**1. Introduction**

*Opuntia* fruit, also known as cactus pear fruit, prickly pear, tuna (Mexico), higo (Colombia), higo chumbo (Spain), and figue de barbarie (France), is harvested from various species of the genus *Opuntia* of the cactus family (Cactaceae). The fruit is a xerophyte, producing about 200–300 species, mainly growing in arid and semi-arid zones, and it is produced and consumed in several countries. The most important species producing edible fruit are *O. ficus-indica*, *O. robusta*, *O. streptacantha*, *O. amyclaea*, *O. megacantha*, and *O. hiptiacantha* [1]. It is native to Mexico and was introduced to Europe by the Spanish conquerors, and the Canary Islands was the first non-American territory where it was planted at different altitudes, thus enabling the extension of the commercialization period from the end of June to February [2].

*Opuntia* is a fruit with high contents of fiber, minerals, vitamins, and antioxidant compounds with functional properties for preventing chronic diseases [3–7]. A few studies have shown that the phytochemicals from *O. ficus-indica* help control hypoglycemic, hypolipidemic, and hypocholesterolemic diseases and are neuroprotective [8]. Moreover, a recent study confirmed that the antioxidants from red, orange, and white prickly pear varieties from the Canary Islands remain stable while traveling through the gastrointestinal

**Citation:** Díaz-Delgado, G.L.; Rodríguez-Rodríguez, E.M.; Dorta, E.; Lobo, M.G. Effects of Peeling, Film Packaging, and Cold Storage on the Quality of Minimally Processed Prickly Pears (*Opuntia ficus-indica* L. Mill.). *Agriculture* **2022**, *12*, 281. https://doi.org/10.3390/ agriculture12020281

Academic Editor: Bengang Wu

Received: 17 December 2021 Accepted: 11 February 2022 Published: 16 February 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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tract and can be easily absorbed by the human body [9]. The presence of glochids together with spines on the fruit´s surface [10] is a grea<sup>t</sup> disadvantage that limits its intake and commercialization in comparison to other fruits. On the other hand, modern consumers are increasingly demanding healthy and ready-to-eat products. Taking advantage of this opportunity, the prickly pear could be minimally processed (fresh-cut) to potentially increase its consumption and open new alternatives for its commercialization. The preparation of minimally processed fruits includes washing, peeling, disinfecting, packaging, and cold storing [11]. These processes cause increases in enzyme activity and the acceleration of physiological reactions, thus promoting microbial growth [12], which is usually the parameter that limits commercialization [13]. The use of bio-materials [14], surface coatings, calcium salt applications, modified atmosphere packaging, gamma radiation, and cold storage are the most used approaches used for quality retention to minimize nutritional losses, sensorial losses, and microbial growth [15]. In fact, under these conditions, one can obtain products with similar characteristics to fresh products with a shelf life of 7–10 days such as pineapple [16], kiwifruit [17], mango [18], and lychee [19].

Peeling is an extremely important step because it exposes large surface areas to air, leading to water loss, oxidation, and microorganism attacks. Moreover, peeling is usually performed by hand, thus increasing the final value of the minimally processed product. However, few studies have compared manual and mechanical peeling. Many processing innovations and automations are being implemented to reduce the amount of manual peeling in order to increase production yields [20–22]. The main factors that affect the peeling process are the mechanical and physical properties of fruit and vegetable tissues, such as skin thickness, firmness, toughness, variety, rupture force, cutting force, maximum shearing force, shear strength, tensile strength, and rupture stress [21,23]. Additionally, the peel obtained in this process is a by-product source of dietary fiber and bioactive compounds [24] that can be used to improve the profitability of manufacturing companies as a novel step in its sustainable utilization. Djeghim et al. [25] and Parafati et al. [26] reported that the use of various by-products, including prickly pear peel and prickly pear seed peel, improved the dough rheology and nutritional properties of bread. Furthermore, Morshedy et al. [27] reported that low levels of prickly pear cactus peel supplementation in the diet of lactating ewes improved the ewes' productive performance and growth, as well as the physiological status of their offspring.

