Effect of Wax Coating on the Quality of Cucumber Fruits during Storage

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Introduction
Cucumber (Cucumis sativus L.) is an important vegetable belonging to the family of Cucurbitaceae. Cucumbers contain approximately 95% water, 3.6% carbohydrates, and 0.65% protein, and are low in calories (150 kcal kg −1 ). They are a good source of the following nutrients (in mg kg −1 ): pantothenic acid (B5) (0.026); vitamin C (0.28); magnesium (1.3). Cucumbers are grown both as a source of pickles, and to be eaten fresh. While consumption of pickles has been waning, the use of cucumbers as a fresh vegetable has been increasing [1]. Cucumber (Cucumis sativus L.) is a favorite commodity exports for markets and local consumption and represents one of the most important and economic vegetables in Egypt. It is grown in Egypt in the open field from March to November and under plastic houses from September to May. The total cultivated area of cucumber in Egypt was about 27989 ha in 2011 according to the statistics of CAPMS [2]. Waxing was used as a preservation technique for fruits since 1900. Wax has been used since pre-historic times. In 4200 BC, the ancient Egyptians kept bees and used the wax from honeycombs to make models and to preserve their dead and in encaustic painting. The Chinese were using waxing processes on fruits, but it was not until 1922. Waxes are used to replace some of the natural waxes that removed in washing and cleaning operations, and this helps to reduce the water loss during handling and marketing. If produce is waxed, the wax coating is allowed to dry thoroughly before further handling. Waxing of the produce is an old age art, which was started in the beginning of the 19 th century [3].
Water loss can be controlled by altering the fruit surface or by proper control of storage conditions. Examples of the first method are the application of artificial wax coatings and heat treatments. Artificial, edible coatings are generally applied by spraying or dipping after removal of the natural wax. The effectiveness of the coating depends on the thickness, concentration, and type of coating [4,5].
To avoid deterioration, cucumbers are often stored and transported at cold temperatures. However, cucumbers are among a number of tropical and subtropical crops that are sensitive to above-freezing cool temperatures. Low-temperature storage of cucumbers may induce damage associated with Chilling Injury (CI), resulting in compositional as well as in morphological (structural) and microbiological changes [6][7][8][9][10][11][12][13][14].
The susceptibility of cucumbers to chilling injury and to yellowing restricts desirable storage temperatures to a narrow range. If held at 10°C or colder, they suffer from chilling injury; if held at 15°C or higher, they rapidly become yellow. Thus, the optimum temperature is 12 to 13. However, for storage of 1 to 2 weeks, 10°C is preferable because chilling is minimal and yellowing is retarded, although [15] claims that yellowing is slower at 13°C than at higher or lower temperatures. The same author found that differences in chilling sensitivity among cultivars are minor.
The susceptibility of cucumbers to chilling injury does not preclude their exposure to temperatures below 5°C as long as they are utilized immediately after removal from cold storage, because symptoms develop rapidly only at higher temperatures. Thus, 2 days at 0°C, 4 days at 5°C, or 8 days at 7°C are harmless under these conditions [16]. They added that cucumbers can be expected to remain in good condition for 10 to 14 days at 10°C. For fruits harvested relatively young, where yellowing is not an immediate problem, 13°C would be preferable and may extend the storage life 2 to 3 days and proper Controlled Atmosphere (CA) conditions may add a further 4 to 6 days, for a total of about 3 weeks.
Cucumber fruits are losing too much moisture, exposed to damage and bruising, losing the good appearance during handling and storage process, so that using wax as coating is the most common practice to prevent moisture loss, protect the fruits from damage and bruising, increase the shelf life and improve the fruit appearance, which in turn increase the marketability of the cucumber. To prolong storage quality and postharvest shelf life, the cucumber was waxed with different solution concentrations and stored at different temperatures. The appearance indices weight loss and cucumber shelf life were determined as the most important factors affecting the final quality of cucumber during storage.

Materials and Methods
The experiment was carried out at Agricultural Engineering Department, Faculty of Agriculture, Moshtohor, Benha University, during the period of June to July, 2012.

