13 days at 5 ◦ C, respectively. The symbols are the average value for 5 replicates and bars ± S.E. () 21 kPa O2 + 0 kPa CO2 ; () 5 kPa O2 + 5 kPa CO2 ; () 5 kPa O2 + 15 kPa CO2 ).
Fig. 2. Ethylene production of intact bulbs (A) and slices of fennel (B) in different atmospheres for 27 and 13 days at 5 ◦ C, respectively. The symbols are the average value for 5 replicates and bars ± S.E. () 21 kPa O2 + 0 kPa CO2 ; () 5 kPa O2 + 5 kPa CO2 ; () 5 kPa O2 + 15 kPa CO2 ).
3.2. Ethylene production 1.2- and seven-fold cited for this kind of produce (Cantwell and Suslow, 2002; Aguayo et al., 2004), depending on the cultivars, maturity stage, O2 and CO2 levels, water vapour pressure, presence of inhibitors, severity of wounding, and temperature. Therefore, the sliced cut did not seriously harm the fennel tissues as recommended for processing of bulbs. According to previous reports, the respiration rates of diced fennel and intact bulbs for 13 days at 0 ◦ C in air were 11–17 mg CO2 kg−1 h−1 and 9–14 mg CO2 kg−1 h−1 , respectively. For this reason, the effect of cutting resulted in CO2 production 50% higher than that of the intact bulbs (Escalona et al., 2005a). Fennel slices showed a slight decrease in respiration when in air storage with rates of 24–20 mg kg−1 h−1 at 5 ◦ C and 16–14 mg kg−1 h−1 at 0 ◦ C (Escalona et al., 2005b). No other studies on minimal fresh processing of fennel have been published so far. Nevertheless, an increased respiration rate of diced onion in air compared to 5 kPa O2 + 0 kPa CO2 after 4 days at 4 ◦ C was found (Blanchard et al., 1996). On the other hand, freshcut carrot showed a decreased respiration rate under 0.5 kPa O2 + 10 kPa CO2 of about 55% at 0 ◦ C, 65% at 5 ◦ C and 75% at 10 ◦ C compared to air (Izumi et al., 1996).
Intact bulbs under air and CA showed a decrease in ethylene production until day 17 of storage (Fig. 2A). At the end of the experiment, the ethylene production slightly rose in all treatments due to the start of decay on the bulb stalks. After 4 days at 5 ◦ C, the bulbs reached a production rate of 0.9 L C2 H4 kg−1 h−1 in air and 0.7 and 0.5 L C2 H4 kg−1 h−1 in a CA of 5 kPa O2 combined with 5 and 15 kPa CO2 , respectively. Therefore, under both CA conditions, the metabolic activity of bulbs was reduced 50% compared to that in air. In ‘Orion’ fennel bulbs an ethylene production of 0.2–0.5 mL C2 H4 kg−1 h−1 at 0 ◦ C was found (Escalona et al., 2004). Low O2 levels reduced ethylene synthesis of vegetables and high CO2 levels delayed sensitivity of bulbs to ethylene (Saltveit, 2003). Consequently, low O2 and high CO2 levels are interesting for storage of intact fennel because very high ethylene could negatively affect their quality. Therefore, a knowledge of respiration rates and ethylene production might be a useful tool to predict rate of decay and development of physiological disorders. The ethylene production of the sliced fennel decreased throughout storage, particularly during the first 6 days, showing a similar behaviour to that of the respiration rate (Fig. 2B).
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Table 1 Mesophilic, psychrophilic, yeast, and mould counts in fennel slices at day 0 and after 14 days at 5 ◦ C under different atmospheres Treatments
Mesophilic
Psychrophilic
Yeast
Initial
5.2 ± 0.3
5.2 ± 0.4
≤2.0
After 14 days at 5 ◦ C 21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
8.9 ± 0.1 8.8 ± 0.2 9.3 ± 0.2
8.9 ± 0.2 8.6 ± 0.2 9.2 ± 0.3
≤2.0 ≤2.0 ≤2.0
Mould 2.2 ± 0.2 ≤2.0 ≤2.0 ≤2.0
Data are mean values of three replicates and are expressed as log cfu g−1 ± S.E.
