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Potassium sorbate effects on citrus weight loss and decay control
Postharvest Biology and Technology 96 (2014) 7–13
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Potassium sorbate effects on citrus weight loss and decay control Javier Parra ∗ , Gabriela Ripoll, Benito Orihuel-Iranzo ∗ Department of Postharvest Technology, Productos Citrosol S.A., Partida Alameda parcela C, 46721 Potries, Valencia, Spain
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Article history: Received 7 February 2014 Accepted 26 April 2014 Keywords: Potassium sorbate Penicillium digitatum Citrus fruit Wax Weight loss Decay control
a b s t r a c t Potassium sorbate (PS) is a well-known and widely used food preservative. Among other applications, it is used as a GRAS fungistatic postharvest treatment for citrus, although its use is not free of significant adverse effects. In this paper, we study in detail the efficacy of wax containing increasing concentrations of PS to control Penicillium digitatum decay in citrus fruit, and its effect on fruit weight loss. Decay control and weight loss increased with the concentration of PS in the wax. Wax with typical amounts of 2–5% PS showed poor decay reduction indices (DRI), between 26% and 32%, whereas fruit weight loss increased compared with non-waxed controls. Waxing of fruit reduced weight loss by up to 40%, depending on wax formulation, but the addition of just 2% PS to the wax caused an increase in fruit weight loss of up to 65% compared with the waxed fruit. Similar results were observed for all the types of wax formulations tested. The hygroscopic effects of PS are even more damaging for citrus fruit with leaves. The leaves lose weight very rapidly when PS is added to the wax and they become desiccated in 24 h. We also present the results of a similar study where PS was applied to citrus as an aqueous treatment. When applied in water, PS was far more effective for decay control than when applied in wax, but there was also a considerable increase in fruit weight loss. A treatment combining aqueous PS with Fortisol® Ca Plus biostimulant completely solved the problem of weight loss, these mixtures being commercially feasible treatments. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Citrus fruit are prone to postharvest decay, and although transport and storage conditions of fresh citrus have improved, mainly because of the use of refrigerated transport and cold rooms (Korsten, 2006), fungal diseases still produce significant economic losses (Smilanick et al., 2006). As a result of the big increase in citrus sold in pre-packings, i.e. in nets that sometimes contain up to 35 fruit, where just one decayed fruit can contaminate or cause decay in the whole package, citrus decay control has now become a harder task than in the past. In this scenario, the postharvest use of synthetic fungicides such as imazalil, ortho-phenylphenol, thiabendazole, or pyrimethanil, among others, is still the most effective way to achieve mold control in citrus fruit (Ismail and Zhang, 2004). Green mold, Penicillium digitatum, and blue mold, P. italicum, are the major fungal pathogens that cause decay in citrus fruit in Spanish citrus shipments (Tuset, 1987), and in all citrus when grown in low summer rainfall areas (Palou et al., 2008b). It is well known that the highest efficacy in postharvest citrus decay control is achieved
∗ Corresponding authors. Tel.: +34 96 280 05 12; fax: +34 96 280 08 21. E-mail addresses: [email protected] (J. Parra), [email protected] (B. Orihuel-Iranzo). http://dx.doi.org/10.1016/j.postharvbio.2014.04.011 0925-5214/© 2014 Elsevier B.V. All rights reserved.
when the treatment is applied promptly after harvest (Chitzanidis, 1986; Wild and Spohr, 1989; Brown, 1999), usually as an aqueous treatment applied by drenching or water tank dipping. This first treatment is usually complemented by a second treatment on the packing line, commonly with wax containing fungicides. However, because of reports about the deleterious effects of some synthetic chemicals on the environment and even on the health of consumers, there is a demand for the commercialization of chemical-free fruit. Fungicide-free decay control methods are needed, and treatments based on low-toxicity compounds could be a suitable alternative. These chemicals should have high decay control efficacy with minimal toxicity and environmental impact (Palou et al., 2008b). The main low-toxicity chemical alternatives for citrus decay control are food additives (Palou et al., 2002b), inorganic salts (Palou et al., 2002a; Deliopoulos et al., 2010; Youssef et al., 2012b; Cerioni et al., 2013a,b), essential oils (Plaza et al., 2004; du Plooy et al., 2009; Combrinck et al., 2011; Perez-Alfonso et al., 2012; Castillo et al., 2014) and phytochemicals (Hao et al., 2010). Among the food additives, potassium sorbate (E-202), PS, is a widely used broad spectrum food preservative (Sofos, 1989; Stopforth et al., 2005). In 1978, it was first proposed to be used in citrus decay control of P. digitatum (Smoot and McCornack, 1978). Since then, aqueous PS has been described many times as an alternative
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postharvest treatment for citrus decay, often combined with synthetic fungicides and/or with heat, with the aim of increasing the efficacy of the treatment (Kitagawa and Tani, 1984; Palou et al., 2008a; Smilanick et al., 2008; Montesinos-Herrero et al., 2009; D’Aquino et al., 2013). Currently, aqueous PS is widely used in Spain and in other citrus exporting countries for the drenching of citrus fruit, and is incorporated into commercial waxes as an alternative method for decay control. PS is considered a GRAS substance (Generally Recognized As Safe) by the FDA (FDA, 1975) and it was approved by the EFSA as a food additive for surface treatment of citrus fruit (EFSA, 2010). Wax coatings create a protective barrier that essentially has four expected properties: appearance improvement (providing shine and gloss), weight loss reduction, aging retardation, and, very frequently, additional decay control by the addition of chemically compatible fungicides to the wax (Kaplan, 1986; Eckert and Eaks, 1989; Hall and Sorenson, 2006). Recently, it has been reported that PS and sodium and potassium carbonate salts decrease the weight loss reduction capacity of waxes (Youssef et al., 2012a). When PS is added to the wax, equivalent weight loss is observed for waxed and non-waxed fruit. If weight loss reduction capacity disappears, then one of the most important properties of the wax is lost. For many citrus fruit shipments, weight loss can have more implications than just the economic loss in terms of weight and size (Hagenmaier and Shaw, 1992). Several authors link an excess of weight loss and peel water status to the advent of various rind disorders (Eckert and Eaks, 1989; Lafuente and Zacarias, 2006). Fruit firmness is also affected (Hagenmaier and Shaw, 1992), and, in general, 5–7% weight loss is considered by several authors a threshold for the fruit to become shriveled, soft, and unmarketable (Hagenmaier, 1998; Grierson and Miller, 2006). The purpose of the present paper is to study in detail the advantages and disadvantages of using PS as an alternative chemical for citrus decay control, exploring its limitations, both in aqueous treatments and when incorporated into the wax, and paying special attention to weight loss and decay control. Studies on the effects of PS on citrus leaves are also reported. The results obtained allow us to envisage an appropriate use of PS, as well as provide a solution to overcome the adverse effects of this molecule when used as an aqueous treatment.
2. Materials and methods 2.1. Fruit samples The citrus fruit (‘Nova’ mandarins, ‘Valencia’ and ‘Navel’ oranges) used in all the experiments were obtained directly from packing houses located in the Comunidad Valenciana (Spain) and they had not received any postharvest treatment. Citrus leaves (from ‘Nova’ mandarins) were collected from the tree in a nearby grove on the same day of the experiment.