Materials
The Cucumber (Cucumis sativus L.) was brought from the Faculty of Agriculture Farm, Moshtohor, Benha University after harvesting.
Paraffin wax is mostly found as a white, odorless, tasteless, waxy solid, with a typical melting point between about 46 and 68°C (115 and 154°F), and having a density of around 0.9 g/cm 3 . It is insoluble in water, but soluble in ether, benzene, and certain esters. Paraffin is unaffected by most common chemical reagents but burns readily.
Refrigeration room used in this study has a dimension of 3.0 m length, 3.0 m in width and 3.0 m height. It made prefabricated insulated panels of thickness of 60 mm. the panels insulation are covered with precoated stainless steel sheet on both sides.

Methods
The cucumber fruits were dipped into a 0, 25, 50, 75, 100% wax solutions. The fruits were then fan-dried at room temperature (approximately 25°C). The waxed fruits were stored at four temperature 5, 10, 15°C and room temperature (25°C). Twenty treatments were applied (five wax solution concentrations and four storage temperatures) and replicated 3 times.

Measurements
The dimensions of fruits (length and diameter) were measured by digital vernier caliper (Model TESA 1p65-Range 0-150 mm ± 0.01 mm, Swiss). The mass was measured by electric digital balance (Model HG -5000-Range 0-5000 g ± 0.01 g, Japan). The surface area was measured by wrapping aluminum foil around the cucumber fruit and then cutting the foil away with scissors into thin strips sufficient to lay the foil flat. A planimeter (Model Placom KP -90 N-Range 0-10 m 2 ± 0.2 %, Japan) was used to measure the surface area of the cucumber. The volume of fruits was measured by graduated cylinder. The hardness was measured by Hardness meter (Model GY-1-Range 2-15 kg cm -2 ± 0.1 kg cm -2 , China). The TSS was measured by Refractometer (Model HR-010-Range 0-10% Brix ± 0.1% Brix, Japan). These measurements were repeated three times and the average was taken before and after storage.

Results and Discussion
Cucumber quality was determined by measuring the change of fruit diameter, length, volume, volume, surface area, mass, hardness and total soluble solids (TSS). Weight loss and shelf life of the fruit during storage were also studied.

Effect of wax coating and storage temperature on the fruit size change
Diameter change: Figures 1 and 2 show the effect of different wax solution concentrations and storage temperatures on change in cucumber fruit diameter. The results indicated that the change of diameter decreases with increasing wax solution concentrations, meanwhile, the diameter change increases with increasing storage temperature, where, it decreased from 0.053 to 0.017 mm h -1 when the wax solution increased from 0 (control) to 100%. On the other hand, loss of diameter increased from 0.021 to 0.049 mm h -1 with the storage temperature increased from 5°C to 25°C (room temperature). These results agreed with those obtained by [16]. The overall average of diameter change decreased from 0.053 ± 0.014 to 0.017 ± 0.007 mm h -1 when the wax concentration changed from 0 to 100%. The overall Regression analysis was carried out to find a relation between the fruit diameter change and both wax solution concentrations and storage temperatures. Equation (1) shows the most appropriate form for the relationship between the fruit diameter change and wax solution concentrations at different storage temperatures. Equation (2) shows the relationship between the fruit diameter change and storage temperatures at different wax solution concentrations. The constants of these equations and coefficient of determination are listed in Table 1.
Where:-DC is the cucumber diameter change (mm h -1 ) WX is the wax coating (%) T is the storage temperature (°C) Length change: Figures 3 and 4 show the effect of different wax solution concentrations and storage temperatures on change in cucumber fruit length. The results indicate that the length decreases with increasing wax solution and it increases with increasing storage temperature, where, it decreased from 0.199 to 0.0054 mm h -1 when the wax solution concentrations increased from 0 (control) to 100%. On the other hand, the length change increased from 0.074 to 0.168 mm h -1 with the storage temperature changed from 5 to 25°C (room temperature). The overall average of length change decreased from 0.199 ± 0.068 to 0.0547 ± 0.018 mm h -1 when the wax concentration changed from 0 to 100%. The overall average of length change ranged from 0.074 ± 0.035 to 0.168 ± 0.081 mm h -1 depending on the storage temperature (5 to 25°C).
Regression analysis was carried out to find a relation between the fruit length change and both wax solution concentrations and storage temperatures. Equation (3) shows the most appropriate form for the relationship between the fruit length change and wax solution concentrations at different storage temperatures. Equation (4) shows the relationship between the fruit length change and storage temperatures at different wax solution concentrations. The constants of these equations and coefficient of determination are listed in Table 2.
Where:-LC is the cucumber length change (mm h -1 )   Regression analysis was carried out to find a relation between the fruit volume change and both wax solution concentrations and storage temperatures. Equation (5) shows the most appropriate form for the relationship between the fruit volume change and wax solution concentrations at different storage temperatures. Equation (6) shows the most appropriate form for the relationship between the fruit volume change and storage temperatures at different wax solution concentrations. The constants of these equations and coefficient of determination are listed in Table 3.  Regression analysis was carried out to find a relation between the fruit surface area change and both wax solution concentrations and storage temperatures. Equation (7) shows the most appropriate form for the relationship between the fruit surface area change and wax solution concentrations at different storage temperatures. Equation (8) shows the most appropriate form for the relationship between the fruit surface area change and storage temperatures at different wax solution concentrations. The constants of these equations and coefficient of determination are listed in Table 4. SAC= a WX 2 + b WX + c