The present study was aimed to cultivate ready-to-eat prickly pears with a shelf life of at least one week using simple and cheap hurdle technologies (mechanical peeling, micro-perforated films, and cold storage).

Our process flow diagram will be accessible for small and medium agro-industries to revalue this fruit in the Canary Islands and other countries in which its consumption is diminishing. Thus, manufacturers can promote the product while assuring its hygienic, nutritional, and sensorial qualities.

#### **2. Materials and Methods**

#### *2.1. Plant Material, Sample Preparation, Packaging and Storage*

Prickly pears (*O. ficus-indica* L. Mill.) were collected from a farm located in Buenavista, Tenerife, Spain (28◦2213 N, 16◦51 1 W, 127 m above sea level) in December 2018. Two types of prickly pears of different colors were selected: white and orange. The white prickly pears were bigger and had thicker pericarps than the orange ones. We harvested 30 kg of both white (W) and orange (O) prickly pears, locally known as "Ariquero" and "Colorado", respectively, in the same fashion as other non-climacteric fruits when fully ripe to ensure good flavor quality and without any damage caused by decay-causing pathogens. Figure 1 shows the process flow diagram for obtaining the minimally processed prickly pears.

**Figure 1.** Process flow diagram for obtaining minimally processed prickly pears.

Fruits were washed with cold chlorinated water (200 mg/kg, pH 6.5–7.5) for 5 min and then air-dried. After washing and drying, the fruits' distal parts were removed and then peeled either by hand with a knife (H) or with an electric peeler machine (P) (Orange Peel, Pelamatic S.L, Valencia, Spain). The main difference between both peeling methods was the amount of pericarp eliminated in the process; the peel and the whole pericarp were removed in the fruits peeled with the knife (which is how it is traditionally performed by consumers; Figure 1), and the electric peeler only removed the peel (Figure 1).

Then, the peeled fruit was washed for 1 min in cold chlorinated water (50 mg/kg) before being packaged in groups of 2 or 3, depending on size, in plastic trays (polypropylene, 172 × 130 × 50 mm, supplied by Technopak Plastics S.L., Barcelona, Spain), and sealed using a heat-sealing machine (Efaman, Efabind S.L., Murcia, Spain) with a microperforated film (polypropylene, 52 μm, supplied by Amcor Flexibles, Burgos, Spain) of different permeability:


A total of 104 trays of each variety were prepared and stored at 7 ± 1 ◦C; they were analyzed at the beginning of experiment and after 1, 3, 6, 8, and 10 days of storage (Figure 1).

Samples were labeled as follows: WH90P = white prickly pear, hand-peeled, packaged in 90PPlus film; OH90P = orange prickly pear, hand-peeled, packaged in 90PPlus film; WH180P = white prickly pear, hand-peeled, packaged in 180PPlus film; OH180P = orange prickly pear, hand-peeled, packaged in 180PPlus film; WE90P = white prickly pear, electrically peeled, packaged in 90PPlus film; OE90P = orange prickly pear, electrically peeled, packaged in 90PPlus film; WE180P = white prickly pear, electrically peeled, packaged in 180PPlus film; and OE180P = orange prickly pear, electrically peeled, packaged in 180PPlus film.

#### *2.2. Technological Parameters*

In order to calculate the technological parameters to determine the yield in a minimal processing industry, the following parameters were measured or calculated using 20 fruits: the whole weight, peel (residue), and edible portion. Peeling time was measured for both peeling methods and varieties, and it is expressed as the mean value of the 20 fruits in seconds. The yield of each peeling method was calculated by the following expression:

$$\text{Yield } (\%) = \left(\frac{\text{EP}}{\text{AP}}\right) \times 100$$

EP: Weight of the product after being peeled; AP: Weight of the product as purchased.