were from Productos Citrosol S.A. (Potries, Valencia, Spain). PS was purchased from Ter Hell & Co. GmbH (Hamburg, Germany). PS samples were prepared by dissolving the appropriate amount of PS directly, with mechanical stirring, either in water or in the various waxes, at room temperature. Similarly, imazalil formulations were prepared by dissolving the appropriate amount of Citrosol 500 or Citrosol 7.5 LS in waxes or water, respectively. 2.3. Fruit treatments and determination of decay reduction index (DRI) and weight loss control (WLC) Fruit were randomized, cleaned with 6% (v/v) Essasol, rinsed with tap water, and dried at room temperature before use in the waxing experiments. In the case of water dipping experiments, fruit were randomized before application of treatments. Citrus fruit decay experiments were performed with 4 replications of 25 fruit per treatment. Fruit were artificially inoculated by wounding each fruit with a steel rod (1 mm diameter wide and 2 mm long) previously immersed in a conidial suspension of P. digitatum (7 × 106 cfu/mL). After 17 h, fruit were either waxed (1 L wax/1000 kg of fruit) in a commercial waxing unit or dipped in the aqueous treatment 15 L tank for 30 s. After the treatments, waxed fruit were dried with hot air whereas water-dipped fruit were allowed to dry at room temperature, simulating industrial operations. Dried fruit were placed in trays, stored for 1 week at 20 ◦ C and 85% relative humidity (RH), and then decayed fruit were counted and expressed as a percentage. Decay reduction indices (DRIs) were calculated as follows: 100 × (no. of decayed fruit in the control − no. of decayed fruit in the treatment)/no. of decayed fruit in the control. Citrus fruit weight loss experiments were carried out with 4 replicates of 5 fruit per treatment. Fruit were selected with absence of defects or injuries, numbered, and treated as previously described with the different waxes or aqueous solutions. In order to study both decay control and weight loss with the same fruit and in the same conditions, each set of non-inoculated fruit was randomly mixed with a set of inoculated fruit and the whole set of fruit was treated at the same time. Treated non-inoculated fruit were placed into trays and stored for 1 week at 20 ◦ C and 60% RH. Weight of individual fruit was recorded just after the treatment, and then after 2, 5, and 7 days. Weight loss for each day was calculated for each individual fruit as % weight loss referenced to the initial weight of the fruit, immediately after treatment [100 × (initial weight − weight)/initial weight]. Weight loss rate was expressed as % weight loss/day and was obtained from the slope of the % weight loss vs. time (days) plot. Weight loss control (WLC) was calculated as: 100 × (weight loss rate in the control − weight loss rate in the treatment)/weight loss rate in the control. Additionally, two industrial PS wax samples from other manufacturers were given to us by packing house managers, analyzed for PS content by HPLC-DAD, and used as described above to determine DRI and WLC. 2.4. Leaf resistance and weight loss measurements
2.2. Preparation of aqueous solutions and wax formulations Citrosol A UE and Citrosol AS UE (both are emulsions of polyethylene oxide, E914, and shellac, E904), Citrosol AK UE (emulsion of carnauba, E903, and shellac, E904), and Citrosol A (emulsion of polyethylene oxide, E914, and rosin) water waxes (Citrosol A wax complies with US and Canadian legislation and waxes with the UE acronym also comply with European Union legislation), Essasol biodegradable detergent (4% sodium dodecylbenzenesulfonate), Citrosol 500 (50% emulsionable imazalil), Citrosol 7.5 LS (7.5% imazalil sulfate), and Fortisol® Ca Plus biostimulant (proprietary formulation of phosphorous, calcium, and potassium salts)
Citrus leaves were randomized, cleaned with 6% (v/v) Essasol, rinsed with tap water, and dried at room temperature before use in the waxing experiments. Experiments were carried out with sets of 15 leaves per treatment. Leaves were waxed manually, using the same dose as in the waxing line, dried with hot air, simulating an industrial operation, and stored for 4 days at 20 ◦ C and 60% RH. The weight of 10 individual leaves was recorded just after waxing, and then after 1, 2, 3, and 4 days. Weight loss for each day was calculated for each single leaf as % weight loss referenced to the initial weight of the leaf, immediately after waxing [100 × (initial weight − weight)/initial weight]. In addition, photographs of the
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waxed leaves were taken daily, with the aim of observing changes in their appearance. The other 5 leaves of each set were used for the daily measurement of resistance to breakage. Briefly, leaves were placed between two plastic discs in which a central hole with a diameter of 1.5 cm had been made, and then pressed with a manual penetrometer (0–13 kg, 11.3-mm-diameter cylindrical probe, TR, Tecnylab C.B., Valencia, Spain) until leaf breakage. Resistance to breakage was expressed in kg/cm2 . Leaves were penetrated avoiding the midrib, in order not to distort the results. 2.5. Statistical analysis Mean values were compared using analyses of variance (ANOVA) and separated by Fisher’s protected least significant difference test (LSD, P ≤ 0.05) on the basis of statistically significant differences. StatGraphics 5.0 Plus software was used. 3. Results and discussion 3.1. Decay control and weight loss in citrus fruit when PS is incorporated into wax PS is frequently added to waxes as an alternative chemical to control citrus decay. Decay control efficacy of wax containing increasing concentrations of PS was studied on ‘Nova’ mandarins artificially inoculated with P. digitatum (Fig. 1). The effects on weight loss rate were also studied with the same batch of mandarins (Fig. 1). Fruit were waxed in a conventional waxing unit with typical doses and conditions, in order to simulate the real waxing step in the packing house. Weight loss rate increased with the concentration of PS added to the wax, whereas decay decreased slightly (Fig. 1). Thus, there was an observable decay reduction, but this control remained poor, compared to the reference treatment with wax + 2000 mg/L imazalil. For instance, the decay reduction index of fruit waxed with a typical 2% PS in wax was only 26% whereas the fruit showed an even higher weight loss rate than non-waxed fruit. Waxing of fruit with just plain wax showed a 25% reduction in the weight loss rate, but with the addition of PS at concentrations of 2% or higher the weight loss reduction properties of the wax disappeared. These weight loss results are in agreement with previous studies (Youssef et al., 2012a). In that case, ‘Comune’ clementines and ‘Tarocco’ oranges waxed with just wax showed weight loss reduction capacities of 20–30%, whereas fruit waxed with a 6% PS wax showed statistically the same weight loss as
Fig. 1. Weight loss rate (% weight loss/day, bottom, black bars) and % decay (top, gray bars) in ‘Nova’ mandarins waxed with Citrosol A UE wax containing increasing concentrations of PS or 2000 mg/L of imazalil and stored for 7 days at 20 ◦ C and 85% RH (60% RH in the case of weight loss experiments). Non-waxed fruit were used as controls.
non-waxed ones. In the present study, not only has this effect been confirmed, but also a relationship has been found between PS concentration and weight loss. As PS increased, firstly wax coating lost its weight loss reduction properties, and then, if the concentration was high enough, it could turn into a desiccating film, making the waxing step damaging rather than useful or recommendable.
Table 1 ‘Nova’ mandarin decay reduction indices (DRI) and weight loss control (WLC) in wax and water PS treatments with different [PS].a,b Waxing
Water dipping c
DRI (%) [PS] (%) 0 0.5 2 5 10 1.6d 1.8d [Imazalil] (mg/L) 2000 450 a b c d
WLC (%)
DRI (%)
0 18 26 32 40 14 22
d cd bc bc b cd bc
25 22 −11 −6 −44 −15 0.2
a a c bc d c b
70
a
17
a
WLC (%)
61 85 99 100
c b a a
−17 −27 −65 −94
b b c d
97
a
2
a
Results for imazalil treatments are shown as reference. ANOVA tests were carried out within each column. Negative values indicate weight loss increase (desiccation); positive values indicate weight loss reduction. PS wax commercial samples from different manufactures.
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Fig. 2. ‘Nova’ mandarins 24 h after waxing with Citrosol A UE wax without PS (a) and with 10% PS (b) and stored at 20 ◦ C and 60% RH.
Thus, although it is possible to increase the decay control efficacy by increasing the PS concentration in the wax, higher PS concentrations increase weight loss rates significantly. Moreover, an increase in PS concentration up to reasonable amounts did not provide acceptable decay control. For instance, 10% PS resulted in a modest 40% decay reduction with an unacceptable weight loss rate and phytotoxicity (brown stains, Fig. 2). In addition, the wax became sticky and unstable. Taking all these findings into consideration, it is therefore important to avoid the use of PS as an alternative chemical for citrus decay control in waxing treatments, as it cancels the wax film weight loss reduction properties in exchange for a low decay control efficacy. It is important to mention that the conventional treatment, wax + 2000 mg/L imazalil, which is a commercial standard, did not alter the weight loss reduction properties of the wax and showed a DRI of 70%. A summary of these results, in terms of DRI and WLC, is shown in Table 1, together with the results obtained with two PS wax samples from other manufacturers. The DRI and WLC results obtained with these samples are in agreement with our studies, giving an overview of the real commercial performance of PS waxes in the packing houses.
Fig. 3. Weight loss rate (% weight loss/day) in ‘Valencia’ oranges waxed with different wax formulations containing no PS (white bars) or 2% PS (gray bars) and stored for 7 days at 20 ◦ C and 60% RH. Non-waxed fruit were used as controls (black bars). Citrosol A: polyethylene oxide + rosin resin. Citrosol A UE and Citrosol AS UE both: polyethylene oxide + shellac. Citrosol AK UE: carnauba + shellac. ANOVA tests were carried out separately with the data originated for each type of wax.
Fig. 4. Weight loss (%) and resistance to breakage (kg/cm2 ) in ‘Nova’ mandarin leaves waxed with Citrosol A UE wax containing increasing concentrations of PS or 2000 mg/L of imazalil and stored for 24 h at 20 ◦ C and 60% RH. Non-waxed leaves were used as controls.