SAC= a T 2 + b T + c (8)
Where:-SAC is the cucumber surface area change (cm 2 h -1 ) Weight Loss: Figures 9 and 10 show the effect of different wax solution concentrations and storage temperatures on change in weight loss of fruit cucumber. The results indicated that the weight loss decreases with increasing wax solution concentrations but it increases with increasing temperature storage, where, it decreased from 0.020 to 0.005% h -1 when the wax solution increased from 0 (control) to 100%. On the other hand, weight loss increased from 00073 to 0.017% h -1 with the storage temperatures increased from 5 to 25°C (room temperature). These results agreed with those obtained by [17]. The overall average of weight loss decreased from 0.020 ± 0.007 to 0.005 ± 0.002% h -1 when the wax concentration changed from 0 to 100%. The overall average of weight loss ranged from 0.007 ± 0.004 to 0.017 ± 0.008% h -1 depending on the storage temperature (5 to 25°C).
Regression analysis was carried out to find a relation between the fruit weight loss and both wax solution concentrations and storage temperatures. Equation (9) shows the most appropriate form for the relationship between the fruit weight loss and wax solution concentrations at different storage temperatures. Equation (10) shows the most appropriate form for the relationship between the fruit weight loss and storage temperatures at different wax solution concentrations. The constants of these equations and coefficient of determination are listed in Table 5.
WL= a T 2 + b T + c (10) Where:-WL is the cucumber weight loss (% h -1 ) Effect of wax coating and temperature storage on hardness change rate of cucumber fruits: Figures 11 and 12 show the effect of different wax solution concentrations and storage temperatures on hardness change rate of cucumber fruits. The results indicated that the hardness change rate decreases with increasing wax solution concentrations but it increases with increasing storage temperature, where, it decreased from 1.079 to 0.530 N h -1 when the wax solution concentrations increased from 0 (control) to 100%. On the other hand, the hardness in cucumber fruits increased from 0.687 to 1.511 N h -1 with the storage temperature increased from 5 to 25°C [18]. The overall   average of hardness change rate decreased from 1.079 ± 0.107 to 0.530 ± 0.032 N h -1 when the wax concentration changed from 0 to 100%. The overall average of hardness change rate ranged from 0.687 ± 0.024 to 1.511 ± 0.093 N h -1 depending on the storage temperature (5 to 25°C).
Regression analysis was carried out to find a relation between the fruit hardness change and both wax solution concentrations and storage temperatures. Equation (11) shows the most appropriate form for the relationship between the fruit hardness change and wax solution concentrations at different storage temperatures. Equation (12) shows the most appropriate form for the relationship between the fruit hardness change and storage temperatures at different wax solution concentrations. The constants of these equations and coefficient of determination are listed in Table 6.
H= a T 2 + b T + c (12) Where:-H is the hardness change (% h -1 ) Effect of wax coating and storage temperature on total soluble solids (TSS) in cucumber fruits: Figures 13 and 14 show the effect of different wax solution concentrations and storage temperatures on TSS change in cucumber fruits. The results indicated that the TSS decreases with increasing wax concentrations but it increases with increasing temperature storage, where, it decreased from 0.008 to 0.003% h -1 when the wax solution concentrations increased from 0 (control) to 100%. On the other hand, TSS change increased from 0.005 to 0.007% h -1 with the storage temperatures increased from 5 to 25°C (room temperature). These results agreed with those obtained by [17]. The overall average of TSS change decreased from 0.008 ± 0.0016 to 0.003 ± 0.0004% h -1 when the wax concentration changed from 0 to 100%. The overall average of TSS change ranged from 0.005 ± 0.001 to 0.007 ± 0.002% h -1 depending on the storage temperature (5 to 25°C).
Regression analysis was carried out to find a relation between the    fruit TSS change and both wax solution concentrations and storage temperatures. Equation (13) shows the most appropriate form for the relationship between the fruit TSS change and wax solution concentrations at different storage temperatures. Equation (14) shows the most appropriate form for the relationship between the fruit TSS change and storage temperatures at different wax solution concentrations. The constants of these equations and coefficient of determination are listed in Table 7.
TSS=a T 2 + b T + c (14) Where:-TSS is the cucumber TSS change (% h -1 ) Effect of storage temperature on the shelf life of the cucumber coated with different wax solution percentage: Figures 15 and 16 show the effect of storage temperature on the shelf life of the cucumber coated with different wax solution percentage. The results indicated that the shelf life increases with increasing wax solution concentrations meanwhile, it increases with increasing temperature storage, where, it increased from 173.4 to 231.6 h when the wax solution concentration increased from 0 (control) to 100%. On the other hand, the shelf life in cucumber fruits increased from 98.40 to 288.48 h when the storage temperature increased from 5 to 25°C (room temperature). These results were in agreement with those obtained by [16].