Ethylene production decreased when CO2 increased from 0 to 15 kPa, reaching on day 3 a rate of 0.9, 0.63 and 0.47 L C2 H4 kg−1 h−1 under air, 5 kPa O2 + 5 kPa CO2 and 5 kPa O2 + 15 kPa CO2 , respectively. Fennel slices had a similar ethylene production to that of intact bulbs until day 5 of cold storage. Subsequently the ethylene production of the slices continued decreasing less than for the intact bulbs. This behaviour was unexpected because after cutting, ethylene production commonly increases (Varoquaux and Wiley, 1994; Ahvenainen, 1996; Cantwell and Suslow, 2002). However, these results confirm our previous report on diced fennel at 0 ◦ C (Escalona et al., 2005a) and also those with sliced tomato at 2 ◦ C (Art´es et al., 1999b) and pears (Rosen and Kader, 1989). Izumi et al. (1996), found that the ethylene production of fresh-cut carrot was lower than 0.1 L kg−1 h−1 in air and in a CA of 0.5 kPa O2 + 10 kPa CO2 . 3.3. Microbiological quality Results for aerobic mesophilic and psychrophilic bacteria counts were similar to each other and increased during the cold storage period of 14 days from 5.2 log cfu g−1 at day 0 to 8.6–9.3 log cfu g−1 in all gas mixtures (Table 1). The bacterial counts were higher than the mesophilic maximum limit (7 log cfu g−1 ) prescribed by the Spanish standard (RD 3484/2000, 2001) and than the psychrophilic limit of 8 log cfu g−1 (Debevere, 1996). After 14 days at 0 ◦ C, fennel dices in an MAP of 11–13 kPa O2 + 9–12 kPa CO2 had mesophilic counts of 5.3–5.4 log cfu g−1 (Escalona et al., 2005a). Similar to this work, mesophilic and psychrophilic counts in diced onion increased from 4 log cfu g−1 at day 0 to 8 log cfu g−1 after 14 days at 4 ◦ C under 2 kPa O2 + 0 or 5 kPa CO2 and air (Blanchard et al., 1996). However, for diced onion, the psychrophilic count decreased slightly in 2 kPa O2 + 10 kPa CO2 due to the antimicrobial activity of high CO2 levels (Kader et al., 1989). Barriga et al. (1991), found a significant increase of mesophilic and psychrophilic growth on fresh-cut lettuce from 4 to 7 log cfu g−1 after 12 days at 4 ◦ C in 3 kPa O2 + 3 or 5 kPa CO2 and air. These bacteria counts slightly decreased in 3 kPa O2 + 10 kPa CO2 at 4 ◦ C. Nguyen-The and Carlin (1994) reported mesophilic counts in fresh-cut vegetables from 3 to 6 log cfu g−1 after processing and from 3 to 9 log cfu g−1 after cold storage. Additionally, in spinach leaves stored under a CA of 0.8 kPa O2 combined with 0 or 10 kPa CO2 at 5 ◦ C, the
mesophilic and psychrophilic growth decreased in comparison with air atmosphere. In low O2 alone or combined with high CO2 levels at 5 ◦ C, the bacterial counts decreased between 10- and 100-fold compared to air. However, these atmospheres did not have any effect at 10 ◦ C (Babic and Watada, 1996). Nguyen-The and Carlin (1994), also reported that an increased CO2 level delayed the microbial growth on chicory leaves at 2 and 6 ◦ C, but not at 10 ◦ C. The aerobic bacterial counts on fresh-cut celery were 7–7.7 log cfu g−1 after 14 days in MAP at 2 ◦ C (Robbs et al., 1996). After 15 days at 4 ◦ C, celery sticks stored under MAP of 6 kPa O2 + 7 kPa CO2 showed a fresh quality better than air reaching