3.2. Weight loss in citrus fruit when PS is incorporated into different wax formulations In order to explore the scope and nature of the problem further, we studied the weight loss rate of ‘Valencia’ oranges using different types of wax, with and without 2% PS. We assessed some of the most common formulations in the citrus waxing industry (polyethylene oxide + rosin or wood resin; polyethylene oxide + shellac, in two different formulations with different emulsifiers, and carnauba + shellac). Fig. 3 shows how PS caused the same negative effect on the weight loss reduction capacity in all the formulations tested. As expected, all the waxes showed a reduction in the weight loss rate, but when 2% PS was added the weight loss rate increased in all cases, removing the weight loss reduction capacity in all cases except for the case of the polyethylene oxide + rosin formulation (Citrosol A). In this case, although the effect was also clearly observed, it was possible to maintain some of the weight loss reduction capacity of the wax, but this kind of formulation has the disadvantage of not complying with EU legislation. Citrus fruit waxed with 2% PS Citrosol A UE and Citrosol AK UE showed statistically the same weight loss rate as non-waxed ones, and the fruit that were waxed with 2% PS Citrosol AS UE showed an even higher weight loss rate than non-waxed ones (the wax became a desiccating film). These results indicate that the negative effects of PS on the weight loss reduction capacity of waxes are quite general and do not depend on the type of wax formulation. All in all, it is important to consider that waxes could change their weight loss reduction
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Fig. 5. ‘Nova’ mandarin leaves just after waxing (a–c) and 24 h after waxing stored at 20 ◦ C and 60% RH (d–f); a and d: non-waxed leaves (control); b and e: leaves waxed with Citrosol A UE wax; c and f: leaves waxed with Citrosol A UE wax containing 2% PS.
properties when they are supplemented with alternative chemicals, both in a positive and in a negative way. For instance, the addition of essential oils into waxes has been described as beneficial for the weight loss control of the conventional wax coatings (du Plooy et al., 2009), whereas on the contrary, the use of carboxymethyl cellulose and chitosan has been described as negative for the weight loss control capacity when they are used as edible coatings (Arnon et al., 2014). 3.3. PS effects on weight loss and resistance to breakage of waxed citrus leaves Mandarin varieties are frequently sold in Europe with leaves as proof of their freshness, and when this is the case the whole product is waxed in the packing line in order to obtain an overall clean and good appearance. Consequently, leaves could also be negatively affected by waxing with PS waxes. We studied weight loss and resistance to breakage of ‘Nova’ mandarin leaves 24 h after waxing with increasing concentrations of PS (Fig. 4). ‘Nova’ leaves lost weight very fast because of their high specific surface area; around 25% of their weight was lost in 24 h. Unfortunately, waxing ‘Nova’ mandarin leaves with Citrosol A UE did not reduce weight loss rate as it did with fruit. In this case, both waxed and non-waxed leaves showed statistically the same weight loss rate. However, as the PS concentration increased in the wax, the weight loss of the waxed leaves also increased, leading to undesired effects. Leaves waxed with a 2% or higher concentration of PS became dramatically desiccated in 24 h, losing almost 40% of their weight, and looked old and brittle. As a result, resistance to breakage decreased (Fig. 4), waxed leaves lost their fresh appearance and became easy to break. It is important to mention that imazalil does not produce any negative effect on citrus leaves when it is added to the wax (reference treatment), as weight loss and resistance remained statistically the same as in the control. Fig. 5 shows a fresh and healthy appearance for ‘Nova’ mandarin leaves a few minutes after waxing; the appearance was the same for non-waxed, 0% PS waxed, and 2% PS waxed leaves (Fig. 5a, b, and c, respectively). However, 24 h after waxing, all the leaves showed some degree of shriveling (Fig. 5d–f), but this shriveling was higher when PS was added to the wax (Fig. 5f). It is clear that PS waxes
Fig. 6. Weight loss rate (% weight loss/day, bottom, black bars) and % decay (top, gray bars) in ‘Nova’ mandarins dipped in aqueous solutions with increasing concentrations of PS or 450 mg/L of imazalil and stored for 7 days at 20 ◦ C and 85% RH (60% RH in the case of weight loss experiments). Fruit dipped in tap water were used as controls.
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Fig. 7. Weight loss rate (% weight loss/day) in ‘Navel’ oranges dipped in aqueous solutions of increasing concentrations of PS (left, black bars) or in aqueous solution mixtures of 0.75% PS plus increasing concentrations of Fortisol Ca Plus biostimulant (right, gray bars) and stored for 7 days at 20 ◦ C and 60%. Fruit dipped in tap water were used as controls.
are not suitable for maintaining the fresh appearance of the leaves. This issue is very important for the fresh packer because, as proof of fruit freshness, it is quite common to sell citrus fruit with leaves at a premium price. 3.4. Decay control activity and weight loss in citrus fruit when PS is applied in water dip treatments Water dip treatments, such as drenching or tank immersion, are the most important treatments to control citrus decay and are normally applied immediately after the arrival of the fruit at the packing house. Aqueous PS dip treatments are often applied as a residue-free alternative to synthetic fungicides. We studied both the decay control efficacy of aqueous solutions containing increasing concentrations of PS on ‘Nova’ mandarins artificially inoculated with P. digitatum and the effects on weight loss rate of the treated fruit (Fig. 6). For the sake of comparison, the same batch of ‘Nova’ mandarins was used for waxing treatments (Section 3.1) and water dip treatments. A comparison, in terms of DRI and WLC, between the two types of treatments is shown in Table 1. When applied in water (Fig. 6), PS was more effective for decay control than when applied in wax (Fig. 1). Considering the same amount of PS, DRIs for water dip treatments were considerably higher than for waxing treatments (Table 1). This difference in performance is well known and established for any kind of active ingredient because of the immobilization of the fungicides in the wax film matrix, among other reasons (Brown, 1984). Fig. 6 shows that decay visibly decreased as the PS concentration increased, up to 85% decay reduction with 2% PS with regard to the control, a value that is close to the 97% decay reduction obtained with the reference treatment with 450 mg/L imazalil (Table 1). The same 2% PS concentration applied with wax only produced a 26% DRI (Table 1). However, weight loss rate clearly increased as PS increased, the same as in the wax experiments, a fact that should be taken into consideration. PS water dip treatments provide sound decay control, but at the expense of significantly increasing the weight loss rate of fruit, leading to economic losses for the industry and to undesirable effects such as shrinking, desiccated appearance, aging acceleration, and rind disorders (Eckert and Eaks, 1989; Grierson and Miller, 2006), which in some cases may affect fruit condition at arrival and sales. At least 5% aqueous PS is needed to obtain statistically the same decay control efficacy as the reference treatment with 450 mg/L of imazalil. But with this amount, the fruit increased its weight loss rate by 65% with regard to the control (Table 1); in
fact, in just 4 days at 20 ◦ C these ‘Novas’ could reach the 5% threshold for the fruit to become unsalable (Grierson and Miller, 2006). It is important to emphasize, again, that the reference treatment with imazalil does not produce any negative effect on the fruit, as weight loss rate remains statistically equal to the control, whereas it provides high protection against decay. The adverse weight loss effect that PS causes could be partially reduced, hypothetically, by heating the aqueous PS solution. Palou et al. (2002b) and Smilanick et al. (2008) showed higher decay control efficacy for heated PS solutions, so PS concentration could be reduced as solution temperature is increased, maintaining similar levels of decay control. However, in Spain and in many other countries, most frequently the first treatment after harvest can only be applied by drenching. In a heated drencher the gradient of the solution temperature at which the fruit is treated, between the top fruit and the bottom fruit in the pallet, is approximately 10 ◦ C (Orihuel-Iranzo, personal communication), probably too high a temperature difference for consistency in the efficacy of the heated PS treatment. 3.5. Counteraction of the PS-provoked weight loss in citrus fruit with Fortisol Ca Plus Fig. 7 (left) shows the weight loss rate tendency with ‘Navel’ oranges treated with increasing concentrations of aqueous PS. Although ‘Navel’ oranges lost weight slower than ‘Nova’ mandarins, the relative increase in the rate of weight loss caused by PS was much higher for ‘Navel’ oranges at similar PS concentrations. ‘Navel’ orange weight loss rate is increased by 80% with only 1% PS. This is an important negative figure that cannot be ignored, because it could lead to the appearance of weight-loss-related aging and rind disorders (Eckert and Eaks, 1989; Lafuente and Zacarias, 2006), specially if these fruit are shipped overseas. In order to prevent the desiccation produced by aqueous PS, we tried using Fortisol Ca Plus biostimulant. Fortisol Ca Plus, a proprietary formulation of Productos Citrosol S.A., has shown, among other benefits, protection against postharvest rind disorders and fungicide-related phytotoxicities (Orihuel-Iranzo and Breto, 2013a,b). We studied the weight loss rate of ‘Navel’ oranges dipped in different mixtures of the same PS concentration (0.75%) and increasing concentrations of Fortisol Ca Plus (Fig. 7, right). Fortisol Ca Plus counteracted the weight loss problem caused by aqueous PS. Weight loss rate of fruit treated with 0.75% PS decreased as the concentration of Fortisol in the mixture increased, approaching the same weight loss rate as in the control. For instance, a mixture with 0.5% PS + 2% Fortisol Ca Plus showed
