Introduction

The preparation of the fresh cut product includes operations peeled, cut and sliced, among others that cause mechanical damage and cell disruption, and induce an increase in the rate of respiration and ethylene production as well as the synthesis of secondary metabolites, reducing the shelf life of these products. Symptoms of damage include fading, hazing in the areas of cutting, flaccidity and decreased their nutritional value (^{Artes et al., 2007}). The nopal fresh cut (NVFC) is a presentation widely accepted in certain markets and for their preparation, the cladodes (modified stem or stalk) of nopal is washed fillet and cuts either in pictures or in strips, and packaged in polystyrene trays covered with plastic layer or plastic bags (^{Rodriguez, 2002}).

The shelf life of NVFC grow COPENA F-1 is 1 to 2 days at 20 °C, extending to seven days when stored under refrigeration (5 °C, 91% H R), the main attributes that limit shelf life they are darkened and mucilage secretion. The latter causes a change in color from bright green to olive green (appearance of cooked nopal) (^{Rodriguez, 2002}). ^{Quevedo et al. (2005)} found that the NVFC cultivating COPENA F-1, diced 3 x 3 cm, showed no darkening nor secretion mucilage for 12 days at 5 °C when treated with ascorbic acid 0.5 M and bagging Cryovac® PD960. Recently, ^{Fortiz and Rodriguez (2010)} noted that the packaging plastic layers, Cryovac® RD 106 and PD 960, modify the atmosphere inside the container, with this increasing shelf life of NVFC during storage at 5 °C, and which he delayed the dimming to 14 and 15 days, respectively. However, these layers favored packaging mucilage secretion initiated at 11 and 10 days, respectively.

They have employed various methods to extend the shelf life of fresh cut products among which are refrigeration (temperature between 0 and 5 °C), the modified atmosphere packaging (EAM), the use of chemical additives and covered edible (^{Baldwin and Bai, 2011}). Most fresh cut maintain better quality products at temperatures close to 0 °C, and all physiological processes take place more slowly at lower temperatures. In addition to a good temperature control, the EAM can increase shelf life. This is because the O₂ acts as the terminal electron acceptor in various metabolic reactions, and the rate of essential metabolic processes are sensitive to the O₂ concentration. As O₂ concentrations below 10% decrease in respiration rate, the rate of degradation and senescence (^{Brandenburg and Zagory, 2009}).

The decrease in O₂ concentration may in some cases reduce oxidative browning reactions (^{Brandenburg and Zagory, 2009}), which may be of particular interest in fresh cut vegetables as the nopal. The EAM with very low concentration of O₂ (2-5%) and 3-10% CO₂, is essential for prolonging the shelf life of cut mainly due to the inhibition of enzymatic browning and physiological aging tissue (^{Brandenburg and Zagory, 2009}), so it is widely used in the industry of fresh cut salads (^{Rojas et al., 2009}). The EAM fresh cut produce is based on the relationship between product respiration rate and gas permeability of the container to alter the atmosphere within this (^{Brandenburg and Zagory, 2009}).

Based on the above, the objective of this study was to evaluate the effect of three plastic packaging layer, PEBD (control), Cryovac® PD960 and polysweat Bolco®, quality of NVFC cultivate COPENA F-1 during storage at low temperature (1 to 5 °C).

Materials and methods

Results and discussion

Concentration of gases (O₂ and CO₂) into the container. Factors layer temperature and had a significant effect (p< 0.05) in the concentration of O₂ within the package of NVFC. The content of O₂ within the package of NVFC stored at 1 °C was higher (p< 0.05) than those kept at 5 °C (Figure 1A). The O₂ content in the container PEBD was higher (p< 0.05) than in layers Cryovac® PD960 and polysweat Bolco®, presenting these similar values. From day 5 at 5 °C the NVFC Cryovac® packaging in bags PD960 and polysweat Bolco® showed values of 0% O₂. This indicates that the O₂ permeability of these materials is very low at this temperature. However, during storage at 1 °C lower values O₂ in NVFC Cryovac® layer were packaged with Bolco® polysweat PD960 and were 4.3% and 5.5 respectively (Figure 1A). This behavior is attributed to a higher temperature is higher respiration rate of the product, resulting in rapidly consume O₂ remaining in the headspace of the bags to be closed and the entrance of the gas to the container is restricted due to low O₂ permeability of this material. Higher levels of O₂ at 1 °C is attributed to the lower rate of respiration of the product at a lower temperature (^{Cantwell and Suslow, 2002}).