mesophilic and psychrotrophic counts of 2.9 and 1.6 log cfu g−1 , respectively compared to 3.3 and 4.6 log cfu g−1 in air (G´omez and Art´es, 2005). In all gas mixtures, fennel slices registered a yeast and mould growth (2.0–2.2 log cfu g−1 ) substantially lower than the Spanish legal limit (5 log cfu g−1 for yeasts and 3 log cfu g−1 for moulds). Similar mould and yeast counts have been reported in sliced fennel (Escalona et al., 2005a), diced onion (Blanchard et al., 1996) and diced kohlrabi (Escalona et al., 2003). In contrast, Barriga et al. (1991) found that 3 kPa O2 + 10 kPa CO2 did not depress the yeast growth on cut lettuce. 3.4. Colour change In all intact bulb treatments a decrease in lightness (L* ) of the external leaves from 80.4 (at harvest) to 77.7–78.9 after cold storage was found (data not shown). A significant interaction between time and atmosphere factors (P < 0.05) was observed. An unexpected increase in L* values was measured after the retail sale period in comparison with 28 days of storage in all gas mixtures. Significant changes in b* , chroma and hue angle values were found at harvest, after cold storage and after retail sale. These colour differences were affected by the increase in temperature during the retail sale period. Therefore, the colour values on external leaves obtained by using a colorimeter were not a good indicator of fennel bulbs as browning does not affect the leaves. A significant interaction between time and atmosphere factors (P < 0.001) was found for all colour parameters on the butt end cut zone of the bulbs (Table 2). Browning was demonstrated by darkening (decrease in L* ), an increase in chroma values and a hue angle change toward red colours.
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Table 2 Colour changes of the butt end cut zone of ‘Orion’ fennel bulbs at harvest and after 28 days at 5 ◦ C under different atmospheres followed by 3 days at 15 ◦ C in air Time (day)
Atmosphere
0
L*
a*
b*
Chroma
Hue angle
81.5
−3.5
12.3
12.8
106.0
28
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
68.1 71.7 73.6
0.9 −1.3 −1.2
26.3 21.6 22.1
26.4 21.7 22.1
88.7 93.6 93.5
28 + 3
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
64.0 73.7 68.8
2.7 −1.8 0.2
29.9 21.6 21.4
30.1 21.7 21.4
85.3 94.9 89.9
Time Atmosphere Time × atmosphere
(2.79)*** (2.79)*** (4.83)***
(2.21)*** (2.21)*** (3.83)***
(2.21)*** (2.21)*** (3.82)***
(3.42)*** (3.42)*** (4.58)*
b*
Chroma
(1.29)*** (1.29)*** (2.24)***
LSD values are in parentheses, probability: ns, not significant. * Significant at P < 0.01. *** Significant at P < 0.001. Table 3 Colour changes on ‘Orion’ fennel slices at day 0 and after 14 days at 5 ◦ C under different atmospheres Time (day)
L*
a*
75.4
−3.1
7.9
8.5
111.5
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
68.1 69.5 70.7
−3.1 −3.9 −3.4
16.9 12.8 11.5
17.2 13.4 12.0
101.2 106.6 106.9
Time Atmosphere Time × atmosphere
(3.33)*** ns ns
(0.27)** ns ns
(1.52)*** (1.87)*** (2.64)***
(1.51)*** (1.85)*** (2.62)***
Atmosphere
0 14
Hue angle
(1.82)*** (2.23)*** (3.15)***
LSD values are in parentheses, Probability: ns, not significant. ** Significant at P < 0.05. *** Significant at P < 0.001.