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a weight loss rate that was statistically the same as in the control, that is, there was no additional weight loss due to the treatment (Fig. 7, right). 4. Conclusions In conclusion, the results obtained demonstrate that PS increases citrus weight loss rate and does not provide adequate decay control in waxing treatments. Consequently, we do not consider PS an appropriate active substance to use in wax. In many cases, waxes with PS completely lose their ability to reduce weight loss, which is one of the properties expected of a citrus wax coating (Hall and Sorenson, 2006). The negative effects of PS have been confirmed for all the most common types of citrus wax formulations. Moreover, waxed citrus leaves lose weight very rapidly when PS is added to the wax. They become desiccated in 24 h, looking old and brittle. This issue is very important for the fresh packer because some citrus fruit are sold with leaves as proof of their freshness at considerably higher prices and profit margins than regular fresh citrus. On the other hand, when applied in water (drencher and dip tank treatments), PS is much more effective for decay control than when applied in wax, but there is also a considerable increase in fruit weight loss rate, which cannot be ignored. The addition of Citrosol Fortisol Ca Plus biostimulant to the treatment mix avoids the problem of weight loss caused by PS in aqueous treatments. References Arnon, H., Zaitsev, Y., Porat, R., Poverenov, E., 2014. Effects of carboxymethyl cellulose and chitosan bilayer edible coating on postharvest quality of citrus fruit. Postharvest Biol. Technol. 87, 21–26. Brown, G.E., 1984. Efficacy of citrus postharvest fungicides applied in water or resin solution water wax. Plant Dis. 68, 415–418. Brown, G.E., 1999. Practices to minimize decay in Florida fresh citrus. Citrus Packinghouse Newsletters No. 186. Castillo, S., Perez-Alfonso, C.O., Martinez-Romero, D., Guillen, F., Serrano, M., Valero, D., 2014. The essential oils thymol and carvacrol applied in the packing lines avoid lemon spoilage and maintain quality during storage. Food Control 35, 132–136. Cerioni, L., Rapisarda, V.A., Doctor, J., Fikkert, S., Ruiz, T., Fassel, R., Smilanick, J.L., 2013a. Use of phosphite salts in laboratory and semicommercial tests to control citrus postharvest decay. Plant Dis. 97, 201–212. Cerioni, L., Sepulveda, M., Rubio-Ames, Z., Volentini, S.I., Rodriguez-Montelongo, L., Smilanick, J.L., Ramallo, J., Rapisarda, V.A., 2013b. Control of lemon postharvest diseases by low-toxicity salts combined with hydrogen peroxide and heat. Postharvest Biol. Technol. 83, 17–21. Combrinck, S., Regnier, T., Kamatou, G.P.P., 2011. In vitro activity of eighteen essential oils and some major components against common postharvest fungal pathogens of fruit. Ind. Crop. Prod. 33, 344–349. Chitzanidis, A., 1986. Post-harvest rots of citrus fruits in Greece. In: Cavalloro, R., Di Martino, E. (Eds.), Integrated Pest Control in Citrus Groves. A.A. Balkema, Rotterdam, pp. 287–294. D’Aquino, S., Fadda, A., Barberis, A., Palma, A., Angioni, A., Schirra, M., 2013. Combined effects of potassium sorbate, hot water and thiabendazole against green mould of citrus fruit and residue levels. Food Chem. 141, 858–864. Deliopoulos, T., Kettlewell, P.S., Hare, M.C., 2010. Fungal disease suppression by inorganic salts: a review. Crop Prot. 29, 1059–1075. du Plooy, W., Regnier, T., Combrinck, S., 2009. Essential oil amended coatings as alternatives to synthetic fungicides in citrus postharvest management. Postharvest Biol. Technol. 53, 117–122. Eckert, J.W., Eaks, I.L., 1989. Postharvest disorders and diseases of citrus fruits. In: Reuther, W., Calavan, E.C., Carman, G.E. (Eds.), The Citrus Industry. Crop Protection, Postharvest Technology, and Early History of Citrus Research in California, vol. V. University of California, Division of Agriculture and Natural Resources, Oakland, CA, pp. 179–260. EFSA, 2010. Authorised Food Additives for Entire Fresh Fruit and Vegetables, https:// webgate.ec.europa.eu/sanco foods/main/?event=category.view&identifier=50 (accessed 22.12.13). FDA, 1975. Alphabetical List of Approved GRAS Substances Evaluated by SCOGS, http://www.fda.gov/Food/IngredientsPackagingLabeling/GRAS/SCOGS/ ucm084104.htm (accessed 22.12.13).
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Grierson, W., Miller, W.M., 2006. Storage of citrus fruits. In: Wardowski, W.F., Miller, W.M., Hall, D.J., Grierson, W. (Eds.), Fresh Citrus Fruits. , second ed. Florida Science Source, Inc., Longboat Key, FL, pp. 547–582. Hagenmaier, R.D., Shaw, P.E., 1992. Gas permeability of fruit coating waxes. J. Am. Soc. Hortic. Sci. 117, 105–109. Hagenmaier, R.D., 1998. Selection of citrus wax coatings on criteria other than shortterm gloss. Citrus Packinghouse Newsletters No. 182. Hall, D.J., Sorenson, D., 2006. Washing, waxing and color-adding. In: Wardowski, W.F., Miller, W.M., Hall, D.J., Grierson, W. (Eds.), Fresh Citrus Fruits. , second ed. Florida Science Source, Inc., Longboat Key, FL, pp. 421–450. Hao, W., Zhong, G., Hu, M., Luo, J., Weng, Q., Rizwan-ul-Haq, M., 2010. Control of citrus postharvest green and blue mold and sour rot by tea saponin combined with imazalil and prochloraz. Postharvest Biol. Technol. 56, 39–43. Ismail, M., Zhang, J., 2004. Post-harvest citrus diseases and their control. Outlooks Pest Manage. 15, 29–35. Kaplan, H.J., 1986. Washing, waxing, and color-adding. In: Wardowski, W.F., Nagy, S., Grierson, W. (Eds.), Fresh Citrus Fruits. AVI Publishing Co., Westport, CT, pp. 379–396. Kitagawa, H., Tani, T., 1984. Effect of potassium sorbate and thiabendazole mixture on the control of green and blue molds of citrus fruit. J. Jpn. Soc. Hortic. Sci. 52, 464–468. Korsten, L., 2006. Advances in control of postharvest diseases in tropical fresh produce. Int. J. Postharvest Technol. Innov. 1, 48–61. Lafuente, M.T., Zacarias, L., 2006. Postharvest physiological disorders in citrus fruit. Stewart Posthar. Rev. 2, 1–9. Montesinos-Herrero, C., Angel del Rio, M., Pastor, C., Brunetti, O., Paloua, L., 2009. Evaluation of brief potassium sorbate dips to control postharvest penicillium decay on major citrus species and cultivars. Postharvest Biol. Technol. 52, 117–125. Orihuel-Iranzo, B., Breto, J., 2013a. Fortisol Ca y Fortisol Ca Plus en la reduccion del manchado “Fondo de Cajon”. Valencia Fruits, 26th February, 2013., pp. 10–11. Orihuel-Iranzo, B., Breto, J., 2013b. El Fortisol Ca y el Fortisol Ca Plus en la reduccion de los manchados postcosecha de los citricos, Valencia Fruits, 19th November, 2013., pp. 54–55. Palou, L., Usall, J., Munoz, J.A., Smilanick, J.L., Vinas, I., 2002a. Hot water, sodium carbonate, and sodium bicarbonate for the control of postharvest green and blue molds of clementine mandarins. Postharvest Biol. Technol. 24, 93–96. Palou, L., Usall, J., Smilanick, J.L., Aguilar, M.J., Vinas, I., 2002b. Evaluation of food additives and low-toxicity compounds as alternative chemicals for the control of Penicillium digitatum and Penicillium italicum on citrus fruit. Pest Manag. Sci. 58, 459–466. Palou, L., Montesinos-Herrero, C., Pastor, C., del Rio, M., 2008a. Evaluation of heated potassium sorbate solutions to control postharvest green and blue molds on commercially important citrus cultivars. Phytopathology 98, S119. Palou, L., Smilanick, J.L., Droby, S., 2008b. Alternatives to conventional fungicides for the control of citrus postharvest green and blue moulds. Stewart Posthar. Rev. 4, 1–16. Perez-Alfonso, C.O., Martinez-Romero, D., Zapata, P.J., Serrano, M., Valero, D., Castillo, S., 2012. The effects of essential oils carvacrol and thymol on growth of Penicillium digitatum and Penicillium italicum involved in lemon decay. Int. J. Food Microbiol. 158, 101–106. Plaza, P., Torres, R., Usall, J., Lamarca, N., Vinas, I., 2004. Evaluation of the potential of commercial post-harvest application of essential oils to control citrus decay. J. Hortic. Sci. Biotechnol. 