Figure 1 Oxygen concentration (A) and CO₂ (B) into the container (PEBD layer (control), polysweat Bolco® and Cryovac® PD960) of fresh prickly pear cut COPENA F-1 during storage at 1 to 5 ° C. Vertical bars indicate the standard deviation. n= 4. DMS temperature oxygen concentration= 1.19*, DMS layer= 1.75 *; DMS layer*temperature= 3.0 (ns) in concentration of CO₂ in DMS temperature= 0.39 *; Layer DMS= 0.57 *; DMS interaction and layer*temperature= 0.99*. ns= not significant (Tukey, 0.05). 

The factors temperature layer and temperature*layer interaction had a significant effect (p< 0.05) in the concentration of CO₂ in the container NVFC. The CO₂ content in the package was similar (p> 0.05) in the three layers evaluated during storage at both 1 and 5 °C, except NVFC PD960 Cryovac® packaging layer presented during storage at 5 °C higher values. Nearby values were detected at 2% at the beginning of storage increased regardless of the type of plastic layer to advance storage time at both 1 and 5 °C, up to a maximum of 7.32% in NVFC packaging layer Cryovac PD960 (Figure 1B). CO₂ concentrations inside the container NVFC in PEBD bags, Cryovac PD960 and polysweat Bolco® stored at 1 and 5 °C were within the recommended range of concentrations (5 to 10% CO₂) for minimally processed or fresh products cut (^{Rojas et al., 2009}).

The CO₂ in concentrations greater than 1 or 2% reduces tissue sensitivity to ethylene, which is one of the main benefits of EAM for some fruits and vegetables (^{Brandenburg and Zagory, 2009}). The reduction of O₂ and CO₂ increase in the atmosphere of the container can extend the shelf life of cut, reducing his breathing, perspiration and ethylene production (^{Rojas et al., 2009}).

Darkening. The temperature, the type of layer used to package the product and temperature*layer interaction had a significant effect (p< 0.05) in the behavior of darkening NVFC (Figure 2A). The NVFC stored for 20 days at 1 °C showed no darkening in any of the three types of bag evaluated. Not at 5 °C where NVFC the packaging PEBD (control) showed a darkening of 0.1 at 13 days of storage, 20 days and reached values of 0.8 (Figure 2A). The packaging NVFC layer Cryovac® PD 960 showed darkening of 0.1 to 14 days at 5 °C, which increased to 0.3 after 20 days storage. These values obscuration are higher (p< 0.05) than those shown by the NVFC packaging layer polysewat Bolco®, which showed no darkening during the 20 days storage at 5 °C, similar to NVFC behavior packaging with three layers evaluated and stored at 1 °C. However, dimming values NVFC packaging layers PEBD and polysweat Bolco® were less than 1 and not exceeding the value of 2 which is the limit consumer acceptability. Therefore, it is considered that this variable did not affect significantly the quality of NVFC during storage for 20 days at 5 °C.

Figure 2 Darkening (A) and secretion of mucilage (B) of fresh prickly pear cut packaging with 3 different layers (PEBD layer (control), polysweat Bolco® and Cryovac® PD960) during storage at 1° to 5 °C. Vertical bars indicate the standard deviation. n= 10. hedonic scale. 0= no change; 1= very light; 2= slight; 3= moderate; 4= severe; 5= very severe. The dotted line in A and B indicates the limit consumer acceptability. In DMS darkening temperature= 0.02*, DMS layer= 0.02* and DMS interaction layer*temperature= 0.04*. Mucilage layer DMS= 0.02*; DMS layer= 0.03* and interaction DMS temperature*layer= 0.06*. 