The colour changes were larger for control bulbs than for both CA treatments. However, the CA-stored bulbs showed no colour changes compared to colour at harvest. Under 5 kPa O2 + 5 kPa CO2 , the bulbs showed less browning than that at harvest. After the retail sale period, both CA treatments delayed colour changes on the butt end cut zone of the bulbs, with the treatment of 5 kPa O2 + 5 kPa CO2 performing the best. This result agrees with that reported previously by Art´es
et al. (2002a,b). In chicory, where a red colouration is the most important disorder on the butt end cut zone, a CA of 10 kPa O2 + 10 kPa CO2 prevents the development of the red colour during 21 days at 5 ◦ C (Vanstreels et al., 2002). The lightness (L* ) of fennel slices diminished during storage at 5 ◦ C for 14 days irrespective of atmosphere treatment (Table 3). A significant interaction between time and atmosphere factors (P < 0.001) was detected increasing the chroma
Table 4 Changes in fructose, mannose, glucose, sucrose levels and in glucose:fructose ratio of ‘Orion’ fennel bulbs throughout 28 days at 5 ◦ C under different atmospheres followed by 3 days at 15 ◦ C in air Time (days)
Atmosphere
0
Fructose
Mannose
Glucose
Sucrose
Glucose/fructose
1.91
0.16
1.46
0.30
0.76
28
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
1.44 1.79 1.44
0.14 0.15 0.16
1.06 1.03 0.90
0.26 0.36 0.20
0.74 0.58 0.63
28 + 3
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
1.90 1.78 1.99
0.26 0.19 0.20
1.05 1.28 1.12
0.18 0.13 0.10
0.56 0.72 0.56
Time Atmosphere Time × atmosphere
ns ns ns
ns (0.08)** ns
ns ns ns
(0.10)** ns ns
ns ns ns
LSD values are in parentheses. Probability: ns. not significant. The mean values (n = 5) are g per 100 mL juice. ** Significant at P < 0.05.
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under air and 2 kPa O2 alone during 12 days at 4 ◦ C could be due to hydrolysis of sucrose toward glucose and fructose. Under 2 kPa O2 + 10 kPa CO2 , the hydrolysis of sucrose was delayed because the respiration rate of diced onion was reduced.
and decreasing the hue angle values on control slices. A CA of 5 kPa O2 + 15 kPa CO2 was found to be the best to delay browning on fennel slices. Escalona et al. (2005a) also reported that MAP of 11–13 kPa O2 + 9–12 kPa CO2 reduced browning on the cut zones of fennel dices for while at 0 ◦ C for 14 days.
3.6. Organic acid contents 3.5. Sugar contents Oxalic acid was the main organic acid of fennel bulbs with an amount of 30.4 mg per 100 mL−1 juice at harvest (Table 6). This value was higher than that reported by Souci et al. (1986) of 5 mg per 100 g fresh weight although our results were expressed per fresh juice. After cold storage and the retail sale period, the oxalic acid decreased to 11.56–14.81 mg per 100 mL−1 juice, independent of the gas mixtures. These results agree with the reduction of malic acid content in green tomatoes after 4 weeks at 12 ◦ C. The citric acid changes were different depending on cultivar. However, storage in air and 5 kPa O2 + 5 kPa CO2 had no effect on acid content (Goodenough et al., 1982). Fennel slices showed a decrease in oxalic acid contents after cold storage (Table 6). A CA of 5 kPa O2 + 5 to 15 kPa CO2 did not delay the oxalic acid loss following the same pattern as for sugar contents (Table 5). After cold storage, the oxalic acid content of slices was similar to that of intact bulbs, possibly because the respiration rate in slices was two-fold higher than in bulbs although their storage time was half. Therefore, no clear influence of gas mixtures on the oxalic acid consumption of fennel stored at 5 ◦ C was found.