#### *2.3. Microbiological Analysis*

Aerobic mesophiles, psychrophiles, and mold and yeas<sup>t</sup> loads were evaluated to ensure microbial safety. Three trays from each treatment were analyzed in triplicate at each storage time. Six grams were homogenized in 54 mL of 0.1% peptone water (Sigma-Aldrich, Barcelona, Spain) using a homogenizer (Stomacher 80 Biomaster, Seward Limited, Worthing, United Kingdom). The serial dilutions were prepared from this original solution and finally inoculated in triplicate. Aerobic mesophiles and psychrophiles were inoculated in plate count agar (PCA) and then incubated at 30 ◦C for 72 h and 5 ◦C for 7 days, respectively, and molds and yeasts were inoculated in Glucose Chloramphenicol Agar (GCA) and incubated at 25 ◦C for 5 days. Microbial load is expressed as colony-forming units per gram (CFU/g) and compared with the values established by the Spanish legislation regarding minimally processed vegetables [28].

#### *2.4. Sensorial Evaluation*

Sensory evaluation was carried out by 15 panelists, who were regular consumers of prickly pears, in order to evaluate whether they were able to appreciate differences between hand-peeled (traditional method) and electrically peeled prickly pears. The number of panelists was similar to that reported by other authors [16,29–31].

Six fruits from each treatment were evaluated in terms of color, smell, taste, and overall acceptability using a linear scale from 0 (non-acceptable) to 10 (very acceptable) points. Additionally, panelists described the fruits' color (pale, normal/bright, or brown), sweetness (tasteless, normal, or very sweet), smell (unpleasant, normal, or pleasant), and texture (hard, normal, or slimy). Finally, they were asked whether they would buy the product. The trays were kept at room temperature for approximately half an hour before the tasting and opened just before it. Fruits were cut into slices between 0.5 and 1 cm thick. Three slices of each type of prickly pear were placed on plastic plates with a white background, labeled with random numbers, and served in an isolated and illuminated area at room temperature (20 ◦C) individually for each taster. Likewise, an unopened tray of each type was placed in the room so that the tasters could evaluate the general appearance of the packaged prickly pear. Panelists were also instructed to drink some water to rinse their palates between each sample [32].

#### *2.5. Gas Composition*

The gas composition (% CO2 and % O2) was determined using a compact PBI Dansensor Checkmate 9900 (Madrid, Spain), the needle of which was fed through the septum fixed on the unopened trays.

#### *2.6. Physico-Chemical Analyses*

Physico-chemical analyses were carried out for unopened trays (evolutionary analyses) and for opened trays (destructive analyses).

Color parameters (L, a\*, and b\*) were measured through the button of the transparent tray with a Minolta Chroma Meter CR-300 (Wheeling, WV, USA). From these data, the following parameters were calculated: hue angle (H◦), chroma (C\*), total color difference (ΔE), and whiteness index (WI) [33]. Thus, five trays from each treatment were analyzed in triplicate during cold storage.

$$\mathbf{H}^{\circ} = \tan^{-1} \left( \mathbf{b}^{\star}/\mathbf{a}^{\star} \right)$$

$$\mathbf{C}^{\star} = \left[ (\mathbf{a}^{\star})^{2} + (\mathbf{b}^{\star})^{2} \right]^{0.5}$$

$$\text{WI} = 100 - \left[ (100 - \text{L}^{\star})^{2} + (\mathbf{a}^{\star})^{2} + (\mathbf{b}^{\star})^{2} \right]^{0.5}$$

$$\Delta \mathbf{E} = \left[ \left( \text{L}^{\star} - \text{L}^{\star} \right)^{2} + \left( \text{a}^{\star} - \text{a}^{\star} \text{i} \right)^{2} + \left( \text{b}^{\star} - \text{b}^{\star} \text{i} \right)^{2} \right]^{0.5}$$

In addition, three trays from each treatment were opened at each storage time, and the following parameters were analyzed in triplicate: texture (N·s/g fresh weight) using a Kramer cell (TA-HD-Plus, Aname, Madrid, Spain) to simulate chewing and hardness (expressed as ◦Durofel) using a durometer (Durofel, Agro-Technologie, Tarascon, France).