79, 935–940. Smilanick, J.L., Brown, G.E., Eckert, J.W., 2006. Postharvest citrus diseases and their control. In: Wardowski, W.F., Miller, W.M., Hall, D.J., Grierson, W. (Eds.), Fresh Citrus Fruits. , second ed. Florida Science Source, Inc., Longboat Key, FL, pp. 339–396. Smilanick, J.L., Mansour, M.F., Gabler, F.M., Sorenson, D., 2008. Control of citrus postharvest green mold and sour rot by potassium sorbate combined with heat and fungicides. Postharvest Biol. Technol. 47, 226–238. Smoot, J.J., McCornack, A.A., 1978. The use of potassium sorbate for citrus decay control. Proc. Fla. State Hortic. Soc. 91, 119–122. Sofos, J.N., 1989. Sorbate Food Preservatives. CRC Press, Taylor & Francis, Boca Raton, FL. Stopforth, J.D., Sofos, J.N., Busta, F.F., 2005. Sorbic acid and sorbates. In: Davidson, P.M., Sofos, J.N., Branen, A.L. (Eds.), Antimicrobials in Food. , third ed. CRC Press, Taylor & Francis, Boca Raton, FL, pp. 49–90. Tuset, J.J., 1987. Podredumbres de los frutos citricos. Generalitat Valenciana, Conselleria d’Agricultura i Pesca, Valencia. Wild, B.L., Spohr, L.J., 1989. Influence of fruit temperature and application time on the effectiveness of fungicides in controlling citrus green mold, Penicillium digitatum. Aust. J. Exp. Agric. 29, 139–142. Youssef, K., Ligorio, A., Nigro, F., Ippolito, A., 2012a. Activity of salts incorporated in wax in controlling postharvest diseases of citrus fruit. Postharvest Biol. Technol. 65, 39–43. Youssef, K., Ligorio, A., Sanzani, S.M., Nigro, F., Ippolito, A., 2012b. Control of storage diseases of citrus by pre- and postharvest application of salts. Postharvest Biol. Technol. 72, 57–63.
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Postharvest Biology and Technology journal homepage: www.elsevier.com/locate/postharvbio
Potassium sorbate effects on citrus weight loss and decay control Javier Parra ∗ , Gabriela Ripoll, Benito Orihuel-Iranzo ∗ Department of Postharvest Technology, Productos Citrosol S.A., Partida Alameda parcela C, 46721 Potries, Valencia, Spain
a r t i c l e
i n f o
Article history: Received 7 February 2014 Accepted 26 April 2014 Keywords: Potassium sorbate Penicillium digitatum Citrus fruit Wax Weight loss Decay control
a b s t r a c t Potassium sorbate (PS) is a well-known and widely used food preservative. Among other applications, it is used as a GRAS fungistatic postharvest treatment for citrus, although its use is not free of significant adverse effects. In this paper, we study in detail the efficacy of wax containing increasing concentrations of PS to control Penicillium digitatum decay in citrus fruit, and its effect on fruit weight loss. Decay control and weight loss increased with the concentration of PS in the wax. Wax with typical amounts of 2–5% PS showed poor decay reduction indices (DRI), between 26% and 32%, whereas fruit weight loss increased compared with non-waxed controls. Waxing of fruit reduced weight loss by up to 40%, depending on wax formulation, but the addition of just 2% PS to the wax caused an increase in fruit weight loss of up to 65% compared with the waxed fruit. Similar results were observed for all the types of wax formulations tested. The hygroscopic effects of PS are even more damaging for citrus fruit with leaves. The leaves lose weight very rapidly when PS is added to the wax and they become desiccated in 24 h. We also present the results of a similar study where PS was applied to citrus as an aqueous treatment. When applied in water, PS was far more effective for decay control than when applied in wax, but there was also a considerable increase in fruit weight loss. A treatment combining aqueous PS with Fortisol® Ca Plus biostimulant completely solved the problem of weight loss, these mixtures being commercially feasible treatments. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Citrus fruit are prone to postharvest decay, and although transport and storage conditions of fresh citrus have improved, mainly because of the use of refrigerated transport and cold rooms (Korsten, 2006), fungal diseases still produce significant economic losses (Smilanick et al., 2006). As a result of the big increase in citrus sold in pre-packings, i.e. in nets that sometimes contain up to 35 fruit, where just one decayed fruit can contaminate or cause decay in the whole package, citrus decay control has now become a harder task than in the past. In this scenario, the postharvest use of synthetic fungicides such as imazalil, ortho-phenylphenol, thiabendazole, or pyrimethanil, among others, is still the most effective way to achieve mold control in citrus fruit (Ismail and Zhang, 2004). Green mold, Penicillium digitatum, and blue mold, P. italicum, are the major fungal pathogens that cause decay in citrus fruit in Spanish citrus shipments (Tuset, 1987), and in all citrus when grown in low summer rainfall areas (Palou et al., 2008b). It is well known that the highest efficacy in postharvest citrus decay control is achieved
∗ Corresponding authors. Tel.: +34 96 280 05 12; fax: +34 96 280 08 21. E-mail addresses: [email protected] (J. Parra), [email protected] (B. Orihuel-Iranzo). http://dx.doi.org/10.1016/j.postharvbio.2014.04.011 0925-5214/© 2014 Elsevier B.V. All rights reserved.
when the treatment is applied promptly after harvest (Chitzanidis, 1986; Wild and Spohr, 1989; Brown, 1999), usually as an aqueous treatment applied by drenching or water tank dipping. This first treatment is usually complemented by a second treatment on the packing line, commonly with wax containing fungicides. However, because of reports about the deleterious effects of some synthetic chemicals on the environment and even on the health of consumers, there is a demand for the commercialization of chemical-free fruit. Fungicide-free decay control methods are needed, and treatments based on low-toxicity compounds could be a suitable alternative. These chemicals should have high decay control efficacy with minimal toxicity and environmental impact (Palou et al., 2008b). The main low-toxicity chemical alternatives for citrus decay control are food additives (Palou et al., 2002b), inorganic salts (Palou et al., 2002a; Deliopoulos et al., 2010; Youssef et al., 2012b; Cerioni et al., 2013a,b), essential oils (Plaza et al., 2004; du Plooy et al., 2009; Combrinck et al., 2011; Perez-Alfonso et al., 2012; Castillo et al., 2014) and phytochemicals (Hao et al., 2010). Among the food additives, potassium sorbate (E-202), PS, is a widely used broad spectrum food preservative (Sofos, 1989; Stopforth et al., 2005). In 1978, it was first proposed to be used in citrus decay control of P. digitatum (Smoot and McCornack, 1978). Since then, aqueous PS has been described many times as an alternative
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postharvest treatment for citrus decay, often combined with synthetic fungicides and/or with heat, with the aim of increasing the efficacy of the treatment (Kitagawa and Tani, 1984; Palou et al., 2008a; Smilanick et al., 2008; Montesinos-Herrero et al., 2009; D’Aquino et al., 2013). Currently, aqueous PS is widely used in Spain and in other citrus exporting countries for the drenching of citrus fruit, and is incorporated into commercial waxes as an alternative method for decay control. PS is considered a GRAS substance (Generally Recognized As Safe) by the FDA (FDA, 1975) and it was approved by the EFSA as a food additive for surface treatment of citrus fruit (EFSA, 2010). Wax coatings create a protective barrier that essentially has four expected properties: appearance improvement (providing shine and gloss), weight loss reduction, aging retardation, and, very frequently, additional decay control by the addition of chemically compatible fungicides to the wax (Kaplan, 1986; Eckert and Eaks, 1989; Hall and Sorenson, 2006). Recently, it has been reported that PS and sodium and potassium carbonate salts decrease the weight loss reduction capacity of waxes (Youssef et al., 2012a). When PS is added to the wax, equivalent weight loss is observed for waxed and non-waxed fruit. If weight loss reduction capacity disappears, then one of the most important properties of the wax is lost. For many citrus fruit shipments, weight loss can have more implications than just the economic loss in terms of weight and size (Hagenmaier and Shaw, 1992). Several authors link an excess of weight loss and peel water status to the advent of various rind disorders (Eckert and Eaks, 1989; Lafuente and Zacarias, 2006). Fruit firmness is also affected (Hagenmaier and Shaw, 1992), and, in general, 5–7% weight loss is considered by several authors a threshold for the fruit to become shriveled, soft, and unmarketable (Hagenmaier, 1998; Grierson and Miller, 2006). The purpose of the present paper is to study in detail the advantages and disadvantages of using PS as an alternative chemical for citrus decay control, exploring its limitations, both in aqueous treatments and when incorporated into the wax, and paying special attention to weight loss and decay control. Studies on the effects of PS on citrus leaves are also reported. The results obtained allow us to envisage an appropriate use of PS, as well as provide a solution to overcome the adverse effects of this molecule when used as an aqueous treatment.