Fructose and glucose were the predominant sugars in fennel bulbs with an initial level of 1.91 and 1.46 g 100 mL−1 , respectively (Table 4). These sugars did not register significant differences after cold storage and the retail sale period. However, sucrose content significantly decreased after retail sale in all gas mixtures. Probably the reduction of sucrose was related to the increased respiration rate caused by the higher temperature during this period. The sucrose consumption could maintain the fructose and glucose levels in the glycolysis pathway. The initial mannose content (0.16 g 100 mL−1 ) was maintained for 28 days at 5 ◦ C and slightly increased after the retail sale due to the weight loss (Table 4). The glucose:fructose ratio was lower than one, indicating a higher synthesis in opposition to glucose. Fennel slices had no change in fructose and mannose contents for 14 days at 5 ◦ C (Table 5). Fructose is the most important sugar and respiratory metabolic substrate of fennel. However, glucose and mainly sucrose could be the sources to generate new fructose for maintaining the energy requirements of the cells. For that reason, glucose and sucrose contents decreased during storage in all gas mixtures. The slices also showed a glucose:fructose ratio lower than one, as in the intact bulbs. Similar results were previously reported in diced fennel stored 14 days at 5 ◦ C (Escalona et al., 2005a). Heimdal et al. (1995), found that the sugar content in shredded iceberg lettuce slightly declined after 10 days at 5 ◦ C under MAP. The glucose and fructose contents decreased about 10–17% and 6–17%, respectively. L´opez-G´alvez et al. (1997), reported similar results in fresh-cut romaine lettuce stored in MAP. The reduction of total sugar content was from 2.1 to 1.7 g/100 g after 15 days at 5 ◦ C. According to Blanchard et al. (1996), the sucrose loss found in diced onion
3.7. Decay and physiological disorders At the end of cold storage, control bulbs developed bacterial soft-rot (Erwinia and Pseudomonas), gray mould (Botrytis cinerea), and watery rot (Sclerotinia spp.) on the end of bulb stalks. According to Snowdon (1991), these kinds of decay are normal in fennel bulbs stored at chilling temperatures. During the retail sale period, the dehydration of the stalks reduced decay growth. The decay of control bulbs was helped by the prolonged storage of 28 days at 5 ◦ C.
Table 5 Changes in fructose, mannose, glucose, sucrose levels and glucose:fructose ratio of ‘Orion’ fennel slices throughout 14 days at 5 ◦ C under different atmospheres Time (days)
Atmosphere
Fructose 1.91
0.16
1.46
0.30
0.76
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
1.46 1.06 1.12
0.13 0.16 0.10
0.82 0.72 0.72
0.06 0.03 0.04
0.56 0.68 0.64
Time Atmosphere Time × atmosphere
ns ns ns
ns ns ns
(0.71)* ns ns
(0.20)*** ns ns
ns ns ns
0 14
Mannose
Glucose
LSD values are in parentheses. Probability: ns, not significant. The mean values (n = 5) are g 100 mL−1 juice. * Significant at P < 0.01. *** Significant at P < 0.001.
Sucrose
Glucose/fructose
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Table 6 Changes in oxalic acid content of whole bulbs and slices of fennel throughout 28 days at 5 ◦ C under different atmospheres followed by 3 days at 15 ◦ C in air and troughout 14 days at 5 ◦ C under different atmospheres Time (days)
Atmosphere
Intact bulbs
Slices
0
30.43
30.43
After cold storage 28
28 d 5 ◦ C
14 d 5 ◦ C 14.81 11.56 14.11
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
24.63 9.97 12.27
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
28 d 5 ◦ C + 3 d 15 ◦ C 13.50 11.97 12.43
After retail sale period
(12.20)*** ns ns
Time Atmosphere Time × atmosphere
(12.61)*** ns ns
LSD values are in parentheses, probability: ns. not significant, the mean values (n = 5) are mg 100 mL−1 juice. *** Significant at P < 0.001.