Finally, samples of each treatment were minced and homogenized for analysis in triplicate. Moisture was determined with the oven-drying method (P Selecta 207, Barcelona, Spain), dry matter was calculated by difference [34] (AOAC 934.06), total soluble solids (TSS) were determined using a hand refractometer (ATC-1, ATAGO, Tokyo, Japan) [20] (AOAC 932.12), pH was measured with an automatic titrator (Titralab AT1000, Germany) [34] (AOAC 981.12), and total acidity (percentage citric acid) was determined via titration with NaOH to an endpoint of pH 8.1 [20] (AOAC 942.15).

#### *2.7. Bioactive Compounds and Antioxidant Capacity Analysis*

All analyses were performed in triplicate. Ascorbic acid content was volumetrically determined with a 2,6-dichlorophenol indophenol reagen<sup>t</sup> [34] (967.21). Total phenolic content (TP) is expressed as milligrams of gallic acid equivalents (GAE) per 100 g of fresh weight (f.w.) and was analyzed with a Folin–Ciocalteu assay after the extraction of 1 g of pulp with 10 mL of 80% methanol. In the same extract, the antioxidant capacity was determined using the free radical DPPH (2,2-diphenyl-1-picryl hydrazyl) [35], and the results are expressed as milligrams of Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) equivalents (TE) per 100 g of f.w. Sugar contents were determined via high-performance

liquid chromatography (HPLC) [36], with a Waters 2690 HPLC module equipped with a differential refractive index detector (Waters Corporation, Millford, MA, USA), using a Waters Carbohydrate Analysis column (3.9 × 300 mm) and acetonitrile/water (80:20) as the mobile phase. The content of each sugar is expressed as grams per 100 g of f.w.

#### *2.8. Statistical Analysis*

Data were analyzed using SPSS version 25.0 (SPSS Inc., Chicago, IL, USA) with a one-way ANOVA (Duncan's multiple range) in homogeneous groups established by the dependent variable (peeling, film type, and color of prickly pear), assuming significant differences when *p* < 0.05.

#### *2.9. Ethical Statements*

All subjects gave informed consent for inclusion before they participated in the study. The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Brazilian Ethics Committee under number 845,894/2014.

#### **3. Results and Discussion**

The white prickly pears were bigger and heavier than the orange ones (140 vs. 87 g, respectively) (Figure 1). The yield was greater when fruits were electrically peeled: 70.7% (WE) versus 44.0% (WH) and 66.5% (OE) versus 40.8% (OH). The times needed by one of the regular consumer panelists to peel the white and orange prickly pear were 11.9 and 8.7 s, respectively, and the times needed by the peeler were 15.1 and 12.4 s, respectively. However, it was found to be faster to use the electric peeler to produce 1 kg of edible pulp for both varieties (214 s (WE) and 186 (OE) vs. 270 s (WH) and 213 (OH)). Thus, e.g., the production in a food processing company of 1000 trays of 250 g per day will need an operator working for 15 and 13 h using an electric peeler or 19 and 15 h using a knife for peeling white and orange prickly pears, respectively.

Moreover, electric peeling has more advantages because factory workers can operate more than one machine at the same time. In fact, using the same example, time can be reduced by 3 times when the operator places three fruits in three peelers (3 h for (WE) and 4 h for (OE)) or by 4 times (4 h for (WE) and 3 h for (OE)) when using four, which would increase company profits.

#### *3.1. Microbiological Analysis*

WE and OE presented significantly higher aerobic mesophile loads than WH and OH once prepared and after 8 days of cold storage, regardless of the type of film used, except for the white prickly pear with the 90PPlus film (Table 1). Psychrophilic values were generally higher when prickly pears were electrically peeled than those of hand-peeled pears until the 6th storage day. On the 8th day, only OE90P showed significantly higher values than the hand-peeled pears. The presence of mold and yeas<sup>t</sup> was greater in orange prickly pears from day 3 than in white prickly pears. After 8 days of storage, WH showed higher levels of these microorganisms than WE under both films, and in orange prickly pear, OH90P had higher (*p* < 0.05) mold and yeas<sup>t</sup> counts than OE90P.