^{Fortiz-Hernandez and Rodriguez-Felix (2010)} reported slight darkening after 7 days at 5 °C (63% H. R.) in NVFC packaged in PEBD bag. In the present study darkening in this type of bag she started until day 12 and is associated with differences in the concentrations of O₂ and CO₂ in the package, due to the variability of plastic material used commercially PEBD.

The delay observed obscuration in this study is attributed to the low O₂ levels (less than 5% observed in most of the storage period) and CO₂ (1.9 to 4.6%) that occurred within this container, as ^{Brandenburg and Zagory (2009)} reported that very low concentrations of O₂ (2 to 5%), combined with CO₂ from 3 to 10% are critical to the inhibition of enzymatic browning and physiological aging tissue, as well as the extension the shelf life of cut. This may be associated with the activity of polyphenol oxidase (PPO) and ascorbic acid oxidase, causing acid enzymatic browning. ^{Kader (1986)} reported that the activity of these enzymes is reduced to low concentrations of O₂ in the package.

The lowest observed darkening NVFC packaging bag Cryovac®PD-960 contrasts with that published by ^{Quevedo et al. (2005)}, who reported that the darkening of NVFC began at 9 days and 16 days at 5 °C showed values greater than 2, which is the limit consumer acceptability. This could be attributed to the study of ^{Quevedo et al. (2005)}, the concentrations of O₂ in the container NVFC ranged from 2 to 8%, whereas in the present study O₂ concentrations were 0% for almost the entire storage period, except for the initial values. These concentrations of O₂ to zero could cause a further decrease in the activity of polyphenol oxidase (PPO) and ascorbic acid oxidase, causing enzymatic browning (^{Kader, 1986}).

Secretion of mucilage. Secretion of mucilage similar to the dimming behavior was observed, being significant (p< 0.05) of the storage temperature, the type of layer used to package the NVFC and temperature*layer interaction secretion mucilage NVFC (Figure 2B). To 1 °C is not mucilage secretion was observed NVFC, excluding packaging with PEBD layer, which showed values of 0.25 to 19 days and reached values of 0.5 to 20 days; however, these values are below the limit consumer acceptability. However when the NVFC stored at 5 °C mucilage secretion increased as time passed storage (Figure 2B) and differences between the various layers evaluated (PEBD, Cryovac PD960 and polysweat Bolco®) were found, though values were low and only in NVFC packaging bags Cryovac®PD960 limit consumer acceptability after 18 days storage at 5 °C was reached. The NVFC packaging layer PEBD (control) had no secretion of mucilage during storage at 5 °C (Figure 2B).

A similar behavior in NVFC packed in bags of PEBD was reported by ^{Fortiz-Hernandez and Rodriguez-Felix (2010)}. However, the behavior of the secretion of mucilage NVFC bags Cryovac® PD960 packed in this study, does not match those reported by these authors who observed that this quality problem started after 11 days at 5 °C and exceeded the limit of consumer acceptability 16 days at 5 °C. These differences could be attributed to differences in the content of nopal mucilage used in different studies as shown by other authors (^{Camacho et al., 2007}).

Nopal mucilage is a heteropolysaccharide water soluble (^{Matsuhiro et al., 2006}), which is stored in special cells, thin- walled, although clorenchyma found in the parenchyma and are most abundant in the latter. Mucilage has the property of retaining strongly osmotic water (^{Sudzuki, 1999}). Mucilaginous cells are in adjacent and parallel to the vascular bundles channels, so that the transverse cut of the cladodes causes the mucilage be secreted from cut channels (^{Mauseth, 2005}), appears as a viscous fluid that is not capable gelling (^{Goycoolea and Cardenas, 2003}).