Browning on the butt end cut zone of the fennel bulbs increased in all treatments. In air, browning was scored as severe after cold storage and retail sale periods (Table 7). However, browning was delayed under CA conditions, this being more effective at 15 than 5 kPa CO2 . A significant interaction was found between time and atmosphere factors (P < 0.001), showing differences between evaluation periods and air compared to CA conditions. After 3 weeks of storage, fennel bulbs stored under 15 kPa CO2 showed little brown spots on the external leaves that affected around 10% of the surface. This disorder did not increase subsequently after the retail sale period. An MAP of 16–17 kPa O2 + 6–7 kPa CO2 within baskets and 6–7 kPa O2 + 10 to 12 kPa CO2 within bags inhibited browning of the butt end cut of fennel for 14 days at 0 ◦ C. However, only the bag treatments showed a residual effect for delaying browning at the end of the retail sale period (Escalona et al., 2004).
After cold storage, fennel slices showed neither physiological disorders nor decay in any treatment. However, browning on the cut was scored as severe (higher than the limit of marketability) in control slices, but it was efficiently delayed when the CO2 level increased from 5 to 15 kPa (Table 8). Hamza et al. (1996), found that after 16 days at 4 ◦ C under 1 kPa O2 + 10 kPa CO2 fresh-cut romaine lettuce showed lower browning compared to air. 3.8. Sensory attributes After cold storage and retail sale periods, the appearance at harvest decreased in all treatments mainly in control bulbs showing decay development (Table 7). At the end of the retail sale, the bulbs stored under 5 kPa O2 + 15 kPa CO2 reached a score slightly lower than 5 due to little brown spots developing on the external leaves. However, under 5 kPa O2 + 5 kPa
Table 7 Changes in appearance, aroma texture, and browning on the butt end cut zone of ‘Orion’ fennel bulbs troughout 28 days at 5 ◦ C under different atmospheres followed by 3 days at 15 ◦ C in air Time (Days) 0 28
28 + 3
Atmosphere
Appearance
Aroma
Texture
Browning
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
8.0 5.0 6.3 5.5
6.8 5.5 5.8 5.8
8.5 5.8 6.9 7.1
1.0 4.3 3.4 2.4
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
2.5 6.0 4.5
5.8 6.0 5.8
5.7 6.8 6.7
4.1 3.0 2.3
Time Atmosphere Time × atmosphere
(0.94)*** (0.94)*** (1.63)***
ns ns ns
(0.74)*** (0.41)** ns
(0.55)*** (0.55)*** (0.95)***
Appearance scored on 1: inedible, 5: fair, and 9: excellent. Aroma and texture scored on 1: complete lacking or soft, 5: moderate, and 9: full characteristic or fresh, respectively. Browning scored on 1: none; 3: moderate, and 5: very severe. LSD values are in parentheses, probability: ns, not significant. ** Significant at P < 0.05. *** Significant at P < 0.001.
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315
Table 8 Changes in appearance, aroma, texture, and browning of ‘Orion’ fennel slices troughout 14 days at 5 ◦ C under different atmospheres Time (days) 0 14
Atmosphere
Appearance
Aroma
Texture
Browning
21 kPa O2 + 0 kPa CO2 5 kPa O2 + 5 kPa CO2 5 kPa O2 + 15 kPa CO2
8.0 2.8 5.4 5.9
6.8 3.3 2.4 2.3
8.5 7.0 7.0 6.9
1.0 4.0 2.1 1.6
Time Atmosphere Time × atmosphere
(0.61)*** (0.75)*** (1.06)***
(1.66)*** ns ns
(1.14)*** ns ns
(0.23)*** (0.28)*** (0.40)***
Appearance scored on 1: inedible, 5: fair, and 9: excellent. Aroma and texture scored on 1: omplete lacking or soft, 5: moderate, and 9: full characteristic or fresh, respectively. Browning scored on 1: none; 3: moderate, and 5: very severe. LSD values are in parentheses, probability: ns, not significant, significant at P < 0.01. *** Significant at P < 0.001.