Differences were detected in the growth of microorganisms depending on the film used (Table 1). Thus, the mesophile load was higher (*p* < 0.05) in the samples packed with 90PPlus film for WE, and it was higher in WH (except for the 8th day) and OE (from the 1st day) samples packed with the 180PPlus film. The WE and OH samples packed with 180PPlus film showed higher psychrophile loads, and the OH samples with the 90PPlus film showed significantly higher mold and yeas<sup>t</sup> counts than those with the 180PPlus film. Likewise, significant increases were observed in all microorganisms´ loads over time, regardless of color, peeling type, and film used. Nevertheless, the number of aerobic mesophiles in both white and orange prickly pear varieties were within the limits (7 log(CFU/g fresh weight)) regulated in Spain for ready-to-eat fruits and vegetables by the Real Decreto 3484/2000 [28] from the day of preparation until day 8 of cold storage.

Likewise, the counts of psychrophilic bacteria, molds, and yeasts were below 6 log(CFU)/g in all the treatments from the preparation until day 8 of storage (Table 1). Cefola et al. [37] detected an increase in mesophile and psychrophile growth after 13 days of storage when prickly pears were packed in a passive modified atmosphere, and the growth was higher when they were stored at 8 ◦C compared to 4 ◦C. Their final values were similar to our data. Furthermore, Palma et al. [38] and Piga et al. [12] reported a remarkable proliferation of microorganisms during storage time.

**Table 1.** Evolution of aerobic mesophiles, psychrophiles, and mold and yeas<sup>t</sup> in white and orange minimally processed prickly pears manually or electrically peeled and packed in two films of different permeability during cold storage at 7 ◦C.


Hand-peeled (H) and electrically peeled (E) white (W) and orange (O) prickly pears packaged in 90PPlus (90P) and 180PPlus (180P) film. Different letters in a row indicate that there were significant differences between storage days (*p* < 0.05), and different numbers in a column indicate that there were significant differences between samples (*p* < 0.05).

#### *3.2. Sensorial Evaluation*

The panelists detected differences in the appearance, color, flavor, and odor at the beginning (0 days) in both prickly pear varieties, reporting higher mean values for the hand-peeled fruits (*p* < 0.05) (Table 2). No significant differences were detected in the sensory attributes at other storage times or between film packaging types.


**Table 2.** Mean values of the sensory attributes at the beginning.

Hand-peeled (H) and electrically peeled (E) white (W) and orange (O) prickly pears. Different numbers in a column indicate that there were significant differences between samples (*p* < 0.05).

In general, as shown in Figure 2, tasters slightly preferred the manually peeled prickly pears to the electrically peeled ones (7.1 and 6.5 on a 10-point scale, respectively), regardless of the studied variety. WH under both packaging films at any storage time were those with the highest purchase percentages (between 75% and 100%), and the electrically peeled pears (especially those packed in 180PPlus film) were the most rejected by the tasters (only 33.3% would buy them) after 8 days of storage.

The acceptance of the minimally processed fruit did not decrease with storage time. The variable that negatively influenced the product rejection was texture, specifically when the panelists found the fruit too slimy or hard.

#### *3.3. Gas Composition*

Figure 3 shows a clear drop in the O2 concentration and an increase in the CO2 concentration of all the trays during cold storage, trends that were more pronounced for those sealed with 90PPlus film.

Thus, according to the film permeability, regardless of the variety and peeling method, the trays with either white or orange prickly pears packed in 180PPlus film (more microperforated) presented higher O2 values and lower CO2 values than those packed in 90PPlus film (less micro-perforated). Likewise, the accumulation of CO2 inside the trays was greater in the OE for both packing films (90P and 180P), although these differences were not significant. Allegra et al. [39], Cefola et al. [37], and Piga et al. [12] detected significant increases and decreases (*p* < 0.05) in CO2 and O2 concentrations, respectively, during cold storage depending on the used film.

#### *3.4. Physico-Chemical Analyses*