2. Materials and methods 2.1. Fruit samples The citrus fruit (‘Nova’ mandarins, ‘Valencia’ and ‘Navel’ oranges) used in all the experiments were obtained directly from packing houses located in the Comunidad Valenciana (Spain) and they had not received any postharvest treatment. Citrus leaves (from ‘Nova’ mandarins) were collected from the tree in a nearby grove on the same day of the experiment.
were from Productos Citrosol S.A. (Potries, Valencia, Spain). PS was purchased from Ter Hell & Co. GmbH (Hamburg, Germany). PS samples were prepared by dissolving the appropriate amount of PS directly, with mechanical stirring, either in water or in the various waxes, at room temperature. Similarly, imazalil formulations were prepared by dissolving the appropriate amount of Citrosol 500 or Citrosol 7.5 LS in waxes or water, respectively. 2.3. Fruit treatments and determination of decay reduction index (DRI) and weight loss control (WLC) Fruit were randomized, cleaned with 6% (v/v) Essasol, rinsed with tap water, and dried at room temperature before use in the waxing experiments. In the case of water dipping experiments, fruit were randomized before application of treatments. Citrus fruit decay experiments were performed with 4 replications of 25 fruit per treatment. Fruit were artificially inoculated by wounding each fruit with a steel rod (1 mm diameter wide and 2 mm long) previously immersed in a conidial suspension of P. digitatum (7 × 106 cfu/mL). After 17 h, fruit were either waxed (1 L wax/1000 kg of fruit) in a commercial waxing unit or dipped in the aqueous treatment 15 L tank for 30 s. After the treatments, waxed fruit were dried with hot air whereas water-dipped fruit were allowed to dry at room temperature, simulating industrial operations. Dried fruit were placed in trays, stored for 1 week at 20 ◦ C and 85% relative humidity (RH), and then decayed fruit were counted and expressed as a percentage. Decay reduction indices (DRIs) were calculated as follows: 100 × (no. of decayed fruit in the control − no. of decayed fruit in the treatment)/no. of decayed fruit in the control. Citrus fruit weight loss experiments were carried out with 4 replicates of 5 fruit per treatment. Fruit were selected with absence of defects or injuries, numbered, and treated as previously described with the different waxes or aqueous solutions. In order to study both decay control and weight loss with the same fruit and in the same conditions, each set of non-inoculated fruit was randomly mixed with a set of inoculated fruit and the whole set of fruit was treated at the same time. Treated non-inoculated fruit were placed into trays and stored for 1 week at 20 ◦ C and 60% RH. Weight of individual fruit was recorded just after the treatment, and then after 2, 5, and 7 days. Weight loss for each day was calculated for each individual fruit as % weight loss referenced to the initial weight of the fruit, immediately after treatment [100 × (initial weight − weight)/initial weight]. Weight loss rate was expressed as % weight loss/day and was obtained from the slope of the % weight loss vs. time (days) plot. Weight loss control (WLC) was calculated as: 100 × (weight loss rate in the control − weight loss rate in the treatment)/weight loss rate in the control. Additionally, two industrial PS wax samples from other manufacturers were given to us by packing house managers, analyzed for PS content by HPLC-DAD, and used as described above to determine DRI and WLC. 2.4. Leaf resistance and weight loss measurements
2.2. Preparation of aqueous solutions and wax formulations Citrosol A UE and Citrosol AS UE (both are emulsions of polyethylene oxide, E914, and shellac, E904), Citrosol AK UE (emulsion of carnauba, E903, and shellac, E904), and Citrosol A (emulsion of polyethylene oxide, E914, and rosin) water waxes (Citrosol A wax complies with US and Canadian legislation and waxes with the UE acronym also comply with European Union legislation), Essasol biodegradable detergent (4% sodium dodecylbenzenesulfonate), Citrosol 500 (50% emulsionable imazalil), Citrosol 7.5 LS (7.5% imazalil sulfate), and Fortisol® Ca Plus biostimulant (proprietary formulation of phosphorous, calcium, and potassium salts)
Citrus leaves were randomized, cleaned with 6% (v/v) Essasol, rinsed with tap water, and dried at room temperature before use in the waxing experiments. Experiments were carried out with sets of 15 leaves per treatment. Leaves were waxed manually, using the same dose as in the waxing line, dried with hot air, simulating an industrial operation, and stored for 4 days at 20 ◦ C and 60% RH. The weight of 10 individual leaves was recorded just after waxing, and then after 1, 2, 3, and 4 days. Weight loss for each day was calculated for each single leaf as % weight loss referenced to the initial weight of the leaf, immediately after waxing [100 × (initial weight − weight)/initial weight]. In addition, photographs of the
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waxed leaves were taken daily, with the aim of observing changes in their appearance. The other 5 leaves of each set were used for the daily measurement of resistance to breakage. Briefly, leaves were placed between two plastic discs in which a central hole with a diameter of 1.5 cm had been made, and then pressed with a manual penetrometer (0–13 kg, 11.3-mm-diameter cylindrical probe, TR, Tecnylab C.B., Valencia, Spain) until leaf breakage. Resistance to breakage was expressed in kg/cm2 . Leaves were penetrated avoiding the midrib, in order not to distort the results. 2.5. Statistical analysis Mean values were compared using analyses of variance (ANOVA) and separated by Fisher’s protected least significant difference test (LSD, P ≤ 0.05) on the basis of statistically significant differences. StatGraphics 5.0 Plus software was used. 3. Results and discussion 3.1. Decay control and weight loss in citrus fruit when PS is incorporated into wax PS is frequently added to waxes as an alternative chemical to control citrus decay. Decay control efficacy of wax containing increasing concentrations of PS was studied on ‘Nova’ mandarins artificially inoculated with P. digitatum (Fig. 1). The effects on weight loss rate were also studied with the same batch of mandarins (Fig. 1). Fruit were waxed in a conventional waxing unit with typical doses and conditions, in order to simulate the real waxing step in the packing house. Weight loss rate increased with the concentration of PS added to the wax, whereas decay decreased slightly (Fig. 1). Thus, there was an observable decay reduction, but this control remained poor, compared to the reference treatment with wax + 2000 mg/L imazalil. For instance, the decay reduction index of fruit waxed with a typical 2% PS in wax was only 26% whereas the fruit showed an even higher weight loss rate than non-waxed fruit. Waxing of fruit with just plain wax showed a 25% reduction in the weight loss rate, but with the addition of PS at concentrations of 2% or higher the weight loss reduction properties of the wax disappeared. These weight loss results are in agreement with previous studies (Youssef et al., 2012a). In that case, ‘Comune’ clementines and ‘Tarocco’ oranges waxed with just wax showed weight loss reduction capacities of 20–30%, whereas fruit waxed with a 6% PS wax showed statistically the same weight loss as
Fig. 1. Weight loss rate (% weight loss/day, bottom, black bars) and % decay (top, gray bars) in ‘Nova’ mandarins waxed with Citrosol A UE wax containing increasing concentrations of PS or 2000 mg/L of imazalil and stored for 7 days at 20 ◦ C and 85% RH (60% RH in the case of weight loss experiments). Non-waxed fruit were used as controls.
non-waxed ones. In the present study, not only has this effect been confirmed, but also a relationship has been found between PS concentration and weight loss. As PS increased, firstly wax coating lost its weight loss reduction properties, and then, if the concentration was high enough, it could turn into a desiccating film, making the waxing step damaging rather than useful or recommendable.