CO2 a better appearance with a score higher than acceptable for consumption was found. The texture of the bulbs diminished during cold storage although all treatments had a high humidity. The aroma did not show any significant change during the experiment. After the retail sale, the bulbs kept in 5 kPa O2 + 5 kPa CO2 showed the best sensory quality in texture and appearance (Table 7). In agreement with our previous results in bulbs stored at 0 ◦ C under CA after the retail sale period, browning of the butt end cut and external leaves was almost inhibited (Art´es et al., 2002a). The sensory quality of the fennel slices decreased after 14 days at 5 ◦ C, appearance being the attribute most affected by cold storage (Table 8). The appearance scores reached 5.4 and 5.9 in slices from 5 kPa O2 combined with 5 or 15 kPa CO2 , respectively, but control slices had a low score. The aroma decreased from 6.8 to 2–3 in all gas mixtures. Rating of the initial texture also was significantly reduced after cold storage, although it was considered as good (about 7) without significant differences between treatments. The texture changes could be more related to the high relative humidity than to O2 and CO2 levels. It was previously reported that 11–13 kPa O2 + 9–12 kPa CO2 resulted in slight browning on the surface of diced fennel, although chemical, sensory, and microbial quality of the dices were suitable for commercial purposes (Escalona et al., 2005a). Additionally, an MAP with 2–6 kPa O2 + 10–20 kPa CO2 at 0 and 5 ◦ C maintained good quality of sliced fennel (Escalona et al., 2005b). However, our results for browning development contrast with those reported by Albenzio et al. (1998), who considered enzymatic browning as the main cause of postharvest deterioration of minimally processed fennel, probably due to different fennel cultivar sensitivities to browning. Shredded iceberg lettuce had a sensory quality that decreased after 10 days at 5 ◦ C maintaining a better appearance under 2 kPa O2 and 7 kPa CO2 . Low O2 levels delayed enzymatic browning because PPO showed a lower affinity for O2 (Heimdal et al., 1994). The visual quality of diced onion was higher under 2 kPa O2 and 10–15 kPa CO2 than air after 12 days at 4 ◦ C (Blanchard et al., 1996). In other reports,
the visual quality of fresh-cut lettuce significantly decreased when the storage conditions were air or 3 kPa O2 + 0–5 kPa CO2 , but the appearance improved in 10 kPa CO2 (Barriga et al., 1991). According to Mateos et al. (1993), the phenol content in cut lettuce decreased under air enriched with 20 kPa CO2 after 10 and 20 days at 2.5 ◦ C, probably due to a reduction of cytoplasmic pH and PAL activity. However, when the cut lettuces were placed in air at 20 ◦ C, the cytoplasmic pH recovered the normal value and PAL activity. 4. Conclusions 4.1. Fennel bulbs The respiration rate of bulbs decreased under CA with 5 kPa O2 compared to air. The increase of the CO2 level from 5 to 15 kPa raised the respiration of the bulbs. A gas mixture with low O2 and moderate CO2 levels was more recommended for storage of bulbs up to 28 days at 5 ◦ C. Browning on the butt end cut of the bulbs was delayed under 5 kPa O2 combined with 5 or 15 kPa CO2 . Fennel bulbs are sensitive to 15 kPa CO2 level from the 3rd week at 5 ◦ C. 4.2. Fennel slices Under CA conditions, the respiration rate of slices was 50% lower than in air. CA with 5 kPa O2 could be recommended for keeping quality of sliced fennel up to 14 days at 5 ◦ C combined with 5 or 15 kPa CO2 . Under CA, chemical and sensory attributes of slices suffered minimal changes in initial values. Browning of the cutting zones was particularly delayed under 5 kPa O2 + 15 kPa CO2 . Acknowledgements The authors are grateful to financial support of the Spanish Ministry for Education and Science, project AGL200408004/ALI, and to CEBAS-CSIC (Murcia) and FRUVEG Soc. Coop. (Torre Pacheco, Murcia) for providing facilities and fennel, respectively.
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