Analyzing together darkening and secretion of mucilage, the two major quality problems NVFC, shows that the most influential factor was the temperature had. This is because, at 1 °C no blackening or mucilage secretion was presented (except for the minimum values after 20 days storage in bags PEBD), regardless of the layer used to package the product. The NVFC packaging layers polysweat Bolco® not presented darkening at 1 and 5 °C, while the secretion of mucilage was minimal during the storage for 20 days at 5 °C.

Colour. At the start of the experiment the NVFC packaged in three layers showed a value of L* of 52.2 to 55.3, an angle of pitch (AT) between 123 and 124° and a chroma or color saturation of 22.1 to 24.2 which represents a green off (Figure 3). No effect (p> 0.05) the storage temperature, type of packaging used for the NVFC layer, or layer*temperature value L* (Figure 3A). The interaction was observed there were no major changes in this variable during storage for 20 days at 1 and 5 °C, above 50 values (on a scale from 0 to 100).

Figure 3 Color: L (A), hue angle (B) and chroma (C) of fresh prickly pear cut packaging with 3 different layers (PEBD layer (control), polysweat Bolco® and Cryovac® PD960) during storage 1 to 5 °C. Vertical bars indicate the standard deviation. n= 6. Temperature value L*DMS= 0.8 (n.s), DMS layer= 1.2 (n. s) and interaction DMS layer*temperature= 2.0 (ns). DMS pitch angle temperature= 0.7*; DMS layer= 1.1 (ns) and DMS interaction layer*temperature= 1.8 (ns). DMS chroma temperature= 0.7*; DMS layer= 1.1 (ns) and DMS interaction layer*temperature= 1.9 *. ns= not significant (Tukey, 0.05). 

The storage temperature significantly (p< 0.05) hue angle (AT) of NVFC. Although higher values (p<0.05) were taken during storage at 1° to 5 °C, 121.3° and 119.9° respectively, these are located in the characteristic green color of this vegetable. No significant differences (p> 0.05) between the three layers evaluated in both storage temperatures were detected (Figure 3B).

Color saturation or chroma NVFC was affected by storage temperature and temperature*layer interaction (p< 0.05) (Figure 3C). The NVFC packaging layer polysweat Bolco® and stored at 5 °C, showed values of 24.5, which is higher (p< 0.05) than the other treatments showed similar values including values around 21 (Figure 3C). However, despite the changes, the values above indicate muted colors.

Overall, taking into account the three attributes of color, brightness or clarity (L*), hue angle and chroma, it was observed that the green off the NVFC packaging with the three tested layers remained during the 20 days of storage at 1 and 5 °C.

The color is used by the consumer and the industry as a selection criterion and an indicator of the overall quality of fresh cut products (^{Montero and Cerdas, 2011}), so it is very important to maintain the normal color of the fabric (^{Beaulieu, 2011}), as was achieved in the present study. This preservation of green NVFC off is due to the positive and synergistic effect of storage temperature and AM generated inside the containers evaluated. ^{Yahia (2009)} reported that the optimum composition of the atmosphere slows the breakdown of chlorophyll and consequent loss of green tissue. Also, ^{Artes et al. (2007)} reported that the use of low temperatures and high relative humidity, combined with atmospheres low in O₂ and moderately enriched in CO₂, delays the color change from green to yellow or bright coffee green, both due to degradation of chlorophyll.

PH and titratable acidity. The initial pH of NVFC was 4.3 (Figure 4A). Aneffect(p<0.05) temperature and temperature storage*interaction layer used to package the NVFC on pH was found (Figure 4A). As it regards temperature storage*layer interaction, only differences (p< 0 05) between the NVFC packaging PEBD layer (commercial control) stored at 5 °C showed higher values (4.43) found that NVFC PEBD layer packaging and Cryovac® PD960 stored at 1 °C, which showed values of 4.26 and 4.31, respectively.