Table 1 ‘Nova’ mandarin decay reduction indices (DRI) and weight loss control (WLC) in wax and water PS treatments with different [PS].a,b Waxing
Water dipping c
DRI (%) [PS] (%) 0 0.5 2 5 10 1.6d 1.8d [Imazalil] (mg/L) 2000 450 a b c d
WLC (%)
DRI (%)
0 18 26 32 40 14 22
d cd bc bc b cd bc
25 22 −11 −6 −44 −15 0.2
a a c bc d c b
70
a
17
a
WLC (%)
61 85 99 100
c b a a
−17 −27 −65 −94
b b c d
97
a
2
a
Results for imazalil treatments are shown as reference. ANOVA tests were carried out within each column. Negative values indicate weight loss increase (desiccation); positive values indicate weight loss reduction. PS wax commercial samples from different manufactures.
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Fig. 2. ‘Nova’ mandarins 24 h after waxing with Citrosol A UE wax without PS (a) and with 10% PS (b) and stored at 20 ◦ C and 60% RH.
Thus, although it is possible to increase the decay control efficacy by increasing the PS concentration in the wax, higher PS concentrations increase weight loss rates significantly. Moreover, an increase in PS concentration up to reasonable amounts did not provide acceptable decay control. For instance, 10% PS resulted in a modest 40% decay reduction with an unacceptable weight loss rate and phytotoxicity (brown stains, Fig. 2). In addition, the wax became sticky and unstable. Taking all these findings into consideration, it is therefore important to avoid the use of PS as an alternative chemical for citrus decay control in waxing treatments, as it cancels the wax film weight loss reduction properties in exchange for a low decay control efficacy. It is important to mention that the conventional treatment, wax + 2000 mg/L imazalil, which is a commercial standard, did not alter the weight loss reduction properties of the wax and showed a DRI of 70%. A summary of these results, in terms of DRI and WLC, is shown in Table 1, together with the results obtained with two PS wax samples from other manufacturers. The DRI and WLC results obtained with these samples are in agreement with our studies, giving an overview of the real commercial performance of PS waxes in the packing houses.
Fig. 3. Weight loss rate (% weight loss/day) in ‘Valencia’ oranges waxed with different wax formulations containing no PS (white bars) or 2% PS (gray bars) and stored for 7 days at 20 ◦ C and 60% RH. Non-waxed fruit were used as controls (black bars). Citrosol A: polyethylene oxide + rosin resin. Citrosol A UE and Citrosol AS UE both: polyethylene oxide + shellac. Citrosol AK UE: carnauba + shellac. ANOVA tests were carried out separately with the data originated for each type of wax.
Fig. 4. Weight loss (%) and resistance to breakage (kg/cm2 ) in ‘Nova’ mandarin leaves waxed with Citrosol A UE wax containing increasing concentrations of PS or 2000 mg/L of imazalil and stored for 24 h at 20 ◦ C and 60% RH. Non-waxed leaves were used as controls.
3.2. Weight loss in citrus fruit when PS is incorporated into different wax formulations In order to explore the scope and nature of the problem further, we studied the weight loss rate of ‘Valencia’ oranges using different types of wax, with and without 2% PS. We assessed some of the most common formulations in the citrus waxing industry (polyethylene oxide + rosin or wood resin; polyethylene oxide + shellac, in two different formulations with different emulsifiers, and carnauba + shellac). Fig. 3 shows how PS caused the same negative effect on the weight loss reduction capacity in all the formulations tested. As expected, all the waxes showed a reduction in the weight loss rate, but when 2% PS was added the weight loss rate increased in all cases, removing the weight loss reduction capacity in all cases except for the case of the polyethylene oxide + rosin formulation (Citrosol A). In this case, although the effect was also clearly observed, it was possible to maintain some of the weight loss reduction capacity of the wax, but this kind of formulation has the disadvantage of not complying with EU legislation. Citrus fruit waxed with 2% PS Citrosol A UE and Citrosol AK UE showed statistically the same weight loss rate as non-waxed ones, and the fruit that were waxed with 2% PS Citrosol AS UE showed an even higher weight loss rate than non-waxed ones (the wax became a desiccating film). These results indicate that the negative effects of PS on the weight loss reduction capacity of waxes are quite general and do not depend on the type of wax formulation. All in all, it is important to consider that waxes could change their weight loss reduction
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Fig. 5. ‘Nova’ mandarin leaves just after waxing (a–c) and 24 h after waxing stored at 20 ◦ C and 60% RH (d–f); a and d: non-waxed leaves (control); b and e: leaves waxed with Citrosol A UE wax; c and f: leaves waxed with Citrosol A UE wax containing 2% PS.
properties when they are supplemented with alternative chemicals, both in a positive and in a negative way. For instance, the addition of essential oils into waxes has been described as beneficial for the weight loss control of the conventional wax coatings (du Plooy et al., 2009), whereas on the contrary, the use of carboxymethyl cellulose and chitosan has been described as negative for the weight loss control capacity when they are used as edible coatings (Arnon et al., 2014). 3.3. PS effects on weight loss and resistance to breakage of waxed citrus leaves Mandarin varieties are frequently sold in Europe with leaves as proof of their freshness, and when this is the case the whole product is waxed in the packing line in order to obtain an overall clean and good appearance. Consequently, leaves could also be negatively affected by waxing with PS waxes. We studied weight loss and resistance to breakage of ‘Nova’ mandarin leaves 24 h after waxing with increasing concentrations of PS (Fig. 4). ‘Nova’ leaves lost weight very fast because of their high specific surface area; around 25% of their weight was lost in 24 h. Unfortunately, waxing ‘Nova’ mandarin leaves with Citrosol A UE did not reduce weight loss rate as it did with fruit. In this case, both waxed and non-waxed leaves showed statistically the same weight loss rate. However, as the PS concentration increased in the wax, the weight loss of the waxed leaves also increased, leading to undesired effects. Leaves waxed with a 2% or higher concentration of PS became dramatically desiccated in 24 h, losing almost 40% of their weight, and looked old and brittle. As a result, resistance to breakage decreased (Fig. 4), waxed leaves lost their fresh appearance and became easy to break. It is important to mention that imazalil does not produce any negative effect on citrus leaves when it is added to the wax (reference treatment), as weight loss and resistance remained statistically the same as in the control. Fig. 5 shows a fresh and healthy appearance for ‘Nova’ mandarin leaves a few minutes after waxing; the appearance was the same for non-waxed, 0% PS waxed, and 2% PS waxed leaves (Fig. 5a, b, and c, respectively). However, 24 h after waxing, all the leaves showed some degree of shriveling (Fig. 5d–f), but this shriveling was higher when PS was added to the wax (Fig. 5f). It is clear that PS waxes
Fig. 6. Weight loss rate (% weight loss/day, bottom, black bars) and % decay (top, gray bars) in ‘Nova’ mandarins dipped in aqueous solutions with increasing concentrations of PS or 450 mg/L of imazalil and stored for 7 days at 20 ◦ C and 85% RH (60% RH in the case of weight loss experiments). Fruit dipped in tap water were used as controls.