Summary and Conclusions
The experiment was carried out to study the effect of wax coating (control, 25, 50, 75 and 100%) and storage temperatures (room temperature, 15, 10 and 5°C) on cucumber fruits quality, (diameter change, length change, volume change, area change, weight loss, hardness change rate and TSS change on cucumber fruits). Shelf life of the cucumber was also studied. The obtained results can be summarized as follows:     • The diameter change decreased from 0.053 to 0.017 mm h -1 when the wax solution increased from 0 to 100%. On the other hand, loss of diameter increased from 0.021 to 0.049 mm h -1 with the storage temperature increased from 5 to 25°C.
• The length change decreased from 0.199 to 0.0054 mm h -1 when the wax solution concentrations increased from 0 to 100%. On the other hand, the length change increased from 0.074 to 0.168 mm h -1 with the storage temperature changed from 5 to 25°C.
• The volume change decreased from 0.349 to 0.113 cm 3 h -1 when the wax solution increased from 0 to 100%. On the other hand, loss of volume cucumber fruit increased from 0.154 to 0.300 cm 3 h -1 with the storage temperatures increased from 5 to 25°C.
• The surface area change decreased from 0.419 to 0.111 cm 2 h -1 when the wax solution concentrations increased from 0 to 100%. On the other hand, change of surface area increased from 0.151 to 0.358 cm 2 h -1 with the storage temperatures increased from 5 to 25°C.
• The weight loss decreased from 0.020 to 0.005% h -1 when the wax solution increased from 0 to 100%. On the other hand, weight loss increased from 00073 to 0.017% h -1 with the storage temperatures increased from 5 to 25°C.
• The hardness change rate decreased from 1.079 to 0.530 N h -1 when the wax solution concentrations increased from 0 to 100%. On the other hand, the hardness in cucumber fruits increased from 0.687 to 1.511 N h -1 with the storage temperature increased from 5 to 25°C.
• The TSS decreased from 0.008 to 0.003% h -1 when the wax solution concentrations increased from 0 to 100%. On the other hand, TSS change increased from 0.005 to 0.007% h -1 with the storage temperatures increased from 5 to 25°C.
The shelf life increases with increasing wax solution but it increases with increasing storage temperature. It increased from 173.4 to 231.6 h when the wax solution concentration increased from 0 to 100%. On the other hand, the shelf life in cucumber fruits increased from 98.40 to 288.48 h with the storage temperature increased from 5 to 25°C.