Figure 4 pH (A), titratable acidity (%) (B) and firmness (C) of fresh prickly pear cut packaging with 3 different layers (PEBD layer (control), polysweat Bolco® and Cryovac® PD960) during storage 1 to 5 °C. Vertical bars indicate the standard deviation. n = 3. Temperature pH DMS= 0.04*, DMS layer= 0.06 (n.s) and DMS interaction layer*temperature= 0.1*. In titratable acidity in DMS temperature= 0.03 *; Layer DM = 0.04* and DMS temperature*layer= 1.07 (ns). DMS firmness temperature= 0.08*; Layer DMS= 0.12 (ns) and DMS interaction layer*temperature= 0.19 (ns). n.s= not significant (Tukey, 0.05). 

The initial titratable acidity was 0.75% and decreased during storage differences were found (p< 0.05) the effect of storage temperature with different plastic packaging layers (Figure 4B). The NVFC stored at 5 °C showed a titratable acidity of 0.71%, which is lower (p< 0.05) to 0.81% of NVFC stored at 1 °C. The NVFC packaging layer Cryovac® PD960 showed values of 0.78% which are higher (p< 0.05) to 0.73% shown by the NVFC packaging with PEBD layer. The titratable acidity greater than 1 °C, could be due to the positive effect of this temperature decrease storage and metabolic activity.

Nopal deterioration because it has been reported that the storage temperature of the cut products affects the respiration rate thereof, which increases as temperature rises (^{Cantwell and Suslow, 2002}). This effect is exacerbated if the product is in EAM with O₂ concentrations relatively low and high CO₂, designed for low temperature storage storage (2 to 10 °C), as these reduce the loss of acidity of the tissue (^{Baldwin and Bai, 2011}).

Generally higher respiration rate, reduced shelf life as breathing substrates, sugars and acids are consumed. Malic acid, the main organic acid nopal, seems to be used as a substrate breathing via malic enzyme, which decarboxylated malate to pyruvate, with the extra carbon fed to the cycle of tricarboxylic acid (TCA), while citrate can enter directly to the TCA cycle (^{Baldwin and Bai, 2011}).

Firmness. The initial strength of NVFC was 3.6 N. This variable decreased (p< 0.05) during storage after 20 days presenting a value of 2.9 N, with higher values in the NVFC stored at 1 °C with respect to the stored 5 °C (p< 0.05) (Figure 4C). The NVFC firmness treatments packaging layers PEBD (control), Cryovac® PD 960 and polysweat Bolco® was similar. These results agree with those reported by ^{Quevedo et al. (2005)} who observed a decrease in the strength of NVFC PD960 Cryovac® packaging layer during storage for 20 days at 5 °C. Firmness is a critical quality attribute that helps the industry and the consumer to determine the acceptability of cut fruits and vegetables (^{Beaulieu, 2011}). The softening of the tissue is a serious problem that occurs during the marketing of fruits and vegetables cut. This occurs largely because during the preparation of these products integrity of fruits and vegetables is impaired, causing tissue damage and altering cellular microstructure, which by enzymatic action accelerates the degradation of the cell wall (^{Artes et al., 2007}; ^{Montero and Cerdas, 2011}). Modifications to the cell wall have been attributed to the β-galactosidase, polygalacturonase enzymes, pectinmetil esterase, cellulase, fenilalaninamonio lyase and peroxidase (^{Montero and Cerdas, 2011}).

Conclusions

The quality variables pH, titratable acidity, firmness and color showed no major changes affecting the quality of fresh prickly pear cut during storage at 1 and 5 °C. The storage temperature 1 °C preserved the quality of nopal cut fresh (NVFC) with respect to variables darkening and secretion of mucilage, because regardless of the layers used to package the product at this temperature did not show darkening in tissue and mucilage secretion was minimal. This reflects the importance of using optimal temperatures during storage of the product. The use of complementary EAM storage temperature seems only be relevant to 5 °C, as during storage at this temperature the layer packaging Cryovac® PD960 promotes the secretion NVFC mucilage.