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Fig. 7. Weight loss rate (% weight loss/day) in ‘Navel’ oranges dipped in aqueous solutions of increasing concentrations of PS (left, black bars) or in aqueous solution mixtures of 0.75% PS plus increasing concentrations of Fortisol Ca Plus biostimulant (right, gray bars) and stored for 7 days at 20 ◦ C and 60%. Fruit dipped in tap water were used as controls.
are not suitable for maintaining the fresh appearance of the leaves. This issue is very important for the fresh packer because, as proof of fruit freshness, it is quite common to sell citrus fruit with leaves at a premium price. 3.4. Decay control activity and weight loss in citrus fruit when PS is applied in water dip treatments Water dip treatments, such as drenching or tank immersion, are the most important treatments to control citrus decay and are normally applied immediately after the arrival of the fruit at the packing house. Aqueous PS dip treatments are often applied as a residue-free alternative to synthetic fungicides. We studied both the decay control efficacy of aqueous solutions containing increasing concentrations of PS on ‘Nova’ mandarins artificially inoculated with P. digitatum and the effects on weight loss rate of the treated fruit (Fig. 6). For the sake of comparison, the same batch of ‘Nova’ mandarins was used for waxing treatments (Section 3.1) and water dip treatments. A comparison, in terms of DRI and WLC, between the two types of treatments is shown in Table 1. When applied in water (Fig. 6), PS was more effective for decay control than when applied in wax (Fig. 1). Considering the same amount of PS, DRIs for water dip treatments were considerably higher than for waxing treatments (Table 1). This difference in performance is well known and established for any kind of active ingredient because of the immobilization of the fungicides in the wax film matrix, among other reasons (Brown, 1984). Fig. 6 shows that decay visibly decreased as the PS concentration increased, up to 85% decay reduction with 2% PS with regard to the control, a value that is close to the 97% decay reduction obtained with the reference treatment with 450 mg/L imazalil (Table 1). The same 2% PS concentration applied with wax only produced a 26% DRI (Table 1). However, weight loss rate clearly increased as PS increased, the same as in the wax experiments, a fact that should be taken into consideration. PS water dip treatments provide sound decay control, but at the expense of significantly increasing the weight loss rate of fruit, leading to economic losses for the industry and to undesirable effects such as shrinking, desiccated appearance, aging acceleration, and rind disorders (Eckert and Eaks, 1989; Grierson and Miller, 2006), which in some cases may affect fruit condition at arrival and sales. At least 5% aqueous PS is needed to obtain statistically the same decay control efficacy as the reference treatment with 450 mg/L of imazalil. But with this amount, the fruit increased its weight loss rate by 65% with regard to the control (Table 1); in
fact, in just 4 days at 20 ◦ C these ‘Novas’ could reach the 5% threshold for the fruit to become unsalable (Grierson and Miller, 2006). It is important to emphasize, again, that the reference treatment with imazalil does not produce any negative effect on the fruit, as weight loss rate remains statistically equal to the control, whereas it provides high protection against decay. The adverse weight loss effect that PS causes could be partially reduced, hypothetically, by heating the aqueous PS solution. Palou et al. (2002b) and Smilanick et al. (2008) showed higher decay control efficacy for heated PS solutions, so PS concentration could be reduced as solution temperature is increased, maintaining similar levels of decay control. However, in Spain and in many other countries, most frequently the first treatment after harvest can only be applied by drenching. In a heated drencher the gradient of the solution temperature at which the fruit is treated, between the top fruit and the bottom fruit in the pallet, is approximately 10 ◦ C (Orihuel-Iranzo, personal communication), probably too high a temperature difference for consistency in the efficacy of the heated PS treatment. 3.5. Counteraction of the PS-provoked weight loss in citrus fruit with Fortisol Ca Plus Fig. 7 (left) shows the weight loss rate tendency with ‘Navel’ oranges treated with increasing concentrations of aqueous PS. Although ‘Navel’ oranges lost weight slower than ‘Nova’ mandarins, the relative increase in the rate of weight loss caused by PS was much higher for ‘Navel’ oranges at similar PS concentrations. ‘Navel’ orange weight loss rate is increased by 80% with only 1% PS. This is an important negative figure that cannot be ignored, because it could lead to the appearance of weight-loss-related aging and rind disorders (Eckert and Eaks, 1989; Lafuente and Zacarias, 2006), specially if these fruit are shipped overseas. In order to prevent the desiccation produced by aqueous PS, we tried using Fortisol Ca Plus biostimulant. Fortisol Ca Plus, a proprietary formulation of Productos Citrosol S.A., has shown, among other benefits, protection against postharvest rind disorders and fungicide-related phytotoxicities (Orihuel-Iranzo and Breto, 2013a,b). We studied the weight loss rate of ‘Navel’ oranges dipped in different mixtures of the same PS concentration (0.75%) and increasing concentrations of Fortisol Ca Plus (Fig. 7, right). Fortisol Ca Plus counteracted the weight loss problem caused by aqueous PS. Weight loss rate of fruit treated with 0.75% PS decreased as the concentration of Fortisol in the mixture increased, approaching the same weight loss rate as in the control. For instance, a mixture with 0.5% PS + 2% Fortisol Ca Plus showed
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a weight loss rate that was statistically the same as in the control, that is, there was no additional weight loss due to the treatment (Fig. 7, right). 4. Conclusions In conclusion, the results obtained demonstrate that PS increases citrus weight loss rate and does not provide adequate decay control in waxing treatments. Consequently, we do not consider PS an appropriate active substance to use in wax. In many cases, waxes with PS completely lose their ability to reduce weight loss, which is one of the properties expected of a citrus wax coating (Hall and Sorenson, 2006). The negative effects of PS have been confirmed for all the most common types of citrus wax formulations. Moreover, waxed citrus leaves lose weight very rapidly when PS is added to the wax. They become desiccated in 24 h, looking old and brittle. This issue is very important for the fresh packer because some citrus fruit are sold with leaves as proof of their freshness at considerably higher prices and profit margins than regular fresh citrus. On the other hand, when applied in water (drencher and dip tank treatments), PS is much more effective for decay control than when applied in wax, but there is also a considerable increase in fruit weight loss rate, which cannot be ignored. The addition of Citrosol Fortisol Ca Plus biostimulant to the treatment mix avoids the problem of weight loss caused by PS in aqueous treatments. References Arnon, H., Zaitsev, Y., Porat, R., Poverenov, E., 2014. Effects of carboxymethyl cellulose and chitosan bilayer edible coating on postharvest quality of citrus fruit. Postharvest Biol. Technol. 87, 21–26. Brown, G.E., 1984. Efficacy of citrus postharvest fungicides applied in water or resin solution water wax. Plant Dis. 68, 415–418. Brown, G.E., 1999. Practices to minimize decay in Florida fresh citrus. Citrus Packinghouse Newsletters No. 186. Castillo, S., Perez-Alfonso, C.O., Martinez-Romero, D., Guillen, F., Serrano, M., Valero, D., 2014. The essential oils thymol and carvacrol applied in the packing lines avoid lemon spoilage and maintain quality during storage. Food Control 35, 132–136. Cerioni, L., Rapisarda, V.A., Doctor, J., Fikkert, S., Ruiz, T., Fassel, R., Smilanick, J.L., 2013a. Use of phosphite salts in laboratory and semicommercial tests to control citrus postharvest decay. Plant Dis. 97, 201–212. Cerioni, L., Sepulveda, M., Rubio-Ames, Z., Volentini, S.I., Rodriguez-Montelongo, L., Smilanick, J.L., Ramallo, J., Rapisarda, V.A., 2013b. Control of lemon postharvest diseases by low-toxicity salts combined with hydrogen peroxide and heat. Postharvest Biol. Technol. 83, 17–21. Combrinck, S., Regnier, T., Kamatou, G.P.P., 2011. In vitro activity of eighteen essential oils and some major components against common postharvest fungal pathogens of fruit. Ind. Crop. Prod. 33, 344–349. Chitzanidis, A., 1986. Post-harvest rots of citrus fruits in Greece. In: Cavalloro, R., Di Martino, E. (Eds.), Integrated Pest Control in Citrus Groves. A.A. Balkema, Rotterdam, pp. 287–294. D’Aquino, S., Fadda, A., Barberis, A., Palma, A., Angioni, A., Schirra, M., 2013. Combined effects of potassium sorbate, hot water and thiabendazole against green mould of citrus fruit and residue levels. Food Chem. 141, 858–864. Deliopoulos, T., Kettlewell, P.S., Hare, M.C., 2010. Fungal disease suppression by inorganic salts: a review. Crop Prot. 29, 1059–1075. du Plooy, W., Regnier, T., Combrinck, S., 2009. Essential oil amended coatings as alternatives to synthetic fungicides in citrus postharvest management. Postharvest Biol. Technol. 53, 117–122. Eckert, J.W., Eaks, I.L., 1989. Postharvest disorders and diseases of citrus fruits. In: Reuther, W., Calavan, E.C., Carman, G.E. (Eds.), The Citrus Industry. Crop Protection, Postharvest Technology, and Early History of Citrus Research in California, vol. V. University of California, Division of Agriculture and Natural Resources, Oakland, CA, pp. 179–260. EFSA, 2010. Authorised Food Additives for Entire Fresh Fruit and Vegetables, https:// webgate.ec.europa.eu/sanco foods/main/?event=category.view&identifier=50 (accessed 22.12.13). FDA, 1975. Alphabetical List of Approved GRAS Substances Evaluated by SCOGS, http://www.fda.gov/Food/IngredientsPackagingLabeling/GRAS/SCOGS/ ucm084104.htm (accessed 22.12.13).
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